Mixed-flow Dryers Research Articles

Optimal lamella geometry for mixed flow dryers

Drying harvested grain crops prior to storage is a crucial task in the prevailing climatic conditions of Europe. Drying is an extremely energy-intensive process. Its inappropriate application leads to environmental pollution, quality deterioration, and ultimately significant financial losses. Various methods are available for conducting drying operations, with the mixed flow dryer being one of the most employed approach. The mixed flow dryer utilizes air blower systems to redirect the flow of the granulate. Previous research has indicated that uneven distribution of grain flow around the air blower lamellae can cause drying irregularities. By leveraging insights from a long-established classical mechanical problem (the Brachistochrone problem) and harnessing the explicit dynamical modelling capabilities offered by contemporary computing technology (Discrete element method), we have successfully devised an optimized lamella geometry that minimizes the non-uniformity of particle flow.

Design, development, and testing of rice-husk fueled mixed-flow rice dryer for small-scale rice producer farmers

In underdeveloped nations, rice is frequently harvested at a relatively high moisture content and then dried under the open sun to a storage moisture content. However, direct UV exposure, rain, dust, premature drying, contamination, and open sun drying are all associated with problems like poor grains, discolorations, and cracking. Commercial mechanical dryer utilization is not practical in remote regions due to the absence of electrical service, high startup costs, and ongoing operating expenses. In this work, a pilot mixed-flow rice drier that ran on rice husk energy and solar energy to power the driving fan was designed, produced, and tested. The major parts of the dryer assembly that were conceived and created were the combustion chamber, heat exchanger, and mixed-flow dryer. The results of the trial revealed that the combustion chamber's average temperature was 347.3 °C, the drying air at the heat exchanger output, where the drying chamber was attached, was 63.3 °C, and the combustion chamber and heat exchanger's respective efficiencies were 52.4% and 14.9%. The mixed-flow dryer powered by rice has a significantly lower operating cost than a comparable dryer powered by electricity, petrol, and diesel. The study makes it evident that such a reasonably priced and effective rice dryer might significantly help small-scale rice producers to extend the storage duration and maintain the quality of their product.

Modeling of dehydration and color degradation kinetics of maize grain for mixed flow dryer

Effects of drying temperature (60, 70, and 80 °C) and air velocity (3.0, 6.0, and 9.0 m/s) on thin layer and color kinetics of maize grain during mixed flow dryer (MFD) were investigated for the present study. Five thin layer grain drying models (Henderson and Pabis, Page, Lewis, Midilli et al. and Wang and Singh) as well as Zero order reaction rate were applied with the experimental data to predict the drying and color kinetics of maize, respectively. Increased drying temperature had significant leverage on drying time whereas proliferation of air velocity (>6.0 m/s) is scarcely imperative. Model fitting analysis revealed that Lewis and Page model is aptly applied in evaluating drying kinetics for mixed flow drying of maize as coefficient of determination (R 2 ) is higher. Effective moisture diffusivity (4.92 × 10 −7 to 8.38 × 10 −7 m 2 /s) was maximum both for higher temperature and air flow whereas specific energy consumption was lowest (2.75 MJ/kg) for lower temperature and air flow. Regression analysis of color attributes (L*, a*, and b*) designates that color kinetics of maize grain occurred according to the zero order reaction. L* and b* indices of color decreases while a* and color difference (ΔE) increases with drying temperature and time. Thin layer and kinetic constant (k) is proportionate with drying temperature while adverse results were noticed for color attributes. Drying and color degradation kinetics of maize can readily be described by Lewis and Page model and zero order reaction, respectively for mixed flow drying. • Effect of temperature & air flow thin layer modeling of maize for mixed flow drying. • Determination of effective moisture diffusivity of maize. • Color kinetics evaluation of maize grain for mixed flow dryer. • Evaluation of specific energy consumption for maize drying.

Energetic and exergetic performances during drying of freshly harvested peanut with industrial mixed-flow dryer

Rapid drying of peanut is very important to maintain quality and reduce the risk of mildew, especially for peanut harvested in high moisture content in China. The modified mixed-flow dryer can dry freshly harvested peanut efficiently and evenly which is not available for traditional fixed-bed drying. However, the performances of energy and exergy of peanut drying using modified mixed-flow dryer are not clear, lacking of relevant data for equipment improvement and process formulation. Therefore the relevant investigation was carried out to evaluate the performance of fresh harvested peanut drying using industrial mixed-flow dryer with heat-pump full power heating and waste heat recovery The results show that the energy consumption is mainly consumed by heat-pump and centrifugal fan, accounting for 73.5% and 24.2% of all energy consumed respectively. The overall specific energy consumption and specific heat consumption are 1494.5 kJ/kg and 4706.1 kJ/kg respectively. The thermal efficiency and the energy efficiency are affected by the change of the temperature of airflow entering the drying chamber, both fluctuate over time, increase earlier then decrease later and rise again until the material temperature reach to a relatively steady temperature. And the energy efficiency of dryer (178.9%) is approximately triple of thermal efficiencies of drying chamber (56.1%) and dryer (59.4%) for the overall drying. The heat utilization ratio decreases gradually over drying time, with the overall value of 50.0% for drying chamber and 52.9% for dryer. The exergy efficiency of drying process earlier increases then decreases in the overall trend with the overall value of 51.3%. The exergy efficiencies of the drying chamber and whole dryer are both increased gradually with the overall value of 34.5% and 31.8% respectively. However, the variation tendencies of energy improvement potential rates are opposite to exergy efficiencies.

Comprehensive exergy transfer analysis of a cyclonic furnace integrated recirculating mixed flow grain dryer

Investigation of exergy transfer is an important aspect for all grain drying industry. Exergy transfer of rice husk fired cyclonic furnace assisted recirculating mixed flow dryer (MFD) equipped with a heat exchanger, was sought in this study. A cyclonic furnace (CF) having capacity 0.5 MW and recirculating MFD were designed and developed to investigate exergy transfer during grain drying. Efficiency of furnace was determined to quantify performance of CF for industrial grain drying system. Exergetic efficiency was also thoroughfare performed for the developed CF, air mixing chamber (heat exchanger) and MFD. Thermal efficiency of the CF varied from 68.96 to 87.01% during drying of high moisture maize grain. Exergy efficiency of the CF, heat exchanger and drying chamber marked between 58.8 and 63.5%, 74.6–93.29%, and 3.03–21.9%, respectively. Exploration of exergy inflow persist between 53.32 and 60.51 kJ/s and 7.58 to 9.81 kJ/s while exergy outflow ranged between 7.58 and 9.81 kJ/s and 1.52 to 3.10 kJ/s, respectively for heat exchanger and drying chamber. Results based on exergy losses indicates that 44.01–51.54 kJ/s and 5.87–7.50 kJ/s of exergy is just loss through heat exchanger and drying chamber, respectively. Exergy improvement potential and sustainability index varied between 1.02 and 1.80 kJ/s and 1.17–1.40 kJ/s, respectively. Quality of final product is in acceptable range as color difference of dehydrated maize kernel varied from 9.19 ± 0.46 to 17.05 ± 0.13. Exergy results indicate that CF can aptly be applied for grain drying industry in commercial scale.

Quality of corn grains subjected to drying using direct-fired furnace fed with eucalyptus chips and firewood

The objective of this study was to evaluate the drying system using a dryer with furnace automatically fed with eucalyptus chips and another dryer manually fed with eucalyptus firewood, as well as assessing the quality of corn grains. Harvest was mechanically performed and the grains were transported by trucks to the storage unit for drying. Corn grain drying was conducted in mixed-flow dryers with a nominal capacity of 100 tons per hour. For air heating, two direct-fired furnaces were used, one automatically fed with eucalyptus chips and the other manually fed with firewood. Corn grains were evaluated for moisture content, apparent specific mass, thousand-grain weight, polycyclic aromatic hydrocarbons (PAHs), electrical conductivity and color. Drying corn grains with direct fire using firewood or chips as fuel causes contamination by PAHs. In general, the use of firewood or chips did not influence the quality of thousand-grain weight, apparent specific mass and color, while the use of chips increased electrical conductivity. It is essential that research advances in this subject, prioritizing the contamination of food, with proven carcinogenic activity, which should be treated as a high risk to public health.

Particle motion in mixed flow dryers

Particle motion in mixed flow dryers

Particle motion in mixed flow dryers

Differences in flow rates of this nature have a significant effect on the unevenness of the moisture content of the dried material, since the material which remains in the drying chamber for an unnecessarily long time is over-dried and the under-drying is a problem for the material remaining in the dryer for too short a time. In this article, I analyzed the effect of increasing particle-wall friction on the unevenness of the particle flow velocity field. The research has shown that dead zones are formed in the vicinity of the rough walls, which reduce the uniformity of the flow. The results show that the tribological properties of the inner wall surfaces of the dryers can have a very significant effect on the efficient operation of the dryers.

Fluid flow simulation of industrial fixed bed mixed-flow grain dryer using k-ω SST turbulence model

Fluid flow simulation of industrial fixed bed mixed-flow grain dryer using k-ω SST turbulence model

Quasi-2D simulation of soya beans flow in mixed flow dryer

The production of soya is very important for the global food security, and the drying process exerts great impact on grain quality. The irregular flow of the grain in a mixed flow dryer can produce a non-uniform drying of the grains. The objectives of this work were to verify the suitability of the use of the discrete element method simulations in a quasi-two-dimensional domain, to perform experiments with the objective of validating the results found in the simulations, and to analyse the main aspects of grain flow in a mixed flow dryer. The software YADE was used in the simulations, and the results obtained by the quasi-2D simulations were similar to those obtained in the 3D case, using considerable less computational resources. The simulations of soya beans flow in MFD show that particle size does not affect the residence time, when considering the typical soya beans size variation. Simulations carried out considering different moisture content values show that residence time increases with moisture content. The results also show that porosity distribution is not uniform, which can impact airflow mathematical models.

Characteristic of Vertical Mixed Flow Dryer in Coffee Bean Drying Process

This paper proposes a new type of vertical mixed flow dryer and evaluates the characteristic of the dryer for coffee bean drying. The specific design of vertical mixed flow dryer was in the drying chamber that equipped with a perforated-sloped tray to retard grain flow while drying. The dryer characteristic was presented in a form of humidity and temperature behaviour during the drying process. This characteristic could be a database to get coffee with standard moisture content (MC). Parameter stated were air temperature (°C), humidity (kg/kg dry air), coffee bean mass flow rate (kg/s), and air velocity (m/s). For air velocity of 2 m/s, this dryer was able to dry coffee bean from 52% initial MC to final 14% MC for 11,5 hours.

Energetic and exergetic investigation of a mixed flow dryer: A case study of maize grain drying

Evolution of energetic and exergetic performance is an important concern for selecting suitable dryer for any drying process. This paper attempts to observe performance evaluation in terms of energy and exergy analysis of a mixed flow dryer for drying of freshly harvested maize grain. Existing models in regard to energy and exergy that were established by investigating the first law and second law of thermodynamics were applied for this study. Grain drying experiments executed for 40–80 °C drying temperature at 3.0 and 6.0 m/s air velocity. Energy utilization ratio decreased with increase of drying temperature at 3.0 m/s air velocity while differing surveillance was noticed for 70 and 80 °C at 6.0 m/s air velocity. Energy efficiency of the research mixed flow dryer vacillated from 26.52 to 71.75% over the temperature range. The magnitude of exergy inflow, exergy outflow and exergy loss varied as increasing trend with the upturn drying temperature. The analysis shows that exergy efficiency and exergetic improvement potential varied between 8.54 and 20.87% and between 0.81 and 11.12 kJ/s, respectively for mixed flow maize drying. Model equations that could aptly illustrate the energetic and exergetic parameters were developed as a function of drying temperature.

Performance evaluation of small scale energy efficient mixed flow dryer for drying of high moisture paddy

Suitable drying technique and energy efficient dryer are important concerns for drying of high moisture paddy. In this study, a small scale mixed flow dryer (MFD), with presenting systematic design approach, was developed and tested for paddy drying. Performance evaluation of the developed dryer in terms of specific energy consumption, instantaneous drying and energy efficiencies, and quality of the milled rice were investigated. A comparative performance analysis between the developed MFD and industrial Louisiana State University (LSU) dryer, which is widely used for paddy drying in Bangladesh, was also studied. In addition, drying kinetics for high moisture paddy was evaluated thorough analyzing outlet air relative humidity. Energy analysis revealed that specific electrical (in terms of primary energy) and thermal energy consumption were 0.97 and 3.17 MJ kg−1 water removed, respectively in reducing paddy moisture content (mc) from initial 25 to 12%, the final mc wet basis. Calculation indicated that 33.1% specific electrical and 21% specific thermal energy consumption can be saved by MFD in contrast to the industrial LSU dryer. The energy and drying efficiency values were found to be varied from 0.85 to 0.90 and from 0.10 to 0.17, respectively. Statistical analysis revealed that paddy samples dried by the MFD yielded 16.01% higher head rice yield than sun drying method. The energy analysis tactics presented in this paper can readily be applied in performance evaluation for heated air drying of different products.

Intelligent control of a grain drying system using a GA-SVM-IMPC controller

ABSTRACTThe grain drying is difficult to control because of its characteristics of long delay, strong nonlinearity, and uncertainty parameters. The aim of the paper is to design a suitable controller for a newly designed grain dryer. First, a nonlinear math model of the wheat mixed flow drying was established and analyzed based on the fundamental laws of simultaneous heat and mass transfer. The simulations for the batch circulating drying process and the continuous grain drying process have been made and the simulation results show that it fits well with the actual drying process. Second, an internal model proportional integral derivative (PID) controller (IMPC) based on the support vector machine (SVM) algorithm and the genetic algorithm (GA-SVM-IMPC) was proposed from the view of the energy loss and the dried grain quality. The structure of the GA-SVM-IMPC controller consists of a SVM prediction model, a SVM inverse model controller, a PID controller and a genetic optimization algorithm. Finally, the effectiveness of this controller was demonstrated by computer simulations, and the comparative study with the other controllers further confirmed the superiority of the proposed dryer controller.

Controlling air flow in recirculating mixed flow batch dryer with double bed mode

Mixed flow dryers (MFD) are one type of devices used to dry grain after harvesting in farms. The moisture is removed from the grain by air convection through the grain bed using air ducts installed in the bed. In the normal mixed flow dryer, every other row of air ducts acts as inlets and every other row for exhaust. In this article, the double bed mode is proposed to be switched on during the batch drying process. By using the double bed mode, the air counter pressure increases by 69% and the travel distance of the air by 82%, based on computational fluid dynamics (CFD) simulations presented in this article. Hence, both the distance air travels in the grain bed is increased and due to the higher counter pressure the air flow rate decreases, allowing the better evaporation of water from the grain to the air transporting the vapor off.

Optimization of mixed flow dryers to increase energy efficiency

ABSTRACTAgricultural driers are used for grain or maize drying with a limited annual operating time of about 100–1,000 h. To compensate the increasing costs of energy, permanent optimization of the drying process and the drying apparatus is necessary to increase energy efficiency. To attain higher energy efficiency, the drying potential of air should be fully utilized. The objective of this study is to investigate the medium flow in mixed flow dryers to identify adverse drying conditions. The investigation is based on experimental and numerical modeling and takes into account the bed motion (discrete element method) and the drying air flow (computational fluid dynamics). The results show superposition of a homogeneous air flow distribution with a particle flow profile, resulting in locally inhomogeneous residence time, different drying conditions, and ultimately uneven grain drying. Uneven drying is one of the main reasons for high energy consumption. Considering the results, a new mixed flow dryer geometry was developed which should equalize the drying process and thus be more energy efficient.

Untersuchung der Trocknungsluftströmung an einer neu entwickelten Geometrie für Dächerschachttrockner

The mixed-flow dryer has been a matter of investigation many times regarding drying efficiency, dryer control, and performance enhancement over the past years. However, there is still considerable demand for optimization in terms of energy efficiency and homogeneity of drying. In order to analyze the specific energy consumption and the homogeneity of the drying process, different thermodynamic process conditions have been investigated for the conventional MFD design using numerical and experimental methods. Based on the results obtained, a novel dryer design has been developed. With this, a considerable increase of efficiency is expected. As the fluid dynamic analysis of the first design draft revealed, further development is required until scaling-up and transfer into practice will be possible. While homogeneous airflow conditions could be demonstrated in the core flow region in the center of the dryer, the configuration must be optimized in the near wall regions.

Solar Drying Kinetics of Cass Ava Slices in a Mixed Flow Dryer

Abstract Drying characteristics of cassava slices was investigated in a mixed mode natural convection solar dryer to obtain a suitable mathematical model describing the drying. The average drying chamber temperature was between 34 ±2 °C and 50 ±1.8 °C, while 10 commonly used thin layer drying models were used for drying curve modelling. Coefficient of determination (R2) and root mean square error (RMSE) were used to determine the models performances. The drying curve of cassava slices showed a reduction of moisture content with increased drying time in the solar dryer, and the variation of moisture ratio exponentially decreased with increased drying time. The Midilli and Logarithmic models showed better fit to the experimental drying data of cassava slices. As compared with other models tested, there were no significant differences (p >0.05) in the R2 values obtained for the Midilli and Logarithmic models; hence, the Logarithmic model was preferable because of the lower RMSE. The diffusion mechanism could be used to describe the drying of cassava slices that was found to be in the falling rate period. A diffusion coefficient (Deff) of 1.22 × 10-8 m2 s-1 was obtained, which was within the established standard for food products.

Experimental and numerical study of the airflow distribution in mixed-flow grain dryers

ABSTRACTThe aim of this study was to investigate the airflow distribution in a mixed-flow dryer (MFD) and to study the effect of different bed materials and air duct arrangements. The results were used to validate the numerical model developed in a previous work based on Computational Fluid Dynamics (CFD). A series of experiments have been conducted at a semi-technical MFD test dryer with horizontal and diagonal air duct arrangement. Wheat and rapeseed were used as bed materials. The experiments were performed under isothermal conditions. Two experimental methods were selected and adapted to the measuring problem—the measurement of the isobar distribution within the grain bed and the residence time analysis using the tracer gas pulse method. As could be shown, the isobar distributions measured for wheat and rapeseed agreed well with the model predictions. The numerical model could calculate the influence of the bed material with its different particle characteristics (e.g., particle shape, particle size, bed porosity). The results obtained from the residence time analysis confirmed the known quartering of the air stream flowing from one inlet air duct to the four surrounding outlet air ducts for the horizontal air duct arrangement; in the diagonal air duct arrangement, the air stream from one inlet air duct was nearly halved flowing to the two adjacent diagonal outlet air ducts. These results were confirmed by investigations of the air velocity distribution within the grain bulk. Further experiments are necessary to refine the model. The residence time and isobar measurements will be extended to study the influence of different air properties under real drying conditions, the effect of structural elements, and dryer designs.

Development of a novel mixed flow dryer design

Development of a novel mixed flow dryer design