2 GENERAL PROCESSES AND TECHNIQUES APPLIED IN THE PRODUCTION OF POLYMERS
HDPE Leistungsvergleich
4.2 Applied processes and techniques in the production of polystyrene
4.2.3 High impact polystyrene (HIPS) process
4.2.3.1 Process description
Generally, the process is very similar to the GPPS process. The main difference is the addition of the rubber component. Polybutadiene rubber received in the form of 35 kg bales is ground into small chips. These rubber chips are added to the dissolving tank by gravity or pneumatic conveyor. There, with strong agitation, the chips are dissolved in styrene to obtain a rubber so-lution that can contain up to 15 % of rubber.
The antioxidant is usually also added in the dissolving tank. In addition, other chemicals can be added there such as white oil, peroxides, recycled styrene, ethylbenzene or chain transfer agents.
The dissolved mixture is then fed continuously to the reactor train where bulk polymerisation occurs. Chemicals not added in the dissolving tank are added into the feed stream or directly into the reactors.
The reactor train usually includes continuous stirred tank reactors (CSTR) and/or plug flow reactors (PFR). The styrene itself acts as the solvent of the reaction. Moreover, up to 10 % of ethylbenzene is added to ensure a better reaction control. The reactors’ temperatures are be-tween 110 and 180 °C. The pressure is up to 1 MPa in a PFR, whereas reactions in CSTR are carried out under atmospheric or sub-atmospheric pressure. At the end of the reactor train, the styrene monomer conversion reaches 60 - 90 % solid.
The process flow then goes through a devolatilisation section where it faces one or two flashes (one or two devolatilisation vessels) to separate the polymer from the unreacted monomers. The devolatilisers are operated at high temperature (220 – 260 °C) and under high vacuum (<40 mbar).
Between the two devolatilisation steps, an injection of water (stripping) can be added to im-prove monomer removal. After condensation, unreacted styrene and ethylbenzene are recycled to the feed line, either directly using a recycle loop or through a storage tank. A purge of unde-sirable components is carried out on this stream.
The melted polymer is then transferred through a dye head to obtain strands that are cut (dry or underwater) by pelletisers. After drying, the pellets are discharged in a pneumatic conveyor and afterwards stored in silos for packaging and/or shipping in bulk.
A tabulated summary of the HIPS process is shown in Table 4.7.
A flow diagram of the HIPS process is shown in Figure 4.5.
MP/EIPPCB/POL_BREF_FINAL Oktober 2006 79
D is s o lv in g T a n k s
R e c o v e re d s ty re n e a n d d ilu e n ts
R e a c to rs R a w m a te ria ls
s to ra g e P B u ru b b e r
D e v o la tiliz e r
S to ra g e
H IP S P u rg e
P e lle tis e r
Figure 4.5: Flow diagram showing the HIPS process
4.2.3.2 Technical parameters
Product type Medium and high impact polystyrene
Reactor type Continuous stirred tank reactors and/or plug flow reactors Reactor dimensions 3 - 50 m3
Polymerisation type Radical polymerisation Polymerisation pressure Up to 1 MPa
Polymerisation temperature 110 - 180 °C
Diluents Styrene, ethylbenzene Catalyst None or organic peroxides
Additives Polybutadiene, white oil, chain transfer agents, lubrication agents Conversion 60 - 90 %
Table 4.6: Technical parameters of HIPS
Preparation Step Reaction Step Final Step
Storage Grinder Dissolving system Reactors Devolatilisation Pelletiser Storage Packaging Purpose Raw materials storage PBu grinding for
HIPS production
additives Process feed
soluti-on PS + unreacted species Final PS PS pellets PS pellets Outlet Raw material 1 or 2 cm size
chips Process feed
soluti-on PS + unreacted
species SM and diluent + PS PS pellets PS pellets Packed PS pellets
Working - Batch/
continuous Batch/continuous Continuous Continuous Continuous Batch/
continuous Batch/
Solution of PBu in styrene; addition of
Parame-ters Temperature Size of chips Temperature, time,
agitation Temperature and/or pressure control
Temperature and pres-sure control. Water
flow if stripping
Pellet size Level control Weight
Abbreviations
PBu polybutadiene rubber PFR plug flow reactor NR not relevant HIPS high impact
polystyre-ne SM styrene, monomer NA not applicable
CSTR continuous stirred
reactor PS polystyrene
Table 4.7: Summary of the HIPS process
MP/EIPPCB/POL_BREF_FINAL October 2006 81 4.2.4 Expandable polystyrene (EPS) process
4.2.4.1 Process description
Suspension polymerisation is still the mostly used process for large volume production of ex-pandable polystyrene. It is a batch process that allows the conversion of styrene monomer into expandable polystyrene beads through radically initiated polymerisation. Typically, styrene is dispersed under agitation in the aqueous phase containing some suspension agents and/or pro-tective colloids and some secondary ingredients such as electrolytes. Both organic and mineral suspension agent systems can be used. Organic peroxides are added to styrene prior to its dis-persion into the water phase.
The polymerisation then follows a schedule that raises the temperature of the reaction mixture stepwise towards nearly total conversion of the monomer into the polymer. Pentane, a mixture of normal and isopentane, is added as a blowing agent during the course of the polymerisation.
The type and the quality of the suspension agent system and the reaction temperature profile are critical to ensure good suspension stability, bead size distribution and a high conversion rate.
These parameters also influence the properties of the final product. An aliphatic brominated compound is specifically added for the production of flame-retardant grades.
At the end of the polymerisation, the suspension is cooled; the expandable polystyrene beads are separated by centrifugation, washed and then dried at a relatively low temperature of 35 °C.
Following this, the beads are then screened into several bead size fractions corresponding to the various commercial needs and applications. They are then coated to improve both processing characteristics and final performance of the product. Finally, these expandable polystyrene beads are packaged in containers or silos for shipment.
A tabulated summary of the EPS process is shown in Table 4.9.
A flow diagram of the EPS process is shown in Figure 4.6.
Figure 4.6: Flow diagram showing the EPS process
4.2.4.2 Technical parameters
Product type Expandable polystyrene Reactor type Batch reactors
Reactor dimensions 20 - 100 m3
Polymerisation type Radical polymerisation in aqueous suspension Polymerisation pressure 1 - 2 MPa
Polymerisation temperature 65 - 140 °C
Diluents Styrene
Catalyst Organic peroxides
Additives Suspension agents, coating additives, brominated compounds
Conversion >99 %
Table 4.8: Technical paramters of EPS
MP/EIPPCB/POL_BREF_FINAL October 2006 83
Reaction Step Final Step
Storage Reactor Centrifuge Drying Sieving Coating Storage Packaging Purpose Raw materials stora
ge Polymerisation o
styrene Separation of slurry Drying of beads Size Separation of EPS
beads Coating surface o
beads Storage of bulk
EPS Packaging o EPS beads Inlet Raw material Process feed solu
tion
EPS + water + unreacted
species Moist EPS beads Dry EPS beads Dry and separate
EPS beads EPS beads EPS beads Outlet Raw material EPS + water Moist EPS beads + wate Dry EPS beads Dry and separated EPS
beads
Dry, separated and coated EPS
beads EPS beads Packed EPS beads Working Batch/continuous Batch Continuous Continuous Continuous Batch/
continuous Batch/
continuous Batch/
continuous Capacity From litres to tonne Up to 100 m3per
reactor 1 to 30 m3/h NR NR NR NR NR
More details NA CSTR,
possible addition of chemicals
Separates water from
slurry Flash dryer, fluid
dryer Several layers of sieves CSTR NA NA
Key Parameter Temperature Temperature
and/or pressure Capacity, speed Temperature, capac-ity, and residence
time Mesh size Temperature,
mixing efficiency Level control Weight
Abbreviations
SM Styrene monomer EPS Expandable polysty
rene NR not relevant
CSTR Continuous stirred
reactor NA not applicable
Table 4.9: Summary of the EPS process