WO2006061167A1 - Two-stage method for the production of trioxane from a highly concentrated aqueous formaldehyde solution, the reaction mixture being stripped from the synthesis reactor in a gaseous form and being transferred into a distillation column - Google Patents

Abstract

The invention relates to a method for producing raw trioxane, comprising the following stages: - a highly concentrated aqueous formaldehyde solution is subjected to an acid catalyzed reaction so as to obtain a trioxane/formaldehyde/water mixture (stage I); and - the trioxane/formaldehyde/water mixture obtained in stage I is distilled in order to obtain raw trioxane (stage II). The inventive method is characterized in that the trioxane/formaldehyde/water mixture is stripped from the synthesis reactor (R) in a gaseous form in stage I.

Description

TWO-STAGE PROCESS FOR THE PREPARATION OF TRIOXAN FROM A HIGH-CONCENTRATED AQUEOUS FORMALDEHYDE SOLUTION, WHERE THE REACTION MIXTURE IS DEVICED FROM THE SYNTHESIS ACTUATOR AND TRANSFERRED TO A DISTILLATION COLON

The invention relates to a process for the preparation of crude trioxane from a highly concentrated aqueous formaldehyde solution.

Trioxane can be prepared by reactive distillation of aqueous formaldehyde solution in the presence of acidic catalysts. The trioxane is then extracted from the distillate containing trioxane formaldehyde and water in addition to halogenated hydrocarbons, such as methylene chloride or 1,2-dichloroethane, or other water-immiscible solvents.

DE-A 1 668 867 describes a process for the separation of trioxane from mixtures containing water, formaldehyde and trioxane by extraction with an organic solvent. In this case, an extraction section consisting of two sections is fed at one end with a customary organic water-immiscible trioxane extractant, at the other end with water. Between the two sections, the distillate to be separated is fed to the trioxane synthesis. An aqueous formaldehyde solution is then obtained on the side of the solvent feed and a virtually formaldehyde-free solution of trioxane in the solvent is obtained on the side of the water feed. In one example, the distillate resulting from the trioxane synthesis from 40% by weight of water, 35% by weight of trioxane and 25% by weight of formaldehyde is metered into the middle part of a pulsation column, at the upper end of the column methylene chloride and at the lower end of the column Supplied with water. In this case, about 25% strength by weight solution of trioxane in methylene chloride is obtained at the lower end of the column and about 30% strength by weight aqueous formaldehyde solution is obtained at the upper end of the column.

Disadvantage of this procedure is the accumulation of extractant, which must be purified. Some of the extractants used are hazardous substances (T or T ⁺ substances within the meaning of the German Hazardous Substances Ordinance), the handling of which requires special precautions.

DE-A 197 32 291 describes a process for the separation of trioxane from an aqueous mixture consisting essentially of trioxane, water and formaldehyde, in which trioxane is removed from the mixture by pervaporation and the trioxane is added to the mixture. enriched permeate by rectification in trioxane and an azeotropic mixture of trioxane, water and formaldehyde separates. In the example, an aqueous mixture consisting of 40% by weight of trioxane, 40% by weight of water and 20% by weight of formaldehyde in a first distillation column under normal pressure is introduced into a water / formaldehyde mixture and into an azeotropic trioxane / Separated water / formaldehyde mixture. The azeotropic mixture is passed into a pervaporation unit containing a membrane of polydimethylsiloxane with a hydrophobic zeolite. The trioxane-enriched mixture is separated in a second distillation column under normal pressure in trioxane and again in an azeotropic mixture of trioxane, water and formaldehyde. This azeotropic mixture is recycled before the pervaporation step.

Disadvantages of this procedure are the very high investments for the pervaporation unit.

German patent application DE 103 61 516.4, which is not prepublished, discloses a process for the distillative separation of trioxane from trioxane / formaldehyde / water mixtures, which does not require any extraction or pervaporation steps. However, for the separation of pure trioxane and pure water from the product mixture from a trioxane synthesis reactor, the process requires a plant with three distillation columns.

In the acid-catalyzed conversion of aqueous formaldehyde solution to trioxane, the following reaction mechanism is assumed, which proceeds over several equilibrium stages:

trioxane

n CH ₂ O ^ - -vw (CH ₂ O) _n -

Paraformaldehyde (n> 10) The location of the equilibrium is far on the side of the starting material formaldehyde. In addition, a variety of side reactions take place, including the accumulation of para-formaldehyde.

In order to shift the equilibrium position in the direction of the desired product trioxane, it is advantageous to work with the highest possible formaldehyde concentrations. Thus, the additional advantage of a lower water load is connected in subsequent steps. However, high formaldehyde concentrations have the disadvantage that formaldehyde, as a function of the temperature, forms by spontaneous polymerization para-formaldehyde, a poorly soluble solid. The solubility limit of para-formaldehyde in aqueous formaldehyde solutions increases with temperature, since the forming long-chain para-formaldehyde molecules remain in solution due to the increased molecular motion.

The reaction of aqueous formaldehyde solution to trioxane proceeds under homogeneous or heterogeneous acid catalysis. Frequently used as catalysts are sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, generally in a concentration of from 1 to 15% by weight, based on the total weight of the aqueous formaldehyde solution, or an equivalent amount of ion exchange resins or zeolites.

The acidic catalysts in the required high proportions by weight generally increase the problem of solid state failure by binding water molecules by dissociation. As a result, with the same formaldehyde concentration, the solubility limit is shifted in the direction of higher temperatures.

In order to avoid the problem of solid precipitation and still be able to implement as highly concentrated aqueous formaldehyde solutions, therefore, the operation at the highest possible temperatures (and correspondingly high pressures) is required.

For the subsequent distillative removal of crude trioxane from the effluent of the trioxane synthesis reactor, on the other hand, exactly opposite operating conditions are advantageous, namely the lowest possible top pressure and correspondingly low temperatures in the column. Although the relationships are not fully understood, this is advantageous because with increasing pressure, a higher concentration of trioxane in trioxane / formaldehyde / water mixtures can be achieved. - A -

It was therefore an object of the invention to provide a process for the preparation of crude trioxane available that optimally takes into account the opposite requirements of the synthesis or to the distillative concentration of the discharge from the synthesis reactor and which is simpler in terms of apparatus and substantially cheaper compared to known methods ,

The problem is solved by a method with the following method steps:

acid catalyzed reaction of a highly concentrated aqueous formaldehyde solution in a trioxane synthesis reactor to give a trioxane / formaldehyde / water

Mixture (process step I) and

Distillation of the trioxane / formaldehyde / water mixture from process step I to obtain crude trioxane (process step II).

which is characterized in that the trioxane / formaldehyde / water mixture is withdrawn from the trioxane synthesis reactor in process stage I in gaseous form.

Highly concentrated herein is an aqueous formaldehyde solution having a formaldehyde content of at least 50% by weight, or at least 60% by weight, or else at least 64% by weight.

Sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, zeolites or ion exchange resins can generally be used as acidic, homogeneous or heterogeneous catalysts, generally in an amount of from 1 to 15% by weight, based on the weight of the highly concentrated aqueous formaldehyde solution

In the present case, a crude trioxane is a stream containing trioxane in a weight fraction of 40 to 80%, and in addition 1 to 30 wt .-% formaldehyde, water and by-products.

Pure trioxane is a stream which contains at least 97.5% by weight, preferably at least 99% by weight or 99.9% by weight or else 99.99% by weight of trioxane.

The trioxane synthesis reactor can be, for example, a fixed-bed, suspended-bed or continuous-bed reactor, a stirred tank or an evaporator, which is preferably operated at superatmospheric pressure. The trioxane synthesis reactor is preferably operated at a pressure in the range from 1 to 10 bar a, more preferably from 1 to 7 bar a. In order to limit the losses in yield by the formation of the by-product formic acid, the trioxane synthesis reactor is preferably operated at residence times of less than 30 minutes, more preferably less than 15 minutes.

For this purpose, in particular the highly concentrated aqueous formaldehyde solution is fed to the trioxane synthesis reactor via a mixing device with high mixing energy, of more than 0.3 W / 1, preferably of more than 3 W / 1, particularly preferably of more than 30 W / 1.

From the trioxane synthesis reactor, a gaseous stream containing trioxane, formaldehyde and water is withdrawn from the upper region thereof. The composition of the gaseous stream from the trioxane synthesis reactor generally corresponds to 1 to

15% by weight of trioxane, 60 to 80% by weight of formaldehyde, the remainder being water and secondary components.

The gaseous stream from the trioxane synthesis reactor is, preferably via a control valve, expanded into the distillation column, in which the trioxane / formaldehyde / water mixture is separated into a top stream containing crude trioxane and a bottom stream, preferably in the Trioxane synthesis reactor is recycled.

This circulating stream is mixed with the freshly supplied highly concentrated formaldehyde solution either before the circulation pump or in another mixing unit, for example a reaction mixing pump of a mixing nozzle, a static mixer or in a reaction mixer. Alternatively, it is also possible to separate the recycle stream and the fresh aqueous formaldehyde stream separately and only submerged into the trioxane synthesis reactor.

The top stream containing crude trioxane can be purified in a further distillation column to pure trioxane.

The process stage I is preferably carried out in the trioxane synthesis reactor at elevated pressure compared with process stage II in the distillation column

The pressure difference between distillation column and trioxane synthesis reactor can be compensated for as an alternative to the control valve by hydrostatic pressure.

Since the trioxane-containing reaction mixture is taken off in gaseous form from the trioxane synthesis reactor, the acids critical for the solid precipitation of para-formaldehyde remain in the trioxane synthesis reactor and are not included in the distillation column. drags. As a result, more cost-effective steel grades can be used for the distillation column, which need not be acid-resistant. In particular, the steel grades 1.4541, 1.4571 or 1.4539 or similar alloyed steels can be used, alternatively other materials, provided they are not more expensive than the aforementioned.

Since no high boilers have to be separated off, the distillation column for crude trioxane is generally designed without a stripping section.

Moreover, due to the absence of acid in the crude trioxane distillation column over prior art columns, it is possible to operate at lower pressures without the column being added by solid precipitation and thus a trioxane-rich crude trioxane according to the ternary boiling line diagrams to get in the top stream of the column.

However, the higher the trioxane content in the crude trioxane stream from the first distillation column, the more economically the further concentration to pure trioxane can be operated.

Solubility tests with and without acid have shown that in the presence of strong pillars acids, such as the aforesaid sulfuric acid, methanesulfonic acid or para-toluenesulfonic acid, the temperature to be selected by at least 10 ⁰ C higher compared to an operation without acid must so that no solid precipitates.

The invention will be explained in more detail below with reference to a drawing.

Figure 1 shows the schematic representation of a system according to a preferred embodiment of the invention

A trioxane synthesis reactor R is fed with a highly concentrated aqueous formaldehyde solution via a pump P and an evaporator V. From the gas space of the trioxane synthesis reactor, a trioxane / formaldehyde / water mixture is taken off in vapor form and depressurized via a control valve RV to the pressure in the distillation column K for crude trioxane. From the distillation column K, a top stream 3 containing crude trioxane and a bottom stream 4 is withdrawn, which is recycled via a pump P in the trioxane synthesis reactor.

Claims

claims 1. Process for the preparation of crude trioxane with the following process stages: - acid catalyzed reaction of a highly concentrated aqueous formaldehyde solution to obtain a trioxane / formaldehyde / water mixture (process step I) and Distillation of the trioxane / formaldehyde / water mixture from process step I to obtain crude trioxane (process step IT), characterized in that the trioxane / formaldehyde / water mixture from the trioxane synthesis reactor (R) in the process step I is withdrawn in gaseous form. 2. The method according to claim 1, characterized in that the process step I is carried out with respect to the process step II increased pressure. 3. The method according to claim 2, characterized in that the process stage I at a pressure in the range of 1 to 10 bar a, preferably 1 to 7 bar a is operated. 4. The method according to any one of claims 1 to 3, characterized in that the residence time in the trioxane synthesis reactor (R) is limited to less than 30 minutes, preferably to less than 15 minutes, more preferably less than 10 minutes. 5. The method according to any one of claims 1 to 4, characterized in that the highly concentrated aqueous formaldehyde solution to the trioxane synthesis reactor (R) via a Mixing element with a mixing energy of more than 0.3 watts / 1, preferably more than 3 watts / 1, more preferably more than 30 watts / 1 is supplied. 6. The method according to any one of claims 1 to 5, characterized in that the process stage II is carried out in a distillation column, which is operated at a top pressure <1.5 bar a, preferably <1.2 bar a. 7. The method according to any one of claims 1 to 6, characterized in that the process stage II is carried out in a distillation column, which is formed from a material which does not have to be acid-resistant. 8. The method according to any one of claims 1 to 7, characterized in that the process step π is carried out in a distillation column without stripping section. 9. The method according to any one of claims 1 to 8, characterized in that the gaseous stream from the trioxane synthesis reactor (R) via a control valve to the operating pressure in the distillation column for performing the process step π is relaxed. 10. The method according to any one of claims 1 to 9, characterized in that the bottom stream from the distillation column for carrying out the process stage II in the trioxane synthesis reactor (R) is recycled.