Continuous Flow Dryer Research Articles

Generalized Mathematical Model of the Grain Drying Process

Convective cereal grain drying is an energy-intensive process. Mathematical models are applied to analyze and optimize grain drying processes in different types of dryers and in different stages of drying to improve final grain quality and reduce energy consumption. The aim of the present study was to develop a generalized mathematical model of the grain drying process that accounts for all drying stages, including loading and unloading of unprocessed grain, drying, and cooling of dry grain. The developed mathematical model is a system of algebraic equations, where the calculated coefficients are determined by the thermophysical and diffusive properties of dried grain. The model was validated for batch drying of wheat, canola, and corn grain, as well as continuous flow drying of wheat grain. The results were compared with published findings. The relationships between energy consumption during drying and drying time vs. air temperature at the dryer inlet and air stream volume were determined. Dryer capacity and drying conditions specified by the manufacturers, as well as loading and unloading capacity, were considered during batch drying. Continuous flow drying simulations were conducted in counter-flow, parallel-flow, and cross-flow mode. Simulation results indicate that the proposed models correctly depicted process flow in both batch and continuous flow dryers.

Mathematical modelling of lemon verbena leaves drying in a continuous flow dryer equipped with a solar pre-heating system

The modelling of the drying process of lemon verbena leaves in a continuous flow dryer equipped with a solar pre-heating system was performed at three levels of drying temperature (50, 40, and 30 °C) and three levels of air velocity (2, 1.5 and 1 m/s). During the experiments, lemon verbena leaves were dried to the final moisture content of 10 from 76% wet basis in the continuous flow dryer. Drying kinetic showed to drying temperature and air velocity exerted significant influence on the drying time. Also, the dried lemon verbena leaves quality was obtained by determining the essential oil content of the product after drying in different conditions in the dryer. Finally, it was observed that the highest essential oil content was maintained at a temperature of 40 °C and air velocity of 1 m/s. Suitability of 10 different mathematical drying models was used to describe drying lemon verbena leaves in this dryer. The results have shown that Midilli and Kucuk’s model can successfully predict the experimental data in all air temperatures and air velocities. In Midilli and Kucuk’s model, the amounts of R2 were above 999 × 10–3 and the amounts of root-mean-square error (RMSE) and chi-square (χ2) were less than 174 × 10–4 and 19 × 10–4.

Investigation of fuel consumption and essential oil content in drying process of lemon verbena leaves using a continuous flow dryer equipped with a solar pre-heating system

Fuel consumption and the drying behavior of lemon verbena leaves were determined in a continuous flow dryer equipped with a solar pre-heating system. In order to reduce the share of fossil fuels and their negative impact to the environment, the total energy during the drying process was supplied using the combination of solar energy (solar water heater) and natural gas. The experiments were carried out at three levels of drying air temperature (30, 40, and 50 °C) and three levels of air velocity (1, 1.5 and 2 m/s). Two working conditions were considered for the drying system: a) solar-gas drying (SGD) mode and b) gas drying (GD) mode. According to the results, in SGD mode, the fuel consumption at drying temperatures of 30 and 40 °C was zero at all levels of the air velocity, while in the GD mode the fuel consumption increased with increasing the inlet air temperature. The highest and lowest essential oil (EO) content was obtained at 40 °C and 2 m/s by 0.60% and at 50 °C and 1 m/s by 0.33%, respectively. The central composite design-Face Centered technique with response surface methodology was used for the optimization of the drying process. The optimum conditions for drying lemon verbena leaves were obtained at drying temperature of 39.54 °C and an air velocity of 2 m/s. In this mode, the fuel consumption was zero, drying time was 131.48 min and EO content was 0.52%.

On the Performance of Various Types of Solar Crop Dryers with Thermal Modelling and Simulated Studies

Various types of solar crop dryers are discussed and classified by passive solar-energy drying systems and active solar-energy drying systems. Solar energy dryers are broadly classified into direct, indirect and hybrid solar dryers. The working principle of these dryers is discussed. Energy balance equations have been written for various interactive components, such as top glazing, absorber plate and inside heated air for computing the collector heat-removal factor and thermal efficiency of selected dryer. Mathematical modelling of solar greenhouse dryer is also performed using energy-balance equations to compute convective heat-transfer coefficient, Nusselt number, Reynolds number and overall heat-loss coefficients. Various important conclusions have been drawn through simulation studies for optimising the performance of solar crop drying. Mechanical dryers are discussed which include fixed batch, re-circulation and continuous flow dryers. Biomass-based crop dryers are also discussed including design and thermal analysis of dryer performance.

Human-in-the-Loop Intelligent Control System for Continuous-Flow Grain Drying

Moisture content is one of the key controlled variables in grain drying processes. However, this variable cannot or is difficult to be controlled accurately, due to grain drying is a nonlinear process with multi-variables and long delay. To solve this problem, the study is focused on a new intelligent control system for continuous-flow dryer. Based on the manual control idea for controlling the outlet moisture content, a nonlinear process intelligent control algorithm is presented, a human-in-the-loop intelligent control model for grain drying is developed, a system of software structure based on Visual Basic 6.0 is designed.

Corn Grain Drying Using Corn Stover Combustion and CHP Systems

Post-harvest drying of shelled corn grain requires large amounts of fossil fuel energy. In 2004, it was estimated that the upper Midwest consumed more than $1.4 billion of fossil fuels to dry $19.7 billion of corn grain. Over the long term, drying corn with fossil fuels may become cost prohibitive due to limited fuel reserves. To address future energy concerns for grain dryers, this study evaluated the potential use of combined heat and power (CHP) systems that use the combustion of corn stover both to produce heat for drying and to generate electricity for fans, augers, and control components. Net present value (NPV) cost estimates were determined for two continuous-flow dryers: a relatively small on-farm dryer (8.9 Mg h-1), and a larger dryer more common to grain elevators (73 Mg h-1). For each dryer, three levels of assumed stover price were used: $15, $25, and $35 per dry Mg for the small dryer, and $30, $45, and $60 per dry Mg for the larger dryer (includes payments to farmer and off-farm transport costs). Compared to equivalently sized fossil fuel-fired dryers, both the small and large CHP dryers were found to be more economical over the long term. Twenty-year NPV cost savings and breakeven points were estimated to be $63,523 and 14.3 years for the small CHP dryer ($25 Mg-1 stover) and $1,804,482 and 7.5 years for the large dryer ($45 Mg-1 stover). Sharing CHP infrastructure with other processes requiring heat that extend seasonal use can reduce payback periods significantly and provide broader efficiency benefits. Sensitivity analysis found cost savings to be most sensitive to fluctuations in fossil fuel costs, followed by annual use of dryer equipment.

High-temperature Treatment for Efficient Drying of Bread Rye and Reduction of Fungal Contaminants

Mycotoxin-producing fungi are natural contaminants of cereals and their toxins are harmful to humans and animals. Ochratoxin A (OTA) is among the most important. Heat treatment by drum drying does not eliminate already formed mycotoxins but the technique can reduce the number of viable fungi on the grain. The aim of this study was to establish a drying regime that kills fungal propagules on rye without reducing its quality for baking. Special attention was paid to some important mycotoxin-producing species. As drying temperatures and retention time in the drum are essential, the drum drier must be equipped with an effective control unit. Two different control systems were tested. The results showed that fungi colonising the grain can be much more efficiently and precisely reduced by controlling the maximum grain temperature (MGT) than the fixed constant drying air temperature (FAT). Drum drying very efficiently reduced the fungal propagules colonising the grain, including the mycotoxin-producing Penicillium verrucosum, Fusarium avenaceum, F. culmorum, F. poae, F. sporotrichioides and F. tricinctum. Both temperature and retention time in the drum affected survival rate of the fungi. Using MGT, a retention time of 10·5 min and a temperature of 64 °C killed 99% of the yeast propagules and 98% of the filamentous fungi. Moisture contents were reduced to about 12%. A similar drying regime in a supplementary trial reduced the number of P. verrucosum contaminated kernels from more than 70% to 12% but confirmed that drum drying did not destroy already formed OTA. The combination of a high drying capacity and a short but efficient heat treatment was obtained by drum drying as opposed to on-floor, batch and ordinary continuous flow drying, and it reduced the risk of mould deterioration to almost zero when the grain was properly stored afterwards. At the same time a high quality for baking was maintained. The highest baking quality in rye was obtained at grain temperatures of about 62 °C and only at grain temperatures above 70 °C visual quality changes were detected.

CURING PEANUTS USING CONTINUOUS FLOW DRYERS

A two–year study was conducted to determine the potential for curing peanuts using continuous flow dryers with minimal detrimental effects on quality. A single–pass continuous flow dryer and a recirculating batch dryer were compared with conventional wagon drying systems. The rate of change of peanut kernel moisture content (% w.b./h) was considerably higher in the single pass continuous flow (2.1%/h) and the recirculating batch (1.0%/h) dryers than observed in conventionally cured peanuts (0.41%/h). Peanuts cured using the single–pass continuous flow dryer had unacceptably high levels of split and bald kernels when compared to those cured in wagons. The recirculating batch dryer resulted in significantly higher percent split kernels and skin slippage. However, the reduction in peanut milling quality may be acceptable in order to achieve the faster drying rate.

QUALIDADE DE SEMENTES DE MILHO NAS SECAGENS INTERMITENTE RÁPIDA E CONTÍNUA COM ALTAS TEMPERATURAS

A procura por sementes de milho tem crescido nos últimos anos pela expansão do cultivo. As sementes vêm úmidas do campo havendo necessidade de secá-las a granel até 13%, que é realizado tradicionalmente com temperaturas que não excedem 40ºC no ar de secagem. Dessa forma, o objetivo deste trabalho foi o de estudar a eficiência de secadores intermitente rápido e de fluxo contínuo na secagem de sementes de milho variedade, empregando-se altas temperaturas, e avaliando-se a qualidade física, fisiológica e sanitária. Assim, dois lotes de sementes de milho IAPAR 51 foram submetidos a dois secadores: a) intermitente rápido, com 60oC na câmara inferior; b) fluxo contínuo, empregando 60oC (câmara superior) e 50oC (inferior). As amostras de sementes, retiradas durante o processo de secagem, foram avaliadas pelos testes de germinação, frio sem solo (vigor); sanidade, pureza e danos mecânicos, em julho e outubro de 1996. As velocidades de secagem foram de 1,1 pph-1 para o secador contínuo e 0,9 pph-1 para o intermitente rápido. Conclui-se: a) os secadores intermitente rápido e de fluxo contínuo são eficientes na secagem de milho com altas temperaturas; b) não há comprometimento das qualidades física e fisiológica das sementes submetidas à secagem contínua e intermitente rápida, empregando-se as temperaturas no ar de secagem de 60ºC e 60/50ºC, respectivamente; c) nas temperaturas utilizadas, os processos de secagem não reduzem a infestação de Penicillium.

Adaptação do método contínuo de secagem para sementes de arroz

Com o objetivo de avaliar a influência da adaptação do método contínuo de secagem sobre a qualidade das sementes de arroz durante o armazenamento, quatro porções de sementes, pesando cada uma 2500 kg, foram secadas em secador de fluxo contínuo Kepler Weber, modelo KW-2, utilizando diferentes temperaturas do ar, respectivamente, nas câmaras de secagem, superior e inferior: 60/40ºC; 60/50ºC; 70/50º C; 70/60ºC e um tratamento testemunha, constituído por quatro amostras submetidas à secagem estacionária em estufa regulada à temperatura constante de 32ºC. A qualidade das sementes foi avaliada através dos seguintes testes: grau de umidade, germinação, envelhecimento artificial e emergência em campo, conduzidos em quatro épocas, correspondentes aos 0, 60, 120 e 180 dias de armazenamento em condições ambientais não controladas. Os componentes do beneficiamento industrial foram determinados na primeira época de avaliação. Os resultados obtidos permitiram concluir que lotes de sementes de arroz com alta porcentagem de grãos quebrados podem apresentar elevada qualidade fisiológica; na secagem contínua adaptada, as sementes de arroz podem ser submetidas a temperaturas do ar de até 60ºC e 50ºC, respectivamente, nas câmaras superior e inferior, apesar de causar redução acentuada na porcentagem de grãos inteiros.

Auger Design for Uniform Unloading of Granular Material: II. Cylindrical Containers

The analysis presented can be used to determine the design equations for augers that produce uniform vertical flow of granular material through containers with cylindrical cross-sections. The augers so designed have a constant outside diameter and a variable root or shaft diameter. Installed in cylindrical grain bins they could be used to convert a batch-in-bin dryer to a continuous flow dryer. The analysis is an extension of the work of Jones and Kocher (1995) who designed and tested such an auger for containers of rectangular cross-section, and determined flow to be uniform for practical purposes. The analysis follows the same concept used by Shivvers (1973) to develop augers with variable outside diameter to provide uniform vertical flow of granular material through containers with cylindrical cross-section.

Optimization of solar grain drying in a continuous flow dryer

The optimum method of using a solar air collector combined with a grain drying system was investigated. Two types of solar collectors, integrated into a roof are considered; one uncovered (flow under absorber) and one covered (flow over absorber). A computer model was developed to minimize the cost of drying and in doing so optimize the geometry of the collector and the specific rate of air flow through the collector. The ratio of the cost of drying in a solar-supplemented system to the cost of drying in a conventional system was minimized. This ratio represents the maximum possible savings for the given drying conditions. Sensitivity analyses were carried out for drying air temperatures in the range 40 to 90°C and for grain initial moisture contents of 16 to 35% d.b.(dry basis). The effect of daily drying times, the length of the drying season and the specific cost of collector and fuel was also investigated. Two types of farm management systems were considered for various drying conditions: use of the dryer to operate for a fixed number of hours per day and, alternatively, drying a given quantity of grain per day. The results show that there is an optimum number of hours of operation per day, optimum drying temperature and optimum initial moisture content for any combination of the other variables. The most critical factors affecting annual savings are the length of the drying season and the specific cost of the collector. The optimum geometry of the collector and the specific flow rate are also functions of these factors.

Present status of solar crop drying

Significant developments of the past decade in the area of solar crop drying are reviewed. Increasing energy prices stimulated research on solar drying in highly mechanized agricultural systems. However, in those countries solar drying has to compete with automated crop dryers using fossil fuels as the energy source. Studies have shown that neither the temperature required nor the quantity of heat necessary in high-temperature batch type or continuous-flow dryers can be reached with solar collectors. Therefore solar energy cannot be adapted economically to high-temperature drying systems. Solar energy is considered more applicable to low-temperature in-storage drying systems which has gained more importance in the last decade for drying grain and hay. Conditions in tropical and subtropical countries make the use of solar energy highly recommendable. The introduction of solar crop dryers seems to be a way to lower mass losses compared to traditional drying methods and improves the quality of the product considerably. Solar drying can be a way to increase the food supply and it also can be a possibility to increase the income of the rural population.

Optimal energy management in grain drying.

Grain drying is very specific to the geographic location, kind of drying system, and the type of grain. Under a given set of conditions, the optimal system can be selected based on careful evaluation. However, a good choice of drying systems, procedures, and management practices can be made from the information already available. The review of several grain-drying procedures has provided some insight in making a quick evaluation of the process and arriving at the most suitable system for a particular application. Despite extensive research efforts, the present knowledge of grain drying is yet insufficient to optimally design each drying process with respect to capacity, quality, and energy requirement. There is a need for incorporating grain and air parameters more accurately. It is also important to develop comprehensive drying simulation models to encompass agronomic practices, such as planting and harvesting. Recent efforts indicate a strong influence of planting and harvesting strategies on optimal drying and storage system selection. Results of the varietal trials at Ohio State University indicate that it is now possible to select midseason varieties, which dry down rapidly, without sacrificing yield. Also, low moisture at harvest is important to the energy management process because it affects total drying time and energy required. It is also important from a quality standpoint because kernel damage increases rapidly at harvesting moisture levels above 25%. The trend in grain-dryer design has shifted from focusing on drying capacity and operation reliability to energy consumption. The development in design of energy efficient continuous-flow dryers has been significant. Multistage concurrentflow dryers are excellent examples. Various aspects of dryer staging for efficient operation and control are yet to be determined. Recirculation of the exhaust air is a proven method of improving energy efficiency. Likewise, in batch-in-bin systems, stirring and intermittent drying are worth considering. Further research is required to formulate the best procedures for exhaust air recirculation and intermittent drying. Low temperature drying has a great potential in the U.S. corn belt. The suitability of low temperature drying at other places and for grains other than corn has not yet been well established. Energy savings resulting from low temperature drying entail careful planning and management on the part of the operator. Poor design and operation can result in a serious deterioration in grain quality.(ABSTRACT TRUNCATED AT 400 WORDS)

PARAMETERS AFFECTING GRAIN DRYING BY IMMERSION IN A HOT PARTICULATE MEDIUM

ABSTRACT High temperature particulate medium conduction grain drying is a very promising technique. However, lack of basic drying characteristic information has resulted in unsuccessful attempts to develop an efficient, economical, continuous flow dryer. In this study, the effects of initial grain moisture content, initial medium temperature, medium particle size, medium to grain mass ratio, and contact time on the drying of shelled corn immersed in hot sand are evaluated. Moisture losses ranged between 0.2 and 5.2 percentage points (wb).

Solar Assisted Rice Drying in a Continuous-Flow Dryer

ABSTRACT A solar collector array occupying a roof area of 96 m2 was installed near a continuous-flow, heated-air rice dryer (3.29 m3 capacity) to determine the extend to which solar energy would reduce fuel requirements of the dryer. The maximum percentages of total thermal energy supplied by the solar collector were 43.5, 38.1, and 32.6, respectively, for air temperature control settings of 49 C, 54 C and 60 C. The cost of the solar collecting installation was $156/m2 panel area. The energy col-lected in 1980 and 1981 was worth lc/h/m2 panel area at an LP-gas cost of $0.19/L.

52—THE DRYING OF CONTINUOUS TOW BY FORCED CONVECTION DURING THE FALLING-RATE PERIOD

The well-known drying equation is regarded as empirical. An argument is developed to show that in certain circumstances, such as fibre-drying, it has a sound basis, and experimental results from a continuous-flow dryer support this. Thus, when the mass-transfer coefficient from drying surface to gas stream may be evaluated, and data on equilibrium moisture contents at a range of humidities are available, the drying time may be calculated for the falling-rate period.