Comparing Composition Control Structures for Kaibel Distillation Columns

Yang Yuan,Lijing Zang,Liang Zhang,Xing Qian,Haisheng Chen,Kejin Huang,Shaofeng Wang

doi:10.3390/pr8020218

Abstract

Although Kaibel distillation columns are superior to conventional distillation sequences owing to smaller equipment investment and operation cost, they display high nonlinearity and this greatly increases the difficulty of achieving their tight control. To overcome this problem, four decentralized composition control structures, i.e., the CSR/QR, CSR/B, CSD/QR, and CSD/B structures, are proposed and compared based on the control of a Kaibel distillation column fractionating a methanol/ethanol/propanol/butanol quaternary mixture. These four composition control structures all include five composition control loops. While the four of them are employed to maintain the purity of the top, upper sidestream, lower sidestream, and bottom products, the remaining one is employed to minimize the energy consumption of the Kaibel distillation column by maintaining the composition of propanol at the first stage of the prefractionator. Dynamic simulation results show the CSR/QR and CSR/B structures can tightly maintain the purity of the controlled products with a small overshoot and short settling time after facing various disturbances in feed conditions, but the CSD/QR and CSD/B structures lead to oscillatory responses (the latter even shows divergent responses under individual disturbances). At the end of the article, some effective guides for developing composition control systems are given.

Highlights

In order to lessen the energy consumption of distillation processes effectively, various process intensification technologies have been proposed and dividing-wall distillation columns (DWDCs, c.f.,Figure 1a) are the most famous of them all [1,2,3,4,5,6]
When disturbances are imposed to the MEPB Kaibel distillation, the CSR/QR and CSR/B structures can quickly suppress the adverse effects of the disturbances on the top product and reduce the possible adverse effects caused by the failed control of the top product to the control of the upper sidestream, lower sidestream, and bottom products
If one of R and D is much larger than the other, the lager one should be selected as the manipulated variable of the composition control loop to guarantee the performance of the derived composition control structure for the control of the Kaibel

Summary

Introduction

In order to lessen the energy consumption of distillation processes effectively, various process intensification technologies have been proposed and dividing-wall distillation columns There are three noteworthy differences between the Kaibel distillation column and the conventional three-component DWDC: (1) The former separates quaternary mixtures but the latter separates ternary mixtures; (2). From the three differences above, it is not difficult to deduce a fact that the nonlinear degree of the Kaibel distillation column is higher than that of the conventional three-component DWDC. For the control of a small experimental Kaibel distillation column built to separate a methanol(M)/ethanol(E)/propanol(P)/butanol(B) (MEPB) quaternary mixture, Dwivedi et al proposed a temperature control structure including the top, upper sidestream, lower sidestream, and prefractionator temperature control loops [17]. To fractionate a benzene/toluene/xylene/heavies quaternary mixture, Tututi-Avila et al developed a Kaibel distillation column using a genetic algorithm and demonstrated its capability in energy saving [8].

Four Composition Control Structures Proposed for the Control of the Kaibel

Derivation of a MEPB Kaibel Distillation Column

Tuning of Controller Parameters

Dynamic distillation column column

Discussion

Conclusions