centrifuge focus - evaluation and optimisation

8/12/2019 Centrifuge Focus - Evaluation and Optimisation

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Fr trat on+Separat on J ulylAugust 2OO5

Centrifuge focus:eval uatitesting anoptimisation

f a centrifuge is providing a satisfactoryfree operation, does that means it is anNigel Day challenges operatives to lookcentrifuges, as well as the interaction ofwith every day.There are around 12 different configurations ofindustriai centrifuge currently availabie to theprocessing industry. Some adopt filtrationtechniques whilst others employ sedimentationtechniques.Experience has found in the vast majority ofcases that operatives consider a centrifuge that isproviding a satisfactory separation and troublefree operation to be an optimised process.Unfortunately this is not necessarily the case.It is the intention of this article to provideguidance as well as challenge operatives to lookmore closely at the centrifuges and theinteraction of the process they work with everyduv.

Figure 1: Typical continuous solid bowl decantercentrifuge - supplied courtesy of Thomas Broadbenl& Sons Ltd.

Philosophy of operationA decanter centrifuge separates solids from aliquid by means of sedimentation. Revolvingthe centrifuge, thus applying gravirationalforces many times more than normal on thesolids and liquid within the centrifuge,creates the sedimentation effect.In order for a sedimentary type centrifuge tofunction correctly the suspended solids in theslurry must have a greater specific gravitythan the liquid phase. From the secriondrawing below it can be seen that themachine consists essentially of two rotatingelements. The outer element is a solid bowl:?:::,n" second, inner element, is a helicalThe inner element rorares ar a slightlydifferent speed to the bowl, usually slower butcan be faster. In effect we have a screwconveyor inside a rotating bowl.The differential speed, as we know it isnormally provided via a mechanical gearbox,though some configurations drive theconveyor using a hydraulic or electric motorback drive mechanism.The gearbox is fitted to one end of themachine. The main body of the gearbox isfitted to the bowl element with the outputdriveshaft from the gearbox connected to theconveyor.

and,f troubleprodess'?ly # theirs tlffiey work

As can be the bowi has a parallelsection k asihe clarification zone and akiown as the drainage orapered secbeach zoneFeed slurry ii introduced through a starionaryfeed pipe and spills into rhe conveyor hub,accelerated as it passes through the feed portsand delivered onto they bowl wall. Feed exitports are so located as nor to spill slurry intothe drainage zone.Clearance between the periphery of theconveyor blades and bowl wall (approx 3mm)becomes packed with solids, the flow ofliquid is back around the screw conveyorwith the blades acting iike baffles, preventingshort-circuiting.Liquid travels towards the discharge ports,the centrifugal force causes the solids tosettle, so that the discharge liquor can beessentially clear. Solids, which are thrown outof suspension against the bowl wall, areconsolidated, picked up by the conveyor rhenconveyed towards the solids discharge end ofthe rotating assembly.Solids are transported up the drainage zone,out ofthe liquid and up the dry beach, beforebeing discharged.Both the liquid and solid phases aredischarged into the stationary outercasing,which is equipped with baffles ro prevent re-mixing of the two separated phases.


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F= ltration+Separation J u lylAu g ust 2 OO5

$olids Discharge Liquor Discharge

Figure 2: Sectional view schematic - supplied courtesy of Thomas Broadblent & Sons LtdGathering the data\Thether considering employing a decantercentrifuge or optimising an existing machine thevalue of preliminary testing and general datacollation cannot be over emphasised. It ispossible with a sample of oniy about 1 litre toobtain a considerable amount of information.On most materials, sufficient information can beobtained to rule out completely the use of acontinuous decanter centrifuge or how best tooptimise an existing centrifuge installation.The following is a brief description of theapproach used for examination of a small sampleto determine:. Can the material in question by successfully

handled in a decanter centrifuge?o \,Vhat size and configuration of decantercentrifuge is required?r What performance can be expected from thechosen cenrifuge?r \Vhat degree of optimisation is possible?

It is essential that the sample evaluated isrepresentative of the material to be processed.Various physical data are then obtained fromthe material, providing iruight relative to thenature of the material. Centrifuging behaviour,i.e. settling and packing characteristics, are thendetermined. Evaluation of the data obtainedprovides the basis for predicting type, size andperformance of equipment for the duty. (dreproperties that need to be determircd cml be seen intable I belaw.)In addition to the feed material properties wealso need to identifii the actual processrequirements, as follows,. Scale ofthe proposed operation?. Which phase(s) of the feed slurry is required?e What properties are required of the solid

product?r rVhat properties are required of the liquorproduct?o ls product washing required?o How will the centrifuge integrate withsunounding plant?

r Reliability of equipment?o \Vhat space is or will be available?r What is the value of the productl. Any special requirements?Settling and packing characteristicsThere are two main requirements that must bemet if a'material is to be handled satisfactorilyin a continuous decanter centrifuge. The first isthat the suspended solids settle in the length oftime that the slurry can be subjected tocentrifugal force in the machine for asatisfactory eflluent is to be obtained. Secondlythe solids so settled rnust pack firmly enough tobe conveyed through the machine.To determine settling and packingcharacteristics a small sample is spun, at 1000 xG, at the process temperature in a test tubecentrifuge. The sample is examined after 10seconds of spinning and the volume of settledsolids noted, also the condition ofthesupematant, i.e. clear, cloudy or turbid. It isthen spun for a further 20 secs, making a totalof 30 secs, and examined as before. Howeveqthis time a thin glass-stirring rod is slowlylowered into the test tube until it touches thesettled solids.The volume of solids that will support theweight of the rod is noted. The sample is spunagain for an additional 30 seconds and

examined as before. Spinning is continued andthe sample examined with the results recordedin a table similar to that on shown page 24.As a rule of thumb if the volume of solids andthe volume of compaction come togetherwirhin 90-120 secs with a clear effluent, asabove, then the material being tested would beconsidered an ideal candidate for processing in acontinuous solid bowl decanter centrifuge.More difficult material may require spinning ata higher G force, up to say 2500 x G. This willnot only influence the type and size of thecentrifuge for the duty but also whichmanufacturer to approach.Adjustments available and their effectsAssuming a centrifuge has been selected andinstalled, we have the following adjustmentsavailable for optimisation.. 1-FeedRate;. Z-BowlSpeed;r 3-PoolDepth;. 4-Poollength;o 5 - Conveyor/Bowl Differential Speed;I 6-FeedConcentration;. 7-FeedTemperature;1 - feed rateThis adjustment is probably the easiest tocarryout and does not involve stopping thecentrifuge. The effect of feed rate onperformance can be generalised as follows.The higher the feed rate the less residence timethe liquid has in the bowl hence the harder tosettle out possible ultra fines. Conversely thelower the feed rate the longer the liquorresidence time and hence the easier to obtainclear liquors and in some instances 100%recovery of solids.The feed rate can be funher adjusted anywherebetween the lower and upper limits to obtaineither complete removal of the solids or to splitand classifir the solids at a particular micron size.


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2 - bowl speedUnless you have some means of changing thebowl speed i.e. fiequency convertor, the usualmethod is by changing the driving pulley on themain drive motor. This of course meansstopping the machine. The speed at which thebowl is driven is related to the'G'forcedeveloped with the bowl. Machine speed can begeneralised by the following staremenrs;. The higher the speed the dryer the solids oncrystalline type solids;o The higher the speed on finer material morefine solids will be captured resulting inwetter solids;o The higher the speed the capaciry of rhemachine solids wise increases due to theincrease of the delta RPM provided by thegearbox. However conveyor-scrolling torqueusually increases and the gearbox could be alimiting factor in how fast the machine can

rotate;r Increased speed leads to higher powerconsumption;However, one must always remember the higherthe speed the more abrasion takes place andmore general wear and tear. Therefore the idealsituation is to operate the centrifuge at thelowest speed possible whilst maintaining theprocess performance.Never exceed the maximum rotationai speedspecified by the original manufacturer of rhecentrifuge3 - pool depthTo adjust the pool the machine normally has tobe stopped, though there are decantercentrifuges available that allow adjustmentwhilst runnihg.The pool is usually adjusted by moving weirplates onto different annulus settings. Byincreasing the pool depth the volume of liquidheld within the bowl is also increased.Consequently the deeper the pool rhe moreclarifiiing capacity you have available.However, the deeper the pool the shorter thedrying beach, hence the werrer the dischargedsolids will be. Depending upon cusromerrequirements, whether they want dry solids orclear liquids derermines at which end of the

pool depth range to work in, but usually thecustomer requires dry as possible solids withclear liquids. In this case you have to aim on acompromise.Deep pools will also lower conveyance torque,as it is easier to convey solids under liquid.4 - pool lengthThe effective pool length can be adjusted byfeeding in one of usually mo feedcompartments. By feeding in the feedcompartment nearest to the liquor dischargeend of the bowl, the effective pool length isshortened hence the least residence time for theliquid.Respectively the longer the pool the longer theresidence time.5 - conveyor to bowl differentialThe conveyor to bowl differential (Delta RPM)is altered by changing the bowl speed due ro thegearbox. The gearbox itself can be changed fora different ratio, though an expensive changeonce in rhe field.The effects of changing the Delta RPM are asfollows:r The lower the differential the dryer thesolids due to longer residence time;r The lower the differential the leastdisturbance of the pool, hence better finesolids recovery and centrate clarity;r The lower the differential the lower the

solids throughput capaciry;r The lower the differential the less wear takesplace on the conveyor;r Increased differential reduces intemal cakepile height;o Increased differential greater agitationaffecting cenff ate clariry ;

Filtration+Separaton July/August 2OO5

o Increased differential reduces convevancetorque;r You should always ensure that thedifferential is not so low as to overloadthe conveyor with solids;6 - feed concentrationA highly concentrated feed may lead to poorseparation efficiency due to hindered settlingofsolids through the liquor phase. A lowconcentration will assist with thesedimentarion of the solids due to bothlowering the feed viscosity and improvedsettling potential through the mother liquor.7 - feed temperatureFeed temperature will affect the cakemoisture to the extent that temperatureaffects the viscosity of the mother liquor.Generally speaking the hotter the slurry, theless viscous and thinner the film of liquidretained on rhe solids.GeneralOver the years decanter centrifugemanufacturers have developed their own'unique' configurations with many majordevelopments being established. Whilst theymay disagree, the general principles andphilosophy of operation though remainvirtually unchanged.Parameters briefly discussed all have theirown effects, to obtain the outright optimisedperformances from the centrifuge couldinvolve adjusting several of these parametersas some directly influence or give similareffects to others.The subject of centrifugal separation ishighly complexed with a high degree ofacademic connotations. However, thecombinarion of application experience,testing and design capabilities of thecentrifuge experts combined with the endusers invaluable knowledge of their materialis what ensures that the centrifuge inquestion will produce a product on schedule,of the desired quantity and quality,economically and above all safely. jContact:Nigel DayNCD Separatlon Solutions Ltd01924 [email protected]

centrifuge focus - evaluation and optimisation

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