Research30Filtration+Separation July/August 2005

Centrifuge focus:

do particle sizechanges affectacetate recovery?B art Peeters, Gert Callaerts, Benno Van den Bogaert and

Jan Crommen from Monsanto Antwerp (Belgium) advocatea detailed investigation of the effect of slurry feedconditions when optimising the performance of a solidbowl decanter centrifuge.

Background to the study

A great number of manufacturing processesdeal with the centrifugation of slurries ofsmall particles in a liquid medium. Thesesuspensions can originate from a chemicalreaction or from non-chemical reactions suchas suspensions in the food/beverages industry.

One particular sub-group of chemicalreactions are those in which a precipitationof a solid reaction product takes place from aliquid phase. Such precipitations arefrequently carried out in stirred tank

reactors* and there are many examples ofliquid phase reactions where solid productsare precipitated from solution, both inorganic and inorganic chemistry, e.g. salts,hydroxides, various organic products andpolymers.

For example, suspension polymerisation fromvinyl chloride monomer in water* results inthe precipitation of poly (vinyl chloride)resin grains of 100 - 180 µm in diameter thatcan be separated from the water by feedingthe slurry to a centrifuge. Another example

is the mass co-polymerisation of vinyl acetateand vinyl chloride, yielding a suspension thatcan be separated either by filtration orcentrifugation.*

When looking for opportunities to optimisethe performance of the centrifuges that dealsuch slurries, a more in-depth exploration ofthe slurry feed conditions should beconsidered. Without making any cost, andjust fine-tuning process parameters,improvements are possible from a productyield, product quality and environmental(less waste) point of view. This was thestarting-point of view for this particular casestudy, with a clear emphasis on a practicalapproach.

During chemical reaction, a suspension offine product particles is formed. Acetates arealso formed as by-product of this reaction. Atthe completion of the reaction, the resultingslurry is fed to a solid bowl decantercentrifuge where the batch liquor is separatedfrom the grains in order to recover theacetates. An internal washing is applied onthe conical beach to improve the recovery ofthe acetates.*

Previous research has proven that very smallchanges in the conditions of thecentrifugation can have an important effecton the recovery of the acetates. It has shownthat there is an optimum pool depth for the

Figure 1: How the experiment worked: by changing reaction conditions, slurries with different PSDs have beenproduced which were fed to the centrifuge. The centrifugation condition (like pool depth) remained constant.

Research 31Filtration+Separation July/August 2005

efficiency of the displacement wash takingplace on the beach of the centrifuge: a smalldifference of only 20 mm in pool depthresulted in a 5% increase in acetate yield anda related 20% reduction in the organic wastestream of the plant*.

As a logical continuation of this optimisationof the equipment, an extensive study wasperformed aimed at investigating the effectsof the product itself in the recovery ofacetates in an attempt to define optimisedproduct conditions. The centrifugationconditions themselves (like the optimisedpool depth, the pitch of the conveyer, thebowl speed and differential speed, theinternal washing and feed flow, etc) remainedconstant during the entire study.

As can be found in various studies*, thesignificant properties of the slurry are theparticle size, the shape of the particles, theporosity of the solids (or resulting cake), andthe density, viscosity and surface tension ofthe suspending medium. From thesecharacteristics, particle size can be consideredas the most important.

Figure 1 on page 30 shows the set-up of thisresearch. Different upstream conditions werechanged to affect the particle size of theslurry, such as the speed of agitation duringthe reaction. In total, 166 test-batches wereproduced. For all these batches, the particlesize distributions (PSDs) of the resultingslurries were measured as well as thecomposition of the cake coming out of thecentrifuge.

Particle size distributions of thegrains

The particle size distributions of all test-batches are shown in figure 2 (above.) The

particles can be considered to be relativelylarge because the particles are larger than 50 µm*. As can be seen, a wide range ofgrain sizes was produced. A preceding gaugerepeatability and reproducibility studyindicated that the variation in the height ofthe curves is caused by the variation for thecoarser fractions. A potential explanation isthe (de-) conglomeration of grains in themeasurement device. The mode (top of thecurve) itself is reproduced with highprecision and the same is true of the medianparticle size.

Figure 3 (below) shows the PSDs of twobatches that have been produced during thetests. As can be seen, the distributions skewto the right (mode < median < average)*.Because the curves are not ideal Gauss-curves, the parameter (D[0,50]-D[0,10]) isused as an alternative measure for the spread

of the distributions, with D[0,X0] indicatingthat X0 volume% of the grains has a particlesize smaller or equal to this value. Theparameter D[0,50] is called the medianparticle size. Using (D[0,50]-D[0,10]) meansthat the left (ideal) side of the distributions istaken as measure of the whole spread of thedistributions. Investigation shows that themeasure of spread of the particle sizedistribution increases exponentially if acoarser grain is produced (R² = 90%). This isshown in figure 4 on page 32.

Effect of particle size on torqueduring centrifugation

A clear correlation for R² (70%) – where R²

is the coefficient of determination – isestablished between the observed torqueduring centrifugation and the median particlesize of the slurries (see figure 5 on page 32.)

It is known that the torque developed by adecanter centrifuge has a number ofcomponents, including the torque to conveythe cake and overcome the frictional forcesinvolved. It seems that, in our case, a coarserpolymer grain and an inherent bigger spreadof the particle size distribution (see figure 4,page 32) result in a higher torque, indicatingthat small particles in between biggerparticles cause higher friction in the cake.

Effect of torque on acetate recovery

The data of the 166 test-batches were dividedinto torque classes, using torque as anindicator of particle size distribution. Theresults are shown in figure 6 on page 32. Datashown are the average losses of acetates (perunit of final product) remaining in the cake(+/- one standard deviation). The lower theacetate concentration in the cake, the morethe acetates are recovered via the effluentand the higher the recovery efficiency of thecentrifuge. A trend is observed that when

Figure 2: Particle size distributions of all 166 test-batches.

Figure 3: Particle size distributions (and the cumulative curves) of two batches that have been produced duringthe tests.

Research32Filtration+Separation July/August 2005

slurry causes higher torque duringcentrifugation, a lower acetate concentrationis observed in the cake.

All data at the different torque classes followa normal distribution, indicated by anAnderson-Darling test p-value of at least0.10. Homogeneity of variances is shown bya Bartlett's test p-value of 0.60 (this test isused in case of data at more than two levels,and for data following normal distributions).

First, in order to see if averages in acetateconcentration (at different torque levels) aresignificantly different, a one-way ANOVAtest was performed. An F-test p-value of 0.00indicates that at least one set of data issignificantly different from the data at theother torque classes. As well as this, aFisher’s pair wise comparisons table was madegiving the 95% confidential intervals for themean of a certain torque class minus themean of another torque class. When for acombination of means, such an intervalexcludes zero, the averages in acetateconcentration can be declared as different.This analysis has shown that the acetateconcentration in the cake is significantlylower for the higher torque classes comparedwith the lower torque classes*.

The results show that when the grains causehigher torque (higher friction), the resultingcake contains fewer acetates. Taking intoaccount the relationship that was foundbetween particle size and torque (figure 5,page 32), a possible explanation could befound in the grain size, with a coarser grainresulting in a better acetate recovery.

Effect of slurry PSD on acetaterecovery

The data of the 166 test-batches have beendivided into classes of the median particlesize and expressed relative versus thestandard process median particle size. Theresults are presented in figure 7 on page 33.Data shown represent the average losses of

acetates (per unit of final product) remainingin the cake (+/- one standard deviation). Atrend is observed with a bigger particle sizecorresponding with a lower loss of acetatesvia the cake.

All data at different median classes follow anormal distribution, indicated by anAnderson-Darling test p-value of at least0,12. The homogeneity of variances is shownby a Bartlett’s test p-value of 0.29. To see ifaverages in acetate concentration (atdifferent median levels) are significantlydifferent, a one-way ANOVA test wasperformed: an F-test p-value of 0.12 indicatesthat there is not quite sufficient evidence toclaim that not all the means of acetateconcentrations (at the different particlesizes) are equal*.

The results show a trend that a coarser grain(and inherent bigger spread of grains) is infavour of a lower loss of acetates via thecake. However, the data are not extensive

enough for this to be stated as a generalprinciple.

Effect of temperature on acetaterecovery

One of the upstream conditions that waschanged to alter the particle size distribution(and the properties of the suspension ingeneral) was the temperature of the reactantsthat are charged in the reactor.

The data of all test-batches were divided intotemperature classes and expressed relativeversus the standard process temperature. Theresults are presented in figure 8 on page 33.The data shown represent the average lossesof acetates (per unit of final product)remaining in the cake (+/- one standarddeviation). It can be seen that a lowertemperature results in a lower acetateconcentration in the cake.

All data at the different temperature classesfollow a normal distribution, indicated by an

Figure 4: (D[0,50]-D[0,10]) as a function of D[0,50] showing that a coarser grainresults in a bigger spread of the grains (N=166).

Figure 5: Torque during centrifugation as a function of the median grain size ofthe slurries (N=166).

Figure 6: Amount of acetates lost via the cake (per unit of cake dryness) as a function of the torque duringcentrifugation. Averages +/- SD are shown (N=166).

Research 33Filtration+Separation July/August 2005

Anderson-Darling test p-value of at least0.15. Homogeneity of variances is shown by aBartlett's test p-value of 0.77. First, in orderto see if averages in acetate concentration (atdifferent temperatures) are significantlydifferent, a one-way ANOVA test wasperformed: an F-test p-value of 0.00 indicatesthat at least one set of data is significantlydifferent from the data at the othertemperature classes. Furthermore a Fisher’spair-wise comparisons table was made givingthe 95% confidential intervals for the meanof a certain temperature class minus themean of another temperature class. This tablehas shown that the acetate concentration inthe cake is significantly lower in case of lowertemperatures of the reactants that arecharged in the reactor.

If we just compare the data at the referencetemperature (average of 6.1 units loss) andthe data for the 4.5% reduced temperature(5.6 units loss) we see a statisticallysignificant lower loss for the lowertemperature (pooled t-test p-value = 0.01)*.

A lower temperature in the early stages of thechemical reaction (when reactants arecharged to the reactor) results in a slurry thatyields a lower loss of acetates via the cake ofthe centrifuge. A reduction of thistemperature with 4.5% compared with thestandard applied temperature results in areduction of the acetate losses from 6.1 unitsto 5.6 units (see figure 8, right) which is areduction of 8% of the remaining losses inthe cake.

Conclusion

The recovery of acetates from a suspensionwith an industrial solid bowl decantercentrifuge has been studied experimentally inthe field. The particle size distribution of theslurry feed has been affected by changes in

upstream (reaction) conditions, whereas thecentrifugation parameters remainedunchanged during the study.

It was observed that slurries causing highertorque (i.e. friction) during centrifugationyield a higher recovery of acetates. Becausea clear correlation was established betweena higher torque and a bigger median particlesize of the slurries, this could indicate thatthe improved recovery (at higher torque) iscaused by bigger grains (and an inherentbigger spread of the grains). Although thisis confirmed by a visual trend, the analysisfailed to prove a statistically significanteffect of grain size on acetate recovery.

It can be concluded from this study that theseparation efficiency of the centrifuge isaffected by the torque that the grains cause

during centrifugation. A higher torqueresults in a 1-2% higher yield. Besides theparticle size, it seems that also anotherparticle-related parameter (such as, forinstance, the porosity or the shape of thegrains) influences both torque and therecovery of the acetates.

It can be expected that this particle-relatedcharacteristic is affected by the initialtemperature of the chemical reaction,because this temperature affects the acetaterecovery. But this needs further investigation.Finally, a new feed condition (e.g. reducedinitial reaction temperature) was defined on acontinuously manufacturing basis.

Based on our findings, we recommend otherprocess engineers dealing with the same kindof centrifuge applications to perform similarstudies: it takes a lot of effort, but it pays itsway. •*For references, please contact the editor.

Contact:Bart Peeters, Gert Callaerts, Benno Van denBogaert and Jan Crommen1 Monsanto Europe NV e-mail: bart.peeters@monsanto.com;gert.callaerts@monsanto.com

[NB: The authors would like to thank alloperators from the plant for taking the hugenumber of slurry and cake samples. They alsogratefully acknowledge the support of theMonsanto Antwerp lab in analysing all thesamples.]

Figure 7: Amount of acetates lost via the cake (per unit of cake dryness) as a function of the median particlesize. Averages +/- SD are shown (N=166).

Figure 8: Amount of acetates lost via the cake (per unit of cake dryness) as a function of the temperature of theinitially charged reactants into the reactor. Averages +/- SD are shown (N=166).