By-products (impurity) removal

Abstract

An electrolytic reactor comprises at least one electrolytic cell with an anode compartment and a cathode compartment separated by a separator, in particular a semipermeable membrane. The anode compartment comprises an inlet and an outlet for anolyte at opposed ends, said inlet and outlet being connected with each other via an anolyte circulation pipe equipped with a storage means for anolyte, an anolyte vessel and at least one adsorption filter for adsorbing molecular impurities. When molecular impurities comes from the cathode compartment through the separator, the electrolytic reactor acts also as a cleaning device for the catholyte.

Claims

1. An electrolytic reactor suitable for reducing a vat dye or sulphur dye, said reactor comprising at least one electrolytic cell with an anode compartment and a cathode compartment, said compartments being separated by a separator, said compartments each comprising an inlet and an outlet for anolyte and catholyte, respectively, at opposed ends, said inlet and outlet of the anode compartment being connected with each other via an anolyte circulation pipe and said inlet and outlet of the cathode compartment being connected with each other via catholyte circulation pipe, said anolyte circulation pipe being equipped with an anolyte vessel to form an anolyte circuit and said catholyte circulation pipe being equipped with a main vessel to form a catholyte circuit, wherein the anolyte circulation pipe is further equipped with at least one adsorption filter, said adsorption filter comprising an adsorption material for adsorbing molecular impurities, said catholyte circuit comprising a catholyte comprising a vat dye or sulphur dye.

2. The electrolytic reactor of claim 1, wherein the adsorption material in the adsorption filter comprises activated carbon or molecular sieves such as zeolites.

3. The electrolytic reactor of claim 2, wherein the adsorption material in the adsorption filter comprises or consists of activated carbon.

4. The electrolytic reactor of claim 1, wherein the adsorption material in the adsorption filter comprises particles with particle size of 1 to 3 mm.

5. The electrolytic reactor of claim 1, wherein the cathode compartment comprises a particulate cathode.

6. The electrolytic reactor of claim 5, wherein the particulate cathode is a conductive carbon cathode.

7. The electrolytic reactor of claim 1, wherein the catholyte comprises indigo.

8. The electrolytic reactor of claim 1, wherein said main vessel is provided with a main suspension circulation loop for a circulating dye suspension.

9. The electrolytic reactor of claim 8, wherein the main vessel is coupled with a first vessel for suspension preparation so that a suspension from the first vessel can be fed to the main vessel and a leucodye solution can be fed from the main vessel back into the first vessel.

10. The electrolytic reactor of claim 9, wherein the first vessel is provided with a first suspension circulation loop.

11. The electrolytic reactor of claim 10, wherein said first suspension circulation loop is equipped with an ultrasound apparatus for circulating the first suspension prior to its delivery to the main vessel.

12. The electrolytic reactor of claim 8, wherein said main suspension circulation loop is equipped with an ultrasound apparatus.

13. The electrolytic reactor of claim 1, wherein the at least one electrolytic cell comprises at least 4 electrolytic cells in the form of at least two stacks of electrolytic cells, wherein the electrolytic cells of one stack as well as all stacks are connected in parallel.

14. The electrolytic reactor of claim 1 wherein at least one of the anode compartment and the cathode compartment are independently from each other provided with cleaning means for providing one or more cleaning solutions and water and cleaning means for removing one or more cleaning solutions and water.

15. The electrolytic reactor of claim 14, wherein at least one of the anode compartment and the cathode compartment are independently from each other provided with cleaning means for circulating one or more cleaning solutions and water.

16. The electrolytic reactor of claim 1 wherein said separator is a semipermeable membrane.

17. A method for improving anode performance in vat dye or sulfur dye reduction, said method comprising continuous cleaning of anolyte in an adsorption filter integrated in an anolyte circulation pipe or anolyte circulation loop, respectively, by continuously flowing the anolyte through the adsorption filter; wherein said anolyte circulation pipe or anolyte circulation loop connects an inlet of an anode compartment with an outlet of an anode compartment and wherein said inlet and said outlet are disposed at opposed ends of said anode compartment, and said anolyte circulation pipe or anolyte circulation loop is equipped with an anolyte vessel to form an anolyte circuit.

18. The method of claim 17 wherein the dye is indigo.

19. A method for removing undesired molecular products and/or by-products and/or impurities from an anolyte in an electrochemical vat dye or sulfur dye reduction process, wherein at least part of said undesired products and/or by-products and/or impurities are reaction products and/or by-products and/or impurities present or formed in the catholyte, and which pass through the semi-permeable membrane into the anolyte where they are adsorbed on an adsorption filter resulting in the purification of the catholyte; wherein said adsorption filter is integrated in an anolyte circulation pipe or anolyte circulation loop connecting an inlet of an anode compartment with an outlet of the anode compartment and wherein said inlet and said outlet are disposed at opposed ends of said anode compartment, and said anolyte circulation pipe or anolyte circulation loop is equipped with an anolyte vessel to form an anolyte circuit.

20. The method of claim 19, wherein the dye is indigo.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 shows the by-product removal in a method wherein leucodye is used as dispersant and preferably as sole dispersant.

(3) FIG. 2a shows schematically six stacks with the relevant supply and withdrawal lines and

(4) FIG. 2b one stack with five electrolytic cells more detailed.

(5) FIG. 3 shows the main parts of a whole electrochemical reactor suitable for by-product removal with the electrolytic cell in exploded view.

(6) FIG. 4 shows in more detail the part of the electrolytic reactor that serves the catholyte preparation in the presence of a first vessel.

(7) FIG. 5 shows the dye suspension preparation in the presence of a first vessel and with use of internally produced leucodye as dispersing agent.

(8) FIG. 6 shows the reactor and method of electrode regeneration by means of electrode washing. The catholyte and anolyte circulation loop are omitted for clarity reason. There is no circulation of anolyte and catholyte in the electrolyte cell during washing.

MODES FOR CARRYING OUT THE INVENTION

(9) The method will now be further illustrated for a dye reduction method, in particular an indigo reduction method.

(10) FIG. 1 shows the basic equipment of an anolyte circuit with an electrolytic cell 4 comprising a cathode compartment 41 and an anode compartment 42 separated from each other by a separator 43, in particular a semipermeable membrane. The anolyte vessel 3 is fed with electrolyte, in particular caustic soda via anolyte supply pipe 31. From anolyte vessel 3 the anolyte is fed to the anode compartment 42 via anolyte outlet 32 into anolyte circulation loop or anolyte circulation pipe 33, respectively equipped with a anolyte circulation pump P02. The anolyte enters the anode compartment via anolyte inlet 421, passes through the anode compartment 42 and is returned from the anode compartment 42 via anolyte outlet 422a and anolyte return pipe 422b into the anolyte vessel 3. Also provided within the anolyte circulation loop is an adsorption filter 332 that can be placed anywhere in the loop, as shown in FIG. 1 e.g. between the anolyte outlet 422a and the anolyte vessel 3 or—as shown in FIG. 3—between the anolyte outlet 32 of anolyte vessel 3 and the anolyte inlet 421 into the anode compartment 42. As also shown in FIG. 1 the catholyte is supplied from a main vessel 1 via catholyte outlet 12 and catholyte supply pipe 151 equipped with catholyte supply pump P01 via catholyte inlet 411 into the cathode compartment 41, through the cathode compartment 41 and back via reduced catholyte outlet 412a, reduced catholyte return pipe 412b and reduced catholyte inlet 13 back into main vessel 1.

(11) As also indicated in FIG. 1, the catholyte suspension can be prepared in a first vessel 2 that—in a preferred embodiment—uses leucodye comprising electrolyte as suspending medium, i.e a medium free of dispersing agent other than leucodye. In a first step, dye is suspended in electrolyte and preferably circulated in the first vessel 2 via an ultrasound apparatus (as shown in FIG. 4) prior to being forwarded to main vessel 1 and then subjected to reduction by circulating it via cathode compartment 41. Once the reduction has been completed, at least part of the leucodye comprising electrolyte solution is returned from main vessel 1 into first vessel 2 by means of pump P04 (as indicated by the semicircled arrow), where it is supplemented with further dye and processed as described before.

(12) If desired, a further circulation loop for improving the suspension prior to feeding it to the cathode compartment 41 can be provided in the main vessel 1, e.g. also equipped with an ultrasound apparatus and optionally also with a heating means (see FIG. 3).

(13) While FIG. 1 (and also the other Figures) only show one electrolytic cell, for industrial purposes it is preferred to have at least 4 electrolytic cells in at least two stacks of two electrolytic cells each.

(14) An arrangement of several stacks 5 of electrolytic cells 4, each stack comprising several electrolytic cells 4, and all stacks 5 and all electrolytic cells 4 of one stack 5 being connected in parallel for easy separation of one stack for cleaning/regeneration, is shown in FIG. 2.

(15) A preferred number of stacks is at least 4, more preferred 6. In case of 6 stacks, in general all are working but in case of more than two hours taking maintenance preferably 5 are working, while 1 is off. In case of 6 stacks 5, in general one at a time is separated for cathode washing/regeneration during leucodye suspension preparation, so that the cathodes of all stacks are cleaned within 6 days and all anodes of one stack are cleaned together on day 7. Since the anodes need less regeneration than the cathodes it proved advantageous to also clean them stackwise after the cleaning of all cathodes, i.e. the cathodes of each stack once a week, all anodes of one stack once all 7 weeks.

(16) Cleaning the electrodes during suspension preparation has the advantage that all stacks remain in leucodye production, and since the preparation of a fresh suspension in the batch procedure takes at least 1 hour while longer, i.e. up to two hours, circulation through ultrasound apparatuses improves the suspension quality, there is almost the same time needed for careful cleaning/regeneration of the electrodes and suspension preparation. Thus, performing the two steps simultaneously does not or only minimally extend the time needed anyway.

(17) At least the cleaning of the cathodes of one stack only at a time has several advantages, namely the water needed for suspension preparation can be supplied via the cathodes of the stack to be regenerated thereby avoiding loss of leucodye, the water needed for suspension preparation is sufficient to remove all leucodye from one stack but might be less efficient in the case of several stacks, since the cleaning solutions are recycled, less cleaning solutions are needed.

(18) Each stack preferably comprises from 1 to 10 electrolytic cells, preferably 4 to 6 electrolytic cells, in particular 5 electrolytic cells. For an electrochemical reactor suitable for producing 1000 kg of leucodye, in particular leucoindigo solutions of a concentration of e.g. 30% within 24 h, an electrochemical reactor with 6 stacks (in general all 6 stacks but at least 5 stacks working) of 5 electrolytic cells each has proved suitable, in particular for electrolytic cells with a cathode compartment having the following dimensions and a particulate carbon cathode: 0.3 m.sup.2 separator area per cell Dimensions of the cathode compartment containing particulate graphite carbon are 0.4 m high, 0.7 m large and 0.04 m thick Dimensions of the carbon granules are between 1 mm to 0.3 mm Dimensions of the stainless steal cathode current collector and the anode are 0.6 m high and 0.9 m large.

(19) Further information on a suitable electrode can be found in WO 2007/147283 A2 the disclosure of which is incorporated herein in its entirety. Such information regards e.g. the determination of the sphericity and the flow properties.

(20) FIG. 3 shows an electrolytic reactor in more detail but without a first vessel 2.

(21) As already shown in FIG. 1, 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 anode compartment 42 via anolyte outlet 32, anolyte pump P02, anolyte heating means 331 and anolyte inlet 421. 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.

(22) An adsorption filter 332 is provided within the anolyte circuit. As already indicated, this adsorption filter 332 can be placed anywhere, however, if a heating means is present, it is preferably placed just before the anolyte heating means 331 since there the temperature is lowest and thus adsorption best.

(23) On the cathode side of the reactor, 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 (see FIG. 4) for circulating the suspension thereby improving its homogeneity.

(24) 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.

(25) As indicated by reference numbers 413a and 413b, in a preferred embodiment the catholyte direction can be inverted. This helps to avoid clogging due to the particulate electrode being used in combination with a suspension. Therefore it is preferred to regularly invert the flow direction, e.g. all 3 minutes. Since the electrolysis shall not be affected by the inversion of the flow direction it is important that the packing of the particulate bed of the electrode in both directions is the same. This is obtained by ensuring that the flow and the particulate bed are so that the bed is always tightly pressed against the upper grid or the lower grid retaining the particles within the electrode compartment.

(26) Dependent on the kind of electrode the electrolyte direction of one or both compartments can be inverted.

(27) All vessels 1, 2, 3 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 leucodye or the anolyte in case of anode cleaning.

(28) FIGS. 4 and 5 show catholyte preparation via a first vessel 2 for suspension preparation. Solid dye from solid dye vessel 21a is supplied via solid dye inlet 21b into a solution of basic electrolyte like caustic soda or leucodye comprising basic electrolyte 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 FIGS. 1 and 3.

(29) As also already indicated above, for starting dye reduction, in a first preparatory step a diluted suspension is prepared 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—much 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.

(30) In a similar way, once the leucodye production has been started, part of the concentrated leucodye is removed and part of the leucodye solution is 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 FIGS. 4 and 5 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 and supplemented with dye. For producing highly concentrated dispersant-free leucodye solutions, it has proved advantageous to add the basic electrolyte and the leucodye in several parts, returning leucodye solution from main vessel 1 to main vessel 2 for each addition as indicated in the Figures like FIG. 1 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%.

(31) Once the dye has been reduced to leucodye in the desired concentration, a main part of the leucodye is removed from the reactor via concentrated leucodye outlet 44 and a minor part retained for the next reduction cycle.

(32) In spite of the adsorption filter, the anode has to be regenerated from time to time. This is also the case of the cathode, in particular for particulate carbon cathodes. In order to keep the reactor working it has proved advantageous to clean/regenerate all cathodes of one stack simultaneously during suspension preparation while catholyte is circulated through the other stacks. In a much preferred cathode regeneration step, the water for the suspension preparation is supplied to the main vessel 1 or to the first vessel 2 via the cathode compartment 41. In a continuously producing reactor, e.g. one batch a day, it is usually sufficient to regenerate the cathodes once a week, while the anodes need less frequent regeneration, e.g. once all seven weeks. For a reactor with 6 stacks this means that the cathodes of all stacks are cleaned after six days, with the anodes of one stack being cleaned on the seventh day, the anodes of a second stack on the seventh day of the second week and so on. In case of more than one stacks also reactor shut down can be kept at a minimum. Even if more time consuming maintenance is needed, in general it is sufficient to separate one stack while the other stacks remain working.

(33) It has now been found that the anode cleaning, in particular if polymerized deposits on the anode are kept to a minimum, can easily be performed by circulating acidic cleaning solution through the anode compartment. The same procedure can also be applied for cathodes, in particular for particulate carbon cathodes.

(34) For this cleaning or regeneration step, as shown in FIG. 6, the electrochemical reactor is also provided with means for supplying cleaning solutions to the stacks of electrolytic cells or rather the electrolytic cells. 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 or the anolyte circulation loop. 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 one or more particle filters and/or adsorption filters for retaining small particles of metallic origin and abraded electrode material or for adsorbing dissolved contaminants placed somewhere in the cleaning cycle, preferably in the cleaning medium circulation pipe 64 just downstream the electrolytic cells.

(35) Cleaning or regenerating, respectively, an anode or both electrodes in an electrochemical reactor of the present invention comprises washing the anode and/or cathode compartments 41, 42 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 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.

(36) This regeneration step e.g. removes deposits from the anode and also metals like iron and/or nickel from the particle surface of particulate carbon based cathodes. Since such metals are assumed to have catalytic effect on H.sub.2-generation that competes the desired reduction such washing step is of significant benefit for the whole performance of the electrolytic reactor besides of the removal of other deposits.

(37) In case of indigo reduction, the acid washing is performed for a suitable time such as 10 to 60 min. followed by washing the bed with a base like caustic soda (to remove the acid and contamination of the electrode) followed by water washing or—less preferred—by washing with water directly. While the washing can be performed for both electrodes, i.e. the cathode and the anode simultaneously, a procedure as described above is preferred.

(38) Suitable acidic solutions have a concentration in the range of 10 to 100 g/l, more preferred 40 to 60 g/l, most preferred about 50 g/l, or 0.25 to 30 M, preferably 1 to 2 M, more preferred 1.3 to 1.4 M (referred to the protons) in deionized water. If a basic solution like caustic soda is used following the acid solution cleaning, the concentration in general is in the range of 10 to 100 g/l, more preferred 20 to 60 g/l, most preferred 40 g/l, or 0.1 to 2.5 M, preferably 0.5 to 1.5 M (referred to hydroxide) in deionized water.

(39) For the regeneration step the strong acid is preferably selected from the group consisting of HCl, H.sub.2SO.sub.4, HNO.sub.3 and mixtures thereof.

(40) 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

(41) 1 main vessel 11a solid dye inlet 11b dye suspension inlet 12 catholyte outlet 13 reduced catholyte inlet 14 main suspension circulation loop P03 main suspension circulation pump 141 ultrasound apparatus in main suspension circulation loop 15 catholyte circulation pipe or catholyte circulation loop 151 catholyte supply pipe P01 catholyte supply pump 152 particle filter in catholyte supply pipe 153 catholyte heating means (heat exchanger) 154 ultrasound apparatus in catholyte supply pipe 16 leucodye outlet 2 first vessel 21a solid dye vessel 21b solid dye inlet 22 first suspension circulation loop 221 ultrasound apparatus in first suspension circulation loop 23 first suspension outlet 231 first suspension supply pipe P05 first suspension supply pump 24 leucodye return pipe 241 leucodye inlet 25 first suspension mixer P04 leucodye return pump 3 anolyte vessel 31 anolyte supply pipe 32 anolyte outlet 33 anolyte circulation pipe or anolyte circulation loop 331 anolyte heating means such as heat exchanger 332 adsorption filter P02 anolyte pump 35 anolyte inlet 4 electrolytic cell with 41 catholyte compartment or cathode compartment 411 catholyte inlet 412a reduced catholyte outlet 412b reduced catholyte return pipe 413a bypass for changing catholyte direction 413b bypass for changing catholyte direction 42 anolyte compartment or anode compartment 421 anolyte inlet 422a anolyte outlet 422b anolyte return pipe 43 separator, semipermeable membrane 44 concentrated leucodye outlet (batchwise) 5 stack of electrolytic cells 61 cleaning medium supply pipe 62 cleaning medium removal pipe 63a acid vessel 63b base vessel 64 cleaning medium circulation means