Process and plant for the purification of raw maleic anhydride recovered from gaseous reaction mixtures

In the purification of maleic anhydride obtained with an oxidation process, by means of absorption with an organic solvent and fractionation of the resulting liquid phase and purification of the crude maleic anhydride thus separated, the above purification being accomplished through two serially placed fractionating columns, the accumulation of polymerisable acrylic acid overhead the first fractionating column of the purification section, and of water as well as of maleic acid overhead the second fractionating column of the purification section, is avoided by carrying the overhead vapours of said first fractionating column to an absorption tower, preferably operating with the same absorption organic solvent used to absorb the crude maleic anhydride from the reaction mixture, and by carrying the overhead vapours of said fractionating column of said purification section to two condensers placed in series, so that the gaseous fraction, coming out of the first condenser, is fed to said second condenser.

The present invention refers to a process for the purification of maleic 
anhydride and, more particularly, to a process for the purification of the 
crude maleic anhydride resulting from the recovery by means of absorption 
with an organic solvent of the same anhydride from gaseous mixtures 
obtained from butane and/or benzene oxidation. 
BACKGROUND OF THE INVENTION 
The maleic anhydride is produced by an oxidation reaction in gaseous phase 
and is obtained as a diluted and raw, gaseous form. 
A first recovery method consists in the water absorption of the maleic 
anhydride, which reacts with water to form maleic acid. 
After the water separation by distillation and the dehydration of the 
maleic acid, maleic anhydride is recovered. 
This recovery type involves, however, a high energy consumption, product 
loss owing to conversion of maleic acid to fumaric acid, high amounts of 
pollutants in the effluents and frequent plant stops for washings. 
Alternatively, for some time the recovery of maleic anhydride by absorption 
in an organic solvent has been proposed. 
To this process, proposed in the U.S. Pat. No. 2,942,005 describing the 
absorption of maleic anhydride and phthalic anhydride from reaction 
mixtures, several operating improvements have been brought. 
In the U.S. Pat. No. 3,891,680, the gaseous reaction mixture (from which 
maleic anhydride is to be recovered) is contacted with an organic liquid 
phase. 
A similar process is disclosed in the U.S. Pat. No. 4,118,403, in which the 
maleic anhydride is recovered by absorption in an organic liquid to which 
phthalic anhydride is added. 
Finally, the U.S. Pat. No. 5,969,687 discloses the maleic anhydride 
recovery by means of an organic solvent and, in particular, includes a 
stripping step of the enriched solvent with a gas of low humidity contents 
to remove water and volatile substances. 
In all the procedures based on the use of an organic absorption solvent, 
the enriched solvent is passed through a separator under vacuum, in which 
the raw maleic anhydride is condensed overhead and the depleted solvent 
leaves from the bottom for being recycled to the absorption phase. 
As already mentioned, this fractionating step is common to all solvent 
recovery processes and leads to produce raw maleic anhydride of a 97-99% 
purity. 
The main residual impurities consist of: 
(i) a small amount of solvent 
(ii) a certain amount of organic by-products formed in the oxidation 
reactor and absorbed together with the maleic arthydride in the absorber, 
and 
(iii) water, either free or combined with maleic anhydride. 
Typical examples of light organic impurities are acrylic acid and traces of 
carbonyl compounds. 
If the solvent is an ester of a light alcohol (e.g. dibutylphthalate) and 
the separation between maleic arthydride and solvent is conducted under 
severe conditions, the alcohol that comes out from the decomposition of 
the organic solvent might also be present as an impurity in the raw maleic 
anhydride, typically in the monoester form. Even though the raw maleic 
anhydride purification can be carried out both in batch and continuous 
processes, the batch process has some major disadvantages, as for example, 
the discontinuous steam demand and the necessity of bringing considerable 
amounts of maleic anhydride to boil, with a consequent, increased risk of 
exothermal reactions of the maleic anhydride. 
The continuous process appears to be preferable and, according to a 
classical scheme, entails a first fractionating column, within which the 
light substances are removed, and a second column from the bottom of which 
the solvent containing a small quantity of maleic anhydride is drawn, that 
solvent being sent to recycling, whilst the recovered maleic arthydride is 
drawn in liquid form from the overhead section. However, also the 
continuous process faces problems and drawbacks not yet solved. 
The first of these relates to the drawing of overhead products from the 
first column where the light substances are separated, including acrylic 
acid. 
The fraction drawn from overhead, and that is sent to a condenser for 
recycling in liquid phase to the column head, contains acrylic acid in a 
high concentration, which tends to polymerize and thus to foul the 
condenser and the column uppermost part in a not negligeable extent. 
The second problem connected to the traditional process is represented by 
water and light or volatile substances present in-the second purification 
column, where the maleic anhydride is recovered in the upper part of the 
column itself. 
This water might originate from atmospheric humidity infiltrating in the 
vacuum operating system, or from the decomposition of maleic acid present 
in the raw maleic anhydride. Water may also be possibly produced by losses 
from condensers or boilers, due to leaking tube-to-tube sheet joints. 
Since such water is being absorbed into the maleic anhydride condensed 
overhead the second purification column and refluxed overhead the column 
itself, an unwanted concentration increase of water, that is, of maleic 
acid, in the maleic anhydride obtained in this column, occurs. 
Purpose of the present invention is to substantially eliminate such 
problems and drawbacks. 
SUMMARY OF THE INVENTION 
This purpose is achieved by the process of the present invention for the 
purification of raw maleic anhydride coming from a recovery unit, of the 
type in which the maleic anhydride present in the gaseous reaction mixture 
is absorbed by means of an organic solvent into an absorber, and the 
enriched organic solvent is sent to a vacuum separation column, from which 
raw maleic anhydride is drawn overhead, whereas the depleted solvent is 
recycled to said vacuum separation column, the maleic anhydride being then 
sent to a purification section, the process being characterized in that: 
(a) the top section of the first purification column is operated under 
stripping conditions, whereby the vapours drawn overhead are fed to an 
absorber which is countercurrently fed with an organic solvent, preferably 
the same solvent used for the absorption of the raw maleic anhydride from 
the reaction mixture, the solvent phase coming out from this absorber and 
containing all the maleic anhydride of the overhead section of the first 
purification column, and most of the acrylic acid being recycled to said 
first absorption column for absorbing with an organic solvent the raw 
maleic anhydride from the reaction mixture, and/or 
(b) the overhead vapour fraction of the second purification column is 
subjected to a first condensation under conditions such as to cause the 
condensation of most of the maleic anhydride contained in those vapours, 
but being kept above the dew point of water present in those vapours, so 
that the liquid phase recycled to the overhead section of said second 
purification column will be enriched with liquid maleic arthydride but 
depleted in water, whilst the vapour phase coming out from said first 
condensation is sent to a second condensation leading to a liquid phase 
containing the residual maleic anhydride, a small amount of water and the 
residual volatile substances, said liquid phase being recycled upstream of 
the second purification column of the process, in particular to said 
absorber of raw maleic anhydride from the reaction mixture, or to the feed 
of raw maleic anhydride at the overhead of said first purification column. 
According to a preferred embodiment of the invention, the vapours drawn 
from overhead the first purification column, before being fed to the 
absorber, are sent to the inlet of a condenser which can be the same 
condenser of the vacuum separation column, the raw maleic anhydride being 
its overhead product, where most of maleic anhydride is condensed. 
In turn, the plant according to the invention for the purification of crude 
maleic anhydride of the type comprising a first column for the absorption 
with an organic solvent of raw maleic anhydride from the reaction mixture 
that comes from the oxidation reactor, vacuum separation column fed with 
the enriched solvent phase coming from said first absorption column, and a 
purification section fed with the overhead product of said vacuum 
separation column, said purification section including a first 
fractionating column and a second purification column, the latter being 
fed with the bottom liquid going out from said first fractionating column, 
said plant being characterized in that it comprises: 
1) a vacuum absorption tower fed with the overhead gaseous fraction 
outgoing from said first fractionating column, possibly after partial 
condensation of said overhead gaseous fraction and/or 
2) a first condenser placed at the outlet of the overhead gaseous fraction 
of said second purification column, and a second condenser placed 
downstream of said first condenser and fed with the gaseous fraction 
outgoing from said first condenser.

DETAILED DESCRIPTION OF THE INVENTION 
More specifically, FIGS. 1, 2 and 3 show the plant section of the maleic 
anhydride recovery and of the first part of the purification section, 
according to possible changes, while FIG. 4 shows the second part of the 
purification section. 
By reference first to the FIG. 3, reference 10 represents the separation 
column by means of absorption with an organic solvent of the maleic 
anhydride that comes from the oxidation reactor. 
Reference 12 precisely indicates the feed of gaseous mixture coming from 
the oxidation reactor, whereas reference 14 stands for the feed of the 
absorption organic solvent. 
The exhausted gases are discharged through outlet 16. From the absorber 10, 
the solvent phase enriched with maleic anhydride is fed, as per reference 
18, to a fractionating column 20, from the bottom of which the depleted 
solvent phase 22 is drawn and recycled to the absorber 10, whilst 
reference 24 indicates altogether and in general the bottom reboiler of 
the column 20. 
The gaseous phase outgoing from head of the tower 20 is sent, as indicated 
by reference 26, to a condenser 28, from which a gaseous phase, consisting 
of uncondensable gases, is obtained and sent for the disposal as shown by 
reference 30, whilst the condensate (line 32) feeds the first 
fractionating column 34 of the purification section. 
From the bottom of the column 34 a liquid phase 36 is obtained, consisting 
of maleic anhydride which is sent to the second purification column (as 
per FIG. 4), while the gaseous or overhead phase, drawn as reference 38 
indicates, contains the previously mentioned impurities and, particularly, 
the acrylic acid that, by polymerisation, solidifies and causes severe 
scale and fouling problems of the column head 34. 
In accordance with the present invention, the above head fraction, instead 
of being subjected to condensation with recycling of the liquid phase 
overhead the column 34, is directly sent to a vacuum absorber 40 which, in 
the embodiment of FIG. 3, is fed by the same organic solvent for the 
absorption of the raw maleic anhydride from the gaseous reaction mixture 
in the absorption tower 10, that is, the depleted organic solvent recycled 
from the bottom of the fractionating column 20. 
The bottom fraction deriving from this absorber is sent through line 42 to 
the first absorbing tower 10 paralleles to the recycle of depleted solvent 
14, whilst the overhead fraction, that includes almost mainly 
uncondensable and volatile products, is sent to the disposal as indicated 
by reference 44, wherein preferably the gaseous fraction resulting from 
the condenser 28 also is fed. 
It is noteworthy that in this manner the problem related to the presence of 
no negligible traces of acrylic acid in the raw maleic anhydride is 
substantially avoided, without however undergoing a loss of maleic 
anhydride, as much the bottom or liquid fraction from the vacuum absorber 
40 passes to the primary absorber 10, where the gaseous reaction mixture 
subjected to the absorption, operates as a stripping gas to remove the 
acrylic acid. 
In the embodiment form illustrated in FIG. 1, the gaseous or head phase 38 
of column 34, before being sent to the vacuum absorption column 40, is 
partially condensed in a condenser 60, the condensate of which is recycled 
(through line 62) to the feed 32 of the first fractionating column 34, 
while the gaseous phase through line 64 is sent to the vacuum absorption 
column 40. 
FIG. 2 shows a change of this embodiment as, instead of a separate 
condenser 60, there is used the same condenser 28 provided for the 
condensation of the overhead. product 26 of column 20. 
With reference now to FIG. 4, the bottom fraction 36 of the first 
fractionating column 34 feeds a second fractionating column 46, wherefrom 
maleic anhydride (as per reference 48), having the desired purity, is 
obtained, whereas the overhead vapours (reference 50) are sent to a first 
condenser (reference 52) wherein most of the maleic anhydride contained in 
these vapours is condensed. 
It is important to point out that this condenser is operated under 
conditions such as not to reach the dew point of water that remains 
therefore in the vapour phase. 
Besides, the apparatus is to be selected of a type with features such as to 
reduce to the maximum possible extent the contact between uncondensed 
vapour and condensed liquid. 
The condensed liquid, consisting of very pure maleic anhydride and having a 
very low water content, is recycled (line 54) overhead the column 46, thus 
reaching the main target, namely to reduce to the maximum possible extent 
the traces of humidity and of other volatile organic products in the 
maleic anhydride so produced, and to avoid that water concentration in the 
purification section increases with the apparent already mentioned 
disadvantages. 
The uncondensed vapours (reference 56), still containing maleic anhydride, 
water, traces of volatile and colouring substances therein, are condensed 
in a second condenser 58, in which the liquid phase is condensed at a 
temperature near to the maleic anhydride melting point, and the liquid 
stream is recycled upstream along the plant, for example to the primary 
tower 10 for the absorption of the maleic anhydride from the gaseous 
reaction mixture or to the feed of the first purification tower 34 
together with raw maleic anhydride 32. 
In FIGS. 1-3 and 4 enclosed herewith, the preferred embodiment forms of the 
two parts of the purification section are shown, being understood that it 
is possible and envisageable to design the purification section with the 
first fractionating column 34 made as shown in FIGS. 1, 2 or 3, and the 
second purification column maintained in traditional form and structure or 
vice versa. 
With an industrial plant built up with the variarious of the two 
purification columns, realized as shown in the drawings, industrial tests 
of the process in accordance with the invention have been carried out. 
The tests have been conducted by checking the concentrations of maleic 
anthydride, acrylic acid and water at the positions that are typical of 
the plant and of the related process. 
The data, referred to 1000 kg/h of treated raw maleic anhydride are 
reported in the following table 
TABLE I 
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Water (as 
free water 
Maleic Acrylic and maleic 
anhydride 
Acid acid) 
______________________________________ 
Raw maleic 1000 2.34 4.78 
anhydride 
purification column (34)- 
200 2.12 4.40 
overhead 
purification column (34)- 
800 0.22 0.34 
bottom 
air leakages entering the 
-- -- 0.37 
column (46) 
gaseous phase from conden- 
25 0.19 0.51 
ser (52) 
liquid from condenser (58) 
23 0.12 0.27 
exhaubt gas from condenser 
2 0.07 0.24 
(58) 
final product (48) 
750 0.03 0.20 
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From the data of the previous table it is clear that in the overhead 
fraction of the first purification column, about 20% of the maleic 
anhydride to be purified is present together with a large part of the 
acrylic acid and water. 
In the subsequent steps, more than 99% of this maleic anhydride is 
recovered (particularly in the vacuum absorption tower 40). In addition, 
the uncondensable vapours from the outlet of the first condenser 52 
contain most of the water and of the acrylic acid which are fed to the 
second purification column 46. 
These vapours are condensed in the second condenser and recycled upstream 
so as to extract the light components. 
With the process and plant of the invention, the purified maleic anhydride 
has an acrylic acid content definitely lower than 0.01% and a maleic acid 
content definitely lower than 0.05%, whereas with the traditional 
processes and plants, the acrylic acid content may be about 0.01% or 
higher, and that of maleic acid may be about 0.05% or more. 
Table 2 indicates the typical values for a traditional purification 
section, operated without using the process of the invention, wherein from 
overhead the first column a liquid and a gaseous phase are extracted and 
the second column is equipped with only one condenser for the overhead 
product. 
Also for this table the reported data refer to a 1000 kg/h feed of raw 
maleic anhydride. 
TABLE II 
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Water (as 
free water 
Maleic Acrylic and maleic 
anhydride 
acid acid) 
______________________________________ 
Raw maleic anhydride 
1000 2.34 4.78 
purification column (34) 
20 2.10 4.35 
overhead (lig. + vap.) 
purification column (34) 
980 0.24 0.39 
bottom 
leakages in (46) 
-- -- 0.37 
exhaust gas from the only 
2.8 0.10 0.24 
condenser of column (46) 
final product 950 0.14 0.52 
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