Study On Process Mechanisms And Membrane Fabrication For Molasses Decoloration By Nanofiltration

As a kind of industrial by-product,molasses has a large yield but lacks high-efficiency utilization methods,which not only reduces the economic benefit of sugar industry but also brings huge environmental pressure.Among the various treatment methods,the recovery of high-value components(sucrose,reducing sugar,pigment,phenolic substances,etc.)can not only solve the environmental problems caused by molasses as a waste liquid but also produce huge economic value,which is the most promising utilization direction.Thereinto,the highly efficient separation of pigments and sugar is the key step of resource recovery.Membrane technology is promising in molasses decolorization due to its easy and convenient operation as well as diverse separation ability.However,at present,the decolorization efficiency still needs to be improved.The trade-off between pigment rejection and sucrose permeation hardly satisfies the need for efficient recovery of sucrose and pigment.Moreover,the complex components and high viscosity cause severe membrane fouling and flux decline.In this study,the molasses decolorization by membrane was systematically studied from membrane process to membrane fabrication in order to guide future industrialization.Firstly,the mechanism of pigment/sucrose separation during decolorization of sugarcane molasses by membrane filtration was studied in detail using a small dead-end filtration set-up and 1812 cross-flow membrane equipment.The separation of pigment/sucrose and the membrane fouling mechanism were systematically analyzed by investigating the effects of different membrane properties(material,membrane pore size),molasses components(sugar,salt,pigment),and membrane process parameters(temperature,flux,pH,cross-flow velocity,etc.).The results showed that:1)the membrane pore size directly determines the rejections of pigment and sucrose,and the membrane fouling affects the solute rejection by changing the membrane pore size.Therefore,suitable pore size and good anti-fouling ability are the two necessary properties for a high-performance decoloration membrane.2)Pigment and salt are the two most important components in the decoloration process.Among them,salt would cause membrane swelling,leading to membrane pore enlargement;while pigments would cause membrane fouling,resulting in the membrane pore narrowing,and the presence of pigments had a "shielding" effect on the salt-induced swelling.Moreover,when the membrane hydrophilicity is poor,the pore narrowing resulting from membrane fouling is more obvious.3)High temperature and low flux can effectively reduce the concentration polarization,improve the permeation of sucrose,and reduce membrane fouling.At the same time,the membrane pore enlargement caused by high temperature can aggravate membrane fouling to some extent.The above study shows that a nanofiltration(NF)membrane with high separation selectivity,anti-fouling ability,and anti-swelling property is the core of realizing efficient decolorization of molasses.Therefore,loose NF membranes were prepared by simple interfacial polymerization and post-treatment.The regulation effect and mechanism of various reagents(organic acid,weak base,organic solvent,ionic liquid)on the post-treatment of nascent polypiperazinamide NF membrane were systematically studied.The loose NF membranes for molasses decolorization were prepared and tested.The results showed that:1)Three mechanisms including hydrolysis,swelling rearrangement,and capping reaction were mainly responsible for the membrane properties variations.All these effects resulted in diverse loose NF membranes with enlarged membrane pore size,increased negative charge,and higher permeance.2)Due to the relatively loose structure,the loose NF membrane shows a better separation of pigment/sucrose with good long-term stability.And,the membrane fouling can be effectively removed by simple alkaline solution cleaning,showing good prospects for the practical application.This study summarized the mechanisms of post-treatment on the interfacial polymerization,provided guidance for the post-treatment of polypiperazinamide NF membrane,and proposed a new idea to prepare loose NF membranes for molasses decolorization.Finally,we conceived a novel strategy to tailor the separating layer through alkaline post-etching following the interfacial polymerization process,where piperazine(PIP)and tannic acid(TA)were used as the water phase monomers while the TMC and Fe(acac)3 were employed as the organic monomers in n-hexane,aiming at easing the membrane swelling during the alkaline cleaning process.The performance,especially the anti-fouling ability,of the prepared loose NF membranes for molasses was investigated.The results showed that:1)Three reactions concurrently occurred at the interfacial zone,including the polyamide formation between PIP and TMC,the polyester formation between TA and TMC,and the chelation between TA and Fe3+,where the polyamide and polyester played critical roles in determining the membrane properties.2)The polyester network formed by TA and TMC could be selectively etched by alkaline treatment,thus obtaining a loose NF membrane,whose structure and performance could be facilely tailored by controlling TA ratio and etching pH.3)The obtained loose NF membrane showed excellent anti-swelling ability during alkaline cleaning,because of network "locking" induced by Fe3+ chelation,carboxyl proportion decrease(more hydroxyl generation due to ester bonds hydrolysis),and the enhanced adhesion between surface layer and sublayer due to the codepositon of PIP and TA.4)The loose NF membrane prepared by etching showed better anti-fouling ability in long-term continuous filtration,while the foulants would accumulate in the membrane pores due to the swelling of the membrane pores caused by alkali cleaning in the case of commercial NF membrane.The performance-enhanced loose NF membrane has higher permeability flux,higher separation selectivity of sucrose/pigment,and more stable separation performance in a long-term filtration.This study not only established a green post-treatment method to regulate the performance of the polyamide NF membrane but also provided a new idea for improving the swelling resistance of polyamide membrane to alkali cleaning in practical application.