Mechanism Of Mass And Heat Transfer For Water Vapor Transporting Through Composite Membranes

The flue gas from thermal power plants contains a huge amount of water vapor,which is a waste of water resource and has an adverse effect on the ecological environment as well.Meanwhile,it is easy to cause the low temperature corrosion in tail heating surfaces of the thermal power unit for the low-temperature flue gas with a high content of water vapor.Therefore,recovering water vapor,sensible heat and latent heat from flue gas is a significant research in the deep energy saving and emission reduction.Using the gas membrane separation technology in water vapor and waste heat recovery from flue gas has gradually drawn wide attention in the industry.As a kind of membrane material in this technology,the ceramic composite membrane has a wide application prospect.This paper has studied the mechanism of mass and heat transfer for water vapor transporting through the ceramic composite membrane,the main contents are as follows.A specific method of water recovery from flue gas with hollow micro-nano porous ceramic composite membranes has been put forward in order to deal with water vapor in exhaust gas from thermal power plants.An experimental study was conducted to illustrate the effect of feed gas and permeate side conditions on water and heat recovery from the simulated flue gas.Hollow micro-nano porous ceramic composite membranes of inner side coating with the selective layer of different pore sizes(20 nm,30 nm.50 nm,100 nm)were characterized and the performance of water vapor recovery was studied and compared based on both theoretical and experimental analyses.In experiments,a binary gas mixture of nitrogen and water vapor was prepared as the simulated flue gas,and the negative pressure in permeate side was provided by a vacuum pump.When the pore size of selective layer in composite membrane is lower than 50 nm,the capillary condensation will appear.For the feed gas with the temperature of 70℃(RH=100%),the water recovery ratio can reach about 20-60%by using four different pore-sized membranes under the different inlet gas flow conditions.Hence,the optimum membrane area and the optimal velocity that gas transporting on the surface of membrane exist for different pore-sized membranes and inlet conditions,which provides the theoretical research for the optimal design of membrane module.The ceramic composite membrane of inner side coating with a selective layer of 20 nm pore size was used in the module.The cooling water was used on the permeate side and a water pump was installed in the outlet of the module in order to build the negative pressure in permeate side.A parametric study was conducted to illustrate the behavior of heat transfer and water recovery in the membrane module by varying operating parameters in experiments.For the hollow micro-nano porous ceramic composite membrane,the recovered water is of high quality by using the cooling water with the temperature higher than the feed gas dew point,and the water recovery ratio can reach above 20%.For the feed gas with saturated water vapor,however,the water recovery ratio and heat recovery efficiency of the module can be up to above 80%and 40%respectively with lower cooling water temperature than feed gas temperature.In proper condition,these values can both reach above 90%.The experimental and simulative studies were carried out to investigate the performance of recycling water vapor and heat with using the microporous ceramic membrane and the membrane module.The mass and heat transfer process was studied and the heat transfer model was built to study the effect of mass transfer on heat transfer on both sides of the membrane and the microporous ceramic membrane with the simulated flue gas flowing outside the membrane and the cooling water flowing in the membrane tube.In this study,the physical model of membrane module and the mathematical model of mass and heat transfer were built.The comparative analysis of the calculated results and the experimental data was conducted to modify the model.On the feed gas side,the percentage of heat transfer due to mass transfer in the total heat transfer increases with the increasing mass transfer flux,and the heat transfer due to mass transfer becomes the major one in the total heat transfer at last.However,the feed gas temperature and the feed gas flow rate have little effect on heat transfer on the permeate side,so the influence of mass transfer on heat transfer can be negligible on the cooling water side.In addition,it is reasonable to think that the heat conduction is the major heat transfer inside the membrane,and the heat transfer due to the mass transfer in the membrane can be neglected.The performance of the self-prepared membrane in water and heat recovery was compared with that of the commercial microporous ceramic membrane.For the simulated flue gas with saturated water vapor or unsaturated water vapor,the recovered water of the self-prepared membrane is similar to that of the commercial α-Al2O3 microporous ceramic membrane under the range of experimental temperatures(50-85℃).At the same feed gas flow rate,the water recovery ratio of the membrane in flue gas with saturated water vapor is higher than that in the unsaturated condition.Therefore,it is better to recycle water vapor from flue gas with water vapor in the saturated condition and the micro pore-sized ceramic membrane is suitable for being installed behind the flue gas desulfurization.The performance of heat recovery of the commercial membrane is slightly better than that of the self-prepared membrane.In summary,the self-prepared membrane can completely replace the commercial microporous ceramic membrane to recycle water vapor and waste heat from flue gas.Research results can provide theoretical guidance for developing low-cost ceramic membranes.The computation model of water recovery was built in the process of water vapor permeating through the nanoporous ceramic composite membrane.The research has revealed the condition of the occurrence of capillary condensation about water vapor permeating through porous ceramic composite membranes and has carried out the experimental verification by using the ceramic composite membrane of outer side coating with a selective layer of 10 nm pore size.In addition,the computation model can be used to study the performance of water recovery in different pore-sized membranes with the capillary condensation for further study,simulate a large number of experimental conditions to reduce the workload of doing experiments,and provide the theoretical support for the design of membrane module.For the ceramic membrane with the layer of less than 50 nm pore size which has a direct contact with flue gas,water vapor can still be recycled when the temperature of the cooling water is higher than the water dew point of flue gas.And the water recovery ratio increases with the decrease of pore size,which has a positive correlation with the intensity of capillary condensation.For the membrane of 10 nm pore size and smaller pore size,the water recovery ratio can be up to above 15%,even more than 25%.However,for the membrane of micro pore size,it is incapable of recycling water vapor from flue gas in the same condition because there is no capillary condensation in the membrane.The ternary gas mixture(nitrogen,water vapor and sulfur dioxide)has been used in experiments,and results show that the presence of sulfur dioxide in feed gas has no significant effect on the pH of the cooling water under the condition of capillary condensation,which means that the sulfur dioxide in flue gas will not produce significant impact on the quality of recovered water with the membrane in capillary condensation mechanism.