Analysis of a reverse osmosis process for concentrating solutions of high osmotic pressure: The low retention method

The use of low retention membranes makes it possible to reach high retentate concentration values by lowering the effective osmotic pressure difference across the membrane. This paper presents a mathematical model of a combined low retention-high retention RO system and discusses its application in the production of fruit juice concentrates. The technique requires very high hydraulic pressures and suitable membranes. The key variable is the membrane retention, related to the solute/solvent permeability ratio. Hollow fibres show better performances than spiral wound modules because of the lower concentration polarization effects. Spiral wound modules are indeed not suitable to reach 60 degrees Brix. The membrane area needed to concentrate juices to 60 degrees Brix with a hollow fibre system is about twice the area needed to concentrate up to 20 degrees Brix with a conventional RO system, while the energy requirement is approximately three times larger.