Application of melt pumps in polyester production lines

The production process for most of today's plastics begins with crude oil. When we think of crude oil, we generally think of the gasoline that comes from it. In reality, only about 40% of the yield of a barrel of crude oil is gasoline. In order to obtain gasoline and many other useful by-products, petrochemical plants heat crude oil in tanks and separate the various components according to their boiling points. These components are called "fractions" and the process is called "distillation". At a temperature of about 150°C, gasoline is separated from the crude oil, and the gas-phase by-products, ethylene and propylene, are "cracked" from the distillate when heated up to 840°C and become the raw materials for plastics.

These gaseous components, also called monomers, are fed into a reactor. The temperature and pressure in the reactor vary depending on the polymer being produced. The molecular chains of the monomers polymerize with each other under heat and pressure to form long chains that we call polymers. As the reaction proceeds, the heavy mass of polymer sinks to the bottom of the kettle, so melt gear pumps for high temperature, high pressure, and high viscosity come into play.

 polyester melt pumps

Because the reactor is mostly under vacuum, the melt pump is often bolted directly to the bottom of the reactor to reduce the required cavitation margin. At this time, due to the polymer still contains a large number of solvents, the material viscosity is not particularly high, about 150,000 cP or so. The kettle bottom discharge melt pumps require a special design to accommodate the low pressure of the inlet and high temperature resistance to offset the thermal expansion of the steel at temperatures up to 200°C. The pumps are designed with a tapered inlet to minimize the thermal expansion of the material. The inlet of the pump is designed as a large conical opening to help the material to enter the pump, and the jacket can be heated with steam or thermal media to heat the pump body.

In some processes, the polymer is pumped into a second reactor or tray tower, where the solvent is separated out, leaving a more viscous material, with viscosities up to 500,000 cP and temperatures up to 340°C, which requires another gear pump to pump the material out of the bottom of the tower. Because the melt needs to be pushed out to other equipment with a higher pressure drop, and then into the pelletizer, this pump needs to have a higher design than the table pump to accommodate more extreme conditions, and higher pressures, with outlet pressures of up to 250kg being not uncommon. As a result, tight and precise clearances between the pump's moving parts must be designed and carefully calculated by the pump manufacturer taking into account factors such as high temperatures, high pressures, high viscosities, different coefficients of thermal expansion of the materials, and the characteristics of non-Newtonian fluid materials.

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