Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1201-1208
1201
Ternary Gas Mixture Separation by Pressure Swing Adsorption: A Combined Hydrogen-Methane Separation and Acid Gas Removal Process Pel-Ling Cen, Wet-Ntu Chen,t and Ralph T. Yang” Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
An experimental and theoretical study was conducted for the bulk separation of a ternary gas mixture by pressure swing adsorption. Each cycle encompassed four steps: pressurization, adsorption, cocurrent blowdown, and countercurrent vacuum desorption. The experimental data were satisfactorily predicted and simulated by an equilibrium model and a lineardriving-force model. A feed mixture containing 49.5/49.5/1 .O H,/CH,/H,S was separated into three useful products: a clean H, (over 99% H, and below 0.01 % H,S), a clean CH, (over 95% CH, and below 0.01% H,S), and an HS , product (over 10%). Cocurrent blowdown was a crucial step for recovering the medium product (CH,). Vacuum desorption and pressurization by H2were important in recovering the heavy product (H2S) and cleaning the bed for a high-purity H, product. These and other fundamental principles for bulk separation of multicomponent mixtures by PSA were discussed.
Since Skarstrom (1960) first proposed a two-column pressure swing adsorption (PSA) cycle for the purpose of air drying, many more sophisticated PSA processes have been developed and commercialized. It has attracted increasing interest more recently because of its low energy requirements as well as low capital investment costs (Stewart and Heck, 1969). State-of-the-art reviews of the PSA processes have been made by Keller (1983), Cassidy and Holmes (1983), and Wankat (1981). In modern PSA processes, three or more beds are used to synchronize and accommodate steps additional to those in the Skarstrom cycle: cocurrent depressurization and pressure equalization. The two major applications of PSA hvae been air drying and hydrogen purification. Other commercial applications are oxygen and nitrogen production from air, octane improvement (using Union Carbide ISOSIV Process), and solvent recovery. In almost all PSA processes, only one component in the feed mixture, generally the weakly adsorbed one, is the desired product. However, for economic reasons, it is increasingly important to recovery all components in the mixture. The literature on PSA separation has been concerned exclusively with binary mixtures, with few exceptions. Nataraj and Wankat (1982) suggested a three-column process for separation of a ternary mixture and predicted the performance by using method of characteristics. Modeling of multicomponent adsorption by numerical methods may be exemplified by the work of Wang and Tien (1982). However, these models were developed for dilute mixtures, and both isothermal and adiabatic conditions were assumed. The problems associated with modeling multicomponent, bulk (as opposed to dilute adsorbates) separation by cyclic processes have been discussed recently by Chen and Yang (1985). The bulk separation of Hz/CH4is an important step in fuel and chemical industries. An experimental and theoretical study of the H2/CH4separation by PSA has been performed (Yang and Doong, 1985). The H2/CH4mixture frequently contains a small amount of H2S,which is usuVisiting Professor from Zhejiang University, People’s Republic of China.
ally removed before the clean mixture is separated, thus involving two separate process steps. In this study, a PSA cycle was used to separate a 49.5/49.5/1.0 H2/CH4/H2S mixture into three useful products: H2 (over 99%), CHI (over 95%), and H2S (over 10%). Both H2 and CH, products contained less than 0.01 % H2S. The H2Sproduct may be further converted to sulfur by commercial processes. An equilibrium model and a linear-driving-force model have been developed for the PSA process, which satisfactorily predicted the performance of the PSA cycle.
Process Description The PSA process used in this study consisted of the following four steps in each cycle: (I) pressurization, (11) adsorption, (111) cocurrent blowdown, and (IV) countercurrent evacuation. The cycle sequence and lengths of time of the steps for a three-column arrangement is illustrated in Figure l. By the arrangement, continuous feed and products are possible. In step I, the bed is pressurized to the feed pressure by either Hz or the feed. Since pressurization by H2 gives a much better separation, as shown by Yang and Doong (1985) and by commercial H2 purification processes, H2 is used in step I. The high-pressure H2can be drawn from the effluent of another column undergoing step 11. In step 11, H2 is the product and CHI and H,S are adsorbed in the bed. Step I11 is divided into two substeps: step IIIa (2 min in most runs) and step IIIb (9.5 min in most runs). Step IIIa is used to exhaust the H, remaining in the voids in the bed near the outlet of the bed. The effluent from IIIa is an H2 product. Step IIIb is used to desorb and recover CHI. Although not necessary, evacuation is also used at the end of step IIIb to assist the desorption of CH,, and the end pressure of step IIIb in this study is 0.5 atm. In step IV, the bed is evacuated from the feed end, hence “countercurrent” to an end pressure of 0.1 atm, to desorb and recover H2S. The evacuation also serves to clean the bed so high-purity H2 and CH, products are possible. The evacuation (or vacuum desorption) step is used here to replace the purge step (by H2 or the weak adsorbate), which is used in most commercial PSA processes. There are several known PSA processes in which vacuum desorption is used, such as the Bergbau Forschung GmbH
0196-4305/85/1124-1201$01.50/00 1985 American Chemical Society
1202
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table I. Parameters for Leading Ratio Correlation and Heats of Adsorption on Activated Carbon
9dcorber I
H, n
Hz
2
CH,
HZS
t
3
a b 87.69 42392 -0.76 40539 29.04 62947
C
-12.336 -10.245 -9.057
d 1219.3 1756.0 1725.2
kcaI/mol 2.6 5.0 5.6
L
Table 11. Parameters for Heat Capacity Equations A B C D -3.289 X 10” 1.826 X H, 6.483 2.215 X 2.86 X 10“ -2.703 X CHI 4.598 1.245 X 5.809 X 10” -2.81 X HzS 7.629 3.431 X
5TEPb I H z Presturizatlon o 5 min 11 Feed 7 5 mln 111 Cocurrent Blowdown 1 1 5 mln I\ Counter current Evacuation
q 0.4 0.97 1.00 1.0 1.00 1.0
o min.
Figure 1. Cycle sequence chart for a three-bed PSA system for bulk separation of an H*/CH4/H2Smixture into three products.
In the equilibrium model, the amount adsorbed is equal to the equilibrium amount
process for N2production from air (Knoblauch, 1978), the Toray process for the same purpose but using a different sorbent and different cycles (Keller, 1983), and the processes commercialized at Air Products invented by Sircar (1977-1979) on Vacuum swing adsorption” for purposes ranging from air separation to separation of multicomponent hydrocarbons.
aqi - aqi* at at
Two Models for Bulk, Multicomponent PSA Separation To simulate the four-step PSA process described in the foregoing, two models are developed: the equilibrium (EQ) model and linear-driving-force (LDF) model. In the EQ model, the adsorbed phase is assumed in equilibrium with the fluid phase, whereas the pore diffusion rate is approximated by a linear mass-transfer equation in the LDF model. In both models, energy balance is considered because the temperature excursion can exceed 50 “C during each PSA cycle, at a fixed point in the adsorber bed. Adsorption of all components from the mixtures is calculated by using the noniterative model, namely, the loading ratio correlation equations. Furthermore, axial dispersion is accounted for by using a finite different backward scheme in numerical computation which is essentially the same as the tanks-in-series model. The simplifying assumptions and approximations made in the models are as follows: (1)the ideal gas law applies (the compressibility factor for the gas mixture was calculated to be 0.99 under our experimental condition of 34 atm and 25 “C); (2) the axial pressure gradient across the bed is neglected; (3) thermal equilibrium is assumed between the fluid and the particles; (4)no variation exists in the radial direction for both concentration and temperature; (5) due to the narrow temperature range of the process, ka in the linear-driving-force model is assumed to be independent of temperature. The value of ka is further assumed to be the same for all components. With the above assumptions, the models are direct extensions of our previous models for binary PSA separation (Cen and Yang, 1985). The total material balance in the column can be written as
whereas in the linear-driving-force model (4)
where i = 1, 2, and 3, denoting C H I , H2S, and H,, respectively. The sum of three qi’s is the total q. Although there are several models which are useful for predicting equilibrium adsorption (si*)from gas mixtures, the only noniterative one is the loading ratio correlation (LRC) (Yon and Turnock, 1971; Chen and Yang, 1985). For the data correlation purpose, the LRC equations can be improved by introducing a fitting parameter, qi, which actually indicates the lateral interaction with coadsorbed molecules. The LRC equations are
(?)/[ p(:)]
qi* = qmiBi
EA aPYi/n R at
AP a(qxJ +--=o v, at
1+
(5)
The use of these noniterative equations substantially simplifies the computation. The constants are qmi = ai + bi/T
Bi = exp(ci + di/T)
(6)
The values of the parameters in the LRC equations for the ternary mixture on activated carbon have been measured in this laboratory by Byer (1982) and are given in Table I. The maximum deviation of the predicted values using the above LRC equations from the experimental data, within the range of conditions in this study, is approximately 30%. Energy balance in the column is given by auC,T az
+--c~R
aC$ at
apC,,T +--Ap v, at
+ApC
pa
-aT + at
where the summation is over three components, and i = 1-3. The molar heat capacity of the adsorbed phase is assumed to be equal to that in the gas phase; thus,
c,
Material balance for component i yields
-auyi a+t -
(3)
=
c,
=
CCPjyi
(8)
and (2)
where i = 1 and 2, denoting CH, and H2S, respectively, and y1 + yz + y 3 = 1.
C,i = Ai
+ BiT + C i P + D i p
(9)
The values of the parameters for heat capacities are given in Table 11.
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 1203
The initial conditions for the PSA process are, a t t = 0 which is the time after step I (H, pressurization) after start up of the process, y1 = x1 = 0.0 y2 =
x2
I Cold
Bath
= 0.0 r
y3 = xg = 1.0 P=Po
To Recordtr
T=To
Q=40
A
(10)
The boundary conditions for the ensuing cycles are as follows. Step 11. At z = L (the feed end), y1 = ylf, YZ Yzf, ~3 = y3f, P = Pf, T = T f , and u = up At z = 0, p = pp Step 111. At z = L, u = 0. At z = 0, u = u ( t ) or P = p ( t ) . Step IV. At z = L , u = u ( t ) or P = p ( t ) . A t z = 0, u = 0. Step I (Hydrogen Pressurization from the End Opposite the Feed). At z = L, u = 0. A t z = 0, u = u ( t ) or P = P(t). The final state of each cycle (step I1 to step I) is at t = A t , P = Pfi The initial conditions for step I1 in the next cycle are the same as the final conditions of step I, thus continuing the cyclic process. Throughout this study, P = P ( t ) rather than u = u ( t ) is used as the boundary condition because accurate and continuous P ( t ) data can be recorded. The heat-transfer term, adh(T - To),although small, was found to be very important to the temperature history and the separation results. The value of h was the overall value across four consecutive resistances: bed-to-steel wall, conduction in steel wall, conduction in the insulation layer, and free convection, with the resistance in the insulation layer being dominant. Since h varied only slightly with bed temperature, a constant value was used in the model. The constant h value was taken as 9.58 X cal/(cm2 K/s), which corresponded to T = 40 "C and To = 20 "C (Cen and Yang, 1985). Method of Solution. A backward finite difference method was used to solve eq 1, 2, and 7, combined with eq 3 (for the EQ model), 4 (for the LDF model), 5 , and 6. In a typical computation, 25 space steps and 1350 time steps were used for each PSA cycle. The P ( t ) data from the transducer/recorder were fitted into polynomials, which were then used as the boundary conditions. In the finite difference scheme, the effluent from each space step, or segment, was the same as that in the segment, which resembled the tanks-in-series model with no back-mixing. A two-loop iteration scheme was used to evaluate the composition, temperature, and flow rate of the effluent. The computation was initiated by assuming a set of values for yi and T for the segment. Equations 5 , 1,and 2 were used to calculate, respectively, qiand xi,u,and yi. Iteration was continued until yi was within of the assumed value. Equation 7 was then used to calculate T,which was iterated until T was within of the assumed value. The convergence was generally rapid for T iteration but slow for the iteration of y;. All computation was performed in a VAX 780 computer. The CPU time was approximately 25 min for 10 PSA cycles, which were adequate for the PSA process to reach steady-state operation, or approximately 2.5 min per cycle. The computation time was rather short as compared to other numerical schemes for multicomponent adsorption processes. For example, for the adsorption from a three-component mixture in the liquid phase, Wang and Tien (1982) used Simpson's rule in iteration, which took approximately 1min for the adsorption step alone. Presentation of Separation Results. The performance of a PSA process is determined by three interrelated
.I-
Figure 2. Schematic diagram of apparatus for pressure swing adsorption of gas separation. SP: sampling port. PG: pressure gauge. CV: check valve. P T pressure transducer. SV solenoid valve. TC: thermocouple. LPR: line pressure regulator. Not shown are the lines connecting the upper end of adsorber the pump and the flow meter downstream the pump.
sets of results: product purity, product recovery, and productivity. The productivity is judged as the rate of feed processed per unit amount of sorbent. These three sets of results were directly calculated from the numerical solution. The product purity was calculated as the volume-averaged concentration of the given product in the effluent from the corresponding step. The product recovery will be defined shortly. The productivity was given as the amount of feed per cycle. All results were expressed as steady-state values. The steady state was generally reached after approximately five cycles from start up. Furthermore, the product purity and recovery were given for all three products. Experimental The experimental apparatus was designed for simulating all basic steps in the PSA process, capable of wide-range conditions (high pressure, temperature, flow rate, adiabatic, isothermal, etc.) for multicomponent, bulk separation. The apparatus was automated and controlled, with the only manual operation being sample collection and analysis and recording the flow rates. A schematic of the apparatus is shown in Figure 2. The adsorption column was a stainless steel (Type 304) pipe 60 cm long and 4.1 cm i.d. The bottom plate of the column was a stainless steel sintered plate glazed to the column. The column was packed with 20-60-mesh activated carbon (designated PCB, manufactured by Calgon Corp.). Glass wool was packed on top of the bed to prevent the carryover of particles. An additional in-line filter (60-pm holes) was installed to keep fine particles from entering the gas lines. All lines were 1/4-in.stainless steel. Three solenoid valves located at the feed, cocurrent end, and countercurrent end points were used to alternately direct the flow into and out of the column. These solenoid valves were activated by electronic timers, which were preset to operate at desired time cycles. A check valve in the purge line prevented any back flow. A pressure transducer outside the bottom of the column was connected to a recorder which provided the pressure history of the process. In addition to the transducer, a pressure gauge connected at the top of the column provided easy reading on pressure. A layer of insulation covering the column helped make the temperature gradient in the radial direction as small as possible and, in the meantime, simulate the adiabatic operation which is nearly the case of commercial PSA processes where large-diameter beds are used. The axial temperature distribution was measured and recorded at three locations in the bed (nominally top, middle, and bottom). This was done by three fine thermocouples sheathed in a l/s-in. thin stainless steel protection tube which was inserted in the center of the packed column.
1204
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table 111. Characteristics of Adsorption Column col ht, L = 64 cm intrapart void fract, 0.43. col Inside radius, r = 2.05 interpart void fract, 0.61. cm tot void fract, e = 0.78 activat C, w = 412 g heat capac of C, C,, = 0.25 cal/(g "C) partic size, d, = 0.055 cm bulk dens, p = 0.498 g/cm3
The apparatus shown in Figure 2 was slightly modified to accommodate the vacuum desorption steps (steps IIIb and IV). The mechanical pump was connected to both ends of the column, and a flow meter was placed in the exhaust line from the pump. Gas samples were taken from two sampling ports which were fitted with septa, by syringes. The syringes were equipped with locks so samples could be collected at short time intervals (ca 30-60 s) and stored for later GC analysis. The characteristics of the adsorber column are given in Table 111, which were used in model computations. Experimental Procedure. The bed was cleaned before each run by degassing with a mechanical pump. Step I, pressurization, was initiated by opening the solenoid valve connected to H2. The desired column pressure was controlled by the pressure regulator connected to the Hz cylinder. Step 11, high-pressure adsorption, started when the bottom solenoid valve was opened by the timer. The flow rate in step I1 was controlled by adjusting a needle valve in the line. Step 111, cocurrent depressurization, was effected by closing the feed valve. Step IV,countercurrent blowdown, was achieved by simultaneously closing the bottom valve and opening the valve in the top exhaust line. The experiments were designed to obtain an understanding of the effects of various operating parameters on separation: feed pressure, feed rate, time of cocurrent depressurization, etc. The productivity of a typical run (run 13) was 360 L STP feed/h/kg carbon, which was in the range of commercial PSA processes. Because of the relatively long cycle time, steady state was reached, both experimentallyand by model simulation, rather rapidly. Generally five cycles were required to reach steady state after start up of the process with a clean bed. Results and Discussion In all commercial PSA processes, only the raffinate, i.e., the weakly adsorbed component which escapes the bed in the adsorption step, is the desired product. The strongly adsorbed components are either unwanted, e.g., in air drying and air separation, or the purity of which is unimportant, e.g., in hydrogen purification. In this study, all components were wanted products. The major problem was, then, to recover high-purity products of the strongly adsorbed components because it was relatively easy to obtain a high-purity raffinate. The experimental goal of this study was to separate a 49.5/49.5/1.0 mixture (mole) of H2/CH4/H2Sinto three useful products: a clean high-purity Hz, a clean high-Btu gas (over 90% CH,), and an H2S product useful as a feed for Claus, Stretford, or other commercial processes for sulfur recovery. A total of 16 runs was made. The PSA cycle sequence and the time allotments depicted in Figure 1were determined as the best possible from the results of the first nine runs. The findings of these nine runs are summarized qualitatively as follows. Vacuum desorption of HzS in step IV was found necessary, as H2 purge could not effectively desorb the strongly adsorbed HzS. Repressurizationof the bed, in step I, by H2 instead of the feed mixture was found important to the product purity of H2. It was further found that still better results for H2 product purity were
achieved when H2 entered the bed in the direction countercurrent to the feed (in step 11), apparently by keeping the portion of the bed opposite to the feed end (in step 11) clean. The purity of the intermediate product (CH,), obtained in step IIIb, was found strongly dependent on the length of time of step IIIb. Slow depressurization was desirable because it sharpened the wave fronts of both CHI and H2S in the bed, which would give a high CH, product purity. It was also found that a slight evacuation at the end of the cocurrent blowdown step (IIIb) would improve both CHI and H2S products. The reason for this was that a large amount of CHI remained in the bed as the adsorbed phase as well as in the voids of the bed (total void 78%), which amounted to approximately 10 L STP at 1-atm end pressure, containing mainly CHI. Evacuation from 1 to 0.5 atm removed a large amount of CH4and improved both CH, and H2S products. Data Treatment and Presentation. The PSA data included pressure history, temperature history and profile, instantaneous flow rate, and composition of the effluent. In a steady-state PSA cycle, the total amounts of the three separated producb were the integrated results of flow rate over time. The amount of feed (in step 11) per cycle was evaluated by CHI balance, based on the amounts and concentrations of effluents from steps 11-IV, which were measured quantities. The amount of Hz used in step I (pressurization) was calculated as the difference between the H2 contained in the effluents in steps 11-IV and that in the feed (in step 11), a check between those in the feed and in the effluents, which were within 6% for all cases. The product purities were evaluated as the volume averaged concentrations. The product recoveries were calculated by the following definitions: H2 recovery = H2 from steps I1 and IIIa - H2 used in step I H2 in the feed in step I1 CH4 recovery =
CHI from step IIIb CHI in the feed in step I1
H2S recovery =
HzS from step IV H2S in the feed in step I1
The amount of a given component in the feed in step 11, i.e., the denominator in the above expressions, was calculated as the total amount of the component contained in the effluents in steps 11-IV, at a steady state. For H2, the amount used in step I was subtracted from the total in the effluents to give the amount in the feed. Presentation and Analysis of Data for Standard Run. The detailed experimental results for run 13 at steady state are presented in Table IV and Figure 3, along with model simulations by EQ and LDF models. Because of the sharp wave front of CH,, as seen in Figure 3 and the concentration-time data in Table IV a clean separation of Hz and CH, was obtained. The H2 product concentration was well above 99% in step 11. The CH, product concentration was 95.19% on a volume-averaged basis (from step IIIb). The EQ model yielded the sharpest CH, concentration wave front because pore diffusion was assumed instantaneous. Due to the high selectivity of adsorption of H2S over CHI and H2, both H, and CHI products were very clean indeed. The H2S contents in these products were below the detection limit of the gas chromatograph, which was in the 0.01-0.0001 % range. The HzS product, from step IV, was about 10%. The recoveries for all products were high compared to commercial PSA separation processes.
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
0
30
15
45
1205
so
Distance from Feed End * c m
TI%lE.MI\
Figure 5. Gas-phase concentration profiles in bed in a steady-state PSA cycle for run 13 as predicted by equilibrium model. Time distribution for steps I, 0.5,11, 7.5, IIIa, 2, IIIb, 9.5, and IV, 3.0 min. Solid: CHI. Dashed: H,S.
Figure 3. Effluent concentrations in a steady-state PSA cycle for separating a 49.5/49.5/1.0H2/CH4/H2S mixture with activated H2 ( O ) , H2S (A),for the carbon (run 13). The symbols are CHI (01, experimental data, and EQ model (solid line), LDF model (dashed line).
”
Distance from Feed E n d . cm
Figure 6. Concentration profiles in adsorbed phase in bed in a steady-state PSA cycle for run 13 as predicted by equilibrium model. The time distribution is given in Figure 5. Solid: CH,. Dashed: HZS. Hz
A
Time. min
Figure 4. Steady-state PSA temperature histograms (for run 13) at 12.7 cm (A), 33.0 cm (B), and 53.3 cm (C) from the top of the bed (60 cm height). Solid: experimental. Dashed: equilibrium model.
The experimental temperature history in a steady-state cycle is shown in Figure 4, at three locations in the bed. The model simulation by the EQ model is compared in the same figure. A peak in temperature was associated with the location of the wave front, in this case the wave front of CHI since H2S was in very small concentration. The temperature rise in the lower bed during step IIIb indicated the readsorption of CH4 (and H2S) which was desorbed from the upper bed. The desorption was accompanied by a sharp temperature drop. These dynamic behaviors in the bed were clearly indicated by the temperature history data. All these features were reasonably predicted by the model. The major disagreement was in the temperature history, at the top of the bed, during step 111. There was no explanation for the diagreement except that the top of the bed was not covered by insulation. Nonetheless, the cold top portion of the bed predicted by the model was responsible for the low H2S concentration in the effluent drawn from the top (Table IV and Figure 3). The increase of H2S concentration with time, as opposite to the experimental trend, was possibly also due to the underpredicted temperature. The compositions of the gas phase and the adsorbed phase in a steady-state cycle in run 13 are given in Figures 5 and 6. They are the values calculated by the EQ model,
k, ,
CHI
,
, 0 5
,
,
,/ H2S
Figure 7. Total bed loading analysis: composition of the total amount including the adsorbed and the gas phases. (A) Composition at the end of step I1 if CH, wave front has reached breakthrough. (B) Composition at the end of step IIIa. (D) Composition at the end of step IIIb at an end pressure of 1 atm. (E) composition of D at 0.5 atm end pressure.
given as functions of location in bed and time in the cycle. The functions performed by each step during the PSA cycle can be understood from these figures. That the voids in the bed (total void fraction = 0.78) play an important role in determining product purities and recoveries has been illustrated by a total bed-loading analysis (Yang and Doong, 1985; Cen and Yang, 1985). Through this analysis, it was shown that the total loading ratio (strongly adsorbed/weakly adsorbed) increases sharply but (1)lowering the total pressure and (2) increasing the gas-phase concentration ratio of the same pair. The total bed-loading analysis for the three-component mixture in a PSA cycle
1206
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table IV. Steady-State Results of Run 13 for PSA Separation of 21.4 atm, Cycle Time = 22.5 min) experimental flow rate, flow rate, step time, s L/min YHZ,% Y C H ~ ,70 Y H ~ S ,70 L/min I1 60 5.0 99.99 0.01 0.00 5.30I 5.0 99.99 0.01 0.00 5.31 180 0.00 5.32 99.98 0.02 5.0 300 6.58 5.33 99.97 0.03 0.00 405 99.61 0.39 0.00 5.95 6.58 I11 480 3.84 3.19 43.47 56.53 0.00 540 0.00 3.16 2.76 10.94 89.06 600 0.00 3.09 3.78 4.95 95.05 660 3.40 0.00 3.09 4.54 95.46 720 4.02 95.98 0.00 3.06 3.08 780 0.00 2.15 2.64 3.35 96.65 840 0.00 1.88 3.36 96.64 2.20 900 1.73 3.37 96.63 0.00 1.63 960 0.00 0.46 0.46 1050 3.16 96.84 3.10 96.82 0.08 6.25 6.70 1125 12.01 1.82 3.09 84.91 3.20 IV 1155 11.13 1.98 2.34 3.13 85.74 1185 2.14 10.21 1215 3.86 85.93 1.86 10.14 2.41 1.60 1245 3.87 85.99 9.45 2.57 1.35 3.97 86.58 1290 recov, '70 94.39 74.74 99.44 -~
30
15
I
20
{lo
30
~
.15
E 2o ?
L
10 O
; 15
30 20 10
0
4
8
12
155
1 9 5 2 2 5
Time.min
Figure 8. Pressure swing histories of runs 10-16.
is given in Figure 7 . The analysis shows the importance of the cocurrent blowdown step (step 111) in the product purities of CH, and HzS. Without step 111, the maximum product purity for CHI, by purge or vacuum desorption, is 75.58% (point B in Figure 7 ) , when the bed is fully utilized in the adsorption step when CH, breaks through. With step IIIa, the bed composition is shifted to point C, where the CH, concentration in the bed is raised to 93.29%. The importance of step IIIb and of the end pressure of this step, in H,S product purity, is also shown in Figure 7 . The maximum possible product purities for HzS as well as CH, are substantially increased by this step. The sharpness of the CH, wave front, shown in Figure 5, made the high H2 purity possible. The wave front of H2S was, however, not sharp, due to the low adsorption selectivity of HzS/CH4 on activated carbon. Effects of Feed Rate. The effects of feed rate at a fixed time cycle in PSA should be more appropriately addressed as the effects of the fraction of bed covered by the CH, wave front in the adsorption step. The fraction of the bed covered by CHI will be referred to as bed utilization. Table V shows the steady-state results of three runs, at identical conditions except feed rate. The pressure histories of these runs are shown in Figure 8. The bed utilizations for the three runs were 68%, 72%, and 84%, respectively,for runs
a 49.5/49.5/1.0 H2/CH4/H2S Mixture (Adsorption Pressure =
LDF model flow rate, Y H ~ ,% Y C H % ~ , Y H ~ S ,70 L/min Y H ~70 , Y C H ~ ,% 0.06 0.00 5.30 99.92 0.08 99.94 0.06 0.00 5.30 99.92 0.08 99.94 0.00 5.31 0.06 99.92 99.94 0.08 0.06 99.92 99.94 0.00 5.31 0.08 0.06 99.92 0.00 5.93 99.94 0.08 9.73 88.04 11.96 90.27 0.00 3.16 0.00 2.78 16.54 83.46 13.20 86.70 4.35 0.00 3.80 4.67 95.65 95.33 1.98 2.15 97.85 98.02 0.00 3.44 1.18 1.24 98.76 0.00 3.10 98.82 1.17 1.23 98.77 0.00 2.17 98.81 0.00 1.87 0.90 0.92 99.08 99.10 0.70 0.00 1.61 0.72 99.28 99.30 0.55 0.00 0.46 0.56 99.44 99.45 0.51 0.00 6.23 0.53 99.47 99.49 0.48 6.89 1.81 0.49 92.50 92.63 0.43 0.45 91.92 92.05 7.52 1.96 9.05 2.13 0.39 0.40 90.55 90.66 0.36 0.36 89.72 89.82 9.91 2.38 0.33 0.34 88.61 88.43 11.23 2.53 EQ model
97.77 75.63 100.00
YH~S. %
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.01 7.63 9.05 9.92 11.05
97.24 74.98 100.00
Table V. Steady-State Results of PSA Separation for a Hz/CH4/Hz5 Mixture at Different Feed Rates step exptl EQ exptl EQ exptl EQ I, input Hz, L STP 17.34 17.56 17.75 17.63 19.98 17.75 11, output, L 31.07 37.23 37.20 39.97 41.16 44.59 yH1, 70 99.99 99.96 99.95 99.94 99.81 99.96 0.05 0.06 0.19 0.04 YCH49 % 0.01 0.04 YH2S* % 0.00 0.00 0.02 0.00 0.00 0.00 IIIa, output, L 12.40 8.94 10.42 9.63 9.58 9.43 yH2, % 99.99 99.90 78.92 91.44 71.90 78.99 YCH4f ?& 0.01 0.10 21.08 8.56 28.10 21.01 0.00 0.00 0.00 0.00 0.00 0.00 $t'o%put, L 21.79 20.45 23.27 21.04 27.13 23.40 YHzs % 3.94 4.85 4.80 3.25 9.13 4.85 YCH4, 96.06 95.15 95.19 96.75 90.86 95.15 yH2S, %
