Evaluation of Three Semi-empirical Models for Superheated Steam Vacuum Drying of Timbers

Superheated steam drying at sub-atmospheric pressure (SSV) has been successfully employed in Europe and Asia for drying some types of timbers, showing that drying time could be reduced by 50% with respect to conventional drying without significant losses in the quality of the final product. This reduction is the consequence of a different heat and mass transfer control mechanism. Since SSV drying is carried out in absence of gaseous air, diffusion of the generated vapor is not a limiting factor and drying rate becomes more dependent on heat transference. Therefore, classical interpretation of timber drying as a process based on moisture migration control is not applicable to SSV. This work is targeting the development and validation of a simplified semi-empirical model for SSV drying of timbers. Mathematical representation of the proposed model is uncomplicated and straightforward to apply, and the comparison between model predicted and experimental data showed a high degree of agreement under variable drying conditions.     

Drying at high temperature of sapwood and heartwood : theory, experiment and practical consequence on kiln control

This paper aims to relate the last results obtained in the field of drying theory with the industrial process. It deals particularly with the problem of the difference between sapwood and heartwood that always occurs in the whole stack of an industrial drier. The experimental procedure and the simulation code TRANSPORE are shortly presented. Experiments have been carried out on sapwood and heartwood (Spruce with superheated steam and pine with moist air). The code is used to compare sapwood and heartwood. In the simulations, only the initial moisture content and the permeabilities differentiate both parts of the tree. Once again, the code proves very efficient. For both experiment and theory, the main results are: i) longitudinal liquid migration occurs up to the end of the board only for sapwood ii) the first drying period disappears for heartwood iii) the drying kinetics cross each other at the end of drying These results are of great importance at the time of optimizing the process: among several possible control devices, the temperature loss through the stack seems to be a good choice combining relevant information and easy-to-use measurement.     

Drying behavior of acrylic latexes

The drying behavior of a series of monodisperse acrylate-based emulsion polymers was studied with respect to two existing drying models. The latexes were all film forming poly(methyl methacrylate-co-butyl acrylate)-based materials, having different polymer morphologies. Crosslinks were introduced via the addition of ethylene glycol dimethacrylate comonomer to the emulsion synthesis, while the molecular weight was reduced by addition of CBr₄ chain transfer agent. Weight loss experiments showed no discernible differences in the drying behavior of the various latexes, but environmental scanning electron microscopy (ESEM) indicated marked differences. It was found that a percolation model developed by Croll describes the drying process well. However, certain experimental observations were not consistent with theory. ESEM revealed that the most ‘fusible’ of the latexes formed a skin during drying, prior to the complete evaporation of the water. It was postulated that the skin remained sufficiently porous to allow unhindered water flux.     

选择性远红外涂料在太阳能干燥器中的应用

从理论上分析了在太阳能干燥器壁上涂选择性远红外涂料,以提高太阳能干燥器热效率的可能性和优越性,并通过对比实验所测得的数据来验证涂选择性远红外涂料的效果,并具体解释各项测试参数差异的原因。实验结果证明,选择性远红外涂料对提高太阳能干燥器的热效率有显著的效果。     

超临界流体干燥法制备MnZn铁氧体超细粉末 Ⅰ.制备参数对粉末织构的影响

本文采用胶体——超临界流体干燥法制得超细ＭｎＺｎ铁氧体粉末。通过Ｘ－射线衍射技术和比表面的测定等对粉末的结构参数进行了测定。其中主要考察了Ｍｎ、Ｚｎ含量、ｐＨ值，溶液浓度以及超临界流体干燥参数（温度、时间和压力等）对粉末织构的影响。并对各种凝胶粉末的织构进行了比较。     

COMPUTER SIMULATION OF HEAT TRANSFER DURING DRYING AND PREHEATING OF WET IRON ORE IN A ROTARY KILN

In the present study, an improved numerical heat transfer model has been developed for a rotary kiln used for drying and preheating of wet iron ore. The present model includes radiation exchange among hot gas, refractory wall and the solid surface, transient conduction in the refractory wall, and mass and energy balances of the hot gas and the solids. The contribution of gas convection has also been taken into account in terms of a fraction of the radiative heat transfer to the inner refractory wall and the solid surface. The computer results show that the present model can predict the length of the kiln as well as axial solid and gas temperature distributions with reasonably good accuracy. A detailed parametric study reveals that a good design of a rotary kiln requires medium gas flow rate, small angle of inclination and low rotational speed of the kiln.     

非传统通心面的研究（下）：烘干工艺对通心面品质的影响

试验表明在通心面烘干过程中,随着温度的提高,产品的色泽得以改善,烹煮重量、烹煮损失减小,煮面强度得以提高。通过三种不同的烘干温度(LT、CT 和 HT)和两种不同的高温试验(HT-A 和 HT-B)比较,本文推荐了适合非传统通心面的适宜烘干条件。本文对通心面干燥过程中水份转移理论及高温烘干过程中的物化变化情况作了探讨。     

Modeling Moisture Transport from a Portland Cement-Based Material During Storage in Reactive and Inert Atmospheres

Monolithic samples of a cement mortar were stored in reactive (100% CO₂) and inert (100% N₂) atmospheres at three levels of relative humidity (23, 48, and 98%). Atmospheric conditions were monitored and the mass of each sample was measured periodically over a period of 88 days. Carbonation depth of split samples was delineated using 1% phenolphthalein solution. In addition, a solid-liquid isotherm was developed by drying smaller monoliths over eight hygroscopic salt solutions in 100% N₂ until a constant mass was obtained. Isotherm experimental data was used to parameterize a two-regime moisture transport model based on previously developed drying approaches. This first-order model accounts for both funicular moisture transport and isothermally controlled drying in a fixed porous matrix. Kinetic drying data for the cement mortar was described adequately by simulation of mean relative saturation as a function of drying time and external relative humidity. Comparison of moisture transport in the inert atmosphere to that in the reactive atmosphere indicates that matrix drying had a substantial effect on the carbonation depth; however, drying was seemingly independent of the carbonation process.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

MULTIVARIABLE PROCESS IDENTIFICATION AND CONTROL OF CONTINUOUS FLUIDIZED BED DRYERS

Dynamic models that rigorously describe fluidized bed dryers based on the fundamental principles of the process are usually so complex to be employed in control system design. To obtain simple reduced-order models for such systems, a sequence of step changes in the manipulated and load variables is introduced into the rigorous model. The obtained input-output dynamic response data are used for off-line model identification. Different types of linear models are generated, which are shown to be adequately representing the fluidized bed drying dynamics. The derived models are useful to develop model-based control algorithms such as Internal Model Control (IMC) and Model Predictive Control (MPC). Performance and robustness properties of these controllers are analyzed. Simulation results demonstrate a good performance in terms of tracking and load rejection capabilities.