MODELISATION PRINCIPLES FOR DRYING OF GELS

When one wants to understand and model the drying of high water content gels, one needs to gather all thermal physical parameters and to derive a physical model based on independently measured parameters. We chose to work experimentally and theoretically on water diffusion and drying of spheres of polyacrylamide gel.

We have demonstrated the strong influence of vitreous transition on the relationship of water diffusivity with moisture content. Values of the diffusion coefficient and temperature shift factor for diffusion have been proposed.

Convective and micro-wave drying kinetics show the presence of a drying phase purely specific to radiation absorption. Finally, we have proposed a mathematical model containing independent physical parameters, which' not only predicts mass fluxes but also analyses critical physical phenomena.     

COMBINED CONVECTIVE-MICROWAVE DRYING OF AGAR GELS: INFLUENCE OF MICROWAVE POWER ON DRYING KINETICS

The use of microwave energy in the drying of deformable material such as gel considerably reduces drying time and enables the control of retraction in the sample. A further advantage is that no hot spots are produced, allowing a dry product of superior quality to be obtained.The aim of this work has been to determine the kinetics of the convective-microwave drying process of agar gel plates. For this purpose, we developed a pilot closed loop, computer-controlled apparatus of convective-microwave drying, that enables the drying air conditions to be changed and the microwave power to be supplied over a wide value range. The equipment also records the sample surface temperature by means of an infrared thermometer. The drying curves obtained for plane geometry present four different drying phases: an initial phase where a rapid increase in the drying rate and in the surface temperature can be observed, as well as a constant rate phase that ends in the so-called convective critical moisture content, a first falling rate phase that concludes in the microwave critical moisture point, and finally a second falling rate phase. Combined convective-microwave drying enables a considerable reduction in drying time compared to convective drying, the time required being inversely proportional to the microwave power supplied. The empirical equation that best represents the kinetics is of the Page type. The absorbed volumetric power in terms of the moisture content was experimentally estimated, with the experimental data fitting an empirical equation.     

COMBINED CONVECTIVE-MICROWAVE DRYING OF AGAR GELS: INFLUENCE OF MICROWAVE POWER ON DRYING KINETICS

ABSTRACT

The use of microwave energy in the drying of deformable material such as gel considerably reduces drying time and enables the control of retraction in the sample. A further advantage is that no hot spots are produced, allowing a dry product of superior quality to be obtained.The aim of this work has been to determine the kinetics of the convective-microwave drying process of agar gel plates. For this purpose, we developed a pilot closed loop, computer-controlled apparatus of convective-microwave drying, that enables the drying air conditions to be changed and the microwave power to be supplied over a wide value range. The equipment also records the sample surface temperature by means of an infrared thermometer. The drying curves obtained for plane geometry present four different drying phases: an initial phase where a rapid increase in the drying rate and in the surface temperature can be observed, as well as a constant rate phase that ends in the so-called convective critical moisture content, a first falling rate phase that concludes in the microwave critical moisture point, and finally a second falling rate phase. Combined convective-microwave drying enables a considerable reduction in drying time compared to convective drying, the time required being inversely proportional to the microwave power supplied. The empirical equation that best represents the kinetics is of the Page type. The absorbed volumetric power in terms of the moisture content was experimentally estimated, with the experimental data fitting an empirical equation.     

COMPARISON OF DIFFERENT DRYING KINETICS MODELS FOR SINGLE PARTICLES

Different kinetic drying models applied to non-shrinking materials are scrutinised and compared. Rigorous mechanistic models are difficult to apply because of the large number of unknown parameters; hence simpler alternatives have been applied instead. This paper focuses on the two most commonly used kinetic models, namely; the diffusion model and the characteristic drying curves. These were compared by plotting the predicted drying curves (moisture content versus time) in the same diagram as a reference curve. The response to different changes in drying conditions and sensibility to extrapolated conditions were tested. The parameters studied were the temperature, velocity and humidity of the gas and the size and initial moisture content of the panicles. The comparison was based on two approaches; one theoretical where the reference drying curves were generated by an rigorous drying model; and one experimental where the reference drying curves were measured using a thin-layer kinetics rig. The materials under study were softwood, ceramic clay, silica gel and purolit. The results were promising in the sense that the characteristic drying curves (CDCs) and diffusion model could both predict the response to most changes in external conditions. The predictions for drying above the boiling point were however less accurate than the ones below it, probably due to internal overpressure effects.     

COMPARISON OF DIFFERENT DRYING KINETICS MODELS FOR SINGLE PARTICLES

ABSTRACT

Different kinetic drying models applied to non-shrinking materials are scrutinised and compared. Rigorous mechanistic models are difficult to apply because of the large number of unknown parameters; hence simpler alternatives have been applied instead. This paper focuses on the two most commonly used kinetic models, namely; the diffusion model and the characteristic drying curves. These were compared by plotting the predicted drying curves (moisture content versus time) in the same diagram as a reference curve. The response to different changes in drying conditions and sensibility to extrapolated conditions were tested. The parameters studied were the temperature, velocity and humidity of the gas and the size and initial moisture content of the panicles. The comparison was based on two approaches; one theoretical where the reference drying curves were generated by an rigorous drying model; and one experimental where the reference drying curves were measured using a thin-layer kinetics rig. The materials under study were softwood, ceramic clay, silica gel and purolit. The results were promising in the sense that the characteristic drying curves (CDCs) and diffusion model could both predict the response to most changes in external conditions. The predictions for drying above the boiling point were however less accurate than the ones below it, probably due to internal overpressure effects.     

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS *

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.     

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS*

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g ). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.

     

TOWARD PREDICTION OF POROSITY IN FOODS DURING DRYING: A BRIEF REVIEW

Four generic trends of pore formation during drying are identified from the literature. The present prediction methods are mainly based on empirical correlations. It is common to correlate porosity with water content by quadratic, polynomial, or exponential forms of equations, which do not provide insight into the physics of the process. The glass transition theory is one of the proposed concepts to explain the process of shrinkage and collapse during drying. However, the glass transition theory does not hold true for all products. Other concepts, such as surface tension, structure, environment pressure, and mechanisms of moisture transport also play important roles in explaining the formation of pores. It is hypothesized that as capillary force is the main force responsible for collapse, so counterbalancing this force causes formation of pores and lower shrinkage.     

A THIN LAYER METHOD FOR THE INVESTIGATION OF BULK DENSITY AND AIR FLOW RESISTANCE DISTRIBUTIONS DURING DEEP BED DRYING

A thin layer experimental method is presented for the investigation of the effect of compression and drying on bulk density and air flow resistance. A thin layer was subjected to a compressive load and ventilated with drying air to simulate the conditions within a deep bed. The technique is analogous to thin layer drying work and in the same way produces thin layer equations which can be incorporated in deep bed simulation programs. It was found that for a bed of germinated barley (malt) the volume change could be considered in terms of three components: initial compaction, transient settling and drying shrinkage. These were correlated with bed pressure, time and moisture content reduction. The effect of initial compression and settling was shown to produce bulk density variations of the order 20% in 1.5 metre beds. Drying shrinkage was found to produce a height reduction of the order 15%. The pressure gradient due to air flow resistance was increased by as much as a factor of six as a result of compression. Moisture content did not have a great effect on air flow resistance.     

DEVELOPMENT OF NEW DRYING TECHNOLOGY AND EQUIPMENT FOR MEDICAL PLASTERS

Study was made of the trends in drying medical plaster leucomass by means of an organic solvent using different heating methods. Based on the research made, a new drying technology and efficient thermal drying equipment have been developed, which are introduced into industry.     

THE INFLUENCE OF COMPRESSION WOOD ON THE DRYING CURVES OF PINUS RADIATA DRIED IN DEHUMIDIFIER CONDITIONS

Measurements of drying rate of Pinus radiata at 55°C and 30% RH are presented. The data, which has been used to establish empirical models for P. radiata under dehumidifier drying conditions, was obtained in four drying runs in a drying tunnel, each yielding detailed drying curves for twelve sample sapwood boards of size 350 × 100 × 50 mm. Compression wood was found to have a significant effect on the drying rate curve, giving lower drying rates at 40-100% MC. This effect is interpreted through the use of a numerical multiple-mechanism two-zone model and quantified by using best-fit diffusion parameters from an isothermal diffusion model. A positive correlation was discovered between the moisture diffusion coefficient and initial moisture content, a strong indicator for the presence of compression wood. In the two-zone model, the compression wood effect was replicated by using a tenfold decrease in permeability to liquid flow. Attributes of compression wood that could cause reduced permeability include an increased proportion of latewood, narrower lumen, and a scarcity of bordered pits on the radial walls of longitudinal tracheids.     

IMPROVEMENT OF DEEP-BED CROSSFLOW MODEL BY A DIFFUSION COMPONENT FOR THE GRAIN PARTICLE AND LOG MODEL FOR EVALUATION OF RECIRCULATING CROSSFLOW DRYER

This paper presents the improved versions of -bed cross now and log simulation models. A diffusion model descries ibing internal diffusion of moisture within a grain kernel to compute moisture removal rate is developed and incur porsted in crossflow model to improve its flexlbillty and versetility. The lag model is suilably modified and employed to evaluate its accuracy and applicability compared to crossflow drying model for evaluating recirculating crossflow drying operation. Experiments were conducted on large scale recirculating crossflow dryer for corn drying. Simulation results are compared with experimental results to test the validity of drying models.     

A MATHEMATICAL MODEL FOR DRYING OF SHRINKING MATERIALS

ABSTRACT

A mathematical model has been developed to describe heat and mass transfer within materials undergoing shrinkage during drying. Both heat and mass transfer equations are solved simultaneously using a numerical technique A beat pump dryer has been used to conduct experiments to validate the model. Several samples were placed in the drver and after the commencement of each drying test one sample was taken oat at rceular time interval: The bone-dry mass of each piece was also determined. This enables to determine moisture distribution within the materials. Temperatures at different locations of the material were measured with thermocouples. The predicted temperature and moisture distribution within the material agreed fairly well with the experimental results.     

STOCHASTIC NUMERICAL MODEL FOR RADIO FREQUENCY VACUUM DRYING OF TIMBERS

A new numeric method to simulate stochastic dispersion (a natural phenomenon that occurs when a magnitude cannot be associated to a specific value, but to the probability of being within a range of values) is proposed and applied to predict Radio Frequency Vacuum (RFV) drying of timber. A theoretical formulation of the method is described and complemented to take into account the frequency distribution of the timber initial moisture content, so that it can be applied to industrial runs. Experimental data obtained from mixed western hemlock and amabilis fir dried in a commercial RFV kiln are used to validate the stochastic model, and the results are compared through moisture content histograms and probability charts. A numerical example is shown in order to provide an idea of the movement of the moisture profiles during RFV drying.     

VALIDATION OF A DYNAMIC MODEL FOR A DEHUMIDIFIER WOOD DRYING KILN

ABSTRACT

The validation of a dehumidifier wood drying kiln model established previously has been conducted by using the performance data for a commercial scale kiln. The good agreement between the modelled and measured performance results shows that the model can be used for the design and analysis of dehumidifier wood drying kilns.     

循环流化床锅炉水冷壁爆管原因与措施

通过对循环流化床锅炉运行过程中多次发生水冷壁爆管事故的原因进行分析,提出了预防措施,改造后效果良好,延长了水冷壁使用寿命,提高了锅炉的运行周期,取得良好的经济效益.