A PROCESS FOR THE PREPARATION OF POLYETHYLENE

A process for preparing high density polyethylene in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a titanated porous silica supported chromium oxide catalyst wherein the chromium is in the oxidation state of plus 6 in a fluidized bed reactor having a recycle gas line, under polymerization conditions, with the following provisos: (i) the atomic ratio of chromium to titanium is in the range of about 0.008:1 to about 2.8:1; (ii) the amount of catalyst is in the range of about 0.002 to about 0.05 part by weight based on 100 parts by weight of the high density polyethylene; (iii) at least about 70 percent of the pores of the silica are larger than 200 Angstroms; (iv) the partial pressure of ethylene is in the range of about 100 to about 350 psia; (v) oxygen and/or another catalyst poison is introduced into the reactor in the range of about 0.05 to about 0.6 part by volume of catalyst poison per million parts by volume of ethylene; (vi) the molar ratio of alpha-olefin, if present, to ethylene is about 0.001:1 to about 0.004:1; (vii) hydrogen is introduced into the reactor in the range of about 0.1 to about 0.6 mole of hydrogen per mole of ethylene; (viii) the polymerization is carried out at a temperature in the range of about 80 to about 120 degrees C; and (ix) a relatively low boiling inert hydrocarbon is introduced into the recycle gas line where it raises the dew point of the recycle gas, which is comprised of ethylene and other reactor gases, and the recycle gas is partially condensed and recycled to the reactor where it promotes cooling by evaporation. L'invention concerne un procédé utile pour la préparation de polyéthylène haute densité en phase gazeuse, qui comporte l'étape consistant à mettre en contact de l'éthylène ou un mélange renfermant de l'éthylène et une ou plusieurs alpha-oléfines, avec un catalyseur d'oxyde de chrome sur un support de silice poreuse modifiée au titanate, le chrome présentant un état d'oxydation de plus 6, dans un réacteur à lit fluidisé qui comporte une ligne de recyclage de gaz, dans des conditions de polymérisation, et selon les prescriptions suivantes: (1) le rapport atomique chrome/titane se situe entre environ 0,008:1 et environ 2,8:1; (2) la quantité de catalyseur se situe entre environ 0,002 et environ 0,05 parties en poids, pour 100 parties en poids du polyéthylène haute densité; (3) au moins 70 pour cent des pores de la silice ont une dimension supér