Dimerization of iso-butene is a key step in the production of the environmentally friendly fuel additive, iso-octane. Iso-octane is obtained from the dimerization of C₄-olefin followed by the hydrogenation of the formed dimer. Commercial process for the dimerization has disadvantages of further side reactions such as trimerization and oligomerization. To suppress the formation of the side products, the process has been carried out in reactive distillation and the enhancement in the selectivity has been proved experimentally.

In this work we present the detailed nonlinear dynamic effect in the reactive distillation process. The simulation of the reactive distillation column using equilibrium stage model has been reported. A 21-staged column (condenser+ reactive stages(3)+non-reactive stages(16)+reboiler) has been used for the purpose. The process involves eight components and five consecutive reactions: dimerization of isobutene to di-isobutene, trimerization of isobutene with di-isobutene to form tri-isobutene, oligomerization of isobutene to form tetramer, reaction of isobutene with 2-butene to form cross-dimer and isomerization of 1-butene to 2-butene. Column configuration, design and operating parameters and the kinetics parameters involved in the process is obtained from our previous work (Talwalkar et. al, 2007). The simulation uses a concentration based kinetics and vapor-liquid equilibrium is based on the UNIQUAC model.

The presence of multiple steady state has been observed in the simulation of the column. Although, the governing kinetics is simple and concentration based, five different steady states have been identified as shown in Figure, which is a bifurcation diagram of concentration and selectivity versus Damkohler number as a parameter. Some of the steady states may not be realized in a real plant, however the studies merit attention in view of the design of suitable control strategy and smooth column operation. The detailed analysis shows that the presence of some of the side reactions causes the self inhibition of dimerized product, causes the multiplicity and affects the selectivity drastically. Broad region of the multiplicity has been seen in the parameter space of the Damkohler number, pressure and reflux rate. To the best of our knowledge this is for the first time that non-linear dynamics of a column with multiple reactions is being studied.

Reference: Talwalkar, S.; Mankar S.; Katariya A.; Aghalayam P.; Ivanova M.; Sundmacher K.; Mahajani S. Selectivity Engineering with Reactive Distillation for Dimerization of C₄ Olefin: Experimental and Theoretical Studies. Ind. Eng. Chem. Res. 2007, 46(10), 3024-3034.