Through Air Drying

Process of through-air-drying is becoming increasingly popular in the manufacture of textiles, non-wovens, tissue, and towel. Very high drying rates, enhanced product properties, i.e., softness, bulk, absorbency, unique 3D structure are the driving forces behind its increasing popularity. In this article, experimental results on convective heat and mass transfer and fluid flow characteristics of tissue and towel products using commercially realistic structures are presented. Comparison with literature data using wet pressed, dried, rewetted sheets indicate significant differences in drying and permeability characteristics confirming that the internal structure of the material does indeed play a significant role in through-air-drying and should be taken into account in modeling, optimization, and control of commercial systems.     

SUPERHEATED STEAM DRYING: A BIBLIOGRAPHY

An experimental setup was developed to study the through air–drying characteristics of permeable grades such as tissue and towel under commercially relevant conditions of basis weight, airflow rate, temperature, and humidity conditions. The experimental setup is capable of evaluating the transient fluid flow, heat, and mass transfer characteristics of relatively larger samples (TAPPI standard hand sheets; 0.1524 m) and is capable of studying the effect of local heterogeneity and structure on convective heat and mass transfer. The system is capable of airflow rates of 0.5–10 m/s with corresponding high-speed data collection and acquisition for measuring important variables such as exhaust air humidity. To study the effect of nonuniformity, local temperature and velocity profiles can also be measured using grid of thermocouples and hot wire anemometers. The instantaneous drying rate and airflow characteristics during through air drying was measured and dry permeability, wet permeability, and convective heat and mass transfer characteristics were then calculated. The experimental results were verified by comparing with the results from literature. Typical experimental results were presented to show the effect of sheet basis weight, initial moisture content, and airflow rates on the drying characteristics for two different types of paper samples.     

Mass Transfer Coefficients and Drying Rates in Through Air Drying: Part II

In this study experimental results and analysis are given of convective heat and mass transfer in the drying of tissue and towel products (25 and 50 g/m² basis weight) under commercially realistic conditions under through flow. Parameter effects on drying rate include process variables such as refining, shaping, commercial forming, initial moisture content, and drying temperature. Drying rate curves display initial increase, plateau, and final decrease as the paper dries. Nondimensional Sherwood numbers (versus Peclet number) generally agreed with and augment the available dataset on through drying.     

Integral Flow Parameters and Material Characteristics Analysis in Through Air Drying: Part I

The extension of the Darcy law (the Forchheimer flow equation) relating second-order nonlinear pressure drop with flow velocity is studied during fast transient through air drying of sheets of porous biobased materials such as paper. A range of the paper materials with open structure consistent with tissue and towel products (basis weights 25 and 50 g/m²) made using different production processes are analyzed for the factor-specific influences with regard to changes in the fluid resistance from the removal of moisture from the material interstices. A characteristic dimension suitable for the drying process is applied from viscous and inertial momentum transport analysis.     

COLLAPSE OF STRUCTURE DURING DRYING OF CELERY

ABSTRACT

Collapse of structure of foodstuffs during air drying affects quality. In many materials the soluble components, mainly sugars, are an important part of the tissue in which case collapse may be related to their glass transition temperature (T_g). It has been speculated that collapse occurs at a temperature (T_c) related to, but greater than, T_g. Plant tissues with high moisture contents, such as celery, have low T_gS. Therefore considerable collapse is expected at drying temperatures.

The aim of this study was to determine how air drying temperature affected the quantity characteristics of the tissue. Celery, air dried at temperatures between 5 and 80°C, was examined for volumetric shrinkage, rehydration characteristics and porosity changes. significant shrinkage occurred at all drying conditions. At low water content collapse was limited, probably due to a higher collapse temperature. porosity development was insignificant during drying until the sample was very dry. Lower air-drying temperatures gave a product with improved quality characteristics.     

Drying of Porous Materials in the Presence of Solar Radiation

ABSTRACT

We present a combined heat and moisture transfer model for predicting the drying characteristics of porous building materials exposed to solar radiation. The model has been validated for convective drying using published data and for radiative drying using results of an experimental study carried out using a solar lamp to simulate solar radiation conditions. Actual and predicted moisture content profiles and the drying rates when compared give favourable results.     

Drying Events in Lódz,Poland

ABSTRACT

Printing and heavier grades paper are dried over steam heated cylinder dryers, a mature process which is characterized by low drying rates and lack of ability for control of moisture uniformity across the sheet. The present study examines factors relating to the applicability of through air drying for such grades of paper. This drying process yields much higher drying rates but now is restricted to high permeability, light weight products such as tissue and toweling as the cost of providing through flow across the sheet is the key economic factor in this technique. With over 400 through drying experiments the parameters characterizing the drying rate curves were determined for 10 quite different types of uncalendered paper produced on a variety of papermachines.

The relation between drying conditions and two characteristics, the moisture content at the end of increasing rate period and the constant drying rate, did not vary significantly between these types of paper. However, the extent of the constant rate and falling rate drying periods varied substantially as the critical moisture content is specific to the type of paper.     

The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 1

Through-air drying is commonly used in the drying of high-quality tissue and towel products. A representative elementary volume method was used to model the fluid flow and heat and mass transfer during through drying in heterogeneous porous biobased materials such as tissue and towel products. Results of flow both upstream and downstream of a modeled porous sheet allowed visualization of the effects of mixing at the top and bottom of the porous medium. The effect of initial nonuniformity on fluid flow and convective heat and mass transfer in heterogeneous porous media was studied. The effect of material nonhomogeneity and associated transport properties on moisture content of the porous material as a function of drying time was studied. Modeling results indicate that for the first time it is possible to simulate the effect of nonuniformity on fluid flow and convective heat and mass transfer in porous media during through-air drying of paper. Moisture and structural nonuniformity contributing to nonuniformity in air flow might contribute significantly to drying nonuniformity. Depending on the moisture regimes and degree of saturation of the convective medium, heat and mass transfer coefficients may have varying effects on the overall drying.     

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.     

A Simulation Study on Convection and Microwave Drying of Different Food Products

Abstract

It is now well recognized that matching the external drying condition with the drying kinetics of a material can lead to substantial savings of energy and in the case of heat-sensitive products, even to higher quality product. In this work, the effect of convection and microwave heat input and other product parameters on the batch drying characteristics of model materials, potato and carrot slabs, whose thermo-physical data are readily available in the literature, was modeled using a one dimensional liquid diffusion model. The influence of various thermo-physical properties of the product in drying of heat-sensitive materials was quantitatively assessed. Heat of wetting, temperature and moisture dependent effective diffusivity and thermal conductivity are considered in this model. The effect of moisture diffusivity on drying using convection and a microwave field is simulated in view of the interest in predicting the drying performance by simplified method. Conditions under which the drying rate is controlled by the external drying conditions and the internal thermo-physical properties of the product are computed and discussed.     

Walnut structure and its influence on the hydration and drying characteristics

Abstract

Fresh harvested walnuts are dehulled, washed, and then dried by hot air (HA) as a continuous process in the industry. The objective of the current work was to study the walnut structure and investigate its effect on the moisture transfer characteristics during the walnut soaking and drying processes. Moisture transport pathways into the walnuts were determined using fluorescence tracer approach, and the hydration kinetics of walnuts under different soaking temperatures (15, 25, and 35?°C) was studied using Peleg model. HA drying experiments in single layer in a self-designed automatic HA dryer at 43?°C and air velocity of 1.41?m/s. The influence of the stem pore (sealed and non-sealed) and the soaking process (0, 2- and 5-min soaking time) on the walnut drying characteristics were investigated systematically. The results indicated that both the presence of the stem pore and the soaking time had significant influence (p < 0.05) on the hydration and drying characteristics of walnuts. Moisture absorptions through the stem pore and the shell were equally important during the soaking process. Two to five minutes soaking process led to 2–4?h additional drying time. This study contributed valuable knowledge for the simulation and prediction of moisture transfer characteristics during the walnut soaking and drying processes. The findings from this study could potentially be applied to the walnut drying industry for more efficient processing     

MICROWAVE FREEZE DRYING CHARACTERISTICS OF BEEF

ABSTRACT

Microwave freeze drying characteristics of unsaturated raw beef were studied experimentally for various levels of electric field strength, vacuum pressure, sample thickness and initial saturation. The results show that drying time is proportional to initial saturation and inversely proportional to the electric field strength and sample thickness. The effect of vacuum pressure on drying time is negligible. Some advantages of microwave freeze-drying over conventional freeze drying with radiant heating are pointed out     

HEAT AND MASS TRANSFER WITH POLYCONDENSATION IN RESIN FILM DURING DRYING

ABSTRACT

In a drying process of dielectric resin films coated on electric conductive substances, phenomena such as polymerization of monomers, by-products yield, shrinkage and stress generation lake place simultaneously in addition to heat and mass transfer. For the enhancement of the drying with high efficiency and high quality, it is important to understand the drying mechanism. In this paper, the characteristics of heat and mass transfer in the resin film including polycondensation reaction are presented. The apparent drying rate of polyamideimide varnish films was measured in two different heating modes of radiation and convection. The reaction rate of polycondensation was analyzed both by the thermogravimetry and the differential scanning calorimetry. The apparent drying rate began to drop remarkably when the reaction rate became significant. It implies that the diffusion of the solvent is inhibited by skinning at the surface. Applying the Vrentas/Duda free-volume diffusion model to the prediction of diffusivity, the heat and mass transfer in the resin film were analyzed theoretically with a reasonable accuracy.     

DRYING IN CYCLONES—A REVIEW

ABSTRACT

This paper presents an overview of the flow, heat and mass transfer characteristics of vortex (or cyclone) dryers. The focus is on the potential of the cyclone configuration for drying of particulates. A selective review is made of the literature pertains to single phase and gas-particle flow in cyclone geometries. Recent data on drying of particulates in cyclone dryers are summarized.     

Coupling CFD and Diffusion Models for Analyzing the Convective Drying Behavior of a Single Rice Kernel

The drying behavior of a single rice kernel subjected to convective drying was analyzed numerically by solving heat and moisture transfer equations using a coupled computational fluid dynamics (CFD) and diffusion model. The transfer coefficients were computed simultaneously with the external flow field and the internal diffusive field of the grain. The model was validated using results of a thin-layer drying experiments from the literature. The effects of velocity and temperature of the drying air on the rice kernel were analyzed. It was found that the air temperature was the major variable that affected the drying rate of the rice kernel. The initial drying rates (in first 20 min) were 7, 12, and 19% per hour at inlet air temperatures of 30, 45, and 60 ^° C, respectively. Important temperature gradients within the grain existed only in the first few minutes of the drying process. The moisture content gradients reached a maximum value of 11.7% (db) mm ^?1 at approximately 45 min along the short axis in the thickness direction. The variation in the inlet air velocity showed a minor effect on the drying rate of the rice kernel. The heat and mass transfer coefficients varied from 16.57 to 203.46 W·m ^?2·K ^?1 and from 0.0160 to 0.1959 m·s ^?1, respectively. The importance of the computation of the transfer coefficients with the heat and mass transfer model is demonstrated.     

STRUCTURE FORMATION OF COATED FILMS WITH DISPERSED PIGMENTS DURING DRYING

ABSTRACT

In the drying of coated films with dispersed pigments, such as floppy disks, the structure of the film is formed during the drying process and depends on the drying condition. It is important to understand the structure formation during drying for the design of the dryer and die better quality of the product. We measured die drying characteristics of the film and determined the structure of dried film experimentally. A qualitative model for the structure formation during drying of the coated film is suggested.     

Drying of Tomato Pretreated with Calcium

Abstract

Pieces of tomato pericarp were pretreated by soaking in CaCl₂ solution before convective drying. Drying was interrupted at prescribed water contents and volume of the piece, its area, perimeter and shape factor were measured. It was found that pretreatment cause shrinkage of tomato pieces by some 15%. Drying causes further shrinkage, which in both nontreated and pretreated tomato is larger than the volume of evaporated water. Microscopic examination showed that pretreatment of tomato with calcium ion causes larger tissue structure alterations in comparison to those observed in nontreated tomato. Calcium concentration gradients cause variable resistance of the tissue to deformation. During dehydration tissue is torn apart and large cavities are formed. It results in faster drying of pretreated tomato in comparison to drying of nontreated tomato.     

THE CURRENT SITUATION OF GRAIN DRYING IN CHINA

Abstract

Drying is an important technology widely used in the processing of such products as grain, food and other agricultural materials. In recent years the drying industry in China is in a fast developing period and the amount of grain dryers is increasing every year. The grain drying technology is also improving. In this paper the present situation of grain drying technology and grain drying equipment is presented. The type, structure and characteristics of most widely used grain dryers are described. Some recommendations for further development are given.     

CONSTRUCTION OF AN ANALYTICAL MODEL OF PAPER DRYING

ABSTRACT

A theoretical model developed is presented to simulate the paper drying process on a production paper machine. The paper sheet is represented as a matrix of pulpfibres which contains free and bound water, water vapour and air. The model is heavily dependent upon a wide range of physical data including pore size distribution, permeability sorptive characteristics, thermal conductivity, specific heat capacity, density, diffusion coefficients and shrinkage characteristics as well as heat and mass transfer behaviour at the interfaces. Theoretical relationships to describe these parameters in terms of the physical pore structure are developed and compared with published data. The model was compared against actual measurements on the Australian Newsprint Mills Boyer PM3 newsprint machine. The comparison with actual machine moisture content values showed the model prediction of moisture change during drying to cylinder No. 38 on PM3 to be 2% less than actual and 0.1% more than actual by the exit from the drying cylinder. In terms of predicting thermal energy consumption of the paper machine a 91% correlation was obtained.     

DEHYDRATION BY DESORPTION AND BY SPRAY DRYING OF DAIRY PROTEINS: INFLUENCE OF THE MINERAL ENVIRONMENT

ABSTRACT

A drying method by desorption in a water activity meter was used to simulate the conditions of spray drying and to determine the water transfer inside dairy concentrates towards the surface and from the surface to the drying air. The concentrates were also spray dried and solubility index of powders were determined during reconstitution. Whey protein concentrates (WPC) and native phosphocaseinate suspensions (NPCS) were used to study the effect of NaCl (420 mM), CaCl₂ (222 raM), sodium phosphate (173 mM) and sodium citrate (238 mM) on the water transfers. The decrease in water transfer during drying was explained by the high hygroscopicity of added mineral salts to WPC. NaCl addition to NPCS decreased the water transfer during drying, but increased the solubility index. Citrate and phosphate addition to NPCS increased the water transfer during drying and reconstitution. CaCl₂ increased the water transfer during drying but the solubility index was always low. Results are discussed as a function of the aqueous environment, of the nature of mineral salts, of the structure of dairy proteins and of protein solvation.