Polymers: Polyolefins: Polyethylene, Polypropylene And Polystyrene (Part - 2) | Chemical Technology - Chemical Engineering PDF Download

Unipol

Process The process produces low density polyethylene and high density polyethylene using low pressure in gas phase. Wide range of polyethylene is produced using proprietary solid and slurry catalyst. The process produces wide range of polyethylene in a gas phase, fluidised bed reactor using proprietary solid and slurry catalyst. Gaseous ethylene, comonomer and catalyst are fed to fluidised bed reactor containing a fluidized bed of growing polymer particles operating at 25kg/cm² and 100^oC. Polymer density is easily controlled from 0.915 to 0.97 g/cm. Process flow diagram for polyethylene manufacture is given in Figure M-VIII 2.1.

Figure M-VIII 2.1: Fluidized-bed Gas Phase PE Process. 

Dupont Sclairtech Process

A broad range of poluethylene with density varying from 0.919 to 0.9605 g/cm² with varying melt index can be made by this process. The process can be divided into three major areas

• Reaction area
• Recycle/recovery Area,
• Extrusion and Finishing
• Dowtherm Vaporiser

The process involves solution polymerization of gaseous ethylene using cycolhexane solvent and comonomer butene or octane comonomer (incase of low density polymers). Zigler catalyst is used to polymerise ethylene using cycloheaxane as solvent. A chain terminator is used to control the molecular weight at the reactor outlet a catalyst deactivator is added to terminate the reaction. The polymer is depressurized to flash off solvent, unreacted ethylene and comonomer from the molten polyethyelene which are separated and recovered using distillation. The polymer after stripping the residual solvents fed to main extruder and resulting polymer pellets are dried and send to blender for homogenizing and finally conveyed to storage silo. In the process Dowtherm is added as heating media. Process flow diagram for the manufacture of polyethylene by sclairtech Process is given in Figure M-VIII 2.2.

Figure M-VIII 2.2: LLDPE Process by SCLAIRTECH Process 

Polypropylene 

Polypropylene is a low density semi-crystalline stereo-regular polymer which exists in three forms- isotactic, syndiotactic and atactic. Polypropylene was discovered in March 1954 by Professor Giulio Natta demand of polypropylene is growing at a much faster rate due to its strong demand per capita consumption of polypropylene is given in Figure M-VIII 2.3. 

Figure M-VIII 2.3: Per capita Consumption of Polypropylene

Process Technology for Polypropylene  

Polypropylene polymerisation process have undergone a number of revolutionary changes since the production of crystalline polypropylene were commercialised in 1957 by Motecatini in Italy and Hercules in U.S. Commercial polypropylene processes are based on low pressure processes using Ziegler-Natta catalyst that produces a product with an isotactic content of 90percent or more. Various processes for polypropylene manufacturing are given in Table M-VIII 2.4. A typical polypropylene process is given in Figure M-VIII 2.4.

 Table M-VIII 2.4: Polypropylene Manufacturing Process 

Source: Petrochemical Processes 2003, Hydrocarbon Processing March 2003 

Figure M-VIII 2.4: Polypropylene Process 

UNIPOL Process: The process produces homopolymer, random copolymer and impact copolymer polypropylene. Polymerisation takes place in a fluidized bed reactor using slurry reactor (TiCl4 supported on MgCl₂ in slurry form in mineral oil. Co-catalyst TEAL, purified propylene and ethylene incase of random PP), purified H₂ and selectivity control agent is continuously fed to the reactor. Temperature 35^oC and pressure 33 kg /cm² is maintained in the reactor. Figure M-VIII 2.5 illustrate the Unipol process for manufacturing of polypropylene. 

Figure M-VIII 2.5: Polypropylene by Unipol process 

Sources: Petrochemical Processes” Hydrocarbon Processing, March 2003, Page 124 

Polystyrene 

Polystyrene is an important thermoplastic. Polystyrene because of itsease of fabrication, low specific gravity, thermal stability and low cost, find wide applications in consumer durable goods, electronics, packaging toys, structural foams, wall tiles, shoe soles, blister packages, lenses, bottle caps, wire and cable sheathing, small jars, vacuum formed refrigerator liners, containers of all kinds, transparent display boxes and automobile interior parts. When styrene is copolymerized with acrylonitrile, the polymer Styrene Acrylonitrile (SAN) resin has a higher tensile strength than polystyrene. Acrylonitrile butadiene styrene (ABS) polymer has special mechanical properties and find application as engineering plastics.

Styrene is produced by dehydrogenation of ethyl benzene which is made by alkylation of benzene. Ethylbenzene by UOP EB One process is produced by liquid phase alkylation of benzene using proprietary zeolite catalyst which can be regenerated repeatedly thereby avoiding significant catalyst disposal problems associated with other aluminium chloride catalyst. The process offers better product quality, better heat integration, low investment and a more rugged and reliable catalyst system.

Lumus/UOP Classic SM process: in this process styrene is made by catalytically dehydrogenating ethylbenzene in presence of steam in multistage reactor system. The reaction is carried out at high temperature under vacuum. The process uses a oxidative rehet technology. Polystyrene is clear transparent resin with a wide range of melting points and good flow properties which make it suitable for injection moulding 

Process Technology

There are two major processes for the production of polystyrene are NOVA’s Polystyrene Technology and UOP Polystyrene Technology

NOVA’s Polystyrene Technology: The process produces a complete range of general purpose (crystal) and impact resistant polystyrene. This is based on bulk continuous polymerization technology.

UOP Polystyrene Technology: this process is based on continuous bulk polymerization to produce a wide range of general purpose polystyrene, high impact polystyrene and SAN resin. A typical plant includes feed preparation, reactor section, devolatilisation section, monomer recovery section, water removal, product pelletizing and bulk resin handling.