Model based analysis of the operation and control of falling film evaporators : a thesis presented for the degree of Doctor of Philosophy in Technology and Engineering, Massey University, Palmerston North, New Zealand
Falling film evaporators are commonly used in the New Zealand dairy industry for the production of milk powders. Milk powder is not susceptible to bacterial damage and it is easy to transport, whereas milk itself has neither of these characteristics. As a result milk powders are an ideal export product for the New Zealand Dairy industry. However, despite their large amount of industrial use, there is a shortage of knowledge on the design, operation and control of falling film evaporators. The work discussed in this thesis was initiated with the aim of improving this situation. The aims of this work are split into three broad areas. Develop Model A first principles model for the Evaporator A plant at Kiwi Co-op Dairies will be developed from the Laws of Thermodynamics. The model is dynamic and includes the evaporator preheat sections. We will concentrate on developing the model for the subsequent analysis of the Evaporator A plant. Specifically, a steady state model will be developed for the optimisation/operation studies and a linear dynamic model for the controllability studies. Optimisation Studies Using the steady state model the operation/optimisation of the Evaporator A plant will be investigated. There are some simple aspects of the evaporator and preheat sections that will be discussed. However, an important aspect will be the evaporator operational optimisation with respect to energy costs, fouling, throughput and milk powder quality. Controllability Studies Using the dynamic model we will investigate the control of the evaporator plant. The dynamic model derived from first principles will be linearised to produce a linear constant coefficient dynamic model. This can be analysed using the standard traditional and some advanced control methods. Specifically we will be interested in disturbance rejection.