EP1051384A1

EP1051384A1 - Method for producing a purified alkaline metal terephthalate, derived from the products of saponification of polyterephthalates

Info

Publication number: EP1051384A1
Application number: EP99901650A
Authority: EP
Inventors: Jacques Thauront; Laurent Bonnamich; Guy Fraysse
Original assignee: Tredi
Current assignee: Tredi
Priority date: 1998-01-30
Filing date: 1999-01-28
Publication date: 2000-11-15
Also published as: WO1999038835A1; AU2168399A; US6657077B1; FR2774374B1; FR2774374A1

Abstract

The invention concerns a method for producing terephthalate of alkaline metal and alkylene glycol from recuperated alkylene polyterephthalate. It comprises saponification of said alkylene terephthalates with an aqueous sodium solution, adding a sufficient volume of water to ensure solubilization of the entire alkaline metal terephthalate produced in the reaction medium liquid phase and crystallisation of the sodium terephthalate by concentration of said liquid phase. The resulting sodium terephthalate can be directly used for producing pure terephthalic acid usable for the renewed production of alkylene polyterephthalates to be used in the food industry.

Description

PROCESS FOR THE PRODUCTION OF TEREPHTHALATE FROM A PURIFIED ALKALINE METAL FROM POLYTEREPHTHALATE SAPONIFICATION PRODUCTS

ALKYLENE RECOVERY

The invention relates to an improved process for recovering alkali metal terephthalate and alkylene glycol from polyalkylene terephthalate waste from used products, for example packaging, films, fibers or bottles, these polyterephthalates recovery which may also contain other impurities, such as fillers, additives, dyes, or even be mixed with other solid materials such as paper labels, closure caps (for bottles) made of polyethylene and / or polypropylene, etc. The terephthalic acid capable of being produced from these polyterephthalate solutions, in particular by precipitation with sulfuric acid, is normally intended for the production of new polyalkylene polyterephthalates, intended for the production for example new packaging usable in the food industry.

Such methods have already been described in the technical literature. Those which the invention aims to perfect implement the alkaline saponification of these polyalkylene terephthalates, most often polyethylene terephthalates (PET), this saponification consequently leading to the production of an alkali metal polyterephthalate and corresponding alkylene glycol.

Among the oldest patents relating to such saponification processes, mention may be made of British Patent No. 822834, according to which this saponification reaction is carried out by reacting polyalkylene terephthalates with a relatively dilute solution of a hydroxide of alkali metal, in particular sodium hydroxide (18% sodium hydroxide solution according to Example 1), the reaction conditions being adjusted so as to promote a maximum release effect of the alkali metal polyterephthalate formed in the reaction solution. Due to the relative solubility of sodium terephthalate, the authors of the patent even recommended increasing the salting-out capacity of sodium terephthalate by adding an alcohol, or even substituting for the aqueous solution a mixture of insoluble alkali hydroxide and alcohol. According to French patents 9101025/2672049 and 9213583/2697839, the alkali metal hydroxide (or alkaline earth metal) is used in the solid state or in the presence of an amount of water at most equal by weight to that alkali or alkaline earth metal hydroxide, or in the molten state in the absence of water, the reaction being carried out at a temperature above 120 ° C and preferably between 140 and 180 ° C. The advantage provided by these methods lies in the possibility of implementing them continuously.

The alkali metal terephthalate solutions resulting from the above saponification operations are often colored and mixed with other dissolved materials; hence the need to carry out significant preliminary purifications of these solutions before treating them, in particular by acidification, in order to obtain a terephthalic acid having a satisfactory degree of purity.

Another difficulty which has not been satisfactorily remedied to date lies in the recovery of most of the polyol, in particular of the polyethylene glycol released by the saponification reaction. According to a solution proposed for this purpose in PCT application WO 95/29952, the saponification reaction is carried out at a temperature above the polyol volatilization temperature, in particular above 200 ° C. The polyol can then be recovered by condensation. According to this request, the quality of the alkaline polycarboxylate, in particular of the sodium terephthalate produced, would not be affected by these high temperatures even if it is advisable to operate in an oxygen-depleted atmosphere, to control the carbonization reactions of the impurities to these high temperatures. The problem of the coloring and of the content of other dissolved materials in the solutions of the alkali metal polycarboxylate is therefore not further resolved by this process. It is then also obvious that the terephthalic acid finally obtained by acid precipitation from the sodium terephthalate solutions thus obtained cannot have the desired degree of purity for new manufacturing of polyethylene glycol terephthalate, for example for their use. for food packaging.

The object of the invention is to further improve the saponification processes of the genus in question, in particular to optimize the reaction conditions, in order to obtain more directly polycarboxylates, in particular alkali metal polyterephthalates having a compatible degree of purity with the requirements of the food industry, while operating in conditions that consume less energy.

Another object of the invention is to ensure that the reactions are as complete as possible, so that all the main other reagents, in particular the polyols released during the saponification operation, are almost recovered total.

The process according to the invention for the production of an aromatic dicarboxylate of alkali metal, in particular of sodium, and of alkylene glycol by alkaline saponification with an alkali metal hydroxide of a polyester of aromatic dicarboxylic acid, itself derived from a dicarboxylic acid whose salt of this alkali metal is soluble in water, is characterized

- By carrying out the saponification step in a liquid medium at a temperature between about 120 ° C and the boiling temperature of the corresponding alkylene glycol, if necessary under a pressure allowing the maintenance of the liquid phase of the reaction medium in the liquid state or at the boiling point at the above temperature, preferably throughout the saponification period,

- By providing a sufficient volume of water to ensure the solubilization, in particular at the end of the reaction, of all of the aromatic dicarboxylate of the alkali metal produced in the liquid phase of the reaction medium and, if necessary after separation of the insoluble materials,

- By concentrating the solution by evaporation of water in order to cause the crystallization of most of the alkali metal dicarboxylate, outside the liquid phase then charged with alkylene glycol.

Applied to the production of sodium terephthalate and alkylene glycols from recovered poly (alkylene terephthalate), the process according to the invention comprises a saponification of these poly (alkylene terephthalates) by an alkali metal hydroxide, in particular in the form of '' an aqueous solution, this process then being characterized:

by using in this saponification reaction a solution of an alkali metal hydroxide having an initial weight content of this hydroxide of the order of 20% to about 45% by weight,

- in that the reaction is carried out at a temperature between approximately 120 and the boiling temperature of the alkylene glycol produced, if necessary under a pressure allowing the liquid phase of the reaction medium to be maintained in the liquid state or at boiling at the above temperature, for the entire duration of the saponification operation,

by adding to the medium a sufficient volume of water to ensure the solubilization of all of the sodium terephthalate produced in the liquid phase of the reaction medium,

- Then by concentration of the solution by evaporation of water, if necessary after filtration of the insoluble materials, to induce crystallization of the alkali metal terephthalate. It goes without saying that the original reaction medium will initially be essentially free of the alkylene glycol corresponding to that which will then be released into the medium, after initiation of the saponification reaction. Besides that an initial addition of alkylene glycol would add nothing to the reaction conditions, it would be of a nature to contribute to a greater salting out of the alkali metal dicarboxylate produced and, consequently, to a subsequent addition of '' a greater volume of water to re-dissolve the above-mentioned salt, before the concentration operation aimed at subsequently inducing its crystallization out of the reaction medium.

Advantageously, the polyalkylene recovery terephthalate is gradually introduced, in the divided state, in particular ground, into the aqueous solution of the alkali metal hydroxide at boiling point.

In a preferred form of the invention the alkali metal hydroxide is sodium hydroxide.

In a particularly preferred embodiment of the process according to the invention, the temperature is between 125 and 160 ° C, for example between 140 and 160 ° C and the concentration of sodium hydroxide in the solution is between 35 and 40 % in weight.

A particularly important parameter of the reaction is the temperature. This temperature must be sufficient for the saponification reaction to be able to be seriously initiated and then maintained. By choosing the intervals of the initial concentration of the alkali metal solution used in the reaction, it is ensured (1) that the reaction medium can effectively be maintained in the liquid state or at boiling in the range of temperatures considered, and this during almost all of the saponification reaction, even in the last part of the reaction, at atmospheric pressure or, if necessary, at slightly higher pressures, when the medium is considerably depleted in alkali metal hydroxide, (2) because the alkylene glycols remain in the reaction medium. It can be appreciated by reference to the diagram in Figure 1, including the curves which illustrate the variations of the vapor pressures (in kilopascals on the ordinate axis) of various soda solutions whose weight proportions are indicated in the upper corner to the right of the figure, as a function of the temperature (in ° C) on the abscissa axis. By operating within the concentration and temperature ranges set out above, it can be seen that the major part of the reaction can advantageously be carried out at atmospheric pressure or at a pressure not exceeding 600 kilopascals. The final concentration step to induce crystallization of the alkali metal polycarboxylate can be carried out under respectively reduced pressure and temperature. Due to the amount of water used to completely redissolve the terephthalate produced at the end of the saponification reaction, the impurities also dissolved, but then in the very diluted state, remain in solution.

An additional decisive advantage of the process according to the invention, more particularly of its crystallization stage, resides in the direct production of dicarboxylate, in particular alkali metal terephthalate which, after redissolution in water leads to a solution which, if necessary after a simple passage over activated carbon has a degree of purity allowing the subsequent direct manufacture of polycarboxylic acid, in particular terephthalic acid, compatible with food applications.

The mother liquors obtained after the separation of the alkali metal dicarboxylate crystals then already constitute concentrated solutions of alkylene glycol.

Preferred embodiments of the invention, as well as a certain number of examples are further described below in order to illustrate the possibilities, however in the absence of any limiting character.

The diagram in FIG. 2 recalls the main steps of such a preferred embodiment of the invention according to which one proceeds in particular in three steps. The polyalkylene recovery terephthalate is saponified by an alkaline solution at a temperature of 120 to 160 ° C, for example 120 to 150 ° C, or preferably 140 to 160 ° C, this saponification being followed by an addition of water to obtain a solution of alkali metal terephthalate and alkylene glycol. This solution is then concentrated to crystallize the alkali metal terephthalate from this solution.

The above-mentioned placing in total solution of the alkaline terephthalate also allows the easy separation of the impurities present in the used bottles (PVC, polyethylene, polypropylene, pigments, etc.) which have not reacted with soda.

As already indicated above, it is therefore possible to directly obtain pure alkali metal terephthalate, essentially free of alkylene glycols, by crystallization from the solution resulting from the saponification step.

The crystallization of pure alkali metal terephthalates from the solution resulting from the saponification stage - where appropriate after an intermediate storage stage outside the reactor and, where appropriate, prior concentration - is advantageously carried out continuously in equipment (crystallizers) which the industry already has.

The crystallization operation of the aromatic dicarboxylate, in particular of the alkali metal diterephthalate outside of the reaction medium is preferably carried out at a temperature of 50 ° C. to 90 ° C., in particular 6 ° C., if necessary under partial vacuum. . Preferably, this operation is preceded by a step of concentrating the solution by evaporation of the forced circulation type, with mechanical recompression of the vapors by volumetric compressor at high temperature (around 100 ° C.), the already concentrated solution then being subjected to the above-mentioned crystallization under partial vacuum, for example at a temperature of the order of 60 ° C. (conditions under which the solubility of the dicarboxylate in the purge of crystallization mother liquors is then reduced). The water thus recovered (condensates) can then, for example, be recycled at the saponification or dissolution stage.

The crystalline alkali metal terephthalate obtained is sufficiently pure and in particular contains only such small quantities of alkylene glycols that it can then be converted into terephthalic acid (TPA), high purity, for example after it has been dissolved in water and acidification, in particular at pH 2.5, of the solution obtained with an acid, such as sulfuric acid. The terephthalic acid obtained, then itself of high purity, is then directly usable for new polymerizations, together with an alkylene glycol, in particular ethylene glycol, to produce plastics of food quality.

It has in fact been discovered that the presence of the alkylene glycols produced in the solution, even in the concentrated state, does not affect the desired degree of purity of the crystallized alkaline terephthalate obtained, which makes it possible not to eliminate the alkylene glycol either during the saponification stage, or before the dissolution of the alkaline terephthalate, whereas in most of the prior processes (for example the patents FR 2 672 049; FR 2 697 839; EP 597 751 A1; WO 95 / 29952) the elimination or separation of the alkylene glycol is carried out prior to this dissolution.

The alkylene glycols can then be recovered from the mother liquors (or purge), after separation of the sodium terephthalate crystals. This purge is preferably first acidified, in particular treated with sulfuric acid, to a pH of the order of 2.5, to remove all traces of soda and to precipitate non-food grade terephthalic acid which still contained this purge. The crystals obtained can be used to neutralize the excess sodium hydroxide at the outlet of the saponification reactor. The final filtrate contains alkylene glycol, in particular ethylene glycol, sodium sulfate and acidic sodium sulfate. If the acidic sodium hydroxide sulfate is neutralized with liquid sodium hydroxide and then the solution is concentrated by evaporation of water, a precipitation of sodium hydroxide is induced. The liquid is then constituted by an ethylene glycol of technical quality (for example for the production of antifreezes) or thermally recovered, since in general it then does not contain more than 5% of water. It is also possible, as a variant, after treatment with sulfuric acid of the crystallization mother liquors, filtration of the precipitated technical terephthalic acid, recovering the alkylene glycol in the purified state from the solution of water, sodium hydroxide and alkylene glycol, for example by vacuum distillation.

At least part of the technical terephthalic acid obtained is optionally returned to the saponification stage or to the solubilization of the sodium terephthalates produced at the end of the saponification stage.

In general, the process of the invention can be implemented in the following manner: an alkaline solution, previously or not heated, having a concentration of sodium hydroxide in water of between approximately 20% and 45% by weight, is introduced into a stirred reactor. This solution is brought to a boil at a temperature between 1 15 and 140 ° C, or more preferably between 140 and 160 ° C. A solution having a sodium hydroxide content of 38% has a boiling temperature of the order of 130 ° C.

The waste of previously crushed films and bottles is introduced little by little into the reactor until the quantity expressed in pure resin is close to the stoichiometry relative to the soda, or even a little less than the latter. In general, this proportion varies from 0.8 times the stoichiometry to 1.5 times the stoichiometry. For example, the proportion of soda - or more generally of alkali hydroxide - does not exceed 1.2 times the stoichiometry.

As the alkylene terephthalate is introduced, the reactor is maintained at the correct temperature by external and / or internal heating and, if necessary, the pressure is adjusted to a value allowing the medium to be maintained at boiling point. The water vapor is condensed in a reflux condenser associated with the reactor so as to keep the initial water content constant. 10

At the end of the saponification reaction, a sufficient quantity of water is introduced to obtain an alkaline terephthalate solution of the order of 10%.

After separation, by successive filtrations, of the impurities present in the initial waste, the solution obtained containing the alkaline terephthalate, the glycols and the excess soda is stored.

The solution thus prepared is sent continuously to a crystallizer. The alkaline terephthalate crystals are separated, washed with water and are in a sufficiently pure state to be directly used for the manufacture of terephthalic acid.

The following examples are given by way of illustration and do not limit the present invention in any way.

EXAMPLES I - SAPONIFICATION OF THE SODIUM TEREPHTHALATE SOLUTION

1.1 Laboratory tests

- Two tests (examples 1 and 2) were carried out at atmospheric pressure. In both cases, 80 g of PET were mixed with 80 g of 45% sodium hydroxide in a flask equipped with a reflux condenser and heated by an oil bath. After three hours of reaction and a temperature of

130 ° C, the respective yields were:

• tests n ° 1 48%

• tests n ° 2 70%. Only the geometry of the agitator changes from test n ° 1 to test n ° 2, the efficiency of the stirring being better in test n ° 2 than in test n ° 1.

A pressure test (Example 3) was carried out with the same quantities of reagents. The temperature of the heating oil bath was maintained at 150 ° C and the pressure at about 200 kPa. 11

After 45 minutes of reaction, the yield observed was 85%.

1.2 Semi-industrial tests

Two tests were carried out in a stirred reactor at atmospheric pressure with a volume of 1400 liters and equipped with a reflux condenser.

Example 4

In the 1,400 liter jacketed reactor, 101.6 kilos of 45% sodium hydroxide were introduced and preheated by steam at 300 kPa circulating in the jacketed jacket of the reactor. When the temperature of the solution exceeded 100 ° C., 110 kg of PET were added over two and a half hours, the temperature falling to 110 ° C. after having risen to 124 ° C. After cooling, water was added. was added (approximately 750 liters) to obtain 1,100 liters of filtrate having the following composition:

NAOH: 1.02% TPNa ₂ : 7.46% glycol: 2.1% the recovery yield obtained, expressed by the ratio of the mass of solution, multiplied by the concentration of TPNa ₂ , to the mass of PET work, multiplied by the 192/210 ratio, being 72%.

Example 5

In order to show the predominant effect of the temperature of the reaction medium on the yield, 102 kilos of 50% sodium hydroxide were introduced into the same reactor as that of Example 4.

After preheating to around 100 ° C, 90 kg of crushed PET were introduced until the temperature started to drop. During this example the temperature of the reaction mixture rose to 130 ° C against 124 ° C in Example 4. 12

After cooling, water was added to obtain 879 liters of filtrate having the following composition:

N _a OH: 1.96% TPNa ₂ : 9.84% glycol: 3%. The yield obtained, calculated as in Example 4, is 90

% versus 72% in Example 4.

Example 6

1.3 Pilot tests In a stirred reactor with a volume of 70 liters, equipped with a reflux condenser and heating resistors, 6,950 g of sodium hydroxide solution at 30% by mass, were heated to 110 ° C. 5,000 g of PET were introduced, which corresponds to stoichiometry. The reactor was closed then heated, the pressure was maintained by the use of the reflux condenser at a value of 500 kilopascals for one hour. After cooling, 42.7 kg of water were introduced into the reactor. The filtrate obtained after sieving had the following composition:

N _a OH: 0.16% TPNa ₂ : 9.74% glycol: 2.85%.

The yield obtained, calculated as in Example 4, is 95%.

It - CRYSTALLIZATION OF A SODIUM TEREPHTHALATE SOLUTION

From 10 liters of a sodium terephthalate solution from the saponification stage having the following main characteristics: TPNa ₂ concentration: 5.02% glycol concentration: 0.19% 13

five charges or "batches" of two liters were concentrated under vacuum at a temperature of 80 ° C up to a factor of 2.

From the mixture of concentrates, two "batches" were concentrated by a factor of 2 under the same conditions and so on until a concentration factor of 16 was obtained.

The analyzes of the various products from the last experiment are as follows:

Food Concentrate Distillate TPNA Crystals ₂ 12.9% 4.6% Traces 98.19%

Glycol 1, 66% 2.64% <0.01% <0.01%

It follows from these results that the increase in the glycol concentration of the solution from which the crystallization is carried out does not affect the purity of the TPNa ₂ finally obtained.

Although we have focused above all on the recovery of alkali metal terephthalate and alkylene glycols from polyalkylene terephthalate, the process according to the invention can also be applied to other polyesters aromatic dicarboxylic acids and reclaimed alkylene, in particular of the type used in the same industries, in particular the food industry, also for the production of packaging, films, fibers, bottles, etc. .. as soon as the alkali metal, in particular sodium, salts of the corresponding aromatic dicarboxylic acids can be crystallized out of aqueous solutions containing them in the dissolved state, under conditions similar to what has been observed with sodium terephthalate .

By way of examples of such aromatic alkylene dicarboxylate polyesters to which the invention can be applied, there will be mentioned alkylene polyesters containing repeating units derived from:

- isophthalic acid 14

- naphthalene dicarboxylic acids, in particular naphthalene 2,6-dicarboxylic acid

- 4,4-oxybis (benzoic) acid

- 5-ter-butyl-1, 3-benzene dicarboxylic acid, etc. In other words, the invention can also be defined as consisting of a process for the production of aromatic dicarboxylates of alkali metal of the sus- type indicated, in particular of sodium, from polyesters of corresponding aromatic dicarboxylic acids of recovery and of alkylene glycols, comprising a step of saponification of these polyesters with an aqueous solution of the alkali metal hydroxide, characterized:

- by using in this saponification reaction a solution of the alkali metal hydroxide, the initial weight content of this hydroxide is of the order of 20 to about 45% by weight, - in that the reaction is carried out at a temperature between about 120 ° C and the boiling temperature of the corresponding alkylene glycol, if necessary under a pressure allowing the liquid phase of the reaction medium to be maintained in the liquid state or at the boiling point at the above temperature, for the whole duration of the saponification, - then by the addition in the middle of a volume of water sufficient to ensure the solubilization of all of the aromatic dicarboxylate of the alkali metal product in the liquid phase of the reaction medium.

The preferential conditions which have been discussed above with respect to polyalkylene terephthalates can also be envisaged for the treatment of polyesters of aromatic alkylene dicarboxylate acids.

Claims

15

1 Process for the production of an aromatic dicarboxylate of an alkali metal, in particular of sodium, and of an alkylene glycol, by alkaline saponification with an alkali metal hydroxide of a polyester of aromatic dicarboxylic acid itself derived from an acid dicarboxylic whose salt of this alkali metal is soluble in water, characterized:

- By carrying out the saponification step in a liquid medium at a temperature between about 120 ° C and the boiling temperature of the corresponding alkylene glycol, if necessary under a pressure allowing the maintenance of the liquid phase of the reaction medium in the liquid state or at the boiling point at the above temperature,

by adding to the medium a sufficient volume of water to ensure solubilization, in particular at the end of the reaction, of all of the aromatic dicarboxylate of the alkali metal produced in the liquid phase of the reaction medium and, if necessary after filtration of insoluble matter,

2. Method according to claim 1, for producing an aromatic alkali metal dicarboxylate constituted by an alkali metal terephthalate, on the one hand, and alkylene glycols, on the other hand, from poly (alkylene terephthalate). recovery, comprising a saponification of these alkylene terephthalates with an aqueous solution of an alkali metal hydroxide, characterized:

by using in this saponification reaction a solution of the alkali metal hydroxide having an initial weight content of this hydroxide of the order of 20% to about 45% by weight, 16

- in that the reaction is carried out at a temperature between approximately 120 and the boiling point of the alkylene glycol produced, under a pressure allowing the liquid phase of the reaction medium to be maintained in the liquid state or at 'boiling at the above temperature, for the entire duration of the saponification operation,

- Then by the addition in the middle of a sufficient volume of water to ensure the solubilization of all of the alkali metal terephthalate produced in the liquid phase of the reaction medium.

3. Method according to claim 1 or claim 2, characterized in that the operation of concentrating the solution by evaporation of water comprises a step of crystallization of the alkali metal dicarboxylate out of its solution at a temperature of 50 ° C at 90 ° C under partial vacuum.

4. Method according to claim 3, characterized in that said crystallization is carried out from said solution, after a preconcentration of the latter at a temperature of the order of 100 ° C.

5. Method according to any one of claims 1 to 4, characterized in that the recovered alkylene dicarboxylate is gradually introduced, in the divided state, in particular ground, in the aqueous solution of the alkali metal hydroxide to boiling, at the above reaction temperature.

6. Method according to any one of claims 1 to 5, characterized in that the alkali metal hydroxide is sodium hydroxide.

7. Method according to any one of claims 1 to 6, characterized in that the temperature is between 125 and 160 ° C and the 17

sodium hydroxide concentration of the solution is between 35 and 45% by weight.

8. Method according to any one of claims 1 to 7, characterized by the use of a proportion of alkali metal hydroxide with respect to the alkylene dicarboxylate of between 0.8 times the stoichiometry and 1 , 5 times the stoichiometry.

9. The method of claim 8, characterized by the use of a proportion of alkali metal hydroxide vis-à-vis the alkylene dicarboxylate between stoichiometry and 1, 2 times stoichiometry.

10. Method according to any one of claims 1 to 9, characterized in that the mother liquors obtained after crystallization of the alkali metal dicarboxylate, which also contain sodium terephthalate in the dissolved state, are acidified with an acid, such as sulfuric acid, to precipitate terephthalic acid.

11. The method of claim 10, characterized by recycling at least part of the terephthalic acid thus precipitated in the step of solubilization of the alkali metal terephthalate formed, following the above saponification.

12. Method according to claim 10 or claim 1 1, characterized by the neutralization of anions, in particular sulfates in excess of the solution obtained after precipitation of terephthalic acid, with a solution of alkali hydroxide, in particular hydroxide of sodium, by concentrating the solution at a temperature allowing most of the water to evaporate but allowing the solution to remain in solution 18

alkylene glycols, by the separation of the salt, in particular sodium sulphate and by the recovery of a polyalkylene glycol of technical quality.

13. Method according to any one of claims 1 to 12, characterized by the recycling of the condensates produced during the concentration step, in the step of solubilization of the sodium terephthalate produced in the reaction medium.