Design and production planning for multiproduct dehydration plants

Abstract

Drying operations constitute an important field of chemical engineering, which is still governed by empiricism. This paper deals with two important aspects related to the construction and operation of dehydration plants. The design problem involves the determination of process flowsheet structure, when a nominal production level is specified for all products processed in the plant. The production planning problem describes the production policy of the plant within a long-range planning horizon, under time-varying product demand and market prices of raw materials and saleable products. The policy adopted assigns production levels and duration of production runs for each product processed, in each one of the plant processors, at a certain time period, within the planning horizon. The objective in each approach is to optimize the total annual profit resulting from the construction of a new plant or the operation of an existing one. The process was described by deducing the mathematical model of conveyor-belt dryers. For the forementioned problems, appropriate formulations were developed and studied. The most general model presented involves numerous integer and continuous decision variables and a large number of space variables and constraints, resulting in cumbersome calculations and tremendous computational load. For the reduction of the computational effort, a shortcut modeling of total annual plant cost was proposed and evaluated from the operational data of a large number of possible flowsheet structures, through a simple analytical equation. The parameters of the proposed shortcut equation were estimated by nonlinear regression over an extensive number of computed points; each one of them was determined by solving an NLP optimization problem. The design and production problems were formulated as MINLP problems in which use of the shortcut cost equation reduced drastically the computational effort involved. Related problems (i.e. process modification and design under production planning criteria) were also taken into consideration. Characteristic examples were presented in order to demonstrate the effectiveness of each proposed approach.