Study On Deeply Concentrating Seawater And Brine By Multiple-effect Membrane Distillation

Many places in our country suffer from a shortage of fresh water. Water shortagelimits the socioeconomic sustainable development. Desalination of seawater is aneffect way to obtain fresh water. Seawater desalination will play an important role inguaranteeing water supply. Membrane distillation (MD) is an emerging separationmethod which combines the advantages of membrane separation and therma ltechnology. The driving force for mass transport in the MD process is supplied by thevapor pressure difference for the evaporating species existing across the poroushydrophobic membrane. Potential advantages of MD process over traditionalevaporation processes include operation at low pressures, lower temperature as wellas ease of process scale-up, avoid of corrosion as a result of inertness of hydrophobicpolymer membrane material, and so on.Ion exchange was integrated with multiple-effect membrane distillation toestablish a desalination process for deep-concentration of seawater. Ion exchange canprevent hydrophobic porous membrane from the fouling of CaSO4at high waterrecovery rate. After de-calcium treatment with ion exchange, the calciumconcentration in seawater could be reduced from410mg/L to35mg/L and thecalcium concentration in the brine could be reduced from695mg/L to64mg/L. Theremoval percentage of calcium ions from seawater or brine was higher than90%while the lost percentage of magnesium was lower than10%. Multiple-effectmembrane distillation (MEMD) process can recover condensation heat of distillate byusing air-gap membrane-distillation-based hollow fiber membrane module with thefunction of internal heat recovery. The influences of various operating variations onthe performance of the MEMD process were studied by single factor experiments,which was indicated by permeate flux (J), gained output ration (GOR). Theexperimental results showed that the maximum J and GOR were6.07L/m2h and13.2,when de-calcium seawater were concentrated to250g/L under optimized operationalconditions, the electrical conductivity of the permeate was still less than200μm/cm, Jand GOR were still up to3.61/m2h and4.96. The results of factorial experimentsshowed that permeate flux increased with the increase of the hot feed-in temperatureand feed flow rate, GOR increased with both the increase of the hot feed-intemperature and the cold feed-in temperature. In addition, both the cold feed-intemperature and feed flow rate had opposite effects to the permeate flux and GOR, thus, there is a trade-off between permeate flux and GOR in the practical application.Also, long-term stabilities of the MEMD process were investigated by using thede-calcium seawater as feed. The stability experiments lasted for about45days, inwhich the MEMD process showed well stability.