Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-02-21
2003-12-30
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S480000, C562S485000, C562S486000, C562S487000
Reexamination Certificate
active
06670503
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a method of recovering terephthalic acid from a material containing poly(ethylene terephthalate). In particular, the present invention relates to a method of obtaining terephthalic acid starting from poly(ethylene terephthalate (P.E.T.) materials, for instance in the form of bottles or other manufactured articles coming from a differentiated salvage dump.
Known in the art is the existence of some chemical methods involving recovery of terephthalic acid starting from materials containing poly(ethylene terephthalate).
Poly(ethylene terephthalate) (P.E.T.) is a saturated polyester resin made from terephthalic acid and ethylene glycol. P.E.T. is widely used in the food-industry packaging sector and in particular in the manufacture of bottles for soft drinks. Due to its large use, an increasingly growing interest in P.E.T. recycling has been developed in the most recent years.
Substantially chemical methods for terephthalic acid recovery are distinguishable from each other due to the mechanism used which can belong to one of the following main categories: alcoholysis, glycolysis, acid hydrolysis, neutral hydrolysis and alkaline hydrolysis.
Obviously, each individual mechanism can find application through various techniques differing from each other due, for example, to a different number of phases, different temperatures, pressures, involved reagents, reaction solvents.
The chemical method utilizing alcoholysis brings to formation of di(alkyl)-terephthalates, whereas glycolysis produces di(hydroxy-alkyl)-terephtalates.
Due to difficulties connected with purging of the obtained reaction products, these methods can only apply to materials containing poly(ethylene terephthalate) (P.E.T.) of high purity. Consequently these methods cannot apply if P.E.T. comes from a material obtained through a differentiated salvage dump as in the case of P.E.T. bottles from said differentiated dump.
The chemical method utilizing acid hydrolysis is carried out by making the P.E.T. react with a large excess of strong acid in solution, concentrated sulfuric acid for example. The sulfuric acid acts in a very short period of time, say some minutes, at a temperature included between room temperature and 95° C., by dissolving the starting P.E.T. with formation of terephthalic acid (TPA). The chemical method utilizing acid hydrolysis is scarcely applied in the industrial field, mainly due to the high corrosiveness of the reaction system and also due to the huge amount of salt solution produced for neutralizing the employed acid.
The chemical method utilizing neutral hydrolysis is conducted by treating the P.E.T. with water or steam, under pressure at a temperature included between 200 and 300° C., in the presence of appropriate catalysts. This method too has some drawbacks. The main disadvantages of this technology are represented by high energy consumptions and the impossibility of eliminating all mechanical impurities from the terephthalic acid (TPA) obtained by precipitation, such as undissolved particulate matter and insoluble polymers originally present in the starting material.
Finally, the chemical method utilizing alkaline hydrolysis is almost always carried out by use of alkaline hydroxides or ammonium hydroxides.
Use of these bases leads to formation of aqueous solutions of the corresponding salt of the terephthalic acid (TPA). These solutions can be easily cleared from mechanical impurities by filtering, flocculating or settling processes. In addition, terephthalic acid (TPA) is recovered from said aqueous solutions by precipitation in an acid medium.
Of all the above mentioned methods, the method utilizing alkaline hydrolysis has recently found many applications. Some of them are reproduced hereinafter:
a first application contemplates treatment of the P.E.T. with a concentrated solution of an alkaline hydroxide, under pressure and at temperatures close to or higher than 250° C. The P.E.T./alkaline solution ratio is greater than 20.
a second application contemplates treatment of the P.E.T. with a stoichiometric amount of an alkaline hydroxide in ethylene glycol (EG) at a temperature included between 100 and 200° C. If an ammonium hydroxide is used as the base, the method is carried out under pressure. In both cases, at all events, the obtained reaction mixture is dissolved in water.
Finally, a third application provides for the P.E.T. to be extruded in the presence of hydroxide at temperatures higher than 250° C. Subsequently, the obtained salt is dissolved in an aqueous solution.
Generally problems resulting from use of alkaline or alkaline-earth hydroxides or from use of concentrated solutions of such hydroxides are well known. For alkaline hydroxides it is meant a sodium hydroxide for example, for alkaline-earth hydroxide it is for example meant a calcium hydroxide.
The main disadvantages are connected with difficulties in manipulating these types of very aggressive reagents by operators. In addition, modifications in the plants are required to be adopted together with a series of technical expedients in plant construction due to the big problems connected with corrosion of these reagents, above all if used in solution.
In the above first application, disadvantages consist in being obliged to heat, filter and recover great amounts of solution. In addition, in this application use of great amounts of hydroxides and adoption of high pressures is provided.
In the above second and third applications, the main disadvantage resides in the fact that it is impossible to obtain terephthalic acid (TPA) free from undesirable yellow-pink colorations. In fact, in the absence of water the hydroxides employed at temperatures higher than 100° C. react with the ethylene glycol (EG) resulting from the hydrolysis reaction. The reaction between hydroxides and ethylene glycol brings to formation of strongly red-colored and water-soluble products. Formation of these colored compounds prevents precipitation of uncontaminated white TPA. Therefore, the terephthalic acid (TPA) containing colored impurities must be bleached.
Methods described in literature for TPA bleaching are long and expensive. Some methods of bleaching for example involve extraction of impurities by use of water-insoluble higher alcohols.
In addition, the above third application requires use of starting material containing P.E.T. in the form of ground, washed and dried scraps, which will involve an increase in the costs of the starting raw material.
Therefore there is a need for a method of recovering terephthalic acid from a material containing polyethylene terephthalate), coming for example from a differentiated salvage dump, which is devoid of the drawbacks of the known art.
In particular, a method is required which enables recovery of a terephthalic acid free from colored contaminations and impurities.
Still more particularly, there is a need for a method of recovering terephthalic acid which does not involve use of alkaline hydroxides, alkaline-earth hydroxides and ammonium hydroxide or concentrated solutions of these hydroxides.
In addition, there is a requirement for a method of recovering terephthalic acid which is particularly cheap. This is very important when a raw material of low value is to be treated, such as the material from a differentiated salvage dump for example. For the above reasons, the volumes of the materials to be heated must be reduced, if possible. In addition, secondary reactions producing by-products must be reduced or eliminated, such as the above mentioned colored contaminations.
Finally, also useful is a reduction in the amount of solvent or water used for the purpose of reducing the amounts of strong acids to employ for acidification of the solution of the metal salt or salts of the TPA so as to obtain precipitation thereof. In addition, a limited use of solvent and water amounts enables the cost of the individual processes to be controlled.
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide a method of recovering terephtha
Barnes & Thornburg
Conte James B.
Richter Johann
Zucker Paul A.
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