Overview of selected case studies

The principles of heat integration have been used in food and drink processing for many years. These techniques have included, for example, vapours bleeding from multiple-stage evaporators in sugar production, and heat recovery during sterilisation and pasteurisation of milk and other food solutions. However, these principles and techniques have not been used as frequently in other branches of food processing industry and there is scope for further research in this area.

The application of pinch technology and heat integration in different sectors of food and drink processing depends on the specific character of the particular production process. As a whole, food processing is characterised by relatively low temperatures of process streams (rarely above 120-140 °C), a low number of hot streams (some with non-fixed final temperatures, for example secondary condensate of multiple-stage evaporation systems), low boiling point elevation of food solutions, intensive deposition of scale in evaporator and recovery systems, and seasonal performance. The development of heat integration is obstructed by some specific technological and design requirements, for example direct steam heating, difficulties in cleaning heat exchanger surfaces and high utility temperatures.

Nevertheless, higher efficiency, optimised heat systems and low energy consumption could be obtained and therefore reduce the quantity of environmentally hazardous emissions. Moreover, heat integration also results in technological improvements such as reduced deposition of scale due to reduced utility temperatures, self-regulation of heat process, etc.

An example of the benefits resulting from the analysis of existing heat systems is provided by the production of refined sunflower oil (Klemes et al., 1998). These production systems operate with a minimal temperature difference of 65 °C at the process pinch and use two types of hot utilities (dautherm steam and water steam) and two cold utilities (cooling water and ice water). As a result of increased heat integration and optimisation the minimum temperature difference was reduced to 8-14 °C, the heat transfer area was increased, but the hot utility and cold utility consumption was reduced considerably. An additional benefit is that there was no necessity to use water steam and cooling water as utilities, considerably simplifying the design.

Similarly, good opportunities for heat integration also exist in sites extracting raw sunflower oil (Klemes et al., 1998). A problem cited frequently with this process was the inappropriate placement of a one-stage evaporation system for separating the solvent, benzine. The problem could not be solved by changes in the pressure because of benzine's flammability and the sharp increase in the boiling point elevation. This disadvantage could be partially compensated for by an appropriately placed indirect heat pump, pumping heat from a condensation temperature level to the utility temperature level in the evaporation system.

The advantages of heat integration can also be illustrated by the case of crystalline glucose production (Klemes et al., 1998). Operating plants in this process widely use vapours bleeding from a multiple-stage evaporation system for concentrating the water-glucose solution. Pinch analysis of these processing systems showed that using vapour bleeding results in the unnecessary over-expenditure of utilities due to the fact that the multiple-stage evaporation system was inappropriately placed across the process pinch. The adjustment of the evaporation system was difficult because of restrictions in maximum boiling temperatures.

A further example of the use of improved heat integration is observed in fruit squash production. During sterilisation, heat recovery between the cold feed and the hot product is difficult because of intensive deposition of scale and the fact that it is impossible to clean the shell and tube-side heat exchange surfaces. This problem can be solved by employing single or multiple throttling of the solution using steam-ejecting compressors for vapours, intermediate utilities, etc.

In tomato concentrate production the main heat consumer is a multiple-stage evaporation system. Appropriate placement of the system is difficult because of the considerable disproportion between the heat consumption of the evaporation and recovery systems. Additionally, the available temperature difference in the evaporator is insufficient for a greater number of stages. Despite this, considerable utility savings can be obtained by increasing the two- and three-stage evaporators to four stages and by using steam-ejecting compressors.

The heat systems involved in alcohol production from grain and potato raw materials are characterised by considerable complexity, for example the availability of a large number of hot and cold streams, several (usually four) fractional distillation columns and a multiple-stage evaporation system. Applying heat integration to these heat systems confirms the efficiency of new three-stage fractional distillation systems. However, their appropriate placement requires heat pumps.

A further example of the use of heat integration and the appropriate choice of utilities is given by a case study of a whisky distillery (Smith and Linnhoff, 1988). In this case it was recognised that steam was being used incorrectly for process heating. The steam use was related to the use of a heat pump and by reducing the size of the heat pump, the steam used below the process pinch was eliminated. However, steam now had to be used for process heating above the process pinch, but overall energy costs were reduced due to the reduction in compressor duty.

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