1724
Ind. Eng. Chem. Res. 1998, 37, 1724-1728
Mass-Transfer Effects in Liquid-Phase Alkylation of Benzene with Zeolite Catalysts C. Ercan,* F. M. Dautzenberg, C. Y. Yeh, and H. E. Barner ABB Lummus Global Inc., 1515 Broad Street, Bloomfield, New Jersey 07003
The alkylation of benzene with light hydrocarbons over an acidic catalyst is an important industrial process. Metal halides and solid phosphoric acid have been the most commonly used acid catalysts for this purpose, while zeolite catalysts are excellent, environmentally clean alternatives. Although liquid-phase alkylation with certain synthetic zeolites has been established commercially (such as in catalytic distillation), little has been published on the fundamental parameters governing the reaction kinetics. This paper examines the effect of external (liquid-solid) and internal (pore diffusion) mass transfer on the main reaction of benzene alkylation with ethylene and propylene using various zeolite catalysts. The results and its implications on the effective catalyst utilization will be presented. This constitutes one of the series of basic studies in an effort to optimize aromatic alkylation reactor systems such as catalytic distillation. Introduction The alkylation of benzene with ethylene and propylene to produce ethylbenzene (EB) and cumene, respectively, is widely used in the petrochemical industry. EB is an important intermediate for the production of styrene while cumene is for the production of phenol and acetone. AlCl3 and “solid phosporic acid” are the common conventional catalysts used in industry for EB and cumene, respectively. The use of both catalysts give rise to many environmental problems such as handling, safety, waste disposal, and corrosion. Much effort has been put into developing alternative environmentally friendly catalyst systems, and zeolite-based catalyst systems have been replacing the conventional catalysts in both processes. The new alternative technologies, commercially proven CD-EB and CD-Cumene processes, for producing ethylbenzene (EB) and cumene by alkylation of benzene with light olefins involve an acidic zeolite catalyst combined with a novel three-phase reactor concept, catalytic distillation (CD). Catalytic distillation, in which gas and liquid in countercurrent fashion are contacted with a solid catalyst, incorporates a threephase catalytic reactor with distillation. The details and the advantages of catalytic distillation are explained by Smith (1981 and 1984) and Smith et al. (1994). Due to the heterogeneous nature of the system, a number of mass-transfer steps have to occur before reaction species are converted into a product. A reliable design and optimization of this complex system enhance an adequate reactor model. The simplest reactor model should recognize the multiphase nature of the system and account for the following phenomenon occurring within the reactor: mass transfer between gas and liquid phases, mass transfer between liquid and solid phases, pore diffusion within the solid, and chemical reaction. This paper summarizes a method of determination of kinetic parameters and examines the effects of liquidsolid mass transfer and pore diffusion, using various * To whom correspondence should be addressed.
Figure 1. Schematic diagram of differential recycle reactor. Table 1. Catalysts Used and Properties catalyst Fp, kg/m3 p τp
cumene
EB
beta and Y zeolite 1000.0 0.5 5.0
Y zeolite 1000.0 0.5 5.0
zeolite catalysts in a liquid phase, laboratory, differential fixed-bed reactor with recycle. Experimental Section The reactor, 0.024 m in diameter, was a differential fixed-bed reactor with recycle. The reaction medium was liquid phase and the reaction was performed at isothermal conditions. The reaction temperature were 190 and 170 °C for EB and cumene, respectively. The amount of catalyst was adjusted such that the conversion per pass was less than 10. The reactor pressure, which was around 25 bar, was kept high enough so that only one phase (liquid) was present in the reactor. Feed mass flow rate was around 0.10-0.11 × 10-3 kg/s, and the recycle-to-feed ratio was about 30. The recycle rate was high enough to give a completely mixed (CSTR) reactor performance. A mixed feed of benzene and olefin (ethylene and propylene) was prepared, measured by both a scale and metering pump and fed into the reactor. Dissolved olefin concentration in the fresh feed was in the range of 3.5-5.0 wt %. A schematic diagram of the reactor with recycle is shown in Figure 1. Commercial zeolite catalysts, which are in 1.6-mm extrudate form, were used. The smaller sizes were obtained by crushing. Two types of zeolite catalysts, Beta and Y zeolite, for cumene and only Y zeolite for EB at three different particle sizes were tested. Further
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Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1725 Table 2. Summary of Tests and Conditions test no.
system
catalyst (zeolite)
dp × 102 (m)
ap (m2/kg)
W × 103 (kg)
Q (m3/s)
R
τ (kg of cat s)/m3)
1 2 3 4 5 6 7 8 9 10
cumene cumene cumene cumene cumene cumene cumene EB EB EB
Beta Beta Beta Y Y Y Y Y Y Y
0.106 0.089 0.034 0.106 0.106 0.090 0.034 0.196 0.126 0.034
5.51 6.74 17.65 5.51 5.51 6.74 17.65 2.97 4.62 17.13
1.00 0.33 0.14 1.00 1.00 0.33 0.14 1.00 1.00 1.00
1.55 1.51 1.51 1.52 1.53 1.52 1.52 1.61 1.62 1.60
31.3 31.9 32.1 31.8 31.6 31.7 31.8 31.7 31.5 31.7
201 67 27 201 201 67 27 191 191 191
Table 3. Assessment of Mass-Transfer Effects test dp × kLsap × rA,app × Mears’ Weisz-Prater no. 102 (m) (m3/(kg s)) (kmol/(kg s)) criterion criterion 103
1 2 3 4 5 6 7 8 9 10
0.106 0.089 0.034 0.106 0.106 0.090 0.034 0.196 0.126 0.034
2.47 3.34 14.16 2.47 2.47 3.34 14.16 1.36 2.63 18.28
106
9.83 20.51 48.48 10.18 9.63 24.63 66.97 6.46 8.64 11.66
0.42 0.37 0.63 0.63 0.65 0.56 0.45 0.18 0.23 0.10
)
(2)
()
dp 2 2 , 1.0 DA,effCA,s
rA,appFp
Only the main reactions for the formation of ethylbenzene and cumene were considered. The role of side reactions was ignored since, in both cases, the selectivities were very high. In the reactor effluent, trialkylbenzene was not observed, and the ratio of dialkylbenzene to monoalkylbenzene was less than 3.0 wt %. It looks like this ratio does not show any trends with particle size. Since benzene was in excess, the reaction rate was assumed to have first-order dependency on olefin concentration. From the olefin mass balance around the reactor, the following equation for the apparent reaction rate was obtained:
(
dp n 2 < 0.15 kLsCA
The Weisz-Prater criterion states that when the following inequality holds, pore diffusion is not important:
Reaction Rate
1 CA,F - CA τ 1+R
()
rA,ppFB
145 111 33 256 268 196 87 78 80 17
information on catalyst properties, which are limited due to the commercial nature, were summarized in Table 1. The reported catalyst diameters in Table 1 are equivalent sphere diameters, and the porosities and tortuasities reported in Table 1 are assumed values. The summary of the tests and conditions were presented in Table 2.
rA,app )
Mears’ criterion states that when the following inequality is satisfied, external mass transfer is negligible:
(1)
The values of R and τ used in the estimation of rA,app were presented in Table 2. Although the relation given above was used to determine the apparent reaction rate, the recycle rate in all the tests reported was high enough to assume perfect mixed reactor (CSTR) behavior. The calculated rA,app values were summarized in Table 3. The initial impression is that rA,app values for cumene is higher than those for EB and for cumene the Y zeolite looks more active than the Beta zeolite. Assessment of Mass-Transfer Effects Before we proceeded with data analysis, first, the importance of the liquid-solid (external) mass transfer and the pore diffusion were checked by using the Mears’ and Weisz-Prater’ criteria, respectively (Fogler, 1986).
(3)
Thus, the liquid-solid mass-transfer coefficient and the effective pore diffusion coefficient are needed to assess the importance of mass-transfer effects. The liquidsolid mass-transfer coefficient were predicted from the correlation given by Thoenes and Kramers (1958). The bed porosity, which is needed for mass-transfer calculation, was assumed as 0.4. The effective pore diffusion was estimated from the following relation:
()
DA,eff ) DAB
p τp
(4)
with the assumed catalyst particle porosity and tortuasity which were tabulated in Table 1. Liquid physical properties, such as viscosity and the binary molecular diffusion coefficient of ethylene and propylene in benzene, were predicted from the correlations given by Reid et al. (1977). Results, summarized in Table 3, show that pore diffusion has strong effects on both the cumene and ethylbenzene reaction rates. The external mass transfer has substantial influence on the cumene reaction, but it has only slight effect on the EB reaction. Since the cumene reaction rate is faster than that of EB, a more pronounced effect of both mass-transfer resistances on cumene is as expected. Prediction of the Contribution of Mass-Transfer Effects The first-order apparent reaction rate constant, k1,app, which was determined from the apparent reaction rate correlation given earlier, is influenced by both external mass transfer and pore diffusion and can be written in the following form:
1 1 1 ) + k1,app kLsap ηk1
(5)
The first term in the right-hand side represents the external mass-transfer contribution and the second term
1726 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 4. Contribution of External Mass Transfer % contribution to overall resistance test k1,app × 103 kLsap × 103 ηk1 × 103 kinetic + ext. mass no. (m3/(kg s)) (m3/(kg s)) (m3/(kg s)) pore diff. transfer 1 2 3 4 5 6 7 8 9 10
0.60 0.68 1.57 0.89 0.92 1.04 3.53 0.14 0.33 1.06
2.47 3.34 14.16 2.47 2.47 3.34 14.16 1.36 2.63 18.28
0.79 0.86 1.76 1.39 1.46 1.52 4.69 0.59 0.38 1.12
76 80 89 64 63 69 75 90 87 96
24 20 11 36 37 31 25 10 13 4
Overall Effectiveness Factor By combining the effects of external mass transfer and pore diffusion, an overall effectiveness factor, which is an indicator of how effectively the catalyst is being utilized in the reactor, can be defined as described below. The observed reaction rate is
Table 5. Particle and Overall Effectiveness Factors test no.
dp × 102 (m)
φs
η
ηo
1 2 3 4 5 6 7 8 9 10
0.106 0.089 0.034 0.106 0.106 0.090 0.034 0.196 0.126 0.034
49 38 12 86 90 66 30 27 28 7
0.060 0.077 0.227 0.034 0.033 0.044 0.095 0.107 0.105 0.382
0.045 0.061 0.227 0.022 0.021 0.031 0.071 0.096 0.091 0.360
represents the intrinsic kinetics combined with the pore diffusion. Using this relation, the external masstransfer contribution, kLsap, was subtracted from the apparent rate constant. The results as well as the percent change in going from k1,app to ηk1 (a combination of the intrinsic rate constant with the pore diffusion), and the share of the each resistance, in the overall resistance were summarized in Table 4. As predicted by Mears’ criterion, the contribution of the external mass-transfer resistance to the apparent reaction rate is substantial for cumene, a little higher for the Y zeolite than Beta zeolite. However, this contribution is small for EB. In both cases, this contribution gets smaller with decreasing particle size. A decrease of 68% in particle size produces a 1.2 times increase in the value of ηk1 for cumene with Beta zeolite while more or less a similar increase (0.0.9 times) in ηk1 for EB with Y zeolite is obtained by an 83% reduction in the particle size. Since, for cumene, Y zeolite is more active than Beta zeolite, a similar decrease (68%) in the particle size produces more change in the value of ηk1 for Y zeolite (more than 2.0 times) than that for Beta zeolite. As mentioned earlier, the Weisz-Prater criterion tells us whether the pore diffusion is important or not. However, the extent of pore diffusion is estimated from the particle effectiveness factor. The Thiele modules and the corresponding effectiveness factor for a spherical particle and for first-order reaction rate are given by the following relations (Fogler, 1986):
φs ) η)
( )x
( )(
dp 2
Fpk1 DA,eff
(6)
)
3 φs coth φs - 1 φs φs
As predicted by the Weisz-Prater criterion, the calculated values for the effectiveness factor are very low. For the cumene reaction, the pore diffusion has strong effect, stronger for Y zeolite than Beta zeolite, while for EB its effect is relatively less, but still significant. The catalyst utilization due to pore diffusion limitation is between 3% and 23% for cumene, 3% and 10% for Y zeolite and 6% and 23% for Beta zeolite, and 11% and 38% for EB, depending on the particle size.
(7)
The calculated Thiele modulus and the effectiveness factor, η, were summarized in Table 5.
rA,app ) (1/kL,sap + 1/ηk1)-1CA
(8)
and the maximum reaction rate theoretically possible in the reactor is given by:
rA,max ) k1CA
(9)
Then, the overall effectiveness factor is defined as
ηo )
rA,app rA,max
(10)
After a simple mathematical manipulation, the following relations for the overall effectiveness factor, in terms particle effectiveness factor and Thiele modulus, are obtained respectively;
(
ηo ) η 1 -
ηo )
( )
ηo kLsap k1
)
( )
(
(11)
ηo 3 (φs coth φs - 1) 1 2 kLsap φs k1
)
( )
(12)
The equations establish the relations between the overall effectiveness factor and particle effectiveness factor or Thiele modulus. In these equations, the ratio of the external mass-transfer coefficient to the reaction rate constant appears to be an important parameter. Table 5 summarizes the results for the overall effectiveness factor. The overall catalyst utilization, relative to the particle effectiveness factor, for cumene drops substantially, even more for Y zeolite than Beta zeolite, while the change for the EB catalyst is only slightly. The objective should be to maximize the catalyst utilization in the reactor by increasing the overall effectiveness factor. The ratio, kLsap/k1, is a very important tool in reaching this objective. Figure 2, which is the plot of ηo versus the Thiele modulus, demonstrates the idea very well. The practical way of increasing the overall effectiveness factor is either to increase the kLsap/k1 ratio or to increase the kLsap/k1 ratio while decreasing the Thiele modulus simultaneously. Increasing only the catalyst activity, k1, is not effective. The Thiele modulus for commercial size EB and cumene catalysts are already a large number and will be larger with increasing catalyst activity. The
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1727
Figure 2. Overall effectiveness factor as a function of Thiele modulus.
reduction of the catalyst particle size is an effective way of increasing the kLsap/k1 ratio while decreasing the Thiele modulus simultaneously, as is shown in Figure 2. Discussion The cumene reaction is relatively fast. Referring to Table 4, as the catalyst activity (ηk1) for cumene is increased (from Beta to Y zeolite), the concentration difference between the bulk fluid and the catalyst surface becomes significant, which means that the external mass-transfer resistance becomes important. Furthermore, as the catalyst activity increases from ethylbenzene (with a Y-type catalyst) to cumene with a Beta catalyst, and to cumene with a Y-type catalyst, the contribution of the external mass-transfer resistance to the overall becomes significant. In the limiting case of an extremely active catalyst, the concentration at the outside surface of the catalyst approaches zero and the external mass transfer becomes the rate-limiting step. For moderately fast reactions, like EB, the rate can be increased by reducing the particle size or by altering the pore structure. Higher fluid velocity will increase the external mass-transfer coefficient. Decreasing particle size within practical considerations might increase the rate. Also, the shape of the catalyst might have influence on the pore diffusion as well as on the external mass transfer. It is mathematically proven that for the first-order isothermal reaction, spherical particles have the lowest particle effectiveness factor of all possible shapes with the same volume. For very fast reactions, the external mass transfer is the rate-limiting step. Here, rate is a strong function of parameters which control the liquid-solid volumetric mass-transfer coefficient, such as fluid velocity and external surface area of the catalyst. It is believed that the cumene case lies somewhere between the moderately fast reaction and very fast reaction case. The cumene rate determined with a large Y-type catalyst is controlled by around 40% external mass transfer. Increasing both the volumetric mass-transfer coefficient and reducing the pore diffusion resistance should have a positive impact for cumene. For the pore diffusion effect, what was said for EB in the previous paragraph is also valid for cumene; in addition, the remedies
discussed for easing the external mass-transfer limitation apply. The cumene process can benefit from particle size reduction in two ways. First, the particle effectiveness factor increases, and second, the liquidsolid volumetric mass transfer coefficient increases, mainly due to the increase in external surface area of the catalyst. For both cumene and EB, the effective catalyst utilization is very low due to the pore diffusion. When the particle effectiveness factor is very low, such as less than 10%, only a small outside shell of the catalyst is effectively utilized (10%). An important issue to keep in mind is that the above analysis is based on the laboratory data which is obtained in a liquid-phase differential fixed-bed loop reactor. The commercial unit involves a multiphase reactor with gas-liquid mass transfer in addition to completely different hydrodynamics. The commercial operation also involves catalysts particles stacked into bales with cloth and wire mesh around them. Thus, it is expected that the liquid-solid mass transfer is lower than the one observed in a single-phase fixed-bed operation. Indeed, analysis of a commercial EB reactor shows that the external mass-transfer resistances contributes about 50% of the total resistance controlling the process, whereas the laboratory data as discussed shows a much lower influence. This is clear evidence that laboratory data should only be used for catalyst screening and for obtaining intrinsic kinetics. This work represents the first step toward developing an understanding of these new EB and cumene processes. Nomenclature ap: external surface area for catalyst (m2/kg) CA: olefin concentration (kmol/m3) CA,F: olefin concentration in feed (kmol/m3) CA,s: olefin concentration at catalyst surface (kmol/m3) DAB: molecular diffusion coefficient (m2/s) DAB,eff: effective diffusion coefficient within the particle (m2/ s) dp: particle diameter (m) k1: intrinsic first-order reaction rate constant k1,app: apparent reaction rate constant (m3/(kg s)) kLsap: liquid-solid (external) mass-transfer coefficient (m3/(kg s)) mF: feed mass flow rate (kg/s) n: reaction order R: recycle ratio, q/Q rA,app: reaction rate for olefin (kmol/(kg s)) rA,max: maximum reaction rate (kmol/(kg s)) q: recycle volumetric flow rate (m3/s) Q: feed volumetric flow rate (m3/s) u: superficial velocity (m/s) W: catalyst weight (kg) Greek Letters p: porosity of the catalyst particle η: particle effectiveness factor ηo: overall effectiveness factor FB: bed density (kg/m3) Fp: density of the catalyst particle (kg/m3) τ: space time, W/(Q + q), ((kg s)/m3) τp: tortuosity factor φs: thiele modulus for sphere
Literature Cited Fogler, H. S. Elements of Chemical Reaction Engineering; PrenticeHall: Englewood Cliffs, NJ, 1986.
1728 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Reid, C. R.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977. Smith, L. A. U.S. Patent 4,307,254, assigned to CR&L, 1981. Smith, L. A. U.S. Patent 4,443,559, assigned to CR&L, 1984. Smith, L.; Chen, J.; Dautzenberg, F. M.; Sy, A. A Greener Technology for Cumene, AIChE Annual Meeting, San Francisco, CA, November 1994.
Thoenes, D.; Kramers, H. Mass Transfer from Spheres in Various Regular Packing to a Flowing Fluid. Chem. Eng. Sci. 1958, 8, 271.
Received for review November 19, 1997 Revised manuscript received February 5, 1998 Accepted February 7, 1998 IE970797B
