FIELD OF THE INVENTION

[0002] This invention relates to the production of aromatic carboxylic acid by the liquid phase oxidation of a corresponding aromatic compound having two or three oxidizable ring substituents. Specifically, this invention relates to a process for the production of aromatic carboxylic acid in by the liquid phase oxidation of a correspond aromatic compound having two or more oxidizable ring substituents wherein the oxidation solvent comprises benzoic acid and water and the process yields aromatic carboxylic acid with reduced impurity levels.

BACKGROUND OF THE INVENTION

[0003] Aromatic carboxylic acids are useful chemical compounds and are raw materials for a wide variety of manufactured articles. The most widely used commercial processes for the production of aromatic carboxylic acids involve the catalytic liquid-phase oxidation of a suitable aromatic feedstock under elevated pressure and temperature conditions. For example, ortho-xylene is oxidized to produced phthalic acid (“PA”), meta-xylene is oxidized to produce isophthalic acid (“IA”), para-xylene is oxidized to produce terephthalic acid (“TA”), 2,6-dimethynaphthalene is oxidized to produce 2,6-naphthalene dicarboxylic acid (“NDA”) and pseudocumene is oxidized to produce trimellitic acid (“TMLA”). These processes may be catalyzed by one or more heavy metal compounds, such as cobalt, manganese, zirconium, cerium or mixtures thereof. In addition, the oxidation reaction is usually promoted one or more promoter compounds, for example elemental bromine.

[0004] TA is likely the most widely produced aromatic carboxylic acid. TA is manufactured on a world-wide basis in amounts exceeding 10 billion pounds per year. A single manufacturing plant can produce 100,000 to more than 750,000 metric tons of terephthalic acid per year. TA is used, for example, to prepare polyethylene terephthalate, from which polyester fibers for textile applications and polyester film for packaging and container applications are made. Although there are competing processes, TA is most often produced by the high pressure, exothermic oxidation of para-xylene in a liquid-phase reaction using air or other source of molecular oxygen as the oxidant and catalyzed by one or more heavy metal compounds and one or more promoter compounds.

[0005] Methods for oxidizing para-xylene and other aromatic compounds using such liquid-phase oxidations are well known in the art. For example, Saffer in U.S. Pat. No. 2,833,816 discloses a method for oxidizing aromatic feedstock compounds to their corresponding aromatic carboxylic acids. Central to these processes for preparing aromatic carboxylic acids is employing an oxidation catalyst comprising a heavy metal component and a source of bromine in a liquid-phase reaction mixture including a low molecular weight monocarboxylic acid, such as acetic acid, as part of the reaction solvent. A certain amount of water is also present in the oxidation reaction solvent. Water is also formed as a result of the oxidation reaction. Although various means can be used to control the temperature of the highly exothermic oxidation reaction, it is generally most convenient to remove heat by allowing the solvent to vaporize, i.e. boil, during the oxidation reaction. Gaseous effluent from the oxidation reaction generally comprises steam, monocarboxylic acid, an ester thereof, carbon dioxide, carbon monoxide and bromine which, depending on the aromatic feedstock compound used, is mainly in the form of one or more alkyl bromide compounds, such as methyl bromide. Methyl bromide is toxic and, if discharged into the atmosphere, is believed to contribute to depletion of atmospheric ozone. It is therefore important to avoid discharge of methyl bromide into the atmosphere. Additionally, when compressed air is used as the source of molecular oxygen, the gaseous effluent contains nitrogen gas and unreacted oxygen.

[0006] In conventional manufacturing processes, TA undergoes catalytic purification to reduce the amount of impurities found therein. Purified Terephthalic Acid (“PTA”), from which fibers, bottles, films etc. are made, is obtained by the catalytic purification of crude terephthalic acid (“TA”) generated by the liquid-phase oxidation of para-xylene.

[0007] Typically, after the TA is formed by oxidation, it is crystallized and separated from its mother liquor which comprises catalyst components, acetic acid and a variety of intermediates and by-products. The crystallized TA contains a number of impurities, such as 4-carboxybenzaldehyde (“4-CBA”) and colored impurities, which are measured by the optical density (light absorption) at 400 nm (“OD400”). These impurities cause undesired effects in the polyester resin. Therefore the TA must be purified.

[0008] In a typical purification process, the crystallized TA is dissolved in deionized water at temperatures of from about 250° C. and upward. The solution is then contacted with molecular hydrogen in the presence of a hydrogenation catalyst. The solution is then cooled to crystallized the purified terephthalic acid which is then recovered, washed and dried. Using conventional processes, TA usually contains about 2000 to about 5000 ppm of 4-CBA and OD400 values of approximately 0.1. And PTA typically contains between less than about 75 ppm of 4-CBA and OD400 values of approximately 0.01.

[0009] Also, in use today are liquid-phase processes that produce Medium Grade Terephthalic Acid known as MTA. MTA can be used in many of the same applications as PTA, for example, fibers and films. MTA usually contains from about 100 to about 500 ppm of 4-CBA and may have OD400 values slightly greater than about 0.01. Although MTA contains more 4-CBA than PTA, it is produced by substantially the same oxidation process with no subsequent purification.

[0010] Conventional processes for the production of IA, PA, NDA and TMLA are similar to that for TA. In each case, the process involves the liquid-phase oxidation of an appropriate aromatic feedstock. Like the TA processes, the aromatic acids obtained from the oxidation contain impurities the level of which is reduced by some type of purification process. In the case of TMLA, the acid is often further processed through dehydration to form trimellitic anhydride.

[0011] In general, an appropriate feedstock is a benzene having two appropriately positioned oxidizable ring substituents in the case of TA, IA and PA. For TMLA a suitable feedstock is a benzene ring having oxidizable ring substituents in the 1, 2 and 4 positions. For NDA production a suitable feedstock is naphthalene having oxidizable ring substituents in the 2 and 6 positions.

[0012] What is needed is a process for the production of aromatic dicarboxylic or tricarboxylic acid in which the production of toxic methyl bromide production is minimized. The current invention provides a process for the production of aromatic dicarboxylic or tricarboxylic acid in which the formation of methyl bromide substantially reduced relative to conventional processes.

[0013] In addition, the current invention provides a process for the production of aromatic dicarboxylic or tricarboxylic acid in which catalytic purification is largely optional. As in one embodiment, to TA produced is suitable for direct conversion to PET without a separate purification step. Other advantages of the invention will become apparent upon reading the following detailed description and appended claims.

SUMMARY OF THE INVENTION

[0014] The current invention provides a continuous process for the production of aromatic carboxylic acid by the liquid phase oxidation of an aromatic feedstock with oxygen in a reaction medium comprising the aromatic feedstock, an oxidation promoter, heavy metal catalyst and solvent, the solvent comprising benzoic acid and water, wherein the oxidation is carried out in the reaction zone of a plug flow reactor and wherein at least a portion of the aromatic acid produced crystallizes in the reaction zone. In one embodiment, the oxidation promoter is bromine. In another embodiment, the heavy metal catalyst comprises cobalt, manganese, zirconium, cerium or mixtures thereof. As much as 10%, 15%, 25% or more, by weight, of the aromatic acid may crystallize from the reaction medium in the reaction zone. The oxygen required for the current process is supplied by an oxygen-containing stream which may comprise air or any other suitable oxygen-containing gas. Importantly, the current invention may be used to produce phthalic acid, terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, trimellitic acid or mixtures thereof depending on the composition of the aromatic feedstock.

[0015] In one aspect of the current invention, the solvent ratio in the reaction medium as it enters the reaction zone is from about 1 to about 40. As used herein the solvent ratio is determined as follows:

SOLVENT RATIO=WEIGHT OF SOLVENT/WEIGHT OF AROMATIC FEEDSTOCK

[0016] Preferably, the solvent ratio in the reaction medium when it enters the reaction zone is from about 2 to about 30.

[0017] The use of benzoic acid as part of the solvent serves to substantially reduce or eliminate the production of methyl bromide relative to conventional process in which an aliphatic acid, e.g. acetic acid, is used. In the current invention, the solvent comprises from about 5% to about 60% water by weight. Preferably, the solvent comprises from about 10% to about 40% water by weight.

[0018] Plug flow reaction conditions are employed to reduce the level of oxidation intermediates, such as 4-CBA, in the reaction zone effluent. By “plug flow reactor” we mean reactor conditions under which the aromatic reactants are prevented from exiting the reaction zone in a residence time significantly shorter than the average residence time of the reactor charge. Importantly, the plug flow reactor of the current invention may comprise a series of two or more continuous stirred tank reactors. The use of a series of continuous stirred tank reactors to achieve plug flow conditions is a common technique recognized and frequently used by those of ordinary skill in the art.

[0019] With regard to the current invention, the residence time of the reaction medium in the reaction zone can be optimized to allow for more complete oxidation of the aromatic feedstock relative to conventional processes. Accordingly, the aromatic carboxylic acid obtained from the reaction zone effluent contains lower levels of oxidation intermediates when compared to the oxidation effluent of a conventional process. When the current process is used to produce TA, the amount of 4-CBA in the TA obtained from the reaction medium after the reaction zone is sufficiently low such that a separate purification step is not needed before the TA is converted into PET. Preferably, the amount of 4-CBA in the TA is less than about 500 ppm.

[0020] As mentioned previously, the oxidation reaction is highly exothermic. The current invention contemplates adiabatic reaction conditions. Accordingly, no heat is removed from the reaction zone external means. Moreover, the reaction medium may boil thereby generating an off-gas stream that may comprise water vapor, benzoic acid, carbon monoxide, carbon dioxide, oxygen and other gaseous components. This off-gas may be processed and treated using a variety of methods known the those of ordinary skill in the art.

[0021] In another embodiment, the current invention also provides a continuous process for the production of a aromatic carboxylic acid by the liquid phase oxidation an aromatic feedstock comprising: (a) providing a reaction medium comprising aromatic feedstock, heavy metal catalyst, a source of bromine, and solvent comprising benzoic acid and water, wherein the aromatic feedstock comprises a benzene having two oxidizable alkyl ring substituents or a naphthalene having two oxidizable alkyl ring substituents and wherein the solvent ratio in the reaction medium is in the range from about 1 to about 30; (b) contacting at least a portion of the reaction medium with an oxygen-containing gas in a first continuous stirred tank reactor thereby generating a product comprising crystalline aromatic carboxylic acid in a liquid medium comprising carboxylic acid, water, heavy metal catalyst, bromine, benzoic acid, oxidation intermediates and by-product compounds; (c) transferring at least a portion of the product to a second continuous stirred tank reactor wherein at least a portion of the product is contacted with an oxygen-containing gas whereby a substantial portion of the oxidation intermediates are oxidized to aromatic carboxylic acid.

[0022] According to this embodiment of the current invention, the liquid phase oxidation takes place in two stages, the second stage being useful to complete the oxidation of a substantial portion of oxidation intermediates to carboxylic acid. The crystallized carboxylic acid may comprise about ten percent or more of the carboxylic acid produced in the first continuous stirred tank reactor. The solvent ratio in the first continuous stirred tank reactor is preferably less than about 20. Preferably, the solvent comprises about 5% to about 60% water, by weight, more preferably from about 10% to about 40% water. The preferable aromatic feedstock are selected from the group consisting of para-xylene, meta-xylene, ortho-xylene, 2,6-dimethylnaphthalene or mixtures thereof.

[0023] Similar to the plug flow embodiment discussed above, this embodiment of the current invention contemplates adiabatic operation of the first and second continuous stirred tank reactors. Therefore, gaseous off-gas streams are generated. These gaseous off-gas streams comprise water, carbon dioxide, oxygen, carbon monoxide and benzoic acid. When air is used and the oxygen-containing gas, these overhead off-gas streams also comprise nitrogen and other non-condensible components. Importantly, the off-gas stream from each reactor may be treated separately or the streams may be combined into one combined stream and treated as such.

[0024] In conventional processes for the production of carboxylic acid in which aliphatic acid, e.g. acetic, solvent is used, the gaseous overhead is stream is treated to remove methyl bromide and other environmental bad actors generated by the oxidation reaction, and to recover desirable components which may be returned to the oxidation reaction. These treatment and recovery operations typically involve fractionation, scrubbing, and catalytic oxidation. In addition, energy recovery schemes, such as those disclosed in co-owned U.S. Pat. Nos. 5,612,007 and 5,723,656 both to Abrams and the teachings of which are incorporated herein by reference, may be employed to recover energy generated by the exothermic oxidation reaction by proper handling of the off-gas. In any event, any off-gas recovery/treatment system necessarily involves separating water from the solvent acid and removing environmentally offensive components by scrubbing or catalytic oxidation.

[0025] The use of benzoic acid as a solvent component serves substantially reduce the complexity of the processes and equipment needed to treat or recover off-gas components. First, water and benzoic acid are more easily separated because of difference in their respective boiling points versus, for example, water and acetic acid. Therefore, the complexity of the fractionation of the acid and water in the off-gas is substantially reduced. Secondly, the amount of bromides, e.g. methyl bromide, generated is minimized thereby reducing the amount of equipment and processes needed to treat the off-gas to remove this component and the risk of environmental damage.

[0026] The operating conditions in each continuous stirred tank reactor may be determined by those of ordinary skill in the art without undue experimentation depending on the level of oxidation intermediates desired in the end product stream. The temperature in the first continuous stirred tank reactor may be in the range from about 160° C. to about 230° C., preferably in the range from about 180° C. to about 220° C. The pressure in the first continuous stirred tank reactor is preferably in the range from about 200 psig to about 500 psig, more preferably from about 300 psig to about 450 psig. The temperature in the second continuous stirred tank reactor may be in the range from about 180° C. to about 260° C., preferably in the range from about 190° C. to about 220° C. The pressure in the second continuous stirred tank reactor is preferably in the range from about 200 psig to about 500 psig, more preferably in the range from about 300 psig to about 450 psig. In any event, the pressure and temperature profiles of both reactors are generally determined in such away as to ensure that the oxidation reaction takes place in the liquid phase.

[0027] When the aromatic feedstock is para-xylene and the aromatic dicarboxylic acid produced is terephthalic acid, the level of the oxidation intermediate 4-CBA in the product is greater than about 3000 ppm. Preferably, at least about 85% of the 4-CBA present in the product is further oxidized in the second continuous stirred tank reactor. More preferably about 90% to about 98% of the 4-CBA in the product is further oxidized to terephthalic acid in the second continuous stirred tank reactor.

[0028] The fluid effluent from the second continuous stirred tank reactor may be sent to a crystallizer wherein most of the dicarboxylic acid in the liquid medium is crystallized thereby forming a crystallizer effluent slurry comprising crystallized solid dicarboxylic acid and mother liquor. The crystallizer effluent stream is then transferred to a liquid/solid separation system whereby the dicarboxylic acid is recovered and subsequently dried. The separated mother liquor may then be handle according to conventional methods.

[0029] In yet another embodiment, the current invention also provides a continuous process for the production of a aromatic tricarboxylic acid by the liquid phase oxidation an aromatic feedstock comprising: (a) providing a reaction medium comprising aromatic feedstock, heavy metal catalyst, a source of bromine, and solvent comprising benzoic acid and water, wherein the aromatic feedstock comprises a benzene having three oxidizable alkyl ring substituents and wherein the solvent ratio in the reaction medium is in the range from about 2 to about 30; (b) contacting at least a portion of the reaction medium with an oxygen-containing gas in a first continuous stirred tank reactor thereby generating a product stream comprising aromatic tricarboxylic acid in a liquid medium comprising water, heavy metal catalyst, bromine, benzoic acid, oxidation intermediates and by-product compounds; (c) transferring at least a portion of the product stream to a second continuous stirred tank reactor wherein at least a portion of the product stream is contacted with an oxygen/containing gas whereby a substantial portion of the oxidation intermediates are oxidized to aromatic tricarboxylic acid. According to this embodiment of the current invention, the liquid phase oxidation takes place in two stages, the second stage being useful to complete the oxidation of a substantial portion of oxidation intermediates to tricarboxylic acid. The solvent ratio in the first continuous stirred tank reactor is preferably in the range from about 2 to about 20. Preferably, the solvent comprises about 5% to about 60% water, by weight, more preferably from about 10% to about 40% water.