Liquid phthalic anhydride recovery process

A process for separating phthalic anhydride from a vapor phase oxidation product by mixing and cooling the vapor phase oxidation product with recycled by-products which have freezing points lower than the freezing point of pure phthalic anhydride, thereby condensing and recovering a liquid phase phthalic anhydride product without the formation of an intermediate solid phase.

The present invention generally relates to a method and system for 
continuously recovering liquid phase phthalic anhydride from a vapor phase 
oxidation product without the formation of a solid phase. In particular, 
phthalic anhydride is condensed and recovered from the vapor phase 
oxidation product by contacting this gaseous oxidation product with 
recycled maleic anhydride and/or other by-products so that the condensate 
has a lower freezing point than pure phthalic anhydride, thereby 
eliminating the need for switch condensers. 
BACKGROUND OF THE INVENTION 
Phthalic anhydride is an important commercial chemical useful in the 
manufacture of plasticizers, polyesters, alkyd resins and dyes. 
Phthalic anhydride is typically produced from raw materials such as 
orthoxylene (o-xylene), petroleum naphthalene, and coal-tar naphthalene. 
The price of these raw materials and, as a direct result, the price of 
phthalic anhydride have fluctuated greatly depending upon supply and 
demand. Because the cost of the raw materials is a major factor in the 
price of phthalic anhydride, it is of great importance that any system 
used to produce phthalic anhydride capture as much of the resultant 
product as possible. 
Phthalic anhydride can be successfully produced from any of a number of 
processes, i.e., (1) air oxidation of o-xylene in fixed-bed reactors, (2) 
air oxidation of petroleum or coal tar naphthalene in fixed-bed reactors, 
(3) fluid bed oxidation of o-xylene, (4) fluid bed oxidation of petroleum 
or coal tar naphthalene, and (5) liquid phase oxidation of o-xylene or 
naphthalene. 
The general process scheme for the various vapor phase routes is to mix the 
hydrocarbon feed (in the vapor form) with compressed air and to feed the 
mixture to fixed-bed reactors which contain tubes packed with catalysts, 
e.g., vanadium oxide and titanium dioxide coated on an inert, nonporous 
carrier. When fluid bed reactors are used, the hydrocarbon feed in liquid 
form can be injected directly into the fluidized bed so that the air and 
the hydrocarbon are mixed in the reactor to produce a reactor effluent gas 
(i.e., the vapor phase oxidation product). The reactors are equipped with 
means for removing the heat of the oxidation reaction. The heat that is 
removed is used to generate steam. 
After the vapor phase oxidation product exits either the fixed-bed or fluid 
bed reactors, it is cooled to cause the phthalic anhydride to condense. 
This allows separation of the phthalic anhydride from the gas stream. The 
phthalic anhydride is typically condensed as a solid. However, a two-stage 
condensation system can be used to first condense a portion of the 
phthalic anhydride as a liquid and then to condense the remainder as a 
solid. 
Expensive switch condensers that operate alternatively on a cooling cycle 
and a heating cycle are used to collect the phthalic anhydride as a solid. 
The solid is then melted 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 
(Healy et al.), issued May 25, 1993, which is incorporated herein by 
reference. The resultant vapor phase oxidation product is cooled close to 
the solidification point (131.degree. C.) of phthalic anhydride and any 
condensed liquid is usually 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. Both the hot condenser oil and cold condenser oil are 
pumped through switch condensers via horizontally disposed heat exchange 
tubes. 
A substantial amount of impurities exit switch condensers as part of the 
vapor stream, whereas the crude phthalic 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. 
This crude phthalic anhydride liquid is collected from the switch 
condensers in surge vessels before being pumped to storage for crude 
finishing to commercial product. 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. This can be done in 
combination with fuel gas to produce steam. 
Unfortunately, switch condensers involve a significant portion of the 
capital and operating costs of a phthalic anhydride plant. The cost of 
each switch condenser, including installation, can exceed a million 
dollars. Also, switch condensers operate in a batch mode on 3-6 hours 
cycles to desublime solid phthalic anhydride on the heat exchange tubes. 
The present inventor has developed a unique process scheme which avoids the 
need to use expensive switch condensers in order to recover the phthalic 
anhydride from the vapor phase oxidation product. This unique process 
continuously condenses and recovers phthalic anhydride in the liquid phase 
without the formation of an intermediate solid phase. 
The continuous liquid recovery process of the present invention provides 
the following advantages over conventional switch condensers: (1) fewer 
pieces of processing equipment; (2) continuous versus batch mode of 
operation; (3) recovery of more than 99.7% of the phthalic anhydride from 
the vapor phase oxidation product versus 99-99.4% for switch condensers; 
(4) typical losses of 0.25 to 0.5% of the crude phthalic anhydride 
production in the light ends distillation following recovery by switch 
condensers are significantly reduced due to the recycle of the light ends 
cut; (5) since the process concentrates the by-products from a low vapor 
concentration (e.g., for the maleic anhydride from less than 0.1 mol % in 
the vapor to more than 50 mol % in the recovered liquid streams), the 
maleic and citraconic anhydrides and the benzoic acid by-products can be 
readily further concentrated and upgraded for commercial sales of these 
by-products; (6) the cost of maleic anhydride recover for sale is lower 
than for typical plants because the impurities such as citraconic 
anhydride can be rejected into the recycled impure maleic anhydride and 
eventually purged with the benzoic acid stream and (7) benefits the 
environment because the waste gas contains less by-products and less 
phthalic anhydride. 
In addition, this process has advantages over solvent recovery processes in 
that only materials already present are used for the recycle. No new 
material is added. In addition to cost of a solvent, some of the solvent 
will escape to the environment. Also the solvent could adversely effect 
the product quality of the phthalic anhydride or recovered by-products. 
Even if the recovery step using an ester is added to this recovery 
process, only the alcohol portion of the molecule is extraneous to the 
process stream because the acid portion of the ester is made from one of 
the acids and/or anhydrides in the process stream. 
SUMMARY OF THE INVENTION 
A process for separating phthalic anhydride from a vapor phase oxidation 
product which comprises the steps of: (a) cooling the vapor phase 
oxidation product to a temperature of about 130.degree. to 165.degree. C.; 
(b) mixing and further cooling the vapor phase oxidation product of step 
(a) with at least one by-product stream having a freezing point which is 
lower than the freezing point of pure phthalic anhydride, thereby forming 
a liquid phase phthalic anhydride product having a freezing point lower 
than the freezing point of pure phthalic anhydride, and a first vapor 
stream; (c) separating the liquid phase phthalic anhydride product from 
the first vapor stream; (d) separating the liquid phase phthalic anhydride 
product into a crude phthalic anhydride stream and a first by-product 
stream; (e) cooling the first by-product stream to a temperature in the 
range between about 40.degree. to 80.degree. C.; (f) recycling at least a 
portion of the first by-product stream to step (b); (g) cooling the first 
vapor stream to a temperature in the range between about 40.degree. to 
80.degree. C., thereby forming a second by-product stream and a second 
vapor stream; (h) separating the second by-product stream from the second 
vapor stream; and (i) recycling the second by-product stream to step (b). 
Optionally, cooling step (g) and separating step (h) may all take place in 
a cooling tower which comprises countercurrent flowing vertically disposed 
tubes and a recirculating coolant, wherein the liquid phthalic anhydride 
crude product is taken out as bottoms and the remaining vapor phase is 
taken overhead. 
This cooling tower may alternatively comprise multiple cooling zones. 
The process for separating phthalic anhydride from a vapor phase oxidation 
product may also include a by-product (i.e. maleic anhydride) recovery 
step which includes the following steps: mixing vapor stream from a second 
flash step with an absorbent to form an absorbent containing by-product 
stream; separating the absorbent containing by-product stream into a 
desorbed by-product stream and a concentrated absorbent stream; and mixing 
the desorbed by-product stream with a by-product stream. 
Optionally, the cooling, separation and maleic anhydride recovery steps can 
be combined within a cooling tower which comprises countercurrent flowing 
vertically disposed tubes, a recirculating coolant, and an ester absorbing 
section, wherein the liquid crude phthalic anhydride stream is taken out 
as bottoms, the ester containing stream is taken out as a side stream, and 
the vapor phase purge is taken overhead.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A continuous process for condensing and recovering phthalic anhydride (PAN) 
in a liquid phase from a vapor phase oxidation product of o-xylene, 
naphthalene or the like, and air is hereafter described. 
The freezing point of pure phthalic anhydride is 268.degree. F. 
(131.degree. C.). Cooling the vapor phase oxidation product below this 
temperature in conventional switch condensers leads to the plating out of 
a solid phase phthalic anhydride on the heat exchange tubes within each 
switch condenser. 
The formation of crude liquid phthalic anhydride product without the 
presence of an intermediate solid phase phthalic anhydride according to 
the present invention is accomplished by contacting the vapor phase 
oxidation product with recycled by-products (e.g., maleic anhydride (MAN)) 
which have a lower freezing point than pure phthalic anhydride, whereby 
the freezing point of the resultant liquid phase phthalic anhydride 
product (i.e., a product which comprises a mixture of phthalic anhydride, 
maleic anhydride and other by-products) is approximately 240.degree. F. 
(115.degree. C.) compared to a freezing point of 268.degree. F. 
(131.degree. C.) for pure phthalic anhydride itself. Therefore, so long as 
the temperature of the liquid phase phthalic anhydride product is 
maintained above this new freezing point, the phthalic anhydride will be 
recovered in the liquid phase even at temperatures which are below the 
freezing point of pure phthalic anhydride. 
That is, the phthalic anhydride recovery process according to the present 
invention uses recycled maleic anhydride together with other by-products 
from its second flash unit and the maleic anhydride fractionation column 
to increase the concentration of by-products in the vapor phase oxidation 
product. As the vapor phase oxidation product is cooled, some of the 
maleic anhydride by-product condenses with the phthalic anhydride to form 
a liquid phase phthalic anhydride product. The condensed maleic anhydride 
lowers the freezing point of the liquid phase phthalic anhydride product 
below the freezing point of pure phthalic anhydride. By careful adjustment 
of the operating conditions, the formation of solid phthalic anhydride can 
be avoided. 
The freezing point of pure phthalic anhydride is 268.degree. F. 
(131.degree. C.) and that of the pure maleic anhydride is 127.degree. F. 
(53.degree. C.). As shown in FIG. 6, the minimum freezing point of a 
PAN/MAN mixture is about 104.degree. F. (40.degree. C.) at the eutectic 
composition of 17 mole % phthalic anhydride and 83 mole % maleic 
anhydride, based on estimates from heat capacity data. The presence of 
other materials such as citraconic anhydride in the vapor phase oxidation 
product further lowers the freezing point of the resultant liquid phase 
phthalic anhydride product. 
FIG. 1 is a schematic diagram of the liquid phthalic anhydride recovery 
process wherein the reactor effluent gas (e.g., a vapor phase oxidation 
product of o-xylene, naphthalene, and/or any other material capable of 
being catalytically converted to phthalic anhydride) is passed via conduit 
6 through heat exchangers 2 and 4. After passing through heat exchanger 4 
the vapor phase oxidation product has an approximate temperature of 
280.degree. F. (138.degree. C.). The cooled vapor phase oxidation product 
is then sent to a mixing vessel 8 where it is combined with recycled 
by-products (e.g., maleic anhydride and citraconic anhydride) via conduits 
10 and 12. This cools the vapor phase oxidation product to approximately 
250.degree. F. (121.degree. C.) and condenses approximately 74-85% of the 
phthalic anhydride present within the gas. Normally, pure phthalic 
anhydride would have solidified at temperatures below 268.degree. F. 
(131.degree. C.), but the mixing of phthalic anhydride and at least one 
recycled by-product stream having a freezing point lower than pure 
phthalic anhydride (such as maleic anhydride) produces a liquid phase 
phthalic anhydride product having a lower freezing point than pure 
phthalic anhydride. 
The liquid phase phthalic anhydride product is then separated from the 
remaining vapor phase which comprises maleic anhydride, residual phthalic 
anhydride and other volatile by-products via flash separation in flash 
unit 14. 
The separated liquid phase phthalic anhydride product is then sent via 
conduit 16 to fractionation column 18 where maleic anhydride and other 
volatile by-products are separated. The overhead from fractionation column 
18 is then passed through heat exchanger 20 and into stream splitter 22 
wherein a portion of the liquid phase maleic anhydride and other 
by-products are recycled at approximately 140.degree. F. (60.degree. C.) 
via conduit 12 to mixing vessel 8. The remainder of the maleic by-product 
stream from splitter 22 is sent via conduit 24 for purge of benzoic acid 
and for downstream maleic recovery. Instead of a splitter, a distillation 
tower could be used to obtain a more concentrated purge stream and a 
higher purity maleic recycle by-product stream. 
A phthalic anhydride enriched stream having less than 0.1% of benzoic acid 
is taken as bottoms from fractionation column 18 and sent to a second 
fractionation column 26 via conduit 27. In fractionation column 26, 99.8% 
pure phthalic anhydride product 28 is separated from higher boiling 
residue 30. 
The vapor phase from flash unit 14 is sent to heat exchanger 32 via conduit 
34 where it is cooled to approximately 116.degree. F. (47.degree. C.) 
before being sent to second flash unit 36. As the vapor is cooled and more 
liquid is condensed, the maleic anhydride concentration in the condensed 
liquid increases. The freezing point of the condensate continues to 
decline as the mixture cools so that no solidification occurs. At 
116.degree. F. (47.degree. C.) the concentration of liquid is that of the 
eutectic which has a freezing point of about 105.degree. F. (40.degree. 
C.). The condensed liquid is separated from the remaining vapor in flash 
unit 36. This condensate is then recycled via conduit 10 to mixing vessel 
8. The vapor phase is sent via conduit 38 to either maleic recovery or an 
incinerator (not shown). The vapor phase from flash unit 36 contains less 
than 0.3% of the phthalic anhydride originally present in the vapor phase 
oxidation product. 
In cases where the entering maleic concentration is less than that in the 
vapor leaving the system, increasing the recycle of the other by-products, 
especially benzoic acid, increases the recovery of maleic anhydride to 
remain in material balance. Lowering the temperature in the flash units 
further increases the maleic anhydride recovery. Alternatively, the maleic 
anhydride leaving the system can be recovered for recycle. 
FIG. 2 depicts a continuous liquid phthalic anhydride recovery process 
which includes a maleic anhydride recovery step. Vapor phase oxidation 
product having a temperature of approximately 282.degree. F. (139.degree. 
C.) and recycled by-products having a freezing point lower than that of 
pure phthalic anhydride are fed via conduits 40 and 42, respectively, into 
mixing vessel 44. The resulting mixture has a temperature of 250.degree. 
F. (121.degree. C.) and condenses approximately 70-90% of the phthalic 
anhydride present in the vapor phase oxidation product. The liquid 
condensate (i.e. the liquid phase phthalic anhydride product) is then 
separated from the vapor phase in flash unit 46. 
The liquid phase phthalic anhydride product from flash unit 46 is then sent 
via conduit 48 to fractionation column 50 where benzoic acid and lighter 
components (i.e., more volatile components) are separated from a crude 
phase phthalic anhydride enriched stream which comprises phthalic 
anhydride and heavier components. The overhead from fractionation column 
50 is then sent via conduit 54 for further distillation in fractionation 
column or tower 52. Fractionation column 52 purges benzoic acid and 
heavier components from the overall system before recycling the 
maleic-rich by-product stream to mixing vessel 44 via heat exchanger 58, 
pump 60 and conduit 56. The bottoms product from fractionation column 50 
comprises a crude phthalic anhydride enriched stream. This crude phthalic 
anhydride enriched stream is discharged via conduit 62 and typically 
comprises less than 0.1% benzoic acid. Optionally, the discharge from 
conduit 62 can be sent to another fractionation column wherein 99.8% pure 
phthalic anhydride product can be separated from higher boiling residue. 
The vapor phase effluent from flash unit 46 is further cooled to 
118.degree. F. (48.degree. C.) via heat exchanger 64 to condense as much 
of the maleic anhydride as possible. The cooled product passes through 
conduit 66 into a second flash unit 68 wherein a condensation product of 
primarily maleic anhydride and smaller amounts of phthalic anhydride is 
recycled via conduit 70 to mixing vessel 44. The concentration of maleic 
anhydride has been adjusted by the operating conditions so that the 
condensed liquid is always above its freezing point. 
In cases where the orthoxylene or naphthalene concentrations in air are low 
in the phthalic anhydride air oxidation reaction, the vapor-phase leaving 
the 118.degree. F. (48.degree. C.) flash unit 68 contains a higher amount 
of maleic anhydride than originally present in the vapor phase oxidation 
product. In these cases a maleic recovery step is added. The following 
describes a method for the recovery and recognizes that there are other 
methods which could be used. The vapor phase is passed from flash unit 68 
to mixing vessel 72 where it is contacted with dihexylphthalate, or any 
other ester having a similar boiling point, supplied via conduit 74. The 
ester absorbs approximately 70% of the maleic anhydride which is present 
in the vapor phase. The maleic/ester mixture from vessel 72 is then passed 
to a flash unit 76. The residual vapor is separated and sent via conduit 
78 to an incinerator (not shown). The liquid from flash unit 76 containing 
the absorbed maleic anhydride is separated from the ester via distillation 
in fractionation column or tower 80. This minimizes the amount of ester in 
the overhead maleic anhydride stream and especially in recycle stream 56. 
Virtually all of the ester remaining in the recovered maleic anhydride is 
removed via purge stream 82. Ester in recycle stream 56 would be purged as 
a heavy along with the phthalic anhydride residue, unchanged, but would 
increase the quantity of residue for disposal. The desorbed ester is 
passed through heat exchanger 90, and recycled to mixing vessel 72 via 
conduit 74. 
Instead of esters, an alcohol such as hexyl alcohol or isopropyl alcohol 
could be used as make-up to the maleic recovery section. An alcohol which 
is capable of forming the monoester in-situ from maleic or phthalic 
anhydride and eventually forming the diester with similar adsorption 
properties to dihexyl phthalate would be a satisfactory substitute for the 
esters in the absorption of maleic anhydride. 
The maleic anhydride recovered from the absorption step in fractionation 
column 80 is sent to fractionation column 52 along with overhead stream 54 
from fractionation column 50. Fractionation column 52 has three product 
streams, i.e., bottoms, overhead and a recycle side stream. The bottoms 
are taken out via conduit 82 and primarily include benzoic acid and any 
heavier components rejected from fractionation column 50. Although not 
identified, trace components in this cut have been shown to cause color 
problems if not removed from the final purge stream. Essentially all of 
the ester make-up is rejected to this purge stream. The side stream 56 is 
an impure maleic anhydride recycle stream (i.e., a by-product stream) 
which contains no significant amounts of phthalic anhydride. Overhead 84 
is a higher purity maleic anhydride stream suitable for upgrading for 
commercial sale. 
overall recovery of the phthalic anhydride from the reactor effluent gas 
(i.e., vapor phase oxidation product) is approximately 99.7%. Unlike other 
recovery systems, essentially all of the benzoic acid in the reactor 
effluent gas is recovered and concentrated in purge stream 82. By 
increasing the amount of benzoic acid purged, the benzoic acid content in 
recycle stream 56 can be significantly reduced. 
It is possible to combine cooling step 64 and flash steps 46 and 68 by 
using the countercurrent cooler tower shown in FIG. 3. Starting at the 
cold end 102 of cooler 100, tubes 104 extend beyond tubes sheet 106 
similar to that of wetted wall column designs. A pumparound circuit 105 
with a trim cooler 107 permits making vernier adjustments to the 
temperature. The recycled maleic anhydride stream is added to this circuit 
105 via conduit 111. Part of the cooled pumparound is used to keep the 
de-entrainment screens 108 wet and provide good contacting with the vapor. 
Not only is entrainment reduced, but also an additional countercurrent 
contacting stage is obtained. 
The main cooling is done by the coolant disposed within shell 118 of cooler 
100. The coolant enters cooler 100 via conduit 110 and exits via conduit 
112. The vapor phase from flash unit 46 enters cooler 100 via conduit 114 
and the liquid phase phthalic anhydride product stream exits via conduit 
116. 
A close approach to the process stream in the cold end 102 is achieved by 
the use of heat transfer enhancers (not shown) in the tubes therein. The 
liquid on the top tube sheet 106 overflows into tubes 104 and is 
distributed evenly using typical wetted wall distributor designs. As the 
liquid moves down tube 104 it is heated by the rising vapor phase effluent 
supplied via conduit 114 and cooled by the coolant. As the stream gets 
hotter the maleic anhydride is recondensed with smaller amounts of 
phthalic anhydride setting up a large internal recycle of maleic 
anhydride. When the stream reaches 250.degree. F. (121.degree. C.), the 
concentrations will be that of the 250.degree. F. (121.degree. C.) flash 
unit. The remaining vapor phase exits cooler 100 via conduit 113 and is 
sent for further maleic recovery or incineration. 
The coolant rate is controlled to produce a liquid phase phthalic anhydride 
product at 250.degree. F. (121.degree. C.)Heat transfer enhancers are not 
used in the bottom section 120 to minimize the possibility of freeze-ups 
by localized overcooling. In practice, the system tends to be self 
limiting. If too cold, solids will form on the tube surface which 
significantly decreases the localized cooling. Decreased cooling will 
reduce the amount of phthalic anhydride condensation which increases the 
concentration of maleic anhydride in the liquid phase which tends to wash 
away the solid deposits. 
The concentration of the phthalic anhydride in the liquid phase at the cold 
end 102 of the exchanger (i.e., the pumparound composition) is monitored 
and maintained by addition of the recycled maleic stream via conduit 111. 
During start-up significantly higher amounts may have to be added until 
steady state is reached. 
Since the organic content of the vapor phase oxidation product is typically 
less than 3 volume %, the volume of vapor phase is very large compared to 
the liquid organic stream condensed therefrom. Transporting such large 
volumes of gases requires the use of large diameter piping to minimize 
pressure drops throughout the process. As such, FIG. 4 depicts another 
embodiment according to the present invention wherein the ester absorption 
step is placed on top of the cooler shown in FIG. 3. This puts all of the 
large diameter equipment into one compact unit which minimizes pressure 
drop losses. In accordance with this embodiment, the vapor from tubes 104 
pass through tubes sheet 106 into vapor channel 130 wherein it is 
contacted with an ester or ester convertible alcohol sprayed into the 
channel via distributor means 132. The resulting maleic/ester mixture 
collects along the bottom 134 of the ester recovery stage where it is sent 
on to fractionation column 80. The vapor phase is then sent via conduit 82 
for disposal by incineration. 
FIG. 5 depicts another embodiment in accordance with the present invention 
wherein a packed or trayed cooling tower 200 is substituted for cooling 
means (32 or 64) and flash unit (36 or 68) and their connecting piping. 
Cooling tower 200 contains low pressure packing or trays to promote heat 
and mass transfer. Cooling tower 200 avoids the large pressure drops 
associated with multiple components and also provides the advantage of 
countercurrent operation which reduces the possibility of plugging and 
provides more efficient heat removal. 
Cooling tower 200 includes cooling coils or trays 202, 204 and 206 which 
reduce the amount of recycle required. The cooled reaction effluent stream 
from cooler 4 is mixed with less recycle in mixer 8 or 44 and enters tower 
200 via conduit 208 at a temperature of approximately 268.degree. F. 
(131.degree. C.). As the gas enters zone 1 of tower 200 it contacts 
cooling coil 202 which has a coolant passing therethrough having a 
temperature of approximately 210.degree. F. (99.degree. C.), thereby 
reducing the temperature of the gas to approximately 250.degree. F. 
(121.degree. C.). The gas then enters zone 2 where it contacts cooling 
coil 204 which has a coolant passing therethrough having a temperature of 
approximately 190.degree. F. (88.degree. C.), thereby further reducing the 
temperature of the gas to about 200.degree. F. (93.degree. C.). Finally, 
the gas enters zone 3 where it contacts cooling coil 206 having a coolant 
temperature of approximately 130.degree. F. (54.degree. C.). The final 
cooling step reduces the temperature of the gas to approximately 
140.degree. F. (60.degree. C.). The remaining vapor phase is then sent via 
conduit 210 for maleic anhydride recovery or incineration. The liquid 
phase which is formed by the condensation product occurring within each 
zone of tower 200 is progressively enriched in phthalic anhydride as it 
moves down the tower. The liquid phase is removed from tower 200 via 
conduit 212 and recycled to the mixing stage (8 or 44). 
It would be cost effective if the conventional switch condensers were 
modified so that they could be adapted to recovering liquid phthalic 
anhydride and maleic anhydride with only small modification. Conventional 
switch condensers cool the effluent gases over about the same temperature 
range as needed for the liquid phthalic recovery process of the present 
invention. FIG. 7 shows a conventional switch condenser 300 which has been 
modified according to the present invention to include three layers of 
crinkled wire mesh screening 302, 304 and 306 (CWMS) or similar devices. 
The CWMS devices have been incorporated into switch condenser 300 in order 
to promote contacting and liquid distribution and to reduce entrainment. 
In one embodiment the switch condenser 300 performs phthalic recovery, 
wherein maleic recycle (i.e., a eutectic mixture of maleic and phthalic 
anhydride) is sprayed over the top heat exchange tube bundle 310 using an 
existing spray wash distributor 308. As the cooled maleic mixture descends 
through each bundle 310 (typically 4 bundles) it contacts the hotter 
incoming gas which enters switch condenser 300 via inlet 312. The hotter 
incoming gas is also cooled by the oil flowing inside the heat exchange 
tube of bundles 310. As the liquid descends, it will increase in 
temperature and will become enriched in phthalic anhydride so its freezing 
point will increase. The phthalic enriched liquid is drained from switch 
condenser 300 via port 314 and has a temperature of approximately 
250.degree. F. (121.degree. C.). This phthalic anhydride enriched liquid 
has the approximate composition of the first flash in the liquid phthalic 
recovery process at 250.degree. F. (121.degree. C.). The vapor phase from 
switch condenser 300 exits via port 316. This vapor phase has a 
temperature of approximately 120.degree. F. (49.degree. C.) and is sent to 
an incinerator or maleic recovery. Thus, the condenser accomplishes the 
phthalic anhydride recovery part of the liquid phthalic recovery process 
by combining the two flashes and intermediate cooling step in one vessel. 
The oil rate in the liquid phthalic recovery process using switch 
condensers is controlled (i.e., limited) to obtain the desired 
temperatures and a countercurrent temperature profile (hotter at the 
bottom) which minimizes solids formation from freezing of the liquid. If 
some freezing were to occur this would self limit the cooling in that 
region due to the insulating effect of the solid. Deposits are of much 
less concern than in existing condensers because there is no temperature 
cycling and because the continuous liquid flow over the deposit would 
dissolve any acid formed as in the current spray washing. 
In a second embodiment, switch condenser 300 can also be used for maleic 
anhydride recovery, wherein dihexylphthalate, or any other ester having a 
similar boiling point, is sprayed over the top of bundles 310 using the 
existing spray wash distributor 308. The vapor phase from the phthalic 
anhydride recovery process is introduced into switch condenser 300 via 
port 316. As the cooled maleic mixture descends through bundles 310 it 
contacts dihexylphthalate, or any other ester having a similar boiling 
point, and is also cooled by the oil flowing inside bundles 310. As the 
liquid descends the ester absorbs approximately 85% of the maleic 
anhydride which is present in the vapor phase. The ester/maleic mixture is 
thereafter forwarded via port 314 to a downstream desorption step and the 
residual gas is sent via port 312 to incineration. 
Alternatively, the maleic anhydride absorption can be done countercurrently 
by introducing the gas in the bottom of the condenser and withdrawing it 
from port 316. 
In still another embodiment, the desorption of the ester can be carried out 
in a condenser by using the hot oil circuit provided to melt out the 
desublimed phthalic anhydride. In this embodiment, the ester containing 
the maleic anhydride from the absorption step is sprayed over the switch 
condenser tubes as in the absorption step. Because the tubes are 
maintained at about 204.4.degree. C. (400.degree. F.) by the hot oil in 
the tubes, the maleic anhydride is desorbed and removed via port 312 or 
port 316. The desorbed ester after cooling is then returned to the maleic 
recovery step. 
The process can then be conducted in three switch condensers using one for 
each of the three aforementioned embodiments. In the first, phthalic 
anhydride is recovered. The remaining vapor leaving the top of the 
condenser is connected via a short jump-over to a second adjacent 
condenser wherein the maleic anhydride is recovered from the vapor by 
absorption with ester. The maleic anhydride is then separated from the 
ester by heating in the third condenser. Thus, all three process steps can 
be accomplished using the existing equipment. This significantly reduces 
capital investment and downtime in conversion to the more efficient 
recovery process. 
While I have shown and described several embodiments in accordance with my 
invention, it is to be clearly understood that the same are susceptible to 
numerous changes apparent to one skilled in the art. Therefore, I do not 
wish to be limited to the details shown and described but intend to show 
all changes and modifications which come within the scope of the appended 
claims.