Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2001-05-24
2002-08-13
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C203S014000
Reexamination Certificate
active
06433194
ABSTRACT:
The invention relates to the removal of trioxane from a liquid mixture comprising trioxane, formaldehyde, alcohol, hemiformals formed from formaldehyde and alcohol, and small amounts of secondary components. In addition or as an alternative to the alcohol and the hemiformals, the mixture may also comprise water and reaction products formed from formaldehyde and water.
For the preparation of the engineering plastic polyacetal, in particular of polyoxymethylene (POM), high-purity trioxane is required. The quality of the plastic, i.e. the achievable degree of polymerization, is, besides the polymerization conditions, determined principally by the purity of the trioxane.
Various processes for the preparation of trioxane are known (for example homogeneously or heterogeneously catalyzed from aqueous formaldehyde solutions (AT 252913) or heterogeneously catalyzed from gaseous formaldehyde on heteropolyacids (EP 0606056). Irrespective of the preparation process, trioxane is generally not produced as a pure substance, but instead always as a mixture with unreacted formaldehyde and other substances, such as alcohol, water and small amounts of other components, so-called secondary components, such as methanol, methyl formate, methylal, formic acid, dioxolane and tetraoxane. For use of the trioxane in the polymerization, this must be separated, in particular from the formaldehyde, and may only contain small amounts of secondary components.
A multiplicity of literature is known relating to the removal of trioxane from aqueous formaldehyde-containing mixtures. The separation of gaseous mixtures of formaldehyde and trioxane is only described in a few places.
The separation from aqueous solution has hitherto been carried out, in particular, by distillation (AT 252913 and JP 83/171278). In the distillation, unreacted formaldehyde is frequently fed back into the reactor, where it is reacted further to give trioxane. A limit for the increase in the concentration of trioxane by distillative removal arises through an azeotrope of trioxane with water which boils at 92° C. and 1 bar and which generally has a trioxane content of about 70% by weight. A further disadvantage of this process is possible solids formation by polymerization of formaldehyde or formation of paraformaldehyde, especially in the region of the column head. In order to avoid same, all apparatus parts must either be heated to temperatures above 100° C. (for example at a formaldehyde partial pressure of 1 bar) or wetted with a liquid.
A further process for the separation of formaldehyde and trioxane from aqueous solutions comprises the extraction of the trioxane with organic solvents in which trioxane has higher physical solubility than formaldehyde. This process has exclusively been employed for the removal of trioxane from the aqueous phase. Examples of organic extractants used are saturated aliphatic or aromatic hydrocarbons or halogenated hydrocarbons (EP 0583907) which are sparingly miscible or even immiscible with water. A disadvantage of the extraction is that an additional substance in the form of the organic solvent is introduced into the process, which makes subsequent work-up of the organic phase necessary too. A further disadvantage is that large parts of the trioxane sometimes also remain in the aqueous phase. Large amounts of trioxane therefore have to be circulated or are lost in the work-up process.
A further possibility described for the selective removal from an aqueous formaldehyde/trioxane phase is crystallization of the trioxane (DE 3508668). The trioxane concentration in the aqueous mixture must be greater than 50% by weight here.
In the preparation of trioxane by trimerization of formaldehyde from aqueous formalin solutions, the above-mentioned processes of azeotropic distillation, extraction and, if desired, crystallization are generally linked with one another in a suitable manner in order to obtain trioxane in the requisite high purity.
For the separation of gaseous mixtures of formaldehyde and trioxane, selective absorption of one species has frequently been employed. In general, either the formaldehyde is chemisorbed and the trioxane left in the gas phase (GB 1245990) or conversely selective physisorption of the trioxane is carried out (EP 0680959). Since no liquid phase in which either only the formaldehyde or only the trioxane is soluble has been found, fractions of the respective other species are also bound here. For this reason, the purities necessary for polymerization cannot be achieved using this process. Furthermore, large losses of the valuable product trioxane occur.
The separation of a mixture of formaldehyde and trioxane in gaseous form with a low water content by absorption of the trioxane in an alcohol followed by crystallization from the alcoholic solution is described in the as yet unpublished German Patent Application No. 19833620.9. This process forms a possible step in a novel process for the preparation of trioxane from methanol, consisting of the steps of nonoxidative dehydrogenation (DE 3920811), removal of formaldehyde (as yet unpublished German Patent Application Nos. 19747647.3 and 19748380.1), formaldehyde trimerization (EP 0606056 and EP 0691388) and removal of trioxane (for example in accordance with German Patent Application No. 19833620.9).
The object was to find an alternative process for the removal of trioxane from a liquid mixture comprising trioxane, formaldehyde, alcohol, hemiformals formed from formaldehyde and the alcohol, secondary components and, in addition or as an alternative to the alcohol and the hemiformals, water and reaction products formed from formaldehyde and water, in which the aim was to obtain the trioxane in high purity. The other valuable products present in the mixture, namely formaldehyde and possibly the alcohol, should if possible be present in other product streams that can be utilized.
This object is achieved in accordance with the invention by transferring trioxane as selectively as possible from the liquid mixture into the vapor phase in a suitable manner, and subsequently removing it selectively in liquid or solid form by cooling and subsequent condensation or desublimation.
The invention therefore relates to a process for the recovery of trioxane in which the trioxane is transferred from a liquid mixture comprising trioxane, formaldehyde, alcohol, hemiformals formed from the formaldehyde and the alcohol, usual small amounts, formed in the preparation of trioxane, of lower- and higher-boiling secondary components and, in addition or as an alternative to the alcohol and the hemiformals, water, into the gas phase by volatilization or evaporation and subsequently converted into a liquid state by condensation and isolated as condensate or converted into a solid state by desublimation and isolated as desublimate.
The invention also relates to the use of the trioxane condensate or desublimate obtained by the process according to the invention, which is characterized by its composition and a trioxane content of at least 80% by weight, for the preparation of polymers and fuels or for obtaining formaldehyde by depolymerization.
Examples of usual secondary components are methanol, methyl formate, tetraoxane, dioxolane, trioxy ether and traces of formic acid. The alcohol present in the liquid starting mixture, which in some cases is able to form hemiformals with the formaldehyde, is preferably a monohydric alcohol, for example cyclohexanol, methanol, propanol or butanol. However, it is also possible to use other alcohols, including polyhydric ones, such as glycerol, diethylene glycol, triethylene glycol, triethanolamine, butanetriol and pentanetriol. If desired, it is also possible to use a mixture of alcohols. The alcohol should preferably have a higher boiling point and a lower melting point than trioxane.
In contrast to German Patent Application No. 19833620.9 and DE 3508668, the process according to the invention achieves, in particular, the object of separating the mixture in liquid form with a low water content, as arises, for example, in the a
Heffels Stefan
Reichl Albert
Schweers Elke
Connolly Bove & Lodge & Hutz LLP
Covington Raymond
Rotman Alan L.
Ticona GmbH
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