experimental and theoretical approach of a multi-stage
TRANSCRIPT
Experimental and Theoretical Approach of a Multi-Stage Membrane Distillation System
P. Boutikos, E.S. Mohamed, E. Mathioulakis and V. BelessiotisSolar 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 separationprocess, in which water vapor molecules or volatile compounds aretransferred from a hot aqueous solution (usually saline water), through amicroporous hydrophobic membrane, because of the partial pressuredifference created due to the temperature difference across themembrane.
Introduction – Membrane Distillation (MD)
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 of a pilot-scale
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 of the produced desalinated water.
The experimental approach of the multi-stage membrane distillation
system.
Aim
Mathematical Model Development
Mathematical Model Development
Mass and Energy Balances
Evaporator (Hot water stream):
, , ,
, , , , , ,
Stage Feedsalinesolution :
, , ,
, , , ,
, , , , ,
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 (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.
Transport Phenomena – Heat Transfer
Stage
In the feed channel the feed saline solution is initially pre-heated to its boilingpoint, Tsat, and then is partially evaporated at the membrane interface, wherenew 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 (Thw,in) at 75 oC. Hot water flow rate (Fhw): 1500 – 3500 L/h .
Feed Loop Feed inlet temperature (Tf,in) at 25 oC. Feed solution flow rate (Ff,sw): 40 – 120 L/h .
Cooling Loop Cold water inlet temperature (Tcw,in) at 30 oC. Cold water flow rate (Fcw): 1500 – 3500 L/h .
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):
Performance and Energy Efficiency Indicators
Influence of Hot Water Inlet Temperature
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 1000,5
1,0
1,5
2,0
2,5
Gai
ned
Out
put R
atio
, GO
RHot Water Inlet Temperature (oC)
300
600
900
1200 GOR STEC
STEC
(kW
h/m
3 )
40 60 800
10
20
30
40
Wat
er P
rodu
ctiv
ity (L
/h)
Hot Water Inlet Temperature (oC)
Rec
over
y R
atio
, RR
(%)
0
20
40
60 WP RR
Influence of Feed Flow Rate
50 100 150
0,5
1,0
1,5
2,0
STEC
(kW
h/m
3 )
GOR STEC
Feed Flow Rate (L/h)G
aine
d O
utpu
t Rat
io, G
OR
0
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 15010
20
30
40
WP RR
Feed Flow Rate (L/h)
Wat
er P
rodu
ctiv
ity (L
/h)
0
20
40
60
80
100
Rec
over
y R
atio
, RR
(%)
Model Validation – Hot Water Inlet Temperature
50 60 70 80 900
20
40
Pure water Experimental data Simulation Curve
Hot Water Inlet Temperature (oC)
Wat
er P
rodu
ctiv
ity (L
/h)
50 60 70 80 900
20
40
60Saline Solution (30 mS/cm)
Experimental data Simulation Curve
Hot Water Inlet Temperature (oC)W
ater
Pro
duct
ivity
(L/h
)
Model Validation – Feed Flow Rate
80 100 1200
20
40
60Pure water
Experimental data Simulation Curve
Feed Flow Rate (L/h)
Wat
er P
rodu
ctiv
ity (L
/h)
60 80 100 12010
20
30
40
50Saline Solution (30 mS/cm)
Experimental data Simulation Curve
Wat
er P
rodu
ctiv
ity (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 valueat 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%.
Conclusions
Thank you for your attention