In this paper, the energy-saving potential of a heterogeneous azeotropic dividing-wall column is investigated by demonstrating an example for the separation of pyridine and water using toluene as entrainer. The original two-column system includes a heterogeneous azeotropic column with top decanter and another column served as preconcentrator column for the fresh feed and also served as recovery column for aqueous outlet stream from decanter. It is demonstrated that this complex two-column system can be thermally coupled into a dividing-wall column with top decanter. By comparing the optimized design of this dividing-wall column with the original design, a significant reduction (29.48%) in steam cost can be obtained. Furthermore, because important control degree-of-freedoms (two reboiler duties) are still preserved in this dividing-wall column, control performance of this proposed design is found to be comparable to that of the original two-column system.

Separation of 1,2-propanediol and ethylene glycol is an important task for coproduction of these two compounds via hydrogenolysis of glycerol for the purpose of utilizing this biodiesel byproduct. These two compounds exhibit close-boiling behavior which requires many stages of a regular column and also large energy consumption to meet stringent product purity specifications. In this paper, several alternative designs are investigated in order to save steam cost and also total annual cost of this separation task. Alternative designs considered include multieffect distillation, heterogeneous azeotropic distillation, and extractive distillation. Significant reductions of 38.3% in steam cost and 30.6% in total annual cost as compared to the regular column can be obtained by a design flowsheet via an extractive distillation system using triethylene glycol as entrainer. Methods for further improving the economics of this extractive distillation system have also been investigated. A simple worthwhile improved design is to preserve the energy from the hot entrainer to preheat the fresh feed via a feed-effluent heat exchanger. With this simple improvement, a further 12.8% reduction in steam cost can be made as compared to the original extractive distillation system.

Manufacturing processes in the semiconductor and pharmaceutical industries often produce alcohol mixture byproducts. Therefore, the esterification of alcohol mixtures may be an important step in reusing wastes from these industries. There are two alternative methods for using the alcohol mixtures as feed for reactive distillation (RD). The first method separates this mixture into pure alcohols first and then follows with esterification using the RD column. The second method uses direct esterification of the alcohol mixture in a RD column, and then separates the mixed-ester products. This paper discusses the esterification of a n-butanol (BuOH) and n-amyl alcohol (AmOH) mixture with acetic acid (HAc). This study presents two important results based on optimizing the total annual cost (TAC). First, the mixed BuOH/AmOH system, with direct esterification with RD followed by product separation, is more economical than the system that first separates the mixture. Second, this study proposes a novel economical indirect-sequence design flowsheet with aqueous reflux. Another important issue in this study is the choice of the relative feed location, because the boiling point of acid lies between that of the two alcohols (i.e., BuOH < HAc < AmOH). Reaction kinetics is an important factor to be considered in determining the feed location of the alcohol mixture.

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Isopropyl acetates are important organic solvents that are widely used in the production of varnishes, ink, synthetic resins, and adhesive agents. Previous studies developed a process for the production of isopropyl acetate incorporating a reactive distillation (RD) column, a decanter, and a stripper. According to the previous study, the rectifying section of the RD column has a prominent remixing phenomenon. Furthermore, the overhead compositions of RD column and the stripper are all within the liquid−liquid equilibrium envelope. Based on the above observations, a thermally coupled design of this process is established. The key points in the thermally coupled design are: to move the location of the decanter to the stripper side, to totally reflux the organic phase outlet stream, and to sidedraw a liquid stream from the stripper to the RD column. Simulation result shows that 23.14% energy savings can be realized using the proposed thermally coupled design. The control strategy of the proposed design flowsheet is also investigated using tray temperature control loops to indirectly control the product composition. The proposed control strategy is capable of maintaining high-purity product, despite changes in feed composition and throughput.