Method for recovering phthalic anhydride/maleic anhydride mixtures by distillation

A process for recovering phthalic anhydride and maleic anhydride from a maleic anhydride-rich vapor phase oxidation product comprising the step of: contacting the vapor phase oxidation product with: (i) at least one by-product stream having a freezing point which is lower than the freezing point of pure phthalic anhydride; and/or (ii) a solvent having a boiling point in the range between about 150.degree. to 350.degree. C. and a freezing point of less than 40.degree. C.; wherein a vapor-to-liquid weight ratio in the range between about 2 to 20 is exhibited within the contacting means, thereby forming a liquid phase phthalic anhydride product having a phthalic anhydride concentration in the range between about 50-100 wt. % and a first vapor stream.

The present invention generally relates to a method and system for 
continuously recovering mixtures of maleic anhydride and phthalic 
anhydride from a maleic anhydride-rich vapor phase oxidation product 
without the formation of a solid phase using distillation with or without 
added solvent. 
BACKGROUND OF THE INVENTION 
Phthalic anhydride (PAN) and maleic anhydride (MAN) are important 
commercial chemicals useful in the manufacture of plasticizers, 
polyesters, alkyd resins and dyes. 
Maleic anhydride is typically produced by air oxidation of butane, butene 
or benzene (e.g., 2-5 mole percent in air) in the presence of a 
vanadium/phosphorus catalyst. Selectivities are typically 60-70 mole 
percent for conversions ranging from 80 to approximately 100%. This 
oxidation process produces organic by-products such as light acids (e.g., 
acetic acid and acrylic acid). 
A phthalic anhydride and maleic anhydride mixture is typically produced by 
oxidation of n-pentane in the presence of a selective oxidation catalyst, 
e.g., a vanadium phosphate oxide catalyst (VPO) or a molybdenum oxide 
catalyst. Vanadium phosphate oxide catalysts can be obtained, for example, 
from precursors prepared by either of two methods: (1) immediate 
precipitation of a solution containing vanadia in isobutanol and H.sub.3 
PO.sub.4 and (2) facilitating, before precipitation, the conditions for 
the intercalation of the isobutanol in the VOPO.sub.4 hydrated phase. 
Catalysts were obtained from the precursors by in situ treatment under 
reaction conditions for the selective oxidation of n-pentane. Control of 
the stage of formation of the precursor is crucial for obtaining a 
selective catalyst for formation of phthalic anhydride (PAN). The 
preparation of VOHPO.sub.4.1/2H.sub.2 O via a full development of the 
VOPO.sub.4.2H.sub.2 O phase, containing intercalated isobutanol, seems to 
favor the adequate structure of the precursor which promotes the formation 
of PAN. By careful control of the preparation of the VPO precursor, e.g., 
controlling the isobutanol/water ratio, the final catalyst can lead to the 
desired PAN/MAN ratio. 
The resulting vapor phase oxidation product from the catalytic air 
oxidation of butane, butene or benzene typically comprises: a reaction gas 
composed of nitrogen, oxygen, water, carbon dioxide, carbon monoxide, 
acetic acid, acrylic acid, maleic anhydride, maleic acid, and partial 
oxidation products. While the vapor phase oxidation product from the 
catalytic air oxidation of pentane typically comprises: a reaction gas 
composed of nitrogen, oxygen, water, carbon dioxide, carbon monoxide, 
maleic anhydride, acetic acid, acrylic acid, phthalic anhydride, and other 
partial oxidation products. 
The aforementioned vapor phase oxidation products are typically first 
cooled to generate steam and then delivered to expensive switch 
condensers, where they are cooled to permit the desublimation of a crude 
phthalic anhydride from the gas. Thereafter, the crude phthalic anhydride 
is sent to a finishing section in order to produce substantially pure 
phthalic anhydride; whereas the crude maleic anhydride is taken overhead 
from the switch condenser as a vapor and then recovery by solvent 
absorption. 
The switch condensers operate alternatively on cooling and heating cycles 
in order to first collect either the maleic anhydride or, in the case of a 
PAN/MAN mixture, phthalic anhydride as a solid and then melt it for 
removal from the condensers. The use of switch condensers to separate 
crude phthalic anhydride from a vapor phase oxidation product is described 
in U.S. Pat. No. 5,214,157, which is incorporated herein by reference. 
Typically, the reactor vapor phase oxidation product is cooled close to 
the solidification point 131.degree. C. (268.degree. F.) of phthalic 
anhydride and any condensed liquid is separated out before the remaining 
vapor enters the switch condensers. The switch condensers desublime the 
vapor phase oxidation product using the cold condenser oil, and then melt 
off the solid phase crude phthalic anhydride product using a hot condenser 
oil heated with steam. 
A substantial amount of impurities exit switch condensers as part of the 
vapor stream, whereas the crude maleic anhydride or phthalic 
anhydride/maleic anhydride product is plated out on the heat exchange 
tubes as a solid during the cooling step and exits the switch condensers 
at the bottom as a liquid during the melting step. The vapor gases from 
the switch condensers are sent to waste gas incinerators where the 
by-products are destroyed by oxidation to carbon dioxide and water. 
Unfortunately, switch condensers contribute to a significant portion of the 
capital and operating costs of a phthalic anhydride plant. Also, switch 
condensers operate in a batch mode on 3-6 hour cycles to desublime solid 
phthalic anhydride on the heat exchange tubes. Another problem associated 
with switch condensers is that they necessitate frequent maintenance which 
requires that designated switch condensers be taken out of service on a 
periodic basis. Maintenance of switch condensers is costly due to the high 
labor requirement and condenser down time. 
The present inventors have developed a unique process scheme which avoids 
the need to use expensive switch condensers to recover either maleic 
anhydride or phthalic anhydride/maleic anhydride mixture from vapor phase 
oxidation products. This unique process continuously condenses and 
recovers phthalic and maleic anhydride by rectification without the 
formation of an intermediate solid phase, wherein the maleic anhydride is 
taken overhead and recovered from the other overhead by-products by means 
of distillation. The rectification tower can be operated with or without 
the aid of a solvent which lowers the freezing point of the mixture 
contained therein so as to avoid freezing of the overhead products in the 
top of the rectification tower and/or condenser. A liquid phthalic 
anhydride recovery process using a rectification tower is disclosed in 
co-pending U.S. patent application, Ser. No. 08/431,647, (Dengler et al.), 
filed on May 2, 1995 now U.S. Pat. No. 5,731, 443, and which is 
incorporate herein by reference. The liquid phthalic anhydride recovery 
process disclosed in U.S. patent application, Ser. No. 08/431,647, 
(Dengler et al.) is based on concentrating the indigenous maleic anhydride 
(MAN) and benzoic acid (BA) co-products produced in the phthalic anhydride 
(PAN) reactor to form a minimum freezing MAN/BA/PAN mixture in the 
rectification tower condenser system. The present inventors have 
discovered that this condensing temperature is critical to the liquid 
phthalic anhydride recovery process and must: (1) be low enough to recover 
sufficient MAN to insure the liquid distillate will not freeze; and (2) be 
high enough to avoid free water from condensing thereby forming excessive 
amounts of acids in the distillate. 
However, Dengler et al. does not pertain to the recovery of maleic overhead 
by means of distillation, rather it discloses a process for recovering 
maleic as a liquid from a vapor phase oxidation product which comprises 
mixing the vapor phase oxidation product with reaction by-products in a 
contacting means such that a substantial portion of the maleic anhydride 
contained within the vapor phase oxidation product transfers from the 
vapor phase to a liquid phase and the by-products contained in the 
reaction by-products which are more volatile than maleic anhydride 
transfer from the liquid phase to the vapor phase. 
Dengler et al. also does not describe or suggest the use of a solvent to 
lower the freezing point of the mixture contained within the rectification 
tower. Therefore, the present invention provides a unique method for 
recovering large quantities of phthalic anhydride without the need for 
expensive switch condensers. 
A similar conventional technique for recovery maleic anhydride is to scrub 
the reaction off-gas with a solvent to remove maleic anhydride before the 
off-gas is exhausted to an incinerator. The rich solvent stream is heated 
and vacuum stripped to release the maleic anhydride. Crude maleic 
anhydride is condensed and sent to purification. Stripped solvent is 
cooled and returned to the maleic anhydride absorber. A solvent slipstream 
is withdrawn for purification. Thereafter, the crude maleic anhydride is 
fractionated to remove light ends. The small quantity of by-product light 
ends is delivered to the incinerator for destruction and waste heat 
recovery. The maleic anhydride is further fractionated to separate any 
solvent and heavies which accompanied it in the stripper overhead. The 
bottoms stream is returned to the maleic anhydride stripper. A special 
step is included to remove any solvent degradation products from the 
slipstream, in order to prevent the build-up of impurities in the solvent 
recycle loop. 
The process for absorption in an organic solvent as discussed immediately 
above is very expensive in terms of both hardware and consumable organic 
solvent. The maleic anhydride recovery using distillation with or without 
a solvent provides a significant advance in terms of cost and processing 
time verses the conventional solvent absorption process. The unique 
process according to the present invention uses distillation as a maleic 
anhydride recovery technique. The process according to the present 
invention recovers maleic anhydride from any gas stream resulting from any 
of the current oxidation processes, without the restrictions of the 
current technology. 
The substantial technical differences between using absorption versus 
rectification for separating out maleic anhydride from a vapor phase 
oxidation gas product without the formation of an intermediate solid phase 
can be understood by comparing the vapor to liquid weight ratios (V/L) in 
the absorbent tower against the V/L for the rectification tower. For 
example, the V/L for the absorbent tower U.S. Pat. No. 4,285,871 (Keunecke 
'871), as calculated from the example provided therein is 0.3 to 0.7. The 
rectification tower of the present invention exhibits a V/L ratio of 
between 5 to 20, more preferably 8 to 15. That is, due to the substantial 
pumparound or recycling of the bottoms stream which is required in any 
absorbent case, its V/L ratio is only a fraction of that which occurs 
during rectification. The low V/L ratio in the absorbent case of Keunecke 
'871 clearly demonstrates that due to these high pumparound rates the 
absorbent tower is not providing any noticeable degree of separation of 
liquid maleic anhydride from a vapor phase maleic anhydride, such as that 
recited in the present invention. 
Therefore, the advantages of the recovery processes of the present 
invention over the conventional solvent absorption processes discussed 
above are in simplification of process concept and elimination of 
adsorption and stripping steps required for the commercial solvent 
absorption process. 
The present invention also provides a unique method for recovering maleic 
anhydride from a vapor phase oxidation product formed from the air 
oxidation of butane, butene and benzene by taking the maleic anhydride 
overhead as a vapor stream. This recovery process utilizes a uniquely 
tailored solvent which enlarges the window of operation for the condenser 
by reducing the freezing point of the condenser reflux, compared to the 
freezing point of maleic anhydride, and allows the condenser to operate at 
a lower temperature; thereby reducing the amount of maleic anhydride in 
the vapor stream exiting the condenser. 
The solvent, taken overhead and condensed with the maleic anhydride in the 
condenser, forms a condensate which provides the liquid reflux to the 
rectification tower. In the rectification tower, the liquid reflux stream 
is stripped of the maleic anhydride by the hot vapor phase oxidation 
product and the residual solvent in the liquid phase is removed as a 
bottoms stream from the tower. The vapor phase maleic anhydride, recovered 
as liquid in the condensate, is then distilled from a portion of this 
liquid phase and the residual solvent from this distilled portion of the 
condensate stream is returned to the rectification tower. The liquid 
condensate from the condenser also contains low boiling, compared to 
maleic anhydride, by-products from the oxidation reaction such as acetic 
acid and acrylic acid. 
Further, the use of a tailored solvent-enhanced recovery process is 
beneficial when maleic anhydride and phthalic anhydride are co-produced as 
a vapor phase oxidation product. The present inventors have discovered 
that the addition of a small amount of a selected solvent to a MAN/PAN 
mixture substantially reduces the freezing point of the total mixture. 
More importantly, the present inventors have discovered that the addition 
of a solvent to concentrations of 5 to 10 mole percent to the MAN/PAN 
mixtures obtained as the liquid reflux to the rectification tower is 
sufficient to reduce the freezing point of the MAN/PAN mixtures and allows 
recovery of both MAN and PAN by means of simple distillation. 
The present inventors have developed a modification to the liquid phthalic 
anhydride recovery process disclosed in co-pending U.S. patent 
application, Ser. No. 08/431,647, (Dengler et al.). The present invention 
provides for the addition of a solvent to the rectification tower reflux 
system to lower the freezing point of the PAN/MAN mixture. In addition, 
the solvent according to the present invention will increase the low 
temperature operating region and broaden the potential application of the 
maleic anhydride or phthalic anhydride/maleic anhydride recovery process 
In summary, using a specifically tailored solvent in the maleic anhydride 
or PAN/MAN mixture recovery process: (1) allows for the choosing of a 
tailored component with properties better than maleic anhydride (MAN) 
reflux to reduce the freezing point of the maleic anhydride/phthalic 
anhydride mixture; (2) provides a low freezing point operating region 
sufficiently large for good commercial plant operation and control; and 
(3) frees the liquid recovery process of the present invention from 
depending on the maleic anhydride and other reaction by-products to lower 
the freezing point of the liquid condensate, the rectification tower 
reflux, sufficiently to recover either maleic anhydride (MAN only) or 
maleic anhydride and phthalic anhydride with high efficiency by simple 
distillation means. 
SUMMARY OF THE INVENTION 
A process for recovering phthalic anhydride and maleic anhydride from a 
maleic anhydride-rich vapor phase oxidation product of C.sub.5 to C.sub.8 
hydrocarbons having a temperature at its dew point temperature or greater; 
the process comprises the step of: delivering the vapor phase oxidation 
product to a contacting means which is capable of causing the vapor phase 
oxidation product to come into contact with: (i) at least one by-product 
stream having a freezing point which is lower than the freezing point of 
pure phthalic anhydride; and/or (ii) a solvent having a boiling point in 
the range between about 150.degree. to 350.degree. C. and a freezing point 
of less than 40.degree. C.; such that a substantial portion of the 
phthalic anhydride contained within the vapor phase oxidation product 
transfers from the vapor phase to a liquid phase and a substantial portion 
of the solvent and/or by-product stream is more volatile than phthalic 
anhydride and is transferred from the liquid phase to the vapor phase and 
wherein a vapor-to-liquid weight ratio in the range between about 2 to 20 
is exhibited within the contacting means, thereby forming a liquid phase 
phthalic anhydride product having a phthalic anhydride concentration in 
the range between about 50-100 wt. % and a first vapor stream. 
The process may further comprise the steps of: (a) removing the liquid 
phase phthalic anhydride product from the contacting means as bottoms; (b) 
removing the vapor phase maleic anhydride product from the contacting 
means as overhead; (c) cooling the vapor phase maleic anhydride product to 
a temperature in the range between about 25.degree. C. to 80.degree. C., 
thereby forming a first liquid stream comprising the solvent, a first 
by-product stream comprising a crude maleic anhydride product and 
light-ends by-products, and a first vapor stream; (d) separating the first 
liquid stream from the first vapor stream; (e) separating the solvent and 
crude maleic anhydride product from the light-ends by-products; (f) 
separating the solvent and the crude maleic anhydride product into a 
solvent-enriched stream and a substantially pure maleic anhydride product; 
and (g) recycling at least a portion of the solvent-enriched stream to the 
upper section of the contacting means. 
A substantial portion of the phthalic anhydride contained within the vapor 
phase oxidation product transfers from the vapor phase to a liquid phase 
and a substantial portion of the by-product stream is more volatile than 
phthalic anhydride. 
The process further comprises the steps of separating the liquid phase 
phthalic anhydride product into a crude phthalic anhydride stream and a 
second vapor phase which comprises the solvent; and condensing the second 
vapor phase and blending it with the first by-product stream and the 
solvent from step (d). 
Typically, between about 0 to 50 wt. % of the liquid phase phthalic 
anhydride product of step (a) is recycled to the contacting means, whereby 
the liquid phase phthalic anhydride product is only used for temperature 
and composition control. 
The solvent is preferably at least one solvent selected from the group 
consisting of: adipates, maleates, phthalates, carbonates, benzoates, 
ketones, aromatics, anhydrides, halogenated hydrocarbons, halogenated oxy 
hydrocarbons, ether acetates, naphthalenes, ethers, and esters. More 
specifically, the solvent is at least one solvent selected from the group 
consisting of: dimethyl adipate, diethyl adipate, dipropyl adipate, 
dibutyl adipate, dihexyl adipate, diheptyl adipate, dioctyl adipate, 
dinonyl adipate, dimethyl maleates, diethyl maleates, propylene carbonate, 
propyl benzoate, isobutyl benzoate, isophorone, e-caprolactone, isobutyl 
heptyl ketone, di-normal amyl ketone, di-isoamyl ketone, hexyl benzene, 
mixed aromatics, n-valeric anhydride, C.sub.9 alkyl acetate ester, 
C.sub.10 alky acetate ester, 1,4-butanediol diacetate, malonic 
acid-dipropyl ester, dimethyl phthalates, and esters of C.sub.5 to 
C.sub.10 neo acids and mono-polyhydric alcohols. 
The solvent may also be formed in situ by the addition of an alcohol to the 
contacting means, wherein the alcohol reacts with the maleic anhydride 
and/or the phthalic anhydride to form the desired solvent. 
The C.sub.5 to C.sub.8 hydrocarbons are preferably selected from the group 
consisting of: pentanes, pentenes, hexanes, hexenes, heptanes, heptenes, 
octenes and octanes. 
Moreover, the vapor phase oxidation product is typically formed by 
oxidation of n-pentane. The oxidation of the n-pentane occurs in the 
presence of a vanadium phosphate oxide catalyst or any other phthalic 
anhydride/maleic anhydride selective oxidation catalyst, thereby forming a 
vapor phase oxidation product comprising maleic anhydride and phthalic 
anhydride. 
Finally, the above process further comprises the step of adding additional 
solvent to the solvent-enriched stream in order to make-up for solvent 
losses. 
The process according to another embodiment of the present invention may be 
used in recovering maleic anhydride from a vapor phase oxidation product 
of butanes, butenes and benzenes having a temperature at its dew point 
temperature or greater; the process comprises the step of: delivering the 
vapor phase oxidation product to a contacting means which is capable of 
causing the vapor phase oxidation product to come into contact with: (i) 
at least one byproduct stream having a freezing point which is lower than 
the freezing point of pure maleic anhydride; and/or (ii) a solvent having 
a boiling point in the range between about 150.degree. to 350.degree. C. 
and a freezing point of less than 40.degree. C.; such that a substantial 
portion of the maleic anhydride contained within the vapor phase oxidation 
product transfers from the vapor phase to a liquid phase to the vapor 
phase and wherein a vapor-to-liquid weight ratio in the range between 
about 2 to 20 is exhibited within the contacting means, thereby forming a 
liquid phase maleic anhydride product having a maleic anhydride 
concentration in the range between about 50-100 wt. % and a solvent-rich 
vapor phase stream. 
The above used in recovering maleic anhydride from a vapor phase oxidation 
product of butanes, butenes and benzenes may also comprise the below 
steps: (a) removing the liquid phase maleic anhydride product from the 
contacting means as bottoms; (b) removing the solvent-rich vapor phase 
stream from the contacting means as overhead; (c) cooling the solvent-rich 
vapor phase stream to a temperature in the range between about 25.degree. 
C. to 80.degree. C., thereby formiing a liquid solvent-enriched stream 
comprising solvent, a crude maleic anhydride product, heavy by-products 
and light-ends by-products, and a first vapor stream; (d) separating the 
liquid solvent-enriched stream from the first vapor stream; (e) separating 
the crude maleic anhydride product and the solvent from the light-ends 
by-products; (f) separating the solvent and the crude maleic anhydride 
product into a first by-product stream and a substantially pure maleic 
anhydride product; and (g) recycling at least a portion of the first 
by-products stream to the upper section of the contacting means. 
The solvents used when only maleic anhydride is recovered from a vapor 
phase oxidation product may be similar to those set forth above; provided 
that the solvent is chosen such that it has the following properties: (1) 
inert or only slightly reactive; (2) a boiling point greater than 
150.degree. C., but preferably a higher boiling point than maleic 
anhydride and low enough to be vaporized by hot oil or high pressure steam 
(for the PAN/MAN recovery discussed above, the solvent should exhibit an 
atmospheric boiling point up to 350.degree. C., but preferably less than 
PAN); (3) a lower melting or freezing point than maleic anhydride; and (4) 
soluble with maleic anhydride or mixtures of maleic anhydride, PAN or 
other oxidation by-products.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A continuous process for recovering phthalic anhydride (PAN) and maleic 
anhydride (MAN) from a vapor phase oxidation product of n-pentane or the 
like. 
According to the present invention, the formation of crude liquid phthalic 
anhydride product without the presence of an intermediate solid phase 
phthalic anhydride is accomplished by contacting the vapor phase oxidation 
product with a solvent having a boiling point in the range between about 
150.degree. to 350.degree. C. and a freezing point of less than 40.degree. 
C., whereby the operating temperatures are always above the freezing point 
of the liquid phase. It is preferable that the contacting occur in a 
rectification tower so that a vapor-to-liquid weight ratio within the 
contacting tower is maintained in the range between about 2 to 20, more 
preferably 3 to 15. 
Thus, it is a primary objective of the present invention to add a 
specifically tailored solvent to the liquid phthalic anhydride process 
described below which is capable of lowering the freezing point of the 
phthalic anhydride/maleic anhydride mixture. In addition, the present 
inventors have discovered that the uniquely tailored solvent will increase 
the low temperature operating region and broaden the potential application 
of the liquid phthalic anhydride process. The present inventors have also 
discovered that maleic anhydride which is taken overhead from the 
rectification tower can be readily separated from the solvent by means of 
distillation without the need for the absorption, stripping and 
fractionation steps disclosed in the conventional maleic recovery 
processes. 
A preferred solvent candidate must exhibit the following properties: (1) 
have good solvency for the mixture so as to achieve maximum freezing point 
depression at low solvent concentrations; (2) have a volatility lower than 
maleic anhydride so as to minimize the solvent lost to the incinerator 
gas, but higher than phthalic anhydride to enable recovery and recycle of 
any solvent that breaks through the liquid phthalic anhydride tower as 
bottoms; (3) have a low pure component freezing point; and (4) be 
chemically inert. 
The most desirable solvents for the liquid phthalic anhydride recovery 
process are those with an aromatic ring and/or oxygen in the structure. 
These types of solvent structures promote solubility of the phthalic 
anhydride, maleic anhydride, and other by-products in the solvent. The 
most preferred solvents are those which have a boiling point in the range 
between about 150.degree. to 350.degree. C., preferably 200 to 
284.degree. C., and most preferably 213.degree. to 270.degree. C., and a 
freezing point of less than 40.degree. C. The best solvent family groups 
are adipates, carbonates, benzoates, ketones, aromatics, anhydrides, 
halogenated hydrocarbons, halogenated oxy hydrocarbons, ether acetates, 
naphthalenes, ethers and esters. 
Set forth below are the more volatile solvents in each of the 
above-identified solvent families along with their boiling and melting 
points: 
______________________________________ 
Boiling Point Melting Point 
Solvent (.degree.C.) (.degree.C.) 
______________________________________ 
ADIPATES 
dimethyl adipate 
235 0 
diethyl adipate 
240 -21 
CARBONATES 
propylene carbonate 
242 -49 
BENZOATES 
propyl benzoate 
229 -52 
isobutyl benzoate 
242 
KETONES 
isophorone 215 -8 
e-caprolactone 235 -2 
isobutyl heptyl ketone 
213-224 -10 
di-normal amyl ketone 
228 14 
di-isoamyl ketone 
226 15 
AROMATICS 
hexyl benzene 226 -61 
narrow cut mixed 
(Aromatic 200 range) 
-26 
aromatics solvent 
ANHYDRIDES 
n-valeric anhydride 
227 -49 
ESTERS 
C.sub.9 alkylacetate ester 
205-234 -60 
C.sub.10 alkylacetate ester 
220-250 -60 
1,4-butanediol diacetate 
229 12 
malonic acid-dipropyl ester 
229 -70 
malonic acid-dibutyl ester 
251 -83 
phthalic acid-dimethyl ester 
282 2 
maleic acid-dimethyl ester 
205 -19 
maleic acid-diethyl ester 
225 -88 
C.sub.5 -C.sub.10 neo acid-polyol ester 
______________________________________ 
The preferred less volatile solvents are the higher boiling homologues of 
the above families, such as dipropyl adipate, dibutyl adipate, dihexyl 
adipate, dioctyl adipate, and dinonyl adipate. 
The preferred embodiment according to the present invention involves the 
configuration depicted in FIG. 1. This figure pertains to the use of a 
contacting or packed tower having either an external cooling/condensing 
system. However, the cooling/condensing system can also be internal to the 
tower. 
FIG. 1 describes a process for recovering maleic anhydride and liquid 
phthalic anhydride from a vapor phase oxidation product. The vapor phase 
oxidation product of n-pentane or any other material capable of being 
catalytically converted to a mixture of phthalic anhydride and maleic 
anhydride is passed via conduit 6 through heat exchangers 2 and 4 wherein 
the vapor phase oxidation product is cooled to its dew point temperature 
or greater. The cooled vapor phase oxidation product is delivered from 
conduit 6 to rectifier or contacting means 401 which is capable of causing 
the vapor phase oxidation product to come into contact with a solvent 
having a boiling point in the range between about 150.degree. to 
350.degree. C. and a freezing point of less than 40.degree. C. so that a 
vapor-to-liquid weight ratio in the range between about 2 to 20 is 
maintained, thereby forming a liquid phase phthalic anhydride product 
having a phthalic anhydride concentration of between about 50-100 wt. %, 
preferably between about 85-100 wt. %, more preferably about 90-100 wt. %, 
and most preferably between about 95-99.8 wt. %, and a vapor phase maleic 
anhydride product stream. Rectification tower 401 separates the liquid 
phase phthalic anhydride product from the vapor phase maleic anhydride 
product stream by means of multiple equilibrium stages, i.e., packing or 
trays, (not shown) disposed therein. Rectifier tower 401 is typically a 
low pressure drop counter-current gas/liquid contactor having at least 2 
low pressure drop equilibrium stages, preferably 3 to 10. 
It should be kept in mind that despite operating below the freezing point 
of phthalic anhydride there is no formation of a solid phase anywhere 
within rectifier tower 401 due to the choice of operating conditions. 
The liquid phase phthalic anhydride product is removed from rectifier tower 
401 as bottoms via conduit 403, while the vapor phase maleic anhydride 
product stream is removed from rectifier tower 401 as overhead via conduit 
405. Rectifier tower 401 has an upper section 407, an intermediate section 
409 and a lower section 411. 
The vapor phase maleic anhydride product stream taken overhead via conduit 
405 is passed through a heat exchanger or low pressure drop gas cooler 413 
where it is cooled to a temperature in the range between about 25.degree. 
C. to 80.degree. C. (77.degree.-176.degree. F.), thereby forming a first 
by-product comprising a crude maleic anhydride product, acetic acid, 
acrylic acid and other heavy oxidation by-products and solvent stream, and 
a first vapor stream. This mixed phase stream is then delivered to a 
vapor/liquid disengaging drum 415 wherein the first by-product stream and 
solvent stream are separated from the first vapor stream. The first vapor 
stream is then taken out overhead via conduit 417 for disposal via 
incineration. The first by-product and solvent stream are taken out as 
bottoms from drum 415 via conduit 419 and delivered to a fractionation or 
distillation tower 501, wherein the acetic acid and acrylic acid are taken 
overhead via conduit 503 and the crude maleic anhydride product, other 
heavy oxidation by-products and solvent are taken as bottoms via conduit 
505. 
The overhead stream taken from fractionation tower 501 via conduit 503 is 
preferably cooled via heat exchanger 507 and recycled to the top of tower 
501 via conduits 509 and 511 and purged from the system via conduits 509 
and 513. The bottoms from fractionation tower 501 are preferably sent to 
fractionation tower 551 wherein substantially pure maleic anhydride is 
taken overhead via conduit 550 and wherein solvent and the heavy oxidation 
by-products are recycled via conduit 553 to top portion 407 of 
rectification tower 401 in order to maintain the proper vapor to liquid 
ratio therein such that the freezing point of the PAN/MAN mixture is 
lowered, thereby avoiding freezing and providing a larger operating range 
in rectification tower 401. If necessary, make-up solvent can be added to 
rectification tower 401 via conduits 515 and 553. 
Alternatively, liquid condensate from drum 415 can be diverted via valve 
555 and conduit 557 directly to rectification tower 401 if additional 
maleic anhydride is needed to maintain the proper vapor to liquid ratio in 
tower 401 and also to prevent freezing of the mixture contained therein. 
The liquid phase phthalic anhydride product which is removed from 
rectification tower 401 as bottoms preferably has a concentration of 
between about 50-100 wt. %, more preferably between about 90-100 wt. %, 
and most preferably between about 95-99.8 wt. %, phthalic anhydride. 
The liquid phase phthalic anhydride product passes via conduit 403, 
optionally, into at least one decomposer 421 which operates under a slight 
vacuum (about 700 mm Hg absolute) and high temperatures (e.g., 260.degree. 
C. (500.degree. F.)) to convert the small amount of phthalic acid that is 
present to phthalic anhydride. Thereafter, the liquid phase phthalic 
anhydride product is pumped from decomposer 421 to a light-ends column or 
fractionation column 423 wherein a second by-product and solvent stream 
comprising low-boiling by-products, e.g., maleic anhydride, along with 
some solvent are removed at the top of fractionation column 423, cooled 
via heat exchanger 433, and at least a part of this stream is optionally 
returned to rectifier tower 401 as a second by-product and solvent stream 
via conduits 424 and 435 with the remainder of the stream being recycled 
to the top of fractionation column 423 via conduits 428 and 519, and 
blended with the first by-products and solvent stream being sent to 
fractionation tower 501 by means of conduits 523 and 419. 
Crude phthalic anhydride is taken as bottoms from fractionation column 423 
and is optionally fed via conduit 425 to fractionation column 427 wherein 
substantially pure phthalic anhydride is removed from the top of 
fractionation column 427 via conduit 429, while heavy products are removed 
from the bottom via conduit 431. If a heavy solvent is used, then it is 
separated from the heavy products via fractionation tower 561, wherein 
heavies are taken out the bottom via conduit 563 and heavy solvent is 
removed overhead via conduit 564 and returned to tower 401. 
Optionally, a small portion (i.e., 0-10%) of the liquid phase phthalic 
anhydride product removed as bottoms from rectifier tower 401 via conduit 
403 is recycled via conduit 435 to rectifier tower 401 for temperature and 
concentration control in the tower. 
Overall recovery of the phthalic anhydride from the reactor effluent gas 
(i.e., vapor phase oxidation product) is approximately 99.7% for process 
described in FIG. 1. 
The vapor phase oxidation product fed into the system via conduit 6 is 
preferably formed by oxidation of n-pentane in the presence of a PAN/MAN 
selective oxidation catalyst, preferably a vanadium phosphate oxide 
catalyst, thereby forming a vapor phase oxidation product comprising 
maleic anhydride and phthalic anhydride. The following is an example 
preparation procedure of a vanadium oxide phosphate catalyst which is 
preferably obtained from precursors prepared by either: (i) by immediate 
precipitation of a solution containing vanadia in isobutanol and H.sub.3 
PO.sub.4 ; or (ii) by facilitating, before precipitation, the conditions 
for the intercalation of the isobutanol in the VOPO.sub.4 hydrated phase, 
as described in Sobalik et al., Influence of the Precursor Formation Stage 
in the Preparation of VPO Catalysts for Selective Oxidation of n-Pentane, 
Study of Surfactant Science Catalysts, Elsevier Science B.V., 1995, pp. 
727-736, which is incorporated herein by reference. 
FIG. 2 depicts a maleic anhydride recovery process according to the present 
invention. The vapor phase oxidation product of butane, butene or benzene 
or any other material capable of being catalytically converted to maleic 
anhydride is passed via conduit 6 through heat exchangers 2 and 4 wherein 
the vapor phase oxidation product is cooled to its dew point temperature 
or greater. The cooled vapor phase oxidation product is delivered from 
conduit 6 to rectifier or contacting means 401 which is capable of causing 
the vapor phase oxidation product to come into contact with a solvent 
having a boiling point in the range between about 150.degree. to 
350.degree. C. and a freezing point of less than 40.degree. C. so that a 
vapor-to-liquid weight ratio in the range between about 2 to 20 is 
maintained, thereby forming a solvent-rich vapor phase and a liquid phase 
maleic anhydride product stream. Rectification tower 401 separates the 
solvent-rich vapor phase from the liquid phase maleic anhydride product 
stream by means of multiple equilibrium stages, i.e., packing or trays, 
(not shown) disposed therein. Rectifier tower 401 is typically a low 
pressure drop counter-current gas/liquid contactor having at least 2 low 
pressure drop equilibrium stages, preferably 3 to 10. 
It should be kept in mind that despite operating below the freezing point 
of maleic anhydride there is no formation of a solid phase anywhere within 
rectifier tower 401 due to the choice of operating conditions. 
The liquid phase maleic anhydride is removed from rectifier tower 401 as 
bottoms via conduit 403, while the solvent-rich vapor phase stream is 
removed from rectifier tower 401 as overhead via conduit 405. Rectifier 
tower 401 has an upper section 407, an intermediate section 409 and a 
lower section 411. 
The solvent-rich vapor phase stream taken overhead via conduit 405 is 
passed through a heat exchanger or low pressure drop gas cooler 413 where 
it is cooled to a temperature in the range between about 25.degree. C. to 
80.degree. C., thereby forming a liquid solvent-enriched stream comprising 
solvent, a crude maleic anhydride product, heavy by-products and 
light-ends by-products and a first vapor stream. This mixed phase stream 
is then delivered to a vapor/liquid disengaging drum 415 wherein the 
liquid solvent-enriched stream is separated from the first vapor stream. 
The first vapor stream is then taken out overhead via conduit 417 for 
disposal via incineration. The liquid solvent-enriched stream is taken out 
as bottoms from drum 415 via conduit 419 and delivered to a fractionation 
or distillation tower 501, wherein the light-ends by-products (i.e., 
acetic acid and acrylic acid) are taken overhead via conduit 503 and the 
solvent, the crude maleic anhydride product and heavy by-products are 
taken as bottoms via conduit 505. 
The overhead stream taken from fractionation tower 501 via conduit 503 is 
preferably cooled via heat exchanger 507 and recycled to the top of tower 
501 via conduits 509 and 511 and purged from the system via conduits 509 
and 513. The bottoms from fractionation tower 501 are preferable sent to 
fractionation tower 551 wherein substantially pure maleic anhydride is 
taken overhead via conduit 550 and wherein solvent and the heavy oxidation 
by-products are recycled via conduit 553 to top portion 407 of 
rectification tower 401 in order to maintain the proper vapor to liquid 
ratio therein such that the freezing point of the contents of tower 401 is 
lower than pure maleic anhydride, thereby avoiding freezing and providing 
a larger operating range in rectification tower 401. If necessary, makeup 
solvent can be added to rectification tower 401 via conduits 515 and 553. 
Alternatively, liquid condensate from drum 415 can be diverted via valve 
555 and conduit 557 directly to rectification tower 401 if additional 
maleic anhydride is needed to maintain the proper vapor to liquid ratio in 
tower 401 and also to prevent freezing of the contents of tower 401. 
The vapor phase oxidation product used in the embodiment of FIG. 2 is 
formed by oxidation of either butane, butene or benzene in the presence of 
an oxidation catalyst, selective for maleic anhydride, such as a vanadium 
phosphorus oxide catalyst, thereby forming a vapor phase oxidation product 
comprising maleic anhydride at 60-100% conversion of the feed. 
It is also possible that solvent can be formed in situ within rectification 
tower 401 by the addition of an appropriate alcohol via conduit 515 (e.g., 
methanol, ethanol, propyl, butyl, hexyl, heptyl and octyl alcohol) which 
reacts with the anhydrides contained therein to form an ester, primarily 
esters of maleic anhydride, which meets the criteria for solvents set 
forth above. 
FIG. 3 describes a process for recovering phthalic anhydride as a liquid 
from a vapor phase oxidation product. The vapor phase oxidation product of 
n-pentane or any other material capable of being catalytically converted 
to a mixture of phthalic anhydride and maleic anhydride is passed via 
conduit 6 through heat exchangers 2 and 4 wherein the vapor phase 
oxidation product is cooled to its dew point temperature or greater. The 
cooled vapor phase oxidation product is delivered from conduit 6, 
following cooling to a temperature no lower than its dew point, to 
rectifier or contacting means 401 which is capable of causing the vapor 
phase oxidation product to come into contact with first by-products stream 
(i.e., maleic anhydride, phthalic anhydride and other reaction 
by-products) a vapor-to-liquid weight ratio in the range between about 2 
to 20 is maintained, thereby forming a liquid phase phthalic anhydride 
product having a phthalic anhydride concentration of between about 50-100 
wt. %, preferably between about 85-100 wt. %, more preferably about 90-100 
wt. %, and most preferably between about 95-99.8 wt. %, and a vapor phase 
maleic anhydride product stream. Rectification tower 401 separates the 
liquid phase phthalic anhydride product from the vapor phase maleic 
anhydride product stream by means of multiple equilibrium stages, i.e., 
packing or trays, (not shown) disposed therein. Rectifier tower 401 is 
typically a low pressure drop counter-current gas/liquid contactor having 
at least 2 low pressure drop equilibrium stages, preferably 3 to 10. 
It should be kept in mind that despite operating below the freezing point 
of phthalic anhydride there is no formation of a solid phase anywhere 
within rectifier tower 401 due to the choice of operating conditions. 
The liquid phase phthalic anhydride product is removed from rectifier tower 
401 as bottoms via conduit 403, while the vapor phase maleic anhydride 
product stream is removed from rectifier tower 401 as overhead via conduit 
405. Rectifier tower 401 has an upper section 407, an intermediate section 
409 and a lower section 411. 
The vapor phase maleic anhydride product stream taken overhead via conduit 
405 is passed through a heat exchanger or low pressure drop gas cooler 413 
where it is cooled to a temperature in the range between about 25.degree. 
C. to 80.degree. C., thereby forming a first by-product comprising a crude 
maleic anhydride product, acetic acid, acrylic acid and other heavy 
oxidation by-products, and a first vapor stream. This mixed phase stream 
is then delivered to a vapor/liquid disengaging drum 415 wherein the first 
by-product stream is separated from the first vapor stream. The first 
vapor stream is then taken out overhead via conduit 417 for either further 
maleic recovery or disposal via incineration. The first by-products stream 
is taken out as bottoms from drum 415 via conduit 419 and delivered to 
upper portion 407 of tower 401 or diverted by means of valve 402 and 
conduit 404 to fractionation or distillation tower 501, wherein the acetic 
acid and acrylic acid are taken overhead via conduit 503 and the crude 
maleic anhydride product and other heavy oxidation by-products are taken 
as bottoms via conduit 505. 
The bottoms from fractionation tower 501 are preferable sent to 
fractionation tower 551 wherein substantially pure maleic anhydride is 
taken overhead via conduit 550 and wherein the heavy oxidation by-products 
are recycled via conduits 553 and 419 to top portion 407 of rectification 
tower 401 in order to maintain the proper vapor to liquid ratio therein 
such that the freezing point of the PAN/MAN mixture is lowered, thereby 
avoiding freezing and providing a larger operating range in rectification 
tower 401. Optionally, bottoms from fractionation tower 501 may be taken 
to a holding tank for treatment to reduce the color thereof prior to 
distillation in fractionation tower 551. 
The liquid phase phthalic anhydride product which is removed from 
rectification tower 401 as bottoms preferably has a concentration of 
between about 50-100 wt. %, more preferably between about 90-100 wt. %, 
and most preferably between about 95-99.8 wt. %, phthalic anhydride. 
The liquid phase phthalic anhydride product passes via conduit 403, 
optionally, into at least one decomposer 421 which operates under a slight 
vacuum (about 700 mm Hg absolute) and high temperatures (e.g., 260.degree. 
C. (500.degree. F.)) to convert the small amount of phthalic acid that is 
present to phthalic anhydride and to reduce color bodies. Thereafter, the 
liquid phase phthalic anhydride product is pumped from decomposer 421 to a 
light-ends column or fractionation column 423 wherein a second by-products 
stream comprising low-boiling by-products are removed at the top of 
fractionation column 423, cooled via heat exchanger 433, and at least a 
part of this stream is optionally returned to rectifier tower 401 as a 
second by-products stream via conduits 424 and 435 with the remainder of 
the stream being sent to incineration or other disposal via conduit 428 
and returned to column 423. 
Crude phthalic anhydride is taken as bottoms from fractionation column 423 
and is optionally fed via conduit 425 to fractionation column 427 wherein 
substantially pure phthalic anhydride is removed from the top of 
fractionation column 427 via conduit 429, while heavy products are removed 
from the bottom via conduit 431. 
Optionally, a portion (i.e., 0-50%) of the liquid phase phthalic anhydride 
product removed as bottoms from rectifier tower 401 via conduit 403 is 
recycled via conduit 435 to rectifier tower 401 for temperature and 
concentration control in the tower. 
Overall recovery of the phthalic anhydride from the reactor effluent gas 
(i.e., vapor phase oxidation product) is approximately 99.7% for process 
described in FIG. 3. 
The vapor phase oxidation product fed into the system via conduit 6 
preferably formed by oxidation of n-pentane in the presence of a VPO 
catalyst, thereby forming a vapor phase oxidation product comprising 
maleic anhydride and phthalic anhydride, as discussed above in FIG. 1.