Modeling, Simulation, and Thermally Optimization of Thermally-Coupled Distillation Columns

Introduction

Distillation is one of the most common and important processing separation techniques. Among all of the common units, distillation consumes about 3% of the world energy (Khalifa & Emtir, 2009; Gadalla et al. 2007). Regarding this disadvantage many ideas have been presented. Optimization of operating parameters such as reflux, pressure and feed stage, perform maintenance programs like using insulation, instrumentation and two-stage condenser, distribution of heating and cooling in column and thermal coupling methods are some solutions to decrease energy requirement in distillation (Saber et al., 2009; Anderson et al., 2000; Engelien, 2003).

Thermal coupling method is one of the most widely used procedures. In this method, heat is integrated over the tower, and at a specific operating condition, various streams are applied for heating and cooling each other. HIDIC, Petlyuk, Kaibel columns, industrial and common configuration of thermally coupled distillation follow this technique (Ghadrdan et al., 2011; Engelien, 2004). In this paper, a thermally coupled distillation column versus two simple columns is studied. The thermally coupled distillation column includes two columns with different pressures where the condenser of a high pressure tower provides the re-boiler duty of low pressure column. In this structure, the boiling point of upstream from high pressure column should be more than the boiling point of downstream from low pressure column (Agrawal. 2000). Figure 1 shows a direct two simple columns distillation. In a Direct Sequence (DS), the volatile component exits from top of the first column while, and for an Indirect Sequence (IS), the heaviest component exits as the bottom product of the first column. Figure 2 shows the Direct Sequence with Forward (DSF) and Backward (DSB) integration of an industry thermally coupled distillation for a ternary separation. In the forward integration, the directions of flow and energy integration are the same and in backward (reverse) integration, the integration of energy is in the opposite direction of flow (Engelien, 2004).

Figure 1.

Direct split column arrangement (Engelien, 2004)

Figure 2.

Multi-effect distillation for direct split, (a) Direct split with forward integration, (b) Direct split with backward integration (Engelien, 2004)

Background

Many researches were studied on optimization of energy in thermally-coupled distillation. Some of them are mentioned here, briefly.