Distillation: processes – separatory – With measuring – testing or inspecting – Of temperature or pressure
Reexamination Certificate
2002-06-26
2004-02-17
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
With measuring, testing or inspecting
Of temperature or pressure
C203S014000, C203S018000, C203S078000, C203S080000, C203S077000, C203S075000, C203S091000, C568S854000, C568S853000, C568S680000
Reexamination Certificate
active
06692616
ABSTRACT:
The present invention relates to the field of industrial chemistry. More precisely, it relates to a process in which trimethylolpropane obtained by the hydrogenation of dimethylolbutanal is purified by distillation. The present invention further relates to a process in which trialkylammonium formate produced as a by-product in the preparation of the alkylolated alkanal, from the trialkylamine used as catalyst and formic acid, can be separated off by simple distillation.
Trimethylolpropane, hereafter abbreviated to TMP, is a trihydric alcohol which has become widely used in the manufacture of surface coatings, polyurethanes and polyesters, for example alkyd resins. Trimethylolpropane is prepared by means of a condensation reaction between n-butyraldehyde and formaldehyde. This reaction can be carried out using different process variants.
Firstly, there is the so-called Cannizzaro process, in which the butyraldehyde is reacted with the formaldehyde in the presence of stoichiometric amounts of a base. 2,2-Dimethylolbutanal is formed in the first step and then reacts with excess formaldehyde, in a so-called crossed Cannizzaro reaction, to give formic acid and trimethylolpropane. The Cannizzaro process is referred to as inorganic or organic according to the type of base. The inorganic procedure employs an inorganic base, usually NaOH or Ca(OH)2. The disadvantage of this procedure is that a quantity of unwanted by-products is formed which are difficult to separate off and which interfere with the subsequent use of the trimethylolpropane. Also, one mole equivalent of formic acid salt is formed; this has to be discarded, thereby increasing the consumption of formaldehyde and contributing to environmental pollution.
In the organic Cannizzaro process, a tertiary amine, generally a trialkylamine, is used in place of the inorganic base. The reaction proceeds as described above to give one equivalent of the ammonium formate of the amine in question. This can be worked up further by appropriate measures, whereby at least the amine can be recovered and recycled into the reaction. The crude TMP obtained can be worked up to pure TMP in a variety of ways.
One further development is the hydrogenation process, in which butyraldehyde and formaldehyde are reacted together in the presence of catalytic amounts of a tertiary amine, generally approx. 5 to 10 mol %, rather than at least stoichiometric amounts. In this case the reaction stops at the 2,2-dimethylolbutanal stage and this compound is then converted to trimethylolpropane by hydrogenation. This method does not give rise to stoichiometric amounts of a formate, and the resulting solution is easier to purify because fewer interfering by-products are formed. In many cases, however, it is necessary to resort to reaction engineering measures to achieve a complete conversion of the educts to dimethylolbutanal. The description of an efficient process can be found in WO 98/28253 in the name of the Applicant.
The state of the art contains a wealth of publications describing different techniques for working up trimethylolpropane. A study of these publications clearly reveals the differences in work-up demanded by the different ways of preparing trimethylolpropane. The following publications relate to the purification of trimethylolpropane obtained by the inorganic Cannizzaro process.
DD-P-45 078 discloses a process in which the crude TMP obtained is treated with a secondary cycloaliphatic alcohol, for example cyclohexanol, water is then distilled off azeotropically with this alcohol and the formates which have precipitated are filtered off. After distillation of the excess alcohol, the crude product obtained is then purified by distillation.
DD-P-287 251 describes a process for freeing TMP of the high-boiling components formed. In the vacuum distillation of crude TMP, the high-boiling components accumulate in the fractions less volatile than TMP. Many of the high-boiling components, which consist of secondary reaction products of TMP, especially formals, can be converted back to TMP by the addition of 0.02 to 0.05 kg acid/kg distillate, thereby increasing the yield.
Also, GB 1 290 036 describes a process for the decomposition of TMP formals in crude batches obtained by the inorganic Cannizzaro process. By the addition of cation exchange resins and heating, formals contained in the crude mixtures, which have a similar boiling point to TMP, are converted to products of different boiling points which can readily be separated off by distillation. Pure TMP can be obtained.
U.S. Pat. No. 3,097,245 describes a process for the preparation of trimethylolpropane with an APHA color index of between 50 and 200. This color index is achieved by maintaining specific reaction conditions in respect of temperature, reaction time, pH and concentration of the starting compounds. The reaction is followed by treatment of the resulting solution with ion exchange resins.
U.S. Pat. No. 5,603,835 discloses a process for the preparation of TMP with APHA color indices of <100. These are achieved by extractive aftertreatment of the crude TMP solutions obtained with an ether or an ester. The TMP solutions used generally originate from the inorganic Cannizzaro process.
By contrast, crude TMP originating from the organic Cannizzaro process is worked up differently.
EP-B-142 090 describes the work-up of such a crude TMP mixture. This crude mixture is worked up by distillation and then hydrogenated and distilled again. Such a process is expensive, demands a high vacuum and gives low yields.
In particular, in the preparation of TMP by the organic Cannizzaro process, there is an interfering secondary reaction which can markedly reduce the yield of TMP. The trialkylammonium formate produced in the reaction reacts under specific conditions, for example dehydration of the solution or heating, to give trialkylamine and trimethylolpropane formates. These compromise the yield of trimethylolpropane and should therefore be cleaved as completely as possible without the simultaneous appearance of unwanted secondary reactions.
WO 97/17313 discloses a process suitable for this purpose. In the first step, trimethylolpropane is prepared in a manner known per se by reacting formaldehyde with butyraldehyde in the presence of stoichiometric amounts of a tertiary amine. In the second step, the crude TMP mixture is freed of excess water, tertiary amine and formaldehyde. In the third step, the residual mixture is heated, causing the cleavage of the trialkylammonium formate into trialkylamine and formic acid (both of which are separated off) and the formation of TMP formates. In the fourth step, the amine which has been separated off is recycled, either into the first step or into the fifth step which follows. In this fifth step, the TMP formate obtained is reacted with a lower alcohol (this reaction being catalyzed by the very amine which has been separated off) to free the TMP with the formation of methyl formate.
A similar process is disclosed in DE-A-198 48 568. The crude TMP mixture obtained after the conventional reaction in the presence of stoichiometric amounts of a trialkylamine is heated, freed of trialkylamine and, before work-up by distillation, treated with either water, ammonia, a primary amine or a secondary amine. The trimethylolpropane formate produced on heating is converted to TMP and formic acid or a formamide. The yield of TMP is increased.
However, the abovementioned processes are only of limited suitability for the efficient work-up of a TMP mixture obtained by the so-called hydrogenation process, in which only catalytic amounts of trialkylamine are used and which consequently also contains only small amounts of trialkylammonium formate.
It is therefore an object of the present invention to provide such a process. This process should furthermore make it possible to prepare TMP with a high purity, preferably >99%, with a low color index of 10 to 100 APHA and in high yield.
We have found that this object is achieved by a process for the purification, by distillation, of trimethylol
Dernbach Matthias
Kratz Detlef
Rust Harald
Schulz Gerhard
Stammer Achim
BASF - Aktiengesellschaft
Keil & Weinkauf
Manoharan Virginia
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