Pumps & Systems, July 2008
Plastic is one of the most widely used materials in the manufacturing of everyday products, including food containers, toys, computers, medical equipment, building materials, household products, tools and the pipes that carry the water in our homes. We take plastic for granted, but most people know little about how it is produced. There are many different types of plastics and just as many processes for making them. The history of plastic goes back to the early 1800s. The first patent for plastic was given to John Wesley Hyatt in 1859 for inventing celluloid, an inexpensive substitute for ivory. He found that a mixture of cellulose nitrate could be "plasticized" by adding camphor. His celluloid was used for making billiard balls, combs, buttons, dentures and other products. Then in 1951, two chemists for the Phillips Petroleum Company discovered polypropylene and polyethylene. Their discoveries became the basis for today's plastics.
The process for making most plastics today begins with crude oil. When we think of crude oil, we think of gasoline. However, a barrel of crude oil will usually only yield about 40 percent gasoline. Although additional gasoline can be obtained by additional, more expensive refining processes. Crude oil in its raw, unprocessed form is not very useful. To get the useful products like gasoline and many other byproducts used in petrochemical plants, the crude oil is heated in a vessel where the components are separated by their boiling points. These components are called "fractions," and the process is known as fractional distillation. Unrefined gasoline is removed from the crude oil at approximately 300-deg F. The gaseous byproducts-ethane and propane-are then "cracked" (further heated to 1,550-deg F) from the remaining crude to break down the molecules into ethylene and propylene gases, which are the main feedstocks for many types of plastic.
These gases, also called monomers, are introduced into a reactor vessel with steam. The pressure and temperature in the reactor depends on the polymer produced and the process used. The molecules of the monomers combine with other like molecules and under the heat and pressure create long chains called polymers. As the reaction takes place and the polymers become heavier, they settle to the bottom of the reactor where they are extracted by a gear pump specially designed for the temperatures, pressures and viscosities encountered.
Since the reactor vessel is under vacuum, the pump is typically bolted directly to a flange on the bottom of the reactor to help with NPSH. At this point, the viscosity is not extremely high, usually in the 150,000 cP range, because a large amount of solvents are still in the polymer. This extraction gear pump must be specifically designed for the very low suction pressure and the high temperature to compensate for the thermal expansion that occurs in the steel at elevated temperatures of +400-deg F. It features a large, tapered inlet to assist in filling the pump and is jacketed for steam or liquid heating to the product temperature.
In some processes, the polymers are pumped into a second reactor vessel or disc ring reactor where the solvents are removed, resulting in a much more viscous product. Now, with the viscosity as high as 5,000,000 cP and temperatures above 650-deg F, another pump is required for extracting the polymer from this reactor.
Because this pump must push the molten polymer through various other equipment-which can have large pressure drops-and then into the pelletizer, the second pump must be designed for even more extreme conditions than the first pump, and it must be capable of delivering a high discharge pressure. Discharge pressures as high as 3,500-psi are not uncommon, so the pump must have tight, precise clearances between its moving parts. The clearances must be precisely calculated by the pump manufacturer, taking into account the extremely high temperature, viscosity, pressure, the differing rates of thermal expansion of the pump parts and the non-Newtonian nature of the product.
The next step of the process is mixing into the molten polymer additives such as stabilizers, UV protection agents or colorants to give the product the qualities desired by the buyer. If the additives are in liquid form, they must be introduced into the melt stream under high pressure. To accomplish this, a third gear pump designed for high discharge pressure at medium viscosity is used. This gear pump is designed for precise metering of the additives into the high pressure stream of molten polymer. It is jacketed for heating by fluid, steam or electric resistance cartridge heaters.
To turn the molten polymer into a sellable form, it must be made into pellets. In the pelletizer, whether it is an underwater pelletizer or a strand pelletizer, the molten plastic is pushed through a die, cooled and cut into pellets which are then dried, packaged and sold to end users (or converters).The process described above is called solution-phase polymerization and is just one of the many processes used.
Another common process, gas-phase polymerization, uses slightly different reactors. In this process, the ethylene and a catalyst are circulated in a loop reactor. This process takes place at a lower temperature (185- to 230-deg F) so an inert solvent is introduced to help dissipate heat, as the reaction is highly exothermic. The polymer is not in molten form but comes out of the reactor as a powder. The powder must then be re-melted and pelletized. The powder is fed into the hopper of a twin screw extruder or mixer where it is melted, homogenized with additives (added with a pump as described and shown in Figure 3) and fed into a compounding gear pump. The compounding pump takes the molten plastic from the extruder or mixer and pushes it through a pelletizer similar to one described. The pellets are then cooled, dried and boxed for sale to converters.
Compounding pumps are sized for the production rates of the plant's other equipment and range from 3 to 100 metric tons per hour. Due to the extremely high temperature in which they operate, the pumps are centerline mounted to compensate for the thermal expansion of the steel. For products with extremely high viscosities or abrasive fillers, pumps typically are installed with timing gears that drive each gear shaft so there is no contact between the gear teeth. Additional output capacities have also been realized with the use of shaft and bearing cooling systems that keep the pump components running cooler, allowing for faster speeds than would otherwise be possible, resulting in higher production rates. Because these pumps operate at such extreme temperatures, pressures and viscosities, the precision of the pump dimensions and clearances is critical. The designer and manufacturer must have engineering knowledge of pump construction materials as well as polymer characteristics.
The final operation is converting the pellets into a usable product. End users, or converters, are companies who buy the pellets and make useful products by melting and reshaping the plastic. The pellets can be melted and molded into intricate shapes in injection molding machines, blow molded into bottles or extruded into a shape, sheet or film.
One extrusion application example is a plastic pipe manufacturer. The pellets are first placed into the feed hopper of a single screw extruder where they are gradually melted and mixed (sometimes with colorants or other additives) into a homogeneous mixture as the plastic is conveyed from one end of the extruder to the other. The melted polymer is forced through a die to create the shape of the pipe. It then runs through a water bath to cool and set the shape.
Although the extrusion process works without the use of a gear pump, adding a pump at the end of the extruder offers several advantages. The main advantage is the consistency and quality of the output. The gear pump, also called a melt pump, is a positive displacement device so the output is a consistent predictable volume per revolution. The gear pump is made for pumping at high pressure; therefore, it is more efficient than the extruder at building the pressure required to overcome the back pressure of the system (die and screenchanger).
The volumetric efficiency of the melt pump is 98 to 99 percent, whereas the extruder's efficiency is only about 45 percent. This means that adding a gear pump to the extruder will usually result in lower total power consumption. By taking the pressure building away from the extruder, wear is also reduced on the components of the very expensive extruder. Extrusions pump systems, including drives and controls, usually pay for themselves in a few months by reduced scrap and raising throughputs.
Pumps for molten polymer are highly engineered positive displacement devices. Given the extreme temperatures, pressures and viscosities involved, knowledge of the characteristics of the materials of construction of the pump and an intimate knowledge of the properties of polymers and how they react at various operating conditions is a must for any polymer pump manufacturer.
