Ind. Eng. Chem. Res. 1993,32, 2365-2372
2365
Gas Separation and Purification by Polymeric Adsorbents: Flue Gas Desulfurization and SO2 Recovery with Styrenic Polymers E. S. Kikkinides and R. T. Yang' Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
High-surface-area polystyrenic sorbents possess all necessary properties for flue gas and tail gas desulfurization: high SO2 adsorption capacity and diffusion rate, high SOdCO2 selectivity, and hydrophobicity. NO chemisorbs on these polymers a t room temperature, but the NO-treated polymers retain all of the above properties. Model simulations show that pressure swing adsorption cycles using these NO-treated polymers can effectively remove well over 9 0 % of the SO2 from the flue gases and, a t the same time, generate SO2-concentrated desorption products containing well over 5 % by volume of S02. The desorption products can be readily converted to elemental sulfur. Aromatic rings comprise the major fraction of the polymer surface area. Infrared spectroscopic analysis indicates that NO chemisorbs by interactions with the ?r electrons in the aromatic rings on the polymer surface. The existence of lone-pair electrons in the SO2 molecule enables it to interact with the a electrons of the aromatic rings much more strongly than C02, leading to the high SOdCO2 selectivity.
Introduction The main process used for SO2 emission control for power plants and industrial tail gases is wet scrubbing. However, this process generates large amounts of solid wastes which are another form of pollutant and lead eventually to the contamination of groundwater and soil. Adsorption processes, on the other hand, are dry, and conceivably could be self-contained, i.e., generating no wastes. The key to success in developing these processes is to find the appropriate sorbent. The search for such a sorbent has been ongoing for nearly 30 years, with essentially no success prior to this work. The ideal sorbent for SO2 emission control must possess the following properties: (1) high S02/C02 selectivity and high SO2 capacity, (2) hydrophobicity, and (3) fast diffusion of SO2 within the sorbent. Property 1 has been met by polymeric anion exchange resins (Layton and Youngquist, 1969; Chen and Pinto, 1990; Kikkinidesand Yang, 1991). These resins, however, fail to meet the other two required properties. Extensive research on other sorbents, such as activated carbon (Stacy et al., 1968; Slack, 1975) has also failed to result in a sorbent that fulfills property 1. This study reports successful results on polystyrenic sorbents, which possess all the required properties. These sorbents were developed mainly by Rohm & Haas Company, resulting from their success in building porosity into polymers which led to high surface areas (Watters and Smith, 1979; Albright, 1986). They are being used for aqueous purification purposes (e.g., Kuo et al., 1987;Rohm & Haas, 1989;Garcia and King, 1989) as well as for catalyst support (Albright, 1986). A series of these sorbents are being studied in our laboratory, and are listed in Table I. They have high surface areas, accessible pores, and good mechanical strength. We will report here adsorption isotherm, diffusion data, gas-polymer surface interactions, and simulated pressure swing adsorption (PSA) results. The goal of PSA operation is to be able to remove over 90% of the SO2 from a simulated flue gas and simultaneously generate a concentrated SO2 desorption product which can be directly converted to elemental sulfur. The SO2-rich stream needs to be over 5 ?6 (by vol) to be fed to
* Author to whom correspondence should be addressed. 0888-5885/93/2632-2365$04.00/0
Table I. Styrenic Polymers (Divinylbenzene, Ethylvinylbenzene. Styrene) trade name surface area, (source) composition m2/a xus-40323 -80% DVB, 650 @ow) -20% EVB XUS-43436 -80% DVB, 1400 (Dow) -20% EVB XAD-2 50% DVB, 350 (Rohm & Haas) 35%EVB, 15%Sty 84%DVB, 790 XAD-4 (Rohm & Haas) 16%EVB XAD-16 75% DVB, 900 (Rohm & Haas) 25%EVB
~
mean pore size, A 100 28 190 60 157
a Claus process for conversion to elemental sulfur. The elemental sulfur is a useful product and, at least, is stable and no longer a pollutant.
Experimental Section Thermogravimetricanalysis (TGA),employing a Cahn System 113 recording microbalance with programmed temperature control, was used to measure uptake curves as well as equilibrium isotherms of S02, C02, NO, and water vapor at two temperatures, 25 and 60 "C. Highpurity He was used as the inert carrier gas and the gas for regeneration. The sorbents tested were XUS-40323 (Dow) and XAD-16 (Rohm & Haas). At the end of each adsorption experiment a desorption experiment was performed to ensure reversibility of the adsorption isotherm and to check whether diffusivity remained the same during adsorption and desorption. In all cases the adsorption isotherms were found to be reversible and the desorption rates were mirror images of the adsorption rates, except for the case of NO on both sorbents. NO uptake rates were unusually slow and,during desorption at the same temperature as adsorption, only a very small amount of the preadsorbed NO was slowly desorbed. These results showed that NO was chemisorbed on these polymeric sorbents, and additional experiments by temperature programmed desorption (TPD) and infrared spectroscopy (IR) were performed in order to understand the structure of the NO-treated polymers. The NO-chemisorbed polymers were used for all subsequent adsorption and diffusion experiments. For the case of 0 1993 American Chemical Society
2366 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 SO2 and CO2, the equilibrium data were fitted using a Langmuir isotherm. From the uptake rates of these gases, their diffusivities were calculated using Crank’s solution as the fitting model (Yang, 1987). For the adsorption of water vapor on these polymers, the uptake rates were initially rapid, followed by a very slow approach to equilibrium. Therefore an exposure time of the order of 1h was considered to construct each point in the water vapor isotherm. The adsorption capacity of the polymer sorbents for Nz, which is the major component in flue gases, was negligible at 1 atm compared to their capacities for the other gases.
(4) with boundary conditions (assuming negligible film diffusion) aqilar = 0 a t r = 0
(5)
qi = qi* at r = R
(6) where qi* is the equilibrium amount adsorbed at the surface of the polymer microsphere and can be calculated using the Langmuir isotherm: (7)
PSA Cycle Description Two major PSA cycles are considered in this study (Yang, 1987). The Skarstrom PSA cycle (using two beds) is considered for the majority of the simulations. A multibed five-step process has also been employed in order to show the effect of the additional cocurrent depressurization step of PSA performance. During the Skarstrom cycle, each bed undergoes the following cycle steps: (I) feed pressurization (i.e., repressurization with feed), (11) high-pressure feed (adsorption), (111) countercurrent blowdown (or evacuation), and (IV) countercurrent purge with clean product (at low pressure). PSA flue gas cleanup process performance is judged by the following factors: (1) step I1 product purity and recovery, (2) desorption product (SO2)purity and recovery, and (3) feed throughput. The desorption product purity will be expressed as the enrichment ratio (Kikkinides and Yang, 19911, i.e., the SO2 concentration ratio of exhaust to feed. Mathematical Model A complete adiabatic model with intraparticle diffusion is used in this study. The mass balance equation for component i in the bed is a2yi ayi R,T -- D, -+ u -+ -(p,,/t)
ayi at
az2
az
P
[
N
where qm and b are the Langmuir parameters and are functions of temperature: qm = k , - k,T; b = k , exp[kJTl
(8) The bed boundary conditions are of the following form:
at Z k = 0, and
at Z k = L. Note that the index k corresponds to the step number in the PSA cycle and for each step we have the following: Step 11, High-pressure adsorption:
Step 111. Countercurrent blowdown:
dqj
-y i z - ]
J = l dt
=0 (1)
Step IV. Countercurrent purge:
The overall mass balance is ZIV
= L - 2 , Y1V.i = YP$
UIV
= upwe;
P = PL
where yp,i is the time-averaged effluent mole fraction of species i, during step 11. Step I. Pressurization with feed: The heat balance is
ZI
=z
at zI= 0, atz,=L,
yi = yf,i u=O
P = P(t) Since the polymeric adsorbent has a bidisperse pore structure similar to that of zeolite molecular sieve, we assume that a solid diffusion model can represent the intraparticle diffusion process. The polymer bead consista of many small microspheres with radius R, and the diffusion rate is controlled by that in the microsphere. For each species i
The initial conditions for each step are the conditions at the end of the preceding step. Note that the pressure history is an input in PSA and in the present work is represented by a linear change with time, from PHto PL during blowdown and from PLto PHduring pressurization. In the above equations the basic assumptions are ideal gas behavior, constant viscosity and heat capacity of the
Table 11. Adsorption Bed Characteristics and Operating Conditions for the Standard PSA Simulation Adsorbent Bed bed length 500 cm bed diameter 100 cm void fraction 0.4 bed density 0.71 g/cma 0.28 cal/(g.K) heat capacity of bed Operating Conditions Na/COa/S02 = 81.5/17.0/0.5 feed composition, % 2.0 X 108 L(STP)/h feed flow rate purge/feed 0.03 PH 1.2 atm PL 0.03 atm 0.23 cal/(g-K) heat capacity of gas mixture totalcycle time (i) Skarstrom cycle 9 min (ii) five-step cycle 20 min 333 K ambient temp, T ~~
gas phase, negligible variation of all dependent variables in the radial direction, and negligible pressure drop along the bed. Note that aulaz does not appear in eq 1not because of any assumptions but because it was eliminated by proper combination of the species and the overall balance (Doong and Yang, 1986). The above equations can be made dimensionless by defining several dimensionless quantities as described elsewhere (Kikkinides and Yang, 1993). The adsorber characteristicstogether with the operating conditions for the standard case are listed in Table 11.In addition, the dimensionless Peclet numbers for mass and heat dispersion,Pel and Pez, respectively (Kikkinides and Yang, 1993), were both set equal to 1000 since in an industrial adsorber the effect of axial dispersion is negligible. In all Skarstrom cycle PSA model simulations, it was assumed that the duration of cycle steps I and I11 was very short compared to the cycle time; therefore the lengths of cycle steps I and 11, and those for cycle steps I11 and IV, were combined where an equal length of time was allotted for each combined set. Two different ambient temperatures were used in the simulations: 60 and 25 "C. For the case where the ambient temperature was 60 "C, a 9-min cycle was used in the simulations with the following distribution: step I, 0.5 min; step 11,4.0 min; step 111,0.5 min; step IV, 4.0 min. For the case where the ambient temperature was 25 "C a 19-min cycle was used in the simulations with the following distribution: step I, 0.5 min; step 11, 9.0 min; step 111, 0.5 min; step IV, 9.0 min. In the five-step cycle, a cocurrent depressurization step is inserted after the adsorption step, and 4 or 9 minis allowed for each step depending on the ambient temperature (60 or 25 "C, respectively). Numerical Solution of the Model. A numerical scheme with Galerkin finite elements for discretizing the equations in the bed in the axial direction and with orthogonal collocation for discretizing the intraparticle diffusion equation in the radial direction was used for the first time in modeling a PSA process. Twenty to thirty quadratic elements were used in the axial direction of the bed, while three to six collocation points were found to be enough for discretizing the radial direction inside the polymer microsphere. After the model equations were written in dimensionless form, they were discretized in space and the resulting system of ordinary differential equations was solved by a semi-implicit method called the 8 method, with a variable time step. In the initial parts of integration during each cycle step we used 8 = 1which corresponded to the fully implicit Euler method, to ensure stability and to avoid
-
1
c
4
40323
b G
-
d p: L
0.2
0.0
L
0
1
2 3 1600 TIME (min)
1800
Figure 1. Uptake curves of C02 (at 0.1 atm), SO2 (at 0.15 atm), and NO (at 0.1 atm) at 26 "C.
oscillatory results, and after that we used 8 = 0.5 which corresponded to the Crank-Nicholson method, in order to keep the same accuracy with less number of time steps. The step size was not constant and was changing in a way similar to the one proposed by Gear (1971). All computations were performed on a Vax 8800 computer. It took approximately 10-20 min of CPU time for each cycle, and it generally took 8-10 cycles to reach cyclic steady state.
Results and Discussion Adsorption Isotherms and Uptake Rates. The uptake curves of COz, SOz, and NO on XUS-40323 are shown in Figure 1. As expected, COZdiffuses faster than SO2 since it has a smaller effective diameter but the difference in their diffusion times is not large. NO is chemisorbed on the surface a t 26 "C, and an infrared spectroscopy study of the chemisorbed NO will be discussed shortly. A temperature programmed desorption study indicated that NO started to desorb a t 80-90 "C, and complete desorption required temperatures near 180 "C which is the temperature limit for the stability of the polymer. The adsorption isotherms of NO on XUS-40323 are shown in Figure 2. It is clear that XUS-40323 is almost saturated with NO at low partial pressures (less than 0.1 atm), characteristic of chemisorption. Consequently, two series of adsorption experimentswere performed where SO2 and COZwere measured on XUS40323 as well as on XUS-40323 saturated with NO. The resulting isotherms for SO2 and COZare shown in Figures 3 and 4, respectively. It is evident that the NO-treated sorbent has a slightly higher capacity for both SO2 and COZ. However, no significant change in terms of the Sod COZ selectivity occurred. Similar results were obtained for the case of XAD-16, the only difference being the higher capacities for both gases as compared to XUS-40323. The reason for this was the difference in surface areas between the two sorbents. It is found that the amount adsorbed was nearly proportional to the surface area for all three gases, SOZ,CO2, and NO. In Figure 5 the uptake curves of SO2,C02,and NO on XAD-16 are shown, and in Figure 6 adsorption isotherms of SO2 and COZon the NO-treated
T=26'C
0 T=60"C A T=26'C 0
6 80 o
t.
I
8
A
0
T=26'C T=60'C
4
20
0
0
0.1
0.0
0.2 PNO
0.3
0.4
0.0
0.2
0.4 PcO,
(atm)
Figure 2. Equilibrium isotherms of NO on XUS-40323.
0.8
0.6
1.0
(atm)
Figure 4. Equilibrium isotherms of COz on XUS-40323.
200
COZ (3.7mg/g)
T=26'C
h
0 T=60"C
bo
2
160
E nw
120
A 0
T=26'C T=6O'C
(FRESH) (FRESH) (NO-TREATED) (NO-TREATED)
v
m
n
12
3
3
T=60"C
(FRESH) (FRESH) (NO-TREATED) (NO-TREATED)
A
NO
(176mg/)
p: O
m
ADSORBENT:
P
5
XAD-16
-
80
b
2 3
0
2
40
5
/
0
0.0
I
0.2
1
1
0.4
I
I
0.6
0.8
1.0
pso, (atm)
Figure 3. Equilibrium isotherms of SO2 on XUS-40323.
XAD-16 are presented. Furthermore, water vapor isotherms on the fresh as well as on the NO-treated sorbent (shown in Figure 7) indicate that the NO-treated sample did not lose its hydrophobicity, although it became more hydrophilic due to the presence of NO on the polymer surface. The amounts of uptake for all gases measured were 2 or more orders of magnitude higher than those due to bulk dissolution in the polymers. This was consistent with the result of the dependence of amount adsorbed on surface area. Thus, the dissolved amounts were negligible and the question of solubility will not be considered. The isosteric heat of adsorption, Q, for each adsorbateadsorbent pair was calculated using the Clausius-Clapeyron equation (Yang, 1987). In Figure 8 the isosteric heats of adsorption of SO2 and C02 on NO-treated XAD16are plotted as functions of the amount adsorbed. Since during the PSA processthe partial pressure of SO2 is always much less than 0.3 atm, we assumed a constant value of
0.0 0
1 TIME
2 1600 (min)
1800
Figure 5. Uptake curves of COz (at 0.1 atm), SO2 (at 0.15 atm), and NO (at 0.008 atm) at 26 OC.
Q = 7.5 kcal/mol for S02. For the case of C02 the value of Q was also assumed to be constant and equal to 5.5 kcal/mol. The hydrophobicity of the sorbent is clearly indicated in Figure 7. Although the water vapor isotherms on XAD16 were measured only at room temperature, there is sufficient information in the literature regarding the capacity of styrenic polymer sorbents for water a t higher temperatures (Gvosdovich et al., 1969;Gassiot-Matas and Monrabal-Bas, 1970). From the literature results, it is seen that the equilibrium amounts of water adsorbed on styrenicpolymers, at constant relative humidities, decrease significantlywith increasingtemperature. It can be found, for example, that the equilibrium capacity of Porapak-Q adsorbent, which is similar to XAD-16, for water at 60 "C and 70 % relative humidity is about 3mg/g (Gassiot-Matas and Monrabal-Bas, 1970). Similarly, the equilibrium capacity of Chromosorb-102 for water near 100% relative humidity drops from 11mg/g at 30 OC to about 6.5 mg/g
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2369 10
A
n
M
2
SO.,
8
T=2B'C
SOs. T=-80'C
200
A
E
6
C O = , T=21'C
0 COa.
T=BO'C
4
v
n w a
5m
160
2
120
n 4
b
z
8o
3
0
2
40
1
4 t
0 0.0
0.2
0.6
0.4
1.0
0.8 (atm)
PRESSURE
Figure 6. Equilibrium isotherms of SO2 and COz on NO-treated XAD-16. 20
I
A
h
M
I
I
I
/
/
l
I
E
/
FRESH NO-TREATED
l5
n w a P;
0
10
v)
n d
i3 0
5
$ 0 0
20
40
RELATIVE
60
HUMIDITY,
80 %
0 04
0.08 q
0.12
0.16
0.20
(mmol/g)
Figure 8. Isosteric heat of adsorption of SO2 and COz on NO-treated XAD-16, as a function of the amount adsorbed.
-2 v
0.00
100
Figure 7. Equilibrium isotherms of water vapor on XAD-16 at 26 "C.
at 100 OC (Gvosdovich et al., 1969). Since water vapor capacity is lower than the capacities for both C02 and SO2, HzO adsorption will be neglected in PSA simulations. The Langmuir parameters together with the isostericheats of adsorption and the diffusivities of COz and SOzon XAD16 sorbent are given in Table 111. Note that the SO2 isotherm does not follow the Langmuir equation in the entire range of pressures. However, the lower-pressure region (which was the region of interest) was fitted by the Langmuir equation, and the resulting isotherm was used in the loading ratio correlation (LRC) (eq 7) for mixture adsorption. Infrared Spectroscopy Analysis of Chemisorbed NO. Infrared spectra of the NO-chemisorbed polymers were taken in order to understand the interactions between NO and the polymer surface. A Fourier transform infrared (FTIR) spectrometer was used to to take spectra of both XAD-16 and NO-chemisorbed XAD-16.Typical results are shown in Figure 9. It should be noted before discussion
of the IR spectra that the majority of the internal surfaces of these polymers are exposed aromatic rings (Albright, 1986). Fresh XAD-16 Polymer. Since XAD-16 is a styrenic type of polymer, it is expected to have characteristic IR adsorption bands similar to the ones that appear in polystyrene or in aromatic monomers such as benzene. However one cannot expect these bands to appear at identical positions with the polymers of the constituent monomers or benzene. The spectrum of the fresh polymer shows a very strong peak at 2926 cm-l followed by weaker peaks in the region 2858-3051 cm-I. All these peaks correspond to the C-H stretching vibrations (Bellamy, 1958; Conley, 1966;Smalley and Wakefield, 1973). The next series of peaks exhibited by the fresh polymer sample is in the region 1400-1600 cm-l. All these peaks correspond to the C=C stretching vibrations (for alkenes as well as for aromatics). Another less intense peak appears at 1358 cm-l which could be due to the C-H in-plane deformation for the -CH=CH- (trans) bond (Conley, 1966). The final series of peaks appears in the region 700-1000 cm-l. Peaks from 700 to 800 cm-1 correspond to C-H out-of-plane bending vibrations (for aromatics), while peaks around 900-1OOO cm-l correspond to C-H in-plane bending vibrations (for aromatics). NO-treatedXAD-16 Polymer. In the case of the NOchemisorbed polymer sample, one should expect the appearance of some new bands and some possible shift of existing bands. Moreover, major changes in the spectrum would indicate that structural changes in the polymer had occurred upon NO chemisorption. However,the similarity between the two spectra in Figure 9, as seen by the coincidence of most of the major peaks, indicates that the chemical bonds were mostly unaltered. Differences between the two spectra and their possible causes are discussed next. Comparison of the two spectra in Figure 9 leads to the observation that all spectral features of the fresh sample are preserved in the spectrum of the NO-treated sample, and that three new major absorption bands have appeared.
2370 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 Table 111. Lanwuir Parameters. Heats of AdsorDtion. and Diffusivities on XAD-16. component ki, m o U g kz, mmol/(g.K) ks, atm-1 k4, K so2 12.850 0.0320 1.15 X 1P 2361.08 coz 2.367 0.0020 4.68 X 1V 2657.89 ~~
a
~
Langmuir parameters correspond to the equation q,* = qmrb,pt/(l+ Zb,p,), where qm = kl
T
r a n 5
rn i t t
a n C
e
t
io00
I 3500
3000
PBOO
ZOO0
1500
$000
I
500
Wevenumbere
Figure B. Infrared spectra of XAD-16 polymer, and XAD-16 with NO chemisorption.
An examination of the magnified spectra (not shown here) reveals some shifts of the peak positions for a number of bands. The major shift due to NO chemisorption is found in the strong band at 1602 cm-1 (with a shoulder at 1628 cm-l) to the new peak position at 1564 cm-' (witha shoulder at 1605 cm-1). The three new adsorption bands caused by NO chemisorption are 1655, 1265, and 1082 crn-'. The band at 1265 cm-' corresponds to the dimeric N=O asymmetric stretching frequency in aromatic nitroso compounds (Rao and Bhaskar, 1969). NO is known to have atendency to form dimers even at room temperature (Kaneko et al., 1987a). Rao and Bhaskar (1969) have reported a single, intense band in the region 1253-1259 cm-l due to trans dimer N=O bonded to aromatic groups. However, it is likely that the band at 1082 cm-' is associated with the symmetric stretching frequency of the same dimeric N-0 that gives rise to the 1265-cm-1 band. The new band at 1655 cm-l appears to be associated with the stretching frequency of monomeric N=O, which has been studied extensively on inorganic surfaces of metals and metal oxides (Little, 1966; Low and Yang, 1974; Enault and Larher, 1977; Hanoch and Folman, 1979; Busca and Lorenzelli, 1981; Kaneko et al., 1987a,b). The bands at 1602 and 1628 cm-1 (as a shoulder) are characteristic of the carbon-carbon bond stretching vibrations in the aromatic benzene rings in polymers such as polystyrene (Conley, 1966). The shift of these bands toward substantially lower frequencies (1564 cm-' and the 1605-cm-I shoulder) indicates that the chemisorption of NO is a result of interactions between NO and the r electrons in the aromatic rings to form a ?r complex. It is known that the intensity of these bands will increase upon interactions with NO (and other gas molecules such as
~~~~~
~~
-AH,kcaVmol 7.8 5.5
DIR2,a-l
7.6 X 1P 1.5 X 10-2
- kzT and b = ks exp[kJm.
C = O and NOZ)(Bellamy, 1958). The results shown in Figure 9 are consistent with this observation since these bands are indeed intensified by NO chemisorption. The interactions are likely to involve the unpaired electron in the antibonding orbital by the NO molecule (Low and Yang, 1974). Gas-Polymer Surface Interactions. As discussed, the equilibrium amounts of adsorbed SOz, CO2, and HzO are not influenced significantlyby NO chemisorption.This result indicates that an abundant amount of the aromatic ring surfaces remains available for interactions with other molecules after NO chemisorption. This can be understood as a simple estimate will show that only less than half of the exposed aromatic rings are occupied by NO dimers and that more than half of the surface aromatic rings are unoccupied. Adsorption of SO2 and C02 on Graphite Basal Plane. The high selectivity for SO2 over COZ by the polymer surface (Figures 3 and 4) indicates strong interactions of SOzwith the aromatic rings. In order to provide further support for this assertion, the adsorptions of SO2 and COz were measured on a graphite sample which has predominately a basal plane surface. The basal plane of graphite has abundant amounts of ?r electrons, two per carbon atom. The equilibrium amounts of SO2 at 22 "C are 4.72 mg/g (at 0.33 atm), 7.20 mg/g (at 0.67 atm), and 8.80 mg/g (at 1.0 atm). Values for COZat 22 "C are 0.21 mg/g (at 0.33 atm), 0.37 mg/g (at 0.67 atm) and 0.51 mg/g (at 1.0 atm) (Chen and Yang, 1993). The S02/COz selectivityratios on the graphite basal plane are strikingly similar to those on the polymer surfaces, Le., near 20 at low pressures, decreasing to about 17 near 1 atm. This result provides strong evidence that the SOZ/COZselectivity is caused by interactions of SO2 with the .rr electrons. For a theoretical understanding of the interactions, semiempirical molecular-orbital theory calculations were performed for SO2 and COZ adsorbed on a graphite substrate as well as a benzene ring. The results show that, indeed, strong interactions are formed between the SO2 molecule and the ?r electrons. The strong interactions are present because the SO2 molecule possesses a lone pair of electrons whereas the COZmolecule does not; the lonepair electrons can interact with the ?r electrons on the polymer surface. Detailed experimental and theoretical results on graphite and benzene rings are published elsewhere (Chen and Yang, 1993). The HzO molecule also has lone-pair electrons. However, the oxygen atom has a high electronegativity,second only to F. The electronegativity of the sulfur atom, on the other hand, is substantially lower than that of oxygen. (Electronegativity, as defined by Pauling, is the power to attract electrons.) As a result, the polymer surface (as well as the graphite basal plane) does not adsorb water strongly. PSA Simulation. The main objective of the PSA simulation is to examine whether and under what conditions it is possible to effectively remove SO2 from flue gas or industrial tailgases and at the same time concentrate SOZ. The SO2 removal should be over 90%. The concentration of SOZin the desorption stream should be over 5 % for converting to elemental sulfur by the Claus process. In this simulation the XAD-16 polymer is used and it is
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2371 Table IV. Effect of Purge/Feed on PSA Performance for 4,= 0.03 atm, Ti = 333 K. Skarstrom Cycle (Feed SO2 = 0.846, COS = 17%)
volume-averaged effluent concn, % product stream desorption stream P/F 0.015 0.020 0.025 0.030
SO2 0.046 0.021 0.010 0.004
C02 15.59 15.67 15.68 15.68
SO2
8.40 8.03 7.56 6.65
C02 46.88 44.31 42.19 38.82
SO2recovery, % 96.0 100.0 100.0 100.0
Table V. Effect of Purge/Feed on PSA Performance for 4, = 0.03 atm, Ti = 298 K. Skarstrom Cycle (Feed SO2 = 0.5%, C02
P/F 0.020 0.025 0.030
17%) volume-averagedeffluent concn, % product stream desorption stream SO2 C02 SO2 C02 0.040 15.82 10.17 46.96 9.60 44.33 0.010 15.84 0.001 15.89 8.87 41.80
SO2recovery, % 97.7 100.0 100.0
Table VI. Effect of PCDon PSA Performance for P/F = 0.03,4, = 0.03 atm, Tr = 333 K. Five-Step Cycle (Feed 902 = 0.5%, C02 17%) volume-averaged effluent concn, % product stream desorption stream Pc~,atm SO2 COz SO2 C02 S02recovery, % 0.60 0.013 15.79 8.59 44.00 100.0 0.30 0.014 15.93 9.19 43.30 100.0 0.10 0.024 16.67 10.59 34.53 95.5 11.33 28.10 92.6 0.06 0.038 16.98
Table VII. Effect of PCDon PSA Performance for P/F = 0.025,4, = 0.03 atm, T r = 298 K. Five-Step Cycle (Feed 8 0 2
O.S%, C02 = 17%)
volume-averaged effluent concn, % product stream desorption stream P c ~ , a t m SO2 COS SO2 COz SOzrecovery, % 100.0 10.18 45.11 0.60 0.015 15.89 44.40 98.8 0.30 0.017 15.98 10.48 33.67 94.3 12.24 0.10 0.031 16.79 26.82 91.0 13.11 0.06 0.048 17.09
further saturated with NO, as NO is present in the flue gas. The adsorptions of N2 and H2O are neglected in the simulation. The SO2 recovery is calculated by SO2recovery =
amt of SO,* from stem I11 and IV amt of SO2in feed used in steps I and I1
the light product purity (or SO2 removal) increases with the purge to feed ratio while the SO2 concentration in the desorption product decreases with it. Increasing the feed temperature significantly decreases the SO2 product concentration. In addition, the cycle time becomes shorter because of the reduction of the adsorption capacity of the bed. A major difference between the Skarstrom cycle and the five-step cycle is that the latter involvesthe additional step of cocurrent depressurization (CD) in the PSA cycle for internal product recovery. To incorporate this step into the Skarstrom cycle, the adsorption step is cut short before the break point; the concentration front is kept far from reaching the exit of the bed. The major function of the cocurrent depressurization step is to increase the concentration of the strong adsorptive in the bed. This is done by lowering the pressure in the voids, which enhances the concentration of the strong adsorptive in both phases (Yang, 1987). The net result of the incorporation of the CD step in the Skarstrom cycle is the product purity enhancement of the strong adsorptive which in turn increases the product recovery of the weak adsorptive. In the simulation of the five-step process, when the feed temperature was at 60 "C, each step was 4 min long resulting in a total cycle time of 20 min. When the feed temperature was 25 "C, each step was 9 min long resulting in a total cycle time of 45 min. The pressure during the CD step was considered to decrease linearly with time, from 1.2 atm at the beginning to a subatmospheric pressure, PCD,at the end of this step. The results in Tables VI and VI1 show that a significant increase in the SO2 enrichment is achieved using the fivestep process compared to the corresponding cases using the Skarstrom cycle. This result was solely due to the additional CD step. Hence by using a more complex process, higher SO2 enrichment can be obtained without significantly lowering the purity of the product stream. Nevertheless, unless there is a superseding need for higher SO2 enrichment, the simple two-bed Skarstrom cycle is considered efficientfor both SO2removal and SO2 recovery.
Acknowledgment This work was supported by NSF under Grant CTS9212279. We appreciate the helpful discussions with Dr. Robert Albright, Mr. Jay Miers, and other staff at Rohm & Haas Co. The polymer samples were kindly donated by Dow Chemical Co. and by Rohm & Haas Co.
Nomenclature b = Langmuir parameter, defined by eq 8, atm-1 (11) cp,g = heat capacity of the gas phase, cal/(g.K) The results with a Skarstrom (two-bed) cycle are cpa = heat capacity of the solid phase, cal/(g.K) compared to the results using a multibed, five-stepprocess De = effective intraparticle diffusivity, cmVs in order to examine the effect of the additional step of DL = mass axial dispersion constant, cm2/s cocurrent depressurization on the PSA process perforL = length of the bed, cm mance. N = total number of components in the mixture The simulation results at two feed temperatures (Tf) P = pressure, atm are shown in Tables IV-VII. The feed pressure is 1.2 atm, q = adsorbed amount, mol/g and the low pressure in the PSA cycle is 0.03 atm. Under p = volume-averaged adsorbed amount, mol/g these conditions, the simulation results show that it is qm = saturated amount adsorbed, defined by eq 8, mol/g possible to achieve well over 90% SO2 removal while Q = (-AH) = heat of adsorption, cal/mol concentrating SO2 to well over 5% in all runs shown in r = radial distance from the center of the microsphere, cm Tables IV-VII. The objective described above can still R = radius of the microsphere, cm be met with an evacuation pressure of 0.05 atm, but not R, = gas constant, atm cm3/(rnol.K) a t 0.1 atm. T = ambient temperature, K The effect of purge/feed ratio follows the same trend as t = time, s described previously (Kikkinides and Yang, 1991); i.e., u = interstitial velocity, cm/s
2372 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
y = mole fraction in the gas phase z = axial position in the bed, cm Greek Letters t
= fractional void in the bed
XL = mass axial dispersion constant, cm2/s Pb = density of the bed, g/cm3 Subscripts b = bed f = feed H = high
i = species i
j = species j k = step number in PSA cycle
L = low p = product Superscript
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Received for review December 18, 1992 Revised manuscript received March 5, 1993 Accepted March 18, 1993' Abstract published in Advance ACS Abstracts, August 15, 1993. @
