Gas-Solid Reactor Models

8/6/2019 Gas-Solid Reactor Models

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GasGas--Solid Reactor ModelsSolid Reactor Models

Quak Foo LeeQuak Foo LeeDepartment of Chemical and Biological EngineeringDepartment of Chemical and Biological EngineeringUniversity of British ColumbiaUniversity of British Columbia


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GasGas--Solid ReactorsSolid Reactors

Packed bedsPacked beds

Bubbling fluidized bedsBubbling fluidized beds

Turbulent fluidized bedsTurbulent fluidized beds

Circulating (fast) fluidized bedsCirculating (fast) fluidized beds


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The Packed Bed ReactorThe Packed Bed Reactor

The flow and contacting can be simplyThe flow and contacting can be simply

represented by the plug flow model.represented by the plug flow model.

In reality, flow can deviate significantly from thisIn reality, flow can deviate significantly from thisideal.ideal.

Near the vessel walls, the voidage is muchNear the vessel walls, the voidage is much

higher than in the vessel interior.higher than in the vessel interior.

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


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Real Velocity Distribution in a G/SReal Velocity Distribution in a G/S

Packed BedPacked Bed


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FirstFirst--Order Catalytic ReactionOrder Catalytic Reaction


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FirstFirst--Order, Plug FlowOrder, Plug Flow


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Dispersed Plug FlowDispersed Plug Flow


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Info Needed to Relate Output toInfo Needed to Relate Output to

I

nput of a Process VesselI

nput of a Process Vessel


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

Class 1 modelClass 1 model plug flowplug flow

Class 2 modelClass 2 model the twothe two--region modelsregion models

Class 3 modelClass 3 model based on the Davidson bubblebased on the Davidson bubble


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Class 1 ModelClass 1 Model Plug FlowPlug Flow

The earliest performance studies on G/S (heatThe earliest performance studies on G/S (heat

transfer, mass transfer, catalytic reactions) alltransfer, mass transfer, catalytic reactions) allassumed plug flow of gas through the BFB.assumed plug flow of gas through the BFB.

However, experiments show that seriousHowever, experiments show that seriousbypassing of fluid occurs and that the plug flowbypassing of fluid occurs and that the plug flow

model should not be used to represent the flowmodel should not be used to represent the flow

of gas in BFBs.of gas in BFBs.


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Class 2Class 2 The TwoThe Two--Region modelsRegion models

The rising bubbles were the cause of the greatThe rising bubbles were the cause of the great

deviation from plug flow model.deviation from plug flow model.

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


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Class 3Class 3 Based on the DavidsonBased on the Davidson

BubbleBubble Each rising bubble dragged a wake of solids upEach rising bubble dragged a wake of solids up

the bed.the bed.


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Gas Flow Around and Within a RisingGas Flow Around and Within a Rising

Gas Bubble in a Fine particle BFBGas Bubble in a Fine particle BFB


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KK--L BFB ModelL BFB Model


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The Turbulent Fluidized Bed, TFBThe Turbulent Fluidized Bed, TFB

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

Distinct bubbles are no longer seen, the bed becomes more uniform withDistinct bubbles are no longer seen, the bed becomes more uniform with

many small scale turbulent eddies.many small scale turbulent eddies. In addition, the pressure fluctuations fall dramatically to a low level. This isIn addition, the pressure fluctuations fall dramatically to a low level. This is

the turbulent bed, the TFB.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 greatlyAt even higher gas velocities, u < 1.5 m/s, solid carryover increases greatly

and the vessel enters the fast fluidization.and the vessel enters the fast fluidization.


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Commercial TFB ReactorsCommercial TFB Reactors


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The Circulating Fluidized BedThe Circulating Fluidized Bed ---- CFBCFB

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

Two arrangement: an upflow of solids and a downflowTwo arrangement: an upflow of solids and a downflowof solids.of solids.


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Reactor Performance of a CFBReactor Performance of a CFB

To determine the reactor performance of a CFB,To determine the reactor performance of a CFB,

we need to know: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 theThe radial distribution of solids at all levels of the

vessel, andvessel, and

How the gas contacts the solids in the vessel.How the gas contacts the solids in the vessel.


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The Two Board Types of CFBThe Two Board Types of CFB


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Various CFB SystemsVarious CFB Systems


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Two Models for the VerticalTwo Models for the Vertical

Distribution of SolidsDistribution of Solids


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The CFB at Various Flow Rates ofThe CFB at Various Flow Rates of

solids, but at Fixed Flow rate of Gassolids, but at Fixed Flow rate of Gas


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The Distribution of Solids in the CFBThe Distribution of Solids in the CFB

KK--L ModelL Model


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FirstFirst--order, Ignores the Annularorder, Ignores the Annular

Flow of Wall SolidsFlow of Wall Solids


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Some Challenge QuestionsSome Challenge Questions

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

InIn BFBBFBs, how do we handle the growing size distribution ofs, how do we handle the growing size distribution ofnonsperical coalescing and splitting bubbles?nonsperical coalescing and splitting bubbles?

InIn CFBCFB, hwhere are the solids, how does the gas contact the, hwhere are the solids, how does the gas contact thesolids?solids?

In all these contactors, how does the gas distributor influence theIn all these contactors, how does the gas distributor influence the

behavior in the reactor?behavior in the reactor?


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Final CommentsFinal Comments

With the need to design real performing units, whetherWith the need to design real performing units, whetherpacked beds, BFBs, TFBs, or CFBs, we find that wepacked beds, BFBs, TFBs, or CFBs, we find that we

often must turn to some of the simpler idealizedoften must turn to some of the simpler idealizedpredictive engineering models. In all cases, we shouldpredictive engineering models. In all cases, we shoulduse good judgment in our choice of models.use good judgment in our choice of models.


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ReferencesReferences

Levenspiel, O., G/S reactor modelsLevenspiel, O., G/S reactor modelspacked beds, bubbling fluidized beds, turbulentpacked beds, bubbling fluidized beds, turbulent

fluidized beds and circulating (fast) fluidized beds, Powder Technology, 122:1fluidized beds and circulating (fast) fluidized beds, Powder Technology, 122:1--99(2002)(2002)

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Gas-Solid Reactor Models