Applications requiring new technologies for atmosphere monitoring and control in the closed environment and their principal functions aboard the Space Station Freedom are described. Oxygen loop closure, involving the conversion of carbon dioxide to oxygen; carbon dioxide reduction and removal; and monitoring of atmospheric contamination are discussed. The Trace Contaminant Monitor, the Major Constituent Analyzer, the Carbon Dioxide Monitor, and the Particulate Counter Monitor are discussed

Humphries, R.

Perry, J.

English

NASA Technical Reports Server

N91-23071
MONITORING AND CONTROL OF ATMOSPHERE IN A CLOSED ENVIRONMENT
By R. Humphries and J. Perry
NASA, George C. Marshall Space Flight Center
Thermal and Life Support Division
INTRODUCTION
NASA is developing a manned orbiting space station for flight by the 21st century. Currently, the
Space Station Freedom (S.S. Freedom), as it has been named, will be manned by an eight-person crew. An
Earth-like atmosphere will be provided in 11 pressurized modules where the crew will five and work. The
Environmental Control and Life Support System (ECLSS) is being developed to provide these surroundings
artificially. The ECLSS is subdivided into six major subsystem groups to accomplish this task. These
subsystem groups are composed of temperature and humidity control (TI-IC), atmosphere control and supply
(ACS, atmosphere revitalization (AR), water recovery and management (WRM), waste management (WM),
and fire detection and suppression (FDS). As a whole, these subsystems supply, revitalize, condition, and
monitor the respirable atmosphere; supply, recover, and condition water for hygiene and potable use; and
collect, process, and store human waste for return to Earth.
Techniques have been adopted for some of these functions which are similar to those used in
ground-based or earlier manned spacecraR applications. And, for the remainder, it has become necessary to
develop new techniques. Those applications requiring new technologies for atmosphere monitoring and
control in a closed environment will be discussed and their principal function onboard S.S. Freedom will be
described.
SYSTEM OVERVIEW
The dosed environment inherent to spacecraft is conducive to contaminant buildup, especially over
the long periods of time the S.S.Freedom is designed to be operational (-30years). In such an
environment, care must be taken to define all contaminant sources and sinks in detail. These sources range
from human to equipment inputs. As part of the S.S. Freedom ECLSS activities, each of these sources is
being characterized and analyzed in great depth via testing and analysis.
In striving to meet these challenges, the common denominators of all space system engineers are to
minimize weight, power, and volume - three of the most valuable commodities of a spacecraft. Also, of great
importance for S.S. Freedom are high reliability, minimization and ease of maintenance, and a low re,supply
penalty. With these in mind, designers are optimizing the S.S. Freedom ECLSS.
Undoubtedly, the greatest challenge the S.S. Freedom ECLSS designer faces is that of loop closure.
For the first time in the history of the United States manned space flight program, both oxygen-and
water-loop closure is planned. Oxygen-loop closure will be discussed because it is pertinent to the topic of
air purity. Also, of special interest are atmospheric contaminant control and monitoring because of the new
technologies necessary to efficiently accomplish these functions onboard S.S. Freedom. Although not
addressed, other technologies are being developed which support these specific ECLSS functions. Examples
of these other functions are electromechanical devices for new motor-driven valves with integral control
devices, smart sensors, new 120-Vdc conversion devices, and others.
OXYGENLOOP CLOSURE
To minimize re,supply demands, recycling is extremely important. Consequently, a string of
equipment which can convert carbon dioxide to oxygen has been developed. Evolution of these technologies
began in the 1960'sand encompassed a number of different techniques. Competitive development of a
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https://ntrs.nasa.gov/search.jsp?R=19910013758 2020-03-19T17:38:41+00:00Z
number of techniques, as shown in table 1, was funded until recently and brought the techniques to a
predevelopment maturity for comparative testing. This equipment includes carbon dioxide removal and
concentration, carbon dioxide reduction, and oxygen generation.
Low concentration carbon dioxide is extracted form the cabin air and concentrated by the removal
subassembly and sent to a carbon dioxide reduction device. In tlfis device, the carbon dioxide is catalytically
reacted with hydrogen at high temperature, producing water vapor and a waste product. Water is condensed
and ultimately fed in the form of hygiene water to a unit which decomposed it into its elemental parts of
oxygen and hydrogen by an electrolytic process. The hydrogen is fed to the carbon dioxide reduction process
for reaction with the carbon dioxide while the oxygen is returned to the cabin, thus closing the oxygen loop.
Nitrogen purge streams for the carbon dioxide reduction and oxygen generation subassemblies are fed
directly to the trace contaminant control subassembly for processing any residual hydrogen during startup
and shutdown of these p_.
The water produced by the carbon dioxide reduction subassembly produces potable water for crew
consumption, this water eventually is introduced into the air where it is removed as humidity condensate and
reclaimed for crew consumption or into the waste management system. The water from the urine introduced
into the waste system is reclaimed and processed for crew use during showering, hand washing, and other
personal hygiene activities. Also, this water is used as a process feed stream to the oxygen generation
subassembly, thus achieving loop closure.
As a consequence of analytical studies combined with evaluation of comparative test results, baseline
subassembly selections have recently been made. As indicated in table 1, the preferred approaches are four
bed molecular sieve (4BMS) for carbon dioxide removal, the Sabatier reactor (Sa-CRS) for carbon dioxide
reduction, and the static feed potassium hydroxide (KOH) electrolyzer (SFWES) for oxygen generation.
Carbon Dioxide Removal
The 4BMS, shown schematically in figure 1, uses a zeolite material (Linde 5A) which has been
further modified by the supplier to produce a material which is extremely efficient in absorbing carbon
dioxide. Unfortunately, this material also has a high affinity for water vapor. Conseqmmtly, the influent
moist air must be dried. This is accomplished by using a two-media desiccant bed. This bed is made of
silica gel and a second zeolite material (Linde 13X) with a high water removal efficiency. The influent air,
carbon dioxide, and water vapor mixture is first passed through the desiccant bed where the dew point is
reduced to a range of -300 F to -900 F. The carbon dioxide is then absorbed in the second 5A zeolite bed
until the sorbent material becomes saturated. The carbon dioxide molecular sieve material is then desorbed
by a combination pressure and temperature swing process which requires first evacuating the bed, then
heating it to 4000 F. The desorbed carbon dioxide is then purged from the bed in a concentrated form. The
initial air passing through the bed after desorption is routed back through the desiccant bed to drive the
absorbed moisture out of the desiccant bed and back into the cabin. During the desorption period of these
two beds, the remaining beds are absorbing so that a continuous carbon dioxide removal capability exists at
all times utilizing this four-bed concept.
Carbon Dioxide Reduction
Reduction of the carbon dioxide to water is achieved by a 950 °F Sabatier reactor process shown
schematically by figure 2. The chemical reaction which takes place in this reactor is the following:
CO 2 + 4H:-> 2I-I20 + CI-I4
In this process, concentrated carbon dioxide at greater than 97-percent purity is fed into the reactor with
hydrogen produced by the oxygen generator. A packed bed reactor containing a ruthenium on alumina
catalyst provides the appropriate conditions for the process to proceed. This reaction forms water vapor and
methane stoichiometrically. However, since the reaction is not 100 percent efficient, not all the carbon
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dioxide is reacted and it exits with the water vapor and methane. A condenser at the reactor outlet
condenses the water vapor to the liquid phase and it is pumped away to the potable water subsystem to be
processed for crew consumption.
Oxygen Generation
A KOH electrolyte-type generator shown schematically in figure 3 is used to separate water into
hydrogen and oxygen. Electricity fed into the unit causes the electrolytic process to occur. At the same
time, water enters a static water compartment where it evaporates through a Gortex membrane into the
hydrogen cavity of the electrolysis cells. The chemical definition of how the water is separated into its
elements by this process is the following:
Cathode: 2e +2H20 --_ H2+2OH
Anode: 2OH --, H_,O + V202 + 2e-
Overall: H20 --> H2+ 1/20_,+2e
By this process, gaseous hydrogen and oxygen is formed in respective compartments of the multicell unit. It
is forced out of the unit by the pressure buildup caused by continuing reaction. The hydrogen produced
from this process is fed to the Sa-CRS and the oxygen is supplied to the cabin for crew respiration.
ATMOSPHERIC CONTAMINATION
Inherently, spacecraft must have closed environments. Since gas lost overboard must be produced at
an expense of power, weight, and volume for closed loop processing equipment or through costly resupply
from the ground, a tight environment is a must to reduce the loss and thus reduce resupply and resource
requirements to make up for these losses. As a consequence, all contaminants evolved onboard a spacecraft
from crew metabolism, material offgassing, and everyday housekeeping and work activities may build up over
time. this buildup must be controlled and monitored to assure that the crew health is not compromised
during their exposure to this environment of 180 days or more. This and a combination of other factors
make contamination control and monitoring onboard S.S. Freedom a necessity. Of special concern is the
large range of payloads and experiments planned over the S.S. Freedom life. This gives rise to the potential
for a large variety of potential types as well as quantities of contaminants. Previous manned space flights by
the United States have all been of short enough duration and/or in a large enough volume (i.e., Skylab) that
metabolic methane buildup was of no concern. Complicating this for the S.S. Freedom is the fact that the
new dosed loop equipment generates contaminants such as hydrogen, methane, and carbon monoxide
internally. All of which, if they leak, are not well removed by previously flown activated charcoal trace
contaminant control media. As a consequence, for the first time a high temperature catalytic oxidation
reactor or catalytic converter is being developed for the S.S. Freedom to be used in combination with
activated carbon adsorption.
The need to monitor unexpected contaminants, long-term buildup of known contaminants, and
contamination leakage gives rise to the need for an onboard real-time trace contaminant monitor. The need
to monitor multiple atmospheric constituents points toward the desire to fly both a major constituent
analyzer as well as a separate carbon monoxide monitor. The concern with long-term buildup of airborne
particulates, where a clean room environment of less than or equal to 100,000 particles/ft 3 for particles
between 0.5 and 100 btm is desired in the station atmosphere, while hazardous aerosols may be generated by
potential payloads such as gallium arsenide (GaAs) and mercury cadmium telluride (HgCdTe) crystal growth
experiments, has stimulated both particulate and aerosol monitoring requirements on S.S. Freedom.
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Airborne Trace Contaminant Control
The trace contaminant control subassembly (TCCS) combines activated carbon adsorption with high
temperature catalytic oxidation to achieve the most efficient long-duration airborne contamination control.
This subassembly is shown schematically in f_mre 4. An activated charcoal impregnated with phosphoric acid
is used to remove ammonia and high molar volume organic contaminants. Other contaminants which are
not easily removed by the charcoal are removed in a 600o F to 7000 F catalytic reactor. This reactor removes
primarily methane, hydrogen, and low molar volume compounds while converting carbon monoxide to carbon
dioxide. The reactor utilizes a palladium on alumina catalyst in a packed bed to achieve its function.
Methane oxidation efficiencies as high as 95 percent per pass have been experienced using predevelopment
and prototype testing. The reactor effluent is scrubbed by a lithium hydroxide (LiOH) packed bed to
remove any acidic reaction products from the air stream before it is exhausted to the spacecraft cabin.
Atmospheric Composition Monitorino
Atmospheric composition is monitored for trace contaminants, major consitutents, carbon monoxide,
and particulates on a continuous basis at multiple locations in the S.S. Freedom. Four separate instruments
provide these functions since it is difficult for any one instrument to monitor the entire spacecraft
atmospheric composition. Table 2 summarizes the S.S. Freedom atmospheric monitoring capabilities. Each
instrument is described briefly.
Trace Contaminant Moniter
The trace contaminant monitor (TCM) instrument utilizes a gas chromatography/mass spectrometry
principle to analyze both species and quality for mass to charge (m/e) ratio of 24 to 250. The TCM gas
chromatograph uses two stages of sample preconcentration to detect low contaminant concentrations. The
analysis is conducted in a cyclic, three-stage process. The sample is concentrated on a large sorbent trap and
then further concentrated on a smaller sorbent trap. A small carrier volume is then used to transfer the
sample to the carbonous fused silica gas chromatographic columns and then to a scanning double focusing
magnetic sector mass spectrometer which scans a mass range from m/e 24 to 250. The unit is capable of
detecting at a resolution of 50 percent of the spacecraft maximum allowable concentration (SMAC) for each
species monitored. Approximately 200 compounds are monitored through use of a software mass spectra fit
library search. Any compounds which are not contained in the library are labeled as stranger compounds
and may be analyzed offline if desired. The typical cycle time for this unit is I hour, and its software allows
direct automated outputs of speciation and quantity. The samples are drawn through long, thin tubing from
several locations within the S.S. Freedom cabin for analysis.
Major Constituent Analyzer
The major constituent analyzer (MCA) is a mass spectrometer which utilizes a single focusing
magnetic sector with a Faraday cup detector. It has a capability of monitoring in the mass to charge range
of I to 48. this unit specifically focuses on oxygen, nitrogen, carbon dioxide, methane, hydrogen, and water
vapor. The instrument's response time is less than 100 milliseconds (ms) for all gases except water which
has a response time of 500 to 800 ms. Vacuum is maintained to the instrument by an ion pump which
operates by surface absorption and chemical reaction of gases with the active metals of titanium and
tantalum. It has a sampling rate of one sample per minute and draws samples from lines similar to those of
the TCM.
Carbon Monoxide Monitor
The carbon monoxide analyzer (COA) is a nondispersive infrared instrument using a dual isotope
fluorescence technique. This instrument is used since carbon monoxide is not easily detected with mass
spectrometry because it has approximately the same molecular weight as nitrogen. An air sample is
introduced into the COA similarly to the TCM and MCA. In the instrument, an infrared source excites a
416
fluorescent cell which in turn produces radiation in the range for the most abundant isotope of carbon
monoxide (carbon-12). The radiation passes through a chopper fdter which contains the abundant and rare
isotopes of carbon monoxide (carbon-12 and carbon-13) which results in the sample gas being exposed to
alternating bursts of radiation which is absorbed by either the carbon-12 isotope or the carbon-13 isotope.
Since carbon-13 is not very abundant, the radiation associated with it is not absorbed significantly by the
sample and is used as a reference. The transmission and adsorption of the fluorescent radiation associated
with the two isotopes is detected by a thermoelectrically cooled lead selenide (PbSe) detector. Since the
degree of absorption is proportional to the carbon monoxide partial pressure, the detector signal is compared
to calibration data to determine the partial pressure.
particulate Counter Monitor
The particle counter monitor (PCM) is a compact commercial model based on light scattering. This
device uses an AIGaAs laser diode to produce a fight beam at a 780-nm (nanometer) wavelength which is
scattered by particles in the air sample and detected by a photodetector. The light beam passes through
several lenses and an aperture to produce a thin plane in the particle sensing zone. A light trap captures the
main light beam after it passes the instrument's sensing zone. As each particle passes through the sensing
zone and scatters light, the photodetector converts the scattered light energy to electrical pulses which have
an amplitude corresponding to the particle size. The air is sampled through a dedicated line to continuously
measure the total particle constituency from 0.5 to 100 _'a.
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Table1. 02 closurecompetitivetechniques
CO 2 Removal
CO 2 Reduction
• 0 2 Generation
* Baselined for SSF
• 4 Bed Molecular Sieve*
• Solid Amine Water Desorbed (SAWD)
• Electrochemical Depolarized Cell (EDO)
• Sabatier/Advanced Carbon Reactor
• Bosch Carbon Reactor
• Sabatier °
• Static Feed Water Electrolysis Subsystem (SFWES)*
• Anode Feed Solid Polymer Electrolyzer (AFSPE)
Table 2. Atmospheric composition monitor (ACM) functional summary.
SUBASSEMBLY
Major Constituent Analyzer (MCA)
Trace Contaminant Monitor ('FCM)
Carbon Monoxide Analyzer (COA)
Partide Counter Monitor (PCM)
INSTRUMENT
MS
GC/MS
NDIR
Light Scattering
SPECIES MONITORED
N 2,O 2, H 20, H 2, CH2, CO 2
trace contaminants
CO
particulates
RANG_
ACCURACY
:L-5% of range
0-50% of SMAC
-t-1% of range
0.5-100p.m
SAMPLE
LOCATIONS
SAMPLE
INTERVAL
1 min/line
30 mirdine
1 mirdine
continuous
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AIR 114.ET
FROM cIIx
m
P4TUI_N
I0 CAIIIII
AIR INLET
SEUECTO4_
<k
/
I_E SlCC_T RED
IOESONW+O!
|_ADSSICO_I_EO I m i CO,$OtMI(NT (_Ct
/ c.+c. I 4J I /,_,/ .o..,
AIR HEAT
EXC_R
ELECTNCAL [] • PI_SS_ SEI_OR
t_ATER [_] • TEMPIEI_;U_E SENSO4_
Figure 1. S.S. Freedom 4BMS CO 2 removal subassembly schematic.
HEATERS
COOUNG AIR OUT
WATER
SEPARATOR
HEAT
EXCHANGER
INSULA11ON
REACTOR
(950 *F) _ t
INN2
[] NITROGEN (PURGE)
[] HYDROGEN
D CARBON DIOXIDE
m COOMNG AIR
Figure 2.
COOLING
AIR IN PRODUCT PURGE
WATER OUT
[] REACTANT MIXED GAS
[]PRODUCT GAS W/WATER VAPOR
W PRODUCT GAS (COa/CH4) W/O WATER VAPOR
[] PRODUCT WATER
S.S. Freedom sabatier CO 2 reduction subsystem schematic.
VENT
CH./CO2
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H2
I
KOH
Bectrolyteand
Retention Matrix
Water
Feed Membrane
1t20
qt,
I
Water
Corn )artment
Anode (+): 40H- _ 0 2 = 2H 20 + 4e-
Cathode (-): 4H 0 + 4e--'-_2H 2 + 40H-
Overall: 2H 20 + Power --)-2H 2 + 0 2 + Heat
Figure 3. S.S. Freedom oxygen generator cross-sectional schematic.
ARS Rack
Bleeds snd Purges
from CO, Reductionend
Oz Oeneret_
Slime Station Module
Urine Pro¢_ Bleed LEGEND
I FM = Flow Meier
V - Valve
T - Temperature ,_msof
S = Speed Sensor
Figure 4. S.S. Freedom TCCS schematic.
1_'U.ILQOVUWNMENT_ OFFK_:t BI -s2"/.0e4/'ze015
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Monitoring and control of atmosphere in a closed environment

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