Adsorption processes in spacecraft environmental control and life support systems

Adsorption and its Applications in Industry and EnvironmentalProtection Studies in Surface Science and Catalysis, Vol. 120 A. Dabrowski (Editor) 9 1998Elsevier Science B.V. All rights reserved.

455

Adsorption processes in spacecraft environmental control and life support systems L. A. D a l l B a u m a n a and J. E. Finn b a NASA Johnson Space Center, Houston TX*, USA b NASA Ames Research Center, Moffett Field CA, USA

The environmental control and life support system on a spacecraft maintains a safe and comfortable environment in which the crew can live and work by supplying oxygen and w a t e r and by removing carbon dioxide, w a t e r vapor, and trace contaminants from cabin air. Although open-loop systems have been used successfully in the past for short-duration missions, the economics of current and future long-duration missions in space will make nearly complete recycling of air and water imperative. A variety of operations will be necessary to achieve the goal of nearly complete recycling. These include separation and reduction of carbon dioxide, removal of trace gas-phase contaminants, recovery and purification of humidity condensate, purification and polishing of w a s t e w a t e r streams, and others. Several of these can be performed totally or in part by adsorption processes. These processes are good candidates to perform separations and purifications in space due to their gravity independence, high reliability, relatively high energy efficiency, design flexibility, technological maturity, and regenerative nature. For these reasons, adsorption has historically played a key role in life support on U.S. and Russian piloted spacecraft. Among the life support applications t h a t can be achieved through use of adsorption technology are removal of trace contaminants and carbon dioxide from cabin air and recovery of potable water from waste streams. In each of these cases, adsorption technology has been selected for use onboard the I n t e r n a t i o n a l Space Station. The requirements, science, and h a r d w a r e for these applications are discussed. H u m a n space exploration may eventually lead to construction of planetary habitats. These h a b i t a t s may provide additional opportunities for use of adsorption This manuscriptwas prepared while L. DallBaumanwas employedat Johnson Space Center. She has sincejoined AlliedSignal Inc., 50 E. AlgonquinRd., Des Plaines IL 60017-5016USA.

456 processes, such as control of greenhouse gas composition, and may have different resources available to them, such as gases present in the planetary atmosphere. Separation and purification processes based on adsorption can be expected to continue to fulfill environmental control and life support needs on future missions. 1.

INTRODUCTION

A spacecraft's environmental control and life support system (ECLSS) provides a breathable atmosphere by supplying 02 and by controlling CO2, water vapor, and trace contaminant levels in cabin air. The system must also supply the crew with water for drinking, food preparation, and hygiene use. Although operating in space offers unique challenges, the ECLSS is essentially a network of unit operations in which chemical reactions, separation processes, and heat transfer play important roles. An understanding of chemical engineering principles and operations is therefore essential to the design process. Technologies commonly used in the chemical process industries, such as distillation and adsorption, have been tailored for use in space. A variety of factors drive ECLSS design. First and foremost, equipment must be safe, reliable and effective in reduced gravity. Certain physical phenomena are strongly gravity-dependent; for example, separation of vapor and liquid phases is considerably more difficult in space than on earth. This increases the complexity associated with any operation in which two-phase flow, condensation, or boiling occurs. Due to the tight constraints placed on spacecraft and system mass and volume, hardware must be lightweight and compact and the use of consumables (e.g., reagents) and expendables (e.g., non-regenerable sorbents) must be minimized. Limited power is available, so equipment must be energy-efficient. In addition, process equipment should not require frequent maintenance or a large stock of spare parts. Determining the best system for a given mission is essentially an optimization problem. The objective is to provide the crew with a breathable atmosphere and potable water while minimizing mass, volume, and power. The relative weights of the constraints are determined by specific mission characteristics; for example, power is the dominant driver for the International Space Station (ISS). For very short missions, an "open-loop" system is generally the best available solution. In such a system, all water and 02 needed for the mission are stowed onboard prior to launch while CO2 and wastewater are discarded or stored. No effort is made to recover useful substances from waste products or to regenerate materials. Power requirements are relatively low and mass and volume requirements can be reduced if waste products are discarded rather than being stored.

457 As mission length and/or crew size increases, use of an open-loop scheme becomes less practical. The problem of supplying water for the crew can be used to illustrate this point. For advanced missions (i.e., missions beyond ISS) the National Aeronautics and Space Administration (NASA) has allocated approximately 23 kg (50 lb) of water per person per day to meet all needs: drinking, food preparation, personal hygiene, and laundry. This is a meager allowance by terrestrial standards; a typical household uses 265 kg (580 lb) per person per day, with up to 95 kg (200 lb) being used for a single shower [1]. However, at a launch cost of thousands of dollars per kilogram, supplying all water at launch quickly becomes prohibitively expensive. In addition, the storage volume available on a spacecraft is limited and does not allow transportation of large quantities of water. On long missions, the mass and volume associated with the process equipment in a regenerative ECLSS can be less t h a n the mass and volume of the 02 and water required by an open-loop configuration. The first step toward closing the ECLSS loop has historically been to reduce reliance on expendable materials. For example, CO2 was originally removed from the atmosphere on all NASA space shuttles by irreversible absorption on LiOH. The two shuttles designated as extended duration orbiters have been retrofitted with a cyclic CO2 removal subsystem in which a solid amine material is used to reversibly sorb CO2. At the end of each half-cycle, the C02 is desorbed to space vacuum so that the amine can be reused. Mass of the LiOHbased subsystem increases linearly with crew size and mission duration, but the mass of the solid amine subsystem is fixed for a crew of up to seven and is essentially independent of mission duration. For a seven-person crew on an eighteen-day mission, replacing LiOH with solid amine and making the necessary h a r d w a r e changes reduces the effective mass of the CO2 removal subsystem by 189 kg (416 lb) [2]. System closure can be dramatically increased by recycling waste streams or recovering useful substances from them. In a fully closed system, the air revitalization system (ARS) retains the CO2 removed from the habitat atmosphere instead of discarding it and ultimately recovers 02 from the CO2. A closed water recovery system (WRS) recycles wastewater streams including humidity condensate, spent hygiene water, and urine to produce water suitable for ingestion and hygiene use. The added mass of equipment required to recover 02 and to purify wastewater is offset by the reduction of mass needed for storage. The point at which an openloop system's mass and volume become equal to a regenerative system's mass and volume is determined by mission p a r a m e t e r s including crew size and mission duration. The source of spacecraft power is another important parameter. The U.S. space shuttle fleet relies on fuel cells to generate electricity. Since clean water is generated as a byproduct, there is no potential benefit in adding a WRS to the space shuttle ECLSS. Because ISS will use solar panels instead of fuel cells, it will not

458 have a built-in water supply and a WRS with a high degree of closure will be necessary. 2.

SPACECRAFT APPLICATIONS

On earth or in space, adsorption processes can be useful in both air and water purification. Three spacecraft life support operations that can employ adsorption are trace contaminant control, CO2 removal, and potable water recovery. The following sections provide historical perspectives for each application, together with descriptions of hardware to be used onboard ISS and h a r d w a r e currently in use onboard the Russian space station Mir. Possible future applications are also described. 2.1. T r a c e c o n t a m i n a n t c o n t r o l In the closed environment of a spacecraft or planetary base, trace quantities of potentially harmful substances can be more deleterious to h u m a n health t h a n similar levels would be in a less confined space. Crew exposure is constant r a t h e r t h a n intermittent and in the future will last for weeks or even months. Atmospheric trace contaminants may be organic or inorganic and may have biological or nonbiological origins. Some of the compounds found in space shuttle air samples [3], such as methane, can be identified as being products of crew metabolism. Others, such as dichloromethane, are likely produced by equipment off-gassing. Still others, such as 2-propanol, arise from evaporation of cleaning solvents. Adsorption has played a role in trace contaminant control throughout the U.S. h u m a n space flight program. Activated charcoal was used to remove contaminants onboard Mercury, Gemini, and Apollo spacecraft as well as on Skylab [4]. In these programs, the primary goal was to control odors rather t h a n to remove specific contaminants. The early Soviet space program also relied on activated charcoal for contaminant control in its Soyuz and Salyut spacecraft [5]. More recently, a list of spacecraft maximum allowable concentrations (SMACs) for some two hundred contaminants has been compiled [6]. The list includes a wide variety of aromatic and aliphatic hydrocarbons, halocarbons, and inorganics. The compilation of this list has driven a search for methods of removing or destroying particular compounds and classes of compounds. For example, the air quality requirements imposed on the space shuttle and Spacelab programs have resulted in the addition of an ambient temperature catalytic oxidizer intended primarily for destruction of CO [7]. Efforts to develop a trace contaminant control subsystem (TCCS) to remove specific compounds as well as odors expected on a space station have been underway since the early 1970s [8]. A contaminant load model was originally derived from

459 Apollo equipment off-gassing data and h u m a n metabolic studies. Based upon that model, a multi-step contaminant removal process was proposed in which an expendable activated charcoal bed would remove irreversibly adsorbed compounds, a regenerable activated charcoal bed would remove more weakly adsorbed compounds, and a catalytic oxidizer would convert reactive compounds to CO2, N2, and water vapor. The contaminant load model was used to optimize the relative sizes and flow rates for the charcoal beds, with the resulting design calling for a flow rate of 129 m3/hr (76 cfm) through the 14.5 kg (32 lb) expendable bed and a flow rate of 5.1 m3/hr (3 cfm) through the 2.2 kg (4.9 lb) regenerable bed. As more complete and accurate information became available, the original contaminant load model was refined and the TCCS design evolved [9-11]. The design ultimately selected for use on ISS is represented schematically in Figure 1 [12]. It includes an activated charcoal bed, a catalytic oxidizer, and a LiOH bed. The charcoal bed removes high molecular weight compounds that cannot be readily desorbed and is therefore an expendable item. It also removes ammonia, which is a potential catalyst poison. The bed contains 22.7 kg (50 lb) charcoal impregnated with 10% (weight) phosphoric acid to enhance its ammonia removal capability.

catalytic

,eater

oxidizer '~1 recuperative heatexchanger

~l activated - - - I ~ Q charcoal bed

from cabi

''

I ~1 LiOHbed I

blower ~

l

return

bypass Figure 1. ISS trace contaminant control subsystem schematic. The activated charcoal bed removes high molecular weight organics and potential catalyst poisons. The catalytic oxidizer destroys low molecular weight compounds that are not captured by the charcoal bed. The LiOH bed removes acid gases produced by oxidation of halocarbons.

460 Cabin air flows through the bed at 15.3 m3/hr (9 cfm). After leaving the bed, the process stream is split so that 4.6 m3/hr (2.7 cfm) is routed through a catalytic oxidizer where low molecular weight compounds are destroyed and then through a post-sorbent bed containing LiOH to remove acid gases produced in the oxidizer [7]. The remainder of the air bypasses the oxidizer and LiOH bed. The trace contaminant control assembly used onboard Mir relies on both regenerable and expendable sorbent beds upstream of a catalytic oxidizer. As is the case with the ISS TCCS, the expendable bed is sized to remove high molecular weight contaminants. The regenerable beds are used to remove more volatile (and therefore more readily desorbed) compounds. The expendable bed contains 1.3 kg (2.9 lb) activated charcoal and each of the regenerable beds contains approximately 7.4 kg (16.3 lb) activated charcoal. Air flow through the assembly is nominally 20 m3/hr (11.8 cfm). Each bed is regenerated after 20 days of operation by exposing it to space vacuum for 60 minutes and maintaining bed temperature between 170 C and 200 C (338 F and 392 F) for 90 minutes [13]. 2.2. CO2 r e m o v a l Regardless of whether it is open-loop or regenerative, an ECLSS must remove CO2 from the habitat atmosphere. NASA has specified that the nominal CO2 partial pressure shall be 5.0 mm Hg on the space shuttles [14] and that the maximum daily average CO2 partial pressure shall be 5.3 mm Hg on the ISS [15]. These values are roughly an order of magnitude higher than the typical atmospheric value of 0.23 mm Hg [16]. In order to meet these specifications, the ECLSS must remove CO2 generated by the crew at a nominal rate of 1.0 kg/person/day (2.2 lb/person/day). On Mercury, Gemini, and Apollo missions, CO2 was removed by LiOH. In these systems, CO2 was absorbed by LiOH and then reacted to form Li2CO3: CO2 + 2 LiOH

> Li2CO3 + H20

This reaction is essentially irreversible. As noted earlier, the mass of LiOH required increases linearly with mission length and crew size. While acceptable for relatively short missions, the mass penalty associated with expendable LiOH canisters was unacceptably large for the longer Skylab missions of the 1970s. For those missions, CO2 was reversibly adsorbed on zeolite material. The Skylab CO2 removal subsystem included two canisters, each containing molecular sieve 13X for dehumidification and 5A for C02 removal. Air entering the adsorbing canister contacted the 13X material before the 5A material. Adsorption of water lowered the dewpoint of the process air and reduced the competition between water and CO2 for adsorption sites on the 5A material. The subsystem operated cyclically so that as one bed adsorbed water vapor and CO2, the other was regenerated by venting to space vacuum [17].

461 The loss of water by the Skylab subsystem is clearly a disadvantage for systems driven toward loop closure. A design for a "water-save" process was proposed even before the Skylab missions [17]. In this configuration, the 13X material in the sorption beds would be replaced with separate desiccant beds containing silica gel. Space vacuum would no longer be used to recover water from the saturated desiccant material; instead, the warm, dry process air leaving the adsorbing 5A bed would be used to strip the water from the desiccant bed before being returned to the cabin. This concept ultimately evolved into the four-bed molecular sieve (4BMS) subsystem that will be used onboard ISS. A schematic representation of the 4BMS is shown in Figure 2. As was true on Skylab, the ISS subsystem operates continuously and works in a cyclic fashion so that as one sorbent bed adsorbs CO2, the other bed is regenerated. Similarly, as one desiccant bed removes water from the saturated air entering the subsystem, the other desiccant bed is regenerated by dry, CO2-depleted air and water vapor is returned to the cabin. The desiccant beds contain zeolite 13X and silica gel, while the CO2 sorbent beds contain zeolite 5A.

~ _ _ _ C02 sorbent i. . . . 4

desiccant 1

process air

II I I I

1

~)-D~~ precooler I blower

return to cabin

9

desiccant3 ]~

Q

()

Q-~ to vacuum

C02 sorbent2

Figure 2. ISS Four-bed molecular sieve subsystem schematic. Darker lines indicate process air flow during half-cycle described in text: air enters desiccant bed 1, continues through CO2 sorbent bed 2, and exits through desiccant bed 3. CO2 sorbent bed 4 is desorbed to vacuum.

As shown in Figure 2, process air laden with water vapor and CO2 enters the adsorbing desiccant bed (bed 1), where the water vapor is removed to protect the downstream CO2 removal bed. Although CO2 is also adsorbed by zeolite 13X, it is displaced by the advancing water front as the desiccant bed approaches saturation. The dry air exiting the bed is drawn through the blower and into the precooler which

462 removes the heat introduced by the blower as well as the heat of adsorption generated in the desiccant bed. The cool, CO2-1aden air passes into the adsorbing CO2 sorbent bed (bed 2), which selectively removes the C02. The air is then directed into the desorbing desiccant bed (bed 3), where the CO2-1ean air stream is rehumidified before being returned to the cabin. During this half-cycle, the second CO2 removal bed (bed 4) is regenerated by a combination pressure/thermal swing method. The bed is pumped down at the beginning of the half-cycle and the residual air is returned to the cabin. The pump is then turned off and embedded electrical heaters are used to heat the bed. As temperature increases, the CO2 is desorbed from the zeolite surface and returns to the gas phase, causing the pressure in the bed to rise. The gas is removed from the desorbing bed by means of a vacuum pump or by exposure to space vacuum. At the end of the half-cycle, the selector valves change position, allowing the newly regenerated beds to become the adsorbing beds and the saturated beds to become the desorbing beds so that the next half-cycle can begin [18]. NASA has gained understanding of and experience with 4BMS operation through extensive testing at Marshall Space Flight Center (MSFC). A combination of subsystem, integrated system, and life testing conducted at MSFC has provided basic understanding of the subsystem and how its performance is affected by various cabin and interface conditions [19-22]. During one series of tests, the 4BMS was challenged by off-nominal flow rates, inlet C02 concentrations, and temperatures [21]. The MSFC program also included a test of the ISS baseline ARS configuration in 1996 [22]. Testing of the 4BMS has also been performed at NASA's Johnson Space Center. In 1992 and 1994, subsystem tests were performed in which inlet CO2 concentration, flow rate, and half-cycle duration were varied [18]. In addition, a 4BMS unit has been used extensively in chamber tests at JSC. In 1996, the subsystem was part of an integrated ARS that provided a breathable atmosphere for a four-person crew living in a 250 m 3 (8824 ft 3) chamber for 30 days. In a second test in 1997, the unit was part of a different integrated ARS that supported a four-person crew living in the same chamber for 60 days. During both tests, the crew participated in normal activities including exercise. The unit's CO2 removal rate varied with the chamber's atmospheric CO2 concentration, which in turn varied with the level of crew activity. The subsystem was able to maintain acceptable C02 levels in the chamber [23,24]. As noted above, the desiccant prevents competition between water and CO2 for the 5A adsorption sites. In order to reduce hardware mass and process complexity, several studies have explored the possibility of using hydrophobic sorbents for CO2 removal. A method of producing amine-functionalized carbon molecular sieves (CMS) was developed in the late 1980s [25]. The untreated CMS material exhibited a high degree of hydrophobicity and the addition of the amine groups increased the material's C02 capacity. The functionalized CMS material has since demonstrated

463 its ability to remove 80% of the CO2 present in a simulated spacesuit air stream without initial dehumidification [26]. Competition between water and COe can also be avoided through use of a material that allows (or even requires) the two compounds to sorb cooperatively. The solid amine used in the extended duration orbiter and described earlier is an example of such a material [2]. The Mir CO2 removal system also relies on a combination of desiccant beds and CO2 sorbent beds. Detailed information on the materials used is not readily available. 2.3. W a t e r r e c o v e r y

As h u m a n space missions have become longer and more complex, the demand for water has increased. During the Mercury program in the early 1960s, the crew for each mission consisted of a single astronaut and missions lasted from 15 minutes to 1.4 days. As NASA progressed through the Gemini and Apollo programs to Skylab and the space shuttle, missions grew longer and included more astronauts. Water supply requirements increased accordingly. On Mercury missions, approximately 2.7 kg (6 lb) local municipal water was supplied for the astronaut's ingestion on each mission [27]. Over the last thirty years, NASA has developed increasingly stringent water quality standards [15]. Allowable concentrations are now specified for ionic species and for classes of organic compounds. Comparison of NASA's standards with the Environmental Protection Agency's (EPA) National Primary and Secondary Drinking Water Regulations [28] shows that for species and compounds appearing in both documents NASA's standards are equal to or stricter than the EPA's. Although wastewater has not yet been recycled for use by crewmembers onboard a U.S. spacecraft, this will occur on ISS. Wastewater streams will include urine, spent hygiene water, and humidity condensate. These streams have distinctly different compositions; urine typically contains high concentrations of solids and ionic species, while spent hygiene water contains large organic molecules contributed by soap and skin oils. Humidity condensate is a relatively clean stream, but it contains volatile, water-soluble organics that can be difficult to remove. The total organic carbon (TOC) concentration of the combined wastewater is expected to be less t h a n 500 ppm. Dozens of species will contribute to the TOC, with concentrations ranging from less than 10 ppb to greater t h a n 10 ppm [29]. In addition to neutral and charged organic species, a number of inorganic ions will be present. In spite of the power penalty associated with phase change operations, they are attractive options for processing waste streams with high solids content. A vapor compression distillation (VCD) unit has been chosen to process urine and flush water onboard ISS. The urine distillate produced by the VCD will be mixed with the hygiene waste and humidity condensate streams before being processed by a

464 combination of technologies, p r i m a r i l y ion exchange, adsorption, a n d catalytic oxidation. These technologies will achieve complete recovery of the w a t e r in the combined w a s t e s t r e a m . A series of ion exchange resins a n d a d s o r b e n t s p a c k a g e d in Unibeds | will remove a p p r o x i m a t e l y 99% of ionic species a n d 95% of organic species [30] p r e s e n t in the w a s t e w a t e r . Each of the Unibed | media h a s been chosen because of its affinity for a specific group of c o n t a m i n a n t s . For example, a w e a k base anion exchange resin is used to remove organic acids. The media c u r r e n t l y baselined for use onboard ISS are shown in Figure 3 and described in Table 1.

influenvt~

200 cm3 MCV-RT

iiiiiii !i !ii,!il i iiii! !i!,o

),;/.. .~i,:.i

:::'ii:;,i: !:",~ii:.....i 695 cma INN-77 275 cma IRN-68

630 cm3 580-26

1325 cm3 APA *****+4 1325 cm3 XAD-4 ~ ~ 4 ............ tl ...... 200 cm3 IRN-150 effiuen

Figure 3. ISS development unit Unibed | media

465 Table 1 ISS Development Unit Unibed | Media Sorbent

Description

Function

MCV-RT (Umpqua Research)

iodinated anion exchange resin

introduces bactericidal iodine into water

IRN-150 (Rohm and Haas)

equal exchange capacity mix of strongly basic anion exchange resin (IRN-77) and strongly acidic cation exchange resin (IRN-78)

removes cations, anions, and some organics

IRN-77 (Rohm and Haas)

strongly acidic cation exchange resin

removes ammonium and other cations

IRA-68 (Rohm and Haas)

weakly basic anion exchange resin

removes organic acids

580-26 (Barneby Cheney)

GAC manufactured from coconut shell

removes organics

APA (Calgon)

GAC manufactured from bituminous coal

removes organics

XAD-4 (Rohm and Haas)

polymeric adsorbent

removes low MW solutes

IRN-150 (Rohm and Haas)

equal exchange capacity mix of strongly basic anion exchange resin (IRN-77) and strongly acidic cation exchange resin (IRN-78)

removes cations, anions, and some organics

note: sorbents are listed in direction of flow Reference: [31]

466 The p r i m a r y a d v a n t a g e of using adsorption for this application is the fact t h a t complete w a t e r recovery can be achieved. Simplicity of operation is an additional benefit. Due to the complexity associated with regenerating the various media in a closed w a t e r recovery loop with limited mass and volume allocations, each Unibed | m u s t be replaced when expended. In this case, the benefits associated with using a variety of media outweigh the mass penalty associated with replacing the beds. Since the selected media have different physical and chemical characteristics, the beds can remove a wide range of contaminants; this gives the Unibeds | the ability to handle variations in the feed s t r e a m with a high degree of reliability and robustness. Since each Unibed | will contain identical media, commonality provides an additional advantage. The Mir hygiene w a t e r processor relies on a system similar to t h a t described above for ISS. The specific n a t u r e and sequencing of the sorbents is proprietary [32].

3.

FUTURE DIRECTIONS

NASA is currently studying the possibility of constructing remote bases on the l u n a r and M a r t i a n surfaces. When the destination of a space mission is a p l a n e t a r y body, it m a y be more economical to process locally available resources into usable products t h a n to carry all consumables from E a r t h or to d e m a n d complete closure of the life support system. This aspect of mission design, called in situ resource utilization (ISRU), could involve extensive use of adsorption processes to achieve separations. The rock and soil on both the moon and Mars are potential sources of oxygen, minerals, and bulk material, but Mars also has a substantial atmosphere. As shown in Table 2, the Mars atmosphere is made up primarily of carbon dioxide and contains significant a m o u n t s of nitrogen and argon. When s e p a r a t e d and compressed, the atmospheric constituents can be used to supply m a n y essential ingredients needed to support life. In addition to being essential for plants, carbon dioxide can also be an i m p o r t a n t source of oxygen. While oxygen's use in life support is obvious, its p r i m a r y use on a M a r t i a n base would be as an oxidant for rockets launched from the planet's surface [34,35]. Nitrogen is needed to make up for leakage of buffer gas from spacecraft and surface structures; argon m a y also be used as a component of buffer gas and in electrical propulsion systems [36]. The atmosphere is presently the only known source of water outside the polar areas, and although it is quite dilute, it m a y have tremendous value both for life support and as a source of hydrogen.

467 Table 2 Model Mars Atmospheric Composition and Conditions Material CO2 N2 Ar 02 CO H20

Composition (vol. %) 95.3 2.7 1.6 0.13 0.07 0.03

pressure: 6 torr (range, 4.5 - 11.5 torr) temperature: 200 - 270 K (range, 130 - 300 K) Conditions and humidity are highly dependent on latitude and season. Reference: [33]

The high costs of electrical power and launched mass give sharp focus to the search for technologies that will perform the separations and compressions needed to utilize the Martian atmosphere. Adsorption technologies are candidates for the same reasons they have been chosen for space missions to date. In addition, it appears possible to take advantage of the large Martian diurnal temperature cycle (70 K on average over much of the year at low latitudes) to effect adsorption separations and compressions that use very little electrical power. Instead, the processes operate as heat engines that take energy from the environment during the relatively warm M a r t i a n day and dump it to the environment during the cold night. Pressure-swing adsorption separations and compressions using the Mars diurnal temperature cycle have been suggested for a variety of potentially important ISRU applications and have been demonstrated in the laboratory [37]. F u r t h e r work in this area could lead to a re-examination of the optimum degree of closure for a surface ECLSS. 4.

SUMMARY

Throughout the history of the U.S. and Soviet (and now Russian) h u m a n space flight programs, adsorption processes have played an important role in controlling atmospheric contaminant concentrations. These processes are currently used in Mir's water recovery, CO2 removal, and trace contaminant control operations and are expected to play similar roles on ISS. Adsorption can also be

468 expected to play a significant role in ECLS systems on planetary bases, where sorbents may be used to process habitat air or to recover useful substances from the local environment. 5.

ACRONYMS

4BMS ARS CMS ECLSS EPA ISRU ISS JSC MSFC NASA SMAC TCCS TOC VCD WRS

four-bed molecular sieve air revitalization system carbon molecular sieve environmental control and life support system Environmental Protection Agency in situ resource utilization International Space Station Johnson Space Center Marshall Space Flight Center National Aeronautics and Space Administration spacecraft maximum allowable concentration trace contaminant control subsystem total organic carbon vapor compression distillation water recovery system

A c k n o w l e d g e m e n t s : The authors thank D. L. Carter, C. Finn, M. Kliss, J. Knox, and J. Perry for their helpful comments on style and content.

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