Biosystems Engineering (2004) 89 (1), 101108doi:10.1016/j.biosystemseng.2004.05.012

Available online at www.sciencedirect.com

PH}Postharvest Technology

Wear of Rice in an Abrasive Milling Operation,Part II: Prediction of Bulk Temperature Rise

Debabandya Mohapatra; Satish Bal

Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur - 721302, India;e-mail of corresponding author: sbal@agfe.iitkgp.ernet.in

(Received 11 June 2003; accepted in revised form 27 May 2004; published online 11 August 2004)

The phenomenon of abrasion of rice grains during milling operations was analysed in Part I. This partincludes modelling of the temperature rise and energy utilisation in an abrasion milling operation and its effecton milling quality of grain. Medium grain brown rice was milled in an abrasive polisher. The rise in the bulktemperature was modelled by energy balance, on the basis of abrasion wear theory. The head rice yield wascorrelated with the nal temperature of the grain and was found to decrease steadily with increase in the bulktemperature of the grain. The developed model accurately predicted well the bulk temperature rise in the ricegrains with milling time. Energy utilised for milling was found to be about 33%, whereas, about 10% of theenergy was utilised to raise the temperature of the grains, and 5560% of the total energy was utilised inrunning the machine in idle conditions.# 2004 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

1. Introduction

Principles of wear nd appropriate application in riceprocessing. The mechanism of abrasive wear wasdiscussed in Part I of this paper and it was found thatwear rate was not only affected by the hardness, lengthof cut and load on the material but also by the shape ofthe material (Mohapatra & Bal, 2004). In this paper, theinvestigations focus is on the effect of abrasion andfriction on the temperature rise in the grain, and itscascading effect on the milling quality.The temperature rise due to dissipation of energy loss

at the peaks of the contacting asperities may be of a highorder of magnitude but is of short duration due to thesmall area of contact. This temperature, normally calledthe ash temperature, has a profound effect on thefriction and wear characteristics of the contactingsurface for the changes in mechanical and thermalproperties (Guha & Roy Chowdhuri, 1996). The energysupplied during polishing is utilised in polishing, heatingof the grain and overcoming the inertial forces of themachine, i.e. it is used for idle running of the machine.Part of the energy for polishing is used in overcomingthe forces of adhesion and cohesion between different

layers, resulting in breaking of bonds between the cells.The cells constituting rice grain include starch, proteinand fat, which are polymeric in nature. These polymericbonds easily break due to dissipation of thermal energy.Once one bond in the polymer has dissociated,degradation of that polymer chain follows. Thermalenergy distributes itself rapidly along the polymer chain,so that all of the backbone bonds are exposed to theenergy at some point (Ernest & Porankiewicz, 1999).In tropical countries, such as India, where high

humidity and temperature conditions prevail in mostrice-growing regions, milling operations yield a veryhigh percentage of broken grains. During milling, whilebran is being removed, the temperature of the grainincreases simultaneously, thereby causing thermal stressinside the grain. With the prevailing average ambienttemperature of about 30 8C, the grain temperature aftermilling increases beyond 45 8C. The situation deterio-rates even more in the summer season, when themaximum ambient temperature in this region variesbetween 40 and 50 8C. This signicantly contributesto the reduction in head rice recovery, posing adetrimental effect on rice quality. It is a well-knownphenomenon that changes in temperature cause change

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1537-5110/$30.00 101 # 2004 Silsoe Research Institute. All rights reservedPublished by Elsevier Ltd

in material properties, increasing susceptibility to crack-ing/ssuring.Therefore, the specic objectives of this investigation

are to: (i) develop a model simulating rice-polishingoperation, (ii) predict the rise in bulk temperature usingthe developed model, and (iii) quantify energy utilisationin different stages of a polishing operation.

2. Theoretical considerations

2.1. Modelling of temperature rise in abrasion polishingof rice grains

Temperature rise of grain was modelled by energyauditing of the rice-milling operation. In a rice-millingoperation, the increase in temperature of the grain aswell as that of the machine occurs due to partialconversion of total input energy to thermal energy. Thetotal electrical energy input to the motor results inrotation of the polishing wheel relative to grains,abrasion, mass loss and increase in the temperature of

the grain. In the present investigation, the milling systemconsists of an abrasive disc of radius R and width B,rotating inside a casing. The gap between the casing/screen and abrasive disc is s (Figs 1 and 3).For development of the model, the following assump-

tions were made.

(i) The system is in steady-state condition.(ii) The heat is uniformly conducted away within the

bulk of solids.(iii) Grain velocity in contact with the rotating disc is

equal to the peripheral velocity of the disc.(iv) Velocity of grain is zero at the screen.(v) There is linear distribution of velocity from disc to

screen.(vi) Clearance between screen and emery element is very

small compared to the radius of the rotating emeryelement.

In the abrasive milling system, the abrasive discrotates inside a milling chamber. The disc has emery/abrasive surfaces on both sides of radius R, peripheralregion of radius R and width B. The grains in the

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Notation

Ap projected area of a single rice grain, m2

b thickness of grain, mB width of polisher disc, mCOD coefcient of determinationCpg specic heat of brown rice, kJ kg

1 8CE total energy input to the system, JE0 energy under no load condition, JEa total abrasion energy, JEc abrasion energy on the periphery of the

abrasive wheel, JEf frictional energy, JEh thermal energy generated to raise the tem-

perature of the grain, JEm energy due to change in momentum, JEp abrasion energy on the abrasive plate

surface, JFc tangential component of the centrifugal

force, NF0c abrasive force associated with the rotating

grain, NFs frictional force, Nk coefcient of abrasionL angular momentum, Nm sl length of rice grain, mL0 change in angular momentum, Nm smg mass of single grain, kgM moment, NmMc torque, Nm

Mg bulk mass of grain, i.e. total mass of grainunder abrasion, kg

n number of grains present in the bulk massn0 number of data pointsNc centrifugal thrust, NNf normal force due to friction, NP relative deviation modulus, %Pw power, Wr radial distance at which grain is abraded, mR radius of abrasive wheel, mR0 radial distance of the grains present on the

outside periphery, mr0 radius of the steel core of the disc, ms gap between abrasive disc and screen, mt milling time, sTf nal bulk temperature of the grain, 8CTfobs observed bulk surface temperature of

grain, 8CTfpre predicted bulk surface temperature of

grain, 8CTi initial bulk temperature of the grain, 8Cv peripheral speed of grain/abrasive disc, m s1

w width of rice grain, mYHR head rice yield, %y angle, 8o angular velocity of disc/grain, rad s1

m coefcient of frictionrg bulk density of grain, kgm

3

D. MOHAPATRA; S. BAL102

immediate neighbourhood, i.e. touching the rotatingemery disc are carried by it and then forced outwardsby centrifugal acceleration. Thus, the velocity of thegrain has both, radial and axial components, andthe mass of grain driven outwards by the centrifugalforce is replaced by an axial motion of the grain withinthe chamber. The direction of the frictional forcesis assumed to be uniformly distributed over thesurface.Analysis of frictional forces on the rotating emery disc

in a grain mass was done in two steps. In the rst step,the energy required to overcome the frictional forces onthe two sides of the disc was calculated. In the secondstep, the energy for overcoming the frictional forcesalong the periphery of the disc was calculated analogousto the principles of the centrifugal pump.

2.2. Energy requirement for the two sides of therotating emery disc

Considering an innitesimal area of rdy dr (Fig. 1),making an angle of dy at the centre, with the verticalcentreline, the grain mass in a single layer just adjacentto the emery disc will be rgbr dy dr, which moves with anangular velocity of o at a radial distance of r from thecentre, so the force dNf in N acting on this small elementis (Shames, 1996)

dNf rgbr dy dro2r 1

where: rg is the bulk density of grain in kgm3; b is the

thickness of grain in m; o is the angular velocity of theemery wheel/grain in rad s1.

The frictional force Fs associated with it is

dFs mdNf mrgbo2r2 dy dr 2

where m is the coefcient of friction.The direction of dFs must oppose the relative motion

between the emery disc and the grains. The relativerotation of concentric circles of grain about the centre-line generates a couple of forces (dFs1 & dFs2), which areequal in magnitude and opposite in direction (Fig. 2).Since the entire milling zone can be subdivided intosmall sections, the couple acting on the shaft of theemery disc can be calculated by integration of themoments of couples within the boundary of the polisher.Considering an area element on the ring at radius r, itsmoment M in Nm will be given by

dM mrgb o2r2 dy drr 3

Solving Eqn (3) for both sides of the emery disc gives

2M 4pmrgo2b

R4

4

r404

4

where: R is the radius of the abrasive wheel in m; and r0is the radius of the steel core of the disc in m.The energy Ef, in J, required to overcome friction for

a time period t in s is given by

Ef pmrgbo3R4 r40t 5

Since at high speed, friction is overcome by abrasion(Mulhearn & Samuels, 1962; Robinowicz, 1965; Xie &Williams, 1996), the nal expression in Eqn (5) can bepresented as the abrasion energy Ep in J:

Ep pkrgbo3R4 r40t 6

where k is the abrasion coefcient, replacing thecoefcient of friction.

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r

R

dNf

d

Polisher screen

Abrasive wheel

Steel core ro

Spindle

Fig. 1. Drawing showing emery disc in a milling chamber: R,radius of the abrasive wheel; r0, radius of the steel core; r, radialdistance at which the grain is abraded; dy, small angle; dNf,normal frictional force acting on the emery wheel by the grain

self-weight

r

Vr V

r

dFS1

dFS2 Rice grain

Abrasive surface

Polisher screen

Abrasive wheel

Fig. 2. Drawing showing frictional forces forming a couple(dFS1 and dFS2) generated in an abrasive disc rotating in a

milling chamber filled with rice, acting at a radial distance r

WEAR OF RICE IN AN ABRASIVE MILLING OPERATION 103

2.3. Energy requirement for the peripheral regionof the rotating emery disc

For a cylinder of radius R, consider the periphery ofthe emery disc of width B (Fig. 3), where maximumabrasion on the grain occurs. Since the emery disc isconned in a milling chamber, where the gap betweenthe disc and the casing s is very small compared to theradius R of the emery disc, the grains will experience acentrifugal thrust Nc in N by the rotating disc of surfacearea 2pRB (Shames, 1996).Since the disc is rotating at a peripheral speed of oR,

the grain mass experiences the following tangentialcomponent of centrifugal force Fc in N

Fc mNc 2pmrgbBR2o2 7

Like the previous section, the coefcient of friction canbe replaced by coefcient of abrasion k.Hence, the abrasion force F 0c in N is

F 0c 2pkrgB bR2o2 8

The torque Mc in Nm exerted by the grain along theperiphery of the emery disc at a distance Rdy is

dMc Z 2p0

2pkBbrgo2R3 dy 9

and integrating

Mc 4p2kB brgo2R3 10

The energy Ec in J, required to abrade the grain alongthe periphery of the emery disc

Ec 4p2kB brgo2R3t 11

Hence, total abrasion energy Ea in J required to abradethe grain on the side and periphery of the abrasionelement/emery surface is given by Eqn (12), by addingEqns (6) and (11)

Ea Ec Ep 12

2.4. Momentum energy

Part of the milling energy is transferred due to changein momentum of the grain. When the bulk of grainrotates inside the milling chamber, the ow behaviourcan be represented as the uid ow inside a boundsystem. Since the gap between milling element andscreen is very small and velocity distribution is linear,the average velocity of the grains moving inside thecasing will be 05 v, where v is the peripheral velocity ofthe disc in m s1 (Douglas et al., 1979). It is seen that thegrain adhering to the emery surface exerts inertial forceto the neighbouring grains, while the other grainrevolving in the annular space travels backward to llthe gap and some tend to fall on the emery surface dueto gravitational force. Therefore, each grain of mass mgin kg moving at a peripheral velocity v, will have anangular momentum L in Nm s.

L mgvR 13

Hence, the transfer of momentum L0 in Nm s betweengrains can be estimated as

L0 mgvR 05R0 14

where R0 is the radial distance of the grains present onthe outside periphery, in m.So power transmitted Pw in W in the process is

Pw mgvR 05R0o 15

Energy transfer for a single grain is

Em mgvR 05R0ot 16

The number of grains n present on the emery surface in asingle layer

n 2pRB=Ap 17

where Ap is the projected area in m2 of a single rice

grain.Taking an ellipsoidal shape of the rice grain, the

surface area of the rice grain of length l in m and widthw in m can be given as (Selby, 1967)

Ap plw=4 18

Hence,

n 8RB

lw19

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2 R 2ro

B

B+2s

Emery disc

Polisher screen

Spindle for rotation

Steel core

Fig. 3. Abrasion on the disc peripheral surface: R, radius of theabrasive wheel; r0, radius of the steel core; B, width of the

abrasive wheel; s, gap between the casing and abrasive wheel

D. MOHAPATRA; S. BAL104

Hence, total momentum energy Em in J caused by grainto grain friction is

Em 8RB

lwmgvR 05R0ot 20

2.5. Energy balance

Since abrasion and friction cause the surface tem-perature elevation, the thermal energy of the grain Eh inJ may be expressed as

Eh MgCpgTf Ti 21

where Mg is the mass of grain in kg; Cpg is the specicheat of the grain in kJ kg1 8C; Ti and Tf are the initialand nal temperatures of the grain in 8C, respectively.Therefore

Tf Ti Eh

MgCpg22

Tf Ti Eh

MgCpg23

Bran is removed at every stage of milling; thereby, alinear equation was used to represent the grain massretained with respect to time of milling in s, during themilling operation in a specied milling machine

Mg dMg

dtt 0197 01879 103t 24

All the energy factors are time dependent, representedwith respect to time. On energy balance

E E0 Ea Em Eh 25

where E is the total energy input energy in J and E0 is theidle energy under no load condition in J.

Eh E E0 Ea Em 26

By using Eqns (23) and (26), the nal temperature of thegrain becomes

Tf Ti E E0 Ea Em

MgCpg27

3. Materials and methods

Freshly harvested, Swarna, medium grain variety(procured from local market) was selected for thisstudy. The variety was dehusked using a Satake ricedehusker (Type THU, Satake Engineering Co., Tokyo,Japan) and stored in double-sealed polythene bags at5 8C in a refrigerator (Quick freezer, 200L capacity,Remi equipments, India) till the experimentation.Samples were removed from the refrigerator 24 h beforethe experiments to equilibrate the temperature to room

conditions. Three principal diameters, viz. length, widthand thickness of brown rice of each variety weremeasured manually by Satake Grain Shape Tester(Model-MK 100, Japan) having 0001mm precision.Measurements of 50 well-distributed, randomly drawngrains from the test samples of each variety were made.Bulk density was determined by weighing 1L of brownrice in the USDA test weight apparatus, as specied bythe equipment (Ohaus, USA, precision 00001 kgm3),in triplicate. The measurement of specic heat wascarried out using a NETZSCH Differential ScanningCalorimetry (DSC) software version 3.6. (Phonics,Germany) (Tang et al., 1991).The measured dimensions and other relevant proper-

ties of brown rice are presented in Table 1. The brownrice samples (200 g), after cleaning and grading werepolished in an abrasive polisher (Model: Satake Pearler-TM05) for 15180 s, at 15 s intervals. The emery sizechosen was 36 grit and at an average rotor speed of1330min1. Power at different durations of milling(15180 s) was measured using a wattmeter (AutomaticElectrical, Mumbai, India, DVS/1065).The head rice yield YHR was expressed as the

percentage of the head rice with respect to the brownrice weight. An average data of triplicate samples wereused for analysis. The temperature rise in the bulk of thesamples during the abrasion was determined using adigital thermometer (Multispan 6 Channel, 1106, J typeSensor, range 0600 8C, precision of 01 8C). An averageof ve data points was considered for calculation. Themodel developed to predict nal bulk temperature ofgrain in an abrasive milling system was validated withthe experimental data.

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Table 1Properties of medium grain brown rice

Property Value

Machine efciency 058Specic heat of brown rice, kJ kg1 8C 1942Abrasion coefcient 0030Grain bulk density, kgm3 794Radius of the abrasive roll, mm 76Width of the abrasive roll, mm 38Length of grain, mm 575Width of grain, mm 226Thickness of grain, mm 171Speed of roll, min1 1330Bulk mass of grain, kg 02Mass of single grain, mg 157Radial distance of the grains present outsideperiphery, mm

791

Radius of the steel core, mm 35Radius of the polisher spindle, mm 12

WEAR OF RICE IN AN ABRASIVE MILLING OPERATION 105

Model accuracy was measured by the per cent relativedeviation modulus P using the following formula(Madamba et al., 1996):

P 100

n0

Xn00

jTfpre TfobsjTfobs

28

where: Tfpre is the nal predicted temperature of thegrain, Tfobs is the observed value of the temperature ofthe grain and n0 is the number of data points.

4. Results and discussion

4.1. Temperature rise during milling

The relationship between the nal bulk temperatureof grain Tf and head rice yield YHR was depicted inFig. 4. As observed from Fig. 4, it is evident that withrise in bulk temperature of the grain, the head rice yielddecreases linearly. The mechanical properties (i.e. hard-ness) change with temperature, thereby making thematerial brittle at high temperatures (Juliano, 1985;Ernest & Porankiewicz, 1999). Since the rice grain is astarchproteinfat complex polymer, it is likely that thegrain property will alter with rise in temperature. Thisresults in crack generation, leading to breakage ofgrains. As evident from Fig. 4, unit degree rise intemperature above 35 8C results in additional 15 to2% broken grains in the experimental range. Thus, a

temperature differential of even 5 8C above ambient canpose a detrimental effect on the head rice yield byreducing its value from 81 to 73%, especially for thefragile indica variety. Therefore, it is essential to checkthe milling temperature by grain cooling or maintaininga lower temperature in the milling environment to checkthese thermal stress cracks.The relationship between the nal grain temperature

and milling duration t under steady-state conditions wasvalidated and presented in Fig. 5. The mathematicalmodel developed was a good t for the experimentaldata. It was evident from the solution that the heatgeneration was greatly affected by the abrasion coef-cient of the grain with the milling material underdynamic conditions. The model gave a maximumdeviation (2 8C) of temperature between the observedand predicted value. The difference between theestimated and corresponding measured temperaturemay be expected because of the time delay (about 10 s)in measuring the temperature after the milling opera-tion. Per cent relative deviation modulus was calculatedfor the predicted and observed values and it was foundto be less than 3%, which is in the acceptable range(Madamba et al., 1996). The average error percentagewas 28% indicating a good t of the predicted modelwith the experimental data (Cleland et al., 1987). Sincethe temperature is a time-dependent factor, the rise intemperature could be known for a given operating timeof a polisher in a batch process. This model may beused to design the cooling devices for the polisher. Asthe breakage was related to the grain temperature,the temperature of the grain as well as polisher may

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YHR = 1.6931Tf + 142.76COD = 0.982

60

65

70

75

80

85

90

95

30 35 40 45 50

Final temperature of grain Tf, C

Hea

d ric

e yi

eld

Y HR,

%

Fig. 4. Variation in head rice yield with final bulk surfacetemperature of grain depicting a linear relationship: ^,measured final temperature of the grain; COD, coefficient of

determination

30

35

40

45

50

0 50 100 150 200Time of milling, s

Fina

l tem

pera

ture

of g

rain

,C

Fig. 5. Measured (^) and predicted (-) final bulk surfacetemperature grain for different milling times

D. MOHAPATRA; S. BAL106

be regulated to reduce breakage during the millingoperation. In the model, the friction factor plays a majorrole in deciding the energy requirement for the operation(Fig. 6).

4.2. Energy balance of milling system

It was observed from the performance test on thepolisher that 5560% of the total energy supplied to themachine is used for rotating the rotor and other machinecomponents under idle run condition. The energyutilised to rotate the machine components in idleconditions in the initial stage was high (60%), whichreduced to 55% on progressive milling conditions(Table 2). Out of the total energy supplied to thesystem, 4512% goes for heating the grain and 3335%

for milling/polishing and momentum transfer to grainsduring milling.

5. Conclusions

Modelling of dynamic abrasion in a rice millingoperation is important in the understanding of the basicphenomena, as well as for design of suitable polishingsystems to obtain better quality-milled rice. During amilling operation there is an increase in the bulk surfacetemperature of the grain due to abrasion/friction. Witha rise in temperature, the grain experiences thermalstress, leading to crack generation. This inescapablyresults in reduction in head rice yield. The developedmodel predicts the rise in temperature of the bulksamples of rice grain in an abrasive milling operation.This shows that the energy dissipated in the grain due tofriction/abrasion causes an increase in the bulk tem-perature in a steady-state condition. The energy balancegave an estimate of energy utilisation in the millingoperation. More than half of the total input energy tothe system is actually utilised for running the machinecomponents in idle conditions, and the balance isutilised for polishing the grain by overcoming friction/abrasion and carrying the grain weight. The rest of theenergy is dissipated into heat energy, thus rising thetemperature of the grain.

Acknowledgements

Assistance received from Dr Amiya Ranjan Mohanty,Mechanical Engineering Department, Indian Instituteof Technology, is gratefully acknowledged.

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Table 2The input energy to the polishing machine under full load condition and no load condition at different milling times

Milling time (t), s Energy under fullload (E), kJ

Energy under noload (E0), kJ

Idle energy,%

Abrasion energy,%

Heat energy,%

15 360 216 600 354 4630 732 432 590 349 6145 1116 648 581 343 7660 1512 864 571 337 9175 1920 1080 563 332 10590 2340 1296 554 327 119105 2730 1512 554 327 119120 3120 1728 554 327 119135 3510 1944 554 327 119150 3900 2160 554 327 119165 4290 2376 554 327 119180 4680 2592 554 327 119

Abrasion ,33.4%

Heating, 10.1%

Idle, 56.5%

Fig. 6. Pie chart showing distribution of energy in a millingoperation

WEAR OF RICE IN AN ABRASIVE MILLING OPERATION 107

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ARTICLE IN PRESSD. MOHAPATRA; S. BAL108

Wear of Rice in an Abrasive Milling Operation, Part II: Prediction of Bulk Temperature RiseIntroductionTheoretical considerationsModelling of temperature rise in abrasion polishing of rice grainsEnergy requirement for the two sides of the rotating emery discEnergy requirement for the peripheral region of the rotating emery discMomentum energyEnergy balance

Materials and methodsResults and discussionTemperature rise during millingEnergy balance of milling system

ConclusionsAcknowledgementsReferences