3) The art of Dehydration

The high energy cost of drying processes forms a strong incentive to search for alternative technologies that consume significantly less energy. This session focusses on two such alternative technologies to remove water or recover compounds form aqueous streams: Super Critical CO2 and Eutectic Freeze Crystallization (EFC). Topics will be discussed in the context of both application and fundamental principles.


Ruben Halfwerk, Wetsus & Wageningen University:

Eutectic freeze crystallization (EFC) is a newly developed crystallization technique that operates at subzero temperatures. The eutectic point of an aqueous solution is the concentration and temperature were both the solvent as the solute starts to crystalize simultaneously. Due to the density difference between the solvent and solute, separation by gravity is possible. A pure stream of ice and solute can then be extracted, the remaining liquid can be separated further or recycled again into the process. In comparison with other separation technologies like evaporation, EFC has a low energy requirement and has the ability of complete conversion of feed in to water and solidified solutes.

Previous research has been focused on separating salts from brines emitted from RO plants. However it is also possible to separate organic substances. This research focusses on the recovery of heat sensitive products and concentrates in the agro and food industry. An EFC setup has been developed by Coolseperations bv. and Wetsus. The first part of this research will look at the recovery of lactose from delactosed whey permeate (DLP), which is a byproduct created by removing lactose from whey. In this project, parameters of interest are crystal growth, ice and solvent quality with respect to size, shape and purity and the influence of impurities on the crystallization behavior.

Shin Yee Wong, Singapore Institute of Technology:
In the dairy industry, lactose crystallization during refining typically generates large number of fines (< 100 µm), which greatly reduces the efficiency of downstream processes, resulting in low recovery. Theoretically, the crystal size distributions of the lactose crystals can be controlled by two major kinetic steps: nucleation and growth. Nucleation is strongly related to the metastable zone width (MSZW), defined as the area between the saturation curve and labile zone. Inside the MSZW, crystal growth is fastest with least secondary nucleation. However, existing data published in the 1920s were not suitable for directing an industrial operation. In this talk, refined metastable limit (ML) will be presented. Then, the talk will focus on applying the newly established reference graph (ML) together with Computational Fluid Dynamics (CFD) to analyze and design a crystallization operation. From all these studies, it was demonstrated that depending on the crystallizer design, large lactose crystals with minimal fines can be produced when operating in the optimal region in the MSZW. Finally, the technology was transferred to the dairy industry where it was successfully tested.

Andrew Shamu, Wetsus & Technical University Eindhoven:
Supercritical CO2 (scCO2) being used in the food industry as a water extraction agent, require dehydration units for regeneration. Previous studies show that membrane-based processes drying scCO2 by selective H2O permeation, are more cost-efficient than current methods based on H2O absorption through zeolites. However, severe concentration polarization (CP) effects in the feed boundary layer were not considered in those studies. This study shows that water fluxes are by a factor of 150 reduced when taking CP into account. Methods to reduce these CP effects were examined. Three processes differing in pressure and temperature within the membrane unit were designed and compared according to their drying costs at pilot plant scale to industrial scale. Among them, the most cost-efficient process requires only half the drying costs of the benchmark process based on zeolite. These costs would be reached using a dense polymeric skin layer with a H2O permeability of 10,000 Barrer or higher and CO2 permeability of 10 Barrer or lower.


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