Modelling, optimisation and control of a falling-film evaporator : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Production Technology at Institute of Technology and Engineering, Massey University, Palmerston North, New Zealand
Falling-film evaporators in the dairy industry are key process units where most of the water is removed in the production of milk and whey powders. Evaporators of two to eight effects are common in the dairy industry. Mechanical vapour recompression is widely used to increase the energy efficiency of the evaporation process. A Thermal vapour recompression is used to control the final total solids concentration exiting the evaporator at Fonterra-Ingredients, Whareroa. Previous research into the performance of dairy evaporators has focused largely on milk evaporators (Choudhary, 1996; Runyon et al., 1991; Quaak & Gerritsen, 1990; Quaak et al, 1994; Winchester, 2000). The performance of the whey evaporator at whey products was completely different (see Chapter-9) to the performance with milk products (Winchester, 2000). This could be due to the difference in the physical and chemical properties (discussed in Chpter-3).The aim of the current study was to apply mathematical modelling to the optimisation and control of a two-effect thermal vapour recompression evaporator for whey products at Fonterra Ingredients-Whareroa, Fonterra Co-operative Ltd. The purpose of this study is to solve the problems (low throughput, fouling due to film break-up, increased energy consumption and unknown running conditions for each product) currently experienced with the evaporator at whey products. Figure 1.0 illustrates a general whey powder process at Whareroa (see section 2.2). The manufacture of protein ingredients from cheese and casein whey has evolved during the last fifty years into an established part of the world dairy industry (Bylund, 1995). Raw milk is variable in its composition (see Appendix A-3) and most dairy products can be produced in a variety of ways from this milk. Therefore, it is not surprising to find significant variations in reported values for the physical properties of dairy products (Bloore et al., 1981; Snoeren, 1982; Murakami and Okos, 1989; Fernandez-Martin, 1971; Jeurnink and Kruif, 1993, Antonio, 1983; Adam et al., 1994; Middleton, 1996). A major problem in the processing of whey protein ingredients is variation in final product properties (foamability and solubility index) due to factors relating to protein denaturation during processing and to the high variability of raw material composition. These interrelated factors include the source of raw material (whey), cheese manufacturing practices, heat treatment history, protein fractionation procedures and storage conditions (Bloore et al., 1981). The processing of whey powder, types of whey products and their properties are discussed in Chapter 2.