WO2000043112A1

WO2000043112A1 - Membrane extraction process

Info

Publication number: WO2000043112A1
Application number: PCT/GB2000/000112
Authority: WO
Inventors: Andrew Guy Livingston
Original assignee: Membrane Extraction Technology Limited
Priority date: 1999-01-21
Filing date: 2000-01-20
Publication date: 2000-07-27
Also published as: GB9901349D0; AU3061800A; GB2346095B; GB2346095A

Abstract

There is provided a process for removing one or more organic compounds from an aqueous fluid, the process comprising the steps of: (a) transferring the one or more organic compounds from the aqueous fluid (1) to a non-condensable gas (6), wherein transfer of the one or more organic compounds from the aqueous fluid to the non-condensable gas occurs across a membrane (3); wherein the membrane (3) is a non porous membrane; (b) treating the non-condensable gas containing the one or more organic compounds with an oxidising medium (11), wherein the oxidising medium is an aqueous oxidising medium (8).

Description

Membrane Extraction Process

The present invention relates to a process for removing one or more organic compounds from an aqueous fluid. In particular the present invention relates to a process for removing one or more organic compounds from an aqueous fluid stream, transferring the one or more organic compounds into a non-condensable gas and then treating the gas stream containing the one or more organic compounds with an oxidising medium where they are degraded by oxidation.

Stripping is a process which may be used to remove volatile compounds from water. The basic concept of stripping is to bring water contaminated with volatile compounds into intimate contact with a stripping gas, so that the volatile compounds are transferred from the water into the stripping gas stream. The stripping gas stream may be a non- condensable gas such as nitrogen, oxygen, methane or air. The stripping gas stream may be a condensable gas such as steam. The stripper is typically a tower column device containing trays, spray jets, or packing material. The stripper may be a membrane module containing microporous membranes, where a gas-liquid interface is established in the pores of the fibres. After passing through the stripping device, the stripping gas stream is preferably treated so that the volatile compounds are removed and are not passed into the environment.

Several processes which combine gas stripping with treatment of the stripping gas are known in the prior art.^'

US-A-4,857,198 discloses a process system for water decontamination by conventional air stripping tower, biological open reactor containing powdered activated carbon, sedimentation/thickening and wet air oxidation. A similar method and system for decontaminating groundwater or other water, is disclosed in US-A-4,892,664. US-A- 4,892,664 relates a process comprising a conventional air stripping tower for groundwater treatment and catalytic oxidation for air purification. US-A-5,246,584 describes a process where steam stripping is utilised to remove organics from a water stream. The steam is then condensed prior to being fed to a bioreactor where the condensed organics are destroyed by biodegradation. US-A-5,273,572 describes a process where an inert stripping gas stream is used to strip organics from a wastewater stream. The stripping gas stream is then treated in a membrane separation unit to recover the organics and allow the gas to be re-used for stripping.

The open literature also describes processes for gas stripping and treatment of off-gases. "Laboratory scale testing of a continuous CLAS process", Bhowmick M. and Semmens M.J., Journal of American Waterworks Association, 1994, Vol. 86, No 8, pp 86-96 describes a stripping system in which a microporous membrane contactor is used to strip volatile organic compounds from water to an air stream. The air stream is then subjected to a gas phase UN photooxidation process to degrade the volatile organic compounds.

Other work describes various combinations of incineration or carbon adsorption used for treating the organic-laden stripping gases exiting a stripping column ("Selection among aqueous and off-gas treatment technologies for Synthetic Organic Chemicals", Dvorak B.I., Herbeck C.J., Meurer C.P., Lawler D.F., Speitel G.E. Journal of Environmental Engineering, July 1996 pp 571-580). Biological treatment of off-gases containing organic contaminants is known. Devices such as biofϊlters and bioscrubbers have been described in, for example, "Waste gas biotreatment technology", Kennes C. and Thalasso F., 1998, Journal of Chemical Technology and Biotechnology, 72, pp 303- 319. However, for high gas flows such as those often employed in stripping operations, these prior art systems require relatively large equipment.

One alternative process to gas stripping is organic-selective pervaporation. In organic- selective pervaporation, an aqueous stream is passed through a membrane module and hydrophobic organics present in the aqueous stream are selectively extracted to the permeate side of the system. In contrast to gas stripping, a non-porous membrane separating layer prevents direct contact of the aqueous stream with the gas stream present on the permeate side of the membrane. These systems are known to those skilled in the art and are described in, for example. US-A-5,266,206, US-A-5, 030,356 and "The use of pervaporation for the removal of organic contaminants from water", Lipski C. and Cote P. 1990, Environmental Progress Nol. 9, No 4. J One problem commonly associated with pervaporation is maintaining the chemical potential gradient across the membrane. Pervaporation processes employing a vacuum pump to remove permeant are energy intensive and so expensive to operate. Some pervaporation processes employ a sweep gas stream in the membrane module (Hoover et al.. J Membrane Science 1982 p 253). Sweep gases create a secondary treatment problem since they must be processed and the volatile organic compound removed before discharge. US-A-5,464,540 disclose the use of a condensable sweep gas stream to improve chemical potential driving force and facilitate organic compound recovery.

US-A-5,464,540 suggests the use of steam as a sweep gas for removing volatile organic components from water. However, the steam must be condensed and the aqueous phase of the condensate treated prior to discharge.

^•NUN gas-phase photooxidation: A new tool for the degradation of VOCs in air^"' Gassiot I, Baus C, Schaber K, Braun AM Journal of Information Recording, 1998, Vol. 24, No. 1-2, pp. 129- 132 report a pervaporation process where a vacuum is used to selectively extract volatile organics from a wastewater stream, and then the organics are oxidised in the gas phase by a vacuum UV oxidation process.

The present invention addresses the problems of the prior art.

According to a first aspect of the present invention there is provided a process for removing one or more organic compounds from an aqueous fluid, the process comprising the steps of: (a) transferring the one or more organic compounds from the aqueous fluid to a non-condensable gas, wherein transfer of the one or more organic compounds from the aqueous fluid to the non-condensable gas occurs across a membrane; wherein the membrane is a non porous membrane (b) treating the non- condensable gas containing the one or more organic compounds with an oxidising medium, wherein the oxidising medium is an aqueous oxidising medium.

In the present specification by the term "removing one or more organic compounds" it is meant that one or more organic compounds present in the aqueous fluid are partially or completely removed therefrom. In the present specification by the term "non porous" it is meant a material and/or a membrane free of pores such that there is no direct interface between the aqueous fluid and the non-condensable gas. Materials and/or membranes having pores which are of a size or density such that there is no direct interface between the aqueous fluid and the non-condensable gas are encompassed by the term "non porous".

In the present specification by the term "non-condensable gas" it is meant a gas which will not condense at atmospheric pressure and temperatures above zero degrees centigrade.

The process of the present invention, which is a pervaporation-oxidation process, may be used for extracting volatile organic compounds from aqueous process streams (an aqueous fluid). Using the present process it is possible to increase the temperature of the aqueous process stream with a relatively smaller increase in the water content (and thus humid heat) of the gas stream (the non-condensable gas) than would be observed for a conventional gas stripping process. This is due to the selective permeation properties of the non-porous membranes. The relatively smaller increase in the water content of the gas stream reduces the cooling load on the reaction system used to treat the organic-laden gas stream. It also reduces the heat requirement necessary to hold the aqueous process stream within a specified temperature range across the module.

The advantages of being able to operate with the aqueous process stream temperature higher than the temperature of the oxidising medium are several, and include better mass transfer across the membrane, and better mass transfer between the gas stream and the oxidising medium, than would be possible if the aqueous process stream and the gas stream were at equal temperature.

Better mass transfer across the membrane at higher temperatures is a consequence of pervaporation relying on the volatility of the organic compounds present in the water stream to be effective. For dilute systems the Henrys law constant is usually employed to characterise the volatility of an organic compound ("Steam stripping for removal of organic contaminants from water. 1. Stripping effectiveness and stripper design" Hwang et al. 1992, Industrial and Engineering Chemistry Research 31 pp 1753-1759). Henrvs law is usuallv written as:

v = H :

where y is a gas phase fraction, concentration or pressure, x is a liquid phase fraction, concentration or pressure, and H is the Henrys law constant. The units of H will depend upon the units used for y and x. In general Henrys law constants for most volatile organic compounds in an aqueous fluid increase with temperature in the range 5-90°C, in some cases passing through a maximum ("Predicting Henrys law constant and the effect of temperature on Henrys law constant" Nirmalakhandan et al. 1997 Water Research 31 pp 1471-1481). For any given aqueous fluid flowrate and concentration, the theoretical minimum flow of gas that must be used to remove the volatile organic compounds to a specified level will depend on the Henrys law constant, and will in general decrease with temperature in the range 20-60°C. Thus a heated aqueous fluid at 60°C will theoretically require less gas to remove the organic contaminants to a specified level than the same aqueous fluid at 20°C.

A further advantage of the present invention over direct stripping processes is the ability of the process to achieve selective removal of organic compounds. Thus, while a conventional stripping process would not select between benzene and other organic compounds with the same vapour pressure but which are less permeable in the membrane, the present process could, for example, selectively remove the benzene while leaving the concentrations of the other organic compounds in the process stream substantially unchanged.

In the present process, use of aqueous oxidising medium instead of a vacuum pump and condenser, a non-condensable gas and condenser, or a condensable sweep gas, is an advantage over prior art pervaporation systems. There is no need to handle pure solvents decanted from a condenser, and because the oxidation reaction may hold the concentration of the organic compound near zero in the oxidising medium, the exiting gas can have very low organic compound concentrations. In conventional pervaporation systems, low exit concentrations in the non-condensable gas leaving the condenser can only be achieved by using very low condensing temperatures which are relatively expensive to realise in industrial practice, or by using complex gas compression loops.

The present invention provides a process for removing organic compounds from an aqueous fluid and degrading them. It has been found possible to combine sweep gas pervaporation with subsequent dissolution/condensation of the organic compounds into an oxidising medium where oxidation takes place. In this improved process, the pervaporation step is used to selectively transport organic compounds from the aqueous fluid into a non-condensable gas.

The aqueous fluid and the gas will preferably flow countercurrently in the pervaporation step of the present invention.

The organic laden gas stream from the pervaporation step of the present invention may pass through one or more reaction chambers containing an oxidising medium. Here oxidation takes place. Oxidation may be achieved by providing an aqueous oxidising medium and dissolving the organic compounds in an aqueous oxidising medium.

The gas exiting the oxidising medium containing reaction chamber(s) has a reduced content of the one or more organic compounds and, preferably, is essentially free of the one or more organic compounds. The gas may be discharged to the environment or compressed and re-used in the process. When the gas is discharged to the environment, preferably the gas is essentially free of the one or more organic compounds.

The aqueous fluid will generally be a water based waste stream. The water based waste stream may arise for example in a manufacturing activity, or as a groundwater stream.

Generally speaking, the one or more organic compounds are volatile organic compounds. The one or more organic compounds may be aromatic or aliphatic, halogenated or non-halogenated. The one or more organic compounds may be selected from chlorinated hydrocarbons such as dichloromethane, trichloroethylene, chloroform, perchloroethylene, dichloroethane, carbon tetrachloride, chlorinated benzenes, chlorinated toluenes, non-chlorinated hydrocarbons such as benzene, toluene, xylene, ethylbenzene, styrene, cyclohexane, hexane, nitrobenzene, derivatives and mixtures thereof.

After treatment the aqueous fluid may be re-used for any purpose or discharged to the environment following treatment.

The membrane comprises at least one non porous layer, which prevents direct contact of the non-condensable gas with the aqueous fluid. As defined above, the membrane is a non porous membrane. This feature is advantageous because if a direct contact stripping device, such as a packed or plate column or microporous membrane contactor, is used, there is significant evaporation of water into the non-condensable gas. The energy for this evaporation comes from the aqueous fluid. This both reduces the temperature of the aqueous fluid, and increases the cooling load required to maintain the temperature of the oxidising medium.

Preferably the membrane is a selectively permeable membrane.

In the present specification by the term "selectively permeable membrane" it is meant a membrane which is permeable to the one or more organic compounds to be removed from the aqueous fluid and which is substantially impermeable to water. Preferably the selectively permeable membrane is substantially impermeable to all components of the aqueous fluid other than the one or more organic compounds to be removed from the aqueous fluid.

The membrane and/or the non porous layer may comprise a material selected from polydimethylsiloxane (PDMS) based elastomers, other modified polysiloxane based elastomers, ethylene-propylene diene (EPDM) based elastomers, polynorbornene based elastomers, polyoctenamer based elastomers, polyurethane based elastomers, butadiene and nitrile butadiene rubber based elastomers, natural rubber, butyl rubber based elastomers, polychloroprene (Neoprene) based elastomers, epichlorohydrin elastomers, polyacrylate elastomers, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), derivative and mixtures thereof.

The membrane and/or the non porous layer may comprise or may be a homogeneous material, preferably a homogenous material selected from the materials listed above.

The membrane and/or the non porous layer may be in the form of a rube or a sheet.

The membrane and/or the non porous layer is preferably a homogenous material in the form of a tube or a sheet.

The membrane and/or the non porous layer of the present invention may be reinforced to increase their burst pressure, for example by overbraiding tubes using fibres of steel or plastic, or by providing a supporting mesh for flat sheets.

In a further preferred embodiment the membranes may be composite membranes comprising a porous support structure and a non porous layer. Suitable materials for the porous support structure are known to those skilled in the art of membrane processing. The porous support structure may be formed from a material selected from polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF) polyethersulfone, and other polymeric material suitable for use in fabricating microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes, derivatives and mixtures thereof. Suitable materials for the non-porous separating layer include by way of non-limiting example polydimethylsiloxane (PDMS) based elastomers, other modified polysiloxane based elastomers, ethylene-propylene diene (EPDM) based elastomers, polynorbornene based elastomers, polyoctenamer based elastomers, polyurethane based elastomers, butadiene and nitrile butadiene rubber based elastomers, natural rubber, butyl rubber based elastomers, polychloroprene (Neoprene) based elastomers, epichlorohydrin elastomers, polyacrylate elastomers, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), derivatives and mixtures thereof.

When the membrane is a composite membrane comprising a porous support, the pores may be arranged to allow entry of either the aqueous fluid or the gas, depending on which configuration provides superior performance. Entry of both the aqueous fluid and the gas is possible. The membranes may be of cylindrical geometry or planar geometry. The membrane may be a hollow fibre. In this aspect, either the aqueous fluid or the gas may be present in the internal volume of the hollow fibre membrane. The membrane may be spiral wound. In this aspect, either the aqueous fluid or the gas may be present inside the membrane leaves. The coiled shell and tube module designs described in UK Patent Application No 9801272.7 "Method" Filing Date 21 January 1998 would be advantageous for use in the present process.

The membrane may be contained in a membrane module. The module may be of any design known to those skilled in the art, such as spiral wound, plate and frame, shell and tube, and derivative designs thereof.

The non-condensable gas may be any non-condensable gas, such by way of non-limiting example air, oxygen, nitrogen, methane, ethane, propane, butane, argon, helium, hydrogen, carbon dioxide, ethylene, propylene, and mixtures thereof.

The non-condensable gas may be a non-condensable permanent gas or a mixture of non- condensable permanent gases.

In a preferred aspect, the process of the present invention is performed in a reactor comprising at least a first zone, and a second zone; wherein the first zone is discrete from the second zone; wherein the first zone and the second zone are separated by the non porous membrane; wherein the first zone contains the aqueous fluid; and wherein the second zone contains the non-condensable gas.

In a further preferred aspect the process of the present invention is performed in a reactor comprising at least a first zone, a second zone, and a third zone; wherein each of the zones is discrete from each other zone; wherein the first zone and the second zone are separated by the non porous membrane; wherein the first zone contains the aqueous fluid; wherein the second zone contains the non-condensable gas; wherein the third zone contains the oxidising medium; wherein the second zone and the third zone are operably connected to each other; and wherein the process further comprises the step of transferring at least a portion of the non-condensable gas to the third zone and contacting the portion of non- condensable gas with the oxidising medium.

In a further aspect the process of the invention is carried out in a reactor comprising at least a first zone, a second zone, and a third zone; wherein each of the zones is discrete from each other zone; wherein the first zone and the second zone are separated by a non porous membrane; wherein the first zone contains an aqueous fluid inlet and an aqueous fluid outlet; wherein the second zone contains a non-condensable gas inlet and a non- condensable gas outlet; wherein the third zone contains a non-condensable gas inlet operably connected to the non-condensable gas outlet of the second zone; and wherein the third zone comprises a means for contacting non-condensable gas from the second zone with an oxidising medium.

In one aspect of the present invention it is preferable to re-use a fraction of the gas exiting the third zone by recycling this fraction and combining it with fresh gas fed to the second zone. In yet a further aspect, it is preferable to recycle all the gas exiting the third zone and to use it to substantially replace fresh gas entering the second zone, thereby reducing the demand for fresh gas.

When the gas is air, oxygen, or an oxygen containing gas, the gas brings oxygen for oxidation to the oxidising medium, preferably to the oxidising medium in the third zone. In this aspect, the rate of supply of oxygen to the oxidising medium from the gas may be less than the demand for oxygen in the oxidising medium. Thus it may be necessary to provide, in addition to the non-condensable gas, an oxygen containing gas to provide oxygen to the oxidising medium. The provision of an oxygen containing gas may also be necessary in cases where the fresh non-condensable gas does not contain oxygen.

The provision of an oxygen containing gas, in addition to the non-condensable gas, may be achieved by any suitable and convenient means. For example it may be achieved by direct sparging of oxygen containing gas to the aqueous oxidising medium, or for example by membrane oxygenation of the oxidising medium.

Oxidation of the one or more organic compounds may be partial or may be complete. Complete oxidation to carbon dioxide, water, and mineral ions is envisaged. Fresh oxidising medium may be fed to the reaction chambers to replenish the oxidising medium.

Material may be withdrawn from the oxidising medium in order to remove any products of oxidation which could accumulate in the oxidising medium, for example chloride ions arising from oxidation of chlorine containing organic compounds, or fatty acids or other organics produced by oxidation, or biomass which might result from biodegradation.

In a preferred aspect the oxidising medium may comprise a biological oxidising means, a chemical oxidising means or a mixture thereof. The oxidising medium may contain a biological oxidising means and a chemical oxidising means. In these aspects the oxidising medium may also contain any other materials necessary for the effective functioning of the biological oxidising means or the chemical oxidising means.

Biological oxidising means include cells, enzymes, other biological material, and mixtures thereof. Cells may use the one or more organic compounds as a source of carbon and energy for maintenance and/or growth.

The temperature of the oxidising medium may be less than 25°C. When the oxidising medium is a biological oxidising means comprising cells, the oxidising means is preferably operated at temperatures in the range 15-35°C.

Chemical oxidising means include hydrogen peroxide, ozone, with or without catalysts such as Fentons reagent, titanium dioxide together with UV light, and combinations thereof.

Preferably the conditions in the oxidising medium are controlled to provide advantageous performance of the oxidation reactions. In a preferred aspect, control of pH, control of dissolved oxygen levels or a combination thereof may be exercised.

In the preferred aspect described above, the third zone may be a reactor selected from any suitable design of gas-liquid contacting equipment. The third zone may be a reactor selected from stirred tanks, including aerated stirred tanks, bubble column reactors, gas- lift columns including air-lift reactors, packed columns, plate columns, spray columns, microporous membrane modules, tray columns, porous membrane contactors and combinations thereof.

In the preferred aspect, the third zone comprises two or more reactors and/or reaction chambers.

In one aspect, the third zone of the reactor may comprise at least two reactors in series or in parallel.

In a further preferred embodiment of the present invention, the non-condensable gas is recirculated within the third zone to enhance the removal and reaction of the organic compounds. When the third zone comprises two or more reactors and/or reaction chambers the non-condensable gas may be recirculated through some or all of the reactors and/or reaction chambers. Recirculation of the non-condensable gas may be via any suitable device known to one skilled in the art, such as by way of non-limiting example a venturi device employing the oxidising medium, a fan, compressor, or blower.

The non-condensable gas may be introduced to the oxidising medium using any suitable sparging device known to those skilled in the art. By way of non-limiting example, these include porous tubes, porous plates, or perforated elastomer or thermoplastic tubes. When the oxidising medium is contained in a third zone, the non-condensable gas may be supplied to the interior region of the third zone. Other modifications will be apparent to those skilled in the art. Furthermore, when the oxidising medium is contained in a third zone comprising two or more reactors and/or reaction chambers, and the non-condensable gas is recirculated through one or more of the reactors and/or reaction chambers, the non-condensable gas may be supplied to the head space of one of the reactors and/or reaction chambers.

Heating the aqueous fluid stream, optionally together with the non-condensable gas stream, will increase the Henrys law constant in the system. This will have advantageous effects. The theoretical minimum amount of gas required to reduce the concentration of organic compound in the aqueous fluid from specified C,_n to specified

Com. while raising the concentration of organic compound in the gas from zero to S_out, can be calculated by assuming the membrane area is infinite in which case the concentration of organic compound in the two streams reaches equilibrium:

^-■min out - U) — L (C_m - C₀ut) where G_mιπ = minimum theoretical gas flow and L = aqueous fluid flow. Since by assumption for infinite membrane area and countercurrent flow equilibrium is reached: Sout ⁼ H C_m _mιn ⁼ ( m - Cout) ' (H C,_π)

In practice, since membrane area is not infinite:

G = G_mιn where G = non-condensable gas flowrate and α = a multiplicative constant, α > 1 So for specified L, C,_n and C_out, increasing H will decrease G_mιn. While in practice G

(non-condensable gas flowrate) will be greater than G_mιn, it is apparent to one skilled in the art that a higher H will lead to lower G, all other things being equal. Lower gas flowrates will lead to more effective removal of the volatile organic compounds in the oxidising medium.

Additionally, higher temperatures of the aqueous fluid and the non-condensable gas will lead to more rapid mass transfer and higher driving forces for pervaporation. and thus to lower membrane area requirements to effect a specified removal

Preferably both the temperature of the aqueous fluid entering the first zone and the temperature of the non-condensable gas exiting the second zone are higher than the temperature of the oxidising medium. This may be achieved in a number of ways. For example, (i) the temperature of supply of the aqueous fluid may be greater than the operating temperature of the oxidising medium; (ii) the aqueous fluid and the non- condensable gas may be heated; (iii) the oxidising medium may be cooled through heat exchange.

If the aqueous fluid entering the first zone is warmer than the non-condensable gas the two streams will tend to exchange heat and this will increase the temperature of the non- condensable gas. It is preferable to operate with the non-condensable gas exiting the second zone at a higher temperature than the oxidising medium because the non- condensable gas is rapidly cooled by direct contact with the oxidising medium as it enters the third zone.

In a preferred aspect organic compounds in the non-condensable gas are dissolved or condensed into the oxidising medium as they seek to reach equilibrium between the non-condensable gas and the oxidising medium. This enhances the speed of removal of the organic compounds from the non-condensable gas.

The temperature to which the aqueous fluid is heated depends upon the characteristics of the aqueous fluid and the properties of the membrane material. In most cases it is desirable to have a temperature in the aqueous fluid entering the first zone of between 30-100°C, more preferably between 40-100°C, yet more preferably between 60-100°C. The temperature to which the non-condensable gas is heated depends on the characteristics of the stream and the properties of the membrane material. In most cases it is desirable to have the non-condensable gas stream exiting the membrane modules at a temperature between 30-100°C, more preferably between 40-100°C, yet more preferably between 60- 100°C.

The temperature maintained in the oxidising medium depends on the characteristics of the oxidising medium and the Henrys law constant of the organic compounds exiting the system. For the case of biological oxidising means the temperature is preferably between 5-50°C, yet more preferably between 15-30°C. For chemical oxidising means the temperature is preferably between 0-60°C, yet more preferably between 30-50°C.

The difference in temperature between the non-condensable gas exiting the second zone and the temperature of the oxidising medium is preferably in the range 0-100°C, yet more preferably in the range 20-80°C.

When the non-condensable gas from the second zone contacts the oxidising medium it is laden with organic compounds. Depending on the rate of cooling of the gas, the rate of oxidation, and the solubility of the organic compound in the oxidising medium, a second phase comprising the organic compound can form in the third zone. This is undesirable because it leads to high concentrations of the organic compound in the exit gas from the third zone. Moreover when the oxidising means comprises biological material, high concentrations of the organic compound may inhibit the oxidation reaction. This may be avoided when necessary by addition of a suitable adsorbent solid or absorbent liquid to the third zone. Adsorbent solids include activated carbon. polymer particles, elastomer segments, derivatives and mixtures thereof. Absorbent liquids include oils which form a non-inhibitory second phase in the aqueous oxidising medium. Suitable oils are, by way of non-limiting example, organic solvents, such as hexadecane, octanol, decanol, tetradecane, hexadecanol, oils such as silicone oil, mineral oil, kerosene, sunflower oil, high molecular weight organic compounds such as polyethylene glycol or polypropylene glycol, mixtures and derivatives thereof. The use of such absorbents improves the rates of gas-liquid mass transfer and avoids formation of solvent phases comprising pure volatile organic compounds.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 shows a system in accordance with the present invention. Figure 2 shows a system in accordance with the present invention. Figure 3 shows a system in accordance with the present invention. Figure 4 shows a system in accordance with the present invention.

Figure 5 shows a system in accordance with the present invention. Figure 6 shows a system in accordance with the present invention.

Figure 7 shows a system in accordance with the present invention.

Figure 1 shows, by way of non-limiting example, a schematic of the improved process. The aqueous fluid (1) is fed to a first zone of a reactor (5) where it is contacted with a non-condensable gas (4) across a membrane (3). The aqueous fluid (2) exits the first zone depleted in the volatile organic compounds. The non-condensable gas (6) exiting the second zone of the reactor is passed into a third zone (7) which contains an aqueous oxidising medium (8). The non-condensable gas (9) is discharged essentially organic- free from the third zone. A stream of fresh medium ( 10) may be fed to the aqueous oxidising medium in the third zone to replenish the oxidising capacity of the aqueous oxidising medium. This creates a constant overflow of material from the oxidising chamber ( 1 1). which also serves to carry away any aqueous soluble products of the oxidation reaction. The temperature of the aqueous oxidising medium in the reaction chamber can be carefully controlled by either heating or cooling through suitable heat exchange devices (12) known to those skilled in the art.

Figure 2 shows, by way of non-limiting example, a second process schematic in which the aqueous fluid is first heated to increase its temperature prior to being fed to the first zone of a reactor (5). Any combination of suitable heating methods can be used, including direct steam injection to the aqueous fluid, and/or use of an indirect heat exchange device (13) such as a finned coil or plate heat exchanger, with steam or some other suitable hot stream on the opposite side. The non-condensable gas may optionally be heated as well using an indirect heat exchange device (14) such as a finned coil or plate heat exchanger, with steam or some other suitable hot stream on the opposite side. For example, the hot aqueous fluid (2) exiting the membrane modules may be diverted through indirect heat exchangers (13) and (14) to efficiently recover heat and reduce energy consumption by transferring heat to the incoming aqueous fluid.

As discussed above, in a preferred aspect, the third zone may be a reactor selected from stirred tanks, bubble column reactors, air-lift reactors, packed columns, plate columns, spray columns, microporous membrane modules and combinations thereof. Figure 3 shows an example of a multiple reaction chamber system in which the first chamber (19) is a stirred tank and a second chamber (15) is a packed column. The non- condensable gas (6) exiting the membrane modules enters the stirred tank and is cooled rapidly in the first chamber. The non-condensable gas (9) exiting the first chamber is cooled to the temperature of the aqueous oxidising medium, but still contains traces of volatile organic compounds. The non-condensable gas (9) is subsequently fed to a packed column chamber (15) where it passes countercurrently upwards against a flow of aqueous oxidising medium (17), which is recirculated from the first chamber (19), to the second chamber (15), and back to the first chamber (18). This arrangement permits rapid cooling of the non-condensable gas in the stirred tank contactor, followed by prolonged contact in the packed bed to ensure low exit volatile organic compound concentrations. In general, reaction chambers of different configurations can be arranged to provide the most advantageous combination of heat transfer and mass transfer between the non-condensable gas stream and the aqueous oxidising medium.

Figure 4 shows, by way of non-limiting example, the process of this invention where a fraction of the non-condensable gas is recycled. As described above, the provision of an oxygen containing gas, in addition to the non-condensable gas, may be achieved by any suitable and convenient means. For example it may be achieved by direct sparging of oxygen containing gas to the aqueous oxidising medium, or for example by membrane oxygenation. Figure 4 shows a system in which an oxygenated gas (25) is added to the aqueous reaction medium in the reaction chamber (7). In this case the fraction of gas recycled (20) and the fraction of gas discharged to atmosphere (24), are controlled so as to maintain the effective operation of the system.

As discussed above, the non-condensable gas may be recirculated within the third zone to enhance the removal and reaction of the organic compounds. Figure 5 shows a process in which the non-condensable gas is recirculated via a venturi device through the reaction chamber. The aqueous oxidising medium (21) is withdrawn from the chamber (7) using a suitable pump, and supplied to the venturi (22) under pressure. This creates a motive force for gas suction, and non-condensable gas (23) is sucked into the venturi and intimately mixed with the flow of aqueous reaction medium passing through the venturi. Various combinations of gas recirculation in the reaction chambers are anticipated as a part of this invention.

Figure 6 is a system based on that shown in Figure 2. The system of Figure 6 differs from that of Figure 2 in that the membrane of Figure 6 is a tubular membrane.

Figure 7 is a system based on that shown in Figure 5. The system of Figure 7 differs from that of Figure 5 in that the membrane of Figure 7 is a tubular membrane.

The invention will now be described further described in the following non-limiting examples. Example 1

An aqueous fluid (1) comprising a caustic liquor containing dichloromethane is supplied at 50°C to the process. The membrane modules (i.e. first/second zones of the reactor) (5) contain a plurality of membrane tubes (3), comprising silicone rubber tubes 3mm i.d. (internal diameter) and 0.6 mm wall thickness, and mounted in a shell and tube arrangement. The aqueous fluid (1) is fed to the shell side of the membrane modules.

Air is used as a non-condensable gas (4) and passes through the boreside of the silicone rubber membranes. The air stream (6) exiting the membrane modules is at 40°C and is sparged into a third reaction chamber (a third zone) (7) which is a stirred tank reactor containing a biological oxidising means (8). Sparging is achieved by supplying said non-condensable gas to the interior of a perforated rubber tube immersed at the base of the stirred tank reactor.

The biological oxidising means is the bacteria Hyphomicrobium sp (ATCC 43129), and is present in an aqueous phase containing appropriate amounts of nitrogen as ammonium and/or nitrates, phosphorous and other essential elements required for microbial growth. The temperature of operation of the aqueous oxidising medium is 20°C. Fresh nutrient medium (10) is supplied to the reaction chamber, and the overflow (1 1) contains chloride ion, evolved as a product of the biological oxidation of dichloromethane .

The pH in the aqueous oxidising medium is controlled within 0.2 pH units of pH 7 by the automatic addition of sodium hydroxide to the aqueous oxidising media. A venturi device (22) is fed with aqueous oxidising media (21) withdrawn from the reaction chamber (7) and recirculated to the chamber as a mixture of non-condensable gas (23) and aqueous oxidising media. The non-condensable gas (9) exiting the reaction chamber is substantially free of dichloromethane and is discharged to the atmosphere.

Example 2 An aqueous fluid process stream (1) comprising an acidic liquor comprising 30wt% aluminium chloride in water and containing benzene is supplied at 50°C to the process.

The membrane modules (5) contain a plurality of membrane tubes (3), comprising ethylene-propylene-diene monomer (EPDM) rubber tubes 3mm i.d. and 1.0 mm wall thickness, mounted in a shell and tube arrangement. The EPDM tubes are overbraided using a stainless steel mesh to provide extra strength. The aqueous process stream (1) is fed to the bore side of the membrane tubes.

Air is used as a non-condensable gas (4) and passes through the shell side of the membrane modules. The air stream (6) exiting the membrane modules is at 40°C and is sparged into a first reaction chamber (19) which is a stirred tank reactor containing a biological oxidising means. Sparging is achieved by supplying said non-condensable gas to the interior of a perforated rubber tube immersed at the base of the stirred tank reactor. The biological oxidising means is a mixed culture comprising Pseudomonas putida (ATCC 700007). and other microorganisms with the ability to metabolise benzene, and is present in an aqueous phase containing appropriate amounts of nitrogen as ammonium and/or nitrates, phosphorous and other essential elements required for microbial growth.

The temperature of operation of the aqueous oxidising medium is 20°C. Fresh nutrient medium (10) is supplied to the reaction chamber. The pH in the aqueous oxidising medium is controlled within 0.2 pH units of pH 7 by the automatic addition of sodium hydroxide or sulphuric acid to the aqueous oxidising media. A venturi device (22) is fed with aqueous oxidising media (21) withdrawn from the reaction chamber (19) and recirculated to the chamber as a mixture of non-condensable gas (23) and aqueous oxidising media. The non-condensable gas (9) exiting the first reaction chamber is supplied to a second reaction chamber (15) which is a packed column. Aqueous reaction medium is withdrawn from the first reaction chamber ( 19) and supplied to the top of the packed column (15). The aqueous reaction medium flows downwards in the packed column in countercurrent flow to the to the non-condensable gas stream, and passes (18) from the base of the packed column back into the first reaction chamber (19). The non-condensable gas exiting the packed column (16) is substantially free of benzene. Example 3

An aqueous process stream (1 ) comprising a caustic liquor containing chlorobenzene is supplied at 20°C to the process. It is heated in an indirect plate heat exchanger (13) using steam, to 70°C. The membrane modules (5) contain a plurality of membrane tubes (3), comprising silicone rubber tubes 3mm i.d. and 0.6 mm wall thickness, and mounted in a shell and tube arrangement. The silicone rubber tubes are overbraided using a stainless steel mesh to provide extra strength. The aqueous process stream (1) is fed to the boreside of the membrane modules. Nitrogen is used as a non-condensable gas (4), and is mixed with recirculating non-condensable gas (20) and subsequently heated to 70°C in a finned coil (14) using steam. The heated non-condensable gas stream then passes through the shell side of the membrane modules. The nitrogen stream (6) exiting the membrane modules is at 65°C and is sparged into a reaction chamber (7) which is a stirred tank reactor containing a biological oxidising means. Sparging is achieved by supplying said non-condensable gas to the interior of a perforated rubber tube immersed at the base of the stirred tank reactor. The biological oxidising means is Pseudomonas JS150 (ATCC 51283), and is present in an aqueous phase containing appropriate amounts of nitrogen as ammonium and/or nitrates, phosphorous and other essential elements required for microbial growth.

The temperature of operation of the aqueous oxidising medium is 30oC. Fresh nutrient medium (10) is supplied to the reaction chamber, and the overflow (1 1) contains chloride ion, evolved as a product of the biological oxidation of chlorobenzene. The pH in the aqueous oxidising medium is controlled within 0.2 pH units of pH 7 by the automatic addition of sodium hydroxide to the aqueous oxidising media.

To provide oxygen to the aqueous oxidising medium, a stream of air (25) is sparged into the aqueous oxidising medium via a perforated rubber tube immersed in the aqueous oxidising medium. A venturi device (22) is fed with aqueous oxidising media (21) withdrawn from the reaction chamber (7) and recirculated to the chamber as a mixture of gas (23) and aqueous oxidising media. The gas (9) exiting the reaction chamber is a mixture of nitrogen and air supplied to the aqueous oxidising medium. A fraction of this is discharged to atmosphere substantially free of chlorobenzene (24). while the remaining fraction is recirculated (20) as non-condensable gas. By varying the ratio of nitrogen (4) to air supplied to the aqueous oxidising medium (25), the relative levels of oxygen and nitrogen in the non-condensable gas can be controlled so as to avoid the formation of explosive mixtures.

Example 4

An aqueous process stream (1) comprising 30wt% sulphuric acid and containing perchloroethylene is supplied at 20°C to the process. It is heated in an indirect plate heat exchanger (13) using steam, to 60°C. The membrane modules (5) contain a plurality of membrane tubes (3), comprising ethylene-propylene-diene monomer (EPDM) rubber tubes 3mm i.d. and 1.0 mm wall thickness, mounted in a shell and tube arrangement. The aqueous process stream (1) is fed to the shell side of the membrane modules. Air is used as a non-condensable gas (4) and passes through the boreside of the EPDM rubber membranes. The air stream (6) exiting the membrane modules is at 50°C and is sparged into a reaction chamber (7) which is a stirred tank reactor containing a chemical oxidising means. Sparging is achieved by supplying said non-condensable gas to the interior of a perforated rubber tube immersed at the base of the stirred tank reactor.

The temperature of operation of the aqueous oxidising medium is 30°C. Fresh aqueous oxidising medium (10) is supplied to the reaction chamber, and the overflow (11) contains chloride ion, evolved as a product of the chemical oxidation of perchloroethylene. The non-condensable gas (9) exiting the reaction chamber is substantially free of perchloroethylene and is discharged to the atmosphere.

All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in chemistry or related fields are intended to be within the scope of the following claims.

Claims

1. A process for removing one or more organic compounds from an aqueous fluid, the process comprising the steps of: (a) transferring the one or more organic compounds from the aqueous fluid to a non- condensable gas, wherein transfer of the one or more organic compounds from the aqueous fluid to the non-condensable gas occurs across a membrane; wherein the membrane is a non porous membrane;

(b) treating the non-condensable gas containing the one or more organic compounds with an oxidising medium, wherein the oxidising medium is an aqueous oxidising medium.

2. A process according to claim 1 wherein the aqueous fluid is an aqueous process stream.

3. A process according to claim 1 or 2 wherein the aqueous fluid is contacted with one side of the membrane and wherein the non-condensable gas is contacted with the other side of the membrane.

4. A process according to any one of the preceding claims wherein the membrane comprises at least one non porous layer.

5. A process according to any one of the preceding claims wherein the non- condensable gas is bought into direct contact with the oxidising medium.

6. A process according to any one of the preceding claims wherein the oxidising medium comprises a biological oxidising means, a chemical oxidising means or a mixture thereof .

7. A process according to claim 6 wherein the oxidising medium contains a biological oxidising means and a chemical oxidising means.

8. A process according to claim 6 or 7 wherein the oxidising medium contains a biological oxidising means selected from cells, enzymes, other biological material capable of reacting with the said at least one organic compound, and mixtures thereof.

9. A process according to claim 6 or 7 wherein the oxidising medium contains chemical oxidising means selected from chemical agents capable of reacting with the said at least one organic compound.

10. A process according to any one of the preceding claims wherein the process is performed in a reactor comprising at least a first zone, a second zone, and a third zone wherein each of the zones is discrete from each other zone; wherein the first zone and the second zone are separated by the non porous membrane; wherein the first zone contains the aqueous fluid ; wherein the second zone contains the non-condensable gas; wherein the third zone contains the oxidising medium; wherein the second zone and the third zone are operably connected to each other; and wherein the process further comprises the step of transferring at least a portion of the non-condensable gas to the third zone and contacting the portion of non-condensable gas with the oxidising medium.

1 1. A process according to any one of the preceding claims wherein the aqueous fluid is at a temperature greater than the temperature of the oxidising medium.

12. A process according to any one of the preceding claims wherein the temperature of the oxidising medium is less than 25°C.

13. A process according to any one of the preceding claims wherein the aqueous fluid is heated prior to contact with the membrane.

14. A process according to any one of the preceding claims wherein the non- condensable gas is heated prior to contact with the membrane.

15. A process according to any one of the preceding claims wherein the membrane comprises a material selected from polydimethylsiloxane (PDMS) based elastomers. other modified polysiloxane based elastomers, ethylene-propylene diene (EPDM) based elastomers, polynorbornene based elastomers, polyoctenamer based elastomers. polyurethane based elastomers, butadiene and nitrile butadiene rubber based elastomers, natural rubber, butyl rubber based elastomers, polychloroprene (Neoprene) based elastomers, epichlorohydrin elastomers, polyacrylate elastomers, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), derivatives and mixtures thereof.

16. A process according to any one of the preceding claims wherein the membrane comprises a homogeneous material.

17. A process according to any one of the preceding claims wherein the membrane is reinforced with an external mesh or support.

18. A process according to any one of the preceding claims wherein the membrane is a composite membrane comprising a porous support and at least one non porous layer.

19. A process according to claim 18 wherein the porous support is formed from a material selected from polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF) or polyethersulfone, any other polymeric material suitable for use in fabricating microfiltration, ultrafiltration. nanofiltration or reverse osmosis membranes, derivatives and mixtures thereof.

20. A process according to claim 18 or 19 wherein the least one non porous layer is formed from a material selected from polydimethylsiloxane (PDMS) based elastomers. other modified polysiloxane based elastomers, ethylene-propylene diene (EPDM) based elastomers, polynorbornene based elastomers, polyoctenamer based elastomers, polyurethane based elastomers, butadiene and nitrile butadiene rubber based elastomers, natural rubber, butyl rubber based elastomers, polychloroprene (Neoprene) based elastomers, epichlorohydrin elastomers, polyacrylate elastomers, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), derivatives and mixtures thereof.

21. A process according to any one of the preceding claims wherein the non- condensable gas is a non-condensable permanent gas or a mixture of non-condensable permanent gases.

22. A process according to any one of the preceding claims wherein the non- condensable gas is selected from air, oxygen, nitrogen, methane, ethane, propane, butane, argon, helium, hydrogen, carbon dioxide, ethylene, propylene, and mixtures thereof.

23. A process according to any one of the preceding claims further comprising providing, in addition to the non-condensable gas, an oxygen containing gas to provide oxygen to the oxidising medium.

24. A process according to any one of claims 10 to 23 wherein the third zone is a reactor selected from stirred tanks, including aerated stirred tanks, bubble column reactors, gas-lift columns including air-lift reactors, packed columns, plate columns, spray columns, microporous membrane modules, tray columns, porous membrane contactors and combinations thereof.

25. A process according to any one of claims 10 to 24 wherein the third zone comprises at least two reactors arranged in series or in parallel.

26. A process according to claim 25 wherein the non-condensable gas is partially or wholly recirculated within the at least two reactors.

27. A process according to any one of the preceding claims wherein the oxidising medium further comprises a solid adsorbent.

28. A process according to any one of the preceding claims wherein the oxidising medium further comprises an absorbent liquid.

29. A process according to any one of the preceding claims comprising the step of passing the aqueous fluid and the non-condensable gas in a co-current manner or in a counter-current manner with respect to each other.

30. A process according to any one of claims 10 to 29 wherein the non-condensable gas is partially or wholly recirculated between the second zone and the third zone.

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31. A process as substantially described herein and with reference to any one of Figures 1-5.