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
Solar and Other Energy Systems Laboratory NCSR «DEMOKRITOS» 14 – 16 September 2016, Athens
Introduction Aim Mathematical Model Development Experimental Approach Model Validation Conclusions
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.
Advantages Disadvantages
Production of high purity distillate. Reduced production of the water vapor flux. The possibility of operating at lower temperatures. The temperature polarization affects 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 supersaturated solutions. High specific energy consumption, mainly due to the heat losses by conduction. The capability of utilizing solar thermal energy or even waste heat from other processes.
Optimized design
unit The development of a mathematical model, which can be used to study the effect of the significant parameters that influence the quality and quantity
The experimental approach of the multi-stage membrane distillation system.
Mass and Energy Balances
Evaporator (Hot water stream):
, , , ,,, ,,,
Stage Feed saline solution:
,
,
,
, ,
, , ,
Condenser (Cold water stream):
, , ,
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).
,
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 (Qconds,st = Qevap).
The latent heat of condensation is transferred through the condensing film and
the foil by conduction and heats up the feed saline stream.
Stage
In the feed channel the feed saline solution is initially pre-heated to its boiling
point, Tsat, 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.
,
Multi-stage membrane distillation unit that employs both the vacuum
membrane distillation and multi-stage distillation concept.
Membrane module Heating Loop
Feed Loop
Cooling Loop
Vacuum system (Vacuum pressure at ~ 800 mbar)
Water Productivity, Fdist (L/h):
∗
Recovery Rate, RR (%): 100 ∗ Gained Output Ratio, GOR: Specific Thermal Energy Consumption, STEC (kWh/m3):
Pure water, Fhw= 3500 L/h, Tf,in= 25 oC, Ff,sw= 80 L/h, Tcw,in= 30 oC, Fcw= 3500 L/h
40 60 80 100 0,5 1,0 1,5 2,0 2,5
Gained Output Ratio, GOR Hot Water Inlet Temperature (
300 600 900 1200 GOR STEC
STEC (kWh/m
3)
40 60 80 10 20 30 40
Water Productivity (L/h) Hot Water Inlet Temperature (
Recovery Ratio, RR (%)
20 40 60 WP RR
50 100 150 0,5 1,0 1,5 2,0
STEC (kWh/m
3)
GOR STEC
Feed Flow Rate (L/h) Gained Output Ratio, GOR
250 500 750
Pure water, Thw,in= 75 oC, Fhw= 3500 L/h, Tf,in= 25 oC, Tcw,in= 30 oC, Fcw= 3500 L/h
50 100 150 10 20 30 40 WP RR
Feed Flow Rate (L/h) Water Productivity (L/h)
20 40 60 80 100
Recovery Ratio, RR (%)
50 60 70 80 90 20 40 Pure water Experimental data Simulation Curve
Hot Water Inlet Temperature (oC) Water Productivity (L/h)
50 60 70 80 90 20 40 60 Saline Solution (30 mS/cm) Experimental data Simulation Curve
Hot Water Inlet Temperature (oC) Water Productivity (L/h)
80 100 120 20 40 60 Pure water Experimental data Simulation Curve
Feed Flow Rate (L/h) Water Productivity (L/h)
60 80 100 120 10 20 30 40 50 Saline Solution (30 mS/cm) Experimental data Simulation Curve
Water Productivity (L/h) Feed Flow Rate (L/h)
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%.