Experimental Determination of the Moisture Content Pattern in Yam During Drying

A.O. Adelaja, O.S. Asemota and I.K. Oshiafi

Department of Mechanical Engineering, University of Lagos, Akoka, Yaba, Lagos, Nigeria

Abstract: In this study, a forced convection solar dryer was designed, constructed and tested for the purpose of drying yam in order to study the moisture removal pattern. Some pieces of yam fillets weighing, 0.52kg, were dried in the dryer while an equal mass was dried in the open sun (control) and the profiles obtained in both cases were compared. Also, evaluating the performance of the dryer; the collector and system efficiencies of 65.6% and 54.8% were obtained respectively. The moisture content removal of 75.0% was achieved as against 61.5% (control), indicating 13.5% difference. The percentage of the difference in final moisture content to that obtained by dryer gave about 53.9% while the average drying rate for the dryer was 0.0481kg/hr as against 0.0447kg/hr for open sun drying. Though the profiles are similar in pattern, a uniform and better drying was achieved with the aid of the developed dryer.

Key words: solar energy, thermal analysis, yam, solar dryer, forced convection

INTRODUCTION employed a photovoltaic (pv) system which powers a d.c to power a dc fan that facilitates forced convection Yam is a widely distributed tuber crop in West while materials were locally sourced. Bala and Africa. The most important species cultivated in Woods[9] did considerable studies on simulation and Nigeria are D. rotundata (white yam), D. alata (water optimization of dryer. Bolaji and Olalusi[10] developed yam) and D. cayenensis (yellow yam). They are a simple and inexpensive mixed-mode dryer from important in the diet and socio-cultural life of people locally sourced materials. Janjai et al[11] presented a in the growing region. More than 95% of the world’s simulation model of pv ventilated system for a solar yam is produced in Africa with the remainder grown in tunnel dryer. Mathematical model for pv module and the West Indies, Asia and Central and South d.c. fan were formulated. Using a computer America[1]. Nigeria contributes 68% of the world’s programme, these models were used to predict the output and 74% of the total production in West performance of pv-ventilated system for different Africa[2] and in 2005, of the 48.7 million tones weather conditions. A pv solar dryer for small scale produced worldwide, West and Central Africa application in the maize industry in Malawi, Central accounted for about 94%, Nigeria being the main Africa was developed by Mumba[12]. Although the producer[3]. “Yam zones” comprising Cameroun, capital cost was high, about $900.00, it was found to Nigeria, Benin, Togo, Ghana and Cote d’ Ivore be cost effective with a payback period of less than produce approximately 90% of the world output[4]. It is one year. Oosthuizen[13] attributed the low correlation an important source of Carbohydrate in the sub-Saharan between the drying rate achieved by a flat plate region especially in the “yam zones”[5] and contributes collector and the drying efficiency to failure to match over 20% of the total dietary calorie intake in the the dryer with design requirements. He also discussed South pacific[6]. Despite its dietary and socio-cultural the use of a model in the selection of a design that importance to people in the regions where it is grown, meet a particular set of requirements. Box type dryer it suffers a high degree of post harvest loss due to its is most suitable for drying fruits and vegetable mass high water content which is between 65 and 85% of between 10 and 15 kg (Sharma et al[14]). Wisniewski[15] the weight of the tuber. To preserve it from perishing, estimated the potentials of solar energy for drying it will require reducing the moisture content by drying. medicinal plants in Polish condition. He also conducted In an attempt to dry various foodstuffs, many an economic assessment on flat plate collector and researchers have developed different solar dryers. application of pv module for driving a fan connected (Abdel-Rehim and Fahmy[7] presented the design and to a dryer. The result showed that the fan ensured a optimization of a cylindrical photovoltaic dryer with control of the drying temperature. Wisniewski and dual packaged beds thermal storage for drying Pietruszko[16] developed pv driven solar heating systems medicinal herbs. Adelaja et al[8] developed a dryer that applied for drying medicinal plant in remote areas.

Corresponding Author: A.O. Adelaja, Department of Mechanical Engineering, University of Lagos, Akoka, Yaba, Lagos, Nigeria E-mail: [email protected] 1171 J. Appl. Sci. Res., 6(8): 1171-1181, 2010

In this study, we have developed a solar dryer in 319K, being the average of the maximum of the inlet order to study the moisture content pattern of yam and outlet temperatures of the system during no load during drying and compare it with the traditional test. method of drying, (sun drying). The forced convection solar dryer designed was evaluated for its performance Reynolds number, Re and also compared with the traditional method. VD MATERIAL AND METHOD Re  h (1)  The study was carried out in October/November, 4Ac 2008. The season was characterized by cloud cover and Where, D h  unpredictable weather conditions. The system set up is P shown in figure 1. The dryer comprises three main components namely the solar collector, the drying Nusselt number, Nu chamber and the pv-extractor assembly. Firstly, the 1 solar collector assembly comprises the collector box, Nu  0.023Re0.8 Pr 3 (2) absorber plate, glazing material and the heat reservoir (a collection of granite stones painted black) set at the The following air properties were used (extracted upper part of the collector. The drying chamber houses from Holman[22]); the loading trays, access door and the extractor hole. Thirdly, the pv-extractor assembly comprises the pv  1.1092kg / m 3 , module, extractor, battery and the charge controller. The framework is made basically from wood, painted o black on the inside except for the insulated sides and CkJkgC p  1.00695 / . , the top of the solar collector which is glazed to allow direct heating of the plate and the heat reservoir. The 5 battery stores excess energy during the sunshine hours  2.1552 10kg / m . s , when the extractor is powered by the photovoltaic system. At this time, the heat reservoir stores its heat both by direct radiation from the sun and through  18.3296 1062ms / , convection from the moving heated air passing over the collector plate until equilibrium is reached with the plate. During protracted cloud cover or non sunshine kWmC 0.02777 / .o , hours, the charge controller reverses the operation of the assembly and the extractor is being powered by the battery while the moving air extracts some heat energy   0.25075 ms 2 / , Pr  0.70382 . from the heated plate and reservoir before getting into the chamber. Expectedly, the plate loses its heat faster Convective coefficient of heat transfer, hc than the reservoir which later contributes a larger percentage of the heat energy in the chamber until Nu* k equilibrium is reached with the environment. Table 1 hc  (3) below shows the design considerations and assumptions Dh while Table 2 contains the technical specifications.

Total heat loss coefficient, UL Design Analysis: In this work we shall focus on the thermal and drying analyses only. The structural 1 analysis of the dryer may not be necessary because it U L  (4) is not a heavy load bearing structure. RRRR1234  Thermal Analysis: The performance of a solar 1 collector and system is described by energy balance where, R  , 1 hh that indicates the distribution of incident solar energy co r pco into useful energy gain and various losses leading to the calculations of the efficiencies. The thermal analysis here follows the procedures as in Duffie and Beckman[21]. The properties of air are evaluated at

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Table 1: Design considerations and assumptions. S/N Parameters Description 1 Location Lagos, (long 3o24’, lat 6o27’N) ------2 Drying period October- November ------3 Solar irradiation, Lagos (October/ November) 15.4 – 16.7 MJ/m2/day [17] ------4 Average relative humidity of Lagos over 10 year period 80% [18] ------5 Food products Yam ------6 Initial moisture content in yam 71% [19], 65-85% [20] ------7 Mode of heating Indirect ------8 Number of glazing 1 ------9 Glazing slope 16.7o ------10 Glazing Materials Glass ------11 Loading provision Door at the back side of chamber ------12 Number of trays 2 ------13 Air outlet provision Vent at top of chamber (extractor hole) ------14 Air circulation mode Forced convection ------15 Drying capacity 10kg ------16 Thickness of yam fillets 3 mm ------17 Construction materials Wood, glass, mild steel sheet ------18 Insulation used Glass fibre ------19 Thickness of glass 4.5 mm ------20 Transmittance of glass 0.81 ------21 Emissivity of cover 0.88 ------22 Emissivity of plate 0.98 ------23 Sky temperature 32 oC ------24 Air velocity at fan speed of 750 rpm 5 m/s ------25 Dimension of collector 1.08 x 0.76 m2 ------26 Insulator thickness 0.0125 m

Table 2: Technical Specifications. S/N Parameters Description

1 Pv module type and rating 55Wp (RSM 50, Solar smart switch) ------2 Solar charge controller SDRC- 10IP (12/24 V auto) ------3 Extractor rating 12 Volts ------4 Extractor speed 702 - 1012 rpm

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Fig. 1: Schematic diagram of solar dryer assembly set up indicating the main components and air

1 x R 2  , R 3  , IUTT  () hh k cLavea wrco a in FR  (5) IUTTcLia () 11 R 4  , hh ccopco  ,  Collector’s efficiency, c hhco, b a co 11 The useful energy rate:  , hhcb,  a w QmACTTuupca  (6) or, 22  TTTTpcopco h  QAFI UTT  rp,  co 11 ucRc Lavea 1 pco useful energy gain over time   c Total Incident solar energy over same time period and

22  coTTTTTT co s co s co s  Qu (7) hrcoa,   c  TTco s Ac HR

Mass flow rate, Heat removal factor, FR m

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Q mh. fg m  u (8)   (17) CTpava() T Gcd

where, GIAt  3600 cd Volume flow rate, V  Experimental Procedure and Testing: The pv module m was tilted such that its surface area became V  (9) perpendicular to the rays of the sun. Five thermometers air were set in place to measure the ambient, Ta, entry, Ti, plate, Tp, collector air, Tc, and chamber, Td Drying Analysis: The drying analysis was carried out temperatures. The experiment was carried out in in the following order: October/November, 2008. The season was characterized Percentage moisture removed from product,  % by rain, cloud cover and fewer hours of sunshine. The load test involved cutting 1.04kg of yam into 100(mm ) slices of about 3mm, washed and weighed. This was  %  wd wet basis (10) divided into two parts, 0.52kg each. One part was mw spread into the first tray of the chamber which had earlier been checked for air tightness so as to avoid Final moisture content of product, m f heat and moisture losses. The second part was spread in the open sun. An hourly measurement of the 100  temperatures at specific locations and mass of the mm fi % (11) specimen was carried out between 10.00 and 17.00 100 hours each day for three days.

Amount of moisture removed, m Results: The no load test result, Table 3, reveals the maximum, average and minimum temperatures measured for T , T , T , T and T before the specimen mmwi() m f a i p c d m  (12) was weighed and loaded into the drying chamber. The 100  m f highest temperature readings were observed at the plate with 62oC maximum, 56.6 +/- 2.09oC average and 50oC Quantity of heat used in evaporating moisture, Q e minimum. This was followed by the collector, dryer, inlet and of course the ambient in that order respectively. This is in agreement with the results [8] [10] Qmhefg (13) obtained by Adelaja et al and Bolaji and Olalusi . For the load test, Table 4 shows the same where, temperature pattern was observed around the plate as in the case of the no load test but this time with a 3 [23] o o hx fgd  4.186 10 (597 0.56 xT ) (14) maximum of 75 C and average of 57.04 +/- 1.81 C. The drying chamber which of course is our focus has a maximum of 55oC. However, the 55oC limit may Efficiency of dryer,  d have been occasioned by the extractor’s passive control Q over the air flow and hence the drying temperature. e [12] d  (15) This was also reported by Mumba and [15] Ac I Wisniewski . Figures 2-4 show the profiles of the temperatures recorded for the three days. The general Average drying rate, m ave patterns show an increase in the temperatures from 10.00 to between 12.00 and 14.00 before a decline. m In the three figures, the highest temperatures were mave  (16) observed between 13.00 and 14.00. These are in good [8] td agreement with Adelaja et al and Bolaji and Olalusi[10]. where, t d = sunshine hours per day The calculated parameters used for the design and evaluation of the system are presented in Tables 5 and Determination of drying time, δ 6. The heat removal factor of 0.9429, useful energy

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Table 3: No load test, the average, maximum and minimum temperatures recorded during no load test.

Time Ta Ti Tp Tc Td Ave 34.60 +/-0.51 35.60 +/- 0.75 56.60 +/- 2.09 49.40 +/- 2.86 46.80 +/- 2.63 ------Max 36.00 37.00 62.00 58.00 55.00 ------Min 33.00 33.00 50.00 40.00 39.00 Source: analysed from data collected

Table 4: The measured temperature’s average and maximum.

Time Ta Ti Tp Tc Td Average 33.67 +/- 0.41 35.00 +/- 0.49 57.04 +/- 1.81 49.65 +/- 1.49 44.00 +/- 1.21 ------Maximum 37.00 38.50 75.00 64.00 55.00 Source: analysed from data collected

Table 5: Calculated values for design parameters. S/N Parameters Values Data or equations used 1 Reynolds number 229539.35 (laminar) Equation (1) ------2 Nusselt number 397.71 Equation (2) ------3 Coefficient of heat transfer, W/m2 12.38 Equation (3) ------4 Total heat loss coefficient, W/m2k 1.803 Equation (4) ------5 Heat removal factor 0.9429 Equation (5) ------6 Useful energy collected, W 391.50 Equation (6) ------7 Collector efficiency, % 65.59 Equation (7) ------8 Mass flow rate, kg/s 0.01944 Equation (8) ------9 Volume flow rate, m3/s 0.01753 Equation (9) Source: calculated from data provided in the equations used

Table 6: Performance evaluation of the system. S/N Parameters Values (Solar dryer) Values (Open Sun dried) Equations used/ Data 1 Moisture removed, % 75 61.54 Equation (10) ------2 Final moisture content, % 17.75 27.3 Equation (11) ------3 Amount of moisture removed, kg 0.337 0.313 Equation (12) ------4 Quantity of heat for moisture evaporation, MJ/kg 0.8088 0.7512 Equation (14) ------5 Dryer efficiency, % 54.76 Equation (13) ------6 Avearge drying rate, kg/hr 0.0481 0.0447 Equation (16) ------7 Drying time, hrs (0.233/days)-1 (0.217/day)-1 Equation (17) Source: calculated from data provided in the equations used rate collected, 391.50 W, collector efficiency, 65.6% Discussion: The percentage moisture contents profiles and mass flow rate of 0.01944kg/s were obtained for in figure 5 reveal that there is usually an increase in the system. The collector efficiency is greater than the the loss of moisture from the early hours to between 57.5% obtained by Bolaji and Olalusi[10] but lower than 13.00 and 14.00 after which a reduction in rate is 80%[8,12]. For the performance evaluation of the system, observed. This can be attributed to low sunshine at the some parameters obtained from the dryer and the early and late hours of the day, hence low heat control are compared. The moisture content removal transfer. This explains the section of the profile indicated a difference of about 13.5%. The percentage between 6.00 and 7.00, 15.00 and 16.00 and 20.00 and of the difference in the final moisture to that obtained 21.00 drying time. The two profiles representing the by dryer gives about 53.9%. The average drying rates moisture content pattern of yam during drying with are 0.0481kg/hr as against 0.0447kg/hr (control). The solar system and open sun, show that the solar dryer is drying efficiency obtained was 54.76%. more efficient compared to open sun drying apart from

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Source: experimental data

Fig. 2: Recorded ambient, inlet, plate, collector and chamber temperatures for day 1.

Source: experimental data

Fig. 3: Recorded ambient, inlet, plate, collector and chamber temperatures for day 2. the contamination and degradation associated with open 65.6% and 54.76% respectively, moisture content sun drying. The moisture content removed by the solar removal of 75% and average drying rate of 0.0481kg/hr dryer was 75% while the control was 61.54% which were recorded during solar drying of yam. The translates to about 13.5% difference. Comparing the appearance of the yam as captured in figure 6a reveals physical appearance of the dried products the end uniform and better drying as compared with open sun product of the solar dryer, figure 6a reveals a uniform drying. The efficiency of the dryer however can be drying compared with the control, figure 6b. improved among other things by using a glass with higher transmittance and a collector plate with higher Conclusion: The photovoltaic powered solar dryer thermal diffusivity like aluminum. designed, constructed and tested here can function on a continuous basis, that is, both during high intense sunshine and non insolation hours especially during cloudy weather. The collector and dryer efficiencies are

1177 J. Appl. Sci. Res., 6(8): 1171-1181, 2010

Source: experimental data

Fig. 4: Recorded ambient, inlet, plate, collector and chamber temperatures for day 3.

Source: experimental data

Fig. 5: Moisture content removal pattern in yam fillet during drying using open sun and solar dryer.

Nomenclature / Notations U L Overall convective coefficient of heat transfer, T Temperature, oC Re Reynolds number W/m2 K Nu Nusselt number τ Transmittance of glass h Coefficient of heat transfer, W/m2 ρ Density, kg/m3 I Solar insolation of collector, W/m2 α Absorptivity of collector plate σ Stefan-Boltzmann constant Q u Useful energy rate, W m  Mass flow rate, kg/s Q e Heat of vapourisation, MJ A Area of collector, m2 m Amount of moisture removed, kg P Perimeter of collector, m Volume flow rate, m3/s

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Fig. 6a: Yam fillets after process using solar dryer

Source: photographs taken after drying process

Fig. 6b: Yam fillets after open sun drying

υ Speed of air, m/s t d Sunshine hrs (Time), s η Efficiency, % o C p Specific heat capacity, kJ/ kg K F R Heat removal factor H Rate of incidence of beam or diffuse radiation on γ Moisture removal, % a unit area of space, W2/m4 K δ Drying time, days

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20. Kordylas, J.M., 1990. Processing and Preservation 23. Youcef-Ali, S., H. Messaoudi, J. Desmons, Y. of Tropical and Sub Tropical Food, MacMilan Abene and M. Le Ray, 2001. “Determination of Publishers Limited, London and Basingstoke, 49- the Average Coefficient of Internal Moisture 71. Transfer During the Drying of a Thin Bed of 21. Duffie, J.A. and W.A. Beckman, 1980. “Solar Potato Slices”, Journal of Food Engineering, pp: Engineering of Thermal Processes”, New York, 95-101. John Wiley and Sons. 22. Holman, J.P., 2002. Heat Transfer, Ninth Edition, Tata McGraw-Hill Publishing Company Limited, pp: 602.

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