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.     

Control of Drying Process in Mechanically Spouted Bed Dryer

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.     

DRYING EXPERIMENTS WITH ALO(OH) SUSPENSION OF HIGH PURITY AND FINE PARTICULATE SIZE TO DESIGN AN INDUSTRIAL SCALE DRYER

ABSTRACT

AIO(OH) suspension of high purity and fine particle size was dried in a Mechanically Spouted Bed (MSB) dryer with inert particles. During drying the maximum required moisture content and the mean particle size of the dried product had to be taken into consideration. Also another important aspect was to avoid contamination of the dried solid with metals. PTFE cylinders of 8 mm in diameter and of 8 mm in height were used as inert particles, and also the whole dryer itself was coated with teflon. As a result of the drying experiments the optimum values of the process parameters were used for scaling-up. The specific rate of evaporation in the drying zone of the inert bed was the basis of the calculations. The suspension with capacity of 125 kg/h was successfully dried in the designed industrial scale MSB dryer with inert panicles and the required quality of the material was produced.     

DRYING EXPERIMENTS WITH ALO(OH) SUSPENSION OF HIGH PURITY AND FINE PARTICULATE SIZE TO DESIGN AN INDUSTRIAL SCALE DRYER

AIO(OH) suspension of high purity and fine particle size was dried in a Mechanically Spouted Bed (MSB) dryer with inert particles. During drying the maximum required moisture content and the mean particle size of the dried product had to be taken into consideration. Also another important aspect was to avoid contamination of the dried solid with metals. PTFE cylinders of 8  mm in diameter and of 8  mm in height were used as inert particles, and also the whole dryer itself was coated with teflon. As a result of the drying experiments the optimum values of the process parameters were used for scaling-up. The specific rate of evaporation in the drying zone of the inert bed was the basis of the calculations. The suspension with capacity of 125  kg/h was successfully dried in the designed industrial scale MSB dryer with inert panicles and the required quality of the material was produced.     

HEAT AND MASS TRANSFER DURING DRYING OF A LIQUID FILM FROM THE SURFACE OF A SINGLE INERT PARTICLE

ABSTRACT

This paper presents the results of theoretical and experimental studies on drying of aqueous suspensions of finely dispersed solids sprayed over the surface of an inert ceramic sphere. The effects of temperature and air velocity on kinetics of heat and mass transfer as well as peeling off the layer of a dry material from the sphere surface are described. The mathematical model of a drying process based on simplified ?gradientless? approach to transfer phenomena is proposed. The adequacy of the model developed for drying of the wet coat from a single sphere to the real drying process taking place in a bed of particulate carrier is confirmed by results of drying of several organic dyestuffs in an industrial spouted bed dryer with inert particles.     

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.     

Drying Of Pastes In Spouted Beds Of Inert Particles: Design Criteria And Modeling

ABSTRACT

The drying of pastelike materials can be performed well in spouted beds (SB) of inert particles. In this work the drying performance of pastes in conical pastes in conical spouted beds is analyzed as a function of column dimensions, fluid flow characteristics and paste properties. imulated data on fluid flow together with the experimental results on drying of different pastelike materials are presented and discussed to provide criteria for the design of a conical spouted bed dryer for suspensions.     

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.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

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

Abstract

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.     

Inert particles as process aid in spray-freeze drying

Abstract

Spray-freeze drying (SFD) is a novel and advanced drying technology in the production of high-value foods and pharmaceuticals. However, long drying time is still a disadvantage for the SFD applications. This constraint could be alleviated using inert particles. This article provides the experimental results on SFD of whole milk in an integrated spray-freeze and vacuum freeze drying equipment. The effects were quantified in terms of the SFD drying time and the properties of the obtained milk powder in relation to the size of stainless steel balls used as inert particles. It is shown that the drying time could be reduced to some extent due to inert particles embedded and the milk powder of reduced size has high porous microstructure.     

An Investigation of Milk Powders Produced by a Laboratory-Scale Spray Dryer

ABSTRACT

A mini spray dryer has been used to investigate morphological changes that occur to milk particles during the spray drying process. We have found that the mini spray dryer is ideal for such investigations, because phenomena such as skin and vacuole formation in particles can be analyzed without the added complication of particle agglomeration, which only occurs in much larger spray dryers where particle number concentrations are higher. We have confirmed observations made by various researchers that the bulk density of spray-dried milk powder is greatly affected by the drying temperature, due to the strong influence of the latter on the porosity of the particles. In addition, we have attempted to explain observations made by various workers that fat accumulates preferentially at the surface of a particle during drying by postulating that fluid fat is transported towards the surface, via a network of cracks and pores, by the development of a vacuole overpressure which is also responsible for the inflation of the particle. Finally, we have shown that milk powders can be spray dried a second time, by reconstitution with water, with no change to the thermodynamic characteristics of the resultant powder. Thus, milk concentrates for spray drying research can be prepared from already-spray-dried milk powders rather than using the more arduous evaporation method to concentrate unprocessed milk.     

STUDY OF HEAT AND MASS TRANSFER DURING IR DRYING OF PAPER

ABSTRACT

A mathematical model has been developed to study the drying of paper using a gas-fired IR dryer. The model accounts for various phenomena : water and vapour mass transfer, conduction, convection and radiation heat transfer. The phenomenological equations are solved with a finite difference scheme, including a modified upwind differencing scheme to account for water migration within the paper sheet. The simulation results illustrate the basic underlying phenomena involved in IR paper drying and can be instrumental to the engineer to make the detailed analyses of such a drying process. A sensitivity analysis has shown that the drying rate is most sensitive to parameters governing the IR beat transfer process whereas the paper sheet temperature is most sensitive to parameters governing the mass transfer process with the surroundings.     

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.     

DRYING OF WOOD BIOMASS AT HIGH PRESSURE STEAM ATMOSPHERE; EXPERIMENTAL RESEARCH AND APPLICATION

ABSTRACT

Imatran Voima Oy together with Technical Research Centre of Finland has carried out experimental research on fuel drying at high pressure steam atmosphere. The pilot dryer is a pressurized flash dryer. Since its commissioning in 1991, the dryer has been used for drying experiments of peat and wood biomass for about 1000 h. The dryer operates at 23 bar pressure steam atmosphere with capacity of abt. 1000 kg/h of wet feedstock.

The developed high pressure steam dryer is planned to be used in a power plant process suitable for wet fuels as peat, biomass, and brown coal. The process is based on the connection of a pressurized fuel dryer, a pressurized gasifier, and a gas turbine. The integration of the high pressure steam dryer to the process increases the power generating efficiency of the process essentially.     

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.     

SIMULATION OF TRANSPORT DYNAMICS IN FLUIDIZED-BED DRYERS

ABSTRACT

A mathematical model for predicting three-dimensional, two-phase flow, heat and mass transfer inside fluidized-bed dryers has been developed. The model consists of the full set of partial-differential equations that describe the conservation of mass, momentum and energy for both phases inside the dryer, and is coupled with correlations concerning interphase momentum-, heat-, and mass-transfer.

It is shown that the model can predict the most important engineering aspects of a fluidized-bed dryer including pressure drop, particle holdup, temperature distribution in both phases as well as drying efficiency all over the fluidized-bed. Plug-flow conditions are predicted for the gas phase, while back-mixing is predicted for the particles.

The effect of particle mass-flow-rate on fluidized-bed dryer performance is evaluated. It is shown that the lower the particle mass flow-rate, the more intense the horizontal moisture gradients, while the higher the particle rate the more uniform the moisture distribution throughout the bed.     

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)     

INSTRUCTIONS FOR PREPARATION OF MANUSCRIPTS FOR DIRECT REPRODUCTION (REVISED, 1992)

ABSTRACT

A software for simulating the drying operation of diluted suspensions in conical spouted beds of inert particles has been developed. This computer program combines the fluid flow and the solids circulation models with the drying kinetic equations. The simulated results for drying animal blood in a spouted bed of inert particles compare with data reported in the literature.     

SNIPPET ON DRYING

ABSTRACT

When regarding the atmospheric contact drying of granular beds wetted with a liquid mixture, both the drying rate and the selectivity of the process, i.e. the change of moisture composition, are of interest. The batch drying of a free flowing ceramic substance, wetted with a 2-propanol-water mixture, is investigated in a rotary dryer with heated wall and air flow.

The theoretical analysis is based on physical models for heat and mass transfer, moisture migration and particle transport, which are presented in examples.

The experimental and theoretical results show that higher selectivities can be achieved by reducing the particle size because of the lower liquid-phase mass-transfer resistance. An increase of the rotational speed leads to a higher drying rate with slightly decreased selectivity if the particles are sufficiently small, since contact heat transfer is enhanced.