HYDRAULIC CONVEYING OF PLASTIC PELLETS (2004)

Today’s plastic production plants can have a yearly capacity of more than 400,000 t. New catalysts enable them to produce a great variety of increasingly softer product grades in the same reactor. Together with logistic concepts that increase conveying distances, the polymer industry has reached the limits of traditional dilute and dense phase pneumatic conveying systems. This paper presents a new concept to convey plastic pellets as water slurry to overcome the limitations of pneumatic conveying. A further benefit of this technology is the gentle handling of attrition sensitive plastics like Polycarbonate and Polyester. Background In general dilute phase conveying with high capacity over long distance is technically possible. Due to the resulting large pipe diameter and therefore high conveying velocity there will be substantial attrition of polymer dust (1). This dust reduces the final product quality, has to be removed and will be lost production. With softer polymers there will be the formation of streamers that can cause production and of course quality problems as they are difficult to separate. Single line dense phase low velocity conveying systems have definite limitations in distance as well. Pipe diameters have to stay below 300 mm in order to keep the pipe shock forces, resulting from the moving product slugs, within a manageable range. Figure 1 shows the limits for product mass flow versus conveying distance for today’s bandwidth of Polyethylene pellets. Basis for this graph is a 315 mm pipe and a conveying pressure of 6 bar which is the practical limit of available rotary feeders. This graph shows that the conveying capacity for softer LLDPE grades and a typical conveying distance of 200 m is limited to 50 t/h. With extruder capacities of 70 t/h and still increasing demand this will be the ideal application for hydraulic conveying. The benefit of using water instead of air as the transport medium is that the specific density of the polymer is relatively close to the specific density of water. The pressure loss is calculated as pressure loss of the conveying medium without product plus the additional pressure loss due to the product. In both, pneumatic and hydraulic conveying, the pressure loss due to the conveying medium is a smooth function of the conveying velocity (Figure 2). The additional pressure loss due to the product in a horizontal pipe is caused by friction or better called “slip”, which is defined as the difference between the medium and the pellet velocity. The “slip” in water slurry and therefore the additional pressure loss due to the product is a much smaller portion of the total pressure loss compared to pneumatic conveying. Figure 2 shows this relationship from test data taken in a 65 mm pipe. This physical effect also helps to make hydraulic conveying more tolerant for product grade changes than pneumatic transport, which normally is a serious influencing factor on pressure loss and therefore system performance especially with softer grades. Air as the conveying medium in pneumatic conveying systems is a compressible gas. As the conveying pressure decreases from the pickup point towards the terminal point, the gas velocity increases. In a hydraulic conveying system the medium water is not compressible. The water velocity will therefore be constant throughout the entire length of the conveying pipe. This constant low velocity and the small “slip” are the reason for an up to 65% lower energy consumption of the hydraulic conveying. Typical slurry velocities in hydraulic conveying systems are in the range of 2.0 – 4.5 m/s. They are related to factors like minimum required conveying velocity and residence time of the product in water. The minimum velocity depends on the polymer density similar to pneumatic conveying (1) and is increasing with increasing density. Table 1 shows feasible capacities in various pipe sizes for LLDPE and a volumetric concentration of pellets in the slurry of 25%. Figure 3 shows the relationship of pellet mass flow versus conveying pipe diameter for hydraulic, dilute and dense phase conveying. Even when reaching the technical limits with the pneumatic conveying, the hydraulic conveying needs just 50% of the pipe diameter.