Control of integrated unit operations

Abstract

As a thermal separation method, distillation is one of the most important technologies in the chemical industry. Given its importance, it is no surprise that increasing efforts have been made in reducing its energy inefficiencies. A great deal of research is focused in the design and optimization of the Divided-Wall Column. Its applications are still reduced due to distrust of its controllability. Previous references studied the decentralized control of DWC but still few papers deal about Model Predictive Control. In this work we present a decentralized control of both a DWC column along with its equivalent MPC schema.Keywords: Process Control, Thermally coupled distillation, Model Predictive Control. Main TextBasically, in every production process some of the chemicals go through at least one distillation column on their way from raw species to final product. Distillation is and will remain the main separation method in the chemical industry (there are more than 50,000 columns in operation around the world). Despite its flexibility and widespread use, distillation is very energy demanding. It can generate more than 50% of plant operating costs and it is the responsible of 3% of the energy usage in the U.S. (notice that the thermodynamic efficiency of a distillation column is between 5-20%). In order to reduce this drawback new approaches and configurations have appeared. The divided-wall column (DWC, the name is given because the middle part of the column is split into two sections by a wall) is an important example of process intensification and integration. DWC is very appealing to the chemical industry, with Montz and BASF as the leading companies. Kenig et al. state that there are more than 125 industrial applications nowadays and if the exponential trend continues there will be more than 350 by 2015 [1]. DWC can separate three or more components in one vessel using a single condenser and reboiler, hence reducing capital and operating costs compared to conventional twocolumn sequences. In fact, DWC can save up to 30% in the capital invested and up to 40% in the energy costs, particularly for close boiling species. DWC is considered to be on the path for energy conservation and green house gases emissions decrease. DWC is not widespread due to distrust of its controllability. Its control is more difficult than the control of a conventional schema with two columns for the separation of ternary mixtures because there is more interaction among control loops. Besides, the absence of controllability could mean the absence of the energy savings if the optimal operation is not accomplished. The remaining of the paper is organized as follows. Section two discusses thermally coupled distillation columns, making emphasis in DWC columns. Section three presents the control strategies used in the paper. Section four applies decentralized (PID controllers) control and MPC to a ternary system separation. Finally, section five draws conclusions and introduces further work.

Control Strategies of DC Microgrids Cluster: A Comprehensive Review

“…In this approach, each MG uses its own local measurements, and control actions are verified through local controls without any communication links. As a result, adding or removing an MG has no impact on the activities of other MGs in the system [65]. Several studies have been conducted to exploit the lack of dependence on communication links to transform information among MGs.…”