Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Patent
1986-11-17
1988-05-10
Sever, Frank
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
203 11, 203 99, 203DIG17, E01D 1300
Patent
active
047433787
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to the rapid transport of vapour through unwetted porous barriers.
The invention may be applied to a wide variety of porous separating devices which may be in the form of sheets, tubes or hollow fibres. Such devices are widely used in microfiltration, ultrafiltration, dialysis, electrodialysis, diffusion, pervaporation, osmosis and reverse osmosis where separations are made by selective permeation through relatively thin porous barriers or layers. For the sake of brevity, such devices shall be referred to as "porous barriers".
BACKGROUND ART
The invention has application to the distillation of aqueous solutions through microporous hydrophobic membranes whose pore sizes usually range from 0.001 to 10 microns in diameter. The present state of the art is that no manufacturer offers a recommended multiple effect water still which depends on hydrophobic porous barriers and transmembrane evaporation beyond laboratory experimental size.
Practical problems which arise in the design of stills include the high energy use of single effect stills, fouling-induced hydrophilic break-throughs, difficulty in applying feed to the hydrophobic pore entrances, solute polarisation and crystallization in the pores as well as the low rates of diffusion through the long pores of the barriers. These problems have impeded commercial development of membrane distillation.
Some progress has been made in overcoming many of the above problems by coating hydrophobic porous hollow fibre barriers with woven or cast porous coatings which are usually very thin and hydrophilic. However, poor energy efficiency due to difficulty in achieving multistage operation and the low rates of vapour transport through the fibres have remained as prime limitations to commercial acceptance.
The multistage countercurrent flow of hot feed which directly distills into the cold condensate stream is most easily achieved by the use of porous, hydrophobic hollow fibres with very thin walls. There is however, still a difficulty in providing a heat exchanger to recover the heat content of the now hot condensate to reheat the now colder evaporation residue stream. This remains a difficulty and continues to necessitate some form of heat exchanger of large area. There is an economic balance between the cost of the heat energy and the costs added by the heat exchanger.
In some circumstances, the heat is available as waste, but the low rate of vapour transfer through the necessarily fine pores limits the utilization of the otherwise favourable application. The slowness of the vapour transfer is explicable and expected.
Commonly, the tortuous pore path of 200 to 600 microns consists of 0.2 micron diameter pores -- a 1000 to 3000 fold ratio of length to diameter. Some non-condensable or permanent gas is always in the pores and the rates of water vapour diffusion through these gases are low.
Hitherto, the best that could be done was to feed a degassed hot aqueous feed. The diffusing water carried the air entrapped in the pores very slowly out into the cold condensate stream where the air dissolved.
European Patent EP No. 94543 (1983) discloses the methods of heating near to boiling to gain enough vapour pressure to increase diffusion rates. Thermolabile liquids such as milk and beer cannot be treated at high temperatures and cannot be boiled at lower pressures without distilling the volatiles out of the system or reducing the partial pressure of the vapour, which defeats the original purpose of heating to boiling.
DISCLOSURE OF INVENTION
In broad terms, the invention is concerned with a method of increasing the rate of transport of volatiles through unwetted porous barriers by trapping a region of low pressure in all the pores, which are substantially freed of permanent gas, by applying a vacuum and then sealing the pores by opposing film or bubbles of non-wetting liquid so that vapour transport is aerodynamic and not diffusive and by the re-application of vacuum and surface treatment as needed to renew hydrophobicity or hydr
REFERENCES:
patent: 4476024 (1984-10-01), Cheng
Memtec Limited
Sever Frank
LandOfFree
Rapid vapor transport through unwetted porous barriers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rapid vapor transport through unwetted porous barriers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rapid vapor transport through unwetted porous barriers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1319209