Control of an Isomerization Column/Reactor Process

The control of a distillation column with a side reactor is studied for the conversion of n-butane into isobutane for use in the alkylation process. Fresh feed of a mixture of propane, isobutane, n-butane, and isopentane is fed to a distillation column. A vapor sidestream is withdrawn in the stripping section of the column and fed to an isomerization reactor in which some of the n-butane is converted to isobutane. Reactor effluent is fed back into the column in the rectifying section. Isopentane is removed in the bottoms, and moderate-purity isobutane is removed in the distillate. The control of the purity of the isobutane product can be achieved in two alternative ways: manipulating sidestream flow rate to the reactor or manipulating reflux ratio. The latter method is demonstrated to be better because manipulating sidestream flow rate presents potential control problems due to the existence of multiple steady states.