Thomas, C. L.. "Catalytic Processes and Proven Catalysts." Academic Press, New York, N. Y., 1970, p 46. Voge, H. H., Morgan, C. 2.. lnd. Eng. Chem.. Process Des. Develop.. 1 1 , 454 (1972). Wolf, C. N.. Bergman, R. I., Sittig, M., Chem. Week, 98, No. 22, 113 (1966).
T h i s p a p e r was presented a t t h e 4 t h J o i n t AIChE-CSChE C h e m i c a l E n g i n e e r i n g Conference. Vancouver, Canada. Sept 1973.
Received for rvripu. May 16, 1973 A c c e p t c d A u g u s t 20, 1972
An Electrochemical Device for Carbon Dioxide Concentration. I. System Design and Performance Jack Winnick,"' National Aeronautics and Space Administration, Johnson Space Center Houston, Texas
Richard D. Marshall, and Franz H. Schubert Life Systems, lnc
Cleveland, Ohio
A system comprised mainly of 90 electrochemical cells has been designed for use a s a carbon dioxide (CO2) concentrator in a manned spacecraft. Cabin gas, with a COz partial pressure of about 3 r n m , I S
passed across t h e air cathode of an electrochemical cell. It is concentrated through the carbonate electrolyte and expelled into the hydrogen-filled anode cavity. The total system, a s well a s the individual cell design, is described. Experimental results are shown for t h e full (90-cell) system and also for smaller scale ( 1 - and 3-cell) tests..Excellent consistency among the tests was found.
Introduction
Manned missions in space for extended periods require recycling of most consumables. The recovery of water ( H 2 0 ) and oxygen ( 0 2 ) becomes attractive for missions of about 30 man-days or longer. Oxygen can be regenerated from the metabolically produced carbon dioxide (COz) by three main paths: using direct electrolysis of COz in a solid electrolyte 0 2 regeneration subsystem forming 0 2 and solid carbon; using the Bosch reaction, converting C 0 2 and hydrogen (Hz) through a series of steps to 0 2 , solid carbon, and H20; and using the Sabatier reaction converting COz and Hz to methane and HzO. The product H 2 0 formed by the Bosch and Sabatier reactions is electrolyzed to form make-up 0 2 and Hz. Both the Bosch and Sabatier reactions require H2, although in different ratios to the CO2. Carbon dioxide concentrating systems developed for current ( e . g . , Skylab) manned spacecraft are cyclic adsorption beds utilizing commercial zeolites (Dell'Osso, et al., 1969) as the CO2 sorbent. Criteria for cabin COz environments for future missions require control a t or below CO2 partial pressures (pCO2) of 3 mm. At these low partial pressures the zeolite systems are inefficient resulting in a high system equivalent weight. An electrochemical C 0 2 removal device developed for aircraft (Wynveen and Quattrone, 1971) proved applicable for spacecraft (Wynveen, et al., 1972) and is presently under development. The electrochemical concept offers these advantages: (1) continuous operation instead of cyclic operation; (2) low pCOz capability with low system equivalent weight; (3) concentrated COz, noncontaminated by cabin air, thus protecting the COz reduction process catalyst (Sabatier) or preventing buildup of nonreacting
'
Permanent Address, Department of Chemical Engineering, University of Missouri, Columbia, Mo. 65201.
gases in C O z reduction recycle loops (Bosch and solid electrolyte); (4) supply of COz premixed at proper ratios with Hz for the Bosch or Sabatier process; ( 5 ) elimination of COz compressors since vacuum desorption is not required. Extensive development experimentation was performed to identify performance parameters using single electrochemical cells, a three-cell unit, and a six-man capacity, 90-cell system. The test results provided a basis for system sizing and performance prediction. The data also served to provide a basis for analytical simulation of' the CO2 removal process (Lin and Winnick, 1974).
Process Description The electrochemical reactions occur on electrodes separated by an aqueous carbonate solution supported in a thin asbestos matrix. The cathode and anode are comprised of fine mesh screens upon which a Teflon and platinum mixture has been applied. Moist air containing COZ at ambient concentrations is passed over the cathode. Oxygen in the air diffuses into the liquid film a t the electrode and reacts with HzO t o form hydroxyl ions (OH-) O? + 2H,O de$OH(1) The COz also diffuses into the liquid, reacting with OH- to form carbonate ions ( C o s 2 - ) CO, OHHC0,('a)
-
+
+
HC0,-
+
OH-
-
CO,,'-
+
HLO
('11)
Moist H2 is passed over the anode. After diffusing through a thin film of solution, it reacts a t the electrode to form hydronium ions (H30+ )
+
H? ~ H ~ -+ O + ?e(3) The lowered p H in the anode region favors higher bicarbonate ion concentrations via the equilibrium reaction Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1, 1974
59
shown as eq 2b. These bicarbonate ions dissociate to COZ which is evolved into the flowing Hz HCO,;
---f
CO,
+
OH-
(4)
A t yet lower pH values, bicarbonate ions react to form carbonic acid, which then also decomposes to evolve COz HCOC
+
H1O - OH-
S H,CO;
--+
CO,
+
/
HjO (5)
Details of the analysis of these reactions are given by Lin and Winnick (1974). The net reaction is that of HzO production, accompanied by the generation of electrical energy (+1.23 V, theoretical) and the transfer of COz from the cathode side to the anode side of the cell. Cell Description
Figure 1 shows a functional schematic of a single cell. The carbonate electrolyte is supported in a porous 30-mil asbestos matrix, compressed to 25 mil. The dimensions of the electrodes are 4.1 X 8.6 in. (0.244 f t z ) . Expanded nickel spacers in the air (80 mil) and Hz gas compartments (60 mil) act to mix the gas streams, transfer heat, and transmit electrical current between the electrodes and current collectors. The anode current collector is a solid nickel plate extending 1.0 in. outside the cell. These extensions are used to dissipate the heat generated in the reaction through forced convection to air blowing over the fins or to liquid coolant flowing through attached tubes. An aqueous solution of potassium carbonate (KzCO3) was first used as the cell electrolyte (Huebscher and Babinsky, 1970). However, a cesium carbonate (CszCO3) solution was substituted to broaden the cell's tolerance to variable air inlet humidities. If slightly lowered humidity air (with respect to the initial electrolyte charge concentration) flows through the cell, drying of the relatively small amount of electrolyte will occur. This drying will eventually bring about precipitation, accompanied by degradation of cell performance, when the solubility limit of any of the species present in the electrolyte is reached. Precipitation of potassium bicarbonate limited the application of cells using KzC03 electrolyte (Huebscher and Babinsky, 1970). Cesium carbonate and, especially cesium bicarbonate, have significantly higher solubilities than the respective potassium salts. Test Apparatus
Experimentation was conducted with a single-cell unit, a three-cell unit, and a 90-cell, six-man C O z removal capacity system. The test apparatus for both the single- and three-cell tests was identical. A schematic of this test setup is shown in Figure 2. A functional schematic and a photograph of the 90-cell system is shown in Figures 3 and 4, respectively. Due to geometry considerations, this system was split into two 45-cell modules as shown in the figures. Test Program
The experimentation reported in this document was conducted to determine the relationship between the COz removal rate, the cell current density, and the pC0z level, as measured at the air inlet to the cell(s), for a single-cell unit, three-cell unit, and a 90-cell system. The single- and multicell units were used to determine the validity of single-cell results for performance prediction of multicell systems. a. Single-cell Operation. A single cell was tested under highly controlled conditions. The cell was loaded with an aqueous solution of 61.5 f 0.57'0, by weight, Cs2C03. For this test, near isothermal conditions were 60
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1, 1974
i1
Lp
Process Air
Figure 1. Single-cell functional schematic.
maintained by flowing liquid coolant through copper tubes attached to the anode current collector plate. The cell temperature was controlled a t 68 f 1°F by controlling the temperature of the liquid coolant bath. The hydrogen flow rate was adjusted to a constant 0.30 f 0.03 standard liter per minute (slpm) (70"F, 760 mm). This flow is sufficient to maintain the electrochemical reaction such that it does not hinder the COz transfer process. Both the air and Hz streams were a t ambient pressure. Air, premixed with COz, was passed through the humidifier (see Figure 2) where the dew point was brought to 50 f 2°F. The COz flow added to the air was accurately monitored using a soap bubble flow meter. Inlet and outlet air dew points were measured with a Cambridge Model 880 hygrometer. The air flow rate was maintained a t 0.44 to 0.50 standard cubic feet per minute (scfm) (70"F, 760 mm). The concentration of COz in the inlet and outlet air streams were measured with a Lira Model 300 infrared spectrometer (0.5% COz in air, full scale). Anode gas outlet COS concentrations were measured by two means; a Lira Model 300 infrared spectrometer (50% COz in HZ full scale) and by absorbing the COZ in a tube of Ascarite, comparing the flow rates upstream and downstream of the tube, and calculating the COz transferred by difference. The estimated error in the measured COz transfer rate is f 5 % using the air or HZ Liras and f2.5% for the Ascarite method. Cell current was maintained constant by an electronic controller. b. Single-cell Test Procedure. Tests for COz removal performance were conducted a t five current densities (total amperage/geometrical electrode area): 5.0, 7.5, 10.0, 20.0, and 30.0 A/ftz. Each current condition was maintained for a t least 24 hr. Three inlet air pC0z levels were run a t each current density: 0.5, 3.0, and 9.0 mm. Each pCOz was maintained for a t least 3 hr to ensure steadystate behavior, confirmed by comparing the COz transfer rates as determined from the air-side mass balance with those from the Hz-side mass balance. In general, these measurements would agree within f2.570 after 2 hr of operation. c . Three-Cell Operation. A single electrochemical cell of the size described above will transfer about 1,5 the COz production of one man a t 20 A/fP. Thus, in a spacecraft, it will be necessary to have multicell units or modules. TO test the performance of a small group of cells, a test was run with three identical cells connected in series electrically. The Hz flow was also connected in series, with air and liquid coolant flows in parallel, each cell receiving air
*-
PrCaIS
,.,,cr,z,,\,. ,\,>I,.
-
, "m Hg
Figure 5. Carbon dioxide transfer rate us. process air inlet pC0z: single-cell unit, 61.5% CszCOa electrolyte, 68 1'F cell temperature, 66 1°F inlet air temperature, 50 + 2°F inlet air dew paint, 0.44-0.50 scfm air flow rate per cell, 0.30 0.03 slDm Hz flow
* *
*
I
7
rate.
test, maintained the cell temperatures a t 62 f 2°F. Air flow rates per cell ranged from 0.5 to 0.8 scfm, with the higher flows used with the higher currents. Hydrogen flow was maintained a t 0.50 0.10 slpm. The same experimental test stand and instrumentation used for the single-cell tests was used for the three-cell tests (see Figure 2). For the three-cell test, however, the COz in the outlet anode gas stream was monitored only with the Lira infrared spectrometer since the higher flow rates prevent effective use of the Ascarite tube method. d. Three-Cell Test Procedure. Tests for COz removal performance were conducted a t four current densities: 16.0, 20.0, 24.0, and 28.0 A/ft2. Four inlet pCOz levels were run a t each current density: 1.5, 2.0, 2.5, and 3.0 mm. The procedures to ensure steady-state behavior during the single-cell tests were also used in the three-cell tests. e. Ninety-Cell Operation. For certain future space missions, in particular the space station, crews will he comprised of six-men. A system (see Figures 3 and 4) designed for such an application containing 90 cells identical with those used in the sinele- and three-cell tests was tested. Air flow was in parailel through all 90 cells while Hz flow was in parallel through six equal paths each consisting of 15 cells connected in series. Air flow per cell and Hz flow per 15 cells averaged 0.5 scfm and 1.67 slpm, re-
*
/--
I'
x
Figure 4. Ninety-cell system.
and coolant of the same inlet condition. The inlet air was maintained a t 56 2°F dIry hulh and 45 1°F dew point temperature. Liquid coolan t flow, similar to the single-cell
*
Air ,"let G O ,
//I/,
II.OI,II/
*
Ind. Eng.
Chem.. Process Des. Develop., Vol. 13,No. 1. 1974
61
Or
/
20
10
0
30
o v
40
0
' I
2
1
3
Current Deoslty, omp/TtZ
Figure 6. Carbon dioxide transfer rate us. current density: singlecell unit, 61.5% CszC03 electrolyte. 68 1°F cell temperature, 66 1°F inlet air temperature, 50 f 2°F inlet air dew point. 0.440.50 scfm air flow rate per cell, 0.30 f 0.03 slpm Hz flow rate.
*
*
4 Process
Air
5 6 7 Inlet pCOz, mm Hg
8
9
IO
Figure 8. Carbon dioxide transfer rate us process air inlet pC0z: 90-cell system. 61.5% C S ~ C O electrolyte, ~ 67 f 0.5"F cell temperature, 55 f 1°F inlet air temperature, 47.5 f 1°F inlet air dew point, 0.49 scfm air flow rate per cell, 10.0 f 0.3 slpm HZ flow rate.
p, 0 Three Cell Test ib N m t v Cell Test1
01 0'
"
'
15
1
20 Process
Air
2.5 Inlet p C O z , mm Hg
3.0
3.5
Figure 7. Carbon dioxide transfer rate us. process air inlet pC0z: three-cell unit, 61.5% CszCO3 electrolyte, 62 f 2°F cell temperature, 56 f 2°F inlet air temperature, 45 f 1°F inlet air dew point, 0.50-0.80 scfm air flow rate per cell, 0.50 f 0.10 slpm HZ flow rate.
spectively. Cell temperature was controlled at 67 f 0.5"F (with air inlet dry bulb and dew point temperatures of 55 & 1°F and 47.5 & 1"F, respectively) by blowing, on demand, 50°F air over the finned portion of the anode current collectors. Demand for cooling was sensed by the process (cathode) air outlet temperature. All process fluids were properly conditioned and metered in ground support equipment for the total test setup. f. Ninety-Cell Test Procedure. Tests were conducted a t one current density, 20 A/ft2, for air inlet pC0z levels of 0.6, 1.75, 2.8. 5.0, and 7.7 mm. The same geneial test procedures used for the single- and three-cell tests were used for the 90-cell tests. Test Results
a. Single-cell Unit. The results for the single-cell tests are shown in Figures 5 and 6. It can be seen that the specific COz transfer rate, expressed in pound mole per day square foot of geometrical electrode area, increases with increasing inlet pC0z and with increasing current density. Figure 5 shows that for each current density a COz transfer rate is reached after which increasing inlet pCOz has negligible effects on the transfer rate. This "maximum effective" p C 0 ~level increases with increasing current density. Similarly, Figure 6 shows that at a specific pCOz level, increasing current density above a certain maximum effective value has negligible effects on COz transfer rate. 62
Ind. Eng. Chern., Process Des. Develop., Vol. 13, No. 1, 1974
0,v
I
2
3
a
5
6
7
Q
9
,b
Process Air Inlet p C O p , m m Hg
Figure 9. Comparison of COZ transfer rate
us. process air inlet pC0z for single-cell, three-cell, and 90-cell tests at 20 A/ft2.
b. Three-Cell Unit. The results for the three-cell test are shown in Figure 7. The tests were conducted over a smaller range in pC0z and current density. The results of the three-cell tests agreed well with the single-cell results. c . Ninety-Cell System. Figure 8 shows the results obtained for the 90-cell system for a current density of 20 A/ft2. The shape of the curve is quite similar to that obtained for the single-cell unit at 20 A/ftZ. d. Test Comparison. A plot comparing results of the single-cell, three-cell, and 90-cell tests is shown in Figure 9. The data presented are for a current density of 20 A/ft2 (baseline for the 90-cell system). The figure shows the results to be in excellent agreement. The differences in cell temperatures among the tests apparently has little net effect on the COz removal rate. The explanation for this behavior is reported by Lin and Winnick (1974). The comparison of the results also shows no effect of scale-up in the number of cells on the CO2 transfer rate. Discussion Cells used for the electrochemical removal of COz from air have been tested for a variety of current densities and inlet air C O z partial pressures for three experimental configurations ranging from a single-cell unit to a 90-cell system. Agreement among the test results is well within experimental error. Variations in COz removal rate as a function of pCOz and current density has been defined for the respective ranges of 0 to 9 mm and 5 to 30 A/ftz. The 90-cell system showed a one-man (seven instead of six men) overcapacity capability at its baseline 20 A/ftZ and for an inlet pCOz of 3 mm.
The results indicate that single-cell experimentation can be used to predict the characteristics of multicell subsvstems. It can also be concluded that an electrochemical COz removal device for handling any size crew can be designed with good assurance of performance.
Huebscher, R. G . . Babinsky, A. D., "Aircrew Oxygen System Development. Carbon Dioxide Concentrator Subsystem Report." NASA CR73397; July 1970. Lin. C. H., Winnick, J . , Ind. Eng. Chem , Process Des. Develop.. 1 3 . 63 (1974). Wynveen, R. A., Quattrone, P. D., ASME Life Support Conference, San Francisco, Calif., July 1971 Wynveen. R. A.. Schubert, F. H., Powell, J . D . , "One-Man, Self-contained CO2 Concentrator System." Final Report No. NASA CR-114426, March 1972.
Literature Cited Receiued for reuieu M a y 29, 1973 Accepted S e p t e m b e r 27, 1973
Dell'Osso, L., Ruder, J., Winnick, J., Ind. Eng. Chem.. Process Des Deveiop., 8, 477 (1969).
An Electrochemical Device for Carbon Dioxide Concentration. I I. Steady-State Analysis. C 0 2 Transfer Chin H. Lin and Jack Winnick*' Nationai Aeronautics and Space Administration. Johnson Space Center. Houston, Texas 77058
The rate equations which govern each of the processes controlling the C O P transfer in an electrochemical COz concentration cell are given. A steady-state, isothermal mathematical model based on these equations is established. The model is a nonlinear boundary value problem which i s solved by a shooting method. Most of the parameters needed for carrying out the numerical simulation of the model are evaluated from independent physical and chemical data. Rates of COn transfer are calculated for inlet C O P partial pressure of 0 to 14 m m and current density from 5 to 40 A/ftz. The simulation results are shown to fit the test data for concentrators of two different designs and configurations within experimental error.
In the previous paper (Winnick, e t al., 1974), the basic mechanisms and the description of one of the cell designs for an electrochemical carbon dioxide (CO,) concentrator have been presented. An analytical model for the steadystate COZ transfer in the concentrator has been developed and is presented herein. The model should be able (1) to describe the plausible reaction and transport processes relevant to the transfer of C o n , ( 2 ) to simulate the existing tests results from two different designs (Winnick, e t al., 1974; Russell, 1973), and (3) t o furnish cell performance predictions for off-design operations and optimum design study. Description of Transport and Reaction Mechanism and Rate Equations
The concentrator is considered to be divided into several subcells in the direction of the air flow such that for each subcell mass transfer may be considered one-dimensional and in the direction perpendicular to the electrodes. A schematic of the subcell developed for the model is shown in Figure 1. The numerical simulations are carried out first for each subcell, and then for the entire concentrator to compute the COz transfer rate for various given inlet COZ partial pressures (pCOz) and current densities. A subcell is divided in the x direction into five zones characterized by different dominating transport or reac-
' Permanent Address,
Department of Chemical Engineering, University of Missouri, Coiumbia, Mo 65201
tion mechanisms. The dominating mechanisms in each zone are discussed separately below. A. Air Zone (Cathode). The transport of COZ from the bulk air flow to the gas-electrolyte interface consists of two stages: convectional mass transfer in the free air stream and diffusional mass transfer in air-filled electrode pores. An overall mass transfer coefficient for C O z , h5', is defined such that the molar flux of C O z , Nj (rate per unit geometric electrode area), is given by where the subscript c refers to the cathode region, PsCand PsCoare the COz partial pressures in the bulk air flow and a t the interface, respectively, R is the gas constant, and T is the temperature of the cell. The coefficient hbcdepends mainly on the air flow rate, the geometry of the air channel. and the structure of the cathode. The pressure PsCois that in equilibrium with the dissolved COz, Cj,, according to the relation
Pi = HC-,, (2) where H is the Henry's law constant which depends on the temperature and the ionic strength of the electrolytic solution but not on CsC. The driving force for the COz transfer in this region is assumed to be the logarithmic mean of that at the inlet and the outlet of the subcell; that is
(P, = ( P , - P,O).
-PLf)j,l
where 2,
( A i --L,))'1n(AI/'A,)
(3) The mass balance for the air zone =
Ind. Eng. Chern., Process Des. Develop., Vol. 13, No. 1, 1974
63
