NEW TWO-STAGE PROCESS CONVERTS BUTADIENE TO ETHYLBENZENE

In recent years, there has been a growing surplus of butadiene. DSM Research B.V. has developed to a semitechnical scale, a two-stage process to produce ethylbenzene from butadiene (Fig. 1). Process engineer Harrie A.M. Duisters and research chemist Johan G.D. Haenen recently described the process. The first step is the liquid-phase dimerization of butadiene to vinylcyclohexene. This step is carried out using an iron dinitrosyl-on-zinc catalyst. Butadiene conversion is more than 95% and

March 29, 1993

5 min read

In recent years, there has been a growing surplus of butadiene. DSM Research B.V. has developed to a semitechnical scale, a two-stage process to produce ethylbenzene from butadiene (Fig. 1).

Process engineer Harrie A.M. Duisters and research chemist Johan G.D. Haenen recently described the process.

The first step is the liquid-phase dimerization of butadiene to vinylcyclohexene. This step is carried out using an iron dinitrosyl-on-zinc catalyst. Butadiene conversion is more than 95% and selectivity is 100%.

In the second step, vinylcyclohexene is dehydrogenated to the end-product ethylbenzene in the gas phase, using a magnesium-oxide-based palladium catalyst. With 100% conversion, selectivity of greater than 95% is achieved for this reaction.

The process design of this new ethylbenzene route is fairly simple, which translates to relatively low investment costs. Based on the 1992 Western European situation, a 100,000 metric tons/year (mty) ethylbenzene plant is economically feasible, according to Duisters and Haenen. And if butadiene prices drop to cracking value, the new process becomes even more attractive.

PROCESS DESCRIPTION

Fig. 2 shows a general process flow diagram.

The ethylbenzene plant consists of two sections. In the first section, butadiene is converted to vinylcyclohexene by catalytic dimerization. In the second section the vinylcyclohexene, purified by distillation, is dehydrogenated and purified to the end-product ethylbenzene.

The liquid butadiene is transported from battery limits to the dimerization reactor together with a 2 wt % solution of the Fe(NO)2C1 catalyst in tetra-hydrofuran, plus zinc powder. The stirred reactor is operated in the liquid phase at approximately 80 C.

The pressure is determined by the partial pressures of the reactants present in the reactor. With a typical butadiene conversion of 95%, the pressure is a maximum of 5 bar. The small amount of catalyst and zinc powder present in the reaction product are separated before the product is further handled by using a decanting centrifuge or a settler.

The liquid phase-consisting of tetrahydrofuran, the main product vinylcyclohexene, and unconverted butadiene-is then further routed to a distillative separator. Only a few trays are needed because of the large difference in volatility of the components being separated.

Vinylcyclohexene is separated as the bottom product. As the butadiene dimerization is 100% selective, the bottom product is on-specification and can be further processed directly.

The top stream of the column consists of tetrahydrofuran and butadiene. The butadiene is recycled to the dimerization reactor while the tetrahydrofuran is reused in the catalyst synthesis.

The spent catalyst is separated in the centrifuge or settler. It must be dried before further processing by a catalyst firm. Fresh catalyst, incidentally, is produced batchwise.

The vinylcyclohexene from the dimerization section is fed to the dehydrogenation reactor as a vapor at approximately 260 C. The reactor consists of a fixed bed of palladium on magnesium-oxide extrudates. The adiabatic temperature rise of the dehydrogenation reaction is 155 C.

For reasons related to the selectivity of the reaction and the lifetime of the catalyst, reaction temperatures greater than about 350 C. should be avoided. This can be done by properly designing the process with an adequate reactor configuration. A good choice, say Duisters and Haenen, would be a cooled multitube reactor, with dilution of the reaction gas using nitrogen or methane.

Because low pressure has a positive effect on the selectivity of the reaction, the reactor is operated at almost atmospheric pressure.

The dehydrogenation reaction is very fast. The conversion of vinylcyclohexene is complete within 10 sec. The selectivity to ethylbenzene is 95-98%, depending on the outlet temperature. The only by-product formed, apart from hydrogen, is ethylcyclohexane.

After cooling and condensing the reactor effluent, the hydrogen is separated in a gas-liquid separator. Next, the ethylcyclohexane (top) is separated from the ethylbenzene (bottom) in a vacuum column with structured packing. The design and the operation of this column are determined by the desired specification of the ethylbenzene (

This top stream can be mixed with the gasoline pool. With a suitable arrangement of coolers and reboilers, substantial heat recovery is possible between the dehydrogenation reactor and the distillation column.

ECONOMICS

Duisters and Haenen cite low raw material price as the major advantage of the new ethylbenzene route over the conventional route, which is based on benzene and ethylene. Over the past several years, the price of butadiene has regularly dropped below $200/metric ton. Moreover, the process design is fairly simple, and this means that investment costs are relatively low.

Butadiene is principally produced as a by-product of steam cracking, and is found in the mixed-C4 stream. Depending on the ethylene plant operating rate, the nature of the feedstock, and the cracking severity, the amount of butadiene produced varies from 15,000 to 75,000 mty. The actual surplus to be used for ethylbenzene production can be much smaller.

Because of the economy of scale, say the researchers, the new ethylbenzene process is economically competitive with the large-scale conventional process (typically 400,000 mty) only when the butadiene surplus of crackers is combined, or when butadiene prices drop to cracking value.

The scale at which butadiene upgrading to ethylbenzene becomes economically attractive depends strongly on the raw material and product prices. With recent butadiene prices in Western Europe, a 100,000-mty plant is now economically feasible.