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Sorption Kinetics of Higher n -Paraffins on Zeolite Molecular Sieve 5At Sorption kinetic measurements on n-paraffins ranging from n-heptane to n-tridecane have been carried out on zeolite molecular sieve 5A. Apparent diffusivity coefficients have been determined from the kinetic data. The effect of carbon chain length, preadsorbed water, and different nonadsorbing solvents on the kinetics of sorption has been examined. Molecular sieve zeolites are increasingly being used in commercial separation and catalytic processes (Csicsery, 1976). Consequently, the study of diffusion and sorption kinetics in these microporous materials has assumed a considerable significance. This fact is also evident from the recent developments in the experimental techniques of measuring diffusivity (Karger and Caro, 1977; Ruthven, 1984). Most of the available data on the diffusivity of hydrocarbons are reported for the lower hydrocarbons in the vapor phase (Barrer, 1984). From the studies so far reported on liquid-phase adsorption, it has been shown that sorption rates significantly depend on the nature of the nonadsorbing solvent and other adsorbable compounds present in trace amounts (Caro et al., 1980; Wolf and Pilchowski, 1971; Aleksandrov et al., 1967). Caro et al. (1980) have concluded, from their study on the adsorption of n-decane on 5A zeolite, that solvent molecules such as ethyl- and butylbenzenes adsorbed on the external surface of the zeolite block the pore openings, thereby affecting the sorption rates. The carbon chain length is another factor which influences the orientation of paraffin molecules within the zeolite cage (Jasra and Bhat, 1987). It is therefore to be expected that diffusivity of the paraffins also will depend on the carbon chain length. In the present paper, we report the measurements on sorption kinetics of n-heptane, n-nonane, n-decane, n-dodecane, and ntridecane on molecular sieve 5A from their solution in p-xylene. The effect of nonadsorbing solvents on sorption kinetics of n-decane also is reported. The choice of the adsorbate-adsorbent system is based on its relevance in the adsorptive separation of detergent grade paraffm from kerosene (Broughton, 1968, 1978). Furthermore, the influence of preadsorbed water on the sorption kinetics is studied, since moisture is often an impurity in the feedstock affecting the performance of the adsorbent in the separation process.
Experimental Section The sample of molecular sieve 5A was activated by heating at 673 K for about 8 h in a glass reactor under a dry nitrogen purge. The sample was cooled to ambient temperaure, and appproximately 2 g was accurately weighed and transferred to the experimental cell under dry nitrogen atmosphere. The experimental cell was a glass tube (5 mL) closed by means of a silicone rubber septum and a screw cap having a hole at the center. A known volume of the n-paraffin solution in the nonadsorbing solvent was injected into the cell, and the mixture was vigorously shaken to dissipate the heat evolved during adsorption. The experimental cell was kept in a water bath maintained at the required temperature (h0.05 K) and was periodically shaken to overcome any surface film effect. The concentration of the paraffin in the solution was determined at regular time intervals by the analysis of the samples withdrawn by means of a microsyringe. All analyses were done by gas chromatography (Shimadzu GC-7AG) having a 6-ft X l/s-in. SE 30 column and thermal conductivity detector with hydrogen (flow rate, 30 mL/ min) as the carrier gas. The errors in GC analysis were within *0.5%. 0888-5885/87/2626-2544$01.50/0
Table I. Apparent Diffusivity Coefficient ( D / r 2 of ) II -Paraffins in Molecular Sieve 5A at 298 K 104D/r2, 10-4D/r2, n-paraffin C no. cm-' n-paraffin C no. cm-' c7 1.3 CIZ 0.2 C9 1.3 c13 0.4 ClO 0.7
The volume and the concentration of the paraffin solution were so chosen that the amount of sorbate in solution was always in a large excess, larger than that which was required to saturate the adsorbent. This was done in order to minimize errors in sorption rates due to a change in paraffin concentration during the experiment. Zeolite samples having different levels of preadsorbed water were obtained by gravimetric adsorption in a McBain balance equipped with a quartz spring (Thermal Syndicate, UK). The zeolite sample with a known amount of adsorbed water was quickly transferred to the experimental cell and the sorption experiments were carried out as described before. n-Paraffins used in the study were of high purity grade (>99.5%). Solvents used were m-xylene, p-xylene, ethylbenzene, and methylcyclohexane (Fluka AG). The molecular sieve 5A was a commercial sample (Associated Cement Co. Ltd., India) having spherical beads of 0.75-mm average diameter. The sample had a water adsorption capacity of 22.4 molecules/cavity.
Results and Discussion Figure 1gives the plots of QJQ- against time for various n-paraffms from p-xylene at ambient temperature (298 K). It is seen from the f i e that although the rate of sorption generally decreases with an increase in carbon chain length of n-paraffins, this dependence is more pronounced from n-decane onward. The influence of solvent on the sorption kinetics of n-decane is presented in Figure 2 which shows that the sorption of n-decane is faster from methylcyclohexane and ethylbenzene as compared to the sorption from m- and p-xylenes. Similar data were also obtained earlier by Car0 et al. (1980) in their study on sorption of n-decane on 5A molecular sieve from ethylbenzene and cyclohexane. Figure 3 shows the influence of preadsorbed water on sorption kinetics on n-decane. It can be seen that adsorbed water even in a very small amount (0.8 molecule/cavity of zeolite) suppresses the sorption rates of paraffins to a significant extent. We have attempted to calculate the apparent diffusivity, D / r 2 ,from the sorption data by employing the well-known equation (Barrer, 1979), Qt - Qo -- 1- 6 E - 1 exp( Q- - 80 a2 n = l n2
7) -Dn2a2t (1)
The reported diffusivities of n-paraffins in molecular sieve 5A (Ruthven, 1984) are low enough to be determined from sorption rate measurements. The applicability of the above equation was ensured by considering the molecular sieve sample as an assemblage of spherical particles. By a simple fitting of the sorption curve to the solution of eq 1,the values of D / r 2 were calculated and are reported in 0 1987 American
Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987 2545 Table 11. Apparent Diffusivity Coefficient ( D / r 2 )of n -Decane in Molecular Sieve 5A from Different Nonadsorbing Solvents at 298 K 104D/r2, 10aD/r2, solvent cm-' solvent cm-' p-xylene 0.7 ethylbenzene 1.9 m-xylene 0.7 methylcyclohexane 2.6
10
08
o -n- HEPTANE A - n NONANE
-
Table 111. Effect of Preadsorbed Water on Apparent Diffusivity Coefficient of n -Decane in Molecular Sieve 5A from p -Xylene presorbed water molecules/cavity 104D/r2, cm-'
-n - DECANE - n - OODECANE -n-TRIDECANE
e 0
A 02
0.0
0.8 5.0
0
120
40
200
360
280
440
520
TIME I N MfNUTES
Figure 1. Sorption uptake curve for n-paraffins from p-xylene solvent on molecular sieve 5A at 298 K.
- METHYLCYCLOHEXANE
A - ETHYLBENZENE
-2
o
04
0
-p-XYLENE
A -m-XYLENE
c
0
83
160
240
320
400
480
TIME I N MINUTES
Figure 2. Sorption uptake curve for n-decane from different nonadsorbing solvents at 298 K on molecular sieve 5A.
1
1
MOI/CAVITY M O L / CAVITY
MOI/CAVITY
O2
t
values are much lower when m- and p-xylenes are used as the solvent. The apparent diffusivity values for n-decane in the zeolite sample containing preadsorbed water are shown in Table 111. The rate of sorption as well as the values of apparent diffusivity shows a sharp decrease when the number of carbon atoms in the paraffin molecule increases beyond 10. Since it is known that the activation energy of diffusion increases with the carbon number (Vavlitis et al., 1981), one expects a gradual decrease in diffusivity with the carbon chain length. The present observation therefore suggests a change in the intracrystalline diffusional behavior of n-paraffins beyond n-decane. In zeolite, 5A, the n-paraffins are adsorbed in the supercage of diameter 11.4 A. Each supercage is separated from the similar adjacent cage by a window of 4.2-A diameter formed by eight oxygen atoms. During the diffusion of a paraffin molecule through these windows, the movement of the sorbate molecule is constrained due to an appreciable energy barrier. It has been shown (Jasra and Bhat, 1987)that paraffin molecules having 10 or more carbon atoms are adsorbed in zeolite 5A in such an orientation that the same molecule extends into the two parallel eight-oxygen windows of the supercage. The molecule thus experiences an additional interaction which probably is responsible for the decrease in diffusivity of the higher paraffins. The influence of presorbed water on paraffin diffusivity also indicates that the movement of higher paraffin molecules through zeolite A cages is hindered even in the presence of trace amounts of presorbed water (Karger and Pfeifer, 1987). The effects of solvents on the sorption uptake and diffusivity appear to be due to a surface barrier effect since other factors such as intercrystalline transport and bulk diffusion are overcome during the experiment by constant stirring and choosing the solvents of similar viscosity, molecular weight, and density. The adsorption of the solvent molecules at the external surface of zeolite crystallites can adversely affect the transport of the paraffin molecule into the internal surface of the zeolites. The interaction of the external surface of the zeolite is stronger with the xylene and ethylbenzene molecules than with the paraffin molecules because of the ir electrons of the aromatic ring. Consequently, the aromatic solvent molecules are expected to be preferentially adsorbed on the external surface, forming an adsorbed layer which may act like a barrier to the diffusion of the sorbate molecules into the zeolite pores. On the other hand, when methylcyclohexane is used as the solvent, n-paraffins will be preferentially adsorbed on the external surface and the solvent molecules will not give rise to any surface barrier. In other words, one should expect greater resistance to the diffusion of paraffin molecules in aromatic solvents than in the me-
r
0
L
&I3
120
200
280
360
440
0.7 0.4 0.14
520
TIME I N MINUTES
Figure 3. Sorption uptake curve for n-decane from p-xylene solvent on molecular sieve 5A containing preadsorbed water.
Tables I and 11. The values of D / r 2 for n-paraffins are seen to decrease with an increase in the carbon chain length. Interestingly, a similar carbon number dependence of diffusivity has been observed by Vavlitis et al. (1981) for the lower hydrocarbons as well. The influence of the nature of the nonadsorbing solvent on the diffusivity of n-paraffins is shown in Table 11. In conformity with the sorption rates, the apparent diffusivity of n-decane is the highest in methylcyclohexane, closely followed by that in ethylbenzene. The apparent diffusivity
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thylcyclohexane, which indeed is observed in the present study. Although this explanation may not take into account all the complex processes occurring at the zeolite surface, one can visualize the surface barrier effect on diffusivity. In conclusion, the diffusivity of n-paraffins in 5A zeolite shows a unique dependence on the hydrocarbon chain length. The presence of those molecules in the adsorption medium which can strongly interact with the external or internal zeolite surface also tends to suppress the sorption rates of the n-paraffins. These observations will have significance in the liquid-phase adsorptive separations of n-paraffins from hydrocarbon feedstocks. Acknowledgment We are grateful to Dr. T. S. R. Prasada Rao for helpful discussions and to A. J. Pate1 for technical assistance. Valuable suggestions by Dr. K. K. Chaudhary and Dr. P. R. Char as well as computational assistance by B. Hegde are gratefully acknowledged. Nomenclature D = diffusivity Dlr’ = apparent diffusivity Qt, Qo = amount of sorbate present in the zeolite at time t and 0, respectively Q m = amount adsorbed at equilibrium r = radius of the zeolite crystallite t = time of sorption, s Registry No. HzO, 7732-18-5; C,, 142-82-5; C9, 111-84-2; Clo, 124-18-5; C12, 112-40-3; C13,629-50-5; 4-H3CCsH,CH3, 106-42-3;
3-H3CC6H4CH3,108-38-3; C6H&H2CH3, 100-41-4; methylcyclohexane, 108-87-2.
Literature Cited Aleksandrov, G. G.; Larinov, 0. G.; Tschmutov, K. V. Zh. Fiz. Khim. 1967, 41, 1511. Barrer, R. M. In Characterization of Porous Solids; Gregg, S., Sing, K. S. W., Stoechli, H. F., Eds.; Society of Chemical Industry: London, 1979. Barrer, R. M. In Zeolite-Science and Technology; Romoa, R. F., Rodrigues, A. E., Rollmann, L. D., Naccache, C., Eds.; Martinus Nijihoff Boston, 1984; p 261. Broughton, D. B. Chem. Eng. Prog. 1968, 64, 60, Broughton, D. B. “Adsorptive Separation-Liquids” In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1978; p 563. Caro, J.; Bulow, M.; Karger, J. AIChE J. 1980,26, 1044. Csicsery, S. M. In Zeolite Chemistry and Catalysis, Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, D.C., 1976; p 615. Jasra, R. V.; Bhat, S. G. T. Zeolites 1987, 7, 127. Karger, J.; Caro, J. J . Chem. SOC.,Faraday Trans. 1 1977, 73,1363. Karger, J.; Pfeifer, H. Zeolites 1987, 7, 90. Ruthven, D. M. AIChE Symp. Ser. 1984,80, 21. Vavlitis, A. P.; Ruthven, D. M.; LoughIin, K. F. J. Colloid Interf. Sci. 1981, 34, 526. Wolf, F.; Pilchowski, K. Chem. Technol. 1971, 24, 672. IPCL Communication No. 103.
Raksh V. Jasra, Thirumaleswar S. G . Bhat* Research Centre Indian Petrochemicals Corporation Limited Vadodara 391 346, India Received for review May 8, 1986 Revised manuscript received April 30, 1987 Accepted July 20, 1987
Operation and Testing of a Novel Catalytic Reactor Configuration for the Conversion of Methanol to Hydrocarbons The experimental testing of a novel pseudoadiabatic reactor for the conversion of methanol to gasoline is described. This reactor concept provides an always increasing temperature profile along the longitudinal axis in a cocurrently cooled packed-bed catalytic heat exchanger/reactor. A mini pilot-plant-scale unit reactor, 2 m long and 2 cm in diameter, was built according to this concept and successfully tested. It was shown that the pseudoadiabatic operation is feasible and compatible with very high methanol conversion. The quality of the hydrocarbon products obtained was satisfactory, the liquid fraction having a research method octane number of 95-97. In the present paper, it is shown experimentally that the pseudoadiabatic operation of a packed-bed catalytic reactor is a feasible approach to the process of conversion of methanol to gasoline. The pseudoadiabatic operation brings about considerable advantages with respect to other operational policies, such as the dual adiabatic reactor scheme used in the industrial-scale MTG process developed by Mobil, and can be easily achieved without penalizing productivity. The novel pseudoadiabatic reactor belongs to the nonadiabatic reactor type, cooled by a heat-transfer medium. These reactors have in common an axial temperature profile normally exhibiting a maximum or “hot spot”. The problem of sensing hot spots becomes critical in multitubular reactors in which a nonboiling coolant is circulated in the shell side of the reactor tube bundle. Particularly when the circulation is of the “cross-flow”type, each tube of the bundle has a hot spot located at a different axial position (de Lasa et al., 1981; Zaman et al., 1985). 0888-5885187 2626-2546$01.50/0
The pseudoadiabatic operation of fixed-bed catalytic reactor develops for a fully cocurrent circulation of reactants and coolant. It was first described by Soria Lopez et al. (1981) and subsequently by de Lasa (1987), and it appears to be an attractive approach to the challenge posed by the hot spot problem. As discussed in recent contributions (de Lasa, 1983; de Lasa et al., 1985, 1986; Ravella et al., 1986), the pseudoadiabatic operation of a tubular packed-bed reactor is a peculiar operating regime in which the axial temperature profie is constantly increasing along the reactor. Therefore, the temperature maximum is located exactly at the reactor outlet. Such a configuration was named pseudoadiabatic operation for all temperatures (or simply PO). It was shown (de Lasa et al., 1986) that the transition between the PO regime and the hot spot operation is progressive rather than sudden. When the limiting temperature and reactant partial pressure are exceeded, the thermal profiles inside the reactor start exhibiting a maximum in the axial di0 1987 American Chemical Society
