Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2002-09-05
2004-10-12
Dentz, Bernard (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06803473
ABSTRACT:
The present invention relates to a process for preparing maleic anhydride by heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms using oxygen-containing gases in the presence of a volatile phosphorus compound over a catalyst comprising vanadium, phosphorus and oxygen in a shell-and-tube reactor unit having at least two successive cooled reaction zones.
Maleic anhydride is an important intermediate in the synthesis of &ggr;-butyrolactone, tetrahydrofuran and 1,4-butanediol, which are in turn used as solvents or are processed further to give, for example, polymers such as polytetrahydrofuran or polyvinylpyrrolidone.
The preparation of maleic anhydride by heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms using oxygen in a shell-and-tube reactor over a catalyst comprising vanadium, phosphorus and oxygen is generally known and described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 6
th
edition, 1999 Electronic Release, Chapter “MALEIC AND FUMARIC ACID—Maleic Anhydride”. In general, benzene or C
4
-hydrocarbons such as 1,3-butadiene, n-butenes or n-butane are reacted. The catalysts comprising vanadium, phosphorus and oxygen, hereinafter referred to as “VPO catalysts”, are used in unpromoted (see U.S. Pat. Nos. 4,429,137, 4,562,268, 5,641,722 and 5,773,382) or promoted (see U.S. Pat. No. 5,011,945, 5,158,923 and 5,296,436) form. According to U.S. Pat. No. 4,562,268, Example 18, a maximum space-time yield of 76 g/lh, i.e. 76 g of maleic anhydride per 1 of catalyst and hour, is achieved. For the purposes of the present invention, the space-time yield is the amount of desired product, i.e. maleic anhydride, in grams produced per hour and volume of the catalyst bed in liters in continuous operation.
The most important objective of the heterogeneously catalyzed gas-phase oxidations of hydrocarbons to maleic anhydride is essentially to achieve a very high space-time yield over a long period of some months.
Thus, it was recognized in U.S. Pat. No. 3,296,282 that deactivation of the VPO catalyst can be suppressed by addition of an organic phosphorus compound. The best results were achieved when the phosphorus compound was fed in after interruption of the oxidation reaction. In the oxidation of 2-butene, a maximum space-time yield of 77 g/lh was able to be achieved (see Example 1 in the document cited).
EP-A-0 123 467 teaches that the addition of the phosphorus compound can also be carried out continuously during the oxidation reaction with addition of steam. In the oxidation of n-butane, a space-time yield of 68 g/lh was achieved (see Example 1 in the document cited).
In U.S. Pat. No. 4,515,899, it was recognized that the activity of the VPO catalyst is damped by addition of a phosphorus compound. This makes possible a higher hydrocarbon throughput at improved selectivity, which leads to an increased space-time yield. In the oxidation of n-butane, a space-time yield of 90 g/lh was achieved (see Example 3 in the document cited).
U.S. Pat. NO. 5,185,455 discloses optimization of the process parameters in the oxidation of n-butane to maleic anhydride-over a VPO catalyst in the continual presence of trimethyl phosphate and water vapor. The maximum space-time yield achieved was 104 g/lh (see Example 2 in the document cited).
The conversion of the hydrocarbons into maleic anhydride is strongly exothermic and is accompanied by many possible parallel and subsequent reactions. Consequently, an increase in the hydrocarbon throughput, which is desirable for achieving a high space-time yield, results in a decrease in the selectivity of desired product formation at the same hydrocarbon conversion due to the increased generation of heat.
EP-A-0 099 431 has proposed countering the abovementioned effect of reduced selectivity at increased generation of heat by use of a structured catalyst bed, i.e. a bed in which the activity varies according to position. The lowest catalyst activity is at the reactor inlet and the highest is at the reactor outlet. In between, it can vary continuously or in steps. For the targeted setting of the catalyst activities, the document teaches essentially dilution of the active catalyst particles with inert material and the use of catalysts of differing activity and possibly mixtures thereof.
U.S. Pat. No. 5,011,945 describes the use of a structured catalyst bed in the oxidation of n-butane over a VPO catalyst in the presence of a volatile phosphorus compound, with the unreacted n-butane being recirculated after maleic anhydride and by-products have been separated off. The lowest catalyst activity is at the reactor inlet and the highest is at the reactor outlet. A space-time yield of at most 95 g/lh is described.
WO 93/01155 discloses a process for preparing maleic anhydride from n-butane over a VPO catalyst in the presence of a volatile phosphorus compound, in which the catalyst activity is varied with temperature and n-butane concentration in the flow direction of the gas in such a way that the reaction rate is promoted by high activity in a region of low temperature and low n-butane concentration within the bed and is damped by low activity in a critical region within the bed where the combination of temperature and n-butane concentration would lead to an excessive increase in conversion and reaction temperature. The maximum space-time yield achieved was 129 g/lh (see Example 6 in the document cited).
Wellauer et al., Chem. Eng. Sci. Vol. 41, No. 4 (1986), pages 765 to 772, describe a simulation model for the oxidation of n-butane to maleic anhydride on the basis of experimental data for a known catalyst. With the objective of a simple modeling concept, the hot spot maximum, i.e. the maximum temperature in the catalyst bed resulting from the exothermic reaction, was identified as critical parameter in terms of yield. The publication teaches two measures for optimizing the process: (a) the use of two catalyst beds with a low activity at the reactor inlet and a high activity at the reactor outlet and (b) the setting of two different salt bath temperatures. Even by mathematical optimization, it was only possible to achieve an increase in the experimentally determined space-time yield from 60 g/lh to 62 g/lh for (a) and 63 g/lh for (b) compared to the comparative method using a catalyst bed at a constant salt bath temperature (see Table 2 in the document cited).
According to the present invention, it was recognized that the n-butane throughput over the catalyst can no longer be increased significantly under the conditions disclosed in Wellauer et al., since the temperature of the hot spot maxima would increase significantly. This in turn would result in a significant reduction in the selectivity and thus a significant decrease in the yield and space-time yield.
It is an object of the present invention to develop a process for preparing maleic anhydride by heterogeneously catalyzed gas-phase oxidation of a hydrocarbon having at least four carbon atoms using oxygen, which makes it possible to achieve, at high hydrocarbon throughput over the catalyst, a high conversion, a high selectivity and a high yield of desired product and thus a significantly higher space-time yield than according to the prior art. A further object of the present invention was to find flexible reaction conditions which make it possible to obtain a high space-time yield over a prolonged period even in the event of fluctuations in the amount, quality or purity of the starting materials or in the event of progressive catalyst deactivation.
We have found that these objects are achieved by a process for preparing maleic anhydride by heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms using oxygen-containing gases in the presence of a volatile phosphorus compound over a catalyst comprising vanadium, phosphorus and oxygen in a shell-and-tube reactor unit having at least two successive cooled reaction zones, wherein the temperature of the first reaction zone is
Ruppel Wilhelm
Storck Sebastian
Tenten Andreas
Weiguny Jens
BASF - Aktiengesellschaft
Dentz Bernard
Keil & Weinkauf
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