Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2001-09-11
2003-09-23
Dentz, Bernard (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C549S256000, C549S257000, C549S258000, C549S260000
Reexamination Certificate
active
06624315
ABSTRACT:
The present invention relates to a process for the preparation of maleic anhydride by heterogeneously catalyzed gas-phase oxidation.
Maleic anhydride has considerable industrial importance. For example, it can be used for both condensation polymerization and addition polymerization, with polyester resins and alkyd resins being the most important industrial secondary products. In addition, maleic anhydride is the starting material for commercially important chemicals, such as succinic anhydride, gamma-butyrolactone, 1,4-butanediol and tetrahydrofuran.
In known commercial-scale processes for gas-phase oxidation to give maleic anhydride, a reaction mixture comprising n-butane, oxygen and further components, such as nitrogen and steam, is converted at from 320 to 480° C. in a catalyst bed consisting of individual particles into a reaction mixture which, besides the principal component maleic anhydride, furthermore comprises steam, carbon monoxide, carbon dioxide, unreacted butane, inert gases, for example nitrogen, and further organic trace components. The enthalpy of reaction liberated during the process heats the reaction mixture. Strong warming of the reaction mixture reduces the yield of the desired valuable product maleic anhydride due to nonselective superoxidation, with very strong warming entailing the risk of the reaction becoming a combustion reaction in the gas phase, with a considerable increase in temperature and pressure.
In order to be able to limit the increase in temperature, some of the enthalpy of reaction is therefore dissipated in the reactors employed via the reactor walls surrounding the catalyst. For this purpose, the reaction space is divided into a large number of parallel individual reaction spaces in the form of tubes having an internal diameter of from 20 to 45 mm. Reaction mixture flows from top to bottom through the vertical individual tubes filled with catalyst bed, with more than 60% of the enthalpy of reaction being released via the tube wall to a heat-exchange medium flowing around the tubes. The heat-exchange medium used is generally an inorganic salt melt, but it is also possible to use organic heat-exchange media, metal melts or gases, such as helium. In order to limit the warming of the heat-exchange medium in the reactor to less than 20° C., high circulation quantities and corresponding pump capacities are necessary. Re-cooling of the heat-exchange fluid is usually carried out by the generation of steam at tubes through or around which the heat-exchange medium flows.
For high selectivity of the reaction, it is necessary to limit the axial and radial temperature gradients in the reaction tube to less than 30° C. For the axial temperature gradient, the internal preheating zone in the reaction tube, in which the reaction mixture is heated to the reaction temperature, and which can also be positioned in a heat exchanger upstream of the actual reactor, remains out of consideration. The maximum permissible radial temperature gradient determines the maximum tube diameter, while the minimum gas velocity necessary for adequate convective heat transfer in the tube filled with catalyst bed, together with the specific reaction rate, determines the minimum tube length. In order to meet these requirements, the reactors for commercial-scale processes generating about 30,000 metric tons per annum of maleic anhydride contain from 12,000 to 40,000 reaction tubes connected in parallel. The minimum tube length and the minimum gas velocity in turn result in a loss of flow pressure over the reactor of greater than 0.4 bar. In order to avoid nonuniform flow into the reaction tubes and thus corresponding selectivity losses and the risk of changeover into a homogeneous combustion reaction in the gas phase due to local overheating, the pressure loss in each of the numerous reaction tubes is therefore usually equalized in a complex manner. The filling of the reaction with catalyst and the removal thereof are therefore very time-consuming and expensive.
Reactors of this type thus have firstly the disadvantage that a very large number of individual reaction tubes are necessary, resulting in a complex design and high costs. In addition, it is disadvantageous that an intermediate circuit containing a heat-exchange medium is necessary, which in turn means that high pump capacities are necessary and additional costs arise for re-cooling of the heat-exchange medium. In addition, a loss of exergy occurs. A further disadvantage is that the high pressure loss in the individual reaction tubes and the equalization necessary therefor result in very complex handling of the catalyst.
DE-C-197 54 185 describes, for example, a reactor having a cylindrical reactor tank, with heat-exchanger plates in the form of thermal plates being arranged alongside one another at a prespecified spacing from one another in the reactor tank in vertical orientation on the perforated base of the reactor. A cooling medium which is fed to the heat-exchanger plates via suitable devices in the region of the tank lid and is discharged from the heat-exchanger plates via suitable devices in the region of the tank base flows through the plates. A gaseous reaction medium is passed between the heat-exchanger plates in countercurrent to the cooling medium, with feeding in the region of the tank base and discharge in the region of the tank lid. The specification gives absolutely no indication that a reactor of this type can be employed for heterogeneously catalyzed gas-phase oxidation to give maleic anhydride.
DE-A-197 19 375 describes a process for the preparation of ethylene oxide by catalytic gas-phase oxidation of ethylene using oxygen in a reactor, where the catalyst is arranged in reaction zones between heat-exchanger plates, and the gaseous reaction mixture flows through the catalyst. In catalytic gas-phase oxidation to give ethylene oxide, a comparatively small amount of heat is developed per volume unit of the catalyst.
It is an object of the present invention to provide a process for the preparation of maleic anhydride which exhibits increased economic efficiency, in particular with respect to the consumption of heat-exchange medium, even at very high conversions and in plants of large capacity.
We have found that this object is achieved by carrying out the heterogeneously catalyzed gas-phase oxidation to give maleic anhydride in a reaction space between heat-exchanger plates and thus in a two-dimensional catalyst bed which extends beyond the reactor cross section. Surprisingly, an unforeseeable increase in selectivity of the formation of maleic anhydride has been found here.
The invention thus relates to a process for the preparation of maleic anhydride by heterogeneously catalyzed gas-phase oxidation in a reactor with feed for the reaction mixture at one end of the reactor and discharge of the product mixture at the opposite end of the reactor, and with devices for dissipating the heat of reaction which are arranged in the reactor interior and through which a heat-exchange medium flows, wherein the devices are heat-exchanger plates.
Preferred embodiments of the invention are defined in the following description, the figures and the dependent claims.
The starting compounds used can in principle be any starting materials known for the preparation of maleic anhydride, in particular n-butane, n-butene or mixtures thereof, and benzene or butadiene. The most preferred is the preparation from n-butane. The catalytic gas-phase oxidation using molecular oxygen or a gas containing the latter, for example air, is advantageously carried out, with air being particularly preferred. In addition, the reaction mixture may also comprise further components, for example nitrogen, steam or other inertizing diluent gases, for example carbon dioxide, carbon monoxide, isobutane and/or methane.
The gas-phase oxidation is preferably carried out at temperatures in the range from 320° to 480° C., in particular from 380° to 450° C., and, where appropriate, superatmospheric pressure, preferably at a pressure of from 1 to 6 bar absol
Hechler Claus
Machhammer Otto
Olbert Gerhard
Stabel Uwe
Weck Alexander
BASF - Aktiengesellschaft
Dentz Bernard
Keil & Weinkauf
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