اثرات تقسیم تغذیه و یکپارچه سازی حرارت در ترتیبات کلاسیک بر روی به حداقل رساندن هزینه در جدایی از مخلوطی سه تایی

Multi-component separation by distillation is known as a conventional method in chemical processes. In this study the effects of feed splitting on three classical arrangements (with and without heat integration) were investigated in order to reduce the total annual cost (TAC). Ternery mixture separation (propane, butane and pentane) by distillation was selected as a case study. An optimization problem was defined based on the effective parameters affecting the total annual cost. Optimum values for decision variables were calculated by applying the genetic algorithm. Calculation results showed that the heat integration procedure in the feed splitting method reduced the TAC significantly. It was also indicated that the prefractionator arrangement with heat integration resulted in the greatest decrease in the total annual cost.

In recent years, significant attention has been focused on saving energy in chemical industries. This can be attributed to the increase in energy cost and environmental issues. The energy recovery procedures can save large amounts of operating costs in chemical processes [1] and [2]. Distillation columns are used for separation as a basic part in many chemical industries. Since distillation consumes significant amount of heat, many activities have been focused on saving energy in distillation columns. From a technical and economical point of view, high energy consumption is one of the important weaknesses of distillation columns. High level of energy consumption leads to increase in operating costs. Process integration has been known as a proper method for reducing the energy consumption and has attracted lots of interest in chemical processes. Heat integration is used in order to design integrated distillation configurations with energy savings in condensers and reboilers of the compound distillation columns. The required duty in the towers can be supplied by the cold/hot streams available from the other parts in the plant [3]. Many techniques have been implemented in chemical industry to save energy such as thermal coupling, pinch method, and feed splitting [2], [4], [5], [6] and [7]. There are three classical arrangements for separation of ternary mixtures; direct split, indirect split and prefractionation [8] and [9]. Integrated prefractionator arrangement gives the highest energy saving compared to the other schemes. The separation process in multi-effect distillation was accomplished in two columns operating at low and high pressure. Appropriate arrangements decreased operating costs in different processes [8] and [10]. Distillation works based on the difference in boiling points of the components. Preheating the feed has been known as a common procedure for saving energy in industrial distillation towers. Heating whole or a portion of the feed with one of the column streams was introduced as an appropriate way for energy saving in distillation columns. Soave and Feliu [7] showed that feed splitting and preheating of the feed by the bottom stream can result in a considerable energy saving in a binary mixture (benzene and propane). Three different arrangements were considered among them was the feed splitting [7]. Feed splitting and preheating (58% of the feed) resulted in decrease in rebolier duty while condenser duty remained almost unchanged. The main objective of this work is to propose an optimum scheme for ternary distillation systems considering all effective variables. The effects of feed splitting in three classical arrangements are investigated. Total annual cost is defined as the objective function to be minimized applying genetics algorithm (GA). Genetic algorithm is an attractive technique used to solve optimization problems in chemical processes. Optimal design of coupled systems for separation of multicomponent mixture was achieved by GA [11] and [12]. In the present study the modified HYSIM inside-out Methods were used by HYSYS® simulator instead of the shortcut methods used in previous investigations to increase the accuracy. As it was indicated in [13] shortcut method has higher errors because of its simple assumptions.

In this study different arrangements for a ternary separation system was considered and optimized by using genetic algorithm. Based on the calculations feed splitting with heat integration gives the lowest TAC among the other arrangements considered in this work. Heat integration was another procedure which was discussed in the paper and the results showed that the feed splitting arrangement with heat integration reduced TAC. 1. The feed splitting prefractinator arrangement with heat integration was selected as the best scheme. TAC in this arrangement compared to the best direct arrangement without integration decreases 49.219%. 2. The feed splitting in the direct arrangement (even without integration) gives better results (lower TAC) compared to the direct arrangement with integration. 3. The prefractionator arrangement without heat integration compared to the feed splitting direct arrangement with heat integration gives more reduction in TAC. 4. TAC reduction in the prefractionator arrangement without feed splitting with heat integration was (44%).