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CFD Analysis of Spacer-Obstructed Channels in Membrane Distillation
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.ORCID iD: 0000-0002-2992-6814
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.ORCID iD: 0000-0002-3661-7016
(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 [en]
CFD, Membrane Distillation, sheer stress, pressure drop
National Category
Energy Engineering Water Engineering
Identifiers
URN: urn:nbn:se:kth:diva-44482OAI: oai:DiVA.org:kth-44482DiVA: diva2:450583
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
In thesis
1. Desalination using Membrane Distillation: Experimental and Numerical Study
Open this publication in new window or tab >>Desalination using Membrane Distillation: Experimental and Numerical Study
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
Air gap membrane distillation, applications, experimental and simulation, CFD, physical modification, performance improvement
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-44405 (URN)978-91-7501-133-2 (ISBN)
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

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