SCALE-UP OF CONTACT DRYERS

ABSTRACT

The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer.

This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions.

The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-hygroscopic in the moisture range investigated.     

SCALE-UP OF SPRAY DRYERS

Abstract

Commercial success of a new spray dryer investment depends upon the dryer meeting its specified performance in all respects, and this puts great importance on the scale-up procedures used in the projecting of the new spray dryer. Scale-up still relies heavily on the experience of the designer. However, as the applications and specifications become more and more complex, so does the need for improved test work in pilot plants, and computational fluid dynamics simulations become more important, so that better scale-up tools are available to minimize the possibility for personal errors that may lead to the new dryer investment becoming an unsuccessful commercial venture.     

SCALE-UP OF SPRAY DRYERS

Commercial success of a new spray dryer investment depends upon the dryer meeting its specified performance in all respects, and this puts great importance on the scale-up procedures used in the projecting of the new spray dryer. Scale-up still relies heavily on the experience of the designer. However, as the applications and specifications become more and more complex, so does the need for improved test work in pilot plants, and computational fluid dynamics simulations become more important, so that better scale-up tools are available to minimize the possibility for personal errors that may lead to the new dryer investment becoming an unsuccessful commercial venture.     

SCALE-UP OF PNEUMATIC CONVEYING DRYERS

Scale-up of pneumatic conveying dryers has until now been largely empirical. A theoretical model is outlined which predicts dryer performance effectively. It depends on some uncertain parameters, notably agglomeration and wall friction, but these can be found more accurately by checking the predictions against results from the small-scale dryer. The model can then be used to scale up to the full size dryer. The required duct length falls as the gas velocity is reduced and the duct diameter increased, subject to satisfactory conveying and dispersion at the feedpoint.     

SCALE-UP OF CASCADING ROTARY DRYERS

Abstract

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.     

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.     

SCALE-UP OF LAYER DRYERS: A UNIFIED APPROACH

Abstract

The concept of Specific Drying Rate is introduced for analysis of experimental drying data for layer dryers and as a convenient scale-up factor. In conjunction with data obtained from a batch laboratory simulation the method is applicable to most types of continuous layer dryers. Three illustrations of the use of this approach for scale-up are presented including a continuous convection heated rotary shelf dryer, an indirectly heated paddle dryer, and an indirectly heated plate dryer with heating zones.     

SCALE-UP OF LAYER DRYERS: A UNIFIED APPROACH

The concept of Specific Drying Rate is introduced for analysis of experimental drying data for layer dryers and as a convenient scale-up factor. In conjunction with data obtained from a batch laboratory simulation the method is applicable to most types of continuous layer dryers. Three illustrations of the use of this approach for scale-up are presented including a continuous convection heated rotary shelf dryer, an indirectly heated paddle dryer, and an indirectly heated plate dryer with heating zones.     

SCALE-UP OF SPOUTED BED DRYERS: CRITERIA AND APPLICATIONS

The criteria required for reliable scale-up of the aerodynamics of spouted bed dryers of grains are stated and discussed. The dryer modeling role for effective scale-up, design and analyses of such dryers will be outlined. Suggestions are made on how small scale drying and aerodynamic data may be scaled up for spouted bed dryers. Appropriate empirical and semi-empirical correlations are listed. A flowchart is provided for design and analysis of a conventional and a two-dimensional spouted bed dryers.     

SCALE-UP OF SPOUTED BED DRYERS: CRITERIA AND APPLICATIONS

Abstract

The criteria required for reliable scale-up of the aerodynamics of spouted bed dryers of grains are stated and discussed. The dryer modeling role for effective scale-up, design and analyses of such dryers will be outlined. Suggestions are made on how small scale drying and aerodynamic data may be scaled up for spouted bed dryers. Appropriate empirical and semi-empirical correlations are listed. A flowchart is provided for design and analysis of a conventional and a two-dimensional spouted bed dryers.     

SCALE-UP OF SPRAY DRYERS WITH THE AID OF COMPUTATIONAL FLUID DYNAMICS

The scale-up of spray dryer chambers is difficult because of the complexity of gas and spray flow patterns. The principal concerns in designing a spray drying chamber are to ensure sufficient residence time for drying and to avoid particle-wall collisions. Dimensionless groups are of limited use because it is practically impossible to achieve dynamic similarity between small and large chambers. In the past, empirical, rather than theoretically based, rules generated by experience with existing plant have been used in the design and scale-up of spray dryer chambers but these models, because of their empirical nature, are limited in their range of applicability. Computational fluid dynamics (CFD) is potentially a powerful tool to aid spray dryer design allowing much more flexibility in design but because of the difficulties of modelling such complex phenomena, especially the gas turbulence, its predictions cannot, at present, be considered absolutely reliable and experimental validation of the results is required. However, by considering the principles of similarity, it is shown that validations carried out on pilot scale equipment under the correct conditions will prove the accuracy of CFD applied to spray dryers of any size.     

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.     

SCALE-UP OF SPRAY DRYERS WITH THE AID OF COMPUTATIONAL FLUID DYNAMICS

Abstract

The scale-up of spray dryer chambers is difficult because of the complexity of gas and spray flow patterns. The principal concerns in designing a spray drying chamber are to ensure sufficient residence time for drying and to avoid particle-wall collisions. Dimensionless groups are of limited use because it is practically impossible to achieve dynamic similarity between small and large chambers. In the past, empirical, rather than theoretically based, rules generated by experience with existing plant have been used in the design and scale-up of spray dryer chambers but these models, because of their empirical nature, are limited in their range of applicability. Computational fluid dynamics (CFD) is potentially a powerful tool to aid spray dryer design allowing much more flexibility in design but because of the difficulties of modelling such complex phenomena, especially the gas turbulence, its predictions cannot, at present, be considered absolutely reliable and experimental validation of the results is required. However, by considering the principles of similarity, it is shown that validations carried out on pilot scale equipment under the correct conditions will prove the accuracy of CFD applied to spray dryers of any size.     

ANALYSIS OF MECHANICALLY AGITATED FLUID-PARTICLE CONTACT DRYERS

Incorporation of agitation to spouted and fluidized bed dryer result in significant increases in the drying capacity (Q_s ), although product retention persist, which is reduced by increasing the air flow. The physical phenomena occurring in these dryers with several liquid substrates was analyzed and the residence time distributions (RTD) were obtained by the use of dye tracers. The residence time (τ) was found to be a function of the rate of agitation (n) and reaches a minimum at n = n _opt, which was characteristic for each type of substrate, and where maxima also appeared for the drying capacity (Q_s = Q _{s max}) and the heat transfer coefficient (Nu _p = Nu _{p max}). The RTD can be modeled by series of consecutive dryers and a modified Vanderschuren and Delvosalle model can be employed to calculated moisture of the dry product.     

ANALYSIS OF MECHANICALLY AGITATED FLUID-PARTICLE CONTACT DRYERS

Incorporation of agitation to spouted and fluidized bed dryer result in significant increases in the drying capacity (Q_s), although product retention persist, which is reduced by increasing the air flow. The physical phenomena occurring in these dryers with several liquid substrates was analyzed and the residence time distributions (RTD) were obtained by the use of dye tracers. The residence time (τ) was found to be a function of the rate of agitation (n) and reaches a minimum at n = n_opt, which was characteristic for each type of substrate, and where maxima also appeared for the drying capacity (Q_s = Q_{s max}) and the heat transfer coefficient (Nu_p = Nu_{p max}). The RTD can be modeled by series of consecutive dryers and a modified Vanderschuren and Delvosalle model can be employed to calculated moisture of the dry product.     

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.     

SCALE-UP OF STEAM-DRYING

Abstract

The possibility of drying in superheated steam or superheated solvent vapour has been realised for a long time but practised very little. Recent advances-have led to predictions of rapid growth of steam-drying applications. Most of these exhibit remarkable energy-savings over conventional plant and reductions in cost which make attractive the installation of steam-drying plant even without “Greenhouse” considerations. Given this growth potential, it is essential to think about those aspects of scale-up particular to steam-drying. This paper looks at the variety of potential steam-drying plant and related scale-up problems, while recognising that each particular, type of plant requires more individual consideration than can be addressed in this brief paper.     

SCALE-UP OF STEAM-DRYING

The possibility of drying in superheated steam or superheated solvent vapour has been realised for a long time but practised very little. Recent advances-have led to predictions of rapid growth of steam-drying applications. Most of these exhibit remarkable energy-savings over conventional plant and reductions in cost which make attractive the installation of steam-drying plant even without “Greenhouse” considerations. Given this growth potential, it is essential to think about those aspects of scale-up particular to steam-drying. This paper looks at the variety of potential steam-drying plant and related scale-up problems, while recognising that each particular, type of plant requires more individual consideration than can be addressed in this brief paper.