b) Gas-Phase Process
Fluidized-bed processes for the production of HDPE were developed in the late 1960s by Union Carbide and later by BP. Many fluidizedbed plants are built as dual-purpose (swing plants) with ability to produce either LLDPE or HDPE according to demand. This process produces a very wide range of MFls (0.01-100) and densities (0.890.97 gm/cm3) It is free of the viscosity constraints of the solution process and solubility constraints of the slurry process. Haxene is a preferred comonomer for LLDPE production.
[ad#aryshi1]
The reactor has characteristics shape with cylindrical reaction section, and an expended section in which the gas velocity is reduced to allow entrained particles to fall back into the bed &he bed diameter is around 4 m with a working height of 10m and an overall height of the reaction unit of 30m. The gas enters the reactor through a distributor plate which provides an even distribution of gas and must also prevent powder falling through when the gas flow is stopped.
Reaction temperature is 80-100GC, depending on the density of the product being made, and the pressure is in the range c6f 0.7-2.0 MPa . The conversion per pass is 2% for HDPE A cyclone or filter prevents fine particles reaching the recycle cooler and compressor. The polymer is removed via a sequenced valve to a powder cyclone, from which residual monomers are removed and recompressed. The main recycle compressor circulates gas at high flow rate, but with small pressure rise. Since the process operates close to the melting point of of the polymer, accurate temperature control is necessary by regulating the rate of catalyst addition. If a runaway reaction is detected, a gas such as carbon dioxide can be injected to poison the catalyst.
Fluidized-bed process
a) Calalyst hopper and feed valve; b) Fluidized-bed reactor; c) Cyclone; d) Filter; e) Polymer take-off system; f) Product recovery ryclone; g) Monomer recovery compressor; h) Purge hopper; i) Recycle compressor; j) Recycle gas cooler
The catalyst particle grows by a process of replication to about 15-20 times its initial size. This affects fluidization characteristics and polymerization rate. Heat transfer must be controlled to prevent fusion of the particles. Catalysts based on microspheroidal silica or MgCI2 with a mean particle size of about 50 ~m are particularly suitable.
[ad#aryshi1]
