MIXED-GAS ADSORPTION AND VACUUM DESORPTION OF CARBON DIOXIDE ON MOLECULAR SIEVE Bed Performance and Data Analysis LUlNO
DELL'OS SO,
JR.'
NASA Manned Spacecraft Center, Houston, Tex. JOSEPH
M.
RUDER
AND
J A C K
WINNICK'
A iResearch Manufacturing Diuision, The Garrett Gorp ., Los Angeles, Calif. A molecular sieve COz-removal system was built, based on a rigorous mathematical analysis and predesigned experimental program. The system was constructed for a specific task-to maintain a n Apollo Spacecraft cabin a t a CO? partial pressure below 7.6 mm. of Hg for a t least 45 days with a Con-removal rate equal to the production rate of three men. A closed chamber test was conducted for 48 days, during which time the system performed according to specification. Analytical techniques developed in connection with the system development accurately described system performance under normal and accelerated operating conditions.
THEdevelopment of a life support system for the Apollo
Applications Program (AAP), of which the regenerative COz-removal unit is a major part, had as main objectives the initial analysis, research, and development required to extend the life of the Apollo command module environmental control system to 45 days for post-Apollo missions. From the initial studies, the use of a regenerative C02-removal system was identified as a major improvement, compared to the required weight and volume of the lithium hydroxide (LiOH) system now used. Basically, the design of a successful gas adsorption system requires the system to remove a given quantity of contaminant from a gas stream. This can be achieved in space vehicles by use of two (or more) primary adsorption beds, if gas is to be continually purified. While one bed adsorbs the contaminant, the other bed desorbs it to space, if collection of the contaminant is not deemed worthwhile. If another component in the gas is adsorbed, to the exclusion of the contaminant, it must also be removed, either during the desorption cycle or in separate secondary beds used for protection of the primary bed from this component. The CO2-removal unit fabricated for the AAP system was based on extensive mission, vehicle, and system analyses as well as the results of a research and development program. I n the research program, data were obtained on the equilibrium characteristics of water on silica gel and of CO? and water on molecular sieves. Related data were also obtained by AiResearch (1967) on the adsorption and desorption mass-and heattransfer rates. A pilot-plant scale unit was built to examine the effects of realistic bed configurations. Concurrent with the research test program, a computer program was developed to predict the transient performance of a composite silica gel--molecular sieve bed. The heat-and
' Present address, Continental Oil Co.. Ponca City, Okla.
' Present address, University of
Missouri, Columbia, Mo.
65201
mass-transfer coefficients used in the computer program were obtained from the research test program; the computer program was then used to design the regenerative C02-removal system (Dell'Osso and Winnick, 1969). The design verification test program included performance tests to verify computed steady-state performance predictions, a life-cycle test of over 45 days' duration to verify water poisoning predictions, and postlife cycle tests to determine regeneration effectiveness. System Development
Because of the lack of basic knowledge in the area of regenerable adsorbents for use in spacecraft, an extensive research and development program was begun. I t was obvious from previous studies that water vapor had a profound effect on the performance of a molecular sieve as a CO? adsorbent. Thus, an effort was directed toward obtaining basic data for the silica gel predryer as well as the molecular sieve itself. These data were then used to develop a laboratory test program to obtain fundamental sorption data and a computer program for the process. Equilibrium adsorption isotherms were determined for water vapor on silica gel and for water vapor and CO? on molecular sieve. Mass-transfer coefficients were obtained using a dynamic sorption apparatus (Fukunaga et al., 1968). I n the next part of the experimental test program, a pilot-plant scale silica gel and molecular sieve bed was used to investigate the influence of bed geometry and heat-transfer surfaces. Utilizing the computer program and these data, a minimum-weight-penalty regenerative C0,-removal system for the 45-day AAP mission was designed and fabricated (Dell'Osso and Winnick, 1969). System Description
T h e regenerative COz-removal system contains a dual composite bed utilizing a thermal-swing silica gel predryer, VOL. 8
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followed by an adiabatic molecular sieve C02-removal section. The systenl is packaged to fit within an area adjacent to the environmental control unit (ECU) in the Block I1 Apollo command module (Figure 1). Interface connections are made to the spacecraft pressure vessel for desorption to space, to the existing ECU gas loop for circulation of the cabin gas through the adsorbing bed, and to the spacecraft heat transport loop for hot and cold fluid for operation of the thermal-swing predryer. Figure 2 is a schematic of the system, showing interface connections as well as the portion of the system subjected to the design verification tests. The adsorbent canister consists of two distinct sections containing the adsorbent materials and the associated inlet and outlet heaters (Figure 3). For the predryer section, silica gel is packed on the air side of a gas-liquid platefin heat exchanger. The heat transport fluid is circulated through the liquid side of the heat exchanger, and the silica gel is held in place by screens located a t each end of the absorbent bed.
The molecular sieve canister is connected directly to the outlet of the silica gel canister, with springs located between the screens in the canisters to maintain the bed packing. The canister consists of a vacuum-insulated housing and of a plate-fin heat exchanger, in which the molecular sieve is located. The heat exchanger contains film-type heaters sandwiched between fin plates, and provides electrical heating for removal of water adsorbed on the molecular sieve C02-removal bed prior to launch and for emergency conditioning in the event of accidental water poisoning. Each bed contained 3.2 pounds of silica gel and 11.2 pounds of molecular sieve. Test Program
The performance tests were conducted to verify the accuracy of the computer program performance predictions and to establish the best combination of cycle time, hot coolant temperature and flow rate, and gas flow rate (within the constraints of the AAP environmental control life support system) for use in the life-cycle test.
GLYCOL PUMPS-
GLYCOL RESERVOIR
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Figure 1. Regenerative C02-removal system integrated with Apollo Block II environmental control system
Figure 3. Adsorbent canister
TEST SYSTEM
Figure 2. Schematic of AAP regenerative Cor-removal system
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These tests are described extensively by AiResearch (1967) and the results are not given here. Optimum performance was indicated a t a cycle time of 30 minutes with the maximum allowable hot coolant temperature and flow of 120°F. and 25 pounds per hour. These values are very nearly those predicted in the design phase, as shown by Dell'Osso and Winnick (1969). The bed performance a t the design gas flow of 10 pounds per hour accurately matched pretest prediction and was chosen for the extended period life test. [The 10 pounds per hour gas flow is calculated on the basis of a 70 to 30 O I - X 2 mixture. I n the life test pure nitrogen was used, so the mass flow was reduced to (28130.7) x 10 = 9.2 pounds per hour to keep the volume flow a t the design value.] Test conditions are detailed in Table I. At the conclusion of the 47-day test period, both molecular sieve beds were baked out by heating the beds to 400°F. with the internal electrical heaters. A vacuum of 10 microns was maintained on each bed to remove the water desorbing from the molecular sieve. Posttest activity included complete CO, breakthrough curves on the molecular sieve bed after various amounts of water poisoning. A breakthrough test was also run immediately following the life test. Since degradation of CO, performance has been assumed by Dell'Osso and Winnick (1969) to be directly attributable to the total amount of water vapor passed through the silica gel predryer to the molecular sieve bed, the same degradation should be simulated with a few hours' flow of high dew point gas (ca. +20"F.) in place of the low dew point (ca. -60'F.I gas experienced over the 45-day test. For this purpose, the bed was given an accelerated water load corresponding to 45 and 60 days' normal operation, following a bake-out. After either loading was established, a C o r breakthrough curve was run. In addition, cyclic CO? performance was observed after accelerated water loads corresponding to 18 and 30 days of normal operation. Test System Description
I n the life-cycle test, nitrogen and CO? were injected directly into the chamber, and water was injected into a metabolic latent load simulator-all a t controlled rates. The process gas flowed from the chamber into the metabolic simulator, through a compressor, condensing heat exchanger, and water separator to the test unit. Approximately 9 pounds per hour of process gas flowed through the adsorbing canister; the remainder flowed through an odor adsorber and orifice valve. All three flow paths dumped the gas back into the chamber. During
Table I. Life Cycle Test, Nominal Test Conditions
CorditLon
Requirement
Process gas circulating rate, lb./ hr. Gas flow to beds, lb./hr. Chamber pressure, p.s.i.a. Gas inlet temperature, F. Gas inlet dew point, F. Cold water flow rate, Ib./hr. Hot water flow rate, lb. / hr. Cold water supply temperature, F Hot water supply temperature, O F. Half-cycle time, min. CO1 injectyon rate, Ib./hr. Leakage simulation rate, lb./hr.
74.5 9.2 5.0 52 to 58 52 167 10 to 32 58 120 30 0.27 0.2
desorption, the desorbed gas flowed out of the canister through a cold trap. Process gas and the hot and cold coolant were switched automatically between the adsorb and desorb mode. Chamber pressure was controlled by a backpressure valve; the gas passing through this valve also served to simulate spacecraft leakage. A detailed schematic of the system is available from the authors. All pressures, except those on the vacuum system, were monitored using mechanical pressure gages. All vacuum system pressures (except downstream of the cold trap) were measured using thermocouple gages. The cold-trap outlet pressure was measured with an ion gage. Laminar flowmeters were used to measure the CO, and nitrogen injection rate and the process gas circulation rate. Area flowmeters were used to measure the coolant flow rates and the nitrogen leakage rate. A Beckman l5A infrared Cor analyzer was used to monitor the C 0 2 partial pressure entering and leaving the test unit. An automatic programmer was used to switch to the adsorbing canister as well as the inlet sample point. The output from the analyzer was recorded continuously. Dew point sense lines were located a t the inlet to the C o r removal unit, between the silica gel and molecular sieve beds within the canisters, and downstream of the molecular sieve beds. An automatic programmer was used to switch the dew point sample point to the adsorbing canister. Two humidity probes (Panametrics) were used t o monitor dew points in the same sample lines as used by the cooled mirror-type dew point analyzer. I n addition to the temperature, dew point, and CO? partial pressure, which were automatically recorded, a large quantity of data was recorded manually. Fifteen minutes after the start of a half cycle for every second full cycle, temperature, pressure, and flow rate data were taken. Vacuum pressure readings were taken 20 minutes after the start of each half cycle and dew point readings were taken every 25 minutes into each half cycle. I n addition, every time the CO, analyzer was calibrated (three times per test day), water tank weight, as well as CO,, nitrogen, and water supply pressure, was noted. During system shutdowns, while the beds were in the desorb mode, bed temperatures and pressures as well as vacuum duct pressures were monitored. Experimental Results
As seen in Figure 4, the CO, partial pressure in the chamber continually increased as the performance of the molecular sieve bed degraded because of increased water adsorption on the molecular sieve. The daily average value of chamber CO, partial pressure, however, was held below the maximum design value of 7.5 mm. of Hg for the entire test period. A line voltage alteration caused an increase in gas flow; this may be seen as a depressed step in the curve shown in Figure 4. The dew point entering the test unit was maintained a t approximately 50°F. The dew points of the process gas leaving the silica gel beds were somewhat different. This was probably caused by a slightly greater amount of channeling in bed 1. A few dew point readings were taken on the process gas leaving the molecular sieve beds; they were consistently lower than -100" F . I t was therefore decided to concentrate the dew point measurement effort on the inlet and silica gel bed outlet dew points. Figure 5 shows the outlet CO? partial pressure from the molecular sieve beds. COP outlet partial pressure inVOL. 8 N O . 4 OCTOBER 1 9 6 9
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Figure 4. Average daily COz partial pressure of chamber
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creased with time, as expected, because of increased water loading in the molecular sieve bed. Bed 2 outlet COz partial pressure increased a t a rate much higher than bed 1. A posttest examination was initiated to determine the cause of this bed performance degradation. Humid air leaking into bed 2 through an instrument line was found to be the cause of the increased degradation rate of bed 2 . Data obtained during typical cycles are described below, compared with analytical prediction. COZ performance curves run on molecular sieve bed 1 under various stages of water poisoning are also compared with predicted curves. Data Evaluation
A computer program was developed to represent completely the transient behavior of sorbent beds. The mathematical assumptions have been described (Fukunaga et al., 1968; Dell’Osso and Winnick, 1969). Mass-transfer resistance was found in earlier studies to be adequately represented by a hypothetical surface or film coefficient. A similar effect was noted by Schumacher and York (1967) for hydrocarbons on molecular sieve. Occurring simultaneously with the mass transfer are energy effects. These are accounted for in fairly rigorous fashion. Heat transfer between all pairs of metal surfaces, 480
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
between sorbent material and gas, and between coolant and heat exchanger was calculated, as well as latent heats of adsorption. The mass and energy balances are solved simultaneously in a finite difference computation which moves completely through both the molecular sieve and silica gel beds in each time increment. Laboratory scale tests in the early phases of the development program established the mass-transfer coefficients. Heat-transfer coefficients were first estimated using standard techniques and later adjusted t o match predicted and observed bed temperatures. Determination of the masstransfer coefficients was reported by Fukunaga et al. (1968) and Dell’Osso and Winnick (1969). The heat-transfer parameters are available from the authors. Water poisoning of the molecular sieve was assumed to take place continuously during the test. The amount of water entering the bed was determined from the outlet dew point of the silica gel bed, as calculated by the computer program. A step-wave adsorption front was assumed to move through the molecular sieve bed because of the strong affinity of the sieve for water. The water loading in the poisoned section was calculated solely from the equilibrium value a t the average bed temperature and entering water partial pressure. I t was assumed that no cyclic desorption of water from the molecular sieve occurred.
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Figure 7. Life test computed a n d measured outlet CO?profiles
Silica gel bed performance, for bed 1, is shown in Figure 6 for two typical cycles. The upper curve was obtained during minimum (10 pounds per hour) flow of heat-transfer fluid during desorption. The lower comparison was made for a time of maximum (32 pounds per hour) flow. The computations show the effect of coolant flow rate in the desiccant heat-exchanger on silica gel performance.
Figure 9. Posttest comparison of computer and test water poisoning data a t 45 a n d 60 days
The molecular sieve performance during the test is shown in Figure 7. Also shown are cyclic performance data following accelerated water loading of the molecular sieve. The computed curves show the gradual effect on predicted CO? performance of the adsorbed water. Breakthrough curves run immediately after bake-out and cool-down are shown in Figure 8. N o degradation occurs after long-term testing, if a bake-out to 400" F . and 10 microns is periormed to remove the adsorbed water. Figure 9 shows the experimental and computed breakthrough curves obtained after the long-term life test as well as the simulated long-term (accelerated) water poisoning tests. VOL. 8 N O . 4 OCTOBER 1 9 6 9
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Conclusions
Literature Cited
T h e regenerative COZ-removal system designed for the three-man AAP command module was tested in a closed chamber for 47 days. N o electrical regeneration of the molecular sieve beds was required during the test. At no time during the 47 days did the average daily level of COZ partial pressure in the test chamber exceed the design limit of 7.6 mm. of Hg. The analytical techniques developed provide excellent description of system performance and accurate prediction of performance degradation because of water poisoning. These predictions were compared with experimental results from both normal and accelerated operation.
AiResearch Manufacturing Co., Rept. 67-2779 (December 1967) Dell'Osso, L., Jr., Winnick, J., IND.ENG.CHEM.PROCESS DESIGNDEVELOP. 8,468 (1969). Fukunaga, Paul, Hwang, K. C., Davis, S. H., Jr., Winnick, J., IND.ENG.CHEM.PROCESS DESIGNDEVELOP. 7 , 269 (1968). Schumacher, W. J., York, R., IND.ENG. CHEM.PROCESS DESIGNDEVELOP. 6,321 (1967).
RECEIVED for review November 1, 1968 ACCEPTED May 23, 1969
METHANATION OF LOW-CONCENTRATION CARBON MONOXIDE FEEDS OVER RUTHENIUM 5 . E.
S . R A N D H A V A , A M l R A L l H . C A M A R A
Institute of Gas Technology, Chicago, Ill.
R E H M A T ,
A N D
60616
The methanation of carbon monoxide a t parts per million levels was studied over 0.5% ruthenium metal catalyst, dispersed on alumina catalyst in a fixedbed reactor. Gas mixtures of 3450, 1090, and 505 p.p.m. carbon monoxide in hydrogen were used. The rate of reaction of carbon monoxide follows simple pseudo-first-order kinetics. The rate constant follows the Arrhenius temperature dependence at low temperatures. Evidence of diffusion control of the reaction rate was found in the higher temperature regions investigated.
METHANE synthesis by the reaction of CO and Hz over metal catalyst was first reported a t the beginning of the century. This reaction was soon recognized to have commercial significance in gas manufacture because it offered a possible alternative to oil carburetion. Akers and White (1948) studied the kinetics of methane synthesis over reduced nickel catalyst a t atmospheric pressures. A detailed study of the synthesis of methane with empty stainless steel tubes and with steel balls was reported by Gilkeson et al. (1953). Nicolai et al. (1946) determined the kinetics of the methanation reaction over ruthenium catalyst a t elevated pressures. Extensive work has also been reported by the U.S. Bureau of Mines (Karn et al., 1965). A detailed survey of the studies in methanation appears in a research bulletin (Dirksen and Linden, 1963). The best catalysts for hydrocarbon synthesis and the hydrogenation reactions belong to the eighth group of the periodic system of the elements that comprises iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Although the metals of the platinum group are all generally active, they differ considerably in their ability to catalyze various reactions between CO and HZ. Ruthenium has been known for many years to be a highly effective catalyst in the Fischer-Tropsch reaction and methane synthesis. Recently, McKee (1967) studied the interaction of H2 and CO with platinumgroup metals and suggested that the highly specific behavior of ruthenium was probably due to its lower affinity for CO than the other noble metals. Many thermodynamic analyses of the methanation reaction system have been conducted to evaluate the influence 482
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
of the major operating variables. Under conditions of the normal chemical equilibrium with a fair degree of CO conversion, the direct methanation reaction is accompanied by the shift reaction. However, in the presence of a highly active methanation catalyst, CO is consumed so rapidly that the CO water-gas shift equilibrium is not approached, and CO concentrations are well below computed equilibrium values. Consequently, to negate any effects of the CO-shift reaction in determining the exact kinetics of the CO methanation, it is advisable to work with feed gases having very low concentrations of CO. At the energy conversion laboratory of the Institute of Gas Technology investigations have been under way to remove traces of CO from the H 2 anode gas, which would otherwise act as poisons in the normal operations of low-temperature fuel cells. In the present study of methane synthesis, parts-permillion mixtures of CO and H z were passed through a tubular reactor containing ruthenium catalyst. The effects of temperature, feed rate, and feed composition were investigated. Experimental
Experimental Apparatus. The diagram of the flow system is shown in Figure 1. T o facilitate the description, the apparatus is considered in operation. The mixture of carbon monoxide and hydrogen passes through a pressure regulator and control valve to the rotameter, which has been calibrated by using a wettest meter, enters the top of the reactor, and flows downward through the catalyst bed.
