Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2003-02-12
2004-08-10
Powers, Fiona T. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06774246
ABSTRACT:
The present invention relates to a process for preparing phthalic anhydride by gas-phase oxidation of xylene, naphthalene or mixtures thereof over two different fixed-bed catalysts arranged in zones in a shell-and-tube reactor which is thermostated by means of a heat transfer medium.
It is known that phthalic anhydride is prepared industrially by catalytic gas-phase oxidation of o-xylene or naphthalene in shell-and-tube reactors. The starting material is a mixture of a gas comprising molecular oxygen, for example air, and the o-xylene and/or naphthalene to be oxidized. The mixture is passed through a large number of tubes located in a reactor (shell-and-tube reactor), with each of the tubes containing 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. Nevertheless, local temperature maxima (hot spots) in which the temperature is higher than in the remainder of the catalyst bed can occur. These hot spots give rise to secondary reactions such as total combustion of the starting material or lead to the formation of undesirable by-products which can be separated from the reaction product only with difficulty, if at all. Furthermore, the catalyst can be irreversibly damaged above a particular hot spot temperature.
The hot spot temperatures are usually in a temperature range from 400 to 500° C., in particular from 410 to 460° C. Hot spot temperatures above 500° C. lead to a severe decrease in the achievable PA yield and in the operating life of the catalyst. On the other hand, hot spot temperatures which are too low lead to an excessively high content of under-oxidation products in the phthalic anhydride (in particular phthalide), resulting in a significant deterioration in the product quality. The hot spot temperature depends on the xylene loading of the air stream, on the space velocity of the xylene/air mixture over the catalyst, on the state of aging of the catalyst, on the heat transfer conditions in the fixed-bed reactor (reactor tube, salt bath) and on the salt bath temperature.
To reduce this hot spot, various measures have been proposed in, for example, DE 25 46 268 A, EP 286 448 A, DE 29 48 163 A, EP 163 231 A, DE 41 09 387 A, WO 98/37967 and DE 198 23 362 A. In particular, as described in DE 40 13 051 A, a change has been made to arranging catalysts of differing activity in zones in the catalyst bed, with the less active catalyst generally being located closer to the gas inlet and the more active catalyst being located closer to the gas outlet. The process is carried out using a two-stage salt bath, with the salt bath temperature of the first reaction zone in the flow direction of the reaction mixture being kept 2-20° C. higher than the salt bath temperature of the second reaction zone. The catalyst volume in the first reaction zone is from 30 to 75% by volume and that in the second reaction zone is from 25 to 70% by volume. The temperature of the hot spot in the first reaction zone is higher than that in the second reaction zone. The difference between the hot spot temperatures in the modes of operation described in the examples is considerably less than 50° C.
DE 28 30 765 A describes a shell-and-tube reactor in which a catalyst is present in two reaction zones and which is suitable for, inter alia, the preparation of phthalic anhydride. The reaction temperature in the second reaction zone from the gas inlet is higher than that in the first reaction zone.
DE 29 48 163 A describes a process for preparing phthalic anhydride using two different catalysts arranged in zones, with the catalyst of the first zone making up from 30 to 70% of the total length of the catalyst bed and the catalyst of the second zone making up from 70 to 30% of the total length of the catalyst bed. This is said to reduce the temperature of the hot spot. However, it has been found that the yield of phthalic anhydride even at the low o-xylene loadings in the starting gas mixture (maximum 85 g/standard m
3
) employed in this publication leaves something to be desired. A similar process is disclosed in DE 30 45 624 A.
DE 198 23 262 describes a process for preparing phthalic anhydride using at least three coated catalysts arranged one above the other in zones, with the catalyst activity increasing from zone to zone from the gas inlet end to the gas outlet end. In this process, the difference in the hot spot temperature from catalyst to catalyst is not more than 10° C.
It is an object of the present invention to provide a process for preparing phthalic anhydride which gives high yields of phthalic anhydride even at high o-xylene or naphthalene loadings and at high space velocities.
We have found that this object is achieved by carrying out the preparation of phthalic anhydride over two catalysts having differing activities and arranged in zones and controlling the process in such a way that the hot spot temperature in the catalyst zone furthest from the gas inlet (in the flow direction) is at least 52° C. lower than that in the catalyst zone closest to the gas inlet.
The present invention accordingly provides a process for preparing phthalic anhydride by gas-phase oxidation of xylene, naphthalene or mixtures thereof over two different fixed-bed catalysts arranged in zones in a shell-and-tube reactor which is thermostated by means of a heat transfer medium, wherein the maximum temperature (hot spot temperature, i.e. localized region of highest temperature in a catalyst zone) in the second catalyst zone furthest from the gas inlet is at least 52° C. lower than the maximum temperature in the first catalyst zone.
The maximum temperature in the second catalyst zone is preferably at least 55° C., in particular at least 60° C., lower than the maximum temperature in the first catalyst zone. However, the reaction is generally controlled so that the maximum temperature in the first catalyst zone is not more than 75° C., in particular not more than 70° C., higher than that in the second catalyst zone. The temperature difference is thus preferably in the range from 52 to 75° C.
Furthermore, the process is carried out so that the hot spot temperature in the first catalyst zone is less than 470° C., preferably less than 450° C.
The difference in the hot spot temperatures can be adjusted in various ways. For example, it can be done by increasing the admission pressure of the starting gas mixture by up to 10% or by lowering the amount of air used for the oxidation by up to 20%. However, the temperature difference is preferably controlled by means of the bed length ratio of the two catalysts or by means of the temperature of the heat transfer medium (hereinafter, reference will always be made to the preferred heat transfer medium, namely a salt bath), in particular when the two catalyst zones are thermostated by means of different salt bath circuits. The bed length of the first catalyst zone preferably makes up more than 60%, in particular more than 70% and particularly preferably more than 75%, of the total height of the catalyst bed in the reactor tube.
If the salt bath temperature is used for control, an increase in the salt bath temperature leads to an increase in the hot spot temperature in the first catalyst zone and to a decrease in the second catalyst zone. For this reason, a slight increase or decrease, e.g. by 1, 2 or 3° C., is generally sufficient to set the desired hot spot temperature difference. If the two catalyst zones are thermostated by means of different salt bath circuits, the upper salt bath circuit, i.e. the salt bath circuit which thermostats the first catalyst zone, is preferably operated at a temperature which is 1-5° C. higher than that of the lower salt bath circuit. Alternatively, the temperature of the salt bath which thermostats the second catalyst zone is decreased by up to 20° C.
The operating life of the catalyst is generally from about 4 to 5 years. The activity of the catalyst generally decreases somewhat over the course of time. As a result, the hot spot temperature difference can drop below the mini
Heidemann Thomas
Reuter Peter
Voit Guido
Keil & Weinkauf
Powers Fiona T.
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