CS250591B1 - Method of formaldehyde production from methanol - Google Patents

Abstract

The solution relates to a process for producing formaldehyde by oxidizing dehydrogenation of methanol in the presence of water steam at elevated temperature on silver the catalyst at which it is achieved increased utilization of reaction heat for own the need for a process. The reaction mixture is indirectly cooled it cools and partly condenses by heating and / or by evaporation of methanol or methanol / water under reduced pressure and the resulting vapor or a mixture of steam and liquid methanol or methanol and water compresses and leads to the air choke and / or a mixer of air and methanol; \ tor. \ t air, methanol and water.

Description

The present invention relates to a process for the production of formaldehyde by oxidative dehydrogenation of methanol with air, optionally in the presence of water vapor at elevated temperature on a silver catalyst, which results in increased utilization of the reaction heat for the process's own use.

The reaction mixture is indirectly cooled in the cooling stage and partially condensed by heating and / or evaporating the methanol or methanol / water mixture under reduced pressure and the resulting steam or a mixture of steam and liquid methanol or methanol and water is compressed and passed to an air choke and / or air mixer. and methanol or air, methanol and water.

The present invention provides a process for the production of formaldehyde by oxidative dehydrogenation of methanol with air, optionally in the presence of water vapor, at elevated temperature on a silver catalyst, which results in increased utilization of reaction heat for the process's own use.

The formation of formaldehyde by oxidative dehydrogenation of methanol with air on a silver catalyst, optionally in the presence of water vapor, proceeds essentially according to the following reactions:

CH3OH --- HCHO + H2; ΔΗ = 82.8 kJ / mol (, a)

CH 3 OH + 1/2 O 2 -> HCHO + H 2 O;

Δ = —163.3 kj / mol (b)

While ^one the first reaction is endothermic, the second reaction is exothermic, and the system can be maintained in a dynamic equilibrium in which the reaction of (bi main source of heat for the reaction (a). To this balance, however, occurs only at temperatures above 500 DEG C., generally the 550 ° C, whereby the preparation of the reaction mixture and the treatment of the reaction products are very energy intensive.

Essentially, two processes are established in industrial mass production. In the reaction of methanol with air in the absence of substantial proportions of water or inert, it is economical to run a process with incomplete conversion of methanol, usually only to 50 to 60%, while achieving high selectivity with respect to the formaldehyde produced. However, the condensed crude formalin contains a considerable amount of unreacted methanol which must be recovered from it by energy-intensive rectification under reduced pressure. According to the second process, a certain proportion of water or inert gases is added to the reaction mixture to allow the reaction to proceed to almost complete methanol conversion, albeit with slightly impaired selectivity. The product obtained contains 2 to 3 wt. % of methanol and is usually no longer rectified, but transmitted as such.

The preparation of the synthesis mixture consists of evaporating the appropriate balance amount of methanol or a mixture of methanol and water, mixing with air and heating to a temperature above the dew point of the mixture. its energy intensity is between 1,2 and 1,6 GJ / t of product produced. Also, rectifying methanol recovery and the associated production of the final commercial product requires 1.5 to 1.7 GJ / t of product produced. In order to cover all or part of these needs, there is a heat source in the process - the reaction fumes leaving the reactor at a temperature above 500 ° C. Their apparent and latent heat is 2.6 to 2.8 GJ / t of formalin produced, and if used well, the plant could be partially or fully heat self-sufficient.

For technological reasons, it is desirable to rapidly cool the reaction mixture to a temperature below 300 ° C to suppress the decomposition reactions of the formaldehyde formed. The reactor therefore includes a cooler or a set of coolers in which the heat of reaction is generally used more or less at the same time.

A known method of utilizing the heat content of the reaction gases is to produce water vapor. So eg. According to French Patent No. 1,569,137, the reaction gases were indirectly cooled by evaporation of water, in a first stage at elevated pressure (about 10 bar, in a second stage under vacuum (at a pressure of 30 to 50 kPa). converts to a usable temperature and pressure level (temperature above 100 ° C, pressure above 0.1 MPa) by means of an ejector, where the high pressure steam obtained in the first cooling stage is used for its compression. The cooling stage can be used for two-stage pre-heating of the inlet reaction mixture and the steam obtained under vacuum, transferred by a multi-stage compressor to a temperature above 100 ° C and a pressure above 0.1 MPa and used to heat the rectifier column.

Another known method of utilizing the reaction heat is based on the fact that the indirect cooling of the reaction gases in the primary cooler is effected by heating and, in part, by evaporating the bottom product from the rectification column. The product is circulated between the bottom of the column and the primary cooler and utilizes some of the apparent heat of the reaction gases to heat the rectification column. After pre-cooling to 100 to 120 ° C, the gas mixture enters the secondary condenser where it is indirectly cooled with a mixture of fresh and regenerated methanol circulating between the secondary condenser and the apparatus where the synthesis mixture is prepared for the reaction. The methanol is heated to 64-69 ° C (according to working pressure) and partially evaporated to cover part of the heat demand (20-30%) to prepare the synthesis mixture. By this arrangement, 55 to 60% of the total apparent and latent heat of the reaction gases can be utilized. The reaction fumes are fed to a absorption column at a temperature of 75-80 ° C where their heat content is no longer used.

Said methods of solution are nothing economical and a part, in particular the condensation heat of the crude formaldehyde, is lost in the absorption column substantially without any benefit. This is because this heat is already available at a low temperature level (75 to 80 degrees Celsius) and its immediate use is not obvious.

According to this method, formaldehyde is produced from methanol in the presence of air and optionally water with increased heat utilization of the reaction mixture by indirectly cooling the reaction mixture in the primary and / or secondary cooling stage and partially condensing by heating and / or evaporating methanol or methanol / water. under reduced pressure, preferably 40 to 60 kPa, and the resulting steam or a mixture of steam and liquid methanol or methanol and water is compressed, preferably to a pressure of 0.14 to 0.18 MPa, and fed to an air choke and / or air-methanol mixer or air, methanol and water.

This process has a number of advantages over known and used processes. In comparison with processes that use reaction heat to produce steam, heat is obtained here already in a usable technological medium - methanol vapors or a mixture of methanol and water vapors.

The direct use of heat in the technological process does not require an additional apparatus (exchanger, heater, etc.) and simplifies the movement of media in production.

Working under reduced pressure, preferably 40 to 60 kPa, makes it possible to maintain a low temperature (typically 40 to 50 ° C) in the cooling space of the first and / or second chillers of the reaction mixture and thereby achieve better utilization of the exchange surface by improving the temperature gradient. At the same time, the outlet temperature of the thermal stream can also be reduced with a better utilization of the apparent and in particular the condensation heat of the reaction products. The energy required for the poor compression of the vapors of methanol or methanol and water to the working pressure is substantially lower than the heat obtained and the whole system thus shows a markedly better use of heat.

The advantages of an embodiment of the invention are illustrated in the accompanying examples, the first of which is comparative.

Example 1 (comparative)

The reaction gases from the formaldehyde production are withdrawn from the catalyst at a temperature of 586 ° C and a heat content of 1.672 GJ / t. They are fed to a primary cooler where they are indirectly cooled to 120 ° C while transferring their heat to produce a 110 ° C steam at a pressure of 0.15 MPa. The total amount of heat transferred is 0.716 GJ / t reaction mixture. In the secondary cooler, the reaction mixture was indirectly cooled to 77 ° C, and a small portion of methanol, formaldehyde and water condensed therefrom. Methanol, which is circulated between the air saturator and the secondary cooler at temperatures of 65 to 69 ° C, serves as a cooling medium and partially evaporates in the cooling space. In this section, 0.231 GJ / t of reaction gases are utilized, and from the total heat content of the reaction gases, it is returned directly to the process of 0.947 GJ / t, t. j. 56.6 percent.

Example 2

The reaction gas was cooled to 120 ° C in the primary condenser as in Example 1 after leaving the reactor at 586 ° C (1.672 GJ / t heat content), but in the secondary condenser methanol was evaporated at 50 kPa and 49 ° C on the cooled side. . In a condenser of the same size as in Example 1, the reaction mixture is cooled to 59-60 degrees Celsius and 0.611 GJ / t of reaction gases are transferred to the evaporated methanol. The evaporated methanol is compressed to a pressure of 0.14 MPa (required power input of 0.084 GJ / t reaction gases), heated to 100 ° C and fed to a choke-mixer, where a synthesis mixture is formed by mixing with the balance amount of air.

The directly utilized heat in this arrangement is 1.243 GJ / t (heat dissipated in both coolers minus the compressor input), t. j. 74.3% of the total heat content of the reaction gases.

Claims (2)

PEEDMET Process for the production of formaldehyde from methanol in the presence of air and possibly water with increased heat utilization of the reaction mixture, characterized in that the reaction mixture is indirectly cooled in the primary and / or secondary cooling stage and partially condensed by heating and / or evaporating methanol or methanol / water at under reduced pressure, preferably 40 to 60 kPa, and the steam or liquid vapor / liquid methanol or methanol / water mixture obtained is compressed, preferably to a pressure of 0.14 to 0.18 MPa, and is fed to an air choke and / or air-methanol mixer, or air, methanol and water.