- Email: [email protected]
Sustainable Seawater Desalination by Permeate Gap Membrane Distillation Technology
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 110 (2017) 346 – 351
1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia
Sustainable seawater desalination by permeate gap membrane distillation technology Farzaneh Mahmoudi,*, Hasham Siddiqui, Mohammadebrahim Pishbin, Gholamreza Goodarzi, Saeed Dehghani, Abhijit Date, Aliakbar Akbarzadeh School of Engineering, RMIT University, Melbourne 3000, Australia
Abstract Membrane distillation (MD) as a novel thermally-driven process with moderate operating temperatures, is an effective technology for salt water desalination, by this process, it becomes achievable to directly utilize low-temperature waste heat or solar energy. This research is aimed to design a lab scale plate-and-frame permeate gap membrane distillation (PGMD) module, with internal heat recovery characteristic which could significantly reduce the energy consumption of the process. In this paper, the PGMD module performance is experimentally investigated for fresh and saline water feed, in terms of permeate water flux, specific thermal energy consumption (STEC) and gained output ratio (GOR). The experimental results show, by increasing the saline feed flow rate in a range of (0.4-1) lit/min, the fresh water flux increase from 3 to 11 kg/m2.hr, however, the thermal energy demand of process also increased by nearly 20 %. As a result, optimization of the MD module performance is achievable, by adjusting the effective membrane surface area and feed flow rate, to improve internal heat recovery and also produce higher fresh water rate. ©©2017 The Authors. Published by Elsevier Ltd. This 2017 The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords:Sustainable desalination; Membrane distillation; Flat module design; Specific thermal energy consumption; Permeate flux
1. Introduction The need for fresh water is considered as a critical international problem, according to the World Water Council, 17% of the world population will be living in short of the fresh water supply by 2020 [1].
* Corresponding author. Tel.: 61-040-232-5294. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.151
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
Nomenclature Cp Ei GOR ∆Hv Jp ݉ሶ οܲ qSTEC S TEi TEo TCi TCo ܸሶ ܸሶ
specific heat capacity (J/kgK) total energy input (J/s) gained output ratio (-) specific heat of vaporization (J/kg) permeate flux (kg/m2.s) feed flow rate (kg/s) pressure drop (Pa) specific thermal energy consumption (kWh/m3) salinity (g/Kg) temperature at evaporator inlet (ºC) temperature at evaporator outlet (ºC) temperature at condenser inlet (ºC) temperature at condenser outlet (ºC) feed flow rate (m3/s) permeate flow rate (m3/s)
The demand for alternative sustainable water sources including ground water, desalinated water and recycled water is increased, in recent years. As a result, the implementation of desalination plants is growing on a large scale. Fresh water can be derived from sea water by evaporation processes (e.g., multi-stage flash) or by membrane based processes, such as reverse osmosis, electro dialysis and membrane distillation. The commercially developed RO technology for desalination requires large amounts of energy in the form of electricity or shaft power to drive the pump, which the electricity is presently being generated from non-renewable and polluting fossil fuels [2]. In contrast, MD use lower top temperature with respect to the traditional distillation processes, making it suitable for using waste heat or solar heat, besides, aqueous solutions of salts with higher concentrations than seawater can be treated by MD. Reducing discharge volumes and increasing the water recovery factor up to 95% which considerably diminish the environmental impact of the brine disposal are advantages of the MD process [3]. However, the MD process is still under evaluation, the lack of experimental data has indicated that there is a need for more intensive research in this field, both experimentally and mathematically. The central issues are the external energy source in MD units, lack of MD membranes and fabrication of modules to suit each MD configuration, difficulties with long term operations with risk of membrane pores wetting and fouling. Thus, the optimization of different MD installations and systems is still to be done in order increase MD performance and decrease energy consumption [4-6]. 2. Experimental work 2.1. Description of setup A novel optimized experimental approach was followed up by a lab scale plate-and-frame PGMD module with two transparent poly acrylic sheets with 25 mm thickness for evaporator and condenser sides and the permeate water sheet with 6 mm thickness (Fig. 1). As it is clear from this figure, two main cylindrical channels with 10 mm hole diameter is milled in each evaporator and condenser plate for the flow inlet and outlet manifold. To have a more optimized flow distribution from the inlet and outlet, a set of 11*3 mm distributor holes, was drilled in each flow channel. The hydrophobic PTFE membrane with 0.22 µm nominal pore size, (140-200) µm thickness and polypropylene (PP) as support, with an effective surface area (760*160) mm^2 was applied, the permeate channel was separated from condenser channel by an impermeable 100 μm clear PP film, which filled by two similar plastic net spacer either as a mechanical support between membrane and condensing polymeric film and turbulence promoter. The schematic diagram of the lab scale experimental setup is shown in Fig. 2. The feed water is pumped from storage tank using a 12 V small DC water pump. A mechanical filter with 0.2 mm pore size have used before the
347
348
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
pump to protect the setup from unwanted solid. To adjust the inlet feed temperature to the condenser channel, the lab cooling circuit used to control the input temperature to the condenser channel. The saline feed water gradually preheats by flowing through the condenser channel by obtaining latent heat of condensation and conduction via the PP condensing film. Then, the condenser outlet temperature increases by flowing through a copper heating coil which is immersed in an insulated electric water tank (2.4 kW URN) to reach the established evaporator channel inlet temperature. By estimating the required external source demand in this stage, the integration of the evacuated tube solar collectors (ETSC) could be evaluated for the sustainable outdoor desalination setup. a
b
Fig. 1. a) Module's SolidWorks assembly design, b) PGMD module arrangement with internal heat recovery
Fig. 2. Schematic diagram of the MD experimental setup
During the evaporator channel the hot water vaporizes at the membrane surface, diffuse through the hydrophobic PTFE membrane pores and condense on the permeate channel film. The evaporator channel outlet saltwater with higher salinity with compare to the inlet feed water to the condenser channel, return to the feed tank. To maintain the
349
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
feed tank salinity at a constant level, the fresh water pumped from permeate tank by applying a floating ball valve. The produced fresh water exits from the top manifold of permeate gap. 3. Results and discussion In this section, the influence of feed water flow rate on pressure drop, required pumping power in condenser flow channel is studied, besides the influence of different feed water flow rate and salinity on the most important characteristic value including permeate flux, STEC and GOR are presented. The qSTEC is the amount of total energy input (ܧ ) to produce 1 m3 of fresh water [1].
ݍௌ்ா ൌ
ܧ ݉ሶ ܥ ሺܶாି ܶ ሻ ൌ ܸሶ ܸሶ
(1)
The GOR is an indication of how well the total energy input to the system utilizes to produce fresh water, where οܪ௩ is the enthalpy of evaporation of water:
ܴܱܩൌ
݉ሶ οܪ௩ ܧ
(2)
3.1. Effect of feed flow rate on pressure drop and pumping power Fig. 3 illustrates the effect of feed flow rate on the pressure drop in the condenser channel. This experiment is done to study the hydrodynamic condition in flow channels for cold fresh water feed. As it is obvious from this graph, by increasing the feed flow rate, the pressure drop increases, so the demanded pumping power which is a function of feed flow rate (ܸሶ ) and pressure drop (οܲ), which can be described by Eq.3, will increase ܲ ݎ݁ݓܲ݃݊݅݉ݑൌ ܸሶ כοܲ
(3)
Pressure drop(bar)
Pumping power (W) 0.8 0.7
0.09
0.6 0.5
0.07
0.4 0.05
0.3 0.2
0.03
Pumping power (W)
Pressure drop (bar)
0.11
0.1 0
0.01 1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
3.25
3.5
3.75
4
4.25
4.5
Fresh Water Flow Rate (l/min)
Fig. 3. Pressure drop and required pumping power in condenser channel at different fresh water flow rate
350
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
3.2. Effect of feed flow rate on permeate flux, STEC and GOR The effect of the feed flow rate on permeate flux for operating condition TCi=15 ºC, TEi=82 ºC and for two different salinities (0 and 30000 ppm) is shown in Fig. 4. As it is indicated by this figure, by increasing the feed flow rate, the permeate flux increase. By increasing fresh water flow rate from 0.14 to 1.03 l/min the permeate flux increase about 22%. By increasing the feed flow rate, the turbulence in the flow channel increase, so the Reynolds number will increase, which improves the heat transfer rate in flow channels and reduce the temperature polarization effect at two sides of the membrane surface and improve the temperature difference across the membrane surface which eventually lead to higher permeate flux. Fig. 4, also presents the effect of salinity on permeate flux, which shows a decrease on permeate water flux for saline water as compared to the fresh water feed. According to the Rault’s law, higher concentration of solutes in the feed causes a decrease of vapour pressure above the solution and the decrease of the MD process partial vapour pressure difference in two sides of the membrane, so lead to the lower permeate flux [2]. S=30 g/Kg
Permeate Flux (kg/m2.h)
13
S=0 g/Kg
11 9 7 5 3 1 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Feed Flow Rate (l/min)
Fig. 4. Permeate flux at different feed flow rate for S=0 and S=30 g/Kg, test condition: TCi=15 ºC, TEi=82 ºC
The effect of feed flow rate on STEC of the system for two different feed salinity (0 and 30 g/kg) which obtained via Eq. (1) is shown in Fig. 5. The results show that by increasing the feed flow rate, the required amount of thermal energy will increase. Higher feed flow rate lead to higher demand of external energy input to reach the constant TEi value with compare to lower feed flow rate besides at high flow rate, the feed residence time in the flow channel decreased and the sensible heat recovery in condenser channel becomes less efficient [7], so for a test operating condition, TCo decreases and the amount of external heat demand to reach the considered value for TEi, will increase. On the other hand, as mentioned in Fig. 4 by increasing the feed flow rate, the permeate flux increases, however, the effect of higher energy demand is not completely satisfying by higher permeate flux, so the STEC values increase at the higher feed flow rate. As it is clear from this figure, for higher feed salinity, the amount of STEC is higher, due to the lower amount of permeate output at higher salinity which is also confirmed by previous studies [8,9]. S=30 g/Kg
2900
S=0 g/kg
STEC (kWh/m3)
2500 2100 1700 1300 900 500 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Feed Flow rate (l/min)
Fig. 5. Specific thermal energy consumption at different feed flow rates for S=0 and S=30 g/Kg, test condition: TCi=15 ºC, TEi=82 ºC
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Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351 S=0 g/Kg
0.8
S=30 g/Kg
0.7 0.6 GOR
0.5 0.4 0.3 0.2 0.1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Feed Flow Rate (l/min)
Fig. 6. Gained output ratio at different feed flow rates for S=0 and S=30 g/Kg, Test condition: TCi=15 ºC, TEi=82 ºC
Gained output ratio values, as an alternative representation of the STEC and to quantify the modules’ capability for heat recovery, illustrate in Fig. 6, which show a decreasing trend by increasing feed flow rate and feed water salinity. Long module flow channel with higher membrane surface area, will provide more efficient sensible heat recovery, lead to a higher GOR value and so the better system performance [10]. 4. Conclusion A lab scale plate-and- frame PGMD module with 0.12 m2 effective membrane area has been developed and tested. A set of experimental test has been performed to investigate designed MD module's main characteristics under different operating condition, including feed flow rate and salinity. Under the defined test condition: (TCi=15 ºC, TEi=82 ºC, feed flow rate (0.1-1.1) l/min) the permeate flux varies from (2-12) kg/m2.h, specific thermal energy consumption was between (1000-2500) kWh/m3 and GOR < 1 The experimental results show that, lower feed flow rate provides higher residence time and lower STEC (higher GOR). Operating at lower STEC, is also achievable by increasing flow channel length and providing more contact time between feed stream and membrane surface, which lead to higher heat recovery and lower external energy demand of the system. However, working at low feed flow rate and high membrane surface area leads to lower permeate flux and higher investment cost, respectively, so it is required to reach an optimal design for heat recovery system in MD module.
References [1] Charcosset C. A review of membrane processes and renewable energies for desalination. Desalination 2009;245:214-31. [2] Bai F, Akbarzadeh A, Singh R. A novel system of combined power generation and water desalination using solar energy. Solar09, the 47th ANZSES Annual Conference. [3] Camacho L, Dumée L, Zhang J, Li J-d, Duke M, Gomez J, et al. Advances in Membrane Distillation for Water Desalination and Purification Applications. Water 2013;5:94-196. [4] Gryta M. Long-term performance of membrane distillation process. J Membr Sci 2005;265:153-9. [5] Khayet M. Membranes and theoretical modeling of membrane distillation: a review. Adv Colloid Interface Sci 2011;164:56-88. [6] Meindersma GW, Guijt CM, Haan AB de. Desalination and water recycling by air gap membrane distillation. Desalination 2006;187:291-301. [7] Xu J, Singh YB, Amy GL, Ghaffour N. Effect of operating parameters and membrane characteristics on air gap membrane distillation performance for the treatment of highly saline water. J Membr Sci 2016;512:73-82. [8] Winter D, Koschikowski J, Wieghaus M. Desalination using membrane distillation: Experimental studies on full scale spiral wound modules. J Membr Sci 2011;375:104-12. [9] Cipollina A, Di Sparti MG, Tamburini A, Micale G. Development of a Membrane Distillation module for solar energy seawater desalination. Chem Eng Res Des 2012;90:2101-21. [10] Winter D, Koschikowski J, Ripperger S. Desalination using membrane distillation: Flux enhancement by feed water deaeration on spiralwound modules. J Membr Sci 2012;423-424:215-24.
ScienceDirect Energy Procedia 110 (2017) 346 – 351
1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia
Sustainable seawater desalination by permeate gap membrane distillation technology Farzaneh Mahmoudi,*, Hasham Siddiqui, Mohammadebrahim Pishbin, Gholamreza Goodarzi, Saeed Dehghani, Abhijit Date, Aliakbar Akbarzadeh School of Engineering, RMIT University, Melbourne 3000, Australia
Abstract Membrane distillation (MD) as a novel thermally-driven process with moderate operating temperatures, is an effective technology for salt water desalination, by this process, it becomes achievable to directly utilize low-temperature waste heat or solar energy. This research is aimed to design a lab scale plate-and-frame permeate gap membrane distillation (PGMD) module, with internal heat recovery characteristic which could significantly reduce the energy consumption of the process. In this paper, the PGMD module performance is experimentally investigated for fresh and saline water feed, in terms of permeate water flux, specific thermal energy consumption (STEC) and gained output ratio (GOR). The experimental results show, by increasing the saline feed flow rate in a range of (0.4-1) lit/min, the fresh water flux increase from 3 to 11 kg/m2.hr, however, the thermal energy demand of process also increased by nearly 20 %. As a result, optimization of the MD module performance is achievable, by adjusting the effective membrane surface area and feed flow rate, to improve internal heat recovery and also produce higher fresh water rate. ©©2017 The Authors. Published by Elsevier Ltd. This 2017 The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords:Sustainable desalination; Membrane distillation; Flat module design; Specific thermal energy consumption; Permeate flux
1. Introduction The need for fresh water is considered as a critical international problem, according to the World Water Council, 17% of the world population will be living in short of the fresh water supply by 2020 [1].
* Corresponding author. Tel.: 61-040-232-5294. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.151
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
Nomenclature Cp Ei GOR ∆Hv Jp ݉ሶ οܲ qSTEC S TEi TEo TCi TCo ܸሶ ܸሶ
specific heat capacity (J/kgK) total energy input (J/s) gained output ratio (-) specific heat of vaporization (J/kg) permeate flux (kg/m2.s) feed flow rate (kg/s) pressure drop (Pa) specific thermal energy consumption (kWh/m3) salinity (g/Kg) temperature at evaporator inlet (ºC) temperature at evaporator outlet (ºC) temperature at condenser inlet (ºC) temperature at condenser outlet (ºC) feed flow rate (m3/s) permeate flow rate (m3/s)
The demand for alternative sustainable water sources including ground water, desalinated water and recycled water is increased, in recent years. As a result, the implementation of desalination plants is growing on a large scale. Fresh water can be derived from sea water by evaporation processes (e.g., multi-stage flash) or by membrane based processes, such as reverse osmosis, electro dialysis and membrane distillation. The commercially developed RO technology for desalination requires large amounts of energy in the form of electricity or shaft power to drive the pump, which the electricity is presently being generated from non-renewable and polluting fossil fuels [2]. In contrast, MD use lower top temperature with respect to the traditional distillation processes, making it suitable for using waste heat or solar heat, besides, aqueous solutions of salts with higher concentrations than seawater can be treated by MD. Reducing discharge volumes and increasing the water recovery factor up to 95% which considerably diminish the environmental impact of the brine disposal are advantages of the MD process [3]. However, the MD process is still under evaluation, the lack of experimental data has indicated that there is a need for more intensive research in this field, both experimentally and mathematically. The central issues are the external energy source in MD units, lack of MD membranes and fabrication of modules to suit each MD configuration, difficulties with long term operations with risk of membrane pores wetting and fouling. Thus, the optimization of different MD installations and systems is still to be done in order increase MD performance and decrease energy consumption [4-6]. 2. Experimental work 2.1. Description of setup A novel optimized experimental approach was followed up by a lab scale plate-and-frame PGMD module with two transparent poly acrylic sheets with 25 mm thickness for evaporator and condenser sides and the permeate water sheet with 6 mm thickness (Fig. 1). As it is clear from this figure, two main cylindrical channels with 10 mm hole diameter is milled in each evaporator and condenser plate for the flow inlet and outlet manifold. To have a more optimized flow distribution from the inlet and outlet, a set of 11*3 mm distributor holes, was drilled in each flow channel. The hydrophobic PTFE membrane with 0.22 µm nominal pore size, (140-200) µm thickness and polypropylene (PP) as support, with an effective surface area (760*160) mm^2 was applied, the permeate channel was separated from condenser channel by an impermeable 100 μm clear PP film, which filled by two similar plastic net spacer either as a mechanical support between membrane and condensing polymeric film and turbulence promoter. The schematic diagram of the lab scale experimental setup is shown in Fig. 2. The feed water is pumped from storage tank using a 12 V small DC water pump. A mechanical filter with 0.2 mm pore size have used before the
347
348
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
pump to protect the setup from unwanted solid. To adjust the inlet feed temperature to the condenser channel, the lab cooling circuit used to control the input temperature to the condenser channel. The saline feed water gradually preheats by flowing through the condenser channel by obtaining latent heat of condensation and conduction via the PP condensing film. Then, the condenser outlet temperature increases by flowing through a copper heating coil which is immersed in an insulated electric water tank (2.4 kW URN) to reach the established evaporator channel inlet temperature. By estimating the required external source demand in this stage, the integration of the evacuated tube solar collectors (ETSC) could be evaluated for the sustainable outdoor desalination setup. a
b
Fig. 1. a) Module's SolidWorks assembly design, b) PGMD module arrangement with internal heat recovery
Fig. 2. Schematic diagram of the MD experimental setup
During the evaporator channel the hot water vaporizes at the membrane surface, diffuse through the hydrophobic PTFE membrane pores and condense on the permeate channel film. The evaporator channel outlet saltwater with higher salinity with compare to the inlet feed water to the condenser channel, return to the feed tank. To maintain the
349
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
feed tank salinity at a constant level, the fresh water pumped from permeate tank by applying a floating ball valve. The produced fresh water exits from the top manifold of permeate gap. 3. Results and discussion In this section, the influence of feed water flow rate on pressure drop, required pumping power in condenser flow channel is studied, besides the influence of different feed water flow rate and salinity on the most important characteristic value including permeate flux, STEC and GOR are presented. The qSTEC is the amount of total energy input (ܧ ) to produce 1 m3 of fresh water [1].
ݍௌ்ா ൌ
ܧ ݉ሶ ܥ ሺܶாି ܶ ሻ ൌ ܸሶ ܸሶ
(1)
The GOR is an indication of how well the total energy input to the system utilizes to produce fresh water, where οܪ௩ is the enthalpy of evaporation of water:
ܴܱܩൌ
݉ሶ οܪ௩ ܧ
(2)
3.1. Effect of feed flow rate on pressure drop and pumping power Fig. 3 illustrates the effect of feed flow rate on the pressure drop in the condenser channel. This experiment is done to study the hydrodynamic condition in flow channels for cold fresh water feed. As it is obvious from this graph, by increasing the feed flow rate, the pressure drop increases, so the demanded pumping power which is a function of feed flow rate (ܸሶ ) and pressure drop (οܲ), which can be described by Eq.3, will increase ܲ ݎ݁ݓܲ݃݊݅݉ݑൌ ܸሶ כοܲ
(3)
Pressure drop(bar)
Pumping power (W) 0.8 0.7
0.09
0.6 0.5
0.07
0.4 0.05
0.3 0.2
0.03
Pumping power (W)
Pressure drop (bar)
0.11
0.1 0
0.01 1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
3.25
3.5
3.75
4
4.25
4.5
Fresh Water Flow Rate (l/min)
Fig. 3. Pressure drop and required pumping power in condenser channel at different fresh water flow rate
350
Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351
3.2. Effect of feed flow rate on permeate flux, STEC and GOR The effect of the feed flow rate on permeate flux for operating condition TCi=15 ºC, TEi=82 ºC and for two different salinities (0 and 30000 ppm) is shown in Fig. 4. As it is indicated by this figure, by increasing the feed flow rate, the permeate flux increase. By increasing fresh water flow rate from 0.14 to 1.03 l/min the permeate flux increase about 22%. By increasing the feed flow rate, the turbulence in the flow channel increase, so the Reynolds number will increase, which improves the heat transfer rate in flow channels and reduce the temperature polarization effect at two sides of the membrane surface and improve the temperature difference across the membrane surface which eventually lead to higher permeate flux. Fig. 4, also presents the effect of salinity on permeate flux, which shows a decrease on permeate water flux for saline water as compared to the fresh water feed. According to the Rault’s law, higher concentration of solutes in the feed causes a decrease of vapour pressure above the solution and the decrease of the MD process partial vapour pressure difference in two sides of the membrane, so lead to the lower permeate flux [2]. S=30 g/Kg
Permeate Flux (kg/m2.h)
13
S=0 g/Kg
11 9 7 5 3 1 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Feed Flow Rate (l/min)
Fig. 4. Permeate flux at different feed flow rate for S=0 and S=30 g/Kg, test condition: TCi=15 ºC, TEi=82 ºC
The effect of feed flow rate on STEC of the system for two different feed salinity (0 and 30 g/kg) which obtained via Eq. (1) is shown in Fig. 5. The results show that by increasing the feed flow rate, the required amount of thermal energy will increase. Higher feed flow rate lead to higher demand of external energy input to reach the constant TEi value with compare to lower feed flow rate besides at high flow rate, the feed residence time in the flow channel decreased and the sensible heat recovery in condenser channel becomes less efficient [7], so for a test operating condition, TCo decreases and the amount of external heat demand to reach the considered value for TEi, will increase. On the other hand, as mentioned in Fig. 4 by increasing the feed flow rate, the permeate flux increases, however, the effect of higher energy demand is not completely satisfying by higher permeate flux, so the STEC values increase at the higher feed flow rate. As it is clear from this figure, for higher feed salinity, the amount of STEC is higher, due to the lower amount of permeate output at higher salinity which is also confirmed by previous studies [8,9]. S=30 g/Kg
2900
S=0 g/kg
STEC (kWh/m3)
2500 2100 1700 1300 900 500 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Feed Flow rate (l/min)
Fig. 5. Specific thermal energy consumption at different feed flow rates for S=0 and S=30 g/Kg, test condition: TCi=15 ºC, TEi=82 ºC
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Farzaneh Mahmoudi et al. / Energy Procedia 110 (2017) 346 – 351 S=0 g/Kg
0.8
S=30 g/Kg
0.7 0.6 GOR
0.5 0.4 0.3 0.2 0.1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Feed Flow Rate (l/min)
Fig. 6. Gained output ratio at different feed flow rates for S=0 and S=30 g/Kg, Test condition: TCi=15 ºC, TEi=82 ºC
Gained output ratio values, as an alternative representation of the STEC and to quantify the modules’ capability for heat recovery, illustrate in Fig. 6, which show a decreasing trend by increasing feed flow rate and feed water salinity. Long module flow channel with higher membrane surface area, will provide more efficient sensible heat recovery, lead to a higher GOR value and so the better system performance [10]. 4. Conclusion A lab scale plate-and- frame PGMD module with 0.12 m2 effective membrane area has been developed and tested. A set of experimental test has been performed to investigate designed MD module's main characteristics under different operating condition, including feed flow rate and salinity. Under the defined test condition: (TCi=15 ºC, TEi=82 ºC, feed flow rate (0.1-1.1) l/min) the permeate flux varies from (2-12) kg/m2.h, specific thermal energy consumption was between (1000-2500) kWh/m3 and GOR < 1 The experimental results show that, lower feed flow rate provides higher residence time and lower STEC (higher GOR). Operating at lower STEC, is also achievable by increasing flow channel length and providing more contact time between feed stream and membrane surface, which lead to higher heat recovery and lower external energy demand of the system. However, working at low feed flow rate and high membrane surface area leads to lower permeate flux and higher investment cost, respectively, so it is required to reach an optimal design for heat recovery system in MD module.
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