Heat And Mass Transfer In The Membrane Modules Used For Liquid Desiccant Dehumidification And Performance Study Of The Dehumidification System

Energy-efficient and eco-friendly air dehumidification technology is crucial in improving the quality of people’s live and economic growth.Membrane liquid dehumidification technology is a promising air dehumidification technology because of its high dehumidification efficiency,absence of droplet entrainment,lack of liquid water condensation,low-grade energy consumption,and energy storage capability.Membrane liquid dehumidification technology uses a membrane with selective permeability to separate the air to be dehumidified from the dehumidification solution,and the air and the solution exchange heat and humidity through the membrane to achieve air dehumidification.The membranes are processed into the parallel plate and hollow fiber tube types,creating parallel plate and hollow fiber membranes,respectively.In the plate membrane module,a cooling device is added to form an internally cooled plate membrane module,which can effectively solve the problem of temperature rise of the dehumidification solution in the process of dehumidification.Although parallel plate membrane module is simple in structure and easy to manufacture,the packing fraction is low.Whereas hollow fiber membrane module is more complex in construction but the packing fraction and dehumidification efficiency are higher.In a word,parallel plate membrane and hollow fiber membrane play a significant role in the process of liquid dehumidification.A single dehumidifier cannot achieve continuous dehumidification,so a comprehensive liquid dehumidification system comprised of regenerators,pumps,and other equipment are required.However,the study on membrane modules and dehumidification systems still need to be improved.The following research is conducted on two types of membrane modules and liquid dehumidification systems:(1)Pure counter flow parallel plate membrane module with cooling tubes inside the solution channels.The momentum,energy,and mass equations of the three fluids(dehumidification solution,wet air,and cooling water)are solved with the corresponding conjugate heat and mass transfer boundary conditions.The criterion numbers describing the characteristics of heat and mass transfer(product of resistance coefficient and Reynolds number,Nussle number,and Sherwood number)are determined.Experiments are used to demonstrate the accuracy of model.Finally,the variation law of the criteria numbers calculated by the mathematical model is studied to disclose the conjugate heat and mass transfer of this membrane module in the process of liquid dehumidification.(2)Spiral hollow fiber membrane module.The spiral-type hollow fiber membrane module is proposed.The momentum,mass,and energy equations of the two fluids are established and numerically solved with the corresponding boundary conditions.The criterion numbers(product of drag coefficient and Reynolds number,Nussle number,and Sherwood number)for the air flow at various tube numbers(4,7,9)and pitch ratios(0.3,0.6,∞)are calculated.The dehumidification performance and conjugate heat and mass transfer characteristics of the spiraltype hollow fiber membrane bundle and the straight-type hollow fiber membrane bundle are compared and analyzed.It is found that the spiral-type hollow fiber membrane bundle has a greater dehumidification capacity than the straight type hollow fiber membrane bundle.The spiral-type hollow fiber membrane bundle is more conducive to heat and mass transfer enhancement,but it also increases the flow resistance to a certain extent.(3)Internally-cooled membrane liquid dehumidification system.Firstly,the steady-state characteristics of the system are analyzed,then the three key components(internally-cooled flat membrane dehumidifier,regenerator,and air-cooled chiller)that make up the system are analyzed in detail,and a mathematical model is established with the corresponding boundary conditions to solve the thermodynamic model of each component.The model is verified by experiment.Consequently,the performances of the system(dehumidification capacity,energy efficiency ratio,etc.)are analyzed under the design and other conditions.Then,the transient performance of the system during operation is considered.Combing the experimental data and the steady-state model and coupling each component,the transient model of the system is established.The transient performance of the system is studied under typical summer conditions.The performance changes of the system during the start-up and steady-state operation periods are analyzed to better adjust weather and load changes.This study lays the theoretical groundwork for engineering applications.Thus,the product of resistance coefficient and Reynolds number,Nusselt number and Sherwood number of the flow channels of the pure counter internally-cooled plate membrane module and the spiral hollow fiber membrane module under the conjugate heat and mass transfer boundary conditions are obtained in this paper.These basic data is the tools for future component structural optimization and system design.In addition,the steady-state thermodynamic performance and transient characteristics of the internally-cooled membrane liquid dehumidification system are obtained.These results will provide important theoretical groundwork for the operation and optimization of the system in practical application.