Study On Purification And Diafiltration Of Mannitol From Kelp Process Industry By Nanofiltration Membrane

Nanofiltration (NF) is a new type of pressure-driven membrane technology located between reverse osmosis (RO) and ultrafiltration (UF), and developed in the late 1970s as a new kind of high technology. Based on NF membrane performance, NF technology has been mainly used in water softening, water purification, and separation, grading & concentration of the substance with Mw 100-1000, and so on. With the fast-growing demands of social development, NF technology become more and more obvious effects in the application of drinking water purification, water softening, grading & concentration, decolorization & removing foreign taste from the products of chemical industry and bioengineering, such as antibiotics, polysaccharoses, polypeptides, and wastewater treatment & resource recovery.The mannitol (M) concentration is about 1.0 wt%, and is relatively high in kelp process industry. Obviously, development of recycling technology in the kelp process is significant economic benefit. Based on the background of comprehensive utilization of kelp resources, developping the technologies of energy saving and efficient utilization of nature resource in the production of kelp food and a series of chemical products from kelp has been listed as National Key Technology R&D Program. Because of the complexity of wastewater constituents, there is still residual high inorganic salts impurity after removed suspended substance, protein, glucide and other macromolecules organic by pretreatment process, especially, the main salt is NaCl. It is difficult to reject NaCl by conventional process, and is common to be high energy consumption, serious environmental pollution. Present dissertation face to the development of extraction technology of mannitol in kelp processing wastewater. For the purpose of removing the main impurity NaCl, the purification experiments of M in mixtures solution were studied by means of a setup pilot scale device equipped two kinds of polyamide (PA) spiral wound NF membrane modules with total effective area of 6.8 m2. The experiments were designed M & NaCl as the main impurity in kelp wastewater, and studied the membrane performance and process parameters in M aqueous solution and M-NaCl mixtrues solution. The results show that effect parameters on NF membrane separation performances were mainly the concentrations of M and NaCl in the feed, operation pressure (P) and temperature (T). The 1# membrane module was preferred the 2# membrane module in terms of permeate flux (J) and selectivity. The J was higher, and the M retention (RM) was more than 80% when separating M aqueous solution system. The RM of 75-80% and J of 13 L/m2-h were stable when separating M-NaCl mixture aqueous solution system. The NaCl penetration (SNacl) was about 50% and 70%, when NaCl concentration (Cf,NaCl) in the initial feed was 0.2 wt% and 0.8 wt%, respectively. The transport model of integrate effect considered preference penetration and space hinder was developed and successfully used to interpret the mechanisms of NF membrane separation of M-NaCl mixture aqueous solution system.In present dissertation, with the preferable 1# NF membrane, this work combined the NF process and DF operation mode, and brought forward the combined process of Pre-concentration (Câ… )-Continuous Constant Volume Diafiltration (CCVD)-Post-concentration (Câ…¡) to meet the need of desalination and concentration of M, namely MDF technique. The effects of main process parameters namely the concentration of M and NaCl in the retention, the pre-concentration factor (CF), the operation time (t) of (CCVD), the consumption of diafiltration solvent (deionized water) (Vw), and purification factor (PF) of membrane diafiltration (MDF) were discussed. The experimental results showed that the suitable process of pre-concentration before CCVD was necessary, and the rejection of NaCl in the feed was about 90%, the concentration of mannitol rose to about 3 times of initial feed concentration, PF come up to 40, the consumption of deionized water was lower when CF was between 2.0 and 2.5. In order to predict M/NaCl concentration and consumption of diafiltration solvent (deionized water) in MDF technological process, present dissertation deduced the following formula based on the related theories research and the characteristics of M/NaCl solution system:Concentration stage:(?)Diafiltration stage:(?) Vw(?) This work reveals the feasibility of new double effect NF process of NaCl penetration and M retention. The result provides the theoretical basis for constructing the engineering application of purification and concentration of M in kelp processing by means of NF membrane.