PTA production process of Sinopec Shanghai Engineering Co., Ltd

Thread Content

The PTA production process of Sinopec Shanghai Engineering Co., Ltd. The PTA production process used by Sinopec Shanghai Engineering Co., Ltd. is based on the hydrorefining method, with PTA units supplied by Mitsubishi Petrochemical Corporation (MPC). This process consists of the PX oxidation section (TA unit), the crude terephthalic acid hydrogenation and refining section (PTA unit), and auxiliary systems; each of these will be described below: 2.2.1 PX Oxidation Section The oxidation unit is the core component of the PTA production facility, where the main oxidation reaction that converts PX into PTA takes place. It primarily includes steps such as material preparation, reaction, crystallization, filtration, and solvent recovery. Using high-purity PX as the raw material, acetic acid as the solvent, cobalt acetate and manganese acetate as catalysts, and hydrogen sulfide or tetraalkylammonium bromide (BST) as a promoter, these components are mixed in a feed mixing tank in specific proportions. After being measured, the mixture is fed into an oxidation reactor, where air is introduced to carry out the oxidation reaction, resulting in the formation of crude terephthalic acid (TA). The reaction heat is removed by solvent evaporation; the exhaust gas is condensed and then washed with acetic acid and deionized water in a high-pressure absorption tower, after which it is used to drive an expansion turbine. A portion of the exhaust gas, after being dried, serves as a pneumatic conveying medium and as an inert shielding gas for the equipment. The oxidized solution coming out of the reactor is cooled by flash evaporation of the solvent in three crystallizers connected in series, thereby allowing TA to gradually precipitate and the crystals to grow. Then, filtration and washing are carried out using a rotary vacuum filter to separate the crystals from the mother liquor and gases. The mother liquor is subjected to solvent recovery and dehydration to obtain acetic acid, which is returned to the system for reuse, while the separated catalyst residue is treated separately. The filter cake from the rotary vacuum filter is dried in a \"porcupine\" type dryer. The resulting TA is pneumatically transported to silo 1221. The reactor equipped with MPC technology, introduced by Sinopec Shanghai Petrochemical Engineering Co., Ltd., replaces the batch-style structure of Amoco technology’s reactors; it changes the shape from round and bulky to tall and slender, reducing the diameter of the equipment while increasing its height. A dehydration tower is directly connected to the top of the reactor. The bottom of the reactor is equipped with a downward-falling uranium-type anchor stirrer; air enters vertically from the lower part of the reactor, creating uniform bubbles that ensure thorough contact between the air and the material. The anchor stirrer at the bottom operates at a speed of only 5-Sr/min, thereby preventing scaling and TA deposition at the bottom. Due to the small reactor diameter and low stirring power, the investment is reduced, the floor space required is minimal, and space is made full use of. The reactor dehydrating tower has a dehydration function, which keeps the water content within the reactor at a relatively low level, thereby facilitating an increase in the reaction rate [2312.2.2 In the hydrogenation and refining stage of crude terephthalic acid, the CTA (terephthalic acid) produced in the oxidation unit contains small amounts of side reaction products (mainly 4-hydroxybenzaldehyde and p-formic acid). The presence of these side products in the product can affect properties such as coloration in subsequent PTA processing steps. The main task of the refining unit is to convert 4-alkylbenzaldehyde into p*-formic acid through a hydrogenation process, and to remove it in the water washing section 1241. The refining unit mainly includes hydrogenation reaction, crystallization, PTA separation, filtration and drying, product transportation, and mother liquor recovery. 2.2.2.1 Hydrorefining: The purpose of the hydrogenation reaction is to remove impurities 1251 from TA, thereby reducing 4-CBA to PT acid; subsequent washing is carried out. Hydrorefining consists of three steps: pulping, dissolution, and hydrogenation reaction. The content of 4-CBA in untreated terephthalic acid is 1600–3000 ppme, while in the terephthalic acid after purification, this amount can be less than 25 ppm. 2.2.2 The removal of 4-CBA. 2. Separation and drying: After the hydrogenation reaction, most of the impurities have been converted into water-soluble substances, which can be removed by methods such as crystallization and washing, followed by drying to produce pure terephthalic acid 1261. Separation and drying involve three steps: crystallization separation, washing, and drying. 2.3 Acetic acid recovery system: The solvent recovery system is an important part of the PTA production process; it mainly includes a mother liquor distillation tower (i.e., a dehydration tower) and an azeotrope recovery tower. During capacity expansion upgrades, many plants encounter problems such as insufficient load on the dehydration tower and suboptimal separation results. Amoco holds a patent1281 that proposes utilizing the reversibility of the acetylation reaction to modify the solvent dehydration tower, converting one of its sections into a packed tower, where methyl acetate is hydrolyzed into methanol and acetic acid, allowing for the recovery of acetic acid. This method is not discussed in this paper; instead, it focuses on providing process data for the modification of the towers in the acetic acid recovery system after expansion, through modeling calculations and discussions, in order to achieve stable operation and optimal separation results. In the reaction process of oxidizing PX to PTA, acetic acid can serve as a solvent for this reaction for the following reasons: (1) Acetic acid can dissolve PX (p-xylene), zinc acetate, manganese acetate, and oxygen; (2) Acetic acid possesses good thermal stability and oxidation stability; (3) Acetic acid can dissolve the intermediate products formed during the oxidation of PX to PTA as well as the reaction by-products; (4) Under the reaction conditions, TA (terephthalic acid) is insoluble in acetic acid. The presence of acetic acid not only enables the oxidation reaction to proceed under nearly ideal conditions, but also facilitates the separation of TA from the reactants. This makes it possible to separate the catalyst from TA easily and recycle it back to the oxidation reactor, thereby reducing production costs. During the reaction process, only about 2% (wt) of acetic acid reacts with oxygen and is lost; therefore, whether the acetic acid solvent can be recovered as intended will directly affect the acetic acid consumption in the PTA plant. During the oxidation of PX to PTA, a large amount of water is generated, which reduces the concentration of the acetic acid solvent. This affects the progress of the oxidation reaction and leads to the formation of excessive by-products. To ensure the smooth progress of the reaction, it is necessary to remove in a timely manner from the system the water generated by the oxidation reaction, as well as the water added during the treatment of process gases in the atmospheric-pressure absorption tower and the high-pressure absorption tower. This water must be removed, while acetic acid should be recovered from it as much as possible in order to reduce the consumption of acetic acid. The function of the solvent recovery system is to purify the acetic acid solvent used in the oxidation process, by removing the water generated during the oxidation reaction as well as the water added during catalyst preparation, so as to obtain acetic acid solvent of the specified purity for reuse within the system [291]. Therefore, the operational condition of the recycling system affects both the cost and quality of PTA production