Note: Descriptions are shown in the official language in which they were submitted.
1055503
The invention relates to the production of carboxylic
acids, and more particularly to the production of carboxylic
acids from nitriles.
Aromatic or heterocyclic nitriles are generally con-
verted to the corresponding carboxylic acid by aqueous hydrolysis
which is catalyzed by an acid or base. Thus, for example,
terephthalonitrile has been hydrolyzed with aqueous ammonia
to produce the corresponding ammonium salt, which is then con-
verted to the acid by steam stripping. Although such hydrolysis
reactions are capable of producing the acid from the nitrile,
there is a need for improvements in the production of carboxylic
acids from nitriles.
In accordance with the present invention, a nitrile and/or
an intermediate hydrolysis product thereof is reacted with
water, in the vapor phase, in the presence of a suitable
catalyst to convert the nitrile and/or an intermediate hydrolysis
product to the corresponding carboxylic acid.
In accordance with the preferred embodiment of the pro-
cess of the invention, the carboxylic acid is recovered from
the effluent by sublimation of the acid.
1055503
The organic materials which are employed as starting
materials for producing carboxylic acids in accordance with the
present invention are either aromatic or heterocyclic nitriles.
The aromatic nitriles contain one or more cyano-groups, pre-
ferably one or two cyano-groups and can-be unsubstituted or
substituted with other substituent groups; e.g., an alkyl group.
The aromatic nucleus is preferably benzene or naphthalene. As
representative examples, there may be mentioned: phthalonitrile,
terephthalonitrile, isophthalonitrile, tolunitrole, l-cyano-
naphthalene, and 2,6-cyanonaphth~lene. Similarly, the hetero-
cyclic nitriles can contain one or more cyano-groups, with the
heterocyclic nucleus generally being pyridine. The preferred
starting materials are nicotinonitrile, isophthalonitrile,
terephthalonitrile and phthalonitrile, As hereinabove noted,
the starting material may be a nitrile intermediate hydrolysis
product, such as the imides, amides, cyano-acids, cyano-amides
and amide-acids, which can be employed alone or in combination
with each other or the nitrile starting material.
The catalyst employed for the vapor phase hydrolysis
of the invention is a solid acid catalyst. As representative
catalysts, there may be mentioned: silica gel, silica-alumina,
supported phosphoric acid, Group III metal phosphates and sul-
fates, e.g., phosphates and sulfates of aluminum, boron and
gallium, transition metal oxides; e.g., one or more oxides of
vanadium, chromium, manganese, iron, cobalt, nickel, etc. The
catalysts are of the type employed for hydration, dehydration and
esterification reactions. The preferred catalyst is supported
phosphoric acid.
1055S03
The nitrile and water are reacted, in the vapor phase
at temperatures which are generally of from 200F to 1,000F,
and preferably of from 400F to 800F. The temperatures which
are employed are generally above the dew point of both the feed
and product components. The temperatures are most preferably
selected to provide at least a 50% conversion of the nitrile at
a contact or reaction time of no greater than one minute.
The water is employed in at least stoichiometric
proportions; however, an excess of water is preferably employed
in that reaction kinetics are more favourable at higher water
partial pressures. The stoichiometric excess of water can be
as much as to provide a water to nitrile mole ratio of up to
500:1 with the water to nitrile mole ratio generally being from
5:1 to 50:1. The use of an excess of water also functions to
maintain vapor phase conditions; however, an inert gaseous
diluent, such as nitrogen, can be used for such purposes.
The total reaction pressure is generally selected to
provide the desired water partial pressure, with such total
pressures generally being from 1 to 10 atm.
The catalytic vapor phase reaction may be effected
by any one of a wide variety of reaction techniques, including
fixed bed, fluidized bed, dilute phase transport, etc., and the
selection of a specific technique is deemed to be within the
scope of those skilled in the art from the present teachings.
The gaseous reaction effluent contains the carboxylic
acid, unreacted starting nitrile, ammonia, and some reaction
intermediates. Thus, for example, in the production of
terephthalic acid from tere-
IOS5503 - Y -
phthalonitrile the gaseous reaction effluent includes tere-
phthalic acid, ammonia, unreacted terephthalonitrile, reaction
intermediates, such as cyanobenzoic acid, terephthalamic
acid, as well as any unreacted water. The reaction mixture
can be cooled to condense the entire effluent and produce a
water solution and slurry of the ammonium salts of terephthalic
acid and intermediate hydrolysis products.
The carboxylic acid and the intermediate hydrolysis
products can be separated from the ammonia by any suitable
means, for example, by the addition of hydrochloric acid or
of another suitable acid. In accordance with the preferred
embodiment of the process of the invention, the carboxylic
acid may be recovered from the effluent by sublimation of
the acid.
The temperature at which the sublimation of the acid from
the effluent is effected varies with the particular carboxylic
acid. As the temperature at which sublimation of the acid
i9 effected decreases, there is an increase in the recovery of
carboxylic acid from the vapor phase; however, as the tempera-
ture decreases below the dew point of the least volatile
component, other than the acid product, in the reaction mix-
ture, there is also an increase in the amount of other products
sublimated from the vapor phase, which decreases the purity
of the sublimated acid product. In general, the sublimation
temperature is not less than 50F below the dew point of the
least volatile component; however, as should be apparent, it
is preferred to effect sublimation of the acid at a temperature
above thedew point of the least volatile component, other than
the acid product, of the vapor effluent. The selection of
a particular temperature to coordinate sublimated acid product
purity with quantity of acid recovery is deemed to be within
the scope of those skilled in the art from the present teachings.
In general, the temperature is selected to provide for at
least 50% recovery of the acid product from the vapor phase.
lOS5503
The sublimation is preferably effected in the presence
of solid carboxylic acid, with such solid carboxylic acid
functioning both as a nucleus for crystal growth and a heat
transfer agent for the sublimation.
Tke invention will be described with reference to the
following examples:
Example 1
A U-shaped, 1" O.D. stainless reactor, 40" total length,
was charged with 200 grams of a pelleted 10% H3P04 on silica-
alumina catalyst. The reaction was heated to 250C in an air
bath and a vaporized 1% ammonia solution passed over the catalyst
for one hour, then flushed with nitrogen and heated to 300C.
Terephthalonitrile was heated to 230C in a steel cylinder,
through which nitrogen was passed at a rate of 1 liter/min.
(~P~)
such that approximately 0.5g of terephthalonitrile ~S per minute
was volatilized and passed through heated tubing into the reactor.
Water, at the rate of 1.3g/min., was also vaporized and passed
into the reactor co-currently with the TPN/nitrogen. The mole
ratio H2O/TPN was approximately 18.
The reactor effluent was condensed in an ice bath. The
resulting product consisted of a white solid slurried in water.
The mixture was acidified with hydrochloric acid to precipitate
terephthalic acid (TPA) dissolved as the ammonium salt. The
resulting solids were filtered, washed with water and analyzed
by liquid chromatography. The solid consisted of a mixture of
58% unreacted TPN, 5% terephthalamic acid/cyanobenzoic acid
and 37% terephthalic acid.
Example 2
The system of Example 1 was operated at a temperature of
350C and a pressure of 35 psig. TPN was fed at a rate of 5
millimoles/min. and steam at the rate of 83 millimoles/min.
The run was efected for a period of 50 minutes.
The product contained 45.7 wt.% TPA; 38.2 wt.% cyano-
benzoic acid; 9 wt.% terephthalamic acid; and 7.1 wt.% TPN.
1055503
The invention will be further described with respect to
a specific embodiment thereof illustrated in the accompanying
drawing wherein:
The drawing is a simplified schematic flow diagram of
an embodiment of the present invention.
The embodiment will be specifically described with
respect to the production of terephthalic acid; however, the
embodiment is not limited to such production.
Referring to the drawing, vaporized terephthalonitrile
in line 10 is combined with vaporized fresh feed water in line
11, vaporized recycle terephthalonitrile combined with a vaporized
organic solvent therefor, such as xylene, in line 12, obtained
as hereinafter described, and vaporized intermediate hydrolysis
products, in line 13, obtained as hereinafter described, and the
combined gas stream in line 14 is introduced into a terephthalic
acid production reactor, schematically indicated as 15. Reactor
15, as hereinabove described, contains a suitable catalyst,
such as phosphoric acid supported on silica-alumina, and in
reactor 15 the water and terephthalonitrile react, in the gas
phase, to produce terephthalic acid.
A gaseous reaction effluent, containing terephthalic
acid, unreacted terephthalonitrile, unreacted water, ammonia,
and reaction intermediate; in particular, terephthalamic acid
and cyanobenzoic acid is withdrawn from reactor vessel 15 through
line 16 and introduced into a terephthalic acid recovery vessel
17 wherein terephthalic acid is separated from the gaseous
effluent by sublimation at a temperature above the dew point
of the remaining components of the gaseous effluent. In this
matter, terephthalic acid is recovered as a solid without
condensation of the remaining components.
1055503
The recovery vessel 17 preferably includes solid
terephthalic acid in a fluidized state to function as nucleation
points for crystal growth and a heat transfer agent. The heat of
desublimation is removed by the use of a cooling coil 18; however,
it is to be understood that other heat exchange means, such as
heat exchange jackets or direct water quench could also be
employed.
In general, the recovery vessel is operated at a temp-
erature of from 400F to 600F and at a pressure of from 1 to 3
atm. in order to effect recovery of the terephthalic acid by
sublimation at above the dew point of the remaining components
of the gaseous effluent.
A gaseous stream, containing water, ammonia, unreacted
terephthalonitrile, p-xylene, cyanobenzoic acid, terephthalamic
~5 acid and a small amount of terephthalic acid, is withdrawn from
recovery vessel 17 through line 19 and introduced into a quench
vessel 21 to separate ammonia from the remaining components.
In quench vessel 21 the gaseous stream is directly quenched by an
aqueous quench liquid introduced through line 33 to condense a
xylene solution of terephthalonitrile and an aqueous solution of
the ammonium salts of terephthalic, cyanobenzoic and tereph-
thalamic acid. By including a solvent for terephthalonitrile,
such as p-xylene in the system the quench circuit is maintained
free of solid terephthalonitrile. Similarly, by allowing the
acids present in the gas stream to combine with ammonia and form
water soluble ammonium salts the system is also maintained free
of solids.
In general, the quench vessel 21 is operated at a
temperature of from 100F to 200F and a pressure of from 1 to 3
atm.; however, it is to be understood that such conditions are
merely illustrative.
1~)55503
A gaseous overhead is withdrawn from quench vessel 21
through line 22, which includes a suitable cooler and separator,
and ammonia is withdrawn from the system through line 23.
Condensed liquid is withdrawn from quench vessel 21
through line 24 and introduced into a separator 25, with an
organic phase of terephthalonitrile dissolved in p-xylene being
withdrawn therefrom through line 26 and vaporized in heater 27
for recycle to reactor 15 through line 12.
An aqeuous phase, containing the ammonium salts, is
withdrawn from separator 25 through line 31, with a first portion
thereof being passed through line 32, including a cooler 34 for
use as quench liquid in line 33. The remaining portion of
the aqueous phase is passed through line 35 and vaporized in
heater 27 for introduction into reactor 15 through line 13.
Solid crude terephthalic acid, containing the partial
hydrolysis product terephthalamic acid, is withdrawn from
recovery vessel 17 through line 41 and introduced into a storage
vessel 42 for ultimate purification.
As particularly shown, crude terephthalic acid with-
drawn from storage vessel 42 through line 43 is repulped invessel 44 in water introduced through line 45. A slurry of
crude terephthalic acid in water is withdrawn from vessel 44
through line 46, heated in coil 47 positioned in the upper
portion of flash crystallizer 48 and further heated in heater
49 to effect dissolution of the crude terephthalic acid.
The solution of crude terephthalic acid is introduced into a
hydrolysis vessel 51 wherein the solution is maintained at a
temperature and a time sufficient to hydrolyze the terephthalamic
acid to terephthalic acid. In general,the solution is main-
~055503
tained at a temperature of from 400F to 600F, with the timebeing in the order of from 30 minutes to 2 hours.
An aqueous solution of terephthalic acid is withdrawn
from vessel 51 through line 52 and introduced into a flash
crystallizer 48 to crystallize terephthalic acid. In general,
the crystallizer 48 is operated at a temperature of from 100F
to 200F and a pressure of from 1 to 5 atm. It is to be under-
stood that such crystallization could be effected in one or more
stages.
A slurry of terephthalic acid is withdrawn from
crystallizer 48 through line 53 and introduced into a suitable
separation vessel, such as centrifuge 54 to recover terephthalic
acid.
A wet terephthalic acid cake is withdrawn from
centrifuge 54 through line 55 and dried in drier 56, with the
final pure terephthalic acid product being recovered through
line 57.
A water wash is introduced into centrifuge 54 through
line 62 and a first portion thereof employed in line 45 for
repulping the crude terephthalic acid. A second portion is
passed through line 63 and vaporized in heater 27 for introduction
into the reactor 15 through line 11.
lOSSS03
The present invention is particularly advantageous in
that the nitrile is converted to the acid in shorter reaction
times and at lower pressures than achieved with conventional
liquid hydrolysis. In addition, less expensive materials of
construction can be employed.
The carboxylic acids produced by the process of the
invention are of high purity. For example, the invention
produces terephthalic acid of fiber grade.
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