SCALE-UP OF CASCADING ROTARY DRYERS

Until now most of the design methods for cascading rotary dryers have been either empirical or purely theoretical. A theoretical model is presented which simulates the operation of both cocurrent and countercurrent rotary dryers. It relies on pilot plant and bench scale tests to determine the values of parameters which describe respectively the transport of solids through the dryer and the drying rate of the feedstock. A procedure is outlined for using the model to scale up from these pilot-plant and bench-scale tests to full-scale dryers.     

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

ABSTRACT

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.     

AIR-SOLIDS DRAG IN CASCADING ROTARY DRYERS

ABSTRACT

This paper describes the results of theoretical and experimental studies on the effect of air flow on particle transport through cascading rotary dryers. Two possible models are formulated. In the first, the drag coefficient between the air and the solids is calculated on the assumption that the particles can be regarded as isolated spheres. This approach has been employed widely in the literature to estimate particle residence times in rotary dryers. The results yielded coefficients which were 1–2 orders of magntiude greater than those measured experimentally; this demonstrates the inadequacy of this approach. In the second model, the curtains are treated as flat. two-sided plates and the drag coefficient calculated accordingly. The predictions of this model are in much closer accord with the     

SOLIDS TRANSPORTATION MODEL OF AN INDUSTRIAL ROTARY DRYER

ABSTRACT

A complete simulation model has been developed for an industrial rotary dryer to account for the heat and mass exchange between the solids and the gas. This simulator is mainly composed of three models: solids transportation model, furnace model, and gas model. The solids transportation model is the modified Cholette-Cloutier model It consists of a series of interactive reservoirs which are subdivided into an active and a dead compartments to account for the characteristic extended tail of the residence time distribution (RTD) curves observed in industrial dryers.

To expand the validity of the model, experiments have been performed in an industrial rotary dryer to obtain RTD curves under different mineral concentrate and gas flow rates. This paper describes these experiments and presents the variation of the average residence time and model parameters as function of solids and gas flow rates.     

MODELLING OF PARTICULATE DRYING IN THEORY AND PRACTICE

ABSTRACT

The range of theoretical models used by SPS for the design and analysis of dryers, especially dispersion dryers, is reviewed. Different levels of complexity are appropriate at different stages of the design process. Models should use an appropriate level of rigour for the available data and the required purpose of the results. Scale-up from experimental design curves often yields better results in practice than a highly complex model requiring many parameters, and the type of drying kinetics measurements required also depends on the type of model to be used. Models at four different levels are presented in general terms and for a number of major types of dryers. Significant developments in the SPS methodology for fluidised bed, pneumatic conveying and cascading rotary dryers are noted. It is also important to consider the dryer in the context of the overall process flowsheet.     

THE DEVELOPMENT AND VALIDATION OF A SYSTEM MODEL FOR A COUNTERCURRENT CASCADING ROTARY DRYER

ABSTRACT

An overall system model for a countercurrent rotary dryer has been developed with the ullimale aim of assessing controller pairings in these dryers. This model is based on heat and mass balances within dryer regions combined with two subsidiary models, one describing the equipment (which determines particle transport and heat transfer)and the other describing the behaviour of the material (the drying kinetics). Six partial differential equations have been set up to evaluate six state variables: solids moisture content, solids temperature, gas humidity, gas temperature, solids holdup and gas holdup as functions of time and rotary dryer length. A control-volume method has been used to reduce the six partial differential equations with respect to time and the length of the rotary dryer to six ordinary differential equations in time.

The drying model has been implemented in the SPEEDUP flowsheeting package (with FORTRAN subroutines) The model has been validated by fifteen experiments-in a pilot scale countercurrent-flow rotary dryer (0.2m in diameter and 2m in length)     

A Three-Dimensional Simulation of a Spray Dryer Fitted with a Rotary Atomizer

Abstract

Spray dryers fitted with rotary atomizers are commonly used in diverse industries to produce engineered powders on a large scale. Scale-up of such units is still largely empirical and based on prior experience and know-how. In the present study, a three-dimensional spray dryer with rotary atomizer is investigated numerically with a commercial CFD code. Continuous-phase, i.e., air, conservation equations are formulated in the Eulerian model while the droplet or particle equations are set up in the Lagrangian model. Two-way coupling between the continuous and dispersed phases is taken into account in the governing equations. The stochastic approach is used to predict the particle trajectories. The RNG k ? ? turbulence model was used. Typical results, viz. air velocity, temperature, humidity profiles, and particle trajectories are presented and discussed. Compared with the pressure nozzle spray dryer, more volume of drying chamber is used effectively by the rotating disc type spray dryer. It is found that evaporation and drying take place mainly in the region and in the vicinity of first contact between air and spray. A parametric study is presented and, where appropriate, comparison is made with experimental data obtained with the simulated spray dryer.     

FLUIDISED BED DRYER SCALE-UP

Abstract

Design methods for fluidised bed dryers have developed to the point where reliable scale-up is now possible for most applications. The model underlying the methods is described in terms of its constituent parts, namely a material model and an equipment model. The material model utilises drying data from laboratory tests together with scientifically based rules for predicting the effect of changes in operating conditions and the main design parameters on performance. The equipment model describes the residence time distribution for the system. This paper focuses on continuous well mixed and plug flow units.

There are a number of factors which must be considered in scale-up beyond the basic design method. In particular, the feed system, distributor, plenum and freeboard regions can introduce operational differences between the laboratory and full scale units which may adversely affect performance. Guidance is given on these practical considerations.     

On-Farm Drying and Storage Systems

ABSTRACT

Rotary dryers are widely used for the continuous drying of minerals and chemicals on a large scale. Hot gases are passed parallel to the flowing solid to achieve the desired product moisture content. Because these dryers are energy intensive, it is mandatory to operate them as efficiently as possible to respond to economic pressures. Using a dynamic rotary dryer simulator for mineral concentrate, five control strategies are evaluated and compared. Two control strategies are based on PI controllers and the others use neural network models. Results clearly show that a feedforward action, in conjunction with a PI controller or incorporated within the structure of a neural network model, led to the best performances provided an accurate measurement of the feed moisture content is available.     

Rotationszerstäuber für große Tropfen mit enger Größenverteilung zur Anwendung in kleinen Sprühtrocknern

A downscaled rotary atomizer, operated in the regime of laminar thread breakup, is presented. The atomizer allows for producing dropsizes of 70 – 200 µm and narrow size distribution. Due to the special design of the rotary atomizer the drops are detaching with low velocity and the spray can easily be deflected. The new technology can be applied in small scale spray dryers for the production of powder samples with quantities of several kg h^?1. Pilot scale spray dryers were equipped with this type of atomizer in order to carry out scale‐up experiments in small driers, but at particle sizes comparable to production‐scale conditions.     

THE DESIGN OF FLIGHTS IN CASCADING ROTARY DRYERS

ABSTRACT

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure....     

THE DESIGN OF FLIGHTS IN CASCADING ROTARY DRYERS

ABSTRACT

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure....     

The Design of Flights in Cascading Rotary Dryers

ABSTRACT

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure....     

The Design of Flights in Cascading Rotary Dryers

ABSTRACT

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure     

DETERMINATION OF RESIDENCE TIME OF GRAINS IN DRUM DRYER

Abstract

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.     

A SIMPLE DYNAMIC MODEL FOR SOLID TRANSPORT IN ROTARY DRYERS

ABSTRACT

The solid particle movement in a rotary drum plays an important role in drying processes. The solid distribution in the drum affects the amount of contact surface between the solid and the gas. The retention time of solids influences the time particles can stay in contact with the gas in order to transfer heat and mass. Any heat and mass transfer model for a solid particle dryer must be able to predict solid flowrate and solid hold-up. There have been several reports in the literature regarding the modelling aspects of solid transport in dryers. If the model is developed for model-based control, it must be simple and yet represent dynamics of the system accurately. This paper addresses solid motion modelling and the effects of different variables involved in solid transport phenomena. Sugar drying process is the case study in this work. A steady state semi-empirical model was modified to predict solid hold-up and flowrate in rotary dryers. This model was incorporated into a heat and mass transfer model ;o predict solid moisture and temperature for inferential and model-based control purposes. Results of several experiments that have been used to investigate dynamics of the system in terms of solid motion and to validate the model are also presented. The approach advocated in this paper is directly applicable to the transport of other solids in rotary drum equipment and can thus be regarded as a generalized model.     

The Design of Flights in Cascading Rotary Dryers

ABSTRACT

In designing cascading rotary dryers, it is important to sire the flights correctly in order to ensure....     

DRYING IN COMMERCIAL RICE PARBOILING PLANTS

ABSTRACT

Two rotary dryers were replaced by one three-stage concurrent flow(CCF) dryer in a commercial rice parboiling plant. Utilization of the CCF dryer has resulted in a 34 percent savings in fuel energy and a significantly higher quality of the final product. Additional advantages include space, power and labor savings.     

THE DESIGN OF FLIGHTS IN CASCADING ROTARY DRYERS

ABSTRACT

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure.…     

THE DESIGN OF FLIGHTS IN CASCADING ROTARY DRYERS

ABSTRACT

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure.…