Gas-Solid Reactor Gas-Solid Reactor ModelsModels

Quak Foo LeeQuak Foo LeeDepartment of Chemical and Biological Department of Chemical and Biological

EngineeringEngineeringUniversity of British ColumbiaUniversity of British Columbia

Gas-Solid ReactorsGas-Solid Reactors

Packed bedsPacked beds

Bubbling fluidized bedsBubbling fluidized beds

Turbulent fluidized bedsTurbulent fluidized beds

Circulating (fast) fluidized bedsCirculating (fast) fluidized beds

The Packed Bed ReactorThe Packed Bed Reactor

The flow and contacting can be simply The flow and contacting can be simply represented by the plug flow model.represented by the plug flow model.

In reality, flow can deviate significantly In reality, flow can deviate significantly from this ideal.from this ideal.

Near the vessel walls, the voidage is Near the vessel walls, the voidage is much higher than in the vessel interior.much higher than in the vessel interior.

Gas slides up close to the wall giving a Gas slides up close to the wall giving a velocity profile (see next slide).velocity profile (see next slide).

Real Velocity Distribution Real Velocity Distribution in a G/S Packed Bedin a G/S Packed Bed

First-Order Catalytic First-Order Catalytic ReactionReaction

First-Order, Plug FlowFirst-Order, Plug Flow

Dispersed Plug FlowDispersed Plug Flow

Info Needed to Relate Info Needed to Relate Output to Input of a Output to Input of a

Process VesselProcess Vessel

The Bubbling Fluidized Bed The Bubbling Fluidized Bed (BFB)(BFB)

Class 1 model – plug flowClass 1 model – plug flow

Class 2 model – the two-region Class 2 model – the two-region modelsmodels

Class 3 model – based on the Class 3 model – based on the Davidson bubbleDavidson bubble

Class 1 Model – Plug Class 1 Model – Plug FlowFlow

The earliest performance studies on The earliest performance studies on G/S (heat transfer, mass transfer, G/S (heat transfer, mass transfer, catalytic reactions) all assumed plug catalytic reactions) all assumed plug flow of gas through the BFB.flow of gas through the BFB.

However, experiments show that However, experiments show that serious bypassing of fluid occurs and serious bypassing of fluid occurs and that the plug flow model should not that the plug flow model should not be used to represent the flow of gas be used to represent the flow of gas in BFBs.in BFBs.

Class 2 – The Two-Region Class 2 – The Two-Region modelsmodels

The rising bubbles were the cause of The rising bubbles were the cause of the great deviation from plug flow the great deviation from plug flow model.model.

This model has dense and lean solid This model has dense and lean solid regions, the lean representing the regions, the lean representing the rising bubbles.rising bubbles.

Class 3 – Based on the Class 3 – Based on the Davidson BubbleDavidson Bubble

Each rising bubble dragged a wake Each rising bubble dragged a wake of solids up the bed.of solids up the bed.

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

particle BFBparticle BFB

Different Combinations of Different Combinations of Assumptions Give a Variety of Assumptions Give a Variety of

ModelsModels

K-L BFB ModelK-L BFB Model

The Turbulent Fluidized The Turbulent Fluidized Bed, TFBBed, TFB

When the gas velocity through a BFB is increased, When the gas velocity through a BFB is increased, bubbling becomes more vigorous and pressure fluctuations bubbling becomes more vigorous and pressure fluctuations become more intense until a point is reached where the become more intense until a point is reached where the character of the bed changes.character of the bed changes.

Distinct bubbles are no longer seen, the bed becomes more Distinct bubbles are no longer seen, the bed becomes more uniform with many small scale turbulent eddies.uniform with many small scale turbulent eddies.

In addition, the pressure fluctuations fall dramatically to a In addition, the pressure fluctuations fall dramatically to a low level. This is the turbulent bed, the TFB.low level. This is the turbulent bed, the TFB.

Here, solid carryover is minor and can be dealt with Here, solid carryover is minor and can be dealt with internal cyclones.internal cyclones.

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

Commercial TFB Commercial TFB ReactorsReactors

The Circulating Fluidized The Circulating Fluidized Bed -- CFBBed -- CFB

For very fine catalyst solids and even For very fine catalyst solids and even higher gas flow rates, these solids are higher gas flow rates, these solids are carried out of the bed by the gas, and the carried out of the bed by the gas, and the bed has to be replenished.bed has to be replenished.

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

Reactor Performance of a Reactor Performance of a CFBCFB

To determine the reactor To determine the reactor performance of a CFB, we need to performance of a CFB, we need to know:know: The vertical distribution of solids in the The vertical distribution of solids in the

vessel, vessel, The radial distribution of solids at all The radial distribution of solids at all

levels of the vessel, andlevels of the vessel, and How the gas contacts the solids in the How the gas contacts the solids in the

vessel.vessel.

The Two Board Types of The Two Board Types of CFBCFB

Various CFB SystemsVarious CFB Systems

Two Models for the Vertical Two Models for the Vertical Distribution of SolidsDistribution of Solids

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

Flow rate of GasFlow rate of Gas

The Distribution of Solids in The Distribution of Solids in the CFBthe CFB

K-L ModelK-L Model

First-order, Ignores the First-order, Ignores the Annular Flow of Wall SolidsAnnular Flow of Wall Solids

Some Challenge Some Challenge QuestionsQuestions

For For packed bedspacked beds, how do we predict and , how do we predict and measure the non-uniform gas/liquid velocity?measure the non-uniform gas/liquid velocity?

In In BFBBFBs, how do we handle the growing size s, how do we handle the growing size distribution of nonsperical coalescing and distribution of nonsperical coalescing and splitting bubbles?splitting bubbles?

In In CFBCFB, hwhere are the solids, how does the gas , hwhere are the solids, how does the gas contact the solids?contact the solids?

In all these contactors, how does the gas In all these contactors, how does the gas distributor influence the behavior in the reactor?distributor influence the behavior in the reactor?

Final CommentsFinal Comments

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

ReferencesReferences Levenspiel, O., “G/S reactor models—packed beds, bubbling Levenspiel, O., “G/S reactor models—packed beds, bubbling

fluidized beds, turbulent fluidized beds and circulating (fast) fluidized beds, turbulent fluidized beds and circulating (fast) fluidized beds”, Powder Technology, 122:1-9 (2002)fluidized beds”, Powder Technology, 122:1-9 (2002)