Field of technology
5 The present invention relates to the manufacture of light acrylates, and
notably relates to a method for continuous preparation of light acrylates by
direct esterification from a stream of crude acrylic acid and a light alcohol, in
particular methanol or ethanol.
More particularly, the present invention relates to a method according to
10 which the thermal dissociation of the Michael adducts present in a stream of
crude acrylic acid, called hereinafter "crude ester grade", and the esterification
reaction of the acrylic acid present in said stream of crude acrylic acid, or
generated in situ by said thermal dissociation, with a light alcohol, are carried
out simultaneously.
15
Prior art and the technical problem
Light acrylic esters (or light acrylates) are prepared from acrylic acid and
light alcohol (such as methanol or ethanol) by simple esterification. The range of
uses for the manufacture of polymers is extensive. Methyl acrylate (MA) is very
20 often used in copolymerization processes for making fibres. Ethyl acrylate (EA)
is used in particular in copolymerization processes for imparting cohesion to
textile fibres.
Obtaining these monomers with a satisfactory degree of purity for the
final industrial application is therefore essential and often challenging, and
25 requires expensive purification techniques. Thus, to meet these purity
requirements, quite particular attention is paid to the quality of the acrylic acid,
which must be free from impurities that could generate by-products in the light
acrylate. That is why the esterification reaction is generally carried out starting
from a purified acrylic acid.
30
For synthesis of acrylic acid (AA), the most widely used industrial route is
the oxidation of propylene. This synthesis of acrylic acid is called petrochemical
WO 2015/015100 - 2 - FCT/FR2014/051928
synthesis and comprises two steps, the first is the oxidation of propylene to
acrolein with simultaneous production of one molecule of water per molecule of
acrolein and the second is the oxidation of acrolein to acrylic acid. This
synthesis is generally carried out in two reactors using two catalytic systems
5 specific to each of the oxidation steps, or in a single reactor having two
separate catalyst beds.
Another route of synthesis of acrylic acid uses glycerol or glycerin as raw
material, which is submitted firstly to dehydration leading to acrolein with
simultaneous production of 2 molecules of water per molecule of acrolein, the
10 latter then being submitted to oxidation to form acrylic acid. This route also
comprises two steps with, as a point in common, the presence of acrolein as
intermediate. It is in the course of industrial development as it is a "green"
process using a renewable natural raw material and not a fossil material such
as propylene.
15
The effluent leaving the reactor after the oxidation of acrolein to acrylic
acid contains, besides the reaction products, acrylic acid and water, a whole
range of by-products constituting impurities, both by the propylene route and by
the glycerol route. These by-products are notably:
20 - light compounds that are incondensable in the conditions of
temperature and pressure usually employed (nitrogen, oxygen, residual
reactant, carbon monoxide and dioxide formed in small amounts by final
oxidation);
- condensable light compounds: in particular water, unconverted acrolein,
25 light aldehydes, such as formaldehyde, acetaldehyde and acetic acid, the main
impurity generated in the reaction section;
- heavy compounds: notably furfuraldehyde, benzaldehyde, maleic
anhydride, benzoic acid.
In the acrylic acid synthesis process, the effluent leaving the reactor,
30 regardless of the route chosen, is submitted to a chain of treatment and
purification that can be summarized as follows:
WO 2015/015100 - 3 - PCT/FR2014/051928
Acrylic acid is recovered from the gas mixture resulting from the 2nd
step, by introducing this gas at the bottom of an absorption column, where it
meets, in counter-current, a solvent introduced at the top of the column. In most
of the processes described, the solvent used in this column is water or a high-
5 boiling hydrophobic solvent. By removing the incondensables, this step allows
"crude" acrylic acid to be formed.
In the case of absorption processes using water as absorbent solvent,
the additional purification steps comprise a dehydration step, generally carried
out in the presence of a non-water-miscible solvent, in an extraction column or a
10 heteroazeotropic distillation column, then a step of removal of the light
compounds, in particular acetic acid and formic acid, this step generally being
called "topping". Finally, a step of separation of the heavy compounds is carried
out by distillation, this step generally being called "tailing" and leading to an
"industrial" acrylic acid. In the case of processes using a hydrophobic solvent,
15 the steps are essentially the same, except for the removal of water, which is
effected at the top-of .the first absorption column.
Recently, novel "solvent-free" technologies for recovery/purification of
acrylic acid have appeared, involving a reduced number of purification stages
and eliminating the introduction of external organic solvent. Patent
20 EP 2 066 613 describes a process for the recovery of acrylic acid without using
azeotropic solvent and while employing only two columns for purification of the
cold gaseous reaction mixture: a) a dehydration column, where the gas stream
distilled at the top is condensed and sent to the dehydration column in the form
of reflux in order to absorb the acrylic acid, and b) a finishing column fed with
25 the bottom stream from the first column, in which, i) the residual water and the
residual acetic acid are distilled at the top and recycled at the bottom of the first
column, ii) a stream comprising the heavy by-products and acrylic acid is
removed at the bottom, in order to optionally be used in the production of acrylic
esters, and iii) a stream of acrylic acid of industrial grade is recovered by side
30 withdrawal in the liquid or vapour form.
During these various steps of treatment/purification, covalent side
reactions of Michael addition on the double bond of the acrylic acid, generating
WO 2015/015100 - 4 - PCT/FR2014/051928
compounds corresponding to the general term Michael adducts, may take
place.
Essentially these are oligomers of acrylic acid, principally composed of
Michael adduct molecules known as acrylic acid dimer (3-acryloxypropionic
5 acid, n = 1) and acrylic acid trimer (3-(3-acryloxypropionylpropionic acid, n = 2).
They are characterized by a boiling point above the boiling point of the products
employed in the reaction, and will be found in the heavy compounds fraction
separated during the final step of tailing.
n+1 H2C=CH-C(0)OH ^ H2C—CH-C(0)O^CH2-CH2-C(0)oVhl
n
10 Other Michael adducts can also coexist with the derivatives from the
addition of acrylic acid to itself, which involve the nucleophilic addition of
compounds present in the medium for purification of the acrylic acid, such as,
for example, water, to the double bond of the acrylic acid or acrylic acid
oligomers:
H20 + nH2C=CH-C(0)OH —^ H04H2C—CH2-C(0)04H
15 n
The process of treatment/purification of acrylic acid therefore leads to
obtaining, on the one hand, a stream of purified acrylic acid generally
designated as being of "industrial" grade, used notably as raw material for
producing light acrylates, and on the other hand a stream comprising heavy
20 compounds, notably including large amounts of oligomers of acrylic acid.
In the conventional process, the heavy fraction is removed, but most
often, in order to avoid loss of products that can be upgraded, it is submitted to
a high-temperature thermal treatment, in the presence or absence of a catalyst,
for dissociating the oligomers and recovering the acrylic acid monomer, the final
25 residue then being removed and the recovered acrylic acid being recycled as
raw material to an esterification process (EP 887 334; FR 1 351 243). The
drawbacks of this type of treatment are the high viscosity of the residue, which
can no longer be conveyed in the pipelines, and fouling of the wall of the
cracking reactor.
WO 2015/015100 - 5 - PCT/FR2014/051928
Moreover, the production of acrylic esters comprising the esterification
reaction of industrial acrylic acid with an alcohol, followed by a purification
treatment, is also accompanied by the formation of Michael adducts: these are
principally acrylic acid oligomers and derivatives resulting from their
esterification with alcohol used for the esterification reaction, and also products
of Michael addition of the alcohol to the double bonds of the compounds
mentioned:
H2C=CH-C(0)o4cH2-CH2-C(0)0-]-H + ROH —>- H2C=CH-C(0)o4cH2-CH2-C(0)0-)-R
H2C=CH-C(0)0-f CH2-CH2-C(0)o4-R + ROH -* RO-H2C-CH2-C(0)O4CH2-CH2-C(0)C>4R
10 V ;n n
In document US 3,868,410, these Michael adducts are advantageously
converted to monomers by thermal treatment at a temperature above 180°C,
with a view to recycling them to the esterification step.
In order to improve the yield in recovery of valuable products generated
15 by thermal cracking of the heavy fractions formed, on the one hand during the
synthesis of acrylic acid, and on the other hand during the synthesis of acrylic
esters, it was proposed, in document EP 2 727 964, to conduct thermal
dissociation on a mixture of these heavy fractions, in the absence of catalyst,
which has the effect of reducing fouling in the dissociation reactor as well as
20 lowering the viscosity of the residue obtained at the end of the operation of
thermal dissociation.
In addition to the fact that the abovementioned methods do not resolve
the problems inherent in the formation of Michael adducts during the steps of
25 synthesis and/or purification of acrylic acid and/or of acrylic esters, such as:
the use of large amounts of polymerization inhibitors for limiting
the radical polymerization reactions;
complexity of the steps of separation of the heavy fractions;
high cost of thermal cracking of the heavy fractions for
30 recovering the valuable compounds;
WO 2015/015100 - 6 - PCT/FR2014/051928
difficulty of removing the heavy residues after cracking and
fouling of the cracking reactor;
the processes for esterification of acrylic acid have drawbacks which result
directly from the use of a purified grade of acrylic acid as raw material for
5 producing the acrylic esters.
To reduce these drawbacks, the method of production of light acrylates
described in document WO 91/01966 uses an aqueous solution of crude acrylic
acid as raw material for the esterification reaction. The aqueous solution is
10 received from the column for absorption of the gas mixture from oxidation of
acrolein in the process for producing acrylic acid, and it comprises from 50% to
70% of acrylic acid and the impurities inherent in its production. In the method in
WO 91/01966, the aqueous solution of crude acrylic acid is not purified in a
specific purification plant, before being fed into the esterification reactor; it is
15 introduced directly at the bottom of the distillation column used for separating
the products from the esterification reactor, to result in a stream enriched in
acrylic acid at the bottom of said column, which is recycled to the esterification
reactor. This method is not, however, economic on an industrial scale.
Document WO 00/78702 describes a method of synthesis of acrylic
20 esters, in particular of butyl acrylate, in which the esterification reaction is
carried out at a temperature and a pressure sufficient to permit cracking of the
Michael adducts formed in situ or introduced into the reactor, in one and the
same reaction zone, and to vaporize the ester produced. Typically, the reaction
is carried out at a temperature in the range from 100 to 16'0°C, and at a
25 pressure in the range from 0.01 to 100 bar. This method allows acrylic acid of
low purity to be used, notably acrylic acid comprising acetic acid and/or dimer
and/or other Michael adducts at a content above 0.5%.
In document US 2004/0236143, the process for producing acrylic esters
can start from a crude grade of acrylic acid, i.e. comprising acetic acid,
30 aldehydes, heavy compounds such as maleic anhydride, this crude grade of
acrylic acid optionally being pretreated by means of an amine compound to
reduce its content of carbonylated compounds, and whose content of dimers is
WO 2015/015100 - 7 - PCT/FR2014/051928
of the order of 0.01-5%. The purification steps result in obtaining a methyl
acrylate with a purety above 99.9%.
In the documents WO 98/52903 and WO 98/52904, provision has been
made to produce butyl acrylate by direct esterification in two reactors placed in
5 series operating at two different temperatures, the temperature of the second
reactor being higher, so as to continue the esterification reaction and to
thermally dissociate the heavy products. The first reactor is fed with pure
reactants (butanol and acrylic acid) and with a recycled stream of heavy
products, in particular a fraction of heavy esters which is separated during the
10 purification of the butyl acrylate. This method requires the use of two reactors
and uses pure acrylic acid as raw material.
The document US 2007/280866 describes the thermal dissociation of
acrylic acid oligomers in the presence of a cleaving reactant, such as butanol.
The dissociation is carried out batchwise. The conditions of the reaction are not
15 suitable for the implementation of an industrial production method continuously.
The problem to be solved by the present invention consists of developing
a method for manufacturing light acrylic esters that does not have the
drawbacks of the existing methods, and permits the continuous preparation of
methyl acrylate or ethyl acrylate of high purity, from a crude grade of acrylic acid
20 having a high content of Michael adducts, designated hereafter in the
description of the invention as being "of crude ester grade". The target objective
is also to achieve a high esterification yield for all of the upgradable acrylic acid
present in the acrylic acid of the crude ester grade, without having recourse to a
specific device for cracking said adducts.
25 This aim is achieved according to the present invention by carrying out,
simultaneously in a single reactor, the thermal dissociation of the Michael
adducts present in a stream of crude acrylic acid and the reaction of
esterification of the acrylic acid present in the stream of crude acrylic acid
and/or generated in situ by said thermal dissociation, the effluent leaving the
30 reactor then being submitted to a chain of treatment and purification leading to
recovery of methyl acrylate or of ethyl acrylate of high purity.
WO 2015/015100 - 8 - PCT/FR2014/051928
One of the aims of the present invention is thus to upgrade, in an
esterification process, all of the valuable products potentially recoverable from a
heavy fraction generated in an acrylic acid synthesis process. Another aim of
the present invention is to reduce the overall amounts of final residue to be
5 incinerated in processes for production of acrylic acid and/or acrylic esters,
under conditions which make it possible to obtain the final residue with a
viscosity acceptable for facilitating its removal.
According to the method of the invention, the Michael adducts generated
during the esterification reaction, such as oligomers of acrylic acid or of
10 acrylate, can be dissociated thermally within the reactor as they are formed,
thus optimizing plant productivity.
The method according to the invention therefore only requires a
moderate number of steps in a simplified apparatus, and produces light
acrylates of high purity with a final residue that is sufficiently fluid to be removed
15 easily.
Summary of the invention
The invention relates firstly to a method for continuous preparation of
light acrylate, by reaction of a light alcohol with a stream of acrylic acid of crude
20 ester grade comprising Michael adducts at a content by weight above 8%,
according to which the following are carried out simultaneously, in a single
reaction zone: the thermal dissociation of the Michael adducts present in said
stream of acrylic acid of crude ester grade, or generated in situ in the reaction
zone, and the reaction of esterification, with a light alcohol, of the acrylic acid
25 present in said stream of acrylic acid of crude ester grade and/or generated in
situ by said thermal dissociation, the effluent leaving the reaction zone then
being submitted to a chain of treatment and purification leading to obtaining a
purified light acrylate, the reaction residue remaining sufficiently fluid to be
withdrawn using a pump.
30 According to the invention, the light acrylate is selected from methyl
acrylate and ethyl acrylate, and the corresponding light alcohol is selected from
methanol and ethanol. Preferably, the light acrylate is methyl acrylate.
WO 2015/015100 - 9 - PCT/FR2014/051928
More precisely, the invention relates to a method for continuous
preparation of light acrylate selected from methyl acrylate and ethyl acrylate by
reaction of the corresponding light alcohol selected from methanol and ethanol
with a stream of acrylic acid of crude ester grade, in the presence of at least
5 one acid catalyst and of at least one polymerization inhibitor, in a reaction zone
comprising a reactor connected to a distillation unit, characterized in that:
the stream of acrylic acid of crude ester grade comprises
oligomers of acrylic acid at a content by weight above 8%;
the molar ratio of alcohol to acrylic acid contained in the form of
10 monomer, dimer or trimer in the stream of acrylic acid of crude
ester grade is between 1.2 and 1.5;
the reactor temperature is above 130°C;
the concentration by weight of acid catalyst is maintained above
2.5% in the reaction mixture;
15 - the concentration of polymerization inhibitor in the reactor is
.adjusted to a value above 50 ppm;
the effluent leaving the distillation unit is submitted to a chain of
treatment and purification leading to obtaining a purified light
acrylate;
20 a residence time of the reaction residue in the reactor longer
than 50 hours is maintained.
The method of the invention is particularly suitable for optimizing the
productivity and the economics of a method of manufacture of light acrylates.
According to one embodiment of the invention, the stream of acrylic acid
25 of crude ester grade is derived from a production process using propylene or
propane as raw material.
According to one embodiment of the invention, the stream of acrylic acid
of crude ester grade is derived from a production process using glycerol or
glycerin as raw material.
30 According to one embodiment, the stream of acrylic acid of crude ester
grade results from a process for the dehydration of lactic acid or ammonium
WO 2015/015100 -10- PCT/FR2014/051928
lactate to give acrylic acid or from a process for the dehydration of 3-hydroxypropionic
acid or of its ammonium salt to give acrylic acid.
Detailed description of the invention
5 The invention will now be described in more detail and non-exhaustively
in the description given hereunder.
In that which follows, the "upgradable acrylic acid" is composed of the
acrylic acid monomer and of the Michael addition derivatives inherent to the
synthesis of acrylic acid, in particular acrylic acid oligomers, which are present
10 in the acrylic acid of crude ester grade.
In that which follows, the expressions "thermal dissociation" and "thermal
cracking" have the same meaning; the expression "between" or "in the range
from" is to be interpreted with limits included.
Reaction residue is understood to mean the fraction enriched in heavy
15 by-products which do not react, accumulated in the reactor, which it is
necessary toperiodically purge from the reactor.
Unless otherwise indicated, the concentrations described in the
description of the invention are concentrations by weight.
20 Stream of acrylic acid of crude ester grade
The method by which the stream of acrylic acid of crude ester grade was
obtained is of no importance for the method according to the invention, provided
it is a stream of acrylic acid having a high content of Michael adducts, notably a
content by weight of oligomers of acrylic acid above 8%, in particular a content
25 of dimers of acrylic acid above 8%, preferably in the range from 8% to 25%, and
a content of trimers of acrylic acid above 0.1%, preferably in the range from 0.5
to 3%.
The stream of acrylic acid of crude ester grade generally has a content of
upgradable acrylic acid above 90%.
30 The stream of acrylic acid of crude ester grade can in addition contain
high-boiling heavy by-products, inherent in the synthesis of acrylic acid, such as
WO 2015/015100 - 1 1 - PCT/FR2014/051928
furfuraldehyde, maleic anhydride, benzaldehyde or benzoic acid, and
polymerization inhibitors.
The content by weight of heavy compounds can typically be:
Furfuraldehyde: 0.03-0.5%
5 Maleic anhydride: 0.3-4%
Benzaldehyde: 0.05-0.5%
Benzoic acid: 0.2-1%
According to one embodiment, the acrylic acid of crude ester grade can
be obtained during purification of crude acrylic acid recovered by means of an
10 absorption column fed with a solvent, such as water or a hydrophobic solvent,
at the outlet of the acrylic acid synthesis reactor.
This purification can notably comprise a first dehydration step, generally
carried out in the presence of a non-water-miscible solvent, in an extraction
column or heteroazeotropic distillation column, followed by a step of removal of
15 the light compounds, in particular acetic acid and formic acid, said step
generally being.called "topping". Finally, a final step of tailing performed by
distillation separates the heavy fraction comprising high-boiling by-products and
Michael adducts, which can be used as acrylic acid of crude ester grade.
Alternatively, the acrylic acid of crude ester grade can be obtained during
20 purification of acrylic acid recovered by means of a dehydration column without
using solvent for extraction or azeotropic distillation, at the outlet of the acrylic
acid synthesis reactor, as described in patent EP 2 066 613. In this type of
process, the acrylic acid contained in the gas from the reaction section is
absorbed in a first column in counter-current with an essentially aqueous liquid
25 stream from the reaction gas, partially condensed and refluxed to the top of the
column. The concentrated stream of acrylic acid recovered at the bottom of the
column is purified in a second column, which carries out additional topping
(removal of light top residues recycled in the first column) and tailing (recovery
of acrylic acid of crude ester grade at the bottom), the purified acrylic acid of
30 industrial grade being withdrawn as a sidestream. The purification steps are
approximately equivalent to those of the process using adsorption in water and
then an azeotropic solvent, the step of topping of the light products being
WO 2015/015100 - 12- PCT/FR2014/051928
carried out in the first column at the same time as the dehydration step, and the
final step of separation of the heavy compounds being carried out in the second
column.
5 According to one embodiment, the acrylic acid of crude ester grade
comprises, or consists of, the heavy fraction separated at the bottom of the last
purification step called tailing in an acrylic acid synthesis process.
According to one embodiment, said stream of acrylic acid of crude ester
grade partly comprises the stream separated at the bottom of the tailing step in
10 an acrylic acid synthesis process.
The operating conditions of the method according to the invention are
adapted so as to dissociate, almost quantitatively, the Michael addition
derivatives and oligomers present in said stream of acrylic acid of crude ester
grade to regenerate the acrylic acid monomer, and carry out the esterification
15 reaction. According to the invention, an acrylic ester is obtained at a yield above
95%, expressed as molar percentage of ester manufactured relative to the
upgradable acrylic acid contained in the acrylic acid of ester grade, essentially
introduced in the form of monomer, dimer or trimer:
Yield = /MM-
\mAAmonomer +,t1AAdnner + mAA(iaier)/
/72
20 where mx = mass of species x and MMX = molar mass of species X
Section for reaction and recovery of the crude acrylic ester
The reaction zone, generally comprising an esterification reactor
connected to a distillation unit, is fed continuously with the stream of acrylic acid
of crude ester grade, a light alcohol (methanol or ethanol), and an esterification
25 catalyst. The average feed flow rate of the reactants is generally between 0.1
and 0.5 T/h per m3 of useful volume of the reactor, preferably between 0.2 and
0.3 T/h per m3 of useful volume of the reactor.
The esterification reaction is carried out in the presence of a molar
excess of alcohol relative to the acrylic acid present in the form of monomer,
30 dimer and trimer.
WO 2015/015100 - 13 - PCT/FR2014/051928
The molar ratio of alcohol to acrylic acid present in the form of monomer
or of oligomer is generally between 1.2 and 1.5, preferably between 1.3 and
1.45. The alcohol is fed into the reactor, alone or mixed with other reactants or
recycled streams, preferably directly in the liquid phase consisting of the
5 efficiently stirred reaction mixture, or through static or dynamic distribution
systems, for example upstream of a pump, permitting dispersion of the reactant
in the form of fine droplets. The known systems of mixers permitting rapid
mixing of 2 liquids or rapid dispersion of a gas in a liquid can be used.
The so-called esterification reaction is carried out in the reactor at a high
10 temperature so as to provide simultaneous dissociation of the Michael adducts,
generally at a temperature above 130°C, preferably in the range from 135°C to
155°C, or from 140°C to 145°C, generally at a pressure from 0.9 to 1.3 bar.
A strong mineral acid, such as sulphuric acid or phosphoric acid, or an
organic acid, such as methanesulphonic acid (MSA), para-toluenesulphonic
15 acid, benzenesulphonic acid, dodecylbenzenesulphonic acid, xylenesulphonic
acid, or mixtures thereof, is generally used as esterification catalyst.
Methanesulphonic acid is preferably used as the esterification catalyst.
The catalyst is advantageously introduced continuously in order to
maintain a concentration in the reactor above 2.5%, preferably in the range from
20 3% to 5% relative to the reaction mixture.
To limit the formation of polymers during the reaction, polymerization
inhibitor(s) is (or are) introduced at the same time as the reactant feed stream.
Examples of polymerization inhibitors that can be used are phenothiazine, or a
derivative of phenothiazine, amine derivatives such as diphenylamine, or
25 diphenylene-amine, phenolic compounds such as hydroquinone, hydroquinone
monomethyl ether, di-tert-butyl para-cresol (BHT), or di-tert-butylcatechol, or Noxyl
compounds such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or
TEMPO derivatives, such as 4-hydroxy-TEMPO, manganese salts, or copper
compounds such as copper carbamates, alone or mixtures thereof in all
30 proportions.
Preferably, phenothiazine or a mixture of phenothiazine and
hydroquinone is used as polymerization inhibitor.
WO 2015/015100 - 14- PCT/FR2014/051928
The polymerization inhibitor can be introduced in the reactor and/or at
the top of the distillation unit in the reaction zone.
The polymerization inhibitor is introduced in the reaction zone in such a
way that the concentration of polymerization inhibitor in the reactor is
5 maintained at a value above 50 ppm, preferably above 100 ppm, more
preferably at a value between 300 ppm and 1000 ppm. The concentration of
inhibitor can be monitored by analysis, for example by liquid chromatography.
This level of concentration makes it possible, on the one hand, to
eliminate any loss of efficiency of the polymerization inhibitor, on the other
10 hand, to maintain an adequate efficiency of the catalyst, due to its possible
reactions with the acid catalyst, and furthermore to reduce the viscosity of the
final reaction residue in the reactor.
It may be advantageous to add phenothiazine as polymerization inhibitor
at different points of the reaction zone, for example in the reactor, in the
15 distillation unit, and at the top reflux of the distillation unit.
It may,be advantageous to introduce an oxygen-containing gas in the
reaction zone, especially when the polymerization inhibitor contains a phenolic
compound.
The reaction is carried out in the reactor for a time such that the recovery
20 of products that can be upgraded generated by thermal dissociation and the
yield in esterification are very high, and such that there is a very small amount
of reaction residue to be removed.
This corresponds to a residence time of the reaction residue in the
reactor longer than 50 hours, preferably longer than 100 hours, the residence
25 time being expressed as the average time during which the reaction residue is
held in the reactor before being purged, calculated from the ratio of the volume
of reaction mixture to the purge rate. In the reaction conditions, the reaction
residue, enriched with heavy products coming essentially from the reaction
stream of acrylic acid, remains sufficiently fluid to be withdrawn using a pump
30 and to be able to be sent to a thermal oxidizer for removal and optional recovery
of energy, or to any other appliance for the purpose of final upgrading.
WO 2015/015100 - 15 - PCT/FR2014/051928
Preferably, the dynamic viscosity of the residue measured at 100°C
should be less than 200 cP, measured with a viscosimeter, such as, for
example, a Brookfield rotary viscosimeter. It may be advantageous to add a
viscosity depressant, for example methanol or acetic acid, at a content in the
5 range from 10 to 30%, to facilitate the subsequent pumping operations after
storage or transport at lower temperature for its removal or recycling as heattransfer
fluid, for example.
Advantageously, the reaction residue is taken at the temperature of the
esterification reaction permitting thermal dissociation of the Michael adducts
10 and it is sent to storage, maintained at a temperature near the reaction
temperature to within 10°C. The gases vented from the storage tank are
collected and recycled to the reactor, thus permitting additional recovery of
acrylic ester from the residue during this storage phase before removal.
The stages of reaction (esterification and cracking) and recovery
15 (removal and purification of the reaction products) are closely related. The
reaction conditions control the composition of the reaction mixture in the
reactor, which conditions the formation of light acrylate/water and light
acrylate/light alcohol azeotropic mixtures. These mixtures are purified in the
distillation unit and also depend on the composition of the reflux imposed at the
20 top of said unit. In return, the mixtures formed have a significant impact on the
effectiveness of the expected reaction.
For example, during the esterification of acrylic acid by methanol to
produce methyl acrylate, a first methyl acrylate/methanol azeotropic mixture
relatively rich in methanol (52% in theory) coexists with a second methyl
25 acrylate/water azeotropic mixture. The first azeotropic mixture has a negative
impact on the progression of the reaction by favouring the removal of a reactant
(the alcohol) from the reaction mixture. The second azeotropic mixture has a
positive effect since it makes it possible to displace the esterification equilibrium
but its removal is put at a disadvantage relative to the first mixture due to a
30 boiling point greater by 9°C and as a result of its relative dearth in water (9% in
theory).
WO 2015/015100 - 16- PCT/FR2014/051928
Consequently, the reaction conditions tending to favour the formation of
the second azeotropic mixture (methyl acrylate/water) rather than the first
(methyl acryiate/methanol) make it possible to improve the yield of the reaction.
They are carried out in a reaction zone comprising a reactor and a
5 distillation unit permitting the simultaneous removal of the water produced by
the esterification reaction, of the ester manufactured, of the excess unreacted
alcohol and also of small amounts of residual impurities or impurities generated
by the reaction, in order to form a mixture referred to as crude ester.
The reaction zone can be a reactor, the gas phase of which is connected
10 to a distillation column, or a reactive column consisting, in the bottom part, of a
reaction section containing the liquid reaction mixture and, in the top part, of a
distillation section.
The reactor can be any type of stirred reactor known to a person skilled
in the art. Preferably, the reactor or the reaction section of the reactive column
15 are fed continuously with the mixture of reactants, a portion of the reaction
mixture is withdrawn and reheated in an external exchanger and the reheated
stream is recycled to the reactor using a pump.
Preferably, the reactor, the exchanger, the pump, the transfer lines and
any equipment in contact with the reaction mixture are made of a corrosion-
20 resistant material or are coated with corrosion-resistant materials.
The distillation column or the distillation section of the reactive column
can be composed of plates and/or random packings and/or stacked packings of
any type available for the rectification of mixtures and suitable for the distillation
of polymerizable compounds. It is equipped with a condenser and with a liquid
25 feed at the top, which provides a liquid reflux in the column.
The number of plates and/or the height and the type of packing of the
column are chosen so as to limit the entrainment of unreacted acrylic acid in the
effluent recovered at the column top.
Optionally, downstream of the condenser, a decanter can be installed in
30 order to separate an organic phase comprising most of the ester and traces of
water and of unreacted alcohol, and an aqueous phase comprising most of the
WO 2015/015100 - 17- PCT/FR2014/051928
water generated by the reaction and of the unreacted alcohol, and also small
amounts of ester.
The removal of the water generated by the esterification is carried out
essentially by entrainment in the form of an azeotropic mixture with the light
5 ester manufactured. In order to limit the entrainment of the unconverted light
alcohol by the azeotropic mixture with the ester manufactured, which would
have the consequence of a reduction in the reaction yield, an excess of ester is
maintained, so as to promote the esterification reaction by removal of the water
formed. This is carried out by virtue of a reflux of an essentially water-free light
10 acrylate phase which, expressed as flow rate by mass relative to the flow rate
by mass of the feed of reactants to the reaction zone, is kept above 0.8,
preferably between 1 and 2.5, indeed even between 1 and 1.2.
Said water-free refluxed light acrylate, preferably containing less than 5%
by weight of water, may come from a portion of the organic phase separated in
15 the decanter and/or from a fraction separated during purification of the distilled
effluent, for example at the bottom of a distillation column for light compounds
or at the bottom of a column for separation of heavy compounds and thus
makes it possible to recycle the light acrylate present in these fractions.
The distilled effluent comprising the crude ester mixture is submitted,
20 either after decanting or directly, to a chain of treatment and purification leading
to obtaining a purified light acrylate.
According to the method of the invention, the light acrylate present in the
crude ester mixture is produced at a yield above 95%, generally in the range
from 95% to 98%, expressed in number of moles of light acrylate produced
25 relative to the number of moles of acrylic acid introduced in the form of acrylic
acid monomer, dimer or trimer.
Purification section
Each of the phases (organic and aqueous) obtained by decanting the
30 crude ester is subjected to a purification treatment, targeted at recovering the
purified ester essentially present in the organic phase by removal of water and
the impurities present at a low concentration and by recovering the alcohol
WO 2015/015100 - 18 - PCT/FR2014/051928
which is found therein, and in recovering, for recycling purposes, the alcohol
and the low concentrations of ester which are present in the aqueous phase.
Advantageously, this is carried out in a liquid/liquid extraction stage
applied to the crude ester mixture after decanting, so as to increase the
5 concentration of alcohol in the aqueous phase and to reduce this concentration
in the organic phase, and thus to improve the recovery of the alcohol for the
purposes of recycling to the reaction stage.
The extraction column is fed at the bottom with the organic phase
resulting from the decanting and at the top with the aqueous stream recovered
10 at the bottom of the column for recovery of light alcohol.
The aqueous phase obtained at the bottom of the extraction column,
enriched in alcohol, is advantageously partially sent to a distillation column for
recovering, at the top, the light alcohol, which is then recycled to the reaction,
and, at the bottom, an aqueous phase depleted of light alcohol that can be used
15 as extraction solvent fed into the top of the extraction column. A portion of this
aqueous phase is removed.
Alternatively, the crude ester effluent distilled from the reaction zone may
be sent directly to the extraction column, without preliminary separation in a
decanter, thus minimizing the equipment necessary for treatment of the distilled
20 effiuent, and facilitating control of the operations of separation and recycling of
the residual alcohol and/or of the scrubbing water.
The scrubbed organic phase is thus essentially free from light alcohol
and comprises the light acrylate required, but still contains light by-products and
heavy by-products as impurities.
25 The scrubbed organic phase is sent to a first distillation column for
removing the light by-products that the light acrylate contains, notably including
traces of alcohol, acetates, dimethyl ether or diethyl ether; the latter are
withdrawn from the top of said column, partly to be recycled to the reaction zone
or to the extraction step, and partly removed.
30 At the bottom of said distillation column, the light acrylate is recovered,
still comprising heavy impurities including notably methyl or ethyl
alkoxypropionate, small amounts of methyl or ethyl acryloxypropionates,
WO 2015/015100 - 19 - PCT/FR2014/051928
dimethyl or diethyl maleate and methyl or ethyl benzoate and polymerization
inhibitors.
This stream is sent to a separating column for final purification. At the
bottom of the separation column, a light acrylate with high concentration of
5 heavy impurities is recovered, which is partly removed, partly recycled as reflux
to the distillation unit in the reaction zone.
At the top of the separation column, a light acrylate is recovered with
purity above 99%.
The advantages of the invention are now illustrated without implied
10 limitation in the following examples.
EXAMPLES
Example 1
The experimental set up is composed of a stirred reactor with a useful
15 volume of 1 litre heated by recirculation in its jacket of hot oil at regulated
temperature, surmounted by a distillation column. The reactor is equipped with
an inlet for the feeding, via a pump, of the mixture of reactants, with a separate
feed of 70% MSA catalyst in water via a second pump, with a temperature
measurement in the liquid and with a withdrawal point at the bottom. The
20 column is equipped with 7 perforated plates comprising weirs, with an inlet at
the column top for feeding the reflux via a third pump, with a vertical condenser
placed over the exiting gas phase at the column top, fed via a fourth pump with
a water mixture comprising 2% hydroquinone, with an intermediate tank
equipped with a level control and with a receiving tank/decanter withdrawing,
25 using a fifth pump, the crude mixture of distilled ester.
In a first phase lasting 3 weeks during which the operating conditions and
the composition of the mixture change, the residue mixture rich in heavy
compounds is formed by gradual enriching of the reaction mixture, in order to
finally achieve the conditions which make it possible to simultaneously carry out
30 the esterification of the acrylic acid and the thermal cracking of the oligomers
present in the reactive acrylic acid and generated during this operation.
WO 2015/015100 - 20 - PCT7FR2014/051928
On conclusion of this first enriching phase, the operating conditions and
compositions are stabilized; the MSA concentration measured in the reaction
mixture is 4.5%.
A feed mixture, consisting of 57.9% of acrylic acid of crude ester grade,
5 of 32.4% of methanol, and of 7.6% of methyl acrylate and of 2.1% of water
(these 2 compounds resulting from the recycling of the stream originating from
subsequent stages) is fed to the reactor with a flow rate of 300 g/h. The acrylic
acid of crude ester quality is composed of 84.4% of acrylic acid, 12.8% of
acrylic acid dimer and 0.6% of acrylic acid trimer, 0.5% of phenothiazine, 0.3%
10 of hydroquinone and 1.5% of other compounds. The 70% MSA catalyst in water
is added with a flow rate of 0.97 g/h. At the top of the distillation column, pure
methyl acrylate comprising 0.1% of phenothiazine is sent as reflux with a flow
rate of 330 g/h.
The reaction is carried out for 196 h at a temperature of 140°C under
15 these conditions, with the following operating parameters:
• the.alcohol/upgradable acrylic acid (sum of acrylic acid monomer,
dimer and trimer) molar ratio is 1.3,
• the feed flow rate per unit of useful reaction volume is 0.3 T/h/m3,
• the reflux/feed of the reactants flow rate ratio is 1.1,
20 • the mean residence time of the residue in the reactor, calculated
by the ratio of the volume of reactor occupied to the residue purge
flow rate, is 116 h,
• the concentration of MSA present in the reaction mixture is 4.5%,
determined by measurement of the acidity,
25 • the concentration of phenothiazine, measured by analysis, in the
reactor is 0.05%.
The crude ester mixture condensed at the column top decants into
2 phases which are separated and separately analysed. Over a withdrawal
period of 16 h, 9532 g of organic phase, composed of 3.2% of methanol, 5.15%
30 of water and 0.26% of acrylic acid, the remainder being essentially methyl
acrylate, and 458 g of aqueous phase, consisting of 13.1% of methanol; 7.33%
WO 2015/015100 - 21 - PCT/FR2014/051928
of methyl acrylate and 0.06% of acrylic acid, the remainder being essentially
composed of water, are obtained.
The reaction yield, determined by the molar ratio of methyl acrylate
produced (subtraction made of the methyl acrylate fed via the reflux and the
5 feed stream) relative to the upgradable acrylic acid fed in (sum of acrylic acid
monomer, dimer and trimer), is 95.2%. This yield is also the mean yield
obtained during 1 week of operation.
The dynamic viscosity of the reaction residue, measured using a
Brookfield CAP1000+ viscosimeter at a temperature of 100°C is 150 cP.
10
Example 2
The reactor is operated in the same way as during Test 1, apart from the
following changes:
• reaction temperature: 143°C,
15 • alcohol/upgradable acrylic acid (sum of acrylic acid monomer,
dimer and trimer) molar ratio: 1.4,
• MSA concentration: 3.3%,
• mixture sent as reflux at the column top, consisting of methyl
acrylate comprising 0.5% of methanol, 2.6% of methyl acetate and
20 2.4% of water, so as to take into consideration the recycling of a
mixture resulting from the following stages of the process,
comprising a few impurities.
Under these conditions debased by the recycling of impurities in the
25 reflux mixture, which are kept constant for 200 h, the mean residence time of
the residue in the reactor is 114 h and the mean reaction yield over a period of
operation of 100 h reaches 97.8%.
The dynamic viscosity of the reaction residue, measured at a
temperature of 100°C, is 160 cP, with a measured phenothiazine concentration
30 of 0.05%.
Example 3 (comparative)
WO 2015/015100 -22- PCT/FR2014/051928
The reactor is operated for 53 h in the same way as Test 1, with the
following operational parameters:
• composition of the acrylic acid of crude ester grade: 88.8% of
acrylic acid monomer, 9% of acrylic acid dimer, 0.3% of acrylic
5 acid trimer, 0.27% of phenothiazine and 0.19% of hydroquinone,
• feed flow rate per unit of useful reaction volume is 0.3 T/h/m3,
• alcohol/upgradable acrylic acid (sum of acrylic acid monomer,
dimer and trimer) molar ratio is 1.3,
• reflux flow rate/feed flow rate of the reactants ratio is 1.1,
10 • MSA concentration: 11%,
• residence time of the residue greater than 300 h.
The reaction temperature is reduced to 128°C and the viscosity of the
residue, measured at 100°C, is 150 cP.
Despite the high concentration of catalyst deployed, the mean yield
15 calculated during the operation is only 92.7%.
Example 4 (comparative)
The reactor is operated for 47 h in the same way, with the same feed
stream and under the same conditions as Test 3, apart from:
20 • an alcohol/upgradable acrylic acid (sum of acrylic acid monomer,
dimer and trimer) molar ratio of 1.45,
• an MSA concentration of 10%,
• a reaction temperature of 135°C.
By virtue of the higher reaction temperature than that of Test 3, the mean
25 yield calculated during the operation is 97%. On the other hand, the viscosity of
the reaction mixture, measured at 100°C, is much greater than 250 cP (limit of
measurement of the viscosimeter) making it very difficult to empty the reactor,
and solids could be observed during this emptying operation. The concentration
of phenothiazine measured in the reaction mixture is less than 10 ppm.
30
Example 5 (comparative)
WO 2015/015100 - 23 - PCT/FR2014/051928
The reactor is operated for 62 h in the same way as during Test 1, with
the following operational parameters:
• composition of the acrylic acid of crude ester grade: 75.5% of
acrylic acid monomer, 18.8% of acrylic acid dimer, 1% of acrylic
5 acid trimer, 0.85% of phenothiazine and 0.42% of hydroquinone,
• feed flow rate per unit of useful reaction volume is 0.3 T/h/m3,
• alcohol/upgradable acrylic acid (sum of the acrylic acid monomer,
dimer and trimer) molar ratio is 1.3,
• reflux flow rate/feed flow rate ratio is 1.1,
10 • reaction temperature: 140°C.
The concentration of MSA in the reaction mixture is reduced to 2.2%.
The mean yield calculated during the operation reaches only 85.9%. Due
to the low reactivity of the reaction, the purge flow rate for providing a constant
level in the reactor is increased, and the residence time which results from
15 these debased conditions is 34 h.
Example 6 (comparative)
The reactor is operated for 46 h in the same way as during Test 1, with
the following operational parameters:
20 • composition of the acrylic acid of crude ester grade: 88.8% of
acrylic acid monomer, 9% of acrylic acid dimer, 0.3% of acrylic
acid trimer, 0.27% of phenothiazine and 0.19% of hydroquinone,
• feed flow rate per unit of useful reaction of volume is 0.3 T/h/m3,
• reflux flow rate/feed flow rate ratio is 1.1,
25 • reaction temperature: 140°C,
• MSA concentration: 4%.
The alcohol/upgradable acrylic acid (sum of acrylic acid monomer, dimer
and trimer) molar ratio is reduced to 1.1.
The mean residence time of the residue in the reactor is 87 h and the
30 mean yield calculated during the operation is only 87.5%.

CLAIMS
1. Method for continuous preparation of light acrylate selected from methyl
acrylate and ethyl acrylate, by reaction of the corresponding light alcohol
5 selected from methanol and ethanol with a stream of acrylic acid of crude ester
grade comprising Michael adducts at a content by weight above 8%, according
to which the following are carried out simultaneously in a single reaction zone:
thermal dissociation of the Michael adducts present in said stream of acrylic
acid of crude ester grade, or generated in situ in the reaction zone, and the
10 reaction of esterification, with a light alcohol, of the acrylic acid present in said
stream of acrylic acid of crude ester grade and/or generated in situ by said
thermal dissociation, the effluent leaving the reaction zone then being submitted
to a chain of treatment and purification leading to obtaining a purified light
acrylate, while the reaction residue remains sufficiently fluid to be withdrawn
15 using a pump.
2. Method for continuous preparation of light acrylate selected from methyl
acrylate and ethyl acrylate by reaction of the corresponding light alcohol
selected from methanol and ethanol with a stream of acrylic acid of crude ester
20 grade, in the presence of at least one acid catalyst and at least one
polymerization inhibitor, in a reaction zone comprising a reactor connected to a
distillation unit, characterized in that:
the stream of acrylic acid of crude ester grade comprises
oligomers of acrylic acid at a content by weight above 8%;
25 - the molar ratio of alcohol to acrylic acid contained in the form of
monomer, dimer or trimer in the stream of acrylic acid of crude
ester grade is between 1.2 and 1.5, preferably between 1.3 and
1.45;
the reactor temperature is above 130°C, preferably between
30 135°Cand 155°C;
the concentration by weight of acid catalyst is maintained above
2.5%, preferably between 3% and 5%, in the reaction mixture;
WO 2015/015100 - 25 - PCT/FR2014/051928
the concentration of polymerization inhibitor in the reactor is
adjusted to a value above 50 ppm, preferably above 100 ppm;
the effluent leaving the distillation unit is submitted to a chain of
treatment and purification leading to obtaining a purified light
5 acrylate;
a residence time of the reaction residue in the reactor longer
than 50 hours, preferably longer than 100 hours, is maintained.
3. Method according to Claim 1 or 2, characterized in that the light acrylate
10 is methyl acrylate.
4. Method according to any one of the preceding claims, characterized in
that the acrylic acid is from a production process using propylene as raw
material.
15
5. Method, according to any one of Claims 1 to 3, characterized in that the
acrylic acid is from a production process using glycerol or glycerin as raw
material, or from a process for the dehydration of lactic acid, of 3-hydroxypropionic
acid or of their ammonium salts.
20
6. Method according to any one of the preceding claims, characterized in
that the stream of acrylic acid of crude ester grade is obtained during
purification of crude acrylic acid recovered by means of an adsorption column
fed with a solvent, such as water or a hydrophobic solvent, at the outlet of the
25 acrylic acid synthesis reactor.
7. Method according to any one of Claims 1 to 5, characterized in that the
stream of acrylic acid of crude ester grade is obtained during purification of
acrylic acid recovered by means of a dehydration column without using solvent
30 for extraction or azeotropic distillation, at the outlet of the acrylic acid synthesis
reactor.
WO 2015/015100 - 26 - PCT/FR2014/051928
8. Method according to any one of the preceding claims, characterized in
that the stream of acrylic acid of crude ester grade comprises, or consists of,
the heavy fraction separated at the bottom of the last purification step called
tailing in an acrylic acid synthesis process.
5
9. Method according to any one of the preceding claims, characterized in
that a light acrylate phase comprising less than 5% by weight of water is sent as
reflux to the reaction zone with a flow rate by mass above 0.8, preferably
between 1 and 2.5, expressed relative to the feed flow rate by mass of the
10 reactants.
10. Method according to any one of Claims 2 to 9, characterized in that the
catalyst is methanesulphonic acid.
11. Method according to any one of Claims 2 to 10, characterized in that
phenothiazine.-or a mixture of phenothiazine and hydroquinone is used as
polymerization inhibitor.
12. Method according to any one of the preceding claims, characterized in
that the dynamic viscosity of the reaction residue, measured at 100°C with a
Brookfield rotary viscosimeter, is less than 200 cp.