Drying Kinetics of Pinus Rqdiata Boards at Elevated Temperatures

ABSTRACT

From the numerical results of a rigorous, mathematical model, the rate–o–drying curves for high–temperature seasoning of Pinus radiata board show that the drying process can be divided into three periods. Based on the mechanistic picture of moisture movement and differences in the movement in each period, this paper simplifies the model by using the concept of the haracteristic drying curve.

In heartwood drying, the first period is very short (5–15 minutes), hence it can be neglected, whereas for sapwood, the first period is substantial with the drying rate being taken as constant in the analysis. Since the physical characteristics of the second and third drying periods are different, separate characteristic drying curves have been developed for each period. It has been found that common normalised drying curves are able to describe the results generated from the earlier model for normal range of kiln conditions over the examined ranges in dry–bulb temperatures of 1 10–140°C, wet–bulb temperatures of 70–90°C and air velocity of 3–7 m s^-1. Therefore. the derived simplified model can directly be used to calculate kiln–wide variations in moisture content and drying rate.     

External Heat Transfer in Moist Air and Superheated Steam for Softwood Drying

In kiln drying of softwood timber, external heat and moisture mass transfer coefficients are important in defining boundary temperature and moisture content at the wood surface. In addition, superheated steam drying of wood is a promising technology but this has not been widely accepted commercially, partially due to the lack of understanding of the drying phenomena occurred during drying. In this work, experimental investigation was performed to quantify the heat transfer between wood surface and surrounding moist air or superheated steam. In the experiment, saturated radiata pine sapwood samples were dried using dry-bulb/wet-bulb temperatures of 60℃/50℃, 90℃/60℃, 120℃/71℃, 140℃/90℃, 160℃/90℃, 140℃/100℃ and 160℃/100℃. The last two schedules were for superheated steam drying as the wet-bulb temperature was set at 100℃. The circulation velocity over the board surface was controlled at 4.2m·s-1. Two additional runs (90℃/60℃) using air velocities of 2.4m·s-1 and 4.8m·s-1 were performed t     

Simulation of Jet Drying of a Moist Cylinder at Low Reynolds Number

A numerical analysis of the process of jet drying of a moist cylinder was conducted. The drying jet was a laminar 2D jet stationed at three different distances (D/H = 0.22, 0.25, and 0.33) from the moist cylinder to investigate the jet effectiveness on heat and mass transfer. The diameter of the object and initial jet height were fixed in all cases. Temperature and mass distributions were obtained inside the object for different jet velocities. A finite volume method was used to solve the governing equations for momentum and energy with a commercial code. Calculations were performed for three different Reynolds numbers, namely, Re = 100, 200, and 300. It was found that heat and mass transfer increased with decreasing the distance, D/H, between the jet and the cylinder. In addition, increasing Reynolds number showed a positive effect on heat and mass transfer. Locally, jet drying was found to be most effective near the stagnation point on the leading side of the cylinder.     

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.     

EFFECT OF THE VARIABILITY OF HEAT AND MASS TRANSFER COEFFICIENTS ON CONVECTIVE AND CONVECTIVE-RADIATIVE DRYING OF POROUS MEDIA

A numerical investigation was conducted to study two-dimensional heat and mass transfer during convective drying of a clay brick. The established numerical code has allowed us to determine the effect of heat and mass transfer coefficients variability on state variables and on the drying kinetic.     

Interfacial Behaviour of Complex Hydrocarbon Fluids at Elevated Pressures and Temperatures

Interfacial tension is unique in the sense that it relates to the interface between two immiscible fluid phases. Hence, interfacial tension between the fluid phases can be used to infer a great deal of information about phase equilibria such as solubility, miscibility and mass transfer interactions between the two bulk fluid phases in contact. In this paper, we examine the utility of interfacial tension to characterize miscibility and mass transfer mechanisms in complex hydrocarbon fluids at elevated pressures and temperatures. This study also provides a new method to estimate the dynamic interfacial tension and miscibility by incorporating counter‐directional diffusivities into the traditional Parachor model.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

Mass Transfer Coefficient for Drying of Moist Particulate in a Bubbling Fluidized Bed

Experiments on drying of moist particles by ambient air were carried out to measure the mass transfer coefficient in a bubbling fluidized bed. Fine glass beads of mean diameter 125 μm were used as the bed material. Throughout the drying process, the dynamic material distribution was recorded by electrical capacitance tomography (ECT) and the exit air condition was recorded by a temperature/humidity probe. The ECT data were used to obtain qualitative and quantitative information on the bubble characteristics. The exit air moisture content was used to determine the water content in the bed. The measured overall mass transfer coefficient was in the range of 0.0145–0.021 m/s. A simple model based on the available correlations for bubble‐cloud and cloud‐dense interchange (two‐region model) was used to predict the overall mass transfer coefficient. Comparison between the measured and predicted mass transfer coefficient have shown reasonable agreement. The results were also used to determine the relative importance of the two transfer regions.     

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.     

Ascorbic Acid Thermal Degradation during Hot Air Drying of Camu-Camu (Myrciaria dubia [H.B.K.] McVaugh) Slices at Different Air Temperatures

Abstract

Air drying of camu-camu slices was performed in order to estimate the effect of air temperature on the kinetics of ascorbic acid thermal degradation. Moisture variation during the air drying process was monitored gravimetrically by weighing the trays at predetermined time intervals. The experimental points were adjusted by Fick's diffusion model and by the Page empirical model. The effective diffusion coefficient (D_eff) ranged from 8.48 × 10^?10 to 1.34 × 10^?9 m²/s.The ascorbic acid content was evaluated in samples taken during the drying process using Iodine titration, and the results modeled by the Weibull equation. Concerning ascorbic acid retention the best drying condition required air at 50°C. The ascorbic acid retention was 78%, when the moisture content of the product reached 10% (wet basis).     

Evaporation of solvents by infrared radiation treatment

The first stages of infrared drying of waterborne coating systems are controlled by the evaporation of their volatile components. In order to analyse the kinetics of drying during IR radiation, the evaporation behaviour of water and other solvents of waterborne coatings are investigated using a combined gravimetric and photoionization technique. The resulting specific evaporation rates under IR radiation are compared with those obtained by thermal annealing. It is shown that in the case of IR radiation heating of water, the mass transfer coefficients are much higher than by thermal annealing at the same driving potential. The dependence of the water absorption rates and the mass losses of different solvents on the humidity of the air was also determined.     

Experimental and Numerical Investigation of the Heat and Mass Transfer for Cut Tobacco During Two-Stage Convective Drying

In this article, a two-stage convective drying strategy was presented for dehydration of flue-cured tobacco. In order to develop the multistage drying method of tobacco, two-stage drying as well as traditional single-stage drying of cut tobacco was experimentally evaluated and accurately simulated by proposed heat and mass transfer models. The experiments were performed in a dual fixed bed dryer. Different air temperature combinations of 120°C/90°C, 110°C/80°C, and 100°C/70°C were employed during two-stage drying. The drying rate and temperature variations of cut tobacco were investigated. The results showed that the average drying rates during two-stage drying were nearly 50% higher than those obtained from lower-temperature single-stage drying. On the other hand, the two-stage drying method, which used high air temperature for the early period and low temperature for the late period, could reduce the exposure of tobacco to high temperature due to the low final temperature of the dried sample. The temperature and moisture evolution of cut tobacco at different air temperature combinations were consistent with simulation results by developed heat and mass transfer models. This indicated that the models had a good prediction precision for two-stage drying of cut tobacco. The model predictions can be useful for the design of a feasible two-stage drying process for flue-cured tobacco.     

The Meaning of Droplet‐Droplet Interaction for the Wet Flue‐Gas Cleaning Process

In most large coal‐fired power plants an absorption process with a limestone suspension is applied today. The flue gas proceeds upwards through a series of spray headers that introduce a uniform liquid flux of droplets of the limestone suspension. These droplets resist the gas flow and provide a large mass transfer surface area required for the SO₂ removal process. During the spray overlapping the collision of the droplets may lead to a coagulation or a separation process depending on certain collision parameters, such as surface tension, impact velocity and collision geometry. A model for droplet collisions was developed and implemented in a two‐phase flow simulation by Euler‐Lagrange. The model is based on experimental investigations with overlapping sprays.     

Drying Schedules for Canelo Wood

Canelo wood is a highly valued native species in Chile that shows delicate marbling patterns with a pinkish soft silver luster. Due to its decorative qualities, canelo wood is dried for the manufacture of furniture and musical instruments. However, canelo wood lacks vessels cells that typically transport the water in hardwoods. Per its drying behavior, canelo wood is considered a transition species between hardwoods and softwoods. Therefore, this article reports drying schedules that were developed for drying 25-mm and 50-mm canelo lumber. In addition, this article reports experimental overall mass transfer coefficients, so that drying times for each of the drying stages can be easily estimated.     

高初始含湿多孔介质喷雾干燥过程的热质耦合传递

多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。     

Computational Simulation for Transport of Priority Organic Pollutants through Nanoporous Membranes

Modeling and simulation of membrane‐based solvent extraction is conducted by computational fluid dynamics (CFD). The process is used for removal of priority organic pollutants from aqueous waste streams in nanoporous membranes. The pollutants include phenol, nitrobenzene, and acrylonitrile extracted by organic solvents. The mathematical model commonly applied to predict the performance of membrane‐based solvent extraction is the conventional resistance‐in‐series model. Here, a comprehensive mathematical model is developed to predict the transport of pollutants through nanoporous media. In order to predict the performance of the separation process, conservation equations for pollutants in the membrane module are derived and solved numerically. The model is then validated through comparing with experimental data reported in the literature. The simulation results were in good agreement with the experimental data for different values of feed flow rates.     

Study of the drying behavior of high load multiphase droplets in an acoustic levitator at high temperature conditions

Experimental data on the drying behavior of suspension droplets is limited, despite its importance in industrial applications for material processing, chemical or the food industry involving spray dryers. This fact is particularly significant for high load and temperature conditions, as found in such industrial applications. In this work, the drying behavior of acoustically levitated multiphase droplets has been experimentally investigated. The acoustic tube levitator has been modified in order to allow experiments to be performed at high temperature conditions. The flow rate, temperature and relative humidity of this air stream can be controlled by an air conditioning system. A CMOS camera and a back-light illumination system are used to measure the droplet cross-sectional area and vertical position of the droplet during the drying process. The experiments have been performed using water–glass particle suspensions. The glass particles have a mean particle size and relative density of 13 μm and 2.5, respectively. The effect of the air temperature (60 °C<T<120 °C), initial volume of the droplet (0.05 μl<V₀<0.7 μl), initial solid mass load (0.01<Y_S<0.5) and relative humidity of the air (0.05<HR<0.45) on the mean porosity of the grain, first drying period duration and liquid evaporation rate has been analyzed by means of a parametric screening matrix and also by means of a central composite design (CCD) experimental design. The most important parameters to be considered for the porosity and the drying behavior in the range of variables analyzed are the initial solid mass load and the initial droplet volume. The relative humidity of the air exerts a moderate influence on the drying behavior of the droplet and the temperature has only a very low impact on the mean porosity. In addition, particular attention should be given to the drying behavior of small droplets, which result in a very low mean porosity values for high solid mass loads. The CCD confirms that the initial droplet volume, the solid mass load and their interaction exert significant influence on the three responses.     

Hydrodynamic‐Analogy‐Based Modeling Approach for Distillative Separation of Organic Systems with Elevated Viscosity

A modeling approach based on hydrodynamic analogies is applied to predict the influence of elevated viscosity on the separation efficiency of distillation processes in structured packed columns. An advantage of this approach compared to traditional models based on the film theory is that separation columns can be described without empirical mass transfer coefficient correlations. The separation efficiency was evaluated for two systems with distinct viscosities at different gas and liquid loads and various operating pressures. Simulated concentration profiles and height of a theoretical plate (HETP) values were compared with the data obtained from own experiments. The developed model is capable of correctly predicting the differences in the HETP values of the two evaluated systems.