Research Of Multi-effect Membrane Distillation Technology Concentrating Aqueous Sodium Carbonate Solution

Sodium carbonate is the basic raw of many industries. It is widely used in dailychemical, chemical industry, food industry, national defense and pharmaceutical fieldsand so on. In the background of petrochemical increasing scarcity, the high energyconsumption of soda ash production process not only improves the production costs,but also does not conform to the concept of energy saving. Therefore, the demand forenergy-saving technology that can replace multi-effect evaporation technology is veryurgent. Multi-effect membrane distillation technology that is a combination ofmembrane processes and distillation technology has higher thermal efficiency. It canplay a significant role in the production of soda ash.Starting from the perspective of reducing the cost of equipment investment, inthis paper, thermally induced phase separation （TIPS） was used to preparepolypropylene hollow fiber membrane. The preparation of hollow fiber membraneswas described. The outer surface and cross-sectional of the membrane were observedby scanning electron microscopy （SEM）. At the same time, testing the relatedperformance of the membrane, such as, inner and outer diameters and wall thickness,porosity, pure water flux, bubbling pressure and average pore size and so on. Resultsare as follows: NO.8,9and10hollow fiber membrane samples’ performance whichhave the corresponding performance with imported membranes are more excellent.Additionally, in the background of concentrating the sodium carbonate solution,multi-effect membrane distillation modules which are composed of hydrophobicmicroporous polypropylene hollow fiber membrane and polypropylene solid wallcapillary is used for the first time. The sodium carbonate solution needs pretreatmentsteps which contains microfiltration process, ultrafiltration process and chelating resinprocess. This paper proposed that using PR and membrane flux to characterize theheat and mass transfer performance of multi-effect membrane distillation process.In the experiment, concentrating the sodium carbonate solution from170g/L to220g/L, by examining the influence of feed temperature, steam temperature, flow rateand feed concentration on PR and membrane flux, conclusions are as follow: whenfeed temperature is40℃, steam temperature is95℃, flow rate is20L/h and feedconcentration is170g/L, the heat and mass transfer achieve optimal, PR is6,38and membrane flux is1.07L·m^-2·h^-1. Simultaneously, using homemade multi-effectmembrane distillation module to examine the performance, when feed temperature is30℃, steam temperature is90℃, flow rate is20L/h and feed concentration is170g/L,PR reach6.92and membrane flux is1.01L·m^-2·h^-1. It further illustrates that thehomemade membrane has considerable performance with imported membrane inaspects of heat and mass transfer.Additionally, two different membrane modules demonstrated good long-termoperational stability. The cleaning cycle of imported membrane module andhomemade membrane module is17days and10days, respectively. The average valueof PR is6.52and6.22, respectively. The average value of membrane flux is0.91L·m^-2·h^-1and0.75L·m^-2·h^-1, respectively. After homemade membrane module has achemical cleaning, both recovery rate of membrane flux and PR are81.2%. It can beseen that after chemical cleaning the membrane module can be restored to good massand heat transfer performance.Study has shown that using multi-effect membrane distillation to concentratesodium carbonate solution can obtain high PR and membrane flux. Compared totraditional multi-effect evaporation, thermal efficiency is greatly improved. Therefore,in order to achieve energy saving effect by the way of multi-effect membranedistillation to concentrate sodium carbonate solution is feasible.