Znd. Eng. Chem. Res. 1995,34, 591-598
591
Comparison of Activated Carbon and Zeolite 13X for COa Recovery from Flue Gas by Pressure Swing Adsorption K. T. Chue, J. N. Kim, Y. J. Yoo, and S. H. Cho* Korea Institute of Energy Research, P.O. Box 5, Taedok Science Town, Taejon, Korea
R. T . Yang Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
For the recovery of high purity C02, two adsorbents, zeolite 13X and activated carbon, are examined. Pressure swing adsorption (PSA) cycle simulations are performed to compare the performance of different adsorbents in separation from two feed mixtures: 16/84(v/v) COdN2 and 26/74C02/"2. Despite high-temperature excursions in the adsorption step, zeolite 13X is a better adsorbent than activated carbon in nonisothermal, adiabatic PSA process due to favorable isotherm shape and equilibrium selectivity. Moreover, a higher temperature excursion for 13X during the product ( C o d purge step reduces the purge gas amount and this actually helps PSA performance. High purity of C02 (over 99%) can be produced by zeolite 13X at higher recoveries (53% from low-CO2-concentration flue gas and 70% from high-COz-concentration flue gas) and higher productivities than carbon. In selecting an appropriate adsorbent, high working adsorption capacity, high equilibrium selectivity, and low purge gas requirement dominate the separation by PSA process whereas the heat effects play a secondary role. A simple criterion is proposed for a priori adsorbent selection.
Introduction The recovery of C02 from flue gases emitted by power plants, steel mills, cement kilns, and fermentation processes is becoming increasingly viable. C02 recovery has been achieved by gas absorption employing solutions of carbonates and alkanolamines. However, this process is energy-intensive for regeneration of solvent and is also plagued by corrosion problems. Recently, the PSA (pressure swing adsorption) process treating high-COz-concentration flue gases (25% C02) has become an alternative to the conventional absorption process. For the recovery of high purity C02 from lowconcentration flue gases (e.g., 15% COS),further work such as development of better adsorbents and more efficient PSA cycle is needed to improve the PSA performance (Kawai, 1991). Prior t o the design of a PSA process, the choice of a preferred adsorbent is most important. The fundamental properties of an adsorbent such as selectivity, effective adsorption amount (or working capacity), mass transfer kinetics, and heat of adsorption should be included for consideration. The effective purge amount to be defined in the next section should be also considered when a PSA process involves a product purge step (Cen and Yang, 1986;Baksh et al., 1990;Sircar, 1988). In C02 bulk separation from low- and high-CO2concentration flue gases, AC (activated carbon), CMS (carbon molecular sieve), and synthetic zeolites are candidate adsorbents. Recently, a PSA cycle for C02 separation from a low-CO2-concentration flue gas containing 17% CO2 has been studied using activated carbon and carbon molecular sieve as the adsorbents (Kikkinides et al., 1993). It was pointed out that the equilibrium selectivity for C02 in activated carbon dominates the PSA separation, and the kinetic selectivity in favor of COz in carbon molecular sieve is less important. However, the wide-pore carbon molecular sieve (CMS-W) was successfully tested in pilot scale ~~~
* Author to whom correspondence should be addressed. 0888-588519512634-0591$09.00/0
plant for recovery of C02 from flue gases containing 11% COS;over 98.9% COS could be produced at over 53%72% recovery, with energy consumption of 0.8-3.0kWh k g COz (Pilarczyk and Schroter, 1990). In general, zeolites have higher adsorption capacities for C02 as well as higher equilibrium selectivities for C02 over N2 than activated carbon. On the other hand, the heat of adsorption of C02 on activated carbon is lower than on zeolite, so that the use of activated carbon in a PSA process may result in less severe heat effect on the PSA performance. The temperature excursion due to heats of adsorption and desorption is detrimental to the separation performance of a PSA process (Cen and Yang, 1986;Lu et al., 1993). In present work two adsorbents, AC and zeolite 13X, are examined for the recovery of high purity C02 by the PSA process from two flue gases (16% and 26% C02, the balance being N2). The equilibrium isotherms of pure CO2 and N2 on AC and zeolite 13X were measured by using a volumetric apparatus. The mathematical model of a nonisothermal adiabatic PSA process is solved to compare the PSA performance. Effects of feed rate and purge-to-feed ratio in the PSA cyclic operation were analyzed from the simulation results and compared for the two adsorbents to select the more promising one to be used for C02 PSA.
Adsorption Equilibria, Heats of Adsorption, and a Simple Criterion for Sorbent Selection The adsorption isotherms of pure C02 and N2 on zeolite 13X were measured at different temperatures (15-70 'C) by using a volumetric apparatus. For the C O D 2 mixture on zeolite 13X,the binary adsorption equilibrium was also measured a t 15 "C and at a total pressure of 830 mmHg. Experimental details can be found in previous work (Kim et al., 1993). The thermogravimetric technique, employing a microbalance (Cahn system 113), was used to measure equilibrium isotherms for BPL activated carbon, supplied by Calgon (Kikkinides et al., 1993). Equilibrium data of pure C02 0 1995 American Chemical Society
592 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 Table 1. Parameters of the Langmuir Adsorption Isotherms of Pure Adsorbatesa Ao
adsorbent adsorbate (mol kg-l) AI (K-l) zeolite 13X zeolite 13X AC AC
C02 N2
COz Nz
2.383 41 0.06355 0.023 81 0.001 25
Bo (Pa-')
-0.028 16 -0.029 34 -0.020 07 -0.020 86
0.12266 6.313 x 5.937 x 1.271 x
A = A0 exp(A1T);B = Bo exp(B1Tj;q = AP/(l (mol kg-'). 3.5
-
,
1
B I W')
-0.023 52 -0.014 19 -0.003 78 -0.014 52
+ BPI; P (Pa); q
7
7 -
I
1
--*--AC[16%]
2.5
--t 13X(16%]
z Q
s
- -0-
2
&
~
AC[26%)
+ 13X[26%]
t
1 0
100
200
1
,
300
400
I
500
600
700
800
PARTIAL PRESSURE (mmHg) Figure 2. Equilibrium isotherms at 15 "C on zeolite 13X (-, pure Langmuir isotherm; - - -, binary equilibrium by IAS theory).
0
15
20
25
30
35
40
45
50
55
TEMPERATURE ("C)
Figure 1. Purge-to-adsorption differential ratio of pure COZon two adsorbents.
and N2 were fitted with the Langmuir isotherm, q = AP/(l+ BPI; the parameters A and B are well correlated by an exponential function of temperature (Table 1). From the equilibrium data, a simple criterion for adsorbent selection is proposed based on the sorbent working capacities. The sorbent working capacity in a PSA cycle is the effective adsorption amount Aq,, i.e., the difference in the adsorption amounts between the high pressure adsorption step and the low pressure evacuation step. However, the working capacity alone cannot be used as the criterion for sorbent selection when a PSA process involves a strong adsorptive purge step for recovering the strong adsorptive component as the desired product. Here the product recovery to obtain a desired product purity depends on the effective purge amount (Aq,), i.e., the difference between the amounts of CO2 adsorbed at the end of the adsorption step and that at the end of the purge step. A higher Aq, will decrease the product recovery. Thus, the ratio AqdAq, may be used as a simple criterion to evaluate the sorbent; a lower ratio favors the PSA separation. A comparison of two adsorbents is made from the adsorption isotherms of the key component COZ. Figure 1shows the purge-to-adsorption differential ratio (AqJ Aqa) of adsorbate C02 a t different temperatures and for two feed compositions (16% and 26% COa). For the particular PSA cycle conditions being considered, the pressures for adsorption, purge and evacuation are, respectively, 830, 760, and 50 mmHg. Thus, Aqa = q(830 x mol % CO2 in feed) - q(50) and Aq, = q(830 x mol % CO2 in feed) - q(760),with all pressure in mmHg. For both adsorbents, the ratio increases with increasing
temperature. AC has higher ratios than zeolite 13X, so when the activated carbon is used as the adsorbent, a larger amount of the C02 product gas has to be used in the purge step. From this criterion, zeolite 13X is the better sorbent. This criterion is used without consideration of heat effects. A second comparison is made based on the binary adsorption isotherms from which the equilibrium selectivity of C 0 2 over N2 form the mixture is calculated. The US (ideal adsorbed solution) theory is used to predict the binary adsorption equilibrium for COD2 on zeolite 13X (Kim et al., 1993). For activated carbon the extended Langmuir isotherm, qr = ALpL/(l ZJBjpj) is employed. As shown in Figure 2, a significant difference between the pure and binary adsorption amounts has been observed for the less adsorptive N2 on zeolite 13X. N2 is hardly adsorbed when its mole fraction is less than 0.8 in C O D 2 mixtures. Conversely, N2 in the C O D 2 mixture does not affect significantly the equilibrium amount of the strong component C02. However, the adsorption of N2 on activated carbon affects more significantly the binary equilibrium amount of C02 (Figure 3) than in the case of zeolite 13X. These binary adsorption behaviors can be represented by the equilibrium selectivity for the two adsorbates. Figure 4 exhibits the variation of the equilibrium selectivity for C02 over N2 at 15 "C and at 830 mmHg total pressure as a function of C 0 2 mole fraction up to 0.25. The equilibrium selectivity on zeolite 13X is much higher than that on AC. For the two adsorbents, it increases with the C02 mole fraction in the gas mixture. The third factor for consideration is concerned with the heat effects. This comparison is based on the isosteric heats of adsorption which are calculated from the pure gas isotherms according to the ClausiusClapeyron equation. The average values of heat of adsorption over the adsorbed concentration range are 36 and 30 kJ/mol for, respectively, CO2 on zeolite 13X and on AC, and 25 and 16 kJ/mol for, respectively, N2 on zeolite 13X and on AC. For the present operating condition where the adsorption occurs at near atmospheric pressure and the
+
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 593
,
Since the heat effects due to temperature rise in the 1 I adsorption step and temperature decrease in the evacu-
2.5
h
I
- 7 - r
ation step are detrimental to the C02 bulk separation in a PSA cycle, it is necessary to compare the two adsorbents in a nonisothermal adiabatic PSA cycle by model simulations.
2 -
P 8
v
B
COz PSA Process Description -
1.5
N2
0 0
100
200
300
400
500
600
700
800
PARTIAL PRESSURE ("Hg)
Figure 3. Equilibrium isotherm at 15 "C on activated carbon (-, pure Langmuir isotherm; - - -, binary equilibrium by extended Langmuir isotherm). 30
,
1 2000
n
/I
I
The PSA cycle considered in the present study consists of three-bed, seven-step cycles. This process is based on the strong adsorptive purge type for recovery of the strong adsorptive, but with further refinements. Figure 5 represents the schematic diagram of one cycle with time for each step. The process operation can start with a cocurrent pressurization step (I)with a fresh feed mixture from low pressure PL t o high pressure PH. Then the fresh feed mixture is passed through the bed at high pressure PH (step 11). During cocurrent depressurization step (III), the less adsorptive N2 in voids and on the adsorbent is withdrawn by pressure reduction to near atmospheric level (PM). Step I11 is then followed by the recovery step (IV)where the feed comes from another bed undergoing purge step (vr)with high purity C02. Step IV is optional depending on the concentration wave front at the end of the product purge step. Nz remaining in the bed can be replaced by flowing through the bed at the end of step IV with a portion of product C02. Product purge can be divided into two consecutive steps: one is continued until the COz concentration wave front breaks through the exit bed end (step V); the other is continued until the full saturation of the bed with product CO2 (step VI). The effluent containing CO2 composition higher than that of the fresh feed mixture is sent to the bed undergoing step W . The bed at the end of step VI is then evacuated in the same direction as the feed flow to the lowest pressure PL to produce the high purity C02 product (step VII). The purge gas is compressed from PLto PM prior to feeding t o steps V and VI.
Mathematical Model
0
0.05
0.1 0.15 0.2 C02 MOLE FRACTION (-)
0.25
Figure 4. Equilibrium selectivity vs mole fraction of COz at 15 "C and total pressure of 830 mmHg.
desorption at subatmospheric pressure, the adsorbent with the lowest AqdAqa ratio, highest equilibrium selectivity, and lowest heats of adsorption should be the best adsorbent for PSA separation. As a result of the above analysis, it can be noted that zeolite 13X is more promising judging from the adsorption capacity and equilibrium selectivity; however, it is inferior with respect t o heats of adsorption. But under some particular operating conditions (Le., adsorption at elevated pressures and desorption at near atmospheric pressure), activated carbon having a lower selectivity and a lower initial capacity can be a better adsorbent for PSA application than zeolite. Depending on the isotherm shape, activated carbon can release much more CO2 than the zeolite system during the pressure reduction (blowdown) step (Kumar, 1989).
A dynamic model for a nonisothermal, adiabatic PSA is employed for this study. The model equations have been given elsewhere. The cell model (30 cells for each bed) was used for the mass and heat balances on the adsorption bed. Axial dispersion, if approximated by using the cell model, and 30 cells seemed to give a reasonable approximation (Cen and Yang, 1986). Several basic simplificationswere made: ideal gas behavior, constant heat capacity of gas and solid phases, and negligible pressure drop. The effect of the column thickness is not considered for adiabatic system. The mass balance equation for component i is
and assuming the thermal equilibrium between the gas and solid phases the heat balance equation is
594 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
Product Purge
Fresh Feed Recovery
[ I I I
Iv
I
T
Product Waste
4
Figure 5. Schematic representation of the basic COz PSA cycle with step times.
for beds 1, 2, and 3, respectively. As the initial conditions, all beds are assumed to be uniformly saturated at the ambient temperature TOwith a fixed fresh feed mixture. Thereafter the final conditions of the previous step become the initial conditions for the next step in PSA cycle. We have the following boundary (feed) conditions on the gas phase for each step. where the coefficient ilis a heat capacity ratio between the solid and gas phases,
A=1+-
yII,i= yF,i; TI, = 0
adsorption (11):
@bCps EbCkCpg
yI,i= Y ~ , TI ~ ;= To; uI,s= 0
pressurization (I):
uIII = 0
depressurization (111):
Assuming the intraparticle diffusion represented by the linear driving force approximation, the mass transfer rate for zeolite 13X is
recovery (W): yw.i = Y ~ . ~ Tw . ~ =; Tvr,s; uw = ~v.s(Pvr/Pw)(Tw/TvI)
(3)
purge (VI:
YV.i
= yvI1.s.i; Tv = To -
Since diffusion of C02 and Nz in activated carbon is instantaneous (Kikkinides et al., 1993), an equilibrium model is used: dqi.k - dqi.k* dt dt
(4)
From the overall mass balance the interstitial velocity within the bed is
purge
(VI):
Y U .= ~~ v I I . s . i ;
evacuation (VII):
UvII
Tm = To
=0
Variation of the bed pressure with time during steps
I, 111, and VI1 is represented by P(t) = CO+ Cle-czt with CO= apt,, C1 = PO- apt,, and C2 = -ln[(l - aY(y a)Yt,, where POand Pt, are the initial and final pressures, y is the ratio of POto PtB,and the values of
coefficient a are 1.1for pressure increasing step and 0.9 for pressure reduction step. The inlet and outlet flow rates are calculated by eq 5 for the pressure reduction and increase steps. The conditions of the effluent of step VI (4yvr.s.iand T v r . ~become ) those of the feed for step IV. The feed composition of the purge step is equal to the average value of the product gas during the evacuation step VI1 of the previous cycle bv11.s.i). In all steps, For the first step, the PSA operation starts with the adsorbed concentration 9 i . k is calculated by the IAS pressurization (I),product purge (VI), and recovery (N) model and the extended Langmuir isotherm for zeolite
[nd. Eng. Chem. Res., Vol. 34, No. 2, 1995 696 13X and activated carbon, respectively. Ordinary differential equations, eqs 1-5, for three adsorption beds are integrated, and for each calculation of PSA operation 50-80 cycles are needed t o reach steady state. Model computation requires about 5 min per cycle on a PC 486DX (66MHz) within a tolerance of PSA performance at steady state can be described in terms of product recovery and productivity at a desired product purity. We have the following definitions.
0.15 0.12
s
0.09
* 0.06 0.03
0
CO, recovery: CO, RC = CO, evacuated in step VI1 - CO, used in steps V and VI CO, fed in steps I and I1
0.2 ds
CO, productivity: CO, PN = CO, evacuated in step VI1 - CO, used in steps V and VI mass of adsorbent x cycle time
As the operation variables of PSA process, the following are defined. feed rate: of feed mixture in steps I and I1 E = amount mass of adsorbent x cycle time
CO, purge/feed (P/F) ratio: CO, P/F ratio = amount of CO, used in steps V and VI amount of CO, in feed in steps I and I1 The total amount of the feed and of the effluent during the pressure variation steps is obtained from the difference in the gas amounts between the initial and final states. Note that the P/" ratio can be greater than 1 because it is defined more like a recycle ratio.
Parametric Studies As a fair basis for comparison of the two adsorbents in PSA process, the bed diameter is 2.54 cm for zeolite 13X PSA and 5.3 cm for AC PSA (with the same height of 1.0 m), which is determined by taking into account the adsorption capacity ratio at 30 "C or [(Aq@b)13~/ ( A ~ Q ~ ) A Cwhere ] ~ ~ ,Aq is equal to the effective adsorption amount. The effects of the feed gas amount during pressurization and adsorption steps as well as the COZ purge gas amount on the productivity and the recovery are studied. The adsorber properties and operating conditions are given in Table 2. The feed conditions, as mentioned, are 16% and 26% CO, in Nz. Oxygen is not considered, although flue gases contain a little 0 2 , for the following reasons. For activated carbon, the adsorbed amounts for 0, and NZ are approximately equal, whereas for zeolite 13X, 0 2 is approximately 1/3 of Nz. Thus, counting 0 2 as NZwould only favor activated carbon, and would not affect the conclusion of this study. Moreover, water vapor in the flue gas is not considered because it must be removed prior to entering the PSA system, particularly for the zeolite 13X. Removal of water vapor can be accom-
bed diameter, D,(cm) bed height, H (m) adsorbent amount, W (kg) bed density, @b (kg m-3) bed porosity, Cb heat capacity, Cps.(J kg-l K-l) specific heat of mixture, Cpg (J mol-' K-I) ambient temperature, TO("0 pressure, PJPMIPH(mmHg)
zeolite 13X 2.54 1.0 0.38 750.0 0.348 920.0 30.7
activated carbon 5.3 1.0 1.06 480.5 0.399 1050.0 30.7
30.0 50l760l830
30.0 5017601830
plished by, for example, presorbers or guard beds (in PSA) containing activated alumina or silica gel desiccants. The mass transfer coefficient ki (=60D&,2) for zeolite was obtained by fitting the breakthrough data (Kim et al., 1993). The breakthrough curves were measured with a nonadiabatic adsorption column of 2.54 cm in diameter and 100.0 cm in height. After fitting the maximum temperature to obtain the heat transfer coeficient through the column wall, the ki value was then determined. The feed contained 14.9%(vol.) COZ in N2, at flow rates of 0.2 m/s (P = 920 mmHg) and 0.132 m/s (P= 820 mmHg), fed to the feed equilibrated with Nz at the same total pressures. The concentration and temperature breakthrough curves are shown in Figure 6, together with the best fit of ki = 0.33 (Kim et al., 1993). Effect of Feed Rate. The first comparison of two adsorbents is to observe the effects of the feed rate on product purity and productivity. A series of simulations is performed by varying the feed rate in steps I and 11. The feed gas amount of the pressurization step I is calculated by eq 5 introducing the pressure change with time. Product purity and recovery curves are plotted as functions of feed rate per unit mass of adsorbent. The ratio of purge amount to total evacuated gas amount is held a t a constant recycle ratio of 0.75 for both sorbents; 75% of the total product gas during evacuation step VI1 is used for the purge steps (V and VI). Figure 7 plots the product COS purity and the
596 Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995
>/
100 r--7 1 L
1
t
1
2 '*-
3.5
I
-
13X __-
Q -b -
-
AC
50
t 1 1 1 1 0.5 70 ~
5
10
15
20
25
30 35 40 FEED RATE, E (Nl/kg/min)
45
5
50
Figure 7. Effect of feed rate for low-CO2-concentration flue gas (16%COz): circles = zeolite 13X and triangles = activated carbon.
productivity as a function of feed rate for two adsorbents and for feed gases (16% CO2). The increase in feed rate at the same adsorption time results in a longer bed coverage with C02. Thus the C02 productivity and purity are increased with feed rate. Further increases in feed rate over a critical feed amount do not affect the productivity, where the adsorption bed is almost saturated with the feed conditions. During the cyclic operation the initial bed condition is not clean with respect to C02; the evacuation occurs at a fixed evacuation pressure of 50 mmHg. Thus the temperature rise between the end of evacuation step VI1 and adsorption step I1 at steady state is much lower than that for a clean bed (Figure 64. At steady state even though the temperature excursions are 16.8and 9.8 "C for zeolite 13X and activated carbon, respectively, the corresponding differential adsorption amounts of C02 are 0.423and 0.27 mol kg-l. This is due to the more favorable adsorption isotherm on zeolite 13X. According to the model simulation results, a more favorable PSA performance and higher purity products at high productivities are obtained with zeolite 13X for a wide range of feed rates, as compared to AC. The simulations were also performed for the high-COz-concentrationflue gas (26% C02). As shown in Figure 8, less of the feed gas is needed, for both adsorbents, to obtain the high purity of product C02 than in the case of the low-CO2concentration flue gases. The reason for the better performance with the high-CO2 feed is clearly seen from Figure 1,where the AqdAq, is lower (favoring PSA) with a higher C02 feed for both adsorbents. Effect of COZ Purge Gas Quantity. The second variable used t o compare the performance of AC and zeolite 13X is the C02 purge gas amount. The product purge at the end of the adsorption step is essential for the PSA operation where the strongly adsorbed component is the desired product. The main function of this step is t o displace (with high purity C02) the weakly adsorbed component Na remaining in the void volume and on the solid phase in the bed. Comparison between AC and zeolite 13X for the low-CO2-concentration (16% C02) flue gas is presented in Figure 9. For the model simulation, the volumetric feed rates during the adsorp-
10
15
20
25
30
35
40
FEED RATE, E (Nykg/min) Figure 8. Effect of feed rate for high-COz-concentration flue gas (26% Cog): circles = zeolite 13X and triangles = activated carbon.
loo
95
-7
!---
1
1 ,
-
L I
6o
55
I
85
65
5
A-.+---
-
I
b
80
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
C02 P/F RATIO (-)
Figure 9. Effect of COz purge-to-feed ratio for low-CO2-concentration flue gas (16%C02): circles = zeolite 13X and triangles = activated carbon.
tion step I1 were 20 and 40 L(NTP)/min (with 0.38kg of 13X or 1.06kg of AC). With these flow rates the bed utilizations for two adsorbents are almost the same as the C02 concentration of the effluent is half that of the feed stream. Other conditions are held constant: identical step times and bed pressures. For both adsorbents, product purity increases with increasing C02 P/F ratio and product recovery decreases: in the case of zeolite 13X,C 0 2 recovery drops precipitously for PA? ratio over 0.8, while for activated carbon it continues to decrease as P/F is increased up to 1.3and the sudden drop will occurs at P/F > 1.3.The bed profiles of the adsorbed concentration and of the temperature at the end of C02 purge step VI (Figures 10 and 11)elucidate clearly the relationship between the P/F ratio and the PSA performance. At higher P/F ratios, more product C02 is used to purge so that the
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 597 4
z P
3*5
1.06
',,
0.8
= : 3
100
80
98
70
96
60
',,
8
h
Y cr 2.5
5 E
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
(-)
',
3
AC
Di
-,\ \
94
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
z (-1 Figure 10. Bed profiles at the end of product purge (VI) (at various P/F ratios as marked) for zeolite 13X. (a) C02 adsorbed concentration; (b) temperature.
1.4
9E" 1.2 v
g1 0.8 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.6
0.7
0.8
0.9
1
z (-) 50
-E +
45 40 35 30
2s
'
0
88
\
\
92
\
1
0.1
0.2
0.3
0.4
0.5
1
(-)
Figure 11. Bed profiles a t the end of product purge (VI) (at various P/F ratios as marked) for activated carbon. (a) C02 adsorbed concentration; (b) temperature.
concentration front of COZpenetrated deeply toward the exit end of the bed. In the case of zeolite 13X, the COS front of the adsorbed phase reaches almost the exit end of the bed at a P/F ratio of 0.8 (Figure loa), but for AC it breaks through the bed a t a higher P/F ratio of 1.26 (Figure lla). Further increases in P/F ratio do not improve significantly the product purity but decrease the product recovery. The largest temperature rise in a cycle operation occurs during the product purge step: 34 and 17 "C for zeolite 13X and AC, respectively (Figures 10b and llb). Unlike the adsorption step, this high-temperature excursion during the product purge step favors the PSA performance: the breakthrough time of the heavier component C02 becomes more
e4
30
20
0
0
40
\
'7
88
2 6
9
\
90
0
50 \
r
70
g
\
5
2
h
0.2
0.4
0.6 0.8 C02 P/F RATIO (-)
1
1.2
Figure 12. Effect of COz purge-to-feed ratio for high-CO2concentration flue gas (26% C02): circles = zeolite 13X and triangles = activated carbon.
precocious with increase in the bed temperature, and consequently it requires less purge gas. Figure 12 shows the simulation result for the highCOz-concentration flue gas: the volumetric feed rates during the adsorption step I1 were held at 15 and 30 L(NTP)/min for 0.38 kg of 13X and 1.06 kg of AC, respectively. Separation by zeolite 13X results in a high purity COZproduct (over 98.5%)from a P/F ratio of 0.2. However, AC allows obtaining the same product purity at CO2 P/F ratio of 0.8, which is much higher than that required for zeolite 13X. By comparing the performance of the two adsorbents, it is evident that zeolite is the more appropriate adsorbent than AC, because a higher purity of product COZcan be obtained at a lower P/F ratio with zeolite. For the case of 16% C02 flue gas, the recycle ratio of 0.5 is the optimal value for zeolite 13X which corresponds to a P/F ratio of 0.8. For high-COz-concentration flue gases, lower P/F ratios are required the optimal values for zeolite 13X and activated carbon are 0.4 and 0.9, respectively.
Conclusions In comparing the performance of COz PSA with two adsorbents, zeolite 13X gives more favorable results than activated carbon. Although the heat of adsorption of COZon zeolite 13X is higher than that of activated carbon, the former adsorbent has a higher working capacity, lower purge requirement, and higher equilibrium selectivity. Consequently it yields higher product purity and recovery from the PSA cycle. The largest temperature rises occur for zeolite 13X rather than for activated carbon. However, the temperature excursion in the adsorption step with zeolite does not offset its more favorable equilibrium properties. On the contrary the high-temperature excursion during the purge step reduces the purge gas amount. Zeolite 13X is a better adsorbent for bulk separation of COz from flue gas. Nomenclature A = product of saturation amount and Langmuir constant, mol kg-l Pa-l
598 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
B
= Langmuir constant, Pa-' C = gas concentration (P/RT),Pa m3 mol-l K-l C,, = specific heat of gas mixture, J mol-l K-l C,, = heat capacity of adsorbent, J kg-l K-l De = effective diffusivity, m2 D, = particle diameter, m H = bed height, m. ki = overall mass transfer coefficient of component i, s-l L(NTP) = liter at 20 "C and 1 atm N = total number of adsorbates in adsorption system P = total pressure of the bed, Pa, or purge qi = amount adsorbed of component i on the solid phase, mol kg-I qi* = amount adsorbed of component i in equilibrium with gas phase, mol kg-l T = temperature, K (or "C where noted) t = time, s t, = step time in a PSA cycle, s u = interstitial velocity, m s-l W = adsorbent amount, kg yi = mole fraction of component i in gas mixture AHi = heat of adsorption of component i , kJ mol-' Az = length of a unit cell, m
Greek Letters Eb = bed porosity y = ratio of initial to final pressure in a pressure-changing step 1 = heat capacity ratio of solid to gas phase @b = bed density, kg m-3
Superscripts
* = equilibrium - _- average
Literature Cited Baksh, M. S. A.; Kapoor, A.; Yang, R. T. A New Composite Sorbent for Methane-Nitrogen Separation by Adsorption. Sep. Sci. Technol. 1990,25 (7&8),845. Cen, P. L.; Yang, R. T. Bulk Gas Separation by Pressure Swing Adsorption. Ind. Eng. Chem. Fundam. 1986,25, 758. Kawai, T. Separation Technology of Carbon Dioxide; NTS Press: Tokyo, 1991. Kikkinides, E. S.; Yang, R. T.; Cho, S. H. Concentration and Recovery of COz from Flue Gas by Pressure Swing Adsorption. Ind. Eng. Chem. Res. 1993,32 (111, 2714. Kim, J. N.; Chue, K. T.; Kim, K. I.; Cho, S. H.; Kim, J. D. NonIsothermal Adsorption of Nitrogen-Carbon Dioxide Mixture in a Fixed Bed of Zeolite-X. J. Chem. Eng. Jpn. 1993,27 (l),44. Lu, Z. P.; Loureiro, J. M.; Rodrigues, A. E. Simulation of a ThreeStep One-Column Pressure Swing Adsorption Process. AlChE J . 1993, 39 (91, 1483. Kumar, R. Adsorption Column Blowdown: Adiabatic Equilibrium Model for Bulk Binary Gas Mixtures. Ind. Eng. Chem. Res. 1989,28, 1677. Pilarczyk, E.; Schroter, H. J. New PSA-Processes with Carbon Molecular Sieves for Recovery of Carbon Dioxide and Methane. In Gas Separation Technology;Vansant, E. F., Dewolfs, R., Eds.; Elsevier: Amsterdam, 1990; p 271. Sircar, S. Separation of Methane and Carbon Dioxide Gas Mixtures by Pressure Swing Adsorption. Sep. Sci. Technol. 1988, 23, (6&7),519.
Subscripts
F = feed in the bed z = component i j = componentj K = cell number S = effluent from the bed I-VI1 = serial number of steps in PSA cycle
Received for review May 2, 1994 Accepted October 26, 1994@ IE940282+ @
Abstract published in Advance A C S Abstracts, January 1,
1995.
