Optimal design of a system of distillation columns

Abstract

Systems of distillation columns for separation of multicomponent mixtures are highly metaland energyintensive. Therefore, in optimal design of dis� tillation units, the optimality criterion is generally economic criteria, including investment and operating costs. The investment cost is proportional to the metal and content of the plant and depends on the number of separation stages and the reflux ratios (reflux flows) in distillation columns. The cost of a column increases with an increase in the number of separation stages (trays) and, hence, in the column height. An increase in the reflux ratio initially leads to reduction in the col� umn cost because the number of trays decreases. How� ever, further enhancement of internal liquid and vapor flows in the column causes an increase in the column diameter, which increases the column cost. The oper� ating cost is constituted by the costs of arrangement of vapor flows in columns and heating and cooling of intermediate and end flows. The operating cost increases with an increase in the reflux ratio. Conse� quently, from the energy standpoint, distillation should be performed at minimum reflux. At the same time, at minimum reflux, the number of separation stages is maximal. Hence, the main problem of opti� mal design of distillation units is to find a tradeoff between the investment (lumpsum) cost of installa� tion and the operating cost of operation (1-3). The issue of optimal design of distillation columns was considered in a large number of works. This issue can be split into at least two problems. The first prob� lem is to find the optimal sequence of distillation col� umns that ensures the minimal cost of separation. The second problem is to optimize a distillation plant of a given topology that minimizes the total (investment and operating) costs. In this work, we propose a new approach to solving the second problem. The optimal design problem is posed as follows. Given are the parameters (flow rate, composition, temperature, pressure) of multicomponent feed sup� plied for separation, the topology (sequence of distil� lation columns) of the separation plant to be designed, and requirements for quality of end products. It is nec� essary to design a system of distillation columns, namely, to determine the number of trays in each col� umn, feed points, and column operation modes at which the reduced cost criterion (sum of operating and investment costs) takes the minimal value and the requirements for quality of end products are met. Let us consider a distillation plant as a single chem� ical engineering system consisting of N distillation columns the optimal design of which is proposed to be performed using a global approach. In this case, the design problem reduces to discretecontinuous opti� mization of the entire system simultaneously, in which discrete variables are the numbers of trays of the exhausting and rectifying sections of each column and continuous variables are operating parameters.