nov05solidsonline particle analysis in wet processes

8/10/2019 Nov05SolidsOnline Particle Analysis in Wet Processes

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Many stable processes can

be accurately trackedand controlled by tak-ing samples every couple

of hours for an offline particle sizeanalysis in the laboratory. However,

for unstable processes or those wheresignificant increases in profit can begenerated by closing the loop, onlineanalysis can be highly beneficial.Where offline measurements maymiss key events that occur outside oftheir once-every-two-hour snapshots,online analysis records all events

nearly like a film, updating the cur-

rent status every few seconds.Online analysis provides a continu-

ous stream of data to the programma-ble logic controller (PLC) or other con-trol system, and ensures that processbehavior can be fully observed andacted upon in a timely fashion. For anumber of years, online analysis and

the automated control that it facili-tates have provided an alternative tooffline analysis and the manual con-trol that typically accompanies it.

For solids processes, particle size isfrequently the key variable, and there-fore online analysis can be highlybeneficial. For dry-solids handling

processes, in industries as diverse aspharmaceuticals and cement, onlineparticle size analyzers based on laser-diffraction technology have been usedsuccessfully for many years (see box,above). These instruments generatesignificant cost benefits in the formof improved process efficiency and en-

hanced product quality. More recently,new developments have made onlineanalysis via laser-diffraction technol-ogy accessible for wet processes as

well. Laser-diffraction particle-sizeanalyzers can now be used reliablyfor a range of wet systems, from emul-sions to highly concentrated slurries,to achieve benefits similar to those en-

joyed by dry processors.

Benefits of online analysisAt plants where a switch to onlineanalysis has been made, it is oftenpossible to achieve the following: Control the plant more effectively

during steady-state and transientoperation, either by improving man-ual control or switching to automaticcontrol

Fully understand the interactionsbetween different process param-eters and/or carry out systematicstudies to fully optimize the process(for an example, see box, p. 3)

More consistently manufacture prod-uct with the required specification

Minimize certain variable costs

such as those associated with wasteor energy consumption during sizereduction

More rapidly identify process upsets,

hence minimizing their impactTo achieve such benefits, however,fully automated particle-size mea-surement technology must addressthe challenges posed by wet processapplications sample extraction andpreparation in particular and in-corporate the appropriate techniques

to overcome them.

Why laser diffraction?A variety of different technologies canbe used for wet-process particle-sizeanalysis, all of which have differentstrengths and weaknesses. Ultrasonictechniques, recently combined with

gamma-ray transmission and sound-velocity measurement, are attractivein that they can be used on slurriesthat are opaque and electrically non-conducting, but have the drawbackof being highly sensitive to the pres-ence of entrained air bubbles. Opti-cal-image analysis methods are in-

valuable for the production of particleshape data and for the detailed analy-sis of individual particles but, beingbased on particle counting, can result

Solids Processing

Online Particle AnalysisIn Wet ProcessesDiscover how laser-diffraction technology can make

these measurements possible, even in tough slurries

David Pugh and Alain Blasco, Malvern Process Systems

FIGURE 1. In laser diffraction, a dif-fraction pattern a series of concentricrings of diminishing intensity areanalyzed to determine the particle-sizedistribution of the sample

HOW DOES LASER

DIFFRACTION WORK?

When a focused beam of light is shonethrough a sample it is scattered by par-ticles present, which interrupt the laserlight beam. Relatively small particlesscatter light at wide angles with low in-tensity whereas larger particles scatterlight at narrow angles with high inten-sity. A diffraction pattern a series ofconcentric rings of diminishing intensity can therefore be detected and ana-lyzed, to determine the particle size dis-tribution of the sample (Figure 1).

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in highly inaccurate mass or volume

size distributions, especially if only alimited number of particles has beentaken account of.

Laser diffraction (or low angle laserlight scattering) is an attractive tech-

nique in that it rapidly generates con-sistent, volumetric particle-size analy-sis without the need for any externalcalibration. It is non-destructive androbust in terms of ambient condi-tions. Using modern systems, particlesacross a broad size range, typically 0.5to 1,000 mm for wet systems, can be

measured accurately. Its drawback is

that it requires the media to be trans-parent to some degree.

It is primarily the constraint of nec-essary transparency that has previ-ously limited the use of online laser-diffraction technology in a range of wetapplications. However, with the devel-opment of mathematical algorithms,

which take account of multiple scatter-ing and extend the concentration rangeover which laser diffraction can be used,and more effective sample extractionand preparation systems, which arecapable of producing a representativesample stream (from a concentratedslurry) that is appropriate for analysis,

the applicability of the technique hasbeen significantly extended.

Sampling andsample preparationFor some dilute-stream applicationsa laser diffraction instrument can besimply installed inline for particle size

measurement (Figure 2). One exam-ple consists of oil-in-water emulsionscommon in oil-rig effluent streams,which must be directly analyzed for

environmental monitoring require-

ments. Dilute flocculating systems inthe pharmaceutical industry have alsobeen studied directly using the laser-diffraction method. For other systems,however, direct analysis is more diffi-

cult, a key challenge being the effec-tive design of the process interfaceand appropriate sample preparation.

Slurry handling, for instance, is a no-toriously difficult issue for the processindustries, particularly at the rela-tively low flows that can be associatedwith sample lines. Thick, hot and sticky

slurries need well-designed sample-ex-

traction systems to avoid the problemof blocking, and subsequently must behandled carefully. Mobile calcium-car-bonate slurries, for example, can setsolid after the loss of only a small pro-portion of the diluting solvent.

Studies have shown that for reliablemeasurement by laser diffraction, a

minimum of 3060% of the laser lightneeds to pass through the sample (de-pending on the mean size and spanof the particle size distribution); thistypically equates to a solids content of0.005-0.2% by volume. For many ap-plications, therefore, sample dilutionis an important step. Other condition-

ing processes may also be required; forexample, additives or ultrasonics maybe required to prevent particle ag-glomeration, or break up aggregates.In summary, the main issues needingcareful consideration in the design ofa sample system are as follows: Reliable extraction of a sample at

the process interface any poten-tial for blockage must be carefullyconsidered and mitigated

The extent of dilution required, to-

gether with the optimum number

of dilution stages investigationof the effects of dilution on the par-ticles is necessary in the laboratorybefore a process solution can be im-plemented. Factors to be considered

include zeta potential of the particle,agglomeration effects and the timedelay before these effects take placein the proposed dilution medium

If a material does tend to aggregateas a result of changing pH, then theappropriate solution might requireimplementation of additives or the ap-

plication of ultrasonics to the system

Any tendency of the material to dis-solve, as a result of changes in su-persaturation, may place limitationson the dilution of the media

Uninterrupted flow through thesample system settling and foul-ing must be avoided

The suitability of either batch or

continuous configurations whilea continuous sampling loop may bepreferred, problems with excessivediluent or sample usage may resultin the need for a batch system. Inthis latter case, line flushing andcleaning is essential to minimizeoperation problems and maintain

data integrity

Sample extraction and dilutionFor free-flowing liquid systems, sam-ple extraction from the process can beachieved using a simple eductor; butfor more-demanding slurries, more-complex systems are required.

Static sampling with tank dilu-

tion.Mineral processors typically needto sample and analyze high-tonnage,

Using Online Measurements to Determine theEffect of Process Parameters

Homogenizers are widely used within the food, dairy, cosmetic and pharmaceu-tical industries to produce emulsions with the required droplet size, and hence,the desired properties; droplet size impacts directly on product taste, consistency,

performance and stability. With offline analysis this type of study is time consuming andprone to error as a result of, for example, operator variability and sample stability. Withonline analysis, however, the effects of processing variables on droplet size are rapidlydetermined and optimal conditions more quickly identified.

This type of systematic study can be carried out during product development or at thebeginning of a production run to optimize the processing response to a change in, forexample, feed-material quality. In either case, the rapid identification of optimal operat-ing conditions and the accompanying development of improved process knowledge,lead directly to better manual or automatic control, enhanced process efficiency and

variable cost savings.

Solids Processing

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concentrated slurry streams with spe-cific gravities in the range of 28. For

many applications in this industry atwo-stage sampling procedure, in com-bination with dilution in a continuous

stirred tank, has proven highly effec-tive. The particles measured typicallyhave a diameter of around 101,000microns in laser diffraction termsthey tend to lie at the coarser end of

the spectrum.In the first stage of this sampling

process a primary flow of 50170 L/min

is removed from the bulk flow,which is typically tens of tons

per hour, using a static sampleroperating under gravity flow. The sec-ondary sampling system then cuts a

representative 0.010.03-L slice fromthe primary flow every 10-30 s by mov-ing the sample line across a stationarycutter. The bulk of the primary flow isreturned to the process and a represen-

tative secondary flow of around 0.020.18 L/min is provided for dilution.

The secondary-flow sample is di-

luted by mixing itwith water in a dilu-

tion tank. The sample-di-lution ratio is typically in the range of10100 and residence time within the

tank around 1 min. The resulting sam-ple is routed to the optical head andanalyzed in its entirety. Since the onlymaterial added during the samplingprocess is water, all of the stream can

be recycled into the process. The con-centration of the sample measuredcan be controlled by altering either

FIGURE 2. A typical configuration foran online wet particle-size measurementsystem is shown here

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FIGURE 3. An effective sample dilutionsystem needs to dilute the sample repre-sentatively, be highly reliable and prefer-ably not require significant manual inter-vention for maintenance, operation, orcleaning purposes

CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2005 57


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the secondary-sample cutting fre-quency or the dilution ratio, allowingthe approach to be tailored to a range

of different applications. Overall mea-surement time using this approach isaround 2 min.

Rotating sampler/diluent powered

diluter. A rotating-type sampler similar in design to a four-way valve can be used to extract slugs of flowfrom a process stream if a continuoussample stream cannot be successfullytaken. The resulting system can ef-

fectively replicate continuous onlinemeasurement. For example, at a clayproducing facility, an analyzer was re-

quired to measure the particle size ofmaterial leaving a ball mill. The slurryto be characterized was hot, sticky, andconcentrated, with relatively fine par-ticles around 10 microns. Continu-

ous sample extraction posed a signifi-cant challenge, and sample dilutionwas also required.

A rotating sphere with a central 8-mlcylindrical channel was used to success-fully extract a sample. As the sphererotated, a slug of slurry was removedfrom the process, and then washedthrough the sampler with water. Thiswater began the dilution process andcleaned the rotating sphere to prevent

blockages. The initial sample was fedinto the pre-diluter tank, where it wasdiluted by a factor of 58 with water;

the relatively low dilution factor of thisinitial stage prevents dilution shock.For further dilution, a two-stage ver-sion of a diluent-powered, commerciallyavailable diluter was used to let down

the sample, firstly by a factor of two andthen by a factor of five. The initial stageprepared the sample for more intense

dilution and eliminated the dependencyof dilution factor on the distance fromthe sample line.

Figure 3 shows a schematic of afive-stage version of the diluter. An ef-fective sample dilution system needsto dilute the sample representatively,be reliable, and preferably not require

significant manual intervention formaintenance, operation or cleaningpurposes. This diluter is mechanically

relatively simple and trouble-free,with has no moving parts. The throatand tip of the unit are designed suchthat the diluent entrains a slurrysample into the central tube. Diluted

sample flows through the tube to fur-ther identical dilution stages.

At each stage, the throat, which ef-

FIGURE 4. Thesedata correspondto a talcum slurry

diluted using theunit illustrated inFigure 3

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fectively forms a venturi mixer, ensuresrapid intermixing with the clean dilu-ent, thereby delivering a homogeneous

sample to later dilution stages and ul-timately the analyzer. The diameter ofnozzle used in the design dictates the

dilution ratio, which can be controlledin the range of 2:15:1. The unit is suit-able for particles with a maximum di-ameter no greater than 150 microns.

The key test of any diluter is the con-sistency of particle-size measurementstaken at different dilutions. Figure 4shows data for a talcum slurry mea-

sured using the diluter shown in Figure3. The extent of dilution has no impacton particle size, confirming the repre-

sentative nature of the dilution processand the effectiveness of the design.

By combination of this diluter de-sign and the rotating sampler, a con-tinuous online analysis system for ball

mill monitoring at the clay productionfacility has been achieved. The systemoperates continuously, day and night,

producing a measurement every 30sec. Currently, the data produced areused to more optimally respond to

changes in feed quality and to moni-tor the performance of the millingmedia in order to determine when

fresh material is required. (The ballsused to promote milling wear out overa period of weeks, reducing milling ef-ficiency, and therefore regular change-out is required.) A long term aim forthe facility is automated mill control,which is now a real possibility.

For alternative applications, the

size of bore, number of rotations of thesampler, volume to which the sampleis initially diluted, and number of

stages of the main diluter, could allbe altered to control the concentrationof solids at the optical head and meetthe requirements of the process. Thisapproach is therefore applicable to a

wide range of processes with hard tohandle slurries.

Edited by Rebekkah Marshall

AuthorsDavid Pugh is Europeanmanager for Malvern ProcessSystems, a division of MalvernInstruments Ltd. (EnigmaBusiness Park, GrovewoodRd, Malvern, Worcestershire,WR14 1XZ, U.K.; Phone: +44(0) 1684 892456; Fax: +44 (0)1684 892789; Email: [email protected]). He hasa B.S.ChE from the Univer-sity of Aston in Birmingham.

He joined Malvern Instruments Ltd. in 1990 assales manager in Europe for laboratory particlesizers, before working as business developmentmanager in the U.S. office of Malvern Instru-ments in Boston, Mass. Since returning to Mal-

vern UK six years ago, he has been in charge ofsales within the European process market.

Alain Blasco is technicalmanager for Malvern ProcessSystems, a division of Mal-

vern Instruments Ltd. (Email:A l a i n . B l a s c o @ m a l v e r n .co.uk). Alain joined MalvernInstruments SA (France) inthe 1990s, working first incustomer support, and laterdesigning new products inresponse to specific customerrequirements. He has first-

hand experience of online solutions. A foundermember of Malverns process department, Alainnow contributes to the development of process-interface solutions as well as working to enhancethe performance of measurement sensors.

Solids Processing


nov05solidsonline particle analysis in wet processes

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