Patent Application
20090203938
The present invention belongs to the field of the synthesis, purification and regeneration of (meth)acrylic monomers, such as acrylic acid and methacrylic acid as well as C1-C8 alkyl acrylates and methacrylates. More particularly, the present invention relates to the suppression or at least the limitation of the phenomena of fouling due to the deposition of insoluble polymers and/or of insoluble heavy products in plants used for the abovementioned synthesis, purification or regeneration operations.
Mention may in particular be made, as reactions for the synthesis of (meth)acrylic monomers, of:
It is in fact well known that one of the problematic points in the manufacture of (meth)acrylic monomers arises from the fact that these compounds are relatively unstable and easily change in the direction of the formation of polymers. This change, caused by a radical reaction promoted by the effect of the temperature, is thus particularly easy in the stages of synthesis and of purification of these monomers, for example during distillation operations. The usual consequence of this process is the deposition, in the items of equipment of a plant, of solid polymers which end up causing blockages and make it necessary to shut down the plant in order to clean it, which proves to be difficult and expensive in unproductive downtime.
Polymerization inhibitors are added to the streams in order to reduce these disadvantages. Numerous polymerization inhibitors are used conventionally in processes for the purification of (meth)acrylic monomers. Mention may be made, among them, of phenolic compounds, phenothiazine and its derivatives, manganese salts, thiocarbamates and dithiocarbamates, N-oxyl compounds, amino compounds and nitroso compounds, examples of these various compounds being indicated later.
A large majority of polymerization inhibitors are compounds which are heavier than the (meth)acrylic monomers to be purified. In the distillation stages, due to their low volatility, they are only to a very slight extent entrained in the vapour phases in equilibrium with the liquid phases of the rectification plates and consequently cannot carry out their role of protecting the monomer against polymerization in the upper parts of the columns. For this reason, these inhibitors are generally introduced at the level of the top items of equipment of columns, condensers, and the like, which can be the site of a liquid-vapour equilibrium resulting in the condensation of streams rich in (meth)acrylic monomer.
The effectiveness of these polymerization inhibitors, used alone or as a mixture, is generally enhanced when they are used in combination with the introduction of oxygen or of a gas stream comprising oxygen at the column bottom. These polymerization inhibitors can be introduced as they are, for example when they are liquid, or in solution in a solvent and, preferably, in solution in the monomer itself.
Despite the use of polymerization inhibitors, the operations for the purification of monomers are often accompanied by the formation of polymers, which are soluble or insoluble in the medium according to the nature of the monomer and the composition of the medium. In the most troublesome cases, the polymer formed is insoluble in the medium and precipitates in the form of a solid deposit.
The precipitation of the polymer from the medium occurs in particular when the length of the polymer chain is sufficient or when polyfunctional compounds generate bridges between certain units of the polymer by a process known under the name of crosslinking.
Another source of fouling of the plants by solid deposition is the presence of heavy compounds generated during the reaction stage of synthesis of the monomer.
For example, the manufacture of (meth)acrylic monomers is accompanied by the formation of compounds resulting from a Michael addition reaction of a molecule comprising a labile hydrogen atom to the double bond of a (meth)acrylic compound:
with:
These addition compounds may, depending on the medium in which they are present and their molar masses (proportional to the number of units n), be insoluble in this medium.
The formation of insoluble polymers or of insoluble heavy compounds can take place rapidly at the spot itself where the polymerization process or the Michael addition reaction are initiated, if the polymer or the heavy compound are insoluble in the medium, or can take place during a stage of the purification process in which these products, dissolved beforehand, become insoluble by modification of the composition of the original medium, for example during an operation in which the heavy compounds are concentrated by separation of the light compounds or during the removal of a polar component responsible for their solubilization.
For example, problems of deposits of polymers frequently occur during the steps in which the monomers are dried. In conventional processes for the recovery and purification of acrylic acid and methacrylic acid, which involve a stage of absorption in water (case of the manufacture of acrylic acid by oxidation of propylene or propane, or of methacrylic acid by oxidation of isobutene or tert-butyl alcohol), the monomers are re-encountered, in an intermediate stage, as a mixture in a medium rich in water (20 to 40% water). In the case of the production of methacrylic acid by hydrolysis of methacrylamide sulphate, the crude methacrylic acid exiting from this stage comprises 1 to 5% water. As water is a good solvent for polymer chains exhibiting a relatively low degree of crosslinking, the insoluble polymers generated in the prior stages, which are dissolved in the starting medium, precipitate during the step of removal of the water (water content <0.5%) and are deposited in the reboilers and the column bottoms.
Problems of troublesome deposits can also occur when attempts are made to recover the maximum of monomer (light compound) from a mixture rich in heavy compounds obtained at the bottom of the final columns for the purification of the monomers. The enriching of the medium in heavy compounds (polymers and addition derivatives) results in the precipitation of solids, which makes it necessary to shut down the plant in order to clean it. In order to prevent this stage from being reached, it is necessary to limit the degree of recovery of monomer. As the monomer could be put to use, this results in a loss.
Likewise, the treatment of the heavy waste from the distillation of the monomers, in order to recover therefrom light fractions which can be exploited by thermal and/or catalytic cracking, results in major phenomena of fouling which are very disadvantageous industrially, which indeed even render the operation impossible in some cases.
The documents Patents EP 839 790, DE 19 851 984 and U.S. Pat. No. 4,440,625 teach the use of derivatives of alkyl- or arylsulphonic acids as antifoulants for the purpose of dispersing the acrylic polymers or the heavy addition derivatives formed during the synthesis or purification of (meth)acrylic monomers, DE 19 851 984 also envisaging the case of the tailing or cracking operations on residues from the distillation of (meth)acrylic monomers. However, the tests carried out in dynamic mode with this known family of products by the Applicant Company have not given satisfactory results from the viewpoint of industrial application.
The Applicant Company has thus sought to solve the problems of fouling due to the deposition of solid materials of polymeric origin or resulting from addition reactions to (meth)acrylic double bonds and has discovered a family of compounds soluble in the medium comprising the (meth)acrylic monomer and exhibiting the property of effectively preventing or limiting the abovementioned depositions in items of equipment for the synthesis, purification or regeneration of the (meth)acrylic monomer.
A subject-matter of the present invention is thus first the use of at least one compound of formula (I):
in which:
The compounds of formula (I) have a dispersing property and are also referred to subsequently as dispersants.
The compounds of formula (I) are chosen in particular from those in which R1 and R2 each independently represent the
radical where p is an integer from 4 to 12, preferably 8 or 9, and q is an integer from 4 to 50, preferably 6 to 20; and R3 represents a hydrogen atom or a counterion; and/or
those in which R1 represents the
radical
where R is an integer from 4 to 12, preferably 8 or 9, and s is an integer from 4 to 50, preferably from 6 to 20; and R2 and R3 each independently represent a hydrogen atom or a counterion.
Mention may be made, as counterion coming within the definition of R2 and R3, of those resulting from the neutralization of the OH functional group, in the case where R2 and/or R3=H, by alkanolamines and alkali metal or alkaline earth metal hydroxides. Mention may be made, as specific examples, of N+(CH2CH2OH)3, Na+ and K+.
The compound or compounds (I) can be introduced as is into the medium comprising the (meth)acrylic monomer. They can also be introduced into the medium comprising the (meth)acrylic monomer in solution in a solvent, the said solvent being chosen in particular from aromatic solvents, such as phthalates, for example diisononyl phthalate and dioctyl phthalate, and glycol dimethyl ethers, or also in solution in the (meth)acrylic monomer.
The compound or compounds (I) can be introduced at a concentration of 0.01% to 1% by weight, in particular of 0.05 to 0.5% by weight, into the medium comprising the (meth)acrylic monomer to be prepared, purified or regenerated.
The medium comprising the (meth)acrylic monomer can also include at least one polymerization inhibitor in a proportion in particular of 0.01% to 5% by weight, especially of 0.05% to 3% by weight, with respect to the medium comprising the (meth)acrylic monomer to be prepared, purified or regenerated, it being possible for the polymerization inhibitor or inhibitors to be chosen from:
in which Z1 and Z2 each independently represent an alkyl, aryl, alkylaryl or arylalkyl radical, an example being N,N′-di(sec-butyl)-para-phenylenediamine.
The invention makes it possible to reduce depositions of solid materials in plants for the purification of (meth)acrylic monomers and thus to reduce the frequency of shutdowns necessary for the cleaning of columns, exchangers, reboilers, condensers, reactors, and the like.
A second advantage may be to reduce the amount of stabilizers introduced into the items of purification equipment, to decrease their cost.
The recovery with better yields of valuable products (monomers, reactants) in the concentration operations, or thermal or catalytic cracking operations, constitutes another advantage.
The operations affected by the present invention are very diverse and are in particular the following:
In the case of the use according to the invention where the operation involves a distillation column, the compounds (i) are preferably introduced at the column top.
The present invention will now be described using Examples, Reference Examples and Comparative Examples.
In all these examples, the percentages are expressed by weight and the following abbreviations are used:
The dispersant used in the Examples of the invention is a neutralized mixture of ethoxylated alkylphenol mono- and diphosphate, corresponding to the definition of the formula (I) as above, in solution in an aromatic solvent.
Use is made of a glass distillation device comprising a distillation column, a reboiler, a condenser and a reflux receiver.
The column, with a diameter of 38 mm, is packed with a Multiknit component made of 316L stainless steel with a height of 14 cm. The vapours are generated at the bottom in a thermosiphon reboiler heated by electrical resistances with a volume of 200 ml into which acrylic acid stabilized by 0.2% of PTZ and 0.2% of PMP is introduced. A continuous throughput of air is additionally provided into the liquid of the reboiler. The vapours are condensed at the top through a water-cooled reflux condenser and collected in a reflux receiver.
The assembly is placed under a reduced pressure of 1.6×104 Pa (120 mmHg) and the temperature measured at the bottom is 89° C. The throughput of liquid condensed at the top is 540 g/h.
PMP and PTZ (10 ppm of each of the stabilizers, with respect to the throughput of distillate) and dispersant (0.3%, with respect to the throughput of distillate), in solution in acrylic acid, are continuously introduced into this receptacle.
The liquid collected in the reflux receiver is returned via a pump to the column top, a constant level being maintained in this receptacle. The level of liquid in the reboiler is kept constant by withdrawing the excess liquid.
After operating for 3 hours, the weight of polymer deposited in the Multiknit packing is 6.5 g.
A Reference Example is carried out under the same conditions as those of Example 1 but without the dispersant: the weight of polymer recovered in the packing is 25.1 g.
A mixture of acrylic acid comprising 5% of water is introduced into the same assembly as Example 1, except for the column packing, composed this time of a Multiknit packing component with a height of 4 cm and of glass Raschig rings over a height of 11 cm. Feeding with acrylic acid comprising the stabilizers PMP and PTZ and the dispersant is provided so as to convey, at the column top, 10 ppm of each of the polymerization inhibitors and 0.1% of dispersant, with respect to the throughput of distillate measured (740 g/h).
After operating for 3 hours, the weight of polymer deposited in the Multiknit packing is 4 g.
A Reference Example is carried out under the same conditions as those in Example 2 but without the dispersant: the weight of polymer recovered in the packing is 28 g.
A Comparative Example is carried out under the same conditions as those of Example 2 but with pure acrylic acid and dodecylbenzenesulphonic acid as dispersant: the weight of polymer recovered in the packing is 15.9 g.
Pure methacrylic acid, replacing acrylic acid, is introduced into the assembly and under the conditions of Example 2.
No solid deposit is observed in the packing of the column.
After testing for 3 hours, in the presence of the dispersant (0.1%, with respect to the throughput of distillate), the liquid solution present in the reboiler is cloudy, with a few fine solids in suspension. After emptying the liquid, a few solid particles remain attached to the wall of the reboiler and disappear by simple washing with water.
Under the same conditions as in Example 3 but in the absence of the dispersant, the solid particles are agglomerated into clusters, partly in suspension and partly stuck to the wall. The deposits attached to the wall, in a greater amount than Example 3 carried out with dispersant, are not removed by washing with water.
This example describes the application of the present invention to the recovery of valuable products by thermal cracking from a stream obtained at the bottom of a column for removing the heavy compounds of a process for the manufacture of butyl acrylate.
The mixture, composed of:
Recovery takes place in two stages: removal of the heaviest compounds (in particular the polymers) in an evaporation stage (Example 4a of the invention and Reference Example 4a), followed by cracking of the evaporated fraction in the presence of a sulphuric acid catalyst (Example 4b of the invention and Reference Example 4b).
The operation is carried out under a reduced pressure of 6.7×103 Pa (50 mmHg) at a temperature of 146° C. measured in the reboiler. At the top, 189 g/h are recovered of a mixture composed of:
In a first test (Reference Example 4a), the mixture is introduced without addition of dispersant. After operating for 5 hours, the wall of the reboiler is completely covered with a solid deposit which is very difficult to remove, which makes this method of recovering in value unusable industrially.
In a second test (Example 4a of the invention), 0.04% of the dispersant is added to the feed medium and it is found that the wall of the reboiler is clean after operating for 10 hours.
In a first test (Reference Example 4b), the mixture is introduced without addition of dispersant. After operating for 2 hours, the wall of the reactor is covered with a solid deposit which is very difficult to remove.
In a second test (Example 4b of the invention), 0.12% of the dispersant is added to the feed medium and it is found that the wall of the reboiler is clean after operating for 6 hours.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0410687 | Oct 2004 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2005/002480 | 10/7/2005 | WO | 00 | 4/11/2007 |
