Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-05-25
2001-05-08
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S422000, C568S449000, C568S457000, C568S458000, C568S465000
Reexamination Certificate
active
06229053
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for purifying formaldehyde, that is in particular for the continuous purification of formaldehyde, the use of formaldehyde purified in this manner for preparing plastics, in particular polyacetals and polyoxymethylenes, and plastics which have been prepared using the formaldehyde thus purified.
Formaldehyde is an important feedstock of the chemical synthesis of many products such as plastics, fertilizers, dyestuffs, and in mirror manufacture and textile finishing. It is therefore synthesized industrially in large amounts. Accordingly, a multiplicity of different processes for preparing formaldehyde are known, the synthesis from methanol having the greatest importance. In this synthesis, by means of an oxidative dehydrogenation, for example on an oxidized copper wire, formaldehyde is synthesized by oxidizing methanol with air. In this synthesis, 30-40 % strength aqueous solutions of formaldehyde are usually obtained, which already, from the preparation process, comprise the methanol necessary for stabilization.
DESCRIPTION OF THE PRIOR ART
German patent application 197 22 774.0 and the international patent application PCT/EP98/03084 corresponding thereto describe the preparation of formaldehyde by direct dehydrogenation of methanol at a temperature of 300 to 1000°C using a catalyst. In this process, a circulated gas stream, which predominantly consists of hydrogen and carbon monoxide, is passed through the reactor. In this manner, a mixture is formed which comprises formaldehyde, hydrogen, carbon monoxide and by-products such as water, methanol, methyl formate, methylal, carbon dioxide and methane at contents <5% by weight.
However, all these processes for preparing formaldehyde have the disadvantage that, using them, formaldehyde is not obtained as a pure substance, but as a mixture containing unwanted by-products and unreacted feedstocks. However, for a plurality of industrial syntheses, such as the preparation of engineering plastics, formaldehyde in a highly pure state is required, since the quality of the product produced using it is also dependent on the purity of the gaseous formaldehyde, in addition to the reaction conditions selected.
Therefore, a multiplicity of processes have already been developed to separate off formaldehyde from an aqueous solution and prepare high-grade products using it. In such cases, formaldehyde is customarily taken off from an aqueous solution and then further purified in the gaseous state. The underlying engineering and material properties of formaldehyde are described in J.F. Walker, “Formaldehyde”, Reinhold Publishing Corporation Chapman and Hall, Ltd., London.
In addition, DE-A 10 90 191 describes a process for the continuous purification of gaseous formaldehyde, in which the formaldehyde is brought into contact with a flowing surface of a cold liquid formaldehyde/hemiacetal solution, which itself was formed by reacting formaldehyde with a primary and secondary alcohol. In this process, 60 -98% of the liquid contaminants from the formaldehyde gas are absorbed by the cold liquid solution. However, removal of substances which are usually gaseous, such as N
2
, O
2
, CO
2
, and CO is not possible by this process, as is expressly emphasized.
U.S. Pat. No.2,848,500 describes a process according to which a formaldehyde/water mixture is reacted with an alcohol whose boiling point is above 95°C to form the hemiformal, the resulting mixture is dehydrated under reduced pressure to form a hemiformal solution of considerably lower water content and is then pyrolyzed, the formaldehyde being released from the hemiformal, whereupon by partial condensation of the pyrolysis products, the alcohol is recovered as liquid and formaldehyde as vapor. However, in this process formaldehyde is removed together with water, as a result of which the yield is considerably decreased. Therefore, further process stages are required in order to recover the formaldehyde removed from the mixture and separate it off from the water. For this reason, DE-A 16 18 772teaches adding at least one alkali metal salt of an organic acid to the mixture before dewatering.
U.S.Pat. No. 3,510,525 describes a process for preparing gaseous formaldehyde, according to which a gaseous mixture of air and aqueous formaldehyde is reacted in countercurrent with a circulating dry mixture of formaldehyde and polyhydric alcohol to give the hemiacetal. The remaining gaseous mixture, which comprises unreacted formaldehyde, is reacted in a second step at a lower temperature with an aqueous formaldehyde/alcohol mixture, and finally, in a third step, the remainder of the gaseous formaldehyde still present is dissolved in water. The hemiformal solution of the first step is then fed to the pyrolysis, if appropriate after an upstream drying using solid desiccants or using dry air or another drying method. DE-A 41 37 846 describes a process for preparing substantially anhydrous and impurity-free formaldehyde from aqueous formaldehyde solutions, the aqueous solution being extracted with an alcohol, the alcoholic hemiacetal solution thus obtained being freed from water and thereafter being thermally cleaved in a subsequent solvent exchange column in the presence of a solvent having a lower boiling point than the alcohol used in the extraction and formaldehyde in a mixture with the solvent being taken off as overhead product. DE-A 12 49 846 describes the thermal decomposition of substantially pure hemiformals in columns having heated plates at temperatures between 20 and 250°C and residence times of 1 to 60 min., the hemiformal being forced along the heating surfaces by spiral internals or internals constructed as baffle plates.
DESCRIPTION OF THE PREFERRED EMBODIMENT
However, all the processes described above have disadvantages with respect to the purity of the formaldehyde obtained or in the yield, or they cannot be carried out continuously. The object therefore underlying the invention was to provide a purification process for formaldehyde which does not have the above described disadvantages and using which, even in the continuous process, formaldehyde is obtainable in a highly pure state, with formation of hemiacetals in a simple manner without significant loss of yield.
This is achieved according to the invention by the formaldehyde being scrubbed with an alcohol in an absorption unit and the formaldehyde here being virtually completely dissolved in the alcohol or converted to the hemiformal and simultaneously in a first purification step, gaseous impurities, which are not absorbed by the scrubbing solution, being separated off. The formalde-hyde/alcohol/hemiformal solution is thereafter passed to a desorption unit, in which the residual impurities absorbed in the absorption unit by the alcohol/hemiformal solution are converted into a gaseous state and thus separated off from the hemiformal. The hemiformal purified in this manner is then cleaved in a pyrolysis unit to form the alcohol and formaldehyde and the alcohol and formaldehyde are recovered. In the process according to the invention, formaldehyde is preferably purified in the gaseous state in which it arises directly on synthesis. However, it is also possible to use formaldehyde in other forms, eg. as formalin, but preferably, here also, it is first converted into the gaseous state.
As absorption unit, use is preferably made of absorption columns, in particular surface rectifiers, such as packed columns or plate columns, plate columns having a high holdup capacity and packed columns having a high surface area. Columns which are particularly preferred are bubble cap tray columns, spray columns, trickle columns and columns with a random packing. To achieve the highest possible absorption of formaldehyde, preferably, use is made of columns having internals which provide the highest possible exchange area.
The process according to the invention is preferably carried out in such a manner that the alcohol is introduced into the absorption unit in countercurrent to the formaldehyde. This is beca
Meister Christine
Schweers Elke
Sievers Werner
Connolly Bove & Lodge & Hutz LLP
Padmanabhan Sreeni
Ticona GmbH
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