The Effect of Drying Air Temperature and Humidity

Applied Mechanics and Materials Vol. 315 (2013) pp 710-714 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.315.710

The Effect of Drying Air Temperature and Humidity on the Drying Kinetic of Kenaf Core Suhaimi Misha 1,2,a*, Sohif Mat 1,b, Mohd Hafidz Ruslan 1,c, Kamaruzzaman Sopian 1,d and Elias@Ilias Salleh1,e 1

2

Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia a

[email protected], [email protected], [email protected], d

[email protected], [email protected]

Keywords: Drying kinetic; Kenaf core; Drying models

Abstract. Drying kinetics of kenaf core was investigated in a Low Temperature and Humidity Chamber Test. The drying experiments were carried out at temperature of 45, 50 and 550C and air relative humidity of 10, 20 and 30% using a constant air velocity of 1.0 m/s. The moisture content data at various drying air conditions were converted to moisture ratio and plotted against time to obtain the drying curves for each experimental data. The curves were fitted to eight different thin layer drying models to determine a suitable model for drying of kenaf core. The fit quality of the models was evaluated based on their coefficient of determination (R2), reduced chi-square (χ2) and root mean square error (RMSE). Among the eight models, Two Term model is the best model for describing the drying behavior of kenaf core. The drying air temperature gave more significant effect on the drying kinetic of kenaf core compared to the drying air relative humidity under the experimental conditions studied. Introduction Kenaf fibres have been commercially used as industrial fibres in various industries such as in the fibre board, paper, mattress, thermoplastic composites, cushion, insulator, wall panels, door and etc. Kenaf can be used as an alternative for timber because it is a short term fibre crop. According to K. Cronin [1] water is present in timber in the form of free water and bound water. The free water, fills the wood cell cavities and intercellular spaces where as bound water is hygroscopically attached to the cell walls. The kenaf stems produce two types of fibre, a coarser fibre in the outer layer (bast fibre) and a finer fibre in the core. In producing the kenaf fibre, both fibres need to be dried. Wan Ramli Wan Daud et al.[2] carried out drying experiment using superheated steam to dry kenaf fibres at atmospheric pressure, temperatures in the range of 110 to 170°C and steam superficial velocity of 0.50 m s-1. The superheated steam can reduce the moisture content by almost 100 % in 15 minutes without steam condensation during the drying process and eliminate the discolored problem. Ooi Ho Seng and Ten Seng Teik [3] investigated the kenaf drying using the flat bed box dryer. At drying air temperature of 85°C, the drying duration was about 20 hours and the overall thermal efficiency was about 33%. Whereas at drying air temperature of 70°C, the drying duration was 24 hours and the overall thermal efficiency was about 46%. Drying kenaf core under open sun drying is time consuming and suffers from high product losses due to inadequate drying, fungal growth, encroachment of insects, birds and etc. The same product through difference drying methods will also produce difference quality of product in texture, colour, taste and nutrient content [4]. Drying at high temperature, low humidity and high flow rate will speed up the drying rate of the product. However, drying at high temperature will

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decrease the quality of the product especially on heat sensitive product. Excessively hot air can also cause dehydrate the solid surface, leading to crust formation and stop the drying process since the water from the interior will not migrate to the surface for evaporation. Drying at low temperature and humidity can be done by using desiccant dryer as reported by Misha et al. [5]. The objective of this study were to investigate the effect of drying air temperature and humidity on the drying kinetics of kenaf core in a Low Temperature And Humidity Chamber Test, and to select a best mathematical model using several thin- layer drying models available from the literature. Material and methods Material. Chipped kenaf core was supplied by Institute of Tropical Forest and Forestry Product (INTROP), Universiti Putra Malaysia (UPM). The average length of the sample is about 4 cm with diameter in between 1 and 1.5 cm. The kenaf core is dried without the outer layer (bast fibre) and cut into two, half of cylinder shape. The initial moisture content was determined by drying in oven at 105oC until constant weight was obtained. The average initial moisture content of the sample was about 60% wet basis. Drying procedures. In this study, the drying of kenaf core fibre was conducted in a Low Temperature and Humidity Chamber Test. The weight of the sample will be recorded in the computer for each 5 minutes using digital balance. Three drying air temperature (45, 50 and 550C) and three drying air relative humidity (10, 20 and 30 %) were selected to investigate the effect of drying air temperature and humidity on kenaf core drying. Air velocity was set at constant value of 1 m/s. Approximately 25 g of wet kenaf core fibre were spread on a wire mesh tray and loaded in drying chamber test. The drying process was continued until there was no weight change for eights successive readings. Mathematical modeling of drying curves. Several simplified drying models that are commonly used to describe the drying kinetics of a product, is shown in Table 1 [7-9]. In these model, moisture ratio (MR) represents the dimensionless moisture ratio expression by using following equation:

=

(1)

where Mt is the moisture content of kenaf core at time, t; M0 is the initial moisture content kenaf core; Me is the equilibrium moisture content of kenaf core. All the moisture contents are calculated based on dry basis. Me is relatively small compared to Mt and M0 [10-12]. Thus, MR can be simplified to MR= Mt/M0. The thin-layer drying models from Table 1 were fitted to find the most suitable model to represent the drying curve of kenaf core. The non linear regression analysis was carried out using online free mathematical software [13] to select the best model. The website has capability to produce quality curve fit and surface fit for 2D and 3D data online. The coefficient of determination (R2) was the main criterion for selecting the best model to describe the drying curves [10,14]. In addition to R2, the statistical parameter, such reduced chi-square (χ2) and root mean square error (RMSE) were calculated to evaluate the fitting of a model to experimental data. The highest values of R2 and the lowest values of RMSE and χ2 were used to determine the best fit [6,11]. These statistical parameters can be calculated as follows:

=

∑

,

=

∑"%

,

(2) !," −

!$ ,"

&

⁄

(3)

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Where MR pre,i is the ith predicted moisture ratio, MR exp,i is the ith experimentally observed ratio, N the number of observation and n is the number of constant in a model. The value of each coefficient in models can be determined using a plot of curve drying models.

No. 1 2 3 4 5 6 7 8

Table 1 Mathematical models of drying curves. Model Name Model Newton MR = exp(-kt) Page MR = exp(-ktn) Modified Page MR = exp[-(kt)n] Henderson and Pabis MR = a exp(-kt) Logarithmic MR = a exp(-kt) + c Two term MR = a exp(-k0t) + b exp(-k1t) Wang and Singh MR = 1 + at + bt2 Midilli et al. MR = a exp(-ktn) + bt

Result and discussion Analysis of drying curve. The drying curve of the kenaf core fibre dried in Low Temperature and Humidity Chamber Test until no significant weight lossess at different drying air temperature and humidity are shown in Fig. 1,2 and 3. It was found that at the early stage, moisture ratio decrease continuously with drying time but as time increase there are constant drying exists. Initially the free water was removed from the sample and then followed by bound water. The free water is easy to be removed since it in the liquid phase, fills the wood cell cavities and intercellular spaces. Bound water is more difficult to be removed since it hygroscopically attached to the cell walls. The drying air temperature has significant effect on the drying kinetics of kenaf core. The drying time to the moisture content value approximately 0% dry basis (db) for drying air humidity of 10% RH were 56, 75, 127 min at 55, 50 and 45oC, respectively. An increase of drying temperature at constant air humidity reduces the drying time of the kenaf core. The drying time should be shorter at lower humidity as presented at drying air temperature of 55oC. However, it showed that the drying air relative humidity give less significant effect on the drying curve of kenaf core under the experimental conditions studied. The result also may effected by the different initial moisture content of the samples.

Moisture ratio

1.0

Relative humidity = 10%

0.8 0.6

45 C 50 C 55 C

0.4 0.2 0.0 0

50

100 150 Drying time (min)

200

Fig. 1 Moisture ratio versus drying time of kenaf core at different temperatures (10% RH)

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Moisture ratio

1.0

713


0.8 45 C 50 C 55 C

0.6 0.4 0.2 0.0 0

50


200

Moisture ratio

Fig. 2 Moisture ratio versus drying time of kenaf core at different temperatures (20% RH) 1.0 0.8 0.6 0.4 0.2 0.0


45 C 50 C 55 C

0


150

Fig. 3 Moisture ratio versus drying time of kenaf core at different temperatures (30% RH) Modeling of drying curves. The moisture content data obtained from the drying experiment were converted into moisture ratio and then were fitted to the eight mathematical models. The average values of R2, χ2 and RMSE for all model are presented in Table 2. All the average value of R2 for each models is below 0.99 due to the constant drying exist in the experiment. The result showed that Two Term model has the highest value of R2 and the lowest value of χ2 and RMSE. Thus Two Term model appears as the best model in describing the drying curves of kenaf core. The predicted moisture content was validated with the measured moisture content. The plots of predicted moisture ratio and experimental moisture ratio by Two Term model showed that the data mainly scattered adjacent to the 45o straight line, thus indicated that this model could represent the drying characteristic of kenaf core accurately. Table 2 Statistical results from eight different thin-layer drying models No. Model Name Variables (Average values) R2 χ2 RMSE Newton or Lawis 0.973729 0.002081 0.044171 1 Page 0.983386 0.001425 0.035550 2 Modified Page 0.983386 0.001425 0.035550 3 Henderson & Pabis 0.981345 0.001558 0.037216 4 Logarithmic 0.983434 0.001375 0.034542 5 Two term 0.988244 0.001152 0.029699 6 Wang & Singh 0.895969 0.008583 0.086615 7 Midili et al. 0.825521 0.013611 0.071068 8

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Conclusion Thin layered kenaf core was dried in a Low Temperature and Humidity Chamber Test at three different drying air temperature (45, 50 and 55) and three different relative humidity (10, 20 and 30% RH). The drying curves of kenaf core show a constant drying rate at certain period before achieved the equilibrium moisture content. It may due to the bound water in the sample. The moisture ratio data was fitted to eight selected mathematical model using online website. The mathematical software provided by this website is very useful to produce quality curve fitting and surface fitting for 2D and 3D data. Among eight of the mathematical models, the model developed by Two Term showed good agreement with the experimental data. The drying air temperature gave more significant effect on the drying kinetic of kenaf core compared to the drying air relative humidity under the experimental conditions studied. Acknowledgements The authors would like to thank the Solar Energy Research Institute and Centre for Graduate Management, Universiti Kebangsaan Malaysia and Universiti Teknikal Malaysia Melaka for sponsoring this work. Thank also to INTROP, UPM for provide the kenaf core sample. References [1]

[2]

[3] [4] [5]

[6] [7] [8] [9] [10] [11[ [12] [13] [14]

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