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Ind. Eng. Chem. Res. 2002, 41, 4936-4940
KINETICS, CATALYSIS, AND REACTION ENGINEERING The Etherification of Methanol and Isobutene in a Catalytic Distillation Column Packed with Zeolite-Beta-Coated Catalytic Packings Yonghong Li,* Shaobing Yu, and Xigang Yuan National Key Laboratory of C1 Chemical Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China
Haitao Wang Tianjin Bohai Chemical Industry Group Corporation, Tianjin 300040, People’s Republic of China
Zeolite-beta-coated catalytic packings were used to improve the etherification of methanol and isobutene in a catalytic distillation column. The steady-state composition profiles and the characteristics of the reaction in the catalytic distillation column were investigated. The zeolitebeta-coated packing has better characteristics and an activity that is comparable to that of the conventional resin catalyst in both catalysis and the separation of reactants from products. The optimal operating conditions for the catalytic distillation process are a pressure of 0.7 MPa, a methanol/isobutene feed molar ratio of 1, and a reflux ratio of more than 3. The effect of the strong nonlinearity in the vapor-liquid equilibrium of the reactive system on the conversion of isobutene was analyzed on the basis of the experimental results for both catalytic distillation and conventional distillation. There is a valley in the boiling-point temperature surface of the ternary mixture of methanol/isobutene/MTBE. This valley, as a separatrix, divides the boilingpoint temperature surface into two regions. Although the conventional distillation profiles are restricted by the valley, the catalytic distillation system is not because the composition profiles in the reaction-separation section can cross the valley. A higher methanol/isobutene feed ratio can result in a lower conversion of isobutene and less MTBE obtained, which is different from what happens in a reactor. In this case, the product in the stripping section will run back up to the reaction zone so that the positive reaction is restrained. To obtain a high conversion of isobutene, a lower methanol/isobutene feed ratio should be used. 1. Introduction The etherification of methanol and isobutene in a catalytic distillation column packed with acidic ionexchange resin catalysts for the synthesis of methyl tertbutyl ether (MTBE) is well-known. Despite the resin’s good catalytic activity for the reaction, its thermostability and selectivity are poor. During the past decade, zeolites have gained attention as suitable catalysts for this etherification reaction.1-6 Acid zeolites, especially zeolite beta, have shown good activity that is comparable to that of conventional resin catalysts. In addition, the thermal stability and selectivity of the zeolites were much higher.4,5 To date, most catalytic distillation columns have been packed with a number of containers filled with catalyst particles, which makes prompt removal of the product from such a reaction zone difficult. In addition, the pressure drop in a column with containers is much higher than that in a column with conventional packing so that the energy consumption is increased. Therefore, * To whom correspondence should be addressed. Telephone.: 022-27405825. E-mail:
[email protected].
it is expected that an configuration involving catalytically active zeolite coating on structured packings (called zeolite-coated catalytic packing) in the reaction zone of the catalytic distillation column could beneficially be used to replace the present operation configuration. In fact, zeolite crystals can be made to grow on the surface of porous ceramic supports to form the zeolitecoated catalytic packings.7-10 Such packings have advantages for catalytic distillation in that (a) optimal vapor-liquid mass transfer is obtained, (b) suitable active sites are provided, and (c) they are not easily broken. It appears that the application of zeolite-beta-coated catalytic packing to catalytic distillation for the etherification of methanol and isobutene is favorable because of its good activity and selectivity for this etherification reaction. Therefore, it is important to know about the relationships between the characteristics of the catalytic packing and the optimal operating conditions for the catalytic distillation, but such issues have not been addressed up to now. This paper presents experimental work on the characteristics of zeolite-beta-coated catalytic packing used
10.1021/ie010755u CCC: $22.00 © 2002 American Chemical Society Published on Web 08/31/2002
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Figure 1. XRD spectrum of the zeolite beta coating.
in a catalytic distillation column for the etherification of methanol and isobutene and a comparison between this packing and conventional resin catalyst. In addition, the effects of the strong nonlinearity in the vaporliquid equilibrium of the reactive system on the conversion of isobutene and the composition profiles in the catalytic distillation column are analyzed. 2. Preparation and Characteristics of Zeolite-Beta-Coated Catalytic Packing First, porous alumina ring supports (5 mm × 5 mm with an internal diameter of 2 mm) were cleaned in boiling aqueous hydrochloric acid solution (0.1 M) for 30 min, then rinsed with deionized water, and dried at 110 °C for 2 h. Next, the structured supports were coated in situ with zeolite in a reactor. The templating agent was tetraethylammonium bromide (TEABr, obtained from Xingfu Chemical Factory, Beijing, China). The silica source was SiO2 particles (0.254-0.318 mm, obtained from Haiyang Chemical Factory, Qingdao, China), the aluminum source was NaAlO2 solution (3.73 M, homemade), and the sodium source was NaOH solution (5.74 M, homemade). The zeolite synthesis solution had a molar composition of Al2O3:30 SiO2:4 TEA+:6 Na2O:260 H2O. The synthesis was performed by submerging the supports in the synthesis solution, heating the reactor to 140 °C at autogenous pressure for 80 h, and then reducing the temperature to 30 °C at a rate of 0.5 °C/ min. After synthesis, the packings were rinsed with deionized water; dried at 100 °C for 4 h; and calcined to remove the templating agent by heating to 550 °C at 1 °C/min, maintaining this temperature for 3 h, and reducing the temperature to 30 °C at 1 °C/min. The samples were treated in an ultrasonic bath for 15 min. The above procedures were repeated three times, to yield a zeolite-beta-coated packing. The thin coating had a SiO2/Al2O3 ratio of 30.5. The zeolite coating was further treated by ion exchange with an aqueous 1 M NH4Cl solution at 90 °C for 4 h, rinsing with deionized water until all Cl- had been removed (checked by AgNO3), drying at 120 °C, calcining at 500 °C for 3 h, and steaming with 0.1 M acetic acid vapor carried by N2 at 350 °C for 4 h. Finally, the structure of the thin coating crystal was determined by XRD (shown in Figure 1). The external surface of the coating was examined using SEM (shown in Figure 2) and compared with the external surface of the support (shown in Figure 3). 3. Catalytic Distillation Experiment using Zeolite-Beta-Coated Catalytic Packings Catalytic distillation experiments were carried out in a laboratory-scale stainless steel column (diameter )
Figure 2. External surface of the zeolite-beta-coated packing.
Figure 3. External surface of the support.
30 mm). The column consists of a total condenser and a reboiler, as well as (1) a rectifying section below the condenser (height ) 0.2 m), (2) a reaction-separation section (catalytic packing zone) below the rectifying section (height ) 1.1 m), and (3) a stripping section below the catalytic packing (height ) 0.5 m). Triangle stainless steel gauze packings were filled in the rectifying section and the stripping section. The zeolite-beta-coated catalytic packings were filled in the reaction-separation section. There were 15 theoretical plates per meter of gauze packing and 11 theoretical plates per meter of zeolite-coated packing, as determined by total reflux distillation with benzene and tetrachloromethane. The composition of the mixture was analyzed using a Abbe refractometer. When the catalytic distillation was carried out, methanol and isobutene were fed continuously into the reaction-separation section at the top and at the bottom of the section, respectively. The liquid streams were sampled from the condenser, the reboiler, and several sampling points at different heights of the column. The compositions of the samples were determined with an HP-4890 gas chromatograph. The composition data were, in turn, linked to obtain a composition profile (catalytic distillation line). The etherification of methanol and isobutene is an exothermic reaction. The exothermicity causes some of the liquid to be vaporized. Hence, it can change the compositions of the liquid and the vapor in the reactionseparation section. Because the temperature of the section depends on both the composition of the mixture and the pressure, the pressure will affect the reaction rate in the catalytic distillation. In addition, the reaction of methanol and isobutene over zeolite packing is a liquid-solid reaction. Sufficient
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Table 1. Results of Catalytic Distillation Experiment by Orthographic Design expt no.
P (MPa)
r (mol/mol)
R
1 2 3 4 5 6 7 8 9 effect of the factor on the conversion of isobutene
0.5 0.5 0.5 0.7 0.7 0.7 0.9 0.9 0.9 25.86
1 2 3 1 2 3 1 2 3 52.97
1 3 5 3 5 1 5 1 3 25.09
X (%) 15.38 19.11 10.72 91.25 20.49 11.06 84.96 19.53 10.88
Figure 4. Vapor-liquid tie lines and the valley of the boilingpoint temperature surface.
Table 2. Azeotropic Data of the System component or azeotrope
mole fraction of methanol in azeotrope (mol %)
boiling temperature (°C)
methanol isobutene methyl tert-butyl ether methanol and isobutene methanol and ether
5.5 32.19
64.51 -6.9 55.06 -8.61 51.23
contact of liquid reactants with solid catalysts is important. Both increasing the reflux ratio and reducing the space velocity can increase the contact. In this paper, the effect of reflux ratio on the conversion of isobutene was investigated at the fixed space velocity of 1 h-1. To study the effects of the operating conditions (e.g., pressure, reflux ratio, and methanol/isobutene feed ratio) on the conversion of isobutene, experiments by orthographic design were carried out at three levels of three factors. The experimental results are shown in Table 1. The data show that the methanol/isobutene feed ratio has a greater effect on the conversion of isobutene than either the reflux ratio or the pressure. Moreover, the effect of the feed ratio on the conversion of isobutene in the catalytic distillation column is different from that in a reactor. In the catalytic distillation process, the conversion of isobutene decreases greatly when the feed molar ratio is increased from 1 to 2, whereas the conversion of isobutene in a reactor always increases with increasing methanol/isobutene feed ratio. The appropriate operating conditions for the catalytic distillation, according to Table 1, are a pressure of 0.7 MPa, a methanol/isobutene feed molar ratio of 1, and a reflux ratio of more than 3. The experimental catalytic distillation results reveal an interesting phenomenon in that the conversion of isobutene was higher than 84% or lower than 21%, but no value in between. This phenomenon will be analyzed in next section. 4. Effects of the Vapor-Liquid Equilibrium Behavior of the Ternary Mixture on the Conversion of Isobutene The system of methanol/isobutene/MTBE is a strongly nonlinear system so that two minimum-boiling binary azeotropes occur in the system. The azeotropic data at 1 atm are shown in Table 2. In this paper, the vapor-liquid equilibrium tie lines at 0.7 MPa (shown by arrows in Figure 4) were obtained using bubble-point and dew-point calculations according to the NRTL equation. The two low-boiling azeotropes
Figure 5. Composition profiles in a continuous distillation column.
are connected by a valley (shown as a dashed line). The valley, as a separatrix, divides the boiling-point temperature surface into two regions. In one region, pure methanol is the least volatile, and the binary azeotrope of methanol/isobutene is the most volatile. If the feed composition is located in this region, the distillate will reach the binary azeotrope, and the bottom will be nearly pure methanol. In the other region, pure MTBE is the least volatile, and the most volatile species is again the binary azeotrope of methanol/isobutene, so the distillate will still reach the azeotrope, but the bottom will be close to pure MTBE. It is possible that feed compositions in different regions will result in very different bottom compositions. Continuous distillation experiments for the mixture of methanol/isobutene/ MTBE with feed compositions in the different regions were performed to examine the effects of the valley (shown in Figure 5). The valley could not be crossed by the conventional distillation profiles. In a catalytic distillation column, the difference in temperature between two plates is not an only factor to influence heat transfer and mass transfer. Because the exothermicity of the reaction can cause some of the liquid to be vaporized, so that the composition of the liquid in the reaction-separation section depends not only on the distillation conditions but also on reactive rate, the distillation profiles in this section are able to cross the valley, and the compositions of the top and of the bottom can fall in different regions divided by the valley. If such a crossing occurs, the stripping line will locate on the MTBE side of the valley. Then, the species in the reboiler will be mostly MTBE. Otherwise, the stripping line will be located on the methanol side, and then MTBE could not be obtained from the bottom of
Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 4939 Table 3. Results of Comparative Experiments autoclave zeolite beta
catalytic distillation column resin D005
zeolite-coated packing
resin packing
T (°C)
X (%)
S (%)
T (°C)
X (%)
S (%)
P (MPa)
X (%)
x (mol %)
P (MPa)
X (%)
x (mol %)
75 80 85
86.36 90.05 93.18
100 100 100
75 80 85
93.55 91.62 81.40
99.5 99.6 99.5
0.60 0.65 0.70
87.09 92.80 94.28
90.35 94.54 96.11
0.55 0.60 0.65
80.57 83.59 73.46
84.40 87.38 77.91
Figure 6. (a) Composition profile in the catalytic distillation column for P ) 0.7 MPa, r ) 1 mol/mol, R ) 3. (b) Concentration profiles of the components in the catalytic distillation column.
the column. Two typical situations of the composition profiles in the catalytic distillation column are shown in Figures 6 and 7, respectively. Figure 6 shows the crossing situation. In this case, the composition at the top of the stripping section locates in the region in which the boiling temperature of MTBE is the highest (see Figure 6a), so that MTBE is concentrated at the reboiler, and the methanol in the stripping section vaporizes and runs up into the reaction-separation section so as to improve the conversion of isobutene. The result is that the fraction of MTBE is increased from the top of the stripping section to the bottom (see Figure 6b). In contrast, Figure 7 shows the noncrossing situation, where the composition at the top of the stripping section is in the region in which the boiling temperature of methanol is the highest (see Figure 7a). Instead of MTBE, methanol is concentrated in the reboiler, while MTBE vaporizes in the stripping section and returns to the reaction-separation section. The returning MTBE restrains the reaction of methanol and isobutene, so that there some MTBE in the reboiler (see Figure 7b). It can be seen that increasing the methanol/isobutene feed ratio in catalytic distillation does not always improve the conversion of isobutene because of the valley. It is not good for the crossing of the catalytic distillation line over the valley to feed too much methanol. The preferred methanol/isobutene feed ratios are those that can make the composition at the top of the
Figure 7. (a) Composition profile in the catalytic distillation column for P ) 0.7 MPa, r ) 3 mol/mol, R ) 2. (b) Concentration profiles of the components in the catalytic distillation column.
stripping section locate in the correct region to concentrate MTBE in the stripping section. According to the experimental results shown in Table 1, this means that the optimal methanol/isobutene feed molar ratio is about 1. 5. Comparative Experiments with Resin Catalyst Comparative experiments were carried out in a 500mL autoclave and in a catalytic distillation column identical to that described for the zeolite-coated catalytic packing. Instead of zeolite-coated packings, the same amount of resin catalyst particles, packed in several bags wrapped by stainless steel corrugated gauze, was filled in the reaction-separation section. Seven theoretical plates were measured in this section. Resin D005 catalyst used in industry for the synthesis of MTBE was selected for the comparison. The experiments in the autoclave were performed at 1.5 MPa, with a methanol/ isobutene mole ratio of 3.5 and a reaction time of 40 min. Both the zeolite catalyst and the resin catalyst were 10% of the total reactant weight. The conversions of isobutene and selectivities for MTBE over the catalysts at different temperatures are reported in Table 3. The experiments in the catalytic distillation column were performed at different pressures (0.6-0.7 MPa for the zeolite packing and 0.55-0.65 MPa for the resin) to provide optimal temperature in the reaction zone for each of the catalysts, while the methanol/isobutene feed ratio and reflux ratio were constant at 1 and 4,
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respectively. The conversions of isobutene and fractions of MTBE in the reboiler are also listed in Table 3. For the liquid batch reaction, the activity of zeolite beta is the same as that of resin D005 at the optimal reaction temperature and a methanol/isobutene ratio of 3.5, but the selectivity of the zeolite is higher than that of the resin. When the zeolite is used as zeolite-coated packing in a catalytic distillation column, its activity is promoted because it has more theoretical trays than the resin does. In addition, the active sites of the zeolite, where the separation of the products and the reactants can take place, are all over the surface of the packing, so the heat can be removed from the reaction center promptly, which is important for an exothermic reaction. The zeolite-coated packing gave a higher conversion of isobutene and a larger bottom fraction of MTBE than did the resin at a methanol/isobutene mole ratio of 1 and a reflux ratio of 4. 6. Conclusions Zeolite-beta-coated catalytic packing was used for the etherification of methanol and isobutene in a catalytic distillation column. The packing has better characteristics and an activity that is comparable to that of the conventional resin catalyst in both catalysis and the separation of reactants from products. The optimal operating conditions for the catalytic distillation process are a pressure of 0.7 MPa, a methanol/isobutene feed molar ratio of 1, and a reflux ratio of more than 3. There is a valley in the boiling-point temperature surface of the ternary mixture of methanol/isobutene/ MTBE. This valley, as a separatrix, divides the boilingpoint temperature surface into two regions. Although the conventional distillation profiles are restricted by the valley, the catalytic distillation system is not because the composition profiles in the reactionseparation section can cross the valley. A higher methanol/isobutene feed ratio can result in a lower conversion of isobutene and less MTBE obtained, which is different from what happens in a reactor. In this case, the product in the stripping section will run back up to the reaction zone so that the positive reaction is restrained. To obtain a high conversion of isobutene, a lower methanol/ isobutene feed ratio should be used. Acknowledgment The financial support of the National Distillation Laboratory at Tianjin University is gratefully acknowledged.
Nomenclature IB ) isobutene Me ) methanol MTBE ) methyl tert-butyl ether P ) pressure, MPa R ) reflux ratio T ) temperature, °C S ) selectivity for MTBE r ) methanol/isobutene feed ratio, mol/mol X ) conversion of isobutene x ) fraction of MTBE in a reboiler, mol %
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Received for review September 7, 2001 Revised manuscript received March 27, 2002 Accepted March 29, 2002 IE010755U
