Gas-Solid Reactor Models

1. Gas-Solid Reactor Models Quak Foo Lee Department of Chemical and Biological Engineering University of British Columbia

2. Gas-Solid Reactors • Packed beds • Bubbling fluidized beds • Turbulent fluidized beds • Circulating (fast) fluidized beds

3. The Packed Bed Reactor • The flow and contacting can be simply represented by the plug flow model. • In reality, flow can deviate significantly from this ideal. • Near the vessel walls, the voidage is much higher than in the vessel interior. • Gas slides up close to the wall giving a velocity profile (see next slide).

4. Real Velocity Distribution in a G/S Packed Bed

5. First-Order Catalytic Reaction

6. First-Order, Plug Flow

7. Dispersed Plug Flow

8. Info Needed to Relate Output to Input of a Process Vessel

9. The Bubbling Fluidized Bed (BFB) • Class 1 model – plug flow • Class 2 model – the two-region models • Class 3 model – based on the Davidson bubble

10. Class 1 Model – Plug Flow • The earliest performance studies on G/S (heat transfer, mass transfer, catalytic reactions) all assumed plug flow of gas through the BFB. • However, experiments show that serious bypassing of fluid occurs and that the plug flow model should not be used to represent the flow of gas in BFBs.

11. Class 2 – The Two-Region models • The rising bubbles were the cause of the great deviation from plug flow model. • This model has dense and lean solid regions, the lean representing the rising bubbles.

12. Class 3 – Based on the Davidson Bubble • Each rising bubble dragged a wake of solids up the bed.

13. Gas Flow Around and Within a Rising Gas Bubble in a Fine particle BFB

14. Different Combinations of Assumptions Give a Variety of Models

15. K-L BFB Model

16. The Turbulent Fluidized Bed, TFB • When the gas velocity through a BFB is increased, bubbling becomes more vigorous and pressure fluctuations become more intense until a point is reached where the character of the bed changes. • Distinct bubbles are no longer seen, the bed becomes more uniform with many small scale turbulent eddies. • In addition, the pressure fluctuations fall dramatically to a low level. This is the turbulent bed, the TFB. • Here, solid carryover is minor and can be dealt with internal cyclones. • At even higher gas velocities, u < 1.5 m/s, solid carryover increases greatly and the vessel enters the fast fluidization.

17. Commercial TFB Reactors

18. The Circulating Fluidized Bed -- CFB • For very fine catalyst solids and even higher gas flow rates, these solids are carried out of the bed by the gas, and the bed has to be replenished. • Two arrangement: an upflow of solids and a downflow of solids.

19. Reactor Performance of a CFB • To determine the reactor performance of a CFB, we need to know: • The vertical distribution of solids in the vessel, • The radial distribution of solids at all levels of the vessel, and • How the gas contacts the solids in the vessel.

20. The Two Board Types of CFB

21. Various CFB Systems

22. Two Models for the Vertical Distribution of Solids

23. The CFB at Various Flow Rates of solids, but at Fixed Flow rate of Gas

24. The Distribution of Solids in the CFBK-L Model

25. First-order, Ignores the Annular Flow of Wall Solids

26. Some Challenge Questions • For packed beds, how do we predict and measure the non-uniform gas/liquid velocity? • In BFBs, how do we handle the growing size distribution of nonsperical coalescing and splitting bubbles? • In CFB, hwhere are the solids, how does the gas contact the solids? • In all these contactors, how does the gas distributor influence the behavior in the reactor?

27. Final Comments With the need to design real performing units, whether packed beds, BFBs, TFBs, or CFBs, we find that we often must turn to some of the simpler idealized predictive engineering models. In all cases, we should use good judgment in our choice of models.

28. References • Levenspiel, O., “G/S reactor models—packed beds, bubbling fluidized beds, turbulent fluidized beds and circulating (fast) fluidized beds”, Powder Technology, 122:1-9 (2002)