Manufacture of phthalic anhydride by gas phase oxidation of a mixture of o-xylol and naphthalene

A process for the manufacture of phthalic anhydride by oxidation in the gas phase of an o-xylol naphthalene mixture. By producing a solution of 1 to 80 parts by mass naphthalene in 99 to 20 parts by mass o-xylol at a temperature of 0.degree. to 80.degree. C., storing the solution at that temperature, heating the solution only shortly prior to the reaction to 110.degree. to 180.degree. C., atomizing the heated solution into a hot airflow, passing the heated solution over metal oxide catalysts in the gas phase and collecting phthalic anhydride product, a problem-free manufacture of phthalic anhydride is attained.

CROSS-REFERENCE TO A RELATED APPLICATION 
Applicants claim priority under 35 USC 119 for application No. P 37 19 
476.3 filed June 11, 1987 in West Germany. 
BACKGROUND OF THE INVENTION 
The field of the invention is the manufacture of phthalic anhydride and the 
invention is particularly concerned with the manufacture of phthalic 
anhydride from a mixture of o-xylol and naphthalene in the gas phase. 
U.S. Pat. Nos. 4,077,984 and 4,472,587; the disclosures of which are 
incorporated herein by reference, disclose the state of the art of 
manufacturing phthalic anhydride by the oxidation of o-xylol and 
naphthalene in the gas phase. 
The oxidation of o-xylol, naphthalene, or mixtures thereof, in the gas 
phase to form phthalic anhydride is known and referred to many times in 
the literature. The oxidation of mixtures of o-xylol and naphthalene has 
also been described. Thus, in accordance with U.S. Pat. No. 4,472,587 the 
feed of o-xylol is substituted by naphthalene, as the age of the catalyst 
increases. This does not involve losses of production or quality. It is 
also known that the oxidation of mixtures of o-xylol and naphthalene 
produces clearly higher yields of phthalic anhydride than the conversion 
of the individual hydrocarbons alone as disclosed in J. Appl. Chem. USSR 
41 (1968), pages 2 223 and 2 224. Due to this fact and other advantages 
the mixed oxidation route is a process of particular commercial interest. 
Phthalic anhydride is manufactured on a large scale from o-xylol, 
naphthalene or mixtures thereof. In the gas phase oxidation of o-xylol the 
preheated hydrocarbon is atomized into hot process air having a 
temperature of about 170.degree. C., and this mixture of o-xylol and air 
is subjected to oxidation. On the other hand, the processes for the 
oxidation of naphthalene employ the so-called evaporator principle. In the 
case of the evaporator principle a primary flow of air is charged with 
hydrocarbon by being introduced into hot molten naphthalene at about 
140.degree. C. After dilution with a further amount of secondary air this 
mixture of naphthalene and air is subjected to catalytic oxidation. The 
two processes accordingly require individually different process lay-outs 
in a plant, depending on the nature of the raw material as disclosed by H. 
Suter in Phthalsaureanhydrid und seine Verwendung, Darmstadt 1972, page 
51. Accordingly, a plant designed for the oxidation of o-xylol is not 
suitable for the employment of naphthalene without additional expedients, 
and vice versa. 
Processes are known for the gas phase oxidation of mixtures of o-xylol and 
naphthalene, wherein the evaporator principle for naphthalene on the one 
hand and of the injection of o-xylol on the other hand have been 
preserved. The mixtures of hydrocarbon and air are generated in separate 
parallel process lines according to the process principles peculiar to 
each. The lines are then combined and fed jointly to the reactor for 
catalytic gas phase oxidation. This procedure for mixed oxidation was 
developed from the process of naphthalene oxidation, supplemented by those 
apparatus parts which permit an additional atomization of o-xylol. 
The flexibility in relation to the raw materials constitutes an important 
criterion when evaluating a process as disclosed by H. Suter, i.b.i.d., 
page 62. A process which can convert o-xylol and naphthalene under process 
conditions which as far as possible are identical and which is reliable in 
operation remains the declared objective of such process commercial 
development. The feeding of naphthalene as a mixture with o-xylol whilst 
dispensing with the evaporator principle complies with this concept also 
in respect to safety, because the evaporator due to its volume and the 
amount of products contained therein, is subject to special risks, 
particularly in the Low Energy Process. 
The possibility of injecting atomized naphthalene as in the o-xylol process 
has been mentioned in the literature such as disclosed by H. Suter, 
i.b.i.d., page 49. However, no process based on such technology exists to 
date. This is due to a number of difficulties in conducting the process 
resulting from the particular chemical and physical properties of 
naphthalene. The problems arise particularly, in conducting this process 
whenever the hydrocarbon mixture injected into the hot process air, 
contains more than 20 parts by mass of naphthalene. With higher 
naphthalene proportions the mixed oxidation results in he formation of 
organic deposits in the catalyst bed which in turn after a very short time 
result in differential pressure rises in the reactor combined with the 
need to reduce the naphthalene injection or to stop the reaction. 
SUMMARY OF THE INVENTION 
Having in mind the limitations of the prior art it is an object of the 
present invention to improve the oxidation process for the manufacture of 
phthalic anhydride from a mixture of o-xylol and naphthalene in the gas 
phase on metal oxide catalysts. 
This oxidation process is improved by: 
(a) producing a solution of 1 to 80 parts by mass of naphthalene and 99 to 
20 parts by mass of o-xylol at a temperature of 0.degree. to 80.degree. 
C.; 
(b) storing the solution at a temperature of 0.degree. to 80.degree. C.; 
(c) immediately prior to carrying out the oxidation process, heating the 
stored solution of (b) to a reaction temperature of 110.degree. to 
180.degree. C.; 
(d) atomizing the heated solution of (c) into a hot air flow of 150.degree. 
to 200.degree. C.; 
(e) passing the atomized mixture over metal oxide catalysts; and 
(f) collecting the phthalic anhydride product. 
The gas phase reactor, catalysts and means for recovery of phthalic 
anhydride disclosed in U.S. Pat. No. 4,077,984 are useful in the present 
invention. Description of the Preferred Embodiments 
In order to produce phthalic anhydride by gas phase oxidation, a solution 
of naphthalene in o-xylol is employed, according to the present invention, 
and this solution is preferably stored at temperatures below the 
solidification point of naphthalene. Periods of residence above 78.degree. 
C. are for brief duration only. Suitable qualities of naphthalene are 
preferably distilled naphthalene, more particularly freshly distilled 
naphthalene and/or crystallized naphthalene, in particular those having a 
solidification point of 78.degree. to 80.degree. C. Other qualities of 
naphthalene can also be used. 
In the process according to the present invention a solution of 
naphthalene, preferably of a naphthalene as obtained by crystallization 
and/or distillation, is made in o-xylol at a temperature of 0.degree. to 
80.degree. C., preferably 30.degree. to 70.degree. C., in particular 
40.degree. to 60.degree. C. Thereafter the solution of o-xylol in 
naphthalene is kept at a temperature of 0.degree. to 80.degree. C., 
preferably at 30.degree. to 70.degree. C., in particular at 40 to 
60.degree. C. In this manner solutions of 1 to 80 parts by mass of 
naphthalene, preferably of 30 to 65 parts by mass, in particular of 40 to 
60 parts by mass of naphthalene in 99 to 20 parts by mass, preferably of 
70 to 35 parts by mass, in particular of 60 to 40 parts by mass of o-xylol 
are produced and stored at such temperatures practically indefinitely, 
preferably up to 10 days. The higher temperatures are selected preferably 
for the solutions rich in naphthalene and the lower temperatures for the 
solutions of low naphthalene content. Any insoluble portions which may 
precipitate during storage are preferably separated off. 
The solution of naphthalene in o-xylol is withdrawn from the storage vessel 
by pumping shortly prior to the reaction, heated to temperatures of 
110.degree. to 180.degree. C., the heated solution being atomized into a 
hot flow of air at 150.degree. to 200.degree. C. and passed over metal 
oxide catalysts. TiO.sub.2 -V.sub.2 O.sub.5 -catalysts, in particular 
doped TiO.sub.2 -V.sub.2 O.sub.5 -catalysts are preferably employed. 
In a special embodiment of the process according to the invention, an 
o-xylol naphthalene solution of higher concentration, up to 80 parts by 
mass naphthalene, is produced. This solution, the temperature of which 
initially amounts to less than 80.degree. C., is pumped out of the storage 
vessel, heated to temperatures of 110.degree. to 180.degree. C. and 
diluted with further amounts of hot o-xylol to concentrations of 1 to less 
than 80 parts by mass of naphthalene before such solution is atomized into 
the hot process air and passed to the gas phase oxidation. Alternatively, 
the heating-up to 110.degree. to 180.degree. C. may take place only after 
mixing with the additional amount of o-xylol. 
In a preferred embodiment of the process according to the invention, 
naphthalene is distilled, the fresh distillate is withdrawn by pumping 
from the distillate collecting vessel preferably within a residence period 
of up to 60 minutes, in particular up to 20 minutes, for example 15 to 45 
minutes at a temperature of 80.degree. to 120.degree. C. The distillate is 
mixed with 99 to 20 parts by mass of o-xylol to form a solution comprising 
1 to 80 parts by mass of naphthalene. The o-xylol naphthalene solution is 
heated to 110.degree. to 180.degree. C., the heated solution is atomized 
into hot process air having a temperature of 150.degree. to 200.degree. C. 
and passed over metal oxide catalysts. 
The process according to the present invention permits the problem-free 
oxidation of an o-xylol naphthalene mixture by direct atomization of the 
hydrocarbons into hot process air. Under the mild conditions of this 
process it has now been possible to overcome the previous problems with 
this novel process. Surprisingly it is found in addition that the 
naphthoquinone content in the raw PSA of the mixed oxidation amounts to 
only 1/10 of the content formed when operating with pure naphthalene.

SPECIFIC EXAMPLES 
The following examples serve for further elucidation: 
EXAMPLE 1 
Naphthalene distillate having a solidification point of 79.degree. C. is 
taken from a distillate receiver and mixed into cold o-xylol to produce a 
solution of the hydrocarbons containing 60 parts by mass of naphthalene 
and having a temperature of 60.degree. C. The o-xylol naphthalene mixture 
is stored under these conditions for 20 days. Thereafter it is pumped out 
of the storage vessel, mixed with further amounts of hot o-xylol at 
160.degree. C. to result in a mixture containing 50 parts by mass 
naphthalene which is heated to a temperature of 160.degree. C., the heated 
solution being atomized into hot process air at 170.degree. C. and passed 
over a TiO.sub.2 /V.sub.2 O.sub.5 catalyst in a 12 000 tube reactor. 
The charge rate amounts to 60 g hydrocarbon per Nm3 air at 3.5 Nm.sup.3 
(cubic meters at normal conditions) per tube per hour, the salt bath 
temperature being set to 371.degree. C. The mixed oxidation proceeds 
smoothly and without problems and without any differential pressure rises 
being noticeable during the period of the example of e.g. 25 days. 
Phthalic anhydride product is collected. 
EXAMPLE 2 
Naphthalene is distilled, the distillate (solidification point 79.degree. 
C.) being pumped out of the receiving vessel after a residence period of 
20 minutes and at a temperature of 100.degree. C. and mixed with a hot 
flow of oxylol of 165.degree. C. so that a solution of the hydrocarbon is 
formed comprising 50 parts by mass of naphthalene. This mixture is heated 
to a temperature of 140.degree. C. and oxidized under the conditions of 
example 1 on TiO.sub.2 /V.sub.2 O.sub.5 catalysts. The mixed oxidation 
proceeds smoothly and without problems without differential pressure rises 
being noticeable. Phthalic anhydride product is collected. 
EXAMPLE 3 
Naphthalene distillate having a solidification point of 79.degree. C. is 
stored at a temperature of 100.degree. to 110.degree. C. The residence 
period under these conditions amounts to 20 days. Thereafter the product 
is pumped out of the storage vessel, mixed with hot o-xylol of 160.degree. 
C. to form a hydrocarbon mixture comprising 50 parts by naphthalene, the 
solution of the hydrocarbons being heated to a temperature of 160.degree. 
C. The heated solution is atomized as in examples 1 and 2 into hot process 
air of 170.degree. C. and passed over TiO.sub.2 /V.sub.2 O.sub.5 catalysts 
of a 12 000 tube reactor. 
The charge rate is once again 60 g hydrocarbons per Nm3 air at 3.5 Nm3 per 
tube and hour, the salt bath temperature being again set at 371.degree. C. 
The differential pressure across the catalyst at the beginning of the 
example amounts to 335 mbar. As the mixed oxidation proceeds it rises 
gradually and continuously, reaching 352 mbar after 3 days, thereafter 
rising very rapidly to beyond 400 mbar. A stabilization of the 
differential pressure is possible only by reducing the naphthalene content 
in the hydrocarbon mixture to 20 parts by mass. A slight reduction to 
lower values is possible by complete withdrawal f the naphthalene feed, 
however, a regeneration of the original differential product takes place 
only after stopping the reaction. Phthalic anhydride product is collected.