Technology

The Spherilene gas-phase technology is designed to produce the full range of linear low density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE).

Solid catalyst, alkyls and donor are brought into contact in the pre-contact vessels, where the catalyst is activated. Activated catalyst stream from the pre-contact vessel flows directly into the gas phase reactor and is dispersed in the fluidized reacting bed. The spherical morphology Avant Z catalysts can be introduced directly into the reactor, even when empty. Chromium or other specialty catalysts are introduced when there is a seed bed in the reactor.

Ethylene, comonomer and hydrogen are fed to the reactor according to the required production capacity and proportions needed for the target product. Propane is used as the inert medium of reaction and provides an independent means of controlling the reaction kinetics while providing a heat removal capability far superior to that of inorganic inerts (such as nitrogen). The heat of the reaction is removed from the fluidizing gas in a water cooled vertical heat exchanger. The polymer is withdrawn continuously from a specially designed outlet at the bottom of the reactor and sent to a degassing vessel where monomer unreacted, comonomer and hydrogen are removed from the polymer with the help of a counter-current propane gas flow. The recovered gas is sent to the hydrocarbon recovery section, where the reaction components are separated from propane and both fractions are recycled inside BL as appropriate.

Polymer from the propane degassing vessel is fed by gravity to a deactivation vessel where the dissolved hydrocarbons are removed from the polymer by counter-current stripping with nitrogen assisted by a small amount of steam. Steam also ensures that any residual activity of catalyst system components is neutralized completely. From the bottom of the deactivation vessel, the polymer is conveyed pneumatically to the powder silo located on top of the extruder. The silo is provided with nitrogen purging to ensure that no residual hydrocarbons accumulate in the silo. Polymer from the silos is mixed with required additives before being fed to the extruder.

The Spherilene process can also be designed with two gas-phase reactors in series to allow production of bimodal products. In addition, a Spherilene process plant built with one reactor can also be expanded in the future by the addition of a second reactor circuit to increase capacity and expand product capability. Thus, Spherilene provides a proven and unique, future-proof technology platform to assure long term success for a plant.

The product density range is very wide, from approximately 0.915 g/cc (LLDPE) to > 960 g/cc (HDPE), including full access to the MDPE range (0.930 to 0.940 g/cc). Melt index (MI) capability ranges from 0.01 to > 100 g/10 min. From a simple cost-effective single reactor configuration, traditional LLDPE, MDPE and HDPE grades for film, blow molding, injection molding, rotomolding, geomembranes, textile, raffia and extrusion applications, a full range of LLDPE products for cast and blown film, extrusion coating are available. Further, the dual reactor configuration enables production of premium bimodal grades (MI, density) in gas phase with “inverse” comonomer distribution, hitherto available only via more investment-intensive slurry technologies. Commercially proven grades include bimodal HDPE for pressure pipe markets with PE100 certification and bimodal HDPE grades for high-strength film markets.

Source: Jack, 17^th Jun 2018, LL/MD/HDPE Process by LyondellBasell, Oil & Gas Process Engineering