Polymer Technology

Copolymers of propene with olefins account for 30% of all PP sales. There are two types of copolymers having different applications. Random copolymers, obtained by copolymerizing mixtures of propene and other olefins have lower melting points and improved clarity. Impact (block) copolymers made in a two-stage polymerization process, are high-impact-strength grades that contain dispersed propene-ethylene elastomers. Stabilizers, antistats, antacids, slip and antiblock agents, crystallization nucleants are further compounded.

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In gas-phase processes, gaseous propene is contacted with solid catalyst intimately dispersed in dry polymer powder. Industry uses two different methods of carrying out this reaction depending on the chosen method of heat removal. The Union Carbide/ Shell process uses fluidized-bed system (Unipol PE Process) BASF and Amoco use mechanically agitated dry powder beds with evaporative cooling in vertical and horizontal autoclaves, respectively.

BASF Novolen Process

BASF gas-phase Novolen process
a) Primary reactor; b) Copolymerizer; c) Compressors; d) Condensers; e) liquid pump; f) Filters; g) Prim.!)’
cyclone; h) Deactivation/purge

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The reactors are equipped with helical agitators, which give excellent mixing. Homopolymerization needs only primary reactor, into which catalyst components are fed. The reaction conditions of lO-80DC and 3-4 MPa ensure that monomer phase is gaseous in the reactor. Low concentrations of hydrogen are used to control molecular mass. The temperature is controlled by removing gaseous propene from the reactor head space, condensing it with cooling water, and then recirculating it back into the reactor. Each tonne of polymer made requires 6 tonne of liquid propene to be evaporated as coolant.
Powder and associated gas discharge continuously from the primary reactor dip tube directly into a low-pressure cyclone (g). Propene carrier gas from this cyclone is recycled to the reactor after compression and liquification. The powder then passes to the purge vessel where a deactivator quenches all residual catalyst activity, and nitrogen strips out traces of propene from the hot powder. From here powder is conveyed into silos for stabilization and extrusion into granules.

Amoco-Chisso Stirred-Bed Process

Collaboration between Amoco and Chisso resulted in joint licensing of this precess from 1985. this process uses horizontally stirred reactor, instead of vertical helical reactor of BASF process. Condensed recycled monomer sprayed into top of the reactor provides cooling, while uncondensed monomer and hydrogen injected into the base maintain the gas composition.

 Amoco – Chisso gas-ph3se process
a) Horizontal reactor; b) Fluidized•btd deactivation; c) Compressor; d) Condenser; e) Hold/separator tank

Unipol-Shell Fluidized-Bed Process

The Unipol-Shell plants, commissioned in 1986, combine technologies from Union Carbide and Shell. They have the tall fluidized-bed reactor with its expanded upper section to reduce gas velocity and powder entrainment. Continuous feeds of catalyst components, comonomer, if any, hydrogen, and propene are thoroughly mixed in the dense-phase fluidized bed of powder. A large cooler in the gas circulation loop removes all the reaction heat from the considerable gas flow. Reaction conditions are 88°C and 4MPa.
Product powder and associated gas are discharged from just above the distributor plate by timed valves into cyclone separator (e) and then directly to a purge vessel (g) to remove residual monomer.

Neither catalyst removal nor atactic polymer extraction IS necessary with modern Shell catalysts used in the Unipol process.

UCC/Shell- Unipol fluidized-bed process
a) Primary fluidized bed; b) Copolymer fluidized bed; c) Compressors; d) Coolers; e), f) Discharge cyclones; g) Purge

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Mitsui Petrochemicals, co-inventor with Himont of the supported catalyst system, has developed similar cheap process using bulk polymerization with its own supported catalysts. It differs from Spheripol system in that batch polymerization is used with washing. Two conventional stirred polymerizers are used in series, with heat removal by evaporation of liquid propene in the reactors. Slurry is then discharged into a stirred, heated flash vessel, where propene is recovered from polymer, as in the Himont system.

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This process is based on Himont (now Bassel) technology uSing supported catalysts. Polymerization takes place 70C and 4 MPa in liquid propene circulating round one or more loop reactors. A single axial flow agitator in each loop maintains high flow rates to ensure good heat transfer to the water-cooled jackets. Typically the PP concentration is 40 wt%.

 
Spheripol Process
a) Loop reactors; b) Primary cyclone; c) Copolymer fluidized bed; d) Secondary and copolymer cyclone;
e) Deactivation; f) Purging

Continuously metered catalyst, triethyaluminum, and a Lewis base stereoregulator such as dialkyldimethoxysilane are fed into the reactor to maintain polymerization and stereocontrol The initial few seconds of polymerization with a new high-activity catalyst particle are quite critical to secure good performance For this reason, some processes have prepolymerization stage in which the catalyst components react at lower temperature and monomer concentration.
 
The prepolymerized catalyst is then fed into the loop reactor. Mean residence time is 1-2 h. two loop reactors can be operated in series to narrow residence time distribution, modify the polymer, and increase output.
A continuous stream of polymer slurry discharges through a heated zone for the first stage of pressure let down on cyclone (b). For homopolymers, this connects directly to the secondary cyclone (d), bypassing the copolymerization unit. Untreated propene flashes off from the first cyclone and is condensed with cooling water and recycled to the reactor. A compressor is required for gas from cyclone (d). Polymer powder from the cyclone is fed into vessel (e) for deactivation with steam and additives. Residual moisture and volatiles are removed by a hot nitrogen purge vessel (f) before conveying polymer to storage silos for finishing as stabilized powder or extruded pellets.

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Suspension Homopolymerization (Early Diluent Processes)
A common practice is to link the reactors in series or in parallel. The maximum output rate of a reactor is determined by the heat removal system. Propylene and hydrogen feeds each reactor. Diluent and catalyst is fed to the first reactor only. Slurry viscosity increases rapidly during the latter stages of polymerization. The polymerization temperature is 50-75°C and reactor pressure 0.5-1.0 MPa.
Propene recovery, catalyst removal, centrifuging, drying, extrusion and pelletizing and diluent recovery stages follow polymerization.