Distillation: processes – separatory – Addition of material to distilland to inhibit or prevent... – For inhibiting or preventing a polymerization reaction
Reexamination Certificate
1999-04-14
2002-03-05
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
Addition of material to distilland to inhibit or prevent...
For inhibiting or preventing a polymerization reaction
C203S059000, C203S029000, C203S071000, C203SDIG002, C562S600000
Reexamination Certificate
active
06352619
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improvement to the standard process for purifying acrylic acid, in particular to steps directed towards obtaining a monomer of very high purity, intended for the manufacture of technical-grade polymers of very high molecular masses.
BACKGROUND OF THE INVENTION
The main route for the synthesis of acrylic acid which is used industrially today involves a catalytic oxidation of propylene, generating acrolein as an intermediate. This reaction also produces side impurities, among which are carbonyl impurities such as aldehydes, for instance furfuraldehyde (or furfural) and benzaldehyde, in addition to the acrolein. These compounds are a great nuisance, even at extremely low contents, since they make it impossible to manufacture polymers of high molecular masses, which are desired in many applications.
The present invention thus relates to a process for purifying acrylic acid, and more particularly to a step of this process in which the carbonyl impurities present in the general flow (aldehydes and ketones) are removed by addition of a compound such as hydrazine during a distillation step, the hydrazine-type compound forming, with these impurities, heavy compounds which are flushed away at the foot of the distillation column.
The standard processes for purifying acrylic acid involve successive distillation steps directed towards removing the light impurities (water, acrolein, acetic acid, etc.) and the heavy impurities (maleic acid, acryloxypropionic acid, etc.) which are formed during the reaction or purification steps.
During these steps of purification by distillation, it often occurs that polymers are formed, under the effect of heat, this occurring even under operating conditions directed, for example by carrying out the distillations under reduced pressure, towards reducing the temperature of the flows rich in polymerizable monomers. In the case of acrylic acid, since the polymer is insoluble in the monomer, it precipitates in the medium and gives rise to deposits on certain parts of the plant, for instance the exchangers.
The formation of these solid deposits is a particular nuisance, since they form an insulating layer which reduces the heat exchange, the consequence of which is to cause an increase in the heating in order to maintain a constant temperature in the boiling vessel, thereby entailing an aggravation of the polymerization phenomenon. In this case, one is limited to rapidly stopping the distillation in order to carry out a difficult and expensive cleaning of the plant.
It is well known that the distillation of acrylic monomers, which readily polymerize under the action of radicals formed, for example, by the effect of heat, necessitates the use of polymerization inhibitors, particularly during the distillation steps. The compounds typically used for this purpose are, for example, phenolic derivatives, for instance hydroquinone or hydroquinone methyl ether; phenothiazine and its derivatives; derivatives of the thiocarbamate family; compounds containing a nitroso group; quinones; or alternatively aromatic amines.
Despite the use of these polymerization inhibitors, polymers can gradually accumulate, more or less quickly depending on the purification step and the operating conditions of the equipment through which the monomer-rich flow passes, in the form of interfering deposits.
These problems are particularly acute during a final step of purification of the acrylic acid.
With the aim of obtaining an acrylic acid of very high quality in terms of purity, the removal of all of these impurities, down to extremely low contents, is not economically achievable by means of a simple separation by distillation. In particular, on account of their volatility which is close to that of acrylic acid, the abovementioned carbonyl impurities cannot be removed efficiently, down to the extremely low contents targeted, by a single distillation operation.
In order to remove the aldehyde impurities, American patent U.S. Pat. No. 3,725,208 describes a chemical treatment which consists in adding amines to the impure mixture and then in distilling the mixture obtained. The amine family is particularly suitable for achieving this aim, since these compounds have the particular feature of forming, with aldehydes, heavy compounds which can readily be separated from the acrylic acid at the foot of the distillation column. Among the reagents which offer the best efficacy, the ones described in particular are those which form part of the hydrazine family, such as glycine (Japanese patent No. J 7,500,014) or hydrazine itself or its derivatives (American patent U.S. Pat. No. 3,725,208 and Japanese patent J 7,430,312) or alternatively aminoguanidine (European patent EP-B-270,999) or salts thereof.
The chemical treatments which are described all have the drawback of generating water during the reaction of the aldehyde with the amino reagent. The presence of this impurity in the acrylic acid is also harmful with respect to the reactivity of the monomer in its most technical applications. For this reason, it may be particularly advantageous to carry out this chemical treatment during a step of distillation directed towards removing the water and the light head-fraction compounds, before the step of distillation of the acrylic acid which is intended to separate out the heavy compounds, as is described in Japanese patent J 7,495,920.
A second major drawback of the chemical treatments for the removal of aldehydes with amines is that they entail a significant reduction in the stability of the medium. The amine function of these compounds gives them the property of reacting not only with aldehydes, but also with the acrylic acid itself, to form salts by reaction with the carboxylic part of the molecule, or Michael addition compounds by reaction of the amine with the acrylic double bond.
The reaction products of amines with acrylic acid result in an exacerbated sensitivity of the reaction medium with respect to polymerization. Despite the use of inhibitors conventionally used for the distillation of this monomer, when this treatment is carried out specifically in this distillation, in order to remove, at the foot of the column, the heavy addition compounds of the aldehydes with the amine, polymer deposits are observed, particularly on the hot wall of the boiling vessel.
The formation of these solid deposits rapidly leads to problems of blockage of the pipes or a change in the heat exchanges described above, making it necessary to stop the plant for cleaning.
In order to reduce these harmful effects, European patent application EP-A1-0,648,732 claims the use of an organic sulphonic acid during the treatment for the removal of the aldehydes with an amine such as hydrazine or aminoguanidine. This improvement has several major drawbacks. Firstly, the sulphonic acids described are corrosive and require the use of expensive equipment for those parts of the plant which come into contact with them. Moreover, large amounts of additives are used, since they must be used in an excessive molar ratio with respect to the amino compound, thus making the treatment expensive.
With the same aim of avoiding the deposition of polymers in this step of the process for purifying acrylic acid, British patent application GB-A-2,285,046 describes an improvement which consists in carrying out the treatment for the removal of the impurities with hydrazines, during a distillation, by adding a copper dithiocarbamate compound. The compounds of this metal thiocarbamate family are well known as polymerization inhibitors for acrylic acid and other acrylic and methacrylic monomers. Unfortunately, they have the drawback of entailing, in the undistillable heavy by-products of the plant, metallic residues which make them difficult to be removed. The reason for this is that these residues can cause the encrustation of the incineration ovens, thus necessitating a high cost for their removal.
In general, the addition of polymerization inhibitors is not sufficient for totally preventing the fo
Augustin Francis
Esch Marc
Fauconet Michel
Atofina
Manoharan Virginia
Millen White Zelano & Branigan P.C.
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