Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2000-11-15
2004-03-02
McKane, Joseph K. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C502S353000
Reexamination Certificate
active
06700000
ABSTRACT:
The present invention relates to a process for preparing phthalic anhydride in which the catalytic gas-phase oxidation of o-xylene and/or naphthalene is carried out over at least three zones of catalysts of increasing activity and in which the increase in activity of the zones is effected in a particular way.
It is known that many carboxylic acids and/or carboxylic anhydrides are prepared industrially by the catalytic gas-phase oxidation of aromatic hydrocarbons such as benzene, the xylenes, naphthalene, toluene and durene in fixed-bed reactors, preferably multitube reactors. This is used to obtain, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride. This is generally carried out by passing a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized through a plurality of tubes arranged in a reactor. Each of the tubes contains a bed of at least one catalyst. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt. Despite this thermostatting, hot spots in which the temperature is higher than in the remainder of the catalyst bed may occur. These hot spots cause secondary reactions such as total combustion of the starting material or lead to formation of undesirable by-products which can be separated from the reaction product only with difficulty, if at all, for example the formation of phthalide or benzoic acid in the preparation of phtahlic anhydride (PA) from o-xylene. Furthermore, the formation of a pronounced hot spot prevents rapid running-up of the reactor, since the catalyst can be irreversibly damaged above a certain hot spot temperature, so that the loading can only be increased in small steps and the increase has to be monitored very carefully (hereinafter referred to as running-up phase).
To decrease the intensity of these hot spots, it has become customary in industry to arrange catalysts having different activities in zones in the catalyst bed, with the less active catalyst generally being located in the fixed bed in a position where the reaction gas mixture comes into contact with it first, i.e. it is at the gas inlet end of the bed, while the more active catalyst is located toward the end where the gas leaves the catalyst bed (DE A 2546268, EP 286 448, DE 2948163, EP 163 231). The catalysts of differing activity in the catalyst bed can be exposed to the reaction gas at the same temperature, but the two zones comprising catalysts of differing activity can also be thermostatted to different reaction temperatures for contact with the reaction gas (DE A 2830765). According to EP 163 231, a plurality of the measures mentioned can be employed simultaneously for setting the activity structure described. It is known from WO 98/00778 that the addition of temporary activity dampers can lead to a shortening of the running-up phase. In EP 676 400, multistructuring in the reaction of tetraalkylbenzenes to give pyromellitic anhydride has been found to have a positive effect in respect of yield and product purity if the activity structuring is carried out such that the catalyst activity first increases and then decreases again in the flow direction of the gas. Finally, EP 99 431 states that, in the reaction of butane to give maleic anhydride, activity structuring of the catalyst bed has a positive effect on the productivity if the catalyst activity increases stepwise (or ideally continuously) in the flow direction of the gas; the activity structuring can be achieved by many different methods, preferably by dilution with inert material. As a result of the stepwise activity increase, a more homogeneous distribution of the energy liberated by the exothermic reaction can be achieved, so that larger amounts of MA can be produced. Since the reaction is carried out at partial conversion, the activity structuring can be achieved in virtually any way. These teachings cannot be applied to the preparation of PA by oxidation of o-xylene and/or naphthalene since, as is known, phthalic anhydride of good quality is obtained only when the reaction is carried out at full conversion (i.e. >99.9% conversion, based on the starting material used) in order to minimize contamination by undesirable, color-producing components such as phthalide or naphthoquinone and to avoid contamination of the waste gas by residual xylene or residual naphthalene.
EP-A 539 878 discloses a method for production of phthalic anhydride from o-xylene and naphthalene over a two-stage catalyst. The catalysts in both stages are so constructed that an inactive carrier is loaded with vanadium pentoxide, anatase titanium dioxide and also niobium, phosphorus, antimony and at least one element oxide of potassium, rubidium, cesium and thallium, the second-stage catalyst differing from the first-stage catalyst in containing less of said at least one element oxide of potassium, rubidium, cesium and thallium.
Despite the abovementioned proposals for improvement, long running-up times of 2-8 weeks or more have hitherto been necessary. “Running-up time” describes the time which is needed to bring the catalyst to the desired final loading, according to the present state of the art 80 g of o-xylene/standard m
3
of air or more, i.e. to bring the oxidation to the steady state, without the catalyst being damaged irreversibly. Here, particular care has to be taken to ensure that the hot spot does not exceed a certain critical value (usually 450-480° C.), since otherwise the PA selectivity, the PA product quality and the life of the catalyst are very adversely affected.
It is an object of the present invention to find a process for preparing phthalic anhydride in which it is possible to simultaneously achieve all desired parameters such as short running-up time, high yield and low formation of by-products and also good product quality, e.g. a low phthalide content, by a combination of particular catalyst zones.
We have found that this object is achieved by a process for preparing phthalic anhydride by catalytic gas-phase oxidation of xylene and/or naphthalene by a gas comprising molecular oxygen in a fixed bed at elevated temperature and using at least three coated catalysts arranged in superposed zones, which catalysts have a layer of catalytically active metal oxides applied in the form of a shell to a core of support material, in which process the catalyst activity rises from zone to zone from the gas inlet end to the gas outlet end and the activity of the catalysts of the individual zones is set such that the least active catalyst comprises a lower amount of active composition and, if desired, additionally more alkali metal selected from the group consisting of potassium, rubidium and cesium as dopant than the catalyst of the next zone and the subsequent even more active catalyst comprises the same amount of active composition and even less alkali metal as dopant or a greater amount of active composition and, if desired, less alkali metal as dopant than the catalyst of the second zone, with the proviso that
a) the least active catalyst on nonporous support material comprises from 5 to 9% by weight, based on the total catalyst, of active composition comprising from 3 to 8% by weight of V
2
O
5
, from 0 to 3.5% by weight of Sb
2
O
3
, from 0 to 0.3% by weight of P, from 0.1 to 0.5% by weight of alkali metal (calculated as metal) and as balance TiO
2
in anatase form having a BET surface area (J. Amer. Chem. Soc. 60 (1938), 309 et seq.) of from 18 to 22 m
2
/g,
b) the next more active catalyst has the same composition as catalyst (a) except for an active composition content which is from 1 to 5% by weight (absolute) higher and an alkali metal content which is from 0 to 0.25% by weight (absolute) lower and
c) the most active catalyst has the same composition as (a) except for an active composition content which is from 1 to 5% by weight (absolute) higher than in (a) and an alkali metal content which is from 0.15 to 0.4% by weight (absolute) lower than in (a).
Accordingly
Heidemann Thomas
Wanjek Herbert
BASF - Aktiengesellschaft
Keil & Weinkauf
McKane Joseph K.
Shameem Golam M M
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