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 Distillation1. 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 Distillation1. 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 colddistillate streams flow in the counter-current mode for a constanttemperature gradient in contact with membrane interfaces.

Figure 1: Configuration of DCMD

I A partial pressure gradient of vapor molecules generates the distillateflux 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

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p3p2 dp n(z)

❤♦t ❢❡❡❞

T1 (> T2)T2

T3z = 0

z = δm

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T4 (< T3)

Figure 3: Schematic diagram of counter-current DCMD system(3) of pore diameter dp andmembrane length δm. Temperature linearly decreases along the membrane pore due to lowpermeate flux and heat conductivity. p2 and p3 are water-vapor pressures of the feed andpermeate 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 Tfeed = 75◦C and ε = 0.75.

Operating Conditions

Table 1: Operating parameters for DCMD simulations

Parameter Operating ConditionLength 20 cmHeight of top water 1 cmHeight of bottom water 1 cmThickness of membrane 0.1 mmThermal conductivity 0.2 W/m KPorosity 0.75Pore diameter 0.1 µmTemperature of feed 75◦CTemperature 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 κs.

Transmembrane heat flux ∆ Q versus Temperature T

Figure 7: Convective heat flux ∆Q as the difference between Q of membrane-only and Q ofpseudo-effective for varying porosities from 0.65 to 0.80 with κs = 0.2 W/m K.

Conclusion

1. A flat sheet DCMD phenomena was successfully modelled using a newalgorithm embedded in OpenFOAM, which is named dcmdFoam.

2. The current simulations can include various parameters, such asthermal conductivity, porosity, fluid speed and temperatures, andmodule geometry.

3. The convective heat flux was calculated as proportional to the vapormass transfer rate.

References

1. Desalination and Water Treatment, 58 (2017) 351–3592. Journal of membrane science, 428 (2013) 410–4243. Journal of Membrane Science, 455 (2014) 168–1864. OpenFOAM: http://openfoam.org and http://oepnfoam.com

5. ParaView: https://www.paraview.org/

Acknowledgements