Vacuum Membrane Distillation Was Used For NMMO Concentration Recovery Studies

At present,the traditional vacuum distillation is widely used in the concentration and recovery of solvent NMMO for Lyocell fiber,which is not only high energy consumption,but also causes the breakdown and destruction of NMMO due to high temperature operation result in low recovery rate,so it is urgent to develop a new concentration and recovery process.As a new separation technology,membrane distillation combines the traditional distillation and membrane separation technology,has the advantages of high rejection coefficient,mild operating temperature and can make use of low-level heat energy such as solar energy,has great potential in the deep concentration and recovery of NMMO.In this paper,a preliminary study on the deep concentration and recovery of cellulose green solvent NMMO by vacuum membrane distillation(VMD)is first investigated.Firstly,PP and PTFE hydrophobic hollow fiber membranes’ characteristics,such as morphology,water contact angle,LEP,porosity,bubble point pressure and maximum pore diameter,average pore size,gas flux and tortuosity were characterized,and the results showed that PP membrane of thermally induced phase separation method has narrower pore size distribution,higher porosity,lower mass transfer resistance and better anti wetting ability than PTFE membrane of extruding-stretching method.Secondly,the solution of UF pretreatment,effects of membrane distillation operating variables including vacuum degree,feed-liquid’s temperature,feed-liquid’s concentration as well as feed flow rate on the performances of VMD,and the feasibility and stability of the VMD for the deep concentration were studied.The results showed that permeate flux increased with improvement of vacuum degree,feed-liquid’s temperature and feed flow rate,and reduced with addition of feed-liquid’s concentration;While permeate water concentration fell with accretion of vacuum degree,but have no obvious dependence on feed-liquid’s temperature,feed flow rate and feed-liquid’s concentration;The NMMO solution of 100g/L could be successfully concentrated to 600g/L by VMD method,the rejection coefficient of PP and PTFE membrane was up to 99.98%during the concentration process,also,the PP membrane’s concentration efficiency is more than 3 times that of PTFE;Although in high feed concentration of 500 g/L,PP membrane’s continuous operation for 60h did not lead to the reduction of rejection coefficient,and the flux retention rate is also higher than that of PTFE membrane.PP membrane has better operation stability.Finally,the temperature polarization under pure water system,membrane fouling and the effect of operating variables on fouling were investigated.The results showed that the effect of temperature polarization could be reduced by lowering the feed liquid’s temperature and increasing the feed flow rate;Membrane fouling of MD process mainly manifested as pore blocked and wetted,but 60h continuous operation of PP membrane in 500g/L of high concentration had not resulted in complete wetting,and its anti-wetting ability is better.The solution of UF pretreatment,improve the vacuum degree and feed-liquid’s temperature,choose the appropriate concentration endpoint could effectively alleviate membrane fouling.