Experimental and Theoretical Approach of a Multi-Stage Membrane - PowerPoint PPT Presentation

Experimental and Theoretical Approach of a Multi-Stage Membrane Distillation System P. Boutikos, E.S. Mohamed, E. Mathioulakis and V. Belessiotis Solar and Other Energy Systems Laboratory NCSR «DEMOKRITOS» 14 – 16 September 2016, Athens

Contents  Introduction  Aim  Mathematical Model Development  Experimental Approach  Model Validation  Conclusions

Introduction – Membrane Distillation (MD)  Membrane Distillation (MD) : A thermal membrane separation process, in which water vapor molecules or volatile compounds are transferred from a hot aqueous solution (usually saline water), through a microporous hydrophobic membrane, because of the partial pressure difference created due to the temperature difference across the membrane.

Introduction – Membrane Distillation (MD) Advantages Disadvantages Production of high purity distillate. Reduced production of the water vapor flux. The possibility of operating at lower The temperature polarization affects temperatures. negatively the flux through the membrane. Operates at relative low pressures. The trapped air in the membrane pores increases the resistance to mass transfer. It can treat high concentration or High specific energy consumption, mainly due supersaturated solutions. to the heat losses by conduction. The capability of utilizing solar thermal energy or even waste heat from other processes.

Aim The development of a mathematical model, which can be used to study the effect of the significant parameters that influence the quality and quantity of the produced desalinated water. Optimized design of a pilot-scale unit The experimental approach of the multi-stage membrane distillation system.

Mathematical Model Development

Mathematical Model Development Mass and Energy Balances  Evaporator (Hot water stream): � ��,�� � � ��,��� � � �,��� �� ��� � �� �� ��� � � ��,�� � �,�� � ��,�� � � ��,��� � �,�� � ��,���  Stage �Feed saline solution�: �� � � �,��� �� � �,�� � � �,��� �� ��� �� � �,�� ���� � � �,��� �� ���� � �,�� � �,��� �� � �,�� � ��� � � � ����,�� �� ��� � � �,�� �,�� � � ����,�� �� ���  Condenser (Cold water stream): � �� �� ���� � � �� � �,�� � ��,��� � � ��,��

Transport Phenomena – Mass Transfer  Mass Transfer :  Feed boundary layer (concentration polarization).  Through the membrane pores.  The mass transfer in the feed boundary layer can be described by the film theory. � � ���� ���� ��� � �,��� � � � � � � �,�  The mass flux through the membrane is proportional to the water vapor partial pressure difference (Darcy’s Law). � � � � � � �,� � � ���� o � � : membrane mass transfer coefficient  Function of membrane structural properties  Knudsen diffusion, Viscous flow, Molecular diffusion or combination

Transport Phenomena – Heat Transfer Evaporator  The total heat is transferred from the bulk feed through the hot water boundary layer to the feed membrane interface by conduction. � ��� � � ��� �� �� � � �,� �  The transferred heat is consumed, at the membrane surface, only by the latent heat of vaporization. � ���� � � �,���� ∆� � Stage  The generated vapor is completely condensed at the surface of the impermeable foil (Q conds,st = Q evap ).  The latent heat of condensation is transferred through the condensing film and the foil by conduction and heats up the feed saline stream.

Transport Phenomena – Heat Transfer Stage  In the feed channel the feed saline solution is initially pre-heated to its boiling point, T sat , and then is partially evaporated at the membrane interface, where new water vapor is produced. � ����,�� � � �,��� ∆� � Condenser  The produced vapor from the last stage is completely condensed.  The latent heat of condensation is transferred through the condensing film and the foil by conduction.  In the boundary of the cold water stream the heat is transported by convection. � ��� � � ��� � ����,�� � � ��

Experimental Desalination Unit

Experimental Desalination Unit  Multi-stage membrane distillation unit that employs both the vacuum membrane distillation and multi-stage distillation concept.  Membrane module  Heating Loop  Hot water inlet temperature (T hw,in ) at 75 o C.  Hot water flow rate (F hw ): 1500 – 3500 L/h .  Feed Loop  Feed inlet temperature (T f,in ) at 25 o C.  Feed solution flow rate (F f,sw ): 40 – 120 L/h .  Cooling Loop  Cold water inlet temperature (T cw,in ) at 30 o C.  Cold water flow rate (F cw ): 1500 – 3500 L/h .  Vacuum system (Vacuum pressure at ~ 800 mbar)

Performance and Energy Efficiency Indicators  Water Productivity, F dist (L/h): � ���� � ���������� ���� ∗ �������� ���� ���������� ���� ����  Recovery Rate, RR (%): �� � 100 ∗ ���� ���� ���� ����������� ����  Gained Output Ratio, GOR: ��� � ���� �����  Specific Thermal Energy Consumption, STEC (kWh/m 3 ): ���� ������ ����������� ���� � ���������� ���� ����

Influence of Hot Water Inlet Temperature 40 60 2,5 WP GOR 1200 RR STEC Gained Output Ratio, GOR Recovery Ratio, RR (%) Water Productivity ( L/h ) 30 2,0 3 ) 40 900 STEC ( kWh/m 20 1,5 600 20 10 1,0 300 0 0 0,5 40 60 80 40 60 80 100 o C ) o C ) Hot Water Inlet Temperature ( Hot Water Inlet Temperature ( Pure water, F hw = 3500 L/h, T f,in = 25 o C, F f,sw = 80 L/h, T cw,in = 30 o C, F cw = 3500 L/h

Influence of Feed Flow Rate 40 100 750 WP GOR 2,0 RR STEC 80 Gained Output Ratio, GOR Recovery Ratio, RR (%) Water Productivity ( L/h ) 3 ) 30 500 1,5 STEC ( kWh/m 60 40 1,0 20 250 20 0,5 10 0 0 50 100 150 50 100 150 Feed Flow Rate (L/h) Feed Flow Rate (L/h) Pure water, T hw,in = 75 o C, F hw = 3500 L/h, T f,in = 25 o C, T cw,in = 30 o C, F cw = 3500 L/h

Model Validation – Hot Water Inlet Temperature 60 Saline Solution ( 30 mS/cm ) Pure water Experimental data Experimental data Water Productivity (L/h) Simulation Curve Simulation Curve Water Productivity (L/h) 40 40 20 20 0 0 50 60 70 80 90 50 60 70 80 90 Hot Water Inlet Temperature (oC) Hot Water Inlet Temperature (oC)

Model Validation – Feed Flow Rate 60 50 Saline Solution ( 30 mS/cm ) Pure water Experimental data Experimental data Water Productivity (L/h) Simulation Curve Simulation Curve Water Productivity (L/h) 40 40 30 20 20 0 10 80 100 120 60 80 100 120 Feed Flow Rate ( L/h) Feed Flow Rate ( L/h)

Conclusions  An experimental multi-stage membrane desalination unit was designed and tested to several operating conditions.  A mathematical model was developed with aim of maximizing the productivity and the energy optimization of the process.  The water productivity and the recovery ratio increases with the increase of the hot water inlet temperature. The GOR also increases and obtains an asymptotic value at high values of the hot water inlet temperature. However, the STEC decreases as the hot water inlet temperature increases.  Increasing the feed flow rate the residence time decreases and the water vapor flux and the recovery ratio decreases. The GOR ratio increases, whereas the specific thermal energy consumption increases.  The model predictions were in a good agreement with the experimental results, presenting low deviations (1 – 15%) from the experimental data for the pure water, whilst for the saline water the deviations were in the range of 5 – 22%.

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