NBSIR 73-311-

STUDY OF A

MOBILE LIQUID OXYGEN-NITROGEN GENERATOR

Peter C. Vander Arend

Stanley T. Stoy

Cryogenic Consultants, Inc. Allentown, Pennsylvania 18103

Thomas R. Strobridge

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

December 15. 1972

Final Report

Prepared for

NAVAL AIR ENGINEERING CENTER

Philadelphia, Pennsylvania 19122

NBSIR 73-311

STUDY OF A

MOBILE LIQUID OXYGEN-NITROGEN GENERATOR

Peter C. Vander Arend

Stanley T. Stoy

Cryogenic Consultants, Inc Allentown, Pennsylvania 18103

Thomas R. Strobridge

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

December 15,1972

Final Report

Prepared for

NAVAL AIR ENGINEERING CENTER

Philadelphia, Pennsylvania 19122

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director
Prefac e

This study deals with the preliminary design of a mobile liquid

oxygen-nitrogen generator. The objective is to increase production

over existing units while maintaining the same overall physical dimensions and a similar air compression system. Because of the sponsor's

request and because almost all data concerning air separation are

given in English units, that system is used in this report. Conversion

to the SI System of units is accomplished with the following conversion

factors=i=:

pound X 0.4536 = kilogram

foot X 3 . 0480 = meter

- 3 3

liter X 10 - m

2

PSl X 0. 6895 = N/cm

Btu/lb X 2. 326 = joule/ gram

Btu/hr X 0. 2931 = watt

See also [l]. (Numbers in brackets designate References at end

of report.

)

ii
CONTENTS

Page

1, Introduction , 1

2, Specification 3

3. Design Parameters 4

3. 1 Compression Equipment 4

3, 2 Expander , . . , . . . 4

3. 3 Generator Dimensions. ......... 4

3. 4 Weight 4

3, 5 Generator Operation 5

4, Process and Design Limitations of the Air Separation

Process 5

4, 1 Introduction 5

4. 2 Allowable Space , . , . 6

4, 3 Allowable Weight 7

4. 4 Condition of Feed Air 8

5. Air Separation Process 9

5. 1 Introduction 9

5.2 Process Description 9

5. 3 Data for Process Calculations 12

6. Generator Operation 12

6. 1 Introduction 12

6.2 Generator Reactivation 13

6. 3 Generator Startup 13

6. 4 Normal Operation 14

6. 5 Liquid Oxygen Production 15

6. 6 Liquid Nitrogen Production 15

6. 7 Controls for Automatic Operation 16

iii
CONTENTS (Continued)

Page

6. 7. 1 Feed Air Flow Rate Control 16

6. 7. 2 Product Purity Control 16

6, 7. 3 Refrigeration Control 18

6.7.4 Miscellaneous Generator Controls. . . 18

6.7.4. 1 Level Controls 18

6.7.4.2 Pressure Controls .... 19

6.7.4. 3 Temperature Controls ... 19

6.7.4.4 Pressure ... ... . 19

6.7.4.5 Temperature 19

6. 7. 4. 6 Purity Analyzer 20

7. Impurity Removal 20

7. 1 Introduction 20

7. 2 Impurity Removal At or Near Ambient

Temperature 21

7. 3 Impurity Removal by Liquefaction and

Solidification in a Heat Exchanger 25

7. 4 Impurity Removal in Regenerators and

Adsorbent Bed . . „ . 27

7. 5 Choice of Purification System 27

8. Distillation Columns . . , 28

8. 1 Introduction 28

8. 2 High Pressure Column 28

8. 3 Low Pressure Column 29

9. Heat Exchanger Design 30

9, 1 Introduction 30

iv
CONTENTS (Continued)

Page

9. 2 Heat Exchanger I , <, , . 31

9. 3 Heat Exchanger II 32

9. 4 Subcooler 33

9. 5 Condenser-Reboiler 33

9. 6 Compressor and Blower Aftercoolers .... 33

9. 7 Refrigerant 12 Condenser and Air Cooler ... 34

9. 8 Defrost Heater 34

10. Machinery 34

10. 1 Introduction 34

10. 2 Expansion Engine and Blower 35

10. 3 Lobe-Type Blower 35

10.4 Refrigerant 12 System 35

10.5 Air Compressor 36

10. 6 Liquid Oxygen Pump 36

11. Insulation 37

11. 1 Introduction 37

11.2 Column Cold Box 38

11. 3 Heat Exchanger Cold Box 39

11. 4 Insulation of Purification System 39

12. Physical Arrangement 40

12. 1 Introduction 40

12. 2 Column Cold Box Arrangement 40

12. 3 Equipment Arrangement 41

12. 4 Instrumentation and Controls 41

12. 5 Power and Weight 42

13. Conclusions and Recommendations 44

14. References 47

V
LIST OF FIGURES

I Page

Figure 1. Process Flow Diagram of the Liquid OxygenNitrogen Generator . , . 49

Figure Z. General Arrangement of the Water Knockout

Drum 50

Figure 3. Drier and CO^ Adsorber . . o » . . . . 51

Figure 4. Impurity Removal through Liquefaction and

Solidification. . 52

Figure 5, Cooling Curve of the Freezeout Heat

Exchanger 53

Figure 6. Impurity Removal in Regenerators .... 54

Figure 7. High Pressure Column Cross Section ... 55

Figure 8. Column Cross Section Typical Tray .... 56

Figure 9. Vapor-Liquid Equilibrium of Oxygen-Nitrogen

for High Pressure Column 57

Figure 10, Vapor -Liquid Equilibrium of Argon- OxygenNitrogen System for Low Pressure Column. . 58

Figure 11. Low Pressure Column Cross Section. ... 59

Figure 12. Cooling Curve Heat Exchanger I 60

Figure 13. Cooling Curve Heat Exchanger II 61

Figure 14. Vacuum Insulation for Column Cold Box. . . 62

Figure 15. Column Cold Box Partial Elevation .... 63

Figure 16. Column Cold Box Elevation 64

Figure 17. Column Cold Box Plan View 65

Figure 18. Equipment Arrangement Plan View .... 66

Figure 19. Equipment Arrangement Elevation .... 67

vi
LIST OF TABLES

Page

Table 2. 1 Contaminants in Feed Air . . . . . , « . 3

Table 4. 1 Saturated Water Content of Air as a Function

of Temperature 8

Table 7. 1 Carbon Dioxide Content of Air as a Function

of Temperature 26

Table 11. 1 Performance of Various Insulation Systems . . 38

Table 12, 1 Equipment Weight 42

Table 12. 2 Power Requirements 43

vii

Study of a Mobile Liquid Oxygen- Nitrogen Generator

Peter C. Vander Arend, Stanley T. Stoy

and

Thomas R. Strobridge

It appears feasible to produce as much as 4 tons per

day of liquid oxygen or liquid nitrogen of 99. 5 percent purity

with the equipment contained in an envelope 8 ft by 8. 5 ft by

15 ft. The process cycle is completely described. Physical

layouts were studied to assure that the equipment required

for the modified process would fit into the allotted space. A

conceptual layout of the cold components, with the required

vacuum insulation, was developed.

Major modifications to the existing 2 ton per day

generator consisted of: a) the use of a blower on the expansion engine to pressurize part of the air stream to 105 psia,

b) the use of a liquid oxygen pump to move liquid from the

sump of the low pressure column to the reboiler side of the

reboiler condenser and c) the purification of feed air in a separate system before it joins the major recycle stream.

It was assumed that a compressor would supply air to the

process at the same pressure and flow rate as the existing

unit.

Key words: Air separation; liquid nitrogen; liquid oxygen;

mobile; purification.

1. Introduction

A number of mobile liquid oxygen-liquid nitrogen generators

are in use by the U.S. Navy [2, 3]. These generators produce approximately 2 tons per day of liquid oxygen. A previous study carried out

by the Cryogenics Division of the National Bureau of Standards [4]

concluded that within the envelope of the existing generator and with

its air supply, a considerable increase in the liquid oxygen or nitrogen

production might be possible.
^ The present study deals in more detail with the engineering

aspects of the mobile liquid oxygen-liquid nitrogen generator. It has

been assumed that the air supply conditions and equipment envelope

have remained unchanged. Various processes were studied to optimize

the available refrigeration. Additional refrigeration has been added

in the form of a Refrigerant 12 (R-12) system.

Available height was found to be a severe limitation on the low

pressure column performance. Because only a limited number of

trays can be installed, it is impossible to separate air completely

into pure streams of oxygen and nitrogen. The nitrogen leaving the

top of the low pressure column contains approximately 5-6 percent

oxygen.

In order to make full use of the limited height, the low pressure

column stretches the full height of the envelope. A liquid oxygen pump

lifts liquid oxygen from the bottom of the low pressure column to the

reboiler- condens er located on top of the high pressure column.

To improve the thermodynamic efficiency of the process, work

available from the expansion engine is used to compress some of the

process air to approximately 105 psia. This makes it possible to

operate a high pressure column, reboiler- condenser, and low pressure

column in accordance with standard industrial practice.

Purification of the feed air to the generator is carried out near

ambient temperature before it joins the recycled air of the process.

Most of the water present in the air is condensed in a R-12-air

heat exchanger. The remaining water vapor and carbon dioxide are

removed from the air stream in a desiccant bed. Trace impurities

consisting of components with low boiling points not removed in the

purification system are carried in the air to regions of low temperature

2
in the plant. These impurities settle out in the liquefied air and

finally find their way to the liquid oxygen sump in the low pressure

column. A guard adsorber located down stream of the sump removes

these impurities.

2. Specification

The oxygen-nitrogen generator must produce liquid oxygen and

nitrogen meeting the following specifications:

Oxygen Purity:

Nitrogen Purity:

99.5 percent as per MIL-O-27210B

99.5 percent as per MIL-N-6011

Air supplied to the plant will be at the following conditions:

Pressure:

Temperature:

One Standard Atmosphere

(-30°F to +125°F)

Contaminants in the feed air to the generator are not to exceed

the limits shown in Table 2.1.

Table 2. 1

Contaminants in Feed Air

Water

Argon

Carbon Dioxide

Nitrous Oxide

Carbon Monoxide

Methane

Sxilfur Dioxide

Acetaldehyde

Saturated

10, 000 ppm

1000 ppm

5 ppm

20 ppm

3 ppm

1 ppm

1 ppm

3
3. Design Parameters

3.1 Compression Equipment

The basic choice of compression equipment is between high

pressure reciprocating or low pressure rotating machinery; the

existing oxygen generator employs rotating machinery. This study

assumes the use of rotating machinery since it is available and

relatively light in weight. It also assumes that the compressor can

recycle purified air in a closed loop and that feed air may be purified

before it joins the air of the closed loop.

/ 3.2 Expander

When low pressure compression equipment is used, the

refrigeration must be provided by a turbo expander. A reciprocating

machine would be too bulky and operate at too high a speed to be

reliable. Turbo expanders are standard devices and it is possible

to design the process before selecting the expansion engine. The

expansion engine specification is determined by the process design

of the generator.

3.3 Generator Dimensions

The generator is to fit an envelope of the following dimensions:

Height 8. 5 ft Width 8 ft Length 15 ft

3.4 Weight

Maximum weight is not specified.

4
3.5 Generator Operation

The generator, after manual startup, will operate in a semi-automatic mode. The generator will operate automatically

in the steady state with product purity and refrigeration control

regulated by plant instrumentation. Major upsets caused by malfunctioning machinery will shut down the generator. Personnel

attendance will be required for startup, shutdown, evaluation of the

reason for shutdown, and to monitor machinery and instruments.

No auxiliary fluids will be required.

4. Process and Design Limitations

of the Air Separation Process

4. 1 Introduction

Separation of air into its components through low temperature

distillation is a well-known process and is treated in a number of

standard texts [5, 6]. The low temperature industry has developed a number of processes by which air is separated into its components.

Historically, major changes have come about through technological

developments in certain areas. In the early days, high pressure

processes were used extensively to produce the necessary refrigeration without expansion engines. Low pressure processes were developed when rotating compressors and turbo expanders became

available

.

Various methods of air purification have been used. Adsorption at high and low temperatures and freezeout by deposition of the solid

material on heat exchanger or regenerator surfaces have all been

employed.

5
In general, the choice of process, including purification of

the feed air, is based on overall economic considerations. Production

of liquid oxygen or nitrogen at lowest overall cost may result in a process which thermodynamic ally is not the most efficient and does

not show the lowest horsepower per unit of production.

The process and design of the mobile oxygen-nitrogen generator

in this study is greatly influenced by the allowable space and weight of

the complete unit, the condition of the feed air to the generator, and

the compression equipment used.

4.2 Allowable Space

Air separation plants normally are not limited in height. In

order to separate air into its components by distillation, a double

column with a reboiler- condenser is used. The equipment is arranged

in a single column with the low press\ire column at the top, the high

pressure column at the bottom, and the reboiler- condenser between

the low and high pressure columns. The number of trays used in

both columns is set by the requirements of the process, and generally

the column assembly is very much taller than the height allowed for

the mobile generator.

In order to stay within the allowable height, the following must

be done:

a) The low and high pressure columns are placed side

by side. The reboiler- condenser may be located on

top of the high pressure column or at the bottom

of the low pressure column.

6
b) Regardless of the location of the reboiler - condenser,

a liquid pump is required. When the reboiler condenser

is at the bottora of the low pressure column, liquid

nitrogen must be pumped from the condensing side to

the top of the high pressure column to provide reflux

for that column. If the reboiler-condenser is at the

top of the high pressure column, then liquid oxygen

must be pixmped from the sump of the low pressure

column to the reboiler side.

c) The trays in the low pressure column must be as close together as is practical and feasible.

4.3 Allowable Weight

Although maximum weight is not specified, the expression

"mobile generator" implies the requirement of moving the generator

with reasonable ease. This means that the weight cannot be indiscriminately high. The selection of rotating machinery results in

lowest weight and unbalanced forces for compressor and expander.

With a rotary compressor, the pressure attainable is relatively low.

This affects the process from a standpoint of refrigeration available

at the lowest temperature levels. Refrigeration through efficient

expansion of air in a centrifugal expander may be obtained at higher

temperature levels.

In order to produce liquid oxygen efficiently, a considerable

amount of refrigeration must be made available to the low pressure

column at the lowest temperature level in the generator. Section 5 deals with this subject in more detail.

7
4.4 Condition of Feed Air

The condition of the feed air to the generator is important for

the determination of weight and size of the purification system. Most

important, the specification of Section 2 states that the air entering the

generator may be as high as 125°F in temperature and saturated with

water vapor. The amount of moisture contained by air in the saturated

condition is shown as a function of temperature in Table 4. 1. The

table shows that air saturated at 125°F will require a large amount of

refrigeration for the condensation and removal of water from the air.

Table 4. 1

Saturated Water Content of Air as a Function of Temperature

Temperature Water Content c Moles per Mole

125 0. 1530

120 0.1310

110 0. 0955

100 0.0695

90 0.0503

80 0.0359

70 0. 0255

60 0.0179

50 0. 0123

40 0. 0084

30 0. 00555

20 0. 00346

10 0. 00218

0 0.00127

8
5. Air Separation Process

5.1 Introduction

A considerable amount of process work was done during the

course of this study in order to increase the output of the liquid oxygennitrogen generator. In order to satisfy the conditions of limiting height

and rotating compression equipment, the process calculations were

repeated a number of times before the final cycle was selected. For

instance, the initial process employed a liquid nitrogen pump to return

the condensed nitrogen from the reboiler-condenser to the top of the

high pressure column. As a result of actual design of the low pressure

column and reboiler-condenser, it became necessary to make the low

pressure column as tall as possible, leaving no space available for

the reboiler-condenser at the bottom of the low pressure column. By

pumping liquid oxygen instead of liquid nitrogen, the reboiler-condenser

may be located at the top of the high pressure column allowing maximum

height for the low pressure column.

The diameter of the low pressure column was increased in the

final stages of the study when it became apparent that the desired

number of trays could be fitted in the low pressure column only by

decreasing vapor velocities and reducing the distance between individual trays

.

5.2 Process Description

Figure 1 shows the process schematic of the oxygen-nitrogen

generator. Air is drawn in by a blower (stream 1) and compressed

(stream 2) sufficiently to overcome the pressure drop in the R-12-air

heat exchanger and adsorber vessels. The air is cooled to approximately

40°F in the R-12-air heat exchanger where most of the water vapor is

9
condensed. The liquid water is separated from the air stream in the

knockout vessel. Water saturated air at 40 °F enters one of the two

adsorber vessels (stream 4). These vessels are filled with a desiccant

in which both water and carbon dioxide are adsorbed. Impurities other

than water and carbon dioxide will pass into the main recycle stream

(6) of the generator. Most of the impurities will liquefy in the air

stream (14) or in the reboiler- condenser of the generator. Finally,

these impurities will find their way into the liquid oxygen and will be

removed in the guard adsorber located in stream (19). The compressor

recycles air made up of a new feed air (stream 5) plus pure air coming

from the expansion engine discharge via the heat exchangers (stream 37).

The air is compressed to 80 psia and the heat of compression is removed

in the aftercooler. The aftercooler may be air or water cooled.

The compressed air is divided into three streams. The largest

part (stream 33) flows through exchanger I to the expansion engine and

returns as a low pressure stream (37) through exchangers I and II to

the suction of the compressor. This stream provides the refrigeration

required for cooling and liquefying the product of the generator. A

second stream (38) flows through exchangers I and II with very little

pressure drop and is liquefied in exchanger II. This stream provides

some of the reflux and all of the refrigeration required to operate the

low pressure column. A third stream (8) flows to the suction of the

blower and is compressed by the blower to a pressure of 105 psia.

This air is cooled in exchangers I and II to its saturation point (-274°F)

and flows to the high pressure column (stream 11).

Separation of the air into its components begins in the high

pressure column. Refrigeration required by the high pressure column

is supplied by an indirect route. Stream 13 is liquefied through heat

10
transfer to the expander discharge stream (35). (Recall that the

expander produces all of the cooling for the entire process.) After

passing through a control valve, the low temperature, low pressure

(stream 14) fluid is fed to the low pressure column. Pure liquid

oxygen is produced in the sump of the low pressure column. Part

of that liquid oxygen is pumped to the reboiler- condenser at the top

of the high pressure column. There, since the expander blower is

used to produce the 100-105 psia operating pressure in the high pressure

column; nitrogen condenses on one side of the exchanger as the oxygen

boils on the other. The high pressure is necessary to condense the

nitrogen at the temperature of the oxygen. The liquid nitrogen flows

down the high pressure column and the oxygen vapor is returned

(stream 39) to the s\imp of the low pressure column and starts to

flow upward.

The liquid stream leaving the bottom of the high pressure column

is essentially in equilibrium with the gaseous air entering the column

(stream 11) and contains approximately 36 percent oxygen (molar basis).

Some of the condensed nitrogen leaves the high pressure column

(stream 25) and becomes reflux for the top of the low pressure column

(stream 27). This liquid is subcooled in the subcooler through heat

exchange with the vapor leaving the top of the low pressure column

(stream 17). Also, when liquid nitrogen is produced by the generator,

the product nitrogen will be taken off after passing the subcooler

( stream 26)

.

Pure liquid oxygen from the bottom of the low pressure column

is removed from the generator after subcooling against cold nitrogen

vapor to reduce the flashing losses in the storage container receiving

the liquid (stream 21). Waste nitrogen gas leaves the top of the low

11
pressure column (stream 17). This waste nitrogen gas contains, in

addition to some oxygen, all of the argon present in the air fed to

the columns. In addition to the impurities listed in the specification,

the air fed to the columns also contains some neon, hydrogen, and

helium. These gases will not condense at -320°F, and if allowed to

accumulate in the generator, will finally prevent normal operation of

the condenser-reboiler . To prevent accumulation, a small bleed

stream is taken off the condenser and vented to the atmosphere.

' 5.3 Data for Process Calculations

Data required for the calculations are:

a) Thermodynamic data for air, nitrogen, oxygen, and

mixtures of nitrogen and oxygen [7, 8].

b) Vapor-liquid equilibria data for mixtures of

nitrogen-oxygen and argon at the pressures of the

high and low pressure columns [9, 10].

c) Distillation column tray design data [11, 12].

d) Heat transfer correlations for gases, boiling

liqmd, and condensing vapors [13].

e) Pressure drop correlations for flow of gases and

liquids [14].

f) Performance data for insulation [15].

6. Generator Operation

6.1 Introduction

Figure 1 shows schematically various control valves and

instruments; their selection and particular location in figure 1 are

the result of "operating" the generator on paper. The following

sections deal with the various phases of generator operation.

12
6.2 Generator Reactivation

The generator will be started only after a thorough drying

of all the low temperature components and reactivation of at least

one of the adsorber vessels for removal of water vapor and carbon

dioxide

.

To begin reactivation of the generator, the lobe blower and

air compressor are started. By closing the valves in streams 9, 33,

and 38, the compressor discharge stream is forced to recycle through

the pressure control valve between lines 32 and 37.

Some gas is taken from the compressor discharge at point A

before it has passed through the aftercooler. This warm gas is fed to

line 9 (point a') and warms successively heat exchangers I, II, and

the high pressure column before venting through the liquid nitrogen

product line. By opening the throttle valve in line 15, warm gas

enters the low pressure column. The flow divides two ways: one

part flows through lines 18, 19, 20, 21, and 39 to a vent and the liquid

oxygen product line; the other part flows through lines 17, 27, and

29 to the nitrogen vent line and the liquid nitrogen product line.

The rate of gas flow during the reactivation process is less

than the feed flow to the generator during steady state operation. The

feed gas is dried in the warm purification section of the generator

before it enters the air compressor system. Reactivation of the other

drier may be achieved in the normal manner by providing some gas to

the heater from stream 31.

6.3 Generator Startup

The generator will be started only after a thorough reactivation

process in accordance with Section 6.2 has been carried out and a reactivated adsorber for water and carbon dioxide removal is available.

13
The lobe blower and compressor are started and air begins to flowto the expansion engine through line 33. The blower of the expansion

engine will compress air from stream 8. This air will be returned

to the suction of the blower through the bypass valve between lines 9

and 32 until flow to the high pressure column is started.

When the discharge of the expander (stream 35) becomes cold,

flow through lines 38, 12, 13, and 14 into the low pressure column

will be started. Gas from the low pressure column is vented through

lines 29 and 31. The high pressure column is cooled with gas flowing

through lines 9, 10, and 11. The gas is vented from the high pressure

column to the low pressure column through lines 15, 25, 26, and 27.

With time, liquid starts to accumulate in the low pressure

column. When sufficient liquid is in the bottom of the column, the

liquid oxygen pump is started and condensation will start in the reboiler- condenser . The flows through lines 15 and 25 are adjusted to

accumulate liquid in the high pressure column.

6.4 Normal Operation

To reach the required purity of the oxygen product in the low

pressure column, the system will be operated at low capacity after

liqxiid levels have been established in both the high and low pressure

columns. The flow of liquid from the high pressure to the low pressure

column is not automatically controlled. The valve in line 15 acts as an orifice and initially this valve will be throttled to a low flow rate.

Next, the temperature controller in line 35 is activated. The temperature of the expander discharge gas (stream 35) will be controlled

manually by venting gas from line 29 directly to the atmosphere. This

will continue until the liquid oxygen in the low pressure column sump

becomes pure. Depending on the rate of venting from line 29, some

14
impure product may have to be delivered to the product line (21) and

vented away.

Once the required oxygen purity in the low pressure column is

obtained, the manual throttle valve in line 15 will be opened to its

normal steady state condition and excess liquid nitrogen will flow as

reflux (line 27) into the low pressure column. This flow rate is

controlled by a liquid level sensor in the sump of the high pressure

coliomn. Product oxygen will flow from the generator through a valve

controlled by the level sensor in the siimp of the low pressure column.

Rate of production is controlled by the temperature controller in line

35.

6.5 Liquid Oxygen Production

The procedure described in Section 6.4 is used to produce

liquid oxygen only. The liquid nitrogen made in the reboiler - condenser

is used completely for reflux to the high and low pressure columns.

6.6 Liquid Nitrogen Production

In order to produce liquid nitrogen, the valve in the generator

product nitrogen line 28 is opened. This will automatically reduce

the flow of liquid nitrogen to the top of the low pressure column and the

level controller in the sump of the low pressure column will reduce

the flow of liquid oxygen to line 21. Product oxygen purity will be

maintained since the liquid- to- vapor flow ratio in the low pressure

colvimn will be reduced. By varying the rate of liquid nitrogen withdrawal, the ratio of liquid oxygen to liquid nitrogen production may be

controlled at will.

15
6.7 Controls for Automatic Operation

The flow diagram in figure 1 shows that only the purification

system at the warm end of the generator is operated manually. Depending on the water content of the feed air and rate of production, the

adsorber vessels will be switched manually at intervals of 2-6 hours.

The length of interval will be determined by checking for breakthrough

of the impurities in the adsorber bed at certain sample points.

All other controls of the generator operate automatically once

the plant has been started and brought to equilibrium and full production.

The controls operate as follows:

6.7.1 Feed Air Flow Rate Control

A valve in line 2 allows air to vent back to the atmosphere

when the pressure in the discharge of the blower becomes too high.

6.7.2 Product Purity Control

Because of the limitation in height of the low pressure column

and the limited number of trays in this column, liquid oxygen purity

control is critical. The low pressure column is designed to produce

the required product purity when the reflux ratio in the low pressure

column is maintained at or below a design level. The reflux ratio in

the bottom of the low pressure column is controlled by the division

of flow between streams 11 and 14. The stream 11 flow rate determines

the rate of oxygen vapor generation for the bottom of the low pressure

column since all of the nitrogen in stream 11 is eventually condensed

at the top of the high pressure column. When the generator produces

liquid oxygen only, all of stream 11 becomes liquid and is fed ultimately

to the low pressure coliimn as streams 15 and 27. The total liquid flow

16
to the low pressure column is the sum of streams 14, 15, and 27.

With stream 11 fixed, only stream 14 may be varied to change the

liq\nd flow rate to the low pressure column.

The reflux ratio of the column is determined by the ratio of

flow of streams 15 + 27 + 14 and 11. If the reflux ratio is too high

and the required purity of the oxygen is not obtained, stream 11 flow

may be increased and stream 14 flow decreased. Ultimately the

flow rate of stream 11 is fixed by the expander -blower capacity,

therefore, variation of the reflux ratio is only possible by variation

of stream 14. If purity of the oxygen product is satisfactory, stream

14 flow will increase until the temperature controller in stream 35

indicates that all of the available refrigeration is being used. This is

the point of maximum production of the generator.

If the required purity cannot be obtained under these conditions,

the differential temperature controller in the low pressure column will

reduce stream 14 in an indirect way by opening a bypass valve in the

nitrogen vent off line 29. When this valve opens, refrigeration is

wasted and stream 14 will be reduced to maintain the proper discharge

temperature of the expander. The signal for operation of the bypass

valve is produced from the measurement of the difference of temperature of two points in the lov/ pressure column. Under normal operating conditions, each tray of the low pressure column has a characteristic

composition, and with it, a characteristic temperature. This temperature is pressure dependent. By measuring the differential temperature

of two points, the pressure dependence is eliminated. A change in

the characteristic temperature will be detected well before the purity

of the oxygen product in the bottom of the low pressure column drops

below the specified value. Corrective action is taken automatically and

the upset will not reach the bottom of the low pressure column.

17
6.7.3 Refrigeration Control

If the generator is equipped with a sufficient number of trays

in the low pressure column to produce the required purity within a range of reflux ratios, refrigeration control becomes the primary

control of the generator. Refrigeration is provided by the expander.

Excess refrigeration manifests itself in lowering the expander discharge temperature, and lack of refrigeration has the opposite reaction.

The valve in stream 14 will be controlled as a function of expander

discharge temperature for maximum liquid production.

6.7.4 Miscellaneous Generator Controls

In addition to the product purity and refrigeration controls,

other controls which make the operation of the generator semiautomatic are:

6.7.4.1 Level Controls

The sump of the high pressure column receives liquid from

the top of the column and delivers liquid to the low pressure column

through line 15 and its manual valve. The manual valve is set and

acts as a fixed orifice. The setting is not very critical, but should

be based on operational experience with the plant running at full

capacity. The valve in line 27 is controlled by the level controller

in the sump of the high pressure column to add the proper quantity of

reflux liquid nitrogen to the top of the high pressure column. The

flow of liquid oxygen from the sump of the low pressure column is

controlled by a level controller which acts on the liquid oxygen product

valve

.

18
6.7.4.2 Pressure Controls

The low pressure column pressure is controlled by a valve

in the discharge of the waste gas (line 31). This control valve operates

the low pressure column at constant pressure and at the same time

provides a bleed stream for the reactivation of the adsorbers. The

air compressor has a bypass control valve activated by a pressure

sensor in the discharge line of the compressor. This controller and

valve allow operation of the compressor without running the expander

or other parts of the plant. A valve in line 2 opens to the atmosphere

if the lobe-type air blower discharge pressure becomes too high;

therefore, the blower can be independently operated.

6.7.4.3 Temperature Controls

The R-12 refrigeration system is controlled by maintaining

a constant temperature of the feed air stream at the inlet to the knockout drum. The control normally is a pressure control in the R-12

evaporator since each temperature has a fixed related pressure.

6.7.4.4 Pressure

Pressure is measured in a number of the process streams to

ascertain that the generator is operating normally. Excessive pressure

drop, especially in the high pressure circuits, will indicate when

impurities break: through the cleanup system and freeze out in the heat

exchanger s

.

6.7.4.5 Temperature

Temperature measurement of the streams in and out of the heat

exchangers will also indicate freezeups.

19
6.7.4.6 Purity Analyzer

The oxygen content of the waste stream (line 29) and the

product streams will be monitored.

7. Impurity Removal

7.1 Introduction

The composition of the air, including impurities, is fixed,

except for the water content. The specification given in Section 2

states that the air entering the generator can be saturated with water

vapor and that the temperature of the air may vary from -30°F to + 125°F. Table 4.1 shows the amount of water in air as a function of

temperature. At the upper temperature limit of 125°F, some 9.72

moles of water need to be removed per hour from the feed air stream

of 63.7 moles/hr.

Refrigeration is required to remove the water and carbon dioxid

from the feed air. When the contaminants are removed as vapor, the

heat of adsorption must be removed. If the air is cooled before adsorption of the water vapor is accomplished, a large part of the water

is condensed in the cooling process. In that case, the heat of vaporization (condensation) must be removed in the cooling process.

There are three basic ways in which the main contaminants

of water and carbon dioxide may be removed from the air:

a) Cooling the air with an R-12 refrigeration system,

followed by physical separation of liquid water and

adsorption of the remaining water vapor and carbon

dioxide on a desiccant bed at approximately 40 ° F

.

20
b) Cooling the air by heat transfer to a low temperature

refrigeration stream and successive removal of

water and carbon dioxide by liquefaction and solidification in the heat exchanger. The liquefied and

solidified material finally will be removed by a purge stream of dry waste gas from the generator.

c) Cooling the air in regenerators so that most of the

impurities solidify, combined with some removal

of carbon dioxide in adsorber beds operating at

-230 to -240°F.

Each of the above methods have advantages and disadvantages.

Sections 7.2, 7.3, and 7.4 deal vidth each of the methods

.

7.2 Impurity Removal at or Near Ambient Temperature

The system chosen to remove water and carbon dioxide from

the feed air is shown schematically in figure 1. The feed air is

cooled to a temperature of 40 °F by Refrigerant 12 evaporating at

30-35°F. The duty of the exchanger (the amount of heat to be transferred) is a function of the air flow rate and the heat content of the

air at inlet and outlet (streams 2 and 3). When air of 125°F is fed

to the generator, the lobe-type blower will discharge the air at a temperature of l6l°F. Cooling the air from a temperature of l6l°F

and a dew point of 125°F to 40°F (saturated) requires removal of

212, 000 Btu/hr (63.7 moles/hr of dry air feed stream). Of this

amount 156, 500 Btu/hr is contributed by the condensation of water.

If the air fed to the generator has a dew point of 90 °F and a temperature of 125°F, the R- 12- air cooler need only remove 97, 600 Btu/hr.

21
It is obvious that the R- 1 2 refrigerator primarily functions to

condense water. The R-12 systera required to dry air with a dew

point of 125°F is approximately twice the size of that necessary to

condense water from air with a dew point of 90 °F. In order to

assure that all generator equipment fit the stipulated maximum

envelope of 8 ft by 8 , 5 ft by 15 ft, it has been assumed that the feed

air will have a maximum dew point temperature of 90° F.

After the air has been cooled to 40° F the liquid water is

separated in the knockout vessel. The diameter of this vessel is

large enough to separate the water droplets from the air stream and

to collect them in the bottom of the vessel. A liquid level controlled

valve will drain water from the vessel.

The diameter of the vessel is determined from laws applicable

to particle motion in a gas atmosphere under the influence of gravity.

A number of textbooks on this subject are referred to in [12]. The

general expression for maximum permissible air velocity in an

upward direction under which spherical fluid particles will settle

is as follows:

L - g U = K - in consistent units (1) V g

where U = air velocity in ft/ sec

L = density of water in Ib/ft^

' 3 g = density of air in lb/ ft

K = coefficient in ft/ sec. V

The value of K is normally chosen to be approximately 0.2

ft/sec. With this value, the diameter of the knockout vessel will

be about 1.3 ft

.

22
The general arrangement of the knockout vessel is shown in

figure 2. The wet gas enters tangentially above the internal baffle.

The water droplets are sl\ing against the wall of the vessel and the

initial motion of air and water is downward. At the bottom of the

baffle the air flow turns upward and flows out of the vessel through

the center exit pipe. Water collects in the bottom of the vessel.

Dimensions other than the diameter of the vessel are chosen on the

basis of practical experience.

After the bulk of the liquid water has been removed, the

saturated air at 40 °F flows to one of two parallel vessels containing

a desiccant. Design practice in industry normally assumes that

some liquid water in the form of very small droplets will be carried

beyond the knockout vessel. The amo\int chosen is set somewhat

arbitrarily, and in this case, was assumed to be 15 percent of the

amount of water carried in the saturated air. Table 4. 1 shows

that saturated air of 40 °F contains 0.0084 moles of water vapor per

mole of dry air. The desiccant beds, therefore, have to remove water

vapor at a rate of 1.15 x 0.0084 x 63. 7 = 0.616 moles/hr.

In order to save space, type 13X, molecular sieve desiccant

will be used; this material will remove both water and carbon dioxide

from the feed air. The capacity of the desiccant for water and carbon

dioxide vapor under various operating conditions is supplied by the

manufacturers of these materials. The design of the adsorber vessel

with regard to diameter, height, and method of reactivation was based

on data also supplied by the manufacturers.

The pressure drop through the bed determines the minimum diameter.

If the flow of air is upwards, then the pressure drop must not exceed

that value which would lift the particles and fluidize the bed. The air

flow, therefore, is normally downward.

23
Not all of the desiccant is fully utilized for the removal of the

water and carbon dioxide. In order to purify the air to a level of

a few parts per million the bed must have a concentration gradient

between inlet and discharge conditions. The length of bed used

for this gradient is a few feet.

Since water is preferentially adsorbed by the molecular sieve,

the entrance of the bed will contain all of the water and the downstream

part of the bed will contain the carbon dioxide. With time, the water

replaces the carbon dioxide, and if the bed is used too long, the first

impurity to appear in the air leaving the bed will be carbon dioxide.

In order to utilize the bed fully, a few sample points will be located

inside the bed at short distances from the discharge end of the bed.

Breakthrough of impurities at these sample points will warn the operator

that a limited amount of time is left in the adsorption cycle of the bed.

After the desiccant has been fully utilized, impurities are

driven off by passing dry gas with a temperature of 500 °F in the

opposite direction to air flow through the bed. The gas used for this

purpose is waste gas from the generator (stream 31). The total

amount of heat required to heat the bed, wall, and desiccant and to

vaporize water and carbon dioxide is some 130, 000 Btu. To supply

this amount of heat in a period of 2 hr, an electric heater with a capacity of 65,000 Btu/hr (20 KW) is required. The rate of flow

of waste gas during the reactivation period is 19 moles per hour.

Approximately 50 percent of the reactivation time is used for heating

and 50 percent for cooling to ambient temperature. Cooling is

accomplished simply by turning off the electric heater and continuing

the flow of waste gas. Figure 3 shows the general arrangement of

the adsorber vessel.

24
7.3 Impurity Removal by Liquefaction and

Solidification in a Heat Exchanger

Figure 4 shows the flow schematic of a set of heat exchangers

in which water and carbon dioxide are removed from the feed air

through liquefaction and solidification. The system is a modification

of the purification system discussed in [ 1 6] . The dark lines show the

flow of gas when exchanger A is being used for removal of the impurities. Exchanger B is being reactivated with waste gas from the

generator. Feed air from the lobe-type blower enters exchanger A

at the bottom. Cooling and liquefaction of water occurs in the bottom

section of the exchanger. Liquid water will, with proper design of

the heat exchanger, rundown into the warm bottom of the exchanger

and accumulate. At temperatures of 32^ F and lower, water will

freeze out and be deposited as snow on the walls of the exchanger.

At still lower temperatures the carbon dioxide v;dll settle out as snow. At the cold end of the exchanger the purified gas is mixed with

the cold gas from the expander discharge, flows through the return

passages of the exchanger and on to the suction of the compressor.

In order to prevent plugging heat exchangers I and II (figure 1),

it is necessary to lower the carbon dioxide content of the air to at

least 5 ppm. Table 7. 1 gives the equilibrium concentration of carbon

dioxide in air at various temperatures showing that the air must be

cooled to -260"F to reach 5 ppm. The refrigeration required to

condense the impurities and to make up for the losses in the heat

exchanger is supplied by cold gas from the expander discharge. The

flow rate needed is determined by the moisture content of the feed air.

The cooling curves of exchanger A or B under normal operating

conditions are shown in figure 5.

25
Table 7.1

Carbon Dioxide Content of Air as a Function of Temperature

Temperature

°F

-225

-230

-240

-250

-260

Curves I and II are for air with a dew point of 125°F and a temperature of 161°F at the inlet of the heat exchanger. The flow

rate from the expander is 1.142 moles per mole of feed air. Curves

III and IV are for air with a dew point of 90 °F and the same inlet

temperature. In this case, 0.442 moles of expander discharge air

are required per mole of feed air. Both the curves in figure 5 and

the cold air flow rates demonstrate the increase in cooling requirement as the dew point increases.

The expander must operate at low temperature so that the

column will function. Using part of the expander discharge stream

for feed air purification is thermodynamically inefficient because the

temperature is lower than necessary. The result, compared to the

R-12 system, is a reduction in liquid oxygen production of 10 percent

if the dew point is 90° F and 22 percent if the dew point is 125°F. The

reduction in generator capacity would actually be somewhat larger

since the heat exchangers would have to be cooled after each reactivation.

CO2 Content of Air

Parts Per Million of CO

320

190

60

17

5

26
7.4 Impurity Removal in Regenerators and Adsorbent Bed

Figure 6 shows the flow schematic of the regenerator cleanup

system of the existing liquid oxygen-nitrogen generator [2].

Successful operation of the regenerator system depends on

some liquid water removal after the air has been compressed. The

regenerator contains a mass of solid material with large surface

area. The amount of mass determines the length of time during

which the regenerator can remove impurities. In order to maintain

the required temperature gradient regardless of the number of cycles,

part of the impure air flowing through the regenerator is removed

before it reaches the cold end of the regenerator. Final purification

of this bleed air is accomplished in an adsorber. A guard adsorber

is used to remove trace quantities of impurities from the air flowing to the cold end of the regenerator.

The refrigeration required to freeze the water is obtained

from the expansion turbine. Therefore, the generator employing

a regenerator purification system will have a lower production

capacity than a generator employing a separate R-12

refrigeration system and warm adsorbers.

7.5 Choice of Purification System

The objective of the study is to maximize the production

capacity of the generator within a given envelope and with the

assumed air supply. The use of the purification system employing

R-12 refrigeration and adsorption equipment operating at 40°F will result in the highest production capacity of the generator.

27
8. Distillation Columns

8. 1 Introduction

Since the compressor flow and pressure were fixed through

the use of an available component, it was necessary to determine

the proper balance between flow required for separation and that

required to provide the necessary refrigeration for the separation

process. In addition, the space requirement imposed by the

envelope size places a serious limitation on the number of trays

that can be used in the low pressure column. Calculations to satisfy

the material and energy balances around the columns and total

generator resulted in the process conditions shown on the flow sheet.

In order to use a conventional double column design utilizing a condenser- reboiler, it is necessary to raise the high pressure column

pressure above the 80 psia available from the air compressor to

provide a sufficient temperature difference between the condensing

nitrogen vapor and boiling liquid pxygen. By using the air blower

which dissipates the work of expansion of the turbine expander, air

for the high pressure column is compressed to 105 psia. The high

and low pressure columns were designed for pressures of 100 and

22 psia, respectively.

8.2 High Pressure Column

Since there is no appreciable argon enrichment in the high

pressure column, it is possible to use the McCabe- Thiele Diagram

[17] for the binary mixtures of oxygen-nitrogen to make the separation

process calculations. The column design is based on sieve tray

columns. Available data for sieve tray performance in low temperature separation processes [18] provide the required information to

28
properly size and design columns. Figure 7 shows a schematic high

pressure column design. Figure 8 shows a typical tray configuration

for the column.

From the vapor-liquid equilibrium data for the oxygen-nitrogen

system (figure 9) and the process conditions, the theoretical number

of trays required for the high pressure column was determined to be

15. Overall tray efficiency for a high pressure column sieve tray

operating with a 1 in unaerated liquid depth will be 100 percent [18].

Consequently, the actual number of trays will be 1 5 . Performance

data available from low temperature sieve columns [18] show that

the column diameter for a 3 in tray spacing is 12 in. Providing

adequate space for the vapor outlet line, the feed line and the liquid

outlet line, and adequate liquid holdup in the bottom of the column

results in an overall column height of 67 in.

8.3 Low Pressure Column

The design of the low pressure column is seriously affected

by available height and refrigeration. The argon- oxygen-nitrogen

equilibrium data in figure 10 show that at high oxygen concentrations

the effect of argon on the separation process becomes very pronounced .

To produce liquid oxygen of the required purity, the reflux

ratio in the bottom of the low pressure column must be low. The

maximum value is determined by the limited number of trays which

can be installed in the available height of the column. In order to

obtain a low reflux ratio, the rate of oxygen vapor generation in the

bottom of the column should be high. This is difficult to do with a process in which the highest pressure available is 100 psia.

29
The low pressure column must be refrigerated with either

liquid air or nitrogen. To maximize the liquid oxygen production,

liquid air and/or nitrogen flow must be high. The two requirements

of low reflux ratio and high refrigeration supply need to be balanced

carefully when the number of trays is limited and the highest pressure

available is 100 psia. Insulation of the low pressure column is very

important. Heat leak will reduce the output of the generator directly.

A maximum of 33 trays can be installed in the height available

for the low pressure column. Tray spacing is then 2.6 in. The

efficiency of the tray with 1 . 0 in of unaerated liquid is of the order

of 80 percent. The theoretical number of trays is then 26 and the

reflux ratio in the bottom of the column is 1.28. The process data

given in the table of figure 1 just allow this reflux ratio.

Figure 11 shows a cross section of the low pressure column.

It is possible to reduce the tray spacing below the 2.6 in used; however, data are not available for smaller tray spacing in air separation

plants. Various packing materials (as in petrochemical separation

processes) may be used in the column to reduce the height of a theoretical tray. Height equivalents to a theoretical tray of 1 . 5 to

2.5 in have been reported [12] but low temperature separation data

are not available for these packings. Future generator development

might consider the use of packed columns for an increase in the

numbers of theoretical trays available in a fixed height.

9. Heat Exchanger Design

9.1 Introduction

The low temperature heat exchangers in the generator will be

the plate-fin type which is used extensively in low temperature process

plants. Such heat exchangers have the following advantages:

30
a) More than two streams can be accommodated in the

same exchanger.

b) Large amounts of surface area per unit length are

available; therefore, the height of the exchangers

will be less than for other types.

c) Total cross sectional area for plate-fin- type ex- changers compares favorably with other types

of exchangers.

d) Pressure drop can be controlled by the proper

selection of plate spacing and fin dimensions.

Heat transfer correlations for the plate -fin- type exchangers

are available from the various maniifacturer s . The correlations

are based on a great amount of actual experience and are, therefore,

quite reliable in predicting the performance of the exchanger. Computer programs are utilized by the manufacturers to arrive at ex- changer size and configuration.

Heat exchangers for the compressor, blower after- coolers,

and R-12 condenser are commercially-available units of the conventional forced- air finned-tube type. For shipboard operations

these would be commercial shell and tube heat exchangers utilizing

seawater for the cooling fluid. The R-12- gas heat exchanger is a commercially- available shell and tube heat exchanger.

9.2 Heat Exchanger I

The design of heat exchanger I of figure 1 is based on the

process data indicated on the flow sheet. In order to permit manufacturers of the plate- fin- type heat exchangers to determine the size

and configuration of the exchanger, a cooling curve of the exchanger

31
was prepared as shown in figure 12, Properties of the gases used

for the construction of the cooling curve have been taken from published thermodynamic properties of gases.

The cooling curve for heat exchanger I is typical of countercurrent flow low pressure gas-to-gas heat exchangers. Since the

specific heat of the gas of the various streams is nearly constant,

the cooling curves are essentially straight lines. The total duty for

5 heat exchanger I is 6. 29 x 10 Btu/hr. Using the process conditions

as given on the flow sheet and the cooling curve, a typical plate -fintype manufacturer's computer program has indicated that the size of

heat exchanger I would be 12 in by 12 in core cross section by 90 in

long. Headers are provided on each side of the ends to minimize

total length.

9. 3 Heat Exchanger II The design of exchanger II of figure 1 is based on the process

data shown on the flow sheet. The cooling curve for exchanger II is

shown in figure 13. The cooling curve for exchanger 11 shows a severe

pinch at the point where air at 78 psia pressure starts to liquefy (stream

12). This particular pinch point will determine the production level

of the generator. As the pressure of the air to be liquefied is reduced

below 78 psia, the temperature difference at the pinch point approaches

zero. In reality the temperature difference will remain positive and

the temperature difference at the warm end of the exchanger will

increase. The net result is a loss of refrigeration and reduced

output of the generator. To insure that the pressure at the point of

liquefaction is at its maximum, the pressure drop in stream 12 from

the compressor through heat exchanger I is kept at a minimum.

32
Total duty for exchanger II is 5.01 x 10 Btu/hr. From the

cooling curve and process conditions, a typical manufacturer's

computer program has indicated that the heat exchanger will have

a cross section of IZ in by 12 in by 90 in long.

9 . 4 Subcoole r

Subcooling the liquid nitrogen feed to the top of the low pressure

column reduces flashing losses when expanding from 100 psia to 22 psia.

Refrigeration for subcooling is supplied by the waste nitrogen from the

low pressure column. In addition, the product liquid oxygen is subcooled to minimize flashing losses. Total duty of this heat exchanger

is 8.81 X 10 Btu/hr. The manufacturer's computer program has

indicated that the exchanger cross section will be 8 in by 6 in by 48 in

long.

9.5 Condenser- Reboiler

The condenser- reboiler condenses the nitrogen from the top

of the high pressure column to provide reflux liquid to both the high

and low pressure columns. The liquid oxygen boils as the warmer

nitrogen vapor is cooled and condensed. The oxygen vapor is then

fed to the bottom of the low pressure column. Total duty for the

5 condens er- reboiler is 1.07 x 10 Btu/hr. The reboiler- condens er must be less than 28 in high to fit into the envelope and will have a core size of 12 in by 12 in.

9.6 Compressor and Blower Aftercoolers

Compressor and blower aftercoolers are commercial forcedair finned-tube heat exchangers. The main air compressor aftercooler duty is 7.92 x 10 Btu/hr. Size of the unit, including fan and

headers, will be 3 ft by 5 ft by 5 ft. The expander blower aftercooler

33
duty is 4,56 x 10 Btu/hr. This aftercooler consists of two units in

parallel, each being 3 ft in diameter by l-l/Z ft high.

9.V Refrigerant 12 Condenser and Air Cooler

The R-12 condenser is a commercially available forced- air

finned-tube heat exchanger. The unit selected consists of two equal

size parallel units, each being 45 in by 42-1/2 in by 39-3/8 in.

The R-12 evaporator is a flooded shell and tube heat exchanger.

5 Expected duty on this heat exchanger is 1.06 x 10 Btu/hr. The

exchanger is a 2 pass shell and tube heat exchanger, 12-3/4 in in

diameter and 108 in long.

9.8 Defrost Heater

A 20 kilowatt electric heater is provided to heat the defrost

gas. The unit is a commercially available electric tubular element

inline heater complete with temperature control. Size of the unit

will be 8-5/8 in in diameter by 60 in long.

10. Machinery

10.1 Introduction

The liquid oxygen- nitrogen generator has a number of moving

components. The number is larger than for the existing generator

in order to make more efficient use of the refrigeration. The work

done with regard to machinery consisted of contacting vendors of the

required components. As stated in Section 10.5, the air compressor

chosen for the generator is a unit which is very similar to the unit

now used. The major difference from a standpoint of process design

is the capability of circulating dry air free of impurities in a closed

cycle

.

34
All other machinery is commercially available in standard

units with a proven operating record. Advancements in the state of

the art are not required.

10.2 Expansion Engine and Blower

A combination expansion engine and blower is available

commercially to satisfy the process conditions shown in figure 1.

The expander is a rather standard unit of a size supplied and in

operation since 1966. The blower uses all of the power available

from the expander. Its efficiency is rather low due to the relatively

small flow rate of air.

10.3 Lobe- Type Blower

Feed air to the generator is compressed to approximately

18 psia, sufficiently high to force the gas through the purification

system into the suction of the main compressor. The blower is a commercially available unit which compresses air by the action of

two figure eight- shaped impellers mounted on parallel shafts inside

a casing rotating in opposite directions, A timing gear on each of

the impeller shafts controls their relative position. The blower

capacity is independent of the discharge pressure and the air is

delivered free of oil. In order to eliminate the need for controls

for the operation of the blower, a pressure controlled valve in the

discharge line will return air to the atmosphere when the discharge

pressure exceeds a certain level.

10.4 Refrigerant 12 System

The R-12 compressor with the evaporator and piping will

form a closed system. All components of the system are commercially

available and will be procured by specification. A deviation from using

35
standard packaged equipment will only be made when it is necessary

to fit the equipment into the envelope.

The size of the R-12 compressor chosen is dictated by the

condition of stream 2 (figure 1). As mentioned in Section 7.2, the

most severe condition chosen for the design of the generator is feed

air of a 125°F temperature and a dew point of 90° F.

The R-12 unit provides 120,000 Btu/hr of refrigeration at

an evaporator temperature of 30°F. The condenser operates in an

environment of 125°F with a 20°F approach. (Refrigerant 12 condenses at 145° F. )

10.5 Air Compressor

The existing generator employs a compressor which supplies

air to the combustion chamber and drive turbine. In this study it has been assumed that a turbo compressor will be used, driven by

an independent power turbine. With the power and compression

systems separated, only the air fed to the columns must be purified.

10.6 Liquid Oxygen Pump

Either a reciprocating or centrifugal pump may be used to

lift the 3 7.2 moles per hour of liquid oxygen from the bottom of the

low pressure column to the reboiler- condenser some 8 feet higher.

The actual volume pumped is only 17 cubic ft/hr (8.15 cubic in per

sec) and is somewhat small for a centrifugal pump. Because of

the limited liquid level in the sump the pump must operate with a very low NPSH (net positive suction head). A centrifugal pump with

inducer capable of operating with a very low NPSH is commercially

available

.

36
There are reciprocating liquid oxygen pumps on the commercial market but most of these are designed for very high discharge pressures. Our low discharge pressure alleviates many

of the design problems. A low speed pump, with rather large displacement and large inlet valve, is best suited when the NPSH is

minimum.

- 11. Insulation

11.1 Introduction

Heat leak to the low temperature equipment reduces production capacity in any liquid oxygen generator but is especially damaging

in smaller units. Selection of the proper insulation, therefore, becomes critical for the cold end of the generator. There are several

types of insulation used extensively in low temperature separation

plants. These include foams, gas-filled powders and fibers,

evacuated powders and fibers, vacuum, and evacuated multilayer

insulations. Choice of insulation is based on insulation effectiveness

and such considerations as application, v/eight, and cost. Performance of the various insulations between ambient and the lowest

temperature encountered in the air separation process are given

in Table 11.1.

When space is limited, evacuated insulations offer the lowest

heat flux. When properly designed and constructed, vacuum insulated

containers have proven reliable and durable. When more space is

available or when higher heat fluxes can be tolerated in parts of the

process operating at higher temperature levels, gas-filled powders

offer reasonable heat flux with simplified construction and low cost.

37
Table 11.1

Performance of Various Insulation Systems

Type of Insulation Heat Flux

Btu/Hr Ft

3

6 in polyurethane foam (2 lb/ft ) 15.3

6 inperlite (5-6 Ib/ft^ filled W/N2) 15.2

High vacuum (e = 0. 02) 2.84

6 inperlite - vacuum (5-6 Ih/ft^) 0.50

6 in vacuum-multilayer insulation 0.01

11.2 Column Cold Box

Figure 14 shows a proposed arrangement of the cold box in

which the lowest temperature components of the generator (i.e.,

columns, subcoolers, liquid oxygen pump) are housed. The cold box

consists of a lower vacuum insulated container with a load bearing

flat plate bottom. This container is 60 in O.D. by 42 in high. An

omega bellows in the outer wall compensates for the thermal contraction of the inner wall and maintains the load bearing capability of the

bottom. The upper vacuum- jacketed cylindrical container is 44 in

O.D. by 82 in high. Again, an omega bellows in the outer wall

provides for thermal contraction. The inverted cylinder upper vessel

is located off center from the lower cylinder and is supported by the

flat nonvacuum- jacketed cover of the lower container. To prevent

excessive heat transfer between the nonvacuum- jacketed cover plate

and the cold bottom of the lower container, the upper vacuum jacket

extends some 2 ft below the plate. With this arrangement both upper

and lower containers are readily removed for access to the cold

equipment.

38
None of the components located inside the vacuum insulated

jacket need to be vacuum tight since a waste gas atmosphere of

1 atmosphere is maintained inside the box. In case of problems

with the vacuum insulation, either the upper or lower container may

be quickly replaced.

In order to prevent heat transfer by convection currents, all

components and piping inside the cold box are wrapped with fiberglass

blankets. The total heat leak to the cold components is estimated to

be approximately 500 Btu/hr.

11.3 Heat Exchanger Cold Box

Insulation for heat exchangers I and II will be gas- filled

powder. The relatively high heat leak of gas-filled powder will

not affect the performance of the generator greatly, since this heat

is directly removed by the return cold gas stream from the expansion

engine

.

The heat exchanger cold box will be made of structural support

members with sheet metal sides. Provisions will be made to add to or remove the powder insulation. A nitrogen purge will be supplied

from the waste nitrogen stream to prevent moisture from entering

the insulation.

11.4 Insulation of Purification System

Insulation is required for the R-12-air cooler and purification

systemi. The operating temperature of the R-12 unit is 30 °F and

pre-formed foam insulation will be used. The drier and carbon

dioxide removal unit, however, will experience temperatures of the

order of 500° F during reactivation of the adsorbent bed. Commercially

39
available magnesium oxide will be used to insulate the adsorbers for

both high and low temperatures.

12. Physical Arrangement

12.1 Introduction

The space available for all of the equipment is 8 ft by 8 . 5 ft by

15 ft. All equipment, with the exception of the components in the

column cold box, are commercially available components. No

attempt was made to optimize the packing arrangement of these

components through rearrangement of piping, instruments, etc.

Rough scale models in the form of cubes, square boxes, and cylinders

were used to fit the equipment in the allotted space.

It appears that the space available is sufficient to accommodate

the 4 tons per day generator equipment when the feed air has a maxim\im dew point of 90°F and a temperature of 125°F. A dew point of

125°F requires a bulkier R-12 system and it is not clear that the

larger R-12 system can be accommodated. Packaging into a minimum

volume of all the components with piping, instrumentation, access

for maintenance, etc. is a major job and outside the scope of this

study.

12.2 Column Cold Box Arrangement

Figure 15 shows the proposed cold box plan view and partial

elevation. The plate between the upper and lower vacuum- insulated

cylinders will be supported from the structure of the generator

envelope. All penetrations into the cold box pass through the support

plate, i.e., all valves, transfer lines to the other cold box and for

products, and the liquid oxygen pump.

40
Figures l6 and 17 show the plan and elevation view of the

equipment inside the cold box. An equipment support plate is

structurally connected to the main support plate between the two

vacuum- insulated cylinders. The columns and heat exchangers will

be equipped with bumpers at the top of the box to insure stability

in the horizontal plane.

12.3 Equipment Arrangement

Figures 18 and 19 are the plan view and elevation view of

all the generator equipment in the envelope. The figures show that

the space occupied by the air separation equipment (heat exchangers,

expander- blower , and columns) occupies only 30 percent of the total

sp ac e

.

The air compressor with its aftercooler and associated

equipment occupies the same space as the existing portable generator

The purification unit (R-12 compressor, condenser, air cooler,

drier, and CO^ adsorbers) occupies 20 percent of the total space.

There is sufficient space for instrumentation and a control panel

located on one side of the generator. The remaining space would

be utilized for piping and valves. A complete and detailed design

of the generator unit, including enclosure, supports, piping, and

valves, may result in some revision of the equipment; however,

sufficient space is available for the required equipment to satisfy

the 4 tons per day design conditions.

12.4 Instrumentation and Controls

Instrumentation and controls for the generator are commercia

available types for temperature, pressure, liquid level, and flow.

Control functions utilize commercial pneumatic controllers. Air for

41
operation of the pneumatic controllers is supplied from, the main

air compressor with high pressure gas cylinders for startup. For

machinery operation and protection, electric controls are provided

as required. Electric power is supplied from the generator unit

operating from the main air compressor driver.

Once the plant is in normal operation, the automatic controls

maintain the proper process parameters. Failure of any process

equipment or controller automatically shuts the generator down in a safe condition and indicates on a master alarm panel the cause of

the shutdown. After corrective action is taken, the plant is manually

placed back in operation and the unit returned to automatic operation.

12.5 Power and Weight

The following tables list the weight and power requirements

of the required equipment.

. Table 12.1

Equipment Weight

Description Weight- Lb.

1. Air Compressor Package

(Includes Driver and Electrical) 1, 000

2. Air Compressor Aftercooler 2, 500

3. Lobe Blower 720

4. H^O Knockout Drum 50

5. Drier and CO2 Adsorber 2,600

6. R-12 Compressor Package 1,060

7. R-12 Condenser 1,100

8. R-12 Air Cooler 1, 350

9. Column Cold Box 1,000

42
Table 12.1 (Continued)

Description Weight-Lb.

10. High and Low Pressure Columns 300

1 1 . Subcooler 60

IZ. Condenser- Reboiler 60

13. Liquid Oxygen Pump 50

14. Heat Exchangers I and II 700

15. Blower Expander 100

16. Blcwer Aftercooler 332

17. Piping, Valves, and Instrumentation 800

18. Electric Heater 195

TOTAL 13, 977 Lb.

Table 12.2

Power Requirements

Description Power-H.P.

1. Air Compressor (Lube System

and Starter Motor) 2.00

2. Air Compressor Aftercooler 15.00

3. Lobe Compressor 10.00

4. R-12 Compressor 30.00

5. R-12 Condensers 1.50

6. Blower-Expander Lube System 0.25

7. Blower Aftercooler 1.00

8. Liquid Oxygen Pump 0.10

9. Electric Heater 27.00

10. Instrumentation and Controls 2.00

11. Lighting 2. 00

TOTAL 90. 85 H. P.

43
From the Equipment Weight Table, 12.1, it can be seen that the

weight of the purification equipment is 6, 880 lb or 49 percent of the

total equipment weight. The weight of the air compressor package

and air separation equipment is 6, 102 lb or 43. 5 percent of the total

weight. The effect of adding the high temperature level R-12 refrige

tion equipment with the warm moisture and adsorbers is to

essentially double the weight of the generator.

' 13. Conclusions and Recommendations

The following conclusions may be drawn from this study:

1 . A capacity of approximately 4 tons per day of liquid oxygen

or liquid nitrogen appears feasible within an envelope of

8 ft by 8. 5 ft by 15 ft, with a maximum weight of 15, 000 lb,

and with an existing air compressor package. The main

features of the generator are:

a) A closed cycle R-12 refrigeration system cools the

feed air to the generator to approximately 40° F and

condenses most of the water in the feed air.

b) A purification system consisting of two desiccant

beds filled with molecular sieve. Type 13X, removes

all of the remaining water vapor and carbon dioxide

at a temperature of 35-45°F.

c) The expansion engine generates the power to compress

some of the feed air to a pressure of 105 psia. This

makes it possible to operate a more or less standard

double column with reboiler- condenser, improves the

utilization of available refrigeration, and increases

the fraction of oxygen separated from the feed air.

44
d) Height limitations of the generator makes it necessary

to use a small liquid oxygen pump.

e) Using vacuiim insulation reduces heat leak to the

distillation columns and reboiler- condenser to a minimum.

The increase in production capacity using the same pressure

and compressor flowrate is due primarily to the addition

of the R-IZ unit, i.e., large amounts of high temperature

refrigeration. The process is thermodynami cally more

efficient at the expense of greater complexity. Many times,

thermodynamic efficiency is a poor criterion by which

to judge cryogenic equipment intended for field use. If

a higher capacity air compressor could be used, then

some of the expander refrigeration capacity could be

employed to chill the feed air for water removal. The

process would be thermodynamically less efficient but

would be simpler and probably more reliable and easier

to maintain.

Within the allowable height of 8 . 5 ft, the columns must

be placed side by side. The low pressure column design

and operation are critical. The reflux ratios are fixed and

the number of trays which can be installed within the 8 . 5 f

t

height is barely enough to produce liquid oxygen of the

specified purity. If, for some reason, the number of

trays proves to be insufficient, the required purity of

the liquid oxygen can be obtained by either drastically

reducing the output of the generator or using packed

c olumn s

.

45
4. The water content of the feed air to the generator is veryimportant. A high water content adds weight and bulk to

the generator through increased requirements for the

purification system. It might be advantageous to plan

for a separate package consisting of an R- 1 2 refrigeration system and water knockout drum for localities of

extremely high humidity and air temperature and to

design the basic generator for operation in more moderate

climates.

5. To obtain reliability and simplicity of operation, the

portable liquid oxygen generator should not be optimized

in terms of production rate relative to input power.

46
14. References

1. ASTM metric practice guide, Nat. Bur. Stand. (U. S. j Handb. lOZ,

(March, 1967).

2. Navair 19075 AAC-8, Operation, service, and repair instructions

with, illustrated parts breakdown; Liquid oxygen /nitrogen

generating plant Model GB-IA (June 1967).

3. BrowTi, Ross, M. , A portable liquid oxygen plant employing a low pressure cycle, Paper No. 51a, AICHE National Meeting

61st, Houston, Texas, Feb. 19-23, 1967.

4. Arnett, R. W. , and Muhlenhaupt, R. C. , Study of oxygen and

nitrogen generating systems. Unpublished NBS Report

(Sept. 1970).

5. Barron, Randall, Cryogenic Systems (McGraw Hill, 1966).

6. Ruheman, M. , The Separation of Gases (Oxford University Press,

1949).

7. Claitor, L. C. , and Crawford, D. B. , Thermodynamic properties

of oxygen, nitrogen and air at low temperatures. Trans. ASME

7J. , 885 (1949).

8. Cryogenic Consultants, Inc. , Proprietory information.

9. Linde Report, AIChE meeting, Sept. , 1956,

10. Cryogenic Consultants, Inc. , Proprietory information.

11. Latimer, R. E. , Private communication, 1958,

12. Perry, J. H. , Chemical Engineer's Handbook, 4th edition

(McGraw Hill, 1963).

13. Manufacturer's computer program for heat exchanger design.

14. Flow of fluids through valves, fittings, and pipe. Crane Tech.

Paper No. 410 (1942).

47
15. Barron, Randall, Cryogenic Systems, p. 497 (McGraw Hill,

1966).

16. Crawford, D. B. , Elliot oxygen process and impurity removal

system, Chemical Eng. Progress 46^ , No. 2 (1950).

17. McCabe, W. L. , and Thiele, E. W. , Ind. Eng. Chem. 17 , 605 (1925).

18. Latimer, R. E. , Private communication, 1958.

48
49
STREAM 4

tl

WATER

WATER DRAIN

FIGURE 2. GENERAL ARRANGEMENT OF THE

WATER KNOCKOUT DRUM

50

STREAM 35

LI

T

>

<

\

> /

<

> <

\ >

/

<

>

<

-CXh

c

TO

STREAM 6

FROM

STREAM 31

FIGURE 4. IMPURITY REMOVAL THROUGH LIQUEFACTION

AND SOLIDIFICATION

52
53

12" ID

FIGURE 7. HIGH PRESSURE COLUMN CROSS SECTION

55
INLET WEIR OUTLET WEIR

DOWNCOMER

DOWNCOMER

PLAN VIEW

LIQUID FLOWING

DOWN COLUMN

VAPOR RISING UP COLUMN

L

n

O O C> Qi 2 0

b « Q

"1

FIGURE 8 . COLUMN CROSS SECTION TYPICAL TRAY

56
57
90

80

70

60

50

40

3C

10 20 30

» NITROGEN

IN VAPOR

ARGON CORRECTION

22 PSIA

20

4! si el

% NITROGEN IN LIQUID

22 PSIA

-45 LINE

OPERATING

LINE

15

OPERATING

'LINE

11

10

% AKGON

IN VAPOR

ARGON CORRECTION

22 PSIA

IC

FIGURE 10. VAPOR-LIQUID EQUILIBRIUM OF ARGON-OXYGEtl-NITROGEN SYSTEM

FOR LOW PRESSURE COLUMN

% ARGON IN LIQUID

4 5 6

58
FIGURE 1 1 . LOW PRESSURE COLUMN CROSS SECTION

59
60

60" OD

LOAD BEARING INSULATION

OMEGA BELLOWS

VACUUM

2'-3"

OMEGA BELLOWS

VACUUM

EXCHANGER SUPPORT PLATE

44" OD

VACUUM

.42"OD_

.44"OD_

rm

.58"OD_

.64"OD_

-60"ODII 1 1 1 n 1 1 1 1

£

5'-0'

3'-6"

LOAD BEARING INSULATION

FIGURE 14. VACUUM INSULATION FOR COLUMN COLD BOX

62
FIGURE 15. COLUMN COLD BOX PARTIAL ELEVATION

63
FIGURE 16. COLUMN COLD BOX ELEVATION

64
FIGURE 17. COLUMN COLD BOX PLAN VIEW
66
67
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1 1. PUBLICATION OR REPORT NO. 2. Gov't Accession

B.BUOGRAPH.C DATA |NBsij,73_3n

3. Recipient's Accession No.

j

4. TITLE AND SUBTITLE

STUDY OF A MOBILE LIQUID OXYGEN-NITROGEN

GENERATOR

5. Publication Date

6. Performing Organization Code

7. AUTHOR(s) Peter C. Vander Arend and Stanley T. Stoy Thomas R. Strobridge

8. Performing Organization

NBSIR 73-311

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

Roulder, Colorado 80302

10. Project/Task/ Work Unit No. 2750556

11. Contract/Grant No. PO 2-8041

12. Sponsoring Organization Name and Address

Naval Air Engineering Center

"PViil arlplnlni a Penn svl vania 1912Z

13. Type of Report & Period

Covered

Final-2/25/72 - 12/31/

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

72

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

It appears feasible to produce as much as 4 tons per day of liquid oxygen

or liquid nitrogen of 99. 5 percent purity with the equipment contained in an envelop^

8 ft by 8. 5 ft by 15 ft. The process cycle is completely described. Physical

layouts were studied to assure that the equipment required for the modified process

would fit into the allotted space. A conceptual layout of the cold components, with

the required vacuum insulation, was developed.

Major modifications to the existing 2 ton per day generator consisted of:

a) the use of a blower on the expansion engine to pressurize part of the air stream

to 105 psia, b) the use of a liquid oxygen pump to move liquid from the sump of the

low pressure column to the reboiler side of the reboiler condenser and c) the

purification of feed air in a separate system before it joins the major recycle

stream. It was assumed that a compressor would supply air to the process at

the same pressure and flow rate as the existing unit.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Air separation; liquid nitrogen; liquid oxygen; mobile; purification.

18. AVAILABILITY STATEMENT

1 1 UNLIMITED.

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(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

[X] FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

20. SECURITY CLASS

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22. Price

USCOMM-DC 66244-P7I