NBSIR 74-631

Analysis of Proposed Air Drying Process

Lewis Greenspan

Humidity Section

National Bureau of Standards

Wastiington, D. C. 20234

December 1974

Quarterly Report

Prepared for Goddard Space Flight Center

Greenbelt, Maryland 20771

NBSIR 74-631

ANALYSIS OF PROPOSED AIR DRYING PROCESS

Lewis Greenspan

Humidity Section

National Bureau of Standards

Washington, D. C. 20234

December 1974

Quarterly Report

Prepared for Goddard Space Flight Center

Greenbelt, Maryland 20771

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

Abstract

NASA Proposal No. 7A01-6-01A by the GCA Corporation describes a proposed

air drying process which has been analyzed. The drying process,

intended to reduce the water vapor content of a stratospheric sample

to below one part per million by weight, is based upon the removal

of the water as It passes through a phosphorous pentoxide absorber.

Our analysis indicates that the method is feasible. Dependence on

ram pressure to maintain flow through the absorber appears questionable

and consideration should be given to other means.

Key Words: Absorption; drying; phosphorous pentoxide, water vapor.

1
Preface

GCA Corporation has made a proposal to the National Aeronautics

and Space Administration for the development of a new technique with

which to measure water vapor in the stratosphere. This technique

is based on the discovery that ion plasma oscillations in coaxial

corona discharges have a frequency dependence related to the

presence of trace gases, including water vapor. The proposed

technique is to determine the frequency difference between two

separate discharges, one in which the stratospheric gas is present and

the other in which the stratospheric gas with the water vapor removed

is present. The frequency difference would then be related to the

quantity of water vapor removed.

The National Bureau of Standards was asked to analyze the proposed

method of water vapor removal with the purpose of determining its

feasibility. Our conclusion was that the proposed method of drying

was feasible within the required restraints, but that a mechanical

blower or other positive means might be necessary to produce sufficient

flow through the required absorber configuration.

2
Introduction

The GCA Corporation proposal does an analysis of a proposed air

stream drying in situ in the stratosphere. Their proposal involves a

3 ^2^^ desiccant with a minimum flow rate of 10 on /s at pressure altitude

equivalents up to 53 fen. The goal is to achieve an effluent gas stream

with a water vapor concentration of less than one part of water vapor

per million parts of air by weight. Their proposal is to operate the

drying system at -13''C. No discussion is given as to how the temperature

would be maintained in the stratosphere.

Rather than doing a critique of their analysis, we did an

independent worst case analysis (53 km pressure altitude equivalent)

of our own. This is a worst case because for a given vapor content

the parts per million are lowest at the highest ambient pressure. We

used the -13° C temperature of their proposal in our analysis. Some

parts of our analysis are the same as the GCA analysis and others

differ considerably. Vvhere appropriate, certain aspects of their

analysis are referred to and commented upon.

3
Discussion

The subject proposal uses 0.25 x 10"^^ g/citi^ as the equilibrium

water vapor concentration for ^2^^ atmosphere of air and 298 K.

This is in agreement with results obtained by Morley'^^^ for a flow

of three liters per hour through absorption tubes filled with ^2^5

and is probably as good a value as is available. At this low flow

rate, it is likely that vapor pressure equilibrium was achieved.

This is equivalent to .02 PPM by weight at one atmosphere and 298 K.

At a pressure altitude equivalent of 53 km this would be 37 PPM and at the

53 IcTO pressure altitude equivalent and -13°C in lieu of 25°C this would

be 32 PPM.

By means of calculations similar to those in the proposal, we

calculate that lowering the temperature of the desiccant from 25*'C to

-13°C would reduce the equilibrium water vapor content by 256 fold

resulting in an equilibrium vapor content of .125 PPM at -13°C and a

pressure altitude equivalent of 53 km.

Since the water vapor equilibrium with P2^5

^ altitude and

-13°C appears to be considerably less than 1 PPM, it is of interest

to determine what flow rates and absorber configurations are appropriate

to dry air at the above condition to a level of less than 1 PPM. The

best basis we have for evaluating such drying techniques is our own

(2) experience with a P-,0 absorption tube train.

4
Our absorption train is composed of three tube?, the first one is

filled with magnesium perchlorate and the second and third tubes are

filled with phosphorous pentoxide. For purposes of this analysis

we shall assume that the action of the magnesium perchlorate is not

very different frocn that of the phosphorous pentoxide. We estimate

that the gaseous volume as sociated with the deslccant in each tube is

approximately 18.4 cm^ per tube. Our flow rate is 2 1pm giving us a

residence time of, 1.66 seconds.

Although we do not know how dry our effluent gas is, we have

evidence that we can achieve an effluent with a level of less than 0.18

PPM by weight with an initial water vapor content of 7000 PPM by weight.

Based on the premise that absorption of moisture is exponential with

respect to the time the gas is in contact with an absorber, we obtain

a time constant at normal temperature and pressure of less than 0.155

seconds.

The time constant should be a function of diffusion and/or chemical

reaction rate. The diffusion, D, at any condition should be, approximately.

where D^p, ^^^-p snd T^^^p are the diffusion, pressure and absolute

temperature respectively at normal temperature and pressure and P and T

are the pressure and absolute temperature at the desired condition.

For -13°C and 53 Km pressure equivalent we have

3/2

D

NTP

D D,NTP

x 1013.25

5.5385

= 1,491 D,NTP

5
Therefore, if the time constant is less with increased diffusion,

it will be greatly reduced at the new conditions. Let us assume the

worst case, that the time constant is a function of the chemical

reaction rate only. We use a method similar to that in the proposal

for estimation of the reaction rate but our results are quite

different from those in the proposal. For a decrease of 38 deg from

25°C to -13°C, we obtain a decrease in reaction rate of almost twelve

fold as opposed to the eight fold in the proposal. This gives us a time

constant of less than 1.8 seconds.

Table 1 is a table of residence times for various moisture levels

of inlet gas, required to obtain an effluent of 1 PPM by weight based

on a time constant of 1.8 s.

Table 1

Required Residence Times

for outlet levels of less than

1 PPM by weight of water

Inlet Moisture Level

PPM by weight

Required Residence Time

Seconds

2 1.4

5 3.1

10 4.4

20 5.6

50 7.3

100 8.5

6
Let us do a typical calculation using a relatively high level of

20 PPM. Providing a residence time of 5.6 seconds at 10 cm^/s flow rate

requires a volume of 56 cm^ and since about 65 percent of the volume

occupied by the desiccant is gaseous volume, approximately an 86 cm^

container of desiccant is required. There is probably a minimum length

for effective desiccation necessary in order to prevent channeling

(let us assume 3 cm) resulting in an area of 29 cm^

.

Our experience has been that we obtain a pressure drop of about

30 dynes/cm^ per cm length of desiccant per cm/s of air velocity. One

might expect some reduction in pressure drop at the reduced pressure

and approximately 12% reduction as a result of the lower viscosity

due to the 38 deg temperature reduction. Let us , therefore, use the

figure of 25 dynes/cm^ per cm length of desiccant per cm/s of air

velocity. With a 3 cm length and a 29 cm^ area, a 10 cm^/s flow would

give a pressure drop of 26 dynes/cm^ which is greater than the pressure

available due to ram pressure. Of course, reducing the length to 2 cm

and increasing the area proportionately would reduce the pressure drop

to 11 1/2 dynes/cm^. Preferably one would wish to increase absorber length

for greater absorption assurance and reduced residence times might be

desirable for faster speed of response, either condition would necessitate

reductions of area. It would therefore seem that the use of ram pressure

to provide the 10 cmVs flow through the desiccant Is marginal and that

consideration should be given to the use of a blower on the downstream side

in order to obtain the flexibility necessary to insure adequate flow

through an optimum desiccant configuration.

7
Conclusions

A worst case analysis of the drying process indicates that a

sample of 10 cm^/s of air at an altitude of 53 kn maintained at

-13°C can be dried to less than 1 PPM by weight by means of a ^2^^

absorption tube. Such drying may require residence times considerably

greater than one second. Use of a mechanical blower or other device

with which to produce the flow through the desiccant appears desirable.

8
References

1. E. W. Morley. Amount of Moisture in a Gas. Araer. Jour.

30, 1887. p. 199.

2. A. Wexler and R. W. Hyland. The NBS Standard Hygrometer.

NBS Monograph 73, p. 3.

USCOMM-NBS-DC 9
NB$-114A (REV. 7.73)

1 U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1l« 11 '1IM>l.|t i li 1 If ItA triONJ 1 V /iV (Ui I 'l\ l\\l \-

1 • \H)U 1 ^ /|\ 1I ,W\(\/ 4 %2a (lov t Aci t'ssion No.

3. RcL ipic-nt 's Accession No.

Analysis of Proposed Air Drying Process

5. Publication Date Dprpmhpr 1Q7^

4» Performing Organization CotJc

7. AUTHOR(S)

Lewis Greenspan

8. Performing Organ. Report No.

T. PhKh OKMlNd OKCiANI/,A 1 ION NAM1-. AND AUlJK l.bS NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro|ect/Task/Work Unit No. 2130A38

11. Contract/Grant No.

12* Spon sor 1 ng Or^ aniZ3tion Name an d C^-ompltrtc AJdrfss (S trcet , dty , Stotc . ZIP

)

National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Md. 20771

13. Type of Report Si Period

Covered

Quarterly

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual surrunary of most sigfyificant information. If document includes a significant

bibliograpfty or literature survey, mention it here.)

NASA proposal NO. 7401-6-OlA by the GCA Corporation describes a proposed air

drying process which has been analyzed. The drying process, Intended to

reduce the water vapor content of a stratospheric sample to below one part

per million by weight, is based upon the removal of the water as It passes

through a phosphorous pentoxide absorber. The analysis indicates that the

method is feasible. Dependence on ram pressure to maintain flow

through the absorber appears questionable and consideration should be given

to other means.

17. KEY UORHS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Absorption; drying; phosphorous pentoxide; water vapor.

18. AVAILABILITY Unlimited

•

19. Si;C URITY CLASS

(Tins REPORT)

21. NO. OF PAGES

1

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UNC L ASSIITED

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