DD211238A3 - PROCESS FOR OBTAINING ACETALDEHYDE FROM RAW ALDEHYDE GAS - Google Patents

Abstract

The invention relates to a process for the recovery of acetaldehyde of high purity from crude aldehyde gas, in particular from a crude gas mixture with an acetylene, water, crotonaldehyde and acetic acid anfaellt in the Fluessigphasenhydratisierung of acetylene or crude aldehyde gas similar composition. The aim of the invention is to reduce the high energy demand and the amount of wastewater incurred in the recovery of pure acetaldehyde from Rohaldehydgas. The object of the invention is to avoid as far as possible by suitable means the washing out of acetaldehyde u. almost completely remove the acetaldehyde from the crude aldehyde gas. According to the invention, the crude aldehyde gas is compressed and in a first rectification step, the separation of the water, the acetic acid and the crotonaldehyde of acetaldehyde and acetylene takes place. In a second rectification stage, the separation of most of the acetaldehyde from acetylene, wherein the distillate is advantageously supplied to a degree of cooling at 0 degrees at 15 degrees C of a gas scrubber and the resulting condensate is returned to the distillation.

Description

Title of the invention

Process for the recovery of acetaldehyde from crude aldehyde gas

Field of application of the invention

The invention relates to a process for the recovery of acetaldehyde of high purity from Rohaldehydgas "in particular from a crude gas mixture with an acetylene content * 35 mol%, water, crotonaldehyde and acetic acid, as z. B. obtained in the Flussigphasenhydratisierung of acetylene, lind simultaneous recovery of acetylene.

Characteristic of known technical solutions

It is known that in the Plüssigphasenhydratisierung of acetylene to acetaldehyde to avoid the formation of polymerization and condensation products of acetaldehyde this is operated with excess acetylene · From the 95-100 ° C hot reaction gases in a cooler mainly water is condensed and fed to the reactor , In

2Q subsequent coolers, especially acetaldehyde-containing water is condensed, which is collected in a crude aldehyde container. The crude aldehyde gas is cooled to 25-35 ° C. In an aldehyde scrubber, the aldehyde becomes acety-

washed out lsättsättigtera water, wherein a _s 8 - 12% aldehyde water is obtained, which is also collected in the crude aldehyde. The gas leaves the scrubber and returns via a gas cleaning, z. B, pressure washing, back into the cycle, where the used acetylene is replaced by fresh gas. The separation of the acetaldehyde from the aqueous solution is carried out by distillation. At the low boiling point of acetaldehyde, sufficient cooling with service water is not possible. The use of cold is often uneconomical, so the distillation is carried out under pressure. This ensures that at a boiling temperature of 44 ° G, cooling water of 20 ° C can be used.

The acetaldehyde water can be fractionated in a single column. However, it is better to work in two columns, especially if crotonaldehyde is to be obtained as a by-product.

In a crude column (or also known as pre-column) is distilled at a pressure of 100-130 kPa and a sump temperature of about 120 ⁰ C, the AIdehydwasser \ , so that the distillate accumulates an enriched in acetaldehyde gas. This is in a rectification column (main column), which operates with a reflux ratio of 1 - 1.3 and at a head temperature of 44 ° G, separated, so that the distillate obtained pure acetaldehyde with acetylene. To the acetylene or dissolved out of the acetaldehyde to be removed NEN _f is boiled it in a degassing column at a boiling temperature of 44 ° G. The released cooking gas passes through a pressure-holding valve on watch towers, in which acetaldehyde is washed out of the cooking gas. The exhaust gas is supplied to the circuit via the acetylene gas purification. The acetic acid is for the most part with the bottom of the crude column in the wastewater. The Crotonaldehyd falls to the largest

Part at the bottom of the main column, which is worked up in a separate distillation. This method has the disadvantage that most of the acetaldehyde dissolved in water and then the acetaldehyde must be distilled off from the water. That means a high energy consumption. The production of 1000 kg of acetaldehyde requires 3800 MJ of water to heat the water, while only 570 MJ are needed for the actual distillation work. Therefore, all efforts have been made so far to obtain the highest possible aldehyde concentrations in the absorption. However, this is not unlimited by the limited solubility of the acetaldehyde in water

 In another known method according to US Pat. No. 3,291,839, crude aldehyde gas is likewise cooled to about 38 ° C. and the resulting condensate, together with the compressed crude aldehyde gas remaining at 119 ° C., is fed to a two-stage rectification which operates at a pressure of approx. 1000 kPa. Due to the high pressure resulting from the self-decomposition of acetylene special safety problems. Rectification requires the use of brine with a temperature below -25 ° C. Under the condition that similar purities are to be achieved, as in the previously described method, an even greater expenditure of energy is required for the separation of substances

An effective recovery of by-products, such as, for example, crotonaldehyde, is not possible by this method.

Object of the invention

The aim of the invention is to substantially reduce the high energy consumption in the recovery of pure acetaldehyde from crude aldehyde gas, At the same time the amount of wastewater generated in the process is to be reduced so that the cost of the wastewater treatment plant is much smaller.

Explanation of the essence of the invention

The technical problem solved by the invention

The invention is based on the object by suitable means to avoid the washing out of acetaldehyde or the high water content at the entrance to the rectification as far as possible and to separate the acetaldehyde almost completely from the crude aldehyde.

- essence of the invention

After lengthy research it has been found surprising that if the cooled crude aldehyde gas is no longer washed but is fed directly to a rectification plant having certain pressure and temperature conditions, considerable energy savings will be achieved.

According to the invention the object is achieved in that from the 95 - 100 ° C hot reaction gases coming from the reactor, condensed in a conventional manner in a cooler mainly water and recycled to the reactor. The dehydrated gas is cooled in other coolers to 20 - 40 ° C, with crude aldehyde consisting of acetaldehyde and

Water, accumulates «

The cooled gas is then compressed to a pressure of 250,300 kPa and fed directly or by cooling to a two-stage distillation while maintaining this pressure at a temperature of 50-90 ° C, in which the water, acetic acid, crotonaldehyde and acetylene from Acetaldehyde is separated. In the first stage, by rectification at pressures of 250-300 kPa and at bottom temperatures in the range of 100 to 130 ° C and head temperatures of 35 to 45 ° G, water, crotonaldehyde and acetic acid separated from acetylene and acetaldehyde, wherein less than 10 Mol % acetaldehyde and almost no acetylene incurred. Without significant effort, the acetaldehyde concentration can be lowered below 1 mol%. The cup temperature can be between 35 and 45 ° C, depending on the acetylene concentration and top pressure. Depending on the permissible water content in the product acetaldehyde is the number of trays required or the height of the column and the reflux ratio is determined in a known manner. The condensate accumulating at the top of the condenser, consisting essentially of acetaldehyde and acetylene, is fed to a second separation stage.

By rectification at 250-300 kPa is at bottom temperatures of 40-60 ° C and a condensation temperature of 0-35 ° C separation of the distillate of the first rectification stage in pure acetaldehyde, which is withdrawn as the bottom product, and distillate, consisting of acetylene and a Proportion of acetaldehyde to about 50 mol% achieved. The number of trays required or the height of the column and the reflux ratio is to be determined in a known manner as a function of the permissible acetylene content in the product acetaldehyde.

The pure acetaldehyde is cooled in a heat exchanger so that it then stored almost without pressure

kaaru To reduce the acetaldehyde in the gaseous distillate of the second rectification this is cooled at 250 - 300 kPa to 0 - 15 ° C This can be done by maintaining a Kemptemperatur the second rectification stage from 0 to 15 ° C with simultaneous cooling with recooling water or by cooling the distillate to 0 to 15 ° C at a higher head temperature wherein the resulting condensate is returned to the second rectification stage. From the distillate of the second rectification stage, the acetaldehyde is advantageously absorbed under pressure of less than 300 kPa with water. The crude aldehyde obtained is worked up together with the crude aldehyde obtained by cooling the crude aldehyde gas to 20-40 ° C. by known processes or fed to a crude column. in which the separation of a large part of the water takes place. The distillate can then be compressed simultaneously with the crude aldehyde gas and returned to the first rectification stage.

In addition, the resulting crude aldehyde can be worked up by the old method. Inert gases from the reactor, such as carbon dioxide, are ultimately removed with the acetylene exhaust gas.

The particular advantages of the method according to the invention are that the absorption or solution in water is eliminated for the major part of the acetaldehyde and thus large amounts of steam for outgassing of the acetaldehyde from the water are not needed and the amount of waste water is substantially reduced.

embodiment

The method according to the invention is explained in more detail below using an exemplary embodiment.

FIG. 1 shows the flow chart of the process according to the invention

The crude aldehyde obtained in the liquid-phase hydration of acetylene to acetaldehyde is cooled in the coolers 1 and 2, with some of the heavier-boiling components condensing. The resulting condensate in the condenser 1 is recycled to the reactor and the crude aldehyde from the condenser 2 is worked up in a plant by known methods. The gas cooled to about 35 ° C. then has the following composition at a pressure of about 100 to 130 kPa :

15-35 mole % acetylene 3 7 mole % water vapor

0.5-1.5 mole % carbon dioxide

0.5-1.5 mole % crotonaldehyde

0.5 - 1.5 mol % acetic acid balance acetaldehyde

According to the invention, this gas mixture is fed via a compressor 3 with a pressure of about 270 kPa of the first rectification stage. This consists of distillation 4, evaporation 5 and condensation By rectification at a reflux ratio of about 1.0 a sump temperature of about 113 ° C. and a bottom pressure of about 280 kPa and a condensation temperature of about 40 ° C. at one pressure of about 265 kPa a separation of the gas mixture in distillate with all acetylene, carbon dioxide and a large part of the acetaldehyde and bottom product with the water, the acetic acid, the crotonaldehyde and a small part of the acetaldehyde achieved. The distillate is gaseous and is fed to the 2nd rectification stage. This consists of the distillation 7, the evaporation 8 and the condensation

By rectification at a reflux ratio of about 2.5, a bottom temperature of about 50 ° C and a bottom pressure of about 270 kPa and a condensation temperature of about 32 ° C at a pressure of cav 262 kPa separation of the Gas mixture in bottom product with pure acetaldehyde and gaseous distillate with all acetylene, carbon dioxide and a proportion of acetaldehyde of about 50 mol % achieved.

The pure acetaldehyde is cooled in a heat exchanger 10 so that it can be stored almost without pressure. In order to reduce the proportion of acetaldehyde in the distillate, this is further cooled in the condenser 11 at approximately 26o kPa to approximately 12 ° C. This results in an acetaldehyde concentration of less than 10 mol % in the outgoing gas. The resulting condensate goes back to the second distillation column 7. From the gas in the gas scrubber 12, the acetaldehyde is absorbed with water. The resulting acetaldehyde and the condensate from the condenser 2 can be worked up in a plant by a known method or it can be fed to a crude column 13, in which the acetaldehyde is completely boiled out of the water, so that an intermediate gas with about 10 mol % of water , The crude column is operated so that the top pressure is adapted to the outlet pressure at the compressor 3, that is between 100 and 130 kPa. By applying this method eliminates the energy consumption for heating the water in distilling off acetaldehyde. This means an energy saving in the form of steam of 3000 MJ in the production of 1000 kg of acetaldehyde, if the workup of the acetaldehyde-containing condensates is taken into account. The required compressor and cooling capacity, on the other hand, is small. The amount of wastewater is reduced to about 20 % compared to the old method.

Claims (3)

invention claims A process for the recovery of pure acetaldehyde from crude aldehyde gas obtained in the liquid phase hydration of acetylene, or Ronaldehydgas similar composition, in particular with high Acetylenz-jWasser-, crotonaldehyde and acetic acid portion by cooling the Rohaldehydgases at 100 to 130 kPa (abs.) 20 - 40 ° C and separation of the condensed acetaldehyde-containing V / assers, compressing the remaining, cooled crude aldehyde gas and two-stage rectification under elevated pressure, characterized in that the remaining crude aldehyde gas to a pressure of 250 - 300 kPa (abs.) Compressed and then a The rectification is carried out in a first stage at a bottom temperature of 100-130 ° C and a top temperature of 35-45 ° C and the resulting overhead product in a second stage at a bottom temperature of 40 oo Is rectified, the distillate of the second St cooled to a temperature of 0 - 15 ° C at a pressure of 250 - 300 kPa, the resulting condensate is recycled to the second rectification stage, while the remaining gaseous distillate is fed to a known Acetaldehydgaswäsche with water. 2. The method according to item 1, characterized in that the aldehyde water obtained in the Acetaldehydgaswäsche distilled together with the obtained in the cooling of the Rohaldehydgases acetaldehyde-containing water, the water separated and the resulting gas is compressed together with the Rohaldehydgas and the first rectification stage is supplied. 3 "method according to item 1 and 2, characterized in that the absorption of the acetaldehyde with water under pressure, but not more than 300 kPa (abs ·) takes place · - For this a sheet drawing -