Systematic approach to batch chemical plant design: application to examples

Abstract

Batch processes are widely used in chemical industry, however, in the academic world there has been traditionally more interest in the design of continuous processes than discontinuous. In the recent years, this trend is being reversed and more articles related with the design of batch chemical plants continue to appear. Along the same lines, the objective of this project is to propose a systematic method for a basic design of discontinuous chemical plants. The first step of the design is to decide the ordered sequence of unit operations that will allow to transform the raw materials into the desired products. In most cases, the laboratory recipe is taken as a reference but usually this step is not immediate, and the operations used industrially differ highly from the ones used in the laboratory. Thus, a good synthesis of the process is a very important first step in the design of the plant. The first section of the project will be a bibliographical analysis to search for patterns regarding the structure of the processes and the operations that form them. These patterns will be of help when performing the process synthesis step. After synthesising the process, a first basic design must be performed. This basic design consists of the allocation of equipment and the dimensioning of it. The introduction of the state of matter concept allows to subdivide the operations into tasks and this, in turn, allows a better treatment of the time variable, something essential to perform a good equipment allocation and dimensioning. The equipment dimensioning step is performed using an iterative calculation method explained in the project. It gives a criterion for the first approximation of the equipment capacity and then modifications on the process must be done to reach the desired production rate. The described steps are applied to an example, a chlorination of 1,2-difluorobenzene, and the dimensioning is used to illustrate the calculation method and to explain and classify the possible modifications and their effects.There are basically two ways to modify the production rate, to act upon the equipment volume and to act upon the occupancy time. These actions are effective when applied to the equipment that limits the size utilization and the one that limits the cycle time, respectively. Modifying the capacity of the limiting equipment, changes the production per batch and thus, the total production rate is modified. Changing the occupancy time of the limiting equipment modifies the cycle time, changing the campaign time and thus, the number of possible campaigns and total production rate. There are many possibilities when playing with these options. It is impossible to find the optimal design through a mathematical model since there are too many variables involved. Therefore, human decisions are essential in every step of the design. The aim of this project is to provide a methodical way to proceed and some tools that can help to make these decisions.