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 AND CONTROL OF AN ALFALFA ROTARY DRYER

ABSTRACT

A dynamic model of an alfalfa rotary dryer was developed and used to test the performance of two different feedback controllers. One controller is a conventional PI (Proportional-Integral) controller with fixed tuning parameters whereas the other is a gain-scheduled PI controller with automatically adjusted tuning parameters. The performance of the two controllers was compared with the performance of the dryer under manual control. The gain-scheduled PI controller was found to be superior in the sense that it used less control action and achieved the same control performance as the fixed tuning parameter PI controller. The use of the gain-scheduled controller was shown to reduce energy consumption, increase dryer throughput and had an estimated pay-back time of nine months.     

MODELLING AND CONTROL OF AN ALFALFA ROTARY DRYER

A dynamic model of an alfalfa rotary dryer was developed and used to test the performance of two different feedback controllers. One controller is a conventional PI (Proportional-Integral) controller with fixed tuning parameters whereas the other is a gain-scheduled PI controller with automatically adjusted tuning parameters. The performance of the two controllers was compared with the performance of the dryer under manual control. The gain-scheduled PI controller was found to be superior in the sense that it used less control action and achieved the same control performance as the fixed tuning parameter PI controller. The use of the gain-scheduled controller was shown to reduce energy consumption, increase dryer throughput and had an estimated pay-back time of nine months.     

DYNAMIC MATHEMATICAL MODEL OF A ROTARY DRYER: METHOD OF CHARACTERISTICS

A mathematical model for the dynamic behavior of a countercurrent rotary dryer has been developed and solved. The model consists of four hyperbolic partial differential equations with split boundary conditions. The equations are solved numerically using an algorithm based on the method of characteristics. The solution is stable and rapid. Sample results of a dryer simulation are presented.     

INFLUENCE OF SOLIDS MOISTURE CONTENT ON THE AVERAGE RESIDENCE TIME IN A ROTARY DRYER

The solids mean residence time in a rotary dryer is influenced by several variables such as dryer dimensions and solids characteristics. One of these characteristics, usually not taken into account in correlations proposed to estimate the mean residence time, is the solids feed moisture content. Although it is well known that the solids moisture content has a major impact on the ability of the solids to move along the rotary dryer, it does not enter as a parameter in available correlations. In this investigation, numerous experiments were performed in a pilot-scale rotary dryer to study the influence of solids moisture content and drying gas temperature on the mean residence time. Sand employed in cement makeup was used to perform these experiments. Results show that the mean residence time for a moisture content in the range of 8% to 12% is four times higher than for dry solids. The moisture content and the drying gas temperature influence significantly the shape of the residence time distribution curve.     

THE DESIGN AND OPTIMIZATION OF AN INDUSTRIAL DRYER FOR SULTANA RAISINS

A mathematical model of a tunnel dryer for sultana grapes is presented and applied for the determination of size and optimal operating conditions of the dryer. The optimum condition is given by the minimization of heat consumption, expressed as the ratio of thermal load to production, with some constraints regarding the production rate of the dryer and the maximum permissible air temperature. The optimization variables are temperature and humidity of the drying air, and product loading thickness on the trays. The optimum condition requires the operation of the dryer on the maximum permissible air humidity, which corresponds to a high degree of recirculation of exhaust air. This can be accomplished using automatic control of fresh air and humid air inlet and exhaust dampers along the length of the dryer, during the entire drying cycle.     

THE DESIGN AND OPTIMIZATION OF AN INDUSTRIAL DRYER FOR SULTANA RAISINS

A mathematical model of a tunnel dryer for sultana grapes is presented and applied for the determination of size and optimal operating conditions of the dryer. The optimum condition is given by the minimization of heat consumption, expressed as the ratio of thermal load to production, with some constraints regarding the production rate of the dryer and the maximum permissible air temperature. The optimization variables are temperature and humidity of the drying air, and product loading thickness on the trays. The optimum condition requires the operation of the dryer on the maximum permissible air humidity, which corresponds to a high degree of recirculation of exhaust air. This can be accomplished using automatic control of fresh air and humid air inlet and exhaust dampers along the length of the dryer, during the entire drying cycle.     

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)     

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

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)     

复混肥干燥机结构的改进设计

对常规转筒干燥机内部结构进行改进设计,增添了二次成球区及抛光区,在提高成球率的同时提高了产品的质量。     

SIMULATION OF A ROTARY DRYER FOR SUGAR CRYSTALLINE

Dynamic heat and material balances were developed, and residence time, heat and mass transfer rates were calculated using literature correlations. The model equations were solved numerically using the Speedup simulation package and tested against industrial data.

Comparison of model predictions with industrial data show that the model is accurate for steady state operation and predicts dynamic trends that are consistent with engineering judgment. Predicted outlet moisture and temperatures differ by about ±10 % from the industrial data.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

COMPUTER ANALYSIS OF INDUSTRIAL BATCH DRYER AERODYNAMICS

One of the fundamental problems encountered in the batch dryer design field is the determination of appropriate equipment configuration that would ensure uniform distribution of air over the dryer trays. Such industrial batch dryer aerodynamics problems can be successfully investigated using computational fluid dynamics techniques. A mathematical model for predicting the two-dimensional air flow inside a typical industrial batch dryer equipment is developed and analyzed. The model consists of the full set of partial differential equations that describe the conservation of mass and momentum inside the dryer. The standard k-E model is used to describe turbulence in addition to the governing conservation equations. Distribution of drying air within the dryer is regulated using adjustable air blast blades in the entrance section of the drying chamber. An appropriate configuration of these flow adjusting devices is proposed so that an adequately uniform drying air distribution pattern inside the drying chamber is achieved. Finally, a characteristic design case is presented to demonstrate the effectiveness of the proposed approach.     

COMPUTER ANALYSIS OF INDUSTRIAL BATCH DRYER AERODYNAMICS

ABSTRACT

One of the fundamental problems encountered in the batch dryer design field is the determination of appropriate equipment configuration that would ensure uniform distribution of air over the dryer trays. Such industrial batch dryer aerodynamics problems can be successfully investigated using computational fluid dynamics techniques. A mathematical model for predicting the two-dimensional air flow inside a typical industrial batch dryer equipment is developed and analyzed. The model consists of the full set of partial differential equations that describe the conservation of mass and momentum inside the dryer. The standard k-E model is used to describe turbulence in addition to the governing conservation equations. Distribution of drying air within the dryer is regulated using adjustable air blast blades in the entrance section of the drying chamber. An appropriate configuration of these flow adjusting devices is proposed so that an adequately uniform drying air distribution pattern inside the drying chamber is achieved. Finally, a characteristic design case is presented to demonstrate the effectiveness of the proposed approach.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

ABSTRACT

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

EXPERIMENTAL STUDY OF RESIDENCE TIME IN A DIRECT ROTARY DRYER

Prediction of residence time in rotary dryers is useful for equipment design as well as for the right selection of operation conditions in order to obtain the optimal unit functioning.

An empirical relationship for residence time estimation that may be applied when the dryer slope is zero or different from it is presented in this work. It was derived from experimental testing carried out with biological origin particulated solids (fish and soya meal, sawdust) and from mineral origin (sand) in a pilot rotary dryer. The fitting degree of the experimental data to those predicted by reported equations was also established here.     

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5 rpm drum speed and quadrupled at 7 rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.     

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

ABSTRACT

A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5?rpm drum speed and quadrupled at 7?rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.     

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.     

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.