DE1115234B - Process for the preparation of acetic acid or its mixture with methyl acetate - Google Patents

Description

Process for the preparation of acetic acid or its mixture with methyl acetate It is known that by reacting methanol or methyl acetate with carbon oxide acetic acid under pressure and at elevated temperature in the presence of catalysts or mixtures thereof with methyl acetate. The reaction is carried out appropriately in the presence of water and uses carbonyl-forming metals as catalysts the iron group or its compounds and elementary or chemically bound Halogen, advantageously iodine or an iodine compound.

Carrying out the reaction on an industrial scale showed that side reactions produce carbon dioxide and methane. These inert gases enrich when the process is carried out continuously in the carbon monoxide, which is in the interest the best possible aeration of the reaction space is usually carried out in a circle, and thereby cause a considerable slowdown in the reaction. To the distance the inert gases must therefore always have a proportion of the reaction gas as exhaust gas from the Reaction chamber can be removed.

It was found that with this exhaust gas due to its high volatility valuable components of the reaction mixture, such as methyl iodide, methyl acetate, dimethyl ether and acetaldehyde, are lost, resulting in lower yields of acetic acid on the methanol used, and higher catalyst losses, especially iodine, has the consequence and also the further use of the exhaust gas, z. B. for power generation, difficult. The substances contained in the exhaust gas can be washed out with water or can only be partially obtained from this by deep-freezing.

It has now been found that you can use acetic acid or a mixture thereof Methyl acetate by reacting methanol or methyl acetate with carbon oxide at increased Temperature and elevated pressure in the presence of catalysts are particularly advantageous and at the same time losses of the valuable components mentioned above the exhaust gases largely avoided if one removes the resulting inert gases withdrawn exhaust gas at low temperature and low pressure with a part or the whole amount of the methanol or methyl acetate required for the reaction in contact brings and introduces this together with the absorbed substances into the reaction chamber. In this way, almost all organic compounds contained in the exhaust gas are in the Dissolved methanol and thus fed back into the synthesis. All the trouble that when the exhaust gas is frozen by ice formation and / or when washing with non-reactive solution average through the then necessary processing of the used Solvents can be avoided.

The scrubbing of the exhaust gas is expedient at a temperature of - 30 to + 30 ° C carried out. However, lower or higher temperatures can also be used be applied. The cooling that may be required can be carried out in a known manner Way, e.g. B. by heat exchange of the methanol with a cooled salt solution or with a low-boiling coolant, e.g. B. ammonia, can be effected. A large part of the energy required for the process to cool the solution can by heat exchange between the washing tower supplied and discharged Liquid can be obtained. The washout can take place at normal pressure or at increased pressure Pressure up to 10 atm. It is advisable to wash at the pressure to which the The reaction mixture is depressurized after leaving the reaction chamber. Is this If the pressure is above 1 atm, the washing solution and the reaction mixture are let down from the pressure prevailing in the washing tower to normal pressure again gas free. Even this gas can be washed with methanol and the solution returned to the reaction vessel will. The exhaust gas leaving the scrubbing tower and expanded to normal pressure contains still methanol vapor. This methanol can then be removed by washing with water removed from the exhaust gas. The methanol can be extracted from the methanol-containing washing water recover by distillation.

Instead of pure methanol or methyl acetate, the washing liquid can also be used Undistilled crude methanol, such as that obtained from methanol synthesis, is used will.

The solutions obtained are introduced into the reaction chamber in the same way introduced like methanol.

The washing tower, in which the exhaust gases by conventional methods, e.g. B. by built-in bubble cap trays or packing, in intimate contact with the washing solution must be brought, can in cocurrent or preferably in countercurrent process operate.

The procedure is illustrated in the following example with reference to the drawing explained in more detail: Example In the reaction tube 1 are with the help of the feed pump 2 hourly 4001 of a mixture of 35 percent by weight methanol, 15 percent by weight Methyl acetate, 8 percent by weight of higher molecular weight esters, 35 percent by weight of water, 5 percent by weight acetic acid, 0.8 percent by weight iodine (in the form of hydriodic acid and methyl iodide) and 0.2 percent by weight cobalt (in the form of cobalt acetate). At the same time so much carbon is injected with the compressor 3 that the pressure in the reactor is about 650 ata. The temperature is kept at 230 "C. That out The mixture flowing off the reaction tube together with excess gas is placed in the cooler 4 cooled to 25 "C and separated from the gas in separator 5. From the gas space of the separator 5 150 m3 of gas are drawn off every hour, still under the operating pressure of 650 ata and pressed back into the reaction tube from below with the aid of the circulating gas pump 8.

The reaction mixture is discharged from the separator 5 via the valve 6 relaxed in the container 7 to 2 ata and then via the valve 9 of the work-up 16 supplied. The acetic acid and the over 1180 ° C are here by distillation boiling by-products separated from the other components of the reaction mixture and fractionated to pure acetic acid. Water, methyl acetate, not smaller amounts converted methanol, by-products boiling below 118 "C and catalyst combined with the methanol flowing out of the washing tower 11 in the mixer 17. To the The solution is the lost amount of catalyst and, if necessary, fresh methanol admitted. The mixture obtained is fed back into the synthesis.

When the reaction mixture is released from the separator 5 in the container 7 is every hour 20 m3 of exhaust gas that was previously dissolved in the mixture, set in freedom. This exhaust gas is still under 2 ata pressure from the container the line 10 initiated at the foot of the washing tower 11. At the same time be on the head this tower with the help of the pump 12 110 1 methanol, which in the cooler 13 to -10 "C was cooled, introduced and with the upward flowing Exhaust in intimate contact brought. The scrubbed gas is depressurized at the head of the scrubbing tower 11 via the valve 14.

The solution of the substances washed out of the exhaust gas in methanol is withdrawn via the valve 15 and fed to the mixer 17.

The following table shows the amount of valuable organic compounds in grams, which before and after washing in a manner according to the invention in 1 m3 of exhaust gas are included.

Content of the exhaust gas in g / Nm3 gas Contains 1 Nm3 exhaust gas before the I after the Wash out | Wash out Methyl iodide 0 7.3 g 0.1 g Methyl acetate 95 g 0 g Acetic acid 6 g 0 g Acetaldehyde 71 g 5 g Dimethyl ether 30 g 3 g In the production of 100 kg of acetic acid, only 20 g of iodine are lost.

If the exhaust gas is not washed out under otherwise identical conditions, the iodine loss is 150 to 200 g per 100 kg of acetic acid produced. as well The iodine loss remains high if water is used as the washing liquid instead of methanol or methyl acetate is used.

Even if the exhaust gas has cooled down to -10 "C without washing only about two thirds of the iodine contained in the exhaust gas can be recovered will.

Claims (4)

PATENT CLAIMS: 1. Process for the production of acetic acid or their mixture with methyl acetate by carbonylation of methanol or methyl acetate at elevated pressure and elevated temperature in the presence of catalysts, thereby characterized in that the exhaust gas withdrawn from the reaction chamber is taken at a low temperature Washed with methanol or methyl acetate and the resulting solution into the reaction chamber returns. 2. The method according to claim 1, characterized in that the Carries out washing of the exhaust gas at 1 to 10 atmospheres. 3. The method according to claim 1 and 2, characterized in that one the washing is carried out at -30 to +30 "C. 4. The method according to claim 1 and 3, characterized in that one the gas released when the washing solution is depressurized also with methanol or methyl acetate washes and the solution obtained is returned to the reaction vessel.