SELECTION OF INDUSTRIAL DRYERS II-India - Copy

Copyright 2006 Arun S. Mujumdar - Selection of Industrial Dryers II

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SELECTION OF INDUSTRIAL DRYERS II

A.S. Mujumdar

National University of Singapore

Singapore


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OUTLINE

Background; basic dryer types Classification of dryers Some key characteristics of common dryers Selection guidelines, feel properties, drying times Examples of selection of dryers (case studies)

based on criteria discussed (expert system) Closing remarks


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DRYER SELECTION

Dryer Selection: A black art or science? Little published work on subject Simple decision trees suggested (SPS) Most textbook and handbook procedures cover very

few conventional types More complex algorithms suggested – Strumillo,

Nevenkin Largely untested in industrial practice – trend is to

“repeat history” Must examine drying system cost rather than dryer

cost for final selection. Also, product quality must be considered


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MAIN DRYER CLASSIFICATION CRITERIAPrincipal Distinctions: Form of material – particle, film, slab, slurry, paste Mode of operation – batch or continuous Mode of heating – conduction, convection, radiation,

dielectric & combinations – steady/unsteady; intermittent

Solids distribution – layer or dispersion (for powders) Gas and solids flow pattern; operating pressure, etc Others – flowsheet features, heater type, solids

conveying intermittent heating etc.


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PRINCIPAL NUMERIC DATA NEEDED

Include as much relevant data as possible, e.g.Solids throughput - mass flow Ws; turndown ratio

Moisture content - inlet X1, outlet X0, variationParticle properties - size, size distribution, density,

p, s

Drying kinetics - drying curves, E.M.C. dataTemperature limits - long-term, instantaneousGas and solvent - identity, physical propertiesOther features - safety, ease of handling, attrition,

etc. Quality aspects, toxicity, flammability

_____________EMC – Equilibrium Moisture Content


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INPUT DATA: ADDITIONAL QUALITATIVE DATA

Rules can be formulated which include effects of:

Fires and dust explosions Toxicity Potential for environmental damage Product value Need for containment Capital cost Attrition, hardness and friability


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INPUT DATA: ADDITIONAL

QUALITATIVE DATA - Continued…Effects continued..

Operating cost Cohesion, adhesion, agglomeration Operating time Need for size reduction/enlargement Special factors Post-drying operations Pre-drying factors


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SMALL SCALE LAB TESTS

Small scale tests give valuable information: Drying kinetics – drying rates (parametric effects) Equilibrium moisture content – effect of T,

(humidity) Microscopic examination – surface, agglomeration Lab-scale rotary evaporator – overheating, balling,

adhesion Rotating drum tester – attrition, dustiness Cohesion and adhesion – handling, sticky point

Vital to have a representative sample of final material Not necessary to carry out all of above tests in all

cases


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STEP 1 – BASIC CHOICEFORM OF FEED AND PRODUCTFeed and product can be in one of these main basic

forms: Particulate solids (bed/layer/or dispersed) Sheet or film Block or slab Slurry or solution (feed only) or paste

Mostly require completely different types of dryer Widest choice available for particulate solids Specification of final product also critical in selection


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STEP 1 – BASIC CHOICESBATCH OR CONTINUOUSBatch dryers favored by: Low throughput (under 50 kg/h) Long residence time (i.e. mainly falling rate

drying) Batch equipment upstream and downstream Requirement for batch integrity Continuous dryers favored by opposite

conditions

Match production made of feed where possible


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STEP 1 – BASIC CHOICESMODE OF HEAT TRANSFERChoice less clear-cut Contact – good for: low moisture removal, low

temperature need for containment, long solids residence time

Convective – good for: high throughputs, high drying rate, high drying rate, high moisture removal ΔX, unhindered drying

Radiation – high-temperature (>500°C) or solar drying

Dielectric – slabs, sheets, large particles, low X (moisture contact)

Combinations of two or more modes


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STEP 1 – BASIC CHOICESINFORMATION FROM KINETIC DATA

Interpretation of drying curves assists choice Unhindered drying period – favors

convective/dispersion Long hindered drying period – favors contact drying Estimate of required solids residence time Maximum likely drying rate Indication of mechanisms controlling drying Difference between initial and final drying rates*

_____

* (If high, favors well-mixed, parallel flow or two-stage)


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STEP 2 – DRYER TYPESCHOICE BETWEEN PRINCIPAL TYPES

Choose one basic classification to examine e.g. continuous convective dryer

Draw up short list of available types of dryer e.g. through-circulation band, pneumatic

conveying, spray, rotary Use chosen criteria to favor or oppose certain

dryers e.g. attrition problems, solids residence time


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SOLIDS RESIDENCE TIME

Required residence times for 500 m solids*: Pneumatic conveying dryer – 1 second (surface

moisture only) Cascading rotary dryer

60 seconds = 1 minute (gas phase) 10-15 minutes (total, both phases_

Fluidized bed dryer – 480 seconds = 8 minutes_________* Undhindered drying of 0.2 kg/kg moisture with gas at

150°C

Is RT variation permissible? (plug flow vs. well-mixed)


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STEP 3 – SUBTYPES AND REFINEMENTS

DETAILED ASSESSMENT METHOD

For given type of dryer e.g. fluidized bed Select subtypes, identify appropriate special

features Again use criteria appropriate to the particular

situation e.g. is material sticky in wet and/or dry state

temperature limitation (wet and dry, long and short term), allowable range of moisture content and particle size


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EXAMPLE – FLUIDIZED BED DRYERS

Sub-types:Batch: well-mixed (WM)Continuous: Well-mixed, plug-flow (PF), multi- stage

(MS)Options: Internal heating coils, split

distributor, vibration, feed end rake, expanded freeboard, special distributor, etc.

Flowsheet options: Gas recycle, backmixing, direct firing, self-inerting

Cost Criteria: Simple types cheapest, most options add to cost vibrofluidized beds have high capital cost/low energy costs

Payback periods (depend on if dryer is new or replacing an existing operating unit)


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Fluidized bed dryers

See power point presentation on classification and selection of fluidized bed dryers


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WEIGHTING FACTORS AND EVALUATIONAt each step in procedure, choices are “good”, “fair”, or

“poor”. Need to evaluate the overall conclusion for each branch.

Suggested computational method: use merit factors factor = 1 if fully acceptable, <1 if potential problems. Users can give greater weight to main factors for their plant

Evaluate combined factor for each type/ subtype at finish. Obtain vendors’ cost quotes for most promising options

Delivery times, warranty service, start-up costs must also be considered

Evaluate drying system total cost (operating & capital)


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EVALUATION METHODLAYOUT OF EXPERT SYSTEM (SPS)

Structure or procedure and knowledge tree is complex- designed by SPS (proprietary)

Decisions not clear-cut; “fuzzy logic” involved Good conditions for an expert system (separation

processes service, Oxfordshire, U.K. has a proprietary system)

Conventional method: questions-and-answer session Combined here with evaluations based on input data User presented with reasons for suggested choices


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STEP 3 – SUBTYPES AND REFINEMENTSCHOICE BETWEEN SUBTYPES

Feed difficult to fluidize – use WM, MS (WM before PF)

If PF used, need rotating rake, backmixing, vibrated feed region

Narrow X0 specification – prefer in order PF > MS > WMX0 much less than X1 – prefer PF and MS to WM

Wide size distribution – use vibration, special distributor, expanded freeboard, fines collection

Small particles, low gas velocity – consider internal coils

__________WM: Well-Mixed; PF: Plugflow


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Fluidized bed dryers

View power point presentation of fluidized bed dryers


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EXAMPLESEXAMPLE 2 – ORGANIC PEROXIDEDrying organic peroxide, maximum temperature 40°C Throughput 40 kg/h batch dryer Low temperature favors vacuum contact drying ΔX = 0.25 kg/kg, mainly surface convective drying Particle size 500 m, fluidizable fludized bed Combustible solid, dust explosion possible inert

gas Organic solid, soft, slightly sticky balling/ sticking

Fluidized bed preferred to rotating batch, conical, etc.


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EXAMPLE 3 – CRYSTALLINE MATERIAL

Crystalline filter cake, drying from 10% to < 0.1% moisture

Throughput 1000 kg/h continuous Agglomerates (observed by microscope), size 10-500

m desirable to preserve use little/ no agitation Lab tests: melting point 150°C, sticky point 130°C Toxic material, emissions limits not dispersion

prefer contact, disfavor rotary (sealing)

Rotating tray dryer or plate dryer (best options)


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EXAMPLESEXAMPLE 1 – SALT DRYER

Drying surface moisture from 400 m salt particles

Throughput 500 kg/h, high drying rate continuous Surface moisture, high Tg allowable convective

Attrition undesirable, particles brittle rotary, pneumatic conveying less suitable

Particles in fluidzable range fluidized bed Other options; horizontal agitated, indirect rotary etc. Good thermal efficiency feasible


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CLOSING REMARKS:CLASSIFICATION/SELECTION:

Very important step (after establishing need to dry and optimal flowsheet for nonthermal dewatering)

Wrong choice leads to severe penalties – start-up costs, downtime and need to replace

User must do “homework” fist; vendors valuable thereafter Several dryers may do the job – same quality, cost etc. Selection does depend on cost of fuel, relative cost of

different energy sources; geographical location; legislative regulations; emission control; safety, etc.

Consider new technologies as well – when available and proven

Expert systems now available (e.g. SPS) to aid in selection – still a combination of art (experience) and science!


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Closing Remarks

Selection may be dominated by just one criterion in some cases e.g. quality for pharma products

Several different dryers can do same job at same cost in some cases

Choice can depend on geographic location, cost of energy etc

Life cycle cost is important to consider- capital cost may be only 10-15% over life of dryer


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THANK YOU FOR YOUR ATTENTION!

QUESTIONS?

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SELECTION OF INDUSTRIAL DRYERS II-India - Copy