Most beers are filtered after fermentation with the objective to obtain a clear beer without turbidity. Fermented beer is turbid due to the presence of yeast cells and haze particles, which are aggregates of proteins and polyphenols. In this case the beer is clarified by means of membrane filtration. The microfiltration membrane used in this process is a porous fiber with a pore size smaller than the yeast cells and haze particles. The membrane provides an absolute barrier, which results in a consistent quality.
During filtration the beer is driven through the membrane by a pressure gradient over the membrane. The retained particles accumulate on the membrane. Consequently, the required transmembrane pressure increases progressively. During filtration a crossflow is applied to limit the build-up of this fouling layer. However, the buildup of the fouling layer cannot be prevented completely. Therefore, periodic cleaning by a backflush is necessary. The remaining irreversible fouling can be removed by cleaning chemicals during a Cleaning In Place (CIP).
Although crossflow, backflush and CIP are effective measures to control membrane fouling, they contribute to the operating costs significantly. The circulation pumps applying the crossflow and the coolers removing their heat are the main energy consumers in the process. The application of backflush and CIP result in lost production time and the consumption of water and chemicals.
The objective of the case is to develop a control policy that allows filtration of a predetermined volume of beer within a certain time, at minimal total costs for a given unfiltered beer quality.
This objective is motivated by two considerations:
(1) the filtration process is one step in the production process of a brewery, which aims to meet customer demand. Therefore the production capacity is determined by exogenous factors and is not a degree of freedom in the optimization;
(2) the quality of the filtered beer depends mainly on the membrane properties and is therefore very consistent. Thus, the product quality does not need to be considered during the optimization. However, the unfiltered beer quality is one of the key aspects of the case.