Falling Rate Period Research Articles

Microwave drying performance of spent coffee grounds briquette coupled with mineral additives

The microwave drying performance of spent coffee grounds (SCG) briquette in a bench-scale setup was evaluated in terms of the influences of power levels (119–700 W) and three mineral additives (10% sodium chloride, 10% sodium sulfate and 10% lignite). During the drying process, a short warm-up period, a first and a second falling rate period were detected. Compared with raw SCG briquette, the required drying times of SCG with Na2SO4 and NaCl additives at 385 W decreased by 38% and 18%, respectively, while the drying time increased by 9% with the addition of the fine lignite. The effective moisture diffusivities of raw SCG in the first falling rate period were 7.89 × 10−9 to 1.75 × 10−8 m2 s−1, while that in the second falling rate period were 1.09 × 10−9 to 1.81 × 10−8 m2 s−1. The average activation energy of raw SCG in the falling rate period was 24.43 W g−1. The apparent activation energy in the falling rate periods for SCG/sodium carbonate and SCG/sodium chloride was less than that of the raw SCG, which indicated that sodium chloride and sodium sulfate could improve the microwave drying performance of SCG briquette. The lignite had a negative effect on the microwave drying kinetics of SCG briquette.

Intensification of cellulosic fiber drying through fundamental insights and process modeling

Balance between heat, mass, and momentum transfer decides effectiveness of cellulose fiber drying in a conveyor dryer such as final product moisture variability aspects and steam consumption. High speed murata vortex spinning (MVS) machines demand precise control over cellulosic fiber moisture and its variability during yarn making. In the present study, cellulosic fiber drying operation at commercial scale is intensified by reducing the moisture variability with the help of drying fundamentals (drying rate curve and critical moisture content) along with mass transfer correlation, and ASPEN based process model development. Cellulosic fiber drying follows falling rate period, where rate of mass transfer decreases with moisture content. Established mass transfer correlation provides connect between Sherwood number with process parameters such as air temperature, inlet fiber moisture content, and absolute air humidity. Developed ASPEN process model for commercial scale cellulosic fiber drying was validated successfully with plant data. Deployment of optimized moderate drying strategy derived based on process modeling and drying fundamentals at commercial scale significantly reduced the moisture variability (80% product moisture falls within 10.5–11.5% moisture band on dry basis) with 9% steam consumption reduction.

Mathematical modelling of drying kinetics of oven dried Hibiscus Sabdariffa seed

Hibiscus sabdariffa or Roselle seed is an agricultural product which possesses a large amount of nutritional content that could be useful in wide application. Drying process can be helpful to conserve the seeds for long storage periods. For this reason, the drying behavior of Roselle seeds was investigated using laboratory oven at various drying temperatures (35, 45, 55, 60 and 65°C). It is found that the drying process of the Roselle seeds consists of constant rate and mostly falling rate period at where the moisture content of Roselle seed decreases as time goes. Adequately suitable drying temperature is found to be 65°C with the drying time of 240 minutes. The drying behavior of Roselle seeds is further understood through the drying kinetics of the Roselle seeds. Five empirical mathematical models were selected to describe and compare the drying characteristics of Roselle seeds at respective drying temperatures. Drying rate constants, coefficients and statistical parameters were determined by non-linear regression analysis, and therefore, the diffusion approach model was selected as the best model with R value obtained is 0.999 while MBE, RMSE and χ2 values of 2.37E-04, 0.01335 and 0.007794 respectively.

Engineering analysis in manufacturing process of Nori made from mixture of Ulva lactuca and Gracillaria sp

A research had been conducted regarding the drying of nori made from local seaweed of Ulva lactuca and Gracillaria sp. The drying process was carried out at 50°C, 60°C and 70°C using mechanical dryer. Drying rate was evaluated with four thin layer drying models, namely Henderson and Pubis, Lewis, Page and Modified Page. The most suitable model is determined from the the highest R2 value and lowest Root Mean Square Error (RMSE) value. The results indicated that nori could be made from local seaweed of Ulva lactuca and Gracillaria sp with a specific method and composition. Drying rate of local nori showed a long constant drying period as compared to it’s falling rate period. Drying rate increased and drying time decreased as the drying temperature increased. The highest drying rate is obtained at 70°C and the Page model was found to be the best suitable model to describe nori drying characteristics.

Heat Transfer Characteristics and Kinetics of Camellia oleifera Seeds During Hot-Air Drying

Abstract The heat transfer characteristics and kinetics of Camellia oleifera seeds under hot-air drying were investigated at different temperatures (40, 60, and 80 °C) and loading densities (0.92, 1.22, and 1.52 g/cm2) with a constant air velocity of 1 m/s. Twelve common drying kinetic models were selected to fit the experimental data. The most suitable model was chosen to describe the hot-air drying process of C. oleifera seeds and help in its optimization. The results showed that the drying temperature has a significant influence on the hot-air drying characteristics of C. oleifera seeds. As the drying air temperature increases, the drying time decreases. The effect of the loading density on the drying characteristics of C. oleifera seeds is much smaller than that of temperature. With the increase in the loading density, the drying time slightly increases. The hot-air drying curve of C. oleifera seeds consists of a very short acceleration rate period at the beginning and a long falling rate period, indicating that the drying of C. oleifera seeds is mainly controlled by the diffusion of moisture inside the material. An effective moisture diffusion coefficient of C. oleifera seeds was estimated to range from 0.81256 × 10−9 to 3.28496 × 10−9 m2/s within the temperature range studied. The average activation energy was 28.27979 kJ/mol. The logarithmic model was found to be the best model to describe the kinetics of hot-air drying of C. oleifera seeds.

Mathematical modelling of infrared-dried kiwifruit slices under natural and forced convection.

In this work, the effect of the radiation intensity, slice thickness, and the distance between slices and infrared lamps under natural drying air and the effect of slice thickness and air velocity under forced drying air on the moisture diffusion characteristics and the drying rate of kiwifruit slices during infrared drying were investigated. The drying of kiwifruit happened in the falling rate period, and no constant‐rate period was observed in the drying curves. One hundred models were fitted to the drying data. Among the models, the exponential dsecay function model and modified two‐term exponential‐V model and the artificial neural networks with 4‐5‐7‐1 and 3‐5‐5‐1 topologies, hyperbolic tangent sigmoid transfer function, and Levenberg‐Marquardt training algorithm presented the best results and showed the goodness of fit with the experimental data for the former and latter systems, respectively. The diffusivities varied between 1.216 × 10−10–8.997 × 10−10 m2⁄s and 2.567 × 10−10–10.335 × 10−10 m2⁄s for natural and forced drying air systems, respectively.

Heat transfer characteristics and drying kinetics of hematite thin layer during hot air convection

Heat transfer characteristics and drying kinetics of the hematite thin layer during hot air convection based on a lab-scale setup were examined at seven temperatures (100, 110, 120, 130, 140, 150, and 160 °C). Four periods during the drying process were detected, a warm-up period, a constant rate period, the first and second falling rate periods. Two isothermal temperature stages were captured during the hot air drying. The average heat transfer coefficients of the whole drying process increased by 28%, 40%, 49%, 56%, 61% and 66% as the hot air temperature rising from 100 to 110, 120, 130, 140, 150 and 160 °C. The effective moisture diffusivities of the hematite thin layer were from 8.37 × 10−9 to 2.05 × 10-8 m2 s−1 for the first falling rate period, and from 1.24 × 10−8 to 4.06 × 10-8 m2 s−1 for the second falling rate period. The isothermal temperature increased with the elevated hot air temperature, which increased by about 56% as the hot air temperature rose from 100 to 160 °C.

Drying kinetics of pomegranate fruit peel (cv. Wonderful)

CITATION: Mphahlele, R. R., Pathare, P. B. & Opara, U. L. 2019. Drying kinetics of pomegranate fruit peel (cv. Wonderful). Scientific African, 5:e00145, doi:10.1016/j.sciaf.2019.e00145.

Comparison of drying kinetics and product quality from convective heat pump and solar drying of Roselle calyx

This study investigated the effect of temperature (40, 50, and 60°C) and sample mass (100, 200, and 300g) on the drying characteristics and product quality of Roselle calyx using heat pump dryer. It was observed that higher drying temperature and lower sample mass led to a shorter drying time (faster drying rate). Higher temperature supplied more heat to the Roselle sample, which increased the activity of water molecules and moisture diffusion. This can be seen where the effective moisture diffusivity increased from 7.87×10−10 to 2.05×10−9m2/s when air temperature of heat pump dryer was increased from 40ᵒC to 60ᵒC. The drying rates for all samples shared the same drying pattern: warming-up period where the drying rate started to increase due to the breakdown of the waxy layer on Roselle and falling rate period where it decreased gradually as the drying progressed. The Roselle sample dried with heat pump dryer at 40ᵒC (100g) possessed the least degradation of quality, maintaining 86.8%, 63.2%, 98%, and 63.3% for color, catechin, protocatechuic acid, and vitamin C, respectively. Overall, heat pump dryer outperformed the sun drying method by recording a much shorter drying period without compromising the product quality.

Effect of drying air temperature on drying kinetics, color, carotenoid content, antioxidant capacity and oxidation of fat for lotus pollen

Hot air drying of lotus pollen at temperatures ranging from 40 to 70 °C was investigated experimentally. Results indicated that the whole drying process occurred in the falling rate period. Color analysis showed that lotus pollen suffered color deterioration. The antioxidant capacity decreased and the fat oxidation activity increased following drying. The carotenoid content exhibited significant loss when dried at 70 °C (p < .05). Zero-order, first-order, and fractional conversion models were employed to fit the experimental data for color profile, antioxidant capacity, and fat oxidation. The first-order fraction model can precisely describe the color change and the antioxidant capacity deterioration, while the kinetics of thiobarbituric acid reactive substances followed the linear model. There is a correlation between color, antioxidant activity, and fat oxidation.

Thin Layer Drying Kinetics of Indian Blackberry (Syzygium cumini L.) Pulp

Drying kinetics of fruit has importance in estimating optimum nutritional retainability, preservation requisites and process economy while processing into useful products. Drying kinetics of pulp of local variety of Indian blackberry was studied in a cabinet dryer. The drying experiment was conducted at 50 ºC, 60 ºC and 70 ºC, and moisture losses were recorded at 30-min intervals. Drying took place in two falling rate periods. Shift of first to second falling rate period was found to be irrespective of temperature, and started from 1.08 g water.g−1 dry matter. The drying data of Indian blackberry pulp was fitted into five commonly used Newton, Page, Peleg, Henderson and Pabis, and Logarithmic thin layer drying models. The logarithmic model adequately described the drying of Indian blackberry pulp. The activation energy values in first and second falling rate periods were 42.20 kJ.mol−1 and 61.62 kJ.mol−1, respectively.

Effect of Temperature on Drying Kinetics of Vegetable Seeds

A study was conducted to investigate the effect of temperature on drying kinetics of onion, carrot and tomato seed. The seeds were dried at drying temperatures of 30o C, 35o C and 40o C in a laboratory hot-air oven dryer. Drying rate was faster at higher temperature (40o C) than at lower temperatures. Drying of the seeds occurred in falling rate periods of drying. Drying data were fitted into Newton, Page, Modified Page, Henderson-Pabis, Logarithmic, two-term exponential, and Wang and Singh models. Based on highest R2 and lowest χ2 , RMSE and mean error values, Page model (R2 &gt;0.9) was found to be the best fit for drying of all selected seeds at the selected temperatures. However, two-term exponential model was the best fit for onion seed at 35o C and carrot seed at 30o C, and was comparable with Page model in the selected temperature range. Effective moisture diffusivity of onion, carrot and tomato seed ranged between 1.59 x 10-10 m2 .s-1 and 3.18 x 10-10 m2 .s-1, 3.55 x 10-10 m2 .s-1 and 7.10 x 10-11 m2 .s-1, 2.65 x 10-10 m2 .s-1 and 5.31 x 10-11 m2 .s-1, respectively, at above drying temperatures. Activation energies of onion, carrot, and tomato seed were 56.85 kJ.mol-1, 60.69 kJ.mol-1, and 61.44 kJ.mol-1, respectively.

Oil–free potato chips produced by microwave multiflash drying

The present work is about the production of oil-free potato snacks using successive cycles of microwave heating-vacuum pulses and was therefore called microwave multiflash drying (MWMFD). Drying kinetics and product physical properties were assessed, as well as the influence of blanching (BL) and vacuum impregnation (VI) as pre-treatments before drying. The drying curves presented three periods: a heating period, a high drying rate period promoted by the vacuum pulses, and a falling rate period. The drying time of 400 g of potatoes was approximately 60 min, reducing gradually the microwave power from 800 to 200 W. The vacuum-pulse step changed the material, resulting in a porous and fragile microstructure, characteristic of crispy products, as well resulting in a product surface with a golden yellow color. BL and VI combined was the best pre-treatment for obtaining very crispy oil-free potato snacks. The results of this study indicated MWMFD as a suitable process to produce crispy potato snacks, free of oil, in only 60 min.

Paraboloid-Based Spouted Bed Drying of Paddy: Aerodynamics, Temperature Distribution, and Moisture Degradation

Aerodynamics, temperature variations in the annulus, and the moisture reduction of paddy in a paraboloid-based spouted bed (PBSB) dryer without draft tube and with solid and porous draft tubes were investigated. Draft tube caused a rapid decrease in the peak pressure drop and minimum spouting velocity when compared with PBSB without a draft tube. Pressure drops with draft tubes were 17 to 30% of the values for the PBSB without a draft tube. Temperature distribution along the bed height in the annular region during drying of paddy was also investigated. Heat-up in the spouted bed without draft tube was more rapid. The experiments were conducted for 70, 90, and 110 °C inlet air temperatures. The required length of drying time to dry the paddy with an initial moisture content of 0.35 db to a moisture content below 0.15 db could be reduced by 50–60% using a temperature of 110 °C instead of 70 °C. No constant rate period was observed. Drying took place in the falling rate period. Drying time decreased in the case of porous draft tube rather than the solid one. The highest drying rates were achieved by spouted bed without draft tube. Drying rates were in the range of 0.62–0.1, 0.51–0.06, and 0.37–0.06 (db.min−1) for the spouted bed without draft tube, with porous, and solid draft tubes, respectively.

Study the effect of microwave power and slices thickness on drying characteristics of potato

In the present work, the influence of microwave power and samples thickness on drying behavior, moisture diffusivity and energy consumption of potato slices was studied. The slices with thicknesses of 3.5, 5, 7 and 9 mm were dried as mono layers at four different power levels (200, 400, 600 and 800 W). The results showed that the process time was significantly (P < 0.05) decreased with increasing power and decreasing thickness. The curves of moisture removal rate included a short accelerating rate period at the beginning followed by a long falling rate period. Seven thin layer models were practiced to simulate the experimental drying kinetics and the Midilli model showed the best performance. The average values of effective moisture diffusivity were obtained to be in the range of 1.15510−8-6.654 × 10−8 m2 s−1, and increased significantly (P < 0.05) with both of the increasing microwave power and the samples thickness. The average activation energies changed from 1.423 W g−1 to 1.621 W g−1, and decreased with increasing samples thickness. Specific energy consumption for the process were ranged from 0.680 MJ kg−1water to 2.591 MJ kg−1water.

Fluidized Bed Drying of Pumpkin (Cucurbita sp.) Seeds.

Pumpkin seeds are a major agricultural waste from the fresh-cut produce industry. The objective of this study was to investigate the drying behavior of untreated, whole pumpkin seeds in a fluidized bed dryer at 50–80 °C (2.87 m/s), with a view to producing a high-quality pumpkin powder from dried seeds. Seeds were dried at 50–80 °C to an average equilibrium moisture value of 0.035 to 0.006 g H2O/g DM (3.4 to 0.6% wb). Drying occurred in the falling rate period only and drying rate constants ranged from 0.0226 to 0.0900 1/min with corresponding diffusivity values for the first falling-rate period ranging from 4.68 to 18.63 × 10−10 m2/s. The activation energy (Ea)—for the first falling rate period was determined to be 43.9 kJ/mol. Of the nineteen thin layer models tested, the Alibas model could be successfully used as a general model to predict the Moisture Ratio (MR) data for all temperatures investigated. After drying, seeds were blended to produce powders, which were found to be high in fat, crude protein and fiber.

Temperature, microwave power and pomace thickness impact on the drying kinetics and quality of carrot pomace

This study investigated the effect of air temperature, microwave power, and pomace thickness on the drying kinetics and quality of dried carrot pomace. The study established that the drying of carrot pomace occurs in the falling rate period, suggesting that drying was driven by molecular diffusion. The microwave-drying moisture diffusivity increased with microwave power and ranged between 1.57×10–8 and 2.61×10–8 m2/s. As regards convective air-drying, the moisture diffusivity values were between 3.38×10–10 and 8.27×10–10 m2/s. The microwave powerto-mass activation energy was 15.079 W/g for 5 mm, 7.599 W/g for 10 mm and 9.542 W/g for 15 mm dried samples. Meanwhile, the temperature-dependent activation energy for carrot pomace was found to be 27.637 kJ/mol for 5 mm, 17.92 kJ/mol for 10 mm and 38.76 kJ/mol for 15 mm thickness pomace. Generally, drying time decreased with increasing microwave power or air temperature. The ascorbic acid content of the fresh carrot pomace reduced after both microwave and convective air-drying. However, microwave power, and sample thickness had significant effect on the β-carotene content of dried products but air temperature did not have a significant effect. The effect of temperature and sample thickness on brown pigment formation was substantial with air temperature compared to microwave. The study has demonstrated that microwave drying, compared to conventional drying, enhances moisture removal, drying time, and preservation of carotenoids and ascorbic acid. Therefore, microwave drying can be considered as an alternative method for obtaining quality dried carrot pomace.

DRYING KINETICS, HEATING UNIFORMITY AND QUALITY CHANGES DURING THE MICROWAVE VACUUM DRYING OF ARTICHOKES (CYNARA SCOLYMUS L.)

In this study, our aim was to investigate the effect of process parameters such as the microwave power (450-800 W), system pressure (12.40–31.20 kPa) and drying time (0-25min) on the drying kinetics, quality characteristics and heating uniformity indices during the microwave vacuum (MV) drying of artichokes. The microwave power was found to be more influential than the system pressure, and variations resulted in different drying characteristics. The effective diffusion coefficients were found to range from 2.279×10-6-3.454×10-6 m2/s and 3.600×10- -6.297×10-m2/s for the first and second falling rate periods, respectively. Reducing microwave power and system pressure resulted in higher uniformity in temperature distributions. However, the average temperatures deviated slightly from the targeted drying temperatures at high microwave power. The highest shrinkage coefficient (0.3705±0.0233) and rehydration ratio (3.82±0.091) values were obtained at 800 W and 12.4 kPa. MV drying preserved the fresh colour characteristics of artichokes, and it could be used as an alternative to conventional drying.

MATHEMATICAL MODELING OF DIFFERENT DRYING METHODS OF DRIED OKRO SLICE

Drying is an essential process that is used in preserving okro sample. This study investigates the modelling of dried okro slice using different drying methods. Freshly harvested okro fruits were cut transversely into slices (0.5, 1.5 and 2.0 cm) and dried using three different drying methods; sun, solar and oven at 40, 45 and 50 ºC. Drying of okro occurred in the falling rate period. Newton, Pabis, and Page and Henderson models were used to describe the drying of okro. An appropriate model was selected based on highest R2, least values of X2, Root Mean Square Error (RMSE) and Mean Biased Error (MBE). Page model described the drying behaviour of okro slice 0.5 cm satisfactorily having the highest R2 of 0.999 and lowest value of X2 (0.152 x10-8), RMSE (4.5 x10-6) and that of MBE (-3.089 x10-4).

Study of the Drying Rate and Colour Kinetics during Stepwise Air-Drying of Apricot Halves

AbstractThe drying rate and colour kinetics of apricot halves were studied during stepwise air-drying compared to continuous drying. The tested stepwise temperature profiles exhibited two successive falling rate periods, which differed from the drying rate period of the constant drying temperature profile. Three chromatic indices, namely browning index, chroma and hue were analysed. The analysis showed that treatment with ascorbic acid was more effective during the step up temperature profile, whilst its antioxidant activity was lost during the step down temperature profile due to the initially used high drying temperatures. The chromatic indices were described by a modified kinetic model, the parameters of which were estimated by theLevenberg–Marquardtoptimization algorithm and in all cases was derived R2adj≥ 0.88 and MRD &lt; 4.88%.