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Feasibility of domestic explosion-proof pat horizontal screw centrifuge

2021-05-28 11:22:03

1、 Process flow and technical characteristics

Purified terephthalic acid (PTA) is the main raw material for the production of polyester (PET) in polyester chain products. Crude terephthalic acid (CTA) is produced by the chemical reaction of p-xylene (PX) and acetic acid, and then the crude terephthalic acid is hydrotreated to remove impurities, filtered, separated and dried to prepare purified terephthalic acid products. Therefore, PTA is the basic material for the production of polyester fiber and packaging industry, and the main raw material of chemical fiber industry.

PTA manufacturing processes are mainly divided into three categories, including hydrofining, precision oxidation and DMT refining. At present, the first of the three types of processes is dominant, and its process is shown in the figure. The whole process is divided into three sections: oxidation, refining and solvent recovery.

In the whole process, there are four processes involving solid-liquid separation, including: ① separation of crude terephthalic acid crystal solution in oxidation process. ② Separation of crude terephthalic acid refining solution. ③ Separation of purified terephthalic acid crystal solution. ④ Terephthalic acid residue recovery. The process conditions of the above processes vary greatly, and the solid-liquid separation equipment used are different.

Drum vacuum filter is mainly used for separation of crude terephthalic acid crystal liquid in oxidation section. Due to the small driving force of vacuum filtration, easy blockage of filter cloth and poor separation effect, pressurized filter and filter centrifuge are used to replace vacuum filter. The separation of crude terephthalic acid refining liquid is required to operate under pressure. Generally, sealed explosion-proof spiral discharge centrifuges are used, mainly including vertical sealed explosion-proof spiral discharge centrifuges and horizontal sealed explosion-proof spiral discharge centrifuges; Purified terephthalic acid crystal liquid can be separated and dehydrated by ordinary spiral discharge centrifuge. Terephthalic acid residue is mainly recovered by horizontal screw centrifuge.

2、 Development status and trend of PTA sealed explosion-proof centrifuge technology

The key solid-liquid separation equipment in PTA unit is the pressurized closed explosion-proof horizontal screw centrifuge in the separation process of crude terephthalic acid refined liquid. The design requires that the centrifuge work under high temperature (200 ℃) and pressure (0.1 ~ 1MPa). At present, the widely used centrifuges mainly include the TRV series vertical spiral discharge centrifuges of Japan's t o m o e company, the horizontal spiral discharge centrifuges of Britain's b r o a D B e nt company and the United States bird / KHD company. The whole machine of the vertical spiral centrifuge adopts the suspension support structure, and the whole rotating parts are suspended and supported on the base by the upper bearing. During operation, the self centering function of the drum and the specially designed rubber vibration isolation element greatly reduce the vibration of the machine; There is only one sealing point between the drum and the casing, which is easier to meet the sealing and explosion-proof requirements. The horizontal screw centrifuge adopts rigid support, and the drum and screw are arranged horizontally. The separation principle of the two is the same, but the horizontal screw centrifuge has two more mechanical seals than the vertical screw centrifuge, which has higher requirements for mechanical seals. The advantage of vertical spiral centrifuge is that it has less vibration and is easier to seal, but the cost is expensive and has higher requirements for manufacturing accuracy and design, so the specification can not be too large; The manufacturing difficulty and cost of horizontal screw centrifuges are lower than those of vertical screw centrifuges of the same specification. Due to the rigid support mechanical structure, the vibration is too large, there are many sealing parts, and the requirements for mechanical sealing are higher, but the specifications of horizontal screw centrifuges are easier to be large-scale.

3、 Localization feasibility analysis

Closed explosion-proof centrifuge is the key equipment of PTA production. Since the separation process is to complete the solid-liquid separation process under the pressure of 0.45MPa and the high temperature of 148 ℃, and the materials entering the centrifuge are composed of 35% ～ 37% crude terephthalic acid (CTA) crystalline particles, water (H2O) and acetic acid (HAC), which are corrosive, the centrifuge must have the function of closed explosion-proof and anti-corrosion under a certain pressure and high temperature. The localization of sealed explosion-proof centrifuge mainly has the following work to do.

1. Localization of large differential

Due to the large processing capacity of the PTA centrifuge and the high slag conveying power of the screw, the torque requirements of the differential for the horizontal screw centrifuge are as high as more than 100000 n · M. at the same time, the machining accuracy of the internal gear of the differential, the indexing accuracy of the planetary carrier and the optimization of the lubrication and cooling structure of the central bearing of the gearbox are the key to the localization of the differential. In foreign countries, the differential, as a component of the spiral centrifuge, is generally manufactured by professional manufacturers, and the centrifuge factory can purchase it directly for supporting assembly. Therefore, we can find a domestic professional gear factory for production, or directly contact with foreign professional factories to purchase supporting equipment.

2. Mechanical seal

PTA sealed explosion-proof centrifuge works under pressure (0.45MPa), and sealing is the key to ensure the performance of the whole machine. There are 4 sealing points for vertical spiral centrifuge and 6 sealing points for horizontal spiral centrifuge. The reliability and service life of the seal directly affect the performance of the whole machine. At present, domestic mechanical seals have been used on imported machines, and the problem of localization has been basically solved. The next step is to improve the reliability and service life.

3. Structural optimization of large centrifuge

PTA sealed explosion-proof centrifuge has a large specification, and the maximum specification of vertical spiral centrifuge currently used in China does not exceed φ 700mm, horizontal spiral centrifuge, the largest specification currently used in China is φ 1200mm。 The largest specification of the industrialized horizontal spiral centrifuge produced by domestic centrifuge manufacturers is φ About 750mm, and it is non hermetic explosion-proof type. Therefore, for a horizontal hermetic explosion-proof centrifuge with a large size of 1200mm, it lacks experience in structural design and fluid mechanism. The structural optimization, material selection, processing and dynamic balance of large drum and screw need to be solved.

4、 Conclusion

With the continuous expansion of domestic PTA industry, China cannot rely on imported equipment for a long time, which is not conducive to the development of domestic petrochemical industry. We should be confident that through the efforts of domestic manufacturing enterprises, we can localize the main PTA equipment, especially the technology introduction and technological progress of domestic separation machinery manufacturing enterprises over the years and the continuous improvement of the technical level of various industries. The localization of PTA centrifuges is completely feasible.