OpenFOAM simulation for direct contact membrane distillation (DCMD) Noelle Chen (UG), Siu Fung Tang, Tyler Tsuchida, Albert S. Kim * Department of Civil and Environmental Engineering, University of Hawai‘i at Manoa, Honolulu, Hawai‘i, USA Introduction I Membrane Distillation (MD): a thermally driven separation process, where vapor is transported across a porous hydrophobic membrane (1) . I Advantages of Membrane Distillation 1. Temperature required for feed solution is below the boiling temperature. 2. Operating hydraulic pressure is (much) lower than that of reverse osmosis. 3. Water production is (almost) insensitive to solute concentrations. I Disadvantages of Membrane Distillation 1. Lower permeate flux compared to reverse osmosis. 2. Dependence on the availability of inexpensive/waste heat sources. DCMD I Direct Contact Membrane Distillation (DCMD) (2) : Hot feed and cold distillate streams flow in the counter-current mode for a constant temperature gradient in contact with membrane interfaces. Figure 1: Configuration of DCMD I A partial pressure gradient of vapor molecules generates the distillate flux as maintained by the transmembrane temperature difference. Recent Research Activities Figure 2: MD and DCMD publication numbers over the years in major journals. Microscopic System Configuration p 3 p 2 d p n(z ) T 1 (>T 2 ) T 2 T 3 z =0 z = δ m T 4 (<T 3 ) Figure 3: Schematic diagram of counter-current DCMD system (3) of pore diameter d p and membrane length δ m . Temperature linearly decreases along the membrane pore due to low permeate flux and heat conductivity. p 2 and p 3 are water-vapor pressures of the feed and permeate interfaces of the pore, respectively, and n (z ) is the number concentration of the gaseous water vapor, non-linearly increasing with T . Macroscopic System Configuration Figure 4: Rectangular mesh structure of computational domain Simulation using OpenFOAM (4) and ParaView visualization (5) Figure 5: Sequential view of simulation results for T feed = 75 ◦ C and ε =0.75. Operating Conditions Table 1: Operating parameters for DCMD simulations Parameter Operating Condition Length 20 cm Height of top water 1 cm Height of bottom water 1 cm Thickness of membrane 0.1 mm Thermal conductivity 0.2 W/m K Porosity 0.75 Pore diameter 0.1 μm Temperature of feed 75 ◦ C Temperature of distillate 20 ◦ C Boundary Conditions Table 2: Boundary conditions for feed (topWater), membrane, and distillate (bottomWater). Q vs. T : pseudo-effective and membrane-only Figure 6: Heat flux Q vs. temperature T varying from 40 ◦ C to 75 ◦ C for various ε and κ s . Transmembrane heat flux Δ Q versus Temperature T Figure 7: Convective heat flux ΔQ as the difference between Q of membrane-only and Q of pseudo-effective for varying porosities from 0.65 to 0.80 with κ s =0.2 W/m K. Conclusion 1. A flat sheet DCMD phenomena was successfully modelled using a new algorithm embedded in OpenFOAM, which is named dcmdFoam. 2. The current simulations can include various parameters, such as thermal conductivity, porosity, fluid speed and temperatures, and module geometry. 3. The convective heat flux was calculated as proportional to the vapor mass transfer rate. References 1. Desalination and Water Treatment, 58 (2017) 351–359 2. Journal of membrane science, 428 (2013) 410–424 3. Journal of Membrane Science, 455 (2014) 168–186 4. OpenFOAM: http://openfoam.org and http://oepnfoam.com 5. ParaView: https://www.paraview.org/ Acknowledgements