Browsing by Author "Hurter, Marielle"
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- ItemA critical evaluation of the mass transfer and fouling behaviour in forward osmosis with integrated flow-reversal(Stellenbosch : Stellenbosch University, 2019-04) Hurter, Marielle; Burger, A. J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Forward osmosis (FO) is a membrane-based technology that can be operated at relatively low mechanical pressures and may be utilised in processes where water needs to be added or removed from process streams. Options for its potential application are diverse and it could, for example, be used in the regulation of water content in fruit juices, or in the augmentation of clean water to high-TDS cooling water circuits. Similar to reverse osmosis (RO) processes, scale formation by sparingly soluble salts can limit the maximum allowed recovery of water, while flux profiles, salt rejection characteristics and cross-flow velocity (CFV) play key roles in the overall behaviour of the system. However, FO systems are more amenable to the utilisation of osmotic backwashing than RO systems. Therefore, this study endeavoured to critically evaluate the mass transfer and fouling behaviour of FO membranes at different operating conditions, including the intermittent switching of the flow path (i.e. intermittently reversing the flux). To support this study, a bench scale FO setup was designed, constructed and commissioned. Subsequent laboratory work entailed: - Evaluate and assess the bench scale setup by comparing the theoretical and measured recovery, based on the measured water flux. - Evaluate the effects of changes in the CFV on the mass transfer of water and solutes over the membrane, while using a feed solution with TDS well below 100 mg·L-1. - Determine the effects of the operational configuration on the mass transfer over the membrane. - Investigate the process realities and limitations of intermittent flow path switching on reducing scale formation. Two operational modes were considered, viz. with the membrane active layer (1) facing the feed solution (AL-FS) or (2) facing the draw solution (AL-DS), with CFVs ranging from 13 cm.s-1 to 52 cm·s-1.Within this CFV range, the water fluxes attained in the AL-FS configuration were on average 40%lower than those in the AL-DS configuration. In the AL-FS configuration, the flux increased from 11.2 L·m-2·h-1 to 20 L·m-2·h-1 when the CFV was increased from 13 cm.s-1 to 37 cm·s-1. However, a further increase in CFV above 37 cm·s-1 did not result in higher fluxes and the limiting flux of 20 L·m-2·h-1 was reached. This is ascribed to the potential increase in dilutive internal concentration polarisation in the support layer of the membrane, thereby limiting the effective driving force (effective osmotic pressure difference) over the membrane. In the AL-DS configuration, this limiting flux was not reached within the defined CFV range. However, it was found that operation in the AL-DS configuration tended to a limiting flux of 20 L·m-2·h-1 when operating at draw solution concentrations above 50 000 mg·L-1 TDS. This is considered to be partly the result of an increased reverse solute flux (RSF) along with dilutive external concentration polarisation on the active layer side of the membrane. During operation with intermittent flow path switching when recovering water from a 1.9 super-saturated gypsum feed solution, ca 15 minutes were required to purge the flow channels of the respective residual solutions in the specific laboratory system under investigation. Operation at a CFV of ca 28 cm·s-1 then proved to enable the most rapid alleviation of internal concentration polarisation (ICP) in the AL-FS configuration (or mostly RSF in the AL-DS configuration). Under the most stable conditions in the AL-FS configuration, the operational flux dropped from 12 L·m-2·h-1 to ca 9 L·m-2·h-1 over a period of only 12 hours. In other words, flux declines of ca 38% were observed over a period of 12 hours when operating in the AL-DS configuration at 15-minute switch-intervals every two hours. This indicated the formation of gypsum scale in the support layer and highlighted the detrimental effects of the support layer in a scaling environment.