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DESCRIPTION OF RAW MEAL BLENDING IN THE

CONTINUOUSLY OPERATING CF-SILO

by ib Finn Petersenand Annette Worre Jørgensen

∫

Description of raw meal blending in thecontinuously operating CF-silo

Ib Finn Petersen and Annette Worre JørgensenF L Smidth & Co. A/S, Copenhagen

SummaryThe blending process of a continuously operating silo isfundamentally a transformation of compositional varia-tions in the feed to the silo into reduced compositionalvariations in the discharge from the silo. The residencetime distribution within the silo is shown to be the causeof this transformation.The operational principle of the so called CF-silo (Con-trolled Flow) is to extract raw meal at different rates fromseveral outlets in the silo bottom and mix these streamstogether. An estimation procedure for the residence timedistribution in the CF-silo is evaluated on the basis offlow pattern observations in small-scale experiments to-gether with general physical principles. This procedureforms the basis of a simulation programme for the CF-silo blending process. Satisfactory agreement is foundwhen results from this simulation programme are com-pared with observations corresponding to actual opera-tional periods for CF-silos.

IntroductionA kiln feed with uniform chemical composition and fine-ness is important in order to obtain good operationalconditions for the kiln and to achieve optimum cementquality on the basis of existing raw materials. In anycase, an intermediate stock of raw meal is necessary inorder to ensure continuous operation of the kiln and toallow for the routine maintenance stops of the raw mill.Today the trend is clearly towards the application ofcontinuously operating silo systems for blending andstorage of raw meal in cement plants.

Continuous blending of raw mealRaw meal from the raw mill can be supplied to a con-tinuously operating silo simultaneously with the extrac-tion of kiln feed. In such through-flow systems,homogenization implies that compositional variationsin the inlet flow to the silo are transformed into reducedcompositional variations in the exit flow. It is commonto evaluate the blending thus achieved in terms ofhomogenizing effect H. This evaluation involves sam-pling under defined conditions a number of samplesfrom the silo inlet and exit, respectively. Based onthese samples the homogenizing effect H is calculatedas the ratio between the standard deviation in one che-mical parameter (for instance % CaO, % CaCO3 or %LSF) in the feed to the silo and the standard deviationin the same chemical parameter in the discharge fromthe silo.

[1]

The index c indicates that H is calculated from the

measured standard deviations San and Sout respective-ly corrected for the analytical accuracy, San- This stan-dard deviation San corresponding to a single deter-mination, can be evaluated from the results (X

M, xi2) of

N double determinations including sample preparationfor analysis:

[2]

Obviously, it is only meaningful to evaluate the homog-enizing effect H by expression [1] in cases where themeasured standard deviation Sout is greater than San. Itis generally acknowledged that when the true inhomo-geneity

of the kiln feed is less than 1 % LSF corresponding to0.2% CaCO3 or 0.1% CaO, no further improvements withrespect to kiln operational stability, run factor orcement quality can be achieved through additionalhomogenization.

Evaluation of the homogenizing effect is a fairly sim-ple method of characterizing the blending achieved ina continuously operating raw meal silo. However, thehomogenizing effect is not only influenced by the silodesign but also by the silo contents, the throughputrate and the character of the compositional variationsin the feed to the silo. Consequently, it is of interest toconsider in greater detail how blending is fundamental-ly affected in such a silo under stationary conditions.From a chemical engineering point of view the generaltreatment by Levenspiel1 concerning the non ideal flowthrough a vessel is relevant in this connection.

Obviously no blending is achieved in a silo of thethrough-flow type if all material leaving the silo has spentthe same time within the silo (plugflow). Blending re-quires that different parts of the input flow take differentroutes through the silo and thereby spend different timespassing through the silo. The distribution of these timesτ is called the exit age distribution E(τ) or simply theresidence time distribution. It is convenient to definethe residence time distribution E(τ) as the normalizeddistribution:

With this representation E(τ) • d t is the fraction ofthe exit flow which has spent a residence time betweenτ and τ+ d τ within the silo. The residence time distribu-tion corresponds to the concentration of tracer as afunction of time which could be measured in the exit

H = =Sin,c

Sout,c

Sin2 -- San

2

Sout2 -- San

2

San =∑ (Xi1 - Xi2)2

2 N

S2out -- S2

an

E(τ)dτ=1∞

ο[3]

3

to be all values smaller than t, i.e., 0 < τ < t. Conse-quently Cout(t), the outlet concentration at time t, becomesthe average of all these contributions:

[4]

Any kind of compositional variations in the inlet flowto a continuously operating silo under stationary condi-tions can, by means of the so-called convolution inter-gral [4], be transformed into corresponding composi-tional variations in the outlet flow provided the residencetime distribution within the silo is known.

Because residence time distribution and blendingare fundamentally interrelated for a continuously operatingsilo, the major challenge in the design of such a silo is toestablish a suitable residence time distribution for theflow of material through the silo. A large part of — prefer-ably the whole — silo contents must be kept moving anddifferent elements of the feed must pass through the siloat different rates in order to obtain a high degree ofdispersion for the residence time distribution. In thedesign of the CF-silo (Controlled Flow) these principleshave been taken into account.

Description of the CF-siloThe CF-silo is a continuously operating blending andstorage silo for raw meal. The operating principle ofthis silo is to extract raw meal at different flow ratesfrom several outlets in the silo bottom and mix thesestreams together.

The silo is laid out as a cylindrical building with araised bottom as shown in Figure 2. Raw meal is con-tinuously supplied to the inlet in the centre of the silotop. Positioned in the silo top deck is the standardequipment including manhole, over- and underpressurevalves and level indicators.

Figure 1. Illustration of the transformation of inlet composition intooutlet composition.

flow after a momentary addition of one unit tracer tothe inlet flow at time zero.The residence time distribution within a silo is fun-damentally responsible for the transformation of com-positional variations in the inlet flow to reduced com-positional variations in the exit flow. In the following,this transformation is considered in greater detail un-der stationary conditions.Let us focus attention on the outlet at time t (seeFigure 1). All material which entered the silo at time (t- τ) and spent the residence time T within the silo willappear in the outlet at time t. Let Cin (t — τ) representthe inlet concentration at time (t - τ) and E(τ)dt is thefraction of the outlet flow which has spent the resi-dence time within the silo. Accordingly the materialwhich entered the silo at time (τ - T) will give the con-tribution Cin(t - τ) E(τ) drto the outlet concentration attime t. In principle the residence time can be assumed

Figure 2. The CF-silo.

Fraction of flow

Cin (t-T)

(t-T) Time

Inlet concentration

Residence time

E (T) x dT

Outlet concentration

Cout (t)

Cin (t-T) x E (T) x dT

Time

Cout ( t ) = Cin ( t _ τ) . E(τ)dτ∫

t

0

4

The silo bottom is divided into seven identical hex-agonal sectors each of which has an extraction outletin the centre covered by a large pressure relief conemade of steel. Each of the hexagonal sectors is subdi-vided into six triangular segments. Accordingly the silobottom consists, in total, of 42 such segments allequipped with porous aeration boxes. Three segmentscan be aerated simultaneously and independently. Theaeration air is supplied from three rotary blowers anddistributed by means of 42 solenoid valves, one foreach segment. Each of the seven extraction outlets isequipped with a flap valve by which extraction from agiven outlet can be started and stopped. From theseoutlets the material is conveyed by air slides to a cen-trally placed mixing tank below the silo. Within the mix-ing tank raw meal extracted from the different outlets ismixed together by strong aeration. The mixing tank isplaced on load cells and the weighing signal from thesecells starts and stops the total extraction process fromthe outlets in the silo bottom, so the material level in-side the tank is kept within predetermined limits. Thisweighing signal can also be applied in the loss-of-weight system, where the mixing tank is used as kilnfeeding installation2.

Typically, seven different extraction rates from thesilo bottom are realized in practice by positioning throt-tle plates with different openings in the outlets. Extrac-tion from the outlets is carried out according to a sequ-ence where three segments positioned at three diffe-rent outlets are aerated at the same time and the flapvalves of these outlets opened. The extraction sequ-ence is cyclic in such a way that typically within 12minutes, corresponding to the contents of kiln feed inthe mixing tank every one of the 42 segments in thesilo bottom is aerated once, together with opening ofthe connecting flap valves.

The CF-silo is equipped with a PC unit (Programm-able Controller) which controls the extraction sequ-ence. This sequence is interrupted when the contentsin the mixing tank reaches maximum level and theprogramme continues from the point of interruptionwhen the contents in the mixing tank reaches minimumlevel.

Blending in the CF-siloIn an attempt to understand how blending occurs in theCF-silo it is relevant to consider the results obtained ina special test. During this test samples were takenevery 30 minutes from all seven outlets in the silo bot-tom and after the mixing tank. The whole samplingconsisted of 25 samples from each sampling spot. The

Figure 3. Flow patterns in model experiment.

results are given in Table 1 in terms of standard devia-tions with respect to CaO concentration. In addition thestandard deviation at the silo inlet prior to this testperiod is included together with the fraction (Qt) of kilnfeed extracted from the different outlets.Considerable blending occurs during the flow ofmaterial through the main silo volume. All standard de-viations measured at the outlets in the silo bottom aresubstantially lower than at the silo inlet. The function ofthe mixing tank can also be elucidated from the resultsin Table 1. The contents of kiln feed in the mixing tankcorresponding to 12 minutes of consumption is small incomparison with the variation period (4-8 hours) oftypical compositional variations in the feed to the silo. Itis therefore to be expected that the function of the mix-ing tank is simply to mix the material from the differentoutlets together without additional blending.Provided the compositional variations are random innature the standard deviation of the composite Scomp

when seven individual streams having the individualstandard deviations Si (i = 1,2,....7) are mixed togetherin the ratios Qi (i = 1,2,....7) can be calculated as:

Scomp = ∑ (Qi .Si)2 [5]

Scomp = 0.07% CaO is obtained when this formula isapplied to the data in Table 1. This result is in closeagreement with the measured standard deviation afterthe mixing tank Sout = 0.08% CaO and in agreementwith the concept that no additional blending takesplace in the mixing tank. The mixing tank simply mixesthe different streams together.The blending occurring as material flows through themain CF-silo volume can be illustrated by means of asimple model experiment. Here the flow pattern isformed in layers of sand of contrasting colour in atransparent half-cylinder equipped with outlets coveredby cones. Figure 3 shows the flow pattern in such an

Table 1. Test results.Standard deviation at silo inlet S

in = 1.97% CaO

Outlet no. i 1 2 3 4 5 6 7

Fraction ofextraction Qi 0.210 0.178 0.154 0.133 0.119 0.108 0.098Standard deviationSi% CaO 0.24 0.09 0.12 0.18 0.06 0.21 0.10

Standard deviation after mixing tank Sout

= 0.08% CaO

i = 1

7

5

experiment. The size of the three outlets close to themodel front increase from left to right. During the ex-periment the degree of filling in the model silo is keptwithin narrow limits through the addition of extra layers ofsand.

The dominating features in the flow patterns shown inFigure 3 are the formation of rather broad flow zones abovethe cones. Within these flow zones predominating mate-rial movement is vertical. The rate of this vertical move-ment has its maximum in the axis of such a flow zoneand decreases with the radial distance from this axis. Inconsequence of this velocity profile within a single flowzone, a residence time distribution will exist for the ma-terial leaving a single outlet. This observation from themodel experiment can at least qualitatively account forthe considerable reduction in standard deviation betweenthe CF-silo inlet and the outlets in the silo bottom whichis documented in Table 1. The key question now is, ofcourse, whether this relationship can be quantified.

Modelling the blending process in the CF-siloIn an attempt to model the blending process in the CF-silo on the basis of the observations concerning flow pat-tern in the model experiment two problems must besolved:1. A procedure for the calculation of the magnitude of theflow zones formed above the cones must be established2. A functional relationship concerning the relative veloc-ity profile of the vertical movement within a flow zone mustbe established.

If an approximate solution can be found to these prob-lems, it is possible to estimate the residence time dis-tribution within a CF-silo.

The first problem was solved primarily on the basis ofthe work of Giunta3 who dealt with the boundaries of flowzones formed above an uncovered outlet in a flat-bottomedbin. These boundaries are influenced by outlet diameter,material level within the bin and the flowability of the ma-terial characterized by the effective angle of friction of thematerial. However Giunta3 did not consider the case wherethe outlet was covered by a cone. Consequently, a cor-rection was needed in order to account for the conescovering the outlets in the CF-silo bottom. This correc-tion was made on the basis of the fundamental work ofJenike4 and Johanson5 with respect to gravity flow of bulksolids and with special reference to the work of Johanson6

concerning the use of flow-corrective inserts in bins.For the second problem concerning the relative veloc-

ity profile within a flow zone a solution analogous withfluid flow was applied. The parabolic velocity profile inPoiseulle flow of a Newtonian fluid in a tube is well known:

where:v(r) = velocity in the distance rfrom the tube axisR = tube radiusv = mean velocity

The velocity has its maximum value 2 • v in the tubeaxis and decreases to zero at the tube wall. Obviouslyvelocity profile and residence time distribution are in-terrelated. The following residence time distributionE(τ) corresponding to Poiseulle flow in a tube is

obtained from expression [6] by a consideration wherethe tube is divided into differential cylindrical shells.

Here T is the mean residence time in the tube. Itappears from expression [7] that in Poiseuille flow nofluid will leave the tube with a residence time less thanhalf the mean residence time.

With the solutions described above it is possible toestimate the residence time distribution of a CF-silo.This estimation involves in principle the following steps:1. Calculation of the boundaries of the flow zonesabove the seven outlet cones. Consideration is givento silo dimensions, dimensions of the cones, filling leveland the effective angle of friction of the raw meal.2. Calculation of the tonnage of raw meal within eachof the seven flow zones. Consideration is given to thebulk density of the raw meal.3. Calculation of the mean residence time within eachof the seven flow zones. This is possible as the totalextraction rate of kiln feed from the silo as well as thefractions of kiln feed extracted from each of the outletsin the silo bottom are known.4. Calculation of the residence time distribution foreach flow zone by means of expression [7].5. Calculation of the residence time distribution for thewhole CF-silo system. As mentioned above no additionalblending takes place in the mixing tank. The totalresidence time distribution is therefore simply obtainedby weighing together the residence time distributions forall the seven flow zones in accordance with thefractions of kiln feed extracted from the seven outlets.

Having estimated the residence time distribution it ispossible to simulate the blending in the CF-silo on thebasis of actual compositional variations in the feed tothe silo. In the simulation the analytical values determinedin samples taken at hourly intervals from a continuousautomatic sampler at the silo inlet are used. Thecorresponding concentration in the material extractedfrom the silo is calculated hour by hour by a numericalevaluation of the convolution integral [4].

Because plant operational conditions of a CF-siloare seldom ideally stationary this calculation takes intoaccount hour by hour the actual tonnage of feed anddischarge together with variations in the silo fillinglevel. The whole simulation procedure has been prog-rammed on a HP-1000 computer.

Simulations resultsResults from a 3-day operational period with a 22. 4

m diameter CF-silo having a capacity of 21 0001 of rawmeal is shown in Figure 4. The average silo filling inthis period was 70% and the average throughput rate260 tph. Figure 4a represents the CaO contents in theraw mill product fed to the silo measured at hourlyintervals in samples from a continuous automatic sam-pler. Figure 4b represents the CaO contents in hourlyspot samples from the silo. A simulation of this oper-ational period has been carried out. The results of thissimulation are shown in Figure 4c as the calculatedCaO contents in the discharged material as a functionof time. Proper results cannot be obtained in thissimulation over the first 24 hours simply because it

v(r) = 2 v ( )(r) - -rR(

2

[6]

for τ ≥ 0.5 τ2 τ3τ2

E(τ) =0 for τ < 0.5 τ{ [7]

6

takes a certain period of time before the variations in-troduced at the silo inlet appear in the discharge.

The standard deviation of the calculated CaO con-tents in the discharged material from hour 25 to 72 is 0.1% CaO, which is in exact agreement with the measuredstandard deviation in the kiln feed spot samples over thesame time interval. However, the mean 42.88% CaO ofthe simulation results is slightly lower than the mean43.02% CaO of the kiln feed spot samples. This differ-ence could arise either due to material moving very slowlyin the silo or in consequence of the filter dust return whichis included in the kiln feed spot samples but not ac-counted for in the simulation. From visual inspection aresemblance between the time record of the curves inFigures 4b and 4c is to be noticed.

It would of course be of interest to add a shot oftracer to the silo inlet and investigate how the silo mod-ifies this signal in comparison with the simulation model.As an alternative to such an extensive tracer testprogramme one could focus attention on an operationalperiod during which a major disturbance occurred in thecomposition fed to a CF-silo. A 4-day operational pe-riod with a 28 m diameter CF-silo having a capacity of35 000 t of raw meal is represented in Figure 5. Duringthis period the average filling was 60% and the averagethroughput rate 260 tph. The CaO contents of the rawmill product determined in hourly samples from a con-tinuous automatic sampler are represented in Figure 5a.A major disturbance starting at hour 32 was introducedin the feed to the silo. Especially the first part of thisdisturbance with a surplus of CaO will be difficult to evenout due to the lack of counterbalance when this materialstarts to appear in the discharge from the silo. The con-tents of CaO in the kiln feed measured in spot samplesevery second hour are recorded in Figure 5b. The distur-bance in the kiln feed composition starts at hour 41. Asimulation of this operational period has been performed.

The results of this simulation in terms of the calcu-lated CaO contents in the extracted material as a func-

tion of time are shown in Figure 5c. Once again no properresults can be obtained in the simulation over the first 24hours. It appears from a comparison between Figures5b and 5c that close agreement exists between simula-tion results and measurements concerning both magni-tude and position in the time record of the disturbancein the composition of the discharge from the silo. Theseresults demonstrate that the estimation procedure for theresidence time distribution used in the simulationprogramme describes the actual conditions within a CF-silo fairly well.

ConclusionsBlending in the CF-silo is mainly affected due to the for-mation of rather broad flow zones above the cones cov-ering the outlets in the silo bottom. The rate profile of thematerial movement down such a flow zone appears tobe parabolic in nature. This rate profile causes a consid-erable degree of blending as material moves through.The function of the mixing tank is simply to mix the mate-rial extracted from the different outlets. This concept ofthe CF-silo blending process has been quantified into amathematical model in the form of a simulationprogramme. Results obtained with this simulationprogramme corresponding to actual operational peri-ods of CF-silos have shown satisfactory agreement withthe corresponding measurements. With the flexible CF-silo blending system this simulation programme forms arelevant tool in the optimization of the raw meal blend-ing process both at the project stage and under plantoperational conditions.

REFERENCES1. LEVENSPIEL, O. Chemical Reaction Engineering. Wiley, New

York, 1972, Chapter 9, 523 ft.2. HOLMBLAD, L P and SCHOU, K. Kiln feed system based on

the loss-of-weight principle, incorporated in the CF-silo. Zement-Kalk-Gips, 7, 1983.

3. GIUNTA, J S. Flow patterns of granular materials in flat-bottombins. Journal of Engineering for Industry, ASME, May 1969, pp 406-413.

4. JENIKE, A W. Steady gravity flow of frictional-cohesive solids inconverging channels. J. Appl. Mech., March 19,64, pp 5-11.

5. JOHANSON, J R. Stress and velocity fields in the gravity flowof bulk solids. J. Appl. Mech., September 1964, pp 499-506.

6. JOHANSON, J R. The use of flow-corrective inserts in bins.Journal of Engineering for Industry, May 1966, pp 224-230.

Figure 5. Time record of 4-day period of operation with 28 mdiameter CF-silo together with simulation results.

Figure 4. Time record of 3-day period of operation with 22.4 mdiameter CF-silo together with simulation results.

7