Method of reprocessing residues containing dihydroxy compounds

Details Method of reprocessing residues containing dihydroxy compounds Method of reprocessing residues containing dihydroxy compounds

1998-08-07

2001-01-16

Acquah, Samuel A. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Removing and recycling removed material from an ongoing...

C526S065000, C526S066000, C526S068000, C528S272000, C528S048000, C528S495000, C528S501000, C528S503000

Reexamination Certificate

active

06174970

ABSTRACT:

The present invention relates to a process for working up residues containing dihydroxy compounds resulting from the preparation of polyesters, where
(1) in a first stage there is esterification or transesterification of a dicarboxylic acid or esters thereof or ester-forming derivatives with a molar excess of a dihydroxy compound,
(2) in at least one second stage there is polycondensation of the esterification product obtained in (1),
(3) the vapors (a) and (b) resulting from the reactions in (1) and (2) respectively are subjected to a treatment to recover the starting materials.
Polyesters, especially polyalkylene terephthalates, are prepared on a large scale in transesterification/polycondensation processes where an esterification or transesterification is carried out in a first stage, and the actual polycondensation is carried out in at least one further stage (cf. Chemiefasern/Textilindustrie 40 (1992), 1058-1062 and Ullmann's Enzyklop{umlaut over (a)}dieder technischen Chemie, 4th Edition, Volume 19, pages 61-88).
This process will be briefly explained taking the example of the preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol.
In a first reaction chamber, terephthalic acid is esterified with a molar excess, preferably 50-120 mol %, in particular 70-100 mol %, of 1,4-butanediol, and the esterified compound is subjected in further steps to the actual polycondensation. The vapors resulting from the esterification are transferred into a column in which the low-boiling components THF/water are removed as distillate, and a bottom product which, besides excess 1,4-butanediol, also contains small amounts of oligomers, polymers and terephthalic acid is obtained.
The esterification product is subsequently polycondensed, expediently in at least two stages, called the precondensation and postcondensation, in continuous processes.
For economic reasons, it is desirable in this connection that as much as possible of the resulting reaction products and dihydroxy compounds present in excess are treated further in order, for example, to recover the 1,4-butanediol and generate as little waste as possible.
A process for treating the vapors from the polycondensation is proposed in DE-A 19509957.5 which is not a prior publication.
DE-A 43 33 929 discloses a process for recovering the starting materials, where a distillation residue consisting of dihydroxy-containing compounds (called “Hex”) is metered into the column to simplify manipulation of the bottom products which are mostly extremely viscous.
The disadvantage of this process is that this residue usually derives from the distillation of butanediol or hexanediol and is thus not available everywhere.
Since the vapors contain oligomeric and polymeric esters, in addition to the starting materials and byproducts, the pH of the vapors is below 7 owing to the content of carboxyl end groups. In addition, renewed fragmentation takes place during recovery, resulting in new carboxyl end groups besides those already present. For these reasons, treatment of the vapors is very costly because it is necessary to employ corrosion-resistant stainless steel containers.
During the treatment and preparation of polyesters there is considerable formation of, for example, THF as byproduct (when 1,4-butanediol is the starting material). Since this side reaction is catalyzed by acids, there are also considerable losses of 1,4-butanediol due to THF formation on recovery by distillation.
It is an object of the present invention to remedy the disadvantages described above and to improve the working up of the vapors so that most of the dihydroxy compounds present therein can be recovered as economically as possible. It is intended at the same time that the quality of the polyester product be maintained.
We have found that this object is achieved by carrying out the treatment to recover the starting materials in the presence of an alkali metal or alkaline earth metal compound in an amount of from 0.5 to 10% by weight, calculated as alkali metal or alkaline earth metal, based on the solids content of the vapors.
The addition, according to the invention, of alkali metal or alkaline earth metal compounds in the treatment of the vapors results in a basic buffering of the vapors so that corrosion of the containers is no longer possible. This makes it possible to employ considerably lower-cost ordinary steel containers for these purposes. In addition, the formation of byproducts, eg. THF, is considerably reduced.
Moreover, at a pH of 7 or above for the vapors, shorter fragments, eg. of the polymeric residues, are present so that, on the one hand, they can be dispersed more easily in the dihydroxy compound, ie. the resulting distillation residue is easier to handle because it has a lower viscosity. On the other hand, the columns can be operated for considerably longer without cleaning (longer service lives) and the solid residues can be removed more easily from the column on cleaning, eg. with water.
In a particular embodiment of the invention, further butanediol, for example, can be recovered in the evaporation apparatus following the distillation treatment, and this working up is independent of the availability of the residue containing dihydroxy compounds (HEX).
In the process according to the invention, from 0.5 to 10, preferably 1 to 8 and, in particular, 2.5 to 6, % by weight, calculated as alkali metal or alkaline earth metal, based on the solids contents of the vapors, are added preferably to the bottom product from columns A, B or C in the treatment of the vapors.
Suitable in principle are inorganic and organic compounds of the alkali metals, preferably of Li, Na, K, particularly preferably Na compounds.
Suitable inorganic compounds of the alkaline earth or alkali metals, preferably of sodium, are, for example, the corresponding silicates, phosphates, phosphites, sulfates or, preferably, carbonates, bicarbonates and hydroxides.
The organic compounds of the alkaline earth or alkali metals, preferably of sodium, include the corresponding salts of (cyclo)aliphatic, araliphatic or aromatic carboxylic acids having, preferably, up to 30 carbon atoms and, preferably, 1 to 4 carboxyl groups. Examples thereof are: alkali metal salts of formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, caprylic acid, stearic acid, cyclohexanecarboxylic acid, succinic acid, adipic acid, suberic acid, 1,10-decanedicaroxylic acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, 1,2,3-propanetricarboxylic acid, 1,3,5-cyclohexanetricarboxylic acid, trimellitic acid, 1,2,3,4-cyclopentanetetracarboxylic acid, pyromellitic acid, benzoic acid, substituted benzoic acids, dimer acid and trimer acids, and neutral or partially neutralized montan wax salts or montan wax ester salts (montanates). Salts with other types of acid residues, such as alkali metal paraffin-, olefin- and arylsulfonates or else phenolates, and alcoholates, such as methanolates, ethanolates, glycolates, can also be employed according to the invention. Sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium salts of mono- and polycarboxylic acids, in particular aliphatic mono- and polycarboxylic acids, preferably those having 2 to 18 carbon atoms, in particular having 2 to 6 carbon atoms, and up to four, preferably up to two, carboxyl groups, and sodium alcoholates having, preferably, 2 to 15, in particular 2 to 8, carbon atoms are preferably used. Examples of particularly preferred representatives are sodium acetate, sodium propionate, sodium butyrate, sodium oxalate, sodium mallonate, sodium succinate, sodium methanolate, sodium ethanolate, sodium glycolate. Sodium methanolate is very particularly preferred and is particularly advantageously employed in an amount of from 2.5 to 6% by weight, calculated as Na, based on the solids content of the vapors. It is also possible to employ mixtures of various alkaline earth or alkali metal compounds.
The alkaline earth or alkali metal or compound thereof can be added in at least one column A, B or C depending on the

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