GB2396625A

GB2396625A - Removal of an acid

Info

Publication number: GB2396625A
Application number: GB0230240A
Authority: GB
Inventors: Andrew Derek Turner; Christopher Peter Jones
Original assignee: Accentus Medical PLC
Current assignee: Accentus Medical PLC
Priority date: 2002-12-28
Filing date: 2002-12-28
Publication date: 2004-06-30
Also published as: GB0230240D0

Abstract

A weak acid that may be corrosive or oxidisable is removed in a plant that includes at least one flow channel (12) for the liquid to be treated, the flow channel (12) being defined between membranes (A, B) and being filled with a fluid-permeable bed (14) of anionic ion-exchange material, and there being a concentrate chamber (20) adjacent to the flow channel (12). Electrodes (16, 18) are arranged to apply an electric field transverse to the flow direction so as to urge anions from each flow channel (12) through an ion-permeable membrane (A) into the adjacent concentrate chamber (20). The membrane (B) at the other side of the flow channel is bipolar, and so permeable to neither anions nor cations; instead water is split into hydrogen and hydroxyl ions within the bipolar membrane, and these emerge from the opposite faces of the membrane. This ensures that there is an adequate concentration of hydroxyl ions in the flow channel (12) to ensure that the weak acid is at least partly ionised. The anode (16) may be separated from an adjacent concentrate chamber (20) by another bipolar membrane, either to protect the anode from corrosion, or to prevent oxidation of the acid.

Description

- 1 2396625

Removal of an Acid The present invention relates to a plant and process for performing electrodeionisation, so as to remove a 5 weak acid, for example a corrosive weak acid, from a fluid - including gases or aqueous or non-aqueous liquids or mixtures thereof. The weak acid may be considered as waste, but is preferably recovered in a concentrated form. The treated gas, aqueous or non-aqueous fluid 10 having been de-acidified can be recycled for closed loop applications. A range of different techniques are known for removing ions from aqueous liquids. For example, the 15 process of electrodialysis enables ions to be removed on a continuous basis, the liquid being passed between anion and cation selective membranes while being subjected to an electric field, but operates efficiently only with

high concentrations of ions in the liquid. It is 20 therefore unsatisfactory for use with weak acids, or dilute streams. Ion exchange is another process used to remove ions, and can successfully treat liquids in which the concentration of ions is low, but it cannot operate continuously, as the ion exchange material will 25 eventually become saturated with absorbed ions and need to be chemically regenerated. Electro-deionisation has been known for over 40 years, and is sometimes referred to as filled bed electrodialysis; it refers to a process in which the liquid to be treated is passed between ion 30 selective membranes while being subjected to an electric field, the flow channel between the membranes being

filled with an ion exchange material. For example US 2 923 674 (Kressman/Permutit Co.) describes a process said to be particularly applicable to the demineralisation of 35 water, in which the water is passed successively through two compartments each being defined between a cation

- 2 selective membrane and an anion-selective membrane, the first compartment being filled with a cation exchange material (to remove cations) and the second with an anion exchange material (to remove anions) . A practical device 5 consists of a stack of membranes which are alternately anion-selective and cation-selective, so defining several such parallel flow channels for the water to be treated, alternating with channels in which the ions become concentrated. US 4 969 983 (Parsi/Ionics Inc.) describes 10 a system for removing weakly ionised substances such as silica from water; the flow path contains anionic ion-

exchange material between an anion-selective membrane and a bipolar membrane, dissociation of water providing hydroxyl ions at the surface of the bipolar membrane and 15 so in the vicinity of the anion resin.

According to the present invention there is provided a process for removing an acid from a fluid stream, the process comprising a passing the fluid stream through an 20 electrodeionisation plant comprising at least one flow channel for the fluid stream, the flow channel being defined between an anion-selective membrane and a bipolar membrane and being filled with a fluid-permeable bed of anion-exchange material, a transfer chamber at one side 25 of the flow channel into which ions are transferred from the fluid stream, electrodes arranged to subject each flow channel to an electric field transverse to the flow

direction so as to urge anions from the flow channel into the transfer chamber, wherein the anode is either of a 30 corrosion-resistant material, or is separated from an adjacent transfer chamber by a membrane that is not permeable to anions.

The fluid stream may be a gas or a liquid, but in 35 the case of a gas stream preferably contains at least sufficient moisture to prevent the membranes from drying

out, and is most preferably an aqueous stream. Where the process is removing and recovering a potentially corrosive acid (so the anode must be protected from the acid), then use of a suitable corrosion-resistant 5 material for the anode is an option, but if it is the acid which is to be protected from electrochemical oxidation at anode then the use of a separating membrane is required.

10 The present invention also provides an electro deionisation plant comprising at least one flow channel for a fluid stream to be treated, the flow channel being defined between an anion-selective membrane and a bipolar membrane and being filled with a fluid-permeable bed of 15 anionexchange material, a transfer chamber at one side of the flow channel into which ions are transferred from the fluid stream, electrodes arranged to subject each flow channel to an electric field transverse to the flow

direction so as to urge anions from the flow channel into 20 the transfer chamber, wherein the anode is separated from an adjacent transfer chamber by a membrane that is not permeable to anions.

Both the membranes and the ion-exchange material 25 must be stable at the operating temperature, and must be resistant to the corrosive effects of the weak acid.

Preferably the flow rates of the fluid to be treated through the flow channel, and of the liquid through the 30 transfer chamber, are such that the concentration of the acid in the transfer chamber during operation is at least ten times that of the acid in the liquid to be treated.

Surprisingly, even in the case of a weak acid, where a significant proportion of the concentrated acid is in un 35 ionised form and so is not affected by the electric field, it has been found possible to obtain such a

- 4 concentration without significant back-diffusion.

A bipolar membrane is permeable to neither anions nor cations. It may be considered as comprising a layer 5 of anion-selective material on one side and a layer of cation-selective material on the other side. In the presence of an electric field, if the potential

difference across the membrane is at least 0.83 V, then water is split into hydrogen and hydroxyl ions within the 10 membrane, and these ions emerge from the opposite faces of the membrane. The bipolar membrane is arranged so that its anion-selective surface is adjacent to the flow channel, and in operation anions (hydroxyl ions) will therefore emerge from the bipolar membrane and help to 15 elute anions from the ion exchange material.

The ion exchange material may be in the form of fibres or foam or other porous solid, but is typically in the form of particles or beads. The thickness of the flow 20 channel is primarily determined by the electrical resistivity of the bed of ion exchange material; it is desirably less than 5 mm thick, for example in the range 2 mm to 4 mm. Clearly the hydraulic flow, that is the flow of the liquid being treated, must be matched to the 25 electrochemical elusion current, taking into account the concentration of ions. Because of the narrowness of the flow channel, the corresponding flow velocity may be high, and the length of the flow channel is desirably such that the pressure drop is no more than 4 atmospheres 30 (400 kPa) and more preferably no more than 1 atmosphere (100 kPa).

A practical electrodeionisation plant comprises several such flow channels and transfer chambers arranged 35 side-by-side between a single pair of electrode chambers.

The fluid flows through all or several of the flow

channels may be in parallel, and similarly liquid flows through some or all of the transfer chambers may also be in parallel. Evidently manifolds are necessary to supply these parallel flows of liquid. The design of the 5 manifolds delivering and collecting the liquid flows must be such that the electrical resistance of the path between adjacent compartments via the manifold is significantly greater than (for example 20 times) that of the main electric current route through the 10 membranes/resins, in order to minimise any bypass current losses. In performing the method, the electrical current density may be limited so as to achieve an overall 15 current efficiency in excess of 50%, and both the current density and the flow rate must be selected to ensure that the temperature does not rise to values such that the membrane materials may be damaged. Prior to performing electrodeionisation, it may be beneficial to pass the 20 liquid to be treated through an absorber column (for example of activated carbon) to remove any organic materials which might foul the electrodeionisation equipment. Similarly, there may be an ion exchange column containing a cation-exchange material (to remove 25 trace levels of strongly-absorbing cations), or containing an anion-exchange material (to remove trace levels of strongly-absorbing anions).

The invention will now be further and more 30 particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic cross-sectional view of a plant for removing and recycling hydrofluoric acid; Figure 2 shows a diagrammatic cross-sectional view

À 6 of a modification of the plant of figure 1; and Figure 3 shows a diagrammatic cross-sectional view of another modification to the plant of figure 1.

Referring to figure 1 a diagrammatic cross-sectional view is shown of a plant 10 for removing hydrofluoric and nitric acids from an aqueous stream, which is then recycled. The plant 10 includes several parallel flow 10 channels 12 (only four are shown) for the solution of the acids in water, each flow channel 12 being defined between a bipolar membrane B and an anion-permeable membrane A (the anionic side of the bipolar membrane B being marked by an asterisk), and containing a packed bed 15 14 of anion-exchange resin beads. The flow channels 12 are arranged sideby-side between a titanium anode 16 completely covered by a pinhole-free platinum foil 17 of thickness in the range 25-50 m, and a graphite cathode 18, the anion-permeable membrane A of each flow channel 20 12 being at the side nearer to the anode 16 in each case, with concentrate channels 20 between the flow channels 12. The cathode 18 is immersed in an electrolyte 19 such as sodium sulphate solution, and the anode 16 is immersed in a concentrate channel 20.

It will be appreciated that the drawing is diagrammatic; in particular the electrodes 16 and 18 which form the ends of the stack may in practice be exposed to liquid only on one face, there being resilient 30 seals around the periphery of the exposed parts of the electrodes so that only the front face of the anode 16 need be provided with the foil 17. In a modification to the plant 10, both cathode 18 and anode 16 share the same electrolyte, which may in this case be the recovered 35 concentrated acid.

- 7 In operation, the solution of hydrofluoric acid and dilute nitric acid in water is passed in parallel through all the flow channels 12, preferably flowing in a vertically upwards direction, and an electric voltage is 5 applied between the electrodes 16 and 18 so that a current flows. Water is supplied to the concentrate channels 20. The anionexchange resin bed 14 is initially in its basic state, that is to say saturated with hydroxyl ions. In each flow channel 12, fluoride and 10 nitrate anions are trapped by the ion exchange resin, and are then eluted by hydroxyl ions generated at the adjacent surface of the bipolar membrane B; these hydroxyl ions ensure that the solution at the surface of the absorber is not acid. The trapped anions then 15 migrate under the influence of the electric field through

the resin bed 14 (undergoing repeated ion exchange steps) and the anionpermeable membrane A, into the adjacent concentrate channel 20. At the side of each bipolar membrane B adjacent to a concentrate channel 20, hydrogen 20 ions are generated, and hydrogen ions are generated by electrolysis at the anode 16. Hence an aqueous solution of hydrofluoric acid and nitric acid emerges from each concentrate channel 20, including the channel 20 in which the anode 16 is immersed, while the aqueous solution 25 containing substantially no anions flows out of all the flow channels 12. Oxygen gas and hydrogen gas are generated by electrolysis at the anode 16 and cathode 18 respectively, and these gases are vented to the atmosphere after dilution with air. It will be 30 appreciated that although hydrofluoric acid is very corrosive, it does not react with the platinum foil 17 covering the anode 16.

For example a solution containing 25 mM HF and 1 mM 35 nitric acid has been treated at a current density of 120 A/m2, the solution flowing at a speed of 0.1 cm/s through

a bed 14 of thickness 2 mm. It was found that 99% of the anions were recovered in a single pass into a concentrate stream of about 0.73 M, the system operating at a current efficiency about 70%. The treated stream had thereby 5 been reduced to a concentration less than 0.05 mM (giving a concentrate to treated solution concentration ratio of >14,000), and the volume reduction factor (comparing feed to concentrate) was greater than 30.

10 If the concentration of the concentrate stream is not sufficiently high, the concentrated stream may be subjected to a second such concentration process either by electrodialysis or by electrodeionisation, for example using a second plant 10. In this case the concentrate 15 from the first plant 10 is passed through the flow channels 12 of the second plant 10. This enables a considerably more concentrated product to be produced, for example for recycling or for crystallization, without requiring a large concentration gradient across any one 20 membrane, so that back-diffusion is minimizer In the embodiment of figure 1 the corrosive effects of the hydrofluoric acid are avoided by selection of material for the anode 16, or rather for the foil 17; it 25 will be appreciated that the substrate material of the anode can be of a different metallic conductor, as it is protected by the foil 17. Alternative materials for the anode 16 would include graphite (or graphitised carbon with a fluoropolymer binder); platinised tungsten 30 (because the platinum prevents passivation of the tungsten); anodised lead (as the lead dioxide is un-

reactive), or Ti4O7 with a fluoropolymer binder. In the case where a common electrolyte of the recovered concentrate is used for both anode 16 and cathode 18, HF 35 resistant electrode materials should be used for the cathode 18 as well. These may comprise graphitised

carbon, lead, nickel-chromium-molybdenum alloys, tungsten and platinum group metals. Alternatively, membranes that are impermeable to anions may be used to separate the anode 16 from the liquid in the adjacent concentrate 5 channel 20, so as to avoid the problems of corrosion. Two such approaches are described in relation to the following figures.

Referring now to figure 2, a modification is shown 10 to the plant 10 of figure 1; the modification only affects the anode, and the other features are exactly as in figure 1 and are referred to by the same reference numerals. In the plant 22 of figure 2 a platinised-

titanium anode 26 (that is to say provided with a layer 15 of platinum by electroplating, typically of thickness 2.5-10 m; it is not possible to guarantee that such a layer does not have a pinhole) is provided with a supply of dilute sulphuric acid solution as an electrolyte (anolyte) 27, while the anolyte 27 is separated by a 20 bipolar membrane B from the adjacent concentrate channel 20. Alternatively a cation-permeable membrane (eg a fluorinated strong-acid material such as Nafion_) may be used to prevent the passage of anions. In operation hydrofluoric acid solution collects in the concentrate 25 channel 20, but no ions pass through the bipolar membrane towards the anode 26. Hydroxyl ions emerge from that bipolar membrane B into the anolyte 27, and oxygen gas is generated at the anode 26. The same electrolyte may be used for the catholyte by combining the feed to channels 30 27 and 19, thus simplifying the plant through the omission of a tank and pump that would have been required for separate electrolytes. By choice of the electrolyte, it is possible to select cheaper electrodes that those based on platinum-group metals - for example lead in 35 sulphuric acid or nickel in caustic soda.

Referring now to figure 3 an alternative modification is shown to the equipment of figure 1.

Again, the modification only affects the anode, and the other features are as in figure 1, and are referred to by 5 the same reference numerals. In the plant 33 of figure 3, a porous carbon anode 36 is separated from the adjacent concentrate channel 20 by a cation-selective membrane C. The anode 36 may for example be of graphite cloth or fibrous graphite felt. The porous carbon anode 10 36 acts as a gas diffusion electrode, and its rear surface may be impregnated with a chemically-resistant non-wettable polymer such as polytetrafluoroethylene (PTFE) so that water cannot pass through it although it is gas permeable. Some water diffuses through the 15 membrane C, so that the porous carbon anode 36 is damp.

The rear surface of the anode 36 is vented to the atmosphere (through a channel, not shown), so that oxygen formed from electrolysis of water can diffuse out of the anode 36; the hydrogen ions generated by electrolysis 20 pass through the membrane C into the adjacent concentrate channel 20.

A similar approach may be adopted for the cathode, where a gas-diffusion electrode may be substituted for 25 the cathode 18 of Figure 3. This comprises a porous carbon electrode with wet-proofing to allow hydrogen to be vented from the rear surface, and with an ion-

permeable membrane (preferably cationic) at the front face in contact with the electrolyte, through which 30 protons can migrate for reduction to hydrogen gas at the electrode. In this case, oxygen evolved from the gas-diffusion anode 36 can alternatively be circulated to the gas 35 diffusion cathode, where it can be reduced back to water, so suppressing the evolution of hydrogen.

The plants 22 and 33 ensure there is no contact between the concentrate and the anode, which not only prevents corrosion of the anode but also prevents 5 electro-oxidation of materials in the concentrate. Such plants would therefore be appropriate when dealing with oxidizable organic acids such as formic acid or amino acids. 10 Where the liquid to be treated contains an oxidising agent such as chlorine, hydrogen peroxide, or peroxydisulphate, the liquid should be pretreated to remove this oxidising agent which would degrade the ion exchange material. In the case of hydrogen peroxide this 15 may be achieved by irradiating the liquid with ultraviolet, or by catalytic decomposition; any oxidising agent may be removed by electrolytic reduction, for example causing the liquid to flow radially outwards through a tubular porous carbon cathode of a cylindrical 20 electrolytic cell with a platinised-titanium anode. It will also be appreciated that if the liquid to be treated contains chloride ions, these will be transferred into the concentrate channel 20 and must be prevented from contacting the anode (as they would form chlorine gas, 25 which is detrimental to the ion exchange material), so that the modified apparatus 22 and 33 of figures 2 and 3 must be used.

Where the liquid to be treated contains an acid and 30 a small proportion of a salt it may be desirable to pretreat the liquid by passage through a cation-exchange column in which the cations of the salt are trapped.

This is of particular relevance when the metal ion is able to precipitate within a subsequent 35 electrodeionisation plant, such as the trivalent cations, Fe(III) and Al(III). It will be appreciated that the

- 12 acid recovery takes place in the same way as described above. When it is necessary to regenerate the cation-

exchange column, this may be achieved using a small amount of the acid that has been recovered, so that no 5 additional chemicals are required.

In the plants 10, 22 and 33 the catholyte 19 may be recirculated to help remove any hydrogen bubbles, and similarly in the plant 22 the anolyte 27 may be 10 recirculated to remove oxygen bubbles. These gases generated at the electrodes may be vented to the atmosphere. If a common electrolyte is used for both anolyte and catholyte 19 and 27, the resultant hydrogen/oxygen mixture will need to be diluted below the 15 lower explosive limit of hydrogen with for example an air stream. Alternatively, if generation of gases is not desired, a common electrolyte may be circulated between the anode chamber and the cathode chamber in the plant 22, the common electrolyte containing two components of a 20 redox couple, having a standard electrode potential between those of H+/H2 and 02/H2O, preferably having a standard electrode potential between 0.2 V and 1.0 V, so that the redox couple is oxidised and reduced in preference to water as it passes over the electrodes in 25 turn. The bipolar membranes B require that the liquid in contact with the anionic surface * should be not substantially less acidic than that in contact with the cationic surface when no voltage is applied, so that it is important that the anolyte 27 is not less acidic than 30 the concentrate 20. In the example given above, where the concentrate is 0.73 M HF and nitric acid, a suitable redox couple would be vanadium(III)/vanadium(IV). In solution in 1 M sulphuric acid, a solution of 0.25 M vanadium (III) and 0.25 M vanadium (IV) has been found to 35 suppress hydrogen evolution to less than 1.5% current efficiency at a carbon felt cathode and at a superficial

- 13 current density of 210 A/m.

For the situations described in the plants 10, 22 and 33 in figures 1-3, where acid is recovered in the 5 concentrate stream, while at the same time the feed becomes depleted, the anion-resin bed 12 in contact with the * side of the bipolar membrane is less acid than the concentrate channel 12. In the unpowered state, this would lead to membrane delamination. In order to prevent 10 this, either a base should be added to the concentrate to bring the pH to less acidic values than the feed, or else on power interruption the electrolyte and concentrate should be able to drain out of the cell stack into a reservoir.

Claims

1. A process for removing an acid from a fluid stream, the process comprising a passing the fluid stream through 5 an electrodeionisation plant comprising at least one flow channel for the fluid stream, the flow channel being defined between an anion-selective membrane and a bipolar membrane and being filled with a fluid-permeable bed of anion-exchange material, a transfer chamber at one side 10 of the flow channel into which ions are transferred from the fluid stream, electrodes arranged to subject each flow channel to an electric field transverse to the flow

direction so as to urge anions from the flow channel into the transfer chamber, wherein the anode is either of a 15 corrosion-resistant material, or is separated from an adjacent transfer chamber by a membrane that is not permeable to anions.

2. An electrodeionisation plant comprising at least one 20 flow channel for a fluid to be treated, the flow channel being defined between an anion-selective membrane and a bipolar membrane and being filled with a fluid-permeable bed of anion-exchange material, a transfer chamber at one side of the flow channel into which ions are transferred 25 from the fluid, electrodes arranged to subject each flow channel to an electric field transverse to the flow

direction so as to urge anions from the flow channel into the transfer chamber, wherein the anode is separated from an adjacent transfer chamber by a membrane that is not 30 permeable to anions.

3. A process as claimed in claim 1 wherein the fluid stream is thereby enabled to be recycled.

35

4. A process as claimed in claim 1 or claim 3 wherein the fluid flow comprises an aqueous liquid.

- 15

5. A process as claimed in claim 1, claim 3 or claim 4 wherein a concentrated solution is generated in the transfer chambers.

6. A process as claimed in claim 1, or any one of claims 3 to 5, wherein a common electrolyte is recirculated between the anode and the cathode, in each case the electrolyte being separated from an adjacent transfer 10 chamber by a membrane.

7. A process as claimed in claim 1, or any one of claims 3 to 5, wherein both the anode and the cathode incorporates a gas-diffusion electrode, and wherein the 15 gas evolved at one electrode is circulated to the other electrode.

8. A process as claimed in claim 1, or in any one of claims 3 to 7, wherein the fluid stream contains an 20 oxidising agent, and is pretreated to destroy the oxidising agent.

9. A process as claimed in claim 1, or in any one of claims 3 to 8, wherein the fluid stream contains cations 25 and is pretreated by contacting it with an ion exchange material to remove those cations, and wherein an acid concentrate generated in the transfer chambers is subsequently used to elute cations trapped in the ion exchange material.

10. A process for removing an acid from a fluid stream substantially as hereinbefore described with reference to, and as shown in, figure 1, or figure 2 or figure 3 of the accompanying drawings.

11. A plant for removing an acid from a fluid stream

substantially as hereinbefore described with reference to, and as shown in, figure 1, or figure 2 or figure 3 of the accompanying drawings.