Heat And Mass Transfer In Membrane Modules Used For Humidity Control

Indoor air humidity is closely related to people’s life and industrial production. It isbeneficial for human health and production safety to manage indoor air humidity in a properrange. In recent years, membrane modules play an important role in humidity control.According to the shape of the flow channel, membrane modules can be divided intoparallel-plate membrane module, plate-fin membrane module, hollow fiber membrane moduleand cross-corrugated triangular duct membrane module. The transport phenomenon isespecially complex in the latter two types. Hollow fiber membrane module has a structuresimilar to a shell-and-tube heat exchanger, water vapor in lumen side and air in shell side canexchange heat and moisture through the membrane. The membrane has a selectivity toprevent liquid water from permeating but allow water vapor to permeate. Cross-corrugatedtriangular duct membrane module has a structure similar to chevron plate heat exchanger.Fresh air and exhaust air exchanges heat and moisture through the membrane. Thecross-corrugated triangular duct can effectively enhance heat and mass transfer in the module.The conjugate boundary condition on the membrane and flow maldistribution in the modulehave significant effect on predicting module’s performance. However, these two elements areusually ignored in the mathematical model in current researches. To reveal the mechanism ofheat and mass transfer in hollow fiber membrane module and cross-corrugated triangular platemembrane module, mathematical models are established in this work:(1) Cross flow hollow fiber membrane module, ordered array tube bank. A periodic cellcontaining inlined or staggered tube banks is selected as research object. Conservationequations in shell side are established, and conjugated boundary condition is applied tomembrane surface. The Nusselt number, Sherwood number and friction factor are obtained.Humidification experimental results are used to verify the model. The results show that masstransfer coefficient and friction factor are higher in staggered tube array due to strongerdisturbance and boundary layer separation. The higher the packing fraction is, the larger themass transfer coefficient and pressure drop is. The pitch to diameter ratio has important effecton mass transfer and resistance characteristic. And the effect caused by transverse pitch todiameter ratio is more evident.(2) Counter-current flow hollow fiber membrane module, random array tube bank. Arepresentative cell containing finite random packed fibers is selected as research object.Conservation equations in shell side and lumen side are established, and conjugated boundarycondition is applied to membrane surface. The Nusselt number, Sherwood number and friction factor are obtained. Humidification experimental results are used to verify the model.The results show that the randomness of the tube array leads to significant flowmaldistribution in shell side. This flow maldistribution deteriorates module’s performancedramatically. Conjugated boundary conditions on the membrane make this deterioration muchhigher.(3) Flow maldistribution in hollow fiber membrane module. To get the flow distributionin the module, hollow fiber membrane bundle is simplified as porous medium. The heat andmass conservation equations are solved to get temperature and humidity distribution in themodule. After that, the effect of flow distribution on sensible cooling and humidificationefficiency can be analyzed. The results show that shell side inlet/outlet effect has effect onflow distribution in the module. The higher the packing fraction is, the more homogeneous theflow distribution is. For a cross flow membrane module whose packing fraction ranges from0.1to0.3, the sensible cooling efficiency can deteriorate by3-30%, and humidificationefficiency can deteriorate by26-58%. For a counter-current flow membrane module whosepacking fraction ranges from0.1to0.3, the sensible cooling efficiency can deteriorate by3-36%, and humidification efficiency can deteriorate by5-39%.(4) Cross-corrugated triangular duct membrane module. A representative cell containingfinite cross-corrugated triangular ducts is selected as research object. Conservation equationsin the flow channel are established, and conjugated boundary condition is applied tomembrane surface. The Nusselt number, Sherwood number and friction factor are obtained.One-step made asymmetric cellulose acetate membrane is used as the exchanger material toenhance mass transfer. The modules’ performance is compared with different flow channelstructures or different membrane materials. The results show that the larger the apex angle ofthe channel is, the more homogeneous the flow distribution is, leading to an enhancement onheat and mass transfer. The smaller the apex angle of the channel is, the more deviationbetween results calculated by conjugated boundary condition and realistic boundary condition.Compared to parallel-plate or plate-fin membrane module fabricated by composite membrane,the cross corrugated triangular duct membrane module fabricated by asymmetric membranecan enhance sensible heat efficiency by20%, and enhance latent heat efficiency by40%.However, due to the complex flow channel structure, its friction factor is about1.5-4times ofthe former two.