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Desalination using Membrane Distillation: Experimental and Numerical Study
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Desalination has been increasingly adopted over the last decades as an option, and sometimes as a necessity to overcome water shortages in many areas around the world. Today, several thermal and physical separation technologies are well established in large scale production for domestic and industrial purposes.  Membrane distillation is a novel thermally-driven process that can be adapted effectively for water desalination or water treatment in industrial applications, due to its potential lower energy consumption and simplicity.

The general objective of this thesis is to contribute to the technical understanding of membrane distillation as a new technology in water treatment for both industrial and drinking water purposes, as a starting point for further improvement. The thesis includes experimental and numerical investigations that highlight some aspects of the technology application and fundamental aspects.

In the field of industrial application, an experimental and numerical assessment has been carried out on an Air Gap Membrane Distillation (AGMD) prototype to assess the utilization of the technology in thermal cogeneration plants; in particular, demineralization of water boiler feed water and treating flue gas condensate. The main assessment parameters were water quality and energy consumption. The results from full-scale simulations of a system of 10 m3/hr production capacity,  connected to the district heating network were as follows: 5 to 12 kWh/m3 specific thermal energy consumption, and  0,6 to 1,5 kWh/m3 specific electricity consumption, depending upon the heat source (district heat supply line or low-grade steam).

For desalination applications, experimental and simulation work was conducted on an AGMD semi-commercial system as part of the EU MEDESOL project. The aim was to evaluate AGMD performance with saline water of 35 g/l NaCl in order to establish an operation data base for simulation of a three-stage AGMD desalination system. Specific thermal energy consumption was calculated as 950 kWht/m3 for a layout without heat recovery, and 850 kWht/m3 for a layout with one stage heat recovery.  The lack of internal heat recovery in the current MD module means that most of the heat supplied to MD system was not utilized efficiently, so the thermal energy consumption is high. This would mean that a large solar field is needed.

In order to analyze the flow conditions in feed flow and cooling channels, CFD was used as tool to analyze a spacer-obstructed flow channel for different types of spacer geometrical characteristics: flow of attack angle, spacer to channel thickness ratio, and void ratio. Velocity profiles, shear stress, and pressure drop were the main assessment criteria. Results show the flow of attack angle has a very minimum effect on the performance of spacers. The effect of spacer to channel thickness ratio was significant in all assessment parameters. Higher void ratios were found advantageous in promoting flow mixing, but resulted in lower sheer stress and hence reduced heat transfer.

Physical modifications were implemented on a semi-commercial AGMD prototype to assess experimentally any improvement in its performance. These modifications were mainly focused on reducing the conductive heat transfer losses by modifying the physical support in the air gap that separates the membrane from the condensation surface. In addition, several feed channel spacers were tested and assessed based on their effect in increasing the mass transfer while maintaining or reducing pressure drop. The modifications yielded a two-fold augmentation: slight increase in the distillate mass flow rate (9-11%), and increased thermal efficiency (6%). The pressure drop in the module was reduced by 50% through selecting the appropriate spacer that would achieve the above mass flow rate increase.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , xiv, 86 p.
Series
Trita-KRV, ISSN 1100-7990 ; 11:7
Keyword [en]
Air gap membrane distillation, applications, experimental and simulation, CFD, physical modification, performance improvement
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-44405ISBN: 978-91-7501-133-2 (print)OAI: oai:DiVA.org:kth-44405DiVA: diva2:450562
Public defence
2011-10-28, M 2, Brinellväg. 64, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111021Available from: 2011-10-21 Created: 2011-10-20 Last updated: 2011-10-21Bibliographically approved
List of papers
1. Membrane distillation and applications for water purification in thermal cogeneration plants
Open this publication in new window or tab >>Membrane distillation and applications for water purification in thermal cogeneration plants
2011 (English)In: Separation and Purification Technology, ISSN 1383-5866, E-ISSN 1873-3794, Vol. 76, no 3, 231-237 p.Article in journal (Refereed) Published
Abstract [en]

Water treatment is an important auxiliary process in all thermal cogeneration plants. In this context membrane distillation (MD) is a novel technology that has potential advantages in: the ability to utilize low-grade heat instead of electricity; reduced sensitivity to fluctuations in pH or salt concentrations. This research is a continuation of a previously conducted theoretical study where the performance of MD-based water treatment was explored via laboratory testing, system simulations of thermodynamic performance, and economic evaluations. The current paper, encompassing field trials, contains details of a test rig deployed at Idbacken Cogeneration Facility (Nykoping, Sweden) with a five-module MD unit capable of producing 1-2 m(3)/day purified water. District heating supply line was employed for heating while municipal water was used for cooling; feed stocks include municipal water and flue gas condensate. A long-term performance evaluation including thorough chemical testing of product water quality is presented. (c) 2010 Elsevier B.V. All rights reserved.

Keyword
Membrane distillation, Low-grade heat, Water treatment, Flue gas condensate
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-30981 (URN)10.1016/j.seppur.2010.09.028 (DOI)000286712600002 ()2-s2.0-78650521877 (Scopus ID)
Note
QC 20110310Available from: 2011-03-10 Created: 2011-03-07 Last updated: 2017-12-11Bibliographically approved
2. Experimental and Simulation of an Air Gap Membrane Distillation System
Open this publication in new window or tab >>Experimental and Simulation of an Air Gap Membrane Distillation System
(English)In: Desalination, ISSN 0011-9164, E-ISSN 1873-4464Article in journal (Other academic) Submitted
Abstract [en]

This paper covers the details and results of experimental and simulation work carried on an Air Gap Membrane Distillation (AGMD) unit, as a part of EU MEDESOL research project. The aim of the experimental work, carried out during a two-week period, was (a) to evaluate the MD performance with saline water (35 g/l NaCl)  establishing an operation data base, and (b) to conduct a system simulation for the design and evaluation of a three-stage MD desalination system. Experimental results shows that production was 30-40% less in the case of using 35 g/l salinity compared with 1 g/l.  Experimental-based simulations of a three step MD system of two arrangement layout were employed to assess the heat demand. Specific thermal energy consumption was calculated as 950 kWht/m3 for a layout without heat recovery, and 850 kWht/m3 for the layout with one stage heat recovery.

Keyword
MEDESOL, AGMD, experiments, Simulation
National Category
Energy Engineering Water Engineering
Identifiers
urn:nbn:se:kth:diva-44476 (URN)
Projects
EU MEDESOL
Note
QS 2011 QS 20120327Available from: 2011-10-21 Created: 2011-10-21 Last updated: 2017-12-08Bibliographically approved
3. CFD Analysis of Spacer-Obstructed Channels in Membrane Distillation
Open this publication in new window or tab >>CFD Analysis of Spacer-Obstructed Channels in Membrane Distillation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Spacers are used in desalination and filtration technologies to enhance the hydrodynamic conditions in flow channels, as well as to provide support and separation of membrane sheets. This paper present the main results of computational fluid dynamics (CFD) simulation for flow in spacer-obstructed flow channel for application in membrane distillation (MD). Flow of attack angle, spacer-to-channel thickness and void ratio were the main geometrical parameters that were studied; velocity profiles, shear stress and pressure drops were the main assessment criteria used for evaluation. Results show the flow of attack angle has a very minimum effect on the performance of spacers. The effect of spacer to channel thickness ratio was significant in all assessment parameters. Higher void ratios were found advantageous in promoting flow mixing, but resulted in lower sheer stress and reduced trans-membrane flux. In practice, the selection of the best case would include a trade-off between the cost of membranes needed to produce the required production and pumping.

Keyword
CFD, Membrane Distillation, sheer stress, pressure drop
National Category
Energy Engineering Water Engineering
Identifiers
urn:nbn:se:kth:diva-44482 (URN)
Note
QS 2011. Manuscript to be submitted to Journal of Membrane ScienceAvailable from: 2011-10-21 Created: 2011-10-21 Last updated: 2011-10-21Bibliographically approved
4. Experimental Evaluation of a Modified Air-gap Membrane Distillation Semi-Commercial Prototype
Open this publication in new window or tab >>Experimental Evaluation of a Modified Air-gap Membrane Distillation Semi-Commercial Prototype
(English)In: Desalination, ISSN 0011-9164, E-ISSN 1873-4464Article in journal (Other academic) Submitted
Abstract [en]

Modifications were implemented on a semi-commercial Air Gap Membrane Distillation prototype to assess experimentally any improvement in its performance. These modifications were many focused on reducing the conductive heat transfer losses by reducing the physical support in the air gap that separate the membrane from the condensation surface. Several feed channel spacers were tested as well and assessed based in their effect in increasing the mass transfer and imposed pressure drop. Results show that the modifications increased slightly the distillate mass flow rate by 9-11 % and reduced the conductive heat losses by 20-24 %. Spacer effect was found to be mainly in imposed pressure drop within the tested types.

Keyword
Membrane distillation, conductive heat losses, experimental work
National Category
Energy Engineering Water Engineering
Identifiers
urn:nbn:se:kth:diva-44483 (URN)
Note
QS 2011 QS 20120327Available from: 2011-10-21 Created: 2011-10-21 Last updated: 2017-12-08Bibliographically approved

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