Industrial & Engineering Chemistry Research, Vol.49, No.1, 189-203, 2010
Temperature Control of the BTX Divided-Wall Column
The control of a divided-wall column is more difficult than the control of a conventional two-column separation sequence for the separation of tertiary mixtures because there is more interaction among control loops. In a previous paper, a control Structure using four composition loops was shown to provide effective control of the purities of the three product streams and also achieve minimum energy consumption for both feed flow rate and feed composition disturbances. The numerical example Studied the separation of benzene, toluene, and o-xylene. The four manipulated variables were reflux flow rate (R), side-stream now rate (S), reboiler heat input (Q(R)), and liquid split (beta(L)) at the top of the wall. In this paper we explore the use of temperatures to avoid expensive and high-maintenance composition analyzers. Two types of temperature control structures are studied. In the first, three temperatures located in the main column and one temperature on the prefractionator side of the wall are used to adjust the four Manipulated variables. Feed How rate disturbances are well handled with this Structure, but product purities start to deviate significantly from their desired values for feed composition changes greater than about 10%. In the second control Structure, four differential temperature control loops are used. Performance is improved and disturbances of 20% in feed composition are well handled with only small deviations in product purities. This Structure also handles large changes in Column operating pressure.