FIELD OF THE INVENTION

[0001] This invention relates to systems and methods to recover heat and scrub the off- gas from oxidation reactors in a process to produce aromatic carboxylic acids.

CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application claims benefit of priority from U.S. Provisional Application No. 61/286957 filed December 16, 2009.

BACKGROUND OF THE INVENTION

[0003] Purified terephthalic acid (PTA) can be produced in a two stage process. In the first stage, the oxidation plant, crude terephthalic acid (CTA) is produced by the air oxidation of paraxylene in a solvent {e.g., acetic acid and water) using a homogeneous catalyst. The reaction temperature is in the range of about 150-210° C. The oxidation is typically carried out in one, two or three vessels in series, and multiple reactors can be used in parallel for each step. Reactor vessels are typically agitated vessels, where the agitation is achieved by a combination of mechanical agitation, and the agitation effect of the air being added.

[0004] The second stage of the production process is the purification of the CTA by catalytic hydrogenation in aqueous solution. Typically, CTA is dissolved in water at high pressure (70-90 barA) and high temperature (275 - 290° C), and hydrogenated over a fixed bed catalyst of palladium supported on carbon. The resulting solution is cooled as it passes through a series of four to six crystallizers, where the purified terephthalic acid (PTA) is crystallized with most of the impurities and by-products, such as paratoluic acid, remaining in solution. The resulting slurry (at a temperature of 140-160° C) is then fed to a suitable continuous solid liquid separation device(s), such as a decanter centrifuge, rotary pressure or vacuum filter, etc where the PTA is separated from the purification mother liquor stream and then dried.

[0005] FIG. 1 illustrates one approach to reacting paraxylene to produce the crude CTA, where two oxidation reactors in series are used. The primary oxidation reactor includes paraxylene, aqueous acetic acid, a catalyst, and air and is operated at a reaction temperature of about 200° C at 16 barA. The slurry from the primary oxidation reactor is transported to a secondary oxidation reactor, where the slurry is reacted with air at about 187° C and 12 barA to convert more of the reactants to CTA. Each of the primary and secondary oxidation reactors has separate off gas systems to recover heat and to scrub the vapor. Since each of the primary and secondary oxidation reactors is operated at significantly different temperatures and pressures, two heat recovery and vapor scrubbing systems are used, which increases capital expenditure. In an attempt to overcome this effect, a compressor (not shown) can be used to boost the pressure of the off-gas from secondary oxidation heat recovery system to feed upstream of a single vapor scrubbing system to reduce duplicated components, such as the secondary oxidation reactor off-gas vapor scrubber system. However, this approach requires capital expenditure for the compressor.

SUMMARY OF THE INVENTION

[0006] There is a need in the industry to find an alternative approach to simplify the systems to recover heat and scrub the off-gas from the oxidation reactors in the process to produce terephthalic acid and also reduce capital expenditure on the equipment used.

[0007] Briefly described, embodiments of this disclosure include systems, methods for the production of terephthalic acid, and the like.

[0008] One exemplary system among others, includes a primary oxidation reactor, wherein the primary oxidation reactor is operated at pressure of about 12 to 18 barA and a temperature of about 180 to 210° C, wherein the primary oxidation reactor is in communication with a primary heat recovery system, wherein the primary heat recovery system is in communication with a vapor scrubbing system; and a secondary oxidation reactor, wherein the secondary oxidation reactor is operated at less than 2 bar below the primary oxidation reactor pressure and a temperature of about 5 - 15° C below the primary oxidation reactor temperature, wherein a slurry produced in the primary oxidation reactor is transferred to the secondary oxidation reactor, wherein the secondary oxidation reactor is in communication with a secondary heat recovery system, wherein the secondary heat recovery system is in communication with the vapor scrubbing system.

[0009] One exemplary method for the production of terephthalic acid, among others, includes: providing a primary oxidation reactor, wherein the primary oxidation reactor is in communication with a primary heat recovery system, wherein the primary heat recovery system is in communication with a vapor scrubbing system; providing a secondary oxidation reactor, wherein the secondary oxidation reactor is in communication with a secondary heat recovery system, wherein the secondary heat recovery system is in communication with the vapor scrubbing system; introducing paraxylene, acetic acid, a catalyst, and air to the primary oxidation reactor; operating the primary oxidation reactor at pressure of about 12 to 18 barA and a temperature of about 180 to 210° C to produce a first slurry; introducing the first slurry produced in the primary oxidation reactor to the secondary oxidation reactor; and operating the secondary oxidation reactor at less than 2 bar below the primary oxidation reactor pressure and a temperature of about 5 - 15° C below the primary oxidation reactor temperature to produce a second slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

[00010] Many aspects of this disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale.

[0001 1 ] FIG. 1 illustrates a schematic block diagram of part of a process for the production of terephthalic acid.

[00012] FIG. 2 illustrates a schematic block diagram of part of an improved process for the production of terephthalic acid.

DETAILED DESCRIPTION OF THE INVENTION

[00013] Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

[00014] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

[00015] Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, chemical engineering, chemical recycling, and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

[00016] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions and compounds disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers {e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C, and pressure is in barA.

[00017] It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a support" includes a plurality of supports. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

Discussion

[00018] Embodiments of the present disclosure include systems for the production of terephthalic acid, methods for processing terephthalic acid, and the like. Embodiments of the systems and methods of the present disclosure are advantageous in that they reduce capital expenditures since certain systems and components are needed to process terephthalic acid. For example, in comparison to one system a vapor scrubbing system can be eliminated, while in comparison to another system a vapor compression system can be eliminated.

[00019] In general, crude terephthalic acid (CTA) is produced by the air oxidation of paraxylene, using a homogeneous catalyst in an aqueous acetic acid solvent. The air used for oxidation contains molecular oxygen that can be enriched or depleted in comparison with atmospheric air, prior to feeding to an oxidation reactor. The resulting slurry of CTA from the oxidation reactor(s), comprising CTA, oxidation catalyst, reaction intermediates and by products, including color compounds, is typically fed to more than one vessel, usually referred to as crystallizers, to reduce the pressure and temperature of the process stream. The CTA solid is separated from the oxidation process mother liquor and, optionally, dried from the oxidation solvent. The CTA solid is then mixed with water to form the CTA purification feed stream, prior to purification of the CTA in the second stage of the purified terephthalic acid (PTA)

manufacturing process. The CTA purification process/system can include a number of stages including, but not limited to, CTA re-slurrying stage, slurry heating and CTA dissolution stage, catalytic hydrogenation stage, crystallization stage, filtration stage, solvent recovery stage, drying stage, a mother liquor stream treatment stage, a mother liquor solid treatment stage, and the like.

[00020] FIG. 2 illustrates an embodiment of a portion of a system for the production of CTA. Embodiments of the present disclosure are improvements of existing processes. The system includes a primary oxidation reactor and a secondary oxidation reactor. In short, the primary oxidation reactor and the secondary oxidation reactor are used to form terephthalic acid by an oxidation reaction including paraxylene, acetic acid, a catalyst (e.g., cobalt and/or manganese compounds or other heavy metals such as vanadium, chromium, iron,

molybdenum, a lanthanide such as cerium, zirconium, hafnium and/or nickel and an oxidation promoter), and air. The relative weight percentage in the liquid phase of each component used is about 10 to 25 of paraxylene, about 74 to 90 of aqueous acetic acid, about 0.1 to 1 of catalyst, and the air flow is greater than about 4.5 times the paraxylene mass flow (for normal atmospheric air, without oxygen enrichment or depletion). The initial oxidation occurs in the primary oxidation reactor and produces an off gas (also referred to as "primary off gas") and a slurry (also referred to as "primary slurry") including the reactants, reaction intermediates and products, is transferred (e.g., flowed or otherwise moved) to the secondary oxidation reactor. Additional oxidation reactions occur in the secondary oxidation reactor to form a slurry (also referred to as "secondary slurry") and an off gas (also referred to as a "secondary off gas"). The secondary slurry is further processed to produce purified terephthalic acid.

[0002 ] The primary off-gas is communicated (e.g., flowed or otherwise transferred using a pipe or transfer structure) to a primary heat recovery system, while the secondary off gas is communicated to a secondary heat recover system. The primary and secondary heat recovery systems remove heat from the off gas and produce steam, by exchanging heat with water or steam as the heat accepting fluid and a condensate stream, as a liquid. The residual off-gas from each of the primary heat recovery system and the secondary heat recovery system is communicated to the vapor scrubbing system. The vapor scrubbing system processes the residual off-gas streams to produce a liquid and vapor that can be further processed.

[00022] The primary oxidation reactor can be operated at a pressure of about 12 to 18 barA and in one embodiment at about 16 barA, while the temperature of the primary oxidant reactor can be about 180 to 210° C and in one embodiment about 200° C. The secondary oxidation reactor can be operated at less than 2 bar below the primary oxidation reactor pressure and greater than 1 bar below the primary oxidation reactor pressure, and a temperature of about 5-15° C below the primary oxidation reactor temperature or in one embodiment about 192° C (when the primary oxidation reactor is about 200° C).

[00023] As a result of the reduction in the pressure drop between the primary oxidation reactor and the secondary oxidation reactor being less than 2 bar, the residual off-gas flowing from the primary heat recovery system can be combined with off-gas flowing from the secondary heat recovery system to flow to a single vapor scrubbing system. However, for this embodiment to operate successfully the pressure drop through the primary heat recovery system must also be increased. A benefit of increasing the pressure drop through the primary heat recovery system is the significant increase in the heat transfer coefficient for the heat exchangers in the primary heat recovery system. This reduces the size of the heat exchangers, thereby reducing capital costs in the primary heat recovery system. As the heat recovery systems are typically constructed from expensive corrosion-resistant material, such as titanium, the reduction in capital cost is significant.

[00024] In general, each of the primary and secondary oxidation reactors can be a vessel that is constructed of or lined with a corrosion-resistant material, such as titanium. Because the oxidation reaction is conducted at an elevated pressure, each oxidation reactor can be constructed to withstand the high pressures used for the oxidation reaction. In addition, the contents of the oxidation reactor are agitated to optimize the oxidation reaction and also to maintain the solid reaction product in suspension. Agitation comprises specific fluid mixing configurations and the oxidation reactor can be equipped with one or more mechanical agitators. Crude terephthalic acid, which is the solid reaction product produced by the oxidation reaction, leaves both oxidation reactors along an outlet line, for example, in the form of an oxidation reaction slurry that comprises a mixture of crude terephthalic acid, water, acetic acid, catalyst metals, oxidation reaction intermediates, and reaction byproducts.

[00025] The primary heat recovery system includes at least one condenser, where each condenser condenses a portion of the off-gas and raises steam. In an embodiment, the primary off gas is communicated into a gas line that delivers the primary off gas to a series of condensers. In an embodiment, a first condenser produces a first pressure of steam (e.g., at about 145°C and 4.5 barA), the second condenser produces a second pressure of steam (e.g., at about 130°C and 3 barA), and the third condenser produces a third pressure of steam (e.g., at about 100°C and 1 barA). Other embodiments can include more or less than three condensers that can be operated at temperatures and pressures that are appropriate for the number of condensers.

[00026] The secondary heat recovery system can include at least one heat exchanger that transfers heat to a heat transfer fluid. In an embodiment, the heat exchanger acts as a condenser and condenses a portion of the off-gas and raises steam. The steam can be utilized elsewhere in the process and also to generate power. In an embodiment, the initial heat exchanger acting as a condenser can generate steam at about 130° C and about 3 barA.

[00027] The vapor scrubbing system operates at about 40-50°C and about 9-15 barA and includes optionally a system to degas the condensate from one or both of the primary and secondary heat recovery systems, a system to recover volatile organics (e.g., methyl acetate, paraxylene, etc.), and a system to retain the liquids used for scrubbing within the vapor scrubbing system and venting residual gases. The components of the scrubbing system are known in the art. The scrubbing liquid can include acetic acid and water, and a combination thereof. The vapor scrubbing system comprises at least one scrubbing zone.

[00028] In general, an embodiment of the method of producing CTA including using a system for processing terephthalic acid as described herein that includes a primary and a secondary oxidation reactor. eactants such as paraxylene, acetic acid, a catalyst, and air are introduced to the primary oxidation reactor. The primary reactor can be operated at a pressure of about 12 to 18 barA and a temperature of about 180 to 210° C (or at other temperatures or pressure described herein) to produce a primary slurry. Then the primary slurry is introduced to the secondary oxidation reactor. The secondary oxidation reactor is operated at less than 2 bar below the primary oxidation reactor pressure and a temperature of about 5 - 15° C below the primary oxidation reactor temperature (or at other temperatures or pressure described herein) to produce a secondary slurry.

[00029] As mentioned above, a primary off gas from the primary oxidation reactor is produced during the oxidation reaction. The primary off gas is flowed to the primary heat recovery system to produce a treated primary off gas. The treated primary off gas is flowed to the vapor scrubbing system, where the treated primary off gas is scrubbed. The primary heat recovery system and the vapor scrubbing system and the operating conditions are described above.

[00030] As mentioned above, a secondary off gas from the secondary oxidation reactor is produced during the oxidation reaction. The secondary off gas is flowed to the secondary heat recovery system to produce a treated secondary off gas. The treated secondary off gas is flowed to the vapor scrubbing system, where the treated secondary off gas is scrubbed. The secondary heat recovery system and the vapor scrubbing system and the operating conditions are described above.

[00031] The primary heat recovery system removes heat from the primary off-gas by heat interchange using one or more heat exchangers. As the off-gas flows through each heat exchanger the temperature of the off-gas is reduced and a primary condensate as a separate liquid phase is formed and comprises volatile organic and aqueous components, such as acetic acid, paraxylene, reaction intermediates and water. The primary condensate liquid is collected and can be recycled to the oxidation reactor, purged to remove by-products and water or returned elsewhere in the production process, as required.

[00032] The secondary heat recovery system removes heat from the secondary off-gas by heat interchange using one or more heat exchangers. As the off-gas flows through each heat exchanger the temperature of the off-gas is reduced and a secondary condensate as a separate liquid phase is formed and comprises volatile organic and aqueous components, such as acetic acid, reaction intermediates and water. The secondary condensate liquid is collected and can be recycled to the oxidation reactor, purged to remove by-products and water or

returned elsewhere in the production process, as required.

[00033] For conventional reaction systems, such as shown in FIG. 1 the treated primary off-gas is scrubbed in a vapor scrubber with the scrubbed treated primary off gas flowing to a power recovery system to beneficially recover work, for example using a turbine to reduce the pressure of the gas. However, the treated secondary off-gas flows to a separate vapor scrubber and is subsequently vented to atmosphere. An advantage of embodiments of the present

disclosure, as shown in FIG. 2, is an improved embodiment to utilize the scrubbed, treated

secondary off-gas, as well as the scrubbed, treated primary off-gas to recover power. In

addition, only one vapor scrubbing system is required to scrub both the primary and secondary off-gas streams, resulting in an a capital cost reduction.

[00034] It should be noted that FIG. 2 may not include all of the various components used in each system, method, or process. For example, one or more fluid pumps can be used to cause the streams to flow through the system or process at one or more flow rates and at one or more pressures.

Examples

[00035] In Table 1 a calculated comparison for an equivalent duty is given of the

significant reduction in total surface area of the primary heat recovery heat exchangers,

indicating the surprising reduction in the quantity of expensive, corrosion-resistant material

required to fabricate the heat exchangers.

Table 1 Comparison of process equipment items

[00036] It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of "about 0.1 % to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also the individual concentrations (e.g., 1 %, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1 %, 2.2%, 3.3%, and 4.4%) within the indicated range. The term "about" can include ±1 %, ±2%, ±3%, ±4%, +5%, ±8%, or ±10%, of the numerical value(s) being modified. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about 'y'".

[00037] Many variations and modifications may be made to the above-described embodiments. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.