Process design: heat integration

Abstract

The principles of thermal balances, heat and mass transfer and fluid flow developed in the previous chapters are well suited to the outline design of flowsheets and the design of individual plant items. It is much more difficult to consider the flowsheet as a design problem, and to decide whether the whole process is optimal. The simple economic analysis developed in Chapter 1 will tell whether a plant will make money, but will not produce better designs that make more money. To a large extent, optimization still depends on the skills of the engineer designing the plant. However, in some areas considerable progress has been made in developing design techniques that search for cost-optimal solutions to problems. These techniques are computer based and complex. However, one of them is conceptually very simple, and the principles can be developed without the use of computers. Any process plant will require some heat input and need some cooling. Some of this can be provided from within the plant, i.e. the cooling need on one process stream can be provided by the heating of another; but how much? It is possible, using thermodynamic principles, to find the maximum amount of heat which can be recovered from a system, and thus to design a process which needs the minimum amount of external heating and cooling. This chapter describes the problem, and then develops the solution using a worked example to demonstrate the method.