Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
1999-11-09
2001-03-13
Lambkin, Deborah C. (Department: 1613)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C203S074000
Reexamination Certificate
active
06201136
ABSTRACT:
The invention relates to a process for separating liquid mixtures comprising formaldehyde, trioxane, alcohol, hemiformal formed from the formaldehyde and the alcohol, usual minor components and up to 5% by weight of water. The liquid mixture is distilled batchwise or continuously at reduced pressure, atmospheric pressure or superatmospheric pressure in suitable apparatuses which are to be connected to one another in an appropriate manner, and the trioxane is produced in very high purity. The other materials of value, formaldehyde and alcohol, present in the mixture can likewise optionally be separated off and recycled or used otherwise, while the minor components are ejected.
To prepare engineering plastics, in particular polyacetals such as polyoxymethylene, high-purity trioxane is required. The quality of the plastic, i.e. the achievable degree of polymerization, depends not only on the polymerization conditions but predominantly on the purity of the trioxane.
Various methods are known for preparing trioxane, e.g. using homogeneous or heterogeneous catalysis from aqueous formaldehyde solutions (AT-252913) or heterogeneous catalysis from gaseous formaldehyde in the presence of heteropoly acids (EP-606056). However, regardless of the preparation process, trioxane is never produced as a pure substance but always in a mixture with unreacted formaldehyde, water and small amounts of other components, such as methanol, methyl formate, methylal, formic acid, dioxolane and tetroxane, which are generally termed minor components. For further use of trioxane, for example for polymerization, this must in particular be separated off from the formaldehyde and may contain only small amounts of minor components.
The separation of trioxane from aqueous trioxane/formaldehyde mixtures which contain more than 10% by weight of water is generally current. However, the separation of gaseous mixtures of formaldehyde and trioxane is also known.
Aqueous mixtures have been separated to date in particular via azeotropic distillation (AT-252913 and JP 83-171278). In this process, unreacted formaldehyde can be recirculated to the reactor and there further reacted to form trioxane. However, the azeotrope of trioxane and water which boils at 92° C. and 1 bar and contains approximately 70% by weight of trioxane sets a limiting value on the achievable purity of the trioxane produced by this method. A disadvantage of this method is the danger of solids formation due to polymerization of formaldehyde or formation of paraformaldehyde especially in the region of the column top. To avoid same, all apparatus components must either be heated to temperatures >100° C. (at 1 bar of formaldehyde partial pressure) or be wetted with a liquid.
A further process for separating formaldehyde and trioxane from aqueous solutions is extraction of the trioxane by organic solvents in which trioxane has a higher physical solubility than the formaldehyde. This process is used solely to separate off trioxane from the aqueous phase. As organic extraction media, for example, saturated aliphatic or aromatic hydrocarbons or halogenated hydrocarbons (EP-583907) which are sparingly miscible with water, or not miscible at all, are used. A disadvantage of the extraction is that using the organic solvent an additional auxiliary is introduced into the process and subsequent work-up of the organic phase also is required. A further disadvantage is that considerable proportions of the trioxane also occasionally remain in the aqueous phase. Large amounts of trioxane must therefore be recycled or are lost in the work-up process.
As a further possible method of selective separation from an aqueous phase, crystallization of trioxane from aqueous formaldehyde/trioxane mixtures is described (DE-3508668). The trioxane concentration in the aqueous mixture in this case must be more than 50% by weight.
In the preparation of trioxane by trimerization of formaldehyde from aqueous formalin solutions, generally, the abovementioned processes of azeotropic distillation, extraction and if appropriate crystallization are linked together in a suitable manner in order to obtain trioxane in the high purity required.
For the separation of gaseous formaldehyde/trioxane mixtures, selective absorption processes are known. In these either the formaldehyde is chemisorbed and the trioxane left in the gas phase (GB-1245990) or, vice versa, there is selective physisorption of the trioxane (EP-A-680959). Since no liquid phase has been found in which either only the formaldehyde or only the trioxane is soluble, in the absorption, proportions of the other respective species are also bound. Therefore, in the selective absorption of formaldehyde, a considerable loss of the valuable material trioxane results, whereas in the selective absorption of trioxane, even with this process, the trioxane purity required for the polymerization cannot be achieved.
A process for separating a formaldehyde and trioxane mixture which is present in the gaseous and low-water state is described in the as yet unpublished German Patent Application No. 198 336 20.9. This process comprises absorption in an alcohol and a downstream separation of the trioxane from the alcohol or from the hemiformal by crystallization. The process forms a possible step in a novel process for preparing trioxane from methanol, consisting of the steps of non-oxidative dehydrogenation (DE-3920811), formaldehyde removal (German Patent Applications No. 197 476 47.3 and No. 197 483 80.1), formaldehyde trimerization (EP-A-606056, EP-A-691388) and trioxane removal. However, even the trioxane produced by this process does not have the purity necessary for a polymerization.
The object is to develop a process by which trioxane can be produced in high purity, in particular purity suitable for polymerization. In this process, in particular, the problem of removing trioxane from a liquid trioxane/formaldehyde mixture should be solved. The other materials of value present in the mixture, formaldehyde and alcohol, should likewise as far as possible be able to be separated off and recycled or used in other ways, while the minor components, some of which are harmful for the polymerization, should be ejected.
This object is achieved according to the invention by a liquid mixture of formaldehyde, trioxane, an alcohol, corresponding hemiformals, small amounts of lower- and higher-boiling minor components and a maximum of 5% by weight of water being distilled in a suitable manner in one or more appropriately connected apparatuses at reduced pressure, atmospheric pressure or superatmospheric pressure and thus the trioxane being produced in a highly pure form, it being able to arise, depending on process procedure, as distillate, bottoms product or sidestream takeoff.
The invention therefore relates to a process for producing trioxane, the trioxane being separated off from a liquid mixture comprising trioxane, formaldehyde, alcohol, hemiformals formed from the formaldehyde and the alcohol and a maximum of 5% by weight of water by distillation and produced in highly pure form.
The invention also relates to the use of the highly pure trioxane thus produced for preparing polymers and fuels or for producing formaldehyde by depolymerization of the trioxane.
It is essential for the process according to the invention that the liquid mixture to be separated is low in water or anhydrous, i.e. it generally comprises a maximum of 5% by weight, preferably a maximum of 3% by weight, of water.
The highly pure trioxane produced according to the invention has a purity of at least 95% by weight, preferably at least 97% by weight, and particularly preferably at least 99% by weight of trioxane. Advantageously, the liquid mixture to be separated here has a trioxane content in the range from 70 to 95% by weight, particularly advantageously in the range from 83 to 88% by weight.
The alcohol which is present in the liquid mixture and can in part form hemiformals with the formaldehyde is preferably a monohydric alcohol, e.g. cyclohexanol, methanol, propanol or butanol.
Kleiber Michael
Nickelfeld Wolfgang
Reichl Albert
Rosenberg Michael
Scheid Dirk
Connolly Bove Lodge and Hutz LLP
Lambkin Deborah C.
Ticona GmbH
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