8/11/2019 Heurastic Rules for Process Plants

1/4

Provided that they are updatedwhen appropriate to keep them

relevant, time-tested heuris-tics rules based upon experi-ence, also referred to as rules

of thumb, can be useful for the design,specifying and operation of severalkinds of equipment used in processplants. The first and foremost heuris-tic rule is that the engineer should not

shy away from using heuristics and

the second rule is that he or she shouldnot rely blindly upon them.

Presented here are a wide rangeof heuristics, classified according tomajor kinds of typical process equip-ment or activity.

Fluid handling Fans are suitable for raising gas pres-

sures moderately (for instance, by3%, or by 12 in. of water); for higherpressures up to about 40 psig, blowersare suitable; for yet higher pressures,employ compressors (however there isoverlap between the operating rangesof blowers and compressors)

Typical polytropic efficiencies forlarge centrifugal compressors areabout 76 to 78%; rotary compressorsnormally have efficiencies around70%, except for liquid-sealed ones,which have efficiencies around 50%

For pipe lines of diameter D ininches, typical fluid velocities and

pressure drops are as follows:(a) for pump discharge (liquid): (5 +

D/3) ft/s, and 2 psi/100 ft; (b) at pump suction (liquid): (1.3 +

D/6) ft/s and 0.4 psi/100 ft; (c) for steam or gas, 20D ft/s and 0.5

psi/100 ft Control valves function best if the

pressure drop through them is atleast 10 psi

Single-stage centrifugal pumps canoperate at rates of up to about 5,000gal/min, (and to maximum heads of500 ft); multistage pumps can oper-

ate to about 11,000 gal/min.

Conveying of particulate solids Screw conveyors:

(a) Can transport solids that areabrasive or sticky

(b) Typical incline is about 20 deg(c) Most are 150 ft or less in length

(d) With a conveyor of 12-in. diam-

eter, throughputs of up to about3,000 ft3/h are feasible; typically,screw rotation rates are up toabout 60 rev/min

(e) Power consumption relatively low Bucket elevators:

(a) Vertical transport of abrasive orsticky materials is feasible

(b) Typically, speeds can reach 100 to300 ft/min; at 100 ft/min, bucketelevators with 20X20-in. bucketscan convey about 1,000 ft3/h

Drag type conveyors:(a) Can convey for relatively short

distances in any direction (b) Have high power requirements

(c) Typical speeds are 30 ft/min (for, e.g.,fly ash) to 250 ft/min (for grains)

Pneumatic conveyors:(a) They offer high capacity

(b) Usually employed with convey-ing distances of 400 ft or less

(c) Can transport simultaneously toseveral destinations

(d) Operate under vacuum or lowpressures

(e) Typical conveying-gas velocities

are 35 to 120 ft/s

Cooling towers In full-scale units, air saturation can

reach 90% To minimize pressure drop (ordinar-

ily a maximum of 2 in. water), em-ploy an open-structured material for

the tower fill

Typical water circulation rates are 1to 4 gal/min per square foot, whereasthe air rates are 1,300 to 1,800 lb/hper square foot, or 300 to 400 ft/min

Countercurrent induced-draft tow-ers, which can cool water to about 2Fabove the wet-bulb temperature, arethe most prevalent version of tower

used in the process industries For a given service, the required size

(volume) of a given tower is a func-tion of the difference between thewet-bulb and the exit temperatures;the smaller the difference, the largerthe required volume

Evaporation losses are typically 1%

of the circulation for every 100Fof cooling range. Windage or driftlosses in mechanical-draft towerstypically amount to 0.1 to 0.3%. Tokeep salt from building up exces-sively, it is typical to blow down 2.5to 3% of the circulation

Heat exchangers; refrigeration In a shell-and-tube exchanger, the

tube side is for corrosive, fouling,scaling and/or high-pressure fluids;

Feature Report

44

CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2006

Engineering Practice

Alejandro Anaya Durand, Josseline Alarid Miguel,Gabriel Gallegos Diez Barroso, Marco Alejandro Leon Garcia,

and Juan Pablo Sierra AngelesNational Autonomous University of Mexico

Heuristics Rules forProcess Equipment

If applied with thought and care, heuristics like these can

make life much easier during project scoping, process

design, equipment specification and similar tasks

8/11/2019 Heurastic Rules for Process Plants

2/4

the shell side is for viscous and/orcondensing fluids

Typical minimum temperature ap-proaches are 20F with normal cool-ants, or 10F or less with refrigerants

Ordinarily, the maximum heat-transfer area for shell and tube heat

exchangers is about 5,000 ft2 When refrigerating to temperatures

below about 80F, it is customaryto use cascades of two or more re-frigeration stages

Evaporators The maintaining of a suitable tem-

perature gradient (for instance,

about 45F) can minimize film-re-lated efficiency losses. From anefficiency standpoint, about 250Btu/(h)(ft2) is a suitable overall coef-ficient of heat transfer

In countercurrent evaporation sys-tems, a suitable temperature ap-proach between the inlet (hot) and

output (cold) streams is about 30F.In multistage operation, the typicalminimum value is 10F

In a well-designed evaporator sys-tem, it should be possible to achieveheat recoveries of more than 75%

Storage tanks For less than 100 gal, it is common

practice to use vertical tanks on legs For between 100 and 10,000 gal,

horizontal tanks on concrete sup-ports are commonly used

For beyond 10,000 gal, consider ver-tical tanks on concrete foundations

Liquids that are subject to breathing

losses may conveniently be storedin tanks with floating or expansionroofs, for conservation

Although the amount of material

inventoried is highly plant-specific,many process plants specify 30 daysworth of capacity, for raw materials

and products alike

Drums Liquid drums usually are horizontal Drums for gas-liquid separation are

vertical A length-to-diameter ratio of 3 is

considered optimal; but in practice,the ratio for drums commonly fallsbetween 2.5 and 5.0

In liquid-liquid separation, reflux

drums are usually kept about halffull, with holdup time of about 5min. (or 5 to 10 min if the drum liq-

uid is fed to a downstream separa-tion tower)

For entrainment removal, meshpads of 4 to 12 in. thickness canachieve 99% removal; a thickness of

6 in. is widely used

Reactors In stirred tank reactors, it is prefer-

able to maintain a liquid level thatis approximately equal to the tankdiameter

Common motives for conducting

batch reactions, in stirred-tank re-

actors, are: the daily production rateis relatively low; reaction times arerelatively long; particular processparameters, such as the feed rateor the vessel temperature, must beprogrammed during the course ofthe reaction

An array of continuous stirred-tank

reactors in series (four or five, for in-stance) is in many cases the systemof choice for slow reactions of liquidsand slurries

Tubular reactors are attractive forshort-residence-time reactions (sec-onds or minutes), high throughputs,and reactions that require a rela-

tively large amount of heat transfer

Distillation and gas absorption Generally speaking, distillation tends

to be the most economical method forliquid-liquid separation; more so, forinstance, than liquid-liquid extrac-tion or crystallization. Flashing can

be more economical than distillation,but is more limited by physical prop-erties of the mixture

The well-known simple equation for

relative volatility relative volatil-ity = (vapor pressure of more vola-tile component)/(vapor pressure of

less-volatile component) is validonly for ideal mixtures

If the system is ideal and there are only

two components, the McCabe-Thielemethod offers a good approximationto the number of equilibrium stages

The most common determinant ofthe column operating pressure iseither the temperature of the avail-able condensing medium (in manycases, cooling water at about 100 to

120F) or the maximum allowablereboiler temperature (for instance,366F for 150-psig steam)

For many separations, the optimalreflux ratio is 1.2 times the mini-mum reflux ratio

In many cases, the economically op-timal number of trays equals twice

the minimum number of trays Reflux pumps should be oversized

by about 25% From a maintenance standpoint,

tray spacings of about 20 to 24 in.are attractive

Typical pressure drop per tray is ofthe order of 3 in. of water or 0.1 psi

For separation of light hydrocarbons

and aqueous solutions, the tray ef-ficiencies are typically 60 to 90% fordistillation, and 10 to 20% for gasabsorption and stripping

For a typical sieve tray, the holes are0.25-0.50 in diameter, and the holearea is about one-tenth of the activecross-section area

For a typical valve tray, the holesare about 1.5 in. diameter, each out-fitted with a liftable cap; there aretypically 12 to 14 caps per squarefoot of active tray cross-section

The typical height of a column weiris 2 in.; the weir length is usuallyabout 75% of the tray diameter;

maximum liquid rate is about 8 gal/min. per inch of weir; for high liquidrates, multipass arrangement areoften the choice

For towers of less than 3 ft diam-eter and where low pressure dropthrough the tower is desirable,packings (random or structured) are

commonly preferred over trays. Ifthe packing is initially distributedwith care and is periodically redis-tributed, the volumetric efficiency

CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2006 45

8/11/2019 Heurastic Rules for Process Plants

3/4

can be greater than that of a compa-rable tray tower

Most reflux drums are horizontal,

kept about half full, and have a liq-uid holdup of 5 min

For 3-ft-diameter towers, about 4 ft

of column height should be addedat the top for vapor disengagement,and 6 ft at the bottom for liquid leveland reboiler return

Due to wind-loading and other struc-tural considerations, towers shouldbe no higher than about 175 ft. Fur-thermore, the ratio of tower height

to diameter should be less than 30

Liquid-liquid extraction

Ordinarily, the phase with thegreater volumetric flowrate shouldbe the dispersed phase; however, inextractors subject to backmixing,the phase with the lower flowrate

should instead be dispersed. It isalso preferable that the dispersedphase be the one that wets the equip-ment less well. Finally, because theholdup of continuous phase is usu-ally the greater, it is desirable thatthat phase consist of the less expen-sive and/or less hazardous material

For separations achievable in rela-

tively few stages (5 to 10 for in-stance), packed extraction towersoffer advantages, unless the surfacetension exceeds 10 dynes/cm. It ispossible to achieve attractive HETS

values (5 to 10 ft, for example). Dis-persed-phase loadings should notexceed 25 gal/(ft2)(min), and the

dispersed phase should be redistrib-uted every 5 to 7 ft

Sieve tray on extraction columnstypically have holes of only 3- to 8-mm diameter. Velocities through theholes should kept below about 0.8 ft/sto minimize formation of excessivelysmall drops. Typical tray spacings

are 6 to 24 in.; typical tray efficien-cies are in the range of 20 to 30%

Crystallization from solution Whether melt crystallization or crys-

tallization from solution is employed,the maximum recovery of solids islimited by the eutectic composition

Crystal growth rates and the finalcrystal size are both controlled bylimiting the extent of supersatura-tion in the liquid

It is good operating practice to holdthe liquid temperature at a fewFahrenheit degrees below the satu-

ration temperature for the prevail-ing concentration

Filtration A convenient way to classify the ease

of a given filtration task is to measurethe rate of cake buildup on a labora-tory vacuum leaf filter: 0.1 to 10 cm/sindicates rapid filtration; 0.1 to 10 cm/min, medium-speed filtration; and 0.1to 10 cm/h, slow filtration

Selection of the filtration method for agiven task depends partly on whetherthe liquid phase or the solid phase is

the one of value. Among the suitablemethods if the liquid phase is desiredare filter presses, sand filters and pres-sure filters. If the solid phase is desired,consider rotary vacuum filters

Drying of solids Continuous tray and belt dryers for

natural or pelletized 315-mm gran-ular material commonly have dryingtimes in the range of 10-200 min

Drum dryers that handle pastes andslurries operate with contact times of

3-12 seconds, generating flakes that

are 1-3 mm thick; common evapora-tion rates are 15 to 30 kh/(m2)(h).Commonly found diameters are 1.5to 5.0 ft; common rotation rates are2-10 rev/min; evaporative capacitiesas high as about 3,000 lb/h are fea-sible in full-sale installations

Fluidized-bed dryers work best on

very small particles, with diametersof a few tenths of a millimeter; butthis technique has also been usedsuccessfully with particles of up to 4mm diameter. A suitable velocity forthe fluidization gas is twice the mini-mum required for fluidization. Inmany continuous operations, drying

times of 1-2 min are sufficient, butsome products (including some phar-maceuticals) require much longer

Most spray dryers complete theirtask in less than one minute; in fact,the surface moisture is usually re-moved within the first 5 s. The wetfeed and the drying air are most com-

monly fed in parallel. The atomizingnozzles typically operate at pressuresof 300 to 400 psi, and have openingsthat measure 0.012 to 0.15 in.

Size reduction Ball or roller mills are commonly

set up to operate in a closed circuit

which includes size classification andthe return of the oversize to the mill

Roll crushers come in two versions:

either smooth or with teeth. A 24-in., toothed crusher can accommo-date pieces of feed as large as 7in. in radius. Smooth rolls achievereduction ratios of about fourfold.Rotation speeds are typically 50 to900 rev/min

To produce particularly small par-

ticles, consider hammer mills. Largeunits operate at 900 rev/min; smallones can reach 16,000 rev/min

Rod mills usually feed on particles ofabout 50-mm size. The material is usu-ally reduced to about 865 mesh, butreduction to 300 mesh is achievable

For fine grinding, consider ball mills,

tube mills or pebble mills. The last-named are the choice when metalcontaminations must be avoided

For jaw crushers, the feed is usuallyunder 4 in. diameter; typically, abouteight to ten strokes are needed toachieve the required size reduction.Gyratory crushers can produce par-

ticles that are more rounded

Mixing and agitation Solids with a low settling velocity

(such as 0.03 ft/s) can be success-fully suspended with either turbineor propeller agitators; suspension ofsolids with settling velocities greaterthan 0.15 ft/s requires vigorous agi-

tation with a propeller When only brief contact time (such

as 1 or 2 s) is required between twocomponents, inline blending shouldprove adequate. Typical power in-puts are 0.1 to 0.2 hp/gal

Small propellers are usually run atabout 1,500 to 1,750 rev/min, large

ones at about 400 to 800 rev/min Paddle agitators typically employ

paddles that extend to about 50 to80% of the inside diameter of the

vessel. Typically, the rate of rotationlies between 20 and 150 rev/min

Agglomeration Some major methods of particle size

enlargement are: compression intoa mold; extrusion through a die fol-lowed by cutting or breaking to size;

Engineering Practice

46

CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2006

8/11/2019 Heurastic Rules for Process Plants

4/4

globule formation from molten ma-terial followed by solidification; andagglomeration under tumbling or

other conditions of agitation withouta binding agent

For rotating-drum granulators typi-

cal length-to-diameter ratios are 2 to3, rotational speeds are 10 to 20 rev/min, and the pitch as great as 10 deg.The size of the produced granules de-pends in part on the rotational speed,the residence time and the amountof binder. Commonly, the producedgranules are 2 to 5 mm in diameter

If uniformity of product size is im-portant, rotary disk granulators

are preferable to drum granulators,other things being equal

For roll compacting and briquetting,

typical rolls measure from 130-mmdiameter by 30 mm wide to about900-mm diameter by 550 mm wide.

The material extruded by the rolls,typically about 1 mm thick, can bebroken into any size desired

Rotary compression equipment,feeding upon powders or granulesand typically operating at around100 rev/min, is suitable for produc-ing tablets of uniform size, at rates

as high as 10,000 tablets/min Fluidized-bed granulation is typi-

cally carried out in beds having adepth of 12 to 24 in. Typical air ve-locities are 0.1 to 2.5 m/s, which is

three to ten times the minimum ve-locity needed for fluidization

Edited by Nicholas P. Chopey

CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2006 47

Principal authorAlejandro Anaya Durand(Parque Espaa 15B, ColoniaCondesa 06140, Mexico D.F.Mexico; aanayadurand@hotmail.com), is a member of theFaculty of Chemistry at theNational Autonomous Uni-

versity of Mexico (UNAM).He has more than 45 yearsof experience in process andproject engineering, duringalmost all of which time he

has also served as a professor at UNAM andother Mexican universities. Also included in thatcareer are 30 years with the Mexican Institute ofPetroleum (IMP), from which he retired in 1998.He is a member of the Mexicos National Instituteof Researchers (SNI), and is a Fellow of AIChE.The Mexican Institute of Chemical Engineers,the National Soc. of Chemistry and the NationalCollege of Chemical Engineers have each hon-ored him with excellence awards. The author ofmore than 220 articles in local and internationaltechnical magazines, he holds a masters degreein chemical engineering from UNAM.

Call for Papers2007 Chem Show ConferenceOctober 30 November 1, 2007 Javits Convention Center New York, NY

Typical topics to be covered:

Energy efciency

Process control and instrumentation

Process-plant safety

Green engineering

Pumps, valves, other uid-ow equipment

Distillation

Solid-liquid separations

Water pollution control

Air pollution control

Process integration

Process-plant water supply

How to submit a paper:

We invite abstracts (less that 200 words) for proposed, non-promotional, technical

presentations on the above conference topics or related elds. Emphasis should be on modern

proven technologies, application of best practices, improvement of return on investment.

Send by February 1, 2007to: Nicholas P. Chopey at nchopey@che.com,

or fax to (212) 621-4694.

Presentations to run for about 25 minutes, followed by 5 minutes for questions and discussion.

Presented by

In conjunction with

10392

Circle XXX or go to www.info.ims.ca/34xx-xx

References1. Anaya Durand, Alejandro, Heuristics Rules

and Criteria for Equipment Process Design,Revista Instituto Mexicano de ingenierosQumicos, November December 1993.

2. Anaya Durand, Alejandro, and oth-ers, Updated Rules for Pipe Sizing,Chem. Eng, pp. 153156, May 1999.

Also recommended3. Peters, Max S., Plant Design and Economics

for Chemical Engineers, 5th Ed., McGraw-Hill, New York, 2003.

4. Cheremisnoff, Nicholas, Handbook of Chem-ical Process Equipment, Butterwith-Heine-mann, 2000.

5. Walas, Stanley, Heuristics in Chemical En-gineering, Butterworh-Heinemann, 1990.