Design of tunnel drier for the non-centrifugal sugar industry

The quality and shelf-life of NCS (Non-centrifugal sugar) mainly depend on the moisture content present in it. NCS formed by the current practice of open sun drying contains moisture substantially greater than the acceptable level of 3%. This paper presents the work taken up to design a tunnel dryer to attain require moisture content in granular NCS for various load conditions. Initially, an experimental investigation had been carried out on a laboratory scale dryer to achieve require moisture content (< 3%) for various load conditions. This experimental data was used for validating two drying models and found that one of the models is best suitable for designing an industrial-scale dryer. For various load conditions on each tray and dryer exit temperature, nine different cases were arrived at. The number of trucks, trays, drying time and energy requirements were computed using the suitable theoretical model. Tunnel dryer with a length of 18 m, a height of 1.2 m, a width of 1 m, number of trucks of 18 and 24 number of trays on each truck was found to be the suitable dryer to dry 1 tone of NCS based on the minimum energy requirement of 176.49 MJ, and a minimum drying time of 68 minutes.


Introduction
Non-centrifugal sugar (NCS), popularly referred to as jaggery, is a traditional and unrefined sugar produced by sugarcane juice concentration. It is a rich mixture of essential nutrients and is therefore considered as the healthiest kind of sugar for human consumption [1]. The processing of NCS is a continuous process of heat and mass transfer involving clarification, boiling and concentration of sugarcane juice [2]. A traditional NCS production unit typically consists of a sugarcane juice crusher, an underground furnace fitted with single or multi-pan systems, where the water is evaporated to concentrate the juice to produce NCS [3] [4]. In India, a large portion of the NCS is manufactured in small-scale cottage level industry [5]. Figure 1 shows the basic production process of NCS production. Figure 1. Basic production process NCS production [3] NCS production in India starts in the month of October and continuous till mid of May and is stored for the rest of the year [6]. The quality of stored NCS depends mostly on the moisture content. The higher moisture in NCS is favorable for the inversion and growth of different types of fungi and bacteria resulting in a change in taste and colour [7]. temperature [8]. The freshly prepared NCS contents moisture about 13 to 15% but preferably is below 3% for NCS storage for longer shelf-life [9]. Further, it has become a prerequisite for the industry to maintain the optimal level of moisture content in NCS to meet the standards and requirements in food hygiene and handling such as packing, transportation and distribution [10]. Drying of NCS is one of the processes to remove the excess moisture content and further increases the shelf-life of NCS during storing. It is a heat and mass transfer phenomenon in which the heat energy is transferred from the surrounding to the NCS surface in different heat transfer modes. Some part of heat supplied is used for sensible heating to increase the surface temperature of NCS and a part is used for the latent heat of vaporisation to remove the moisture present in the interior of NCS. The amount of moisture removed from the NCS depends upon the induced vapour pressure difference between the NCS surface and the surrounding medium [11].
Depending upon the method of drying, drying may be classified as open sun drying or controlled drying [12].  [13]. Every year, it is noted that more than 10% of NCS produced in India worth $0.6 million is lost due to moisture deterioration [14]. Therefore, to achieve a preferable moisture content (that is below 3%), the drying rate should be controlled instead of depending on the weather conditions. To have better control over the drying rate, a control dryer is to be designed and analysed as per the requirement to remove moisture in NCS.
Controlled dryers are available in a wide variety to meet the requirement of the food industry.
The selection of dryers for a particular application depends mainly on the amount of moisture to be removed, the scale of operation, method of heat energy supplied and cost [15]. The dryers that are commonly used in food industries are tray dryers, tunnel dryers, drum dryers, fluidized bed spray dryers, flash dryers, rotary dryers, belt dryers, vacuum dryers, and freeze dryers.
Among these dryers, the tunnel dryer is most extensively used because of its simple and economic design and most importantly it produces uniform drying [16]. It can remove moisture uniformly from the material with low moisture content to a material with high moisture content without any deterioration. Unlike other dryers, the operation is simple and produces slow and uniform drying [17].
Tunnel dryers are considered as the development of a tray dryer. Figure 2 shows the schematic diagram of tunnel dyer. The trays arranged on the trolleys called trucks. The dryer contains several such trucks/trolleys, each of which behaves as a separate batch tray dryer. The material to be dried is placed uniformly on each tray. Then the air is blown over the trays in the tunnel using the blower for proper circulation [16]. Forced convection heating takes place to remove moisture from the material placed in trays. A truck of dried material is removed from the dry end of the tunnel and the remaining trucks are pushed forward along the truck length and the truck of wet material is rolled into the vacant space at the wet end of the dryer [18]. This paper presents the works taken up to design a tunnel dryer to attain moisture content below 3% in granular NCS for various load conditions. Initially, a laboratory-scale dryer is considered and the experimental analysis is carried out for drying NCS. This experimental data is used for validating the drying models that were considered to design a suitable industrial tunnel dryer for removing excess moisture in NCS, to produce quality NCS and can be retained during storing.

Materials and methods
The schematic diagram of the methodology adopted for designing an industrial dryer to achieve a moisture content below 3% in granular NCS is shown in Figure 3. The following sections describe the methodology adopted. Figure 3. Methodology adopted for designing an industrial tunnel dryer.

Experimental analysis for drying NCS
Initially, the experimental analysis was carried out on a laboratory scale forced convective tray dryer with a tray area of 0.128 m 2 , to achieve the required moisture content in NCS granules and to determine the drying time. Figure 4 represents the laboratory scale forced convective tray dryer. The dryer is attached with the mass balance that ranges from 0 to 10 kg to measure the change in mass of the material placed in the tray. It is also equipped with necessary software and hardware support for data acquisition related to a reduction in mass with respect to time. The tray with NCS was covered with an empty tray of the same area at a distance of 0.05m to form an air duct and mimic the configuration in the real dryer with a number of trays and trucks. The hot air required for drying the NCS granules is supplied by means of blower and heater mounted at one end and along the air duct respectively. Air temperature and velocity are the most important factors that influence the drying rate during the drying process [19]. The temperature of air considered for the drying process should not exceed the glass transition temperature of NCS which is 45 o C, beyond which the NCS granules will fuse [20]. The air velocity beyond 2m/s has no significant effect on the drying rate [21].
Therefore, inlet parameters such as velocity and temperature of the air to achieve required moisture content in NCS are considered to be 2m/s and 45 o C respectively.   Figure 5 shows the uniform distribution of the NCS sample on a tray area of 0.128 m 2 . The moisture content of the NCS sample was found to be 4.57% using the thermogravimetric analyser, which should be reduced to 3%. In the process of drying, removal of moisture from the surface and also from the interior of the material is essential, in which drying rate plays a key role [22]. The percentage of moisture removed is assessed by weighing the samples for every 30 seconds. Further, the drying rate is computed by equation 1.
where mi is the mass of the NCS sample before drying, mf is the mass of the NCS sample after drying and t is the drying time.

Theoretical drying models and their validation
To make use of the data obtained through the experiments to get a generalized theoretical model that could be used for the design of an industrial dryer, the following two theoretical models were considered to obtain percentage moisture removal rates.
Model I: Figure 6 represents a typical drying curve applicable to granules. The initial moisture reaches critical moisture at a constant rate of drying during the constant rate period represented by CB. The stage CE represents the falling rate period, during which the moisture reaches equilibrium moisture content and the drying rate is directly proportional to the moisture content [23] [24]. Figure 6. Drying rate curve for a constant drying condition [24] As explained, in the results and discussions section, the experimental data when looked in conjunction with the drying rate curve can be observed that the drying of NCS comes under the falling rate period. Under these conditions, the following classical equation (equation 2) suggested by Dincer [23] could be used for obtaining the drying time (dt).
Using the above two theoretical models, the drying time with respect to the percentage of moisture removal was computed and the results are compared with the experimental results to ensure the applicability of these models for developing an industrial-scale dryer. As presented in the results and discussion sections, the comparison ensured that the theoretical model I could be used for the development of industrial-scale tunnel dryers.

Industrial tunnel drier design using theoretical models
The above verified theoretical model I is used to develop a tunnel dryer capable of drying 1 ton of NCS that brings down the moisture from 4.75% to 3% in an estimated time of around 1.5 hours (same as the drying time in conventional NCS plant). As depicted in Fig. 1, a two steps process is followed for this.

Application of drying model I for tray area of 1m 2
In the first step, the above theoretical model I was applied for of tray area of 1m 2 , initially, to ensure the applicability of this theoretical model I.

Development of tunnel dryer
The tunnel dryer is considered to be the improvement of the tray dryer. It contains multiple trucks with each truck having several trays as shown in figure 2. The main parameters to be arrived at, for this designing process, are (i) the number of trays that can be arranged in the dryer (ii) energy required for the drying process. The actual number of trucks that can be accommodated in the dryer is equal to the number of trucks obtained when the outlet temperature of the last truck is equal to the final exit temperature of the dryer. To achieve optimum dryer design among these nine case, it is necessary to estimate energy for each case to dry 1 ton of NCS granules The total energy required for the drying 1 ton of NCS granules can be estimated by computing the energy required by the blower and heater for supplying air at inlet of 2 m/s and 45 o C respectively. The energy required by the blower (Eblower, kJ) and heater (Eheater, kJ), can be estimated from the following equations 8,9 with heater and blower efficiencies as 90% and 70% respectively [25].
The air flow rate can be determined by taking into account the cross-sectional area of the tray, over which the air flows at a constant velocity. The optimum dryer design can be obtained = based on the minimum energy required by the heater and blower and minimum drying time to remove moisture present in one tone of NCS granules.

Results & discussions
The analysis associated with the design of an industrial tunnel was carried out based on earlier presented experimental data and the theoretical models. The following sections discuss the findings to design an optimum tunnel dryer for achieving the required moisture content in NCS.

Experimental results
The experimental analysis was carried out for various loads of NCS samples (0.     It is also observed that the drying time and the number of trucks are comparatively higher when the dryer exit temperature is at 38 o C. Similarly, the drying time and the number of trucks are relatively minimum when the dryer exit temperature is at 42 o C. Figure 11, shows the variation of temperature with respect to each truck. It is observed that temperature is uniformly decreasing from one truck to another truck in all the cases and the number of trucks is increasing with decreasing in the exit temperature of the dryer.  Figure 13 shows the total energy required for drying 1 ton of NCS granules for varied input conditions. It is observed that 150KJ of energy is required to blow the air at 2m/s and  Therefore, based on the minimum energy required, the cases viz. C7, C8 & C9 are considered for designing the optimum tunnel dryer to achieving the required moisture content in 1 ton of NCS granules. Table 3. show the three optimum tunnel dryer designs for drying one tone of NCS with air inlet temperature and velocity as 45 o C and 2m/s respectively.

Conclusions
In the present paper, an attempt has been made to design a tunnel dryer to attain moisture content below 3% in NCS granules. Initially, the experimental analysis was carried out on a laboratory scale forced convective tray dryer with a tray area of 0.128 m 2 for various loads of NCS. During this experimental study, the drying of the NCS sample is found to be in the falling rate period with average equilibrium moisture of 2.87%. It is also observed that the rate of drying is decreasing with an increase in a load of NCS samples on each tray. This experimental data is used for validating two drying models and found that one of the models is best suitable

Acknowledgements
The research work presented in this paper is a part of "Sustainable technological solutions for energy efficiency in jaggery industry (STEEJ)" project, funded by Royal academy of