WO1998031653A1 - Process for the recovery of polycarboxylic acids - Google Patents

Abstract

Aromatic polycarboxylic acids are recovered from the reaction mixtures resulting from the liquid phase oxidation of polyalkyl aromatics. The reaction mixture is spray-dried to obtain crude solid polycarboxylic acid which in turn is slurried in e.g. water, filtered, and optionally further purified by recrystallization. The process is especially suitable for the recovery of terephthalic and isophthalic acids which are important starting material for the production of polyesters.

Description

Process for the recovery of polycarboxylic acids

The present invention relates to a process for the recovery of aromatic polycarboxylic acids from the reaction mixtures resulting from liquid phase oxidation of polyalkyl aromatics.

Aromatic di- and polycarboxylic acids (e. g., isophthalic acid, terephthalic acid, 2,6-naph- thalenedicarboxylic acid, and trimellitic acid, hereinafter called polycarboxylic acids) are very important products, the present worldwide production of which amounts to several millions of tons per year. In the common processes for the production of these polycarboxylic acids, polyalkyl aromatics are oxidized with molecular oxygen in the liquid phase in a solvent (usually acetic acid) containing a catalyst (usually a soluble transition metal compound containing bromide). The separation of the crude oxidation product is usually performed by crystallization. The liquid reaction mixture feeds a series of crystallizers (up to 5 crystallizers are normally used), the precipitated crystals of crude polycarboxylic acid are separated from the mother liquor (acetic acid containing byproducts and catalyst) by filtration or centrifu- gation, and the cake is dried prior to purification. This method is not completely satisfactory because the crystallizers are bulky and expensive (the corrosivity of the bromide- containing acetic acid requires titanium-lined equipment) and their operation is somewhat delicate.

The problem to be solved by the present invention was to provide a method to avoid the use of crystallizers for the separation of the crude product.

According to the present invention the problem is solved by the process of Claim 1.

The mixture resulting from the oxidation reaction is fed to a spray dryer wherein the solvent and other low-boiling components (e. g. water of reaction) of the reaction mixture are evaporated in a stream of hot drying gas and thus are separated from the crude polycarboxylic acid which is recovered as a dry solid. In a second step, the dry crude polycarboxylic acid is transferred to a washing vessel. There it is slurried in a suitable solvent wherein the polycarboxylic acid is poorly soluble and the catalyst and possible byproducts are soluble, to remove the catalyst and the byproducts formed in the oxidation reaction. Subsequently, the undissolved polycarboxylic acid is separated from the slurry by filtration and/or centrif- ugation and the resulting cake of polycarboxylic acid is optionally subjected to further pu- rification steps according to methods known in the art.

The oxygen content of the drying gas is advantageously below the MOC (Minimum Oxygen for Combustion) in order to prevent inflammation and dust explosions. Preferably inert gases (e. g. nitrogen), mixtures of air and inert gas, or the off gases from the oxidation reaction are used.

The temperature of the drying gas entering the spray dryer is preferably in the 200-500 °C range while that of the solvent-laden exhaust gas leaving the spray dryer is preferably in the 100-200 °C range. It has been found that under these conditions the construction material for the spray dryer can be common stainless steel and only the parts being in direct contact with the liquid reaction mixture (feed pipeline and nozzle/atomizer) should have increased corrosion resistance.

The spray dryer can be operated at atmospheric (ambient) pressure or at slightly elevated pressure, e. g. in the range of atmospheric up to 5 bar gauge.

In a preferred embodiment the exhaust gas stream leaving the spray dryer is cooled to condense the solvent, reheated, and recycled to the spray dryer while the recovered solvent is recycled to the oxidation reaction (after being reprocessed, if necessary).

Suitable solvents (washing liquids) for use in the washing vessel are water and aqueous mixtures, e. g. the aqueous acetic acid obtainable by condensation of the spray dryer exhaust gas. The preferred solvent is water. The washing step is suitably performed at ambient or slightly elevated temperature, the actual temperature and the amount of solvent depending on the polycarboxylic acid, its solubility, and the solubility of the byproducts.

After separating the polycarboxylic acid from the slurry by filtration and/or centrifugation, the resulting wet cake may be subjected to further purification according to methods known in the art. As this is usually accomplished by recrystallization from hot (e. g. 250-300 °C) water, no intermediate drying step is required. If, however, the purity of the polycarboxylic acid is already sufficient for the envisaged application, no further purification is necessary and the cake may be dried and shipped in this form. After separation of the polycarboxylic acid, the washing liquid (mother liquor) is advantageously subjected to a further separation step (e. g. liquid/liquid extraction or reverse osmosis) to recover the catalyst and, if necessary, to remove the dissolved byproducts. The remaining liquid can be recycled to the washing vessel, thus minimizing the consumption of solvent, while the recovered catalyst is recycled to the oxidation reactor.

The drawing depicts a preferred embodiment of the invention. For the sake of clarity, it includes the oxidation reactor. The invention is further described in the following examples with reference to the drawing, however, it is not intended to be limited to the embodiments so described.

Examples:

Example 1

Recovery of terephthalic acid

The oxidation ofp-xylene to terephthalic acid is carried out according to known methods in the reactor 1 in the liquid phase in a solvent, conveniently acetic acid, using molecular oxygen (air) as oxidation agent. The oxidation takes place in the presence of a standard catalyst comprising soluble compounds of cobalt and manganese and bromide as an activator. The reactor 1 is preferably a stirred reactor. The oxidation reaction is highly exothermic and the heat of reaction is removed by evaporation of the solvent and water (of reaction), which are condensed in the reflux condenser 2. The off-gases are sent to an off-gas treatment unit via the vent line 103. The oxidation is conveniently performed in a continuous manner by continuously feeding the reactor with a mixture containing -xylene, solvent (the weight ratio solvent/p-xylene being in the 3 to 7 range) and catalyst through the solvent/feedstock entry 101 and air through the air entry 102 and discharging the reaction mixture containing terephthalic acid through the oxidation product exit line 104. The reaction temperature is in the 160-240 °C range and the pressure is between 15 and 30 bar (absolute). The oxygen concentration in the off-gas is typically between 1 and 8 vol%. The liquid reaction mixture is fed at essentially the same temperature and pressure into the spray dryer where it is atomized and flashed down to a pressure of 0-2 bar gauge in a hot drying gas stream at ca. 400 °C. As a drying gas, nitrogen is conveniently used. The solvent-laden exhaust gas leaves the spray dryer through the exhaust gas line 115 at a temperature of ca. 130 °C. It passes through the dust filter 8 to remove entrained fine particles of terephthalic acid and then through the vapor condenser 9 where it is cooled down to 30-40 °C. At this temperature, acetic acid containing 5-6% water is condensed. The acetic acid is led through the condensate line 117 into the solvent storage vessel 10 and eventually recycled to the oxidation step via the solvent recycling line 123. The greater part of the cooled exhaust gas is recycled through the drying gas recycling line 118, ^'then reheated to 400 °C in the heat exchanger 11 and fed into the spray dryer through the drying gas entry 105. A small stream of gas is purged through the purge gas line 119 and washed in the scrubber 12 before being discharged to the atmosphere through the clean gas exit 120. A corresponding make-up stream of fresh nitrogen is added to the recycled drying gas stream through the drying gas make-up line 121. The dried terephthalic acid containing the components of the catalytic system and reaction byproducts is discharged at a temperature of ca. 130 °C to the storage hopper 4. Subsequently, it is transferred by pneumatic conveying through the conveying pipe 106 and the cyclone 13 into the washing vessel 5. The collected fines from the dust filter 8 are also fed into the washing vessel through the filter dust line 116. The washing step can be performed continuously or batchwise. The washing vessel is fed with cold or warm water through the washing liquid entry 107 and the acid is dispersed in the water at a weight ratio water/tere- phthalic acid of 4 to 6. The washing removes the catalyst and most of the byproducts like ?-toluic acid etc.. The resulting aqueous slurry of terephthalic acid is withdrawn through the slurry exit 108 and fed into the filtration/centrifugation unit 6. The wet filter cake 109 is transferred to the final purification step or dried (not shown). The final purification can be carried out according to known methods, e. g. by dissolving in hot water (typically 250-300 °C/70-120 bar) to produce a 10 to 25 wt% aqueous solution and cooling to precipitate pure terephthalic acid which is separated by filtration and/or centrifugation and dried. If required, the hot solution can be subjected to a hydrogenation treatment to eliminate 4-carboxybenzaldehyde which may occur as a byproduct of the oxidation reaction. The liquid effluent of the filtration/centrifugation unit 6 is fed into the mother liquor treatment (e. g., reverse osmosis) unit 7 through the mother liquor line 110. It is split into a permeate (essentially pure water) that is recycled to the washing vessel 5 through the washing liquid recycling line 111 and a concentrate containing the catalyst, which is withdrawn through the catalyst solution line 112. The concentrate can be recycled to the oxidation reactor through the catalyst recycling line 113 or transferred to a catalyst recovery unit through the line 114.

Example 2

Recovery of isophthalic acid

The process for the production and recovery of isophthalic acid is similar to that descibed in example 1, except for the following differences: -Xylene is used as a starting material instead of -xylene. The acetic acid//»-xylene weight ratio is somewhat higher than in the case ofp-xylene, usually in the 3 to 10 range.

This results in a higher amount of heat required for evaporating the solvent in the spray dryer 3. This can be accomplished by either increasing the flow rate of the drying gas or increasing its temperature (up to 500 °C). The solubility of isophthalic acid in water is higher than that of terephthalic acid. The weight ratio water/isophthalic acid in the washing vessel 5 should therefore preferably be in the 3 to 5 range. List of reference numerals used:

oxidation reactor 1 reflux condenser 2 spray dryer 3 storage hopper/pneumatic conveyor 4 washing vessel 5 filtration/centrifugation unit 6 mother liquor treatment (reverse osmosis) 7 dust filter 8 vapor condenser (solvent recovery) 9 solvent storage vessel 10 heat exchanger (drying gas heating) 11 off-gas scrubber 12 cyclone 13 solvent/feedstock entry 101 air (oxygen) entry 102 vent line (to off-gas tratment) 103 oxidation product exit line (to spray dryer) 104 drying gas entry 105 conveying pipe (to cyclone) 106 washing liquid entry 107 slurry exit 108 filter cake (to final purification or drying) 109 mother liquor line 110 washing liquid recycling line 111 catalyst solution line 112 catalyst recycling line (to oxidation reactor) 113 line to catalyst recovery 114 exhaust gas line (to dust filter) 115 filter dust line (to washing vessel) 116 condensate line (reaction solvent) 117 drying gas recycling line 118 purge gas line (to scrubber) 119 clean gas exit 120 drying gas make-up line 121 conveying gas line 122 solvent recycling line (to oxidation process) 123

Claims

Claims:

1. A process for the recovery of aromatic polycarboxylic acids from the reaction mixtures resulting from the liquid phase oxidation of polyalkyl aromatics in a solvent containing a catalyst, characterized in that

(i) the reaction mixture is fed to a spray dryer operated with a stream of hot drying gas,

(ii) the thus-obtained dry solid polycarboxylic acid is slurried in a solvent wherein the polycarboxylic acid is poorly soluble and the catalyst and possible byproduct are soluble,

(iii) the undissolved polycarboxylic acid is separated from the slurry by filtration and/or centrifugation and, optionally,

(iv) the resulting cake of polycarboxylic acid is subjected to further purification steps.

2. A process as claimed in Claim 1, characterized in that an inert gas is used as a drying gas.

3. A process as claimed in Claim 2, characterized in that nitrogen is used as an inert gas.

4. A process as claimed in any of Claims 1 to 3, characterized in that the temperature of the drying gas entering the spray dryer is 200-500 ┬░C.

5. A process as claimed in any of Claims 1 to 4, characterized in that the spray dryer is operated at a pressure in the range of atmospheric pressure to 5 bar gauge.

6. A process as claimed in any of Claims 1 to 5, characterized in that the exhaust gas stream leaving the spray dryer is cooled to condense the solvent, reheated, and recycled to the spray dryer and the recovered solvent is recycled to the oxidation reaction.

7. A process as claimed in any of Claims 1 to 6. characterized in that water is used as a solvent for slurrying.

8. A process as claimed in any of Claims 1 to 7, characterized in that terephthalic acid is used as an aromatic polycarboxylic acid.

9. A process as claimed in any of Claims 1 to 8, characterized in that isophthalic acid is used as an aromatic polycarboxylic acid.