High-density polyethylene (HDPE) prodn. - with procedure for changing process parameters to enable changeover from high-mol. wt. to low-mol. wt. HDPE without shut-down or cleaning

摘要

A process is claimed for the prodn. of HDPE by catalytic gas-phase polymerisation of a mixt. of ethylene, hydrogen and comonomer in a fluid-bed reactor incorporating a gas distributor, with recirculation of unreacted gas mixt.. The special feature is that, from the steady-state condition for prodn. of a polymer suitable for the mfr. of large tubular prods. (HDPE-I), the system can be changed over directly to reaction conditions for the prodn. of a polymer suitable for the mfr. of film, fibres and injection mouldings (HDPE-II), by changing the catalyst and the comonomer but without shutting down or cleaning the system. This process comprises: (a) raising the hydrogen concn. in the gas circulation to a level corresp. to a hydrogen/ethylene mole ratio of 45-55 x 10 power(-3) mole/mole 36-42 hrs. before changeover, at a temp. of 84-96 deg.C and with continuous removal of off-gas at the rate of 1.7-5.1 x 10 power(-3) Nm3/m3 circulating gas, (b) stopping the addn. of catalyst, (c) emptying the catalyst metering system including instrument parts, washing with the new catalyst and filling with new catalyst, (d) stopping addn. of comonomer (e.g. butene-1) when monomer conversion has dropped to 20-30% of the normal level, (e) preparing for the change of comonomer, e.g. from butene-1 to propene, (f) adding a mixt. of N2/O2 to kill any residual reaction (at the same time as (d)), (g) flushing the reactor 2-4 times with ethylene, (h) establishing the required new hydrogen/ethylene ratio (the propene content is adjusted only after the start of polymerisation), (i) starting addn. of the new catalyst, with the circulating gas contg. less than 1 ppm oxygen and less than 3 ppm CO2, (j) lowering the reactor fill height to 90-94% of the normal level gradually over 1.4-1.8 hrs. (simultaneously with (b)), (k) raising the reactor temp. gradually to 94-96 deg.C starting with (b), (l) increasing the reactor level to 110-115% of normal for 15-35 mins. after stage (i), and (m) re-establishing the normal reactor level. USE/ADVANTAGE - The invention makes it possible to convert a continuous gas phase HDPE prodn. system directly from prodn. of high-mol. wt. polymer to prodn. of low-mol. wt. polymer (mol. wt. 300,000) by changing the catalyst and the comonomer but without shutting down or cleaning the system. This process reduces the amt. of transitional polymer with pinholes and inhomogeneities which is normally obtd. after changeover.