ReactorsCHE 7083CHEM€’S: CAMERON, CONOR & CURTYFEB 24 , 2016

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Fluidized BedFluid flow upward

Particulates enter controllable fluid state

Intimate fluid-solid contact & uniform T distributions (ΔT < 5⁰C)

Well-stirred when L/D < 2

When L/D > 4 they approach plug-flow behavior

Flooding may decrease mass transfer

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Packed BedFluid Flows downward and solid is stationary or moves slowly downward

Fluid motion is similar to plug-flow

“Hot spots” develop from poor gas-solid contact & inefficient heat transfer in exothermic reaction

Less severe ΔP, easier to control, and less sensitive to particle properties

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General Design HeuristicsSpecify inlet/outlet conditions

Define Thermodynamic limitations

Determine kinetic parameters

Energy & material balances req. to determine sizing

Large ΔP may req. reactor design to be done in increments

Well-stirred in series may approach plug-flow behavior

Isothermal Processes: Vol. req. is larger in well-stirred than plug-flow for given X Isothermal performance & general T/x (or y) control are easier in well-stirred reactors

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Batch ReactorNo more feeding after operation start;

No removal of products until process completion;

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Continuous ReactorContinuously fed and having products removed from it;

The process is sensitive to influence from several factors: risk of contamination biomass washout changes in the media

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Comparison - CapacitySizing:Batch: small processes

(<500ton/yr);Continuous: large outputs

(>5,000ton/yr);

Flexibility:Batch: same equipment for

multiple operations;Continuous: less flexibility, made

for specific products;

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Comparison - FunctionMultiple products:Batch: same reactor makes

different products;Continuous: optimized for one

single product;

Maintenance and Operation:Batch: high operation costs;Continuous: low operation costs,

almost no operation needed;

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Comparison - Efficiency & FeedFeedstock: Batch: limited or seasonal feedstock

favors a batch reactor; Continuous: continuous feed needed;

Efficiency: Batch: strict schedule control is

needed, an error can have ripple effects;

Continuous: more efficient and with less energy loss;

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Comparison - Use & KineticsProduct Demand:Batch: favors seasonal demand,

and other products may be made in off-season;

Continuous: only similar products might be made in off-season;

Reaction Rate:Batch: slow reactions or with long

residence times;Continuous: fast reactions;

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Comparison – Risk AssessmentSafety:Batch: high chemical exposure,

requires special training;Continuous: usually safer than

batch;

Fouling/Cleaning:Batch: easier and part of the

process;Continuous: problematic and

expensive;

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Comparison – Risk AssessmentAccountability: Batch: Quality can be verified and

certified, off-spec discarded; Continuous: off-spec can be reworked

and quality is tested periodically;

Controllability: Batch: Schedule is fundamental for

proper control; Continuous: low flexibility, but high

quality control;

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Qualitative Comparison

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CriteriaReactor Type

Batch ContinuousSize (Production) < 500 ton/yr > 5,000 ton/yr

Operational Flexibility Same equip, different operations Little flexibility for operationsMultiple Products Same equip, different products One equip, one product

Maintenance Higher cost Lower costOperating Labor Need a operator constantly Almost no need of operator

Feedstock Favored when feedstock is limited Continuous feed neededProcess Efficiency Less efficient More efficientProduct Demand Favored by seasonal demand, other products

off-seasonNot favored by seasonal demand, other

products off-seasonReaction Rate Slow reactions Fast reactions

Fouling Easy, part of process Hard, problematic, expensiveSafety High exposure, chemical/biological Usually safer

Batch Accountability Quality certified, off-spec discarded Periodically tested, off-spec reworkedControllability Fundamental, errors have ripple effects High quality control, besides low flexibility

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Making the choice

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References

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1. H. S. Fogler (2014), Elements of Chemical Reaction Engineering, 4 ed., Delhi: PHI Learning.

2. G. D. Ulrich e P. T. Vasudevan (2004), Chemical Engineering Process Design and Economics, A Practical Guide, 2 ed., Durham: Process Publishing, pp. 258-276.