Design, construction and evaluation of a vacuum evaporation system for the concentration of aqueous whey protein solutions

Khetni, Munir (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY: Vacuum evaporation is an old and established technology for the industrial concentration of high water content streams. It is a clean, safe and versatile technology with low management costs. The aim of this study was to design, construct and evaluate a vacuum evaporation system, to increase the concentration of aqueous whey protein solution from 5 wt% to the highest possible solution concentration. The system functions to concentrate aqueous whey protein in order to reduce the volume of transportation and storage, thereby minimising costs. The raw material is pre-heated in the feed tank using a heating jacket at temperatures between 65oC and 70oC. It is then pumped by a centrifugal pump to a flash separator at 160 kPa abs through a throttling valve to obtain a pressure drop of up to 149 kPa abs, forming a two-phase mixture of vapour and liquid. The concentrated solution is recirculated until a required concentration is achieved. The vapour is condensed and the condensate, which may itself be a useful product in some applications, leaves the process. The variables affecting the product composition include operating temperatures of between 65oC and 70oC, vacuum level (12 – 15 kPa abs), type of separator internals (half pipe or multi-cyclone), feed flow rate (275 to 350 ml/min), liquid retention time inside the flash separator (2 to 4 minutes) and cooling water flow rate (about 750 ml/min). The design for the two-phase vertical flash separator was done at a ratio (L/D) of 4.8. It was found that the half-pipe internal device was not applicable to the WPC solution since it was foaming. By substituting the half-pipe for the multi-cyclone, the efficiency of the evaporator improved but still foaming persisted due to the laminar flow regime of the solution (inlet momentum of 38 kg/m-sec2). The foaming was finally eliminated by adding antifoam. The initial design of the VES employed direct preheating of the solution in the feed tank with a heating element. This resulted in fouling of the element as the WPC burnt and stuck to the element in the first hour of evaporation, thereby reducing the solids concentration from 10.8 wt% to 8.6 wt%. A modification was done to the preheating system by introducing a heating jacket for indirect heating. This modification managed to eliminate the fouling. The correlation between condensate recovery and solids concentration during evaporation improved with the modification from a coefficient of determination (R2 = 0.6404 to R2 = 0.9942) for direct and indirect preheating respectively. The viscosity measurement was done at temperatures between 59oC to 70oC at a constant shear rate of 23/s. For WPC solutions with concentrations between 4.2 and 14.5 wt%, the viscosity remained constant with increasing temperatures until 62oC, when it started increasing. For 17 wt% solutions of WPC, the viscosity increased with temperature from 59oC. The designed VES managed to concentrate the WPC solution to a maximum concentration of approximately 17 wt% at 65oC and vacuum pressure of 13.3 kPa abs. The viscosity then began to increase and the solution became difficult to recirculate whilst attempting to further evaporate it beyond 17 wt%. Evaporating at 70oC, although giving a higher evaporation ratio than 65oC, caused burning of the solution when the concentration reached 11 wt%. The heat transfer coefficient of the WPC solution with similar initial concentrations (around 5 wt%) was found to be higher at 70oC (433.6 W/m2.K) than at 65oC (431.8 W/m2.K). It was also found to be reduced with increasing solution concentration. When compared to sugar solution with similar initial concentration at 65oC and 13.1kPa abs, the WPC solution was found to have a higher heat transfer coefficient (431.8 W/m2.K and 396.3 W/m2.K for WPC and sugar solution respectively). Due to the differences in heat transfer coefficient, the WPC solution had a higher evaporation ratio than the sugar solution at similar conditions.

AFRIKAANSE OPSOMMING: Vakuum verdamping is 'n ou en gevestigde tegnologie vir die industriële konsentrasie van hoë-water-inhoud strome. Dit is 'n skoon, veilige en veelsydige tegnologie met lae bestuurs koste. Die doel van hierdie studie was om 'n vakuum verdamping stelsel te ontwerp, bou en evalueer, en om die konsentrasie van ‘n waterige wei proteïen-oplossing van 5 wt% na die hoogste moontlike oplossings konsentrasie te verhoog. Die stelsel funksioneer deur waterige wei-proteïene te konsentreer om die volume vir vervoer en stoor te verminder, en so ook koste. Die rou materiaal is vooraf verhit in die voertenk deur 'n verwarmingsbad by temperature tussen 65°C en 70°C. Dit word dan deur 'n sentrifugale pomp na 'n spoel skeier by 160 kPa abs deur 'n klep gestuur om ‘n druk daling van tot 149 kPa abs te verkry en 'n twee-fase mengsel van vloeistof en dampe te vorm. Die gekonsentreerde oplossing hersirkuleer tot 'n vereiste konsentrasie bereik word. Die dampe kondenseer en die kondensaat, wat opsig self ‘n gebruikbare produk is, verlaat die prosedure. Die veranderlikes wat die produk samestelling beïnvloed sluit in ‘n temperatuur van tussen 65°C en 70°C, die vakuum vlak (12 – 15 kPa abs), tipe interne skeiding (halwe pyp of multi-sikloon), stroomsnelheid van die voering (275 – 350 ml/min), vloeistof retensie tyd binne-in die spoel skeier (2 tot 4 minute), en verkoelings water vloei tempo (ongeveer 750 ml/min). Die ontwerp vir die twee-fase vertikale spoel skeier is teen ‘n verhouding (L/D) van 4.8 gedoen. Dit was gevind dat die halwe pyp interne toestel nie van toepassing op die WPC oplossing was nie as gevolg van skuiming. Deur die halwe pyp vir die multi-sikloon te verruil, was die doeltreffendheid van die verdamper verbeter maar skuiming het volhard as gevolg van die laminare vloei van die oplossing (inlaat momentum van 38 kg/m-sec2). Die skuiming was uiteindelik uitgeskakel deur die toevoeging van antiskuimingsmiddel. Die aanvanklike ontwerp van die VES het gebruik gemaak van direkte voorverhitting van die oplossing deur ‘n verhittingselement in die voertenk. Dit het gelei tot aanpakking van die element as gevolg van die aanbranding van WPC in die eerste uur van verdamping, en lei tot vermindering van die vastestof konsentrasie van 10.8 wt% tot 8.6 wt%. ‘n Verbetering van hierdie sisteem was gevind in 'n verwarmingsbad vir indirekte verwarming. Hierdie verandering het daarin geslaag om aanpaksels te elimineer. Die korrelasie tussen kondensaat herstel en vastestof konsentrasie was verbeter met 'n koëffisiënt van bepaling (R2 = 0.6404 na R2 = 0.9942) vir direkte en indirekte voorverhitting onderskeidelik. Viskositeit was by temperature tussen 59°C en 70°C gemeet teen 'n konstante wrywings spoed van 23/s. Vir WPC oplossings met konsentrasies tussen 4.2 en 14.5 wt%, het die viskositeit byna konstant gebly met toenemende temperature tot 62°C. Vir 17 wt% oplossings van WPC, het die viskositeit verhoog vanaf 59°C. Die ontwerpde VES kon die WPC oplossing konsentreer tot 'n maksimum konsentrasie van ongeveer 17 wt% by 65°C en ‘n vakuum druk van 13.3 kPa abs. Die viskositeit het begin toeneem en die oplossing het moeilik geword om te sirkuleer terwyl daar gepoog is om verder te verdamp verby 17 wt%. Alhoewel daar 'n hoër verdamping verhouding by 70°C as 65°C was, het die hoër temperatuur verbranding van die oplossing veroorsaak vanaf ‘n konsentrasie van 11 wt%. Die hitte oordrag koëffisiënt van die WPC oplossing met soortgelyke aanvanklike konsentrasies (ongeveer 5 wt%) was hoër by 70°C (433.6 W/m2. K) as by 65°C (431.8 W/m2. K). Dit is ook gevind om af te neem met toenemende oplossing konsentrasie. In vergelyking met suiker oplossings met soortgelyke aanvanklike konsentrasies by 65°C en 13.1 kPa abs, is daar gevind dat die WPC oplossing 'n hoër hitte oordrag koëffisiënt bevat (431.8 W/m2.K en 396.3 W/m2.K vir WPC en suiker oplossings onderskeidelik). As gevolg van die verskille in hitte oordrag koëffisiënt, het die WPC oplossing 'n hoër verdamping verhouding as die suiker oplossing in soortgelyke toestande gewys.

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