Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-06-05
2004-07-06
Keys, Rosalynd (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06759560
ABSTRACT:
The present invention relates to a process for preparing alkali metal methoxides from aqueous alkali metal hydroxide, which may have been admixed with methanol, and methanol.
The preparation of alkoxides from alkali metal hydroxides and alcohols is known per se.
DE-C 968 903 has already described the continuous treatment of high-concentration aqueous alkali metal hydroxide solutions with a countercurrent of alcohol in a packed column, with the aqueous alkali metal hydroxide being able to be fed in at various places on the column in order to prevent blockage of the column by deposited alkali metal hydroxide. The alkoxide is formed by reaction of the alkali metal hydroxide located on the packing with the alcohol stream additionally passed through the column from the top downward.
However, low-boiling alcohols (methanol) can be used in this process only when a suitable auxiliary liquid is used for removing the water. This process also has the disadvantage that the deposits of solid alkali metal hydroxide which settle on the packing of the column can lead to blockage of the column. The procedure described in DE-C 968 903 does mention removal of the alcohol by azeotropic distillation using an entrainer, but in the procedure indicated the alcohol of the alcohol/water azeotrope which has been distilled off constantly has to be replaced, requiring a complicated recovery step.
SU-A 165 691 discloses a process for preparing sodium methoxide, which starts from a methanolic sodium hydroxide solution which is conveyed from a stirred vessel into the upper part of a continuously operated packed column in countercurrent to gaseous methanol.
The process requires a high energy input to achieve the appropriate product purity. It has the further disadvantage that solid sodium hydroxide is employed.
EP-A 0 299 577 describes a process for preparing alkoxides in which the water of reaction is removed by pervaporation. It is known that complete dewatering of the alcohol cannot be effected in this way.
DE-C 34 13 212 and DE-C 37 01 268 describe processes for preparing potassium tert-butoxide from aqueous potassium hydroxide and an excess of tert-butanol in a distillation column using an entrainer which, in contrast to the auxiliary liquid described in DE-C 968 903, does not form an azeotrope with water.
Furthermore, RO-A 60485 describes the isolation of sodium methoxide from anhydrous technical-grade methanol and technical-grade sodium hydroxide in a three-stage process with the aid of a hydrocarbon, in which process the water of reaction formed is removed by azeotropic distillation. The anhydrous hydrocarbon is recovered by fractional distillation with addition of Na
2
SO
4
. The sodium methoxide is obtained as a suspension in the hydrocarbon from which it can be filtered off.
These processes require additional separation operations and also leave extraneous materials in the alkali metal alkoxide solution. The latter interfere in further processing.
It has been found experimentally that the theoretical number of theoretical plates alone is not a sufficient criterion for an effective process for preparing substantially water-free alkoxide solutions. Thus, known processes for preparing alkoxides give only unsatisfactory qualities in which the water content is from about 0.5 to 2% based on the alkoxide, even in columns which have a high number of from 35 to 40 theoretical plates. These water contents are above those of commercial grades having a water content of about 0.1%.
It is an object of the present invention to provide a process for preparing alkoxide solutions which makes it possible to obtain a substantially water-free end product with justifiable energy input and in which no deposits are formed on the packing when a packed reaction column is used.
We have found that this object is achieved by the known process for preparing alkali metal methoxides from aqueous alkali metal hydroxide, which may have been admixed with methanol, and methanol in a reaction column having at least 5, preferably from 15 to 30, theoretical plates between the feed point for the aqueous alkali metal hydroxide and the feed point for the methanol, where in the case of a reaction column configured as a bubble cap tray, valve tray or sieve tray column, the trays are selected so that not more than 5%, preferably ≦1%, of the liquid rains through the respective trays.
For this embodiment of the process of the present invention, suitable columns are essentially bubble cap tray, valve tray and sieve tray columns. Specifically in the case of valve trays and sieve trays, the trays should be configured so that the raining-through of the liquid is minimized. A person skilled in the art will be familiar with the constructional measures required for this. Particularly tightly closing valve types are selected and thus, in particular, the vapor velocity into the tray openings is increased to double the value which is customarily set. This is achieved by a reduction in the number of valves. In the case of sieve trays, it is particularly useful to reduce the diameter of the openings in the tray and to maintain or even increase the number of openings.
In a variant of the present invention, the reaction column is provided with random packing elements or ordered packing, with ordered packing being preferred over random packing elements with a view to uniform distribution of the liquid. In this embodiment of the invention, the average ratio of liquid flow to vapor flow must not be exceeded by more than 15%, preferably not by more than 3%, in all subregions of the column cross section which correspond to more than 2% of the total column cross section. This low amount of liquid to be maintained according to the present invention obviously makes it possible for the capillary effect on the wire meshes to prevent local peak values of the liquid trickle density.
Suitable methods of achieving this are known from EP A 0 684 060. The desired characteristics of the liquid distribution can be achieved when using columns with random or ordered packing by the liquid trickle density in the marginal region of the column cross section next to the column wall, which region corresponds to about 2-5% of the total column cross section, being reduced by up to 100%, preferably by from 5 to 15%, compared to the remaining cross section. This can be achieved in a simple manner by targeted distribution of the drip points of the liquid distributors or their holes.
In this mode of operation, it is advantageous for the internal wall of the reaction column to be at a temperature which is from 3 to 10° C. above the temperature of the reaction column.
Owing to the very unfavorable position of the chemical equilibrium, breakthrough of even very small amounts of still water-containing liquid which have not come into contact with a sufficiently large amount of vapor obviously has to be prevented with certainty. Local breakthrough of water-containing liquid can obviously have a serious adverse effect on the quality of the product due to backreaction.
The combination of the process known per se for preparing alkali metal methoxides with the features of the invention which ensure that passage at the margins or stream formation or raining-through of liquid occurs at no point of the reaction column cross section results, in a surprising and synergistic fashion, in a purity of the alkali metal methoxide solution which has hitherto only been able to be achieved in the amalgam process or by the use of alkali metals.
It may be stated, and is advantageous for industrial implementation, that there are no additional constructional requirements in respect of the uniformity of the gas flow over the cross-sectional area of the column. It is not disadvantageous if the ratio of gas flow to liquid flow locally assumes higher values than the average value. Rather, increased gas flows locally improve water removal.
When carrying out the process of the present invention, no blockage occurs in a reaction column provided with random or ordered packing, and the methanol used can be recycled to the
Burkart Kirsten
Friedrich Holger
Guth Josef
Hoffmann Elke
Kaibel Gerd
BASF - Aktiengesellschaft
Keil & Weinkauf
Keys Rosalynd
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