Background Art Processes for producing aromatic carboxylic acids are known in the art. Much work has been focused in'particular on the production of 2,6-naphthalene dicarboxylic acid, because there is such a demand for it in the synthesis of polymeric materials. Some methods involve oxidation. See, for example, U. S. Patent Nos. 2,833,816; 3,870,754; 4,933,491; and 4,950,786. Other processes utilize the step of disproportionation. It is known in the art that naphthalene monocarboxylic acid and naphthalene

dicarboxylic acids other than 2,6-naphthalene dicarboxylic acid can be converted to 2,6-NDA using a disproportionation reaction in the case of the monocarboxylic acids or a rearrangement reaction in the case of other napthalene dicarboxylic acids. Henkel and Cie first patented a reaction of naphthoic acid salts to 2,6-NDA in the late 1950s. (See U. S. 2,823,231 and U. S. 2,849,482). After a disproportionation or rearrangement reaction the dialkali metal salts have to be separated and converted into the desired 2,6-NDA.

There have been different approaches to the separation of the dialkali metal salt products of disproportionation reactions and conversion of them into 2,6-NDA.

In U. S. 2,828,231, the method used to separate the dialkali metal salts of 2,6-naphthalene dicarboxylic acid comprises dissolving the disproportionation conversion product mixture in water, filtering off insoluble impurities from the resulting solution, acidifying the filtrate with mineral or organic acid, such as sulfuric or hydrochloric acid, and separating the precipitated naphthalene-2,6-dicarboxylic acid from the acid solution.

In U. S. 2,823,231 the dialkali metal salt of naphthalene 2,6-dicarboxylic acid formed is converted into free naphthalene 2,6-dicarboxylic acid by acidification of said di-alkali metal salt with a strong mineral acid.

U. S. 2,849,482 teaches acidifying an aqueous solution of the crude reaction product of the disproportionation or converting the crude alkali metal salt directly into the dichloride or into esters of naphthalene-2,6-naphthalene dicarboxylic acid in accordance with known methods.

In U. S. 3,631,096, to Phillips, salts formed by the reaction can be transformed into the corresponding free acids by acidifying the solution with organic or inorganic

acids or by introducing carbon dioxide into the solution at atmospheric or elevated pressure, and then separating the free acids from the acidified solution. The individual reaction products may be separated from each other and isolated in pure form by methods that are based upon their different solubilities or volatilities and may thereafter, if desired, be transformed into their derivatives. The salt mixture produced by the reaction may also be transformed directly into derivatives of the acids, for example, into their esters or halides, and these derivatives may be purified, if desired, by fractional distillation.

U. S. 3,671,578, to Teijin, discloses that the monoalkali salt of 2,6-naphthalene dicarboxylic acid is easily disproportionated when heated in water or water- containing organic solvent, to form free dicarboxylic acid and by-product dialkali salt, and the former acid is precipitated.

U. S. 3,888,921, to Teijin Ltd., discloses a method for purifying a dialkali salt of crude 2,6-naphthalene dicarboxylic acid comprising precipitating 40 to 97 mol percent of the dialkali 2,6-naphthalene dicarboxylate dissolved in an aqueous solution substantially as a monoalkali salt of the 2,6-naphthalenedicarboxylic acid while maintaining the pH of said aqueous solution at a value not lower than 6.3, and separating the precipitate, and converting the separated precipitate to a 2,6- naphthalene dicarboxylic acid.

Canadian Patent 864587 discloses a process for the preparation of 2,6-NDA which comprises heating a monoalkali salt of 2,6-NDA in water or water-containing organic solvent causing disproportionation thereof into 2,6-NDA and a dialkali salt and separating the 2,6-NDA by a method that includes dissolving a rearrangement reaction product

containing dialkali salt of 2,6- naphthalene dicarboxylic acid in warm water, filtering off the insoluble matter therefrom, concentrating the remaining solution, whereby the filtrate is concentrated to such a degree that the precipitation yield of the dialkali salt precipitated when the concentrated liquid is cooled to room temperature reaches at least 70% and the purity of said precipitate exceeds 99%, passing gaseous carbon dioxide through the aqueous solution of the precipitate recovered from the concentrated liquid, and recovering the resulting precipitate, and the mother liquour containing the side product dialkali salt of 2,6-naphthalene dicarboxylic acid is recycled into the carbon dioxide reaction step.

Various methods for separating the reaction medium from the aromatic dicarboxylic acids are disclosed in the art. For example, benzene extraction, evaporation, and sublimation are known. Water extraction is vulnerable to problems which are often associated with the settling and phase separation of large amounts of aqueous solutions mixed with large amounts of organic solutions. Evaporation or sublimation, on the other hand, is a slow and costly method, both with respect to the apparatus and to the energy necessary.

As noted, many of these methods rely on filtration at least in part to separate the dipotassium salt products of disproportionation, but filtration is expensive and unreliable. An improved method of separating the disproportionation solids from the remainder of the effluent materials would ultimately allow for better conversion and yield of the desired 2,6-NDA.

In U. S. 3,952,052, to Phillips, there is disclosed a process for separating a disproportionation reaction product by forming a slurry, comprising alkali metal salts

of aromatic polycarboxylic acid and dispersant, and mixing the slurry with the gaseous effluent, and then lowering the pressure, flashing the dispersant, and recovering said alkali metal salts of said polycarboxylic acids as solids from said separation zone.

The instant invention, while somewhat similar to U. S.

052, provides significant improvement in overall reliability and cost, which will be discussed in the Detailed Description of the Invention.

Disclosure of the Invention In accordance with the foregoing the present invention is a process for separating disproportionation reaction product solids, comprising isomers of the dipotassium salts of NDA (2,6 K2NDA and 2,3 K2NDA), unreacted KNA, catalyst and trace coke, from the reaction medium of naphthalene, said solids exiting said disproportionation reactor in a liquid/solid slurry at a pressure of about 700 to 1200 psig which comprises: a) directing said reactor effluent in the liquid/solid slurry into a fluidic valve with second injection of naphthalene to control the slurry flow rate, b) directing said the combined effluent into a tapered bore of pipe section with decreasing diameter where the liquid/solid mixture is accelerated, causing pressure to drop and part of the liquid to flash, leading to increased acceleration, a further pressure 'drop and additional liquid flashing, thereby gradually flashing all naphthalene in the tapered pipe section, resulting in a change in said reactor effluent from liquid/solid slurry to gas/solid mixture; c) directing said gas/solid mixture into at least one stripper cyclone where solids are separated and

stripped with carbon dioxide, and gases exit and are recycled, and thereafter d) recovering the 2,6-NDA from the solid dipotassium salts of 2,6-NDA.

Detailed Description of the Drawings Figure 1 is a flow diagram showing the preferred embodiment.

Figure 2 shows the details of the fluidic valve and the pressure letdown pipe.

Figure 3 is a flow diagram showing in the prior art of U. S. 3,952,052.

Detailed Description of the Invention The disproportionation reactor product solids to be separated in the present invention can comprise any alkali metal salt of an aromatic dicarboxylic acid along, with unreacted feed, catalyst, and trace coke.

These alkali metal salts are intermediates in the production of aromatic dicarboxylic acids, which are useful in the production of various polymers. Examples include alkali metal salts of terephthalic acid and naphthalene dicarboxylic acid. A particularly desirable aromatic dicarboxylic acid is 2,6-naphthalene dicarboxylic acid, because of its superior properties when reacted with ethylene glycol to form poly (2,6-ethylene naphthalate). The dipotassium salt of 2,6-naphthalene is a precursor of 2,6- naphthalene dicarboxylic acid formed by a process that incorporates a disproportionation reaction.

The feed for the disproportionation reaction is an alkali salt of a mono-or dicarboxylic acid which is changed by the disproportionation reaction to an isomer of a dialkali salt of 2,6-NDA with injection of carbon dioxide. The alkali metal salts of the aromatic mono-or dicarboxylic acids are introduced into a disproportionation

reactor along with a dispersant, and a catalyst. This is typically part of a larger integrated process, as described in copending U. S. Ser. No. 60/151,498, filed of even date and incorporated herein by reference in the entirety. Said alkali metal salts of mono-or dicarboxylic acids, carbon dioxide and catalyst are reacted at elevated temperature and high pressure and the effluent of the reactor is a slurry containing said alkali metal salts of aromatic dicarboxylic acids, unreacted feed, catalyst, trace coke, and dispersant.

The reaction medium can be any dispersant with sufficient thermal stability. It is not restricted to aromatic compounds, however aromatic compounds are suitable.

Examples of suitable dispersants include a single compound or mixture of compounds selected from a variety of aprotic polycyclic aromatic compounds, such as, for example, naphthalene, methylnaphthalene, dimethylnaphthalene, diphenyl ether, dinaphthyl ether, terphenyl, anthracene, phenanethrene, and mixtures thereof. The polycyclic aromatic compound is used in an amount of 1 to 6 times, preferably 2 to 4 times, the amount of the starting material based on weight.

In the preferred embodiment of the present invention the alkali salt is potassium naphthoate which is introduced into the disproportionation reactor along with napthalene as a dispersant and a catalyst comprising a zinc compound, such as zinc oxide or zinc napththoate. Carbon dioxide is also fed into the reactor.

Typical operating conditions for the disproportionation reactor are disclosed in U. S. Pat. Nos.

3,751,457 and 3,781,341, incorporated by reference herein.

Temperatures in the range of about 420°C to 460°C and pressures of about 700 to 1200 psia are preferred. The

residence time of the components in the disproportionation reactor is about 0.3 to 1 hour. The dispersant, e. g. naphthalene, is present in the reaction slurry in an amount of 25 to 80%, preferably to %, by total weight of the slurry.

The disproportionation reaction is carried out either batchwise or continuously in any suitable reactor or sequence of reactors in which the desired temperatures and pressures can suitably be maintained and which can adequately be used for charging, stirring, and discharging of slurries.

The hot effluent slurry from the disproportionation reactor comprises disproportionated alkali metal salts of dicarboxylic acids, and dispersant, together with small amounts of other materials. In the case where potassium naphthoate is introduced into the disproportionation reactor, the effluent comprises isomers of the dipotassium salt of 2,6-NDA (2,6 K2NDA and 2,3 K2NDA), unreacted KNA, catalyst and trace coke. Said effluent is in a liquid/solid slurry. In the present invention the liquid/solid slurry exits the disproportionation reactor where the pressure is in the range of about 600 to 1500 psi, preferably about 800 to 1200 psi, with the most preferred pressure about 800-950 psi.

The following description of the preferred embodiment in connection with the drawing will enable a person skilled in the art to better understand and practice the invention.

The drawing is described in terms of preparing the dipotassium salt of 2,6-NDA using naphthalene as a reaction medium. The specific details given are, however, not intended to limit the scope of the invention. In the drawing many details of the apparatus, such as valves, pumps, controlling means, etc have been omitted. These

items, however, are well known to one skilled in the art.

Any suitable apparatus of this nature can be used.

Referring now to Figure 1, the effluent slurry of solids in naphthalene, consisting of 2,6 K2NDA, 2,3 K2NDA (isomer intermediate), unreacted KNA, catalyst, and trace coke exits the disproportionation reactor in line 1 and enters a fluidic control valve 19 which has a second inlet of hot naphthalene 20 to control the product slurry flow rate. The combined slurry 21 then enters a pressure letdown pipe 2 through which the pressure is reduced, leading to gradual flashing of all liquid naphthalene and CO2, thus converting a liquid/solid slurry 21 to a gas/solid mixture that exits in line 3. The mixture in 3 first enters a stripper cyclone 5 where most of solids in the gas/solid mixture are separated and kept in the lower part of 5 for stripping. Naphthalene and C02 vapor with some entrained solids then exit at 11 and enter a second stage cyclone 13.

Additional solids are separated in 13 and returned to the lower part of the stripper cyclone 5 via 14 for stripping.

Naphthalene and C02 vapor exit 13 at 18. The solids in the lower part of 5 are stripped with C02 via 6. The stripped solids exit 5 in line 7 and enter the second stage stripper 8 where additional stripping gas of C02 is introduced via 9.

The stripping gas and entrained solids exit 8 at 12 and enter cyclone 15 where most entrained solids are separated and returned to stripper 8 via 16. The stripped K2NDA solids exit at 10. The stripping gas exits 15 at 17 which combines with 18 to become the recycled naphthalene and COs stream.

The naphthalene and CO2 vapor 18 exiting the pressure letdown section is recycled, used to evaporate water from the aqueous salt of naphthoic acid, and used as a carrier and dispersant in the disproportionation reaction as discussed in

copending Ser. No. 60/151,577. The net naphthalene produced by the disproportionation reaction is either condensed for production of additional methylnaphthalene via transalkylation with dimethylnaphthalene or sold to the merchant market.

Following the separation and stripping, the solid product 10 comprising dipotassium salts of NDA, K2NDA (2,6- and 2,3-isomers), unreacted KNA, catalyst, heavy by- products, trace coke and entrained C02 vapor are washed, followed by a series of steps for isolation and purification of 2,6-NDA as described in copending U. S. Ser.

No. 60/151,577. The solids are contacted with water to create a mixture of aqueous potassium salts comprising potassium naphthoate, and the dipotassium salt of 2,6- naphthalene dicarboxylic acid and its isomers, and solid catalyst. This mixture is filtered to separate the solid catalyst which is recycled. Then aqueous potassium bicarbonate is added to the mixture of aqueous potassium salts and a portion of the water is evaporated to selectively crystallize the dipotassium salt of 2,6-NDA as a solid. The solid dipotassium salt of 2,6-NDA is dissolved in water and optionally passed through an activated carbon bed to remove impurities. The aqueous dipotassium salt of 2,6-NDA is contacted with carbon dioxide to create a mixture of the solid monopotassium salt of 2,6,-NDA and aqueous potassium bicarbonate. The monopotassium salt of 2,6-NDA is contacted with water, optionally in the presence of CO2, to form solid 2,6-NDA, aqueous dipotassium salt of 2,6-NDA, and potassium bicarbonate. The solid 2,6-NDA is contacted with water in a pipe reactor to remove traces of potassium ion impurity.

One key element in the present invention is the control of pressure letdown of the effluent slurry from the

disproportionation reactor. Figure 2 shows the details of the fluidic valve 19 and the pressure letdown pipe 2 in the preferred embodiment of Figure 1. The fluidic valve is shown as a simple tee, which can be one of several different types known in the art, with two inlets; one for disproportionation product slurry 1 and the other for hot naphthalene 20. The combined steam 21 exits the fluidic valve to feed the pressure letdown pipe 2 through which the pressure is gradually reduced. The result of the pressure letdown is that all naphthalene in the combined liquid/solid slurry 21 is gradually flashed and the liquid/solid slurry is changed to gas/solid mixture 3 at the outlet with no additional heat input.

The combination of the fluidic control valve 19 and the pressure letdown pipe 2 controls the flow rate of the disproportionation reactor products stream 1 without the need of a mechanical control valve, which is unreliable in highly erosive slurry service. The control mechanism of the fluidic valve is based on the fact that the letdown pipe 2 can only flush a certain amount of naphthalene at a given dimension and pressure differential. By increasing the flow rate of hot naphthalene 20 into the fluid control valve 19, the flushing rate of naphthalene through the letdown pipe 2 remains constant, but the flow rate of the disproportionation reactor products 1 can be reduced to any amount, including no flow.

Referring to Figure 2 for details of the pressure letdown. pipe 2, the tapered bore can be one of several different types known in the art. The pipe is shown to have a tapered bore of pipe section with decreasing diameter where the liquid/solid mixture is accelerated, causing pressure to drop and part of the liquid to flash, which leads to increased acceleration, a pressure drop and

additional liquid flashing. Generally the letdown pipe should have: a) a straight pipe section having a length of at least 20 pipe diameters with an opening at the end for receiving the liquid/solid phase product exiting the disproportionation reactor and an opening at the other end for discharging the liquid/solid phase product; b) a concentric tapered bore converging in the direction of flow with an inlet diameter identical to the preceding straight section, an included angle of 3° or less and an outlet diameter identical to that of the following straight section ; c) a concentric straight bore section having an inside diameter and length extending in the direction of its longitudinal axis selected to produce the required pressure drop over the range of operation; d) a concentric, tapered bore section diverging in the direction of flow with an included angle of not more than 14° ; and e) terminating in a pipe or line which receives the resulting gas/solid phase product and transports said gas/solid into the first stripper cyclone.

Another key element of the present invention is the use of stripper cyclone to strip out naphthalene from the solids products. The stripper cyclone in the present process is similar to that disclosed in U. S. 4,455,220, incorporated by reference herein in its entirety, but preferably comprises two stages of stripping, as shown in Figure 1.

This method is allows for maximizing the recovery of solids which, in turn, contributes to improved yields of 2,6-naphthalene dicarboxylic acid in subsequent steps where

the 2,6-naphthalene dicarboxylic acid is isolated and purified.

From the description above, it becomes clear that the present invention provides significant improvements in overall reliability and cost, compared to U. S. 3,952,052 which is shown in Figure 3. Distinctions are as follows: Handling of effluent gas-In 052 the product slurry from the disproportionation reactor is combined with the effluent gas before lowering the pressure. In the instant invention, the effluent gas from the disproportionation reactor is treated separately, which makes the separation equipment more compact, permitting lower capital cost.

Means of pressure letdown and flow control-In 052, "the separation zone can comprise any suitable flash evaporation chamber and related means by which feeds are sprayed into a chamber which is maintained at a reduced pressure." (column 3, Line 48 to 52). Two major improvements in the present invention are that the combination of a fluidic valve and a pressure letdown pipe, instead of a spray, is used as the means of pressure letdown and flow control, and no flashing chamber is required. We have discovered in coal gasification process development that a sudden pressure letdown using a control valve or spray nozzle, as in '052, cannot last for more than a few days or weeks of continuous operation with fluid containing substantial amount of erosive particles. The control valve or spray nozzle will be so severely eroded that it can no longer control the pressure letdown and has to be replaced. The renders the system taught in 052 highly unreliable. In the present invention, pressure reduction occurs more gradually in a section of tapered pipe, instead of sudden

pressure reduction through a valve or spray, thus significantly reducing the severity of erosion. We have demonstrated in coal gasification process development (TPR WTC 72-96 Shell Coal Gasification Process- Equipment Development-Fly Ash Letdown Bench Scale Test Facility; TPR WRC 26-93 Shell Coal Gasification Process- Equipment Development-Fly Ash Letdown-Field Test:; 4,838,898 Method for Removal and Disposal of Fly Ash from a High-Temperature, High-Pressure Synthesis Gas Steam; 4,877,419 Stripping and Depressurization of Solids and Gas Mixture) that the pressure letdown pipe can last for more than 6 months in highly erosive pressure letdown operation which significantly improves the reliability of the separation system. We have also demonstrated that the fluidic valve can control the slurry flow over the same period. Another improvement is that the present invention does not require a flashing vessel. The first stripper cyclone is used as the receiving vessel after the pressure letdown, which makes the system much more compact, compared to'052, thus reducing capital cost.

Product stripping-In 052"The gas stream introduced into the flash evaporation chamber exerts a fluidizing action upon the solids accumulated in the lower portion of said chamber. This fluidizing action prevents the solids from caking together and enables continuous removal of the solid products for the process". The gas stream consists of carbon dioxide, benzene and terphenyl in 052'. From the above description, it is clear that in 052 the gas stream introduced into the flush chamber is only for fluidization purpose to assist product removal.

A major disadvantage of this system is that hydrocarbon vapor, such as benzene and terphenyl, will be discharged when the solids are removed from the flashing chamber

which significantly complicates the downstream product purification process. In the instant invention, two stages of stripping using only carbon dioxide to remove all hydrocarbon vapors, such as naphthalene from solid products. This improvement significantly reduces downstream separation cost, because only carbon dioxide gas is removed with the solids in the subsequent step of dissolving the product solids in water.

The following example is provided to illustrate the present invention as applied to the production of dipotassium naphthoate using naphthalene as a reaction medium and zinc oxide as catalyst in an apparatus such as that schematically shown in the drawing.

EXAMPLE Potassium napthoate and zinc oxide are introduced together with naphthalene and carbon dioxide into a disproportionation reactor. The disproportionation reaction is carried out at about 800 psia and 460°C. The slurry comprising 2,6 K2NDA, 2,3 K2NDA (isomer intermediate), unreacted KNA, catalyst, trace coke and naphthalene exits the disproportionation reactor where the pressure is about 800 psi and enters a fluidic valve with hot naphthalene injection, followed by a tapered bore of decreasing diameter where the liquid/solid mixture is accelerated, causing pressure to drop and part of the liquid to flash, which leads to increased acceleration, a further pressure drop and liquid flashing. The result of the pressure letdown is that all naphthalene is flashed gradually and the liquid/solid slurry becomes gas/solid mixture. This gas solid mixture is directed to two stages of stripping and cyclone separation, as described in the detailed description of the drawing.

Reasonable variation and modifications, which will be apparent to those skilled in the art, can be made in this invention without departing from the spirit or scope thereof.