CA2214082C - Plant for producing hydrogen peroxide in situ in a paper mill - Google Patents

Abstract

Plant for the production of hydrogen peroxide on the site of a paper mill, comprising a first stage (2) of catalytic hydrogenation of at least one anthraquinone derivative in solution in a mixture of organic solvents, a second stage (3) of oxidation of the hydrogenated working solution obtained, by injection of a gaseous mixture comprising oxygen, such as air, into the working solution in order to obtain an oxidized working solution, and a third stage (5) of extraction of the hydrogen peroxide from the oxidized working solution by injection of water counter-currentwise to the oxidized working solution, means being provided for injection of a gas comprising hydrogen into the said first stage and emergency air into the second stage; the said means for injection of the gas containing hydrogen comprise an ejector (21) composed of a constriction manifold connected to a conduit (24) for introduction of hydrogen and to means (23) for supplying this ejector with pressurized steam originating from the paper mill (1), so that the steam carries the gas containing hydrogen towards the first stage while being reduced in pressure after passing through the constriction.

Description

Plant for the production of hydroaen peroxide on the site of a paper mill The subject of the present invention is a plant for the production of hydrogen peroxide on the site of a paper mill, of the type comprising a first stage of catalytic hydrogenation of at least one anthraquinone derivative in solution in a mixture of appropriate organic solvents, a second stage of oxidation of the hydrogenated working solution obtained, by injection of -a gaseous mixture comprising oxygen, such as air, into the working solution in order to obtain an oxidized working solution, and a third stage of extraction of the hydrogen peroxide from the oxidized working solution, by injection of water countercurrentwise to the oxidized working solution, means being provided for injection of a gas comprising hydrogen in the said first stage and emergency air in the second stage.
Such a plant is known as "on site", that is to say that the hydrogen peroxide is produced on the very site of its use, in this case a factory for the manufacture of chemical pulps, according to the process for the autooxidation of a working solution comprising at least one anthraquinone derivative.
The constituent anthraquinone derivative of the working solution is preferably chosen from 2-alkyl-9,10-anthraquinones in which the alkyl substituent comprises from 1 to 5 carbon atoms, such as methyl, ethyl, sec-butyl, tert-butyl, tert-amyl and iso-amyl [sic] radicals, and the corresponding 5,6,7,8-tetrahydro [lacuna], or from dialkyl-9,10-anthra-quinones in which the alkyl substituents, which may be identical or different, comprise from 1 to 5 carbon atoms, such as methyl, ethyl and tert-butyl radicals, for example 1,3-dimethyl, 1,4-dimethyl, 2,3-dimethyl, 2,7-dimethyl, 1,3-diethyl, 2,7-di-tert-butyl or 2-ethyl-6-tert-butyl, and the corresponding 5,6,7,8-tetrahydro derivatives. The constituent organic solvent of the working solution is preferably a mixture of a non-polar compound and of a polar compound. The non-polar compound is preferably chosen from petroleum fractions with a boiling point greater than 140 C
mostly containing aromatic-hydrocarbons containing at least 9 carbon atoms, such as isomers of trimethylbenzene, isomers of tetramethylbenzene, tert-S butylbenzene, isomers of methylnaphthalene or isomers of dimethylnaphthalene. The polar compound is preferably chosen from saturated alcohols preferably containing from 7 to 11 carbon atoms, such as diisobutylcarbinol, 3,5,5-trimethylhexanol or isoheptanol, carboxylic acid esters, such as cyclohexylmethyl acetate marketed under the name "Sextate", heptyl acetate, butyl benzoate or ethyl heptanoate, phosphoric acid esters, such as tributyl phosphate, tri(2-ethylbutyl) phosphate, tri(2-ethylhexyl) phosphate or tri(n-octyl) phosphate, or tetrasubstituted ureas, such as tetra(n-butyl)urea.
The hydrogenation takes place at a temperature generally of between 50 and 70 C, the pressure of the gas in the headspace, which controls the hydrogen flow rate, being from 0.5 to 4 bar relative, preferably 0.8 to 1.5 bar. An equivalent of between 5 and 15 g/l, and preferably from 7 to 10 g/l, is obtained at the outlet of the hydrogenator.
"Hydrogen peroxide equivalent" is understood to mean the amount of hydrogen peroxide, expressed in grams, which one litre of working solution is capable of providing on conclusion of the oxidation stage, if the yield of this stage in the oxidizer is 1000. This potential concentration by mass of peroxide corresponds to a molar concentration which is equal to the molar concentration of the combined reoxidizable anthrahydro-quinone forms in the working solution.
The hydrogen is advantageously introduced via a distributor composed of a pipe which is pierced or of a plate which is pierced with holes with a diameter of 0.5 to 3 mm, preferably I to 2 mm, making possible a rate of ejection of the gases of 20 to 100 m/sec, preferably 40 to 50 m/sec.

The hydrogenated solution obtained is first cooled and then introduced at the upper end of the second stage constituting the oxidizer, whereas a gas containing oxygen, for example compressed air, is injected at the base of the latter. The air rises in the oxidizer countercurrentwise to the working solution, which falls through an appropriate packing or perforated plates.
Finally, the oxidized working solution is cooled and then introduced at the base of the third stage constituting the hydrogen peroxide extractor. The working solution is extracted countercurrentwise with water, preferably demineralized water, introduced at the upper part of the extractor. The hydrogen peroxide is recovered at the outlet of the extractor, whereas the working solution is recycled to the hydrogenator.
At the outlet of the extraction column, the hydrogen peroxide assay of the aqueous solution is less than 450.
The hydrogen is introduced by means of an electrical compressor. In point of fact, these compressors contain moving parts which are demanding as regards maintenance and which raise problems of safety, very particularly at the level of the seals, which are capable of allowing the passage of hydrogen leaks or of air admissions.
In the event of shutdown of the plant, inerting nitrogen is injected into the hydrogenator and the gaseous fluid removed at the top of the hydrogenator is recirculated via the same compressor and reinjected at the bottom of the hydrogenator, in order to make it possible to keep the catalyst in suspension.
Moreover, according to the prior technique, hydrogen is introduced into the hydrogenator via a diffuser generally composed of a porous plate in which the holes have very small diameters, of the order of a few tens of microns. This results, on this cold surface, in risks of crystallization of the working solution in the holes by the effect of evaporation. It is therefore necessary beforehand to reheat the gas containing hydrogen and to saturate it with the mixture of the solvents, which requires an additional investment.
Finally, the process for the manufacture of chemical pulps in a paper mill is reflected by the continuous production of a large amount of compressed steam, which until now has remained partially recovered.
The aim of the invention is thus to provide a plant for the production of hydrogen peroxide on the site of a paper mill in which the injection of hydrogen or of inerting nitrogen is ensured by means which increase the reliability of the plant and its safety, in particular in the event of shutdown of the electricity supply of the plant.
In accordance with the invention, the means for injection of the gas containing hydrogen comprise an ejector composed of a constriction manifold, which ejector is connected to a conduit for introduction of hydrogen and to means for supplying this ejector with pressurized steam originating from the paper mill, so that the steam carries the hydrogen towards the first stage.
In the context of the present invention, "gas containing hydrogen" denotes either a gas flow consisting of pure hydrogen or a gas mixture comprising hydrogen and a gas which is inert with respect to the working solution, such as nitrogen, for example.
The steam is produced by the paper mill at an average pressure of 10 to 15 bars and is always available on the site. For this reason, in the event of a sudden shutdown of the plant because of an electrical failure, the recirculation of the inerting nitrogen always remains assured by virtue of the use of the ejector.
Other specific features and advantages of the invention will become apparent in the course of the following description, made with reference to the appended drawings in which an embodiment is illustrated by way of a non-limiting example.
Figure 1 is a simplified diagram of a plant in accordance with the invention for the production of hydrogen peroxide on the site of use.
Figure 2 is a view in longitudinal section and elevation of an ejector with which the plant of Fig. 1 can be equipped_ The plant represented diagrammatically in Fig.
1 is intended for the production of hydrogen peroxide on the site of a paper mill 1("on site" production).
This plant comprises a first stage 2 of hydrogenation of a working solution comprising at least one anthraquinone derivative in a mixture of organic solvents and a second stage 3 of oxidation of the hydrogenated working solution obtained, this oxidation being carried out by injection of air at the base of the column constituting the oxidizer 3 (arrow F), via a compressor 4, in order to obtain an oxidized working solution. Finally, this production unit contains a third stage 5 of extraction of the hydrogen peroxide by injection of water countercurrentwise to the working solution rising from the bottom upward in the extraction column S. The hydrogen peroxide is collected at the lower part of the extractor 5(arrow H) and recovered at a concentration of less than 450, preferably less than 20%. After passing into a coalescer 6, the extracted working solution is recirculated via the pump 39, reheated in the heat exchanger 13 and introduced at the bottom of the hydrogenator.
Fresh extra hydrogen is injected at the base of the hydrogenator 2 (arrow K) and mixed in a pipe 7 with the recirculated gaseous fluid, containing hydrogen, which is recovered at the upper part of the hydrogenator 2, with which it constitutes a closed circulation loop 8. The hydrogenated working solution is collected at the base of the hydrogenater 2, passes through a system of filters 11, is then conveyed via a pump 12 into the exchanger 13 and a cooler 14 and is then introduced into the upper part of the oxidizer 3.
The latter contains a gas-liquid contact device 15, such as random packings, for example Raschig rings, or perforated plates. The exhausted air is extracted at the upper part of the oxidizer 3, passes through a condensation system 16 and is discharged to the atmosphere, optionally after an additional purification. The working solution is collected at the base of the oxidizer 3 via a pipe 17, injected via a pump 18 into a cooler 19 and then introduced at the lower part of the extraction column 5, which it ascends countercurrentwise to demineralized water introduced via the upper part (arrow G).
Means are provided for injecting the hydrogen in the first stage 2. These means comprise an ejector 21, for example of the type marketed by the Company Bertin. This ejector 21 is composed of a constriction manifold, a chamber 22 (Fig. 2) of which is in communication with a pipe 23 for introduction of the pressurized steam originating from the paper mill 1 and with a conduit 24 for supplying with recirculated hydrogen originating from the top of the hydrogenater 2. The steam available on the site of the paper mill originates from the boiler for recovery of the smelt from the black liquors and is normally at an average pressure of between 10 and 15 bars, as already indicated. The chamber 22 terminates at its end opposite the pipe 23 with a constriction 22a extended by a divergent nozzle 25 which is connected via a pipe 26 to a condenser 27, for example a water condenser.
The latter is itself connected via a conduit 28 to a condensate separator 29, from where the fluid containing hydrogen exits in order to be introduced via the pipe 7 at the base of the hydrogenater 2.
The chamber 22 of the ejector 21 comprises, in a manner known per se, a central sleeve 31 which is coaxial with the introduction conduit 23 and in which the latter emerges. The introduction of pressurized steam into the conduit 23 and the sleeve 31 sucks up the hydrogen surrounding the sleeve 31, the two gases mixing just after the constriction 22a in a mixer 32.
The mixture is reduced in pressure after passing through the constriction and is driven by the divergent nozzle 25 into the pipe 26.
A pipe 34 provided for in the top part of the first stage 2 of hydrogenation is equipped with a valve 35 and makes possible the injection of an inerting gas, such as nitrogen, into the gaseous headspace of the hydrogenater 2. If need be, this valve 35 can be opened, whereas the valve 33, situated on the extra hydrogen supply pipe upstream of the pipe 7, is closed.
Thus, in the event of shutdown of the hydrogen peroxide production unit, it is possible to maintain the safety of the plant and to prevent the catalyst contained in the hydrogenater 2 from sedimenting, which facilitates the restarting thereof.
The hydrogen is injected at the base of the first stage 2 via a distributor 36, produced in a manner known per se, via a plate or via a pipe pierced with holes each having a diameter of approximately 0.5 to 3 mm, and preferably 1 to 2 mm, and not via a porous plate as in the prior technique. These holes make possible a rate of ejection of the gases of 20 to 100 m/sec, preferably 40 to 50 m/sec.
In the event of mechanical failure of the compressor 4 or of a shutdown in the electrical supply of the unit, the air is no longer introduced into the oxidizer. In the event of prolonged failure, provision is therefore made for an emergency air supply, via a pipe 37, from an air source which is not represented.
It is in fact necessary then to introduce emergency air into the oxidizer 3 in order to oxidize the reduced working solution present in the loop. In point of fact, in the event of shutdown of the plant as a result of an interruption in the electrical supply, it is not economically possible to have available an auxiliary electrical generator in order to supply the air compressor. An additional characteristic of the invention then provides for carrying out this injection of emergency air via an ejector device composed of an ejector 21', of a condenser 27' and of a condensate separator, such as 29', connected to the pipe 37, in which the steam condenses and from where the gas can be injected into the associated stage 3. This device can provide the emergency air supply via the pipe 37 by virtue of the use of the pressurized steam which originates from the paper mill 1 via a connection, not represented, as far as the ejector 21'.
The working solution recovered via the coalescer 6 is recycled, via a pump 39 and the cooler.
13, to the hydrogenator 2, as represented in Fig. 1.
According to a simplified embodiment of the invention, the plant can be equipped solely with a hydrogen ejector 21, without the second ejector providing the emergency air supply.
In normal operation, the valve 35 is closed. If the plant is shut down, the hydrogen injection is replaced by an injection of inerting nitrogen by opening the valve 35 and by closing the valve 33.
In the event of a sudden shutdown of the electrical supply, the two hydrogen and emergency air ejectors improve the safety and the reliability of the plant, because the pressurized steam is always available on the site of the paper mill 1 and can therefore provide a continuous supply of inerting nitrogen to the hydrogenater 2 and of emergency air to the oxidizer 3.
Moreover, the arrangement of the hydrogen distributor 36 described above prevents any risk of crystallization of the working solution in the holes by the effect of evaporation, as can take place with the porous diffusers used until now. Thus, it becomes possible to dispense with the system for saturating the extra hydrogen with solvents which was necessary until now with the usual diffusers. This results in savings in the construction of the plant.

Finally, it should be noted that the same vapour [sic] ejector system 21 can be used either for supplying the hydrogenater 2 with hydrogen or for supplying it with inerting nitrogen.
The invention is not limited to the embodiments described above and can comprise various alternative implementational forms.

Claims (6)

1. Plant for the production of hydrogen peroxide on the site in a paper mill comprising a first stage (2) for catalytic hydrogenation of at least one anthraquinone derivative in solution in a mixture of organic solvents, a second stage (3) of oxidation of a hydrogenated working solution obtained, by injection of a gaseous mixture comprising oxygen into the hydrogenated working solution in order to obtain an oxidized working solution, and a third stage (5) of extraction of the hydrogen peroxide from the oxidized working solution by injection of water counter currently to the oxidized working solution, means being provided for the injection of a gas containing hydrogen into the said first stage and back up air into the second stage, characterized in that said means for injection of the gas containing hydrogen comprise an ejector (21) composed of a constriction manifold connected to a conduit (24) for introduction of hydrogen and means (23) for supplying this ejector with pressurized steam originating from the paper mill (1), so that the steam carries the gas containing hydrogen toward the first stage while being reduced in pressure after passing through the constriction..

2. Plant according to claim 1, characterized in that the gaseous mixture comprising oxygen is air.

3. Plant according to claim 1 or 2, characterized in that the first stage (2) of hydrogenation is equipped in its top part with a pipe (34) provided with a valve (35) connected to a source of inert gas, in order to make possible the injection of an inert into the gaseous headspace of the first stage (2).

4. Plant according to any of claims 1 to 3, characterized in that the said means for injection of back up air comprise a second ejector (21') connected to a conduit (37) for introduction of back up air into the second stage (3) and means for supplying this ejector with pressurized steam originating from the paper mill (1), so that in case of need the steam can carry the back up air toward the second stage (3).

5. Plant according to any one of claims 1 to 4, characterized in that each ejector (21, 21') emerges at its outlet into a condenser (27, 27') itself connected to condensate separators (29, 29') in which the steam condenses and from where the gas is injected into the associated stage (2 or 3).

6. Plant according to any one of claims I to 5, characterized in that the first stage (2) is provided with a hydrogen distributor composed of a plate or a pipe (36) pierced with holes with a diameter of approximately 0.5 to 3 mm.