Terephthalic acid (PTA) is one of the important raw materials of polyester chemical fiber. With the development of polyester chemical fiber in China, in order to meet the increasing demand of PTA, PTA process production technology is introduced in the terephthalic acid plant in order to meet the requirements of vacuum degree, and rotary vane vacuum pump is generally used. Due to the large amount of air, it is necessary to equip several high efficiency vacuum pumps to meet the requirements. This paper analyzes the working principle of rotary vacuum pump and its application in terephthalic acid plant.
Terephthalic acid, also known as p-phthalic acid, is the most productive dicarboxylic acid, which is mainly made from p-xylene and the main raw material for polyester production. Solid at room temperature. Heating does not melt, sublimation above 300 ℃. If heated in a closed container, it can be melted at 425 ℃. It is difficult to dissolve in water at room temperature. It is mainly used for manufacturing synthetic polyester resin, synthetic fiber and plasticizer.
In order to achieve vacuum suction filtration, PTA vacuum filtration process also needs to be equipped with corresponding vacuum equipment. The process flow is as follows: PTA slurry is delivered to the rotary vane vacuum pump through the filter feed delivery pump; PTA filter cake is dried by the screw conveyor to the rotary dryer, and then enters the subsequent process section. The PTA filtrate and gas mixture extracted by the rotary vacuum filter enter the vacuum filtrate separation tank; the separated filtrate is pumped to other sections by the vacuum filtrate delivery pump; the separated gas is cooled by the vacuum filtration gas-phase condenser, pumped by the rotary vacuum pump to the vacuum gas-liquid separation tank for separation and replenishment to the rotary vacuum filter.
rotary vane vacuum pump working principle:
The vane of the vane pump divides the crescent space enclosed by the rotor, pump cavity and two end covers into three parts a, B and C. when the rotor rotates in the direction of the arrow, the volume of space a connected with the suction port increases gradually and is in the suction process. And the volume of space C connected with the exhaust port is gradually reduced and is in the exhaust process. The volume of the middle space B is also decreasing gradually, and it is in the process of compression.
Because the volume of space a is gradually increased (i.e. expansion), the gas pressure is reduced, and the external gas pressure at the pump inlet is greater than the pressure in space a, so the gas is inhaled. When space a is isolated from the suction port, i.e. it turns to the position of space B, the gas starts to be compressed, the volume gradually shrinks, and finally it is connected with the exhaust port. When the compressed gas exceeds the exhaust pressure, the exhaust valve is pushed open by the compressed gas, and the gas is discharged into the atmosphere through the oil layer in the oil tank. The continuous operation of the pump can achieve the purpose of continuous air extraction. If the discharged gas is transferred to another stage (low vacuum stage) through the air passage, it is pumped out by the low vacuum stage, compressed by the low vacuum stage and then discharged into the atmosphere, which constitutes a two-stage pump. At this time, the total compression ratio is borne by two stages, so the limit vacuum degree is increased.
Application effect of rotary vacuum pump in terephthalic acid plant:
1. Greatly reduced the operation energy consumption
With the vacuum filtration process, only two rotary vacuum filters and two vacuum pumps are needed. Even if the energy consumption of cooling water and pumps of auxiliary facilities is included, it is far less than that of centrifuges. Because the moisture content of the filter cake of the vacuum filter is about 8% ~ 10%, which is far lower than that of the centrifuge filter cake by 13% – 20%, the steam consumption of the dryer downstream can be greatly saved, so as to achieve the purpose of energy saving and consumption reduction.
2. Production capacity can be effectively adjusted
The working characteristic of centrifuge is that when the output is increased, the separation quality is reduced, so as to increase the working load of subsequent and auxiliary sections. Because when the processing capacity of centrifuge is increased from low load to high load, the moisture content of filter cake is increased from 13% to 20%, and the solid content of mother liquor is increased from 2.5% to 5%, so the operation load of downstream dryer is increased, and the loss of PTA product from mother liquor is accelerated. Therefore, centrifugal separation is often one of the main bottlenecks to improve the capacity of the plant. The rotary vane vacuum pump can effectively control the production capacity through the mechanical adjustment of the machine itself, for example, by increasing the height of the overflow baffle to increase the thickness of the filter cake.