The continuous reactors will have both inlet and outlet streams.
These reactors require smaller and less expensive equipments and give more uniform operating conditions and products than batch processes. The process requires quality instrumentation for process control. These are suitable and economical for large-scale productions. The continuous reactors are of two types, tubular and stirred tank. Tubular reactors are also known as plug-flow type.
Tubular-Flow or Plug-Flow reactors (PFR): In these reactors, reactants while passing through the reactor get converted into polymers without back mixing. These are commonly used for highpressure polymerization of ethylene to produce LDPE.
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Production of LDPE in Tubular Reactor
Plug-Flow reactors can be of fixed catalyst bed type or moving catalyst bed type. Schematically these can be represented as following.
Simple Fixed Bed reactors
Union Carbide Unipol process for the production ‘of HDPE and LLDPE.
Continuous Stirred-Tank reactors (CSTR):
These reactors are also known as autoclave. The reactor design is such that the monomer feed and other agents are mixed throughout the volume of the reactor. The nature of mixing affects the molecular weight distribution
Mechanically Stirred Tank reactor
Some reactors, which are called as Semi-batch or Semicontinuous, have inlet flow during reaction but no outlet flow and nothing is withdrawn from the reactor. This mode of operation allows controlling the temperature and concentration in the reactor simultaneously. This reactor offers more flexibility than batch reactor.
These reactors are simplest in design. Batch reactors do not have inlet or outlet streams for reactants and products during the reaction. The reactant quantities are measured carefully, and placed in the reactor. The lid is then closed and we wait for the reaction to finish. Once desired conversion is achieved, we open the reactor and take the product out.
These reactors are useful for small quantities of production, laboratory production or manufacture of some specialty polymers. Temperature control and mixing is difficult in these reactors due to high viscosity of products.
Different cooling or heating designs are possible for these reactors, some of which are shown in following figures.
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Plastics are new and economical materials of construction to the engineer and designer. Plastics replace metals and other materials in some cases. The use of plastic material for specific application IS dependent upon its composition, properties, and design of part.
Like any chemical manufacturing plant, designing of plastics or polymers plant involves the consideration of following factors.
1. Basic chemical data, material balances, energy balances
2. Reactor designing involves selection between batch and continuous reactor.
3. Chemical process and equipment selection, design and
operation.
4. Process control and operation.
5. Market evaluation and costing.
6. Packaging, sales and customer service.
7. Plant location, safety hazards, and environment pollution.
8. Research and development
Some aspects like solids handling, storage, packaging and transportation in plastics industries are very specific. They differ from the petrochemical products.
Selection and designing of reactors for polymerization is main and important aspect of polymer manufacturing plant. Reactor selection depends upon the quantity of production desired, phases of reactants and products (gas, liquid or solid).
The polymerization reactors are classified as batch and continuous reactors. The continuous reactors are further classified as TubularFlow or Plug-Flow reactors and Continuous Stirred- Tank reactors.
Polymerization Reactors
- Batch Reactors
- Continuous Reactors
- Plug-Flow Reactors (PFR)
- Continuous Stirred-Tank Reactors (CSTR)
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