DE1253259B - Process for the production of pure gaseous formaldehyde - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

C 07c

German class: 12 ο -7/01

Number: 1253 259

File number: S 97999IV b / 12 ο

Filing date: July 5, 1965

Open date: November 2, 1967

The invention relates to a process for the production of pure gaseous formaldehyde by absorption of formaldehyde from gases of formaldehyde synthesis in a polyhydric alcohol, chemical Decomposition of the semi-formal formed and recycling of the polyhydric alcohol recovered into absorption.

The production of pure anhydrous formaldehyde gas is of great technical interest because the pure formaldehyde can be polymerized to thermoplastics with good properties. The pure gaseous Formaldehyde is also an important chemical intermediate.

Normally, formaldehyde is produced by the catalytic oxidation of methanol with air in the gas phase manufactured. In a conventional procedure, a reaction gas of 5 to 6 percent by weight is obtained Formaldehyde, 7 to 8% water, small amounts of formic acid and other impurities, The remainder is oxygen and nitrogen.

The formaldehyde is normally obtained from the reaction gas by absorption in water, whereby one arrives at formaldehyde solutions with a concentration between 37 and 60%.

Different cleaning methods for the formaldehyde are also known. These deal with the conversion of formaldehyde solutions 37 to 60% formaldehyde to paraformaldehyde or a-polyoxymethylene, which then after removal is thermally decomposed by water. This process is expensive, especially since there are also further cleaning steps for Removal of the remaining water after decomposition is required, for example by means of solid desiccants or suitable washing liquids.

Another method is to combine formaldehyde with an aliphatic alcohol Semi-formal order, which is then cleaned, dried and thermally converted into pure gaseous formaldehyde and free alcohol is decomposed. The alcohol can then be reacted with further formaldehyde be, etc. All of these methods relate to the purification of formaldehyde solutions and are e.g. B. in the USA. Patent 2,848,500 and German Patent 1151 250. These procedures are all accompanied by high energy costs as large amounts of water have to be removed.

According to the method of Belgian patent 615 778, the formaldehyde gas obtained by methanol oxidation is first passed directly through a packed column which has a temperature of 30 to 100.degree. A water solution containing formaldehyde circulates in the column
gaseous formaldehyde

Applicant:

Perstorp Aktiebolag, Perstorp (Sweden)

Representative:

Dr.-Ing. E. Hoffmann, Dipl.-Ing. W. Vain
and Dr. rer. nat. K. Hoffmann, patent attorneys,
Munich 8, Maria-Theresia-Str. 6th

Named as inventor:

Per Gustaf Magnus Flodin,

Dr. phil. Carl-Axel Edvard Sjögreen, Perstorp

(Sweden)

Claimed priority:

Sweden of August 27, 1964 (10 290)

manure with a vapor pressure of less than 2 Torr at 100 ° C, preferably pentaerythritol or trimethylolpropane. The water content in this solution is about 8 to 15%. Part of the formaldehyde content of the reaction gas is absorbed in this solution, while the residue is absorbed in the water in the top of the column or in a separate column. In addition to the water-formaldehyde solution, the residue from the decomposition stage, ie alcohol with smaller amounts of dissolved formaldehyde, is also passed into the top of the packed column. The solution withdrawn from the bottom of the packed column contains about 8 to 15% water. Because of this high water content, which cannot be reduced without high losses in such a two-stage process, the mixture must be dehydrated by evaporation in a vacuum before thermal decomposition. The dryness half Formal is then decomposed in a vacuum at 120 to 150 ⁰ C in a Pyrolysesäule and yields pure gaseous formaldehyde and a dilute formaldehyde-alcohol solution, which is recycled to the absorption stage.

This process also uses large amounts of energy to get water out of the water Eliminate absorption of the recovered solution. as

709 680/433 I '

Vacuum distillate becomes an approximately 20% formaldehyde solution gained, which has only a very limited application and must be viewed primarily as a loss. Evaporation in a vacuum also requires expensive equipment.

It is therefore the aim of the invention to create a method of the type described above which does not adhere to the disadvantages of known methods and which can be carried out in a relatively simple and operationally reliable apparatus and which allows the production of a pure gaseous formaldehyde of high concentration. According to the invention, this object is achieved with a process which is characterized in that the gas mixture in a first stage in countercurrent with a circulating in this stage, of 0.5 to 3 percent by weight of water, 30 to 60 percent by weight of formaldehyde, the remainder alcohol existing liquid at 80 to 130 ⁰ C, then treated in a second stage in countercurrent with a formaldehyde-alcohol mixture circulating in this stage, more water-containing than the first absorption ^{liquid, at 40 to 80 0} C and the residual gas in a third stage in countercurrent Water washes at 35 to 55 ^° C., the absorption liquid from stage 2 being passed to stage 1 and the washing liquid from stage 3 - together with the residue from the thermal decomposition of the absorption liquid withdrawn from stage 1 - being fed to the second stage.

Is preferably carried out the absorption in the first stage at a between 90 and 120 ° C, in the second stage at a between 50 and 65 ° C and in the third stage at a Hege ends between 40 and 5O ⁰ C by temperature.

According to the invention, a part of the in the first Step, the circulating alcohol mixture is removed for further purification, while the The formaldehyde-alcohol mixture of the second stage passes into the mixture circulating in the first stage and the formaldehyde-water equipment of the third stage together with the formaldehyde-containing Alcohol residue from the decomposition stage passes into the mixture circulating in the second stage. at this process produces a formaldehyde-alcohol mixture with such a low water content, that in certain cases it goes directly to the decomposition stage can be directed. The water content in this outgoing mixture is approximately only 0.5 to 3% JThis is an advantage over previous methods considerable advantages in which the removal of water is one of the most difficult and expensive work processes and which also included large losses of formaldehyde. In certain cases the half acetal obtained according to the invention should also be dried. This drying can e.g. B. done with the help of solid desiccants or a countercurrent of dry air.

Further details of the invention emerge from the following description of the illustrated in the drawing Reaction schemes.

The reaction gas from the formaldehyde reactor (not shown) is passed into the packed column 1 via line 2. The temperature at the bottom of the column is 80 to 130 ° C, preferably 100 to HO ⁰ C.

A formaldehyde-alcohol-water mixture 3, which consists of 0.5 to 3 percent by weight of water, 30 to 60 percent by weight of formaldehyde, the remainder being alcohol, circulates in the column. The gas 4 escaping from the top of the column 1 is passed into a second column 5, for example a column packed with random packing in its bottom area, which is kept at 40 to 80 ^° C., preferably 50 to 65 ^° C. In this column, part of the formaldehyde in the gas mixture is absorbed in a circulating formaldehyde-alcohol-water mixture 6 that is more water-containing than the first absorption liquid. That from the

ίο head escaping gas 7 is passed into a column 8, z. B. a soil column, the top temperature of which is 35 to 55 ⁰ C, preferably 40 to 50 ⁰ C, is. Water 9 is added to the top of this column 8, and a formaldehyde solution 10 is drawn off at the bottom, which is passed to the top of the column 5 together with the remaining liquid 11 from the decomposition stage 12. The air, which contains water vapor and traces of gaseous formaldehyde, flows from the top of the column 8 into the outside air. Part of the mixture 3 circulating in the column 1 is passed either directly or after drying in the dryer 13 to the decomposition stage 12, where pure formaldehyde 14 is obtained. With this three-stage absorption, a semi-formal is obtained with a low content of water and a nevertheless high content of formaldehyde.

So there is one in the high temperature column 1 Drying of the semi-formal mixture from column 5 without significant escape of the formaldehyde instead of. Even with a small amount of column effects, it is easy to check the water content of the gas close to equilibrium vapor pressure above the circulating semi-formal. The equilibrium vapor pressure of the formaldehyde above the circulating semi-formal is quite high 200 to 250 torr.

With regard to formaldehyde, however, there appears to be some resistance in the transition from the to give liquid in the gaseous phase. The pillar effect, which is created as a relationship between the Change in concentration and driving force (difference between the equilibrium vapor pressure and the actual partial pressure of the gas) is three to four times greater for water than for the formaldehyde. This can be explained by the assumption that the speed of chemical Decomposition of the semi-formal at the prevailing temperatures is so low that it slows it down restricts with which the formaldehyde to the

So gas phase can be passed.

As mentioned above, the semi-formal obtained by absorption can go straight to the decomposition stage are performed, whereby a 96- to 97% formaldehyde gas is obtained. When a purer formaldehyde gas is desired, the semi-formal must be dried, e.g. B. in contact with solid Desiccants or in countercurrent contact with dry air. The dry semi-formal is then pyrolyzed in a manner known per se.

Those alcohols are particularly suitable for the process which have a high boiling point, preferably above about 200 ^° C. at normal pressure, and which have a vapor pressure of preferably less than 2 Torr at 100 ^° C. The following alcohols can be mentioned as examples: glycerine, higher polyglycols such as diethylene, triethylene or dipropylene glycol, triethanolamine, 1,2,3-butanetriol, 1,2,4-butanetriol, 1,2,3-pentanetriol, 1 , 2,4-pentanetriol, 2,3,4-pentane

triol, 1,2,5-hexanetriol, 2,2-dimethyl-1,3-propanediol, Trimethylolpropane, trimethylolethane, erythritol, ramnitol, Sorbitol, mannitol and pentaerythritol.

example

At the bottom of the first packed column J, 6000 l / h of gas, consisting of 6% formaldehyde, 7% H ₂ O, the remainder air, are delivered. In the column circulate 6 kg of a solution consisting of 50 ° / ₀ formaldehyde, 1.6% water, balance trimethylolpropane, consists. From this solution, 1 kg / h was taken out as a bottom product. The column worked at 102 "C.

The gas emerging from the top of column 1 was introduced at the bottom of the next packed column. The main part of formaldehyde in the gas was here in the circulating semi-formal water mixture, consisting of 48% formaldehyde, 22% water and the remainder trimethylolpropane, absorbed, of which ao 162 kg / h were fed into the head of column 1. The column worked at 60 "C.

The gas emerging from the top of the column S was fed into the lowest floor of the column 8, into which 0.12 kg / h of H ₂ O were fed from above. In this column, the rest of the formaldehyde was absorbed in the water. The gas coming from the top of column 8 contained only traces of formaldehyde and 0.42 kg / h of H ₂ O in addition to air.

At the bottom of the column 8, a formaldehyde-water solution was drawn off and into the top of the Column 5 together with that obtained as a residue after pyrolysis of the dried semi-formal Liquid fed. The pyrolysis residue contained 10% formaldehyde, the remainder trimethylolpropane and was fed at a rate of 0.54 kg / h.

The bottom product from the column 1 is passed either directly or via a dewatering step 13 to the pyrolysis stage 12 where it is thermally decomposed at 130 to 15O ⁰ C.

If the liquid from the column 1 is pyrolyzed directly, a gas with a purity of about 96%.

The three absorption columns shown can be used to a single column including the three mentioned absorption stages.

Claims (2)

Patent claims: 1. A process for the production of pure gaseous formaldehyde by absorption of formaldehyde from gases of the formaldehyde synthesis in a polyhydric alcohol, chemical decomposition of the semi-formal formed and recycling of the recovered polyhydric alcohol in the absorption, characterized in that the gas mixture in a first stage in countercurrent with a liquid circulating in this stage, consisting of 0.5 to 3 percent by weight water, 30 to 60 percent by weight formaldehyde, remainder alcohol at 80 to 130 ⁰ C, then in a second stage in countercurrent with one circulating in this stage, compared to the first Absorption liquid containing water-containing formaldehyde-alcohol mixture is ^{treated at 40 to 8O 0} C and the residual gas is washed in a third stage in countercurrent with water at 35 to 55 ° C, the absorption liquid from stage 2 to stage 1 and the washing liquid from stage 3 - together with the residue from the thermal Decomposition of the absorption liquid withdrawn from stage 1 - fed to the second stage. 2. The method according to claim 1, characterized in that the absorption in the first stage at 90 to 120 ⁰ C, in the second stage at 50 to 65 ⁰ C and in the third stage at 40 to 50 ⁰ C is carried out. 1 sheet of drawings 709 680/433 10.67 © Bundesdmckcrei Berlin