Design of tunnel drier for the non-centrifugal sugar industry

International Journal of Low-Carbon Technologies 2021, 16, 407–416© The Author(s) 2020. Published by Oxford University Press.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), whichpermits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.doi:10.1093/ijlct/ctaa075 Advance Access publication 15 October 2020 407

Design of tunnel drier for the non-centrifugalsugar industry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S. P. Raj1, M. Srinivas1, B. Sravya1,*, T. S. O’Donovan2 and K.S. Reddy3

1Department of Mechanical Engineering, BITS Pilani, Hyderabad Campus,Shamirpet-Keesara Road, Jawahar Nagar, Shameerpet, Hyderabad, Telangana 500078,India; 2Department of Mechanical Engineering, Heriot-Watt University, Room 2.36, JamesNasmyth Building, Edinburgh, EH14 4AS, UK; 3Heat Transfer and Thermal PowerLaboratory, Department of Mechanical Engineering, IIT Madras, Gajendra Circle,Chennai, Tamil Nadu 600036, India. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AbstractThe quality and shelf-life of NCS (non-centrifugal sugar) mainly depend on the moisture content presentin it. NCS formed by the current practice of open sun drying contains moisture substantially greater thanthe acceptable level of 3%. This paper presents the work undertaken to design a tunnel dryer to achievethe required moisture content of granular NCS for various load conditions. An experimental investigationwas conducted on a laboratory-scale dryer to achieve the required moisture content for various loads. Thisexperimental data was compared with the output of two drying models and a validated one that could beused to design an industrial-scale dryer. For various load conditions on each tray and dryer exit temperature,nine different cases were determined. The number of trucks, drying time, and energy requirements werecomputed using the validated theoretical model. A tunnel dryer with a length, height and width of 18, 1.2and 1 m respectively and 18 trucks with 24 trays on each truck is shown to be dry 1 tone of NCS based onthe minimum energy requirement of 176.49 MJ, and a drying time of 68 min.

Keywords: non-centrifugal sugar; drying; tunnel dryer

*Corresponding author:[email protected]

Received 31 May 2020; revised 13 July 2020; editorial decision 17 September 2020; accepted 17 September2020

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1 INTRODUCTIONNon-centrifugal sugar (NCS), popularly referred to as jaggery, isa traditional and unrefined sugar produced by sugarcane juiceconcentration. It is a rich mixture of essential nutrients and istherefore considered as the healthiest kind of sugar for humanconsumption [1]. The processing of NCS is a continuous processof heat and mass transfer involving clarification, boiling and con-centration of sugarcane juice [2]. A traditional NCS productionunit typically consists of a sugarcane juice crusher, an under-ground furnace fitted with single or multi-pan systems, where thewater is evaporated to concentrate the juice to produce NCS [3, 4].In India, a large portion of the NCS is manufactured in small-scalecottage level industry [5]. Figure 1 shows the basic productionprocess of NCS production.

NCS production in India starts in October and is continuousuntil mid-May and is stored for the rest of the year [6]. Thequality of stored NCS depends mostly on the moisture content.The higher moisture in NCS is favourable for the inversion and

growth of different types of fungi and bacteria, resulting in achange in taste and colour [7]. The growth ofmicroorganismsand deterioration of NCS is highest at 10% moisture content and30◦C temperature [8]. Freshly prepared NCS can have a moisturecontent of between 13 and 15%, but it is preferable for a longershelf-life if it is below 3% [9]. Further, it has become a prerequisitefor the industry to maintain the optimal level of moisture content,i.e. below 3% in NCS to meet the standards and requirementsin food hygiene and handling such as packing, transportationand distribution [10]. Drying of NCS is one of the processes toremove the excess moisture content and further increases theshelf-life of NCS during storing. Drying is a heat and mass transferphenomenon in which the heat energy is transferred from the sur-rounding to the NCS surface; the supplied heat is used for sensibleheating to increase the surface temperature of the NCS and for thelatent heat of vaporization to remove the moisture present in theNCS. The amount of moisture removed from the NCS dependsupon the induced vapour pressure difference between the NCSsurface and the surrounding medium [11].

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Figure 1. Basic production process NCS production [3].

Drying may be classified as open sun drying or controlleddrying [12]. To have better control over the drying rate, controlleddrying is a better option than open sun drying which dependson weather conditions. However, in most of the conventionalNCS plants, open sun drying is the major practice for dryingNCS. In this process of drying, the NCS syrup formed after theconcentration of sugarcane juice is moved from the furnace and iscontinuously stirred with flat wooden stirrers for uniform coolingby natural air. Then, the obtained semi-solidified syrup is trans-ferred into aluminium moulds or an aluminium tray to form solidor granular NCS. The granular NCS is formed by the action ofsevere shearing of the semi-solidified syrup in the aluminium trayusing wooden or stainless steel scrappers. The moisture contentin solid NCS and granular NCS formed by this process rangesabove 3%, which is not preferable for longer storage as it affects thequality and shelf-life of NCS [13]. Every year, it is noted that morethan 10% of NCS produced in India worth $0.6 million is lost dueto moisture deterioration [14]. Therefore, to achieve a moisturecontent below 3%, the drying rate should be controlled instead ofdepending on the weather conditions. To have better control overthe drying rate, a control dryer is to be designed and analysed asfor the requirement of an NCS process.

A wide variety of controlled dryers are available for applicationsin the food industry. The selection of dryers for a particularapplication depends mainly on the amount of moisture to beremoved, the scale of operation, method of heat energy sup-plied and cost [15]. The dryers that are commonly used in foodindustries are tray dryers, tunnel dryers, drum dryers, fluidizedbed spray dryers, flash dryers, rotary dryers, belt dryers, vacuumdryers and freeze dryers. Among these dryers, the tunnel dryer ismost extensively used because of its simple and economic designand most importantly it produces uniform drying [16]. It canremove moisture uniformly from the material with low moisturecontent to a material with high moisture content without anydeterioration. Unlike other dryers, the operation is simple andproduces slow and uniform drying [17].

Tunnel dryers are considered as the development from a traydryer. Figure 2 shows the schematic diagram of a tunnel dryer.The trays arranged on the trolleys are called trucks; the dryercontains several such trucks/trolleys, each of which behaves asa separate batch tray dryer. The material to be dried is placeduniformly on each tray. Air is blown over the trays in the tunnelusing the blower for effective circulation [16]. Forced convectionheating also results in the removal of moisture from the materialplaced in trays. A truck of dried material is removed from the dryend of the tunnel, and the remaining trucks are pushed forwardalong the truck length and the truck of wet material is rolled intothe vacant space at the wet end of the dryer [18].

This paper presents the research undertaken to design a tunneldryer to reduce the moisture content to below 3% in granular NCSfor various load conditions. Initially, a laboratory-scale dryer isconsidered, and the experimental analysis is conducted dryingNCS. This experimental data is used to validate drying modelsthat were considered to design a suitable industrial tunnel dryer,to produce quality NCS and store.

2 MATERIALS AND METHODSThe schematic diagram of the methodology adopted for design-ing an industrial dryer to achieve a moisture content below 3%in granular NCS is shown in Figure 3. The following sectionsdescribe the methodology adopted.

2.1 Experimental analysis for drying NCSInitially, experimental analysis was carried out on a laboratory-scale forced convective tray dryer with a tray area of 0.128 m2,to achieve the required moisture content in NCS granules andto determine the drying time. Figure 4 represents the laboratory-scale forced convective tray dryer. The dryer is attached withthe mass balance (Kern & Sohn GmbH KB10000-1) which hasa measuring range from 0 to 10 kg with a resolution of 0.1 gto measure the change in mass of the material placed in thetray. The inlet temperature of hot air is measured by the Galltec-Mela (KZC2/5-L65) temperature sensor which has an accuracy of±0.2 K. The inlet flow velocity of hot air is measured by using aSchmidt flow sensor (SS 20.260) which has an accuracy of ±5%.It is also equipped with necessary software and hardware supportfor data acquisition related to a reduction in mass with respect totime. The tray with NCS was covered with an empty tray of thesame area at a distance of 0.05 m to form an air duct to mimic theconfiguration in the real dryer with a number of trays and trucks.The hot air required for drying the NCS granules is supplied bymeans of a blower and heater mounted at one end and along theair duct respectively. Air temperature and velocity are the mostimportant factors that influence the drying rate during the dryingprocess [19]. The temperature of air considered for the dryingprocess should not exceed the glass transition temperature of NCSwhich is 45◦C, beyond which the NCS granules will fuse [20].An air velocity in excess of 2 m/s has been shown to have no

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Design of tunnel drier for the non-centrifugal sugar industry

Figure 2. Schematic diagram of tunnel dyer to designed for drying NCS granules.

Figure 3. Methodology adopted for designing an industrial tunnel dryer.

significant effect on the drying rate [21]. The control cabinet of theforced convection drier contains the control unit for the blowerspeed and the heater. The maximum power that can be suppliedto the resistance heater circuit is 6750 W. Depending upon thetemperature of the air to be maintained in the drier, the amountof power sent to the resistance heater circuit is governed by thecontrol unit. The maximum speed of the blower is 950 rpm, whichcan be adjusted by selecting the speed adjustment dial. Initially,before conducting the experiment the temperature of the forcedconvection drier is set to 45◦C and velocity of air as 2 m/s. Oncethe steady-state operating conditions are reached in the forcedconvection drier, the trays are filled with the non-centrifugalsugar for conducting experiments. Moreover, while conductingthe experiment even though there are natural fluctuations in theambient atmospheric properties, the temperature in the forcedconvection drier is maintained in the range of 45±0.1◦C and thesteady flow velocity of 2 m/s is maintained throughout by thecontrol unit.

For the experimental study, the NCS samples were sourcedfrom an NCS plant at Erode, Tamil Nadu, India. The initial mois-ture content of these samples was 4.57%. The study was conductedfor a range of NCS loads of 0.3, 0.4 and 0.5 kg on a 0.128-m2 tray.These loads were selected since NCS samples below 0.3 kg do notutilize the tray area completely and those beyond 0.5 kg exceed a

drying time of 1.5 h (the drying time in the current conventionalprocess). The selected samples are distributed on the tray witha uniform thickness of 1.5, 2 & 2.5 mm for 0.3, 0.4 and 0.5 kgrespectively. Figure 5 shows the uniform distribution of the NCSsample on a tray area of 0.128 m2.

The moisture content of the NCS sample was measured usinga thermogravimetric analyzer. In the process of drying, removalof moisture from the surface and also from the interior of thematerial is essential, in which the drying rate plays a key role [22].The percentage of moisture removed is assessed by weighing thesamples every 30 s. Further, the drying rate is then computed byEquation 1.

m = ΔmΔt

(1)

where Δm is the change in mass of the NCS sample and Δt is thefine amount of drying time.

2.2 Theoretical drying models and their validationThe experimental data is used to form a generalized theoreticalmodel that could be used for the design of an industrial dryer;the following two theoretical models were considered to obtainpercentage moisture removal rates.

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Figure 4. Laboratory-scale forced convective tray dryer.

Figure 5. Uniform distribution of NCS sample on a tray area of 0.128 m2.

Figure 6. Drying rate curve for a constant drying condition [23].

Model I: Figure 6 represents a typical drying curve applicableto granules. The initial moisture reaches critical moisture at a con-stant rate of drying during the constant rate period represented byCB. The stage CE represents the falling rate period, during whichthe moisture reaches equilibrium moisture content and the dryingrate is directly proportional to the moisture content [22, 23].

As explained in the results and discussions section, theexperimental data when looked in conjunction with the dryingrate curve can be observed where the drying of NCS comesunder the falling rate period. In the ‘falling rate period’, Equation2, proposed by Dincer [22], can be used to obtain the drying

time (dt).

Drying time (dt) = mNCS (XC − X1) hfgh (T∞ − Ti)

ln(

XC − X1

X − X1

)(2)

For this model, the drying time mainly depends on the mass ofNCS (mNCS), critical moisture content (XC), equilibrium mois-ture content (X1), instantaneous moisture content (X) and size ofthe air duct. The critical moisture content (XC) and equilibriummoisture content (X1) can be obtained from experimental resultsusing a forced convection dryer. The critical moisture content isthe initial moisture content present in the sample before drying,and equilibrium moisture content is the final moisture contentthat is left in the sample after drying. The values of instantaneousmoisture content (X) could be the intermediate values betweencritical moisture content and equilibrium moisture content. Thelength (l) and width (b) of the duct are derived from the laboratorytray dimensions (0.4 & 0.32 m). Similarly, the height (a) is takenas the distance between the trays (0.05 m).

Model II: As an alternative to the first model, in this secondmodel, the moisture present in the NCS is considered as a lumpwhich in turn suggests that the drying time could be calculatedbased on Equation 3 [24].

Drying time (dt) =(mNCS + mwj

)CNCS

hAln

(Ti − T∞

T(t) − T∞

)(3)

For this model, the rate of air mass flux (G = ρair Vair, =7992 kg/hr-m2), wherein, ρair = 1.1 kg/m3 and Vair = 7200 m/hr,the heat transfer coefficient can be determined by considering asper Equation 4 [23].

h = 0.020G0.8 (4)

Also, the drying time mainly depends on the mass of NCS(mNCS), the equivalent mass of moisture in terms of NCS (mwj),the area of the tray and the temperature of NCS at a particulartime, T(t). The equivalent mass of moisture in terms of NCS can


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be estimated based on the energy balance (Equation 5):

mwhfg = mwjCNCS (Ti − T∞) (5)

Using the above two theoretical models, the drying time withrespect to the percentage of moisture removal was computed andthe results were compared with the experimental results to ensurethe applicability of these models for developing an industrial-scaledryer. As presented in the results and discussion sections, thecomparison ensured that the theoretical model I could be used forthe development of industrial-scale tunnel dryers as results are inagreement with experimental results.

2.3 Industrial tunnel drier design using theoreticalmodelsModel I is used to develop a tunnel dryer capable of drying 1 t ofNCS from a moisture content of 4.75 to 3% in an estimated timeof around 1.5 h; this is the same drying time in a conventionalNCS plant. As depicted in Figure 1, a two-step process is followedfor this.

2.3.1. Application of drying model I for tray area of 1 m2

Model I was applied for a tray area of 1 m2, initially, to ensure theapplicability of this theoretical model. The NCS loads for the 1-m2

tray are obtained by linearly scaling the loads used in experimentalanalysis. Accordingly, the NCS loads for the 1-m2 tray area are2.46, 3.28 & 4.11 kg with corresponding layer thicknesses of 1.5, 2& 2.5 mm respectively. Similar to the experimental analysis, thetemperature and velocity of air are considered to be 45◦C and2 m/s respectively. The drying time and percentage of moistureremoval were then computed, and these results were comparedwith the experimental and theoretical results that are obtainedwith the tray area of 0.128 m2. As presented in the results anddiscussion sections, for the increased tray area, the results are inagreement with the results obtained with the tray area of 0.128 m2.Therefore, the same theoretical model I could be used for thedevelopment of industrial-scale tunnel dryers.

2.3.2. Development of tunnel dryerThe tunnel dryer is considered to be an improvement of the traydryer. It contains multiple trucks with each truck having severaltrays, as shown in Figure 2. The main parameters to be arrivedat, for this designing process, are (i) the number of trays that canbe arranged in the dryer and (ii) energy required for the dryingprocess.

The number of trays to be accommodated in the dryer dependson the vertical and horizontal arrangement of the trays. Thehorizontal arrangement of trays is called a truck, and each truckhas several trays arranged vertically one over the other with agap of 0.05 m forming a duct to pass hot air over it. A typicaltunnel dryer can accommodate a minimum of 5 to a maximumof 20 trucks with each truck having 24 trays [18]. The number of

trucks to be arranged in the dryer also depends upon the exit airtemperature of the dryer. The dryer exit temperature is found tobe 42◦C with the minimum truck arrangement (five trucks), asper Equations 4 and 5 (when air at 45◦C is passed at the inlet ofthe dryer). It is observed during the experimental analysis thatrequired moisture removal (from 4.49 to 3%) does not happenwhen the air temperature falls below 38◦C. For example, when theair temperature is 37◦C, the sample moisture content is decreasedfrom 4.75 to 3.12%, which is not meeting the criteria set by thefood standard agency, i.e. the moisture content in NCS shouldbe 3% [9]. Hence, the upper and lower bound temperatures wereset for the tunnel drier, i.e. 42 and 38◦C. Since the temperaturedifference between the upper bound and the lower bound tem-perature is very low, we have considered an average intermediatetemperature i.e. 40◦C. Therefore, the three exit temperatures ofthe tunnel drier are considered to be 38, 40 & 42◦C.

The hot air inlet of the dryer increases the temperature ofthe NCS granules from 25◦C by absorbing the moisture presentin each truck. Due to this phenomenon, the air temperaturedecreases as it passes from one truck to another and the amountof moisture removed (mw) from the NCS will increase. Thisdecrease in air temperature in each truck (Tox) will be the inletair temperature (T∞x) for the subsequent truck. The decrease inair temperature after each truck can be estimated from an energybalance (Equation 6). The mass of the air (mair) can be estimatedfrom Equation 7; the required drying time to calculate the mass ofthe air can be computed by theoretical model I. The airflow area(Aflow) above the NCS in the tray can be estimated by using thelength of the tray (L = 1 m) and the distance between the trays(a = 0.05 m).

mairCair (T∞x − Tox)

= mNCSCNCS (T∞x − Ti) + mwhfg + mwCp (T∞x − Ti) (6)

mair = ρairAflow Vair dt (7)

The actual number of trucks that can be accommodated in thedryer is equal to the number of trucks obtained when the outlettemperature of the last truck is equal to the final exit temperatureof the dryer. Table 1 represents the nine cases of various combina-tions of 2.46, 3.28 & 4.11 kg of NCS granules placed on each traywith various dryer exit air temperatures viz. 38, 40, & 42◦C.

To achieve optimum dryer design among these nine cases, it isnecessary to estimate the energy required in each case to dry 1 tof NCS granules. This can be estimated by computing the energyrequired by the blower and heater, supplying air at the inlet of2 m/s and 45◦C. The energy required by the blower (Eblower, MJ)and heater (Eheater, MJ), is estimated using Equations 8 and 9 withheater and blower efficiencies as 90 and 70% respectively.

EHeater = mairCair (45 − T∞) ηheater (8)

Eblower = Qair Δpηblower

(9)


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Table 1. Different cases for drying 1 t of NCS granules.

Case NCS power loaded on eachtray (kg)

Dryer exit temperature(◦C)

C1 2.46 42C2 3.28 42C3 4.11 42C4 2.46 40C5 3.28 40C6 4.11 40C7 2.46 38C8 3.28 38C9 4.11 38

The airflow rate can be determined by considering the cross-sectional area of the tray, over which the air flows at a constantvelocity. The optimum dryer design is obtained based on theminimum energy required by the heater and blower and mini-mum drying time to remove moisture present in one tone of NCSgranules.

3 RESULTS & DISCUSSIONThe analysis associated with the design of an industrial tunnel wascarried out based on earlier presented experimental data and thetheoretical models. The following sections discuss the findingsto design an optimum tunnel dryer for achieving the requiredmoisture content in NCS.

3.1 Experimental resultsThe experimental analysis was carried out on a laboratory-scaleforced convective tray dryer for a range of NCS loads of 0.3, 0.4and 0.5 kg on the tray area of 0.128 m2, to achieve the requiredmoisture content in NCS granules and to determine the dryingtime. Inlet parameters such as velocity and temperature of theair were set at 2 m/s and 45◦C respectively. Experimental resultspresented in Figures 7 and 8 show that the rate of drying of theNCS sample is decreasing linearly, which indicates that the rate ofdrying is in the falling rate period with the average equilibriummoisture content of 2.87%. As the load on the tray area increases,the time required for drying also increases. The time requiredfor drying 0.3, 0.4 & 0.5 kg of NCS samples to reach equilibriummoisture content was found to be 42, 65 & 81 min respectively.It is also shown that 30, 35 and 40% of the moisture is removedfor the 0.3-, 0.4- and 0.5-kg cases respectively. It is also observedthat the rate of drying decreases with an increase in a load oneach tray.

3.2 Validation of drying models with experimentalresultsDrying models, I & II have been used to calculate the percentageof moisture removal with respect to drying time with the same

Figure 7. Percentage of moisture content w.r.t drying rate for the varied massof NCS.

Figure 8. Drying rate for the varied mass of NCS w.r.t percentage of moisturecontent.

air inlet conditions considered for the experimental analysis. Asindicated in Figure 9, it is observed that the drying times requiredto reach equilibrium moisture content in 0.3, 0.4 & 0.5 kg of NCSsamples are 42, 65 & 80 min respectively with 30% of moistureremoval as per the drying model I. Similarly, according to dryingmodel II, the drying time required to reach equilibrium moisturecontent in 0.3, 0.4 & 0.5 kg of NCS granules was found to be45, 71 & 90 min respectively with 35% of moisture removal.These theoretical results indicate that the experimental resultsare consistent with drying model I. In model II, the change inmoisture level in non-centrifugal sugar is modelled based on thelumped heat capacity analysis. The decrease in moisture is directlyproportional to the increase in temperature during the falling rateperiod. In model II, a linear relationship is established betweenthe change in moisture content and the change in temperatureof the non-centrifugal sugar. However, when the assessed modelII values are compared with the experimental results, the rateof change of moisture content during the intermediate stages ishigher, which leads to overprediction with a percentage errorranging from 25 to 30%. Therefore, model I is considered for thedesign of an industrial dryer.


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Figure 9. Comparison of theoretical analysis with experimental results for (a) 0.3 kg, (b) 0.4 kg and (c) 0.5 kg of NCS.

Figure 10. Theoretical analysis for scaled-up tray area of 1 m2.

3.3. Scale-up tray area of 1-m2 using drying modelsDrying model I was used to estimate the percentage of mois-ture removal with respect to drying time for the scale-up trayarea of 1 m2 with the same air inlet conditions considered forthe experimental analysis. According to this theoretical model,drying time and percentage of moisture removed for 2.46, 3.28 &4.11 kg of NCS granules to reach equilibrium moisture contentwere found to be 42, 64 & 81 min and 30, 32 & 35% respec-tively. As indicated in Figure 10, the drying time and percentageof moisture removed are predicted by the present theoreticalmodel considered (for increased tray area) are in agreement withthe results obtained from experiment and theoretical analysesfor a tray area of 0.128 m2. Therefore, the same tray area andtheoretical model I are considered for designing the optimumtunnel dryer for achieving the required moisture content in NCSgranules.

3.4 Design of optimum dryer to achieve requiredmoisture content in 1 t of NCS granulesThe tunnel dryer is designed by estimating the number of trucksto be placed in a dryer and the total energy required for drying

1 t of NCS granules. For an effective drying process to happen,the velocity and temperature of the airflow must be as high aspossible. However, in the case of drying of food products, thetemperature and velocity of air are maintained in such a way thatfood is not degraded. For non-centrifugal sugar powder, if thetemperature is exceeded above 45◦C, the powder particles fusetogether and form a solid lump; this temperature is known as glasstransition temperature [20]. If air velocity is increased beyond2 m/s, the powder particles tend to flow away with the hot airflow;this velocity phenomenon is known as dust carry over velocity. Airvelocity >2 m/s has been shown to have no significant effect onthe drying rate [21]. By considering the above points, the studyis done for air in the tunnel drier which enters at 2 m/s and at atemperature of 45◦C.

The number of trucks to be placed in the dryer is estimated bycomputing the drying time for various dryer exit temperatures.Theoretical model I was used to compute the drying time withcorresponding air temperatures at each truck and various NCSloads placed on each tray. Table 2 shows the drying time required,the number of trucks to be placed and the holding capacity ofthe dryer for various cases. Drying time is found to be minimumat 42◦C dryer exit temperature with 2.46 kg of NCS granulesloaded on each tray having 295.2 kg of holding capacity. It is alsoestimated that a maximum of 18 trucks are required when eachtray is loaded with 3.38 kg of NCS granules at a 38◦C dryer exittemperature.

It is also observed that the drying time and the number of trucksare comparatively higher when the dryer exit temperature is at38◦C. The drying time is less and the number of trucks is fewerwhen the dryer exit temperature is at 42◦C. Figure 11 shows thevariation of temperature with respect to each truck for differenttruck loading. It is observed that temperature uniformly decreaseswith increasing truck progression.

From simulation results of Figure 11, we can also get the num-ber of trucks that can be accommodated in the tunnel drier, whenthe truck inlet temperature values are varying within a range of45 to 38◦C. For an instance, in order to calculate the number oftrucks that can be accommodated in the tunnel drier when the


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Table 2. Different cases for drying NCS w.r.t to Model I.

Case NCS power loaded oneach tray (kg)

Dryer exittemperature (◦C)

Number of trucks Time (min) Holding capacity ofthe dryer (kg)

C1 2.46 42 5 55.02 295.2C2 3.28 42 6 56.12 472.3C3 4.11 42 6 61.60 591.8C4 2.46 40 10 74.29 590.4C5 3.28 40 11 83.82 865.9C6 4.11 40 11 96.48 1085.2C7 2.46 38 15 93.60 885.6C8 3.28 38 18 105.98 1416C9 4.11 38 17 119.20 1676.8

Figure 11. Temperature variation w.r.t each truck by the model I for exit temperatures of air (a) 42◦C, (b) 40◦C and (c) 38◦C.

Figure 12. Drying time variation w.r.t each truck by model I for (a) 42◦C, (b) 40◦C and (c) 38◦C exit temperatures.

inlet temperature is 42◦C and the outlet temperature is 38◦C fora tray-loading condition of 2.46 kg. From Figure 11c, for the airtemperatures of 42 and 38◦C, note down the corresponding trucknumber values (i.e. 7 and 15), the number of trucks that can beaccommodated for the following instance in the tunnel drier isgiven by the difference of truck number values (i.e. 15–7 = 8).

Figure 12 shows the variation of drying time with respect toeach truck. It is observed that, for a given number of trucks, asthe mass of the NCS is increasing, the drying time increases. Thedrying time increases progressively along the length of the dryer,indicating that the last truck is at lower temperature resulting inlonger drying time for that truck, in comparison with the truckscloser to the inlet.

Further, design parameters such as the length of the dryer andtotal energy required for drying 1 t of NCS are estimated based onthe above results where the length of the dryer is estimated basedon the number of trucks.

Figure 13 shows the total energy required for drying 1 t of NCSgranules for different air inlet conditions. It is observed that 175–185 MJ of energy is required to blow the air at 2 m/s and 45◦Cfor cases C7, C8 & C9, that is, the minimum energy requiredwhen dryer exit temperature decreases to 38◦C with 2.46, 3.28 &4.11 kg of NCS granules loaded on each tray of 15, 18 and 17 trucksrespectively.

It is also observed that 350 to 450 MJ of energy is requiredfor cases C1, C2 & C3. That is, the amount of energy required


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Figure 13. Energy required for drying 1 ton of NCS granules with respect to varied input conditions.

Table 3. Optimum tunnel dryer design.

Case Mass of NCSgranules loaded on

each tray (kg)

Exit temperature(◦C)

Drying time(min)

Number oftrucks

Energy(MJ)

Length of thedryer (m)

C7 2.46 38 69.5 15 180.29 15C8 3.28 38 68 18 176.49 18C9 4.11 38 71.08 17 184.26 17

is maximum, when the dryer exit temperature decreases to 42◦Cwith 2.46, 3.28 & 4.11 kg of NCS granules loaded on each tray of5, 6 and 6 trucks respectively.

Based on the minimum energy required, C7, C8 & C9 areconsidered the optimal design configuration for the tunnel dryerto reduce the moisture content in 1 t of NCS granules. Table 3shows the three optimum tunnel dryer designs for drying 1 t ofNCS with air inlet temperature and velocity as 45◦C and 2 m/srespectively.

A tunnel dryer of length 18 m, height 1.2 m and width 1 m wasfound based on the minimum energy requirement of 176.49 MJand minimum drying time of 68 min to achieve the requiredmoisture content of 3% in 1 t of NCS granules (case 6).

4 CONCLUSIONSIn the present paper, a numerical model has been experimentallyvalidated and used to design a minimal energy-intensive tunneldryer to reduce moisture content in the NCS granule to below3%. An experimental analysis was conducted on a laboratory-scale forced convective tray dryer with a tray area of 0.128 m2 forvarious loads of NCS. During this experimental study, the dryingof the NCS sample is found to be in the falling rate period withaverage equilibrium moisture of 2.87%. It is also observed thatthe rate of drying is decreasing with an increase in a load of NCS

samples on each tray. This experimental data is used to validatethe two drying models and found that model I is best suitablefor designing an industrial-scale dryer since the results of dryingmodel I are very much closer to experimental results with a per-centage error of below 10%. Nine different design configurationswith different load conditions and dryer exit temperatures weremodelled. It has been found that a tunnel dryer with a length of18 m, a height of 1.2 m and a width of 1 m, with 18 trucks and 24trays per each truck was found to be the optimal configuration,requiring the least energy input of 176.49 MJ, and a minimumdrying time of 68 min to achieve the required moisture content of3% for 1 t of NCS granules.

5. ACKNOWLEDGEMENTSThe research work presented in this paper is a part of “Sustainabletechnological solutions for energy efficiency in jaggery industry(STEEJ)” the project, funded by Royal Academy of Engineering(RAE) vide project reference # IAPP1R2\100083. The authorsgratefully acknowledge the RAE for funding this project.

CONFLICT OF INTERESTThe authors declare no conflicts of interest.


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REFERENCES[1] Kumar R, Kumar M. Upgradation of jaggery production and preservation

technologies. Renew Sustain Energy Rev 2018;96:167–80.[2] Singh J, Singh RD, Anwar SI, Solomon S. Alternative sweeteners produc-

tion from sugarcane in India: Lump sugar (jaggery). Sugar Tech 2011;13:366–71.

[3] Srinivas M, Sravya B, Raj SP. Crushing methodselection for non-centrifugal sugar production byFAHP – ELECTRE I. Inter J Low Carbon Tech 2020;1–8.

[4] Sai PV, Reddy KS. 4-E ( energy-exergy-environment-economic ) analysesof integrated solar powered jaggery production plant with different panconfigurations. Sol. Energy 2020;197:126–43.

[5] Velásquez F, Espitia J, Mendieta O et al. Non-centrifugal cane sugar pro-cessing: A review on recent advances and the influence of process variableson qualities attributes of final products. J Food Eng 2019;255:32–40.

[6] Chand K, Shahi NC, Lohani UC, Garg SK. Effect of storage conditions onkeeping qualities of jaggery. Sugar Tech 2011;13:81–5.

[7] Uppal SK. Storage of jaggery under low temperature for longer duration.Sugar Tech 2002;4:177–8.

[8] Farooque DP., M., & Srivastava, A new physico-chemical concept of thehygroscopic property of gur (Jaggery). in In Proc. II Bien. Conf , 1954,SRDW: 602–607.

[9] Kumar A, Tiwari GN. Effect of shape and size on convective mass trans-fer coefficient during greenhouse drying (GHD) of jaggery. J Food Eng2006;73:121–34.

[10] Ramya HN, Gowda MC, Jayamala GB et al. Effect of ambient atmosphericconditions on change in physico-chemical properties of stored jaggery. IntJ Appl Agr Res 2010;5:9–18.

[11] Prakash O, Tiwari GN. Empirical expressions for convective and evapo-rative heat transfer coefficients for the drying of concentrated sugar-canejuice. Int J Ambient Energy 2005;26:45–55.

[12] Taylor P, Prakash O, Kumar A. Application of artificial neural net-work for the prediction of jaggery mass during drying inside thenatural convection greenhouse dryer. Int J Ambient Energy 2014;37–41.

[13] Jagannadha Rao P, Das M, Das S. Jaggery – a traditional Indian sweetener.Indian J Tradit Knowl 2007;6:95–102.

[14] SRM, MANDAL GCDED, Tudu S. Effect of common packing materials onkeeping quality of sugarcane jaggery during monsoon season. Sugar Tech2006;8:137–42.

[15] Chua KJ, Chou SK. Low-cost drying methods for developing countries.Trends Food Sci Technol 2003;14:519–28.

[16] Suhaimi M. Review on the application of a tray dryer system for agricul-tural products. World Appl Sci J 2013;22:424–33.

[17] Mabrouk SB, Khiari B, Sassi M. Modelling of heat and mass transfer in atunnel dryer. Appl Therm Eng 2006;26:2110–8.

[18] Kilpatrick PW, Lowe E, Van Arsdel WB. Tunnel dehydrators for fruits andvegetables. Adv Food Res 1955;6:60126–30.

[19] Krokida MK, Karathanos VT, Maroulis ZB, & Marinos-Kouris D. Dryingkinetics of some vegetables. J Food Eng 2003;59(4):391–403.

[20] Jagannadha Rao PV, Das M, Das SK. Effect of moisture content onglass transition and sticky point temperatures of sugarcane, palmyra-palm and date-palm jaggery granules. Int J Food Sci Tech 2010;45:94–104.

[21] Putra RN, Ajiwiguna TA. Influence of air temperature and velocity fordrying process. Procedia Eng 2017;170:516–9.

[22] Dinçer I, Zamfirescu C. 2016. Drying Phenomena: Theory and Applications.John Wiley & Sons.

[23] Geankoplis CJ. Transport Processes and Separation Process Princi-ples:(Includes Unit Operations). Prentice Hall Professional Technical Ref-erence. 2003.

[24] Bergman TL, Incropera FP, Lavine AS, DeWitt DP. 2011. Introduction toHeat Transfer. John Wiley & Sons.


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Design of tunnel drier for the non-centrifugal sugar industry

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