Jochen Mellmann

The transverse motion of solids in rotating cylinders - Forms of motion and transition behavior

Mathematical models were developed to predict the transitions between the different forms of transverse motion of free-flowing bed materials in rotating cylinders: sliding, surging, slumping, rolling, cascading, cataracting and centrifuging. Model calculations of the limits between these types of bed motion compare well with measurements of experimental rotating cylinders as well as published results from industrial practice. The motion behavior can be represented on a Bed Behavior Diagram that plots wall friction coefficient and Froude number against the filling degree. From this study, scaling criteria for the bed behavior were found to be the Froude number, filling degree, wall friction coefficient, ratio of particle to cylinder diameter, angle of internal friction, and static and dynamic angles of repose. The transition criteria worked out and the Bed Behavior Diagram provides the user of rotary kilns the possibility to estimate the type of motion of the bed materials used from measured particle characteristics. As a result, the bed behavior can be influenced through selection of operating variables such as rotational speed and filling degree or installation of lifting bars and flights.

Advances in the Application of a Rotary Dryer for Drying of Agricultural Products: A Review

Agricultural products are highly perishable and drying of the product after harvest has been proved as one of the methods to minimize postharvest losses. Different studies have been conducted to evaluate the performance of a rotary dryer for drying of agricultural products. The advantages and the challenges of rotary dryers in drying of agricultural products are discussed. This study discusses the effects of product and drying air properties, and dryer design and operation parameters, on dryer performance and product quality. Rotary dryers are capable of processing a variety of agricultural products with a wide range of thermo-physical and flow properties. Rotary dryers have been used for drying of grains, beans, nuts, vegetables, herbs, woody biomass, animal feeds, agricultural wastes, and by-products. This review paper summarizes the advances in the application of rotary dryers in drying of different agricultural products and recommends future research into the use of rotary dryers to improve the efficiency of the drying process.

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.

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.

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.

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

ABSTRACT The 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.

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.

Research on procedural optimization and development of agricultural drying processes

Abstract Drying is the most important post-harvest process for fast and safe preservation of agricultural products, but it is also energy-intensive at the same time. Agricultural drying is particularly energy-demanding for several reasons – high differences between harvest and storage moisture content, low drying temperatures, and low levels of pre-treatment due to sensitivity to thermal and mechanical stress. Well-established methods for increasing heat and mass transfer, such as surface enlargement and agitation of the bed material, are not applicable for many products. Therefore, farmers depend on optimum harvest windows and weather conditions as well as powerful drying systems in order to attain high product quality, to avoid deterioration, and to process large amounts of produce at the same time. Consequently agricultural process engineers have to compromise between gentle drying, energy efficiency and high drying capacity in order to approach optimum process quality for a specific crop. The current research activities of the Drying Group at ATB Potsdam addressing agricultural drying of grain are presented in this paper.

Optimization of mixed flow dryers to increase energy efficiency

ABSTRACT Agricultural 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.

Kontrollierte Entleerung mit ganzflächigen Austrageinrichtungen

While emptying complex bulk solids apparatuses and silos, the discharger at container bottom can significantly affect the energy demand of the process and the product quality. At practice, a variety of discharge devices has been developed which differ in the closing mechanism and the design of their fixed inserts. Experimental studies with wheat have shown that some whole-area dischargers cause a distinctive flow profile. Due to their design, these systems offer an unexploited potential to locally influence the bulk solids motion and to realize a controlled emptying. Based on a prototype, a new type of discharge device has been developed at the Leibniz Institute for Agricultural Engineering and Bioeconomy e.V. (ATB). First results show that fixed baffles at the walls lead to a decelerated emptying at the sides of the discharge device. On the other hand, a complete renunciation of these installations results in a significant acceleration in the same regions. In future, it is intended to locally control the bulk solids movement by using an optimized design of the fixed inserts of the closing mechanism. Furthermore, it is intended to investigate how the drying process in mixed-flow grain dryers can be influenced and homogenized by using the newly developed discharge device.