HIGH-INTENSITY DRYING OF PAPER

High-intensity contact drying denotes drying under suf- ficiently intensive heating conditions that, following a brief warmup period, the mist paper web operates at internal tem- peratures in excess of the ambient boiling point. A simplified, two-zone analytical model is first presented. The paper is depicted as having a dry layer, of ever-increasing thickness, adjacent to the hot surface. Heat conduction through this layer (the rate-limiting step) causes evaporation at the interface with the “wet zone.” The vapor is then considered to flow through the wet zone into the ambient. Results of bench-scale experiments are discussed. Drying rates as much as twenty times conventional rates are indicated. The drying rate increases with hot surface/boiling point tem- perature difference and applied mechanical pressure. The instsn- taneous drying rate decreases continuously after a brief warmup period. The vapor pressure at the hot surfacelpaper interface rises quickly to a peak value, followed by a continuously- diminishing level. The heat flux shows a rapid rise to boiling- like conditions, followed by a drop to a range compatible with measured drying rates. The temperature of the open side of the sheet is constant during drying, after its initial rise to the boiling point. The experimental results are compatible with pre- dictions of the model.     

Short-Time Drying of Beatsensitwe, Biologically Active Pulps and Pastes

Tomato concentrate and baker's yeast suspension were dried in the laboratory scale Mechanically Spouted Bed dryer with inert packing. Due to a large contact surface provided by the inert particles, very fast drying process can be achieved, ifthe thickness of the wet coating formed on the surface of the inert packing is film-like. The diffution resistance can be then considered negligible and drying can be performed at “quad-constant” rate. Derived fiom hydrodynamic characteristics the contact time between hot air and the wet coating is very short, therefore gentle drymg process can be performed. The quality of the dried yeast was controlled by counting the living cells (relative viabilny) and by measurement of the dough rise time. During continuous drying of thermoplastic tomato pure the “case hardening” periods could be shortened and tomato powder with insignificant quality loss was produced.     

THE RELATIVE INFLUENCE OF DIELECTRIC AND OTHER DRYING TECHNIQUES ON THE PHYSICO - MECHANICAL PROPERTIES OF A PHARMACEUTICAL TABLET EXCIPIENT. PARTI: COMPACTION CHARACTERISTICS.

The relative Influence of nine techniques for drying wet granulated microcrystal-lline cellulose (MCC) on the subsequent compaction characteristics was studied In terms of the tensile strength and corrected work of failure of the tablets. Wet granulation resulted in a substantial decrease in compatibility. However, the drying technique used was found to affect the degree of loss in compatibility. In general, microwave-vacuum drying using the “high” process type resulted in the production of granules with the highest compatibility followed by freeze drying and fluidized bed drying. Granules dried under ambient conditions, and granules tray dried to “just dry” or “over dried” conditions resulted In tablets possessing approximately comparable compatibilites, with the poorest compaction characteristics being exhibited by vacuum dried granules, in addition It was found that use of a “low” drying process type during microwave- vacuum drying yielded granules with inferior compaction characteristics to those dried by the “high” process type. Radio frequency drying was found to yield granules which produced tablets having slightly inferior tensile strength to tray dried material, although the tablet work of failure values were comparable. The effect of drying technique on the subsequent compaction characteristics was not found to be directly related to the moisture content of the granules.     

AN ANALYSIS OF FLOW FILM BOILING OVER AN INCLINED SURFACE EMBEDDED IN POROUS MEDIA

The rate of heat dissipation during flow film boiling from inclined surfaces embedded in porous media is investigated both theoretically and experimentally. The theoretical model relies on the well established Darcy model for the flow both inside the vapor layer and inside the forced convection layer surrounding the vapor film. The main focus is to study the effects of wall temperature, liquid subcooling, wall suction, and orientation of the heated surface on the heat transfer phenomenon. The resulting similarity equations are integrated numerically by use of the fourth-order Runge Kutta method. Systematic “shooting” is required to satisfy the boundary conditions at the liquid-vapor interface and at infinity. Results are reported for the behavior of the vapor film thickness and both the local and average heat transfer coefficients as a function of vapor superheat and liquid subcooling. Experiments were conducted in Freon-113 and a porous medium consisting of 2.8 mm glass beads to verify the theoretical findings, and excellent agreement was found between theory and experiments.     

HIGH INTENSITY DRYING

A mathematical model simulating the heat and mass transfer process during high intensity drying of paper and board has been developed. The model is successful in predicting the vapor pressure developments, pressure driven bulk flow of liquid and vapor, and increased drying rates during high-intensity drying, closely matching the experimental determination.

The model predicts substantial amounts of water removal in the liquid form during high-intensity drying being pushed out of the web by pressurized vapor zone. Water removal by pressure flow of liquid could account for as much as one-third of the total water removed.

Similar to drying under conventional conditions, the existence of a dry zone, wet zone and an intermediate zone with accompanying advancing heat pipe has also been shown for drying under high intensity conditions.     

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5 rpm drum speed and quadrupled at 7 rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.     

Drying of Porous Material During the Constant and the Falling Rate Period: A Critical Review of Existing Hypotheses

During drying of porous material a so-called “falling rate” period is observed, where the drying rate decreases as the moisture content decreases. This behavior is usually described by the well known “shrinking core model.” This model, however, contradicts experimental findings and violates basic laws of multiphase mass transfer in porous media as well. A new model, named “wet surface model,” is suggested which eliminates those discrepancies.     

Drying Enhancement of Clay Slab by Microwave Heating

The effect of internal heating by microwave on the drying behavior of a slab was studied. A wet sample of kaolin pressed into a slab was subjected in microwave irradiation of 2.45 GHz. The absorption of microwave energy into a wet slab can be expressed by a function of the moisture content and the pathway length, which is a similar form to Lambert-Beer's law. The drying behavior was compared among three modes: microwave irradiation, hot air heating and radiation heating in an oven. Microwave heating with a constant power resulted in breaking the sample when the internal temperature achieves at 373 K. However, if the power was controlled to maintain the temperature less than the boiling point of water, the drying succeeded without any crack generation until the completion with a significantly faster drying rate than in convective heating or in the oven. It is also noted that the transient behavior of the temperature is quite different from the conventional drying.     

Moisture Distribution Changes and Wetwood Behavior in Subalpine Fir Wood during Drying Using High X-Ray Energy Industrial CT Scanner

In some species, such as subalpine fir (Abies lasiocarpa [Hook] Nutt), the water content of the confined zones in heartwood is as high as or greater than that of sapwood. Such wet zones of heartwood are referred to as “wetpocket” or “wetwood.” Wood products from subalpine fir forests are adversely affected by the wetwood-associated problems, particularly during the drying process. The objectives of the study were as follows: (1) to investigate feasibility of a high X-ray energy industrial computed tomography (ICT) scanner for imaging wetwood; and (2) to determine changes of the 2-D and 3-D moisture profiles (from core to shell) at different drying times.

Although medical CT scanning has been used for attaining signal intensity profiles of typical wood at different drying times, the technology has not, to date, been used for the study of wetwood phenomenon. This study presents, for the first time, results from the ICT imaging of the wetwood phenomenon. The results indicate that the ICT imaging system provides a powerful technique for imaging wetwood at different drying times. In addition, the results show that during the initial phase of drying, almost flat moisture profiles were observed in all wood types except for the wetwood, which showed a relatively higher moisture profile. A much slower (sluggish) drying development pattern at each increment from core to shell was found within the wetwood zone than normal wood regions along the width, thickness, and length of the board.     

STRATIFIED VAPOR EXPLOSION EXPERIMENTS

An experimental investigation has been conducted on vapor explosions in a stratified contact mode, where the cold liquid and the hot liquid are initially separated by a stable vapor film. A series of experiments were conducted in a narrow, rectangular vessel (20 cm long, 2.54 cm wide, 65 cm high). One face of the vessel wall is made of transparent material for the visual observation using a high speed camera. The explosion is externally triggered at the one end of the vapor film layer by a pressure shock produced by a magnetic hammer. Four quartz pressure transducers are mounted horizontally at the vessel wall. The mechanical energy release is measured by the displacement of a slug which is initially held at the middle of the vessel. The results with water (10 ∼ 85°C) and saturated liquid nitrogen (- 196°C) show that the explosion propagates along the vapor film interface at a speed of 100∼250m/s. The mechanical energy release was up to 33 joules. The variation of the triggering shock pressure revealed that there exists a triggering threshold. Both the higher initial water temperature and the higher triggering shock pressure increase the mechanical work release. High speed cine film shows the course of the explosion: stable liquid-liquid film boiling, triggering, explosion propagation, and expansion.     

Cathodic disbondment resistance with reactive ethylene terpolymer blends

Adhesion to metallic substrates can be improved through the addition of polar functional groups, which bond with surface groups on the metal substrate. Additionally, polar interactions have been shown to increase adhesive strength even in wet environments (such as in the case for cathodic protection). A polymer blend is proposed as a coating material to provide adequate protection against the diffusion of moisture and air to the metallic surface along with superior adhesion even in the presence of wet and corrosive environments to resist cathodic disbondment. A reactive ethylene terpolymer (RET) of ethylene/n-butyl acrylate/glycidyl methacrylate (E/nBA/GMA) was compounded with HDPE to develop a potential coating material. The HDPE component offers high chemical and moisture resistance to permeation, while the RET component provides the material with high polarity and reactivity, which enhances adhesion to the substrates to be coated. The introduction of the reactive ethylene terpolymer decreases the magnitude of cathodic disbondment area of polyethylene coatings. After applying a cathodic potential to the coating substrate, the adhesive strength was observed to remain the same for silane-pretreated steel dollies. Without silane pretreatment, post-CD adhesive loss resembles that of the open circuit “wet” condition. EDAX data in conjunction with oxygen and water vapor transmission rates suggest an initial stage of disbondment where interfacial oxide is dissolved resulting in the delamination of coating around the initial defect. This initial disbondment zone acts like a moving crack tip creating larger areas of disbondment where interfacial bonds are degraded by the ingress of moisture and ions along the interface.     

PREDICTION AND RATIONAL CORRELATION OF THERMOPHORETICALLY REDUCED PARTICLE MASS TRANSFER TO HOT SURFACES ACROSS LAMINAR OR TURBULENT FORCED-CONVECTION GAS BOUNDARY LAYERS†

An approach originally developed to predict and correlate the thermophoretically-augmented submicron particle mass transfer rate to cold surfaces is shown here to account extremely well for the thermophoretically reduced particle mass transfer rate to “overheated” surfaces experiencing either a forced boundary layer (BL)-flow of laminar or turbulent dusty gas. This laminar BL/hot wall situation occurs, e.g., in hot surface/cold envelope chemical reactors used for growing epitaxial silicon layers from mainstreams containing, say, silane vapor and inadvertent submicron dust particles. “Thermo-phoretic blowing” is shown to produce effects on particle concentration BL-structure and wall mass transfer rates identical to those produced by real blowing (transpiration) through a porous wall. Indeed, a “blowing parameter additivity” relationship is proposed to account for the simultaneous effects of both phenomena should they be acting in concert (or in opposition). Exact numerical BL calculations covering the parameter ranges: l≤T_w/T_e6, (particle thermophoretic-/gas thermal- diffusivity ratios between )0·1 and 0·8 and particle Schmidt numbers between 10⁰ and 2 × 10³ are used to establish the validity of the basic forced convection mass transfer correlations for self-similar laminar BLs and law-of-the-wall turbulent BLs. This includes parametric combinations of immediate engineering interest for which the deposition rate is thermophoretically reduced by no less than 10-decades! The applicability of our correlations to developing BL-situations is then illustrated using a numerical example relevant to wet-steam turbine technology.     

Predicting Thermal Efficiency in Timber Radio Frequency Vacuum Drying

In this work, the efficiency of transforming dielectric energy into evaporated water is analyzed for the case of timber radio frequency vacuum drying. Based on well-known heat and mass transfer equations, a simplified mathematical model is proposed that estimates the drying efficacy in regards to the thermo-physical properties of wood. Although not exact, the theoretical results are close to the experimental observations and elucidate some phenomena like the tendency of the timber to dry from inside to outside, and the drying rate increase with the rise of the timber gas permeability. The theoretical efficiency model also predicts a range of wood permeability values for which the drying efficiency changes from 100 to 0%, thus providing a quantitative scale for classifying the spectrum of “difficult-to-dry” all the way to “easy-to-dry” wood species when using radio frequency vacuum technology.     

URTHER DEVELOPMENT OF A COMPUTER PROGRAM SIMULATING HEAT PIPE FUNCTIONING IN CONDEBELT PAPER DRYING

A previously developed computer program for depicting the heat and mass transfer behavior of heat pipe conduits, slightly open to a large ambient at their far ends, inside moist paper webs is developed further. The addition consists of accounting for the thermal dynamics of a hot metal slab from which the latent heat of evaporation corresponding to vapor generation in the hot end of the heat pipe is drawn. Values are used for the input parameters representing as closely as possible an approximately 40 % dry (by total weight) printing paper sheet being suddenly pressed against a hot steel or iron slab at l50°C or 200°C. These are conditions that could occur in Condebelt drying. Temperature distributions in the metal and inside the web, as well as the pressure distribution inside the web, are determined as functions of the distance from the hot surface and time. Important parameters are the initial metal and web temperatures, the interface Biot number, the structural geometry of the web, and the metal Fourier number. It is shown that there is a S to 25°C metal surface temperature drop in the first 100 μs after contact with the web. This can be significant in reducing possible web surface damage.     

ANALYSIS OF HEAT AND MASS TRANSFER DURING DRYING OF PAPER/BOARD

A mathematical model has been successfully developed to study the heat and mass transfer process during paper drying. This model takes into account the consective transfer of vapor and liquid apart from the known transport mechanisms of capillary flow of liquid, diffusion, vaporization-condensation, and heat conduction. The partial differential equations describing temperature, saturation and pressure change within the web during drying with associated boimdary conditions and initial conditions were solved using finite difference method. The model predictions show that during the drying process the web can be conveniently divided into three different zones, namely dry zone, wet zone and an intermediate zone. The movement of liquid and vapor in opposite directions in the intermediate zone is similar to the action of a heat pipe. Also, as drying proceeds the location of the intermediate zone and hence the heat pipe advances progressively through the thickness of the web.     

A GRAPHICAL DESIGN METHOD FOR A PARTIAL SOLAR DRYER FOR CORN

This paper presents the application of a design method for a partial solar heating system of polyvalent modular dryers called “GJ-ABAQUE” to the drying of thick layers of grains.

This method is based on the use of charts or polynomial correlations. In the actual case where the drying air is not recycled, we only need one chart which allows one to determine the fraction of the monthly heating load supply by solar energy as a function of two dimensionless parameters. The latter implies the use of monthly average radiation data, the collector surface and estimates of drying loads.

The “GJ-ABAQUE” method was applied for drying 777 kg of corn, corresponding to 1 m³ of fresh product, in a thick layer in each modular dryer.     

ANALYSIS OF HEAT AND MASS TRANSFER DURING DRYING OF PAPER/BOARD

ABSTRACT

A mathematical model has been successfully developed to study the heat and mass transfer process during paper drying. This model takes into account the consective transfer of vapor and liquid apart from the known transport mechanisms of capillary flow of liquid, diffusion, vaporization-condensation, and heat conduction. The partial differential equations describing temperature, saturation and pressure change within the web during drying with associated boimdary conditions and initial conditions were solved using finite difference method. The model predictions show that during the drying process the web can be conveniently divided into three different zones, namely dry zone, wet zone and an intermediate zone. The movement of liquid and vapor in opposite directions in the intermediate zone is similar to the action of a heat pipe. Also, as drying proceeds the location of the intermediate zone and hence the heat pipe advances progressively through the thickness of the web.     

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。     

Destabilization of leidenfrost boiling by a sudden rise of ambient pressure

The contact of two liquids, one at a temperature significantly above the boiling point of the other, can lead to fast vapor formation on an explosive time scale causing mechanical damage. Such an event, generally termed a “vapor explosion”, has been recognized as the cause of accidents in a wide variety of smelting and liquefied natural gas (LNG) industries. The increase of awareness of the problem, especially in hypothetical accidents in liquid metal fast breeder reactors (LMFBR), has resulted in a broad research effort to investigate the sequence of events and the necessary conditions leading to a vapor explosion.A key step in the explosion scenario has been identified as the destabilization of film boiling, resulting in liquid-liquid contact over a large area. It is found experimentally that, when the ambient pressure is suddenly increased around an evaporating volatile drop suspended above a hot liquid surface, the vapor film may or may not collapse. A theory is developed here in which instability corresponds to resonance between the oscillations in the creeping-flow vapor film thickness and the capillary waves on the underside of the drop surface.Two separate estimates are made of the critical vapor film thickness at which instability occurs, one by a creeping flow model of the film thickness oscillation, and the other by a Taylor-Helmholtz instability model. The two models are in agreement with each other and with the data.     

多层桨搅拌槽内非常温体系局部气含率分布

引　言局部气含率是气 -液分散的重要特征参数 ,它广泛应用于反映器的分析、选择及设计 .局部气含率与槽内的流型密切相关 ,它能够反映槽内局部的分散及传质特性 ,因此受到研究者的广泛关注[1] .由于受测试手段的限制 ,仅有部分学者对常温下单层桨通气搅拌槽进行了研究[2～ 6] ,而对于工业中广泛应用的多层桨搅拌槽则研究甚少 .许多工业过程是在较高的温度下进行的 ,而已有的研究几乎都是在常温下进行 ,忽视了液体的温度对气 -液两相体系流体力学性能的影响 .常温下的气 -液分散体系 (气体 -常温液体 )主要常见于耗氧性生物发酵、水处理等工…