DE4244582A1 - Trioxane prodn. from form-aldehyde gas using vanadyl hydrogen phosphate - Google Patents

Abstract

In the prodn. of trioxane (I) from HCHO in the gas phase, vanadyl hydrogen phosphate hemihydrate (II) is used as catalyst.Pref. (II) is in water-depleted form and is activated with steam (24 h to 14 days at 100-200 deg.C). Reaction is carried out with dry HCHO at 80-150, esp. 100-120 deg.C and 0.5-5, esp. 0.5-2 bar, opt. with addn. of a carrier gas (N2). In an example, the selectivity has 1 (no by-prods. formed) if 0.43 mole/h HCHO + 1.8 mole/h N2 were passed through a reactor contg. 82 g (II) pellets at 80 deg. C; the conversion was 22.1%, i.e. 62.8% of the equilibrium conversion, and throughput 35.3 kg/cu.m.h.

Description

The invention relates to a process for the continuous production of trioxane in the gas phase by heterogeneous catalysis with vanadyl hydrogen phosphate hemihydrate (VO) HPO _{4 *} 1/2 H ₂ O as a catalyst.

Trioxane can be obtained from aqueous formaldehyde solutions with acidic catalysts getting produced. The high level is characteristic of these processes Energy consumption for the evaporation of water caused by the educt flows in the process is entered.

There are various proposals for producing trioxane by gas-phase trimerization of formaldehyde, but this always involves working with formaldehyde streams with different water contents. When working with water-containing formaldehyde, problems arise due to deposits of polyoxymethylene on the catalyst surface. A process for the production of trioxane by means of an acidic resin ion exchanger is known (DE-C 15 93 990). Also known is a catalyst for gas phase trimerization in the form of phosphoric acid and sulfuric acid on SiO ₂ supports (AT-B 252 913).

The task was to eliminate the disadvantages mentioned.

The task was solved by a procedure that differs from the previous ones Process by the catalyst, the composition of the educt stream and the high selectivity of the catalyst, especially at high Differential formaldehyde pressures.

The invention thus describes a process for producing trioxane from Formaldehyde in the gas phase in the presence of a catalyst in which as Vanadyl hydrogen phosphate hemihydrate catalyst is used.  

The following advantages are particularly noteworthy

• 1. achievement of high space-time yields (RZA) [kg / m ³ _* h]
• 2. No by-products
• 3. Moderate heat development in the synthesis reactor

The catalyst is generally used in bulk, ie without a support or pressing aids. Its molecular formula is (VO) HPO _{4 *} 1/2 H ₂ O. Due to a loss of water under reaction conditions and the formation of various meta-phosphates up to the vanadyl pyrophosphate, (VO) ₂ P ₂ O ₇ , ie lower water forms of the compound the activity and selectivity don't.

By using the pure catalyst, deposits that are common when using phosphoric acid on SiO ₂ can be prevented.

The formaldehyde that can be used for the process can vary Be water content, d. H. contain up to 5 wt .-% water. Is preferred anhydrous formaldehyde.

The temperature range for the reaction is 80 ° C to 150 ° C. Is preferred the range from 100 ° C to 120 ° C.

The reaction is influenced by the partial pressure of formaldehyde. Of the The catalyst has a high selectivity for the over a large pressure range Formation of trioxane. The input partial pressure of formaldehyde is in generally 0.5 to 5 bar, preferably 0.5 to 2 bar.

The apparatus for producing trioxane according to the invention consisted of three parts (see Fig. 1):

1. Dosage
2. Education reactor
3. Analytics.

For the investigation, formaldehyde is introduced into the apparatus and - if desired - mixed with a carrier gas. As a carrier gas, the Noble gases helium, argon, krypton or xenon are used, but is preferred Nitrogen.

The formation reactor (R) - for the present experiments - consisted of one Stainless steel tubular reactor 150 mm long and 30 mm in diameter. The heat supply and removal was done by a thermostat, as Silicone oil was used for heat transfer. The use of heat transfer media like mineral oils is also possible. In three different places of the The temperature of the reactor was measured radially in the reactor. This Temperatures were recorded during the tests and provided information about the reactor stability during the test period.

The output stream emerging from the reactor with the resulting ones Reaction products were collected by absorption in water. The The trioxane formed can be extracted therefrom in a known manner become.

The quantitative studies on the formation of trioxane in the gas phase were carried out using online analytics. There were two samples taken from various points of the apparatus during operation and analyzed in a gas chromatograph.

The arrangement of the apparatus for the method according to the invention and the Dimensions can of course be adapted to the respective conditions.

Experiment description

Individual parameters that have been varied are shown in the individual examples explained.

Anhydrous formaldehyde was used in the tests. With the help of a mass flow controller, a carrier gas, preferably Nitrogen, the educt stream are added. However, it can also be done without  Carrier gas to be worked. By using a carrier gas, the Flow rate of the educt stream varies, e.g. B. increased, and so that the residence time in the reactor R affects, for. B. be lowered.

Different amounts of catalyst were tested in the reactor R, the Catalyst is poured into the reactor, for example in the form of pellets. Before and after the reactor R, the gas composition was online with a Gas chromatograph determined. From the compositions of the gases sales calculated. The temperature was controlled by a thermostat and measured at three different points along the catalyst bed.

Sampling for examining the input and output currents was done made automatically and took place at regular intervals, e.g. B. all seven minutes, with the shut-off devices (pneumatic ball valves) connected to a gas chromatograph controlled by a computer becomes.

The information in the tables of the examples is explained as follows: The mole fraction X _shape is based on the gas composition at the synthesis reactor inlet. It is calculated according to equation (1).

X _form = n ° _form / n _tot = n ° _form / (n ° _form + n _N2 ) (1)

n ° _form : formaldehyde input flow [mol / h]
n _total : total current [mol / h]
n _N2 : nitrogen flow [mol / h]

The experimentally determined conversion U _exp is calculated according to equation (2).

U _exp = (n ° _shape - n _shape ) / n ° _{shape *} 100 (2)

n ° _form : formaldehyde input flow [mol / h]
n _Form : Formaldehyde output stream [mol / h]

The relative conversion U _rel (equation (3) is the ratio of the experimentally determined conversion U _exp and the equilibrium _conversion U _glge , determined from the equilibrium constant according to Busfield and Merigold, ("The Gas-Phase Equilibrium between Trioxane and Formaldehyde", J. Chem Soc (A), 1969 p. 2975).

U _rel = U _exp / U _{glge *} 100 (3)

U _exp : experimental turnover [%]
U _glge : equilibrium _turnover [%]

Examples

• 1) 82 g of the catalyst vanadyl hydrogen phosphate hemihydrate in the form of cylindrical pellets (diameter 5 mm, height 5 mm) were used in the reactor R. The temperature of the reactor was 80 ° C. The following conversions could be achieved with a selectivity of 1, ie no by-products were obtained. The nitrogen flow was 1.8 mol / h.

  Table 1

  Turnover at various input mole breaks of formaldehyde and a temperature of 80 ° C

• 2) Example 1 was repeated, the temperature of the reactor being 90 ° C. The following sales were achieved with a selectivity of 1. The nitrogen flow was 1.8 mol / h.

  Table 2

  Turnover at various input mole breaks of formaldehyde and a temperature of 90 ° C

  It shows that the equilibrium conversion can be achieved when the temperature rises. The selectivity is retained when the temperature rises.
• 3) 64 g of the catalyst vanadyl hydrogen phosphate hemihydrate in the form of cylindrical pellets (diameter 5 mm, height 5 mm) were used in the reactor R. The temperature of the reactor was 110 ° C. The following sales were achieved with a selectivity of 1. The nitrogen flow was 0.1-0.2 mol / h.
  The total pressure was 1700 mbar and the inlet partial pressure of formaldehyde was kept constant at 1100 mbar.

  Table 3

  Turnover at various input mole breaks of formaldehyde and a temperature of 110 ° C

• 4) Example 3 was repeated using 21.5 g of catalyst. By reducing the catalyst mass, the residence time in the reactor was shortened. The total pressure was 1350 mbar, the input partial pressure of formaldehyde was 900 mbar.

  Table 4

  Turnover at various input mole breaks of formaldehyde and a temperature of 110 ° C

• 5) Example 3 was repeated with the difference that the total pressure was 1350 mbar and the inlet partial pressure of formaldehyde was 900 mbar.

  Table 5

  Turnover at various input mole breaks of formaldehyde and a temperature of 110 ° C

Examples 1 to 5 show that the catalyst has a high selectivity for the formation of trioxane. At a temperature of 90 ° C, example 2, the highest relative sales can be achieved. However, only low partial pressures of formaldehyde are then possible. From a temperature of 105 ° C, the partial pressure of formaldehyde can be increased to over 1 bar, example 3. It turns out that with longer dwell times, e.g. B. 33.0 seconds, a relative conversion of up to 79.5%, Example 5 can be achieved. However, the space-time yield is reduced. By optimizing the dwell time and the partial pressure of formaldehyde, space-time yields of over 30 kg / m ³ _* h can be achieved (example 3: dwell time 25.4 seconds, partial pressure of formaldehyde 1100 mbar, space-time yield 31.9 kg / m ³ _* h H).

When the dwell time, example 4, is reduced to 7 to 8 seconds the space-time yield, but the relative turnover decreases considerably.

Claims (7)

1. A process for the preparation of trioxane from formaldehyde in the gas phase in the presence of a catalyst, characterized in that vanadyl hydrogen phosphate hemihydrate is used as the catalyst. 2. The method according to claim 1, characterized in that the catalyst is used in a lower water form. 3. The method according to claim 1 or 2, characterized in that anhydrous formaldehyde is used. 4. The method according to one or more of claims 1 to 3, characterized characterized in that the reaction at 80 to 150 ° C, preferably at 100 to 120 ° C is carried out. 5. The method according to one or more of claims 1 to 4, characterized characterized in that the input partial pressure of formaldehyde 0.5 to 5 bar, preferably 0.5 to 2 bar. 6. The method according to claim 1 to 5, characterized in that the Reaction in the presence or absence of a carrier gas is carried out. 7. The method according to claim 6, characterized in that nitrogen as Carrier gas is used.