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A Superheated-Steam Fluidized-Bed Dryer forParboiled Rice: Testing of a Pilot-Scale andMathematical Model DevelopmentSomchart Soponronnarit a , Somkiat Prachayawarakorn b , Wathanyoo Rordprapat c , AdisakNathakaranakule a & Warunee Tia aa School of Energy and Materials, King Mongkut's University of Technology , Thonburi,Bangkok, Thailandb Faculty of Engineering, King Mongkut's University of Technology , Thonburi, Bangkok,Thailandc Department of Post-Harvest and Food Processing Engineering , Rajamangala University ofTechnology Tawan-Ok , Bangpra, Sriracha, Chonburi, ThailandPublished online: 06 Feb 2007.

To cite this article: Somchart Soponronnarit , Somkiat Prachayawarakorn , Wathanyoo Rordprapat , Adisak Nathakaranakule& Warunee Tia (2006) A Superheated-Steam Fluidized-Bed Dryer for Parboiled Rice: Testing of a Pilot-Scale and MathematicalModel Development, Drying Technology: An International Journal, 24:11, 1457-1467, DOI: 10.1080/07373930600952800

To link to this article: http://dx.doi.org/10.1080/07373930600952800

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A Superheated-Steam Fluidized-Bed Dryer for ParboiledRice: Testing of a Pilot-Scale and MathematicalModel Development

Somchart Soponronnarit,1 Somkiat Prachayawarakorn,2 Wathanyoo Rordprapat,3

Adisak Nathakaranakule,1 and Warunee Tia1

1School of Energy and Materials, King Mongkut’s University of Technology, Thonburi,Bangkok, Thailand2Faculty of Engineering, King Mongkut’s University of Technology, Thonburi, Bangkok, Thailand3Department of Post-Harvest and Food Processing Engineering, Rajamangala University ofTechnology Tawan-Ok, Bangpra, Sriracha, Chonburi, Thailand

This article describes the testing of a pilot-scale superheated-steam fluidized-bed dryer for parboiled rice along with developmentof a mathematical model for predicting the changes in temperatureof steam and moisture content of parboiled rice during drying.Based on the obtained results, it was found that the superficial velo-city of steam from 1.3 to 1.5 times of the minimum fluidization velo-city had no significant effect on the drying rates of rice. The energyconsumption for reducing the moisture content of paddy from 0.43to 0.22 kg/kg dry basis was approximately 7.2 MJ/kg water evapo-rated. Drying temperature caused the appreciable change ofparboiled rice qualities as characterized by water adsorption, white-ness and pasting viscosities, white belly, and hardness. Soakingpaddy at a temperature of 70�C for 7–8 h before drying was suffi-ciently enough for producing parboiled rice, with no white belly.The gelatinization of starch during drying resulted in higher headrice yield of the product as compared to that of raw paddy.

Keywords Fluidization; Gelatinization; Parboiling; Simulation;Starch

INTRODUCTION

Drying in superheated steam offers important advan-tages over hot air drying. These include lower net energyconsumption and higher rates of drying under some certainconditions. The quality of the dried products is also gener-ally superior when compared with hot air–dried product.Accordingly, the use of superheated steam as the dryingmedium in fluidized bed, which is an excellent unit oper-ation for contacting fluid and particulate materials, is anattractive alternative for drying for it and can improve

the thermal efficiency of the drying process as well as leadto product of higher quality.

For starch-based products such as paddy, which con-tains high amylose, superheated steam drying providesmany advantages for it, yielding dried product with rela-tively higher head rice yield compared with the hot air–dried product.[1,2] Higher head rice yield is due to relativelyfaster development of starch gelatinization,[3] resulting instrengthening of intermolecular binding forces amongstarch granules; the dried rice also has the characteristicsof parboiled rice. The evidence of faster rates of starchgelatinization during superheated steam drying is alsoreported in other materials, e.g., tortilla chips[4] andpotatoes.[5] Starch granules are uniformly gelatinizedthroughout the material with superheated steam whilenon-gelatinized starch granules still appear on the surfaceof hot air–dried samples.[5]

The finding of the parboiling characteristics of rice whendrying with superheated steam might be a new break-through in the parboiled rice processing, which generallyis composed of many steps; i.e., soaking, steaming, anddrying using a conventional method.[3] In the conventionalprocess the time required for both steaming and drying riceto obtain a moisture content of 0.22 kg=kg dry basis isapproximately [3–4] h.

With the new alternative process, a superheated-steamfluidized-bed dryer acts as both a steamer and a dryerand, consequently, the total processing time is muchshorter, as will be seen in the present work. The qualityparameters of dried paddy, i.e., pasting viscosities, whitebelly, hardness, whiteness, and head rice yield were quanti-tatively determined. In addition to the design and testingof a pilot-scale dryer, a mathematical model was also

Correspondence: Somkiat Prachayawarakorn, Faculty ofEngineering, King Mongkut’s University of Technology, Thonburi,Suksawat 48 Road, Bangkok 10140, Thailand; E-mail: somkiat.pra@kmutt.ac.th

Drying Technology, 24: 1457–1467, 2006

Copyright # 2006 Taylor & Francis Group, LLC

ISSN: 0737-3937 print/1532-2300 online

DOI: 10.1080/07373930600952800

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developed to predict the changes in temperature and moist-ure content paddy while steam was in contact with thekernel. The developed model was validated with the experi-mental results in a lab-scale dryer; the results were thenapplied to a pilot-scale superheated-steam fluidized beddryer, which operated in a continuous mode with a100 kg=h drying capacity.

EXPERIMENTAL SETUP, MATERIALS AND METHODS

Lab-Scale Dryer

Figure 1 shows a schematic diagram of a lab-scale super-heated-steam fluidized-bed dryer, which consists of fivemain components, viz. (1) fluidized-bed chamber, 15 cmin diameter and 100 cm in height, (2) a 13.5-kW electricalheater for heating up the saturated steam to the super-heated steam, (3) a backward-curved blade centrifugalfan which is driven by a 2.2-kW motor, (4) a reverse-flowcyclone, and (5) a boiler with a capacity of 31 kg=h. Fordetailed experimental setup, the reader is refereed toTaechapairoj et al.[3]

Pilot-Scale Dryer

For a pilot scale setup, the designed system is similar tothat of the lab-scale, except for its continuous-mode nature

with a capacity of producing 100 kg=h parboiled rice. Aschematic diagram of the pilot-scale dryer is shown inFig. 2; a boiler was fueled with rice husk and was capableof producing saturated steam at 160 kg=h. To operate theboiler, foreign materials, e.g., small pebbles, were removedfrom rice husk by a cyclone. After cleaning, the rice huskentered the boiler at the top and then moved in a circularmotion to the bottom, where the husk was combusted.Saturated steam generated from the boiler then flowedthrough a 30-kW electrical heater, after which it becamesuperheated steam. Superheated steam was then forced,by a backward-curved blade centrifugal fan driven by a4-kW motor, to flow through a rectangular chamber witha dimension of 0.25� 0.85� 0.8 m3 in the cross directionwith paddy. The bed depth of paddy in the drying chamberwas controlled by a weir and the feed rates at the inlet andoutlet of the dryer were controlled by rotary type feeders. APID type controller was used to control the inlet steamtemperature. The temperature at other positions in the sys-tem, i.e., inlet and outlet was monitored using a datalogger, with an accuracy of �1�C.

METHODS

Long grain rough rice of Suphanburi 1 variety, providedby the Rice Research Institute, Pathumthani Province,

FIG. 1. Schematic diagram of a superheated-steam fluidized-bed drying system. (1) fluidized-bed dryer, (2) heater, (3) fan, (4) cyclone, (5) boiler, (6)

bypass line.

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Thailand, was used to study the drying characteristics ofrice in the lab-scale dryer. The sample was soaked in hotwater, with an initial temperature of 80�C for 3 h.[1] Aftersoaking, the water was drained out and the sample was leftin the container for one hour. Moisture content of thesoaked paddy was approximately 0.43 kg=kg dry basis.The experiments were carried out at superficial steam velo-cities of 1.3 and 1.5 times the minimum fluidizing velocity(Umf) of 2.6 m=s,[1] an inlet steam temperature of 150�C anda bed depth of 0.1 m. The paddy samples were loaded intothe dryer through valve V.10 and were taken out from thedryer through valve V.9 at 60, 120, 180, 240, 300, 360, 420,and 600 s to determine the moisture content. To avoid theeffect of reduced bed mass on moisture fluctuation, newsample was placed after the original sample was pulledout. After superheated steam drying, the sample was driedby ambient air until its moisture content reached0.16 kg=kg dry basis. The moisture content of the samplewas determined using a gravimetric method at a tempera-ture of 103�C for 72 h.

For the experiments with the pilot-scale dryer, two vari-eties of long grain paddy, Suphanburi 1 and Chainat 1,both containing high amylose content, were used. Theoperation was carried out at the following conditions: inletsteam temperatures of 120 to 160�C and feed rates from 95

to 130 kg=h corresponding to the residence time of 4–5 min.Bed depth was fixed at 0.07–0.08 m and the superheatedsteam flowed through the drying chamber at a velocity of3.8 m=s, which was relatively higher than that used in ahot-air fluidized bed (2.5 m=s).[6] The higher steam velocitywas needed to break up the agglomerated wet particlescaused by condensation of steam in order to maintain thefluidizing state.

The dried sample was kept in a plastic bag for 2 weeksbefore testing the quality parameters; i.e., head rice yield,pasting viscosities, white belly, hardness, water absorption,and whiteness.

Head Rice and Color Measurements

To test the head rice yield, a 250-g sample was firstdehusked by a rubber roll rice husker (Thu, Japan). Thiswas followed by polishing with a Satake rice polisher(Model TM05, Japan). In the last step, the milled ricewas graded by a rice sieving machine to obtain a nearly fullkernel (at least 75% of its total length). The head rice yieldwas eventually determined by dividing the mass of thenearly full kernels of milled rice by the total mass of paddy.The milled rice color was measured using a Kett digitalwhiteness meter (Model C-300, Japan).

FIG. 2. Shematic diagram of a pilot-scale superheated-steam fluidized-bed. (1) Boiler, (2) superheater, (3) blower, (4) drying chamber, (5) cyclone, (6)

rotary valve.

PILOT-SCALE AND MATHEMATICAL MODEL OF FLUIDIZED-BED DRYER 1459

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RVA Measurement

The samples were prepared for an RVA analysis follow-ing AACC Method 61-02.[7] Rice flour samples were pre-pared by grinding rice with a hammer mill (RetschMuhle, type SK1, Germany) that has a mesh opening of800 mm. Pasting properties of rice flour were analyzed usinga Rapid Visco Analyser (model-4 Newport Scientific PtyLtd., Australia) . Three grams of rice flour at a moisturecontent of 0.14 kg=kg dry basis was mixed thoroughly withdistilled water (25 mL) in an RVA aluminum canister. Themixture was stirred at 900 rpm for 10 s and then changed to160 rpm. The mixture was held at 50�C for 1 min and thenheated to 95�C at the rate of 12�C=min. After that the mix-ture was held at 95�C for 2.5 min followed by cooling to50�C at the rate of 12�C=min and holding for another2.1 min. A plot of paste viscosity in arbitrary RVA unit(RVU) versus time was used to determine the peak vis-cosity (PV), temperature at peak viscosity (Ptemp), trough,final viscosity (FV), breakdown viscosity (BKV ¼ PV�trough), and setback viscosity (SBV ¼ FV� trough).

White Belly Measurement

White belly, referring to the part of ungelatinized starch,has a characteristic of opaque whiteness, which is oftenfound at the kernel centre. A kernel with 50% opaquewhite area falls in the category of white belly.[8] From acommercial view point, white belly is a very importantparameter for evaluating the parboiled rice quality pro-duced from the process; a value of less than 1.5% presentsthe premium quality. In this study, visual observation ofwhite belly was performed. One hundred kernels were ran-domly selected from white rice and the results werepresented in percentage of kernels with white belly.

Water Adsorption Measurement

Twenty milliliters of water was added to 10 g of whiterice and cooked at a control temperature of 95�C using awater bath. Before determining the water adsorption, itwas necessary to know the cooking time. The cooking timewas determined by removing a few kernels from a cooker atdifferent time intervals during cooking and pressing thembetween two glass plates. If no white cores were left, itmeant the rice was completely cooked. After cooking wasfinished, the surface water of the cooked rice was blottedand weighed using a digital balance with an accuracy of� 0.001 g. The difference in weight between raw white riceand cooked rice was reported as water adsorption uponcooking (g water=g rice).

Hardness Measurement

The hardness of cooked rice was determined using abench-top texture analyzer (model TA-XT2i, Stable Micro

Systems Ltd., England) with a 500 N load cell. An alumi-num cylindrical cup having a diameter of 12.63 mm and aheight of 50 mm with a cylindrical plunger was used to con-duct a back extrusion test. A 4 g portion of cooked rice wasplaced into a test cup and then pressed. The initial height ofthe compression probe was set at 120 mm. The pretestspeed, test speed, and post speed of the probe were 1.5,0.5, and 10 mm=s, respectively. The maximum forcerequired to press cooked rice to 90% of its initial heightwas indicated as the hardness of cooked rice. The hardnessvalue was represented by the means of 5 replications andthe average value was expressed.

Development of Mathematical Model

A mathematical model describing the changes of tem-perature and moisture content of product with time duringbatch operation was first derived based on the fundamentaltransport equations together with energy and mass bal-ances on the beds. Later, the developed model, with simpli-fied assumptions, was extended to predict the changes inthe continuous fluidized bed. During batch and continuousoperations, the wet kernel with initially low temperaturewas contacted thoroughly with superheated steam. Thisnaturally led to steam condensation and increases in theproduct moisture content and temperature. This initialcondensation period is described in the mathematicalmodel as the heating-up period. After the kernel tempera-ture reached 100�C at an atmospheric pressure, conden-sation stopped. Once the period of initial condensationwas over, evaporation of moisture started. In the periodof evaporation, drying was divided into two sub-periods:constant rate and falling rate periods.

Heating-Up Period

For the heating-up period, it was assumed that the con-densed water formed a layer of liquid uniformly over theentire external surface of each particle in the bed; this layeracted as a resistance to heat transfer between the steam andthe surface. Heat transfer to the surface can thus bedescribed by:

q ¼ hf ApðTsat � TpÞ ð1Þ

This rate of heat transfer is equal to the rate of heatreleased by the latent heat plus the cooling of the conden-sate, which is expressed by[9]:

q ¼ _mmcond

�Cp;steamðTsteam

inlet � TsatÞ þ hfg

þ 3

8Cp;waterðTsat � TpÞ

�ð2Þ

or

q ¼ _mmcondh00fg ð3Þ

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The derivation of Eq. (2) was obtained by assuming a lin-ear temperature distribution in the liquid film and laminarflow of the liquid film. It was also assumed that the tem-perature of the condensed water was equal to that of theparticles in the bed. Thus, from this assumption, the con-densation rate of steam, _mmcond, could be calculated andcompared with the mass flow rate of steam entering thebed. If the condensation rate was higher than the steamflow rate, one would expect that the steam flowing throughthe bed was completely condensed. In this case, the warm-ing up of particles can be calculated using Eq. (2), with_mmcond replaced by the mass flow rate of steam.

For an energy balance of the solid phase, heat resultedfrom the steam condensation warmed up both the particlesand the condensed water, thus yielding:

q ¼ mp;sCp;s þmfwCp;water

� � dTp

dtð3Þ

The specific heat of paddy is related to the moisture con-tent in a linear form, Cp�s ¼ 1292þ 4200M,[10] where Mis the average moisture content (decimal dry basis). Theinitial condition for solving Eq. (3) is the particle tempera-ture being equal to the ambient air temperature.

The heat transfer coefficient of the film condensationcan be calculated from an empirical equation for sphere[9]:

hf ¼ 0:815qfðqf � qstÞgh00fgk3

f

lf DpðTsat � TpÞ

" #ð4Þ

The physical properties were evaluated at the filmtemperature,

Tf ¼Tsat þ Tp

2:

The resulting steam condensation in the bed increasedthe moisture content of the particles and this could bemathematically described by the diffusion equation:

@M

@t¼ Deff

@2M

@r2þ 2

r

@M

@r

� �ð5Þ

where Deff is the effective diffusion coefficient (m2=s)determined directly from the drying experiments.[3] Itsrelationship with temperature is given by the followingcorrelation[3]:

Deff ¼ 3:02087� 10�7 exp � 24318:4

RT

� �ð6Þ

where R is the universal gas constant (J=mol�K) and T thedrying temperature (K). Initial and boundary conditionsassociated with the above liquid diffusion equation aregiven by:

Mðr; tÞ ¼Min at t ¼ 0 ð7Þ

dM

dr

����r¼0

¼ 0 at t > 0 ð8Þ

MðR; tÞ ¼Meq;c at t > 0 ð9Þ

Equations (8) and (9) imply, respectively, minimum moist-ure content at the center and that the surface moisture ofthe particle is in equilibrium with the condensed water.Meq;c corresponds to the maximum moisture content forwhich paddy could hold. This maximum value is about0.5 kg=kg dry basis and was obtained by soaking paddyin water overnight, prior to the drying experiment.

The average moisture content of paddy particle, M, isgiven by:

MðtÞ ¼ 4pVp

Z ro

0

r2Mðr; tÞ dr ð10Þ

where Vp is the particle volume (m3). The amount of thecondensed water adsorped by the particles in the bed attime t, madsorp (t), is:

madsorpðtÞ ¼ mp;s½MðtÞ �Mðt� 1Þ� ð11Þ

The amount of remaining condensate in the bed at time t,mfw, is thus:

mfw ¼Xt

0

½ _mmcondDt�madsorpðtÞ� ð12Þ

Initial condensation of steam enabled faster develop-ment of particle temperature. Subsequent condensationrate, however, decreased while the rate of water adsorbedby paddy particles increased. These opposite behaviorsmight result in the adsorption rate being faster than thecondensation rate. If such a case occurred, the gain inmoisture content came from its condensation at presenttime plus the remaining condensed water at the surface,resulting in the smaller amount of condensed water. In thiscase, the remaining water at the present time can be calcu-lated by:

mfw;net ¼ _mmcondðtÞDtþXt�1

0

mfw �madsorpðtÞ ð13Þ

As there was no available condensed water left at the par-ticle surface before condensation stopped, the increase inmoisture content cannot be calculated directly by Eq. (5),but is simply determined by

Mt ¼Mt�1 þ_mmcondDt

mpð14Þ

Such an event usually takes place with the sample havingan initial moisture content far below its saturation value.In this study, however, the moisture content of paddy after

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soaking had a moisture content closed to the saturationpoint, leading to a slow adsorption of water during the per-iod of steam condensation.

Drying Period

A mathematical description of temperature changes ofsteam and particles within the bed was derived from anenergy balance in the solid and gas phases. The solid-phaseenergy balance can generally be written as:

hApðTbed � TpÞ ¼ _mmvap½hfg þ Cp;steamðTbed � TpÞ�

þmpCpdTp

dtð15Þ

A correlation of heat transfer coefficient between the fluidstream and solid proposed by Ranz and Marshall[11] wasused. Equation (15) represents the energy utilized forvaporizing the water and heating up that water to thebed temperature (indicating the steam within bed beinggaseous state) as well as for heating up the bed particles.

As observed from the experiments, while paddy wasbeing dried, the bed temperature was lower than the steamtemperature at the drying chamber inlet, but it was insignif-icantly different from the exhaust steam temperature. Thegas-phase energy balance can thus be written as:

_mmsteam;inCp;steamTsteam;in � _mmsteam;outCp;steamTsteam;bed

¼ hAðTbed � TpÞ þUAdðTbed � T1Þ ð16Þ

The first term on the left-hand side of Eq. (16) representsthe enthalpy of the inlet steam and the second term is theenergy carried by the steam leaving the fluidized-bed cham-ber. These enthalpy changes are equivalent to the heat uti-lized for drying plus the heat lost to the environment. Theoverall heat transfer coefficient for the drying chamber is1.25 W=m2�K. This value was estimated by calculating theseries of heat resistances of the components of the dryingchamber.

The gas phase mass balance is now considered. Themass flow rate of steam leaving from the drying chamberat any instant, _mmsteam;out, can be calculated by:

_mmsteam;out ¼ _mmsteam;in þ _mmvap ð17Þ

Equation (17) represents an evaporation leading to themass flow rate of steam at the outlet being higher than thatat the inlet under normal operation. Condensation, on theother hand, decreased the steam flow within the bed.Under severe conditions, the decreased steam flow mightbecome lower than the minimum value required for fluidiz-ing the bed. This problem is inherent in a superheated-steam fluidized bed. The use of higher steam flow rateand higher steam temperature as well as warming the dry-ing particles before loading them into the system could, ofcourse, alleviate this problem.

As mentioned earlier, when the period of condensationfinished, the particle temperature reached the steam satu-ration temperature and the dying started with the removalof condensed water. During the removal of condensedwater, the temperatures of both the particle and the con-densed water were kept at the steam saturation tempera-ture, and Eq. (15) is reduced to:

hApðTbed � TsatÞ ¼ _mmvap hfg þ Cp;steamðTbed � TsatÞ�

ð18Þ

As no condensed water was left, Ap is equal to As. Theenergy balance for the gas phase is still similar toEq. (16), but with Tp in Eq. (16) is being replaced by Tsat.

After the condensed water was removed, moisture con-tent of the paddy particle was started to be removed. Thereduction of moisture content of paddy, as previouslyreported by Taechapairoj,[3] could be separated into twoperiods. The reduction of moisture content during the firstperiod was be approximated by a linear decrease in moist-ure content with time; this was then followed by an expo-nential decrease of the moisture content. The changes ofinternal moisture content of paddy in both periods canbe calculated using Eq. (5), but the initial and boundaryconditions are different. In the first period, the initial andboundary conditions can be written as:

Constant Drying Rate Period

Mðr; tÞ ¼Mðr; t1Þ at t ¼ t1 ð19ÞdM

dr

����r¼o

¼ 0 at t > t1 ð20Þ

6Deff

pDp

dM

dr

����r¼Dp

2

¼ k at t > t1 ð21Þ

where t1 is the time at the end of condensed water removalperiod (s) and k is the drying constant (kg evaporatedwater=s � kg dry matter). Taechapairoj[3] found that thedrying constant relates to the steam temperature and thespecific mass velocity according to the following equation:

k ¼ 0:08991V� 1:1886� 10�5 Tp

� 2:268� 10�4 VTp þ 0:00314 ð22Þ

where V is the specific mass velocity of steam (kgsteam=s�kg dry matter). Equation (19) represents the exist-ing moisture gradients at the beginning of the constant dry-ing rate period. This moisture profile should be obtainedfrom the profiles at the end of the condensed water removalperiod. In this study, however, the simplification was madein the calculation by assuming that the moisture content atany radial positions of particle at the end of the condensedwater removal period was the same as that at the end ofsteam condensation period, because the time for removingthe condensed water was very short.

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Falling Drying Rate Period

After the moisture content of paddy dropped below0.25 kg=kg dry basis, further reduction of moistureoccurred in the falling rate regime. The initial and bound-ary conditions associated with the moisture diffusion inthis regime are:

Mðr; tÞ ¼Mðr; t2Þ at t ¼ t2 ð23Þ

MDp

2; t

� �¼Meq at t > t2 ð24Þ

dM

dr

����r¼0

¼ 0 at t > t2 ð25Þ

where t2 is the drying time at the end of the constant dryingrate period (s).

The calculation of the bed temperature, the particle tem-perature, and the steam flow still follows Eqs. (16) and (17)for the constant drying rate period, with particle tempera-ture equal to the steam saturation temperature.

RESULTS AND DISCUSSIONS

Comparison of Simulated Results with ExperimentalData at Lab Scale

The velocity of steam flowing through the bed of parti-cles is an important parameter to design the drying system.The influence of the superficial steam velocity on the dry-ing characteristics of paddy, as observed in a lab-scaledryer, is shown in Fig. 3. A smaller figure inserted inFig. 3 provides the detail of the calculated moisture contentof paddy at an early stage of drying, corresponding to thecondensation period. An increase of moisture content with

the drying time could be approximated by a linear relationfor a certain time. At the end of condensation period, themaximum moisture contents obtained from the calcula-tions were 0.4373 and 0.4371 kg=kg dry basis for paddydried at the 1.3 and 1.5 times Umf, respectively. The timerequired for such peak moisture contents was 2.3 s whenusing the velocity of 1.3 times Umf and 2 s when using thevelocity of 1.5 times Umf. According to these calculatedresults, it can be seen that the condensation of steam inthe bed of particles being fluidized occurred shortly andwas difficult to observe in reality. However, condensationtime would be longer if the steam velocity used was verylow. Pronyk et al.[12] investigated the drying characteristicsof foodstuffs with superheated steam using steam velocitiesfrom 0.2 to 0.35 m=s and steam temperatures from 125 to165�C and found that the condensation time was in therange 6 to 7 s for Asian noodle.

The simulated results also indicate that soaked paddyadsorbed condensed steam at a slower rate than the rateof condensation. This led to the condensed water remain-ing on the surface of paddy; this portion of water wasremoved afterwards as shown by the period constantmoisture content in Fig. 3. The time for removing thisadditional water was around 12 s. This period of the con-stant moisture content was longer at lower inlet steam tem-perature (calculated results were not presented for the sakeof brevity). The steam condensed on the paddy surfacealong with initially available condensed water favored thegelatinization of starch within paddy. Rordprapat et al.[1]

indeed compared the quality of soaked paddy drying andfound that gelatinization occurred more readily in paddydried by superheated steam than that dried by hot air.Higher head rice yield obtained in the case of superheatedsteam drying was a subsequent result of gelitinization.Iyota et al.[5] also reported that potatoes subjected tosuperheated steam drying experienced more gelatinizationthan air-dried samples.

After the condensed water was removed the moisturecontent of paddy was further dropped. The reduction ofmoisture content was slightly faster at the superficial steamvelocity of 1.5 times Umf than at 1.3 times Umf. However,insignificant different rates of moisture content reductionwere noted after the moisture content of the sample wasreduced below 0.21 kg=kg dry basis.

Comparison of Simulated Results with ExperimentalData at Pilot Scale

More than 10 experiments were carried out in the pilot-scale superheated-steam fluidized-bed dryer; the runningtime for each drying condition was around 3 hours. Whenthe steady flow of paddy was reached, as visually observedfrom the bed of paddy particles being distributed uniformlyalong the dryer length, the data were recorded at every10 minutes interval. At the beginning of drying, visual

FIG. 3. Comparison between predicted and experimental data of moist-

ure content of sample at superficial velocitites of 1.3 and 1.5 times of mini-

mum fluidization velocity and inlet steam temperature of 150�C (lab scale).

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observation via the side glass of the drying chamberrevealed that the condensation occurred throughout thebed and the paddy particles in the bed likely acted as apacked bed. After a short certain time condensationstopped, except for the region near the inlet. The particleswere fluidised in a similar manner to the bubbling bed andmoved toward the dryer outlet. As determined from theexperiments, the thermal energy consumption for reducingthe moisture content of paddy from 0.43 to 0.22 kg=kg drybasis was approximately 7.2 MJ=kg water evaporated.

To predict the outlet temperature of superheated steamand outlet moisture of paddy from the continuous dryer,the aforementioned mathematical model, which wasinitially developed for the batch operation, was appliedby considering the particle bed as a whole. Hence, the out-let moisture and outlet steam temperature can be calcu-lated from the following equations:

Moutlet ¼Min �Z �tt

0

dM

dt

����t

dt ð26Þ

Toutlet ¼R�tt

0 TðtÞdt

�ttð27Þ

where �tt is the mean residence time (s). The predictions ofthe outlet moisture of paddy and outlet steam temperatureare shown in Figs. 4a and 4b, respectively. Each data pointpresented was obtained from different operating con-ditions, i.e., different inlet initial moisture contents, feedrates, and inlet steam temperatures. As shown in Fig. 4,the predictions agreed reasonably well with the experi-ments, with �0.02 kg=kg dry basis prediction error ofmoisture content and �4�C error for the temperature.

Physicochemical Properties of Parboiled Rice

Table 1 shows the physicochemical properties of par-boiled rice (Suphanburi 1) obtained from the pilot-scaledryer. Before drying, paddy was soaked at a temperatureof 80�C for 3 h. The control samples after soaking had aninitial moisture content of 0.4025� 0.009 kg=kg dry basisand were gently dried to a moisture content of 0.165 kg=kgdry basis before determining their qualities. It was foundthat fresh paddy had an amount of chalky grains of about5.4%. Chalkiness in rice, characterized by an opaque whitespot around the center of grain that is similar to whitebelly, occurs during the growth period of paddy becauseof the adverse environmental conditions[13] such as highenvironmental temperature.[14] Chalky grains inherentlypossess low resistance to milling forces, causing a decreasein head rice yield. This defect disappeared, however, whenpaddy was dried by superheated steam, as indirectly indi-cated by the value of white belly being less than that atthe beginning; the amount of white belly dropped from5.4 to 1.4–1.7% after drying at temperatures of 128–160�C.

After soaked paddy was dried by continuous super-heated steam dryer, the values of head rice yield was higherthan that of the control samples. As shown in Table 1, thehead rice yield was in the range of 66–67% for the samplesthat had the outlet moisture content of 0.22–0.23 kg=kg drybasis but reduced to 59% for the sample with the outletmoisture content of 0.176 kg=kg dry basis. These resultsare similar to those found by several other workers,[1,3]

showing the strong dependence of head rice yield uponthe final moisture content of paddy; the final moisturecontent of 0.18 kg=kg dry basis is considered critical forparboiled rice. For nonparboiled paddy, however, themoisture content should not be reduced below 0.25 kg=kg

FIG. 4. Comparison of predicated and experimental date of moisture

content and outlet-steam temperature during continuous operation at dif-

ferent conditions (pilot scale).

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dry basis under similar drying conditions.[15] Theseinformative results suggested that parboiled rice was morecapable of resisting moisture-induced stresses than nonparboiled rice so that the moisture content of parboiled ricecould be reduced to lower values. The higher resistance tostresses might also be indicated by higher hardness valuesof parboiled rice in comparison with that of raw rice.

As depicted in Table 1, hardness of samples increasedwith an increase in drying temperature; soaked paddy driedat 164�C had a hardness value of 58.9� 0.2 N, which wassignificantly higher than those dried at lower temperatures.Oppositely, the values of whiteness became lower at highertemperatures. The rates of water adsorption, however,changed insignificantly, as indicated by a statistical analy-sis using the T-test with p < 0.05. Although the otherphysicochemical properties of dried paddy show the char-acteristics of parboiled rice, the values of white belly wereapproximately 1.4 to 1.7%, with the lower values at164�C. The evidence of white belly, with values slightlyhigher than the standard value for premium grade rice,led to a question of why the gelatinization of rice starchwas not complete. The results from the developed math-ematical model and experiments confirmed that conden-sation and subsequent rapid rise of paddy temperature aswell as the availability of condensed water satisfied the con-ditions for the gelatinization of starch. Hence, the cause ofincompleted gelatinization should not come from such

factors but it might come from the soaking stage; the soak-ing time might possibly be insufficient to obtain highenough moisture content to facilitate complete gelatinis-ation, particularly that of starch granules that laid insidekernel. To prove the above hypothesis, paddy from thelocal factory, that was already soaked at 70�C for 7–8 hwas tested; after soaking, the moisture content of paddyreached 0.456� 0.8 kg=kg dry basis. The new batch ofexperimental results are shown in Table 2, indicating theabsolutely nil white belly for paddy dried at an even lowtemperature of 128�C or higher. Other properties changedwith the temperature in a similar fashion as that reportedin Table 1. This finding emphasizes the importance ofsoaking for producing parboiled rice.

Viscosity

Rice flour obtained from the paddy samples dried bycontinuous superheated-steam fluidised bed was taken todetermine its viscosity during heating and cooling cycle.This determination reflects how the developed processaffects the starch properties. Figure 5 shows the typical vis-cographs of superheated steam–dried rice flour. Dryingtemperature significantly affected the patterns of visco-graphs of the rice flour. The viscosity of the samplesubjected to drying decreased and the magnitude of itsdecrease significantly increased with an increase in the dry-ing temperature. As shown in Fig. 5, for all temperature

TABLE 1Values of physicochemical property indices for Suphanburi 1 paddy soaked at 80�C for 3 h and dried at different inlet

steam temperatures

Dryingcondition

Feed rate(kg=h)

Outlet moisture content(kg=kg dry basis)

Head riceyield (%) Whiteness

White belly(%)

Hardness(N)

Water adsorption(g water=g rice)

After soaking 0.403� 0.009 53.7� 1.6a 42.2� 0.6�a 5.4� 1.3�a 36.0� 0.9a 3.29� 0.03a

128�C 109 0.232� 0.009 65.9� 1.0b 36.1� 0.3b 1.7� 0.6b 48.2� 0.9b 2.43� 0.03b

147�C 105 0.218� 0.01 66.8� 0.6b 33.8� 0.7c 1.8� 0.6b 49.2� 1.7b 2.35� 0.10b

164�C 101 0.176� 0.009 59.0� 2.5c 29.8� 0.8d 1.4� 0.6b 58.9� 0.2c 2.34� 0.05b

Note: Same letters in the same column indicate that values are insignificantly different at p < 0.05.�Chalky grain.

TABLE 2Values of physicochemical property indices for Chainat 1 paddy soaked at 70�C for 7–8 h and dried at different inlet

steam temperatures

Dryingconditions

Feed rate(kg=h)

Moisture content(kg=kg dry basis)

Head riceyield (%) Whiteness

White belly(%)

Hardness(N)

Water adsorption(g water=g rice)

After soaking 0.456� 0.008 56.6� 1.1a 39.0� 0.2a 5.7� 0.6�a 37.9� 1.3a 3.51� 0.07a

128�C 106 0.290� 0.008 63.5� 0.6b 32.2� 0.8b 0b 48.6� 0.9b 2.82� 0.05b

144�C 98 0.23� 0.004 66.9� 0.6c 29.4� 0.5c 0b 51.9� 0.6c 2.19� 0.01c

160�C 120 0.218� 0.004 67.9� 0.6c 28.0� 0.6c 0b 55.0� 1.3d 1.99� 0.05d

Note: Same letters in the same column indicate that values are insignificantly different at p < 0.05.�Chalky grain.

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treatments, with the exception of FV, PV, and BKV, disap-peared. These curves are characterized by a C-type pastingprofile as defined by Schoch and Maywald.[16] Lai[17] alsofound this trend for the hydrothermally-treated TaichungSen 17 rice, which contains the amylose content of 28.8%which is close to that of the paddy variety studied. It is alsoshown in Fig. 5 that PV and FV of the dried rice flour arelower than those of the raw rice flour.

As already mentioned that the viscosity of dried riceflour was dependent on the drying temperature, the ques-tion arose about the suitable operating temperature. Toanswer this question, the parboiled rice flour sample, whichwas produced by the conventional process, was taken toexamine its viscosity. The experimental result is shown inFig. 5b, indicating that an approximate temperature of130�C is suitable for producing parboiled rice because thevalues of viscosities, i.e., peak, final, and breakdown, fromsuperheated-steam rice flour are comparable to those ofcommercially produced rice flour.

CONCLUSIONS

A pilot-scale continuous superheated-steam fluidized-bed dryer with a drying capacity of 100 kg=h was fabricatedand installed at a parboiled rice factory. A mathematicalmodel that includes the effect of initial steam condensationand subsequent drying has been developed to describe thechanges of temperature and moisture content of soakedpaddy during drying. The predictions agreed reasonablywell with the experimental data, within � 0.02 kg=kg drybasis for moisture content and � 4�C for temperature.From the pilot-scale tests, the thermal energy consumptionof the dryer was approximately 7.2 MJ=kg water evapo-rated when using to reduce paddy moisture content from0.43 to 0.22 kg=kg dry basis; the drying time was approxi-mately 4–5 min. Regarding the quality of dried rice, thewhite belly was not obviously observed if a soaking timeof 7–8 h was used. The physicochemical properties of par-boiled rice, i.e., water adsorption, whiteness, and viscosityof rice flour were decreased with increase in the steam tem-perature, whereas the hardness value was increased. Thehead rice yield was improved, as compared to that of rawrice, and obtained in the range higher than 60%, exceptfor the sample dried to lower moisture content of0.18 kg=kg dry basis of which the head rice yield was in alower level. The pasting properties of parboiled riceobtained from the superheated-steam drying were similarto those obtained from the conventional process whenthe paddy was dried by steam at 130�C.

NOMENCLATURE

Ad Heat transfer area of drying chamber (m2)Ap Total heat transfer area of particles in the bed,

including the thickness of liquid film coveringthe kernels (m2)

As Total heat transfer area of particles (m2)Cp,s Specific heat of solid (J=kg�K)Cp,steam Specific heat of steam (J=kg�K)Cp,water Specific heat of water (J=kg�K)Deff Effective diffusion coefficient (m2=s)Dp Particle diameter (m)g Gravitational constant (m2=s)h Convective heat transfer coefficient (W=m2�K)

FIG. 5. Pasting viscosities of rice flour of samples dried at different

steam temperatures.

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hf Convective heat transfer coefficient of filmcondensation (W=m2�K)

hfg Latent heat of vaporization (J=kg)h00fg Modified latent heat of vaporization (J=kg)k Drying constant (kg evaporated water=s�kg dry

matter)kf Thermal conductivity of steam determined at

film temperature (W=m�K)M Moisture content (kg=kg dry basis)Meq Equilibrium moisture content (kg=kg dry basis)Min Initial moisture content (kg=kg dry basis)M Average moisture content (kg=kg dry basis)madsorp Amount of condensed water adsorped by

particle (kg)_mmcond Rate of steam condensation (kg=s)_mmvap Rate of water vaporization (kg=s)

mp,s Total mass of paddy in the bed (kg)mfw Amount of remaining condensate in the bed (kg)R Universal gas constant (J=mol�K)q Total heat transfer rate (W)r Distance along the radial direction (m)Tbed Bed temperature (K)Tp Particle temperature (K)Tsat Saturated steam temperature (K)T1 Ambient temperature (K)t Drying time (s)U Overall heat transfer coefficient (W=m2�K)V Specific mass velocity of superheated steam

(kgsteam=s�kg dry matter)Vp Particle volume (m3)

Greek Letters

lf Viscosity of steam (kg=m�s)qf Density of water (kg=m3)qst Density of steam (kg=m3)

ACKNOWLEDGMENTS

The authors express their sincere appreciation to theThailand Research Fund for financial support. Thanksare also due to the Pathum Thani Rice Research Centerfor testing physical qualities of rice and to the Institute of

Food Research and Product Development, KasetsartUniversity for allowing the RVA test.

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