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1. Introduction to the Slurry Loop Process

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Polymerisation of Ethylene
This chapter is in the book Polymerisation of Ethylene
1 Introduction to the Slurry Loop ProcessThe polymerisation products of ethylene are among the largest commodity plastics.Low density polyethylene (LDPE) is a highly branched polymer, generated from themonomer at conditions of extremely great pressure and high temperature. High dens-ity polyethylene (HDPE) is a linear polymer, and is obtained by catalysis, at moderatepressures and temperatures. The two types of polyethylene are so different in proper-ties that they can hardly be said to be competitors.Two main processes are known to produce HDPE. In thegas phase process,thepolymer particles are formed on catalyst particles that are fluidized in a fluidized bedreactor by a gas stream carrying the monomer. In theslurry loop processthe polymergrowsoncatalystparticlesthataresuspendedina liquid, whereinthemonomerisdis-solved, thus forming a slurry that is circulated in a recycle tubular reactor, commonlycalledloop reactor.The slurry loop reactor is a long tube, about 600mmin diameter and a few hun-dred metres long, of which the ends are connected to make it one closed loop. Itsvolume is of the order of 100m3. It is totally filled with the slurry, which is circulatedthroughtheloop by onesinglecirculationpump. Thelarger partof thereactor consistsof an even number of vertical tube sections, orlegs, connected by short horizontal el-bows at the top and the bottom.The outer aspect of the reactor is that of a tubular reactor, one of the two main modelreactors known in chemical reactor engineering, where the reagents are fed at oneside of the tube and leave it at the other end. However, since the loop is closed, andthe reactor contents are circulated through the loop at a high speed, the behaviour ofthe reactor is very close to that of the continuous stirred tank reactor, that other modelknown from reactor engineering textbooks.Actually, the loop reactor can be regarded as a stirred tank, where the circulationpumphastakenovertheroleoftheagitator,andwherethevesselhasbeenlengthenedto obtain more external cooling surface, and a higher production capacity. Indeed,the heat of reaction of the polymerisation of ethylene is quite high¹, and the coolingsurface of the reactor is one of the limiting factors to its production capacity. Since aconventional vessel is rather spherical, the ratio of its external surface to its volumeis inversely proportional to its diameter. In a loop reactor, the cooling of the reactorcontents is effected by cooling water flowing through a cooling jacket, concentric withthe tube, and so its external surface is simply proportional to its volume. When thereactortubeismadelonger,bothvolumeandcoolingsurfaceareincreasedtothesamedegree. Due to the fact that the slurry and the cooling water are flowing at very highspeeds, high heat transfer coefficients are obtained at both sides of the reactor wall.1The heat of reaction of the polymerisation of ethylene is about 3720kJperkgof ethylene.

1 Introduction to the Slurry Loop ProcessThe polymerisation products of ethylene are among the largest commodity plastics.Low density polyethylene (LDPE) is a highly branched polymer, generated from themonomer at conditions of extremely great pressure and high temperature. High dens-ity polyethylene (HDPE) is a linear polymer, and is obtained by catalysis, at moderatepressures and temperatures. The two types of polyethylene are so different in proper-ties that they can hardly be said to be competitors.Two main processes are known to produce HDPE. In thegas phase process,thepolymer particles are formed on catalyst particles that are fluidized in a fluidized bedreactor by a gas stream carrying the monomer. In theslurry loop processthe polymergrowsoncatalystparticlesthataresuspendedina liquid, whereinthemonomerisdis-solved, thus forming a slurry that is circulated in a recycle tubular reactor, commonlycalledloop reactor.The slurry loop reactor is a long tube, about 600mmin diameter and a few hun-dred metres long, of which the ends are connected to make it one closed loop. Itsvolume is of the order of 100m3. It is totally filled with the slurry, which is circulatedthroughtheloop by onesinglecirculationpump. Thelarger partof thereactor consistsof an even number of vertical tube sections, orlegs, connected by short horizontal el-bows at the top and the bottom.The outer aspect of the reactor is that of a tubular reactor, one of the two main modelreactors known in chemical reactor engineering, where the reagents are fed at oneside of the tube and leave it at the other end. However, since the loop is closed, andthe reactor contents are circulated through the loop at a high speed, the behaviour ofthe reactor is very close to that of the continuous stirred tank reactor, that other modelknown from reactor engineering textbooks.Actually, the loop reactor can be regarded as a stirred tank, where the circulationpumphastakenovertheroleoftheagitator,andwherethevesselhasbeenlengthenedto obtain more external cooling surface, and a higher production capacity. Indeed,the heat of reaction of the polymerisation of ethylene is quite high¹, and the coolingsurface of the reactor is one of the limiting factors to its production capacity. Since aconventional vessel is rather spherical, the ratio of its external surface to its volumeis inversely proportional to its diameter. In a loop reactor, the cooling of the reactorcontents is effected by cooling water flowing through a cooling jacket, concentric withthe tube, and so its external surface is simply proportional to its volume. When thereactortubeismadelonger,bothvolumeandcoolingsurfaceareincreasedtothesamedegree. Due to the fact that the slurry and the cooling water are flowing at very highspeeds, high heat transfer coefficients are obtained at both sides of the reactor wall.1The heat of reaction of the polymerisation of ethylene is about 3720kJperkgof ethylene.

Chapters in this book

  1. Frontmatter i
  2. Preface vii
  3. Contents ix
  4. 1. Introduction to the Slurry Loop Process 1
  5. 2. The Loop Reactor Circulation Pump Power 4
  6. 3. The Functioning of the Settling Legs 19
  7. 4. The Settling of the Polymer in the Settling Legs 36
  8. 5. Catalyst Activity and Productivity 51
  9. 6. The 1/1 Hypothesis 62
  10. 7. Catalyst Residence Time Distribution 73
  11. 8. Catalyst Activity Profiles from Full Scale Loop Data 88
  12. 9. Conversions of the Reactants 98
  13. 10. The Ethylene Concentration Profile in the Loop Reactor 120
  14. 11. A Simple Model for the Slurry Loop Reactor 132
  15. 12. Establishing Correlations from Loop Reactor Data by Linear Regression 147
  16. 13. Scaling-Up from Bench Reactor to Loop Reactor 161
  17. 14. The Operation of Two Loop Reactors in Series 177
  18. Index 193
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https://doi.org/10.1515/9783110292190.1

Chapters in this book

  1. Frontmatter i
  2. Preface vii
  3. Contents ix
  4. 1. Introduction to the Slurry Loop Process 1
  5. 2. The Loop Reactor Circulation Pump Power 4
  6. 3. The Functioning of the Settling Legs 19
  7. 4. The Settling of the Polymer in the Settling Legs 36
  8. 5. Catalyst Activity and Productivity 51
  9. 6. The 1/1 Hypothesis 62
  10. 7. Catalyst Residence Time Distribution 73
  11. 8. Catalyst Activity Profiles from Full Scale Loop Data 88
  12. 9. Conversions of the Reactants 98
  13. 10. The Ethylene Concentration Profile in the Loop Reactor 120
  14. 11. A Simple Model for the Slurry Loop Reactor 132
  15. 12. Establishing Correlations from Loop Reactor Data by Linear Regression 147
  16. 13. Scaling-Up from Bench Reactor to Loop Reactor 161
  17. 14. The Operation of Two Loop Reactors in Series 177
  18. Index 193
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