Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by Subject "Adsorpsion"

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    Designing and evaluating the technical, economic and environmental performance of an adsorption cooling system operating using bioresources from waste streams of mango processing
    (Stellenbosch : Stellenbosch University, 2019-12) Dzigbor, Aaron; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: This study sought to improve the technical performance (coefficient of performance (COP) and specific cooling power (SCP)), environmental impacts and economic viability of employing the adsorption working pairs produced from waste streams of mango processing in the adsorption cooling system (ACS). The specific objectives were: to produce and characterize mango seed husk activated carbon (AC) using NaCl as the activation agent and compared with commercial AC; assess the performance (in terms of COP and SCP) of the mango seed husk AC (with commercial AC as the control) paired with both high-grade and low-grade ethanol as refrigerants; improve the heat and mass transfer performance of commercial AC paired with both high-grade and low-grade ethanol as refrigerants through composite formation; and evaluate the environmental and economic impacts of integrating adsorption cooling system (ACS) in dried mango chips processing in both grid and off-grid power conditions. Mango seed husk AC was produced through slow pyrolysis method using NaCl as the activation agent. About 100 g of dried mango seed husk was soaked in 250 ml of NaCl solution of concentrations (10 w/v%, 20 w/v%, and 30 w/v%) to obtain impregnation ratios of 0.25, 0.5 and 0.75 at 25 °C. The carbonization temperatures were 400 °C, 450 °C, and 500 °C. The experimental design was based on a 33 (impregnation ration, soaking time, and carbonization time) Box-Behnken fractional factorial optimization method with three center runs, giving total runs of 15. The responses analyzed were bulk density, ash content, and surface area. The optimized mango seed husk AC produced was tested in an ACS constructed in-house and its performance compared with commercial AC. The composite AC were also formed by soaking commercial AC in NaCl solution at varying concentrations of 10 w/v %, 15 w/v %, 20 w/v %, 25 w/v %, 30 w/v % and 35.7 w/v %, for 24 hours at 25 °C, dried at 105 °C for 24 hours and then tested in ACS constructed in-house with high purity (99.7%) and low-grade (60%) ethanol to evaluate the effect of ethanol grade on the performance of the composite formed. Finally, three scenarios for each power setting (on-grid and off-grid) were studied, on-grid: coal as boiler fuel and conventional chiller for cooling (Scenario 1), mango seed as boiler fuel and adsorption chiller for cooling (Scenario 2) and mango seed as boiler fuel and ACS for cooling (Scenario 3). Off-grid scenarios 4, 5 and 6 corresponded to on-grid scenarios 1, 2 and 3, respectively. Environmental impacts and economic viability for each scenario were based on material and energy balances and South African economic conditions, respectively. The results showed that mango seed husk AC had comparable ash content (6.92%) to the commercial AC. The SCP, COP and temperature drop recorded in ACS for mango seed husk AC when paired with high purity (99.7%) ethanol reduced from 40 W/kg, 0.050 and 4.46 °C to 37.3 Wkg-1, 0.048, and 4.5 °C, respectively, when paired with low-grade ethanol (60%). Moreover, the COP and SCP of commercial AC paired with high purity ethanol were 0.099 and 84.5 Wkg-1, which reduced to 0.091 and 75.5 W/kg, respectively, when paired with low-grade ethanol. In addition, the COP of the composite AC containing 20%, 25% and 30% NaCl paired with low-grade ethanol were 0.121, 0.160 and 0.146, respectively, which were higher than when paired with high purity ethanol, thus 0.082, 0.080, and 0.076, respectively. In terms of environmental and economic impacts, on-grid scenario 3 showed the greatest potential for reducing emissions and improving economic viability by emitting 7.10×105 kgCO2eq/yr and internal rate of return (IRR) of 25.33% compared to scenario 1 that had the GHG emission of 7.89×105 kgCO2eq/yr and IRR of 17.48%. In off-grid, scenario 6 had the least GHG emission of 6.90×105 kgCO2eq/yr and IRR of 24.84%while scenarios 4 had the highest GHG emission of 7.67×105 kgCO2eq/yr and IRR of 16.09%. Overall, it is possible to improve the heat and mass transfer of activated carbon paired with low-grade ethanol. The improvement in heat and mass transfer when AC + NaCl was paired with low-grade ethanol suggests that low-grade ethanol can be used as an alternative refrigerant. However, in areas where silica gel is accessible, forming composite with silica gel + NaCl paired with pure water as refrigerant would eliminate the mass transfer challenges associated with using AC+NaCl composites paired with ethanol. Furthermore, the replacement of vapour compression cooling technology with ACS and boiler fuel with mango seed has led to the reduction in GHG emission and improvement in the economic viability of dried mango chip processing. Thus, the study has improved the technical, economic and environmental performance of ACS in terms of temperature maintenance, resource consumption, and emissions.