Simulation Study on Direct Contact Membrane Distillation Modules for High-Concentration NaCl Solution

Abstract

Membrane distillation technology, as a new membrane-based water treatment technology that combines the membrane technology and evaporation process, has the advantages of using low-grade heat, working at atmospheric pressure with simple configuration, etc. In this study, heat and mass transfer were coupled at the membrane surfaces through the user-defined function program. The effects of feed temperature, feed velocity and permeate velocity on temperature polarization were mainly investigated for a high-concentration NaCl solution. The temperature polarization was increased with the increase of feed temperature and the decrease of feed and permeate velocity. The effects of temperature, inlet velocity and solution concentration on the evaporation efficiency of the membrane module for co- and counter-current operations were investigated in detail. The counter-current operation performed better than co-current operation in most cases, except for the condition where the NaCl concentration was relatively low or the module length was long enough. In addition, the optimal membrane thickness for both PVDF and PTFE was studied. The optimal membrane thickness was found in the range of 10 to 20 μm, which corresponded to the highest permeate flux for the selected materials, pore size distribution, and operation conditions. Membrane material with lower thermal conductivity and larger porosity was prone to get higher permeate flux and had larger optimal membrane thickness. Increasing feed velocity or feed temperature could decrease the optimal membrane thickness.

Research Progress in Computational Fluid Dynamics Simulations of Membrane Distillation Processes: A Review

“…(1) The temperature polarization increases with the increase of the feed temperature and the decrease of the feed velocity and penetration velocity; The counter-current operation is better than the co-current operation in most cases, except for relatively low NaCl concentrations or sufficiently long module lengths [15].…”

“…(2) The optimal membrane thickness is between 10-20 μm, which corresponds to the selected materials, pore size distribution and the maximum permeability flux under the operating conditions. The membrane materials with low thermal conductivity and high porosity are easily obtain higher permeation flux and larger optimal membrane thickness, while the optimal membrane thickness can be reduced by increasing the feeding speed or temperature [15].…”

“…The effects of feed temperature (T feed ), feed velocity (v feed ), and permeate velocity (v permeate ) on temperature polarization were presented [ 15 ]. The authors established a simulation model of heat and mass transfer coupling on the membrane surface through User-defined function (UDF), and studied in detail the effects of temperature, inlet velocity ( Figure 3 and Figure 4 ) and solution concentration on the evaporation efficiency of the membrane module under co-current and counter-current operations.…”

“…In this structure, the probability of membrane fouling is minimal and the heat loss is less than the other MD types. However, due to disadvantages including the complex structure, membrane module being too difficult to design, mass transfer resistance having been increased by adding an air gap, GOP being the lowest versus the other MD types, multiple model variables, and modeling being too difficult [ 15 ], relevant studies on AGMD are much fewer than those on DCMD and VMD.…”

Development of mass and heat transfer coupled model of hollow fiber membrane for salt recovery from brine via osmotic membrane distillation

“…Ahmad et al [13] studied the RIS model for osmotic membrane evaporation of fruit juices and found that physical properties like viscosity and density variations during the process had a significant effect on water flux. Ni et al [22] studied the effect of temperature polarization in direct contact membrane distillation for NaCl concentration and observed that temperature polarization was enhanced with the increase of feed temperature and the decrease of feed and permeate velocity. Similarly, Lou et al [23] investigated coupled temperature and concentration polarization in the membrane distillation treatment of brines with the help of the CFD heat and mass transfer model and observed the effect of coupled temperature and concentration polarization.…”