The effects of feed water temperature and dissolved gases on permeate flow rate and permeate conductivity in a pilot scale reverse osmosis desalination unit

Abstract

Feed water temperature is an important parameter in determining the optimum conditions for an efficient sea water reverse osmosis (SWRO) process. Increased feed water temperatures are known to increase the permeate flux rate in commercial SWRO systems. There are several factors which link feed water temperature to the operational efficiency of the fundamental membrane desalination process. In this study we have obtained precise data on these effects using two different types of RO membranes in a small scale pilot unit with feeds of sea water, brackish water and pure water. The mechanisms involved have been examined in this work. Pre-heating the feed water to enhance RO efficiency may also lead to greater cavitation within the RO membrane. Cavitation is the formation of cavities in solution upon sudden pressure release, such as across a Reverse Osmosis membrane. These cavities have the potential to hinder permeate flow by blocking sections of the polymer matrix in the skin layer of the membrane. Previously, the effects of de-gassing on the permeate rate in a small scale pilot SWRO system was reported. It was identified that the presence of dissolved atmospheric gases in sea water leads to a potential for cavitation within the porous membranes used in high pressure RO processes. It was also established that the almost complete removal of the initial dissolved atmospheric gases prevented this cavitation. This work has been extended here to include more hydrophobic membranes, which are more likely to produce cavitation. In addition, there is new evidence to support the view that de-gassing the feed water can remove/reduce vapour cavities in the membrane for improved flow, which is maintained even when the feed water is re-gassed. An initial study of a novel method using 2 dissolved Helium as a 'carrier gas' in the feed water, to assist membrane transport, is also reported.

Feed Temperature Effects on Organic Fouling of Reverse Osmosis Membranes: Competition of Interfacial and Transport Properties

“…Only a few studies have been devoted to this subject, ,− despite the increasing use of membrane-based desalination and wastewater reuse in arid, near-equatorial latitudes where seawater temperatures can reach 35.5 °C . Previous work on the connection between RO membrane transport properties and the feed temperature has shown that water permeability , increases with increasing temperature, due to lower water viscosity , and higher water diffusivity. , As a result of increasing temperature, permeate recovery increases ,− and energy consumption decreases due to lower pressure requirements. − Similarly, the salt permeability coefficient, B s , is directly proportional to the solute diffusivity, D s , and partition (solubility) coefficient K s , ,, both of which increase with temperature, leading to a higher salt flux and lower salt rejection. ,,, In one study, Goosen et al observed an increase in the permeate flux at a fixed applied pressure as the temperature was increased from 20 to 40 °C for NaCl concentrations ranging from 0 to 5% (w/v) NaCl, suggesting that the membrane undergoes morphological changes such as an increase in the polymer free void volume . Sharma and Chellam observed that the network pore size of nanofiltration (NF) membranes increased with increasing temperature (5–41 °C).…”

“…35 Previous work on the connection between RO membrane transport properties and feed temperature has shown that water permeability 32,36 increases with increasing temperature, due to lower water viscosity 37,38 and higher water diffusivity. 31,32 As a result of increasing temperature, permeate recovery increases 36,[38][39][40] and energy consumption decreases due to lower pressure requirements. [39][40][41] Similarly, the salt permeability coefficient, Bs, is directly proportional to the solute diffusivity, Ds, and partition (solubility) coefficient Ks, 31,42,43 both of which increase with temperature, leading to a higher salt flux and lower salt rejection.…”

“…Only a few studies have been devoted to this subject 20,[31][32][33] , despite the increasing use of membrane-based desalination and wastewater reuse in arid, near-equatorial latitudes 34 where water temperatures can reach 35.5 ºC. 35 Previous work on the connection between RO membrane transport properties and feed temperature has shown that water permeability 32,36 increases with increasing temperature, due to lower water viscosity 37,38 and higher water diffusivity. 31,32 As a result of increasing temperature, permeate recovery increases 36,[38][39][40] and energy consumption decreases due to lower pressure requirements.…”

Solar-Intensified Ultrafiltration System Based on Porous Photothermal Membrane for Efficient Water Treatment

“…The decreased rejection can be explained by the fact that most of the polymeric membrane is sensitive to the changes of feed temperature. With raising interfacial temperature, the possible polymer expansion or swelling will slightly increase the effective pore radius and increase the flow rate, while the larger effective pore diameter of the membrane causes a reduction in rejection. , …”

Degradation of pharmaceuticals by ultrasound-based advanced oxidation process

“…Similarly as the thermal conductivity of dissolved gas increases, the amount of heat loss due to thermal dissipation also increases. Thus, gases with lower thermal conductivity provide better cavitation (Brotchie et al 2010;Francis and Pashley 2011).…”