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

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

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

Real Velocity Distribution in a G/S Packed Bed

First-Order Catalytic Reaction

First-Order, Plug Flow

Dispersed Plug Flow

Info Needed to Relate Output to Input of a Process Vessel

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

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. 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. 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.

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

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

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

Different Combinations of Assumptions Give a Variety of Models

K-L BFB Model

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. 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. 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. 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. 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. At even higher gas velocities, u < 1.5 m/s, solid carryover increases greatly and the vessel enters the fast fluidization.

Commercial TFB Reactors

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. 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. Two arrangement: an upflow of solids and a downflow of solids.

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

The Two Board Types of CFB

Various CFB Systems

Two Models for the Vertical Distribution of Solids

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

The Distribution of Solids in the CFB K-L Model

First-order, Ignores the Annular Flow of Wall Solids

Some Challenge Questions For packed beds, how do we predict and measure the non- uniform gas/liquid velocity? 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 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 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? In all these contactors, how does the gas distributor influence the behavior in the reactor?

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.

References Levenspiel, O., G/S reactor modelspacked beds, bubbling fluidized beds, turbulent fluidized beds and circulating (fast) fluidized beds, Powder Technology, 122:1-9 (2002) Levenspiel, O., G/S reactor modelspacked beds, bubbling fluidized beds, turbulent fluidized beds and circulating (fast) fluidized beds, Powder Technology, 122:1-9 (2002)