Chemistry of inorganic compounds – Oxygen or compound thereof – Peroxide
Reexamination Certificate
2000-08-09
2002-09-10
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Peroxide
Reexamination Certificate
active
06447743
ABSTRACT:
The present invention relates to a catalytic process and to a device for preparing, in perfect safety, aqueous hydrogen peroxide solutions at high concentrations directly from hydrogen and oxygen. More particularly, a subject-matter of the invention is a process in which hydrogen and oxygen are injected into the aqueous medium in proportions corresponding to the flammability range of the hydrogen-oxygen mixture and are present in proportions outside the flammability range in the continuous gas phase. Another subject-matter of the invention is a device for the implementation of the process.
The hydrogen and oxygen gas mixture is known to be flammable, even explosive, when hydrogen is present at molar concentrations of between 4 and 94% under standard temperature and pressure conditions, that is to say when the ratio of the hydrogen molar concentration to the oxygen molar concentration is greater than 0.0416 (Encyclopédie des Gaz [Encyclopedia of Gases], Air Liquide, page 909).
To avoid any risk of explosion or fire, it is recommended either to operate with a hydrogen/oxygen ratio below the lower flammability limit or to use an inert gas, such as nitrogen, argon, helium or neon (U.S. Pat. Nos. 4,681,751, 4,009,252, EP 0,787,681).
In point of fact, to obtain satisfactory results, it is necessary to work with a hydrogen/oxygen ratio situated in the flammability range. Thus, the document U.S. Pat. No. 4,009,252 discloses a hydrogen/oxygen molar ratio of between 1/20 and 1/1.5 and preferably of between 1/10 and 1/2. Likewise, the document U.S. Pat. No. 4,336,239 teaches the reader to operate in the presence of a hydrogen/oxygen molar ratio of less than 0.2 and preferably of between 1/15 and 1/12.
The term “direct synthesis of an aqueous hydrogen peroxide solution” is understood to denote the synthesis of hydrogen peroxide from hydrogen and oxygen in an aqueous medium comprising a catalyst.
The direct synthesis of an aqueous hydrogen peroxide solution, continuously or batchwise, in a stirred reactor has formed the subject of many studies. The reactor generally comprises an aqueous region, occupied by the working solution and the catalyst, and a region, occupied by the gases, situated above the aqueous region. It is equipped with a stirring system which makes it possible both to stir the aqueous region and to disperse the gases in the aqueous phase. The reactants, namely the hydrogen and the oxygen, and the inert gases are injected into the region of the gases.
The term “working solution” is understood to denote the aqueous medium, comprising water, acids and optionally decomposition inhibitors or stabilizers for hydrogen peroxide, in which the hydrogen peroxide is formed.
It has been observed that when the direct synthesis of an aqueous hydrogen peroxide solution is carried out in a stirred reactor as described above, the catalyst, thrown under the effect of the stirring on to the walls of the reactor and the shaft of the stirrer which are situated in the region of the gases, would be in direct contact with the reactants. During the synthesis, the catalyst particles in the region of the gases will dry out and will spontaneously bring about the ignition of the hydrogen-oxygen gas mixture if the molar concentration of the hydrogen is greater than 0.04.
This is why, in Example 1 of document U.S. Pat. No. 4,279,883, which illustrates the direct continuous synthesis of an aqueous hydrogen peroxide solution in a stirred reactor, a hydrogen, oxygen and nitrogen gas mixture are introduced continuously into the gaseous region of the reactor, so that the hydrogen, oxygen and nitrogen partial pressures in the gases collected at the outlet are maintained respectively at 5, 49 and 113 atmospheres, that is to say a hydrogen molar concentration of 3%. The industrial manufacture of an aqueous hydrogen peroxide solution under the safety conditions according to the document U.S. Pat. No. 4,279,883 is economically out of the question, however, given the low concentration of the aqueous hydrogen peroxide solution obtained.
In order to be usable, this aqueous solution requires an additional concentration stage.
The direct synthesis of an aqueous hydrogen peroxide solution can also be carried out in a tubular reactor composed of a long pipe (pipeline) filled with working solution in which the catalyst is suspended and into which gaseous oxygen and hydrogen are injected in the form of small bubbles in proportions above the lower flammability limit of the hydrogen-oxygen mixture (U.S. Pat. No. 5,194,242). The safety of such a process is only assured provided that the gaseous reactants are maintained in the reactor in the form of small bubbles. According to the document U.S. Pat. No. 5,641,467, the latter can only be obtained with a high rate of circulation of the working solution.
A catalytic process and a device have now been found which make it possible to prepare aqueous hydrogen peroxide solutions at high concentrations directly from hydrogen and oxygen in a stirred reactor in perfect safety and economically.
This process is characterized in that hydrogen and oxygen are injected, in the form of small bubbles, into the lower part of the aqueous reaction medium, which has been rendered acidic by the addition of a mineral acid and which comprises a catalyst in the dispersed state, with molar flow rates such that the ratio of the hydrogen molar flow rate to the oxygen molar flow rate is greater than 0.0416 and in that oxygen is introduced into the continuous gas phase and/or into the upper part of the aqueous reaction medium in an amount such that the molar ratio of hydrogen to oxygen in the continuous gas phase is less than 0.0416.
The term “small bubbles” is understood to denote bubbles with a mean diameter of less than 3 mm.
The injections of hydrogen and of oxygen in the form of small bubbles into the lower part of the aqueous reaction medium are preferably situated at the bottom of the stirred reactor and are preferably contagious, in order for the H
2
and O
2
bubbles to mix together as quickly as possible.
Mention may be made, as mineral acid, of, for example, sulphuric acid and orthophosphoric acid.
The aqueous reaction medium can additionally comprise stabilizers for hydrogen peroxide, such as, for example, phosphonates or tin, and decomposition inhibitors, such as, for example, halides. The bromide is the particularly preferred inhibitor and it is advantageously used in combination with bromine in the free state (Br
2
).
According to the invention, the oxygen injected, in the form of small bubbles, into the lower part of the aqueous reaction medium and the oxygen introduced into the continuous gas phase and/or into the upper part of the aqueous reaction medium can additionally comprise hydrogen in an amount such that the ratio of molar concentration of hydrogen to molar concentration of oxygen is less than 0.0416.
According to the present invention, the operation can be carried out just as easily continuously as semi-continuously.
The oxygen feed in the form of small aqueous into the lower part of the aqueous reaction medium can be provided in all or in part by the gaseous effluent at the outlet of the reactor.
It is also possible to use the gaseous effluent at the outlet of the reactor to feed the continuous gas phase and/or the upper part of the aqueous reaction medium. In this case, the composition of the gaseous effluent can be adjusted by addition of oxygen and optionally by removal of hydrogen, so that the ratio of the molar concentration of hydrogen to the molar concentration of oxygen in the continuous gas phase is less than 0.0416.
The catalyst generally used comprises at least one element chosen from metals from Group IB and VIII of the Periodic Table. Advantageously, gold, platinum, palladium and ruthenium are chosen. Use is preferably made of palladium, platinum or the palladium-platinum combination or better still palladium or the palladium-platinum combination.
In the case of a palladium-platinum composite catalyst, the platinum preferably represents between 1 and 50% of
Delais Lionel
Devic Michel
Atofina
Johnson Edward M.
Pennie & Edmonds LLP
Silverman Stanley S.
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