EP2027066A1 - Method for separating carbon monoxide from a gas containing hydrogen chloride - Google Patents

Abstract

The invention relates to a method for separating carbon monoxide from a gas containing hydrogen chloride, whereby the carbon monoxide is reacted with chlorine to form phosgene, and the phosgene is then separated from the gas containing hydrogen chloride. The gas used, containing hydrogen chloride, originates preferably from the phosgenation or isocyanate-forming reaction.

Description

Process for the separation of carbon monoxide from a hydrogen chloride-containing gas

The present invention relates to a process for the separation of carbon monoxide from a hydrogen chloride-containing gas, which comprises the reaction of the carbon monoxide with chlorine to form phosgene and the subsequent separation of the phosgene from the hydrogen chloride-containing gas. The hydrogen chloride-containing gas used preferably originates from the phosgenation or isocyanate formation reaction. The process according to the invention is preferably used as part of the chlorine cycle of the isocyanate formation reaction.

A variety of chemical processes for the reaction with chlorine or phosgene, such as the production of isocyanates or chlorination of aromatics, lead to a forced attack of hydrogen chloride. Typically, this hydrogen chloride is converted back to chlorine by electrolysis (See, e.g., WO9724320A1). Compared to this very energy-consuming method, provides the direct oxidation of hydrogen chloride with pure oxygen or an oxygen-containing gas to heterogeneous catalysts (the so-called Deacon process) according to

4 HCl + O ₂ "2 Cl ₂ + 2 H ₂ O

distinct advantages in terms of energy consumption (see for example WO 04014845).

In most processes, such as in particular phosgenation, a greater amount of carbon monoxide (CO) may be present as an impurity in the HCl exhaust gas. In the generally used liquid-phase phosgenation, CO contents in the range from 0 to 3% by volume are generally found in the HCl offgas of the phosgene wash-off column. In the case of gas-phase phosgenation (DE 42 17 019 A1, DE 103 07 141 A1), higher amounts of CO (0 to more than 5% by volume) are to be expected, since in this process preferably no condensation of phosgene, and one with it Connected separation of carbon monoxide, before the phosgenation is performed.

In conventional catalytic HCl oxidation with oxygen, various catalysts are used, for example based on ruthenium, chromium, copper, etc. However, these can simultaneously act as oxidation catalysts for possibly present secondary components such as carbon monoxide or organic compounds. The catalytic carbon monoxide oxidation to carbon dioxide, however, is extremely exothermic and causes uncontrolled local temperature increases at the surface of the heterogeneous catalyst (hot spots) in the way that deactivation can take place. In fact, the oxidation of 5% carbon monoxide in an inert gas (N ₂ ) at an inlet temperature of 250 ⁰ C (operating temperatures of the Deacon process: about 200-450 ⁰ C), a temperature increase of well over 200 degrees at an adiabatic Cause implementation. A cause for the Catalyst deactivation lies in the known microstructural change of the catalyst surface, for example by sintering processes, due to the hot-spot formation. Furthermore, the adsorption of carbon monoxide on the surface of the catalyst can not be excluded. The formation of metal carbonyls can be reversible or irreversible and thus be in direct competition with HCl oxidation. In fact, carbon monoxide with some elements, such as osmium, rhenium, ruthenium (see Chem. Rev. 103, 3707-3732, 2003), can form very stable bonds even at high temperatures and thus cause an inhibition of the desired target reaction. Another disadvantage could arise from the volatility of these metal carbonyls (see Chem. Rev., 21, 3-38, 1937), whereby not inconsiderable amounts of catalyst are lost and, in addition, require a complicated purification step, depending on the application.

Thus, JP-A-62-270404 (EP-A-0233773) describes a method for HCl oxidation with oxygen, wherein the carbon monoxide content of the gas used is previously adjusted to less than 10 vol .-% by

^■ palladium-catalyzed combustion to carbon dioxide,

^■ Distillative separation of the HCl gas, or

^■ Washing the gas with a solution of copper chloride

to increase the life of the catalyst used.

A similar oxidation process for removing carbon monoxide is described in JP2003-171103.

In another known method, the exhaust gas containing hydrogen chloride is passed into an aqueous alkaline absorption system and the exhaust gas freed of hydrogen chloride and phosgene is fed to a combustion plant.

A disadvantage of all previous methods is, in particular, that with the removal of the CO, a precious raw material is destroyed.

Therefore, on the one hand there was the desire to separate the carbon monoxide from the HCl-containing exhaust gases, thereby avoiding disadvantages caused in particular in a subsequent Deacon process, and on the other hand there was the desire to supply the carbon monoxide as economically as possible use. The inventors have now found that it is extremely advantageous to convert the carbon monoxide present in an HCl exhaust gas, in particular an isocyanate synthesis or a chlorination reaction, to phosgene with chlorine, to separate off the phosgene formed and, in particular, to recycle the isocyanate synthesis. The essentially CO-free waste gas is supplied in particular to a Deacon process, it being possible in turn for the resulting chlorine to be used for the preparation of the phosgene. The inventive method dispenses in particular with the separation of CO from the phosgene, in whose synthesis it is used in excess, by the particularly energy-consuming condensation of the phosgene. The carbon monoxide can be left in the phosgene during the isocyanate formation, and is subsequently separated from the exhaust gas before the HCl oxidation by the process according to the invention. In the Deacon process, the formation of hotspots and the associated catalyst deactivation due to the exothermic formation of CO from CO are not to be feared. Furthermore, there is no accumulation of carbon dioxide in the recycle stream in the Deacon process.

The present invention thus provides a process for separating carbon monoxide from a hydrogen chloride-containing gas, which comprises reacting the carbon monoxide with chlorine to form phosgene.

In a preferred embodiment of the method according to the invention, the separation of the phosgene then takes place from the gas containing hydrogen chloride, which is subsequently subjected to the hydrogen chloride oxidation by the Deacon process.

In principle, all gases containing hydrogen chloride (HCl) and carbon monoxide (CO) can be used in the process according to the invention. Preference is given to process gases resulting from isocyanate production by reaction of organic amines with phosgene or gases resulting from the chlorination of hydrocarbons. The hydrogen chloride-containing gases used have, for example, from about 0.1 to about 20% by volume, preferably from about 0.5 to 15% by volume, of carbon monoxide. The content of hydrogen chloride is, for example, from 20 to 99.5% by volume, preferably from 50 to 99.5% by volume. The remaining gases of the hydrogen chloride-containing gas are, for example, nitrogen, oxygen, carbon dioxide and noble gases. They form, for example, from about 0.5 to 80% by volume of the hydrogen chloride-containing gas.

The reaction of the carbon monoxide in the hydrogen chloride-containing gas used takes place in a manner known per se, in particular by the reaction of the carbon monoxide with chlorine to form phosgene, for example on an activated carbon catalyst. Alternative catalysts can also be used. Reference may be made to the prior art (eg DE 3327274; GB 583477; WO 97/30932; WO 96/16898; US 6713035), the content of which belongs to the Offenbamngsgehalt the present patent application.

Particularly preferred parameters of the reaction of CO with Cl ₂ to COCl ₂ are:

■ Catalyst: activated carbon

^■ slight molar excess of chlorine (about 1, 0 to 1, 5 moles of Cl ₂ per mole of CO),

^■ Temperature range from 20 to 600 ⁰ C

^■ pressure range from 1 to about 20 bar;

^■ Working under pressure allows the size of the reaction ^vessel to be reduced and facilitates the subsequent separation of the phosgene which is generally formed,

^■ Apparatus: fixed bed reactor.

In contrast to the "normal" preparation of the phosgene, the separation of the carbon monoxide can be carried out with a molar excess of chlorine in order to remove the carbon monoxide as completely as possible In "normal" phosgene production, an excess of carbon monoxide is used to prevent residual chlorine in the phosgene formed.

After reaction of the CO with the CI ₂ , the phosgene formed is generally separated by at least one operation selected from the group consisting of:

^■ liquefaction or condensation of the phosgene,

^■ The liquefaction (with cooling and / or pressure) may optionally after previous drying of the gas mixture, such as in DE-A-1,567,599, GB 737 442 describes take place (the content of which is part of the disclosure of the present application).

^■ It should be emphasized that the amount of phosgene liquefied at this point is, of course, much smaller than the amount of phosgene which would be liquefied after the actual phosgene production to separate off the CO.

^■ Distillation or rectification and / or ■ Washing out the phosgene with solvents, such as monochlorobenzene, ortho-dichlorobenzene).

Preferably, the separation of the phosgene by condensation or distillation.

The thus separated phosgene is preferably recycled to a phosgenation reaction, in particular the isocyanate production. Particularly preferably, the separated phosgene is recycled to the same phosgenation reaction in which the hydrogen chloride-containing gas used according to the invention was formed.

The resulting hydrogen chloride-containing gas has a CO content which is in particular less than 1% by volume, more preferably less than 0.5% by volume.

The hydrogen chloride-containing gas is preferably subjected to the catalytic oxidation with oxygen in a conventional manner after the separation of the phosgene. This process is commonly referred to as the "Deacon process." Reference may be made to the relevant prior art for the performance of HCl oxidation.

Preferred parameters are:

^■ Catalyst: ruthenium, chromium, copper, bismuth compounds

^■ Molar ratios HC1 / O ₂ : 4/1 to 1/1

^■ Temperature range from 200 to 450 ⁰ C

^■ pressure range from 1 to about 100 bar;

^■ Apparatus: fixed bed, fluidized bed, microreactor

^■ Reaction: isothermal or adiabatic.

A particularly preferred embodiment of the method according to the invention comprises the following steps:

^■ production of phosgene by reaction of CO with CI ₂ ,

^■ subsequent use of the phosgene in the synthesis of organic isocyanates (this step is carried out according to the invention particularly preferably without prior separation of the CO),

^■ separation of the obtained in the phosgenation of organic isocyanates, Separation ^{according to the} invention of the carbon monoxide from the HCl-containing resulting exhaust gas of the isocyanate synthesis by reaction with chlorine to form phosgene,

^■ separation of the phosgene formed,

^■ recycling of the formed phosgene into the isocyanate synthesis,

^■ subjecting the HCl-containing, CO-depleted gas from the HCl oxidation and, optionally, recycling the CI ₂ formed in the production of phosgene, this being both the initial phosgene may be as well as the place later phosgene under the CO removal from the HCl process gas.

Alternatively, the phosgene can be removed from the gas after HCl oxidation.

Illustrations 1 and 3 described below illustrate the method performed according to the invention. In contrast, FIG. 2 illustrates a conventional process in which the CO formed in the phosgene synthesis is first separated by condensation of the phosgene and converted into phosgene in a subsequent combination with CI ₂ . The disadvantage of this process, as already explained above, is that the entire phosgene is condensed, which is very energy-consuming.

In contrast, Figure 1 shows schematically the method according to the invention. The HC1 / CO feed gas, which preferably originates from a phosgenation or isocyanate preparation process, is first preferably reacted on an activated carbon catalyst using chlorine to form a HCl / phosgene gas mixture. Subsequently, the separation of the phosgene, which is preferably fed again into the phosgenation or isocyanate production process takes place. Subsequently, the reaction of the remaining HCl gas in the HCl oxidation according to the Deacon process expediently takes place in a manner known per se, the process gas optionally being able to be returned to the Deacon reactor after removal of the chlorine.

Figure 3 shows the incorporation of the process according to the invention in the course of an isocyanate synthesis. In this case, the CO used in excess in the phosgene synthesis does not need to be separated initially, as a result of which an energy-consuming condensation of the phosgene is eliminated. Also no Nachvereiniger is needed. The CO-containing phosgene will be so in the

Isocyanate synthesis (or other synthesis) used. After separation of the formed

Isocyanate, the resulting CO / HCl-containing exhaust gas is subjected to the erfϊndungsgemäßen separation process to form phosgene, which is separated and in the

Phosgenation reaction can be attributed. The CO-depleted HCl gas, which preferably has less than about 0.5% CO by volume, is then preferably added to the deaconate. Process, ie, subjected to the catalytic oxidation of hydrogen chloride with oxygen to form Ch. The formed Cl ₂ is separated and recycled to the phosgene synthesis process. If appropriate, the residual gas can be recirculated to the Deacon process. The isocyanate synthesis is carried out in a conventional manner. Phosgene obtained by the process according to the invention can then be used according to the processes known from the prior art for the preparation of TDI or MDI from TDA or MDA. The hydrogen chloride which is formed in turn during the phosgenation of TDA and MDA can then be converted to chlorine by the processes described.

By the method according to the invention, the carbon monoxide content is significantly reduced in the HCl stream, whereby a deactivation of the Deacon catalyst is slowed down at the next stage by uncontrolled temperature increase. At the same time, the precious carbon monoxide will be reused by conversion to phosgene.

Claims

claims A process for separating carbon monoxide from a hydrogen chloride-containing gas comprising reacting the carbon monoxide with chlorine to form phosgene. 2. The method of claim 1, which comprises the subsequent separation of the phosgene formed from the hydrogen chloride-containing gas. A process according to claim 1 or 2, wherein the hydrogen chloride-containing gas contains from 0.5 to 15% by volume of carbon monoxide. 4. The method according to any one of claims 1 to 3, wherein the hydrogen chloride-containing gas contains from 20 to 99.5 vol .-% of hydrogen chloride. 5. The method according to any one of claims 1 to 4, wherein the reaction of the carbon monoxide with chlorine to phosgene on a catalyst, preferably activated carbon catalyst occurs. 6. The method according to any one of claims 1 to 5, wherein the separation of the phosgene is carried out by at least one operation which is selected from the group consisting of: liquefaction or condensation of the phosgene, distillation or Rectification and / or washing out of the phosgene with solvent. A process according to any one of claims 1 to 6, wherein the separated phosgene is recycled to a phosgenation reaction. A process according to claim 7, wherein the separated phosgene is recycled to the same phosgenation reaction in which the hydrogen chloride-containing one is used Gas was formed. A process according to any one of claims 1 to 8, wherein the hydrogen chloride-containing gas is subjected to catalytic oxidation with oxygen after separation of the phosgene.