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Polystyrene

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Mohamed Mobarak

on 26 October 2015

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Transcript of Polystyrene

E.Styrenics, as part of the Egyptian petrochemical integrated plan is the only plant set to produce Polystyrene in Egypt

Production capacity: 200,000 Metric Tons/year based on 8000 operating hours

Products: General Purpose Polystyrene (GPPS) and High Impact Polystyrene
(HIPS)

Plant is located at Dekhila port in Alexandria.
Estyrenics
Introduction


The initial, rubber-dissolving step for high impact polystyrene.

Using an initiator for the HIPS production.

Activated alumina column is used to remove the styrene inhibitor (TBC) when producing GPPS to avoid yellow color of the crystal GPPS.

Differences between GPPS and HIPS production:
GPPS
Chemicals
Raw Material
Mineral Oil Plasticizer
Zinc Stearate Lubricant
Blue Dye Color adjustment

Additives
Poly Butadiene Impact modifier
IRGANOX 1076 Antioxidant
PIB ESCR
Triganox 117 Initiator

HIPS
Styrene Monomer
Common Additives in HIPS and GPPS Production
Additives
In HIPS Production only
Solvent and Chain transfer Agent
Additives
Ethyl Benzene
Product
Main Product
Applications
Polystyrene
Side Product
Oligomers
SPI resin identification coding system
Properties
Viscosity
Mn : number average molecular weight

Total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample.

Molecular Weight
Mw :weight average molecular weight

Based on the fact that a bigger molecule contains more of the total mass of the polymer sample than the smaller molecules do


GPC
A measure of the resistance of flow due to internal friction when one layer of fluid is caused to move in relationship to another layer.

Is the viscosity of the polymer solutions to the viscosity of the pure solvent
Relative Viscosity
The difference in the efflux times of the solution and the pure solvent, relative to the efflux time of the pure solvent.
Specific Viscosity
We divide specific viscosity by the concentration of the solution in question
Reduced Viscosity
Intrinsic Viscosity
Intrinsic viscosity reflects the capability of a polymer in solution to enhance the viscosity of the solution.

Temperature
Polystyrene has no specific melting point as it is a non crystalline polymer

Instead of melting point, vicat softening temperature is the closest indication to the melting point (i.e if vicat softening temp is 80 C then the melting range is around 160-180 C)

Vicat softening temp of the polymer is an important factor for determining the temperature of the injection, extrusion and molding machines during further processing

Feed Preparation
TBC removal Columns
Rubber dissolving unit
Process
Reaction Section
Run away Reaction
The polymerization reaction control is based on the following parameters:

Temperature
Residence time
Conversion

Reaction key Parameters
Polymerization consists of a series of reactors.

The first two reactors, for pre-polymerization, are CSTR-type with proprietary agitator designs.

The last reactor, for polymerization, is a plug-flow type, again of proprietary design.
Reactors
Overview
Process
GPPS Production
Plant Areas
HIPS Production
Two columns operating with a lead and lag

Each column contain Activated Alumina to adsorb TBC (inhibitor)

Styrene feed passes through TBC removal columns in case of GPPS production only to prevent the yellow color of the polystyrene pellets
Polybutadiene rubber, in bale form, is dissolved in Styrene monomer in
preparation of producing High Impact PS.

Rubber is used to increase the impact resistance of polystyrene

It is very important to control temperature in polymerization reactors as:

Temperature impacts reaction rate

Temperature impacts product quality

Temperature
Average time a reactant molecule stays in the reactor




Example:
Reactor Weight : 20T
Total flowrate : 10 T/h
Residence time 2h

Residence Time
Indicate how much styrene has been converted into polystyrene




S : Styrene (Monomer)
PS : Polystyrene (Polymer)

Conversion = Reaction rate (%/hr).Residence time (hr)

Note for plug flow reactor:
When inlet flow rate is fixed, conversion depends only on temperature in each zone.

Conversion
Self-accelerating chemical reaction (e.g. styrene polymerization).

During a runaway the temperature and pressure will rapidly increase
and get out of control.

Styrene polymerization is an exothermic reaction (~680kJ/kg)

Styrene is a reactive monomer and polymerization occurs slowly
even at ambient temperatures

Run away reaction can occur in reactors
- Loss of cooling power.
- Loss of agitation in reactor.

Run away reaction can occur in vessels or tanks.
- Mechanical heat input (stirrer or pump).
- Initial temperature higher then normal.
- Residence time much higher then normal.
- Tanks and vessels often have no agitation
- Tanks can have a limited heat removal capacity
.

Perfect mixing assumes that there are no spatial gradients, then composition and temperature are the same in the reactant volume.

3 residence time needed to renew the reactor content

Continuous Stirred Tank Reactor
Typical values of temperature and pressure in the reactor are 104°C, 0,31kg/cm²abs.

Control of residence time, temperature and pressure in the reactor leads to given conversion.

HIPS: 7-8% conversion while GPPS: 22% conversion

HIPS: Thermally and Chemically initiated while GPPS is thermally initiated
1st Prepoly Reactor
Typical values of temperature and pressure in the reactor are 117°C, 0,46kg/cm²abs.

Control of residence time, temperature and pressure in the reactor leads to given conversion.

Total Conversion after 2nd Prepoly:
HIPS 35% while GPPS 49%
2nd Prepoly Reactor
The key assumption is that as a plug flows through reactor, the fluid is perfectly mixed in the radial direction but not in the axial direction, so that composition is changing in the flow direction.

Residence time depends on the inlet flow rate only,

1 residence time needed to renew the reactor content.

Plug Flow Reactor
Polymerization Reactor is a plug flow reactor consisting of two towers in series

These towers consist of heat transfer bundles of proprietary design

Conversion after polymerization reactor:
HIPS: 75% while GPPS: 87%

Polymerization Reactor
Separation Section
Conversion in last reactor is typically 75 -90 %, Unreacted monomer and solvent need to be flashed off from polymer.

It is a two-step operation, which is enhanced with the stripping effect of water in the 2nd stage in order to vaporize the residual unreacted styrene and impurities from the polymer melt

The process take place under vacuum and at a high temperature

Polymer melt having a residual volatile concentration of about 200 ppmw.

Devolatilization
Unreacted Styrene and diluents from the devolatilization operation
are distilled, under vacuum, and recycled to reaction section.

Two purges are provided: a lights purge consisting of styrene and ethylbenzene and a heavy purge consisting of
oligomers and other heavy organics.

Heavy purges are used as fuel for the Hot Oil Heater
Recovery
Process
Process
Pelletization and Storage
Two identical pelletizer lines are used to produce pellets.

The polymer from the 2nd Stage
Devolatilizer is pumped to the Pelletizing Section.

Here, an internal lubricant is introduced to the melt

Pelletization
The pellets are then air conveyed to QC Holding Silos

The pellets are accumulated and held in one silo for quality control checks, while batch transferring acceptable product out of the other silo.

From the QC Holding Silo, the pellets are subsequently air conveyed to the Product Silos.

Recycle Silos are provided to recycle acceptable transition or mildly offspec material to the Product Classifier

Normally no pellets are recycled.
Storage
Process
Process
Bagging
Pellets from storage silos are conveyed towards the bagging silos

Bagging silos feed the 3 identical bagging lines

Pellets are collected in 25 Kg bags then arranged on a pallet (5 bags X 12 rows)
Not very soluble in water, soluble in acetone, ethers and alcohols

Styrene easily evaporates

The molecule of styrene, possessing a vinyl group (double bonds carbon-carbon), can polymerize

The liquid styrene begins to polymerize at ambient temperature. It can be stabilized by the addition of hydroquinone (TBC: 4-tertio butyl-catechol)

The polymerization is very clearly accelerated by the light. The styrene has therefore to be stored in opaque receptacles
Styrene
Reaction
Kinetics

Total Styrene feed flow rate (Kg/h) + Polymer Generation = Reactor product flow rate + Material accumulation in reactor (Kg/h)


where:
polymer generation = conversion (%) * Styrene feed flow rate (Kg/h)
Mass Balance
Total Reactor feed heat (KJ/s) + Reaction heat = Reactor product heat (KJ/s) + Heat accumulation in reactor (KJ/s) + Heat removed by cooling (KJ/s)

Heat Balance
Reaction Rate
Rate at which the monomer reacts to form the polymer (%polystyrene/ unit of time), depends on :
1. Monomer concentration
2. Temperature
3. Initiator concentration (if any)

Polymerization rate is low at low temperatures but increases significantly as temperature increase
Reaction Rate
Rate of thermal polymerization can be calculated by:
ki ,kp ,kt : the rate constants of initiation, propagation and termination

[M]: Monomer concentration

Many existing models consider gel effect, oligomer formation and chain transfer.
Reaction
Chemistry

Thermal initiation (GPPS)

Styrene can undergo self initiated “thermal” polymerization

Process parameter: temperature

N.B: high temperature generates more radicals leading to:

- an increase in reaction kinetics
(runaway risk),

- shorter polymer chains (molecular
weight decrease).
Initiation
Successive addition of monomers on the active chain

Propagation
Termination
Two possible termination mechanisms: recombination (or coupling) or disproportionation

Polystyrene mainly ends up with recombination as styrene chains radicals are accessible to directly link together.

Chemical reaction (coupling):
Chain transfer
Reaction termination for the active chain leading to an “inert” polymer chain
Chain transfer agents (XA): initiators, styrene, ethylbenzene....

N.B:
Transfer reaction decreases molecular weight (product quality).
Initiated process (HIPS)

Decomposition of organic peroxide



Use: Maintaining an overall rate of polymerization at a lower temperature

Process parameters: temperature, type of initiator, amount of initiator
Molecules of very short chain length

Only a few monomers bonded together:
Dimers: two monomers (decompose to styrene)
Trimers: three monomers (relatively stable)

Low molecular weigh, volatile molecules

have a plasticizing effect on final product

The use of initiator significantly reduces oligomer formation
Mol. weight control
In polymerization plug flow reactors pressure drop across the reactor can be estimated by:





Where :
v : velocity (m/s)
L : length (m)
d : tube diameter (m)
μ : viscosity (Pa.s)
E : void fraction (0.75 – 0.80) for SMR reactor

Polymerization Reactor
Polystyrene production using INEOS technologies
Prepared by: Mohamed Mobarak
Supervised by: Dr. Ehsan Nassif

Receiving Styrene
Styrene shipments are received by sea via unloading arm and stored in continuously circulated storage tanks
Full transcript