CS245087B1 - The method of separating formaldehyde from the reaction gases - Google Patents

Abstract

The solution relates to the separation of formaldehyde from the reaction gases of the oxidative dehydrogenation of methanol on a silver catalyst with increased utilization of the condensation heat of the reaction products in the condensation stage, for the evaporation of methanol into the process. The stated purpose is achieved by the solution to the rectification separation stage being taken at the outlet of the condenser, with the ratio of the separated stage conducted solution to the solution in the condensation stage circulated being from 0.25 to 4.

Description

245087

The present invention solves the process of separating formaldehyde from the reaction gases of the oxidative dehydrogenation of methanol on a silver catalyst.

Formaldehyde can be produced by reducing carbon oxides with hydrogen, oxidizing methanol or higher hydrocarbons, or from methanol. In view of the world-wide production of formaldehyde, it is apparent that, in the present, formaldehyde is produced predominantly from methanol. It is carried out by oxidative dehydrogenation with an oxygen-containing gas in the gas phase on metal catalysts, preferably on silver, or by oxidation containing oxygen in the gas phase on the oxide catalysts, preferably on molybdenum and iron oxides.

In the process for the production of formaldehyde from methanol using an oxide catalyst (CSSR Pat. No. 182,449, US Pat. No. 408,504, NS Pat. No. 2,442,311, U.S. Pat. No. 2,450,931, U.S. Pat. No. 2,608,823) conducted at a temperature of 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 The formaldehyde is washed out of the reaction gases by absorption into water and a formaldehyde solution is formed with a small amount of methanol. The heat of reaction is used for the production of 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 step (Germanpat. 228 687), or in two stages (U.S. Pat. No. 2,462,413, U.S. Pat. No. 3,959,383, U.S. Pat. No. 076,754).

A silver catalyst is used in various forms and compositions. In addition to the silver mesh, silver is used on the carrier (ČSSR pat. 167 763, USSR pat. 175 043), sponge silver (Jap. Pat. 56 * 6720), silver alloy (USSR Pat. No. 358 310) and silver ( GB Pat. No. 1,337,783, German Pat. No. 1,905,563, NSR Pat. No. 1 231 229, NSRpat. 2 201434, Czechoslovak Socialist Republic No. 219 193} The catalyst can be stored in a contact basket (NSR Pat. German Patent Specification No. 2,545,104, US 2,745,722) or on a metal backing directly onto a tube plate of a heat exchanger (NSR Pat. No. 1,642,955, CSSR Pat. No. 225 301, CSSRpat. 227 155). at a temperature of 820 to 970 K and proceeding in the main reaction of the dehydrogenation reaction of methanol: CH 3 OH CH 2 O + H 2 = 85.2 µmol 1 and the methanol oxidation reaction: 4

CH 3 OH + 0.5 O 2 -> CH 2 O + H 2 O ΔΗ = -156.3 kjmol "1 The heat exchange is carried out after the catalyst layer.

The combined 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). Generally, the silver catalyst production process is carried out by introducing an oxygen-containing methanol-gas mixture, or a methanol-water-gas mixture containing oxygen, heated to 360 - 390 K, through a heated catalyst bed at a temperature of 820-970 K at pressure 50 - 200 kPa. The autothermity of the process at the desired temperature is achieved by regulating the methanol oxygen ratio and the inert components. The converted gas is 0-cooled downstream of the catalytic bed. From the cooled converted gas, formaldehyde is absorbed with water while the reaction water and unreacted methanol are condensed. It obtains an aqueous solution of formaldehyde containing methanol, which, depending on the amount, remains in the product and separates the albumin and recycles it into the process.

Cooling of the converted gas takes place immediately after the catalytic bed directly with water or a solution obtained in the absorption system (NSR Pat. No. 0110176, CSSR Pat. No. 184,646), or indirectly with water for steam production (U.S. Pat. No. 946,640, GB Pat. No. 1,567,921, U.S. Pat. No. 2,002,789, German Pat. No. 2,546,104), also vaporized into methanol-guided process (CSSR Pat. 184 646, U.S. Pat. the finished product using heat to distill off methanol from the solution obtained by absorption. The cooled gas is conducted either directly to the absorption or is cooled in the condenser to condense part of the condensate components. Cooling is carried out directly with the product and indirectly with water without the use of condensation heat or directly with the solution obtained in the absorption and with the indirect liquid containing methanol, circulating it between the carbonation device and the condenser, using condensation heat to prepare the mixture into the reaction. The absorption system may be one apparatus (CSSRpat. 181,239, RSR Pat. 65,503) or multiple cameras (U.S. Pat. No. 3,174,911, U.S. Pat. No. 3,606,732, G. B. Pat. 970,855).

The known process for the production of formaldehyde with non-complete conversion of methanol uses a mixture of methanol - an oxygen-containing gas where excess methanol is maintained in order to maintain the process in an autothermal mode. The mixture is prepared in a saturator where air, fresh methanol and recycled methanol are added. The air vaporized liquid is heated by circulating it through an intermediate washing device and a condenser. The required temperature is heated indirectly by steam in the carbonator. The resulting mixture is heated to 360-390 K and led to a crystalline silver catalyst bed at a temperature of 820-920 K. Quickly cooling the converted gas through the catalyst bed prevents side reactions. It is carried out with an aqueous solution of formaldehyde from the rectification column using heat for the rectification separation of unreacted methanol and product and recycling of methanol to the process. At a temperature of 360 - 390 K of cooled reaction gas is fed to a condenser not directly cooled by circulating the methanol from the carbonator. The cooled gases together with the condensed liquid lead to an absorber where formaldehyde and methanol are washed from the gases by water and cooling. The absorption is carried out in three stages, with the third mite circulating in the first two stages, with fresh water. The inert gases leaving the absorber leave the process or can be further processed.

The absorption solution is passed through a dorectification column to separate the methanol from the product. On the rectification separation of the methanol recycled to the process, the heat of reaction of the reaction gases immediately downstream of the catalyst is utilized by circulating the solution between the reactor and the rectification column. The residual energy required is indirectly supplied to the rectification by means of water vapor. Depending on the type and position of the pump, the process is carried out by a pressure or vacuum process. Depending on the pressure, there is a significant difference in temperature in the saturator and hence in the condenser, and the utilization of condensation heat of the condensable components also depends on this. The prime process is the temperature of the vaporized liquid in the carbonation plant and the efficiency of the condensation of the liquid components from the reaction gas is better, which makes the utilization of the condensation heat in the step of preparing the gas mixture into the reaction better. In the improved process for the production of formaldehyde, a solution obtained in absorption is added to the reactant gas in the condensation step. The condensed product together with the added solution is fed to the gases for absorption. The intermediate formed in the absorption is introduced into the rectification to separate the product methanol. A disadvantage of this, although improved method, is that the condensed liquid phase obtained in the condenser, the conducted absorption availability, adversely affects the composition of the solution in absorption by its higher formaldehyde content and its thermal content.

SUMMARY OF THE INVENTION The present invention relates to a process for the formaldehyde reaction from the reaction gases of methanolic dehydrogenation on a sterile catalyst using the condensation heat to prepare a gaseous reaction mixture of condensed liquid phase in a condenser with a circulating contacting gas reaction mixture in a cone. - Densification with the solution obtained in the formaldehyde absorption step. It is characterized in that the solution is removed from the condensation step to the degree of rectification separation of the unreacted methanol from the product, with the ratio withdrawn to the reaction step of the lead solution to. The solution of the condensation step circulated is from 0.25 to 4. An advantage of the process of the present invention is the increased amount of condensed form aldehyde, water and methanol in the condensation step.

This proportionally increases the heat dissipation of the condenser to the saturation stage 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. For example, methanol and allowing the production of a more concentrated formaldehyde solution without increasing the harmful emissions, as shown in 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 cheese mill. For the evaporation of methanol to air, 1.74 MJ was obtained by circulating the methanol through the condenser and 0.67 MJ was removed indirectly by steam. The resulting mixture is passed through a catalytic bed of 863 K crystalline silver. 5.7 kg of the solution obtained in the absorber are added to the reactor-cooled reaction mixture. Gases and condensed liquid are fed to an absorber where 1.25 kg of water is added countercurrently. 5.7 kg is passed through a condenser and 3.68 kg is fed to a rectification column where 0.98 is separated. kg of methanol which is recycled to the process to give about 2.7 kg of an aqueous solution of formaldehyde of 37% by weight. The average methanol consumption is 0.442 kg per cubic gram of said solution. Example 2

To produce 1 kg of formaldehyde, 1.83 kg of air and 2.16 kg of fresh and recycled methanol are fed to the sieving machine. To evaporate the methanol in the air, 2 MJ is obtained by circulating the methanol through the condenser and 0.41 MJ is removed indirectly by water vapor. The resulting mixture is passed through a catalytic bed

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.