| Seawater desalination was an important approach to solve the shortage of freshwater resources.As an new technology which combined distillation and separation,membrane distillation was based on gas-liquid equilibrium and phase transition was realized by evaporation latent heat.Driven by the vapor pressure,the vapor passed through the membrane to achieve desalination.This technology had the advantages of large membrane flux,simple equipment,high desalination rate,easy operation and so on,and had a broad prospect in the field of desalination.This paper investigated the influences of temperature,vacuum degree,flow rate and water quality on membrane flux and water production TDS.The enhancement effect of gas-liquid two-phase flow on membrane flux was studied.This paper proposed calcium and magnesium pretreatment and membrane cleaning technology.First of all,membrane flux increased with the increase of temperature,the increase of vacuum would cause the membrane flux to increase,the increase of water flow rate would increase the membrane flux.Temperature,vacuum and water flow would not affect the desalination effect,and TDS kept below 20 mg/L.The effluent met the standard of“Sanitary standard for drinking water”(GB 5749-2006).The system had a desalination rate of over 99.9%.In the range of seawater TDS of20000?60000 mg/L,membrane flux would not change.However,in the range of seawater TDS of 60000?100000 mg/L,the membrane flux decreased.After considering membrane flux,energy and fouling,the conditions of 80?,0.09 MPa,0.84m/s was a suitable membrane distillation operating condition.Secondly,after 24 hours of continuous operation,the membrane flux decreased by 17.5%without removing calcium and magnesium,the membrane flux decreased only by 4.5%after pretreatment.The cleaning effect of diluted hydrochloric acid was better than citric acid,which could restore the membrane flux from 8.3 L/(m~2·h)to17.5L/(m~2·h),and the recovery effect reached 98.8%.Then,in the range of 0?18 L/min,flux would increase with the increase of bubbling,but in the range of 18?24 L/min,flux decreased.Under the conditions of18 L/min and 75?,the strengthening coefficient was 1.39.The blast flow was in the range of 0?12 L/min,the two-phase flow pattern was bubbly flow,and in the range of 15?24 L/min,the two-phase flow pattern was slug flow.When the on/off time of intermittent air blowing was 1min/3min,the water production flux was 23.2L/(m~2·h).Finally,from the perspective of economy,the water production cost of the 1200m~3/d vacuum membrane distillation seawater desalination project was 2.885 yuan/m~3,which was lower than RO. |
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