Forward Osmosis for the concentration of liquid streams in the food industry: insights into concentration polarisation and critical flux
Creators: name: ARTEMI, ANNA
Publisher: name: University of Surrey
Year: 2021

description: "Various liquid streams in the food and beverage industry often require the removal of water as a preliminary step prior to storage, transport, or the ultimate production of powders. A process that can achieve this concentration effectively and at a low cost is highly valuable. Forward osmosis is a potential method for concentrating such liquids, owing to its reported lower fouling tendency compared to reverse osmosis and the lower primary energy requirements compared to thermal methods, which has shown promising results. However, existing literature on the concentration of liquid food streams using forward osmosis has primarily focused on bench scale studies, with limited reports on pilot scale studies. The thesis has addressed several key research gaps in the fundamental aspects of forward osmosis in the concentration of liquid foods: (i) the extent and severity of concentration polarisation and its effect to reduce the flux, (ii) the conditions when fouling occurs in these systems and impact of reverse solute flux, and (iii) whether forward osmosis provides a lower fouling environment for this concentration process.
description: In this thesis, three liquids have been selected to represent streams that require concentration in the food and beverage industry, namely cheese whey, skim milk and orange juice. A pilot scale study of a forward osmosis concentration process of whey and skim milk was successfully demonstrated, which to the author’s best knowledge is the first to be reported in field of food and beverage concentration. Through this pilot scale study, a short-cut method was developed to evaluate the internal and external concentration polarisation effects as a function of the permeate flux, without the knowledge of the channel dimensions and mass transfer parameters of the feed channels in the membrane module. In the concentration of skim milk and whey satisfactory fluxes were demonstrated and the maximum total solids content obtained was comparable to that of reverse osmosis concentrates.
description: Forward osmosis has been widely claimed to have a lower fouling tendency compared to reverse osmosis. In this work, the critical flux of skim milk was found to be 5.1 ± 0.2 L/m2h for the forward osmosis mode and 7.5 ± 0.4 L/m2h for the reverse osmosis mode at the same cross-flow velocity of 0.2 m/s, which are comparable. Thus, it was concluded that the severity of fouling depends predominantly on the deviation of the initial flux from the critical flux value, irrespective of whether this flux is produced by osmotic or hydraulic means. As a comparison to skim milk, the critical flux of orange juice in the forward osmosis mode was found to be slightly lower at 4.2 ± 0.1 L/m2h and this was attributed to the slightly higher total solids content of orange juice.
description: Sodium chloride and magnesium chloride draw solutions gave a similar critical flux for orange juice of 4.8 ± 0.4 L/m2h and 5.3 ± 0.3 L/m2h respectively, indicating a negligible effect of the reverse solute flux in altering the onset of fouling. For glucose, no critical flux was attained, as the slow diffusivity of glucose and the elevated viscosity of the draw solution severely limited the flux. The external concentration polarisation was intensified by a slower transfer of the back-diffusing solute from the boundary into the bulk orange juice, caused by the solid fraction of the juice. Extremely high amounts of each draw solute of up 10 g/100 g juice, demonstrated no indication pectin gelation for neither of the draw solutes, which would otherwise cause fouling in the filtration system.
description: This work has provided insights into the effect of concentration polarisation and fouling in the concentration of liquid food streams by forward osmosis, both at bench and pilot scales. The severity of these concentration polarisation effects has been discussed and this analysis can prove useful in future experimental work toward improving this process. The critical flux was determined for the complex feed streams in the forward osmosis mode, which provides a guide with respect to the selection of flux, draw solution and feed hydrodynamic conditions in industrial processes. Overall, these understandings will advance and improve future implementation of forward osmosis in the food industry."