FI87191C - Process for the production of oxalic acid - Google Patents

Description

1 87191

The present invention relates to a process for the preparation of oxalic acid from a dialkyl oxalate, in which a dialkyl oxalate and an aqueous liquid are fed to a hydrolysis reactor to hydrolyze a dialkyl oxalate. .

Oxalic acid is used e.g. as a component of dyes, as a raw material for technochemical products, in metallurgy 15 and as an analytical reagent. Oxalic acid has been prepared, for example, by heating sodium formate to form sodium oxalate and treating the latter with sulfuric acid so that oxalic acid is released.

JP-54-98715 also discloses a process for producing oxalic acid in which a dialkyl ester of oxalic acid is partially hydrolyzed to form oxalic acid and an alcohol. The preparation process is carried out in such a way that the hydrolysis yields an organic phase and an aqueous phase, which are separated. The organic phase, which contains the completely unhydrolyzed ester and alcohol, is returned to the hydrolysis step, while the aqueous phase is passed to a crystallizer, where the resulting oxalic acid separates as a crystalline dihydrate. Part of the aqueous phase is separated before crystallization into a distillation column in which the alcohol formed in the hydrolysis is recovered and the distillation residue is returned to the solution entering the crystallizer. In addition, alcohol may have been recovered directly from the portion of the reaction mixture separated from the hydrolysis reactor.

35

The above method known per se only works for higher dialkyl esters, since the lower dialkyl esters are not even soluble in the organic phase, so the lower 2 87191 alcohols would be separated by this process, although Chemical Abstracts 92 (1980) 146274p, which refers to the hydrolysis of diethyl oxalate. This prior art process could not even treat the dialkyl oxalate soluble in the hydrolyzable mixture used in the process of the present invention.

In J. Chem. Soc. 1937, pages 876-883 generally discusses the hydrolysis of esters by the liquid phase reaction at high pressures and generally states that esters can be hydrolyzed at 500-6500 atmospheres using long periods of time. No industrially applicable process is disclosed in this publication. Both the pressures 15 and the times used are far too long for an industrially usable process. The experiments have been carried out in batches and the results show that the hydrolysis is not complete.

Chemical Abstracts 107 (1987) 39209c discloses a process for the preparation of oxalic acid in which dimethyl oxalate has been hydrolysed at 104 ° C, i.e. the boiling point of the hydrolysis mixture, for 40 min. The hydrolysis time is far too long for industrial use. This publication does not disclose the crystallization of the solution or the recirculation of the aqueous mother liquor from the crystallizer to the hydrolysis reactor, whereby the hydrolysis reaction can be autocatalysed with the oxalic acid contained in the mother liquor. In this case, the hydrolysis reaction can be carried out at a temperature below the boiling point of the reaction mixture and in a substantially substantially shorter time than that described in the reference. Also, the reference does not disclose any continuous process in which the hydrolysis would be completed, but the process was performed with a batch system.

35

Articles B 1969, 42 (3) pp. 136-142 and B 1971, 44 (10) pp. 291-300 of the Finnish Journal of Chemistry only generally describe the kinetics of hydrolysis of dimethyl oxalate in water 3,871 91 using a buffered system. No industrially useful process for the preparation of oxalic acid as a continuous process according to the present invention is apparent from these references.

The object of the present invention is to eliminate the drawbacks of the above-mentioned methods and to simplify the method based on hydrolysis so that said phase separation becomes unnecessary.

The invention is characterized in that the process uses a dialkyl oxalate soluble in the hydrolyzable mixture so that the hydrolysis results in a substantially homogeneous aqueous solution from which the liberated alcohol is removed by transferring it to the gas phase, after which the solution is transferred to a crystallizer.

An essential advantage of the invention is that alcohol can be separated from the mixture obtained from the hydrolysis step and the oxalic acid obtained thereafter without having to carry out the separation of the mixture, the separation of the phases and the recycling of the organic phase back to the hydrolysis reactor. This avoids a significant decrease in the volume yield of the reaction due to recycling. The separation of the alcohol by evaporation or distillation can be carried out at once to form a substantially completely hydrolysed mixture of the evaporation or distillation residue as such, which is subjected to crystallization of oxalic acid.

The dialkyl oxalate used in the process according to the invention is preferably dimethyl oxalate, which, as a low molecular weight, is highly soluble in the mixture to be hydrolysed and from which methanol is released during hydrolysis. Due to its low boiling point, methanol is easily separated from the aqueous reaction mixture by distillation.

According to a preferred embodiment of the invention, the mother liquor of the oxalic acid crystallization step with water and the non-crystallized oxalic acid is recycled to the hydrolysis reactor as the liquid phase of the hydrolysis of the dialkyl oxalate. Breast fluid. 87191 Pure water is added to the oxide for the hydrolysis step to replace the water leaving the process with oxalic acid dihydrate. The recycling of the mother liquor improves the oxalic acid content and allows the process to be carried out as a continuous process.

The temperature of the hydrolysis reactor is suitably at least 30 ° C where the dimethyl oxalate is sufficiently soluble, and at most about 100 ° C where the aqueous solution boils. The most preferred temperature range for 10 is 50-80 ° C. The dialkyl oxalate is preferably fed to the hydrolysis reactor as a liquid, either as a concentrated solution or in the molten state. When molten dimethyl oxalate is used, the reactor temperature is preferably at least 54 ° C, which is the melting point of dimethyl oxalate.

15

The average residence time of the reaction mixture in the hydrolysis reactor should be at least 15 minutes. The amount of dialkyl oxalate fed to the hydrolysis reactor is preferably such that the concentration of oxalic acid in the solution entering the crystallizer is between 20-60%, most preferably between 30-50%.

The invention will now be described in more detail by way of example with reference to the accompanying drawing, which is a diagram of an oxalic acid production process according to the invention, and then to describe the production of oxalic acid by means of embodiments.

The apparatus for producing oxalic acid according to the drawing comprises a hydrolysis reactor 1, an evaporator 2, a crystallizer 3 and a crystal separator 4, which devices are connected in series in said order. In the hydrolysis reactor 1, recycled aqueous mother liquor from line 5, dimethyl oxalate from line 6 and water 35 are added from line 7 to recycle line 5. In the hydrolysis, dimethyl oxalate and water react to form oxalic acid and methanol. The hydrolysis results in a homogeneous aqueous solution which passes from reactor 1 up to line 87191 to evaporator 2. Evaporator 2 separates methanol from the solution, which according to the drawing exits line 9, and the aqueous solution obtained as evaporation residue , which is separated from the remaining mother liquor, e.g. by filtration in a separator 4, to which the mixture has passed along line 11. The separated oxalic acid dihydrate is discharged according to the drawing into line 12 as the mother liquor returns to the recycling line 5 to the hydro-10 lysis step.

Example 1

At a temperature of 60 ° C, 590 g / h dimethyl oxalate-15 and 1380 g / h water were passed simultaneously to a well-stirred 2500 ml reaction vessel. The average delay time in the reactor 011 was 30 minutes. From this reaction vessel, the mixture was further passed at a constant rate to an average of 325 g / h and an average of 1640 g / h of oxalic acid-water as a substrate.

Oxalic acid was allowed to crystallize. from the mixture at 25 ° C to give an average of 482 g / h of 98.6% by weight of oxalic acid dihydrate as a filtered product. This gives an average yield of oxalic acid in the separated product of 75.4% of dimethyl oxalate. The oxalic acid content of the filtrate averaged 11.1% by weight.

Example 2

590 g / h of dimethyl oxalate and 1380 g / h of the filtrate of the experiment of the previous example, to which 230 g / h of water have been added, were simultaneously introduced into a well-stirred 2500 ml reaction vessel at 60 ° C. The average delay time in the reactor was 30 min. Ko. from the reaction vessel, the mixture was further passed at a constant rate to a distillation apparatus, from which an average of 320 g / h of methanol distillate was recovered 35 and an average of 1645 g / h of oxalic acid-water mixture as a substituent. Oxalic acid was allowed to crystallize. from the mixture at 25 ° C to give an average of 630 g / h of 99.1% by weight of oxalic acid dihydrate as a filtered product. This gives, 87191

O

the average yield of oxalic acid in the separated product is 99.0% of dimethyl oxalate. The oxalic acid content of the filtrate averaged 11.2% by weight.

5 Example 3

236.2 g / h of diethyl oxalate and 551.1 g / h of water were introduced into a well-stirred 1000 ml reaction vessel at 60 ° C. The average delay time in the reactor was 20 min. Ko. from the reaction vessel, this mixture was further passed at a constant rate to a distillation apparatus, from which an average of 235.3 g / h of ethanol distillate was recovered and an average of 550.4 g / h of oxalic acid-water mixture as a substituent. Oxalic acid was allowed to crystallize. from the mixture at 25 ° C, yielding an average of 158.4 15 g / h of 99.9 wt% oxalic acid dihydrate as the filtered product. This gives an average yield of oxalic acid in the separated product of 77.0% of diethyl oxalate. The oxalic acid content of the filtrate averaged 8.3% by weight.

It will be apparent to those skilled in the art that the various embodiments of the invention are not limited to those exemplified above but may vary within the scope of the appended claims. Thus, it is possible that, instead of recycling, the method according to the invention is carried out as a batch process.

Claims (5)

A process for preparing oxalic acid as a continuous process from dialkyloxalate, in which process dialkyl oxalate and aqueous liquid are fed into a hydrolysis reactor to hydrolyze the dialkyloxalate, the alcohol released is removed from the obtained hydrolyzed mixture, and the residual oxalic acid solution is added. in a crystallizer for separating the obtained oxalic acid as crystalline dihydrate, characterized in that the process 10 uses dialkyloxalate which dissolves in the mixture to be hydrolyzed, so that as a result of the hydrolysis a substantially homogeneous aqueous solution is obtained, from which the liberated alcohol is removed transferring it to the gas phase, after which the solution is transferred to the crystallizer, and fed into the hydrolysis reactor from the crystallizer recirculated aqueous mother liquor to catalyze the hydrolysis reaction with the mother liquor content of oxalic acid so that the hydrolysis reaction can be n is carried out at a temperature lower than the boiling point of the reaction mixture, at most 100 ° C. 20 Process according to claim 1, characterized in that the dialkyloxalate is dimethyloxalate. 3. A process according to claim 1 or 2, characterized in that the amount of dialkyloxalate fed to the hydrolysis reactor is such that the oxalic acid content of the solution added to the crystallizer is 20-60%, preferably 30-50%. Process according to any of the preceding claims, characterized in that the temperature of the hydrolysis reactor is at least 30 ° C, preferably 50-80 ° C, and that the dialkyloxalate is fed into the hydrolysis reactor in the form of a liquid. Process according to claim 4, characterized in that the dialkyloxalate is fed into the hydrolysis reactor in the molten state.