CS245087B1 - Method of formaldehyde separation from reactive gases - Google Patents

Abstract

The solution relates to the separation of formaldehyde from reaction gases of oxidative dehydrogenation methanol on a silver catalyst with increased condensation utilization of heat of reaction fumes in the degree of condensation, to evaporate methanol into the process. This purpose is achieved by having the solution to the degree of re -ification separation is taken at the outlet of the condenser, whereby the ratio of the separated stage conducted solution to the solution in the condensation step circulating is from 0.25 to 4.

Description

The present invention provides a method for separating formaldehyde from the reaction gases of oxidative methanol dehydrogenation on a silver catalyst.

Formaldehyde can be produced by reducing carbon oxides with hydrogen, oxidizing methanol or higher hydrocarbons, or from methanol. An overview of world formaldehyde production suggests that nowadays formaldehyde is predominantly produced from methanol. It is carried out by oxidative dehydrogenation of a gas-phase oxygen-containing gas on metal catalysts, preferably on silver, or by oxidation of a gas-phase oxygen-containing gas on oxide catalysts, preferably molybdenum and iron oxides.

In a process for the production of formaldehyde from methanol using an oxide catalyst (USSR Pat. 182 449, USSR Pat. 408 504, German Pat. 2 442 311, German Pat. 2 450 931, German Pat. 2 608 823) carried out at 530 to 650 K the oxidation reaction takes place mainly:

CH 2 OH + 0.5 O 2 - CH 2 O + H 2 O

ΔΗ = —156.3 kjmol ^-1

Formaldehyde is scrubbed from the reaction gases by absorption into water to form a formaldehyde solution with a small amount of methanol. The heat of reaction is used to produce steam, the heat exchange being carried out in the catalyst bed.

The process for the production of formaldehyde from methanol using a silver catalyst can be carried out in one stage (German Pat. 2) 28 687, or in two stages (U.S. Pat. No. 2,462,413, U.S. Pat. No. 3,959,383, U.S. Pat. No. 4,076,754).

A silver catalyst is used in various forms and different compositions. In addition to silver mesh, silver is used on a carrier (CSR pat. 167 763, USSR pat. 175 043), sponge silver (Jap. Pat. 56 * 6720), silver alloys (USSR pat. 358 310) and in particular crystalline silver (GB pat. 1 337 783, German Pat. 1 905 563, German Pat. 1 231 229, German Pat. 2 201434, Czechoslovakia Pat. 219 193). The catalyst can be stored in a contact basket (German Pat. No. 2,137,938, German Pat. No. 2,545,104, US Pat. No. 2,745,722) or on a metal support directly on the heat exchanger tube sheet (German Pat. No. 1,642,955, Czechoslovakia Pat. No. 225). 301, Czechoslovakia Pat. 227,155). The conversion of methanol to formaldehyde is carried out at a temperature of 820 to 970 K and is carried out side-by-side in the main reaction of methanol dehydrogenation:

CH 3 OH CH 2 O + Hz ΔΗ = 85.2 kmor ¹ and methanol oxidation reaction:

CH 2 OH + 0.5 O 2 -> CH 2 O + H 2 O

ΔΗ = —156.3 kjmol ¹

The heat exchange takes place after the catalyst layer.

The combined production process uses a silver catalyst in the first reactor and an oxide catalyst in the second reactor. (U.S. Pat. No. 2,519,788, U.S. Pat. No. 3,987,107).

The silver catalyst production process is generally carried out by passing an oxygen-containing methanol-gas mixture or an oxygen-containing methanol-water-gas mixture heated to 360 - 390 K through a heated immobilized catalyst layer at a temperature of 820 - 970 K and at a pressure 50 to 200 kPa. The autotericity of the process at the desired temperature is achieved by controlling the ratio of methanol to oxygen and inert components. The converted gas is cooled downstream of the catalyst bed. From the cooled converted gas, formaldehyde is scrubbed with water while condensing the reaction water and unreacted methanol. It obtains an aqueous formaldehyde solution containing methanol which, depending on the amount, remains in the product or is separated by distillation and recycled to the process.

Cooling of the converted gas is carried out immediately downstream of the catalytic bed directly with water or a solution obtained in the absorption system (NSR Pat. No. 2 011 676, ČSSR Pat. 184 646), or indirectly with water for steam production (USSR Pat. No. 946 640). GB Pat. 1,567,921;

002 789, German Pat. 2,546,104), also vaporized by methanol (CSSR Pat. 184,646, USSR Pat. 536,158), respectively, to the finished products using heat to distill methanol from the solution obtained by absorption. The cooled gas is either led directly to the absorption or is cooled in a condenser to condense some of the condensable components. The quenching is carried out directly by the product and indirectly by water without the use of condensation heat or directly by the solution obtained in absorption and indirectly by the liquid containing methanol, circulating it between the carbonator and the condenser, using condensation heat to prepare the mixture for reaction. The absorption system may be a single apparatus (CSR Pat. 181 239, RSR Pat. 65 503) or more (U. S. Pat. 3174 911, U. S. Pat.

606,732, G. B. Pat. 970 855).

The known process for producing formaldehyde with incomplete methanol conversion utilizes an oxygen-containing methanol-gas mixture wherein the excess methanol is maintained to maintain the process in an autothermal mode. The mixture is prepared in a carbonator, where air, fresh methanol and recycled methanol are supplied. The liquid vaporised into the air is heated by circulating it between the carbonator and the condenser. The desired temperature is indirectly heated in the carbonator by water vapor. The resulting mixture was heated to 360-390 K and led to a crystalline silver catalyst bed at a temperature of 820-920 K. The rapid indirect cooling of the converted gas downstream of the catalyst bed avoided side reactions. It is carried out with an aqueous solution of formaldehyde from the rectification column using heat for rectifying the separation of unreacted methanol from the product and recycling the methanol into the process. The reaction gases cooled to a temperature of 360 - 390 K are fed to an indirectly cooled condenser by circulating methanol from the carbonator. The cooled gases together with the condensed liquid are fed to an absorber where formaldehyde and methanol are pumped out of the gases with water and cooling. The absorption is carried out in three stages, with a solution circulating in the direction of the gas flow in the first two stages and a third spraying with fresh water. The inert gases leaving the absorber leave the process or can be further processed.

The solution obtained in the absorption is fed to a rectification column to separate the methanol from the product. The specific heat of cooling of the reaction gases immediately downstream of the catalyst is used to rectify the separation of methanol, which is recycled into the process from the product, by circulating the solution between the reactor and the rectification column. Other necessary energy is supplied indirectly by rectification using water vapor.

Depending on the type and position of the gas pump, the process is carried out by pressure or vacuum mode. Depending on the pressure, there is a significant temperature difference in the carbonator and thus in the capacitor, and the use of the condensation heat of the condensable components is also dependent on this. In the vacuum process, the temperature of the vaporized liquid in the carbonator is lower and the efficiency of condensation of the liquid components from the reaction gas is better, which corresponds to a better utilization of the condensation heat in the gas mixture preparation step.

In an improved process for the production of formaldehyde, the solution obtained in the absorption is also added to the reaction gases in the condensation step. The condensed product together with the added solution is passed with the gases until absorption. The intermediate formed in the absorption is led to rectification to separate the methanol from the product. A disadvantage of this, although an improved method, is that the condensed liquid phase obtained in the condenser, led to the absorption stage adversely affects the composition of the absorption solution by its higher formaldehyde content and its thermal content.

SUMMARY OF THE INVENTION The present invention provides a method for separating formaldehyde from the reaction gases of the oxidative methanol dehydrogenation on a silver catalyst using the condensation heat to prepare a gaseous mixture into a condensate liquid phase condenser with circulating contacting the gaseous reaction mixture. It is characterized in that the solution to the rectification separation of unreacted methanol from the product is withdrawn from the condensation step, wherein the ratio of the withdrawn to the reaction step of the guided solution and the solution in the condensation step circulated is from 0.25 to 4.

An advantage of the process of the invention is an increased amount of condensed formaldehyde, water and methanol in the condensation step.

In proportion to this, the amount of heat from the condenser to the degree of carbonation is increased to prepare the gas mixture for reaction. The result is a reduction in the amount of energy supplied to the process at the saturation stage. This will also result in an improved degree of absorption, which translates into a specific consumption of methanol and allows the production of a more concentrated formaldehyde solution without increasing harmful emissions, as will be seen from the following examples.

Example 1 (comparative)

To produce 1 kg of formaldehyde, 1.83 kg of air and 2.16 kg of fresh and rectified methanol are fed to the carbonator. To evaporate methanol into air, 1.74 MJ is obtained by circulating methanol through a condenser and 0.67 MJ must be supplied indirectly with water vapor. The mixture thus formed is passed through a crystalline silver catalyst bed at 863 K. To the reactor-cooled reaction mixture, 5.7 kg of the absorber solution are added at the inlet to the condenser. The gases and the condensed liquid are passed to an absorber where 1.25 kg of water is added countercurrently. From the obtained solution, 5.7 kg is fed to a condenser and 3.68 kg is fed to a rectification column where 0.98 kg of methanol is recovered and recycled to the process to give about 2.7 kg of a 37% aqueous formaldehyde solution. % wt. The average methanol consumption is 0.442 kg per (kilogram of the above solution).

Example 2

For the production of 1 kg of formaldehyde, 1.83 kg of air and 2.16 kg of fresh and recycled methanol are fed to the carbonator. For methanol evaporation into air, 2 M) is obtained by circulating methanol through a condenser and 0.41 MJ must be supplied indirectly with water vapor. The resultant mixture is passed through a crystalline silver catalyst bed at 863 K. To the reactor-cooled reaction mixture, 5.7 kg of the absorber solution is added at the inlet to the condenser. 3.68 kg is withdrawn from the solution leaving the condenser and fed to a rectification column. The remainder of the solution, together with the gases, is fed to an absorber where 1.25 kg of water is added countercurrently.

From the obtained solution, 5.7 kg is fed to a condenser. 0.98 kg of methanol are separated from the condenser withdrawn and fed to the rectification column and recycled to the process to give about 2.7 kg of 37% by weight aqueous formaldehyde solution. The average methanol consumption is 0.441 kg per kilogram of said solution.

Example 3

To produce 1 kg of formaldehyde, 1.83 kg of air and 2.16 kg of fresh and recycled methanol are fed to the carbonator. To evaporate methanol into air, 2 MJ is obtained by circulating methanol through a condenser and 0.41 MJ must be supplied indirectly with water vapor. The mixture thus formed is passed through a crystalline silver catalyst bed at 863 K. To the reactor-cooled reaction mixture, 5.7 kg of the absorber solution are added at the inlet to the condenser. 3.68 kg is withdrawn from the solution leaving the condenser and fed to a rectification column. The remainder of the solution, together with the gases, is fed to an absorber where 0.93 kg of water is added countercurrently. From the obtained solution, 5.7 kg is fed to a condenser. 0.98 kg of methanol is recovered from the condenser withdrawn and fed to the rectification column and recycled to the process to give about 2.38 kg of 42% by weight aqueous formaldehyde solution. The average methanol consumption is 0.501 kg per kilogram of said solution.

Claims (2)

A process for separating formaldehyde from the reaction gases of oxidative methanol dehydrogenation on a silver catalyst using the condensation heat of the condensed liquid phase in a condenser for preparing a gas mixture to react with circulating the gas reaction mixture in a condensation step with a solution obtained in the formaldehyde uptake to water step The process according to claim ¹ , characterized in that the solution is removed from the condensation step to rectify the separation of unreacted methanol from the product. 2. A process for the separation of formaldehyde from the reaction gases of claim 1, wherein the ratio of the condensation degree taken to the rectification stage of the feed solution to the solution in the condensation degree circulated is from 0.25 to 4.