text: Basics of Polystyrene Production - Process and Reactions - Articles - Chemical Engineering - Frontpage - Cheresources.com
  Jump to content
  Other Content:
  Most Active Topics
  ChE Links
  Legacy Articles
  New Releases
  Popular Articles
  Process Engineering
  Process Design
  Process Safety
  Legacy Newsletters
  Contact Us
  Sign In
  Create Account
  Search Advanced
  Search section:
  This database
  Forums
  Members
  Help Files
  Calendar
  Blogs
  Pages
  Downloads
  New Content
  Home
  Forums
  Industrial Forum
  Process Heat Transfer
  Student Forum
  Refining Forum
  Simulation Forum
  Relief Devices Forum
  Tank Venting Forum
  Sign In
  Register
  Quick Nav
  Members
  Calendar
  Status Updates
  Blogs
  ChE Express
  Community Blog
  Ankur's Blog
  Articles
  Separation Technology
  Calcs and Tips
  Fluid Flow
  Heat Transfer
  Maint/Repair
  Utilities
  Safety
  Bulk Solids
  Processes
  Energy
  Other Topics
  For Students
  Physical Properties
  File Library
  Excel Spreadsheets
  Article Supplements
  Hall of Fame
  ChE Plus
  FAQ
  Site FAQ
  Technical FAQ
  Search
  FB
  More
  Cheresources.com Community
  → Pages
  → Chemical Engineering - Frontpage
  → Articles
  → Process and Reactions
  → Article: Basics of Polystyrene Production
  Chemical and Process Engineering Resources
  Basics of Polystyrene Production
  Nov 08 2010 01:40 PM | Chris Haslego in Process and Reactions
  Polystyrene is a widely used polymer.Â After production of the monomer, from one of a few processes, the monomer proceeds to further processing to form polystyrene.
  Styrene Monomer Production
  The energy needed for the reaction is supplied by superheated steam (at about 720 Â°C) that is injected into a vertically mounted fixed bed catalytic reactor with vaporized ethylbenzene.Â The catalyst is iron oxide based and contains Cr2O3 and a potassium compound (KOH or K2CO3) which act as reaction promoters.Â
  Typically, 2.5-3 kg steam are required for each kilogram of ethylbenzene to ensure sufficiently high temperatures throughout the reactor. Â The superheated steam supplies the necessary reaction temperature of 550-620 Â°C throughout the reactor.Â Ethylbenzene conversion is typically 60-65%.Â Styrene selectivity is greater than 90%.Â The three significant byproducts are toluene, benzene, and hydrogen.
  After the reaction, the products are cooled rapidly (perhaps even quenched) to prevent polymerization.Â The product stream (containing styrene, toulene, benzene, and unreacted ethylbenzene) is fractionally condensed after the hydrogen is flashed from the stream.Â The hydrogen from the reaction is used as fuel to heat the steam (boiler fuel).Â After adding a polymerization inhibitor (usually a phenol), the styrene is vacuum distilled in a series of four columns (often times packed columns) to reach the required 99.8% purity.Â The separation is difficult due to the similar boiling points of styrene and ethylbenzene. Â Typical capacity per plant ranges from 70,000 to 100,000 metric tonnes per year in each reactor and most plants contain multiple reactors or units.
  Polystyrene Production
  In 1996, world production capacity for styrene was near 19.2 million metric tonnes per year.Â Dow Chemical is the world's largest producer with a total capacity of 1.8 million metric tonnes in the USA, Canada, and Europe (1996 figures).Â The main manufacturing route to styrene is the direct catalytic dehydrogenation of ethylbenzene (above).
  The reaction shown above has a heat of reaction of -121 KJ/mol (endothermic).Â Nearly 65% of all styrene is used to produce polystyrene.
  The overall reaction describing the styrene polymerization is:
  This reaction is carried out in an inert organic solvent environment which provides the reaction medium for this cationic polymerization reaction.Â The most common solvent used for this reaction is 1,2-dichloroethane (EDC).Â Other suitable solvents may include carbon tetrachloride, ethyl chloride, methylene dichloride, benzene, toluene, ethylbenzene, or chlorobenzene.Â The preferred initiator is a mixture of boron trifluoride and water.
  The initiator solution is prepared by incorporating 1.5% by weight boron trifluoride gas into the organic solvent (EDC) containing 280 ppm water.Â This solution is continuously prepared in a holding vessel and will then be injected into the reactor system.
  Figure 1: Block Diagrame for Polystyrene Process
  Typical feed to the first reactor would consist of 50 weight percent styrene monomer, 100 ppm water (based on styrene weight), 2000 ppm boron trifluoride (based on styrene weight), with the balance being organic solvent. Â The polymerization reaction gives off heat that is carried away from the reactors by jacketing them with a heat transfer fluid.Â The temperature of the reactants should not vary by more than 15 Â°C throughout the reactor series. Â Temperature control is very important in this reaction because as the reaction temperature increases, the average molecular weight of the polystyrene decreases. Â The reaction temperature range is 40-70 Â°C.Â Temperature can also be controlled by intermediate shell and tube heat exchangers.
  The reaction vessels are typically elongated vessels made of stainless steel.Â The initiator is introduced as shown below:
  Figure 2: Typical Reactor Overview
  References
  Weissermel, K., Industrial Organic Chemistry, 3rd Edition, VCH, New York, 19972.Â
  US Patent #4161573, assigned to Dow Chemical
  Processes and Reactions Articles
  More Content on this Subject... Statistical Process Control: Process and Qualit... Ammonia: The Next Step Fume Control and Scrubbing on Hydrochloric Acid... Methanol Plant Capacity Enhancement Basics of Polystyrene Production
  3 Comments
  joker1
  Nov 26 2012 06:09 AM
  - Industrially used initiators are organic peroxides (ex. Tert Butyl Peroxy Octoate or tBPO, Tert Butyl Peroxy Acetate or tBPA...). Initiators decompose quickly at high temperature and are added in the first reactor (lowest temperature)
  - Solvents are usually feed impurities such as Ethyl benzene, xylenes making it easier to recycle spent monomer.
  - each reactor should have its own temperature control loop with an increasing temperature profile as reaction rate decrease drastically with increasing conversion
  - before removing residual monomer/ solvents the polymer syrup from the last reactor must be heated to ca. 230-250 C. Residence time in the heater should be minimized to reduce degradation
  wins888
  Jan 15 2013 01:33 AM
  Hi,
  We manufacture Crystal PS but our color has degraded to light grey because of long residence time of material in heater to remove residual monomers (volatiles). If we lower residence time the volatiles will be higher. What can we do to stabilize the color? Thanks.
  Zubair Exclaim
  Jun 10 2013 01:38 PM
  Hi,
  We manufacture Crystal PS but our color has degraded to light grey because of long residence time of material in heater to remove residual monomers (volatiles). If we lower residence time the volatiles will be higher. What can we do to stabilize the color? Thanks.