Chemical Engineering Science 63 (2008) 5410 -- 5419

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Chemical Engineering Science

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Measuring theparticle sizeof aknowndistributionusing the focusedbeamreflectancemeasurement technique

David Greavesa, John Boxalla, James Mulligana, Alberto Montesib, Jefferson Creekb, E. Dendy Sloana,Carolyn A. Koha,∗

aDepartment of Chemical Engineering, Colorado School of Mines, 1600 Illinois Street, Golden, CO 80401, USAbChevron Energy Technology Company, 1600 Smith Street, Houston, TX 77002, USA

A R T I C L E I N F O A B S T R A C T

Article history:Received 17 December 2007Received in revised form 4 July 2008Accepted 24 July 2008Available online 29 July 2008

Keywords:Particle size analysisGas hydratesAgglomerationEmulsionInstrumentationParticle

The accuracy of the focused beam reflectance measurement (FBRM) probe, which measures a chord lengthdistribution, from Mettler–Toledo Lasentec� has been explored. A particle video microscope (PVM) probe,which provides in situ digital images, was used as a direct visual method to test the reliability of the FBRMresults. These probes can provide in situ particle characterization at high pressures. The FBRM has beenused to study emulsions and ice and clathrate hydrate formation. The ability of the FBRM to accuratelycharacterize unimodal and bimodal distributions of particles and droplets and to measure agglomerationevents was investigated. It was found that while the FBRM can successfully identify system changes,certain inaccuracies exist in the chord length distributions. Particularly, the FBRM was found to oversizeunimodal distributions of glass beads, but undersize droplets in an emulsion and was unable to measurefull agglomerate sizes. The onset of ice and hydrate nucleation and growth were successfully detectedby the FBRM, but quantitative analysis of the particle and agglomerate sizes required simultaneous PVMmeasurements to be performed.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Accurate particle size analysis is a key to study hydrate for-mation from a water-in-oil (W/O) emulsion. During the hydrateformation process it is proposed that two main phenomena occurin which size information is important: (1) water droplets in theemulsion crystallize to form hydrate shells (the inner core of theshells being unconverted water), and (2) these shells aggregate toform larger hydrate agglomerates (Palermo et al., 2005; Turner,2005). For more information about hydrate formation and agglom-eration the reader is referred to Sloan and Koh (2008). Two particlesize measurement techniques have been implemented to investi-gate hydrate growth and agglomeration: a focused beam reflectancemeasurement (FBRM) probe and a particle video microscope (PVM)probe. The purpose of this study is to determine how well the FBRMcan measure the original water droplet size in the emulsion, detecthydrate nucleation and agglomeration, and measure the actual ag-glomerate size, including the measurement of a potential bimodaldistribution between distinct particles and large agglomerates.

* Corresponding author. Tel.: +13032733237; fax: +13032733730.E-mail address: ckoh@mines.edu (C.A. Koh).

0009-2509/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2008.07.023

The FBRM probe used in this study is a Mettler–Toledo Lasentec�

D600X particle size analyzer. A Class I, 3mW laser with wavelength791.8nm (near infrared) is transmitted through fiber-optics to theprobe tip. A rotating optical lens at the probe tip deflects the laser asshown in Fig. 1a. When the probe is inserted into a system of dropletsor particles, the laser emitted is reflected if it scans across the surfaceof a particle, as shown in Fig. 1b. The probe measures the reflectancetime and determines the chord length by the product of the timeand the laser scan speed (scan speed in this work is 4m/s, but itcan range from 2 to 16m/s). For more information on the probe andthe technique the reader is referred to the FBRM Hardware Manual(Mettler–Toledo Lasentec�, 2001).

Previous studies have focused on evaluating the chord lengthdistribution measured using the FBRM technique compared with al-ternative sizing techniques, such as laser diffraction and microscopy(Heath et al., 2002; Li et al., 2005; Pons et al., 2006). A complementaryparticle size analyzer, the PVM, also fromMettler–Toledo Lasentec�,was used in this work. The PVM device was installed in the samethermostated reaction vessel with the FBRM probe at a 180◦ orien-tation (for experimental set-up see description below and Fig. 5).Unlike most alternative measurement techniques, the PVM can beused simultaneously with the FBRM. The PVM consists of six laserswhich illuminate a small area in front of the probe face (Fig. 2a).The probe records digital images of the illuminated area with

D. Greaves et al. / Chemical Engineering Science 63 (2008) 5410 -- 5419 5411

FBRMDetector

Laserdiode

Fiber optic

Beam splitter

Laser beam

Sapphirewindow

Optic rotatingat a fixed highvelocity

Probeat approx.45 angle to turbulentwell-mixed flow.

Laser Beam

Scan Direction

Typical Particle

Intensity Profile

Chord Length

IR

t0 t1 t

Fig. 1. (a) The focused beam reflectance method (FBRM) probe technique. (b) Measurement of a particle chord length using the FBRM technique (figure provided byMettler–Toledo Autochem, Inc.).

Fig. 2. (a) Particle video microscope (PVM) probe with six illuminating lasers. (b) Example images given by the PVM probe (top: water-in-oil emulsion, bottom: hydrateparticle suspension).

a field of view of 826�m × 619�m. Droplets and particles largerthan 20�m can be clearly identified and measured from the PVMimages in these experiments; below this size scale, it can be difficultto distinguish individual drops (Fig. 2b). For more information onthe probe and the technique the reader is referred to the PVM UserManual (Mettler–Toledo Lasentec�, 2002).

Li et al. (2005) compared the FBRM technique with three al-ternative sizing techniques, including laser diffraction (using aMastersizer S), ultrasonic attenuation, and image analysis of mi-croscope photographs. Their study concluded that the chord lengthdistribution (CLD) measured by the FBRM technique is complex,depending on not just the size distribution of the particles butalso on the optical properties and shape, especially for very smallparticles (<20�m) where backscattering becomes important. A

similar study by Heath et al. (2002) analyzed the effects on themeasured FBRM CLD of weighting the distribution, adjusting thefocal point, and varying the agitation speed, the particle properties,and the suspension volume fraction. Heath et al. (2002) concludedthat the mean of square-weighted chord lengths, for objects inthe size range of 50–400�m, compares well with the actual meandroplet diameter obtained using other (conventional) sizing tech-niques. They also suggest applying length and cube weightings tobetter model smaller (<50�m) and larger (>400�m) particles, re-spectively. Work by Pons et al. (2006) also helped to elucidate therelationships between particle size, shape, and the CLD in order tomake the best use of data from the FBRM distributions. In the workof both Heath et al. (2002) and Pons et al. (2002) a linear relation-ship was reported between the median size of the sieve used to

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obtain the particles studied and the mean chord length found usingthe FBRM.

The FBRM probe measures chord lengths, grouped in a chordlength distribution, rather than the actual sizes (i.e. diameters); thusthe CLD must be transformed into a particle size distribution (PSD)to model the actual surface area for hydrate formation. The subjectof transforming or unfolding a CLD into a PSD has gained muchattention, and multiple mathematical unfolding techniques exist totranslate the CLD into its corresponding PSD (Langston et al., 2001;Ruf et al., 2000; Wynn, 2003).

The ability of the FBRM measurement to characterize particlesize is of interest in this work and not the unfolding method; thusthe direct approach (PSD → CLD) is applied and compared with themeasured FBRM CLD. The measured PSD acquired from analysis ofthe PVM images or by microscopy is used to create an appropriateCLD. The calculated CLD (from the measured PSD) can then be com-pared with the CLD measured with the FBRM probe. The mathemat-ical expression for a chord length distribution for spherical particlesis given in Eq. (1) (Ruf et al., 2000). A theoretical chord length dis-tribution for each system was generated by inputting the measuredparticle size distribution into Eq. (1):

q(s,dp) = s

dp√d2p − s2

for 0 < s < dp, (1)

where s is a chord length, dp is the actual particle diameter, andq(s,dp) is the relative probability of obtaining a chord s given thediameter dp.

Known size distributions in the size ranges 25–45�m (small),45–75�m (medium), and 125–250�m (large) were used to investi-gate the effects of the experimental conditions and fluid propertieson the FBRM results for unimodal distributions. Three bimodal dis-tributions were also studied using the FBRM and PVM to identifysystem behavior when two distinct (though perhaps overlapping)distributions are present in the system. Using the FBRM to distin-guish bimodal systems is invaluable for monitoring agglomeratingor flocculating systems where individual droplets or particles ag-glomerate while some droplets or particles remain dispersed. Forexample, a bimodal distribution may appear during hydrate forma-tion and agglomeration if large aggregates in the system co-exist

200 µm 200 µm 200 µm

50 µm 50 µm 100 µm

Fig. 3. Images of the different particle size ranges used in this study: Microscope—(a) 25–45�m particles at 20× magnification, (b) 45–75�m particles at 20× magnification,(c) 125–250�m particles at 10× magnification; PVM (1.3 vol%)—(d) 25–45�m, (e) 45–75�m, (f) 125–250�m.

with unconverted droplets and singular hydrate particles. The PVMprobe provides a visual confirmation of the bimodal dispersion.

Two FBRM probe adjustments were considered for this study:changes to the focal point setting and to the electronics discrimina-tion setting. The focal point of the FBRM laser can be adjusted intothe fluid (+) or inside the probe (−) with the standard FBRM focalpoint position being −20�m (Mettler–Toledo Lasentec�, 2004). Thestandard focal point setting was used at all times unless otherwisenoted. Heath et al. (2002) explored the impact of the focal pointposition on the FBRM measurements, observing that adjusting thefocal point more into the fluid increased the number of measuredlonger chords due to larger particles being less able to approach themeasurement window.

The electronics setting can be switched during operation betweenthe fine and the coarse discrimination settings. Turner (2005) sug-gested that the coarse setting would be “more sensitive to agglom-erate sizes than primary particle sizes” compared to the fine setting,bypassing less detectable edges. Heath et al. (2002) also suggest us-ing the coarse setting for flocculated systems. The fine setting wasused in these experiments unless noted.

2. Experimental procedure

Trelleborg Fillite� cenospheres were used for all unimodal andbimodal distribution experiments. Cenospheres are hollow micro-spheres of low density (<0.8 g/ml) commonly used as fillers in emul-sions, epoxies, cements, etc. These alumino-silicate spheres have lowoil adsorption (<16goil/100g) and low reactivity, making them idealfor particle size analysis (Trelleborg, 2007). The cenospheres weresieved to make the three standard distribution group sizes: small,medium, and large.

Images of these distributions of particles, shown in Figs. 3a, b,and c, were recorded using a Carl Zeiss Axiovert 100 inverted lightmicroscope and sized using ImageJ, an image analysis program. Thedistributions measured from the microscope images (>300 particlesmeasured) shown in Fig. 4 confirmed that 99% of the particles wereless than the maximum particle size according to the sieve meshsize used. With the exclusion of the unmeasurable fine particles, thedistributions show reasonable agreement with the expected particlerange from sieving, suggesting sufficient sieving. Figs. 3(d–f) give

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PVM images for the same particles given in a–c with a concentrationof 5 vol%.

Fig. 4 shows the comparison between the particle size (diame-ter) distributions measured using the PVM with 5vol% of the parti-cles in water with those distributions obtained using the microscopewith the particles dispersed on a glass slide. In both cases more than300 particles were sized to obtain the distribution. This comparisonshows that the PVM provides a good representation of the parti-cle size distribution; the only major observed difference is that theparticle sizes measured with the PVM are slightly larger than thosemeasured using the microscope (with 4.3–7.3% difference in theirmeans).

Single mode size distribution studies were performed by mix-ing 0.23vol% increments of the desired particle size into 300ml ofdeionized (DI) water in the beaker test apparatus shown in Fig. 5a.The beaker is an open top 9 cm deep, 9 cm diameter cylinder. Baffles(not shown) were used to limit gas entrainment. The dispersion wasformed incrementally to test the FBRM response to increasing par-ticle additions. The system was mixed at 400 rpm by a four-pitchedblade impeller, each blade having a length of 2.4 cm. The FBRM andPVM probes were inserted into the fluid at approximately 80◦ an-gles to ensure proper sampling in the system, given the location ofthe pitched blade impeller. The images generated by the PVM andthose from the microscope were used to generate a comparison forthe chord length data provided by the FBRM.

Fig. 4. Comparison of the particle size distributions measured using the PVM andthe microscope.

FBRMPVM

FBRM

Oil

Water

Submersed in water/glycol

bath

Fig. 5. Apparatus set-up for the FBRM/PVM experiments: (a) beaker test apparatus set-up; (b) temperature controlled ice/hydrate formation apparatus (Greaves, 2007).

For the bimodal distributions, the first distribution size group wasadded incrementally up to 1.37vol% particles before a second particleset was added incrementally in the same manner to create a bimodaldistribution. Two bimodal systems were initially studied: 45–75�mwith 125–250�m added and 125–250�m with 45–75�m. A thirdbimodal system was studied to test the response using more sepa-rate, distinct distributions: 25–45�m with 125–250�m added. Theexperiments were performed using both the fine and coarse FBRMprobe setting. Additionally, three focal point positions on the FBRMwere used to see if changing the position improved detection of thebimodal distributions: +50�m into the fluid, −20�m behind theglass (standard focal position), and −90�m behind the glass.

A study of tetrahydrofuran (THF) hydrate and ice formation wasperformed using the FBRM and PVM to evaluate the ability of theFBRM to detect and measure agglomeration. An emulsion of 10vol%water dispersed in crude oil and mixed in the reaction vessel (shownin Fig. 5b) at 400 rpmwas used to form ice. The FBRMand PVMprobesin Fig. 5b were again positioned 180◦ across from each other but atan insertion angle of 45◦ to the vertical. Two 6 flat blade turbineimpellers were used to emulsify the water into the oil. An internalthermocouple was used to measure the vessel temperature, whilebaffles were again used to prevent gas entrainment. The emulsionwas cooled to−10 ◦C and held at this temperature until ice nucleated.

THF hydrate was formed in a similar manner in the same reac-tion vessel; however, the fluids differed with the addition of THF.The water phase was a 19wt% THF in water mixture, which is thestoichiometric mixture for complete conversion of water to hydrate(complete large cage occupancy with THF). The crude oil was pre-mixed with THF tomake a 60:40 crude-THFmixture. This pre-mixingwas performed to lower the driving force for the transfer of THF fromthe water to the oil phase. In this case the emulsion was cooled to−1 ◦C to form THF hydrate. THF–water forms sII hydrate at ambientpressures below 4.4 ◦C. Temperature measurements during dissoci-ation confirmed that hydrate, not ice, had formed from the THF sys-tem. Temperature measurements were also used to identify the nu-cleation points for the ice and the THF hydrate (in which formationgives rise to exotherms).

3. Discussion and results

3.1. Single size distributions

A comparison of the particle size distribution from the PVM andthe chord length distribution using the FBRM probe is given in Fig. 6.It is interesting to note that the FBRM chord length cumulativedistribution extends well beyond the size of the particles actually

5414 D. Greaves et al. / Chemical Engineering Science 63 (2008) 5410 -- 5419

Fig. 6. Comparison of the particle size distributions from the PVM and the chord length distributions of the FBRM: (a) 25–45�m particles (small); (b) 45–75�m particles(medium); (c) 125–250�m particles (large). Calculated CLD from PVM-PSD using Eq. (1).

Fig. 7. FBRM CLD and PVM PSD comparison with Conroe crude oil as the continuous phase: (a) 45–75�m cenosphere particles (1.4 vol%) in crude oil and in DI water(1.4 vol%) for comparison; (b) water-in-oil emulsion at different water cuts and temperatures.

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in the system for the two smaller distributions. Chords should bea fraction of the particle size; hence, there should be no chordsabove the maximum particle diameter in the system. Additionally,the small size end of the FBRM-CLD is exaggerated compared withthe theoretical CLD for the particles present in the system. Such an

Fig. 8. Comparison of the 25–45�m cenosphere particles in both the beaker appa-ratus (shown in Fig. 5a) and the ice/hydrate formation apparatus (shown in Fig. 5b).

Fig. 9. Total, small, and large bin size FBRM chord count response: (a) medium (45–75�m) added to large (125–250�m) particles; (b) large (125–250�m) added to medium(45–75�m) particles; (c) large (125–250�m) added to small (25–45�m) particles. First particle size added incrementally up to 1.37vol%, after which the second particlesize is added.

exaggeration may be due to the detection by the FBRM of smallerparticles not measurable from the PVM images used to generate thecalculated CLD.

Fig. 7 shows two distributions in a crude oil; the first is a dis-persion of 1.6 vol% cenospheres, the second is a W/O emulsion of10 and 20vol% water. The CLD measured for the cenosphere parti-cles (Fig. 7a) shows the same over-sizing as in the water systems.However, the CLD measured for the water-in-oil emulsion (Fig. 7b)shows that the droplet size distribution, measured using the PVM, isvastly undersized by the FBRM technique. The PVM measured sizerange of the particles in Fig. 7a and the droplets in Fig. 7b is simi-lar; however, two key experimental variables differ—the refractiveproperties of the dispersed droplets/particles and the volume frac-tion of the dispersed phase (1.6 vol% for the particles, 10 and 20vol%for the droplets). Such a difference between FBRM results in Fig. 7,despite similar size scales measured from the PVM images, suggeststhat any calibration of the FBRM may be highly dependent on phys-ical properties such as refractive index and also dispersion quantity.

It is also important to determine whether the two different appa-ratuses (given in Fig. 5) have an effect on the resulting size distribu-tions measured. Fig. 8 compares the FBRM chord length distributionsand the PVM particle size distributions measured in both appara-tuses for a dispersion of 6.5 vol% cenosphere particles (25–45�msieve size) in DI water. The results show that the FBRM and PVMmeasured sizes are similar in the two apparatuses, respectively.

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Fig. 10. PVM and FBRM distribution comparison for (a) medium (45–75�m) added to large (125–250�m) particles; (b) large (125–250�m) added to medium (45–75�m)particles; (c) large (125–250�m) added to small (25–45�m) particles.

3.2. Bimodal distributions

Fig. 9 shows the change in total measured chord counts for thethree bimodal distributions on the FBRM fine setting on a volumepercent basis. Results were similar using the coarse electronics set-ting (not shown).

Fig. 9a demonstrates that the total counts (solid circles) in-crease as particle additions increase, as expected. In this experimentmedium-sized particles (45–75�m) were added to large particles(125–250�m). The chord counts increase first as large particles aloneare added and then increase more rapidly as medium particles areadded. The curvature change, switching from large to medium par-ticles (>1.37vol%), is expected as the smaller size additions willhave more particles than the larger particle distribution per volumepercent.

In contrast, Fig. 9b shows an unexpected behavior. As large par-ticles are added to the system of medium particles (after 1.4 vol%),the chord counts first drop very slightly, even though there are moreparticles now in the system for the FBRM to detect, before the countsagain rise. The drop in counts is more pronounced in Fig. 9c whenlarge particles are added to small particles (after 1.4 vol%). In fact,for the third bimodal distribution shown in Fig. 9c the counts neverrecover but drop further with each large particle size addition.

Binning or grouping the FBRM chord data into various bin sizeshelps to explain why the FBRM appears to lose counts (demonstratedby both Figs. 9b and c) when larger particles are added to smaller

particles. As seen in Fig. 9b, at the point when total counts drop(coincident with the first addition of larger particles), the numberof small bin size counts (25–50�m) starts to fall, and the numberof large bin size counts (125–300�m) starts to rise. This behavior iseven more apparent when the two distributions are more distinctas demonstrated in Fig. 9c when 125–250�m particles are added to25–45�m particles. The drop in counts for small particles and rise incounts for large particles in Figs. 9b and c suggest that there is somepreferential measurement of the large particles which precludes anaccurate measurement of the small particles.

Two proposed effects may explain the FBRM response to bimodaldistribution formation:measurement time and hydrodynamics. First,the drop in counts when larger particles are added to smaller parti-cles may be attributed to the time requisite to measure large parti-cles within a given measurement (Hukkanen, 2007). Essentially, theFBRM can only scan so many particles per measurement cycle, soif the probe is measuring a large particle that is twice the size ofthe smaller particles, then the time cost of the laser beam travel-ing across that single large particle to give one count can be up to2 counts of the smaller particles. Hence, while the number of parti-cles in the system increases, the measured number of particles, re-lated to the total measured counts, can decrease. This effect wouldbe more pronounced for the more distinct particle distributions inFig. 9c (125–250�m added to 25–45�m) where a measurement of alarge particle would effectively decrease more overall small particlemeasurements.

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Fig. 11. Agglomeration images from the PVM: (a) water and ice droplets dispersed in Conroe oil 40min after nucleation; (b) ice agglomerates 4h after nucleation; (c) THFhydrate agglomerate 15min after nucleation; (d) THF hydrate 4h after nucleation.

Second, hydrodynamics may promote the measurement of smallparticles at the sacrifice of large particles because small particles maymore easily approach and pack at the FBRM probe window (Heath,2002). Hence, when the medium particle size was added to the largeparticle size, the large counts decreased (Fig. 9a). The combined ef-fects of measurement time and hydrodynamics may explain the lo-cal minima around 2vol% in the total counts in Fig. 9b when largeparticles are added to medium particles. The two effects may com-pete such that there is little change observed in the total counts withlarge particle additions. In fact, both proposed effects may impactall size distribution measurements with the FBRM. The dominant ef-fect will depend on the change to the system (small added to largeparticles or vice-versa) and the similarity in distributions.

Fig. 10 shows the FBRM distribution and PVM measurements forthe three systems in Fig. 9 for three different additions: 1.4 vol%of the original size (large 125–250�m, medium 45–75�m, small25–45�m), addition of a further 1.4 vol% of the different size (cor-responding to 2.8 vol% in Fig. 9c), and 1.4 vol% of the original with4.4 vol% of the different size (corresponding to the final point inFigs. 9a–c).

The qualitative trends shown in the PVM measured size distribu-tions in Fig. 10 agree with the expected result; i.e. an increase in thecumulative frequency of medium sized particles in Fig. 10a, and a de-crease in the cumulative frequency of medium/small sized particlesin Fig. 10b/c. The corresponding FBRM distribution, at least qualita-tively, shows the same trends as the PVM measurements (increasein smaller chords in Fig. 10a, increase in larger chords in Fig. 10band c).

Quantitatively, the PVM favors the measurement of larger parti-cles. For the system shown in Fig. 10a the actual proportions of par-ticles with 1.4 vol% medium-sized particles added to 1.4 vol% largeparticles in the system comprise a 92%medium and 8% large bimodalparticle system. The PVM results showed that 11% of the total parti-cles counted were large for this system (approximately 450 particleswere counted). With 4.4 vol% medium-sized particles added, the ac-

tual proportion of large particles falls to 2%, whereas the PVM resultsgave 6% of the total particles counted were large. The discrepancyin counted large particles (from the PVM) to actual number of par-ticles added is similar for the other two systems. In Fig. 10b (largeadded to medium) the proportion of large particles is 8% (actual)compared with 11% (PVM) for 1.4 vol% large in 1.4 vol% medium, and18% (actual) compared with 29% (PVM) for 4.4 vol% large in 1.4 vol%medium. In Fig. 10c (large added to small) the proportion of largeparticles is 2% (actual) compared with 5% (PVM) for 1.4 vol% large in1.4 vol% small, and 7% (actual) compared with 21% (PVM) for 1.4 vol%large in 4.4 vol% small. This shows that there was a preferential mea-surement of the large particles using the PVM (as the proportionof large particles measured always exceeded the proportion of totallarge particles in each system).

3.3. Hydrate agglomeration study

Both the ice formation and THF hydrate formation showed a sig-nificant degree of agglomeration after nucleation. Typical PVM im-ages of the agglomerates are shown in Figs. 11a and b for the ice andFig. 11c and d for the THF hydrate.

These images show distinctly different agglomeration behavior.The ice appears to maintain its droplet shape forming “grape-like”clusters, while the THF hydrate forms more amorphous agglomer-ates.

The FBRM qualitatively detects the ice and hydrate nucleationpoints, perfectly coincident with the exothermic temperature re-sponse. Figs. 12a and b are plots of the FBRM total chord count andmean square weight statistics before and after nucleation. Figs. 12cand d show the FBRM distributions at different points during theice/hydrate formation process, respectively: pre-formation (thestarting cooled emulsion), soon after nucleation (15min after the ex-othermic peak is detected), and after agglomeration (4h after nu-cleation).

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Fig. 12. Total chord count FBRM statistics and mean square weight for (a) ice nucleation and (b) THF hydrate nucleation. The vertical dotted line indicates the time at whichnucleation occurs. FBRM chord length distributions for (c) ice formation and (d) THF hydrate formation.

Fig. 13. (a) PVM/FBRM comparison for ice formation before nucleation and 7h after nucleation, (b) typical PVM image used to create ice size distributions.

The plots in Fig. 12 reveal several key aspects of agglomerationdetection using the FBRM. First, the FBRM can be effectively used todetect hydrate and ice nucleation. The total chord counts increaseand the mean square weight rises with ice nucleation in Fig. 12a. THFhydrate nucleation is more dramatic with the mean square weight

rising immediately and the total chord count initially decreasingrapidly as in Fig. 12b.

Second, the FBRM can distinguish the extent of ice and hydrateagglomeration. The THF hydrate system fully agglomerates in justover 15min, as can be seen in the mean square weight spiking to

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approximately four times its initial value in the first 3min after nu-cleation then returning to a value approximately double the initialvalue after 15min and remaining relatively constant over the re-mainder of the experiment. The similarity between the 15min andthe after 4h FBRM distributions in Fig. 12d also confirms the imme-diate agglomeration in the system (close to 15min). PVM images inFigs. 11c and d further confirm the existence of large agglomerateswithin 15min of nucleation.

Conversely, the FBRM distribution for ice formation in Fig. 12cchanges gradually, with only a slight increase from before to afternucleation for the first 15min after nucleation. The mean squareweight (Fig. 12a) continuously increases throughout the first 4h afternucleation. After this point themean square weight does not increasein value any further, except for a few fluctuations. The PVM imagesalso show that there is very little additional agglomeration after 4hand only large agglomerates, like in Figs. 11b and 13b, are presentin the system. Such an interpretation of the FBRM results duringice and hydrate formation is only made possible with the in situ(simultaneous) visualization provided by the PVM.

The FBRM cannot be used to measure the actual size of the ag-glomerates. Despite recognizing a qualitative change following hy-drate and ice formation, the FBRM shows no chords within the rangeof the actual agglomerate sizes observed in the PVM images. Themean square weight more than doubles for both ice and hydrateformation (Figs. 12a and b); there is also a clear broadening of thedistribution (Figs. 12c and d), but there are no measured chords nearthe size scale of the agglomerates.

Fig. 13a provides a comparison between the ice PVM sizes mea-sured and the corresponding FBRM distribution. Thirty PVM images(with 1–3 agglomerates per image) were used to create the distri-bution for the ice agglomerates measured in both the vertical andhorizontal directions through the middle of the particle (as shown inFig. 13b). The comparison in Fig. 13a shows a significant change inthe sizes measured compared with the initial droplets, but the FBRMonly gives a slight increase in size and does not measure sizes closeto the actual agglomerate sizes (225–580�m). A similar compari-son was not possible for the THF hydrate as all agglomerates weremuch larger than the field of view of the PVM probe, however, themaximum size measured via the FBRM was only 200�m. The initialdroplet sizes are also underestimated by the FBRM compared to thePVM.

This severe underestimation of hydrate agglomerate size by theFBRM could be due to several factors. First, some agglomerates arelarger than the maximum measurable size of the FBRM (1000�m).Second, and most likely the major reason, the FBRM may be detect-ing only smaller portions of the agglomerate surface rather thanthe whole agglomerate. The FBRM does undersize the droplet dis-tribution as discussed above, but the increase in PVM median sizemeasurements from droplets (63�m) to ice agglomerates (333�m)is over an order of magnitude greater than the increase in the FBRMdistribution median (4.8–6.8�m). Both the THF hydrate and ice ex-periments were repeated using the coarse FBRM setting to investi-gate whether the FBRM could measure the chord lengths equivalentto the actual size of the agglomerates using the coarse setting. Itwas believed that by using the coarse setting, smaller portions ofthe agglomerates (like bumps and crevices) would be ignored andthe whole agglomerate would be measured. Such was not the casefor either the ice or the hydrate agglomerates as the largest chord

size measured with the FBRM did not increase when the probe wasswitched to the coarse setting.

4. Conclusions

The applicability of the FBRM probe for typical measurements ofemulsion and hydrate characterization has been studied. FBRMmea-surements tested include droplet and agglomerate size, the detec-tion of hydrate nucleation, and the response to a potential bimodalsystem. It was found that the FBRM can precisely detect systemchanges, such as hydrate or ice nucleation, or the introduction of adifferent particle size. However, certain limitations exist in using theFBRM: (1) the measured chord length distribution can be mislead-ing in terms of the actual particle size and a calibration specific tothe system of interest is often required, (2) in bimodal systems largeparticles will often diminish the ability of the FBRM to quantitativelymeasure small particles, (3) for rough hydrate or ice particles theFBRM is unable to measure full agglomerate size.

The FBRM response to droplets or particles can be widely differ-ent. For example in this work, the FBRM measured size � PVM forthe systems containing glass particles, but the FBRM measured size� PVM for droplets in an emulsion. The FBRM may be used to mon-itor hydrate events, but a secondary tool like the PVM is necessaryto understand these changes and provide accurate characterizationof the hydrate agglomerates.

References

Greaves, D., 2007. Hydrate formation from high water content emulsions. M.S. Thesis,Colorado School of Mines, Golden, CO.

Heath, A.R., Fawell, P.D., Bahri, P.A., Swift, J.D., 2002. Average particle size byfocused beam reflectance measurement (FBRM). Particle and Particle SystemsCharacterization 19, 84–95.

Hukkanen, E., 2007. Personal communication. Technology and ApplicationConsultant, Mettler–Toledo AutoChem, Inc. Golden, CO, 24 January 2007.

Langston, L.P.A., Burbidge, A.S., Jones, T.F., Simmons, M.J.H., 2001. Particle anddroplet size analysis from chord measurements using Bayes' theorem. PowderTechnology 116, 33–42.

Li, M., Wilkinson, D., Patchigolla, K., 2005. Comparison of particle sizedistributions measured using different techniques. Particle and Particle SystemsCharacterization 23, 265–284.

Mettler–Toledo Lasentec� Product Group, 2001. Lasentec� D600 Hardware Manual.Mettler–Toledo AutoChem, Inc., Redmond, WA.

Mettler–Toledo Lasentec� Product Group, 2002. Lasentec� PVM User Manual.Mettler–Toledo AutoChem, Inc., Redmond, WA.

Mettler–Toledo Lasentec� Product Group, 2004. FBRM Control Interface User'sManual, V. 6.0 (003-16-1 Rev. J). Mettler–Toledo AutoChem, Inc., Columbia, MD.

Palermo, T., Arla, D., Borregales, M., Dalmazzone, C., Rousseau, L., 2005. Study ofthe agglomeration between hydrate particles in oil using differential scanningcalorimetry (DSC). Proceedings of 5th International Conference on Gas Hydrates,vol. 1. pp. 332–339.

Pons, M.-N., Milferstedt, K., Morgenroth, E., 2006. Modeling of chord lengthdistributions. Chemical Engineering Science 61, 3962–3973.

Ruf, A., Worlitschek, J., Mazzotti, M., 2000. Modeling and experimental analysis ofPSD measurements through FBRM. Particle and Particle Systems Characterization17, 167–179.

Sloan, E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gas. third ed. CRCPress–Taylor and Francis Group, Boca Raton, FL.

Trelleborg, 2007. Advantages of Fillite� Products. 〈http://www.fillite.com/advantages〉 (accessed 21 June, 2007).

Turner, D., 2005. Clathrate hydrate formation in water-in-oil dispersions. Ph.D. Thesis,Colorado School of Mines, Golden, CO.

Wynn, E.J.W., 2003. Relationship between particle-size and chord-lengthdistributions in focused beam reflectance measurement: stability of directinversion and weighting. Powder Technology 133, 125–133.