PROCESS DESIGN: HEAT INTEGRATION-CONCLUSIONS

The ways of calculating energy requirements, that is of setting up energy balances around processing operations, and their basis in the first law of thermodynamics were introduced in the first chapter of this book. Anyone faced with designing or operating a food processing plant will have come answer. Two such problems are:

• how can we make best use of the various hot streams throughout a plant?

• what is the absolute minimum amount of energy or work that is needed for a particular set of operations?

The second law of thermodynamics is needed to answer these questions, but the first of them can in fact be solved with little new theory beyond that developed on Chapter 1. This chapter has focused on that problem, that is, of energy integration, and has outlined the principles and techniques of 'pinch' technology which can be employed in its solution. The thermodynamic principle underlying the technique is that heat energy cannot flow 'uphill': in other words, heat is only transferred down a temperature gradient. Since the rate of heat transfer (and ultimately therefore the size of any heat exchanger) is proportional to the driving force it is not economically between a hot and cold stream to zero: in reality a minimum temperature approach (typically 15 to 20 degree) is chosen. The problem is then addressed by preparing composite curves representing the temperature versus heat energy or enthalpy of the hot and cold streams respectively, and adjusting these to maximize the heat recovery from one to the other. The pinch point is the point at which the composite curves come closest to each other, i.e. the minimum temperature approach. The method is illustrated graphically by a simple example, and its extension to more complex and more realistic problems is briefly outlined.

You should now appreciate the difference between the first and second law of thermodynamics, and be able to analyse plant so as to recognize where opportunnities for heat recovery might arise. Whilst the methods of handling complex problems are beyond the scope of this text, we believe that an understanding of the principles behind the technique is of great importance to anyone involved with designing or operating real processes, where energy conservation is economically and environmentally more and more important.

--Chemical Engineering for the Food Industry, D. Leo Pyle (Author), Peter J. Fryer (Author), Chris D. Reilly (Author), Page 248-249