Simulation of Wood Particle Motion Through a Concurrent Triple-Pass Rotary Dryer

The retention time of solids in a drum is an important parameter for the design of rotary dryers, since it directly influences the mass and heat transfer rates. If it is too short, the wood particles do not become adequately dried. If it is too long, they become over-dried. Therefore, having an appropriate retention time is useful in terms of both energy and plant capacity. Wood particle mean retention time in a rotary dryer is affected by several variables, such as dryer dimensions, solid characteristics, and operational parameters. The purpose of this work was to simulate the effects of some wood particle characteristics and operational parameters on the mean retention time, drum holdup, and velocity of the wood particles during drying in a pilot-scale, closed-loop, triple-pass rotary dryer by means of a computer code. The simulation results of wood particle motion can be used for modeling, design, and optimization of closed-loop, triple-pass rotary dryers.     

INDIRECT THERMAL DRYING OF LIGNITE: DESIGN ASPECTS OF A ROTARY DRYER

An investigation of the thermal drying of lignite has been carried out, by using an indirect heat pilot rotary drum. The process aims at the production of dry lignite and clean steam as part of a gasification procedure. Both flighted and bare drum modes have been employed. Temperature profiles along the dryer length, the amount of evaporation (moisture conversion) and the solids residence time distribution (RTD) were measured. A non-isothermal model was tested under three different regimes of solids flow. Model integration, by taking account of experimental amount of evaporation at dryer exit and temperature profiles along the dryer length, has been utilized in the validation of drying kinetics and heat transfer correlations. Model predictions compare satisfactorily with the operating data of an indirect heat industrial lignite dryer. Overall heat transfer coefficients of the pilot rotary dryer were found to agree well with those reported for direct heat dryers.     

Improvement of fluidized‐bed dryers for drying solid waste (olive pomace) in olive oil mills

Olive pomace from the two‐phase method of olive oil extraction (two‐phase olive pomace) must be dried from about 65% [wet basis (wb)] to about 8%, in order to extract part of the remaining pomace oil (about 3 wb‐%). An innovative dryer based on a fluidized bed is developed in this study. The objective is to improve olive pomace drying with low energy cost and high product quality by using optimal operating conditions, e.g. temperature and air flow rate, feeding solid moisture, and a control system. The bed operating temperature was set at 125 °C to obtain a good olive pomace oil quality and to reduce the thermal power consumption and drying time. The dried material is rather homogeneous and contains a negligible amount of polycyclic aromatic hydrocarbons. The fluidized bed was further improved with a moving bed joined by a conical device to the fluidized‐bed freeboard. This is a powerful combination in which the moving bed acts as a pre‐dryer of the wet solid and also as a filter of the output gas, with more than 99.9% of fines retention. The mean power consumption of the improved fluidized‐moving‐bed plant is 1 kWh/kg_water; this means a significant reduction of power cost with respect to the rotary dryers, which require about 1.4 kWh/kg_water.     

几种常用干燥设备的简捷计算方法

为简化干燥设备的设计步骤,根据工业生产实践和经验公式,提出了流化床、振动流化床、内热流化床、惰性粒子流化床、离心喷雾干燥、压力喷雾干燥、旋转闪蒸干燥及气流干燥8种常用干燥设备的主体尺寸的简捷设计方法,并介绍了所有干燥设备都适用的基础计算。所述简捷计算采用了许多来自生产实践的经验数据,对设计工业应用的干燥器有重要的指导作用。除了设备主体尺寸之外,尚有诸多结构尺寸需要确定,设计者需根据自己的经验或参考更为详尽的资料才能完成。     

INDIRECT THERMAL DRYING OF LIGNITE: DESIGN ASPECTS OF A ROTARY DRYER

ABSTRACT

An investigation of the thermal drying of lignite has been carried out, by using an indirect heat pilot rotary drum. The process aims at the production of dry lignite and clean steam as part of a gasification procedure. Both flighted and bare drum modes have been employed. Temperature profiles along the dryer length, the amount of evaporation (moisture conversion) and the solids residence time distribution (RTD) were measured. A non-isothermal model was tested under three different regimes of solids flow. Model integration, by taking account of experimental amount of evaporation at dryer exit and temperature profiles along the dryer length, has been utilized in the validation of drying kinetics and heat transfer correlations. Model predictions compare satisfactorily with the operating data of an indirect heat industrial lignite dryer. Overall heat transfer coefficients of the pilot rotary dryer were found to agree well with those reported for direct heat dryers.     

Aerodynamic Separation and Fractional Drying of Alfalfa Leaves and Stems—A Review and New Concept

This article examines the state-of-the art on aerodynamic separation and drying of leaves and stems. Relevant aerodynamic and drying characteristics of alfalfa leaves and stems, important in the design and functional performance evaluation of appropriate drying and separation systems, are presented. General features and design parameters of rotary drum dryers are discussed. A new efficient approach to combined drying and separation in a rotary drum dryer is described in which fresh or pre-wilted alfalfa mixture is dried at a moderate temperature, and in the same operation the dry leaf fraction is aerodynamically separated from the stem fraction. Preliminary test data obtained from the dryer indicated that the separated product stream had comparatively high leaf purity, confirming the feasibility of the new approach.     

Aerodynamic Separation and Fractional Drying of Alfalfa Leaves and Stems—A Review and New Concept

Abstract

This article examines the state-of-the art on aerodynamic separation and drying of leaves and stems. Relevant aerodynamic and drying characteristics of alfalfa leaves and stems, important in the design and functional performance evaluation of appropriate drying and separation systems, are presented. General features and design parameters of rotary drum dryers are discussed. A new efficient approach to combined drying and separation in a rotary drum dryer is described in which fresh or pre-wilted alfalfa mixture is dried at a moderate temperature, and in the same operation the dry leaf fraction is aerodynamically separated from the stem fraction. Preliminary test data obtained from the dryer indicated that the separated product stream had comparatively high leaf purity, confirming the feasibility of the new approach.     

DRYING OF OILSEEDS

Drying of oilseeds is a post harvest operation required for safe storage of harvested seeds. Oilseeds have gained importance in India over the last few years. Fluidized bed (FB) drying and Spouted Bed (SB) drying of oilseeds were studied as potential dryers for these seeds. Experimental pilot fluidized bed dryers were developed and a 2 TPH capacity (mustard seed) FB dryer plant was designed and installed in the lab. Experimental units and a pilot spouted bed dryer were also developed. The dryer units and the plant have been extensively tested and satisfactory drying performance has been achieved. Drying data and drying characteristics have been generated for different oilseeds such as mustard, sunflower, soybean and groundnut etc. The drying rates in different regimes of drying characterized by constant rate and falling rate periods have been evaluated. The salient results of the work are briefly outlined.     

Drying of biofuels from the forest—A review

The literature during 2000–2016 about drying of biofuels from the forest has been reviewed. Biofuels constitute a low-cost energy resource that is likely to continue to increase and the dryers for such products should be simple, robust, and easy to operate. In 1970s and 1980s, rotary dryers and flash dryers were the most common types, and in 1990s, superheated steam (SHS) dryers became common. Maintenance costs and use of medium pressure steam for the SHS dryers are important topics to consider and one drawback for the rotary dryers is that high-temperature heat sources are used. The development during the last 15 years has moved toward moving bed dryers because of the possibility to use cheap low-temperature energy sources, robust design, and direct capacity control that is achieved by controlling the air temperature in the dryer. A price for the dry biofuel of 15–20 Euro/MWh has been indicated to make a dryer installation profitable based on no cost for the thermal energy and 40 Euro/MWh as the cost for the electrical energy. Shrinkage and the internal transport of moisture and heat in large particles of biofuels will need more considerations in the future. Fractionation of the biofuels, codrying with other products, the total cost for the drying process, environmental issues, and development of drying processes operating at high dew points are the other things to consider.     

THE DESIGN AND PERFORMANCE OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS - AN ANALYTICAL APPROACH

In a previous study, a unified approach to the calculation of the size and energy consumption of well-mixed fluidized bed dryers was developed. A computer code was written and a parametric study undertaken to determine the effects of the principal operating variables. The results were shown to depend on the form of the drying kinetics. The present paper describes the formulation of an analytical model of a well-mixed fluidized bed dryer, which applies for materials that exhibit linear falling-rate drying. These materials are typified by small hygroscopic particles, which are frequently dried in such equipment. Equations are presented that enable the drying time, specific energy consumption, heater duty and bed area to be calculated. The predictions of these equations agreed well with previously published experimental findings and with the results of computer simulations for a typical solid (ion exchange resin) exhibiting the required drying characteristics.     

草甘膦节能干燥技术的研究与开发

分析计算了内加热对流化床干燥过程热效率的影响。实验测定了草甘膦流化床干燥机中内加热传热系数,设计开发了用于草甘膦干燥的内加热流化床干燥机,并与普通流化床干燥机和气流干燥机进行了分析对比。     

BATCH VIBRATING FLUID BED DRYER FOR SAWDUST PARTICLES: EXPERIMENTAL RESULTS

In a batch experimental equipment, the behavior of a sawdust dryer in a vibrating fluidized bed is analyzed. Empirical data concerning fluidization velocities, pressure drops and drying kinetics was obtained, and advantages of using vibration in the drying chamber, relative to a conventional fluidized bed, are shown. This technique is presented as an alternative to solve problems of solid agglomeration and bed defluidization. Results show that it is possible to dry sawdust with more than 2 kg/kg moisture, in a vibrating bed keeping a high degree of bed homogeneity and high quality of fluidized state.     

BATCH VIBRATING FLUID BED DRYER FOR SAWDUST PARTICLES: EXPERIMENTAL RESULTS

ABSTRACT

In a batch experimental equipment, the behavior of a sawdust dryer in a vibrating fluidized bed is analyzed. Empirical data concerning fluidization velocities, pressure drops and drying kinetics was obtained, and advantages of using vibration in the drying chamber, relative to a conventional fluidized bed, are shown. This technique is presented as an alternative to solve problems of solid agglomeration and bed defluidization. Results show that it is possible to dry sawdust with more than 2 kg/kg moisture, in a vibrating bed keeping a high degree of bed homogeneity and high quality of fluidized state.     

THE DESIGN AND PERFORMANCE OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS - AN ANALYTICAL APPROACH

Abstract

In a previous study, a unified approach to the calculation of the size and energy consumption of well-mixed fluidized bed dryers was developed. A computer code was written and a parametric study undertaken to determine the effects of the principal operating variables. The results were shown to depend on the form of the drying kinetics. The present paper describes the formulation of an analytical model of a well-mixed fluidized bed dryer, which applies for materials that exhibit linear falling-rate drying. These materials are typified by small hygroscopic particles, which are frequently dried in such equipment. Equations are presented that enable the drying time, specific energy consumption, heater duty and bed area to be calculated. The predictions of these equations agreed well with previously published experimental findings and with the results of computer simulations for a typical solid (ion exchange resin) exhibiting the required drying characteristics.     

Modeling Asd Optimization of Fluidized Bed and Rotary Dryers

Fluidized bed and rotary dryers are two popular types of dryers utilized in almost every area of dryine. In this study, both dryers are analysed regarding their design and operational performance, so that they could be compared in a straightforward way. The mathematical models describing the convective drying process were evaluated for both types of dryers. Design procedures aiming at the determination of optimum sizing and operational characteristics for each type involved were carried out by appropriately optimizing the total annual cost of each structure for a given production capacity. Both dryer types were compared explicitely by evaluating optimum configurations for a wide range of production capacity values. Once the dryer sizing parameters are defined, its operational performance can be evaluated by comparing the optimum operational cost versus production capacity for predefined optimum designed structures. Rotary dryers turn out to be rather expensive compared to fluidized bed dryers, regarding design. On operational grounds, however, it is the other way around, due to the understandably favored heat transfer achieved in rotarv dryers. Characteristic examples covering a wide range of materials - from food products to inorganic minerals -are included in order to demonstrate the performance of each process as well as the effectiveness of the proposed approach.     

DRYER DESIGN PROJECT - A TOOL TO TEACH DRYING AND DESIGN TO UNDERGRADUATES

A summary is presented of a long teaching experiment to incorporate principles of process design in a chemical engineering science course. Students are assigned real world dryer design problems following a series of lectures clarifying the basic terminology, psychrometry and principles of drying. Selected references relevant to specific dryer design problems are also provided. The instructor and teaching assistants act as consultants to the design groups consisting of up to six students. This paper will refer to authors' recent experiences with this assignment using design of spouted bed dryers for grains as illustration.     

DETERMINATION OF RESIDENCE TIME OF GRAINS IN DRUM DRYER

On the basis of the theoretical analysts of the forces applied to grains and moving locus of grains in rotary drum dryer, residence time and contact heating time of grains in the dryer were calculated. The values of calculation conformed to the measured values. These results supplied reliable foundation in theory and practice for further studying and improving the dryer. They also provided designers with references to construct similar type of dryers.     

PSEUDO TWO-DIMENSIONAL MODEL FOR A PNEUMATIC DRYER

Pneumatic drying of chemical products has been frequently used in chemical industries. The increase in the use of this unit operation requires the knowledge of the dynamic of the gas-solid flow in tubes. The mathematical models of vertical pneumatic conveying found in the literature mostly consider the flow steady and one dimensional. However, experimental evidences suggest that radial profiles of the basic variables of the flow exist. In this work a model is proposed for vertical pneumatic conveying considering axial and radial profiles for gas and solids velocities, porosity and pressure. The conservation equations for energy and mass of water were written to extend the model to a pneumatic dryer. The equations of the model were solved using finite difference method and the results show the axial and radial variations of gas and solid temperatures, gas humidity and particle moisture content in the dryer.     

PSEUDO TWO-DIMENSIONAL MODEL FOR A PNEUMATIC DRYER

ABSTRACT

Pneumatic drying of chemical products has been frequently used in chemical industries. The increase in the use of this unit operation requires the knowledge of the dynamic of the gas-solid flow in tubes. The mathematical models of vertical pneumatic conveying found in the literature mostly consider the flow steady and one dimensional. However, experimental evidences suggest that radial profiles of the basic variables of the flow exist. In this work a model is proposed for vertical pneumatic conveying considering axial and radial profiles for gas and solids velocities, porosity and pressure. The conservation equations for energy and mass of water were written to extend the model to a pneumatic dryer. The equations of the model were solved using finite difference method and the results show the axial and radial variations of gas and solid temperatures, gas humidity and particle moisture content in the dryer.     

SIMULATION AND SCALE-UP OF PNEUMATIC CONVEYING AND CASCADING ROTARY DRYERS

This paper presents a unified model for simulation of cocurrent and countercurrent dispersion-type dryers. The main industrial applications are to pneumatic conveying (flash) dryers and cascading (direct) rotary dryers. The basic model is a one-dimensional incremental (stepwise) simulation, which has been developed over a number of years Equations for particle motion, heat and mass transfer, heat and mass balances and local gas conditions are solved simultaneously over a small increment along the dryer. All workers have previously had considerable difficulty in obtaining a good fit between simulations and actual results from pilot-plants or large-scale industrial dryers. A new “fitting mode” calculation overcomes this by identifying the parameters which need to be adjusted, concentrating on those which cannot be measured accurately. Excellent agreement has been obtained between the model and experimental data by this method. The paper also presents revised formulations for particle motion and heat transfer in rotary dryers. The model has been incorporated into two computer programs for flash and rotary dryers respectively, and results from the former are shown for a case study.