Chemistry: physical processes – Physical processes – Crystallization
Reexamination Certificate
2001-11-30
2004-08-24
Langel, Wayne A. (Department: 1754)
Chemistry: physical processes
Physical processes
Crystallization
C423S584000
Reexamination Certificate
active
06780206
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention is directed towards a process for the preparation of very highly concentrated hydrogen peroxide of concentration c
P
from aqueous hydrogen peroxide of concentration C
E
, C
E
being at least 80 wt. % and C
P
being greater than C
E
, comprising continuous suspension crystallization with subsequent washing of the hydrogen peroxide that has crystallized out. The process is directed especially towards the preparation of over 98 wt. % hydrogen peroxide from approximately 90 wt. % hydrogen peroxide, as well as at least 99.9 wt. % hydrogen peroxide of very high purity obtainable according to the process.
In the known processes for the preparation of hydrogen peroxide, such as the anthraquinone cyclic process, electrolytic processes and methods of direct synthesis, hydrogen peroxide is obtained in the form of an aqueous solution. In order to convert such solutions into marketable products, the preparation is usually followed by concentration of the solution by distillation. By means of distillation it is possible to obtain aqueous hydrogen peroxide solutions having a content of up to approximately 90 wt. % hydrogen peroxide. Cost and safety considerations stand in the way of further concentration by distillation.
More highly concentrated hydrogen peroxide, that is to say products having an H
2
O
2
content in the range of from equal to/greater than 90 wt. % to approximately 98 wt. %, are increasingly gaining importance commercially owing to their high energy content and their high purity. It is known that hydrogen peroxide can be concentrated by repeated recrystallization. Fundamental aspects regarding the crystallization of hydrogen peroxide, as well as the solid-liquid phase diagram of the hydrogen peroxide/water system, are known from the reference book “Hydrogen Peroxide” by Walter C. Schumb et al., Reinhold Publishing Corp. (1955), p. 210-220. According to that book, a prominent property of highly concentrated aqueous hydrogen peroxide is the tendency to extreme supercooling, which makes concentration by crystallization more difficult. According to Schumb et al. (page 215), crystal formation is not affected in a reproducible manner either by stirring or scraping or by the cooling rate. However, freezing of supercooled highly concentrated hydrogen peroxide is induced by seeding with hydrogen peroxide crystals. The low efficiency of fractional batch crystallization is regarded as being a disadvantage of concentration by crystallization (Schumb et al., p. 215), since an increase in concentration of only approximately 2% and never more than 4% is achieved per crystallization stage. That problem is evidently the result of the high boundary solubility, that is to say mixed crystal formation, of water in crystallized hydrogen peroxide, which is also shown in the phase diagram (Schumb, page 211). Because of that boundary solubility, it has hitherto not been possible, according to the experience of the present inventors, to obtain hydrogen peroxide having a content of at least 99.9 wt. % by fractional crystallization.
DE-PS 10 41 479 teaches a continuous process for the concentration of aqueous hydrogen peroxide solutions, which process also allows very highly concentrated hydrogen peroxide to be obtained. The apparatus for concentration comprises a crystallizing vessel and a rectifying column arranged directly beneath it. The hydrogen peroxide starting material is supercooled in the crystallizing vessel until there forms a 2-phase system consisting of a solid crystal phase and a liquid mother liquor, which phases differ according to the equilibrium with regard to the hydrogen peroxide concentration. The temperature along the rectifying column is so controlled that it increases slowly from the upper to the lower end. Owing to the higher density of the crystallized hydrogen peroxide, the crystals sink downwards in the rectifying column, a solid-liquid exchange of material taking place until the respective equilibrium has been reached and the H
2
O
2
concentration in the crystals increasing towards the bottom; the H
2
O
2
concentration in the mother liquor falls towards the top. Because there is only a slight difference in density between the crystallized hydrogen peroxide and the hydrogen peroxide mother liquor, the process is characterized by a low space-time yield. A further very considerable disadvantage that prevents the prior-known process from being applied on an industrial scale consists in scale-up problems (application of the process on an industrial scale): as the diameter of the rectifying column increases, unforeseeable back-mixing occurs, as a result of which both the function is impaired and the process risk is increased considerably (see Wellinghoff et al. in Chem.-Ing.-Tech. 63 (1991), 881-882).
The rectifying column according to DE-PS 10 41 479 is in principle operated as a gravity washing column. In the lowermost portion of the column, some of the crystals melt again and the melt thus acts as the washing medium. In such washing columns, working on the countercurrent principle, solid-liquid separation and further purification of the solid take place in one apparatus, but the scale-up problems remain, even when the suspension crystallization according to the invention described below is carried out.
Although the above-indicated scale-up problems can be avoided, as has been described in the as yet unpublished patent application DE 100 54 742.7, by means of a layer crystallization process with a subsequent sweating operation, the advantage of the simple separation of solid and liquid that can be carried out without problems even on a large scale is, however, offset by the disadvantages of discontinuous, multi-stage operation and the higher energy requirement.
Accordingly, an object of the present invention is to provide a continuous process for the preparation of very highly concentrated hydrogen peroxide from at least 80 wt. % aqueous hydrogen peroxide, which process can be carried out without problems even on an industrial scale.
According to a further object of the invention it is desired to obtain over 98 wt. %, preferably at least 99.9 wt. %, hydrogen peroxide from approximately 90 wt. % hydrogen peroxide in one step.
A still further object, in addition to concentrating and hence obtaining substantially anhydrous hydrogen peroxide, is directed towards lowering the content of impurities, such as organic carbon and conventional stabilizers. Conventional impurities are process-dependent secondary constituents, and stabilizers added during/after the preparation of the H
2
O
2
starting material to be concentrated.
SUMMARY OF THE INVENTION
The above and other objects can be achieved in a simple manner and with an unforeseeably high degree of efficiency by means of the process according to the invention.
Accordingly, the invention provides a process for the continuous preparation of very highly concentrated hydrogen peroxide of concentration C
P
from aqueous hydrogen peroxide of concentration C
E
, C
E
being at least 80 wt. % and C
P
being greater than C
E
, comprising suspension crystallizing aqueous hydrogen peroxide of concentration C
E
and after-treating the resulting hydrogen peroxide crystals contained in the suspension. In further detail, the process is characterised in that the after-treatment takes the form of countercurrent washing in a hydraulic or mechanical washing column with a packed crystal bed, and molten hydrogen peroxide of concentration C
P
is used as the washing medium. The dependent claims are directed towards preferred embodiments of the process according to the invention.
The process is especially suitable for concentrating aqueous hydrogen peroxide having a concentration C
E
in the range from 85 to 95 wt. %, especially from 88 to 92 wt. %, in a single step, preferably to a concentration C
P
of over 98 wt. %, but especially equal to or greater than 99.9 wt. %. In addition to concentration of the H
2
O
2
starting material, the content of impurities is at the same time lowered to surprisingly
Creutz Matthias
Gross Stefan
Nordhoff Stefan
Wagner Rudolf
Degussa - AG
Langel Wayne A.
Smith , Gambrell & Russell, LLP
Wright, Sr. William G.
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