CS260271B1 - A method for recovering cobalt catalyst in the production of dimethyl terephthalate - Google Patents

Abstract

The solution concerns a method of regenerating a cobalt catalyst from distillation residues arising in the production of dimethyl torophthalic acid ester. The essence of the solution is that the cobalt catalyst is separated from the distillation residue by extraction with water or aqueous solutions of lower aliphatic acids at temperatures of 50 to 150 °C and pressures of 0.02 to 0.5 MPa. In order to prevent the formation of emulsions, the distillation residue flows in a thin film during extraction or is dispersed into droplets having dimensions larger than 0.1 millimeter. The cobalt component of the catalyst passes into the extract in the form of cobalt salts of lower organic acids together with other organic substances. The majority of soluble organic substances are separated from the extract by distilling off volatile organic substances from the extract and cooling the remaining aqueous solution to a temperature of 0 to 50 degrees Celsius. The precipitated organic substances are separated from the solution mechanically and the extract thus treated is used for the preparation of the catalyst. The described method of regeneration of the cobalt catalyst can be used in the production of dimethyl terephthalate.

Description

The invention relates to a process for the regeneration of a cobalt catalyst from the production of dimethyl terephthalate (DMT) by oxidation of p-xylene and methyl p-toluylate in the presence of cobalt salts.

Several methods of cobalt recovery from distillation residues in the production of DMT are described in the literature and in industrial practice. One group may include processes in which the distillation residues are incinerated and the cobalt oxides trapped in the electrofilters. The captured metal oxides are dissolved in acids and a catalyst is prepared from the resulting solutions.

The disadvantages of these processes are lower cobalt yields compared to cobalt extraction processes, but mainly the difficult separation of cobalt salts from solutions containing other inorganic substances.

Another group may include processes in which cobalt cation is extracted from distillation residues with aqueous acid solutions in intermittent or continuous operation of extractors. Usually, stirred vessels or columns with rotating parts are used for this purpose.

Blending emulsions are very often produced by mixing. In order to reduce the likelihood of emulsions, the properties of the distillation residues are adjusted prior to extraction by the addition of solvents which blend perfectly with the distillation residue and do not dissolve in water. However, this increases the cost of cobalt recovery since the solvent usually has to be recovered from the raffinate after extraction.

When using most of the solvents suggested, the raffinate density is greatly reduced compared to the raffinate density from the original undiluted distillation residues, and there are difficulties in gravitational separation of the extract and raffinate.

By-products of the oxidation of p-xylene and methyl p-toluylate are also extracted from the distillation residues in addition to the cobalt and possibly manganese cations. If an extract containing significant amounts of these organic substances is used to prepare the catalyst, the substances will cause a disruption to the oxidation process and the yields of the reaction will be greatly reduced while forming undesired by-products.

For separating the cobalt cation from the organic matter, the literature describes either the use of ion exchangers in which cobalt is collected and subsequently washed out of it with aqueous solutions containing a Na ⁺ or K ⁺ cation, or the cobalt salt extract is obtained by evaporation and by crystallization, or the cobalt precipitates out of solution in the form of an insoluble salt, for example a carbonate. A catalyst is then prepared from the cobalt thus obtained. These methods of separating cobalt from the extract greatly increase the cost of catalyst recovery.

The above-mentioned disadvantages are eliminated by the regeneration of the cobalt catalyst, which consists in extracting the distillation residue after distilling off the lower boiling fractions from the esterification product at temperatures of 60 to 150 ° C with water or an aqueous solution of lower aliphatic carboxylic acids. Both cobalt cation and organic matter are passed into the extract. The content of undesired organic substances in the extract can be reduced by using distillation residues after methanol esterification for n- and extraction, then the low-boiling organic substances are distilled off from the extract, and the non-evaporated aqueous solution s-a is cooled to 0 to 50 ° C.

The precipitated solids are separated mechanically from the extract by, for example, sedimentation, filtration, centrifugation and the like. The treated extract containing only small amounts of undesirable organic substances is used to prepare a catalyst for the oxidation of p-xylene and methyl p-toluylate.

The composition of the distillation residue to be extracted is only partially known. Except for cobalt, the content of which is 0.05 to 0.8% by weight. the residue contains 5 to 15 wt. % dimethyl terephthalate and dimethyl isophthalate, 1 to 2 wt.%, methyl p-toluylate, 1 to 5 wt. % paratoluylic acid, 5-10 wt. % monomethyl terephthalate, 2 to 8 wt. % para -formylbenzoic acid and up to 75 wt. other higher molecular unspecified substances.

The composition of the distillation residue is not contiguous and varies as technological conditions change, whether the oxidation of p-xylene and methyl p-toluylate, or the distillation of oxidation products.

As the composition of the distillation residue changes, its susceptibility to emulsion formation also changes. Therefore, in order to prevent the formation of an emulsion, it is desirable to extract the cobalt from the running film of the distillation residue or to use such an extractor type, optionally to select such extraction conditions that no part of the distillation residue is dispersed into very small drops. Several types of extractors switch such requirements. They are mainly shower and packed column, RTL extractor and pulse and vibration extractors.

The improved effect of the process described above is that the solution of cobalt salts and aliphatic acids prepared by the process contains only small amounts of organic substances which do not interfere with the oxidation of p-xylene and methyl p-toluylate. After replenishment of cobalt losses, the solution is immediately used as an oxidation catalyst. In addition, costly regeneration of the distillation residue diluents is avoided or, in the case of non-use of diluents, formation of hardly separable emulsions is avoided.

Example 1

The pump 30 is metered into the manifold at the top of the packed extraction column

S 0.2 7 1 kg / h of distillation residue from vacuum distillation of esterification products in the production of dimethyl terephthalate. The residue contains 0.2 wt. cobalt bound in the form of complex salts. The probable organic content is given above. The temperature of the residue is 95-100 ° C. The residue and the extender, which is acetic acid, flow through the column continuously and for flow. The pressure in the column is 0.11 MPa. An extracted distillation residue containing 0.03 wt. cobalt.

An extract containing 0.85 wt. % cobalt cation, 5 to 10 wt. % organic compounds including acetic acid and 89-94 wt. water. The extract is drained into a container. The volatile fractions are distilled off from the contents of the container. The remaining aqueous solution was cooled to 0 ° C and left at this temperature for 24 hours. The separated organics were separated from the solution on a tissue filter at 1 ° C. The filtrate contains, in addition to cobalt acetate and acetic acid, 1 to 3 wt. water-soluble organic substances which do not interfere with the oxidation of p-xylene. A catalyst solution of the desired composition is prepared from the filtrate by adding the necessary amount of cobalt acetate. The cobalt recovery yield is 85%.

Example 1 and d 2

The distillation residue was passed through the same processes and apparatus as in Example 1 except that it was extracted with an aqueous acetic acid solution at 150 ° C and 0.5 MPa. After the volatile organic materials were distilled off, the aqueous solution of cobalt salts and non-volatile organic substances was cooled to 50 ° C. The precipitated solids are filtered off at this temperature. The recovery yield of cobalt salts is 82%.

Example 3

The distillation residue, which was freed from the free organic acids by esterification with methanol, was passed through the same processes and apparatus as described in Example 1 except that it was extracted at 60 ° C. In order to be extracted at 60 ° C, the distillation residue must be separated, for example with 1: 1 p-xylene. The aqueous solution of the cobalt salts from which the volatile organic compounds were distilled was cooled to 0 ° C and the precipitated organic solids were filtered off. The recovery yield of cobalt salts is 86%.

Example 4

The distillation residue is passed through the same processes and apparatus as in Example 1, except that water is used as the extractant. Moreover, the extraction conditions remain unchanged. The cobalt recovery yield is 80

The cobalt catalyst regeneration process described can be used in dimethyl terephthalate plants.

Claims (3)

0.25 1 kg / h of distillation residue from the vacuum distillation of the esterification products in the production of dimethyl terephthalate. The residue contained 0.2 wt. cobalt bound in the form of complex salts. Probable inorganic substances are given in the previous text. The temperature of the residue is 95-100 ° C. The residue and the extractant, which is the acetic acid flow through the column continuously and pro-stream. The column pressure is 0.11 MPa. Extracted distillation residue containing 0.03 wt. cobalt. Extract containing 0.85 wt. % of a cobalt cation, 5 to 10 wt. % of organic matter including acetic acid and 89 to 94% by weight. The water is extracted into the reservoir. The volatiles were distilled off from the contents of the flask and the remaining aqueous solution was cooled to 0 DEG C. and kept at this temperature for 24 hours. The excreted organic matter is warmed to 1 ° C of the solution on the tissue filter. The filtrate contains, in addition to cobalt acetate, 1 to 3% by weight. however, they do not interfere with the oxidation of β-xylene. A catalyst solution is prepared from the filtrate with the desired composition of addition of the required amount of cobalt acetate. The cobalt regeneration yield is 85%. EXAMPLE 2 The distillation residue is passed through the same processes and apparatus as in Example 1, and is extracted with an aqueous solution of acetic acid at 150 DEG C. and a pressure of 0.5 MPa. After the volatile organic substances have been distilled off, the aqueous solution of cobalt salts and non-volatile organic liquids is cooled to 50 ° C. The resulting solids are filtered off at this temperature. Recovery of cobalt salt recovery is 82%. EXAMPLE 3 The distillation residue, which was freed of free organic acids by esterification with methanol, was passed through the same processes as described in Example 1, except that it was extracted at 60 ° C. In order to extract at 60 ° C, the distillation residue must be separated, for example by 1-1 xylene. The aqueous solution of cobalt salts, from which the volatile organic matter was distilled off, was cooled to 0 ° C and the precipitated solids were filtered off. The recovery of co-salt salts is 83%. EXAMPLE 4 The distillation residue passes through the same processes and apparatus as in Example 1 except that water is used as the extractant. Moreover, the conditions of extraccession do not change. Cobalt regeneration recovery 80 The described cobalt catalyst regeneration process can be used in the production of dimethyl terephthalate. OBJECT A process for recovering a cobalt catalyst from a distillation residue resulting from the distillation of the volatiles from the esterification product in the preparation of dimethyl terephthalate by oxidation of p-xylene, and methyl p-toluyl, characterized in that lower aliphatic acids at temperatures of 50 to 150 rC and pressures of 0.02 to 0.5 MPa extract the distillation residue from the extract which is an aqueous solution of co-salts of aliphatic acids and volatile organic compounds are distilled off; the aqueous solution is cooled to 0 ° C to 0 ° C, the precipitated organic matter is mechanically separated therefrom, and the cobalt salt solution used to prepare the oxidation catalyst. 2. Process according to claim 1, characterized in that the distillation residue flows into the thin film in contact with the aqueous phase during the extraction or is dispersed into droplets having larger dimensions than 0.1 mm, wherein the extraction can be discontinuous or uninterrupted . 3. A process according to claim 1, wherein the distillation residue is freed from free organic acids by esterification with methanol.