Polymer Technology

Solution processes are developed by various companies including DuPont, Dow, DSM and Mitsui for manufacture of LLDPE or HDPE/LLDPE on a swing basis. These processes readily handle wide range of comonomer types and product densities but are unable to handle high-viscosity products.

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Solution polymerization process
a) Reactor feed pump; b) Temperature control; c) Reactor; d) Catalyst adsorber; e) First separator; f) Lowpressure separator and melt-fed extruder; g) Purged prodnct hopper; h) Recycle cooler; i) Diluent and monomer purification unit

The flowsheet shows the Sclairtech process of DuPont Canada”. The ethylene is dissolved in a diluent such as cyclohexane and pumped to the reactor at 10 MPa. The reaction step is adiabatic and the temperature is in the range 200-300°C The feed contains 25% wt ethylene, of which 95% is converted to polyethylene in the reactor.

The residence time is 2 min. The catalyst is a mixture of VOCL3 and TiCI4 activated by an aluminum alkyl.
The polyethylene solution leaving the reactor is treated with a deactivating agent and the mixture then passes through a bed of alumina where the deactivated catalyst residues are adsorbed. Two depressurization stages follow, similar to the high-pressure process, in which solvent and unreacted monomers are volatilized After extrusion into pellets, further removal of solvent residues is achieved by passing a heated stream of gas through the bed of pellets.

Uses and Market of Polyethylene

Following Table shows general analysis by end-use of polyethylene. The numbers indicate the % consumption by application.

The pattern is different for the two classes LDPE together with LLDPE is used predominantly for films. Because of greater rigidity and better creep properties, HOPE is used in more structural applications as well as packaging.
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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.
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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.

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The suspension process has been used extensively for the production of HOPE, and in many cases these polymers incorporate a small amount of comonomer to increase the toughness or resistance to stress cracking The suspension process is used in autoclave reactors as well as loop reactors.

Autoclave Process

The pressure employed in this process is between 0.5 and 1.0 MPa and temperature is 80 – 90C. The reactors are as large as 100 m3 The diluent is a low-boiling hydrocarbon such as hexane The catalyst compounds and aluminum alkyl are slurried with diluent in the catalyst mixing vessel before being fed to the reactor at a rate sufficient to maintain the required polymerization rate In case of bimodal mol wt distribution two or more reactors are used in cascade.
The reaction mixer is passed to a rundown reactor where dissolved ethylene is consumed almost completely, avoiding the need for ethylene recycle. The slurry concentration is an important parameter.

in the process A high concentration allows higher outputs from a given reactor volume. The slurry concentration ranges from 15- 45%.

Hoechst suspension polymerization process
a) Catalyst preparation vessel; b) Polymerization reactor; c) Run-down reactor; d) Centrifuge; e) Fluidized-bed drier; f) Diluent condenser; g) Nitrogen circulator; h) Powder-fed extruder
The slurry from rundown reactor then passes to a centrifuge to remove the bulk of the diluent, which is recycled. The polymer is dried in a continuous fluidized-bed drier in a stream of hot nitrogen to remove residual diluent. Before extrusion into pellets, stabilizers are added to neutralize the catalyst residues, and other additives such as antioxidants may be added at this point.

Loop Reactor Process

The novel double loop reactor constructed from a jacketed pipe was developed by Phillips engineers to avoid deposits, which had been troublesome in a stirred autoclave. It also has high surface-to-volume ratio, facilitating heat removal and allowing short residence times.

Flowsheet of the Phillips Particle Form process
a) Catalyst hopper and feed valve; b) Double loop reactor; c) Flash tank; d) Purge drier; e) Powder-fed extruder; f) JmpeUer; g) Sedimentation leg
The impeller (f) forces the reaction mixture through the pipework in a turbulent regime with a velocity of 5-10 m/s The reaction conditions of 100°C and 3-4 MPa with chromium-based Phillips catalyst are employed The diluent used is isobutane which facilitates the subsequent flash operation It is a poor solvent for polyethylene and permits higher operating temperature than the alkanes The catalyst is flushed into the reactor with diluent from the metering device at the base of the catalyst slurry tank The polymer is taken off from a sedimentation leg which enables the slurry to be passed to the flash tank at concentration of 55-65% instead of 30-35% circulating in the loop reactor. The isobutane diluent evaporates in the flash tank and is then condensed and recycled. Residual isobutane is removed in a nitrogen-flushed conveyer. Pelletization is carried out in a powder-fed extruder.

Low-pressure processes can be classified as following:

a) Suspension (Slurry) process
b) Gas-phase process
c) Solution process

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In high-pressure process the reactor may take two forms a highpressure autoclave or a jacketted tube, but otherwise processes are similar. The reactor pressure is in the range of 150-200 MPa for autoclave process and 200-350 MPa for a tubular reactor. Such high pressures call for very specialized technology. Fatigue is a major design consideration for pumps and compressors. Specialized forms of sealing joints in vessels and pipework have been developed which make use of the pressure itself to increase the sealing forces.

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High-pressure autoclave process

a) Ethylene stock tank (5 MPa); b) Primary compressor; c) Secondary compressor (200 MPa); d) Autoclave reactor; e) Initiator pumps; f). Product cooler; g) Separator (25 MPa); h) Recycle’ cooler; i) Low-pressure separator and melt extruder; j) Low-pressure stock tank (0.2 MPa); k) Booster compressor

The fresh ethylene enters from refinery at 5 MPa, mixes with the lowpressure recycle gas and is compressed to 25 MPa After mixing with the intermediate-pressure recycle, the pressure is raised in the secondary compressor to 150-350 MPa for feeding to the reactor. The pressure in the reactor is controlled automatically by a flow control valve at the reactor outlet The reaction mixture then passes through a cooler to reduce the polymer temperature to a value suitable for feeding the pelletizing extruder. The polyethylene is separated from the majority of the unreacted monomer in the intermediate separator at 27 MPa. Remaining monomer is removed in the low-pressure separator that feeds the pelletizing extruder. The extruder is pelletized underwater by a die-face cutter, and the pellets are then dried and conveyed to temporary storage hoppers to await quality clearance Finally the pellets are transferred to silos for blending and storage, before off-loading to tankers or sacks.

Autoclave Reactor:

The autoclave volume is chosen to give an overall residence time of 30-60 s, with corresponding volumes in larger plants of 1 m3 A novel feature is the internal stirrer motor. The DuPont process uses an external motor.

a) Stirrer motor
b) Stirrer shaft
c) Bursting disk ports

Bursting disks or other relief devices are mounted directly into the reactor walls to provide unrestricted passage for the reactor contents in the event of a pressure rise due to decomposition.
 
Autoclave functions as an adiabatic continuous stirred tank reactor (CSTR), with heat of reaction being removed by a fresh ethylene entering the reactor. The conversion of monomer to polymer is thus related to the difference in temperature of feed gas and the final reaction temperature. For practical purposes percentage conversion IS given as,

% Conversion = 0.075 x delta T

Modern reactors have two or more zones with increasing temperatures The reaction temperatures are maintained constant by controlling the speeds of the pumps feeding initiators into the respective zones. The first zone is typically 180°C and the final zone 290°C. For adequate control the initiators must have decomposition half-lifes of 1s under the reaction conditions in the zone.

Tubular Reactor:

A tubular reactor consists of several hundred meters of jacketed high- pressure tubing arranged as a series of straight sections connected by 180° bends. Inner diameter is 25 – 75 mm (generally 60 mm). A ratio of outer to inner diameters of about 2.5 is used to provide the necessary strength for the high-pressure involved. At many of the pipe junctions thermocouples are introduced to follow the course of the reaction, and initiator and gas inlets or pressure relied devices may also be incorporated. Unlike the autoclave process, no after-cooler is required for the secondary compressor, but the first section of the tubular reactor must function as a preheater to raise the ethylene to a sufficiently high temperature for the reaction to start.

This temperature depends on the initiator employed, ranging from 190°C for oxygen to 140°C for peroxydicarbonate. The latter part of the reactor functions as a product cooler.

The tubular reactor acts as a plug flow reactor. More sophisticated temperature control is required in these reactors When oxygen is used as initiator, the temperature control acts on the rate of addition of oxygen in the lower pressure part of the system When peroxide initiators are used, the speeds of the high-pressure pumps are controlled Oxygen is still widely used in the tubular reactor process, either alone or along with peroxides. (The conversions of 35% are claimed in the tubular reactors (compared with 20% for autoclave)) but the cost of compression energy is high. The maximum useful conversion depends on the product quality required, since quality deteriorates with increasing conversion.

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