Temperature Difference Control of a Four-Product Dividing-Wall Distillation Column

Abstract

The four-product extended Petlyuk dividing-wall distillation column (FPEP-DWDC) can save about 50% energy as compared with the conventional direct and indirect sequences. However, the complex structure of FPEP-DWDC results in strong interactions among different control loops and highly nonlinear behaviors. For the control of the FPEP-DWDC, temperature control (TC) scheme cannot work to handle the disturbances inserted into the process and none of the proposed control schemes can avoid the employment of composition controller (which is not recommended in industrial processes) in the opening articles. The proposed TC scheme in the current work cannot handle the disturbances either. Therefore, a kind of temperature difference control (TDC) scheme for the FPEP-DWDC and simplified temperature difference control (STDC) are proposed. In the proposed TC, TDC, and STDC schemes for the FPEP-DWDC, only temperature controllers are needed for the product quality control. In order to be less complicated than the TC scheme in the opening literature, minimum allowable controllers are used in the proposed TC, TDC, and STDC schemes. One TC/TDC loop for each split is used to reduce interactions among different control loops. One TC/TDC loop is installed in the prefractionator to ensure the A/D sharp split and keep the impurity components leaving from the top and the bottom of the first divided wall. Two TC/TDC loops are arranged in the intermediate distillation column to guarantee the A/C sharp split and the B/D sharp split and keep the impurity components leaving from the top and the bottom of the second divided wall and the third divided wall. Three TC/TDC loops are settled in the main distillation column to ensure the A/B sharp split, the B/C sharp split and the C/D sharp split. An illustrative example of the FPEP-DWDC separating a mixture of methanol, ethanol, n-propanol and n-butanol are adopted to evaluate the effectiveness of the proposed TC, TDC, and STDC schemes while a wide range of feed disturbances are confronted. The outcomes show that the dynamic performances of the FPEP-DWDC are substantially enhanced in most cases by using the proposed TDC and STDC schemes, which is an encouraging result for the industrialization of the FPEP-DWDC.