Purification of carbonylation products

A method is provided for the removal of contaminating alkanes from the vaporized carbonylation products of a process wherein carboxylic acids are produced by the reaction in the liquid phase of an alcohol or an ester, halide or ether derivative of said alcohol with carbon monoxide in the presence of a catalyst system containing a rhodium or iridium component and an iodine or bromine component. The method involves distillation of the alkane-containing vaporized carbonylation products, phase separation of the overhead from said distillation, further distillation of a slipstream of the resulting heavy phase using carbon monoxide as stripping gas and removal of the alkanes as the bottoms stream from the latter distillation.

BACKGROUND OF THE INVENTION 
The present invention relates to a carbonylation process improvement. More 
particularly, this invention relates to an improved process scheme wherein 
alkanes are removed from carbonylation products. 
Recently, processes for producing carboxylic acids and esters by 
carbonylating olefins, alcohols, esters, halides and ethers in the 
presence of homogeneous catalyst systems that contain rhodium or iridiium 
and halogen components such as iodine components and bromine components 
have been disclosed and placed into commercial operation. These recently 
developed processes represent a distinct improvement over the classic 
carbonylation processes wherein such feed materials have been previously 
carbonylated in the presence of such catalyst systems as phosphoric acid, 
phosphates, activated carbon, heavy metal salts and metal carbonyls such 
as cobalt carbonyl, iron carbonyl and nickel carbonyl. All of these 
previously known processes require the use of extremely high partial 
pressures of carbon monoxide. These previously known carbonylation systems 
also have distinct disadvantages in that they require higher catalyst 
concentrations, longer reaction times, higher temperatures to obtain 
substantial reaction and conversion rates that all result in larger and 
more costly processing equipment and higher manufacturing costs. 
The discovery that rhodium or iridium and iodine- or bromine-containing 
catalyst systems will carbonylate such feed materials as olefins, alcohols 
and ester, halide or ester derivatives of the alcohols at relatively mild 
pressure and temperature conditions was a distinct contribution to the 
carbonylation art. In spite of the vast superiority of these newly 
developed catalyst systems, it was found that conventional processing 
schemes for separation of the carbonylation products from the liquid 
reaction mass posed problems of catalyst inactivation and precipitation. 
In U.S. Pat. No. 3,845,121, an improved process scheme is described 
wherein carbonylation products can be recovered from a carbonylation 
reaction zone without resulting in decomposition of the catalyst system 
when the liquid-phase active catalyst and unreacted feed components are 
recycled to the reaction zone. According to this process, the 
carbonylation rection is carried out in the reaction zone at a temperature 
from about 50.degree. to 150.degree. C. and a pressure from about 50 to 
1500 psia and at least a portion of the liquid reaction mass is passed to 
a separation zone without the addition of heat, said separation zone 
having a pressure of at least 20 psi less than the pressure in said 
reaction zone to vaporize at least a portion of the carbonylation 
products, the vaporized carbonylation products being withdrawn and the 
remaining liquid reaction mass being recycled to said reaction zone. 
The vaporized product obtained in the process of U.S. Pat. No. 3,845,121 is 
passed to a conventional purification system of multiple distillation 
columns for ultimate recovery of the carboxylic acid in pure form. 
However, certain difficulties arise during such purification which are 
attributable to the presence in the system of alkanes typical among which 
are those having from 6 to 12 carbon atoms and which are straight-chain 
compounds such as hexane, heptane, octane, decane and the like. Alkanes 
can enter the carboxylic acid production facilities from several sources 
among which are the carbon monoxide and the methanol feed streams. The 
alkanes azeotrope or steam distill with water and are soluble in methyl 
iodide. They create problems in the reaction zone in that a second phase 
containing alkanes, methyl iodide and acetic acid is generated therein. In 
addition, the alkanes appear in the heavy phase from the overhead of the 
column used to remove the light ends from the reaction effluent. This 
heavy phase is normally recycled to the reactor. In addition, to the fact 
that the presence of alkanes therein has a tendency to overload the heavy 
phase pump due to dramatic density changes, these unwanted compounds tend 
to load down the light ends column itself thus limiting plant capacity. 
There is no way out of the system for these alkanes except by actual 
spills and thus a need for a method for their removal is created. 
It is an object of the present invention, therefore, to provide a method 
for the removal of alkanes in a process wherein carboxylic acids are 
produced by the reaction of an alcohol or olefin with carbon monoxide in 
the presence of a rhodium- or iridium-containing catalyst system and 
recovered therefrom by distillation. 
Other objects and advantages of the invention will become apparent from the 
following discussion of the invention. 
SUMMARY OF THE INVENTION 
The present invention relates to an improvement in the process wherein an 
alcohol or an ester, halide or ether derivative of said alcohol is reacted 
with carbon monoxide in the liquid phase in the presence of a catalytic 
system that contains a rhodium or iridium component and an iodine or 
bromine component, a portion of the liquid reaction mass is passed to a 
separation zone without the addition of heat, said separation zone having 
a pressure of at least 20 psi less than the pressure in said reaction 
zone, to vaporize at least a portion of the carbonylation products, the 
remaining liquid reaction mass is recycled to the reaction zone, and the 
vaporized carbonylation products are withdrawn and subjected to 
distillation for recovery of carbonylation products therefrom. The 
improvement comprises removing alkanes from the purification system by 
introducing said vaporized carbonylation products into a first 
distillation zone to remove an overhead product and a bottoms product, 
separating said overhead product into a light phase and a heavy phase, 
removing a slipstream of said heavy phase, introducing it into a second 
distillation zone and distilling so as to remove an overhead stream free 
of alkanes and a bottoms stream consisting substantially of alkanes, and 
recycling said alkane-free overhead stream to the separation zone. The 
bottoms product from the first distillation zone can be recycled to the 
separation zone, while the light phase and the major part of the heavy 
phase from the overhead product of the first distillation column can both 
be recycled to the reactor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To further describe the present invention, reference is made to the 
accompanying drawing, which represents a schematic diagram of the process 
of the invention wherein methanol is carbonylated in the presence of a 
rhodium- or iridium-containing catalyst system. By way of example, the 
catalyst system can be formed by introducing rhodium iodide and hydrogen 
iodide into reactor 10 that has been partially filled with acetic acid and 
water as a reaction medium. Carbon monoxide can be sparged into the 
reactor through line 11. Methanol feed is introduced into the reactor 
through line 12. The reactor is maintained at a temperature of from about 
160.degree. to about 220.degree. C. and the pressure in the reactor is 
maintained from about 200 to 750 psig. Unreacted carbon monoxide, along 
with any gaseous impurities or by-products can be withdrawn from the 
reactor through line 13. 
A portion of the liquid reaction mass is withdrawn from reactor 10 through 
line 14. Pressure let-down valve 15 is disposed in line 14 to let the 
pressure down at least 20 psi as it enters separation zone 16. As the 
reaction mass enters separation zone 16, a portion of the carbonylation 
products vaporize and can be withdrawn from the separation zone through 
line 17. The remaining liquid reaction mass containing the catalyst system 
in separation zone 16 can be recycled to reactor 10 via line 18. The 
vapors withdrawn from the separation zone through line 17 are introduced 
into distillation zone 20 via line 19 and distilled therein to remove an 
overhead product and a bottoms product. The bottoms product consisting 
mainly of acetic acid with some water and a small amount of hydrogen 
iodide is recycled to the separation zone via line 22. The overhead 
product which contains water, acetic acid, a major proportion of methyl 
iodide and alkanes is withdrawn through line 21, condensed and passed to 
separator 23 where it is allowed to settle to form two phases. The light 
phase consisting essentially of acetic acid and water is returned to the 
reactor 10 via line 24. The major part of the heavy phase consisting of 
methyl iodide, acetic acid, water and alkanes is returned to the reactor 
10 via line 25. To effect removal of the alkanes, a slipstream 26 
representing about 1% or less by weight of the total heavy phase is 
withdrawn through line 26 and introduced into distillation zone 27 at the 
upper end thereof while a stream of carbon monoxide is fed through line 28 
to facilitate stripping of the methyl iodide from the alkane mixture. An 
overhead product substantially free of alkanes and consisting 
predominantly of methyl iodide with some acetic acid is withdrawn through 
line 29 and recycled to the separation zone 16. The bottoms product 
consisting essentially of alkanes, some acetic acid and traces of methyl 
iodide leaves the system through line 30 and is sent to waste disposal 
facilities. Crude acetic acid leaves the system through line 31 and is 
passed to downstream distillation for further purification. 
The basic process for the production of carboxylic acids and esters to 
which the present invention applies is described in full detail and 
claimed in U.S. Pat. Nos. 3,769,329 and 3,772,380, both of which are 
incorporated herein by reference. The separation of the carbonylation 
products from the reaction mixture without catalyst deomposition as has 
been mentioned earlier is described and claimed in U.S. Pat. No. 
3,845,121, which is likewise incorporated herein by reference. The process 
of the present invention constitutes an improvement whereby alkanes are 
removed from the product obtained when preparing carboxylic acids using 
the process resulting from combination of methods described in the 
above-mentioned patents. 
In the process of the present invention briefly described above and 
illustrated in the drawing, the distillation zones can comprise any 
distillation columns normally used for separation of fluids and can be 
either the packed or tray type or they can be a combined packed-tray type. 
Distillation column 20 will contain from about 10 to about 20 plates and 
preferably about 15 plates. Distillation column 27 requires only about 15 
plates. Distillation column 27 requires only about 5 plates for 
satisfactory operation. However, from 5 to 10 plates can be employed if 
desired. The associated condensers employed with either or both of the 
distillation columns described are of generally conventional design and 
manufacture. Various pumps, compressors, reboilers etc. normally employed 
in carrying out distillations in chemical processes are employed in the 
process described herein. Since these do not form part of the invention, 
the details of their use in various phases of the process description have 
not been included. 
The temperatures and pressures employed in the distillation zones of the 
present invention as described above will vary considerably depending on 
the particular carboxylic acid being produced. As a practical matter, the 
distillation zones are most often operated at pressures from about 
atmospheric to about 100 psig. The pressure employed in column 20 for 
example, when acetic acid is the carboxylic acid produced is from about 5 
to about 20 psig and in column 27 it is about 30 - 35 psig with this acid. 
However, sub-atmospheric pressures may be employed if desired as well as 
superatmospheric pressures well in excess of 100 psig in either or both of 
these columns. 
Temperatures within the distillation zones will normally lie between 
approximately atmospheric temperature and at or slightly above the boiling 
point of the particular carboxylic acid being recovered and purified. When 
employing the process in the manufacture of acetic acid, the bottoms 
temperature of column 20, for example, will generally be within the range 
from 120.degree. to 135.degree. C. but preferably will be maintained at 
about 130.degree. C. The bottoms temperature of column 27 will generally 
be higher and will be in the range from about 130.degree. to about 
140.degree. C. and preferably is maintained at about 136.degree. - 
139.degree. C. The temperatures at the top of the distillation zones can 
likewise vary. Overhead temperatures in column 20, for example, in the 
process wherein acetic acid is produced can be from about 100.degree. to 
about 120.degree. and preferably are maintained at about 112.degree. C. to 
116.degree. C. In column 27, over head temperatures are lower being in the 
range of 75.degree. C. to 85.degree. C. and preferably about 80.degree. C. 
The point of introduction of the feed stream to the first distillation zone 
(column 20) can be anywhere intermediate the ends of the zone but the feed 
stream preferably is introduced into the lower half of that zone. The feed 
stream to the second distillation zone (column 27) can be introduced 
anywhere in the upper half of that zone. Generally, this feed is 
introduced at a point about two-thirds of the height of that distillation 
zone or into the upper one-third thereof. 
The slipstream 26 removed from the heavy phase overhead of the first 
distillation zone may vary in the size from about 0.1 to about 1.0% by 
weight of the total heavy phase. For most efficient removal of alkanes, 
however, this stream constitutes from about 0.3 to about 0.5% by weight of 
the heavy phase. 
The carbon monoxide introduced into the distillation zone to facilitate the 
removal of alkanes is generally fed at a rate to provide from about 0.01 
to about 1 lb. of CO per pound of feed introduced into the zone. Other 
gases can be used for stripping if desired such as hydrogen and CO.sub.2, 
for example. 
To demonstrate the effectiveness and to illustrate the application of the 
process improvement of the present invention, the following non-limiting 
example is set forth. Unless otherwise indicated, all parts and 
percentages given are on a weight basis. 
EXAMPLE 1 
Methanol was carbonylated in the presence of a catalyst system that formed 
on mixing rhodium iodide with methyl iodide in the presence of carbon 
monoxide in an acetic acid-water reaction medium using apparatus 
substantially the same as is presented in the drawing. Approximately 267 
parts/hr. of methanol were charged to the reactor 10 through line 12 while 
244 parts/hr. of carbon monoxide were charged to the reactor through line 
11. The reactor was maintained at a temperature of about 187.degree. C. 
and a pressure of about 400 psig. Unreacted carbon monoxide was withdrawn 
from the reactor through line 13 and passed to a flare. Approximately 5545 
parts/hr. of liquid reaction mass was withdrawn from the reactor through 
line 14 and passed into separation zone 16. The pressure in separation 
zone 16 was about 20 psig and the liquid temperature was about 127.degree. 
C. No heat was added to transfer line 14 or separation zone 16. The 
overhead vapor product containing 8% of acetic acid, 15% H.sub.2 O, 34% 
methyl iodide, 2% alkanes, and 0.02% hydrogen iodide, leaving separation 
zone 16 via line 17 is introduced at a rate of about 1748 parts/hr. into 
distillation column 20 having about 15 plates at about the second plate. 
About 3843 parts/hr. of unvaporized liquid reaction mass containing the 
stable homogeneous catalyst system was recirculated through line 18 to the 
reactor 10. 
In distillation column 20 operated at about 17 psig, an overhead 
temperature of about 118.degree. C. and a bottoms temperature of about 
129.degree. C., acetic acid, water and some hydrogen iodide were separated 
from the other products in the reaction effluent and removed as the 
bottoms stream for return to the separation zone 16 via line 22 at a rate 
of about 28 parts/hr. The overhead from column 20 containing 52% methyl 
iodide, 3% alkanes, 22% acetic acid and 23% water was condensed and 
introduced into a separator 23 where it formed two phases. The light phase 
consisting essentially of 44% acetic acid and 49% water and 7% methyl 
iodide was returned to the reactor 10 by way of line 24. 
The major part of the heavy phase containing approximately 87% methyl 
iodide, 3.64% alkanes, 3.5% acetic acid, 5% methyl acetate and 0.9% 
H.sub.2 O was returned to the reactor 10. A slipstream 26 of this heavy 
phase constituting about 0.4% of the total heavy phase was withdrawn 
through line 26 and introduced in column 27, a five-plate column at about 
the fourth plate at a rate of about 2.4 parts/hr. A stream of carbon 
monoxide was sparged into column 27 at the lower end via line 28 at a rate 
of 0.075 parts/hr. The overhead temperature in column 27 was maintained at 
about 75.degree. C. while the bottoms stream was kept at 142.degree. C. 
The mid column temperature was 90.degree. C. The overhead product 
consisting of 94% methyl iodide, 5% methyl acetate, and 1% water was 
withdrawn at a rate of about 2.3 parts/hr. and recycled to the separation 
zone. The bottoms product containing 43% alkanes, 52% acetic acid and 
traces of methyl iodide was continously removed from the system through 
line 30 at a rate of 0.1 part/hr. and burned. 
It will be seen from the above data that the alkanes present in the reactor 
effluent are continuously removed by the process of the invention thus 
preventing their build-up in the system and obviating the problems arising 
therefrom.