Applied Basic Research In Clarification And Decolorization Of Cane Molasses By Membrane Technology

Cane molasses,a main by-product in plantation white sugar production,contains a mass of sucrose,reducing sugar,pigments and inorganic salts.And it is mainly utilised as substrate for the production of ethanol and yeast,which generates a large amount of high-strength wastewater with inorganic salts and pigments.After the pilot-scale production white sugar from sugarcane juice by membrane technology was succeed,we make attempt to recover sucrose.pigments and reducing sugar from cane molasses by membrane technology.Thereinto,clarification and decoloration of molasses are the key issues for this new technology,determining the quality and output of white sugar.Therefore,this thesis focuses on the membrane fouling mechanism and its effect on sucrose retention as well as fouling control strategies in the clarification and decoloration of molasses,which can provide theoretical support for the development and optimization of this new technology.Firstly,five commercially-available polyethersulfone(PES)ultrafiltration membranes were selected for the clarification of molasses.It was found that UH050 membrane was the most suitable for the purpose,because it could remove suspended solids and pectin completely,and its sucrose retention and membrane fouling were the lowest.In order to control membrane fouling,the threshold flux for differently diluted molasses was evaluated by associating permeate flux with average transmembrane pressure,average membrane fouling rate or specific energy consumption respectively in dilution-concentration mode,where the amount of raw molasses treated by the membrane should be the same.According to the variation of threshold flux and irreversible fouling,it was deduced that the optimal dilution factor of molasses for clarification by UH050 was 8 times and the threshold flux value was around 26-32 L/(m2 h),below which the membrane fouling could be minimized.Finally,the hydrophilicity and charge pattern of the PES membrane were manipulated via a mussel-inspired coating and subsequent polymer grafting in order to explore membrane fouling mechanism.It was found that the membrane fouling was low when operated at threshold flux,and that the membrane surface hydrophilicity and charge pattern only had significant effect on membrane fouling when the operating flux was greater than threshold flux.Besides,the sodium alginate(ADA)grafting could obviously increase the negative charge on the membrane surface,which generated stronger electrostatic repulsion between foulants and membrane,thus resulting that the threshold flux increased by 14.3%and the total membrane fouling decreased by 29.1-64.9%.Next,eleven commercially-available ultrafiltration and nanofiltration membranes were selected for the decoloration of mo asses.It was found that UP005 ultrafiltration and NF5 nanofiltration membranes were suitable to achieve the high selectivity of sucrose and pigments,whose colour retention were both over 85%.However,their separation mechanisms are different.For UP005 membrane,the high pigment retention was mainly caused by the pore narrowing effect due to membrane fouling.While for NF5 membrane,its own dense separation layer resulted in the high pigment retention.Through systematically investigating the effects of different components in molasses on sucrose retention and membrane fouling behaviours,it was found that reducing sugar had little effect on the sucrose retention;while inorganic salts resulted in pore swelling,lowering the sucrose retention;but the pore swelling effect induced by inorganic salts was eliminated by the pigments fouling,regaining the high sucrose retention.Finally,through analysing the effect of operating parameters on the membrane fouling and sucrose retention,it was found that high temperature and low permeate flux could reduce sugar retention,and high temperature and high permeate flux could aggravate membrane fouling,and the stirring speed had little effect on sugar retention but could reduce membrane fouling at 60 ?.UP005 membrane has strong resistance to high temperature and chemical cleaning,but the serious membrane fouling would influence its long-term and stable operation.Therefore,we attempted to decrease the membrane fouling of UP005 by membrane modification technology,that is,the pore narrowing and hydrophilicity enhancement were realized with nearly negligible effect on pigments retention.Through investigating the membrane fouling resistance under static adsorption and dynamic filtration modes,it was proved that hydrophobic adsorption was mainly responsible for the serious membrane fouling.Then,the membrane surface and pores were modified by mussel-inspired membrane modification methods.It was found that the irreversible fouling under static adsorption was greatly mitigated,while the irreversible fouling under dynamic filtration was only reduced marginally,which was caused by the exposed hydrophobic sites of the small molecules(benzene ring),low accessibility of large molecules(ADA)into the membrane pores and the instability of the super-hydrophilic mineralized layer(TMOS).Therefore,the modification of UP005 membrane should not only increase its hydrophilicity and negative charges of membrane surface and pores,but also narrow the membrane pores and increase the modification layer's stability.Only by this way,the serious hydrophobic adsorption in the membrane could be avoided,and the separation of pigments and sucrose was able to be achieved by its own dense modified separation layer.