Evaluation of convective mass transfer coefficient during drying of jaggery

Transcription

1 Evaluation of convective mass transfer coefficient during drying of jaggery G.N. Tiwari *, Sanjeev Kumar, Om Prakash Center for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi , India Received 25 February 2003; accepted 25 July 2003 Abstract In this paper an attempt is made to evaluate the convective mass transfer coefficient during drying of jaggery in a controlled environment. In this method, different masses of jaggery were used. The jaggery was dried in the roof type even span greenhouse with a floor area of 1.2x0.8 m 2 in natural and forced convection mode at atmospheric pressure till it attained almost no variation in mass. The experimental data of mass evaporated, temperatures of jaggery, greenhouse room air and relative humidity were measured and the data used to evaluate the convective mass transfer coefficient by regression analysis. It was found that the convective mass transfer coefficient is a strong function of mass of jaggery, temperatures and relative humidity for a given size of greenhouse. Keywords: Convective mass transfer; Drying of jaggery; Greenhouse 1. Introduction Jaggery is a traditional concentrated sugarcane juice and is a good source of minerals like calcium, phosphorous, iron and has useful medicinal properties. In India, the jaggery is mainly produced in the months of November to mid March and stored for the rest of year. The quality of stored jaggery mostly depends upon the moisture content which is favorable for inversion and development of different types of fungi and bacteria in the jaggery resulting in changes in tests and colors because of the formation of organic acid and complex decomposition of products (Uppal & Sharma, 1999a, 1999b). Drying is basically a heat and mass transfer phenomenon. The heat energy supplied to the jaggery surface is utilized in two ways i.e. to increase the jaggery surface temperature in the form of sensible heat and to vaporize the moisture present in jaggery through provision of latent heat of vaporization. The removal of moisture from the interior of the jaggery takes place due to induced vapor pressure difference between the jaggery and the surrounding medium. The desired difference in vapor pressure may be obtained either by increasing the vapor pressure of the jaggery surface or by decreasing the vapor pressure of the surroundings or by both. While drying under controlled conditions, all the above means may be employed with better control over the drying rate. This is not so in open sun drying, where it depends upon the weather conditions. The convective heat transfer coefficient depends on the temperature difference between the jaggery surface and air and the thermal physical properties of the humid air surrounding the jaggery. To find the convective heat transfer coefficient for jaggery, dried under different operating conditions, an experimental setup was formulated and data for various parameters were recorded at regular interval for the following cases. (i) Jaggery drying under natural convection mode (Fig. 1(a)) (ii) Jaggery drying under forced convection mode (Fig. 1(b)) The objective of present study is to determine the convective mass transfer coefficient for the design of a dryer for jaggery to retain its quality for storage. 2. Literature review Jaggery is prepared for human consumption as a sweetening base for food in rural as well as urban areas

2 220 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) Nomenclature A t C C v g Gr K K v M j m_ ev n Nu Pr PðTÞ & area of jaggery tray (m 2 ) experimental constant specific heat of humid air (J/kg C) acceleration due to gravity (m/s 2 ) Grashof number = gbx 3 qv 2 DT 0 =l 2 v convective heat transfer coefficient (W/m 2 C) thermal conductivity of humid air (W/m C) mass of the jaggery (kg) moisture evaporated (kg) experimental constant Nusselt number = h c X=K v Prandtl number of humid air = lvc v =K v Partial vapor pressure at temperature T (N/m2 ) rate of heat utilized to evaporate moisture (J/m 2 s) Re t Tc1 T c2 AT' X Greeks P y a k qv Reynolds number = q v vd=l v time (s) greenhouse temperature (in upper side), ( C) above jaggery surface temperature in greenhouse ( C) temperature of jaggery ( C) effective temperature difference ( C) characteristic dimension (m) coefficient of volumetric expansion (1/ C) relative humidity (dec.) surface tension of liquid-vapor interface (N/m) latent heat of vaporization (J/kg) dynamic viscosity (kg/m sec) density (kg/m 3 ) Fig. 1. Jaggery drying in a greenhouse under (a) natural convection mode, (b) forced convection mode. of India since times immemorial. The government of India established the Indian Institute of Sugarcane Research (IISR) in Lucknow (UP) in the year 1952 for all- India coordinated research on processing, handling and storage of jaggery (gur) and khandsari (the local product name from sugar cane juice). Roy (1951) and Khanna and Chakravarti (1955) prepared a monograph on the jaggery industry of India. The neutralization of sugarcane juice acidity during boiling for preparation of jaggery for storage has been studied by Shinde, Marathe, Jabvaleker, and Kadam (1983). They concluded that the sugar content and color is not much affected by neutralization. A study of the design of a godown for the storage of jaggery during the rainy period has been carried out by Agrawal and Ghosh (1984). Gunjal and Galakatu (1986) have recommended that the jaggery can be stored at 40-45% relative humidity. Javalekar, Shinde, and Randive (1986a, 1986b) have studied the effect of physical and chemical properties of jaggery. The first national seminar on jaggery manufacture, quality and storage was organized by Baboo (1985) at IISR, Lucknow. Baboo (1990) has also prepared a bibliography on jaggery (gur) research during in India. Patil and Singhte (1992) have found that color intensity, degrees brix and moisture content of jaggery remained unchanged due to various levels of phosphorous and potassium fertilization. Further, the research work carried by various scientists during in the area of juice extraction machines (crushers), production, nutritive value and post-harvest losses, etc. was reviewed by Baboo and Anwar (1995). Uppal and Sharma (1999a, 1999b) have suggested that the physicochemical properties, color, crystalline properties and sweetness of jaggery is unaffected by keeping jaggery in an airtight container during the rainy season. Jain and Singh (2000) presented a method to determine the moisture content of jaggery by using a microwave oven. Recently, the IISR has prepared a jaggery in the form of

3 cubes (size 2.5 cm 3 and weight, 20 g) by moulding frames which is more popular in the Lucknow (UP) region. 3. Methodology of jaggery drying G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) due to extraction of moisture from the greenhouse air and then started loosing moisture to the greenhouse air. (iv) The mass of jaggery used was 800 and 2000 g respectively. Thus, the dried jaggery can be stored during the rainy period with proper packing. The jaggery is a cash crop in India. In order to store jaggery for a longer period, the Indian Council of Agricultural Research (ICAR), sponsored a research project on jaggery drying at the Indian Institute of Technology, Delhi. The following methodology was considered for drying of jaggery: (i) The jaggery of different shape and mass were dried under open sun conditions and indoor conditions with hot/cold air under free and forced convection. It was observed that there was an increase in the mass of the jaggery during the drying period due to transfer of moisture from the atmosphere to the jaggery (Table 1a-c). This may be due to the fact that the relative humidity surrounding the jaggery remains higher during the drying, (ii) After drying jaggery under above conditions, the same procedure was repeated in a large greenhouse with a size of 6 m x 4 m x 3 m. In this case too, there was an increase in the mass of the jaggery during the drying period (Table 1d). (iii) It was then decided to design a greenhouse of smaller size as described below. It was observed that there is an increase in the mass of the jaggery at the beginning and then there was transfer of moisture from the jaggery to the greenhouse air. This is due to the fact that the jaggery was first saturated 4. Design of experimental greenhouse and procedure 4.1. Experimental setup A roof type even span greenhouse with an effective floor coverage 1.2x0.8 m 2 was constructed of PVC pipe and a UV film covering. The central and walls were 0.6 and 0.4 m high respectively. An air vent was provided at roof level with an effective opening of m 2 for natural convection. A fan of 225 mm sweep diameter and 1340 rpm with a rated air velocity of 5 m/s was provided on the sidewall of the greenhouse during the forced convection experiments. The greenhouse was kept at an east-west orientation during the experiments. Wire mesh trays of 0.26 mx0.31 m were used to accommodate 800 g and 2000 g samples of jaggery for single layer drying. A photograph of a controlled environment greenhouse is shown in Fig. 1 for natural and forced convection respectively. A six-channel digital temperature indicator with a least count of 0.1 C (accuracy ±0.1%) having a 125 C range and using copper constantan thermocouples was used to measure the jaggery and air temperatures at different points. A dial type hygrometer with a least count of 1 was used for relative humidity measurement just above the jaggery. An electronic balance of 5 kg capacity with a least Table 1 Observations on jaggery (gur) drying under various open conditions in the months of February/March (Relative humidity ss 70-80%) Time (min) Surface temperature ( C) Inside temperature ( C) Room/ambient temperature ( C) (a) Indoor forced convection by hot air drying Weigh (b) Indoor forced convection by cold air drying (c) Open Sun drying free convection (d) Drying in greenhouse

4 222 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) count of 0.1 g was used to measure the mass at hourly time intervals. The jaggery was kept on the electronic balance during drying. The dial type hygrometer was kept hanging over the jaggery surface with the sensing part facing towards the surface. For the forced convection a fan was fitted in the sidewall of the greenhouse as shown in Fig. 1(b). The solar radiation was measured by a pyranometer (for beam and diffuse radiation) having the least count of 2 mw/cm 2. The diffused radiation was measured by providing a shade over the solar cell detector. Beam radiation was calculated by subtracting the diffuse from total radiation. A digital temperature meter (make Lutron HT-3003, a least count of 0.1 C with accuracy ±1%) was also used to measure the relative humidity and surface temperature of the jaggery pieces. (length = breadth = 11 cm, depth = 7.5 cm) jaggery was also taken for drying. The experiments were conducted for natural and forced circulation mode. The hourly data of intensity (total and diffuse), ambient air temperature, relative humidity inside the greenhouse, jaggery surface temperature, greenhouse air temperature and weight of jaggery were recorded during the experiments. It is observed that the jaggery initially increased in mass but later it started loosening mass as explained earlier. The observations were taken after it started loosening mass and the results are given in Tables 2-4. The m_ ev has been calculated by taking the difference of mass of jaggery between two consecutive reading at 1 h intervals. The data on solar intensity and ambient temperature has not been given due to its non-utility in the model Experimental observations The sample of jaggery was collected from the IISR, Lucknow (cube size = 2.5 cm 3, weight = 20 g). In addition to the sample from IISR, a half brick size 5. Thermal modeling The convective mass transfer coefficient (h c ) under natural convection can be defined as (Tiwari, 2002; Anwar & Tiwari, 2001) Table 2 Experimental data for jaggery (800 g) drying during (Panel A) March 15-17, 2002 under natural convection mode and (Panel B) March 18-20, 2002 under forced convection mode Day of drying Time (pm) Gr Pr C Panel A xlo xlo xlo xlo xlO xlo xl xlo xlO xl xlo xlo xl xlo Panel B xl xlo xlO xlo xlo xl xlo xlO xlo xlO xlO xlO xlo

5 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) Table 3 Experimental data for (Panel A) jaggery drying (half size brick, wt. 800 g) during March 26-28, 2002 under natural convection mode, (Panel B) jaggery (half size brick, 800 g) drying during April 1-6, 2002 under forced convection mode and (Panel C) jaggery (broken half size brick pieces, 800 g) drying during April 1-6, 2002 under forced convection mode Day of drying Time (pm) Gr Pr C Panel A xlo xlo xlo xlo xlo xlo xlo xlo xlO xlo xlo xlO Panel B xlo xlo xlo xlO xlo 7 5.1OxlO xlo xlo xlo xlO 7 3.3OxlO xlo IVth xlo xlO 7 3.3OxlO xlo Panel C xlo xlo xlo xlO xlo xlo xl xlo xlO and under forced convection can be defined as ð1aþ The rate of heat utilized to evaporate moisture is given as (Malik, Tiwari, Kumar, & Sodha, 1982) Q_ e = 0.0l6h c [P(Tj) -yp(t c )} (2) On substituting h c from Eqs. (1) and (2) becomes Q_ e = 0.016^C(GrPr)"[P(7j) - yp{t c )] (3) Evaporated moisture can be determined by dividing Eq. (3) by the latent heat of vaporization (k) and multiplying by the area of the jaggery drying tray (A t ) and the time interval (t). K v = 0 : 016^ - yp(t c )]A t t (4)

6 224 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) Table 4 Experimental data for jaggery (2000 g) drying during (Panel A) May 2-6, 2002 under natural convection mode and (Panel B) April 26-May 1, 2002 under forced convection mode Day of drying Time m_ ev Gr Pr C Panel A am am pm 1.30 pm 2.30 pm 3.30 pm 4.30 pm 5.30 pm xlo xlO xl xlo xlo xl xlo xlO am 1 n pm pm pm pm xlo xlo xlo xlo xl xl xlo am 1 n pm pm pm pm xlO 7 LlOxlO xlo xlo xlo xlo xlo IVth 1 am 1 n pm pm pm pm xlo xlo xlo xlO xlo xlo xlo Vth 1 am 1 n pm pm pm pm xlo xlo xlo xl xlo xlo xlo Panel B 1 am 1 n pm pm pm pm xlo xl xlo xlo xlo xlo n pm pm pm pm xlo xlo xlo xl xlo 7 xlo am 1 n pm pm pm pm xlo xl xlo xlo xlo xlo xl

7 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) Table 4 (continued) Day of drying Time Gr Pr IVth am pm 1.30 pm 2.30 pm 3.30 pm 4.30 pm 5.30 pm xlo xl xlo xl xlo xl xlo Vth am pm 1.30 pm 2.30 pm 3.30 pm 4.30 pm 5.30 pm xlo xlo xlo xlo xlo xlo xlo Let 0 : 016Ji [P(Zj) - yp(t c )]ta t = Z ^P = CðGrPrÞ n Taking logarithm on both sides Eq. (5) can be written ð5þ ln m_ ev = ln C þ n lnðgrprþð 6aÞ This is in the form of a linear equation, y = mx þ c ð6bþ where y = In [^f-], m = n, x = ln[grpr] and c = ln C. Thus, C = e c. Similarly in the case of forced convection mode, jaggery temperature (Tj) and the air above the jaggery surface (T c ). The convective mass transfer coefficient calculated from Eq. (1) for different conditions have also been tabulated and are given in the same tables. Fig. 2(a-c) shows the hourly variation of convective and evaporative mass transfer coefficient under natural and forced convection modes. From these figures, it is clear that the rate of moisture evaporated in the natural 1.6 v = ln m = n ; x = ln[repr], c = ln C and By using the data of Tables 2-4, the values of y and x can be evaluated for different time intervals and then the constants 'C and '«' can be obtained from the above equations. The constants 'C and '«' will be further used to evaluate convective mass transfer coefficient from Eq. (1) under natural and forced convection. Knowing the convective mass transfer coefficient (h c ), the evaporative mass transfer coefficients (h e ) can also be evaluated from Eq. (2) as follows: o 5= W/r o hc-n 12hc-f A he-n Eq.he-f = 16 : 273 x 10 3 /J, fp(t i )-yp(t c )\ T T ) (7) 6. Results and discussion The results of'c and'«' for different masses of jaggery (800 and 2000 g) under natural and forced convection modes are given in the same tables (Tables 2-4). The range of Grashof numbers for each case has also been given. The physical properties of the moist air above the jaggery surface were obtained at an average value of Fig. 2. Hourly variation of convective and evaporative mass transfer coefficient under natural and forced convection mode for (a) first day of drying (Mj = 800 g), (b) second day of drying, (c) third day of drying (Mj = 800 g).

8 226 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) convection mode of drying is slow in comparison with that of the forced drying mode for a given drying period (3 days). In both cases, the rate of moisture removal initially increases and then decreases for a given day. The total moisture removal decreases with day of drying as expected. This can be explained in terms of the convective mass and evaporative heat transfer coefficients obtained by the present model as given in the same figure and in Table 2. The convective mass transfer coefficient varies from 1.29 to 1.41 W/m 2 K for fist day in the natural convection mode of drying. This value is marginally increased to 1.3 to 1.46 W/m 2 K in the forced convection mode due to changes in the rate of moisture removal and the drying temperature. hc -n 2hc-f he -n he-f o E g o hc-n 12hc-f A he-n -fhe-f (c) oo "E g o hc-n - hc-f A he-n -A-he-f "E g hc-f 8he-f 2 3 (d) 0.8 Fig. 3. Hourly variation of convective and evaporative mass transfer coefficient under natural and forced convection mode for (a) first day of drying (half size brick, Mj = 800 g), (b) second day of drying, (c) third day of drying, (d) fourth day of drying (broken half size brick pieces, Mj = 800 g) oo o hc-n 20 "E -fhc-f g^ A he-n 10 A he-f hc-n hc-f he-n 0he-f 40 u 30 « hc-n 30hc-f A he-n 00he-f (b) hc-n hc-f 0he-n he-f Fig. 4. (a) Hourly variation of convective and evaporative mass transfer coefficient under natural and forced convection mode for (a) first day of drying (Mj = 2000 g), (b) second day of drying, (c) third day of drying, (d) fourth day of drying, (e) fifth day of drying.

9 G.N. Tiwari et al. / Journal of Food Engineering 63 (2004) Fig. 3 shows the results for a half size brick in natural as well as forced convection mode of drying. In forced convection drying the experiment was conducted in two parts namely (i) the jaggery brick was dried first for four days and (ii) the brick was broken into small parts and was dried for two days for complete removal of moisture from the jaggery. In this case also, the behavior of the convective mass transfer and evaporative heat transfer coefficients was similar to those explained earlier (see Fig. 2). The results for convective and evaporative mass transfer coefficients for 2000 g of jaggery have been reported in Fig. 4. It is observed that the convective and evaporative mass transfer coefficients in forced convection drying are higher than in the natural convection mode of drying, as expected. However, the rate of moisture removal in the forced convection mode is less than in the natural convection mode due to the low drying temperature. The same results have also been summarized in Table Conclusions and recommendations On the basis of results reported Figs. 2-4 and Tables 2-4, the following conclusions can be drawn: The complete drying of jaggery under forced convection is faster than under natural convection as expected. The convective mass transfer coefficient in forced convection is higher than in the natural convection mode. For a given mass of jaggery ( gm), the size of a greenhouse will be same as shown in Fig. 1(a). Initially the convective mass transfer coefficient is higher and decreases as drying proceeds in both the cases as expected (Figs. 2-4). Similar results were also observed in the case of evaporative mass transfer coefficient. The values of experimental constants C and n vary from 1.39 to 0.20 and 0.32 to 0.10 respectively. Acknowledgements The financial assistance received from the Indian Council of Agricultural Research (ICAR), Government of India for carrying out this work is gratefully acknowledged. The authors are also grateful to Dr. S.I. Anwar, Senior scientists, IISR, Lucknow (UP) for providing the information for the literature review in this paper. References Agrawal, M. P., & Ghosh, A. K. (1984). Designing and evaluation of godowns for storing jaggery by small farmers. Journal of Indian Sugar, Anwar, S. I., & Tiwari, G. N. (2001). Evaluation of convective heat transfer coefficient in crop drying under open sun drying. Energy Conversion and Management, 42(5), Baboo, B. (1985). National seminar-cum-group discussion on jaggery manufacture and storage. Division of Agriculture Engineering, IISR, Lucknow (UP), India. Baboo, B. (1990). Bibliography of researches on jaggery (gur) in India, Technical Bulletin No. 28, IISR, Lucknow (UP), India. Baboo, B., & Anwar, S. I. (1995). Recent development in jaggery (gur) research, Technical Bulletin No. IISR/JKS/94/9, IISR, Lucknow (UP), India. Gunjal, B. B., & Galakatu, P. D. (1986). Water vapour adsorption by jaggery. Journal of Indian Sugar, Jain, P. C., & Singh, P. (2000). Moisture determination of jaggery in microwave oven. Journal of Sugar Technology, 2(3), Javalekar, D. V., Shinde, B. N., & Randive, S. J. (1986a). Use oforthophosphoric acid in jaggery making I, effect on physical properties. Journal of Indian Sugar, Javalekar, D. V., Shinde, B. N., & Randive, S. J. (1986b). Use oforthophosphoric acid in jaggery making II, effect on chemical properties. Journal of Indian Sugar, Khanna, K. L., & Chakravarti, A. S. (1955). Scientific monographresearches on technical aspects relating to gur industry in Bihar. New Delhi, India: ICSC (pp ). Malik, M. A. S., Tiwari, G. N., Kumar, A., & Sodha, M. S. (1982). Solar distillation. UK: Oxford Pergamon Press. Patil, J. P., & Singhte, A. K. (1992). Effect of application of phosphorus and potassium to sugarcane crop and its effect quality of jaggery. Journal of Indian Sugar, Roy, S. C. (1951). Monograph on the Jaggery (gur) Industry of India. New Delhi: ICSC (pp ). Shinde, B. N., Marathe, A. B., Jabvaleker, D. V., & Kadam, S. K. (1983). Effect of neutralization of sugarcane juice acidity on the keeping quality of jaggery during storage. Journal of Indian Sugar, Tiwari, G. N. (2002). Solar energy fundamental design, modeling and applications. New Delhi, India: Narosa Publishing House. Uppal, S. K., & Sharma, S. (1999a). Evaluation of different methods of jaggery (gur) storage in subtropical region. Indian Journal of Sugarcane Technology, 14(1), Uppal, S. K., & Sharma, S. (1999b). Evaluation of new sugarcane varieties for jaggery (gur) quality and their self-life in airtight containers during rainy season. Journal of Indian Sugar,