Ind. Eng. Chem. Res. 1988,27,1103-1109
(Santa Fe, Argentina). T h i s work was presented at the 1984 AIChE Annual Meeting in San Francisco. Nomenclature a = parabola characteristic constant, m A, = interfacial area per unit volume, l / m C, = molar concentration of t h e j t h species, mol/m3 d = diameter, m F = molar flow rate, mol/s H = Henry's constant, (Pa.m3)/mol kl = chemical absorption coefficient, m / s k r = physical absorption coefficient, m / s 1 = distance from vertex of parabolic reflector t o the reactor,
m
L = length, m N = impeller speed, l / s P = pressure, P a r = radius, m t = time, s T = temperature, K
1103
Registry No. Clz, 7782-50-5; trichloroethylene, 79-01-6; pentachloroethane, 76-01-7; hexachloroethane, 67-72-1.
Literature Cited ALCOA, Aluminum Company of America, Technical Bulletin, 1964; ALCOA, Alcoa Center, PA. Alfano, 0. M. Ph.D. Dissertation, Universidad Nacional del Litoral, Santa Fe, Argentina, 1984. Alfano, 0. M.; Cassano, A. E. Ind. Eng. Chem. Res. 1988,in press. Astarita, G. Mass Transfer with Chemical Reaction;Elsevier: Amsterdam, 1967. Astarita. G.: Savage. D. W.: Bisio. A. Gas Treating with Chemical So1uents;'WileG 'New York, 1983. Benson, S. W. The Foundations of Chemical Kinetics;McGraw-Hill New York 1960. Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, I
1966.
y j = molar fraction of t h e j t h species, dimensionless Greek Symbols r = reflection coefficient, dimensionless c = volume fraction, dimensionless p = attenuation coefficient, l / m 7 = t / t R , dimensionless time ( t =~7200 e ) @ = primary quantum yield, mol/einstein qj = Cj/CR,dimensionless concentration of the j t h species (CR = C,) Subscripts b = bubble property D = diffusion value g = in the gas phase in = inlet condition j = for the j t h species: Clz 0' = 2), CzHCls0' = 5), and C2C& 0' = 6) L = lamp property o = initial value p = relative t o t h e reactor bottom plate ps = relative t o t h e time scale for physical saturation r = reaction value R = reference value or reactor property Rf = reflector property A = wavelength dependence Superscript * = effective property
Chiltz, G.; Goldfinger, P.; Huybrechts, G.; Martens, G.; Verbecke, G. Chem. Reu. 1963,63,355. Daiton, F. S.; Lomax, D. A,; Weston, M. Trans. Faraday Soc. 1957, 53,460. Dickinson, R. G. Chem. Rev. 1935,17,413. Gebhard, T. J. Ph.D. Thesis, Northwestern University, Evanston, IL, 1978.
General Electric Co., Technical Bulletin, 1959; General Electric, Cleveland, OH. Gibson, G. E.; Bayliss, N. S. Phys. Reu. 1933,44,188. Hill, F. B.; Reiss, N. Can. J. Chem. Eng. 1968,46,124. Huybrechts, G.;Meyers, L.; Verbeke, G. Trans. Faraday Soc. 1962, 58, 1128.
Irazoqui, H. A.; Cerdi, J.; Cassano, A. E. AIChE J. 1973,19,460. Koller, L.R. Ultrauiolet Radiation; Wiley: New York, 1966. Kurtz, B. E. Ind. Eng. Chem. Process Des. Deu. 1972,11, 332. Linke, W. F. Solubilities of Inorganic and Metal-Organic Compounds; American Chemical Society: Washington, D.C., 1958; VOl. 1. Otake, T.; Tone, S.; Higuchi, K.; Nakao, K. Kagaku Kogaku Ronbunshu 1981,7,57. Otake, T.; Tone, S.; Higuchi, K.; Nakao, K. Int. Chem. Eng. 1983, 23, 288. Ramage, M. P.; Eckert, R. E. Ind. Eng. Chem. Fundam. 1975,14, 214.
Ramage, M.P.; Eckert, R. E. Ind. Eng. Chem. Fundam. 1979,18, 216.
Walling, C. Free Radicals in Solution; Wiley: New York, 1957. Yokota, T.; Iwano, T.; Deguchi, H.; Tadaki, T. Kagaku Kogaku Ronbunshu 1981a, 7,157. Yokota, T.; Iwano, T.; Saito, A,; Tadaki, T. Kagaku Kogaku Ronbunshu 1981b,7, 164. Yokota, T.; Iwano, T.; Saito, A.; Tadaki, T. Int. Chem. Eng. 1983, 23,494. Received for review May 8, 1986 Revised manuscript received November 4, 1987 Accepted November 30, 1987
Special Symbol ( ) = averaged value
Relationship between Properties of Various Zeolites and Their C02-AdsorptionBehaviors in Pressure Swing Adsorption Operation Tomoyuki Inui,* Yoshitaka Okugawa, and Masaki Yasuda Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto Uniuersity, Sakyo-ku, Kyoto 606, Japan
In order to search proper zeolites for COz separation from C 0 2 - c o n t a i n i n g gas mixtures b y pressure swing adsorption (PSA), relationships between properties of zeolites and their adsorption-desorption behaviors f o r COPwere investigated. Highly crystallized zeolites having a h i g h surface area and the three-dimensional pore connection structure were suitable for C 0 2 separation. A m o n g m a n y kinds of natural and synthetic zeolites, chabazite and 13X were most proper for the purpose. Pressure swing adsorption (PSA) method has recently been used for the purpose of various kinds of gas separation. From t h e view point of thermal efficiency, t h i s 0888-5885/88/2627-1103$01.50/0
m e t h o d would be b e t t e r t h a n conventional cryogenic fractionation (Stewart and Heck, 1969). However, frequent repetition of compression a n d expansion for mixing of gas 0 1988 American Chemical Society
1104 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table I. Physical Properties of Zeolite Samples
pore size: sample
3
name chabazite chabazite clinoptilolite
5
clinoptilolite 80%, Idaho smectite, Opal C.T. clinoptilolite Idaho, another layer
6
mordenite
Oregon
7
mordenite
Nevada
8
ferrierite
Nevada
mordenite, ferrierite, oeilite eribnite MS-5A MS-4A MS-13X H-ZSM-5'
Nevada
1 2
10 11
12 13 14
A
origin Oregon Oregon, another layer Oregon
Oregon
ucc ucc ucc
Inui's lab
3.6 3.6
(:
(:
X X
3.7 3.7
: i:;} : : i:: 5'7
2'9
{::: : 6.7
X
SEM crystal PCb figure shape 3 3a cubic 3 3a cubic 1 3a monoclinic 1
3a
hexagonal
1
3a
small particle
1
3a
small rod
7.0
{i:;: ;:}
'3.6 X 5.2' 5 4 7.4 5.1 X 5.8
1
3b
fine pole
1
3b 3b 3c 3c 3c 3c
fine pole fine fibrous sphere sphere sphere sphere
3 3 3
3 3
m2/g 532 409 27
(D,/d,2p,)108, mS/(s g) 1.9 2.0 3.7
(NH&=, mg of NH3/g 114 92 54
369
3.0
81
41
2.5
58
127
2.7
29
51
1.9
23
43
2.3
48
64 189 460 7 527 387
2.7
34 48 81 79 54 18
S,"
2.1
0.9 1.3 1.3 2.0
Pore size of major zeolite contained in a natural zeolite. Dimension of pore structure connection for the main pore. Apparent BET surface area calculated by N2 adsorption at liquid nitrogen temperature. dTotal amount of ammonia desorbed. e Si/Al atomic ratio was 40.
needs huge amounta of energy and complicated mechanical operations. Therefore, much room still remains for improvement in the PSA method. The most important factors for the improvement must be the following properties for the adsorbent: (i) a separation ability for the objective gas from a gas mixture is prominent, (ii) the capacity for reversible adsorption of the objective gas is large, and (iii) the adsorption and desorption rates of the objective gas are rapid. In this study, to assist in the basic understanding for searching and improving the adsorbent for C02separation from various gas mixtures containing C02,the relationships between the properties of zeolites and their adsorption characteristics in PSA operation under moderate pressure conditions were investigated. The properties of the adsorbent for C 0 2separation by the PSA method have not been studied extensively; therefore, various kinds of natural and synthetic zeolites having different crystal and pore structures were employed as samples, and the relationships mentioned above were examined.
Experimental Section Zeolite Samples. The zeolite samples used are listed in Table I. Ten kinds of American natural zeolites (samples 1-10) were collected by the senior author during the Zeo-Trip in 1983, which was held by the International Committee on Natural Zeolites as the optional tour of the 6th International Conference of Zeolites a t Reno. Identification of crystal structures and chemical composition of the natural zeolites was precisely determined (Sheppard and Gude, 1983). Three kinds of synthetic zeolites (samples 11-13) were commercial ones produced by Union Carbide Co., and typical H-ZSM-5, which had a Si/A1 atomic ratio of 40 (sample 14), was prepared by ourselves using the modified preparation method (Inuiet al., 1984a). A piece of natural zeolite block was first pulverized and ground in a mortar t o 0.7-0.5-pm size. The pulverized powder was then calcined in an air stream a t 350 OC for 1 h. This calcined powder was formed by compression using a tablet machine and crushed to 8-15 mesh to provide the adsorption measurement. Any other treatment such as washing with aqueous acid was not conducted before the adsorption experiment.
Measurements of Physical and Chemical Properties of Zeolite Samples. The amount of nitrogen adsorption for zeolite samples was measured a t liquid nitrogen temperature by means of the continuous flow method using a gas chromatograph. In order to avoid any effect of adsorbed water, before the measurement the sample was treated in situ by a helium stream of 50 mL/min at the following temperatures: the sample was heated with a constant heating rate of 10 OC/min from room temperature to 350 "C and then maintained a t that temperature for 10 min, followed by cooling down to room temperature. The apparent BET surface area was calculated from the amount of adsorbed N2 assuming one adsorbed N2 molecule occupied 16.2 A2. Morphology of the zeolite samples before and after grinding in a mortar was observed by a scanning electron microscope, HitachiAkashi-102. X-ray diffraction patterns of zeolite samples were obtained with Cu KLYradiation by using a Rigaku Denki Gigerflex 2013. Effective diffusivity of the crystalline, De, was measured by the desorption rate analysis of the adsorbed C 0 2 (Inui et al., 1977). The analysis is based on the linear relationship between the desorption rate and the logarithmic time of elution as follows. Plots of C 0 2 concentration against the time of elution in logarithmic scale gave a straight line in a wide range after ca. 4 min of elution. The gradient of the straight line, p, can be regarded as the extent of mass transfer of the intracrystalline pores. The magnitude of the intracrystalline space in a 8-15-mesh granule would be of the same order as the unit particle, i.e., average of 0.75 pm; therefore, this space is wide compared to intracrystalline pores. Undoubtedly, resistance against the mass transfer is determined by the intracrystalline pores. During the early ca. 4-min elution period, the gas involved in the intracrystalline space was eluted, and since the magnitude of this space was similar, the elution period was also the same for each sample. By assuming that the unit particle has a space of diameter d, and density pp, the number of crysWines per 1g of sample is expected to be 6/7rdp"pp. When 5ll2 d, is adopted as the mean pore length (Inui, 19851, then the effective diffusivity of the crystallines can be calculated by
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1105
--
l 5
I8A
6?- u
Figure 1. Schematic diagram of apparatus for PSA. 1, GC carrier gas cylinder (He); 2, adsorption gas cylinder (5.4% CO,/He); 3, four-way valve; 4, adsorption column; 5, electric furnace; 6, pressure gauge; 7, thermocouple; 8, pyrometer; 9, flow control valve; 10, TCD cell; 11, soap film meter.
In this paper, the value of De/dp2ppwas used for discussion for the diffusivity of each zeolite sample. The amount and strength of solid acid sites were measured by temperature-programmed desorption (TPD) of adsorbed ammonia (Inui et al., 1984b), with a Shimadzu thermal analyzer, DT30. A 30-mg portion of the zeolite sample was placed in the sample pan. After the sample was dried by heating it at 450 "C in a nitrogen stream, the gas flow was switched to a 5% NH,-contained N2 stream. After the breakthrough of NH3 was confirmed and following the attainment of a constant level, the supply of adsorption gas was stopped and the concentration of NH, in the elution gas was recorded. When the elution curve again became constant, the temperature was then elevated from 60 to 600 "C a t a constant rate of 10 "C/min. The TPD profile was obtained from the differential of the weight-loss curve. PSA Measurement for C02. The apparatus for PSA measurements is shown in Figure 1. A 600-mg portion of zeolite sample was packed in a tubular column of stainless steel 316 having 6-mm inner diameter. The packing length was 40 mm. Carbon dioxide diluted with He was used as the adsorption gas. The concentration of C02was 5.4%. The adsorption pressure was varied from 2 to 11 kg/cm2. Desorption was operated a t room temperature under atmospheric pressure, irrespective of adsorption pressures. The inlet flow rate was consistently maintained a t 50 mL (STP)/min. A change in C02 concentration through PSA operation was continuously detected by a TCD-type gas chromatograph. A typical PSA profile for C02concentration is illustrated in Figure 2. The adsorption gas is introduced in a stepfunction manner a t time ti under each pressure. During the total adsorption of C 0 2 ,the recorder reflection is in accordance with the base line. The correction for dead space of the flow pass was obtained by a blank test. The breakthrough of C02concentration is detected a t time tb. After the reflection of the detector is attained to the same level (hi)as the feed, the adsorption gas supply is stopped at time t,. The compressed gas in the adsorption tube is reduced to atmospheric pressure until time tel. Helium is then introduced with a flow rate of 50 mL/min and C02 elution allowed to be continued. When the C 0 2 concentration in the elution gas is reduced to 0.45%, the adsorption gas is introduced again and the same operation cycle is repeated. Areas 1and 2 correspond to the amounts of C02 adsorption at the first and second cycles, respectively. Each area is converted into the volume of adsorbed C02 per unit weight of the zeolite sample. The amount of C02 adsorbed in area 1is defined as the total amount of adsorption, AT That in area 2 is apparent reversible adsorption, AR. Accordingly, the apparent irreversible
ti
t b
t o tez
ti
Time on stream
Figure 2. Illustrative gas chromatogramsof the PSA measurements: 1, total amount of adsorption; 2, apparent reversible adsorption.
adsorption, AI, is calculated by AT - AR. Surface Treatment to Zeolites. To investigate the effect on the C02adsorption property, treatment of various metal salts by immersing the zeolite into the aqueous solutions was conducted. A 2-g portion of the zeolite sample was immersed into 100 mL of solution a t 80 "C for 1 h. The C02adsorption measurement was observed under 11 kg/cm2. C 0 2 Separation Experiment. The separation experiment of C02-containingN2 gas by the PSA method was carried out by using chabazite (sample 2) and MS-13X (sample 13), which showed the high efficiency a t the C02 adsorption experiment by He-diluted C02gas. The same apparatus shown in Figure 1was used. In this experiment, the TCD outlet gas was lead to the sampling valve which sampled 1mL of the gas every 1min. The composition of the sampled gas was analyzed by another TCD (Shimadzu GC-3BT, Porapak Q, 3 X 2 m). A 600-mg portion of the zeolite was used, and the bed length was 40 mm. The adsorbent was treated a t 350 "C for 10 min in He carrier gas before the experiment. Nitrogen-diluted 2.7 % C02gas was used and maintained a t a flow rate of 75 mL (STP)/min and a pressure of 4 kg/cm2. After the breakthrough was observed, compressed gas was decompressed to atmospheric pressure and then elution was done by He gas stream. When the C02 concentration in the elution gas was reduced to 1.4%, the adsorption gas was introduced again and the second adsorption cycle was repeated.
Results and Discussions Properties of Zeolite Samples. Morphology of the zeolite samples before grinding is shown in Figure 3. The shape and size of the crystallines were markedly different from each other. Samples 1and 2 (chabazites) were cubic crystals of 4-7-pm size. Sample 4 (clinoptilolite) was hexagonal crystals of 2-4 pm; however another clinoptilolite (sample 5) was an agglomerate of fine (0.4-pm) spherical particles. Samples 6 and 7 (mordenites) showed a small rod shape (0.2-pm diameter X 0.9-pm length) and a fine pole shape (0.5-pm diameter X 5-pm length), respectively. Sample 8 (ferrierite) was also fine pole shaped but finer and longer (0.4-pm diameter X ca. 14-pm length) than sample 7. Sample 10 (erionite) was the fine fibrous shape (0.2-pm diameter X 18 pm). All synthetic zeolites (samples 11-14) had a highly crystallized uniform shape of about 2 pm. The shapes of the crystals are surveyed in Table I with apparent BET surface area, effective diffusivity, and amount of NH, desorbed. Pore size and the dimension of the pore connection were cited from the literature (Smith, 1976).
1106 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
Sample 2
1
H5p1n
2 U \ c3 al
0
Sample 4
Sample 9
i -1
i1 -
5p111
~ ~ I I I
,
,
,
I
I
I
1
2
3
4
5
6
7
8
HZpm
Sample 8
-t
Sample 10
H1 0 pm
Sample 1 2
Sample 13
I
Pore s i z e ( A Figure 4. Effective diffusivity term versus mean pore diameter of zeolites: (0)natural zeolites, ( 0 )synthetic zeolites. Each number stands for the sample number shown in Table I.
!
Sample 7
I
5p1u
H5pi11
H 5 p i i 1
Figure 3. SEM photographs of zeolite samples 1-14.
In Figure 4, the apparent effective diffusivities ((De)a#e/d:pp) of zeolite samples are plotted against their pore diameters. (De)epp)sfor natural zeolite samples are roughly proportional to the pore diameter. For synthetic zeolites, except for sample 14, a different proportional relationship with a lower incline was observed. Compared with natural zeolites, (De)app's were small. Samples 11,12, and 13 were commercial zeolites and were pelletized with a binder, which would narrow the intracrystalline space and blockade the pore opening of the crystalline. On the other hand, all natural zeolites and sample 14, which was H-ZSM-5, prepared by ourselves, were pilled by compression by using a tablet machine without binder. By this method, the blockade of pore opening of crystallines would not be so serious, and consequently, larger (De)ap
