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Water Pollution and its Control
Published in Danny D. Reible, Fundamentals of Environmental Engineering, 2017
where αo is the osmotic coefficient, N is the normality of the solution (gram equivalent weight per liter), R is the ideal gas constant (in units consistent with the concentration units in normality), and T is absolute temperature. The osmotic coefficient depends upon the electrolyte and its concentration. The osmotic pressure of sea water (salt concentration of 35,000 mg/L) is 2740 kPa at 25°C. If a linear dependence of concentration on the osmotic coefficient is assumed, this means that a change in salt concentration of 1000 mg/L changes the osmotic pressure by 78 kPa.
Water desalination by forward osmosis: draw solutes and recovery methods – review
Published in Environmental Technology Reviews, 2019
Imane Chaoui, Souad Abderafi, Sébastien Vaudreuil, Tijani Bounahmidi
Forward osmosis, can be defined as the movement of water from a feed solution with lower osmotic pressure to a draw solution with higher osmotic pressure across a semi-permeable membrane which rejects salts and allows the passage of water. In contrast with reverse osmosis, which uses hydraulic pressure as the driving force, forward osmosis uses the osmotic pressure gradient between the feed and draw solution [11,13,14,31,32]. The estimation of osmotic pressure, π, is of great importance as in a FO process it’s the driving force allowing water drawing from a feed stream to the draw solution. The osmotic pressure of a solution is generally measured using different experimental devices such as freezing point depression method, membrane-based method and vapour pressure measurements [28,29,33–35] or estimated using thermodynamic models [36]. The osmotic pressure can also be calculated by software such as OLI Stream analyzer that uses thermodynamic models for solutions properties estimation [13,37]. For very dilute ideal nonelectrolyte solutions the osmotic pressure is usually described by the Van’t Hoff Equation (1). A parameter specifying the number of ionic species ‘i’ is added for very dilute ideal solutions that contain strongly dissociated electrolyte solutions as shown in Equation (2) [13,33,38].where M, R and T are, respectively, the molar concentration (mol/L), the universal gas constant (J/K mol) and the absolute temperature (K). Osmotic pressure is related to the solution concentration. In FO operation, high osmotic pressures are required, hence, highly concentrated draw solutions are used. As highly concentrated streams do not behave as ideal solutions, the osmotic pressure is estimated using equations with a correction factor Φ, the osmotic coefficient as described by Equation (3). Osmotic pressure can also be estimated using a virial expansion where c is the mass concentration and B2, B3, B4 the virial coefficients determined experimentally [13,33,38,39]: