CS245086B1 - 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 improved condensation utilization of heat of reaction fumes. That purpose will be achieved by having in the degree of condensation at a pressure of 0.04 - 0.1 MPa is added to the reaction gases per kg of formaldehyde 1.5 to 15 kg of solution contained therein of absorption.

Description

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

The formation of formaldehyde from methanol is carried out by oxidative dehydrogenation with a gas-containing oxygen gas on metal catalysts, preferably on silver, or by oxidation of a gas-containing oxygen gas on oxide catalysts, preferably molybdenum and iron oxides.

In the process for the production of formaldehyde from methanol using an oxide catalyst (USSR Pat. 182 499, USSR Pat. 408 504, German Pat. 2,442,311, German Pat. 2,450,931, German Pat.

608 823) the oxidation reaction takes place at temperature 530 - 650 K

CH 3 OH + 0.5 O 2 - CH 2 O + H 2 O

AH = -156.3 kjmol-i

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

In the process for the production of formaldehyde from methanol using a silver catalyst in a different form and a different deposit (CSR Pat. 167 763, USSR Pat. 175 043, USSR Pat. 358 310, German Pat. 1,231,229, German Pat. at 820-970 K side-by-side in the main measure of methanol dehydrogenation reaction:

GH3OH CHa -I-H2

AH = 85,3 kjmol ^-1 and methanol oxidation reaction:

CH 3 OH + 0.5 O 2 -> CH 2 O + H 2 O

AH = -156.3 kjmol-i

The combined process uses a silver catalyst in the first reactor and, after adding air to the converted mixture, uses an oxide catalyst in the second reactor (U.S. Pat. No. 2,519,788, U.S. Pat.

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 a temperature of 360 - 390 K through a heated immobilized catalyst layer at a temperature of 820 to 970 K and pressure 50 - 200 kPa. The autothermality of the process is achieved by controlling the ratio of methanol, oxygen-containing gas and inert. The converted gas is cooled downstream of the catalyst bed. The cooled converted gas is cooled downstream of the catalyst bed. The cooled converted gas is absorbed into water to obtain an aqueous solution of formaldehyde with a small amount of unreacted methanol, or methanol is rectified 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). U.S. Pat. No. 1,567,921, U.S. Pat. No. 2,002,789, U.S. Pat. No. 2,546,104, also by evaporation into a methanol-guided process (Czechoslovakia Pat. No. 184,646; into the process.

The cooled converted gas is fed either directly to an absorption system, which may be from a single apparatus (USSR Pat. 181 239, RSR Pat. 65 503), or from multiple apparatuses (US Pat. 3174 911, US Pat. 3 606 732, GB 970 855), optionally cooling indirectly utilizing for evaporation of methanol by circulating methanol between the saturator and the condenser.

The formaldehyde production process with incomplete methanol conversion uses an oxygen-containing methanol-gas mixture with an excess of methanol. The mixture is prepared in a saturator where air, fresh methanol and recycled methanol are supplied. The liquid vaporised into the air is heated by circulating it between the saturator and the condenser. It is indirectly heated with water steam to the required temperature in the saturator. The resulting mixture was heated to 360-390 K and passed to a crystalline silver catalyst bed of 820-920 K.

The rapid indirect cooling of the converted gas immediately downstream of the catalyst bed prevents side reactions. It is carried out with an aqueous solution of formaldehyde from the rectification column using heat to rectify the separation of unreacted methanol from the product and recycle 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 choke. The cooled gases together with the condensed liquid are passed to an absorber where formaldehyde and methanol are scrubbed from the gases with water and cooling. The absorption is carried out in three stages, with a solution of 320-330 K circulating in the flow and gas directions in the first stage, a solution of 300-310 K in the second stage and a fresh water of 276-298 K in the third stage.

The inert gases leaving the absorber leave the process or can be further processed.

245088 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 rectally separate the methanol to be recycled from the product from the product by circulating the solution between the reactor and the rectification column. The rest of the energy required is recombined indirectly with water vapor.

Depending on the type and position of the gas pump, the process is carried out in a pressure mode, wherein the pressure in the apparatus varies from 0.1 MPa upstream of the gas flow or vacuum mode, where the pressure varies from 0.04 MPa to 0.1 MPa. For this reason, there is a significant difference in temperatures in the saturator and hence in the capacitor. The vaporized liquid temperature in the vacuum mode is about 316 K and in the pressure mode is about 338 K. The gases cooled in the condenser leaving for absorption have a temperature of about 330 K in the vacuum mode and a temperature of about 348 K in the pressure system. Condensation efficiency of the liquid components contained in the reaction gases, i. of formaldehyde, methanol and water is superior to the pressure mode in a vacuum mode.

In an improved process for the production of formaldehyde, 0.15 to 1.5 kg of the solution obtained in the absorption stage are added to the reaction gases in the condensation step per kg of formaldehyde contained in the reaction gases.

In the pressure mode of formaldehyde production, more than 1.5 kg of the solution obtained in the absorption stage are added to the reaction gases per kg of formaldehyde contained therein in the condensation step. Since the absorption solution added is colder than 330 K and the temperature in the heated methanol condensation stage is about 338 K, the addition of the absorption solution in the condensation stage does not indicate a higher effect.

SUMMARY OF THE INVENTION The present invention provides a process for separating formaldehyde from reaction gases of oxidative methanol dehydrogenation on a silver catalyst using condensation heat to prepare a gaseous mixture to react a condensed liquid phase in a condenser with circulating contacting the gaseous reaction mixture with a solution obtained in the degree of formaldehyde absorption into water. 2. The method according to claim 1, wherein 1.5 to 15 kg of the solution obtained in the step of absorbing the formaldehyde into water are added to the reaction gases per kg of formaldehyde contained therein in a condensation step at a pressure of 0.04 to 0.1 MPa.

An advantage of the process according to the invention is an increased amount of condensed formaldehyde, water and methanol in the condensation step. To this end, the amount of heat used in the saturation stage to evaporate the methanol from the condensation stage is increased proportionally, and thus the amount of energy supplied to the process in the saturation stage is reduced accordingly. Máο> also results 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 shown in the following examples:

Example 1 (comparative)

To produce 1 kg of formaldehyde, 1.83 kg of air are sucked into the carbonator and 2.16 kg of fresh and recycled methanol are added. To evaporate methanol into air, 1.33 MJ is obtained by circulating methanol through a condenser and 1.08 MJ must be supplied indirectly with water vapor. The mixture thus obtained is passed through a crystalline silver catalyst bed at a pressure of 0.08 MPa and a temperature of 863 K. 1.2 kg of the absorber solution are added to the reactor cooled reaction mixture at the inlet of the condenser. The gases and the condensed liquid are fed to an absorber where 1.25 kg of water is added countercurrently. The solution obtained is fed to a rectification column, where 0.98 kg of methanol is separated and recycled to the process to obtain about 2.7 kg of a 37% by weight aqueous formaldehyde solution. The average methanol consumption is 0.444 kg per kilogram of said solution.

Example 2

To produce 1 kg of formaldehyde, 1.83 kg of air are sucked into the carbonator and 2.16 kg of fresh and recycled methanol are added. 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 obtained is passed through a crystalline silver catalyst bed at a pressure of 0.08 MPa and a temperature of 863 K. 5.7 kg of the solution obtained in the absorber are added to the condenser-cooled reaction mixture at the inlet of the condenser. Gases and condensed liquid. is fed to an absorber where 1.25 kg of water is added countercurrently. The resulting solution is fed to a rectification column, where 0.98 kg of methanol is separated and recycled to the process to obtain about 2.7 kg of a 37% w / w aqueous formaldehyde solution. The average methanol consumption is 0.442 kg per kilogram of said solution.

Example 3

To produce 1 kg of formaldehyde, 1.83 kg of air are sucked into the carbonator and 2.16 kg of fresh and recycled methanol are added. For evaporation of methanol to

245 air is obtained by 1.74 MJ 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 bed at a pressure of 0.08 MPa and a temperature of 863 K. To the reactor-cooled reaction mixture, 5.7 kg of absorber solution is added at the inlet to the condenser. The gases and condensed liquid were fed to the absorber where 0.93 kg of water was added countercurrently. The solution obtained is fed to a rectification column, where 0.98 kg of methanol are separated and recycled to the process to obtain about 2.38 kg of an aqueous formaldehyde solution having a concentration of 42% by weight. The average methanol consumption is 0.502 kg per kilogram of said solution.

Claims (1)

Method of 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 to prepare a gas mixture for reaction, circulating contacting the gas reaction mixture fe / Μ OF THE INVENTION in the step of condensation with a solution obtained in the step of absorbing formaldehyde into water, characterized in that in the step of condensation, at a pressure of 0.04 - 0.1 MPa, 1.5 to 15 kg of solution contained therein are added to the reaction gases per kg of formaldehyde from the degree of absorption of formaldehyde into water.