Fundamental Research on Drying Process in Institute of Engineering Thermophysics, CAS

Some results of our fundamental research on drying processes are summed up in this article. It consists of three parts: (1) Multistage fluidized bed drying, including particle flowing characteristics, heat and mass transfer between particles and drying medium, drying characteristics of drying materials; (2) Impinging stream drying, the flowing and drying characteristics of a vertical impinging stream dryer, one-stage and multistage semi-circular impinging stream dryer and combined vertical and semi-circular impinging stream dryer are discussed; (3) The effects of rapid transient heat and mass transfer on drying processes, such as time and space scales for nonFourier or nonFickian and influence of extreme heat and mass transfer are also discussed.     

对撞流干燥的实验与理论研究

The experiments of one-stage semi-circular and two-stage semi-circular impinging stream drying as well as the vertical and semi-cricular combined impinging stream drying were carried out.The velocity distribution and the mean residence time of particles,and the influence of various factors on drying characteristics were studied.A mathematical model of granular material drying in a semi-circular impinging stream dryer was proposed,in which the flow characteristics as well as the heat and mass transfer mechanisms were considered.Reasonable numerical methods were used to solve the equations.Under various conditons,the calculated results of drying rate and moisture content versus time were obtained.The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer.Appropriate semi-cricular stage number and curvature radius,flow-rate ratio,air velocity,and higher inlet air temperature should be used for enhancing the drying process.     

SOME CRITERIA OF SPRAY DRYER DESIGN FOR FOOD LIQUID

Abstract

Spray drying has many applications for a wide variety of dried food powders. The quality of spray dried food is quite dependent on the atomization characteristics and the heat and mass transfer on drops inside the spray dryer. This paper deals with some design criteria of the spray dryer such as the atomizer, the drop trajectory and the heat/mass transfer between the drop and the drying air.     

Drying Process on Inert Particles in Mechanically Spouted Bed Dryer

ABSTRACT

Suspensions, slurries and paste-like materials can be dried in the Mechanically Spouted Bed ( MSB) dryer with men packing. The circulation of the men particles characteristic of classical spouted beds is provided with a houseless conveyor screw mounted in the vertical axis of the bed. Radioactive isotopic tracer technique was used for measurement of the cycle time distribution ( CTD) of the spherical inert particles as a function of the operational parameters of drying. The variances ( σ²) of the CTDs and the particle velocity m the various zones of the MSB dryer were calculated. The circulation of the mert particles can be characterised by nearly plug flow. According to the physical model of drying on inert packing the heat and mass transfer coefficients were calculated. Que to the relativeiy uniform film-like, wet coating formed on the surface of the spheical inert particles, the drying process may be characterised with the constant rate of drying. A method has been elaborated for calculation of drying time, hereby the partial processes of drying on inert particles can be synchronised.     

STEAM PRESSURE FILTRATION: MECHANICAL-THERMAL DEWATERING PROCESS

Abstract

Grain drying is a typical heat and mass transfer process with characteristics of multivariables, long time delay and nonlinearity. Thermodynamic modeling and control have been a subject of extensive research. Fuzzy logic provides a means for converting a linguistic control strategy, based on expert knowledge, into an automatic control strategy and is suitable for such process. In this article, the thermal characteristics of the grain drying process and the key factors influencing the final moisture content of the dryer are analyzed. On the basis of the analysis, an on-line measurement and fuzzy control scheme of the grain dryer are proposed. Finally, an on-line measurement and intelligent control software is developed and put into industrial application in a grain dryer. The practical control results show that the on-line measurement and intelligent control system of the dryer product satisfying control performance.     

CLosed-Cycle Drying of Solids

ABSTRACT

The influence of the recovery system and other system parameters upon drying selectivity was studied in a closed-cycle dryer. A continuously-worked unit consisting of a single stage dryer and a ondenser was simulated. In the dryer, a thin solid wetted with a liquid mixture was dryed in contact with an inert gas flowing either parallelly or as an impinging jet Partial recycle of the exhaust gas was allowed. Two recovery systems were studied, a scrubber condenser and an indirect condenser. Both drying and condensation were described by a mathematical model considering a s-phase-controlled process. The inlet liquid composition in the scrubber condenser has the greatest influence on drying selectivity. Degree of vapour recovery, recycle ratio and other operational parameters ffect selectivity and equipment dimensions in a variable extent. The combined use of these variables constitutes a powerful method to control selectivity. The present models may be useful to aid the optimization of the system with regard to product quality in gas-phase-conirolled processes.     

Two-Fluid,Two-Dimensional Model for Pneumatic Drying

Abstract

Pneumatic drying is a widely used process in the chemical industries and includes simultaneous conveying and heat and mass transfer between the particles and the heat gas. The increase in the use of this unit operation requires reliable mathematical models to predict processes in the industrial facilities. In the present study a Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a two-dimensional steady-state condition and considering axial and radial profiles for the flow variables. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The numerical procedure includes discretization of calculation domain into torus-shaped final volumes, solving conservation equations by implementation of the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm and controls over coupling of phases by IPSA (Interphase Slip Algorithm). The developed model was applied to simulate a drying process of wet PVC particles in a large-scale pneumatic dryer and to a drying process of wet sand in a laboratory-scale pneumatic dryer. The numerical solutions are compared successfully with the results of independent numerical and experimental investigations. Following the model validation, the two-dimensional distributions of the flow characteristics were examined.     

Experimental Study of the Heat and Mass Transfer During Drying in a Fluidized Bed Dryer

ABSTRACT

Drying curves obtained in a pilot-scale fluidized bed dryer using biological source solids (sawdust, soya and fish meal) were used to estimate the parameters involved in heat and mass transfer phenomenas: heat transfer coefficient and moisture diffusivity coefficient. Parameters involved in mass transfer were estimated from drying models based on diffusional mechanisms and others that in addition consider internal and external resistance to the mass transfer. The estimate ef ective diffusivity coefficient was between 2x10-11 to lx10 (m2/s) for the considered products. Heat transfer coefficient was estimated from drying data points in the constant drying rate period when the external resistance to the mass transfer controls the process.     

IMPINGING JET DRYER

ABSTRACT

In coating and gravure printing, an impinging jet nozzle with high thermal efficiency for drying of coated film was developed.

Trial production 0f 40 kinds of nozzle enables to develop a high-performance impinging jet nozzle with heat transfer coefficient 1.5 times larger than that of current slit nozzle, through measurement of heat transfer coefficient, visualizations of air flow and heat transfer, and measuremenu of jet velocity and turbulence distribution. The purpose of the trial production was to expand a range of high heat transfer and promote turbulence compared with the current nozzle.

Paying attention to mass transfer within gravure ink coated film, drying characteristic of the film was analyzed by numerical solution of a set of equations governing the drying process in which concentration dependencies 0f the diffusion coefficient and the equilibrium vapor pressure were considered.

Applying these analyses. an industrial scale dryer with excellent drying efficiency has finally been developed.     

On The Drying of Food Products in a Tunnel Dryer

ABSTRACT

We present a dryer model for simulating the drying of hygroscopic-porous food products in a tunnel dryer. The model employs an improved receding-front formulation by taking into consideration the material volumetric shrinkage and the variation of the heat and mass transfer coefficient during drying. Predicted results show close agreement when compared with experimental data. We report a parametric analysis using the dryer model to study the drying transient and the need to cascade the drying process so as to maximise the drying potential of the air stream.     

DRYING CHARACTERISTICS OF SLOT JET REATTACHMENT NOZZLE AND COMPARISON WITH A SLOT JET NOZZLE

ABSTRACT

Slot Jet Reattachment (SJR) nozzle is an extension of the Radial Jet Reattachment (RJR) concept used to provide high heat and mass transfer while minimizing flow exerted forces on the reattachment surface. The SJR is a slot jet nozzle with a bottom plate attached to it, which is machined to direct impinging flow at different angles to the surface. The drying characteristics of the SJR nozzle with four exit angles on a paper sample were studied for three Reynolds numbers, three temperatures and two initial moisture contents. Dry air was used as the jet fluid. Correlations to predict drying rates and moisture content for the SJR nozzle as a function of exit angle, temperature, Reynolds number and drying time, for a given initial moisture content, were developed. A comparison of the drying characteristics and net forces of the slot jet and SJR nozzles was also performed under the same flow power and surface peak pressure.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

ABSTRACT

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

Drying in a Pulsed-Fluid Bed with Relocated Gas Stream

ABSTRACT

Results of studies on cooling and drying of powdery and granulated material in a pulsed-fluid bed ( PFB) system with relocated gas stream presented here are related to hydrodynamics and kinetics of these processes. Generalized results in the form of dimensionless equations can be used to determine basic process parameters such as drying time, heat transfer coefficient, pulsed-fluidzation velocity. pressure drop, etc. Results of experimental investigations, supplemented with experiments carried out in a prototype industrial equipment, can be useful for dryer scale-up and are recommended for designing.

A comparison of technical and economic parameters of these dryers with the parameters of classical fluid-bed dryers shows many advantages of PFB systems, including, among othen. reduced gas consumption, uniform bed structure and stability of final parameters of the product.

Positive estimation of the operation and technological parameters suggests that PFB dryers with relocated gas stream can be applied successfully in industrial practice.     

Control of Drying Process in Mechanically Spouted Bed Dryer

ABSTRACT

The continuously operated Mechanically Spouted Bed (MSB) dryer of high evaporative capacity can be advantageously used to produce fine powder from paste-like materials, slurries, suspensions and sludges. Due to the thin layer formed on the surface of the spherical inert particles intensive heat and mass transfer occur and the drying process takes place in the constant rate period. Steady state drying conditions can be achieved when the total operational time of partial processes of inert bed drying does not exceed the cycle time of the inert particles.

A laboratory scale MSB dryer has been equipped with a computerised measuring, data acquisition and control system. In the knowledge of the hydrodynamic characteristics of the MSB and giving the enthalpy and mass balances over the dryer a calculation method has been developed for control of drying process.     

SIMULATION OF DRYING IN A BATCH LUMBER KILN FROM SINGLE-BOARD TESTS

ABSTRACT

A deterministic model was developed to perform a board-by-board simulation of a forced convective batch lumber kiln. Individual board properties may be input and dryer operating parameters varied. The drying rates are empirical correlations based on single-board laboratory tests. The model incorporates the thermodynamic properties of the wood and gas, as well as mass and energy balances within the lumber stack. It also accounts for differences in heat and mass transfer resulting from position and changing gas properties throughout the dryer. The rate of drying predicted by the model and the final moisture content distribution were verified by weighing boards in a batch kiln before, during, and after drying. The application of the model is illustrated by simulating four common scenarios.     

Numerical Simulation of the Drying of a Deformable Material : Evaluation of the Diffusion Coefficient

ABSTRACT

We are interested in the simulation of heat and mass transfer processes accompanying the drying of a deformable agricultural product. We have used an implicit method with classical finite differences to resolve the set of equations. The identification of the diffusion coefficient within a thin carrot layer is carried by trial and error on two analytical forms. These two forms take into account moisture content, temperature and shrinkage of the product.

The drying rates of thin layer of carrot pieces are measured in a laboratory tunnel dryer. These drying curves are established by varying some airflow parameters such as velocity, temperature and relative humidity. The internal and surface temperature of the slab were recorded by means of therrnosensors. The simulated solutions are in agreement with the experimental results.     

MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

ABSTRACT

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

A Learning Process Simulator for Fluidized Bed Dryers Application to Bentonite Drying

ABSTRACT

A fluid bed dryer simulator was developed under Excel 5 wlth Visual Basic for Applications environment The simulator iS based on a mathematical model describing heat and mass transfer in the dryer. The total model incorporates empirical models for the Drying Constant and the Residence Time. These empirical models are crucial in the total model efficiency. Thus a procedure for updating the parameters of the empirical models is provided. This procedure constitutes the ‘learnhg’ property of the simulator. Two databases are Supplied. The first contains laboratory drying data and it is used for tuning the Drying Constant empirical model. The second contains industrial drying data from the real operation of the dryer, and it is used for tuning the Residence Time empirical model. The experience from the industrial application of the simulator proved that the simulator is a powerful tool for flexible operation of an industrial dryer. This paper is presents the total mathematical model of the dryer, the learning concept, and the databases, including useful information concerning the drying kinetics of bentonite. A simulator outline is presented and typical capabilities and uses are briefly described. A case study for flexible operation of an industrial dryer is discussed.     

Heating/Drying Using Particulate Medium : A Review Part I. General and Heat transfer parameters

ABSTRACT

This paper presents an Overview of particulate medium drying and heat treating of cereal grains. While the conventional air drying of grains is well documented, studies on the drying of grains using heated granular medium do not appear to exist. The scientific study of the different aspects of drying with a heated granular medium began in the early 1970's. Progress on the utilization of particle-to-particle heat transfer was slow as evidenced by the fact that there is no commercial dryer using the method as of today.

The first section of this paper deals with conduction heating and how it led to the use of granular medium in heating the grain. Starting with the earliest work on conduction heating reported by Kelly ( 1939), the developments in the heating of grain using granular media is discussed. For decades since Kelly's report, work in the subject area dealt mostly with the theoretical aspects of solid-to-solid heat transfer. Thus, in the succeeding section of the paper, heat transfer parameters and mechanisms involved in the process are thoroughly investigated.     

CROSS-EFFECT OF HEAT AND MASS TRANSFER OF LU1KOV EQUATION: MEASUREMENT AND ANALYSIS

Abstract

This paper mainly focuses on cross-effect of heat and mass transfer of capillary porous media which A.B.Luikov set up on irreversible thermodynamics principle. On the basis of perfecting the equations of heat and mass transfer, the heat and mass transfer parameters are determined during drying processes, and thermal gradient coefficient δ and moisture gradient coefficient ξ are obtained which show the cross-effect of heat and mass transfer. Thus the fundamentals are provided for quantitative analysis of cross-effect of heat and mass transfer. The convective drying mathematical model under the first unsteady boundary condition is therefore proposed. By the application of Henry transform, the theoretical solution of unsteady drying process is given and its validity is verified