Browsing by Author "Bührmann, Erika Isabel"

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    Seeding as a mitigation strategy for gypsum scaling in membrane distillation desalination
    (Stellenbosch : Stellenbosch University, 2022-12) Bührmann, Erika Isabel; Burger, Andries Jacobus; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Membrane distillation (MD) is a temperature driven separation technology that can desalinate water at higher salinities than can be treated by means of reverse osmosis (RO), and can operate at lower temperatures than multistage flash distillation (MSF) or multi-effect evaporation (MEE), while having a smaller physical footprint. MD presents an alternative to conventional desalination technologies for the treatment of highly saline water especially where solar or low-grade waste heat is available or where a small footprint is desirable. As is typical of membrane processes, MD is susceptible to scaling, especially by calcium scalants, such as gypsum, which has a low solubility in water at elevated temperatures. Gypsum scaling can block the membrane pores and restrict the flux, as well as penetrate the pores and cause the membrane to become wetted, thereby reducing its separation efficiency (SE). It was hypothesised that gypsum scaling in MD can be mitigated by adding gypsum crystals (seeds) to the supersaturated calcium sulphate solution. This procedure (crystal seeding) can restrict crystallisation on the membrane in favour of growth of suspended crystals in the bulk solution. This study endeavoured to demonstrate that seeding can retard the flux decline and delay the onset of pore wetting in an MD process. Furthermore, this study demonstrated the effect of feed agitation and seed loading on the effectiveness of scaling mitigation by seeding. Finally this study investigated the reversibility of gypsum scaling in seeded membrane distillation by rinsing with water. In an experimental investigation a saturated calcium sulphate solution was desalinated using a lab-scale MD process consisting of a membrane fitted to the base of a stirred feed vessel and exposed to cold water that was pumped over the permeate side of the membrane. The MD system was run at 61 °C with a trans-membrane vapour pressure of 16 kPa and a bulk permeability coefficient of 2.3x10-7 s/m. It was demonstrated that the addition of 5 g/L seed crystals agitated to a G-value of 630 s-1 resulted in the flux declining ten times slower than when no scaling mitigation was implemented. However, when crystals were only agitated to a G-value of 210 s-1, seeding accelerated the rate of scaling ostensibly via crystal deposition and secondary nucleation on the membrane. It was demonstrated that increasing the level of agitation (from 210 to 420 s-1 and from 420 to 630 s-1) resulted in a retardation in flux decline and a delayed risk of pore wetting as the crystal growth rate on the membrane and the crystal deposition rate onto the membrane were inferred to be decelerated. Similarly, increasing the seed loading (from 1 to 3 g/L and from 3 to 5 g/L) was also inferred to decelerate the crystal growth rate on the membrane but to accelerate the crystal deposition rate. Therefore a greater seed loading delayed the risk of pore wetting, but did not necessarily retard the flux decline. The scaling that occurred during seeded membrane distillation appeared to be completely reversed by rinsing with and soaking in water. However, only rinsing with water was not sufficient in removing the scale and restoring the membrane properties. It was therefore surmised that while seeding is a feasible gypsum scaling mitigation strategy in MD, it should be paired with a more thorough cleaning sequence than merely periodically rinsing with water.