ELECTROCHEMICAL REACTOR AND ITS CLEANING OR REGENERATION

Abstract

An electrochemical reactor suitable for reducing dye to leucodye, comprises at least four electrolytic cells, wherein the electrolytic cells are provided in the form of at least two stacks of at least two electrolytic cells each such that one stack at a time can be separated for cathode or anode regeneration during suspension preparation.

Claims

1. An electrochemical reactor, suitable for reducing a dye suspension to its leucodye solution, comprises at least four electrolytic cells, wherein the electrolytic cells are provided in the form of at least two stacks of at least two electrolytic cells, said stacks are connected in parallel and allow the separation of at least one stack at a time for washing/regeneration of the cathode during a dye suspension preparation while catholyte solution is circulated through the remaining one or more stacks, wherein the electrolytic cells of each stack are connected in parallel and wherein the electrolytic cells comprise a cathode compartment and an anode compartment separated by a separator, the separator comprising a semipermeable membrane, said cathode compartment comprises a multitude of freely suspended conductive, non-spherical granules in contact with a conductor forming at least the side wall opposite to the separator, said cathode compartment is provided with an upper grid and a lower grid, the width of the mesh and the positioning of said grids are chosen such that the liquid catholyte can pass through them but the granules are prevented from passing through them so that they are retained in the cathode compartment, wherein the anode compartment has a wall opposite to the common side wall formed by an electrode, wherein said cathode compartment is connected to a main vessel via a catholyte supply pipe and a reduced catholyte return pipe for circulating catholyte through the cathode compartment, and said anode compartment is connected to an anolyte vessel via an anolyte supply pipe and an anolyte return pipe, wherein said cathode compartment and said anode compartment are also provided with means for supplying acidic cleaning solutions and water and optionally basic solution to the cathode compartment and the anode compartment, said means comprise at least one cleaning medium supply pipe and at least one cleaning medium removal pipe at opposite ends of the cathode compartment and of the anode compartment for supplying and removing acidic cleaning solutions and water and preferably optionally also a basic solution, said cleaning medium supply pipes and said cleaning medium removal pipes can be bypasses of the catholyte supply pipe and/or the anolyte supply pipe, or independent pipes with own inlets and outlets adjacent to the ones of a catholyte circulation loop or an anolyte circulation loop, said at least one cleaning medium supply pipe is connected to at least one vessel for supplying acid and optionally base and is also connected to a water line supplying deionized water, and the at least one cleaning medium removal pipe is connected to a waste water treatment plant or to a waste water vessel.

2. The electrochemical reactor of claim 1 comprising at least 6 stacks and each stack comprising from 2 to 10 electrolytic cells, the at least 6 stacks being connected such that at any time at least one of them can be separated for being supplied with cleaning medium or water while the remaining stacks are connected such that the catholyte solution comprised in the stacks and the connecting pipes can be circulated through them.

3. The electrochemical reactor of claim 1 comprising a first vessel upstream of the main vessel.

4. The electrochemical reactor of claim 1, further comprising circulation means allowing the acidic cleaning solutions and optionally the basic solution to be circulated via the cathode compartment and/or the anode compartment prior to being removed, said circulation means.

5. The electrochemical reactor of claim 1, wherein the cleaning medium supply pipes are connected with the catholyte supply pipes and/or the anolyte supply pipes feeding each stack.

6. The electrochemical reactor of claim 1, wherein the anode compartments of each stack are connected with the cleaning medium and water supply pipes such that they can be individually controlled and washed simultaneously with or independently from the cathodes of the respective stack.

7. The electrochemical reactor of claim 1, wherein at least the catholyte direction can be inverted, such that the inlet becomes the outlet and vice versa.

8. The electrochemical reactor of claim 1 wherein the anode compartment is placed in an anolyte circulation loop, said anolyte circulation loop being provided with an adsorption filter for removing molecular impurities.

9. The electrochemical reactor of claim 1, wherein the main vessel is for preparing a catholyte to be reduced by circulating a main suspension through a main suspension circulation loop equipped with a main suspension circulation pump and optionally an ultrasound apparatus and optionally a heating means, for raising the temperature to about 50° C. to 65° C.

10. The electrochemical reactor of claim 1, wherein the main vessel for preparing a catholyte comprises a catholyte outlet, a catholyte circulation loop with a catholyte supply pipe equipped with a catholyte supply pump and optionally at least one of an ultrasound apparatus, a particle filter for removing particles of >50 to 100 μm and a catholyte heating means for raising the temperature to about 50° C. to 65° C., said catholyte circulation loop also comprising a cathode compartment and a reduced catholyte return pipe returning the reduced catholyte via reduced catholyte inlet into main vessel.

11. The electrochemical reactor of claim 3 comprising a first vessel upstream of the main vessel, said first vessel comprising a first suspension circulation loop equipped with a first suspension circulation pump and an ultrasound apparatus.

12. A method for cleaning the electrochemical cells comprising a particulate cathode of the electrochemical reactor of claim 1, said method comprising removing the catholyte solution except the amount needed for suspension preparation and the amounts filling the electrochemical cells and their connecting pipes, separating at least one stack at a time for washing/regeneration of the cathodes during catholyte suspension preparation, circulating catholyte solution through the not separated one or more stacks, supplying deionized water through the cathode compartments of the separated at least one stack of electrolytic cells into the main vessel or the first vessel, isolating the separated at least one stack from any catholyte suspension preparation and/or circulation, washing at least the cathode compartments of the separated at least one stack with acidic solution, optionally washing at least the cathode compartments of the separated at least one stack with basic solution, then rinsing the acid and optionally base washed cathode compartments of the separated at least one stack with deionized water until neutral, joining the separated at least one stack with the other stacks and starting catholyte reduction, wherein the washing and rinsing is performed by supplying the washing solution or the rinsing water to one end of the electrochemical cell and removing it at the opposite end and wherein the washing solutions and the rinsing water removed from the compartments are forwarded to a waste water treatment plant (WWTP) or to a waste water vessel.

13. The method of claim 12, wherein all anodes of one stack are cleaned together during the preparation of a new batch of catholyte suspension but separately from cathode cleaning, wherein anode washing comprises the following steps: removing the catholyte solution except the amount needed for suspension preparation and the amounts filling the electrochemical cells and their connecting pipes, separating at least one stack at a time for washing/regeneration of the anodes during catholyte suspension preparation, circulating catholyte solution through all stacks, washing the anode compartments of the separated at least one stack with acidic solution, optionally washing the anode compartments of the separated at least one stack with basic solution, then rinsing the acid and optionally base washed anode compartments of the separated at least one stack with deionized water until neutral, joining the separated at least one stack with the other stacks and starting catholyte reduction, wherein the washing and rinsing is performed by supplying the washing solution or the rinsing water to one end of the anode compartments and removing it at the opposite end and wherein the washing solutions and the rinsing water removed from the anode compartments is forwarded to a waste water treatment plant (WWTP) or to a waste water vessel.

14. The method of claim 12, wherein the washing is performed with strong acids (pK<1) by circulating the acidic solution through at least the cathode compartments of the at least one separated stack followed by rinsing the compartments with water, optionally after washing the compartments with a base.

15. The method of claim 14, wherein the strong acid is selected from the group consisting of HCl, H.sub.2SO.sub.4, HNO.sub.3 and mixtures thereof.

16. The method of claim 12, wherein the catholyte suspension is an indigo suspension and the catholyte solution is a leucoindigo solution.

17. The electrochemical reactor of claim 1, optionally comprising a first vessel, wherein the first vessel and/or the main vessel used for catholyte preparation are/is connected with the cathode compartments of the electrolytic cells of each stack such that the first and/or main vessel can be supplied with water through each one of the stacks independently.

18. The electrochemical reactor of claim 4, wherein said circulation means comprises at least one particle filter and/or at least one adsorption filter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0120] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. This description makes reference to the annexed drawings, wherein:

[0121] FIG. 1 schematically shows a reactor with one electrolytic cell and a cleaning cycle for both electrode compartments. The catholyte and anolyte circulation loops are omitted for clarity reason. There is no circulation of anolyte and/or catholyte in the electrolytic cell during washing.

[0122] FIG. 2a shows schematically six stacks with five electrolytic cells each with the relevant supply and withdrawal lines and

[0123] FIG. 2b shows one stack with five electrolytic cells more detailed.

[0124] FIG. 3 shows the main parts of a whole electrochemical reactor with the electrolytic cell in exploded view. For clarity reason, the first vessel and the first suspension loop is not shown.

[0125] FIG. 4 schematically shows dye introduction in powder form directly into the main vessel.

[0126] FIG. 5 schematically shows dye suspension preparation in the first vessel and supply to the main vessel.

[0127] FIG. 6 shows in more detail the part of the electrochemical reactor that serves the catholyte preparation in the presence of a first vessel.

[0128] FIG. 7 shows the dye suspension preparation in the presence of a first vessel and with use of internally produced leucodye as dispersing agent.

[0129] FIG. 8 shows the reactor and method with continuous cleaning of the anolyte with an adsorption filter.

MODES FOR CARRYING OUT THE INVENTION

[0130] The invention is now further described based on the Figures for a preferred dye reduction plant, in particular an indigo reduction plant.

[0131] In spite of all the improvements described in the scope of the present invention that result in a cleaner process and thus longer intervals between maintenance activities, cleaning or regeneration, respectively, is nevertheless necessary since the quality of the electrolytic cells 4 declines with continued use. The optimal number of stacks 5 in view of such regeneration depends on the dimensions and numbers of the electrolytic cells 4 per stack 5, the volume of the electrolytic cells 4 and the pipes, the amount of dye to be reduced in a certain time etc.

[0132] If a reactor e.g. comprises six stacks 5 as shown in FIG. 2a, one of them can be separated for cathode or anode regeneration at a time, as shown in FIG. 1. In particular in case of dye reduction, this regeneration can be performed during the usual process, namely during dye suspension preparation, so that reduction can be performed with all stacks together. Nevertheless, it may be advantageous to provide a reactor with so many stacks 5 that it can be operated at full production capacity even if one of the stacks 5 is separated for maintenance, i.e. for a time exceeding the suspension preparation.

[0133] The separation and regeneration of one of the stacks 5 at a time only is preferred for the reasons already indicated above.

[0134] Although in particular the particulate electrodes, in general the cathodes, have to be regenerated also the anode quality may suffer from use. Therefore also the anodes are cleaned although preferably separate from cathode cleaning and at longer intervals. Separate cleaning of cathodes and anodes allows better adaptation of the cleaning parameters.

[0135] For this cleaning or regeneration step, the electrochemical reactor is provided with means for supplying cleaning solutions to the stacks 5 of electrolytic cells or rather the electrolytic cells 4 themselves. These means comprise at least one cleaning medium supply pipe 61 for supplying cleaning/regeneration solutions to the cathode or anode and—on the side of the electrolytic cell opposite to the inlet—at least one cleaning medium removal pipe 62 for removing acidic cleaning solutions and water and preferably also a basic solution. These cleaning medium supply pipes 61 can be bypasses of a catholyte supply pipe 151 and an anolyte supply pipe 31, i.e. using the same inlets and outlets, or independent pipes with own inlets and outlets adjacent to the ones of the catholyte circulation loop 15 or the anolyte circulation loop 33. In general, the one or more cleaning medium supply pipes 61 are connected to acid vessels 63a for supplying acid, and optionally to base vessels 63b for supplying base, as well as to a water line supplying deionized water. The one or more cleaning medium removal pipes 62 are either directly fed to a waste water treatment plant (WWTP) or to waste water vessels for storing waste water. In a preferred embodiment, the cleaning solutions are circulated for some time, i.e. until their pollution reaches an undesired level. In case of circulation, the cleaning medium removal pipe 62 is connected to a vessel 63a, 63b via cleaning medium circulation means 64. In this case it is preferred to have the cleaning medium circulation means equipped with one or more particle filters and/or adsorption filters, preferably just downstream the electrolytic cells.

[0136] Cleaning or regenerating, respectively, a particulate electrode or both electrodes in an electrochemical reactor of the present invention comprises washing the cathode and/or anode compartments 41, 42, in particular the bed or the particulate electrode, respectively, with strong acids (pK<1) by circulating the acidic solution through the compartments 41, 42, in particular the electrode bed, followed by washing the compartment(s) 41, 42, in particular the bed with water, optionally and preferably after having washed the acid treated compartment(s) 41, 42 with a basic solution, preferably caustic soda, for more efficient acid and electrode contamination removal.

[0137] In case of indigo reduction, the acid washing is performed as already described above.

[0138] FIG. 3 shows the core of an electrochemical reactor of the present invention with only one electrolytic cell 4 for clarity reasons and with an inlet 11 for dye in solid or suspended form, i.e. transferred from a first vessel 2. An assembly of six stacks is shown schematically in FIG. 2a, and one stack more in detail with five electrolytic cells is shown in FIG. 2b. All stacks 5 and all electrolytic cells 4 are connected in parallel.

[0139] The main vessel 1 is provided with a main suspension circulation loop 14 comprising a main suspension circulation pump P03 and preferably an ultrasound apparatus 141 for circulating the suspension thereby improving its homogeneity.

[0140] When the suspension has been circulated for some time (dependent on the quality of the dye, i.e. its particle size and particle distribution) the main suspension circulation loop 14 is closed and the valve to the catholyte outlet 12 is opened. The catholyte is then circulated through catholyte circulation loop 15 by pumping it by the catholyte pump P01 via catholyte supply pipe 151 through a further optional ultrasound apparatus 154, a particle filter 152 for removing oversized particles if still present and a catholyte heating means (heat exchanger) 153 via catholyte inlet 411 into the cathode compartment 41 of the electrolytic cell 4 separated from the anode compartment 42 by a separator 43, preferably a semipermeable membrane. After having passed the cathode, the catholyte is returned to the main vessel 1 via reduced catholyte outlet 412a, reduced catholyte return pipe 412b and reduced catholyte inlet 13. As indicated by reference numbers 413a and 413b, in a preferred embodiment the catholyte direction can be inverted.

[0141] An anolyte is supplied to anolyte vessel 3 via anolyte supply pipe 31 or—once the reduction has been started—anolyte inlet 35 and supplied to the anolyte compartment via anolyte outlet 32, anolyte pump P02, anolyte heating means 331 and anolyte inlet 421 into the anode compartment 42. Having left the anode compartment 42 via anolyte outlet 422a, the anolyte is recirculated to anolyte vessel 3 via anolyte return pipe 422b and anolyte inlet 35.

[0142] In a preferred embodiment an adsorption filter 332 is provided within the anolyte circuit. This filter 332 can be placed anywhere, however it is preferably placed just before the anolyte heating means 331 since there the temperature is lowest and thus adsorption best.

[0143] All vessels are in addition provided with supply means for nitrogen, caustic soda and optionally further supply means as well as with degassing means and solution withdrawal lines for the withdrawal of the leuco dye or the anolyte in case of anode cleaning.

[0144] FIG. 2a shows schematically 6 stacks, all connected in parallel. Due to independent anolyte and catholyte supply to each of the stacks, any one thereof can be independently shut down for regeneration. In case one stack 5 is separated for cathode washing, water is supplied to the separated stack 5 via catholyte inlet 411, removed through reduced catholyte outlet 412a into reduced catholyte return pipe 412b and forwarded to the main vessel or to the first vessel for suspension preparation.

[0145] FIG. 4 shows catholyte preparation in the main vessel 1 directly. Dye in solid form is introduced into main vessel 1 containing caustic soda in desired concentration as electrolyte (for indigo e.g. caustic soda of 2 to 10%, preferably about 4% w/v) via solid dye inlet 11a. The main vessel 1 and all pipes and apparatuses that are in contact with leucodye once the reduction process is started are purged with nitrogen (or other inert gas), e.g. introduced into main vessel 1, in order to prevent oxidation of the once reduced dye. After having been circulated as described with regard to FIG. 3 (circulation loop not shown in FIG. 4), the thus formed catholyte is circulated via the cathode compartment 41, driven by pump P01, until all dye has been reduced to leucodye.

[0146] In an alternative catholyte preparation method, shown in FIGS. 5 and 6, the dye suspension is prepared in a first vessel 2. As best shown in FIG. 6, this preparation comprises supplying solid dye from solid dye vessel 21a via solid dye inlet 21b into a solution of caustic soda with desired concentration in first vessel 2. The first vessel can be provided with a first suspension circulation loop 22, optionally equipped with an ultrasound apparatus 221. Once the dye suspension has reached desired homogeneity it is supplied to the main vessel 1 via suspension outlet 23 and dye suspension inlet 11b, driven by pump P05. Once the suspension is in main vessel 1, the procedure is as described with regard to FIG. 4.

[0147] Most dyes cannot be suspended in the desired high amounts without the addition of substantial amounts of dispersing agent. It has now surprisingly been found that leucodye such as leucoindigo can act as dispersing agent thereby allowing a purer leucodye and thus a purer dying process. A dye reduction method using leucodye as dispersing agent is schematically shown in FIG. 7.

[0148] In order to start a dye reduction, in a first preparatory step a diluted suspension is prepared as described above and subjected to electrolytic reduction in electrolytic cell 4. Once the dye has been reduced to leucodye, it can either be supplemented with further dye in the main vessel 1 or—most preferred—some of the leucodye produced can be transferred from the main vessel 1 via leucodye outlet 16, leucodye return pipe 24 equipped with leucodye return pump P04 and leucodye inlet 241 to the first vessel 2 for being supplemented with further dye. The leucodye comprising suspension can then be first processed in the first suspension circulation loop 22 in order to improve its homogeneity before being fed to main vessel 1 and finally to the electrolytic cells as described above.

[0149] In a similar way, once the leucodye production has been started, part of the concentrated leucodye is removed and part of the leucodye solution in left in the reactor. This remaining leucodye solution can then be either left in the main vessel 1 or—preferably—fed to first vessel 2 as shown in FIG. 7 via leucodye return pipe 24. In the main vessel 1 or preferably in the first vessel 2 the leucodye solution is diluted with additional electrolyte such as caustic soda. Since the leucodye acts as dispersing agent, a higher concentrated dye suspension can be prepared directly. Nevertheless, it has proved advantageous to add the leucodye in several parts, indicated in the Figures, like FIG. 7, as semicircular arrow. In the case of leucoindigo it has been found that concentrations of 5 to 20% are suitable for stabilizing suspensions comprising indigo in amounts of up to 20%. Once the indigo has been reduced to leucoindogo in the desired concentration, the leucoindigo is removed from the reactor via concentrated leucodye outlet 44.

[0150] As already indicated above and as shown in FIGS. 3 and 8, an adsorption filter 332 can be provided in the anolyte circulation loop 33. If the anolyte circulation loop 33 comprises a heating means 331, the adsorption filter 332 preferably is provided close to but upstream of the heating means 331 and the heating means 331 is preferably provided upstream of the anode compartment. This adsorption filter serves the removal of small molecules that may be present and in general are present in the dye and have been found to migrate through the separator 43, e.g. a semipermeable membrane, and directly or after reaction like polymerization deposit on the anode thereby affecting its activity.

[0151] Suitable adsorption filter materials are e.g. activated carbon and molecular sieves like zeolites. For good anolyte flow through the adsorption filter particle sizes of about 1 to 3 mm, in particular about 2 mm are preferred.

[0152] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

LIST OF REFERENCE NUMBERS

[0153] 1 main vessel [0154] 11a solid dye inlet [0155] 11b dye suspension inlet [0156] 12 catholyte outlet [0157] 13 reduced catholyte inlet [0158] 14 main suspension circulation loop [0159] P03 main suspension circulation pump [0160] 141 ultrasound apparatus in main suspension circulation loop [0161] 15 catholyte circulation pipe or catholyte circulation loop [0162] 151 catholyte supply pipe [0163] P01 catholyte supply pump [0164] 152 particle filter in catholyte supply pipe [0165] 153 catholyte heating means (heat exchanger) [0166] 154 ultrasound apparatus in catholyte supply pipe [0167] 16 leucodye outlet [0168] 2 first vessel [0169] 21a solid dye vessel [0170] 21b solid dye inlet [0171] 22 first suspension circulation loop [0172] 221 ultrasound apparatus in first suspension circulation loop [0173] 23 first suspension outlet [0174] 231 first suspension supply pipe [0175] P05 first suspension supply pump [0176] 24 leucodye return pipe [0177] 241 leucodye inlet [0178] 25 first suspension mixer [0179] P04 leucodye return pump [0180] 3 anolyte vessel [0181] 31 anolyte supply pipe [0182] 32 anolyte outlet [0183] 33 anolyte circulation pipe or anolyte circulation loop [0184] 331 anolyte heating means such as heat exchanger [0185] 332 adsorption filter [0186] P02 anolyte pump [0187] 35 anolyte inlet [0188] 4 electrolytic cell with [0189] 41 cathode compartment [0190] 411 catholyte inlet [0191] 412a reduced catholyte outlet [0192] 412b reduced catholyte return pipe [0193] 413a bypass for changing catholyte direction [0194] 413b bypass for changing catholyte direction [0195] 42 anode compartment [0196] 421 anolyte inlet [0197] 422a anolyte outlet [0198] 422b anolyte return pipe [0199] 43 separator, semipermeable membrane [0200] 44 concentrated leucodye outlet (batchwise) [0201] 5 stack of electrolytic cells [0202] 61 cleaning medium supply pipe [0203] 62 cleaning medium removal pipe [0204] 20 63a acid vessel [0205] 63b base vessel [0206] 64 cleaning medium circulation means