NATL INST. OF 5TAND & TECH

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PUBLICATIONS

IMBSIR 75-910

The Role of Chemical Kinetics in

Energy Conservation

H. M. Rosenstock

D. Garvin

J. T. Herron

W. Tsang

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

October 1975

Final

U S. DEPARTMENT OF COMMERCE

fOO

NATIONAL BUREAU OF STANDARDS

1973
I

I

i

I
NBSIR 75-910

THE ROLE OF CHEMICAL KINETICS IN

ENERGY CONSERVATION

H. M. Rosenstock

D. Garvin

J. T. Herron

W. Tsang

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

October 1975

Final

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Table of Contents

Executive Summary Page

Objectives of the Study

Findings

Recommendations

1. Combustion Modeling and Data Needs 1

1.1 Introduction 1

1.2 The Role of Modeling In this R&D Area 3

*• 1.3 Kinetics and Thermochemlcal Data Needs 4

References to Section 1 8

2. What Is Combustion? 9

2.1 The Hydrogen-Oxygen Flame 9

2.2 The Methane Oxygen Flame 10

2.3 A General Combustion Scheme 14

2.4 Relation to Industrial Processes 19

References to Section 2 20

3. Pyrolysls and Hydrogen Atom Chemistry 21

3.1 Mechanism 21

3.2 State of the Rate Data 23

References of Section 3 28

4. Oxidation Processes 30

4.1 Reactions of Atomic Oxygen 30

4.2 Reactions of Hydroxyl Radicals 40

4.3 Reactions of Hydrocarbon Free Radicals with

Atomic and Molecular Oxygen 40

References of Section 4 49

5. Pollutant-Forming Reactions 50

5.1 Formation of Nitric -Oxide in Combustion 50

5.2 Formation of Nitric Oxide from Fuel Nitrogen 51

5.3 Formation of Oxides of Sulfur from Fuel

Sulfur 51

References to Section 5 53
6. Resources for Combustion Kinetics Data Base: '

Evaluated Data and Prediction Schemes 54 . 6.1 Introduction 54

6.2 Major Data Compilations and Evaluations

Pertinent to Combustion Chemistry 56

6.3 The Prediction of Reaction Rate Data-General 57

6.4 Specific Methods for Estimating Rate Data 62

6.5 Evaluation of Experimental Rate Data for

i Use at High Temperatures 78

6 . 6 Summary 83

References to Section 6 84

7. Recommendations for an Approach 90

7.1 Introduction 90

7.2 Construction of a Combustion Kinetics Data

Base , 92

7.3 Pyrolysis and Hydrogen Atom Chemistry 95

7.4 Oxidation Processes 103

7.5 Pollutant Forming Processes ' ' -j^iQ [ 7.6 Scale and Scope of the Recommended Program 110

Appendix - Organizations Carrying on Combustion Modeling

Attachments - 1. I. Classman and W. A. Sirignano

"Summary Report of the Workshop on Energy

Related Combustion Research"

2. A. K. Oppenheim and F. J. Weinberg

"Combustion R&D - Key to Our Energy Future"
THE ROLE OF CHEMICAL KINETICS IN ENERGY CONSERVATION

EXECUTIVE SUMMARY

OBJECTIVES

Combustion is the primary means for transforming chemically stored

energy into useful forms. A major part of a strategy for energy conservation

is the improvement of performance of combustion based processes and

devices. Present day advanced design techniques make major use of

mathematical modeling of the devices and processes. These models require

substantial amounts of chemical data as input. The use of these modeling

techniques will play a major role in the R&D process in the near future.

It is the purpose of this study to:

1. Define the needs for chemical data by the user community of

equipment designers and chemical modelers, with special emphasis on chemical kinetics.

2. Assess the quality, completeness and usefulness of the existing

body of chemical data pertinent to this user group.

3. Formulate a plan for developing a carefully evaluated, extensive,

and consistent body of chemical kinetics data, i.e. a kinetics data

base, that will provide input information appropriate to the needs of

present-day and future combustion equipment and process modeling.

FINDINGS

Modeling

Mathematical modeling of various combustion processes and combustion

devices is a very active field. Twenty-eight organizations have so far

been identified that are active in this field. They include government,

non-profit, academic and industrial laboratories. The systems modeled

include flames, turbines, engines, rockets and chemical reactors.

Data Needs

Among the most important input data required for these models are information on the detailed chemical mechanisms and the rates of the

individual molecular processes comprising the chemical mechanisms. The

data required must be appropriate to the high temperatures generally

encountered in these systems. The chemical kinetics data required can be categorized in four classes:

a. Pyrolytic processes

b. Hydrogen atom reactions

c. Oxidative processes

d. Pollutant-forming processes
This body of kinetics data will be widely applicable to a diversity

of specific models.

In addition to this chemical information, data are required on pertinent thermochemical and transport processes.

State of the Present-Day Data Base

Although many resources are available, there does not exist at

present an adequate, publicly available data base for combustion modeling.

Many of the existing data have not been compiled and evaluated. Most of

the pertinent data is only valid for low temperature processes and

cannot be simply extrapolated to the high temperature region of combustion

processes. And, last, there are large gaps in the available information,

particularly in the area of hydrogen atom reactions and hydroyl radical

reactions. i

Improvement of the Data Base

In the near future the quality of the present data base can be

significantly improved by a systematic program of kinetics data compilation

evaluation and dissemination. However, numerous information gaps will

remain.

In order to provide the substantial amount of additional needed

information, an extensive experimental program must be undertaken as well. Experimental techniques are available for accurate measurement of

all classes of reactions of importance here. One technique, the shock

tube technique, is directly applicable to the temperature range of

importance. Two other techniques, kinetics-mass spectrometry and

flash-photolysis resonance fluorescence, require modification to carry

out studies in the appropriate temperature range.

Numerous empirical and semi-empirical schemes are available for

estimating rate constants for a number of the types of pertinent chemical

reactions. The reliability of the schemes is often good enough to fill some of the gaps provisionally, and to serve as a screening tool for

deciding which reactions must be accurately measured. The entire group

of estimation schemes needs more careful evaluation and testing, using

an appropriate data base.

RECOMMENDATIONS

It is recommended that a program be initiated, consisting of three

parallel activities.

1. An activity of data compilation, estimation, evaluation and

dissemination, leading to periodic publication of an improved data base.

2. An activity of accurate experimental measurement of mechanisms

and rates of selected pyro lysis and hydrogen atom reactions.
3. An activity of accurate experimental measurement of selected

oxidation processes.

4. An activity of experimental measurements directed at elucidating

the mechanisms and rates of formation of oxides of nitrogen and sulfur

in combustion.

The data activity would ensure the effective utilization of all

experimental data forthcoming from other laboratories.

The experimental activities would provide information not only for

filling important gaps but also key information for improving the usefulness

of selected estimation schemes, thus minimizing the need for additional

measurements

.

An effective level of effort for the above program would require 5-

10 man years per year for a period of 3-5 years.

/

The Role of Chemical Kinetics in

Energy Conservation

1. Combustion Modeling and Data Needs

1. 1 Introduction

Many strategies and approaches to energy conservation have

been and are being developed. The approach proposed here is a program

to systematically develop an accurate, comprehensive and consistent bo4y

of kinetic and thermochemical data pertinent to the quantitative description,

control, and engineering development of devices and processes based on

the combustion and pyrolysis of fossil fuel.

This approach is based on the following premises:

(a) A significant energy saving will result from improvements

in the efficiency of high temperature devices required for transportation

and heating.

(b) The improvement in these devices and engineering development

of alternative devices must be carried on in such a way as to meet

present and future restrictions on pollutant emissions.

(c) It is likely that the restrictions on emissions will pose

more and more acute problems as dirtier and more complex fuels are

employed. In fact, the safe and efficient use of dirty fuels is largely

equivalent in effect to energy conservation.

(d) The entire process of research and development in this

area is becoming more and more strongly dependent on mathematical modeling

of the complex physical and chemical phenomena occurring in these devices.

(e) There is, in consequence, an urgent need for an appropriate

kinetics and thermochemistry data base to provide essential input information

for these models.

1
(f) This single body of data can serve simultaneously in

support of a great variety of engineering developments and for similar

developmental activities carried on in various establishments, such a

single body of information will vastly improve the reliability and

comparability of various modeling approaches.

1.1.1 Transportation and Heating

One of the major points of emphasis today is the effort to

improve the fuel economy of automobiles while at the same time retaining

performance and meeting pollutant emission restrictions. To this end,

active research and development efforts are undeirway ranging from minor

modifications of the existing engines to evaluation and development of

other engine concepts including stratified charge, rotary engines,

turbines, variable displacement, improved performance sensing and control,

etc. An essential element in this R and D process is the measurement,

quantitative description and control of the combustion process in such a

manner as to meet most economically the sometimes conflicting requirements

of efficiency, performance, emission control and noise (Oppenheim and

Weinberg, 1974; Glassman and Sirignano, 1974). It has been pointed out

that each one-percent increase in automobile engine combustion efficiency

leads to an annual saving of about 14.5 million barrels of oil per year.

An analogous problem area is the improvement of the efficiency

of combustors such as are employed in furnaces, steam generators and

heating systems. About 22% of the nation's energy consumption is for

space heating, almost all of it supplied by gas and oil (Hottel and

Howard, 1971). This class of devices is now being subjected to detailed

mathematical analysis in which much of the same physical chemistry

information is required (Palmer and Beer, 1974).

2
1.1.2 Future Emission Restrictions

The continuation of restrictions on pollutant emissions is

generally accepted. Details are given by Hottel and Howard (1971).

1.1.3 Dirty Fuels

One environmental problem caused by dirty fuels is SO^ pollution

associated with use of high sulfur coal or petroleum products. A

second is the generation of nitrogen oxide pollutants from combustion of

organic nitrogen compounds which are very likely to be present in, for

example, shale oil. In addition, there is presently a question concerning

the formation of so-called "prompt nitrogen oxide" at unexpectedly early

times in the combustion process. The mechanism of formation is still

not understood.

1.2 The Role of Modeling in this R and D Area

Mathematical modeling is not new. It is one and the same as

the quantitative considerations that have been employed by process

designers and design engineers for decades. The thing that is new is

that mathematical and computer capabilities have reached a stage of

development such that real processes and real systems are now beginning

to be quantitatively described in terms of the real physical and chemical

processes that are going on (Cornelius and Agnew, 1972). It is now

possible to describe or simulate real systems with a degree of faithfulness

that makes these models an essential complement to, and to an ever

increasing extent, a substitute for costly experimental pilot studies.

Simply stated, modeling is not a curiosity. It is a very cost effective

tool. Examples of real systems which are being modeled include chemical

reactors, jet engines, stratified charge engines, combustors, flames,

3
etc. (Spalding, 1972; Mellor, 1972; Edelman, Fortune and Weilerstein,

1972). Modeling studies are being carried out in many organizations In

the private sector, in the public sector, and in universities. In the

universities the work is often done in engineering departments.

A partial list of organizations carrying out modeling activities

pertinent to this energy utilization problem is given in the appendix.

1 . 3 Kinetics and Thermochemical Data Needs

Any meaningful model of a reacting chemical system must incorporate

appropriate kinetic and thermochemical data for the species and reactions,

and, in addition, the transport properties of the species, a description

of the flow conditions and flow fields, and a statement of the boundary

conditions. Considerable advances have been made in the computer solution

of realistic flow problems of at least a prototype character, and data

and estimation schemes exist for pertinent transport properties. We

turn now to the chemistry.

Many models of combustion processes treat the multiplicity of

detailed chemical events in a global phenomenological fashion. This is

partly a matter of computational convenience and partly a matter of

ignorance. The detailed mechanism of, say, isooctane combustion is not

yet known. Even the oxidation mechanism of methane to CO^ and is

not fully worked out. Reaction schemes including up to several

dozen steps have been proposed and utilized with varying degrees of

success. The point we wish to make is that for any carbon-containing

combustion system from methane on up the detailed sequences and loops of

reactions remain to be firmly established. The apparent complexity

4
of this situation need not, however, lead to grim pesslssism for the

following reasons. When viewed at the level of the elementary

processes, many combustion situations involving a diversity of hydrocarbon

fuels will proceed in large part through the same set of chemical reactions.

Information on mechanisms, energetics and rates of elementary processes

will have a transfer value to many combustion situations, and

the same information is completely applicable to the elucidation of the

mechanistic schemes as well. It is thus to be expected that the value

and range of applicability of the elementary process information will

increase rapidly with time. Lastly, the earlier sketch of modeling has

been given in terms of description of engineering prototype systems.

There is also a significant area of experimentation and modeling of

semi-complex chemical reactors such as pyrolyzers and oxidizers. These

devices are not intended to serve as engineering prototypes but rather

as tools to develop phenomenological data. Here, too, the modeling of

these semi-complex phenomena in terms of the actual chemical processes

is essential in assessing the range of applicability of the phenomenological

data to new engineering situations. In fact studies in this area will

be essential to validating the correctness of the complex reaction

schemes and the associated thermochemical and rate information.

It is evident from the above that the thermochemical and

* kinetics data needs cannot be completely specified at this time. The

needs depend on the details of the exact mechanisms, and the elucidation

of the exact mechanism is aided by the available data. We remark further

that a mechanism states not only which reactions occur, but also which

reactions do not occur and, in a more qualitative way, which reactions

5
dominate the chemical picture and which are side issues. And the side

issues assume a central importance when one is concerned with pollutant

emissions.

As for the kinetics data, at this point we note that the

classes of processes that must be considered include thermal decomposition

reactions of fuel molecules and their reactive intermediate products,

the reaction of all these with oxygen atoms and molecules and with

hydrogen atoms and hydroxyl radicals. Also, in considering pollution

aspects the rates of processes leading to the formation of oxides of

nitrogen and sulfur will have to be considered.

We now turn to the thermochemical data. In order to describe

the energy transport of the real (or semi-complex) system it is necessary

to have available the enthalpies of the species involved. Since the

unstable reactive intermediates are present only in small concentrations

they affect this picture only through their reaction rates and mechanisms.

However, the reactive species enthalpies and free energies are of crucial

short term importance for the kinetics aspect of this problem.

As will be discussed below, the body of available reaction

rate data is at present very incomplete both in regard to specific

elementary processes and to applicable temperature range. Tables of

reactive species enthalpies and free energies are very useful to establish

rapidly preliminary estimates of specific reaction rates. They are also

of considerable use in delineating the complex reaction schemes and

providing some guidance as to which rates need more accurate experimental

definition. Needless to say, they are also extremely useful in deciding

which reaction rates are negligibly small, i.e. they serve a screening

function. A second and important use of reactive species thermochemical

6
data is to provide equilibrium constant data. These are used to compute

rates of a particular reaction where only the rate of the reverse process

is known. This is a frequent occurrence. Although the data are scattered

and not fully evaluated there appears to be no urgent need for a comprehensive

compilation at the moment. This situation may change in the future.

In the following sections we will outline the current state of

the information in these areas, along with an assessment of rate constant

estimation schemes. Finally, we will outline the data, data estimation,

measurement and measurement technique development needs appropriate to

this problem area and suggest a possible program for meeting these

needs.

7
REFERENCES TO SECTION 1

Cornelius, W., and Agnew, W. G,, editors, "Emission Prom

Continuous Combustion Systems," (Plenum Press, 1972)

Edelman, R. B., Fortune, 0., and Wellersteln, G., "Some

Observations on Flows Described by Coupled Mixing and

Kinetics," In "Emission From Continuous Combustion

Systems," W. Cornelius and W. G. Agnew, editors (Plenum

Press, 1972)

Glassman, I., and Slrlgnano, W. A., Summary Report of the

Workshop on Energy-Related Basic Combustion Research,"

Rept. #1177, Energy-Related General Research Office,

Research Directorate, National Science Foundation,

Guggenheim Labs., Dept. Aerospace and Mechanical Sciences,

Princeton University, Princeton, NJ (197^)

Hottel, H. C, and Howard, J. B., "New Energy Technology —

Some Facts and Assessments," (MIT Press, 1971)

Mellor, A. M., "Current Kinetic Modeling Techniques for

Continuous Plow Combustors," In "Emission From Continuous

Combustion Systems," W. Cornelius and W. G. Agnew, editors

(Plenum Press, 1972)

Oppenhelm, A. K., and Weinberg, P. J., "Combustion R&D

Key to Our Energy Future," Astronautics Aero. 12, 22 (197^)

Palmer, H. B., and Beer, J. M., "Combustion Technology: Some

Modern Developments," (Academic Press, 197^)

Spalding, D. B., "Mathematical Models of Continuous Combustion,

in "Emission From Continuous Combustion Systems,"

W. Cornelius and W. G. Agnew, editors (Plenum Press, 1972)

8
2. What Is Combustion ?

2 . 1 The Hydrogen-Oxygen Flame

From the standpoint of generating usable thermal energy by

means of oxidation of a fuel the very simplest example Is the oxidation

of hydrogen. There are fifteen elementary reactions which must be

considered to account for the properties of the hydrogen-oxygen flame

containing excess hydrogen. These are listed below along with their

heats of reaction. When two moles of hydrogen react with one mole of

oxygen to produce two moles of water, there are 485 kJ/mol of energy

released. The rate at which a flame will propagate is determined primarily

by the rate at which this energy can be released. In a rich hydrogenoxygen flame the predominant radicals are hydrogen atoms. Reactions 1,

2, and 3 are important radical generating steps. Although they do not

release heat to the flame they provide a major channel for the consumption

of hydrogen and oxygen. For an atmospheric pressure flame, the most

important heat releasing reactions are 10 followed by 11, 14, and 15.

Even though 10 is a three-body combination reaction it is very fast

simply because oxygen is at a much higher concentration throughout most

of the flame zone than that of any other radical. While 11 and 14,

which rapidly consume the HO^ produced by 10 also generate radicals,

reaction 15 removes radicals from the flame. The other important radical

destroying reaction is 7 which also releases a large amount of energy.

The key reactions in the system are therefore 1, 2, and 3, which produce

radicals but which release little energy, reactions 10, 11, and 14 which

also produce radicals but in addition release energy, and finally reactions

15 and 7 which release energy but destroy radicals.

9
AH(298 K)

1. + OH = + H -oJ KJ/moi

2. H + 0^ = OH + 0 /i . U

oJ. + 0 = OH + H Q 0/1 0 . /4

H„0

z + 0 = OH + OH 71

c H + OH + M = H^O + M — HyO

0 . 0 + 0 + M = 0^ + M AO/.

7

.

H + H + M = H^ + M /. T C

oo

.

0 + H + M = OH + M /. 0 7

H^ + 0^ = OH + OH 7Q 1 /y . i

10. H + 0^ + M = HO^ + M

1 Q 7 -ly /

11

.

TM2 + H = OH + OH -ijy

«°2 + 0 = OH + 0^ — Z JU

1 J. "°2 + OH = H^O + 0^ -jUi

14. »°2 + H = H^C + 0 -230

15. »°2 + H = H^ + 0^ -238

Using these it has been possible to model quantitatively this

flame; to compute temperature profiles, radical concentration

profiles, the energy flux, and the flame speed; and to gain an understanding

of the key processes and cycles through which the flame may be inhibited.

2.2 The Methane-Oxygen Flame . .

The next most simple flame is the methane-oxygen flame. The

following reactions are typical of the sets used in various models of this

flame. From a chemical point of view it is highly unlikely that

more than about one half of these are really significant in any given

model. And it is probable that some other reactions will have to be

added in order to arrive at a complete description of a specific combustion

situation of this system. It should be noted that this set includes the

reactions of the hydrogen-oxygen flame.

10
1. CH + CHO ^ CH^ + CO

2. CH + CH^O -> CH^ + CHO

3. CH + CH^ -y CH^ + CH^

4. CH + CH, ^ CH^ + CH_

4 z J

5. CH + CO^ ^ CHO + CO

6. CH + H + M t CH^ + M

7. CH + OH -> CHO + H

8. CH + OH CH^ + 0

9. CH + OH -> CO + H2

10. CH + HO^ CHO + OH

11. CH + HO^ CH^ + 0^

12. CH + "2 CH^ + H

13. CH + H2O -> CH^ + OH

14. CH + 0 CO + H

15. CH + 0 + M CHO + M

16. CH + 0^ CHO + 0

17. CH + 0^ -> CO + OH

18. CHO +M^CO+H+M

19. CHO + CHO -y CH^O + CO

20. CHO + CH^ -> CH^ + CO

21. CHO + CH^ CH^ + CH^O

22. CHO + CH^ -> CH, + CO

3 4

23. CHO + CH, -> CH^O + CH^

4 2 3

11
24. CHO + H -> CH^ + 0

25. CHO + H + M ^ CH^O + M

26. CHO + H CO + H^

27. CHO + OH CH2O + 0

2.8. CHO + OH CO + H^O

29. CHO + H2 CH^O + H

30. CHO + H^O -V CH^O + OH

31. CHO + 0 -V CO + OH

32. CH^ + CH, ^ CH_ + CH.

4 3 3

33. CH2 + OH CH3 + 0

34. CH^ + H^ -> CH3 + H

35. CH^ + H^O -V CH3 + OH

36. CH2 + O2 -> CH2O + 0

37. CH2O + H CH^ + 0

38. CH^O + H + M -> CH^O + 1

39. CH^O + OH CH^ + 0^

40. CH3 + H + M -> CH, + M

4

41. CH3 + OH -> CH, + 0

4

42. CH3 + HO^ CH^ + O2

43. CH3 + H^ -V CH, + H

2 4

44. CH3 + H^O -y CH, + OH

2 4

45. CH3 + 0^ ^ CH3O + 0

46. CH,

4

+ OH CH3 + H^O

47. CH,

4

+ 0 > CH3 + OH

48. CH,

4

+ H -> CH3 + H^

49. CH,

4

+ HO2 -> CH3 + H^O^

50. CH,

4 + O2 -> CH3 + HO2
51. CO + OH -> CO + H

52. CO + HO- > C0_ + OH

z z

53. CO + H-0 -V C0_ + H_

Z Z z

54. CO + 0 + M -> CO + M

z

55. CO + 0- -> CO + 0

z z 56. H + H + M->H +M z

57. H + OH -> H„ + 0

Z

58. OH + H + M H-O + M

Z

59. H + HO- ^ OH + OH

Z

60. H + HO- ^ H- + 0-

z z z

61. H + HO- -> H-0 + 0

z z

62. H + H-0 -> OH + Hz z

63. H + 0 + M->OH+M

64. H + 0- -V OH + 0

Z

65. H + 0. + M -> HO- + M

z z

66. OH + OH ^ H- + 0-

z z 67. OH + OH H-0 + 0

Z 68. OH + HO- ^ H-0 + 0-

z z z

69. OH + 0 + M -> HO + M

Z

70. OH + 0 HO + 0

z z 71. 0 + 0 + M-»-0. + M

z

72. H_ + OH -V H^O + H

z z 73. H„ + 0 -)- OH + H

Z

74. H_0 + 0 OH + OH

Z 75. H^ + 0^ OH + OH

z z 76. HO^ + 0 -> OH + 0^
2 . 3 A General Combustion Scheme

We arrive at a general scheme by noting that more complex

fuels introduce more thermal decomposition reactions and subsequent

reactions of the radicals produced with the oxidizers. The scheme

illustrated in figure 2.1 shows the chemistry leading to the formation

of CO. The cyclic nature of the combustion process should be noted. In

particular, each cycle of the process has the effect of reducing the

length of the hydrocarbon chain of the fuel molecule. Ultimately carbon

monoxide is formed.

The great number of individual reactants has made necessary

their grouping into four major categories, hydrocarbons hydrocarbon

radicals, oxygenated organics and oxygenated organics radicals. Furthermore

the even larger number of possible reaction paths dictates the consideration

of classes of reactions. These include, abstraction, addition,

decomposition, combination disproportionation and isomerization. We

note that the necessity of considering reactant and reaction classes

at this early stage of the discussion suggests that the only visible

strategy at establishing the required data base must follow the course

of carrying out of a selected number of key experiments and then the

development of correlational and prediction schemes so as to render

such data applicable to all systems of interest.

14

ABSTRACTION (1) (2)

Hydrocarbons

Oxygenated Organics

t 1 R02' ( \ RO^H

RO- ROH

HO^* HO^H

R.

OH"

>^ <

RH

0* OH

H- «2

.0, , .HO ,

> + radicals

ADDITION (1)

Hydrocarbons + ^

Hydrocarbon radicals

(unsaturated) IHADDITION (2)

Hydrocarbons

(unsaturated)

, 0* ; ^ epoxide

ADDITION (3) (4)

Hydrocarbons

(unsaturated)

Oxygenated Organics

(unsaturated)

r •> RO ' ROT

<H02->

0-

°2

OHoxygenated

organic

radicals

I

16
COMBINATION AND DISPROPORTIONATION (1)

R • + •

R^H + olefin

Hydrocarbon

Radicals

COMBINATION AND DISPROPORTIONATION (2)

R^(0)- + R2'

Oxygenated Organic Radicals

+

Organic Radicals

R^(0) R2

R^(0)H = olefin

R2H + Unsaturated Oxygenated Hydrocarbon

Oxygenated Organics

Oxygenated Organics + Hydrocarbon

COMBINATION AND DISPROPORTIONATION (3)

R^(0). + R2(0)

Oxygenated Organic Radicals

R^(0)R2(0)

R^(0)H + Unsaturated Oxygenated Hydrocarbon

Oxygenated Organics

DECOMPOSITION (1)

Hydrocarbon Hydrocarbon Radicals

DECOMPOSITION (2)

R* (Hydrocarbon Radical) olefins + Hydrocarbon Radical

17
DECOMPOSITION (3)

R^R^CO) -> 1 ^2

*

R^COR^CO) -> R^(0) • + R^CO) •

'

Oxygenated Organics Hydrocarbon, Oxygenated Organic Radicals

DECOMrObi i iUIN (.4;

-

' - , k

D D f <W X D • 1 12 3

Oxygenated Organic -> Oxygenated Hydrocarbon + Organic Radical

Radicals R^(0) • + olefins , 2 i! Oxygenated

Organic Radicals

ISOMERIZATION (1) (2)

R

'

R(0)- R(0)-'

Radicals Radicals (from H-shift and cyclization)

MOLECULAR DECOMPOSITION

V2 Unsaturate + Unsaturate

Unsaturated

Compound

18
2 . 4 Relation to Industrial Processes

The discussion above has been given in terms of combustion

only. The data base as envisioned here is obviously applicable to a

wide range of Industrial pyrolytlc processes. These processes are

undergoing rapid development to adjust to new feedstock and product

requirements imposed by our energy and petroleum situation. For example,

total U.S. production of ethylene in 1974 was 23.6 billion pounds (Ponder,

1974). At a unit price of 7c/lb., this is a contribution of $1.6

billions to the U.S. economy. Furthermore the coproducts of the ethylene

production process such as propylene and butadiene also make significant

contributions. All these compounds are important feedstocks for the

chemical industry. Their costs influence the costs of an enormous

variety of consumer products.

Substantially all the ethylene and significant quantities of

the propylene and butadiene are currently produced through the thermal

decomposition of light paraffins (ethane, propane and butane) or naphtha

(higher moleculer weight paraffins, including cyclic compounds and

aromatics) (Woodhouse et al., 1974). However, the shortage of natural

gas and uncertainty with respect to the availability of naphtha has led

to the consideration or in some cases the use of heavier fractions such

as gas oil or vacuum distillates. In addition, the general increase in

the price of all feedstocks makes imperative the optimization of all

such processes. Important constraints are thus set on the design and

operation of these plants. It is true that the chemical industry does

have a vast store of experience and empirical data which can be brought

to bear on this problem. However, the availability of kinetic data for

a realistic model should prove to be invaluable as a guide for the

extension and application of the existing results. Two recent symposia

of the American Chemical Society have addressed this general problem.

19
REFERENCES TO SECTION 2

Ponder, T. C, "U.S. Olefins through I98O," Hydrocarbon

Processing 51, 104 (1974)

Woodhouse, G., Samols, D., and Newman, J., "The Economics

and Technology of Large Ethylene Projects," Chem. Engr. 8I,

73 (1974) . . —
3. Pyrolysls and Hydrogen Atom Chemistry

3.1 Mechanism

The pyrolytic decomposition of hydrocarbons proceed for the

most part via formation of free radicals. (Leathard and Purnell, 1970).

There are a limited number of hydrocarbons which decompose via molecular

mechanisms. In general these can be identified on the basis of a few

simple rules. For the others it is possible to visualize the decomposition

process in terms of the formation, decomposition, isomerization, transfer,

addition and combination processes involving the appropriate free radicals.

(a) Formation of free radicals : (Kerr, 1973; O'Neal and

Benson, 1973; Benson and O'Neal, 1970; Leathard and Purnell, 1970; Frey

and Walsh, 1969) . The initiating reactions in the chain decomposition

of hydrocarbons must involve bond breaking reactions. Carbon-carbon

bond (R^CH2 - CH2R2) splits will in general be favored over carbonhydrogen (R^R2R2C-H) cleavage. The most labile of the former are those

joining radicals which possess some degree of resonance stabilization

(R^CH=CHCH2 - CH2R2). In a second category are the highly substituted

paraffinic C-C bonds, (R^R2R2C - CR^R2R2) followed by those without

substitution. Similar considerations apply to carbon-hydrogen bonds.

The strongest are the carbon-carbon bonds adjacent to multiple bonds

(R^CH=CH - R2) and the multiple bonds (R^CH=CHR2) themselves. These

need be considered only in very high temperature situations.

(b) Decomposition of free radicals ; (Kerr, 1973; Kerr and

Trotman-Dickinson , 1964; Kerr and Lloyd, 1968; Kerr and Parsonage,

1972) . The hydrocarbon radicals created as a result of bond splits can

also be classified in order of their thermal stability. Starting with

the least stable, there are first those alkyl radicals with C-C bonds

21
once removed from the free electron (R-j^CH2 - CH2CH2*). Cleavage reactions

of these C-C bonds have very low activation energies. Molecules undergoing

these reactions have lifetimes at 1000 K of 1 ijsec or less. Thus except

at low temperatures or under conditions where concentrations of reactive

species are unusually large this will be the main reaction path. In the

cases where there is no 6 C-C bond ((CH^)^C*) cleavage of the analogous

C-H bond can occur. Here activation energies are of the order of AO-60

kJ/mol higher. Thus such radicals have longer lifetimes with regard to

decomposition and a greater tendency to undergo other reactions. Resonance

stabilized radicals are less labile. (Tsang, 1973) They are however, not

particularly reactive except with respect to combination. This distinguishes

them from small radicals such as H, CH^, CH2, etc. which do not decompose

but are also very reactive. ;

(c) Isomerization of free radicals : (Kerr, 1973; Leathard and

Purnell, 1973). Large organic radicals isomerize by 1, 4 and 1, 5

hydrogen atom transfer (RCHCH2CH2CH2 -> RCH2CH2CH2CH2 • ; RCH(CH2)2 ^

R(CH2) ^CH2 * ) . There have been reports of 1, 2 and 1, 3 hydrogen shifts,

but it is suspected that these arise from misinterpretation of experimental

data. Unsaturated radicals can also undergo cyclization reactions. As

before preference appears to be for reaction paths which involve 5 or 6

membered cyclic transition states.

(d) Transfer and addition processes : (Kerr, 1973; Kerr and

Parsonage, 1972; Trotman-Dickinson and Milne, 1967; Kerr and TrotmanDickinson, 1964). Free radicals can abstract hydrogen atoms from stable

compounds or add to unsaturates. The general order of reactivity is Hatoms, alkyl radicals and resonance stabilized radicals. With respect

to abstraction by methyl and ethyl at low temperatures, the relative

rates are tert-H > sec-H > primary-H > methane. _ This is probably also

the case for hydrogen-atoms. Hydrogen atom addition to unsaturates

leads to vibrationally excited radicals. This means that at sufficiently

22
low pressures the newly formed radicals will decompose at a rate faster

than the thermal rate.

(e) Combination and disproportionation of radicals : (Kerr,

1973; O'Neal and Benson, 1973; Benson and O'Neal, 1970). These represent

the termination steps in hydrocarbon decomposition. Rate constants are

characteristically large. The most rapid are the reaction of H atoms

with organic radicals. Radical-radical reactions are somewhat slower.

Most recent studies suggest that highly substituted alkyl radicals may

have rates of combination lower than hitherto expected. There are

empirical correlations relating disproportionation and combination

rates.

From the above discussion it is clear that we have a very good

mechanistic picture of how large hydrocarbons decompose into smaller

olefins. There remain however large gaps in our knowledge with respect

to the formation of soot or carbon in these systems. That is, at some

stage polymerization and dehydrogenation must occur. Unfortunately at

the present time there is no convincing explanation for the processes

which precipitate these reactions.

3.2 State of the Rate Data

3.2.1 Bond-Breaking Reactions ; Work at NBS using a single-pulse

shock tube technique has resulted in the collection of a large volume of

data on these processes (Tsang, 1969, 1972). Furthermore correlations

and extrapolation techniques have been developed and the results have

been corroborated from independent studies. The overall situation is

satisfactory. The required rate data exist or can be generated as the

need arises. One area which does need some immediate attention is the

rate and mechanism of decomposition of 5 and 6 membered cyclic alkanes.

These are important components in naphtha and the higher boiling oils.

It should be noted that these rate data are of broader significance
than merely their immediate application to chain initiation. Thus if

the thermal properties of organic radicals are known, then an application

of the principle of detailed balancing yields the appropriate rate of

combination, i.e. chain termination. Furthermore the activation energy

is directly relatable to the enthalpies of the radicals (Benson and

O'Neal, 1970; O'Neal and Benson, 1973). These form the basis for

extrapolation and estimation schemes for the determination of the rates

of a variety of other processes. There is a need for a general review

of the subject. Earlier reviews are obsolete.

3.2.2 Radical Decomposition : Practically all of the existing data

are from studies on alkyl radical decomposition (Kerr, 1973; Kerr and

Trotman-Dickinson , 1964). The principal methods used to determine the

rates for these processes are:

(1) Direct Studies: These yield thermal rates. Radicals are

generated from the photolysis of a variety of precursors (aldehydes,

ketones, azo compounds), from the photosensitized decomposition of '

paraffins, or from the radical sensitized decomposition of aldehydes and

paraffins, and the rate of decomposition is compared with the rate of

combination.

(2) Chemical Activation Studies: (Spicer and Rabinovitch,

1970). These yield lifetimes of vibrationally excited alkyl radicals.

Aside from their own intrinsic importance rate parameters for the thermal

reaction can be derived on the basis of the thermochemistry and calculations

using the Rice-Ramsperger-Kassel-Marcus theory of unimolecular reactions.

(3) Detailed Balance: (Kerr and Parsonage, 1972). There

exists a large volume of data on the addition of methyl radicals to

olefins (see below). Combining this with the thermochemistry permits

calculation of the decomposition rate.

24

)
The overall quality of these data is uncertain since there

appear to be considerable discrepancies. On the other hand at the

higher temperatures this may be immaterial since rates must be extremely

fast. What is needed is a set of very accurate numbers for the branching

ratios in the cases where there are multiple reaction paths. Some of

the data may be extractable from the chemical activation experiments.

With regard to resonance stabilized radicals there is a single

pulse shock tube study of the decomposition of isobutenyl radical

(Tsang, 1973) . This indicates that although it is more stable than

alkyl radicals its rate of decomposition is appreciable at temperatures

in excess of 1000 K. Thus at high temperatures the rate of decomposition

of such radicals must play an important role in the overall kinetics.

Clearly there is a need for more data on such systems and for other

unsaturated radicals such as those with the free electron adjacent to

the multiple bonds.

3.2.3 Isomerization of Free Radicals : Data on such processes are

limited and may not be of high accuracy. The methods used are similar

to those described in Section 3.2.2. The overall rate is fast and it is

suspected that the key factor is not the absolute rate but the competition

between isomerization and decomposition. More data are needed.

3.2.4 Transfer and Addition Reactions : There exists a vast store of

kinetic data on hydrogen atom transfer and addition reactions of methyl

radicals with hydrocarbon molecules (Kerr, 1973). Data on other alkyl

radicals are somewhat more limited but this may not be of serious consequence

except at the lowest temperatures since their short lifetimes preclude

their taking part in subsequent reactions. Practically all of these data

are determined relative to the recombination rate. This is not a serious

problem and in some cases may be an asset since in many physical situations

this ratio is the important quantity. More serious is the fact that all

of the data are taken at or close to room temperature. Thus for use at

25
combustion or pyrolysis temperatures extensive extrapolations are required.

In recent pyrolysis experiments 800°K) (Leathard and Purnell, 1970)

activation energies for abstraction by methyl have turned out to be

considerably higher than the low temperature (A55 K) numbers. Straightforward

Arrhenius extrapolation appears to be unjustified. With respect to Hatoms there are satisfactory numbers for H addition to unsaturated

compounds at room temperature. For higher temperature the question of a

proper extrapolation procedure becomes important again. With respect to

abstraction there is a review of the data on H + CH^ (Walker, 1968).

For all other systems there is little worthwhile data. It is suspected

that in any realistic model the relative rates of hydrogen atom reactions

are of crucial importance. This is because they are very fast and the

hydrogen atoms will not have an opportunity to recomblne. Thus in a

combustion system, for example, the choice is between H +

0^ HO^ and

H + 0^ -> OH + 0 (which ultimately produce water) , or reaction with the

hydrocarbon substrate leading to pyrolysis, or an endothermlc reaction.

3.2.5 Combustion and Disproportlonatlon Reactions : There are extensive

measurements of methyl radical combination at room temperature. At the

present time there is a spread of a factor of 2 in the available results

(Kerr, 1973). The recent determinations are by kinetic spectroscopy

(Bass and Laufer, 1973) while the earlier ones are by the rotating

sector technique. The sector technique has also been used to measure the

rate of combination of methyl, ethyl, Isopropyl and t-butyl radicals

(Kerr and Trotman-Dlckinson, 1964). All investigations were carried out

at room temperature. The measurements on the three large alkyl radicals

all yielded numbers close to the collision rate. These numbers are now

regarded with suspicion because rate constants calculated

26
from those for bond breaking reactions of the appropriate alkane are on

order of magnitude or more smaller (Tsang, 1969) . This conclusion is

supported by measurements on the rates of combination of these radicals

as compared with those of methyl radicals (Hiatt and Benson, 1972).

Nevertheless there remain many unexplained effects. The real need is

for direct measurements of radical combination rates at a variety of

temperatures.

The brief survey given above demonstrates that despite the

gaps there is a considerable volume of data available for use. Unfortunately

they have not been acquired with the present application in mind. In

general there has been no attempt at acquiring data in a systematic

manner; thus it is difficult to devise correlations or extrapolations.

Particularly striking is the absence of high temperature data. However

this is not due to any intrinsic problems with the measurement technique,

and a systematic program for the accumulation of the needed data can be

expected to yield all the desired results.

27
REFERENCE? TO SECTION 3

Bass, A. M., and Laufer, A. H., "The Methyl Radical Combination

Rate Constant as Determined by Kinetic Spectroscopy,"

Intern. J. Chem. Klnet. 5, 1053 (1973)

Benson, S. W., and O'Neal, H. E., "Kinetic Data on Gas Phase

Unimolecular Reactions," U.S. National Bureau of Standards,

NSRDS-NBS-21 (1970) U.S. Government Printing Office,

Washington, DC 20^02

Prey, H. M., and Walsh, R. W., "The Thermal Unimolecular

Reactions of Hydrocarbons," Chem. Reviews 69., 103 (I969)

Hiatt, R., and Benson, S. W., "Rate Constants for Alkyl Radical

Recombination. I. Isgpropyl Radicals," Intern. J. Chem.

Klnet. 4, I5I (1972)

Kerr, J. A., "Rate Processes in the Gas Phase," in "Free

Radicals," J. Kochi, editor (John Wiley and Sons, New York,

1973) 1

Kerr, J. A., and Parsonage, M. J., "Evaluated Kinetic Data on

Gas Phase Addition Reactions," (Butterworths , 1972)

Kerr, J. A., and Lloyd, A. C, "Decomposition Reaction of

Radicals," Quaterly Review 5^9 (1968)

Kerr, J. A., and Trotman-Dlckenson, A. F,, "The Reactions of

Alkyl Radicals," in "Progress in Reaction Kinetics, Volume 1,"

(Pergamon Press, Oxford, 1964)

Leathard, D. A., and Purnell, J. H., "Paraffin Pyrolysis,"

Ann. Rev. Phys . Chem. 21, 197 (1970)

O'Neal, H. E., and Benson, S. W., "Thermochemistry of Free

Radicals," in "Free Radicals," J. Kochi, editor, volume 2

(John Wiley and Sons, New York, 1973) 275

Spicer, L. D., and Rabinovitch, B. S., "Elementary Gas Reactions,

Ann. Rev. Phys. Chem. 21, 197 (1970)

28
Trotman-Dlckenson, A. P., and Milne, G. S., "Tables of Gas

Phase {^molecular Reactions," U.S. National Bureau of Standards

NSRDS-NBS-9 (1<)67) U.S. Government Printing Office,

Washington, DC 20402

Tsang, W., "Thermal Decomposition of 3

, '4-Dimethy lpentene-1

2, 2, 3-Tr.imethylpentane, 3 , B-Dimethylpentane and Isobutylbenzene in a Single Pulse Shock Tube," Intern. J. Chem.

Kinet. 1, 2^5 (1969)

Tsang, W., "Thermal Deocmposl tion of 3 > ^-Dimethy Ihexane

,

2, 2 , 3-TrlTaethy Ipentane-ter t-Buty Icyclohexane and Related

Hydrocarbons," J. Phys. Chem. 7^, 143 (1972)

Tsang, VJ., "Pyrolysls of 2, 4-Dimethylhexene-l and the Stability

of Isobutenyl Radicals," Intern. J. Chem. Kinet. 5, 929 (1973)

Walker, R. W., "Activation Energies of the Reversible Reaction

between Hydrogen Atoms and Methane to Give Hydrogen and

Methyl Radicals," J. Chem. Soc., 2391 (1968)

29
4 . Oxidation Processes

In the oxidation of hydrocarbons and their 0, N, and S derivatives

the primary step is either direct attack by a free radical species or

pyrolysis followed by subsequent reaction of primary products (stable or

free radical in nature)

1. RH + 0,H,0H,H02, ^ R + OH .H^O ,H202 ,

2. RH + M Radicals, olefins, , + M

3. R + 0^,0, OH, -> Radicals, olefins,

This section is concerned with discrete elements of reactions

(1) and (3). The class of reactions represented by (2) has been discussed

in section 3.

4.1 Reactions of atomic oxygen

The reactions of atomic oxygen with hydrocarbons and their

derivatives have been systematically studied at NBS over the past seven

years, and a large body of rate data has been accumulated over the

approximate range 250-650 K. Reviews of the available rate data (Herron

and Huie, 1973) and the general nature of the reactions have been given

(Huie and Herron, 1975). Recommended values for rate constants are not

given at temperatures greater than 1000 K, since they are dependent

on extrapolations of data from lower temperature measurements. Aside

from the question of using these data for accurate modeling, it is

doubtful in many cases if they are adequate even for the purpose of

deciding which are the principal reaction paths.

In terms of high temperature rate measurements the data on 0 +

methane (all kinetic data and figures in this section are from Herron

and Huie, 1973) shown in Figure 4.1, are the most extensive. The data

30
n

0 + Methane

T

10 14

I0'3

• Westenberg and deHaas [75]

O Brown and Thrush [29]

Wong and Potter [4]

Cadle and Allen [5] 1 Azatyon et al [I]

A Avramenko et al [6]

Froben [84]

I0>2 -

7 10"

o

^ iO'O

E

o

10^

10 8

10^

10^

0 1.0 2.0

lO^/T, K"'

3.0

4.1 Arrhenius plot for the reaction of atomic oxygen with

methane (Herron and Huie , 1973)

31
look good and there is no indication of curvature. However, the best

data in this plot (Westenberg and de Haas) were taken by a technique

(electron spin resonance) that requires knowledge of the overall chemist

and this introduces a note of real uncertainty into the meaning of the

rate measurements. (See Herron, 1969, for a discussion of this problem.)

For ethane, illustrated in Figure 4.2 there is again a good

linear fit, although in this case careful examination of the data of

Westenberg and de Haas suggests a small degree of curvature. As in the

case of the methane data, there is a question as to the meaning of the

rate measurements. The flame data of Azatyan, et al. clearly suggest

curvature starting near 1000 K.

For larger alkanes, for example n-butane, shown in Figure 4.3,

the data are scattered and extrapolation to higher temperatures becomes

a very risky procedure. This is typical of the kind of data available

for the larger alkanes.

In the course of the oxidation and pyrolysis of hydrocarbons

significant quantities of olefins will be formed and subsequently react

with atomic oxygen and other free radicals.

Figure 4.4 shows the data for ethylene. The very accurate

resonance fluorescence data of Davis, et al. give a very good linear

Arrhenius plot over the range 230 to 500 K. At higher temperatures the

available data strongly deviate from the extrapolation of the lower

temperature resonance fluorescence data. There is no way of deciding if

this effect is real or due to experimental error.

The plot of the 1-butene data, on the other hand. Figure 4.5

is clearly curved well below 1000 K. The question, in this case, is

whether the curvature is due to a change in mechanism (competition

between abstraction and addition), to experimental error, or to a real

temperature dependence of a specific reaction channel.

32
o

£

fO

E

o

lOVT, K"

4.2 Arrhenius plot for the reaction of atomic oxygen with

ethanr (Herron and Huie , 1973).

33
4.3 Arrhenlus plot for the reactions of atomic oxygen with

n-butane (Herron and Huie, 1973).

34
35
toO

•H>G

(UMOa

•Heo0

cnC

m o r>. •H cyi U iH O

CO

CU

m

u •H3

01 ffi u 13C u CO o

1+1

CO

uo l-i

aw 3

•H c c 0)

0) 4J3 u 1 <m

36
To compound the problem. Figure 4.6 shows data for cls-2-

butene In which a negative slope is observed. At this time it is

impossible to even estimate the rate of this reaction at T > 1000 K. I

should be noted that other reactions with small activation energies may

behave in a similar manner, e.g., 0 + NO^ and N + NO.

The other reaction to be considered in this section is 0 +

acetylene, shown in Figure 4.7 . The data appear to be linear, but do

not go beyond 700 K. Curvature at higher temperatures is a possibility

37
1^

to

4,12

C <D

0) "o k.

«» o O o

a>

*> *>

CD1 CJ

0) oc oc

if) (/) o o > g g o > > Q UJ UJ o o O • + iQ

CVJ

togXo o

00 o

CD S .JOLU

I- I ujo •>! oOO

cJO

go

roO

<u00oo • •H /-^ e <^ OU 0^ O QJ •H c m o

CO

0) o

0) )-i ^ (U

4-1 o <u Mm C

(1)

O 3

rH ^

CU I CM

<:

38
I

1 1 1 r 0 + Acetylene

I I I I I I I 0 1.0 2.0 3.0 4.0 5.0 6.0

lO^/T.K"'

A. 7 Arrhenius plot for the reactions of atomic oxygen

with acetylene (Herron and Huie, 1973).

39
Not only are the rate constants for the atomic oxygen reactions

of great uncertainty at T > 1000 K, but the reaction mechanisms are not

well established In many cases. It Is probably safe to assume that the

alkanes react only by hydrogen atom abstraction. However, the unsaturated

compounds are known to have many available reaction channels (Hule and

Herron, 1975). These are known only at room temperature. It Is not

known which reaction paths are dominant at high temperatures. In particular,

the ratio of abstraction to addition is of fundamental importance.

4.2 Reactions of hydroxyl radicals

The kinetics of hydroxyl radical reactions have been reviewed

(Wilson, 1972). As noted in section 6.5 the recommended value for OH +

CH^ appears to be much greater than the one based on more recent experimental

data. The same may be the case for the other alkanes, although great

caution must be taken in so interpreting the available data. The discrepancies

may arise due to curved Arrhenlus plots as may be seen from examination

of Figures A. 8 and 4.9. At this point there is no single set of reliable

data to make such a determination for any alkane.

For ethylene and acetylene the situation is worse, as shown in

Figures 4.10 and 4.11. Even allowing for recent additional measurements,

it is not certain whether these plots are curved or whether the slopes

are positive or negative.

In terms of mechanism, the same questions arise as for atomic

oxygen. There are several competing reaction channels active at room

temperature. The nature of the products formed at high temperature are

as yet not known.

4 . 3 Reactions of hydrocarbon free radicals with atomic and

molecular oxygen

^ i ; 40
T(K)

2000 1000 700 500 400 300

0 12 3

ioVt(k)

4.8 Arrhenius plot for the reaction of hydroxyl radicals

with methane (Wilson, 1972).

41
8

OO

ro

8

in

O

O

o

8

O

o

o

CM

CVJ ^

O

fO CM

(HO + *H^O)°'M Bo|

42;
a

•H

13

03U

Ou

o

•H

J-l ^O CN

t-l

0) " ^ C u o

(Si u <-\ O -H

O OJ -H C

a (U w >^

•H 4-1 C tU

u u

U -H < 5
oooooo

lOoooooooo

ro

CVJ ^

ro CVJ

/->.

rC

t-H

Oc c o o

•H C/3 4-1 iH u H

TO

0) ^

0)C

(U

4-1 rH

>s

u

U o (Uo

TO

4-1 o

rHu

•H> 3 CO •H rH c TO

(U

O

•H•n

u TO < u

rH

44
The kinetics and mechanisms of reaction of hydrocarbon free

radicals with atomic and molecular oxygen at combustion temperatures are

essentially unknown. The rate constant for 0 + CH^ is known only at

room temperature (Huie and Herron, 1975). For other radicals no direct

data are available. The mechanisms of radical-oxygen atom reactions

have been reviewed recently (ibid). For most reactions, several competing

reaction paths have been postulated, as illustrated in Table 4.1, but

there are few reliable data. This is an area in which application of

unimolecular rate theory could make a major contribution in predicting

branching ratios as a function of temperature.

The reactions with molecular oxygen are not significantly

better known. Baldwin and Walker (1973) have recently reviewed the data

from the point of view of oxidation of hydrocarbons at moderate temperatures

(300 to 800 K) . The reactions considered by them are:

4. R + 0^ + M -> RO^ + M

5. R + 0^ -> olefin + HO^

6. R + 0^ carbonyl + OH

7. R + 0^ cyclic + OH

for which rate constants or rate constant ratios are known in a few

cases at a limited number of temperatures. See, for example. Table 4.2

which lists data on reaction (5) (Baldwin and Walker, 1973). This

appears to be the most important reaction in the R + O2 series. Reaction

(6) may have a high activation energy ("^ 80 kJ mol "'") and there is

considerable uncertainty as to whether this reaction is of any importance

in combustion, and if so, at what temperature. Reaction (7) involves

addition and isomerization followed by decomposition:

45
Table 4.1 Exothermic Reactions of Atomic Oxygen with

Some Free Radicals (Huie and Herron, 1975).

Radical Intermediate Prodvicts kJ inol

CH,

CH,F

CHF2

CF3

HC2O

CH3CO

C2H5

n-CaHv

[CH3—O]^

[CH2F—O]*

[CHF2—O]*

[CF3—O]*

LH—C=C=OJ

[

' ]

LCHa—C-=0J

H

CH3—C—

O

I H

H

I CH3—CH2—C—

O

1 H

H3CO

H2+CHO

H+CH2O

H+H2+CO

CH2FO

HF+CHO

H+CHFO

CHF2O

HF+CFO

H+CF2O

CF3O

F+CF2O

H+2CO

CH3CO2

CH3+CO2

CH3CH2O

H+CH3CHO

CH3+CH2O

OH+C2H4

CH3CH2CH2O

H+CH3CH2CHO

C2H5+CH2O

OH+C3H6

-375

-361

-291

-284

(-377)"

(-477)"

(-377)"

(-431)"

-328

(-435)

-478

-385

-305

-332

-266

-377

-310

-346

-277

iso-CsH? O

I CH3-C—CH3

I H

CH3CHOCH3 -375

H+(CH3)2CO -323

CH3+CH3CHO -347

OH+C3H6 -264

n-C4H9 H

CH3-CH2-CH2-C-O

I H

CH3CH2CH2CH2O -377

H+CH3CH2CH2CHO -302

C3H7+CH2O -345

OH+CH3CH2CH=CH2 -276

46
u

rHO

<u rHo

•Hw

toO

•H

a; p«. ON

0) rH

ft,co

TOUo

•V

T5

rH

CO BQB

Situ rH

Xi«

CO

.2

-2 IS

Oho o o oo oo oo oo o o o o

iiO lOs X XO

III oo oo O CO X Xo

00 ^

•4 o+wo X

Xo oo

1^ 00 o o oX

00

COoo X

CO CO »-H lO

OSot-i

1m ooo

00

0)

«4 O

X

CO ooo•a ^Mo o o XXX

00 1-1

X

CO

1-H C<l C^'

"I

11 ± % I

«4 o

Ok oI •<«» ^7
7'. R + 0^ RO^ -> R'OOH -> cyclic hydrocarbon + OH

The rate of this reaction depends on the strengths of the various C-H

bonds In the alkyl radical and the ring strain in the transition state.

Here again the experimental data are sparse. These classes of reaction

all require a great deal of both experimental and theoretical effort to

gain a satisfactory knowledge of the kinetics and mechanisms.

A8
REFERENCES TO SECTION 'I

Baldwin, R. R., and Walker, R. W., "Problems and Progress

in Hydrocarbon Oxidation," Symp . Combust, l^lth (Combustion

Institute, Pittsburgh, 1973) 241

Herron, J, T., "An Evaluation of Rate Data for the Reactions

of Atomic Oxygen (OP) with Methane and Ethane," Intern. J.

Chem. Kinet. 1, 527 (1969)

Herron, J. T., and Huie, R. E., "Rate Constants for the

Reactions of Atomic Oxygen (OP) with Organic Compounds in

the Gas Phase," J. Phys. Chem. Ref. Data 2, 467 (1973)

Huie, R. E., and Herron, J. T., "Reactions of Atomic Oxygen

o (0-^P) with Organic Compounds," Prog. Reaction Kinet. 8_, 1

(1975)

Wilson, W. E., Jr., "A Critical Review of the Gas-Phase

Reaction Kinetics of the Hydroxyl Radical," J. Phys. Chem.

Ref. Data 1, 535 (1972)

49
5. Pollutant - Forming Reactions

The formation of combustion generated pollutants such as NO

and SO^, is to a large extent a kinetically controlled process. For this

reason, modeling of such systems, particularly in the case of NO, represents

one of the most active areas of combustion modeling.

5 . 1 Formation of nitric oxide in combustion

It is generally accepted that the formation of NO in post

flame gases is governed by the extended Zeldovich mechanism

1. 0 + N^ ^ NO + N

2. N + 0^ NO + 0

3. N + OH ^ NO + H I *r- Because of the high activation energies of these reactions and the short

time scale of combustion processes, the NO level never reaches its

equilibrium value.

However, the observations of Fenimore that rates of formation

of NO at the combustion zone (so-called "prompt" NO) were much greater

than could be accounted for by this mechanism has led to considerable

new research in this area. Two different suggestions have been made.

One is that the excess rate is due to a superequilibrium concentration

of free radicals in the combustion zone (Bowman, 1973) and the other

that other reactions, i.e., those involving fuel derived species generate

NO, (Iverach, et al 1973) e.g.,

A. CH + N^

:J HCN + N

5. + N^ ^ 2CN

6. C + N^ -V CN + N

In fact both mechanisms may contribute depending on the fuel/air

ratio.

50
An even more complex picture has been suggested by Merryman

and Levy (1974) on the basis of flame probe experiments. They find NO^

to be the dominant NO^ species in the flame zone, whereas NO dominates

downstream. They suggest the reactions below as an explanation.

7. NH and/or CN + 0^ ^ NO + OH and /or CO

8. NO + HO^ -> NO^ + OH

9. NO^ + 0 ^ NO + 0^

Clearly, the mechanisms whereby NO is formed in combustion are

not well understood. An added complication is that NO can be formed

from fuel nitrogen.

5 . 2 Formation of nitric oxide from fuel nitrogen

The range of compounds associated with fuel nitrogen is not

well defined. Most researchers have picked fairly simple compounds such

as ammonia, pyridine or an amine to study the formation of NO from fuel

nitrogen. The complete mechanism must involve pyrolysis for some nitrogen

containing fuels, followed by reaction with atomic oxygen, hydroxyl

radicals, molecular oxygen, etc. However, many heterocyclic type

compounds will not readily pyrolyze, and can be expected to enter the

combustion zone and react with free radicals. The problems here are

identical to those described earlier in reference to the reactions of

hydrocarbons with atoms and free radicals. The difference is that there

is almost no base of information on the chemistry of such processes.

These classes of reactions have yet to be studied.

5.3 Formation of oxides of sulfur from fuel sulfur

The general approach to the control of SO^ produced in the

combustion of coal or sulfur rich oil has been to remove the sulfur

51
before burning, or to use stack gas cleaning processes. The basic

question as to how SO^ or SO^ is formed has not been addressed. There

is an inadequate data base from which to carry out modeling studies on

sulfur oxidation, and to consider the importance of chemical kinetics in

the formation and potentially, the control of SO^. The review by Levy,

et al. (1970) discusses the role of various reactive intermediates such

as SO, S2O, and (SO)^. The role of SO in the oxidation process has yet

to be established although it is readily observed in the combustion of

sulfur containing fuels and may be the key intermediate in the sulfur

oxidation process.

The sources of sulfur in the fuel will vary from mineral bound

sulfur in coals to organic bound sulfur in both coal and oil, but if the

initial processes lead to the formation of SO, a common kinetic base can

be used for subsequent modeling and control strategies.

52
REFERENCES TO SECTION 5

Bowman, C. T., "Kinetics of Nitric Oxide Formation in Combustion

Processes," Symp. Combust. 14th (Combustion Institute,

Pittsburgh, 1973( 729

Iverach, D., Basden, K. S., and Kirov, N. Y., "Formation of

Nitric Oxide in Fuel-Lean and Fuel-Rich Flames," Symp.

Combust. 14th (Combustion Institute, Pittsburgh, 1973) 767

Levy, A., Merryman, E. L., and Reid, W. T., "Mechanisms of

Formation of Sulfur Oxides in Combustion," Environ. Sci.

Techn. j4, 653 (1970)

Merryman, E, L., and Levy, A., "Nitrogen Oxide Formation in

Flames: The Roles of NO^ and Fuel Nitrogen," Symp. Combust.

15th, Abstracts of papers (Combustion Institute, Pittsburgh,

1974) 209

53
6. Resources for Combustion Kinetics Data Base: Evaluated

Data and Prediction Schemes

6.1 Introduction

The resources now available are summarized here. They are of

several types.

(a) Evaluations of experimental data with recommended rate

constants. The chemical kinetics program of the Office of Standard

Reference Data, NBS , is devoted to the preparation and publication of

such work. There are analogous and complementary programs in the UK and

the USSR. The reference data programs in these three countries have

supported almost all of the modern chemical kinetics data evaluation

work.

The two principal on-going data centers for evaluation of

chemical kinetics measurements are the High Temperature Reaction Rate

Group at the University of Leeds, England and the Chemical Kinetics Data

Center, Physical Chemistry Division, NBS. Continuing liaison is maintained

between these centers.

(b) Methods for predicting rate data. These have been

developed by individuals and published as regular scientific articles.

A group at the Stanford Research Institute has made many important

contributions

.

(c) Original measurements. These are published as regular

scientific articles. A substantial fraction of these articles has been

collected by the Chemical Kinetics Information Center, NBS. This collection

is available for use in developing a data base.

54
These resources are viewed as a starting point for the development

of a comprehensive publicly available combustion kinetics data base.

They will make it possible for an initial version of the data base to

include values for many of the reactions of interest.

Reliance on existing evaluations in the early stages of the

development of a data base means that essentially all of the output of

the world-wide data evaluation effort will become readily available to

designers and engineers.

Expansion of the data base beyond that point will require work

of several types in many laboratories.

(a) More evaluations - principally for major reactions that

have not been examined by evaluators and for cases where new data are

available.

(b) New measurements - to resolve conflicts in the data, to

fill gaps and extend the usefulness of existing data.

(c) Major use of prediction schemes to extend existing data

to reactions that have not been studied.

6.1.1 Topics treated in this chapter

In section 6.2 are listed the major data compilations upon

which any data base must draw. This forms a major resource. Sections

6.3 and 6.4 are a review of methods for predicting rate data. There are

many methods. They are described in some detail here because many

chemists are not aware of the extensive developments in this area of

kinetics - developments that suggest that prediction schemes will be of

increasing importance in the future. Section 6.5 is a discussion of a

major data evaluation problem - how to extrapolate low temperature

kinetics data up to combustion temperatures. A summary concludes the

chapter.

55
I

6.2 Major data compilations and evaluations pertinent to combustion

| chemistry. These are listed by author and topic. Unless otherwise

noted the compilations include recommended values and cover data applicable

in the range 300-1000 K.

Benson and O'Neal, (1970). Evaluated data on over 500 unimolecular

reactions. This is the basic source for rate data on the pyrolysis of

organic compounds.

Herron and Huie, (1973). Compilation and evaluation of rate

data for reactions of 0 atoms with 107 organic compounds in the gas

phase.

Wilson, W. E. , (1972). Reactions of the hydroxyl radical.

Herron, J. T., (1969). Evaluation of rate data for reaction

of oxygen atoms with methane and ethane.

Baulch, Drysdale, Home and Lloyd (1972, 1973). Two volumes

of evaluated rate data for the hydrogen/nitrogen/oxygen system. Data

for temperatures from 300 K to the upper end of the experimental range

are included. Often this includes the combustion regime.

Drysdale and Lloyd, (1970). Hydroxyl radical reactions.

Kerr and Parsonage, (1972). A comprehensive survey of evaluated

kinetic data on gas phase addition reactions.

"Chemical Kinetics Data Survey" (1973-75). NBS rate data

tables and evaluations, principally for reactions pertinent to atmospheric

chemistry near room temperature but including many radical-molecule

reactions. r ,

Allara, D. , (1973). Rate data evaluation and estimates for

475 reactions of interest in combustion studies, for temperatures near

700 K. ' .... ,

56
Other compilations are listed in NBS List of Publications No.

73 which is a catalog of the world-wide activity in the area of kinetics

data evaluation.

6. 3 The Prediction of Reaction Rate Data - General

6.3.1 Introduction

In a complex chemical system such as combustion many chemical

reactions will occur or will have to be considered in developing suitabl

models for equipment. These reactions may be important in energy

conversion, pollutant formation and control, blow-out phenomena, soot

formation, effects of fuel additives etc.

Inevitably many of the reactions of interest will not have

been studied in the laboratory under conditions suitable for application

to the problem at hand. Prediction schemes offer a method for filling

the gaps prior to making accurate measurements.

Why should prediction schemes by considered an important

resource? As more accurate measurements of kinetics data are made it

becomes apparent that similar chemical reactions have similar mechanisms

and similar rate parameters. Prediction schemes reproduce this "normal"

behavior and in many cases provide accurate means for extending the

existing information to similar molecules. In many cases this should be

sufficient for screening purposes for deciding which reactions should be

included in a model. In some cases the predictions may be good enough

to eliminate the need for measurement. This is most likely to be true

for reactions of large organic molecules.

Some cautionary remarks are in order. First, the object of

prediction schemes is to come close to reproducing good experimental

data but not to do this exactly. It is more important to predict

57
k to within a factor of 2 to 5 for a type of reaction than to pin down

an accurate value for an important reaction. That must be measured.

Second, no one scheme does the job for all reactions. Third, some types

of reactions are not treated well by any method. Fourth, the comparison

with experiment for each method have been done separately and at different

times. There has been no systematic cross-comparison of methods. Good

high temperature data for such comparisons are rare. Fifth, activation

energy predictions are less important for reactions at high temperatures

than are predictions of entropies of activation. That is, errors of + 8

kJ/mol have much less effect above 1000 K than do errors in entropies of

activation. f

A general characterization of prediction methods is given in

section 6.3.2. This is followed by a summary of terms and definitions

used in kinetics section 6.3.3. Then twenty methods that have been

proposed are described in section 6. A. It should become apparent that

the prediction of rate parameters is an active and successful, although

small, branch of chemical kinetics. But it must be emphasized, in line

with the general remarks above, that much more systematic work is needed

in this area. This must go hand-in-hand with experimentation on reactions

that can extend and delimit the utility of the various methods.

6.3.2 Types of methods for the prediction of rate constants

These should all be considered to be empirical. But almost

always they have some tie to a theoretical model (often very tenuous)

and frequently they are calibrated against experiment. They fall into

several classes. •

(a) Correlations of rate constants within a series of related

reactions. Examples:

(1) Reactions of oxygen atoms with alkenes

:

0 + RCH=CHR' -> products.

58
(2) Simple bond fissions: R-R' -> R* + R- ' These can

be entirely empirical and based entirely on kinetic

measurements.

(b) Correlations of rate constants or rate parameters with

* thermodynamic quantities (or other physical properties). Example: E =

11.5 - 0.25 AH (The Evans-Polanyi-Semenov relation).

(c) Theoretical (?) relationships relating the rate constants

of reactions within a class.

Example: Prediction of the rate constant for

R- + R' -> RR' from those for R- + R- RR and R- ' + R- ' R'R'

(d) Establishment of a typical transition-state structure for

t t a class of reactions and then calculation of AS and AH according to a

set rule.

Example: Unimolecular decomposition reactions using

group additivity techniques.

(e) Definition of a reaction coordinate (path from reactants

to products) and then calculation, according to rule, of the geometry of

the transition state, its properties and the energy barrier. Example:

Bond-energy bond-order method for atom transfer reactions.

(f) Generalized rate expressions of an empirical nature.

Needless to say there are some methods that could be placed in

more than one of the classes described above. Also there are some that

do only part of the job described for a particular class.

6.3.3 Terminology and Principles

A brief statements are given here defining important terms and

summarizing principles that are invoked in the estimation of rate data.

(a) Rate of reaction

Rate = k(T) • f (c)

where k(T) is the rate constant, a function of temperature and f(c) is

59
a function of reactant and product concentrations.

(b) The function of concentrations

f (c) may be complex for a complex chemical process consisting

of many steps. However, for each of the individual steps it is a very

simple product of reactant concentrations.

(c) Elementary reactions

These are the individual molecular transformations that

combine to make up the mechanism of a complex process, such as combustion.

The rate data for elementary reactions are transferable from one complex

system to another. Overall rate data are not.

It is a primary function of kinetic studies to determine the

mechanism, the elementary steps, that occur in a complex process. Only

after this is done are rate data for elementary reactions useful.

(d) The rate constant, k(T)

Experimentally this is usually represented by the Arrhenius

equation, shown here in extended form

k(T) = A T^ exp(-E*/RT)

b ^ where AT is the "pre-exponent ial factor" and E is the "activation

energy".

The terms. A, b^ and E^ are usually considered as constants, b^

often is preset at zero, but may be in the range -2.5 < b < 2.5. E

usually is positive (it is an energy barrier) but may be zero or, for

curve fitting, slightly negative.

(e) Absolute rate theory form of k(T)

This formulation is more informative and is used in most

prediction schemes. For a bimolecular process

A + B -> [A B]^ -> C + D

k(T) = ^ exp (AS"^/R) exp (-AH^/RT)

60
where AS is the "entropy of activation" a measure of molecular configuration

change in going from reactants to the "transition state" [A B]^,

AH is the "enthalpy of activation", the potential energy

barrier to be overcome in forming the transition state,

K is the Boltzman constant,

and

h is the Planck constant.

In principle, AS^ and AH^ are weak functions of temperature.

Other types of reactions such as unimolecular decompositions

and addition reactions have similar rate constant expressions.

(f ) "Intrinsic" activation energy or enthalpy of activation

This term means the activation energy for an exothermic

reaction, one that releases energy. It is entirely kinetic in origin.

This is the quantity estimated in prediction schemes. The activation

energy for an endothermlc (heat absorbing) reaction is the intrinsic

activation energy plus the heat of the reaction.

(g) Rate constants and equilibria

For the reaction system

A + B i C + D

at equilibrium the rate of the forward reaction (arrow running to the

right) equals that of the backward reaction.

= k^[A][B] = k^[C][D] = R^

kf/k^ = [C][D]/[A][B] = K^^

This expression is often used to calculate k from k^ (or

r f

vice-versa) In prediction schemes.

61
0 . 4 Specific Methods tor Estimating Rate Data

6.4.1 Unimolecular Reactions: A ^ B, A -> B 4- C at "high" pressures

This class of reactions Includes almost all of the types of

processes that occur in pyrolysis of organic molecules: simple bond

fissions, free radical decompositions, isomerization , molecular elimination

(complex fission) and ring opening.

The prediction method has been developed by S. W. Benson and

co-workers. It has been reviewed by Golden (1973), is developed in

detail in Benson (1968) and is exploited in Benson and O'Neal (1970).

These references and individual papers give examples of the application

of the method. .,

The method uses absolute rate theory as a model. For each

class of reactions a general transition-state configuration is determined

whose calculated entropy of activation is consistent with experimental

data. Then, for individual reactions detailed calculations of the

entropies of activation are made using the class model and molecular

parameters. Enthalpies of activation are not calculated by this method.

In the special case of simple bond breaking reactions AH^ is taken as

the enthalpy of reaction. Thus this method, closely tied to experiment,

scales the estimates to match a class model and properties of individual

reactions. The estimates are made using a detailed "group additivity"

method that is an extension of the very successful technique used for

estimating thermochemical quantities developed by Benson. (Benson et

al. 1969; O'Neal and Benson, 1973).

The accuracy of the estimates of rate constants is difficult

to assess because many of the available data are poor. Pre-exponential

factors based on the entropies of activation are probably good to a

factor of two in most cases. The accuracy of the rate constant estimates

* See Table 6.1 for a summary of the methods described here.

62
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64
is also limited by the inaccuracies of our knowledge of bond strengths,

or, equivalently , the enthalpies of formation of free radicals.

6.4.2 Unimolecular reactions at lower pressures

These decomposition and rearrangement reactions, formally

written A -> B, all involve the transfer to A of sufficient energy to

split a bond, expel a molecular fragment, or cause isomerization. That

is, there is an energy transfer component. The simplest mechanism

showing this is

-y * A + M A + M (energy transfer)

* A B (reaction)

where M is any molecule in the system. Thus the overall rate constant

shows a pressure dependence and, ultimately there is a change from first

order to second order kinetics at very low pressure.

The experimental dependence of k on pressure (or total gas

concentration) is not simple. Since the 1920' s theories have been

developed to account for it. The current theory, the Rice-RamspbergerKassel-Marcus theory has been reasonably successful in reproducing the

behavior of many reactions. It has been applied as an empirical correlation

of data in evaluation work (Tsang, 1973). Such procedures will be

needed in combustion kinetics.

6.4.3 Radical combinations R" + R* • -v rr'

Almost all such measurements have been made at room temperature.

There is a major question about the temperature coefficient of such

reactions that must be resolved by measurement.

Prediction For combustion kinetics the most reliable technique

is calculation of the rate from the reverse, decomposition reaction and

the equilibrium constant.

A "cross-combination ratio" holds for the rate constants of

the related reactions

65
^11 R + R ^ R-R

2. R' + R' ^ R'-R'

1/2

and 3. R + R' ^ R-R' where k ^ 2 (k 'k )

Deviations from the factor of 2 are no more than 25% in comparisons with

experiment

.

This is a very powerful technique for extending measurements.

Given n rate constants of the type reactions (1) and (2) then n(n-l)/2

rate constants of the reaction type (3) may be calculated. With suitable

thermochemistry for the stable molecules this correlation can be applied

to the reverse bond-breaking reaction (Tsang, 1969).

A set of these rate constants that is sufficient for combustion

kinetics now exists for alkyl radicals.

Measurements may be required on other classes of radicals.

However many of them will undergo decomposition or oxidation in preference

to combination.

There are few data and no established methods for reliable

predictions for the important class of reactions: R + 0^ -> RO^ •

Ulitskii and Stepukovich (1963) used a model that set the

activation barrier for combination reactions as the difference between

a centrifugal energy and the polarizability of the molecule formed in R

+ R' -> RR' . The R-R' distance at the potential maximum is determined as

well as E , hence pre-exponential factors can be obtained. Only limited

comparisons were made with experiment, k'

s

shown agree to a factor of 2

to 5, - .

6.4.4 Addition reactions : RCH=CHR' + HX -> RCH^CHXR'

Predictions The method is to calculate the rate constant from

that for the decomposition reaction and the equilibrium constant. These

are "four center reactions". The basic model is a "semi-ion pair"

treatment (Benson and Bose, 1963; Benson and Haugen, 1965; Haugen and

66
Benson, 1970). It has been modified recently to overcome failure when

polar groups were present in the alkene (Maltman et al, 1974; TschuikowRoux and Maltman, 1975). In 55 comparisons with experiment, 28 activation

energies agreed to better than 4 kJ/mol and 19 more to better than 12

kJ/mol.

There is also a completely empirical correlation for dehydrohalogenation reactions that predicts rate constants to within 20-30%. It

is based on constancy of the pre-exponential factors and a correlation

between E and the ion dissociation energy of the alkyl halide (Tsang,

1964).

6.4.5 Addition reactions of radicals and atoms

H + RCH=CHR' -V RCH^CHR'

0 + RCH=CHR' ^ addition complex

HO + RCH=CHR' ^ addition complex

There are extensive compilations and evaluations of data for

reactions of this type (Kerr and Parsonage, 1972; Herron and Huie,

1973). The subject has been reviewed frequently over the past decade.

(For a recent review on 0-atom additions, see Huie and Herron, 1975).

Most of the measurements have been at relatively low temperatures,

< 500 K. Extrapolation to higher temperatures is very uncertain. The

model for the transition state has not been established unambiguously.

Prediction schemes of wide applicability are not available,

although correlations have been made with various molecular parameters

for limited sets of reactions. A correlation of activation energies

with the ionization potential of the alkene is fairly effective. Kerr

and Parsonage (1972) give a general discussion of the question.

Much more work is needed in this area, both measurements and

correlations. For high temperature chemistry perhaps the most important

67
kinetics problem to be solved is determining the relative efficiencies

of addition and abstraction.

6.4.6 Bimolecular atom transfer reactions (abstraction reactions);

These are of the type

R + A-R' ^ R-A + R'

where A is an atom and R and R' may be either atoms or groups.

A substantial number of techniques have been proposed for

predicting rate constants and activation energies. The major emphasis

has been on hydrogen atom transfer reactions, i.e., A = H. This is the

largest class of transfer reactions and the most important for combustion

kinetics. There is very little information on the transfer of oxygen

atoms, also very important. An experimental data base must be accumulated

for such reactions.

Methods that predict rate constants are described first, then

those that treat only activation energies.

(a) A Generalized Method for three atom systems where the

atoms are H, N and 0

This method (Benson et al, 1974) treats bimolecular atom

transfer reactions and combination reactions. It is intended to be used

only for high temperature reactions.

Universal rate constants are established that apply to an

entire class. For example, for the eight reactions

0 + OH ^ 0^ + H

H + HO ^ H^ + 0

'

N + NO i + 0

N + 0^ ^ NO + 0

the same pre-exponential factor is used. The enthalpy of activation

(for the exothermic direction) is either 29 kJ/mol or -4 kJ/mol.

68
The latter is used if the transition state has a stable molecule as its

analog. Stable means AH^^ the sum of the heats of formation of the

reactants minus 12 kJ/mol.

This method has been compared, successfully, against the high

temperature rate data evaluated by Baulch, Drysdale, Horne and Lloyd

(1972, 1973). In some cases the predictions are easily as representative

of the data as the evaluations. This may reflect more on the quality of

the data than on the evaluations.

Similar simple rules apply to the combination reactions.

These authors are extending the method to cover all the threeatom reactions in the H/C/N/0 system.

(b) Bond-energy bond-order method

This technique treats

R + A-R' RA + R'

where R and R' may be atoms or groups and A usually is H but, in variants

of the method, may be a halogen atom or an oxygen atom. No kinetic data

are used. The rate constant is calculated using molecular properties

and the formalism of absolute rate theory. Many of the features of this

method are found in other techniques to be discussed later. The scheme

(Johnston and Parr, 1963; Johnston, 1966) is as follows.

1 2 (1) A linear transition state is assumed: R H R'

.

(2) Three potential energy interactions are assumed, each

being of the form known for H-H and of a strength

controlled by intemuclear separations. Those

between R H and H R' are attractive and that

between R R' is repulsive.

69
(3) As one bond, H-R' , weakens, the other bond, R-H

forms in a manner such that the "orders" of

the bonds add up to unity (a single bond) . This

order relationship is based on a general rule

developed by Pauling.

(4) A closed algorithm embodying the three points above

permits calculation of the energy barrier and the

configuration (bond distances) in the transition

state. Given this configuration, the entropy of

activation is calculated using simplified absolute

rate theory.

(5) The calculation is repeated for several temperatures

over the range of interest. (The pre-exponent ial

factor is temperature dependent). Activation energies

calculated by this method agree with experiment to

within 8 kJ/mol in most cases. Rate constants

are often within a factor of 2. A factor of five

discrepancy is unusual.

The importance of this method is the complete

absence of kinetics data in the expressions used

for the calculation. Spectroscopic and thermochemical

t

-

1 data (bond strengths) are needed.

I ..

! One variant of this method is applicable to halogen

i

.

I

1

.

I atom transfer reactions. Another treats oxygen

i atom transfers and introduces several general

I postulates about reaction paths. (Mayer, 1969;

i

I Mayer et al, (1967)).

i

' - ^

.

70
Clark and Dove (1973) have introduced a method for

correcting for effect of the end groups R and R'

being multi-atom groups, e.g., CH^CH^' , and have

obtained good fits to experimental data for three

reactions when the repulsion term for R-R' is a

(slightly) adjustable parameter. Curvature in the

Arrhenius plots is noticeable. Arthur et al (1974)

also have developed a new repulsive energy approximation that leads to somewhat better agreement with

experiment for reactions of CH^ and CF^.

About 400-500 BEBO calculations have been reported

in the scientific literature (See, for example,

Mayer and Schieler, 1968; Carr, 1972). A systematic

comparison of the results of this method against the

evaluations of Baulch et al (1972, 1973) should be

made. It would give an indication of reliability

and a comparison with the estimation method of

Benson, et al. , described earlier,

(c) Activation energies from bond strengths and vibrational

frequencies

This is a linear transition state model for hydrogen atom

transfer reactions that bears a family relationship to BEBO but uses

a different definition of the transition state and different, non-kinetic,

input data. It has been presented and tested only as a method for

computing activation energies. However rate constants could be

calculated using standard absolute rate theory methods for

the pre-exponential term, since the geometry of the transition state

is defined. How well, or poorly, the rate constants would match with

experiment is not known.

71
Two formulations have been presented (Zavitsas, (1972);

Zavitsas and Melikian, (1975). The latter is described here.

(1) A linear transition state is assumed.

(2) For the transition state A—H B

bonding energies for A-H and H-B are calculated from Morse curves

based on observed dissociation energies and vibrational frequencies.

Antibonding between A-B is estimated using a Sato repulsive "Morse

Curve". A fixed delocalization energy term (for the odd electron

being on any of the three groups) is included. Its value is based

on that for the allylic radical, CH2=CH-CH2*.

(3) The bond distances in the transition state are

defined as those with equal bonding energy in A-H and H-B that produce

a minimum total energy for the three group system (measured from the

dissociation energy of reactant A-H). •

Comparisons are made with experiment for 83 reactions.

The overall statistics show the following performance.

No. of activation energies Range

26 Within spread of reported measurement

39 Less than 4 kJ/mol from range of

measurements.

7 Between 4 and 8 kJ/mol from

' measurements

.

11 - ' Greater than 8 kJ/mol from

measurements.

The authors discuss the causes of some of the discrepancies and apply

the method to revise bond strengths.

This is a method that should be studied in detail in order

to define its limits. .,

72
6.4.6 Methods for estimating activation energies for bimolecular

reactions

Techniques have been proposed and tested since the 1930' s.

Some are correlations of kinetics data and some are correlations of

activation energies with molecular properties. They all work best

for closely related reactions. Those that correlate E with

molecular parameters rely on the fact that molecular properties

vary in a regular manner for a series of closely related molecules.

(a) The Evans-Polanyi-Semenov correlation . This is

E* = A - b • q

where q = -AH, the heat released by the reaction (Evans and Polanyi,

1938) . Semenov (1958) showed that agreement with experiment to + 8

kJ/mol could be obtained for the complete set of atom-molecule reactions

available at that time. Various workers have correlated very closely

related sets of reactions to better than + 2 kJ/mol. It remains a very

useful method, although many alternatives have been proposed.

(b) The BEBO correlation . An empirical equation was

developed (Johnston, 1966, page 209, 347)

E*/E2 = (1 - q/E^) {1 -[1 + (l-q/E^)^^^"-^]^"^}

where q is the energy released, E2 is the energy of the bond formed

and p is an adjustable constant related to the "bond order". The

parameter p. is set using E and E2 for one of a related set of

* reactions. Then E 's for others are estimated using the known E2 s.

Trends in reactivity are reproduced in a reasonable manner. Rate

constants can be calculated since the geometry of the transition state

is defined when p and E /E^ are known. Performance equal to or better

than the Evans-Polanyl-Semenov relation is claimed.

73
This technique should be tested on cases where close

agreement has been found for the EPS relation and also to find out

where it fails. It is important because only one well defined reaction

is needed to set the scale.

(c) Marcus' generalized method . An expression is presented

for calculating activation energies of electron-transfer reactions in

solution and atom transfer reactions in the gas phase (Marcus, 1968).

It contains BEBO as a special case. The most interesting feature

is its ability to predict the activation energy of

A + HB AH + B

from

A + HA -> AH + A

and

B + HB > BH + B

Forty five comparisons were made with BEBO calculations.

* Marcus' method yields slightly lower E . Eighteen cases agreed

to < 2 kJ/mol, 12 to 'V 4 kJ/mol, 13 to 8 kJ/mol and two to 10-12 kJ/mol.

(d) Correlation with references . Splrln (1962) proposed

a correlation for the reaction A + BC -> AB + C

E* = 0.066 [(0.751)^^ - Q] [1/P^ + 1/P^] -1.6

where D. is the bond strength of the A-C molecule, Q is the

heat of reaction, and the P's are modified molar refractions. The

Evans-Polanyi-Semenov relationship is, formally, a special case.

For 40 cases tested, 32 agreed with experiment to within + 8 kJ/mol.

(e) Correlation of activation energies with electron

affinities .

Alfassl and Benson (1973) produced a two constant

correlation

74
i< "k A E = a - bl for E > 0, otherwise E = 0

where I is the sum of the electron affinities of R and R' in

R + A-R* ^ RA + R' and a and b are adjustable constants. Based on

30 comparisons with experiment the average fit is + 1.7 kcal. The

biggest limitation of the method is the dearth of electron affinity '

data.

(f) Empirical end-group correlations . Alfassi and Benson

(1973) also proposed and tested two empirical expressions

E* = X + X'

and

E = F • F'

for the process R + AR' -> RA + R' where the X's or F's are set

by comparison with experiment. Note that these approaches ignore

the transferred atom, as do others. Both models were tested against

50 measurements (38 of which were hydrogen atom transfers). The

average agreement in both cases in about + 1 kcal.

Reactions of highly polar groups are not fit well. A

point-dipole model is presented and applied to CF^ as a correction

on the estimates.

(g) Correlation of activation energies and entropies by

class of reaction . In a series of papers Szabo and

Berces present a formal classification of gas phase reaction based

on types of transition state (Szabo, 1967) and develop a calculation

formula for activation energies for each class (Szabo and Berces, 1968a).

Later they give activation entropies for classes of unimolecular

reactions (Szabo and Berces, 1968b).

The activation energy formula is

E = ZD(broken) - ctED(formed)

75
where the D's are bond strengths, either ijieasured or obtained

from an estimation scheme. £ is a parameter fitted to the general

type of reaction. Comparisons with experiment are given for 28

atom transfer reactions (among others) . Seventeen of these fit to

< 1 kcal and ten more to < 2 kcal. It is not apparent how many of

the reactions presented were used to derive the a's used in the

correlation. In a sense this technique is an elaboration of the

EPS method described earlier. This point is discussed by Szabo and

Konkoly-Thege (1973).

(h) Group addit ivies f or activation energies . Moin (1967)

estimates the activation energies for both blmolecular abstraction

reactions and unimolecular reactions using a group addit ivity

approach. Only the former is described here. For the transition

state

R X R'

(where the transferred atom is H in practice) the energy of

dissociation of the complex is postulated to be the sum of the dissociation

energies of the stretched bonds R X and X R' . In addition,

the dissociation energy of R X is postulated to be Independent

of R' . Then the activation energy is

E* = D(R-X) - (D(R X) + D(R' X))

Tables of D(R X) are constructed using selected reactions and

activation energies predicted for others. For 29 reactions tested

agreement to < 1 kcal was found for 24 cases. There should be a

strong correlation between this method and that of Alfassl and Benson

described above.

(1) Activation energies from superposition of Morse Curves

Kalga et al. (1969a) defined a reaction path for the

process A + XB AX + B whose energy was specified by the sum of the

76
energies of Morse type potential energy curves for the reactant and

product molecules.

The bonding energies of the two partial bonds in the transition

state are set equal to each other and the parameters of the Morse curves

are defined by an empirical relationship involving the energy of activation.

(Compare: Zavitsas model, above). Agreement to + 8 kJ/mol is found in

many cases.

From the model given above Kaiga et al (1969b) developed an

empirical two parameter relationship expressing E in terms of a linear

combination of the dissociation energies of the reactant and product

model. The Evans-Polanyi-Semenov relation is a special case. Agreement

to + 8 kJ/mol was found for most tests. The method is also used to

interpret Hammett type reactivity correlations.

6.4.7 Miscellaneous

(a) Johnston and Birks (1972) present a conceptual model to

explain why activation energies for the dissociation of diatomic molecules

are less than the dissociation energy. This is not a method for estimating

rate data. The principle is that the dissociating molecule can be

treated as a truncated harmonic oscillator with all vibrational energy

states contributing equally to the rate of dissociation.

(b) Denisov and Kosarev (1964) have made transition state

calculations for radical reactions important in liquid phase oxidations.

The types are

R + 0^ ^ RO^

RO^ + RH ^ ROOH + R'

RO + RO products

77
The potential importance of this work is that the authors define transitii

state geometry that may be applicable to similar reactions in the gas

phase.

(c) Specific correlations. Most of the techniques discussed

above are intended to be applicable to large classes of reactions. In

general their performance is reasonable but not outstanding. There is

another approach that often does very well for a restricted group of

reactions. These highly specific correlations may prove to be more

useful than the more general ones. Two examples are given here.

Greiner (1970a) develops a formula correlating rate constants

for the abstraction by OH of a hydrogen atom from straight and branched

chain alkanes from methane to 2,2,4 trimethyl pentane. Although problems

may arise in extrapolating to n-decane, the approach should be very

useful for the lighter hydrocarbons. Greiner (1970b) has also correlated

the activation energies for abstraction by OH with those for abstraction

by H. In both of these correlations the essential factor is the number

of primary, secondary and tertiary C-H bonds in the alkane.

Rate constants and activation energies for H-atom abstraction

from aliphatic organic compounds have also been correlated by Shteinman

(1974) . These correlations relate the rate parameters to the reactivity

parameter o developed by Taft and widely used for solution reactions.

This work is of interest because it connects the attempts to predict gas

phase rates to the much larger body of correlations developed by physical

organic chemists.

78
6.5 Evaluation of Experimental Rate Data for use at

High Temperature

A quite different aspect of the problem of constructing a

kinetics data base Is examined here. The problem Is how to make best

use of data taken at lower temperatures.

There exists a large body of data for many of the elementary

reactions of Interest to combustion scientists. The vast majority of It

was obtained at temperatures well below 1000 K. Two questions arise:

are the same products formed at higher temperatures, and how should the

low temperature rate data be extrapolated to temperatures beyond 1000 K.

The first question - about what products are formed - Is a

matter of mechanism. The klnetlclst speaks of the branching ratio, the

measure of the relative Importance of two or more possible reaction

paths starting with the same reactants. An example Is

0 + C_H, -y CHO + CH^

2 4 3

At present branching ratios must be determined by experiment for the

most Important cases.

The second question - about extrapolating data - may be

Illustrated by an unresolved problem In the combustion of methane. Here

a basic question stands out: what reactive species are responsible for

the removal of CH^? Peters and Mahnen (1973) argue that methane Is

principally removed by OH,

1. CH, + OH -> CH^ + H_0

4 3 2 On the other hand Dryer and Glassman(1973) argue that the OH + CH^

reaction is less important than

2. 0 + CH, -> CH^ + OH

4 3

The argument turns on the values of the rate constants for reactions (1)

and (2) which are derived at flame temperatures by extrapolation

79
of lower temperature data. Figure 6.1, taken from Dryer and Glassman

(1973), illustrates the problem. The older rate evaluations for reaction

(1) by Drysdale and Lloyd (1970) and Wilson and Westenberg (1967) give

rate constants much higher at flame temperatures than those obtained by

extrapolation of the more recent experimental data of Greiner (1970).

The former results in k(OH + CH, ) >> k(0 + CH,); the latter k(OH + CH,)

< k(0 + CH^) . The new data does not completely resolve the problem

however, as the whole question of the validity of extrapolating Arrhenius

plots to higher temperatures is open to serious question.

For the vast majority of reactions, the dependence of the rate

constant on temperature can be expressed by means of the Arrhenius

—E /RT equation, k = Ae . For all practical purposes this is an empirical

expression which has had a remarkable success in fitting the vast majority

of rate data. Unfortunately, the success of the Arrhenius expression

had led to the discounting of data which could not be fitted in the

linear log k vs 1/T form. In particular shock-tube data have been

discounted since the rate constants have often been found to be considerably

higher than would be predicted by linear Arrhenius extrapolations. There

is now reason to believe that the extrapolation procedure is incorrect,

and that the shock-tube data cannot be rejected out-of-hand. Figure

6.2, taken from Baulch, Drysdale and Horne (1973), gives the data for CO

+ OH, a key combustion reaction. Linear extrapolation of the data below

1000 K clearly leads to considerable error in the rate constant at T > 1000 K.

The curved Arrhenius plot for CO + OH has been justified on the basis of

transition state calculations (Dryer et al, 1971). Evidence for nonArrhenius behavior has now been reported for OH + H^ and OH + OH, (Gardiner

et al, 1973) and for CH^ + C^H, and CH^ + H^. (Clark, 1973).

J z b J Z

80
10'^

I \ \ I i i 4 .6 .8 1.0 1.2 1.4

6.1 Arrhenius plots for the reactions of H, 0, and OH with methane (Dryer and Glassman, 1973).

81
130

2000 1000 500 t/k 300

20 30

io't7k-'

40

6.2 Arrhenius plot for the reaction of hydroxyl radicals with

carbon monoxide (Baulch et al, 1973).

82
It should be emphasized that curved Arrhenius plots can arise

for reasons other than the formalism of the rate expression. For

example, the reaction may Involve competing reaction channels which have

different temperature dependencies. A further source of complication In

blmolecular reactions arises when the Arrhenius activation energy, E,

has a negative value. For reactions with activation energies of several

kJ or less, this may be more common than is realized at present.

Aside from the fundamental problems of reaction kinetics,

there is an additional problem encountered in extrapolating Arrhenius

type plots. This is a data problem. It arises from the procedure of

deriving "consensus" values of rate constants by the statistical treatment

of large bodies of data from different sources. The result of this

procedure is to wash out fine structure in any particular study. The

basic problem here is the lack of criteria for the rejection of less

accurate data. The very great improvement in experimental technique

over the past decade, if applied in a consistent and methodical manner

to combustion reactions, should allow us to overcome this problem.

For certain classes of combustion reactions involving reactive

species such as 0, H, and OH there now exist highly accurate (+ 10%)

techniques for measurement of rate constants. These techniques include

flash photolysis-resonance fluorescence, modulation photolysis, shocktube, laser paramagnetic resonance spectroscopy, and mass spectrometry.

They have not been (aside from shock-tube) applied at temperatures much

in excess of 500 K. A primary objective of combustion research should

be the application of these techniques at temperatures greater than 500

K, i.e., in the range 500-1300 K to provide the basis for a fundamental

understanding of the temperature coefficients of combustion reactions.

83
Such data should then be used in conjunction with theoretical expressions

to extrapolate the rate expressions to higher temperature and overlap

the shock tube data. The BEBO (bond energy-bond order) approach (Johnston,

1966) appears to be well suited to a solution of this problem.

6 . 6 Summary

(a) The present resources in the form of evaluated rate data

are large and are immediately applicable to high temperature kinetics

problems

.

(b) Additional evaluations of data for important reactions

will be required.

(c) New experimental and theoretical work is needed to

resolve the problem of how best to extrapolate low temperature kinetics

data to temperatures above 1000 K.

(d) Inevitably there will be many reactions of interest for

which there are no reliable rate data but for which rate constants will

be needed in the near future. Methods for predicting many of these, but

not all, exist. '

(e) There is a wide variety of prediction schemes, most of

which do only part of the job of estimating rate constants as a function

of temperature. Many of them have been demonstrated to have reasonable

performance. However, it is not now known which are the most reliable

and the most widely applicable. There is a need for a detailed study

and cross comparison of the methods, particularly those that estimate

rate constants and entropies of activation. Methods that estimate or

correlate activation energies are useful but less important for high

temperature studies.

84
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Mayer, S. W., "Estimation of Activation Energies for Nitrous

Oxide, Carbon Dioxide, Nitrogen Dioxide, Nitric Oxide,

Oxygen, and Nitrogen Reactions by a Bond-Energy Method,"

J. Phys. Chem. 73, 39^1 (1969)

Mayer, S. ¥., and Schieler, L., "Computed Activation Energies

and Rate Constants for Forward and Reverse Transfers of

Hydrogen Atoms," J. Phys. Chem. 72, 236 (I968)

Mayer, S. W., Schieler, L., and Johnston, H. S., "Computation

of High-Temperature Rate Constants for Blmolecular Reactions

of Combustion Products," Symp. Combust. 11th (Combustion

Institute, Pittsburgh, I967) «37

Marcus, R. A., "Theoretical Relations Among Rate Constants,

Barriers, and BrjzJnsted Slopes of Chemical Reactions,"

J. Phys. Chem. 72, «91 (1968)

Moin, F. B., "Calculation of the Activation Energy of

Chemical Reactions on the Basis of the Additlvlty Principle,"

Russ. Chem. Reviews 36, 511 (1967)

88
O'Neal, H. D., and Benson, S, W., "Thermochemistry of Free

Radicals," in "Free Radicals," editor Jay K. Kochi

(John Wiley and Sons, New York, 1073) volume ?, ?7[~^

Peeters, J., and Mahnen, G., "Reaction Mechanisms and Hate

Constants of Elementary Steps in Methane-Oxygen Flames,"

Symp. Combust, l^th (Combustion Institute, Pittsburgh,

1973) 133

Semenov, N. N., "Some Problems in Chemical Kinetics and

Reactivity," Volume 1, translated by Michel Boudart

(Princeton University Press, 1958) pp. 29-^19

Shteinman, A. A., "Correlation Analysis of the Reactivity

of C - H Bonds in Homolytic Reactions," Kinet. Catalysis

15, 7^5 (197^)

Splrln, Yu. L., "The Activation Energy of Radical Reactions,"

Russ. J. Phys. Chem. 3^, 636 (1962)

Szabo, Z. G., "The Transition State and the Arrhenius

Parameters. I. Classification of Homogeneous Gas

Reactions," Zeit. Physik. Chem. N.F., 5^, 1 (1967)

Szabo, Z. G., and Berces, T., "The Transition State and the

Arrhenius' Parameters, II. Evaluation of Activation

Energy," Zeit. Physik. Chem. N.P., 57, 113 (1968)

Szabo, Z. G., and Berces, T., "The Transition State and the

Arrhenius' Parameters. III. Evaluation of Preexponential

Factor on the Activation of Homogeneous Gas Reactions,"

Zeit. Physik. Chem. N.P., 57, 123 (1968)

Szabo, Z. G., and Konkoly Thege, I., "The Transition State and

Arrhenius' Parameters. IV. On the 'Constants' of the

Pol^nyi-Evans Equation," Zeit. Physik. Chem. N.F., 8j^,

62 (1973)

Tsang, W., "Thermal Decomposition of 3 , ^-Dimethy lpentene-1

2 , 3 , 3-Trimethy Ipentane , 3, 3-Dimethylpentane, and

Isobutylbenzene in a Single Pulse Shock Tube," Intern. J.

Chem. Kinet. 1, 245 (1969)

Tsang, W., "Thermal Decomposition of Some Alkyl Halides by a Shock

Tube Method," J. Chem. Phys. _41> ^487 (1964)

89
Tsang,,W., "Comparisons between Experimental and Calculated

Rate Constants for Dissociation and Combination Reactions

Involving Small Polyatomic Molecules," Intern, J. Chem.

Kinet. 5, 9^7 (1973)

Tschuikow-Roux, E., and Maltman, K. R., "Application of the

Modified Semi-Ion Pair Model for the Evaluation of Activation

Energies for HX(X = F, CI, Br, I, OH) Addition to Olefins,"

Intern. J. Chem. Kinet. 7, 363 (1975)

Ulitskii, V. A., and Stepukhovlcli , A. D., "Steric Factors

and Rate Constants for Recombination Reactions of Alkyl

Radicals," Russ. J. Phys. Chem. 31, 359 (1963)

Wilson, W. E., Jr., "A Critical Review of the Gas-Phase

Reaction Kinetics of the Hydroxyl Radical," J. Phys. Chem..

Ref. Data 1, 535 (1972)

Wilson, W. E., and Westenberg, A. A., "Study of the Reaction

of Hydroxyl Radical with Methane by Quantitative ESR,"

Symp . Combust, 11th (Combustion Institute, Pittsburgh,

1967) 11^3

Zavitsas, A. A., "Activation Energy Requirements in Hydrogen

Abstractions. Quantitative Description of the Causes in

Terms of Bond Energies and Infrared Frequencies," J. Amer

.

Chem. Soc. 9ii, 2779 (1972)

Zavitsas, A. A., and Mellkian, A. A., "Hydrogen Abstractions

by Free Radicals. Factors Controlling Reactivity," J. Amer.

Chem. Soc. 97, 2757 (1975)

90
7. Recommendations for an Approach

7.1 The preceding sections have developed a picture of the information

that is required for the useful modeling of combustion phenomena and of

the behavior of combustion devices. These are rate data on pyrolysls of

stable compounds and free radicals, rate data and mechanisms for oxidation

reactions, and rate data on free radical reactions, all at high temperatures.

A large body of information exists on such processes but much of it is

not usable by modelers, because the data are fragmentary, unevaluated

and often not applicable at high temperatures, as well as not being assembled

into a form usable by modelers. This situation is the basis for our

recommendation of a technical program proceeding along three

parallel lines:

(a) An activity leading to the development of a printed,

published, up-to-date body of pertinent data for the user community.

This would be periodically up-dated and refined on the basis of latest

available information.

(b) An activity concerned with the measurement of rates and

mechanisms of oxidation processes in the appropriate high temperature

regime, leading to specific information currently lacking and also

directed at development of prediction schemes for additional information.

(c) An activity concerned with the measurement of rates and

mechanisms of pyrolysls and hydrogen atom reactions in the appropriate

high temperature regime with similar objectives.

Although an activity concerned with pollutant aspects is also

desirable, we feel that the initial emphasis should be directed to the

primary combustion aspects. Specific recommendations for pollutant

kinetics studies could be readily formulated.

91
The nature, quality, and scope of the published data base is

of necessity determined by the data that is available or can be estimated

as well as by the extent to which specific data needs are defined by the

users. The needs as outlined above are general. They are only an

approximation, albeit a good one. This activity will proceed in an

iterative fashion, leading to a succession of output documents of increasing

quality, scope and appropriateness.

The evolution and tactics of the experimental subprograms will

be determined by the data needs and the availability of appropriate

experimental techniques. Whereas there are significant data needs

in all classes of processes described above, the reactions of hydrogen atoms

and hydroxyl radicals at high temperatures represent areas of greatest

ignorance.

There are three types of experimental techniques, all

developed at NBS, which can be brought to bear on these measurement

problems. One of these, the single pulse shock tube technique is already

suited to obtain information on certain rates and mechanisms at the

appropriate high temperatures. The two other techniques, kinetics mass

spectrometry and flash photolysis resonance fluorescence will require

modification to permit high temperature measurements. With these

modifications, a systematic program of measurements can be undertaken.

One Important feature of this set of capabilities is that in a number of

instances the same measurement problem can be attacked by several different

types of experiments. This will be extremely useful in minimizing the

possibility of systematic errors in the measurements.

More detailed description of the technical program is given in

the following sections under the three categories of technical activities

given above. The apparent duplication of selected experimental objectives

92
is intentional and is meant to indicate where a measurement problem

could be solved by a variety of experimental approaches. In selected

instances, this should in fact be done for reasons discussed above. An

equally close coordination is also required between the activity aimed

at providing a data base and the true experimental activities in order

to ensure the most rapid and effective improvement of the quality of the

data base and to make available additional expert opinion for the critical

evaluation of selected data.

7 . 2 Construction of a Combustion Kinetics Data Base

7.2.1 Introduction and Design

A plan is given in this section for providing the designer of

equipment and the modeler of combustion phenomena with the chemical

kinetics data needed in his solution of his particular problem.

This data base will consist of entries for elementary gas

phase reactions.

Each entry will contain:

(a) The chemical reaction.

(b) It rate constant as a function of temperature, and, where

necessary, also as a function of pressure.

(c) As estimate of the reliability of the rate constant,

where possible.

(d) Comments as needed.

(e) References.

7.2.2 Objectives

(a) To provide tables of chemical kinetics data oriented

toward the solution of combustion problems. They are to include reactions

of major importance for every conversion, for pollutant control and for

the explanation of characteristic phenomena of flames. They are also to

include less important reactions about which information is needed for

93
screening purposes, i.e. setting the minimum chemical mechanism needed

for the study of a particular problem.

(b) To distribute these tables periodically to engineers,

designers and other students of combustion systems and high temperature

industrial processes.

7.2.3 Detailed Plan

The steps described here emphasize the rapid development of a

kinetics data base useful for current problems. They also allow for

refinement of the data base and expansion to include new problem areas.

First Round (steps a through g)

(a) Analyze the current problems of interest in terms of the

chemistry contained in them.

The early stages of this phase are displayed in other sections

of this report. ' . ;! ; . r

(b) Develop a catalog of elementary chemical reactions for

which rate data should be provided.

Some examples are given elsewhere in this report. But the

catalog should be expanded and refined working together with the ultimate

users of the data.

(c) Compile the evaluated rate data available in current

tables, data reviews and monographs.

(d) Annotate the catalog mentioned above to show the reliability

of the data and its suitability for use in combustion studies.

This is an analysis to be made by kineticists.

(e) Provide estimates, using one or another of the schemes

described in Section 6, for rate data for reactions that

(1) are not covered in a satisfactory manner in current

' evaluations

,

94
(2) have not been studied,

(3) are judged not to be important,

or, (4) are needed only for screening purposes.

(f) Start a chemical kinetics measurement program that

emphasizes

.

(1) reactions of major importance for which accurate

data are mandatory,

(2) gaps in the data base, extension and refinement of

predictive schemes and correlations.

(g) Publish and distribute the tables of data and the catalog

of reactions of interest.

Second and subsequent rounds

(h) Reanalyze the field to determine new problems of interest.

This is to be done working with selected users of the data.

(i) Expand the catalog of reactions and annotate it to indicate

those reactions for which the data are in reasonable shape and those for

which new measurements are needed.

(j) Compile and evaluate measurements for important reactions

not satisfactorily treated in the earlier rounds.

(k) Make new measurements as needed for major reactions, to

fill gaps and improve predictions.

(1) Estimate rate data for newly added, less important

reactions.

(m) Publish and distribute an up-dated chemical kinetics data

base and catalog of reactions.

7.2.4 Comments

(a) Combustion is a very broad, applied subject. It has been

the subject of much research. Other sections of this report show

95
the need for more combustion oriented research. This means that not all

the goals addressed by the tactics described above will be reached in

the near future. A combustion kinetics data base will be dynamic. It will

change as new more accurate data become available, as new interpretations

of older data come to light, and as new data needs are defined.

(b) Because combustion is a very large subject, it is unlikely

that one laboratory could or will provide all the needed data for the

many problems within a reasonable time-frame. Cooperative progress of

measurement and data evaluation are desirable. The tactics described in

the preceding section are a suitable guide for a funding agency in

planning its overall program in the area of kinetics.

(c) Combustion is an applied subject. Measurements and data

evaluation programs should have built into them feedback from the users.

This will keep current work in line with current problems and those

extensions likely to become important in the next few years.

/ (d) Experience with the development of a kinetics data base

for atmospheric pollution studies showed that tables of rate data (and

possibly other physical properties) were sufficient. The user group

included chemists who decided what mechanisms were appropriate for

particular problems. This probably is true for the field of combustion.

If it is not true, it will become necessary to supplement the data base

with mechanistic information.

7. 3 Pyrolysis and Hydrogen Atom Chemistry

One class of experiments that is immediately applicable to

the measurement of high temperature data is based on shock tube techniques.

They yield comparatively accurate results and provide the easiest means

of generating combustion and pyrolysis temperatures and pressures.

96
7.3.1 Shock Tube Techniques

Shock tubes have been used for over 75 years as a means of

producing high temperatures. The basic apparatus and process can be

seen in figure 7.1. The procedure consists of separating two sections

of a closed tube by a diaphragm, one side of which is maintained at high

pressure (driver) while the other side containing the test gas is at a

lower pressure. When the diaphragm is ruptured a shock wave propagates

through the test gas, bringing about a sudden increase in temperature.

Upon reflection of the shock wave by the end wall a further increase in

temperature can be achieved. The physical condition of the shocked gas is relatable to

the shock velocity if ideal gas dynamic behavior is assumed. This is a

pulse technique. The advantages are that extreme condition can be

achieved without consideration of material stability. Since the reaction

time is short, reactions are truly homogenous and much less sensitive to

impurity effects. The course of the reaction can be followed either by

kinetic spectroscopy or, through the use of a single pulse shock tube

(see figure 7.2), by classical analytical techniques such as gas chromatography.

The latter is capable of detecting only stable products. On the other

hand with kinetic spectroscopy one can follow only one or two transient

species simultaneously. The real need is for the meshing of both techniques.

The principal problem in shock tube studies is the pulse nature of the

excitation. Thus for the detection of transients one cannot use signal

averaging techniques. There is also the uncertainty with regard to the

calculated reaction temperature. This can be circumvented by the use of

an internal standard. Such a procedure has been successfully applied to

the interpretation of single pulse shock tube experiments. Finally it

should be pointed out that although the discussion or shock tube techniques

97
aKii
99
is focused on the study of pyrolytic decompositions involving hydrocarbon

compounds, there is no reason why the technique cannot be applied to

oxidation processes as well.

7.3.2 Single Pulse Shock Tube Experiments

The earlier work on the mechanism and rates of the initial

steps in hydrocarbon decomposition provides a base for the study of

several of the subsequent steps in the pyrolytic process. These include

the reactions of hydrogen atoms with hydrocarbons and the thermal decomposition

isomerization and cyclization of alkyl and unsaturated radicals. It is

suspected that the former is of prime importance in pyrolytic and

combustion processes. Hydrogen atoms are among the most important chain

carriers in high temperature systems. This is due to their extreme

reactivity. Of critical significance in the modeling of such systems is

the relative importance of each of the multiplicity of reactive channels

that are available. The strategy will be to generate hydrogen atoms

through the decomposition of a labile hydrocarbon and by proper adjustments

of concentrations to enable it to react with the appropriate substrates.

The aim will be to establish the order of reactivity with respect to

hydrogen atom abstraction from methyl, primary, secondary, tertiary,

allylic, and other resonance stabilized groups; to compare the rate of

these reactions with hydrogen additions to unsaturated systems (aromatic,

olefinic and acetylenic compounds) and the development and testing of

appropriate correlations. With respect to unimolecular decomposition of

hydrocarbon radicals, the rationale and plan of attack will be similar

to that for hydrogen atoms. Once again, the processes are fast and the

important factor is the identification of the important reaction channels.

The radicals will, as before, be generated from appropriately labile

100
hydrocarbons. Concentrations will be adjusted so that the newly formed

radicals will undergo only unimolecular decompositions. Extensive gas

chromatographic analysis will then pinpoint the Important reaction

paths.

Finally there is the necessity of completing the work on

hydrocarbon pyrolyses. As mentioned before this involves the mechanism

and rates of decomposition of cyclic alkenes. Although this has been

neglected in the past, no serious experimental problems are anticipated.

It is expected that several cyclic pentyl, hexyl, and bicyclic compounds

will need to be studied before general trends can be established.

7.3.3 Heated Single Pulse Shock Tube :

The experimental program outlined above is limited to studies

of the kinetic properties of volatile organic compounds. This is a

serious limitation if one wishes to study the full range of substances

that are of importance as fuels and feedstocks. Thus at some stage of

the program it will be necessary to extend the experimental capabilities

by constructing a heated shock-tube, and the analytical capabilities by

the use of gas chromatographic-mass spectrographic detection. The

immediate targets are the complex aromatic and heterocyclic compounds.

Such a facility will be an absolute necessity for the study of the

oxidation reactions of complex hydrocarbons.

7.3.4 Studies Using Shock Tube for Kinetic Spectroscopy

This facility will be needed to complement and supplement the

single pulse shock tube work. The limitation of the latter is that it

cannot handle reactions with small temperature dependences or slow

bimolecular or termolecular reactions. This encompasses the whole range

of recombination reactions. As mentioned earlier, these are the element

101
steps in the chain reaction and there is a great need for data at

combustion temperatures. Rate constants that need to be measured are

recombination rates of CH^ + CH^; resonance stabilized radicals + CH^,

resonance stabilized radicals + resonance stabilized radicals, and

organic radicals + H. Another avenue of research is the reactions of

organic radicals with hydrocarbon substrates. These are important chain

transfer steps in thermal decomposition processes. It should be noted

that kinetic spectroscopy is the traditional means of doing kinetics

with a shock tube. Thus no breakthroughs in technology are involved.

The only new features in this program are the application to larger

molecules and the use of some of the spectrocopic and instrumental

techniques that have been developed in recent years for kinetic spectroscopy.

7.3.5 Studies of Global Kinetics

We now turn to other pertinent application of the results of

the research program outlined above. In particular we will want to join

this data with appropriate kinetic models so as to simulate realistic

processes. For the present purposes a particularly relevant problem is

the cracking of naphtha. The industrial process involves passing the

naphtha in steam through hot tubes and cracking the large organic molecules

into smaller olefins and various other products. It thus lends itself

admirably to bench scale experimental work. Such data can than be used

to experimentally verify the prediction of appropriate models or

from another point of view to "fine tune" the models. The

ultimate long range aim is to define the optimum conditions for the

production of desired mixtures of products as a function of initial

hydrocarbon concentration. For the more immediate future the aim will

102
be to begin an experimental and modeling program on the pyrolysls, first

of various Intermediate sized straight chained hydrocarbons such as,

hexane, octane, etc. This will be followed by studies on the effect of

side chains, cycllzatlon and aromatlzatlon. At the same time the study of

mixtures will be slowly phased In. It should be noted that even the

study of the decomposition of Individual hydrocarbons are of potential

industrial significance since there have been proposals to separate the

components of the naphtha prior to cracking. Finally it must be emphasized

that this proposal as the others do not require any breakthrough in

technology. Modeling and verification of the cracking of small compounds

such as propane and butane has been carried out during the past several

years. The present plan is thus a logical extension of the past work.

7.3.6 Detailed Plan

la. Measure rates of hydrogen atom addition to saturated and

unsaturated hydrocarbons using the existing shock tube facility.

lb. Measure rates of decomposition of hydrocarbon radicals

with existing shock tube facility.

Ic. Measure rates of decomposition of cyclic alkanes with

existing shock tube faculty.

2a. Construct heated single pulse shock tube.

2b. Measure rates of hydrogen atom reaction with aromatic and

heterocyclic compounds.

2c. Measure rates of oxygen atom reaction with hydrocarbon

substrates.

2d. Measure rates of hydroxyl radical reaction with hydrocarbon

substrates

.

3a. Construct shock tube for kinetic spectroscopy.

103
:i 3b. Measure rates of hydrocarbon radical recombination at

elevated temperatures.

3c. Measure rates of hydrocarbon radical reactions with 0^

and NO at high temperature.

4. Carry out experimental and modeling studies on hydrocarbon

decomposition.

7.4 Oxidation Processes

In order to study reaction mechanisms and measure rate constants

of oxidation processes at elevated temperatures, it Is planned to adapt

two successful experimental approaches for high temperature work. These

are photolonlzatlon mass spectrometry, and flash photolysis resonance

fluorescence. ^ . , :

7.4.1 Photolonlzatlon Mass Spectrometry

Mass spectrometry has been one of the most extensively and

successfully applied methods for the study of elementary reactions. NBS

has pioneered in the application of these techniques to high temperature

studies, specifically as part of a long-range contribution to combustion

chemistry. Recent advances in technique, including beam sampling mass

spectrometers and photolonlzatlon mass spectrometers have been Incorporated

into the existing NBS experimental capabilities. The basic system is

illustrated in Figure 7.3. As shown, it consists of a chemical reactor

from which a small fraction of the reacting gas flows through a "leak"

into the first vacuum chamber. By means of a collimatlon cone, the

central portion of that flow is selected and enters the second and third

chambers as a molecular beam. The advantages of this method are that

the reacting gases are "frozen" after passing through the leak, and the

absence of wall collisions preserves the Integrity of the sample. By

chopping the beam mechanically and using phase sensitive detection,

104
LUCO>

_l

<

z

<

UJO

105
background contributions to the signal are minimized. The reactor may

be of a variety of configurations. Figure 7.4 shows a temperature

controlled flow reactor; Figure 7.5 a temperature controlled stoppedflow reactor. For high temperature studies reactors would be constructed

of quartz and resistive electrical heating used. This will considerably

extend the range of temperatures previously used in NBS studies of

oxygen atom chemistry using borosilicate glass reactors.

The use of photoionization rather than electron impact ionization

provides a powerful tool for the identification of free radicals, and

the study of their reactions. The method has been applied by Bayes to

the study of oxygen atom reactions at room temperature. Extension to

very high temperatures is feasible. ,

In addition to studies using chemical reactors, crossed beam

studies will be employed to determine the nature of the primary products

in atomic and free radical reactions. The work of Gutman on oxygen atom

and hydroxyl radical reactions with olefins is a beautiful demonstration

of the power of the method. Here again extension to very high temperatures

is feasible and desirable.

In addition to the approaches outlined here, there are several

other techniques of proven reliability which can be adapted and greatly

expanded in application through coupling to a photoionization mass

spectrometer. These include flash photolysis, mercury photosensitized

photolysis and modulated photolysis. These are all potentially powerful

methods for studying kinetics at high temperature.

7.4.2 Flash Photolysis Resonance Fluorescence

This technique, developed at NBS, is one of the most accurate

methods available for the study of atomic and free radical kinetics. A

106

I -b J

108
typical experimental arrangement is shown in Figure 7.6. The atom or

free radical under study is produced by flash photolysis of a suitable

source compound. The reaction cell is irradiated with resonance radiation

which excites the reactant species into an upper state from which it

fluoresces. The rate at which the fluorescence decays is a direct

measure of the time dependence of the atom or radical concentration, and

hence, a measure of the rate of decay of the reactant.

The maximum temperature accessible at present is about 700 K,

primarily because of limitations on the design of the optical windows of th

reaction cell. Extension to much higher temperatures will require

redesign of the apparatus to protect the windows from high temperature

and from condensation of high temperature species. Success in this

endeavor would represent a major breakthrough in high temperature chemical

kinetics.

7.4.3 Detailed Plan

This plan consists of three lines of work which will parallel

and overlap to some extent depending on level of effort.

la. Design and construct a quartz flow reactor to be used in

conjunction with the photoionization mass spectrometer.

lb. Measure the rate constant for the reactions of molecular

oxygen with methyl and other selected alkyl radicals.

2a. Design and construct a high temperature crossed nozzle

beam apparatus for use in conjunction with the photoionization mass

spectrometer.

2b. Study the mechanism of reaction of molecular oxygen with

methyl and other selected alkyl radicals.

2c. Study the mechanism of reaction of atomic oxygen with

alkanes, alkenes, and acetylene at different temperatures.

109
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110
2d. Study the mechanism of reaction of hydroxyl radicals with

alkanes, alkenes and acetylene at different temperatures.

3a. Design and construct high temperature photolysis cells

for use in flash photolysis resonance fluorescence studies, or in conjunction

with the photoionization mass spectrometer. Construct a flash photolysis

resonance fluorescence apparatus.

3b. Measure the rate constants for the reactions of atomic

oxygen with methane and selected alkanes.

3c. Measure the rate constants for the reactions of atomic

oxygen with ethylene, acetylene, and selected alkenes.

3d. Measure the rate constants for the reactions of hydroxyl

radicals with methane and selected alkanes.

3e. Measure the rate constants for the reactions of hydroxyl

radicals with ethylene, acetylene, and selected alkenes.

7 . 5 Pollutant Forming Processes

The two principal classes of pollutants formed in combustion

are the oxides of nitrogen and sulfur. We recommend the following

program to elucidate the mechanisms whereby these oxides are formed,

using experimental approaches described in 7.2.

1. Study the mechanism of formation of nitric oxide in

combustion arising both from fixation of atmospheric nitrogen, and from

fuel nitrogen.

2. Study the mechanism of formation of oxides of sulfur in

the combustion of sulfur containing fuels.

7 . 6 Scale and Scope of the Recommended Program

The program of activities recommended above implies a substantial

manpower commitment. An appropriate NBS manpower commitment would

111
consist of 5-10 man year per year effort for up to five years. A more detailed outline

of an appropriate NBS program will be submitted separately.

It should be noted that a certain amount of pertinent kinetics

data will also be developed by workers outside NBS. The recommended

program is so designed that the results of those activities will be

incorporated into the data base in very effective fashion. Also the

publication of successive editions of the body of data will be a pQsitive

force in guiding outside work towards the most effective contribution to

this problem area.

0 ' I

112
Acknowledgment

We are grateful to the Combustion Institute for permission

to reproduce Table 4.2 and Figures 6.1 and 6.2.

>
Appendix

List of organizations known to be earring on modeling of combustion, flames,

turbines, engines, rockets, and chemical reactors.

Organization

Acurex Corp.

Aerochem Corp.

Bell Aerospace Co.

Creare Inc.

Exxon Research and Engineering Co.

^ Ford Motor Co.

General Applied Science Laboratories

General Electric Co.

General Motors

IKOR, Inc.

KVB Engineering Co. •

Phillips Petroleum Co. -

Ultrasystems Inc.

United Aircraft ..

Westinghouse

In addition to the companies carrying on this work, there is a comparable

effort in engineering schools. It is recognized that there are strong interactions between engineering schools and industry through consulting activities.

i Schools

California State University, Northridge

Caltech

MIT •
Princeton

Purdue University

University of California, Berkely

University of Pennsylvania

University of Wisconsin

Washington State University

Government Laboratories and Private Research Institutions

Battelle

Bureau of Mines, Pittsburgh

Lawrence Livermore Laboratory

NBS

[in addition to the companies mentioned above, sixty-five other companies

appear to be sufficiently interested in the combustion area to sponsor the

lUth Annual Symposium on Combustion.]

The above listing includes only US organizations. There is a comparable

number of similar organizations elsewhere.

It should be noted that there is at least a comparable number of groups

carrying out experimental work on prototype systems of all types.

USCOMM-NBS-DC
NB$-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 75-910

2. Gov't Accession

No.

3. Recipient's Accession No.

4. Tl'n.i; AND SUBTITLK

The Role of Chemical Kinetics in Energy Conservation

^

5. Publication Date October 1975

6. Performing Organization Code

7. AUTHOR(S)

H. M. Rosenstock, D. Garvin, J. T. Herron and W. Tsang

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Addres.s (Street, City, State, ZIP)

Same as No. 9.

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

14. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Many of the technological areas of pertinence to energy independence

involve chemical transformation of gas phase materials at high temperatures.

The design, control and optimization of such devices and processes have

come to depend increasingly on mathematical modeling. Among the most

important input data required for there models are information on the

detailed chemical mechanisms and the rates of the individual processes.

It is concluded that although many resources are available there does

not exist at present an adequate, publicly available data base for the

modeling of high temperature systems. A detailed program involving a combination of experimentation and compilation, evaluation and dissemination

of gas kinetic data is proposed.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Combustion; kinetics; pyrolysis; oxidation; data base; modeling;

estimation schemes; energy conservation

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UNCL ASSIFIED 122

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Washineton. D.C. 20402. SD Cat. No. C13

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USCOMM-DC 29042-P74