DOI: 10.4225/03/5897b4e0e954f
title: Operation of falling film evaporator involving viscous liquid

description: In this thesis, the working principles of falling film evaporator used in dairy industries in relation to the evaporation of skim milk have been explored with a focus on the effect of milk solids content. The changes in rheological behaviour of skim milk and heat transfer within the evaporator during the concentration process have been investigated. With the better understanding of the rheological behaviour of skim milk and the operation of a falling film evaporator, the possibility of improving the performance of commercial falling film evaporator from its current configuration can be assessed.

The use of falling film evaporator is a common and economical practice in the food and beverage industries to remove water from liquid products, e.g. juices, milk, etc., especially when the product is temperature sensitive. In dairy industries, falling film evaporators are used to evaporate water from dairy products such as milk, protein concentrate, etc. Falling film evaporator is a well established technology but the understanding of rheological influence of milk on the performance and operation of falling film evaporator remains relatively unexplored which formed the motivation of this project.

In milk powder production, milk is normally concentrated from around 10wt% to 50wt% or 52wt% (this depends on the type of milk) using multi-effect falling film evaporator prior to the spray drying process. The viscosity changes of milk during the concentration process are evident, especially the exponential rise in viscosity as solids level reaches 50wt%. With the increase in viscosity, the flowing characteristics of the milks within the tubes in the evaporator change with the concentration of milk. Consequently, the performance of the evaporator, in terms of heat transfer, is considerably affected. The current research project has been divided into 3 main parts into context of viscosity and evaporation of milk using falling film evaporator. Firstly, viscosity models of milk are established based on stringent viscosity measurement procedures that ensure the repeatability and reliability of the measurements. These models are used for process simulation, model based control and production planning. The second part is to evaluate the performance of falling film evaporator under the influences of various operation conditions. In the last part, findings from the first two sections are merged into a pilot evaporator mathematical model that is able to predict the solid content, flow characteristics and residence time of milk concentrate in a falling film evaporator.

The viscosity of milk is influenced by several factors such as solids content, temperature, ageing and shearing, etc. The focus in this thesis is on the instantaneous milk viscosity during the operation of the falling film evaporator. Therefore, viscosity measurements and modelling of the milk viscosity have been mainly focused on only on the effect of solids content, temperature and, to some extend, shear rate. A 2m steam-heated pilot evaporator was designed and constructed to commercial grade based on a falling film evaporator design. This pilot evaporator is able to operate under vacuum conditions (up to -85kPa gauge) in both shell and tube side so as to mimic the operation conditions in the dairy industries. The viscosity models in this thesis are predominately formulated based on the fresh and reconstituted medium heat-treated skim milk, unless otherwise stated.  A comparison of viscosity models were made between the reconstituted and fresh skim milk. Significant differences were found and reported between the two types of milks.

The heat transfer coefficient (HTC), a common method of quantifying the performance of an evaporator, was measured based on temperature difference between the heat transfer surface and the processed fluid (e.g. milk) and the energy transfer within the evaporator. Another 1m electric-heated pilot evaporator was designed and built to facilitate the measurement at such temperatures and to have known power inputs which was not accessible in the steam-heated device. The influence of different operating conditions, such as varying heat flux, flow characteristics (e.g. Re number) and protein content, on the heat transfer coefficient is thoroughly investigated in this thesis. The study into the HTC on the product side found that the HTC measured from the evaporation of reconstituted skim milk is unresponsive to some of the operating conditions it was subjected to as compared to the fresh skim milk. Generally, HTC improves with greater Re number, heat flux and protein content (milk protein concentrate was added). Visual examination of the evaporation conditions within the evaporator also indicates that the amount of bubble formed during the evaporation process appears to increase with increasing flow rate, heat flux and protein content.

During the formulation of the pilot evaporator model in this thesis, the viscosity models and heat transfer coefficients established in the current project were incorporated so as to model the steam-heated batch pilot evaporator. Several assumptions were introduced in order to create a working model. The pilot evaporator model has been verified against the actual experimental data and is proven to be accurate. This model is able to predict the solids content of the skim milk at any given time provided that the operating conditions are available. This model can also be applied onto a commercial falling film evaporator with minor modification to the calculation sequence. A scale-up version has been developed but due to commercial nature, it is not reported in this thesis.

The research into the operation of falling film evaporator has been seen to enhance the knowledge of the mechanisms and interactions between the process fluid and the evaporator. Some of the results obtained here have already benefited an industrial operation.