REACTOR SYSTEM FOR MULTIMODAL POLYETHYLENE POLYMERIZATION

The present invention relates to a process for producing a multimodal polyethylene composition in the reactor system according to the invention, comprising; (a) polymerizing ethylene in an inert hydrocarbon medium in the first reactor in the presence of a catalyst system, selected from Ziegler-Natta catalyst or metallocene, and hydrogen in an amount of 0.1 -95% by mol with respect to the total gas present in the vapor phase in the first reactor to obtain a low molecular weight polyethylene or a medium molecular weight polyethylene; (b) removing in the hydrogen removal unit 98.0 to 99.8% by weight of the hydrogen comprised in a slurry mixture obtained from the first reactor at a pressure in the range of 103- 145 kPa (abs) and transferring the obtained residual mixture to the second reactor; (c) polymerizing ethylene and optionally C4 to C12 a-olefin comonomer in the second reactor in the presence of a catalyst system, selected from Ziegler-Natta catalyst or metal locene, and in the presence of hydrogen in an amount obtained in step (b) to obtain a first high molecular weight polyethylene or a first ultra high molecular weight polyethylene in the form of a homopolymer or a copolymer and transferring a resultant mixture to the third reactor; and (d) polymerizing ethylene, and optionally a-olefin comonomer in the third reactor in the presence of a catalyst system, selected from Ziegler-Natta catalyst or metal tocene, and hydrogen, wherein the amount of hydrogen in the third reactor is in a range of 1-70% by mol, preferably 1-60% by mol with respect to the total gas present in the vapor phase in the third reactor or optionally substantial absence of hydrogen to obtain a second high molecular weight polyethylene or a second ultra high molecular weight polyethylene homopolymer or copolymer; and a multimodal polyethylene composition obtainable this way. La présente invention concerne un procédé de production d'une composition de polyéthylène multimodal dans le système de réacteur selon l'invention, comprenant : (a) la polymérisation d'éthylène dans un milieu hydrocarboné inerte dans le premier réacteur en présence d'un système catalyseur, choisi parmi un catalyseur de Ziegler-Natta ou un métallocène, et d'hydrogène en une quantité de 0,1 à 95 % en moles par rapport au gaz total présent dans la phase vapeur dans le premier réacteur pour obtenir un polyéthylène de faible poids moléculaire ou un polyéthylène de poids moléculaire moyen ; (b) l'élimination dans l'unité