GB2399039A

GB2399039A - Removal of undissociated organic acids or bases from a fluid stream

Info

Publication number: GB2399039A
Application number: GB0305287A
Authority: GB
Inventors: Andrew Livingston; Andrew Boam
Original assignee: Membrane Extraction Technology Ltd
Current assignee: Evonik Membrane Extraction Technology Ltd
Priority date: 2003-03-07
Filing date: 2003-03-07
Publication date: 2004-09-08
Anticipated expiration: 2023-03-07
Also published as: GB2399039B; GB0305287D0

Abstract

Removal of one or more undissociated organic acids or bases dissolved in a fluid steam comprises separating the fluid stream into two fractions, a first fraction 2 and a second fraction 3. A base or acid 4 is then contacted with the first fraction 2 to provide a stripping solution 5, in sufficient quantity such that at least one of the organic acids or bases present in the first fraction 2 becomes dissociated. A selectively permeable membrane 6 is provided, having a first surface and a second surface. One or more undissociated organic acids or bases is transferred from the second fraction 3 to the stripping solution 5 across the membrane 6 by contacting the stripping solution 5 with the first surface and the second fraction 3 with the second surface. The volume and/or strength of the stripping solution 5 contacted with the first surface is regulated relative to the volume of the second fraction 3 contacted with the second surface so that a driving force for organic acid or base permeation is maintained.

Description

PROCESS

The present invention concerns a process for removing one or more undissociated organic acids or bases dissolved in a fluid stream s Many organic compounds with acid or base functionalities can be found in fluid streams in industry. By way of non-limiting example, many organic acids, such as phenolic compounds, including phenols, cresols, nitrophenols, chlorophenols, and many organic bases, such as anilines, toluidines, nitroanilines, pyridines, and aliphatic amines enter aqueous process streams in chemical processing. These molecules are in many cases toxic. Methods for removing toxic organic molecules from aqueous process streams are well known. Some of these methods use membranes.

Membrane solvent extraction using microporous membranes to provide a phase contacting between aqueous and organic streams is well known. For example Kiani, Bhave and Sirkar Journal of Membrane Science 20 (1984) pp 125-145 report the use of microporous membranes for immobilizing solvent interfaces during solvent extraction.

Tompkins, Micheals and Peretti Journal of Membrane Science 75 (1992) pp 277-292 report using microporous polypropylene fibres to stabilise phase interfaces during extraction of nitrophenol from an aqueous solution into octanol. US 5,512,180 describes a process wherein polypropylene glycol MW 4,000 was used to extract nitrophenol in a microporous membrane contactor.

Gonzalez-Munoz, Luque, Alvarez, and Coa Journal of Membrane Science 213 (2002) 181-193 report a process in which decanol is used as a solvent which is recirculated between two hollow fibre membrane modules and is used to extract phenol. In the first membrane module solvent extraction of phenol from an aqueous stream into decanol takes place, while in the second membrane module the phenol is extracted from the decanol into an NaOH solution. In a similar process and Cichy and Szymanowski Environmental Science and Technology 36 (2002) 2088-2093 show that kerosene loaded with a complexing agent such as trioctlyamine can also be used as a recirculating solvent 2 1 between two hollow fibre modules. It is evident to one skilled in the art that the solvent phase in these prior art process arrangements acts as a selectively permeable liquid membrane between the two aqueous phases.

A continuing problem with membrane supported solvent extraction with microporous membranes is the breakthrough of one phase into the other due to pressure imbalances.

To overcome this problem, various improvements have been suggested such as using composite membranes comprising a thin layer of non-porous organic-permeable polymer bound to a mieroporous membrane to avoid phase breakthrough, for example US 4,960,520.

Contacting twoaqreous streams with opposite sides of a membrane to effect extraction of organic pollutants from one side to the other is also known in the art. Supported liquid membranes have been applied in this mode. For example US 5,507,949 describes a process wherein the pores of a microporous hydrophobic membrane are filled with a hydrophobic polyamphiphilic oligomeric or polymeric liquid to allow mass transport of various organics across the membranes. In this application the driving force for extraction across the supported liquid membranes may be provided by a stripping solution. The driving force produced by a stripping solution may rely upon conversion of an organic acid to its corresponding salt using a basic stripping solution, or conversion of an organic base to its corresponding salt using an alkaline stripping solution.

Biologically active stripping solutions may also be utilised, for example US 4,988,443 to Michaels et al. discloses a method for contacting an aqueous waste stream containing organic toxicants with a nutrientcontaining aqueous stream using hollow fibre membranes with ater immiscible solvent filled pores. The two streams do not mix but the organic toxicants are transferred from the waste stream across the membrane to the nutrient stream. Microorganisms growing associated with the outside of the hollow fibres utilise the nutrients and organic toxicants as growth substrates which provides the driving force for continued transport.

In further applications non-porous membranes have been employed to effect extraction of 3 1 organic molecules from one aqueous stream into another. US 5,552,053 discloses solid polyamphiphilic polymer films used for keeping separate two aqueous phases, one being a waste stream and the other a stripping solution in which the organic pollutant can be concentrated by conversion into an ionised form at controlled pH. GB 1, 480,018A S describes a process for separating a phenolic material from an aqueous mixture by contacting said mixture with a first surface of a nonporous membrane selectively permeable to the phenolic material, while the second surface of the membrane is contacted with a phenol solvent or solution of a reagent that forms a salt or complex with the phenolic material. Complexing agents such as the hydroxides of the alkaline earth and alkali metals in solution readily form phenates in solution and provide a satisfactory solution sink. US 4,082,658A discloses a process for removing undissociated phenolic compounds dissolved in an aqueous fluid comprising the steps of transferring said phenolic compounds from the aqueous fluid across a selectively permeable membrane to an alkaline stripping solution and maintaining the pH of the alkaline stripping solution in contact with the membrane at a value of about 10 or more to ensure that the phenolic compound(s) is (are) dissociated, ie are in the phenate form. GB 2,207,064A describes a method for extracting a selected species of organic molecule from a test solution, such as phenol from water, by acidifying said solution on one side of a non-porous silicone rubber membrane and making alkaline a receptor solution on the other side of the membrane such that the phenol moves across the membrane to the receptor solution.

In the above prior art, membranes are substantially rigid and are employed in shell and tube modules, in plate and frame modules, or in spiral wound modules. These modules are designed to generate good mass transfer and fluid distribution around all of the membrane surfaces.

In some cases, tubular elastomeric non-porous homogeneous membranes for example silicone rubber (cross linked polydimethoxysiloxane) tubes have been disclosed. The tubular elastomeric membranes provide separation by allowing specific chemical species (for example, hydrophobic organic molecules such as benzene, toluene, or their derivatives) to preferentially dissolve in the membrane and permeate across the membrane by diffusion under the influence of a chemical activity driving force. For example, US 5,585,004 to Livingston discloses a system of apparatus and method wherein a waste stream containing toxic organic compounds is fed to the inside of selectively permeable silicone rubber membrane tubes suspended in a bioreactor receptacle filled with a biologically active medium. The toxic organic compounds diffuse across the silicone rubber membrane and into the biologically active medium where they are destroyed by the microbial culture.

Further examples of the use of tubular elastomeric membranes are oxygenation of microbial systems (Cote et al, Journal of Membrane Science 1989 47 plO7), and pervaporation (Raghunath and Hwang, Journal of Membrane Science 1992 65 pl47). In the field of chemical analysis, silicone rubber membranes have been used to extract organics from aqueous streams prior to analysis (US 4,715,217; US 4,891,137).

GB 2,355,455 describes a process for removing and recovering one or more undissociated phenolic compounds dissolved in aqueous fluid, the process comprising the steps of: (a) transferring the undissociated phenolic compound from the aqueous fluid to an alkaline stripping solution, wherein transfer of the undissociated phenolic compound from the aqueous fluid to the alkaline stripping solution occurs across a membrane; wherein the membrane is a non porous, selectively permeable membrane; (b) regulating the volume of alkaline stripping solution employed relative to the volume of aqueous fluid treated so that the total phenolic compound concentration in the alkaline stripping solution, comprising the sum of the dissociated and undissociated phenolic compound concentrations, is above the solubility of the phenolic compound in water; (c) regulating the pH of the alkaline stripping solution in contact with the membrane to a value at least 0.5 pH units above the acidic dissociation constant of the phenolic compound; (d) adjusting the pH of the phenolic compound containing alkaline stripping solution to a value below the acidic dissociation constant of the phenolic compound and (e) separating the resulting phenolic compound rich phase and the acidified alkaline stripping solution.

GB 2,352,715 describes a process for removing and recovering one or more undissociated aromatic amines dissolved in aqueous fluid, the process comprising the steps of: (a) transferring the one or more undissociated aromatic amines from the aqueous fluid to an acidic stripping solution, wherein transfer of the one or more undissociated aromatic amines from the aqueous fluid to the acidic stripping solution occurs across a membrane; wherein the membrane is a non porous, selectively permeable membrane; (b) regulating the volume of acidic stripping solution employed relative to the volume of aqueous fluid treated so that the total aromatic amine concentration in the acidic stripping solution, comprising the sum of the dissociated and undissociated aromatic amine concentrations, is above the solubility of the aromatic amines in water; (c) regulating the pH of the acidic stripping solution in contact with the membrane so that the membrane remains selectively permeable; (d) adjusting the pH of the aromatic amine containing acidic stripping solution to a value above the acidic dissociation constant of the aromatic amine and (e) separating the resulting aromatic amine and the acidic stripping solution.

In the prior art utilising membranes for organics removal from a fluid stream, the use of a stripping solution which is either acidic or alkaline is known. For example GB 2,355,455 teaches by way of example the addition of 20 to 47wt% NaOH to an initially dilute caustic solution to maintain the stripping solution pH alkaline, while GB 2,352,715 teaches the addition of 37wt% HCl to an initially dilute acid solution to maintain an acidic stripping solution pH. US 5,552,053 teaches the use of 0.1N NaOH solutions, while GB 1,480,018 teaches the use of either sodium or potassium hydroxide solutions.

US 5,507,949 teaches the use of 0.1N NaOH and 0.1N nitric acid as stripping solutions.

In the open literature, Castelo Ferreira F., Han S., Livingston A.G. "Recovery of Aniline from Aqueous Solution using the Membrane Aromatic Recovery System (MARS)" Ind&Eng.Chem.Res 41 (2002) 2766-2774 teach the use of 10.4% HC1 for aniline recovery, while Han S., Castelo Ferreira F., Livingston A.G. "Membrane Aromatic Recovery System (MARS) - A New Membrane Process for the Recovery of Phenols from Wastewaters" J Mem.Sci. 188 (2001) 219-233 teach the use of 12.5 wt% NaOH for recovery of phenol from an aqueous stream.

These prior art disclosures do not reveal how the acids or bases used to form the stripping solutions have been prepared.

In commercial practice, sodium hydroxide and potassium hydroxide solutions are generally sold as 40-50wt% solutions of the metal hydroxide in water. Acids are generally sold as concentrated solutions, such as hydrochloric acid (28-37wt% in water), nitric acid (>80'vt% in water), and sulphuric acid (>9Owt% in water). These solutions constitute a concentrated form in which these commodities can be economically transported. However, these concentrated forms of acids and bases are generally not used directly as stripping solutions in the prior art membrane processes. This may be because they tend to destroy materials used as selectively permeable membranes. This effect can be avoided by careful pH control which matches the quantity of base or acid added to the stripping solution to the quantity of organic acid or base which permeates across the membrane, thereby holding the stripping solution pH at some fixed or controlled value.

However, even with careful neutralization the concentrated forms of bases or acids may not be practical in application due to the concentration of undissociated organic acid or base in the resulting stripping solution, which tends to increase as the concentration of acid or base used to form the stripping solution increases. This effect is known to those skilled in the art and is elaborated on in the open literature, for example Han S. , Castelo Ferreira F., Livingston A.G. "Membrane Aromatic Recovery System (MARS) - A New Membrane Process for the Recovery of Phenols from Wastewaters" J. Mem.Sci. 188 (2001) 219-233. Another reason that concentrated forms of base and acid are not used is because when neutralised with organic acids and bases, the resulting solution viscosity tends to increase with the concentration of the acid or base used. Hence, it is normal practice for those skilled in the art to dilute these acids and bases.

In membrane processes, when a gradient in concentration of inorganic ions such as sodium chloride or potassium chloride exists between two solutions of ions dissolved in a solvent in contact with respective sides of a membrane which is not readily permeable to the ions, an osmotic pressure is said to exist across the membrane by those skilled in the art. This osmotic pressure creates a tendency for the solvent for the ions, which might be water or an organic liquid, or a mixture of water and organic liquids, to permeate from the side of the membrane in which the ion concentration is lower to the side of the membrane where the ion concentration is higher. Thus in the prior art membrane processes mentioned above, if the fluid stream in contact with one face of a membrane impermeable to ions and dissociated species has a higher concentration of ions than the fluid stream in contact with the other face of the membrane, solvent will tend to penneate across the membrane from the solution where ions are lower in concentration to the solution where they are higher in concentration. This migration of solvent can have adverse affects on the operation of a such a membrane process.

The present invention addresses the problems of the prior art.

In one aspect the present invention provides a process for removing one or more undissociated organic acids or bases dissolved in a fluid stream, the process comprising the steps of: (a) separating the fluid stream into two fractions, a first fraction and a second fraction; (b) contacting a base or an acid with the first fraction to provide a stripping solution, and in sufficient quantity such that at least one of the organic acids or bases present in the first fraction becomes dissociated; (c) providing a selectively permeable membrane having a first surface and a second surface; (d) transferring one or more undissociated organic acids or bases from the second fraction to the stripping solution across the membrane by contacting the stripping solution with the first surface and the second fraction with the second surface, wherein the volume and/or strength of the stripping solution contacted with the first surface is regulated relative to the volume of second fraction contacted with the second surface so that a driving force for organic acid or base permeation is maintained.

We have found that addition of a first fraction of the fluid stream from which organic acids and bases are to be separated using the above prior art membrane processes, to a concentrated base or acid used to form the stripping solution for these prior art processes, has advantages, including; (i) reducing the osmotic pressure gradient across the membrane when the fluid stream contains salts; (ii) reducing the required amount of organic compound which must cross the membrane to achieve a given target for removal of the organic compound from the fluid stream, hence reducing the required membrane area, and; (iii) lowering the viscosity of the stripping solution and hence improving mass transfer performance in the stripping solution.

By the term "selectively permeable" it is meant a membrane which is permeable to the undissociated organic acid or base and which is impermeable to the dissociated organic acid or base.

In the present invention, organic acids or bases present in a fluid stream in undissociated form are recovered by means of membrane extraction across a membrane into a stripping solution. The membrane contains at least one selectively permeable layer. The organic acids or bases pass into a stripping solution prepared by mixing a fraction of the fluid stream with a base or an acid in which the organic acids or bases undergo dissociation.

The stripping solution may then be further processed by adjusting the pH upwards or downwards until the organic acids or bases re-associate and precipitate out of solution as organic rich liquids or solids.

An organic acid will undergo a dissociation reaction when the pH of the stripping solution is above the pKa of the organic acid, where pica is the acidity constant and is i defined as follows (see for example "Organic Chemistry" third Edition by T.W.G.Solomns, John Wiley and Sons, p 680): i R-OH + H20 ' R-O- + H3O+ (1) K 1 (I - 0: ]: (2) where R is a group containing organic structure.

Organic bases will undergo a dissociation reaction when the pH of the stripping solution is below the pKa = (14-pKb) of the aromatic amine, where pKb is the basicity constant and is defined as follows (see for example "Organic Chemistry,' third Edition by T.W.G.Solomns, John Wiley and Sons, pp 836-837): s R-NH2 + H2O ' RNH3+ + OH- (3) [RNH+ TOH- 1 glow [RNH2] ) ( ) where R is a group containing organic structure.

In the present indention the separation is effected by causing the undissociated form of an organic acid or base to permeate across a membrane into a stripping solution, where it is caused to dissociate. Dissociation is bought about by the presence of a base in the stripping solution when organic acids permeate across the membrane, and by an acid in the stripping solution when organic bases permeate across the membrane.

Preferably the fluid stream is an aqueous process stream.

The membrane,of the present invention can be configured in accordance with any of the designs known to those skilled in the art, such as spiral wound, plate and frame, shell and tube, and derivative designs thereof. The membranes may be of cylindrical or planar geometry.

For shell and tube designs, the membrane comprises one or more tubular membranes. In this aspect either the second fraction of the fluid or the stripping solution is held within the internal volume of the tubular membrane(s) and the other of the second fraction of the fluid or the stripping solution is in contact with the external surface of the tubular membrane(s). For spiral wound designs, either the second fraction of the fluid or the stripping solution is within the membrane leaves and the other of the second fraction of the fluid or the stripping solution is in contact with the external surface of the membrane leaves.

The membrane of the present invention is formed from or comprises a material suitable to provide a selectively permeable membrane. The membrane may consist of a homogeneous membrane such as a tube or sheet of material, or a composite membrane.

The composite membrane may comprise a non-porous, selectively permeable layer and one or more further materials or may comprise a mixture of materials, including a selectively permeable supported liquid membrane layer. The selectively permeable layer or material prevents direct contact of the second fraction of the fluid with the stripping solution. If a direct contact stripping device such as a packed or plate column or microporous membrane contactor is used, the two streams would mix and there would be no resulting separation.

In a preferred aspect the membrane or the non-porous, selectively permeable layer thereof is formed from or comprises a material selected from modified polysiloxane based elastomers including polydimethylsiloxane (PDMS) based elastomers, ethylene- I propylene diene (EPDM) based elastomers, polynorbornene based elastomers, polyoctenamer based elastomers, polyurethane based elastomers, butadiene and nitrite butadiene rubber based elastomers, natural rubber, butyl rubber based elastomers, polychloroprene (Neoprene) based elastomers, epichlorohydrin elastomers, polyacrylate elastomers, or polymers such as polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyimide, polyamide, cellulose based materials, and mixtures thereof.

In a preferred aspect the membrane comprises a reinforcing material selected from an external mesh and support. This is particularly advantageous for homogenous tubes or sheets. Such tubes or sheets may be reinforced to increase their burst pressure, for example by overbraiding tubes using fibres of metal or plastic, or by providing a supporting mesh for flat sheets.

When the membrane comprises a non-porous layer and an additional component, the additional component may be a supporting layer. The supporting layer may be a porous support layer. Suitable materials for the open porous support structure are well known to those skilled in the art of membrane processing. Preferably the porous support is formed from or comprises a material selected from polymeric material suitable for fabricating microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes, including polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyethersulfone, polyacrylonitrile, polyamide, polyimide, cellulose acetate, and mixtures thereof. The porous support can be made by any technique known to the art, including sintering, stretching, track etching, template leaching, interracial polymerization or phase inversion. Yet more preferably the porous support is prepared from an inorganic material such as by way of non-limiting example silicon carbide, silicon oxide, zirconium oxide, titanium oxide, or zeolites, using any technique known to those skilled in the art such as sintering, leaching or sol-gel processes.

In yet another preferred aspect, the membrane comprises a supported liquid membrane and an additional component. The supported liquid membrane may comprise organic liquids which promote selective permeation of the undissociated organic acid or base, such as hydrocarbons, alcohols, aromatics, polyethyleneglycols, polypropyleneglycols, polybutyleneglycols, and may contain one or more species of carrier molecules to facilitate transport of the organic acids or bases, for example Schlosser S. and Sabolova E. Journal of Mernbranne Science 210 (2002) 331-347 show that trioctylamine and other organic bases may be used as carriers in an n-alkane liquid membrane to facilitate transport of organic acids such as phenol, and Cichy and Szymanowski Environmental Science and Technology 36 (2002) 2088-2093 show that Cyanex 923, Amberlite LA-2 and trioctlyamine are all suitable carriers applied in kerosene. The use of such carrier systems in liquid membranes is known by those skilled in art as facilitated transport.

When the membrane comprises a supported liquid membrane and an additional component, the additional component may be a supporting layer. The supporting layer may be a porous support layer. Suitable materials for the open porous support structure are well known to those skilled in the art of membrane processing. Preferably the porous support is formed from or comprises a material selected from polymeric material suitable for fabricating microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes, including polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyethersulfone, polyacrylonitrile, polyamide, polyimide, cellulose acetate, and mixtures thereof. The porous support can be made by any technique known to the art, including sintering, stretching, track etching, template leaching, interracial polymerization or phase inversion. Yet more preferably the porous support is prepared from an inorganic material such as by way of non-limiting example silicon carbide, silicon oxide, zirconium oxide, titanium oxide, or zeolites, using any technique known to those skilled in the art such as sintering, leaching or sol-gel processes.

In yet a further preferred case the process may be applied using two discreet membrane modules to first extract an organic acid or base from the second fraction into an organic solvent phase acting as a liquid membrane in the first membrane module, and then back- extract the organic acid or base from the organic solvent into the stripping solution. In these cases a suitable organic solvent may be decanol, methyl isobutylketone, toluene, and the solvents may have a complexing agent such as trioctyl amine added to enhance organic acid or base distribution.

Preferably the tubular membranes have a high length to diameter ratios for example the tubular membranes may have internal diameters from 0.5 to 5.0 mm, and/or a wall thicknesses between 0.1 and 1.0 mm and/or a length of from 50 to 500 metros. The.

length to diameter ratio of the tubular membrane may be from 1x104 to 1x106.

In a further preferred aspect of the present invention a pH control system is used to regulate the flow of acid or base to the stripping solution which contacts the membrane.

Control of pH in the stripping solution is important. Upon contact with the membrane the stripping solution will tend to be neutralized by the dissociation of the organic acids or bases permeating the membrane and dissociating in the stripping solution, and it is advantageous for the process efficiency that the pH of the stripping solution is kept at least 0.5 pH units above the pKa of an organic acid, and at least 0.5 pH units below the pKa of an organic base. s

Preferably the stripping solution in contact with the selectively permeable membrane is well mixed so that its composition is well mixed throughout the volume, or it is substantially uniform throughout the volume, operably in contact with the membrane.

Preferably the pH of the stripping solution in contact with the membrane is controlled so that it is substantially the same throughout the stripping solution in contact with the membrane separating layer.

Preferably the fluid contains an organic acid selected from phenol, cresols, chlorophenols, fluorophenols, bromophenols, iodophenols, dichlorophenols, chlorocresols, dimethylphenols, nitrophenols, benzenediols, benzoquinones, isothiazolones, and mixtures thereof.

Preferably the fluid contains an organic base selected from aniline, chloroanilines, fluoroanilines, bromoanilines, iodoanilines, dichloroanilines, toluidines, dimethylaniline, nitroaniline, benylamine, phenylaniline, methylaniline, ethylaniline, anisidine, benzylamine, pyridine, picoline, methyl pyridines, chloropyridines, bromopyridines, fluoropyridines, i odopyri dines, ethyl amines, propylamines, butylamines, octylamine, other alkylamines, piperidine, cyclohexylamine, and mixtures thereof.

When the process is used to remove organic acids from the fluid, the stripping solution is formed by mixing the first fraction of the fluid with a base, preferably a mineral alkali selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof.

When the process is used to remove organic bases from the fluid, the stripping solution is formed by mixing the first fraction of the fluid with an acid, preferably a mineral acid selected from hydrochloric acid, sulphuric acid, phosphoric acid, nitric acid, and mixtures thereof.

In a further preferred aspect the second fraction of fluid is contacted with one side of a plurality of membranes in series, in parallel or in a combination thereof, and wherein the stripping solution is contacted withthe other side of each of the plurality of membranes.

In an alternative explanation, in a further preferred aspect a plurality of selectively permeable membranes is provided, each having a first surface and a second surface; wherein the second fraction is contacted with the second surface of each of the plurality of selectively permeable membranes in series, in parallel or in a combination thereof, and wherein the stripping solution is contacted with the first surface of each of the plurality of membranes.

In a further preferred aspect the ratio of the volume of the first fraction to the volume of the second fraction is less than 1.0, yet more preferably less than 0.5, and yet more preferably less than 0.2.

The process may be performed in a continuous, semi-continuous or discontinuous (batch mode) manner. In the latter aspect the flow of at least one of the second fraction of fluid, or the stripping solution, is discontinuous.

The second fraction of fluid and/or the stripping solution of the present invention may be heated before or during contact with the membrane. The second fraction of fluid and/or the stripping solution of the present invention may have a temperature above room temperature (25 C). This may increase the rate of mass transfer across the selectively permeable membrane. In a further preferred embodiment, the temperature of the second fraction of fluid andlor the stripping solution may be above 60 C. In yet a further preferred embodiment, the temperature of the second fraction of fluid and/or the stripping solution may be above 70 C.

In a further preferred aspect the fluid contains substantial quantities of dissolved inorganic or organic materials. By the term "substantial quantities" it is meant greater than 0.1 wt%., preferably greater than lwt%, preferably greater than 5wt%, preferably greater than lOwt%, and preferably greater than 20wt%. The inorganic materials may include salts, such as sodium chloride, potassium chloride and mixtures thereof. The organic materials may include solvents, such as methanol, ethanol, acetone, acetate and mixtures thereof In the present invention, when the fluid is divided into two fractions and the first fraction is mixed with base or acid to form the stripping solution, inorganic materials present in the fluid will then be present in both the stripping solution and the second fraction of fluid. This acts to reduce the osmotic pressure across the membrane compared to the case when another fluid such as water is used to dilute the acid or base to make a stripping solution, and will act to reduce osmotic pressure and flux of solvents such as water or methanol across the membrane.

The invention will now be described, by way of example only, with reference to the accompanying drawing, In whch: Figure 1 is a schematic of an apparatus operating the process of the present invention.

Figure 2 is a schematic of an apparatus operating the process of the present invention.

Figure 3 is a schematic of an apparatus operating the process of the present invention.

Figure 4 is a schematic of an apparatus operating the process of the present invention.

Figure 1 shows a schematic of one embodiment of the process applied to removal of organic acids. A fluid stream (1) containing undissociated organic acids is divided into two fractions, a first fraction (2) and a second fraction (3). The first fraction (2) is mixed with a base (4) to create an alkaline stripping solution (5). The alkaline stripping solution passes on one surface of a membrane containing at least one selectively permeable separating layer (6), optionally mounted in a membrane module (7). The second fraction (3) passes on the opposite face of the membrane. Undissociated organic acids in the second fraction of the fluid permeate across the membrane into the alkaline stripping solution (5), whose pH is such that at least one of the organic acids is converted into its corresponding salt. The second fraction exiting the membrane (8) has a reduced concentration of organic acids relative to the initial fluid (1). The organic acid laden alkaline stripping solution (9) leaves the membrane module (7) containing dissociated organic acids.

Figure 2 shows a schematic of one embodiment of the process applied to removal of organic bases. A fluid stream (11) containing undissociated organic bases is divided into two fractions, a first fraction (12) and a second fraction (13). The first fraction (12) is mixed with an acid (14) to create an acidic stripping solution (15). The acidic stripping solution passes on one surface of a membrane containing at least one selectively permeable separating layer (6), optionally mounted in a membrane module (7). The second fraction ( 13) passes on the opposite face of the membrane. Undissociated organic bases in the second fraction of the fluid permeate across the membrane into the acidic stripping solution (15), whose pH is such that the at least one of the organic acids is converted into its corresponding salt. The second fraction exiting the membrane (17) has a reduced concentration of organic bases relative to the initial fluid (1). The organic base laden acidic stripping solution (18) leaves the membrane module (7) containing dissociated organic bases.

In a preferred embodiment, the membranes may comprise a bundle of tubular membranes with suitable head piece connections for allowing flow of the second fraction or the stripping solution to pass through the interior of the membranes. This bundle of tubular membranes may be suspended in a tank or other vessel so that the outside surface of the fibres is substantially immersed in the stripping solution or the second fraction. In this case the liquid an the outside of the membrane bundle will be mixed or agitated using a stirrer or pump or some other suitable device to ensure that the liquid is well mixed at all times and the composition of the liquid in contact with the membrane will be the same as the concentration of the liquid leaving the tank. Figure 3 shows this general arrangement applied to removal of an organic acid from a fluid stream, where a bundle of tubular membranes (23) are connected at each end to allow wastewater flow through headpieces (24), and are immersed in a tank (25) of alkaline stripping solution (26) containing dissociated organic acids.

Figure 4 shows yet another preferred embodiment, in which the second fraction of fluid (3) is supplied to the outside surface of a tubular membrane, and pH is controlled using a suitable control system. Figure 4 shows the application to recovery of organic acids, in which one or more elastomeric tubular membranes (31) connected using suitable headpieces (32) are suspended in a well mixed tank (33) to which is fed the second fraction of fluid (3). The elastomeric tubular membranes can be coiled, stacked or otherwise arranged in the tank so that they have their surfaces substantially immersed in the second fraction of fluid (3). It is advantageous in this embodiment to use elastomeric tubular membranes which have high length to diameter ratios for example the elastomeric tubular membranes might have internal diameters from 0.5 to 5.0 mm, wall thicknesses between 0.1 and 1.0 mm and lengths from 50 to 500 metros, i.e. length to diameter ratios of 1x104 to 1x106. The first fraction of fluid (2) together with a base (4) is added to a tank (34) to create a stripping solution (5). The organic acid laden stripping solution (9) is recirculated to the inside of the membrane tubes and back to a tank (35).

The free hydroxide ion concentration in organic-acid laden stripping solution (9) is measured by a suitable instrument (36), for example a pH controller or a conductivity probe have both been found to be useful, and controlled at some fixed value by a controller (37) using a suitable device (38) such as valve or pump to regulate flow of stripping solution (5) from the make up tank (34) to the stripping solution recirculation tank (35).

The processes described above may be operated continuously, semicontinuously or in batch mode. The tanks may be single tanks or multiple tanks. Mixing of one or all of the tanks may be achieved by using any device known to those skilled in the art, such as mixers, pumps, or air lift devices. The applications described for removal of organic acids may be adapted to removal of organic bases by application of an acid in place of a base in the stripping solution, and vice-versa. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

The invention will now be described in further detail in the following non-limiting

Examples.

EXAMPLES

EXAMPLE 1

The following example shows the effect of the concentration of sodium phenolate on the viscosity of stripping solution. A series of solutions were prepared by diluting 50wt% sodium hydroxide with water to a final required value, and then adding phenol to the sodium hydroxide solutions to provide controlled pH values. The viscosities of the solutions were then determined, with the following results: À À 2 -1 Vscosty(mm s) pH 10 wt% Phenol 20wt% Phenol 30 wt% Phenol (5% NaOH) (10 wt% NaOH) (30 wt% NaOH) 12.5 1.5 2.8 6.7 13.0 1.5 2.9 7.4 13.5 1.6 3.0 7.8 14.0 1.9 3.8 11.0 These results clearly show that as the initial concentrations of sodium hydroxide, and so concentrations of phenol present at neutralization to required pH, increase, so does the viscosity of the stripping solution. Increasing viscosity lowers the film mass transfer rates and may hare an adverse effect on the rate of mass transfer across the membrane.

EXAMPLE 2

A process was operated in continuous mode, to recover phenol from a wastewater. A membrane composing a bundle of silicone rubber tubes was immersed in a stripping solution provided on the outside of the membrane tube. Wastewater was pumped through the inside of the membrane tubes. The stripping solution was well mixed and the pH of the stripping solution was controlled using a conductivity probe linked to a controller. This controller dosed 10wt% NaOH to the stripping solution to control stripping solution pH at around 13. The wastewater to be treated contained 5 wt% phenol dissolved in water.

In the first operating mode, the 10wt% NaOH was prepared by adding water to 50wt% NaOH to make a 10wt% NaOH solution. This was then added to the stripping solution via the controller to neutralise the phenol from the wastewater that crossed the membrane. The wastewater was pumped through the membrane tube so that 90% of the phenol was removed from the wastewater, resulting in generation of 0.225 kg of stripping solution containing 20wt% phenol. In this operating mode, 45 grams of phenol was extracted across the membrane for each kg of wastewater treated.

In the second operating mode, wastewater was used to dilute the 50wt% NaOH solution to form a 1 0wt% stripping solution. For each kg of wastewater fed to the process, 0.15 kg was used to dilute the 50wt% NaOH. The remaining 0.85 kg of wastewater was pumped through the membrane tube so that 90% of the phenol was removed, resulting in generation of 0.19 kg of stripping solution containing 20wt% phenol. In this operating mode, 38 grams of phenol was extracted across the membrane for each kg of wastewater treated.

This example shows that by using the untreated wastewater to dilute the base or acid used to prepare stripping solution, relatively less of the compound to be extracted (in this case phenol) has to moss the membrane. This arises since some of the compound to be extracted is provided directly to the stripping solution through the dilution step, and so does not have to be extracted across the membrane.

EXAMPLE 3

A wastewater containing 0.4 wt% of a phenolic compound and 20 wt% KCI was treated for phenolic compound removal using a membrane process in which the wastewater was treated continuously. The wastewater was fed at a rate of 5 L do into a 10 L volume extraction tank in which a silicone rubber membrane tube 3mm i.d. x 0.5 mm wall, area 1 m2 was suspended, so that the wastewater was in contact with the outer surfaces of the membrane tube. The phenolic compound was extracted across the membrane and into a stripping solution made up of a 5wt% KOH solution neutralized to pH 11 with the phenolic compound. The stripping solution was pumped from a stripping solution reservoir, through the inside of the membrane tube, and back to the reservoir. The pH of the stripping solution was controlled at 11 using a conductivity probe immersed in stripping solution in this reservoir and linked to a controller. This controller dosed 5wt% KOH to the stripping solution reservoir to control pH. The wastewater overflowed from the extraction tank and into a treated wastewater container, typically with a removal of the phenolic compound of 40-60%. The process was operated at 80-90 C. The 5wt% KOH solution was prepared using water to dilute a concentrated 50wt% KOH solution.

Over 5 days of operation, the level of the stripping solution reservoir decreased, although wt% KOH was being added to the reservoir to control pH. The reason for this was found to be the osmotic pressure induced by the high concentration of KC1 in the wastewater, which resulted in a flux of water from the stripping solution to the wastewater in the extraction tank. To make up for the losses of water, an additional water flow of 80 g per day was added to the stripping solution reservoir. This addition was not able to match the loss due to osmotic pressure. The addition of water also has practical problems associated with controlling the process.

To overcome this difficulty the process of the present invention was applied. The wastewater itself, containing 20wt% KCl, was used to make up the 5 wt% KOH solution, starting from a 50wt% KOH concentrate. This gave a 5wt% KOH solution with a KCI content of 1 9wt%, and this was used to control stripping solution pH. Conductivity was no longer suitable for measurement of the stripping solution pH due to the presence of the KC1, and a pH probe was employed instead to control addition of the 5wt% KOH solution to the stripping solution. Using this process, the loss of stripping solution volume was reversed, and stripping solution was generated from the process as phenolic compound rich stripping solution overflowed from the stripping solution reservoir.

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 undissociated organic acids or bases dissolved in a fluid stream, the process comprising the steps of: (a) separating the fluid stream into two fractions, a first fraction and a second fraction; (b) contacting a base or an acid with the first fraction to provide a stripping solution, and in sufficient quantity such that at least one of the organic acids or bases present in the first fraction becomes dissociated; (c) providing a selectively permeable membrane having a first surface and a second surface; (d) transferring one or more undissociated organic acids or bases from the second fraction to the stripping solution across the membrane by contacting the stripping solution with the first surface and the second fraction with the second surface, wherein the volume and/or strength of the stripping solution contacted with the first surface is regulated relative to the volume of second fraction contacted with the second surface so that a driving force for organic acid or base permeation is maintained.

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

3. A process according to claim 1 wherein the fluid stream is an organic fluid stream.

4. A process according to claim 1, 2 or 3 wherein the volume and strength of the stripping solution contacted with the first surface is regulated relative to the volume of second fraction contacted with the second surface so that a driving force for organic acid or base permeation is maintained.

5. A process according to any one of the preceding claims wherein the membrane is mounted in a plate and frame configuration, a shell and tube configuration, or a spiral

6. A process according to any one of claims 1 to 4 wherein the membrane comprises one or more tubular membranes and one of the second fraction or the stripping solution is held within the internal volume of the tubular membrane(s) and the other of the second fraction or the stripping solution is in contact with the external surface of the tubular membrane(s).

7. A process according to any one of the preceding claims wherein the composition of the stripping solution in contact with the selectively permeable membrane is substantially uniform throughout its volume.

8. A process according to any one of the preceding claims wherein the pH of the stripping solution in contact with the selectively permeable membrane is controlled so that it is substantially the same throughout the volume of stripping solution in contact with the selectively permeable membrane

9. A process according to any of the preceding claims wherein the fluid stream contains an undissociated organic acid.

10. A process according to claim 9 wherein the fluid stream contains an organic acid selected from phenol, cresols, chlorophenols, fluorophenols, bromophenols, iodophenols, dichlorophenols, chlorocresols, dimethylphenols, nitrophenols, benzenediols, benzoquinones, isothiazolones, and mixtures thereof.

11. A process according to claim 9 wherein the stripping solution is prepared by adding a base to the first fraction of the fluid stream.

12. A process according to claim 9 wherein the stripping solution is prepared by adding the first fraction of the fluid stream to a mineral alkali selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, and mixtures thereof.

13. A process according to any of claims 1 to 8 wherein the fluid stream contains an undissociated organic base.

14. A process according to claim 13 wherein the fluid stream contains an organic base selected from aniline, chloroanilines, fluoroanilines, bromoanilines, iodoanilines, dichloro ani line s, to luidines, dimethylani line, nitro aniline, b enzylani line, phenyl ani line, methylaniline, ethylaniline, anisidine, benzylamine pyridine, picoline, methyl pyridines, chloropyridines, bromopyridines, fluoropyridines, iodopyridines, ethylamines, propylamines, butylamines, octylamine, other alkylamines, piperidine, cyclohexylamine, and mixtures thereof.

15. A process according to claim 13 wherein the stripping solution is prepared by adding an acid to the first fraction of the fluid stream.

16. A process according to claim 13 wherein the stripping solution is prepared by adding the first fraction of the fluid stream to a mineral acid selected from hydrochloric acid, sulphuric acid, phosphoric acid, nitric acid, and mixtures thereof.

17. A process according to claim 6 wherein the tubular membrane(s) is elastomeric.

18. A process according to claim 6 or 17 wherein the tubular membrane(s) has a length to diameter ratio of from 10,000 to 1,000,000.

19. A process according to any one of the preceding claims wherein the membrane is formed from or comprises a material selected from modified polysiloxane based elastomers including polydimethylsiloxane (PDMS) 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, or polymers such as polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyimide, polyamide, cellulose based materials, and mixtures thereof

20. A process according to any one of the preceding claims wherein the membrane comprises a reinforcing material selected from an external mesh and support.

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

22. A process according to claim 21 where the porous support is formed from or comprises a material selected from polymeric material suitable for fabricating microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes, including polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyethersulfone, polyacrylonitrile, polyamide, polyimide, cellulose acetate, or inorganic materials such as silicon carbide, silicon oxide, zirconium oxide, titanium oxide, or zeolites and mixtures thereof.

23. A process according to any of claims 1 to 16 wherein the selectively permeable membrane comprises a liquid membrane phase in simultaneous contact with both the second fraction and the stripping solution, such that the stripping solution and second fraction do not mix.

24. A process according to claim 23 wherein the liquid membrane is a supported liquid membrane comprising an organic liquid contained within the pores of a micorporous support material.

25. A process according to claim 23 or 24 wherein the liquid membrane phase is selected from liquid hydrocarbons, alcohols, aromatics, polyethyleneglycols, polypropyleneglycols, polybutyleneglycols and mixtures thereof.

26. A process according to claims 23, 24 or 25 wherein the liquid membrane contains carrier molecules to facilitate transport of undissociated organic acids and bases.

27. A process according to claim 26 wherein the carrier molecule is an alkyl amine species. s

28. A process according to any one of claims 24 to 27 wherein the porous support layer support is formed from or comprises a material selected from polymeric material suitable for fabricating microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes, including polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene dichloride (PVDF), polyethersulfone, polyacrylonitrile, polyamide, polyimide, cellulose acetate, or inorganic materials such as silicon carbide, silicon oxide, zirconium oxide, titanium oxide, or zeolites, and mixtures thereof.

29. A process according to any one of the preceding claims wherein a plurality of selectively permeable membranes is provided, each having a first surface and a second surface; wherein the second fraction is contacted with the second surface of each of the plurality of selectively permeable membranes in series, in parallel or in a combination thereof, and wherein the stripping solution is contacted with the first surface of each of the plurality of membranes.

30. A process according to any one of the preceding claims wherein the process is performed in a continuous manner.

31. A process according to any one of claims 1 to 29 wherein the flow of the second fraction of fluid and/or the stripping solution is discontinuous.

32. A process according to any one of the preceding claims wherein the second fraction of fluid and/or the stripping solution has a temperature above 25 C.

33. A process according to any one of the preceding claims wherein the second fraction of fluid andlor the stripping solution has a temperature above 60 C.

34. A process according to any one of the preceding claims wherein the second fraction of fluid and/or the stripping solution has a temperature above 70 C.

S

35. A process according to any one ofthe preceding claims wherein the second fraction of fluid contains dissolved inorganic or organic materials in an amount of more than 0.1 wt%, preferably more than lwt%, preferably more than 5wt%, preferably more than lOwt%, and preferably more than 20wt%.

36. A process as substantially described herein and with reference to any one of Figures 1-4.