Concentrating Aqueous Solutions Of Semi-volatile Organic Acids By Using Multiple-Effect Membrane Distillation

Membrane distillation (MD) is an innovative and efficient membrane-basedseparation technique arising several decade years ago. As it has a wide range oftechnical benefits such as mild operation condition, high rejection, large evaporationarea, avoiding corrosion problem and utilizing low-temperature thermal energy, muchattention was paid to the research and application of MD process. However, MD isnot yet implemented at industrial scale. One of the biggest barriers for preventing MDbeing a viable separation technology in industry is extremely lower thermal energyefficiency in traditional MD process than that in conventional multiple effectdistillation (MED) or multi-stage flash (MSF) process.Triggered by recovery of the latent heat of vaporization in conventional MEDand MSF processes, multiple-effect operation was proposed for MD process. So it isof significance to study the MD process with recovering of the latent heat ofvaporization for the industrial application of MD. Meanwhile, there are manydisadvantages in the industrial technology for concentrating dilute aqueous solutionscontaining the semi-volatile organic acids. Therefore, in this paper, multiple-effectmembrane distillation (MEMD) process for concentrating semi-volatile organic acidsfrom their individual aqueous solutions was examined by using a hollow fiber-basedAGMD module and an external heat exchanger. Aqueous solutions of glyoxylic acid,glycolic acid, lactic acid, pyruvic acid, malonic acid and glutaric acid were used asmodel feed. For a feed of1wt%, the final concentrations of each organic acid exceptpyruvic acid were at least15times of the initial feed concentration in the reservoir.The operation performance of MEMD process was characterized with permeation flux(J), performance ratio(PR) and acid rejection rate (R). A series of experiments wereconducted to evaluate the effects of heated feed-in temperature, cold feed-intemperature, feed-in volumetric flow rate and feed-in concentration on theperformance of MEMD process. The experimental results showed that in all theexperiments, R for all the organic acids tested was all above97%and R for glutaricacid was as high as99%; with the increase of heated feed-in temperature, thepermeation flux and performance ratio increased; with the increase of cold feed-intemperature, the permeation flux decreased while PR increased; compared with theheated feed-in temperature, the effect of cold feed-in temperature on the permeation flux and performance ratio is smaller; when the feed-in volumetric flow rate increased,the permeation flux increased, but the performance ratio declined sharply; when thefeed-in concentration increased, the water vapor pressure over the feed decreased andthe viscosity of the solution increased, thus both of the permeation flux andperformance ratio decreased. Maximum values of J, PR and R obtained were4.8L h-1m-2,9.84and99.93%, respectively. Moreover, MEMD process demonstrated a fairlygood stability in a long-term experiments lasting for30days when aqueous solutionof4wt%lactic acid...