fig 1.: plant diagram for hydraulic conveying with the conticon system: a extruder with underwater pelletisation; b: Pellet water mixture; c: pre-dehydration; d: water filtration; e:Water holding vessel; f: Agitator hopper; g: centrifugal pump; h:Concentrator; i:Conveying line over 800 m; k: Centrifugal drier; l: pellet discharge (to the pneumatic distribution system for the storage silos); m: water holding vessel; o: recirculation pump; p: water holding vessel; q: steam injector
Pre-dehydration in the underwater pelletisation water circuit increases the pellet content from approximately 6 to 15 per cent. During pelletisation the low concentration is necessary to prevent doubled pellets and for speedg cooling of the melt temperature (approx. 250'C) to approximate 70'C. In addition, it is also nececessary to keep the water pressure and temperature constant in this circuit within a very narrow range to prevent cavitation on the die plate and to ensure clean pellet cutting.
The pellet water mixture, concentrated during pre-dehydration and at the same time freed from agglomerates, is fed through a gravity pipe into an agitator hopper in which mixture paddles produce a homogenous suspension. It must be taken into account that the specific density of polyolefins is less than that of water, and it must be ensureed that no air is incorporated by the agitator.
A centrifugal pump draws the mixture out of the agitator hopper through a suction pipe and creates the necessary conveying preassure, in this case around 7 bar. In its test centre Coperion Waeschle optimused the impeller of this pump in close collaboration with the pump manufacturer so thag it neither damages the plastic pellets nor generates any fines, and at the same time achieves a high conveying efficiency. Today pumps with different impellers and from various manufacturers are avaliable, depending on the type of plastic and the required product quality. In addition Coperion Waeschle has developes a process for which a pateng has been applied, which permits extremely gentle conveying of plastic pellets through the centrifugal pump