Method for the purification of a liquid by membrane...

Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...

Reexamination Certificate

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C203S010000, C203S022000, C203S025000, C210S650000, C210S774000

Reexamination Certificate

active

06716355

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a method for the purification of a liquid by membrane distillation, in particular for the production of desalinated water from seawater or brackish water or process water, comprising:
passing a relatively warm vaporising steam of liquid (retentate stream) over a porous membrane, vapour flowing via the pores of the membrane to the other side of said membrane, and
condensing said vapour on a relatively cool condenser surface to give a distillate stream, said condenser surface forming the non-porous separation between a feed stream to be purified and said distillate steam, which feed stream is in counter-current with the retentate son o that an appreciable proportion of the latent heat will be transferred via vapour to the feed stream, and a gas gap with a width of less than 5 mm being present between the porous membrane and the condenser surface.
Membrane distillation differs from known distillation techniques such as multi-stage flash, multiple effect distillation and vapour compression in that a non-selective, porous membrane is used. This membrane forms a separation between the warm, vaporising retentate stream and the condensed product, the distillate stream. As a consequence of a suitable choice of material (usually polypropylene, polyethylene or polytetraflorethene), the pores (diameter of between 0.00001 and 0.005 mm, usually between 0.0001 and 0.0005 mm) are not wetted by the liquid; only vapour passes through the membrane.
Membrane distillation was first described in U.S. Pat. No. 3,334,186 from 1967. The intention was to improve the efficiency of seawater desalination by the use of an air-filled porous hydrophobic membrane The method concerned here was so-called direct contact membrane distillation: the warm seawater stream and the cold distillate stream are in direct contact with the membrane.
Substantial interest in membrane distillation was generated in the mid 1980s when a new generation of hydrophobic, highly porous membranes became available. However, research showed that membrane distillation is no less expensive than competitive techniques and therefore there was no commercial application.
A distinction can be made between four types of membrane distillation:
1. Direct contact membrane distillation (DCMD), where both the warm, vaporising stream and the cold condensate stream (distillate stream) are in direct contact with the membrane.
2. Air gap membrane distillation (AGMD), where the condenser surface is separated from the membrane by an air gap.
3. Sweeping gas membrane distillation, where the distillate is removed in vapour form by an inert gas.
4. Vacuum membrane distillation, where the distillate is removed in vapour form by vacuum. This method is described only for the removal of volatile components from aqueous streams and the point at issue is not the production of a liquid distillate.
Up to now direct contact membrane distillation has attracted the most attention.
U.S. Pat. No. 4,545,862 describes a spirally wound module (with flat membranes). This was seawater stream fed in counter-current to the vaporising retentate and the seawater stream thus effectively absorbed the heat of condensation. In this patent an example is described in which a relatively high flow rate of 5.3 liters per m
2
per hour is achieved with a temperture difference &Dgr;T between the warm retentate and the seawater of 4° C., with an energy consumption of only 212 kiloJoule per kg distillate produced.
In addition to the use of flat membranes, the advantages of hollow fibre membranes for direct contact membrane distillation are known. As a result of the compact packing of membrane fibres, a surface area of up to 500 m
2
per m
3
can be obtained, which makes lower equipment costs possible. Furthermore, it has been proposed (see K. Schneider, T. J. van Gassel, Membrandestillation, Chem. Ing. Tech. 56 (1984) 514-521) to couple a direct contract membrane distillation module with a heat exchanger module in a cycle and thus to recover heat of condensation. It is found that for seawater desalination a distillate flow rate of approximately 8.5 liters per m
2
per hour is obtained for a &Dgr;T of 14-16° C. and a specific energy consumption of above 1,000 kJ per kg water. Since 1984 there has been little discernable progress in the state of the art in respect of DCMD.
Air gap membrane distillation was first described in 1971 in British Patent Application GB 1 225 254 A (Henderyckx). In addition to the use of an air gap, counter-current flow of feed and retentate (and thus recovery of latent heat), is already proposed In addition, AGMD was described in 1982 in German Patent Application 3 123 409 (Siemens). This application relates to the use of a gap (with a thickness of 3 mm), filled with air, or optionally a lighter gas such as hydrogen, between a flat porous membrane and a cold condensation surface. The aim was to reduce the transport of perceptible heat by conduction through the membrane. It was established experimentally that heat transport by conduction was approximately equal to that by evaporation. Moreover, it was proposed to feed incoming seawater in counter-current to the vaporising stream and thus to recover heat. The use of solar heat as a source of heat was also claimed. A theoretical case was described in which a distillate flow rate of 3.36 kg per m
2
per hour was achieved with a temperature difference &Dgr;T of 5° C., with a recovery of approximately 4.9% and an energy consumption of over 850 kJ per kilogram water produced.
European Patent Application 0 164 326 describes the use of an air gap with membrane distillation, the various features being constructed in the form of concentric tubes. A variant of this in which packets of flat membranes were used is described in the article Design and filed tests of a new membrane distillation desalination process (Desalination 56 (1985), pp. 345-354). It is striking that the principle of counter-current flow of seawater and retentate is abandoned, as a result of which no recovery of heat of evaporation is possible. Energy consumption figures are then also not given.
International Patent Application WO 8607585 A (1986) is based on the same model data but deduces from these that an air gap thickness of 0.2 to 1.0 mm is needed in order to achieve both a high flow rate and a low loss of perceptible heat (300 -800 kJ/kg water). No account is taken in the model of temperature falls at and in the hot and cold wall, as a result of which a far too optimistic picture is painted.
In U.S. Pat. No. 4,879,041 air gap membrane distillation is described specifically for the preparation of ultra-pure water for the semiconductor industry. Here the effect of the thickness of the air gap, when using flat membrane sheets, on mass transport and heat transport was investigated in the region between 3 and 10 mm. It was concluded from these investigations that transport is determined by diffusion at thicknesses of less than 5 mm and by free convection at thicknesses of more than 5 mm. The performances measured were moderate: maximum distillate flow rates of 3.6 kg per m
2
per hour for a vapour pressure difference of approximately 20 kPa Here again no heat of condensation is recovered and it is therefore also not surprising that a few years later there was a switch back to conventional multi-stage evaporation without membranes.
The attention paid to membrane distillation decreased in the 1990s and was in the main restricted to direct contact membrane distillation and to research into sweeping gas membrane distillation and vacuum membrane distillation for the removal and extraction of volatile components from aqueous streams.
On the basis of the literature, a system without an air gap is required for membrane distillation systems with a low energy consumption. On the basis of the prior art, it is not possible to achieve an energy consumption of less than 850 kJ per kg if an air gap is used or heat recovery is employed. This is related to high temperature differences (&Dgr;T frequently to more than 40°) an

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