Overall Energy Requisite and Quality Feature of Industrial Paddy Drying

Energy consumption and rice quality are the main concerns of millers and must be assessed to ascertain suitable industrial drying strategy. In this article, industrial paddy drying methods as usually practiced in the BERNAS paddy drying complexes of Malaysia have been evaluated. The analysis showed that the specific electrical and thermal energy consumption varied between 16.19 kWh to 22.07 kWh and 787.22 MJ to 1015.32 MJ, respectively, in single-stage paddy drying (SSPD) using an inclined bed dryer (IBD) to dry each tonne of freshly harvested paddy with average moisture content of 23.35 ± 0.86% wb. On the other hand, the energy consumptions for two-stage paddy drying (TSPD) with a fluidized bed dryer (FBD) followed by IBD were 21.37 kWh/t to 30.69 kWh/t and 666.81 MJ/t to 1083.42 MJ/t, respectively. SSPD at 35–39°C and TSPD using FBD at 120°C as the first stage, followed by IBD as the second stage at lower temperature of 35–39°C yielded 2–3.6% higher head rice yield than paddy-dried by a single stage with IBD using comparatively higher temperature of 40–44°C. Therefore, IBD is recommended to be operated using a temperature of 35–39°C both in single-stage drying and second-stage drying of paddy after fluidized bed drying to obtain quality rice.     

Study of hybrid dryer prototype and its application in pregerminated rough rice drying

In this study, a hybrid dyer, combining heat pump drying (HPD) with fluidized bed drying (FBD) concepts were designed and fabricated. The pregerminated rough rice (pre-GRR) was dried in multistage using this hybrid dryer to compare with the single-stage drying by hot air dryer. The objectives were to test the application of this hybrid dryer and determine the suitable drying condition for pre-GRR. The result indicated that the punched plate distributor was the most suitable distributor. The pre-GRR should be dried by the three-stage drying method using either FBD or HPD at 45°C in the last stage to obtain higher head rice yield, lower fissure grain, and better color values than their counterparts. The scanning electron micrographs proved that starch gelatinization occurred when applying FBD at temperatures between 100 and 140°C causing the adhesive connections inside the kernels and subsequent decrease in fissures.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

Abstract

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80–90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Drying kinetics study of irregular fibril materials in a “differential” laboratory rotary dryer: Case study for cut tobacco

Rotary dryers are commonly used in the modern large-scale tobacco drying industry that consumes huge amounts of energy. In fact, rotary dryers are commonly used in chemical industry in general. It is difficult to investigate the drying behavior at industrial scale. A “differential” laboratory rotary dryer was therefore designed and tested. The large diameter of the industrial dryer was preserved, but the width was a section of the industrial dryer. The drying characteristics of cut tobacco from top leaves and bottom leaves with initial moisture contents (22.5?±?1.0% on the wet basis) were studied in the “differential” dryer at air temperatures of 65, 85, 105, 125, and 145°C, respectively. The results show that increasing drying temperature accelerated the drying process, whereas the surface temperatures of the cut tobacco samples stayed in the temperature range of 48–71°C when their moisture contents were reduced to 12.0?±?1.0% (wb). This 12.0% (wb) was required by commercial operations. Uniquely, the drying kinetics was captured using the reaction engineering approach (REA). Although different settings were applied, the model can be used to describe all the data well. The unique relationship between the normalized activation energy and the moisture content is approximated which is independent of the drying air temperature and the tobacco origin. The different drying behaviors for the cut tobacco from top leaves and bottom leaves can be attributed to their different equilibrium isotherms. Through controlling the drying time as predicted by REA model, the outlet moisture contents of cut tobacco from top leaves dried at 95°C/RH0.034/320?s and 115°C/RH0.017/250?s were shown to be 12.3 and 11.8% (wb), with the relative deviations of 2.5 and 1.7%, respectively, and these were within the industrial permissible range.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80-90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Evaluating a Solar Dryer for In-Shell Drying of Split Pistachio Nuts

Abstract

A thin-layer forced air solar dryer was designed to study the feasibility of drying pistachio nuts. The dryer was tested during the 2001 and 2002 drying seasons. The maximum temperature in the solar collector reached 56°C, which was 20°C above the ambient temperature. The required drying time was 36 h. During the first day of drying (0800 to 1700 h) the moisture content dropped to about 21% (wb). The final moisture content of the dried nuts was 6% wb, which was 1% below the recommended storage moisture. The drying constant of the pistachio nuts during solar drying was determined using two mathematical models, a one-term series solution of Fick's diffusion equation and an exponential decaying model. There was no significant difference between the two models (α = 0.05). In general, the quality of solar dried nuts was better than the conventional heated air due to slower drying rates.     

An Experimental Study of Wheat Drying in Thin Layer and Mathematical Simulation of a Fixed-Bed Convective Dryer

Drying of wheat (Algerian cultivar: Hadba03) in thin layers was studied and mass flux phenomenon was used to characterize the thin-layer drying process. Thin-layer drying of wheat was determined for drying air temperature range of 40–60°C, relative humidity of drying air from 10 to 30%, air velocity of 0.7 m/s, and initial grain moisture from 26 to 31% (dry basis). Equilibrium moisture content of wheat was determined using desorption isotherms obtained from the thin-layer drying data. An equilibrium model for a stationary deep bed with drying air moving vertically upward was developed using mass and energy balance between grain and drying air in the bed and drying air characteristics obtained from thin-layer drying experiments. The developed model was validated by drying wheat in a laboratory dryer using different drying air temperatures and initial moisture contents.     

A Quasi-stationary Approach to Drying Kinetics of Fluidized Particulate Materials

A modified quasi-stationary method has been proposed to describe drying kinetics of particulate materials dried in convective dryers with active hydrodynamic regimes. Both our own results and literature data were used. These include sliced celery, cranberry, diced carrot, wheat and polystyrene granules dried in four types of dryers: pulsed fluid bed dryer, fluid bed dryer with a mixer, spouted bed dryer with a draft tube, and vortex dryer. The method was validated by comparing experimental data with results of modeling in terms of a reduced moisture content and material temperature. A new form of the generalized drying curve has been proposed with the reduced time and the index of hydrodynamic intensity as parameters. The equations developed can be used to calculate the total drying time and determine the temporal variation of the moisture content and material temperature.     

The Development of Ginger Drying Using Tray Drying,Heat Pump–Dehumidified Drying,and Mixed-Mode Solar Drying

Mature ginger was pretreated by soaking in citric acid prior to drying in a single layer in a tray and heat pump dehumidified dryer at three temperatures of 40, 50, and 60°C and in a mixed-mode solar dryer at 62.82°C and a radiation intensity of 678 W/m². The drying data were applied to the modified Page model. Diffusivities were also determined using the drying data. Quality evaluation by color values, reabsorption, and 6-gingerol content showed best quality for ginger with no predrying treatment and dried at 40°C in a heat pump–dehumidified dryer. At drying temperature of 60 to 62.82°C, no pretreated dried ginger from mixed-mode solar dryer provided the shortest drying time and retained 6-gingerol as high as heat pump–dehumidified dryer.     

Drying Behavior of Carrots Dried in a Spout–Fluidized Bed Dryer

The effect of water blanching treatment and the inlet air temperature on drying kinetics as well as the quality attributes of carrot cubes dried in a spout–fluidized bed dryer at 60, 70, 80, and 90°C were analyzed. The material shrinkage and the rehydration potential were calculated to assess the changes in quality of dried carrots. It was found that the value of the air velocity during the drying of carrot cubes in a spout–fluidized bed dryer should be related to the moisture content of the carrot particles. A high value of air velocity at the beginning of the drying cycle and a lower value for the later stages were also required. The linear equation was correlated to the data of shrinkage of raw and blanched carrots. Blanching significantly influenced the coefficients in the shrinkage model derived for drying of carrot cubes in a spout–fluidized bed dryer, while drying temperature did not influence the shrinkage of carrot particles. The intensity of heat and mass transfer during spout–fluidized drying of carrot cubes was dependent on the drying temperature. A correlation was developed to calculate the values of effective moisture diffusivity of dried carrot cubes as a function of the moisture content and temperature of the material. It was observed that for any given time of rehydration, both the moisture content and the rehydration ratio calculated for samples dried at 60°C were higher than for samples dried at temperatures of 60, 70, 80, and 90°C.     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

AMARANTH GRAIN DRYING IN A FLUIDIZED BED

ABSTRACT

An empirical equation as a function of drying time and temperature was developed to calculate the moisture ratio required Tor balch fluidized bed dryers design for amaranth grain drying

The relative deviations of the moisture ratio values calculated with the proposed equation with regard to the experimental ones ranged between 0.014 and 0.095 for a drying air temperature between 60 and 100 ° C, and a grain initial moisture content between 23.5 and 16.6 % wb.     

Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

Microwave vacuum drying of lotus seeds: Effect of a single-stage tempering treatment on drying characteristics,moisture distribution,and product quality

This study evaluated the effects of a single-stage tempering treatment during microwave vacuum drying (MVD) on drying characteristics and quality of lotus (Nelumbo nucifera Gaertn.) seeds using two parameters: intermediate moisture content (IMC) and tempering temperature (4 and 25°C). Magnetic resonance imaging (MRI) was used to examine moisture migration and distribution in individual lotus seed during tempering. Results from MRI showed tempering could reduce the moisture gradient in lotus seeds during MVD. The tempering treatments led to increased moisture diffusivity (3.96–43.56%) and a shortened drying time (6.25–31.25%) when compared with continuous MVD. Furthermore, tempered samples exhibited a greater rehydration capacity, a limited overall color change, and increased amounts of taste-active amino acids when compared with nontempered lotus seeds. High IMC improved rehydration ratios of dried samples. Low tempering temperature provided favorable free amino acid content and desirable product color.     

Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

Crystallization and Drying of Milk Powder in a Multiple-Stage Fluidized Bed Dryer

The rationale of this study has been to use fluidized beds to crystallize amorphous spray-dried skim milk powders with multiple stages of processing at different temperatures and humidities with the aim of rapidly making mostly crystalline powders. This paper discusses the performance of a multiple-stage fluidized bed dryer, and a combination of crystallization of lactose in spray drying at high humidity (lactose nuclei formation) and subsequent fluidized bed drying. Two different combinations of spray dryer and multi-stage fluidized-bed dryer have been suggested to crystallize lactose in skim milk powder. The results show significant improvements in the crystallinity of the powders. Moisture sorption test and X-ray diffraction analysis were used to assess the crystallinity of the powders. The processed powders that were crystallized in a humid-loop spray drying combined with a two-stage fluidized-bed dryer/crystallizer showed 92% improvement in lower amorphicity by processing at different stages of 70°C, 50% RH and 80°C, 50% RH for 15 minutes. The conventionally spray-dried powders that were crystallized in a three-stage fluidized-bed dryer/crystallizer showed 87% improvement in lower amorphicity (less moisture sorption) by processing at different stages of 60°C, 50% RH; 70°C, 40% RH; and 80°C, 40% RH for 20 minutes. The multiple-stage fluidized bed system showed distinctive potential to crystallize lactose significantly in skim milk powder using an industrial-feasible process.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Germinated Brown Rice Drying by Hot Air Fluidization Technique

Germinated brown rice containing moisture content of 52% dry basis (db) needs to be dried in a reasonable time in order to prevent the growth of microorganisms. Fluidized bed drying is a possible method because this drying technique provides a high mass and heat transfer rate and high-temperature drying can be used. However, such a high-performance dryer may affect the quality of the finished product. The effect of fluidized bed drying temperatures (90, 110, 130, and 150°C) on the quality of germinated brown rice, that is, cooked rice textural property, γ-aminobutyric acid (GABA) level, fissured grain, and microorganisms was therefore investigated. The germinated brown rice was dried to the moisture contents of 18–20% (db), tempered for 30 min, and ventilated by ambient air until the sample moisture content reached 13–15% (db). The experimental results showed that the drying rate increased with increasing drying temperature. The high drying rate, in particular drying at 130°C or higher, caused severe fissuring on the kernel and this fissuring subsequently affected the cooked rice shape and textural property of rice; that is, hardness. The GABA contents of germinated brown rice insignificantly changed with drying temperatures and did not depend on the rice varieties. The populations of attached bacteria, yeast, and mold on the surface of the dried samples were less than 10⁴ colony-forming units (CFU)/g, which is safe for food.     

A Small-Scale Pneumatic Conveying Dryer of Rough Rice

This article studies the possibility of reducing the high initial moisture content of wet rough rice using a small-scale low-cost pneumatic conveying dryer as a first stage dryer. The parameters investigated are final moisture content, surface temperature of rough rice, head rice yield, drying rate, power consumption per unit mass of evaporated water, and physical characteristics of rice. Parametric effects of the following variables are examined: velocity of drying air from 20 to 30 m/s, feed rate of rough rice from 150 to 350 kg/h, initial moisture content from 22 to 26% (wet basis), and drying air temperature from 35 to 70°C. From the experimental results, it is found that this drying method can be used for fresh rough rice with an initial moisture content of over 24% (wet basis). The drying process is able to lead to very rapid drying without any grain quality problems such as cracks in the rice kernel. The moisture content can be reduced to approximately 18% (wet basis) or about 5–6% of the initial moisture content within 3–4 s. The optimal drying air temperature is in the range of 50 to 60°C. A comparison of pneumatic conveying drying data obtained from the present study with fluidized bed drying data reported in the open literature is also discussed.     

Effects of High-Temperature Drying on Physicochemical Properties of Various Cultivars of Rice

Abstract

Three varieties of paddy rice, namely Langi and Amaroo from Australia and Chainart I from Thailand, were dried from high initial moisture content of about 27% down to 13–14% wet basis using a two-stage drying system. A fluidized bed dryer reduced the moisture content down to 18%. Drying experiments were carried out at 100, 125, and 150°C. Further moisture content reduction down to 14% was achieved by shade drying. As a result of these treatments, head rice yield increased proportionally with the drying temperature. In contrast to that, the yellowness, measured by colorimeter in terms of b value, showed an opposite trend. Starch characteristics were studied by Rapid Visco Analyser (RVA), x-ray diffraction, and differential scanning calorimetry (DSC). Pasting properties were affected by the drying temperature. The peak viscosity and break down were decreasing with the increase of drying temperature in all varieties while the setback values were increasing in Langi and Amaroo only. All starch samples displayed the typical A type x-ray diffraction pattern. The apparent crystallinity determined by x-ray diffraction was reduced with increasing drying temperature. The gelatinization peak shifted to higher temperature while the endothermic enthalpy of gelatinization decreased with increasing drying temperature.