Browsing by Author "Dorrington, Bella Ann"
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- ItemTechno-economic evaluation of organic solvent nanofiltration : recovering dewaxing solvent from lube-oil(Stellenbosch : Stellenbosch University., 2020-03) Dorrington, Bella Ann; Burger, A. J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: To recover the solvent from dewaxed lube-oil, classical separations such as evaporation and distillation are typically used. In the past decade separations that consume less energy and utilities have been investigated. One such technology is organic solvent nanofiltration (OSN). OSN is the size-exclusion of species over a membrane. Many studies have shown that OSN separation consumes less energy than evaporation and distillation. However, few have compared the cost of OSN to classical separations. Therefore, the aim of this investigation was to simulate and compare the cost and energy demand of a conceptual OSN unit with classical separation for the recovery of solvent from dewaxed lube-oil. Cost was expressed as the annualised capital plus total operating costs per year. Energy demand is the total energy consumed per volume of solvent recovered. Max-Dewax was the first and largest OSN membrane unit to recover solvent from lube-oil. For this investigation, a conceptual OSN unit was developed based on Max-Dewax to treat a 480 m3/h feed containing 19% lube-oil, and 46% methyl-ethyl ketone (MEK) and 35% toluene as solvents. Membrane permeate rates for MEK, toluene and lube-oil were 360, 91 and 1 mm/h. Long-term membrane stability was based on a 28% average decline in flux over a membrane life of 24 months. At an operating pressure of 42 bar, the OSN unit could achieve a 51% recovery of solvent at a 99% purity and average flux of 11 L/m2/h. A sensitivity analysis revealed that specific membrane cost, membrane life, flux decline over membrane life, operating pressure and membrane permeate rates had the biggest influence on the cost and energy demand of the OSN unit. The solvent remaining in the concentrated product from the OSN unit was further recovered by evaporation and distillation. The complete solvent recovery unit is referred to as hybrid-OSN and could recover 98% solvent at a 99% purity. The simulated cost of the hybrid-OSN unit for a specific membrane cost of 3200 ZAR/m2 effective area was 127 ZAR/m3 solvent recovered per year. The energy demand was 177 kWh/m3 solvent recovered. Compared to a classical separation including only evaporation and distillation, the energy demand of the hybrid-OSN unit was 64 kWh/m3 less. However, the hybrid-OSN unit was 40 ZAR/m3 more than the classical separation. Process simulations revealed that for a membrane cost of 990 ZAR/m2 and higher membrane permeate rates for MEK and toluene of 540 and 137 mm/h, the energy demand of the hybrid-OSN unit was 140 kWh/m3 and the cost equal to the classical separation. Extending membrane life to 72 months at a membrane cost of 1600 ZAR/m2, the cost of the hybrid-OSN and classical separation were also equal. Moreover, for a membrane cost of 630 ZAR/m2 and 14% decline in flux after 24 months, the energy demand of the hybrid-OSN unit was 163 kWh/m3 and the cost was equal to the classical separation. Typical results from process simulations showed that under certain conditions, OSN membranes bare the economic and energy potential to assist classical solvent recovery units. However, the profitability of a hybrid-OSN unit largely depends on the cost of membrane modules and long-term membrane performance such as flux decline over membrane life.