Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing organic compound
Reexamination Certificate
2000-02-04
2001-11-06
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing organic compound
C205S440000
Reexamination Certificate
active
06312582
ABSTRACT:
The invention relates to a process for recovering aromatic polycarboxylic acids such as terephthalic acid (hereinafter abbreviated to H
2
TP) from a solution of alkali metal salts of such aromatic polycarboxylic acids, in particular from a solution of sodium terephthalate (Na
2
TP).
Na
2
TP is a product which is available or potentially available on the market in large quantities, for example by alkaline saponification of industrial polyol polyterephthalates, in particular glycol. Polyterephthalates (PTEs) are used in large quantities in the food industry, in particular for the production of plastic bottles, or in other industries, for example for the production of support membranes for photographic films or the like. Examples of such saponification processes which can be mentioned are those described in British patent GB 822 834, U.S. Pat. No. 3,544,622 and French patents FR-A-2 672 049 and FR-A-2 697 839. The Na
2
TP produced (obtained as a solution or as an aqueous solution) must then be transformed to H
2
TP which can in its turn be recycled to the step for producing new batches of PTE by polymerisation with fresh batches of polyol.
The transformation of Na
2
TP to H
2
TP is not without its difficulties, however, even if it is obtained in the practically pure state, either because of a modification in the conditions under which saponification is carried out or as a result of previous purification operations. Clearly, care must be taken in all cases to avoid the use of excessive amounts of water. Even when very pure, Na
2
TP solutions are highly alkaline, if only because of their concentrations. They can contain more than 120 g/l of Na
2
TP. The pH can be in the range 10 to 12.5 depending on the concentration of Na
2
TP.
The process which is most frequently recommended for recovering Na
2
TP consists of acidifying the medium, in particular with sulphuric acid. The reduction in pH then causes H
2
TP to precipitate out, in a very considerable amount at pHs of less than 4.8, it being substantially complete at values of the order of 4.
However, recycling operations are only of interest industrially if the processes carried out can both recover the major portion of the reactants previously used in the polymerisation steps, in this case terephthalic acid (H
2
TP) and the alkali metal hydroxide, in particular sodium hydroxide in the case of PTE recycling, and can produce them in a purity which is sufficient for direct re-use in new polymerisation steps. This is not often the case when H
2
TP is obtained by precipitation using sulphuric acid because of the presence in that H
2
TP of an excess of SO
4
2−
ions which is found in the form of sodium sulphate. Finally, the presence of traces of alkali metal salts, in particular Na
2
TP and Na
2
SO
4
, in the H
2
TP obtained constitutes a barrier to subsequent re-polymerisation of H
2
TP.
The foregoing observations regarding the importance of recycling is of greater importance for Na
2
TP solutions such as those obtained by processes of the type described in the patents cited above, as they often contain an excess of the sodium hydroxide used in the previous saponification reaction. Thus such solutions must be neutralised first, in particular using sulphuric acid, so that the solution can initially be “screened” and filtered to eliminate solid waste. It is then generally purified further, for example by passage over activated charcoal. Thus the H
2
TP is recovered from such a “pre-purified” solution, usually by supplementary acidification of the medium, usually again with sulphuric acid as mentioned above. This process is thus accompanied by a loss in the total quantity of Na
+
species initially present in the form of sodium hydroxide, and their transformation into sodium sulphate which has only a low added value, let alone none not simply sent to refuse.
Is it actually possible to use the process proposed, for example, in U.S. Pat. No. 4,093,528 which employs electrolysis of such Na
2
TP solutions? A priori, the answer to that question can only be negative, as the yields on converting Na
2
TP to H
2
TP described in some of the examples of that patent are very low. Further, on repeating the experiment, insoluble non-conducting products are observed to form which rapidly block the access of other terephthalic ions to the anode and as a result, rapidly puts a stop to electrolysis.
In short, no economical process exists at present which can directly recover the major portion of the terephthalate and sodium ion species in a high degree of purity, in particular in the form of terephthalic acid and sodium hydroxide, from solutions of Na
2
TP, especially when these are charged with impurities.
Thus the aim of the invention is to overcome these difficulties and provide a process for producing H
2
TP from solutions of its alkali metal salts, in particular Na
2
TP, at a high degree of conversion.
A further aim is to produce other weak aromatic polycarboxylic acids under analogous conditions, which acids, like H
2
TP, are insoluble in an aqueous acidic solution, and wherein the alkali metal salts of which, in particular the sodium salts, are soluble in an aqueous alkaline medium, and which can be polymerised in the presence of polyols, in particular ethylene glycol.
REFERENCES:
patent: 4092230 (1978-05-01), Norton
patent: 4093528 (1978-06-01), Wynkoop et al.
patent: 5290404 (1994-03-01), Toomey et al.
patent: 5545746 (1996-08-01), Benzaria et al.
patent: 0573042 (1993-12-01), None
patent: 2697839 (1994-05-01), None
Durecu Sylvain
Lam Bach-Tuyêt
Thauront Jacques
Wiaux Jean-Paul
Gollin Michael A.
Tredi
Venable
Wong Edna
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