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BACKGROUND OF THE INVENTION
Field of the Invention:
This invention, in a method for the production of acrylic
acid comprising steps for absorbing with water an acrylic
acid-containing gas obtained by the reaction of catalytic
gas phase oxidation, removing low boiling substances and high
boiling substances, and a step for thermally decomposing an
acrylic acid oligomer contained in the high boiling
substance-containing solution obtained in the step for
removal, relates to a method for the production of acrylic
acid which allows to prevent the polymerization more
efficiently and enjoys exalted productivity.
Description of the Related Art:
Acrylic acid is used in coating, textile processing,
leather products, and building materials, as well as for
copolymers destined to produce acrylic fibers and for
emulsions to produce tackiness agents and adhesive agents.
The demand for acrylic acid is now increasing. With a view
to realizing mass production of acrylic acid by using an
inexpensive raw material, therefore, it is common for acrylic
acid to be produced by the reaction of catalytic gas phase
oxidation of propylene, for example. Since the reaction of
catalytic gas phase oxidation gives rise to by-production
of low boiling substances and high boiling substances besides
yielding acrylic acid, various processes are relied on to
separate and remove such by-products and purify acrylic acid.
The official gazette of JP-A-09-157213, for example,
discloses a method for producing acrylic acid by introducing
a mixed gas obtained by the catalytic gas phase oxidation
of propylene, for example, to an acrylic acid absorption column,
causing the gas to contact an aqueous absorbing solvent
containing acrylic acid, acetic acid, and sparingly
water-soluble solvent thereby obtaining an aqueous acrylic
acid solution, dehydrating the aqueous acrylic acid solution
in an azeotropic separation column and obtaining through, the
bottom of this column acrylic acid substantially free from
acetic acid, water, and sparingly water-soluble solvent,
meanwhile expelling through the top of the column a mixture
consisting of acetic acid, acrylic acid, water, and sparingly
water-soluble solvent by distillation, separating the
expelled mixture in a storage tank into an organic phase formed
substantially of a solvent and a water phase formed of acrylic
acid, acetic acid, a solvent, and water, and circulating the
organic phase in the azeotropic separation column.
The solution, the exhaust gas, and the like which emanate
from such purification processes possibly contain raw
material compounds, produced compounds, and other useful
compounds. With the object of exalting the efficiency of
production, the practice of putting these materials to
circulatory use in the process of production is continuing
in use.
The official gazette of JP-A-11-012222, for example,
discloses a method for recovering acrylic acid from acrylic
acid containing acrylic acid dimer and maleic acid, which
is characterized by introducing the acrylic acid containing
acrylic acid dimer and maleic acid into an acrylic
acid-recovering column, expelling acrylic acid by
distillation through the top of the column to recover the
acrylic acid, meanwhile introducing the bottom liquid (A)
from the acrylic acid-recovering column into a thermal
decomposition tank and decomposing the acrylic acid dimer
in the bottom liquid (A), and then circulating at least part
of the bottom liquid (B) from the thermal decomposition tank
to the acrylic acid-recovering column. This method is
directed toward effective use of the acrylic acid dimer and
maleic acid and, by circulating the acrylic acid produced
by the decomposition to an acrylic acid-recovering column,
is enabled to obtain acrylic acid as a finished product.
Acrylic acid is an easily polymerizing compound and is
liable to generate an acrylic acid polymer during the process
for absorption of acrylic acid and the subsequent process
for purification. Various purification columns, therefore,
have been used to produce acrylic acid while preventing the
occurrence of polymerization by adjusting distillation
pressure, temperature, feed rates of liquids, and the like.
The official gazette of JP-A-2000-355570, for example,
discloses a method for distilling an easily polymerizing
compound by the use of a distilling device, specifically a
method for preventing an easily polymerizing compound from
being polymerized, characterized by supplying a liquid
substantially identical in composition with the liquid
existing in the periphery of component member disposed in
the distilling device to the component member by an introducing
means by spray. The concept of spraying a liquid substantially
identical in composition with the liquid existing in the
periphery of component member throughout the entire surface
of the component member has originated in the discovery that
an easily polymerizing compound begins to polymerize when
it is left stagnating on the surface of a component member
inside a distilling device and the subsequent discovery that
the polymerization within the distilling device would be
effectively prevented by performing distillation while
allowing no stagnation of the liquid on the surface of the
component member in the distilling device. The term "liquid
identical in composition" as used herein embraces a feed liquid,
a liquid extracted from the interior of the column, a reflux,
and a circulating liquid of bottoms (a purified liquid). This
liquid diluted to a low concentration with water, alcohol,
azeotropic solvent, or extracting solvent can also be used.
Since acrylic acid is an easily polymerizing compound,
however, the process for the absorption of acrylic acid and
the subsequent process for purification are liable to form
acrylic acid polymers. Various purification columns have
been used to produce acrylic acid while preventing the
occurrence of polymerization by adjusting distilling pressure,
temperature, and amount of feed liquid. The control of these
factors is not easy because the pressure and the concentration
change simultaneously with a change in temperature. The
occurrence of an acrylic acid polymer results in lowering
the yield of the product.
In the process for the purification of acrylic acid,
not only the acrylic acid polymer but also by-products
generated by the reaction of catalytic gas phase oxidation
adheres to such devices as the distilling column and the
occurrence of this adherence entails such harmful effects
as blocking the devices and possiuiy impairs lasting stable
production of acrylic acid.
In the high boiling substance-containing solution
resulting from the separation of high boiling substances,
the so-called Michael type adduct of acrylic acid exists
besides the acrylic acid dimer and forms a cause for degrading
the efficiency of raw material for the process of acrylic
acid production. When the Michael type adduct accumulates
in the process, it inflicts a serious hindrance on the process
for purification and the process of production as well and
entails elevation of temperature and formation of by-products
possibly to the extent of degrading the quality of product.
When such compounds are recovered as acrylic acid, the recovery
possibly results in degrading the quality of acrylic acid.
SUMMARY OF THE INVENTION
The present inventor has discovered that in a method
for the production of acrylic acid which comprises absorbing
in water the acrylic acid-containing gas obtained by the
reaction of catalytic gas phase oxidation, separating a low
boiling substance, separating a high boiling substance, and
thermally decomposing an acrylic acid oligomer, the
efficiency of production of acrylic acid can be improved by
effecting either (i) a step for introducing a polymerization
inhibitor to a stage other than a stage for supplying raw
material(what is called "step for feeding") to a distilling
column and a stage for supplying a reflux thereto, or (ii)
a step for supplying to a step for dehydration the acrylic
acid recovered by thermal decomposition of the oligomer. The
method for producing the acrylic acid may incorporate therein
a step for producing an acrylic ester from the produced acrylic
acid or a step for further purifying the produced acrylic
acid into acrylic acid of a higher purity. It may further
incorporate therein a step for producing a polyacrylic acid
(or salt thereof) by using the acrylic acid of a high purity
mentioned above.
This invention, particularly by providing a tank and/or
a cooler between each the step for absorption in water, the
step for separation of low boiling substances, the step for
separating high boiling substances, and the step for thermal
decomposition of the acrylic acid oligomer, cooling the feeds
to the subsequent steps, and then performing the subsequent
step's, prevents the occurrence of polymers to the subsequent
steps and exalts the eventual yield of production.
This invention, in the method for the production of
acrylic acid which comprises steps for absorbing in water
the acrylic acid-containing gas resulting from the reaction
of catalytic gas phase oxidation, removing low.-boiling
substances and high boiling substances, and thermally
decomposing the acrylic acid oligomer contained in the high
boiling substance-containing solution resulting fromthe step
for removal, can prevent the polymerization more efficiently
and exalt the productivity.
Accordingly, the present invention provides a method for
the production of acrylic and which comprises the steps of :
(a) supplying one or more gas components selected from the
group consisting of propylene, propane and acrolein to a
reactor for catalytic gas phase oxidation,(b) obtaining an
acrylic acid-containing gas by catalytic gas phase oxidation,
(c) introducing said acrylic acid-containing gas and
supplying an aqueous absorbing solvent into an acrylic acid
absorbing column, whereby an aqueous acrylic acid-containing
solution is absorbed onto said acrylic acid absorbing column,
(d) obtainingsaid aqueous acrylic acid-containing solution
absorbed onto said acrylic acid absorbing column,(e)obtaining
crude acrylic acid from said aqueous acrylic containing
solution in an azeotropic dehydration column by dehydration,
(f) introducing a polymerization inhibitor to said azeotropc
dehydration column at any point between a point for supplying
said aqueous acrylic acid containing solution and a point for
supplying a reflux and not including the point for supplying
said aqueous acrylic acid containing solution material and the
point for supplying the reflux; (g) optionally removing a low
boiling substance from said aqueous acrylic acid-containing
solution by using an azeotropic distillation column,
(h)obtaining acrylic acid and a high boiling substance-
containing solution by removing the high boiling substance
from said crude acrylic acid, subsequently (I) recovering
acrylic acid by thermally decomposing an acrylic acid oligomer
contained in said high boiling substance-containing solution,
and (j) supplying the acrylic acid recovered by thermally
decomposing said acrylic acid oligomer from step (i) to said
azeotropic dehydration column.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a flow diagram illustrating schematically a
process for the production of acrylic acid including a step
for the production of an acrylic ester and a process for the
production of polyacrylic acid (salt).
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The first aspect of this invention relates to a method
for the production of acrylic acid which comprises (a) a step
for obtaining an acrylic acid-containing gas by catalytic
gas phase oxidation, (b) a step for obtaining an aqueous acrylic
acid-containing solution by absorbing the acrylic
acid-containing gas with an aqueous absorbing solvent, (c)
a step for obtaining crude acrylic acid by dehydration and/or
removing low boiling substance from said aqueous acrylic
acid-containing solution, (d) a step for obtaining acrylic
acid and high boiling substance-containing solution by
removing high boiling substances from the crude acrylic acid,
and (e) a step for recovering acrylic acid by thermally
decomposing the acrylic acid oligomer contained in the high
boiling substance-containing solution, which method is
characterized by performing at least either of (i) a step
for introducing a polymerization inhibitor to a stage other
than the stage for supplying a raw material and the stage
for supplying a reflux to the distilling column or (ii) a
step for supplying the acrylic acid recovered by thermally
decomposing the oligomer to the step for obtaining crude
acrylic acid by dehydration.
The term "low boiling substance" as used in this invention
refers to as a substance having a lower boiling point than
acrylic acid under standard conditions and the term "high
boiling substance" refers to as a substance having a higher
boiling point than acrylic acid under standard conditions.
The term "acrylic acid oligomer" refers to the Michael
type adduct of acrylic acid which is represented by the
following formula [I].
CH2=CHCOO-(-X-COO)n-H [I]
(wherein n is an integer of 1 - 5 and -X- stands for -CH2CH2-
or -CH(CH3)-, providing that the plurality of -X-'s may be
identical or different where n is 2 or more.)
The term "polyacrylic acid (salt) " refers to as a polymer
containing acrylic acid and/or an acrylic acid salt as the
main component of the monomer thereof, more specifically in
a ratio of not less than 70 mol%, more preferably not less
than 90 mol%, and most preferably substantially 100 mol%.
By using such polyacrylic acid (salt), it is made possible
to produce a water-soluble polyacrylic acid (salt) and a
water-absorbent resin. As the polyacrylic acid salts,
preferably univalent salts and more preferably alkali metal
salts and ammonium salts may be cited. Such acrylic acid may
be copolymerized with other monomer. For example, the acrylic
acid (salt) monomer may be either cross-linked with
approximately 0.01 - 5 mol% (based on the acrylic acid) of
a cross-linking agent or graft polymerized to such other
hydrophilic polymer as starch and polyvinyl alcohol. The term
"water-soluble polymer" used herein refers to as such a polymer
as exhibits solubility of substantially 100 % in water. The
term "water-absorbent resin" used herein refers to as such
a polyacrylic acid (salt) as possesses a cross-linked
structure and exhibits a property of swelling with water and
water-insolubility.
The term "distilling column" as used in this invention
embraces a wide range of devices, regardless of designations ,
such as collecting column, absorbing column, dehydrating
column, azeotropic dehydrating column, low boiling substance
separating column, high boiling substance separating column,
acetic acid separating column, purifying column, and
thin-film evaporator, which are used for separating
components by virtue of difference in boiling point.
The term "purifying" embraces distillation, stripping,
crystallization, extraction, and absorption, for example.
The term "distillation" means a method for separating volatile
components contained in a solution by heating the solution
to its boiling point, the term "stripping" refers to as a
method for transferring a target substance in a solution to
a gas phase by supplying a stripping gas into the solution.
The term "crystallization" means a method for separating a
target substance in the form of crystals.
One example of the preferred mode of embodying this
invention will be described below with reference to Fig. 1.
The method of this invention for the production of acrylic
acid comprises supplying a raw material gas 1 containing raw
material component, inert gas, molecular oxygen, steam, and
the like to a reactor 10 for catalytic gas phase oxidation
and subjecting the raw material to the reaction of catalytic
gas phase oxidation with a molecular oxygen-containing gas.
Specifically, the raw material gas is supplied to the reactor
10 such as, for example, a shell-and-tube type reactor packed
with an oxidizing catalyst 11. The oxidation of propylene
as a raw material component, for example, results in forming
acrolein. The acrolein, when further subjected to the
reaction of catalytic gas phase oxidation, produces acrylic
acid. The reaction conditions such as raw material gas,
oxidizing catalyst, inert gas, molecular oxygen-containing
gas, and reaction temperature which are used for this
production of acrylic acid may be any of the sets of conditions
which are used in the heretofore known processes for the
reaction of acrylic acid.
The raw material gas is composed of 1 to 15 vol. % of
such a raw material component as one or more of propylene,
propane, and acrolein, 1 to 3 times the amount of the raw
material components a of a molecular oxygen, and the balance
of such inert gas as carbon dioxide or steam. Though the
reactor for performing the reaction of catalytic gas phase
oxidation does not need to be particularly restricted, a
shell-and-tube type reactor can be used advantageously in
respect that it excels in the efficiency of reaction. The
acrylic acid is produced by the one-stage reaction when
acrolein is used as the raw material component or by the
so-called two-stage reaction of catalytic gas phase oxidation
when propylene is used as the raw material component. The
former-stage catalyst and the latter-stage catalyst to be
used in the two-stage reaction of catalytic gas phase oxidation
do not need to be particularly restricted.
The former-stage catalyst is required to produce
acrolein from propylene. As typical examples of this catalyst,
those represented by the general formula: Moa-Bib-Fe
c-Ad-Be-Cf-Dg-Ox (wherein Mo, Bi, and Fe respectively stand
for molybdenum, bismuth, and iron, A stands for at least one
element selected from the group consisting of nickel and cobalt,
B stands for at least one element selected from the group
consisting of alkali metals and thallium, C stands for- at
least one element selected from the group consisting of
phosphorus, niobium, manganese, cerium, tellurium, tungsten,
antimony, and lead, D stands for at least one element selected
from the group consisting of silicon, aluminum, zirconium,
and titanium, 0 stands for oxygen, a, b, c, d, e, f, g, and
x respectively represent atomic ratios of Mo, Bi, Fe, A, B,
C, D, and 0 such that b = 0.1 - 10, c = 0.1 - 10, d = 2 -
20, e = 0.001 - 5, f = 0 - 5, and g = 0 - 30 are satisfied
when a = 12 is fixed, and x assumes a numerical value which
will be fixed by the oxidizing states of the relevant elements)
may be cited.
The latter-stage catalyst is required to effect gas phase
oxidation of ?. reaction gas containing acrolein to produce
acrylic acid. As typical examples of this catalyst, those
represented by the general formula: Moa-Vb-Wc-Cud-Ae-Bf-Cg-Ox,
(wherein Mo stands for molybdenum, V stands for vanadium,
W stands for tungsten, Cu stands for copper, A stands for
at least one element selected from the group consisting of
antimony, bismuth, tin, niobium, cobalt, iron, nickel, and
chromium, B stands for at least one element selected from
the group consisting of alkali metals, alkaline earth metals,
and thallium, C stands for at least one element selected from
the group consisting of silicon, aluminum, zirconium, and
cerium, 0 stands for oxygen, a, b, c, d, e, f, g, and x
respectively represent atomic ratios of Mo, V, W, Cu, A, B,
C, and 0 such that b=2-14,c=0-12,d=0.1-5, e
=0-5, f = 0 -5, and g = 0 - 20 are satisfied when a =
12 is fixed, and x represents a numerical value which is fixed
by the oxidizing states of the relevant elements) may be cited.
The acrylic acid-containing gas which is obtained from the
reactor 10 generally contains 10 - 20 wt. % of acrylic acid,
0.2 - 1*0 wt. % of acetic acid, and 5 - 15 wt. % of water.
The acrylic acid-containing gas which is obtained by
the reaction of catalytic gas phase oxidation is supplied
to an acrylic acid absorbing column 20. The process which
ensues therein is directed to absorbing the acrylic acid
contained in the gas obtained by the process for reaction
with an aqueous absorbing solvent. The reaction conditions
such as the composition of gas components in the reaction
gas, the composition of the aqueous absorbing solvent, and
the temperature of the absorption which are used for this
process may be any of the sets of conditions which are used
in the heretofore known processes for the reaction of acrylic
acid. When the acrylic acid-containing gas contains
unaltered acrolein, the acrylic acid-containing gas may be
supplied to the acrylic acid absorbing column 20 after the
acrolein has been removed as by distillation or diffusion.
It may be otherwise supplied to the absorbing column 2 0 after
the gas has been cooled. This is because the degree with which
the absorption efficiency is exalted increases in accordance
as the gas temperature is lowered.
The absorbing column 20 to be used herein may be any
of such known absorbing columns as plate column, packed column,
wetted wall tower, and spray tower. Generally, this absorbing
column 20 is preferred to be a plate column or a packed column.
In the case of the packed column, the interior thereof is
packed regularly or irregularly with a filler having a large
surface area and exhibiting air permeability. The gas-liquid
contact is effected on the surface of the packed bed filled
with the filler.
In the absorbing column 20, while the acrylic
acid-containing gas is introduced thereto, an absorbing
solvent 21 capable of absorbing acrylic acid is introduced
from the upper part into this column to bring into counter
current contact with the gas mentioned above and to effect
absorption of acrylic acid.
As the absorbing solvent 21 so supplied, an aqueous
absorbing solvent may be used. This solvent is at an advantage
in being inexpensive and allowing exhaust water emanating
from the process for the production of acrylic acid to be
reused. The aqueous absorbing solvent is only required to
contain at least 80 - 100 wt. % of water. One example of such
an aqueous absorbing solvent may be composed of 0.1 - 5.0
wt. % of acrylic acid, 0.1 - 10 wt. % of acetic acid, and
80 - 99.8 wt. % of water. The absorbing solvent 21 to be used
herein may be prepared in advance as formulated in the
composition mentioned above. For example, a water phase in
an oil-water separator 32 annexed to an azeotropic dehydrating
column 30 may be circulated as the absorbing solvent 21 for
acrylic acid to an acrylic acid absorbing column and used
as such.
The absorption efficiency of the absorbing solvent 21
increases in accordance as the solvent temperature decreases .
The absorbing solvent 21 is preferred to be supplied at a
fixed temperature in the range of 0 - 35°C, particularly 5
- 30°C. The amount of the solvent expressed in the liquid-gas
ratio, namely the amount of the solvent (L) to the amount
of the feed gas (m3), is set in the range of 2 - 15L/m3, preferably
3 - 12 L/m3, and more preferably 5 - 10L/m3. The polymerization
of acrylic acid occurs most readily when the mass ratio of
acrylic acid and water is approximately 50:50. The absorption
of acrylic acid can be effected efficiently by maintaining
the mass ratio in the range mentioned above and preventing
the polymerization.
This invention, for the purpose of preventing the
polymerization of such a polymerizing substance as acrylic
acid, prefers the absorbing solvent 21 to contain therein
at least one compound selected from the group consisting of
N-oxyl compounds, phenol compounds, manganese salts such as
manganese acetate, copper dialkyldithiocarbamates such as
copper dibutylthiocarbamate, nitroso compounds and amine
compounds, and phenothiazine. The nitroso compound includs
such compounds as N-nitrosophenyl hydroxylamines or the salts
thereof, for example, ammonium salts of N-nitrosophenyl
hydroxylamine, p-nitrosophenol, N-nitrosodiphenyl amine and
ammonium salts thereof which are decomposed by the conditions
of the distilling column and the decomposed components thereof
manifest an effect of inhibiting polymerization of easily
polymerizing substance. The polymerization inhibitor which
is contemplated by the present invention does not embrace
such a substance as undergoes decomposition in the distilling
column and gives such a product of decomposition as manifests
an effect of inhibiting polymerization.
The N-oxyl compound does not need to be particularly
restricted. Any of the N-oxyl compounds which have been
generally known heretofore as agents for inhibiting the
polymerization of a vinyl compound may be used. Among other
such N-oxyl compounds, 2,2,6,6-tetramethyl piperidinoxyls
represented by the following formula (1):
(wherein R1 stands for CH2, CHOH, CHCH2OH, CHCH2CH2OH, CHOCH2OH,
CHOCH2CH2OH, CHCOOH, or C=O and R2 stands for a hydrogen atom
or CH2OH) are used advantageously. It is preferable to use
one or more compounds selected among 2,2,6,6-tetramethyl
piperidinoxyl, 4-hydroxy-2,2,6, 6-tetramethyl piperidinoxyl,
and 4,4',4"-tris-(2,2,6,6-tetramethyl piperidinoxyl)
phosphites which give good effects in preventing
polymerization, although any of N-oxyl compounds can be used
without any limitation. Particularly when
2,2,6,6-tetramethyl piperidinoxyl or
4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl is used as the
N-oxyl compound, since it forms a stabilizing agent without
requiring to include any metal in the components, there is
no possibility of corroding the metallic material of the
equipment due to the presence of stabilizer and the vaste
liquid can be easily treated.
In this invention, the N-oxyl compound may be used in
combination with an N-hydroxy-2,2,6,e-tetrame^hyl
piperidine compound and a 2,2,6,6-tetramethyl piperidine
compound.
As representative examples of the
N-hydroxy-2,2,6,6-tetramethyl piperidine compound,
l,4-dihydroxy-2,2,6,6-tetramethyl piperidine and
l-hydroxy-2,2,6,6-tetramethyl piperidine may be cited.
These N-hydroxy-2,2,6,6-tetramethyl piperidine compounds
may be used either singly or in the form of a mixture of two
or more members.
As typical examples of the 2,2,6,6-tetramethyl
piperidine compound. 2,2,6,6-tetramethyl piperidine and
4-hydroxy-2,2,6,6-tetramethyl piperidine may be cited.
These may be used either singly or in the form of a mixture
of two or more members. Incidentally,
N-hydroxy-2,2,6,6-tetramethyl piperidine compounds and
2,2,6,6-tetramethyl piperidine compounds are possibly
contained as impurities in commercially available products
of N-oxyl compounds . The use of such a commercially available
product of N-oxyl compound equals the use in combination with
N-hydroxy-2,2,6,6-tetramethyl piperidine compound and
2,2,6,6-tetramethyl piprdidine compound mentioned above.
As typical examples of the phenol compound, hydroquinone,
methoquinone (p-methoxy-phenol) may be cited. The
methoquinone proves favorable in respect that it excels the
hydroquinone in the effect of preventing polymerization when
it is used in combination with an N-oxyl compound and a
phenothiazine compound. These phenol compounds may be used
in the form of a mixture of two or morp members.
As typical examples of the phenothiazine compound,
phenothiazine, bis-(a-methylbenzyl)phenothiazine,
3,7-dioctylphenothiaz ine, and
bis-(a-dimethylbenzyl)phenothiazine may be cited.
The copper salt compound does not need to be particularly
restricted. Either copper inorganic salts or copper organic
salts can be used. As typical examples, copper
dialkyldithiocarbamates, copper acetate, copper napthenate,
copper acrylate, copper sulfate, copper nitrate, and copper
chloride may be cited. These copper salt compounds are usable
in the form of monovalent or divalent compounds. Among other
copper salt compounds mentioned above, copper
dialkyldithiocarbamates prove favorable from the viewpoint
of effect.
As typical examples of the copper dialkyldithiocarbamate,
copper dimethyldithiocarbamate, copper
diethyldithiocarbamate, copper dipropyldithiocarbamate,
copper dibutyldithiocarbamate, copper
dipentyldithiocarbamate, copper dihexyldithiocarbamate,
copper diphenyldithiocarbamate, copper
methylethyldithiocarbamate, copper
methylpropyldithiocarbamate, copper
methylbutyldithiocarbamate, copper
methylpentyldithiocarbamate, copper
methylhexyldithiocarbamate, copper
methylphenyldithiocarbamate, copper ethylpropyl
dithiocarbamate, copper ethylbutyldithiocarbamate, copper
ethylpentyldithiocarbamate, copper
ethylhexyldithiocarbamate, copper
ethylphenyldithiocarbamate, copper
propylbutyldithiocarbamate, copper
propylpentyldithiocarbamate, copper
propylhexyldithiocarbamate, copper
propylphenyldithio-carbamate, copper
butylpentyldithiocarbamate, copper
butylhexyldithiocarbamate, copper
butylphenyldithiocarbamate, copper
pentylhexyldithiocarbamate, copper
pentylphenyldithiocarbamate, and copper
hexylphenyldithiocarbamate may be cited. These copper
dialkyldithiocarbamates may be a monovalent copper salt or
a divalent copper salt. Among other copper
dialkyldithiocarbamates cited above, copper
dimethyldithiocarbamate, copper diethyldithiocarbamate,
and copper dibutyldithiocarbamate prove favorable in respect
of its effects and easy acquisition, and copper
dibutyldithiocarbamate proves especially favorable.
As typical examples of the manganese salt compound,
manganese dialkyldithiocarbamates (wherein the two alkyl
groups may be identical or different and each may be methyl,
ethyl, propyl, or butyl), manganese diphenyldithicarbamate,
manganese formate, manganese acetate, manganese octanoate,
manganese naphthenate, manganese permanganate, and manganese
salt compounds of ethylenediamine tetraacetic acid may be
cited. These manganese salt compounds may be used either
singly or in the form of a mixture of two or more members.
This invention prefers the absorbing solvent to contain
one or more compounds selected from the group consisting of
N-oxyl compounds, phenol compounds, manganese salts, copper
dialkyldithiocarbamates, nitroso compounds, and amine
compounds or one or more compounds mentioned above in
combination with phenothiazine. Naturally, when the
absorbing solvent can be prepared as a system of three or
more components by the addition of one or more of these six
kinds of compounds and phenothiazone compound, it will
manifest an effect in inhibiting polymerization equal to or
better than the effect produced in a two-component system.
The amount of the polymerization inhibitor to be used
does not need to be particularly restricted but may be properly
selected to suit the operating conditions to be involved.
It is preferable to set the total amount of the polymerization
inhibitor to be used in the range of 3 - 3500 ppm (by mass)
based on the mass of the acrylic acid in the reaction gas
to be absorbed. As regards the preferred amount of the
individual polymerization inhibitor to be used, this amount
of the N-oxyl compound is in the range of 1 - 500 ppm based
on the mass of the acrylic acid in the reaction gas, that
of the manganese salt compound or the copper salt compound
is in the range of 1 - 200 ppm based on the mass of the acrylic
acid in the reaction gas, that of the nitroso compound is
in the range of 1 - 500 ppm, that of the phenol compound is
in the range of 1 - 500 ppm, that of the phenothiazine compound
is in the range of 1 - 500 ppm,' that of
N-hydroxy-2,2,6,6-tetramethyl piperidine compound is in the
range of 1 - 500 ppm, and that of 2,2,6,6-tetramethyl piperidine
compound is in the range of 1 - 500 ppm.
In this invention, the polymerization inhibitor is
preferred to be introduced into the reaction system in the
form of a solution having the polymerization inhibitor
dissolved in a solvent, namely in the form of a polymerization
inhibitor-containing solution. The site for supplying the
polymerization inhibitor-containing solution and the method
for the introduction thereof do not need to be particularly
restricted. The solution may be introduced into the
absorption column at any stage other than the stage for supply
of the raw materia] and the stage for supply of the reflux
to the distilling column. Intheabsorptioncolumn, the "stage
for supply of the raw material" means a stage for supplying
an acrylic acid-containing gas and the "stage for supply of
the reflux" means a stage for supplying a absorbing solvent.
When the polymerization inhibitor is mixed with a solvent
to prepare a polymerization inhibitor-containing solution
and then this solution is to be supplied, the polymerization
inhibitor is consumed effectively because it is uniformly
dispersed in the acrylic acid absorption column. As the
solvent for the preparation of the solution mentioned above,
the acrylic acid-containing solution is available. When the
absorbing solvent 21 which is used in the acrylic acid
absorption column 20 contains acrylic acid, for example, the
absorbing solvent 21 itself, part of the crude acrylic acid
obtained in other process, a solution resulting from the
thermal decomposition of an acrylic acid oligomer to be
described specifically hereafter, or a bottom liquid of the
absorption column may be used as an acrylic acid-containing
solution. In the absorption column 20, it is particularly
favorable to use waste liquid from a steam ejector used in
the process for the production of acrylic acid as the acrylic
acid-containing solution. This is because the waste liquid
from the steam ejector is an aqueous solution containing
acrylic acid, which has nearly the same composition ratio
as that of the liquid inside the absorption column and induces
no decrease in the absorption efficiency in the absorption
column. If the acrylic acid-containing solution to be used
has a higher acrylic acid concentration than the acrylic acid
composition in the absorption column, this excess would
possibly result in decreasing the absorption efficiency or
induce polymerization.
The conditions for the operation of the absorption column
2 0 vary with such factors as the temperature of the acrylic
acid-containing gas to be supplied to the absorption column,
the amount of the gas supplied per unit time, and the volume
of the absorption column. Generally, the temperature of the
top of the absorption column is in the range of 40 - 85°C.
If this temperature is lower than 40°C, the shortage would
be at a disadvantage in necessitating plant investment for
cooling and entailing consumption of cooling energy,
increasing the condensation of a substance having a lower
boiling point than acrylic acid, and consequently degrading
the purity of acrylic acid in the bottom liquid of the
absorption column. Conversely, if the temperature exceeds
80°C, the excess would result in increasing the loss of acrylic
acid through the top of the absorption column and possibly
lowering the yield of product.
The pressure at the top of the absorption column 20 is
in the range of 0 - 30 kPa (gauge pressure). If this pressure
is lower than 0 kPa (gauge pressure), the shortage would result
in necessitating a vacuum device and entailing plant
investment and cost of energy. If the pressure exceeds 3 0
kPa (gauge pressure), the excess would be at a disadvantage
in necessitating a big capacity blower used for supplying
a raw material gas to a reactor for catalytic gas phase
oxidation and consequently entailing plant investment and
cost of energy. When an exhaust gas from the top of the column
is circulated to the realtor 10, diluting gas and unaltered
raw material components can be effectively utilized.
In this invention, the aforementioned adjustment is
preferred to set the amount of the liquid wetting the absorption
column per cross-sectional area of the column at a level of
not less than 0.3 m3/m2-h, preferably not less than 1 m3/m2-h.
The term "amount of wetting liquid" is referred to as a value
obtained by dividing the amount of the liquid [m3] supplied
per unit time onto one shelf by the cross-sectional area of
the column. When this condition is satisfied, the interior
of the absorption column is infallibly given a wetted state
and the amount of the wetting liquid proves proper. Since
the liquid is stored in a proper amount on the gas-liquid
contact device, the state of the column wetted with the liquid
and the state of the column avoiding drift and stagnation
of gas or liquid can be both realized without fail.
The bottom liquid of the absorption column 20 is cooled
with a cooler (not shown) annexed to the bottom part of the
column and then circulated to the absorption column to increase
the concentration of acrylic acid in the bottom liquid.
Generally, in the bottom liquid of the absorption column 20,
namely the aqueous acrylic acid-containing solution, such
by-products as propylene remaining in an unaltered state,
such by-products as formaldehyde, acrolein, furfural,
benzaldehyde, formic acid, acetic acid, maleic acid, and
acrylic acid oligomer, and additives such as polymerization
inhibitor are present in addition to acrylic acid.
In this invention, the aqueous acrylic acid-containing
solution mentioned above is led to the azeotropic dehydration
column 30 and subjected, in conjunction with an azeotropic
solvent supplied thereto, to azeotropic distillation.
Subsequently to the process for obtaining the aqueous acrylic
acid-containing solution, this invention prefers to install
a tank and/or a cooler to cool the substance destined to be
transferred to the subsequent process before the subsequent
process is carried out. For example, the aqueous acrylic
acid-containing solution, when necessary, is supplied to a
distillation column 22 and deprived of such low boiling
substance as acrolein therein, then the bottom liquid of the
column is transferred via a pump 23 to a cooler 24, and the
aqueous acrylic acid-containing solution which has been
cooled herein is stored in a tank 25. By cooling this solution
before it is transferred to the tank 25, it is made possible
to cool the solution infallibly, decrease the retention time
in the high-temperature state, and repress the amount of an
oligomer to be formed. Optionally, the bottom liquid of the
absorption column may be transferred to the tank without being
passed through the distillation column 22, transferred via
the pump 23 to the cooler 24, and then circulated to the tank
25 and transferred to the subsequent process as well. As
typical examples of the cooler, shell-and-tube type heat
exchangers, plate type heat exchangers, and spiral type heat
exchangers which have been heretofore known to the art may-
be cited. Low boiling substances which can be removed in the
distillation column 22 can be removed even in the azeotropic
dehydration column 30. The separation of such low boiling
substance can be effected as with a light-ends cut column
which is separately installed. In this respect, theaqueous
acrylic acid-containing solution contemplated by this
invention broadly involves the water-containing acrylic acid
prior to the transfer to the azeotropic dehydration column
or the light-ends cut column and equals to the bottom liquid
of the absorption column and the bottom liquid of the column
obtained after the subsequent distillation. The cooling
temperature for the aqueous acrylic acid-containing solution
in the tank is preferred to be in the range of 20 - 50°C. Then,
the aqueous acrylic acid-containing solution in the tank 2 5
is supplied to the azeotropic dehydration column 30.
As the azeotropic dehydration column 30, such known
columns as plate column, packed column, wetted wall column,
and spray column are usable. The azeotropic column 30f
similarly to -che absorption column 2 0 mentioned above, is
generally preferred to be a plate column or a packed column.
Incidentally, the preferred number of theoretical plates in
the azeotropic dehydration column 30 is in the range of .5
- 30.
As typical examples of the azeotropic solvent to be used
in this invention, solvents containing at least one member
selected from the group consisting of heptane, dimethyl
cyclohexane, ethyl cyclohexane, toluene, ethyl benzene,
chlorobenzene, xylene, and mixtures thereof;
solvents containing at least one member selected from
the group consisting of diethyl ketone, diisopropyl ketone,
methylpropyl ketone, methylisobutyl ketone, methyl-t-butyl
ketone, n~propyl acetate, n-butyl acetate, ethyl acrylatae,
methyl methacrylate, ethyl methacrylate, vinyl acrylate,
n-propyl acrylate, allyl acetate, isopropenyl acetate, vinyl
propionate, propyl propionate, methyl crotonate, methyl
valeate, ethyl butyrate, dibutyl ether, and mixtures thereof;
and
mixed solvents formed of a solvent containing at least
one member selected from the group consisting of heptane,
dimethyl cyclohexane, ethyl cyclohexane, toluene, ethyl
benzene, chlorobenzene, xylene, and mixtures thereof with
a solvent containing at least one member selected from the
group consisting of diethyl ketone, diisopropyl ketone,
methylpropyl ketone, methylisobutyl ketone, methyl-t-butyl
ketone, n-propyl acetate, n-butyl acetate, ethyl acrylatae,
methyl methacrylate, ethyl methacrylate, vinyl acrylate,
n-propyl acrylate, allyl acetate, isopropenyl acetate, vinyl
propionate, propyl propionate, methyl crotonate, methyl
valeate, ethyl butyrate, dibutyl ether, and mixtures thereof
may be cited.
More preferably, solvents containing at least one member
selected from the group consisting of heptane, toluene, and
mixtures thereof; solvents containing at least one member
selected from the group consisting of ethyl methacrylate,
methylisobutyl ketone, n-propyl acrylate, n-butyl acetate,
and mixtures thereof; and a mixed solvent formed of a solvent
containing at least one member selected from the group
consisting of heptane, toluene, and mixtures thereof with
a solvent containing at least one member selected from the
group consisting of ethyl methacrylate, methylisobutyl ketone,
n-propyl acrylate, n-butyl acetate, and mixtures thereof may
be cited.
The amount of the azeotropic solvent to be used cannot
be uniquely defined because it is fixed by such factors as
the water content of the aqueous acrylic acid-containinq
solution supplied to the azeotropic dehydration column and
the kind of azeotropic solvent to be used. It can be used
at the known proportion adopted for the purpose of azeotrope.
The amount of the azeotropic solvent is preferred to be large
particularly from the viewpoint of preventing polymerization
of acrylic acid. If it is unduly large, however, the excess
would be at a disadvantage in necessitating a large amount
of energy for distillation.
The temperature of the top of the azeotropic dehydration
column 30 may be properly selected, depending on such factors
as the water content and the amount of by-product present
in the aqueous acrylic acid-containing solution to be supplied,
the amount of feed liquid per unit time, the temperature of
the feed liquid, the degree of dehydration aimed at, the kind
of other component to be separated and the content thereof,
and the kind of distillation column incorporated in the process
for purifying acrylic acid. Generally, the pressure at the
top of the column is in the range of 20 - 200 hPa (abs.) and
the temperature of the top of the column is decided by the
azeotropic composition which is proper for this operating
pressure. The operation of the azeotrop.LC dehydration column
30 is rendered efficient by providing it with an oil-water
separator, introducing the distillate from the top of the
column into the oil-water separator, separating the
distillate into an oil phase (azeotropic solvent phase) 31
and a water phase 32, refluxing the oil phase 31 at a reflux
ratio in the range of 0.5 - 10 to the azeotropic dehydration
column 30, and circulating the water phase 32 to the absorption
column 20 and used therein as the absorbing solvent 21.
Consequently, the bottom liquid of the azeotropic dehydration
column 30 acquires a composition having a water content of
not more than 0.05 wt. % and an acetic acid concentration
in the range of 0.02 - 3 wt. %.
The azeotropic dehydration column 30 prefers proper
addition thereto of a polymerization inhibitor for the purpose
of preventing acrylic acid from undergoing unwanted
polymerization. As the polymerization inhibitor, the
typical examples cited in the paragraph dealing with the
absorption column 20 may be used either singly or in the form
of a mixture of two or more members.
This invention prefers the polymerization inhibitor to
be supplied in conjunction with acrylic acid. While water
and the solvent are vaporized in the part of the distillation
column above the stage for supply, acrylic acid escapes the
vaporization and transfers to the bottom side of the column.
Thus, the presence of acrylic acid is considered to be effective
in preventing the polymerization inhibitor from being
precipitated because the acrylic acid entrains the
polymerization inhibitor. When the product of the thermal
decomposition of the acrylic acid oligomer which will be
described specifically herein below is used as the acrylic
acid, it serves as an effective utilization of acrylic acid
and contributes to the improvement of the productivity. When
the acrylic acid is supplied to the azeotropic dehydration
column 30, it will be at an advantage in enhancing the quality
of product and preventing the polymerization inhibitor from
being precipitated. It is inferred that while the by-produced
maleic acid, when absorbed in water, is present in the aqueous
solution in the form of hydrous maleic acid, it is gradually
anhydridized when heated by repeating the work of distillation.
Also in the acrylic acid recovered by the thermal decomposition,
the water formed by this anhydridization is contained. By
circulating this water to the azeotropic dehydration column
and dehydrating it therein, therefore, it is made possible
to lower the water content in the product. This dehydration
process is at a further advantage in preventing the
polymerization inhibitor from undergoing precipitation.
According to this invention, the efficiency of production
can be further improved by repressing the concentration of
the acrylic acid oligomer (acrylic acid dimer and trimer)
in the bottom liquid of the azeotropic dehydration column
to a level of not more than 5 wt. %, more preferably to a
level of not more than 3 wt. %.
Though the treatment of azeotropic dehydration removes
water and low boiling substance contained in the aqueous
acrylic acid-containing solution, the process for hydration
and the process for separation of the low boiling substance
may be carried out separately of each other. Generally, after
the treatment for dehydration, the product by this dehydration
can be refined by performing the process for the separation
of high boiling substance either alone or in combination with
other heretofore known method for purification. Not solely
by distillation, the purification of acrylic acid may be
effected by crystallization. Owing to the wide range allowed
for the selection of a process of purification, this invention
designates what is obtained by removing water and low boiling
substances from the aqueous acrylic acid-containing solution
as the crude acrylic acid and transfers it to the process
for separation of high boiling substance.
In this invention, it is preferable to install a cooler
or a tank between the process for azeotropic dehydration and/or
the process for separation of low boiling substance and the
process for separation of high boiling substance to cool the
crude acrylic acid. The crude acrylic acid is cooled by
transferring the bottom liquid with a pump 34 to a cooler
35 and the crude acrylic acid cooled therein is stored in
a tank 36. By this transfer similarly to the transfer to the
tank 25, it is made possible to cool the liquid infallibly
by the cooling, decrease the time of retention in the high
temperature part, and repress the amount of an oligomer
suffered to occur. The cooling temperature of the crude
acrylic acid in the tank 36 is preferred to be in the range
of 20 - 50°C. As the cooler, shell-and-tube type heat
exchangers, plate type heat exchangers, and spiral type heat
exchangers which have been heretofore known to the art are
usable. Then, the crude acrylic acid in the tank 3 6 is supplied
to a heavy-ends cut column 40. The heavy-ends cut column 40
constitutes a process for heating the liquid under treatment
and expelling acrylic acid by distillation through the top
of a distillation column. From the viewpoint of the thermal
efficiency, the feed liquid to the column 40 is preferred
to have as high a temperature as permissible. If the
temperature of the crude acrylic acid is unduly high, the
excess of temperature would result in increasing the speed
of oligomer formation, enlarging the possibility of
polymerization, rendering easy the occurrence of a polymer
in the heavy-ends cut column, degrading the final yiao.d of
acrylic acid, and possibly hindering the continuous operation
in consequence of the occurrence of a polymer. If the
temperature falls short of 20°C and approximates closely to
the freezing point, the shortage would be at a disadvantage
in possibly freezing the content of the column and increasing
the amount of heat to be applied during the process for the
removal of high boiling substance. In consideration of the
final efficiency of production based on the comparison with
the amount of a polymer suffered to occur and the thermal
efficiency, this invention has elected to cool the treated
liquid between the subsequent processes. In the present
invention, the yield can be improved at a highest level by
cooling the treated liquid between the treatment for
azeotropic dehydration and the treatment of high boiling
substance.
As the heavy-ends cut column 40, such known columns as
plate column, packed column, wetted wall column, and spray
column are usable. The heavy-ends cut column, similarly to
the azeotropic dehydration column mentioned above, is
generally preferred to be a plate column or a packed column.
These columns may contain a packing or stepped plates. The
number of theoretical plates is in the range of 3 - 30,
preferably 5-20.
The distillation in the heavy-ends cut column 40 may
be performed under the conditions of distillation which have
been heretofore known to the art. Specifically, the pressure
at the top of the column is in the range of 20 - 200 hPa (abs. )
and the temperature of the bottom of the column is not higher
than 12 0°C.
The heavy-ends cut column 40, similarly to the azeotropic
dehydration column 30 mentioned above, prefers addition
thereto of a proper amount of polymerization inhibitor with
the object of preventing acrylic acid from undergoing unwanted
polymerization.
In this invention, the polymerization inhibitor is
preferred to be introduced to any of the distillation columns
at a stage other than the stage for supply of the raw material
or the stage for supply of the reflux. More preferably, a
polymerization inhibitor which conforms to the composition
of the content of the column is injected at any of the stages
which is present away from the stage for supply of the raw
material and before the stage for supply of the reflux.
Specifically, the polymerization inhibitor is supplied in
conjunction with the acrylic acid-containing solution by an
atomizing means through one or more spraying nozzles disposed
in advance in the distilling column. The reason for the
atomization is that it enables the solution containing the
polymerization inhibitor to be sprayed in a wide range inside
the distillation column and allows the polymerization to be
prevented effectively. Even when the injection is made at
the stage for supply of a raw material or the stage for supply
of a reflux, the polymerization inhibitor may be injected
as separated from the raw material and the reflux through
another spraying nozzle or the polymerization inhibitor may
be mixed in advance with the raw material and the reflux and
the resultant mixture is injected through the spraying nozzle.
Since the acrylic acid concentration does not vary very much
in the heavy-ends cut column, the polymerization inhibitor
may be supplied at a stage other than the stage for supply
of the raw material and the staae for supply of the reflux
or it may be injected at the stage for supply of the rawmaterial
and/or the stage for supply of the reflux. In this case, it
is preferable to use as the acrylic acid part of the distillate
obtained through the top of the column. The reason for the
use of part of the distillate is that since the heavy-ends
cut column is a device for obtaining acrylic acid, the use
of the distillate substantially identical in quality with
the product (raw material as ester and high purity acrylic
acid) results in stabilizing the quality of product.
In this invention, it is permissible to condense the
acrylic acid-containing distillate gas obtained through the
top of the heavy-ends cut column 40 and supply at least part
of the resultant acrylic acid-containing condensate liquid
to the azeotropic dehydration column 30. Incidentally, the
condensate liquid generally is a finished product of acrylic
acid used as the raw material for the ester and high purity
acrylic acid and the bottom liquid of the column contains
the polymerization inhibitor, acrylic acid oligomer, and
other high boiling substances . In this invention, the bottom
liquid is designated as a high boiling substance-containing
solution and is subjected to a process for thermally
decomposing the acrylic acid oligomer contained therein and
consequently recovering acrylic acid.
The thermal decomposition of the acrylic acid oligomer
is carried out in a thermal decomposition tank 51. The thermal
decomposition tank 51 does not need to be particularly
discriminated on account of its form. Since the oligomer has
high viscosity, possibly shows precipitation of a solid
substance, and displays an inferior liquid property, the tank
is preferred to be endowed with an inclination toward the
liquid outlet and provided with a liquid circulating and/or
stirring device capable of uniformizing the composition
inside the tank. The L.oncentration of maleic acid contained
in the decomposed liquid obtained by the thermal decomposition
is set so as to be not more than 5 wt. %, preferably in the
range of 0 - 3 wt. %, and more preferably in the range of
0 - 1 wt. %. The reason for restricting the malic acid
concentration to not more than 5 wt. % is that the maleic
acid readily converts into fumaric acid which is an isomer
and the fumaric acid having a high melting point precipitates
as a solid.
For the purpose of obtaining the decomposed liquid of
this description, it is preferred to provide the decomposition
tank 51 in the lower part thereof with a distilling device
such as, for example, a maleic acid separation column 46.
The high boiling substance-containing solution is supplied
to the maleic acid separation column 46, the bottom liquid
of the column is concentrated with a thin film evaporator
50 , and the oligomer is decomposed in the thermal decomposition
tank 51. The liquid obtained from the thermal decomposition
tank 51 may be again concentrated in the thin film evaporator
50 and the acrylic acid obtained in consequence of the thermal
decomposition may be recovered. Since the acrylic acid is
vaporized in the thin film evaporator 50, this acrylic acid
may be recovered through the top of the maleic acid separation
column 46.
The maleic acid separation column 46 has a number of
theoretical plates of 1 - 10, preferably 1 - 5. The
distillation in this column is preferred to be performed at
a column top pressure in the range of 10 - 150 hPa (abs.)
and a column bottom temperature of not higher than 120°C. The
thin film evaporator 50 is preferred over a shell-and-tube
type heat exchanger in respect that it is capable of
concentrating even a liquid of high viscosity. This device
does not need co be discriminated between a horizontal type
and a vertical type.
The thermal decomposition temperature in the thermal
decomposition tank 51 is generally in the range of 120 - 2 2 0°^
and particularly preferably in the range of 120 - 160°C. While
the retention time (amount of liquid reserved in the thermal
decomposition tank/amount of waste oil) is not generally
defined because it varies with the temperature of thermal
decomposition, it is generally required to be in the range
of 20 - 50 hours. Thus, the thermal decomposition tank 51
needs to provide with a heating means . It suffices, however,
to maintain the temperature of the thermal decomposition by
externally jacketing the tank and/or internally (or
externally) disposing a heat exchanger and utilizing such
a heat medium as steam or oil.
This invention permits addition of a polymerization
inhibitor to the high boiling substance-containing solution
prior to subjecting this solution to thermal decomposition
in the maleic acid separation column 46, the thin film
evaporator 50, or the thermal decomposition tank 51. This
addition results in efficiently preventing the polymerization
and possibly promoting the thermal decomposition. As the
polymerization inhibitor capable of promoting the thermal
decomposition, 4,4',4"-tris-(2,2,6,6-tetramethyl
piperidinoxyl)phosphite and one or more of the compounds,
namely, 2,2,6,6-tetramethyl piperidinoxyls represented by
the following formula (1):
(wherein R1 stands for CH2, CHOH, CHCH2OH, CHCH2CH2OH, CHOCH2OH,
CHOCH2CH2OH, CHCOOH, or C=O and R2 stands for a hydrogen atom
or CH2OH), one or more of the N,hydroxy-2,2,6,6-tetramethyl
piperidine compounds such as, for example,
1,4-dihydroxy-2,2,6,6-tetramethyl piperidine and
l-hydroxy-2,2,6,6-tetramethyl piperidine, and
2,2,6,6-tetramethyl piperidine compounds such as, for example,
2,2,6,6-tetramethyl piperidine and
4-hydroxy-2,2,6,6-tetramethyl piperidine may be used in
combination among other examples cited above.
In this invention, the acrylic acid which is recovered
by thermally decomposing the oligomer mentioned above is
preferred to be supplied to the process for dehydration. This
is because the purification by elimination of such impurities
as water to be contained in the subsequent process will be
attained and the polymerization inhibitor will be put to
effective utilization. To be specific, the dehydration
proves favorable, both in terms of lowering the water content
in the product thereby heightening the quality of the product
and preventing the polymerization inhibitor from being
precipitated.
Incidentally, the acrylic acid expelled by distillation
through the top of the heavy-ends cut column 40 is enabled
to produce an acrylic ester when it is supplied to a process
for the production of the acrylic ester.
Now, a method for producing an acrylic ester from acrylic
acid will be explained below as one mode of embodying this
invention.
To an esterif ication reactor 60 packed with a strongly
acidic cation-exchange resin as a catalyst, the acrylic acid
obtained in the heavy-ends cut column 40 is supplied and then
an alcohol and other are charged to the reactor 60 to form
an ester. Then, the reaction solution is introduced into an
acid separation column 80, which by distillation expels an
acrylic ester, unaltered alcohol, water, and other low boiling
substance through the top thereof. Subsequently, the
distillate emanating from the top of the acid separation column
80 is introduced into an oil-water separator and is separated
therein into an oil phase 81 containing an acrylic ester and
a water phase 82 having water and alcohol as main components.
The water phase 82 is transferred to an alcohol recovery column
90 like a water phase 72 in the oil-water separator which
is the distillate through the top of the water-separation
column 70, and the oil phase 81 is supplied to a light-ends
cut column 100. In this while, part of the oil phase 81 may
be refluxed to the acid separation column 80. In the
light-ends cut column 100, the acrylic ester is separated
through the bottom and supplied to a refining column 110 and
the produced acrylic ester 111 is expelled by distillation
through the top. Incidentally, the alcohol which has been
expelled by distillation through the top of the alcohol
recovery column 90 may be circulated to the oil phase 81 in
the oil-water separator annexed to the acid separation column
80. The water, alcohol, and other low boiling substances
expelled by distillation through the top of the light-ends
cut column 100 are circulated to the esterif ication reactor
60 through the column provided in the upper part of the
esterification reactor 60.
In this process for the production of an acrylic ester,
the bottom liquid of the acid separation column 80 eventually
contains acrylic acid dimer and acrylic acid dimer ester and
Michael type adducts such as alkoxypropionic acids and
alkoxypropionic esters represented by the following formula
[ II ] together with the raw material components such as acrylic
acid.
(wherein m is an integer in the range of 1 - 5, R1 and R2
independently stand for a hvdrogen atom or an alkyl group,
and -X- stands for -CH2CH2- or -CH(CH3)-, providing that when
m is not less than 2, a plurality of -X-'s may be identical
or different).
Thus, the bottom liquid of the acid separation column
80 may be circulated to the esterification reactor 60 as
illustrated in Fig. 1 or it may be supplied to a separately
provided thin film evaporator and decomposition tank (not
shown) to decompose the acrylic acid oligomer contained
therein. The components contained in the bottom liquid are
decomposed and further converted in the thin film evaporator
into an alcohol, acrylic acid, and an acrylic acid ester.
When these components are introduced again into the
esterification reactor 60, they constitute effective
utilization of components. Incidentally, the decomposition
of the bottom liquid may be promoted by the addition of the
aforementioned N-oxyl compound.
The method for the production of an acrylic ester cons ists
in obtaining an ester by subjecting acrylic acid and an alcohol
to a reaction of dehydration. As typical examples of the
preferred alcohol, various species of alcohol such as methanol,
ethanol, n-butanol, isobutanol, sec-butanol, t-butanol,
1-pentanol, 2-pentanol, 3-pentanol, cyclopentanol,
1-hexanol, 2-hexanol, 3-hexanol, cyclohexanol, 1-heptanol,
2-heptanol, 3-heptanol, 1-octanol, isooctanol,
2-ethylhexanol, isononyl alcohol, and lauryl alcohol may be
cited. They may be in a linear form or in a branched form.
They may be used either singly or in the form of a combination
of two or more members . Incidentally, the reaction conditions
and distillation conditions for each of the processes
mentioned above may be arbitrarily selected from the known
conditions.
The acrylic acid which is obtained from the heavy-ends
cut column 40 may be further refined as with a distillation
column to obtain acrylic acid of high purity. For example,
a conventional primary amine such as hydrazine hydrate and
phenyl hydrazine and/or the salts thereof is added to the
acrylic acid in an amount in the range of 1.0 - 10.0 moles,
preferably 1.0 - 5.0 moles, per mol of the aldehyde contained
therein, and after a treating agent is further added thereto,
theresultant mixture is subjected to vacuum distillation
with a known distillation column. This distillation is
performed, for example, in a flash column fitted with a mist
separator under a column top pressure in the range of 10 -
150 hPa (abs.) at a column top temperature in the range of
35 - 90°C. By this treatment, the concentrations of such
aldehydes as furfural, acrolein, and benzaldehyde can be
decreased to less than 10 ppm. Acrylic acid of equal purity
can be obtained by using a crystallization device. When a
water-absorbent resin is produced from acrylic acid, it
possibly proves unfavorable for a given use because of its
odor or stimulus to the skin. Acrylic acid of high purity
obtained by such purification can be preferably used in this
case. In this invention, the acrylic acid of high purity thus
obtained is supplied to a polyacrylic acid (salt) production
process 120 to produce polyacrylic acid (salt), which may
be used to produce a water-absorbent resin, for example.
When polyacrylic acid is produced from acrylic acid of
high purity, this invention prefers the purified acrylic acid
to be transferred with the pump 42 to the cooler 43 and the
acrylic acid cooled in the cooler 43 to be stored in the tank
44. The reason for commending this process is that by this
cooling, it is made possible to cool the liquid infallibly,
decrease the retention time in the high temperature part,
and repress the amount of the oligomer suffered to form. The
cooling temperature of acrylic acid in the tank is preferred
to be in the range of 20 - 50°C.
The polyacrylic acid (salt) production process 120 is
capable of producing polyacrylic acid (salt) by sequentially
introducing the acrylic acid mentioned above to a
neutralization process 121, a polymerization process 122,
a drying process 123, and a cooling process 124 and subjecting
it to the relevant treatments therein. When acrylic acid is
not neutralized, polyacrylic acid is obtained. The
neutralization process mentioned above is performed
optionally. The acrylic acid may be given a treatment adapted
to improve a varying physical property. A cross-linking
process may be additionally performed during or after the
polymerization process.
The neutralization process is an arbitrary additive
process. Forexample, a method which comprises mixing acrylic
acid or a produced polyacrylic acid (salt) with a prescribed
amount of a powder or aqueous solution of a basic substance
may be cited. This method does not need to be particularly
restricted but may be properly selected from the known methods .
This neutralization process may be carried out prior to
polymerization (neutralized in the form of a monomer), during
the polymerization, or after the polymerization (neutralized
in the form of a gel), or both before and after the
polymerization. Though the diagram depicts a process which
performs polymerization after neutralization, the
neutralization may be performed, when necessary, after the
polymerization. In this case, the conf iauration of equipment
and the sequence of component processes may be properly altered
to suit the occasion. A polymerization device and a
neutralization device may be identical or different.
The basic substance to be used for neutralizing acrylic
acid may be properly selected among the known basic substances
such as, for example, (hydrogen) carbonates, hydroxides of
alkali metals, ammonia, and organic amines. The radio of
neutralization of acrylic acid does not need to be particularly
restricted but may be properly selected in the range of 3 0
- 100 mol% andpref erably 50 - 80mol% . When the heat of reaction
which is generated during the neutralization is required to
be removed, it suffices to introduce the product emitting
the heat of reaction into a proper cooling means such as,
for example, a cooling device represented by a cooling tower.
The acrylic acid salt solution resulting from the
neutralization, when necessary, is introduced into a
polymerization process. The method of polymerization in this
process does not need to be particularly restricted. When
the polymerization needs to use a radical polymerization
initiator, it may be performed by any of the known methods
of polymerization such as stripping polymerization, electron
stripping polymerization, and photosensitized
polymerization. In the polymerization process, acrylic acid
may be neutralized as occasion demands and then subjected
in the form of an aqueous solution of acrylic acid (salt)
of a concentration preferably of not less than 10 wt. %, more
preferably not less than 2 0 wt. % and preferably of not more
than 80 wt. %, more preferably not more than 7 0 wt. %.
This invention allows various conditions such as the
kind of polymerization initiator and the conditions for
polymerization to be arbitrarily selected. Optionally,
various known additives such =>s cross-linking agent, other
monomer, and even water-soluble chain transfer agent, and
hydrophilic macromolecular substance may be added. For the
polymerization process, reactors and devices selected
arbitrarily may be used. Any of the polymerization devices
in common use may be used without any particular restriction.
The polyacrylic acid (salt) resulting from the
polymerization is generally a polymer of the form of a hydrogel
and, therefore, is further subjected to a drying process for
the purpose of removing water. The method for this drying
does not need to be particularly restricted. The polymer may
be dried with any of the known drying devices such as hot
air drier, fluidized bed drier, drum drier, and Nauter type
drier at a proper drying temperature preferably in the range
of 70 - 230 °C. As the heat medium to be supplied to a drying
process 123, the vapor discharged in the process for the
production of acrylic acid, particularly the heat of reaction
emitted from the catalytic gas phase oxidizer may be utilized.
The hydrogel, namely the hydrous polymer, of polyacrylic
acid (salt) is thermally dried with a varying type of drier.
The drying may be attained, for example, by exposing the
hydrogel to the heating surface of such a conducting heat
transfer drier as a drum drier or a paddle drier which has
been heated with steam. From the viewpoint of decreasing the
residual monomer content and exalting the drying efficiency,
the hot air transfer drying which expose the hydrogel directly
to the steam proves particularly preferable. Preferably, the
hydrogel is dried with a steam-containing gas such as, for
example, a hot air having a dew point preferably of not lower
than 50°C, more preferably not lower than 60°C and preferably
not higher than 9 0°C, more preferably not higher than 80°C
and having a temperature preferably of not lower than 100°C,
and more preferably not lower than 150°C and preferably not
higher than 200°C, more preferably not higher thanl80°C becauba
this drying promotes the decrease of the residual monomer
content and the exaltation of the water-absorption ratio of
the polyacrylic acid (salt). Incidentally, the duration >)£
drying is generally in the range of one minute to three hours,
preferably five minutes to one hour.
The polyacrylic acid (salt) which is obtained after the
drying process is still hot at the time of its release from
the drying device. Preferably, therefore, it is cooled in
a cooling process 124 at a suitable temperature in the range
of room temperature to 90°C, preferably 40 to 80°C. The method
for cooling this polyacrylic acid (salt) does not need to
be particularly restricted. It may be cooled, for example,
by being blown with cold air or introduced into such a cooling
device as a refrigerator.
The polyacrylic acid (salt) which has been cooled to
a prescribed temperature may be put to use in its unaltered
form. Optionally, it may be further molded in a prescribed
shape as by granulation or pulverization and then made to
incorporate therein various additives such as reducing agent,
flavoring agent, and binder so as to suit the purpose of
application.
This invention prefers the dried polyacrylic acid (salt)
to be cooled. When the hydrogel is finely divided to a particle
size in the approximate range of one - several mm and dried,
the dried polyacrylic acid (salt) is in the form of dry
particles measuring about one - several mm. Generally, the
dried particles assume the form of an agglomerate. Thus , the
dried polyacrylic acid (salt) may be optionally pulverized
or further classified to obtain a polyacric acid (salt) powder
having a weight average particle diameter in the range of
10 - 1000 jum, preferably 100 - 800 |Am. When this powder and
various modifying agents such as, for example, an aqueous
solution of a surface cross-linking agent, pelletizing binder,
and deodorant are added together, the application of a cooling
process enhances the efficiency of pulverization and sharpens
the particle diameter distribution and allows the various
modifying agents to be uniformly added to the powder. Thus,
the cooling process in this case can exalt various physical
properties of the water-absorbent resin such as, for example,
a water-absorption ratio under pressure while restraining
dispersion among individual particles of powder.
For this invention, it is preferable to introduce a
polymerization inhibitor to any of the distillation columns
at a stage other than the stage for supply of the raw material
and the stage for supply of the reflux. Particularly, the
site for supply of the polymerization inhibitor to the
azeotropic dehydration column is preferred to be higher than
the stage for supply of the raw material markedly different
in composition and to be lower than the stage for supply of
the reflux. The polymerization inhibitor in this case is
preferred to be supplied together with the acrylic
acid-containing solution by the use of an atomizing injection
means. Particularly in the azeotropic dehydration column,
the acrylic acid which is recovered by thermally decomposing
an acrylic acid oligomer is used efficiently as the acrylic
acid-containing solution.
Examples
Now, this invention will be more specifically described
below with reference to working examples.
Example 1
Acrylic acid was produced by following the process flow
illustrated in Fig. 1. First, by subjecting propylene to
catalytic gas phase oxidation with a molecular oxygen in the
presence of an oxidizing catalyst, a mixed gas containing
7.1 vol. % of acrylic acid, 0.3 vol. % of acetic acid, and
14.7 vol. % of water was obtained at
This gas was introduced into an absorption column
(cascade miniring 3P 10m) to obtain a bottom liquid at a rate
of 8050 kg/h. This absorption column was operated with the
top thereof kept under 1100 hPa abs. at 62°C. Through the
top of the column, water obtained by mixing hydroquinone as
a polymerization inhibitor, separated water occurring in an
azeotropic dehydration column, and waste water generated from
a vacuum generating device in a distillation column and
containing 1.5 wt. % of acrylic acid and 5.4 wt. % of acetic
acid was supplied as a absorbing water at a rate of 2720 kg/h.
Part of the exhaust gas emanating through the top of the column
was circulated to an oxidation reactor and the remainder
thereof was released as the waste gas from the system. The
bottom liquid of the absorption column was further distilled
to obtain an aqueous acrylic acid solution containing 7 0 wt. %
of acrylic acid, 3.4 wt. % of acetic acid, and 0.3 wt. % of
maleic acid.
The aqueous acrylic acid solution thus obtained was
passed through a cooling device annexed to a tank interposed
between the absorption column and the azeotropic dehydration
column to be cooled to 40°C and the cooled aqueous solution
was supplied together with part of the liquid at the top of
a maleic acid separation column to the middle stage of the
azeotropic dehydrating column provided with 50 sieve trays.
The azeotropic dehydration column was operated under
the conditions of 190 hPa abs. in column top pressure and
1.0 in reflux ratio (total number of moles of the reflux per
unit time/total number of moles of the distillate per unit
time) to effect azeotropic beparation with toluene. The top
liquid of the separation column was led together with the
waste water from a steam ejector which was a vacuum generating
device to a storage tank and separated therein into an organic
phase and a water phase. As the polymerization inhibitor for
the column, copper dibutyldithiocarbamate and hyqroduinone
monomethyl ether were dissolved in the reflux, then the
resultant mixture was introduced into the column together
with the reflux. And hydroquinone dissolved in water were
injected by spraying into the column together with part of
the top liquid of a maleic acid separation column containing
the acrylic acid recovered by thermally decomposing the
oligomer from a stage intervening between the stage for supply
of an aqueous acrylic acid solution and the stage for supply
of the reflux. The concentration of the acrylic acid oligomer
(acrylic acid dimer and trimer) in the bottom of the column
was found to be 2 wt. %.
The bottom liquid of the column was passed through a
cooler annexed to a tank interposed between this column and
a heavy-ends cut column to be cooled to 40°C and then supplied
to the heavy-ends cut column provided with 45 sieve trays
through the intermediary stage thereof. This column was
operated under the conditions of 45 hPa abs. in tower top
pressure and 1.4 in reflux ratio. Through the top of the column,
acrylic acid was obtained at a rate of 5120 kg/h. The bottom
liquid of this column which contained 31 wt. % of acrylic
acid oligomer and 5 wt. % of maleic acid was supplied to a
maleic acid separation column provided with five sieve trays
through the bottom thereof.
The column was provided in the bottom thereof with a
thin film evaporator and a thermal decomposition tank and
operated under the conditions of 45 hPa abs. in pressure and
0.5 in reflux ratj.o to obtain acrylic acid containing 0.5
wt. % of maleic acid at a rate of 400 kg/h through the top
of the column. The acrylic acid thus obtained was used as
the raw material for ester and high-purity acrylic acid. The
high-purity acrylic acid was further used for the production
of polyacrylic acid. Separately, the bottom liquid of the
thin film evaporator was introduced into a thermal
decomposition tank and subjected therein to thermal
decomposition under the conditions of 150°C in temperature
and 40 hours in retention time. Part of the bottom liquid
formed in the tank was circulated to the thin film evaporator.
To the heavy-ends cut column and the maleic acid separation
column, a solution of copper dibutyl dithiocarbamate and
hydroquinone monomethyl ether in acrylic acid was introduced
as a polymerization inhibitor by spraying to a condenser.
The waste oil containing 5.5 wt. % of acrylic acid and 39
wt. % of acrylic acid oligomer (acrylic acid dimer and trimer)
was discarded from the thermal decomposition tank at a rate
of 170 kg/h. The plant was stopped after about three months'
continued operation and then opened to test the interior.
The test failed to detect any sign of problem. The results
of working examples and a comparative experiment are shown
in Table 1. In Table 1, (i) represents the case of injecting
the polymerization inhibitor containing solution to stages
other than the stage for supply of a raw material and the
stage for supply of the reflux, (ii) the case of supplying
the acrylic acid recovered by thermally decomposing the
oligomer to the process of dehydration, (iii) the case of
supplying the polymerization inhibitor containing solution
together with an acrylic acid-containing solution with an
atomizing injection means, and (iv) the case of adjusting
the concentration of maleic acid contained in the acrylic
acid solution recovered by thermally decomposing the oligomer
contained in the high boiling substance-containing solution
to below 5 wt. %. The symbol O represents actual application
and the symbol - represents the absence of actual application.
Example 2
An operation was carried out by following the procedure
of Example 1 while supplying hydroquinone dissolved in water
together with an aqueous acrylic acid solution as part of
the polymerization inhibitor to the azeotropic dehydration
column at the stage for supply of the aqueous solution. The
pressure loss in the azeotropic dehydration column showed
a sign of rise in about one week' s continued operation. When
the plant was opened and tested after about two months'
continued operation, the deposition of a polymer was found
on the sieve trays higher than the stage for supply of the
aqueous acrylic acid solution.
Example 3
An operation was performed by following the procedure
of Example 1 while supplying hydroquinone as part of the
polymerization inhibitor directly to the azeotropic
dehydration column without using the top liquid of a maleic
acid separation column. On the day following the start of
the operation, the azeotropic dehydration column showed a
sign of an increase in the pressure loss. When the plant was
opened to check the interior thereof after about one month's
continued operation, the precipitation of the polymerization
inhibitor occurred in the neighborhood of the stage for
introduction of the polymerization inhibitor into the column
and the holes in the trays were found to be blockage.
Example 4
An operation was performed by following the procedure
of Example 1 while supplying the polymerization inhibitor
directly to the column without using the cooler annexed to
the tank. As a result, the concentration of the acrylic acid
oligomer (acrylic acid dimer and trimer) in the waste oil
from the thermal decomposition tank increased to 43 wt. %
and the amount of the waste oil was increased to 200 kg/h.
When the plant was stopped after about three months ' continued
operation to test the interior thereof, no sign of trouble
was detected.
Comparative Example 1
An operation was performed by following the procedure
of Example 1 with the exception of the following modifications .
First, hydroquinone as one of the components of the
polymerization inhibitor supplied to the azeotropic
dehydration column was dissolved in water and supplied
together with the aqueous acrylic acid solution to the step
for supply of the aqueous solution. The bottom liquid of the
heavy-ends cut column was directly supplied to the thin film
evaporator and the acrylic acid recovered from the evaporator
was circulated to the heavy-ends cut column. Further, this
process was directly shifted to the subsequent process without
using the cooler annexed to the tank.
The maleic acid concentration in the acrylic acid
recovered from the thin film evaporator was about 6 wt. %.
Thus, the water content and the concentration of such high
boiling substances as maleic acid in the acrylic acid were
higher than those obtained in Example 1. The amount of the
waste oil was 210 kg/h and the oligomer concentration in the
waste oil was 43 wt. %. The azeotropic dehydration column
began to show a discernible sign of rise of pressure on the
day following the start of the operation. Though the operation
could be continued for about one month, it had to be stopped
owing to the rise of the pressure during the refining process .
When the plant was opened to test the interior thereof, the
azeotropic dehydration column was found to have a discernible
sign of precipitation of the polymer and the polymer inhibitor.
In the neighborhood of the heavy-euus cut column and the thin
film evaporator, precipitation of fumaric acid inferred to
be formed by the transfer of heat of the maleic acid and the
polymer were confirmed.
The concentration of the acrylic acid oligomer (acrylic
acid dimer and trimer) in the bottom of the azeotropic
dehydration column was 6 wt. %.
Example 5
An operation was performed by following the procedure
of Comparative Example 1 while introducing hydroquinone as
the polymerization inhibitor to the azeotropic dehydration
column at a stage between the stage for supply of the raw
material and the stage for supply of the reflux. The plant
was opened to test the interior thereof after one month's
continued operation. Though the amount of the polymer
suffered to occur in the azeotropic dehydration column was
small as compared with Comparative Example 1, the
precipitation of the polymerization inhibitor was found be
occurred in the proximity of the site for injection of the
polymerization inhibitor and blockage also was found in the
holes in the trays. The neighborhood of the heavy-ends cut
column and the thin film evaporator was not notably different
from that in comparative Example 1.
Example 6
An operation was performed by following the procedure
of Comparative Example 1 while circulating the acrylic acid
recovered from the thin film evaporator to the azeotropic
dehydration column. When the plant was opened to test the
interior thereof after one month's continued operation, the
results of test were not notably different from those of
Comparative Example 1. The water content of the produced
acrylic acid, however, was decreased.
Example 7
An operation was performed by following the procedure
of Comparative Example 1 while introducing hydroquinone as
the polymerization inhibitor to the azeotropic dehydration
column by spraying together with part of the acrylic acid
obtained through the top of the heavy-ends cut column at the
stage intervening between the stage for supply of raw material
and the stage for supply of the reflux. The plant was opened
one month's continued operation to test the interior thereof.
Though the azeotropic dehydration column showed no sign of
any particular problem, the results of test in the neighborhood
of the heavy-ends cut column and the thin film evaporator
were not notably different from those of Comparative Example
1.
WE Claim:
1. A method for the production of acrylic acid which
comprises the steps of :
(a) supplying one or more gas components selected
from the group consisting of propylene, propane and
acrolein to a reactor for catalytic gas phase oxidation,
(b) obtaining an acrylic acid-containing gas by
catalytic gas phase oxidation,
(c) introducing said acrylic acid-containing gas and
supplying an aqueous absorbing solvent into an acrylic
acid absorbing column (20), whereby an aqueous acrylic
acid-containing solution is absorbed onto said acrylic
acid absorbing column,
(d) obtaining said aqueous acrylic acid-containing
solution absorbed onto said acrylic acid absorbing
column (20),
(e) obtaining crude acrylic acid from said aqueous
acrylic acid containing solution in an azeotropic
dehydration column (30) by dehydration,
(f) introducing a polymerization inhibitor to said
azeotropc, dehydration column (30) at any point between a
point for supplying said aqueous acrylic acid containing
solution and a point for supplying a reflux and not
including the point for supplying said aqueous acrylic
acid containing solution material and the point for
supplying the reflux;
(g) optionally removing a low boiling substance from
said aqueous acrylic acid-containing solution by using
an azeotropic distillation column,
(h) obtaining acrylic acid and a high boiling
substance-containing solution by removing the high
boiling substance from said crude acrylic acid,
subsequently
(i) recovering acrylic acid by thermally decomposing
an acrylic acid oligomer contained in said high boiling
substance-containing solution, and
(j) supplying the acrylic acid recovered by
thermally decomposing said acrylic acid oligomer from
step (i) to said azeotropic dehydration column.
2. A method as claimed in claim 1, which optionally
comprises performing the step of ;
thermally decomposing the oligomer contained in
said high boiling substance-containing solution thereby
lowering a concentration of maleic acid contained in the
recovered acrylic acid solution to a level of not higher
than 5 wt.%.
3. A method as claimed in claim 1, which optionally
comprises the steps of;
(j) for esterfying the acrylic acid obtained in said
step (i) thereby producing an acrylic ester, or
(k) for optionally purifying the acrylic acid
obtained in said step (i) thereby obtaining acrylic acid
of high purity.
4. A method as claimed in claim 3, optionally comprising
the step of cooling the aqueous acrylic acid-containing
solution in a tank (36) and/or a cooler (35) between
said steps (b) - (k) and the subsequent step.
5. A method for the production of a polyacrylic acid or
salt thereof characterized by producing said polyacrylic
acid or salt by using the acrylic acid of high purity
obtained at the step (k) set forth in claim 3 in a known
polymerization process.
6. A method as claimed in claim 5, optionally comprising
the step of cooling the aqueous acrylic acid-containing
solution in a tank (36) and/or a cooler (35) between
said step (k) and a step for producing the polyacrylic
acid or salt.
7. A method for the production of a polyacrylic acid or
salt thereof, characterized by producing said
polyacrylic acid or salt by using the acrylic acid of
high purity obtained at the step(k) set forth in claim 4
in a known polymerization process.
8. A method as claimed in claim 1, wherein said
distillation column is at least one member selected from
the group consisting of the azeotropic dehydration
column, the heavy-ends cut column and the maleic acid
separation column.
9. A method as claimed in claim 1, wherein said
distillation column is at least one member selected from
the group consisting of the azeotropic dehydration
column and the heavy-ends cut column.
10. A method as claimed in claim 1, wherein said thermal
decomposition of the acrylic acid oligomer to acrylic
acid in the step (I) is carried out at a temperature of
120°-220°C.
11. A method as claimed in claim 1, wherein said thermal
decomposition of the acrylic acid oligomer is carried
out in a thermal decomposition tank.
12. A method for production of acrylic acid,
substantially as herein described, particularly with
reference to the foregoing examples.
ABSTRACT
METHOD FOR THE PRODUCTION OF ACRYLIC ACID
The invention discloses a method for the production of acrylic and which comprises
the steps of :(a) supplying one or more gas components selected from the group
consisting of propylene, propane and acrolein to a reactor for catalytic gas phase
oxidation,(b) obtaining an acrylic acid-containing gas by catalytic gas phase
oxidation, (c) introducing said acrylic acid-containing gas and supplying an
aqueous absorbing solvent into an acrylic acid absorbing column (20), whereby an
aqueous acrylic acid-containing solution is absorbed onto said acrylic acid absorbing
column,(d) obtainingsaid aqueous acrylic acid-containing solution absorbed onto
said acrylic acid absorbing column (20),(e)obtaining crude acrylic acid from said
aqueous acrylic containing solution in an azeotropic dehydration column (30) by
dehydration, (f) introducing a polymerization inhibitor to said azeotropc dehydration
column (30) at any point between a point for supplying said aqueous acrylic acid
containing solution and a point for supplying a reflux and not including the point for
supplying said aqueous acrylic acid containing solution material and the point for
supplying the reflux; (g) optionally removing a low boiling substance from said
aqueous acrylic acid-containing solution by using an azeotropic distillation column,
(h)obtaining acrylic acid and a high boiling substance-containing solution by
removing the high boiling substance from said crude acrylic acid, subsequently (I)
recovering acrylic acid by thermally decomposing an acrylic acid oligomer
contained in said high boiling substance-containing solution, and (j) supplying the
acrylic acid recovered by thermally decomposing said acrylic acid oligomer from
step (i) to said azeotropic dehydration column. |