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Modelling of a direct osmotic concentration membrane system : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Engineering at Massey University
Direct osmotic concentration (DOC) is a novel continuous membrane process. Two co-current streams, separated by a semi-permeable membrane, are recycled through a DOC module. The turbulent-flow dilute juice stream is concentrated by osmotically extracting water across the membrane into a laminar-flow, concentrated osmotic agent (OA) stream. The semi-permeable membrane is asymmetric, with a non-porous active layer (15 μm) and a porous support layer (150 μm). Membrane solute rejection was greater than 99%. Normal operation orients the active layer towards the juice stream. For this study, water (osmotic pressure = 0) was used in the juice channel. The relationship between water flux rate and the osmotic pressure of the bulk OA stream was asymptotic, reaching a maximum flux of 1.75 x 10-3 kg m-2 s-1, when using fructose OA at 15 MPa osmotic pressure and 20°C. Flux rates doubled when NaCl replaced fructose as OA. A doubling in temperature to 40°C resulted in a 50% increase in flux rate. OA solution properties, particularly viscosity and factors affecting diffusion coefficients had a strong influence on flux rates. When the membrane was reversed, with the active layer facing the OA channel and the support layer filled only with water, flux rates were 40 to 60% higher than the normal orientation. There were three resistances to water flow associated with: osmosis across the membrane active layer (R1), diffusion and porous flow across the support layer (R2), and; diffusion across the boundary layer in the OA channel (R3). For fructose OA at 0.50 g (g solution)-1 (osmotic pressure = 15 MPa), R1 contributed 9% of the total resistance to water flux in the DOC module, R2 contributed 64% and R3 contributed 27%. For an iso-osmotic concentration of NaCl OA (0.15 g (g solution)-1) the relative resistances were. R1 = 17%, R2 = 44% and R3 = 39%. It was clear that the water flux from the dilute to concentrated stream was more strongly influenced by the support membrane and OA solution properties than the active semi-permeable membrane itself. This accounted for the asymptotic relationship between bulk OA stream properties and flux rate. The mathematical model successfully incorporated these resistances and solution properties. Data calculated using this model agreed well with experimental results.