Studies On Plastic Heat Transfer Element Used In The Dewvaporation Unit

The dewvaporation is a novel desalination process, in which air is used as a carrier gas to carry water vapor from feed saline water and to produce fresh water by the following condensation, and the condensation latent heat is transferred to the evaporation side to provide evaporative latent heat for the saline water.In this work, two baffled shell-tube desalination columns (one was equipped with carbon-filled-polypropylene heat transfer tubes and the other was polypropylene tubes) were designed and built to carry out the dewvaporation process. The inside surfaces of these tubes were roughened, which greatly improved their wetting states.The effects of several operating parameters (feed water temperature, water flow rate, carrier air flow rate and external steam flow rate) on the productivity of the two columns were investigated experimentally. The fresh water productivity of the carbon-filled-polypropylene column was usually 0.05~0.30 kg m-2h-1, and the experimental results showed that the feed water temperature had an positive effect on the productivity, while the flow rates of feed water, carrier air and external steam should be optimized in the range of 0.008~0.012 kg m-1s-1, 0.66~1.00 kg m-2s-1and 0.006~0.014 kg m-2s-1, respectively. As for the polypropylene column, the productivity could be achieved about 0.10~0.68 kg m-2h-1, and the suitable operating ranges for the flow rates of feed water, carrier air and external steam were 0.008~0.012 kg m-1s-1, 0.70~1.30 kg m-2s-1 and 0.006~0.014 kg m-2s-1, respectively.The salinity of the produced water was determined to be fewer than 10 mg L-1, which was comparable to that of the traditional thermal desalting processes.The discharged heat energy of the humid air was recovered by recycling or condensing the exhaust gas. When the carrier gas was condensed, it was found that the gained out ratio of the whole desalination system had been improved to above 4.The modeling investigation of such a desalination process was also presented. A heat-mass transfer model is established in order to study the correlations among productivity, thermal efficiency, physicochemical parameters (gas/liquid phase temperatures, heat/mass transfer coefficients, Reynolds number, etc) and operating conditions (the temperature of feed water, the flow rates of external steam, feed water and air); at the same time, the effects of operating conditions on the productivity and thermal efficiency of the column are investigated both theoretically and experimentally. Furthermore, the heat transfer coefficient of the carbon-filled-polypropylene dewvaporation column is in the range of 90~430 W m-2°C-1, and that of polypropylene column is 250~1100 W m-2°C-1, which demonstrates that the heat transfer performance of the plastic element is not lower than that of the coppery element.