Thread Content
The production of PTA is divided into two major processes: Oxidation process and hydrorefining process. At present, the designs of various PTA processes in terms of hydrorefining are basically the same. The difference is mainly in the oxidation part, which is divided into high-temperature processes (Dupond-ICI: 201～205℃ ； BP-AMOCO: 190~196℃), medium temperature process (Mitsui 185℃), low temperature process (Lurgi-Eastman: 155～165℃) three categories. In addition to different oxidation reaction conditions, various processes have significant differences in the design of oxidation reactor units, exhaust gas treatment systems, and slurry treatment systems, and they also have their own strengths and weaknesses. The slurry treatment process of the BP-AMOCO process is reasonably designed, but there are bottlenecks in the exhaust gas dehydration and heat exchange devices. ； The tail gas treatment process of the Mitsui process is well designed, but in terms of slurry treatment technology, there are bottlenecks such as small TA particle size and excessive drying load. ； Although there is no obvious design bottleneck in the Dupond-ICI process, the reaction temperature is too high, resulting in increased consumption and equipment investment. At the same time, there is also considerable room for improvement in reactors and energy utilization. The process design of the new localized process is based on the long-term digestion, absorption and transformation of various imported PTA devices, fully absorbing the advantages of each process while avoiding its disadvantages. According to the current requirements for the increasingly large-scale PTA equipment, the designed production capacity of the domestic PTA process is 600,000 to 800,000 tons/year, of which two oxidation reactors are operated in parallel, and other equipment uses a single series. Figure 1 shows a simplified flow chart of the domestic PTA process. The process can be divided into four modules.: Oxidation reactor unit, tail gas treatment unit, slurry treatment unit, hydrorefining unit. The raw material paraxylene, solvent acetic acid, catalyst, and air are added to the oxidation reactor for oxidation reaction. The reaction heat is converted into steam through solvent evaporation and is removed from the top of the tower together with the tail gas, and enters the tail gas treatment unit. The tail gas undergoes multi-stage heat exchange, absorption, gas purification, distillation and dehydration, recovers energy and solvent, and then discharges. The TA slurry generated by the reactor is discharged from the bottom and enters the slurry processing unit. The slurry is separated from liquid and solid through multi-stage crystallization and supplementary oxidation, filtration, and drying. Most of the separated mother liquor is returned to the oxidation reactor, and a small part is extracted for purification. The dried crude terephthalic acid (CTA) is sent to the hydrogenation unit for purification to obtain purified terephthalic acid (PTA). 1) Oxidation reactor unit: The domestic PX oxidation reactor adopts an integrated reaction/distillation design, with a non-stirring bubble tower reactor in the lower part and a distillation tower in the upper part. The distillation section uses structured packing (Patent 2003101078895, 200320108899.6) or multiple high-efficiency trays (Patent 200510048977.1, 200520100244.3) to directly utilize the heat of reaction for solvent dehydration. The lower part of the reaction section is equipped with a gas distributor, and the material is discharged from the bottom. The height-to-diameter ratio of the reaction section is 4 to 7. The production capacity of a single reactor is designed to be 300,000 to 400,000 tons of TA/year, with a diameter of 4.7 to 5.2 meters and a total height of 30 to 35 meters. The reaction temperature is between 185 and 190°C, and the solvent ratio, residence time, water content and other conditions are set taking into account the requirements of the oxidation reaction, crystal particle size, solvent consumption and other aspects. Laboratory studies and industrial experiments have shown that the reactor can meet the various needs of oxidation reactions. Compared with the introduced stirred tank reactor, the volume of the bubble tower reactor with the same capacity is 25 to 30% larger. However, due to its slender structure and no moving parts, the cost and operating cost are much lower than that of the stirred tank reactor. At the same time, since the reaction heat is directly used for distillation, it has greater advantages than the stirred tank in terms of energy saving and consumption reduction. 2) Exhaust gas after-treatment unit: After the reactor exhaust gas is combined, it enters the multi-stage condenser for parallel flow condensation. The condenser simultaneously produces steam of different energy levels. Most of the steam is passed into the steam turbine to recover energy, and part of it is used for heating the dehydration tower. ； The condensed tail gas passes through the two-stage absorption tower to recover PX and acetic acid, and then purifies the tail gas to remove organic components, and then enters the tail gas expander to recover energy and discharge. Most of the condensate at the top of the tower refluxes to the oxidation reactor, and a small part is pumped to the dehydration tower for dehydration. The dehydration tower uses azeotropic distillation technology, which is beneficial to energy saving and reducing acetic acid consumption. The compressor, steam turbine, and tail gas expander are coaxially connected, and no external energy is required except for startup. This design of the tail gas treatment process draws on the strengths of the Dupond and Mitsui processes, which not only eliminates the bottlenecks of dehydration and heat exchange, but also maximizes the use of reaction heat. 3) Slurry post-processing unit: The slurry at the outlet of the oxidation reactor is combined and enters the three-stage series crystallizer. A small amount of air is introduced into the first crystallizer for supplementary oxidation. ； The slurry is evaporated and concentrated under reduced pressure in the subsequent crystallizer and then enters the vacuum filter to separate the solids. Most of the filtered mother liquor is returned to the reactor, and a small part is extracted for impurity removal and catalyst recovery. ； The solid enters the dryer and is dried to produce CTA powder, which is then hydrogenated and refined to obtain PTA products. The design of the slurry treatment process mainly draws on the ideas of the Amoco process, which is conducive to crystallization and drying and can eliminate bottlenecks in particle size and drying. It needs to be emphasized that the above-mentioned designs of the domestic PTA process adopt mature industrial technologies. Among them, structures similar to the bubble tower oxidation reactor have been successfully operated in industry for many years, and have been tested by industrial tests under various conditions during the development process. ； The tail gas treatment unit mainly draws on the Mitsui process, and the slurry treatment unit mainly draws on the Amoco process. After long-term development and technological transformation practice, the advantages and disadvantages of these processes have been clearly understood. The characteristics of relevant equipment and devices have also been thoroughly understood and mastered in China. The risk factors of using domestic processes are no longer significant. The new domestic process only systematically integrates the above-mentioned various partial technologies that have been industrialized and have obvious advantages. The resulting new process eliminates the bottlenecks of various imported processes and is currently the best PTA process. 3． Technical indicators The use of this technology can save the total investment of new PTA projects by 15 to 25% compared with imported equipment. The investment savings are mainly reflected in: Increase in patent fees, technical service fees, and equipment localization rate (including localization of oxidation reactors and main unit equipment, savings in moving equipment such as centrifuges, etc.). The energy consumption of the localized process is more than 20% energy-saving compared to the currently imported PTA device, and the material consumption is equivalent to the indicators of the latest imported device. Energy saving is a major advantage of domestic technology. Compared with imported devices, energy saving is reflected in the following aspects:: The oxidation reactor directly uses reaction heat for dehydration, uses azeotropic distillation technology, the compressor-turbine-expander coaxial drive uses reaction heat, and the slurry heat is recovered for distillation, etc. In terms of social benefits, the industrialization of domestic PTA technology is a reflection of my country's scientific and technological competitiveness and development strength in the petrochemical field. It is another major step after the localization of polyester technology. Therefore, the use of this technology will significantly increase the visibility and influence of project-initiating enterprises, design institutes, and development units at home and abroad. At the same time, it can also gain * * Support and multiple policy benefits.