GB1583294A - Purification of glycolic acid - Google Patents

Description

(54) PURIFICATION OF GLYCOLIC ACID (71) We, CHEVRON RESEARCH COM- PANY, a corporation duly organised under the laws of the State of Delaware, United States of America, of 525 Market Street, San Francisco, California 94105, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns an improved process for removing hydrogen fluoride from glycolic acid.

Recently hydrogen fluoride has been found to be a surprisingly effective catalyst for glycolic acid production. For instance, U.S. Patent 3,911,003, granted October 7, 1975, describes a process for preparing glycolic acid by contacting formaldehyde, carbon monoxide and water in the presence of a hydrogen fluoride catalyst. For economy of process and for subsequent processing of the crude product, it is necessary to separate the hydrogen fluoride catalyst from the crude glycolic acid product. It is usually necessary to reduce the hydrogen fluoride content of the product to a few parts per million, or even to less than one part per million, before the product can be used.

It would be expected that, as hydrogen fluoride is quite volatile, it would readily be removed by distillation or by stripping with inert gas at moderate temperatures. For example, U.S. Patent 2,534,017 shows that hydrogen fluoride can readily be removed from aromatic aldehydes by vacuum distillation at 55"C to 85"C; and U.S. Patent 3,962,343 shows rapid removal of hydrogen fluoride in a similar system by stripping with a hydrocarbon diluent or by decomposing in a film-evaporator at 80"C to 1000C. It has been found, in agreement with this, that most of the hydrogen fluoride can be removed from the crude glycolic acidhydorgen fluoride product formed by the process of U.S. Patent 3,911,003 by distilling or stripping with gas at temperatures at or below 1 100 C. However, when the concentration of hydrogen fluoride in crude glycolic acid has been reduced to about 4% by weight, i.e., about 40,000 pm, further stripping with an inert gas or distillation at these temperatures does not significantly reduce the hydrogen fluoride concentration.

Accordingly, in order to obtain a glycolic acid product having less than about 4%, by weight, of hydrogen fluoride, an improved process is desirable.

Thus in accordance with the invention the hydrogen fluoride catalyst content of a crude glycolic acid is reduced to below 4% by weight by removal of hydrogen fluoride in the vapour state at a temperature in excess of 1300C.

It has been found that the concentration of hydrogen fluoride in crude glycolic acid may be reduced to less than about 0.1% by weight by heating the crude glycolic acid to a temperature of at least about 140"C during the purification step.

The Figure compares the purification of crude glycolic acid by hydrogen fluoride stripping at a continuous temperature of about 110 C to stripping at an increased temperature of about 140"C. Both runs were carried out at atmospheric pressure using nitrogen as an inert gas. As described in Example 1, stripping at 93"C for over 8 hours did not reduce the hydrogen fluoride concentration below about 4%. In contrast, stripping at 110 C for about 6 hours followed by only 1 hour of stripping at an increased temperature of about 140"C reduced the hydrogen fluoride concentration to less than 2 ppm. Thus, while both temperatures were satisfactory for removing the bulk of the hydrogen fluoride, to obtain a usefud product, for example one containing less than about 5 ppm of hydrogen fluoride, purification had to -be carried out at a temperature in excess of about 130"C, preferably 140"C.

This invention provides an improved pro cess for purifying a crude glycolic reaction product. The term "crude glycolic acid" referes to the reaction product of the hydrogen-fluoride-catalyzed reaction of formaldehyde and carbon monoxide to form glycolic acid. This reaction is described in U.S. Patent 3,911,003. The crude product generally contains in excess of 40% by weight hydrogen fluoride.

While the purification of crude glycolic acid can be carried out in a variety of ways, the central feature of the - invention is based on the discovery that, in order to reduce the hydrogen fluoride concentration to less than about 4%, the final stages of the removal of hydrogen fluoride must be carried out at a temperature in excess of about 1300C, preferably in excess of 140"C.

Lower temperatures can be used during the initial stages to reduce the hydrogen fluoride content to about 4%, but the final reduction to less than about 4% requires a temperature in excess of about 1300C. Thus, the process may comprise heating crude glycolic acid containing hydrogen fluoride to a temperature above about 90"C until the concentration of hyrogen fluoride is about 4% by weight, and subsequently heating the crude product to a temperature in excess of about 1300C. It desired, the entire purification process can be conducted at temperatures above 1300C. In this way, the quantity of HF remaining in the product can be easily reduced to a value less than 100 ppm, preferably less than 10 ppm.

The physical removal of hydrogen fluoride can be done by any known method. One method is to distil off the hydrogen fluoride at reduced pressure in a still or an evaporator. Another method of removing hydrogen fluoride is to pass an inert gas through the crude product, thereby stripping out the hydrogen fluoride.

Inert gas stripping and distillation are well-known methods of refining crude organic products. In general, inert gas stripping is carried out by passing an inert gas through a liquid mixture at a given temperature and pressure which are know to effect transfer of a volatile liquid component into the inert gas. In this way, a refined product having a reduced concentration of the transferred liquid is obtained. A thorough discussion of gas-stripping principles is contained in Perry's Chemical Engineers' Handbook 4th, Chapter 14.

The inability to remove hydrogen fluoride by continuous purification at temperatures of about 110 C is apparently due to solvent difference at these temperatures. The vapor pressure of. pure hydrogen fluoride is only twice as high at about 1500C as at about 1000 C; so, if the solvent were unchanged, hydrogen fluoride removal would be only twice as fast at 1500C as at 100"C. At the lower temperature, the solvent is mainly glycolic acid. The 4% weight hydrogen fluoride retained in the solution corresponds to about 1 mol of hydrogen fluoride for every 6 mols of glycolic acid. A strong acid such as hydrogen fluoride would be expected to be solvated by 4-8 mols of a hydroxylic solvent. Thus, it is believed that the surprising increase in reduction of hydrogen fluoride content which can be obtained by purification at a temperature in excess of about 140 C is due to the tendency of glycolic acid to form polyglycolides.

Apparently the conversion of glycolic acid to polyglycolide at about 140"C not only decreases the content of free hydroxyl and carboxyl groups, but also forms solvent molecules which are too bulky to permit solvating protons with a large number of hydroxyl groups. This change in solvent properties then results in a greatly increased volatility for hydrogen fluoride. As a result, continuous stripping or distillation at about llO"C will not remove the last 4% of the hydrogen fluoride, while an increase in temperature to about 1300C, preferably about 140"C, causes an almost logarithmic reduction with - time. This suggests that the hydrogen fluoride vapor pressure at tem- peratures above 140"C is essentially proportional to the hydrogen fluoride concentration in the solution.

In a preferred embodiment of the invention, purification of crude glycolic acid is carried out by inert gas stripping using synthesis gas or carbon-monoxide-depleted synthesis gas as the inert gas. As thoroughly described in U.S. Patent 3,911,033, crude glycolic acid is prepared by the reaction of formaldehyde and carbon monoxide in the presence of a hydrogen fluoride catalyst. A preferred source of carbon monoxide in that process is synthesis gas -comprising carbon monoxide and hydrogen. This synthesis gas can also act as a source of inert gas for the purification step. Alternatively, the carbon monoxide-depleted synthesis gas which is recovered after the reaction of carbon monoxide with formaldehyde to form glycolic acid can be used as a source of inert gas for the purification step.

The following example further illustrates practice of the present -invention.

EXAMPLE In the following example, crude glycolic acid containing about 40% by weight hydro gen fluoride was subjected to nitrogen stripping for about 2 hours at 23"C to 939C and then for -about 6 hours at 93"C to prepare a product containing 4.3% weight hydrogen fluoride. Stripping for 5 additional hours at 112 C removed only a minor amount of the hydrogen fluoride from this mixture. However, after stripping the mixture for 1.5 additional hours at 1430 C, 'the hydrogen fluoride content of the product decreased to about 2 ppm. Further gas stripping at 143"C reduced the hydrogen fluoride content of the product to 0.1-0.2 ppm of hydrogen fluoride. These results are plotted in the Figure. The Figure is a plot of the percent hydrogen fluoride remaining in the reaction mixture after nitrogen stripping for a given number of hours at either 112"C or 143"C. The plot is on a log scale in order to cover the entire concentration range of 1 ppm to 5% (50,000 ppm). As seen from the Figure, the influence of temperature on crude glycolic acid purification is extremely significant.

WHAT WE CLAIM IS:- 1. A method of reducing to below 4% by weight the hydrogen fluoride catalyst content of a crude glycolic acid (as hereinbefore defined), which comprises effecting purification of the crude glycolic acid by removal of hydrogen fluoride in the vapour state at a temperature in excess of 1300C.

2. A method according to Claim 1, wherein purification is effected by inert gas stripping.

3. A method according to Claim 2, wherein the inert gas stripping is effected with a mixture of carbon monoxide and hydrogen as the inert gas.

4. A method according to Claim 1, wherein purification is effected by distillation.

5. A method according to Claim 1, 2, 3 or 4, wherein the purification is effected at a temperature of from 90 to llO"C. for a period of time sufficient to obtain a glycolic acid product having about 4% by weight hydrogen fluoride and subsequent purification is effected at a temperature in excess of 1300C.

6. A method according to any preceding claim, wherein the purification is effected at above 1300C for a time sufficient to reduce the hydrogen fluoride content to less than 100 parts per million.

7. A method according to any preceding claim, wherein the purification is effected at a temperature of at least 1400C.

8. A method in accordance with Claim 1 for purifying crude glycolic acid, substantially as described in the foregoing Example.

9. Glycolic acid containing less than 4% by weight of hydrogen fluoride catalyst, whenever obtained by the method claimed in any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. mixture. However, after stripping the mixture for 1.5 additional hours at 1430 C, 'the hydrogen fluoride content of the product decreased to about 2 ppm. Further gas stripping at 143"C reduced the hydrogen fluoride content of the product to 0.1-0.2 ppm of hydrogen fluoride. These results are plotted in the Figure. The Figure is a plot of the percent hydrogen fluoride remaining in the reaction mixture after nitrogen stripping for a given number of hours at either 112"C or 143"C. The plot is on a log scale in order to cover the entire concentration range of 1 ppm to 5% (50,000 ppm). As seen from the Figure, the influence of temperature on crude glycolic acid purification is extremely significant. WHAT WE CLAIM IS:-

1. A method of reducing to below 4% by weight the hydrogen fluoride catalyst content of a crude glycolic acid (as hereinbefore defined), which comprises effecting purification of the crude glycolic acid by removal of hydrogen fluoride in the vapour state at a temperature in excess of 1300C.

2. A method according to Claim 1, wherein purification is effected by inert gas stripping.

3. A method according to Claim 2, wherein the inert gas stripping is effected with a mixture of carbon monoxide and hydrogen as the inert gas.

4. A method according to Claim 1, wherein purification is effected by distillation.

5. A method according to Claim 1, 2, 3 or 4, wherein the purification is effected at a temperature of from 90 to llO"C. for a period of time sufficient to obtain a glycolic acid product having about 4% by weight hydrogen fluoride and subsequent purification is effected at a temperature in excess of 1300C.

6. A method according to any preceding claim, wherein the purification is effected at above 1300C for a time sufficient to reduce the hydrogen fluoride content to less than 100 parts per million.

7. A method according to any preceding claim, wherein the purification is effected at a temperature of at least 1400C.

8. A method in accordance with Claim 1 for purifying crude glycolic acid, substantially as described in the foregoing Example.

9. Glycolic acid containing less than 4% by weight of hydrogen fluoride catalyst, whenever obtained by the method claimed in any preceding claim.