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reactors
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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.