Mixed-flow Dryers Research Articles

Simulation of soya bean flow in mixed-flow dryers using DEM

Soya bean movement in mixed-flow dryers (MFDs) is explored. A three-dimensional model of soya beans flow was developed applying the discrete element method (DEM) implementation of the Yade software package. To validate the developed model the experiments of the soya beans discharge in small MFD were carried out and the results compared with simulations. Considering the soya bean seeds as single spheres, and using the values of the principal material properties (Poisson coefficient, Young modulus, fraction angle and rolling stiffness) from the literature, satisfactory agreement was obtained between simulated and experimental results with a damping factor value of 0.05. Simulations of continuous flow detected significant non-uniformity in particle velocity and residence time.

Experimente zum Partikelfluss an einer neu entwickelten Geometrie für Dächerschachttrockner

To preserve large mass flows of grain for long term storage, mixed-flow dryers (MFD) are increasingly used worldwide. Design elements which are unfavorably constructed or arranged can cause broad residence time distributions. Hence, locally different drying conditions occur followed by inhomogeneous drying. As a result, the specific energy consumption increases accompanied by economic and quality losses. With the objective of saving product quality and increasing energy efficiency a new dryer geometry was developed. To compare and evaluate the new design with the traditional geometry regarding solids transport, a series of semi-technical particle flow experiments were performed using wheat as bed material and colored tracer particles.

Experimental Studies on a Newly Developed Mixed-Flow Dryer

Mixed-flow dryers are of great importance in worldwide agriculture for the drying of grain, corn, and rice. Unfavorable dryer designs can result in uneven particle and air flow distributions and, thereby, cause inhomogeneous gas–solids contact and drying conditions. As a consequence, the grain drying can locally be very uneven with high fluctuations of the moisture distribution over the dryer cross section. The main reasons are design and construction of the dryer apparatus and the discharge device. A new mixed-flow dryer design has been developed that promises more homogeneous drying, higher energy efficiency, and increased product quality. Firstly, the new dryer design was proved with respect to particle flow. For this purpose, a new test dryer was constructed. A series of particle flow experiments was performed using colored tracer particles. The flow of the tracer particles was observed through a transparent acrylic front wall by image analysis. Based on a comparison with the traditional design, the advantages and disadvantages of the new design were evaluated. The experimental investigations were accompanied by numerical simulations of the particle flow pattern using the discrete element method. The effects of design properties and different air duct arrangements were studied. The present results show that we are at the beginning of a new development concerning the optimization of mixed-flow drying apparatuses.

NUMERICAL SIMULATION OF AIRFLOW DISTRIBUTION IN MIXED FLOW DRYER

Mixed-flow dryers are widely used to preserve large mass flows of grain, rice and maize. Although well established, there is still a need to improve the dryer design in order to increase energy efficiency and safe product quality. The objective of this work is to study the airflow patterns in mixed-flow dryers with different air duct arrangements (horizontal and diagonal) using Computational Fluid Dynamics (CFD). The CFD model developed was validated based on measurements from the literature. The calibrated model was used to analyze the air flow distributions in the traditional dryer geometries. From the numerical results, design deficits could be indicated. Pros and cons of the different air duct arrangements were discussed. It could be shown, that the horizontal arrangement leads to a more homogeneous airflow distribution than the diagonal arrangement.

Investigation of grain mass flow in a mixed flow dryer

The numerical modeling of grain drying is a topic of great relevance to post-harvest engineering. The required type of drying process depends on the quantity of grain to be dried and the required quality of the grain. The choice of the drying system depends on the operating parameters of the drying process. The granular flow pattern of the material exerts a significant influence on the drying process. Post-harvest drying of grain is essential for better storage, handling, and processing. Therefore, it is important to know the material behavior that controls the particle flow patterns of grain in the drying equipment to guarantee the product quality and to optimize the drying process conditions. The discrete element method (DEM) was applied to investigate the particle flow pattern of wheat through a mixed-flow dryer (MFD) without airflow, and the findings were compared with experimental results in this work. The investigations were performed using dry wheat with 14wb% moisture content.

Investigation of Particle and Air Flows in a Mixed-Flow Dryer

Even though the mixed-flow dryer is well established on the commercial market for the drying of grain, maize, and rice, there further potential as well as a need to optimize the dryer apparatus and to improve product quality. Unfavorable designs can cause uneven mass flow and air flow distributions, resulting in locally different drying conditions and, hence, uneven grain drying. The aim of the present article is to evaluate traditional designs of mixed-flow dryers by numerical and experimental investigation of particle and air flows and to discover design deficits. For this purpose, the dryer geometry and different air duct arrangements (horizontal and diagonal) were studied using the discrete element method (DEM) and computational fluid dynamics (CFD). Drying experiments were performed to evaluate the grain moisture and temperature distributions. With regard to particle flow, a typical core flow was detected as in silos with a retarded particle flow at the dryer walls and a fast flow region in the center of the dryer. This was caused by the wall friction effect and the half air ducts fixed at the side walls. With regard to the air flow, dead zones were discovered for the diagonal air duct arrangement. Based on the design deficits identified for the traditional geometry, a new geometry for the mixed-flow dryer that is still under development is discussed.

Berechnung des Schüttgutmassenstroms in Dächerschachttrocknern

Mixed-flow dryers are delivered in various modes of recirculation or through-flow dryers and predominantly used for grain drying. To design and control the dryer, the knowledge of the solids mass flow rate or discharge rate is essential. Fundamentals for its calculation are not known from the literature so far. Hence, this work was aimed to derive appropriate fundamentals. Based on experiments on the discharge characteristic at a test dryer equipped with a discharge gate, equations for the solids mass flow rate for the continuous and the interrupted mode of operation were developed, respectively.

Grain velocity distribution in a mixed flow dryer

For corn drying one of the most popular possibilities in Europe is to use a mixed flow dryer. Large differences in the vertical grain particle velocity in this type of dryers are causing differences in the residence time. Hence, uneven drying occurs causing under-drying or over-drying of grain portions. To investigate the influences of the dryer walls and the air ducts on the particle velocity distribution experiments have been carried out in industrial and laboratory sized dryers and the measurements were compared with discrete element models. The particle flow velocity distribution between the air ducts is modelled analytically based on arching hypothesis. Using discrete element models the effects of different possible constructional modifications causing more even vertical grain particle velocity distribution were analyzed.

Residence Time Distribution in Mixed-Flow Grain Dryers

Unfavorable design of mixed-flow dryers can affect broad residence time distributions, causing nonuniform drying. To simulate particle movement, a two-dimensional model based on the discrete element method (DEM) was developed using the commercial software Particle Flow Code PFC-2D (Itasca Consulting Group, Inc., Minneapolis, MN). The residence time analysis was employed to detect flow inhomogeneities and design deficits. The influences of structural parameters were considered by evaluating particle velocity and residence time distributions. Model validation in a semi-technical test dryer revealed that the 2D DEM model is very successful in predicting the particle flow patterns, so that it can be used to improve the dryer design.

Moisture content and residence time distributions in mixed-flow grain dryers

Unfavourable designs of mixed-flow grain dryers can cause inappropriate residence time distributions, locally different drying conditions and, hence, inhomogeneous drying. To study the effect of design elements and different air duct arrangements, residence time experiments, grain drying experiments and discrete element simulations were conducted both at pilot and industrial scale. These investigations revealed that present dryer designs give broad distributions of residence time, moisture content, and grain temperature. This is a consequence of different drying histories for individual grains. This variation is mainly influenced by the air duct arrangement applied but is affected by other structural and operating parameters as well. Uneven drying is one of the main reasons for elevated energy demands which are still comparatively high in mixed-flow dryers. A future goal will be to develop improved designs of mixed-flow dryers.

Particle velocity profiles and residence time distribution in mixed-flow grain dryers

Non-uniform moisture content distribution of grains at the discharge of Mixed-Flow Grain Dryers is one of the sources of product quality loss during subsequent storage. Unfavorable design of this kind of dryers may cause uneven residence times of single grain portions resulting in non-uniform drying. It is then of paramount importance to understand the physical phenomena that control the flow of grains in a mixed-flow dryer to guarantee their quality and minimize the risk of quality loss and waste of energy, thereby optimizing the process drying condition. With this objective, a two dimensional simulation model for the grain mass flow in a mixed-flow dryer based on Discrete Element Method (DEM) has been developed. The influences of the side walls and air ducts on solids mass flow were studied by evaluating the residence time distribution (RTD), particle velocity profiles and particle trajectories. The simulation results were validated with experiments using a semi-technical dryer test station with transparent Plexiglas front wall. The obtained results revealed the complexity of the drying process, the influences of the wall friction and half air ducts positioned directly on the wall on the bulk particle movement. Grains in mixed-flow dryers have different vertical velocities resulting in different residence times of every single portion of grains. The experimental validation confirms and verifies the DEM calculation ability for predicting particle flow.

Solids transport in mixed-flow dryers

Mixed-flow dryers are broadly used in worldwide agriculture for the drying of grain, corn and rice but are also applied in industry. Although this drying process is well established, there is still a need to optimize the dryer apparatus. Unfavorable design can cause uneven mass flow and air flow distributions, broad residence time distributions and, hence, inhomogeneous drying histories of the particles resulting in non-uniform drying. The transport of solids in mixed-flow dryers has not yet been sufficiently considered and investigated. Therefore, the objective of this study is to derive basic equations on particle flow in mixed-flow dryers which are practically operated in the interrupted flow regime and equipped with discharge gates. The function of the discharge gate, the discharge characteristic and the solids mass flow rate were studied by varying the discharge and standstill times, respectively. The experiments were conducted at a semi-technical dryer test station with a transparent acrylic glass front wall using wheat as bed material. The fundamentals developed serve as a basis for further theoretical and experimental investigations. The future goal is to improve apparatus design and process control so as to homogenize the drying process, to increase energy efficiency and to save product quality.

A model for air flow control in a mixed-flow grain dryer

Mixed-flow dryers are continuous dryers for fluxionary bulk material as grain, granulates from renewable materials etc. These types of dryers are segmented in a number of air inlet and outlet sections which are regularly operated. Principally, the bulk material can not be ventilated homogeneously. Therefore, a non-homogeneous temperature and moisture distribution inside the bulk is developing during ventilation under normal operation. Partially over dried and under dried material is the result. A mathematical model has been developed based on Potential Theory of the Complex Analysis for the air flow distribution through the bulk material. With this model a control strategy can be developed and analyzed to control air inlet flow and air temperature to equalize the temperature distribution inside the dryer.

ANALYSIS OF GRAIN MASS FLOW EXPERIMENTS IN A MIXED-FLOW DRYER

In Central Europe approximately 30% of the grain must be dried after harvest. The most popular continuous crop dryers are mixed-flow dryers. Although this type of dryer is widely used it is still necessary to optimize many segment processes during the drying. For example there are big differences in the vertical grain particle velocity causing differences in the residence time. The aim of the work was to carry out experiments for the grain mass flow measurements in a semi technical dryer. Based on the results a mathematical and 3D computer model for the grain mass flow will be developed in the future enabling improvements in the prediction of the drying process which does not form part of present study.

Experiments on Hot-Air Drying of Wheat in a Semi-Technical Mixed-Flow Dryer

Although mixed-flow grain dryers are widely used, there is still a need to optimize the process control as well as the dryer apparatus. Fluctuations of the grain moisture content at the dryer entrance are still a major problem resulting in quality and economic losses due to under- or overdrying. Therefore, a mathematical model for heat and mass transfer in a mixed-flow dryer has been developed. Practical drying experiments were carried out at a semi-technical dryer test station that was operated quasi-continuous. The measurements reveal the complexity of the mixed-flow drying process. First predicted results are in satisfactory agreement with data.

Research on Modeling and Simulation of Mixed Flow Grain Dryer

Mathematical modeling and computer simulation of grain drying are the major emphasis of grain drying research in CAU (China Agricultural University). Since 1988, research on simulation of concurrent flow, counterflow, cross-flow, and mixed-flow grain dryers has been accomplished. The developed mixed-flow grain drying software has been used in analysis of dryer performance, design, and optimization of new grain dryers and management of existing ones. Effect of size, shape, number of rows, and arrangement of air duct on the performance of a mixed-flow grain dryer has been studied.

Process control based on principal component analysis for maize drying

Producing the grain with equilibrium moisture content is an important process control objective. However, achieving this objective can be very difficult in grain drying process because of its multi-variables, nonlinearity and long delay. In this paper, a control approach based on principal component analysis (PCA) is presented to achieve this objective. A PCA model which incorporates time lagged variables is used, and the control objective is expressed in the score space of this PCA model. A controller is designed in the model predictive control framework, and it is used to control the equivalent score space representation of the process. The score predictive model for the model predictive control algorithm is built using neural network partial least squares (NNPLS). The process control system with NNPLS was tested on a commercial mixed-flow dryer and showed excellent accuracy and stability.

Fuzzy Control of Mixed-Flow Grain Dryer

Grain drying is a typical heat and mass transfer process with characteristics of multivariables, long time delay and nonlinearity. Thermodynamic modeling and control have been a subject of extensive research. Fuzzy logic provides a means for converting a linguistic control strategy, based on expert knowledge, into an automatic control strategy and is suitable for such process. In this article, the thermal characteristics of the grain drying process and the key factors influencing the final moisture content of the dryer are analyzed. On the basis of the analysis, an on-line measurement and fuzzy control scheme of the grain dryer are proposed. Finally, an on-line measurement and intelligent control software is developed and put into industrial application in a grain dryer. The practical control results show that the on-line measurement and intelligent control system of the dryer product satisfying control performance.

Two-Dimensional Simulation Model of Steady-state Mixed-flow Grain Drying. Part 2: Experimental Validation

Abstract A model of a mixed-flow grain dryer at steady state, described in a previous paper, was used to simulate a laboratory-scale dryer in which all input conditions and the grain moisture, temperature and viability loss had been recorded at the discharge and, for most runs, at points in the grain bed. The model was used to calculate grain flow rate for the observed moisture reduction, and the air and grain conditions throughout the grain bed for 11 drying experiments. A unique feature of the model allows it to calculate two drying regimes, one for the grain which passes near the inlet ducts, the other for the grain flowing near the exhaust ducts. Although grain input was overestimated in all cases by the model, by a mean of 10·9%, the loss of viability was predicted with an absolute average error of only 3·1% points of germination. The model underpredicted the viability loss where it was small, but predicted the loss at 125°C very accurately. The moisture difference between the two drying regimes was predicted well in one run and underpredicted in another, and the magnitude of the observed moisture difference between inlet side and outlet side, 0·08 dry basis, showed the importance of the phenomenon in mixed-flow dryers. Predicted air exhaust temperatures were higher than measured, but balances for energy and mass performed on the dryer suggest that heat losses were partly responsible. The model predicted that cross-flow elements of the bed resulted in rapid heating of the grain, showing that this aspect of mixed-flow dryers is important in achieving accurate calculation of viability loss. A simpler model of mixed-flow drying predicted the grain input within 6·1% but because it did not represent the intense heating near inlet ducts, its predictions of viability loss were very poor.

MODELING AND ANALYSIS OF MIXED-FLOW GRAIN DRYER

The nature of the mixed-flow drying process was investigated experimentally and by simulation. Experimentsproved that non-uniform drying occurs among the grain kernels; they pass through the dryer along different pathsbecause of the non-uniform airflow and grainflow patterns in mixed-flow dryers. A two-dimensional drying model wasdeveloped to simulate the drying process. The model confirms the non-uniform drying characteristics of the mixed-flowdryer.