A Kinetic Model for the Isomerization of n-Butene to lsobutene Vasant R. Choudhary and L. K. Doraiswamy* Communication No. 1684, National Chemical Laboratory, Poona, india
The catalytic isomerization of n-butene to isobutene over fluorinated q-alumina (containing 1.0 wt % F) has been investigated in a continuous stirred gas-solid (rotating basket type) reactor in the temperature range 300-435°C and at total pressure of 0.92 atm. The use of the stirred reactor helped in the elimination of external mass (and heat) transfer resistances. In order to eliminate pore diffusional resistance, small size catalyst particles (40-60 mesh) were used. "Perfect mixing" in the reactor was confirmed by studying the mixing characteristics of the reactor both under reacting and nonreacting conditions, by following the conversion as a function of stirring speed and by a tracer (step-up) technique, respectively. The external mass (and heat) transfer effects were quantitatively estimated by using the Ford-Perlmutter method and from the mass (and heat) transfer correlations developed for this reactor. Analysis of the kinetic data was done using both the power law and the Langmuir-Hinshelwood (Hougen-Watson) models. The provisionally proposed Hougen-Watson model for the temperature range 300-400°C is based on the adsorption of n-butene (single site). controlling, while at 435°C the model based on the desorption of isobutene (single site) holds good. The models reduce to first-order kinetics after removing the adsorption constants which are not significant. Both the models (i.e., proposed Hougen-Watson models and first-order power law model) fit thk kinetic data adequately. A carbonium ion based mechanism of the catalyst action has also been discussed.
Introduction The isomerization of n-butene to isobutene over solid catalysts is an industrially important process and has been carried out commercially for the past 30 years. Many patents and publications are available describing the catalyst and processes for the production of isobutene from n-butene. A comprehensive review based on the literature published during the past 30 years on this reaction (including its thermodynamics) has been prepared by Choudhary (1971). Surprisingly little information is available in the literature on the kinetics of this reaction which could form the basis for the rational design of a commercial reactor. Pis'man and coworkers (1965, 1968) reported the kinetics of this reaction over fluorinated y-alumina containing 0.36 wt YO fluorine, but their work was limited to the determination of activation energy (21 kcal/mol for the forward reaction, 22.7 kcal/mol for the reverse reaction) only. There has been no attempt made so far to explain this reaction on the basis of a heterogeneous model. The present work was therefore undertaken to propose a plausible Hougen-Watson (Langmuir-Hinshelwood) type rate model for this reaction over fluorinated 7-alumina containing 1 wt % fluorine, selected in an earlier study by the group screening technique (Choudhary and Doraiswamy, 1971). The data were obtained in a rotating basket continuous stirred gas-solid reactor developed by Choudhary and Doraiswamy (1972) for determining the precise values of the rates without the influence of heat and mass transfer resistances. The advantages of continuous stirred gassolid reactors for studies in kinetics and catalyst evaluations over the other laboratory reactors have been discussed in detail by Carberry (1964) and Choudhary and Doraiswamy (1972). Experimental Section Starting Materials. n-Butene (99.5% purity) was prepared by the catalytic dehydration' of 1-butanol over basic alumina. The procedure for the preparation of n-butene has been described elsewhere (Choudhary and Doraiswamy, 1971). The catalyst, fluorinated ?-alumina containing 1 w t % F, was prepared by impregnating 7-alumina (activated a t
650°C in air in a fluidized bed reactor) with ammonium fluoride from a solution containing the appropriate amount of fluorine, drying at 110°C for 10 hr, and activating at 450°C in the presence of air. 7-Alumina was prepared by precipitating aluminum trihydrate (P-trihydrate, the crystal phase of which was confirmed by ir and X-ray diffraction studies) from aluminum nitrate solution by ammonium hydroxide at pH 8.2-8.5 at room temperature (30°C), washing, filtering, drying the precipitate at 110°C for 72 hr, and then igniting it in air at 650°C. Some of the important physical properties of the catalyst have been evaluated and are given in Table I. Reactor Assembly. A diagrammatic sketch of the reactor assembly is shown in Figure 1. The reactor used for carrying out the kinetic runs was a rotating basket continuous stirred gas-solid reactor made of stainless steel. Essentially it was a cylindrical vessel, 10 cm i.d. and 10 cm high, provided with inlet and effluent parts and a cooled agitator shaft and seal (Teflon packing gland). Three vertical baffles were placed 120" apart close to the inner wall. The basket unit, affixed to the agitator shaft, consisted of four cylindrical paddles provided with stainless steel wire mesh (120 mesh) and was suitable for holding catalyst of any form and size. Two single propellers, one above and the other below the catalyst basket unit, were also secured to the shaft. A thermocouple well was situated within the reactor close to the inner wall. Detailed designs of this stirred reactor and the catalyst basket have been described by Choudhary and Doraiswamy (1972). The other parts of the system consisted of capillary flow meters to measure gas flow rates, a water condenser to condense liquid products, and a gas collector with mercury seal and water displacement arrangement for collecting the gaseous products. Experimental Procedure. The temperature of the reactor was controlled by adjusting the power input to the heating element (Kanthal ribbon wound on the body of the reactor) by a powerstat and was measured by a chromel-alumel sliding thermocouple. The stirrer speed was raised to 2660 rpm by changing the pulleys of the drive motor and the stirrer shaft. The catalyst (40-60 mesh), freshly activated at 450°C in air, was placed inside the basket and the heating started while stirring in the presence of nitrogen, and after attainInd. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
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6 THERMOWEU 7 DRIVE WTER
e
WATER COMENSER
9 LlPUlO PRODUCT COLLECTOR
IO GAS COLLECTOR
Figure 1. Experimental set-up.
Table I. Physical Properties of the Catalyst [Fluorinated ?-A1203 Containing 1Wt % F] 1. Particle size 2 . Bulk density
3. Particle density 4 . External surface a r e a 5. BET surface a r e a 6. Pore volume 7. Porosity 8. Average pore radius
+ 60 mesh [av. 0.034 cm] 0.795 g/ml 1.507 i 0.008 g/ml 67.8 cm2 150.1 m2/g 0.385 0.005 ml/g 0.590 51.7 A
-40 t o
ing the required temperature nitrogen was switched off and n-butene was introduced. Sufficient time (2.5-3.0 hr) was allowed to reach steady state with respect to temperature, flow rate, and conversion (or outlet concentration), which was confirmed by analyzing the exit gas periodically during the course of the reaction. The runs were conducted by keeping the temperature constant and varying the n-butene flow, while allowing sufficient time (about 2.0 hr) for attaining steady state after every change in the flow rate. The liquid products, mostly due to polymerization of isobutene, and the gaseous products were collected in a liquid receiver and a gas collector, respectively. The analysis of the products has already been discussed (Choudhary and Doraiswamy, 1971). The catalyst was found to maintain its full activity for nearly 12 hr (half-life 62.5 hr), but as a matter of routine it was replaced after every 8 hr. In the organization of the experimental program for determining the rate parameters, the following ranges of the main variable were covered: temperature, 300-435°C; flow rate, 0.20-0.70 g-mol/hr; time factor ( W / F ) , 3.0-20 g hr/g-mol. Check for Homogeneous Reaction. Before arriving at a plausible catalytic mechanism for the reaction, it is necessary to ascertain the extent of homogeneous reaction, if any. Several runs were carried out a t the temperatures (300-435°C) and in the range of flow rates employed in the kinetic runs without the catalyst a t 2660 rpm. Homogeneous isomerization of n-butene to isobutene and polymerization of isobutene (except a little cracking a t higher 228
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
temperatures) were found to be absent. The data obtained by using a catalyst can, therefore, be directly used for the correlation of catalytic reaction rates. Calculation of Kinetic D a t a . The rate of reaction was calculated from the relation y=-
X [WPI
where r is the reaction rate corresponding to the concentration of the exit gas, X is the fractional conversion. and W/F is the time factor. The partial pressures of the reactant and products were directly obtained from their analysis in the exit gas. Flow rates through the reactor were based on the exit gas flow and were determined from the measured volume of the exit gas collected in the gas collector but making necessary corrections due to liquid product. The conversions were calculated from the feed and the product analysis. Mass (and Heat) Transfer An absolute requirement of the stirred gas-solid reactor for obtaining accurate kinetic data is that it should be operated under gradientless conditions. This means that there should be the minimum possible partial pressure drop (Aplp,) and temperature drop ( A T ) between the bulk gas phase and the catalyst surface. That the above requirements are met should be confirmed before employing the stirred reactor for studying kinetics. In the present study, the fulfillment of the above requirements was established under both reacting and nonreacting conditions. Homogeneous Mass Transfer in the Gas P h a s e (Mixing). Mixing in the bulk gas was found to be perfect by using a tracer technique (step response) within the range of flow rates and at the stirring speed (2660 rpm) employed in the. kinetic runs (Choudhary and Doraiswamy, 1972). A residence time distribution study of this reactor has been described in detail by Choudhary (1971). “Perfect mixing” was further confirmed under reaction conditions by carrying out conversion runs as a function of stirrer speed at different temperatures. Figure 2 shows the effect of stirrer speed on the conversion of n-butene to isobutene a t different temperatures. The initial portions of the plots show appreciable dependence of conversion on stirrer speed due to imperfect mixing and/or interfacial mass transfer resistances between the gas and catalyst
221
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1
,
1 4
0 -FOR
A p / p OBTAINED BY FORDPERLMUTTER METHOD AT 300*,360* AND 425.C RESPLY.
20
-FOR A p / p OBTAINED FROM MASS TRANSFER CORRELATIONS AT 42S.C 0 -FOR
A T OBTAINED BY HEAT TRANSFER CORRELATIONS AT 42S.C
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f
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500
1000
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2000
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3000
Figure 2. Dependence of the conversion of n-butene to isobutene on stirring speed.
phases, which is considered in the next section. It can be seen that the minimum speed required to eliminate mass transfer resistance (homogeneous and heterogeneous) is higher at higher temperatures, which is in agreement with the observation of Ford (1963). From Figure 2 the mass transfer resistances appear to be negligible above 1500 rpm. Mass Transfer at Gas-Solid Interface. The resistances to mass transfer from the bulk gas to the internal catalyst surface are due to external film diffusion and pore diffusion. These resistances must be eliminated (or minimized) to the maximum extent possible. The resistance due to pore diffusion was overcome by using small size catalyst particles (-40 to +60 B.S. mesh). In order to be sure about the absence of pore diffusional resistance, the rate data have been tested by using the Weisz-Prater criterion (Weisz and Prater, 1954). The results showed that the rate data are free from pore diffusional effects. The mass transfer effect due to gas film resistance was evaluated by two independent methods: (1) from the mass transfer correlation j , = 2.0 x 10-3[Re]'0.70
I
I
I
500
1000
1500
STIRRING
S P E E G , RPM
( 2)
developed for this stirred reactor by Choudhary and Doraiswamy (1972), and ( 2 ) by following the conversion as a function of stirrer speed (Figure 2 ) . The partial pressure gradients ( A p / p g ) estimated from the above mass transfer correlation (eq 2) are plotted as a function of stirrer speed in Figure 3. At the stirrer speed employed for the kinetic runs (2660 rpm), A p / p g was found to be negligibly small ( (E,, Ei) (EcoveralIr = E, + Ei - Et) and the overall activation energy is negative.” The decrease in the net rate of polymerization as temperature increases may also be due to unfavorable polymerization equilibrium,
+
Heterogeneous Modelling of the Isomerization Reaction The heterogeneous modelling in the present study was restricted to the isomerization reaction because the partial pressures of the polymer (liquid) product could not be determined accurately due to its molecular weight distribution. Further, the rate constants of the polymerization reaction (Table 111) are very large compared to those for the isomerization reaction (i.e., 1221 < 1212
