An application of controllability assessment in process engineering

An example is presented of a plant which was built but did not work and had to be expensively modified. Subsequent investigation using multivariable control analysis highlighted fundamental problems and suggested how the plant might have been simply modified to render it more controllable. The feed to the plant consists of mixed hydrocarbons which are pre-heated, vaporized and partly superheated by heat exchange with the reactor outlet stream which has to be reduced in temperature prior to separation in a distillation train. The feed is further superheated by the furnace and then passed into the reactor where a catalytic isomerisation reaction takes place. The reaction is substantially thermally neutral and the reactor itself has a large thermal time lag for temperature changes. The delay for flow changes is short, of the order of 5 minutes