摘要:

The concept of steam drying originates from the mid of the last century. However, a broad industrial acceptance of the technique has so far not taken place. The paper deals with applications of steam drying within certain industrial sectors where the technique has been deemed to have special opportunities. The applications discussed involve drying of fuels with high moisture contents, cattle feed exemplified by sugar beet pulp, lumber, paper pulp, paper and sludges.

ISSN:0737-3937    

DOI:10.1080/07373939508917009

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Different drying configurations (convective drying with moist air and superheated steam, microwave drying and vacuum drying) on different materials (isotropic and anisotropic) were experimentally studied in order to model and visualise the evolution of internal pressure and temperature. To be able to do so, in addition to measuring the average moisture, a method which can determine internal-local pressure and temperature simultanously by using specially designed sensors was developed. In combination with the experiments, the numerical code TRANSPORE has been used to simulate drying processes. A less comprehensive but more comprehensible analytical model was also provided to facilitate the better understanding of internal phenomena. Based on the results gained from measurement and numerical analysis, the dynamic distribution and development of local temperature and pressure inside seasoned medium are coupled together by a temperature-pressure graph, which is herewith called “Identity Drying Card” (IDC), a new concept initialid in the paper. By using IDC, the internal profile of temperature and pressure, the dominant transport properties (penncability and difisivity), the mechanism of transport (diffusion, convection or both) and the phase transitions during drying can be visualised. More specifically, the amount of dry air, the moisture content in the hygroscopic rcgion or the danger due to internal mechanical loads of handled materials can be figured out with the aid of IDC.

ISSN:0737-3937    

DOI:10.1080/07373939508917010

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

For batch fluidized bed drying of bioproducts the distributed parameter model is presented, in which the internal diffusion equation is coupled to the air balance equations. In practice a large part of the process takes place at high relative humidity of the drying air, equivalent to low driving forces. The results of a number of lumped-parameter methods for the estimation of drying times are compared with those of the full model for the case of constant external conditions around a particle. For low relative humidity of the drying air predictions of drying times with the Regular Regime Power Law method correspond well with the full solution, for higher ambient RH large deviations occur. The other methods are shown to give large deviations in all cases.

ISSN:0737-3937    

DOI:10.1080/07373939508917011

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This article focuses on the concept of a ‘drying characteristic function,’ which is an effective way to correlate drying rate curves for convective drying of homogeneous nonporous, hygroscopic porous and non-hygroscopic porous materials. The characteristic function, obtained bv a certain transformation of a drying rate curve, is independent of drying conditions and hence characterizes the transport kinetics in the material. The principle and some applications of the functions are reviewed. The first application is estimation of dryinge rate curves. Because any drying rate curve can be transformed into the characteristic function and vice versa, the drying rate can be estimated for various drying conditions from a single drying experiment. Another aoolication is determination of the moisture diffusivity. Using the ‘flux ratio method’ an analytical expression of the characteristic function can be obtained for any aiven moisture diffusivity. The exprcssion enables one to determine the moisture diffusivity for a wide range of moisture content from a single drying experiment.

ISSN:0737-3937    

DOI:10.1080/07373939508917012

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The drying behaviour of paticles ( purolit and silica gel) was studied using the thin-layer method described by Langrish et al ( 1). The experiments covered inlet air temperatures between 100 and 150°C, inlet air humidities from 0.02 to 0.052 kgkg1superficial air velocities between 3.8 and 10.8 ms-1, with layer thicknem of 2 – 10mm. No constant mle period war observed. Characteristic drying curves were found to fall within a narrow band fur these ranges of process variables, for material of uniform size and shape and with relative moisture content defined in terms of the end of the induction period. Small changes in panicle shape, particle size distribution and uniformity of particle layers had negligible infuence on the drying kinetics. However, reduction in particle size from 5.2mm diameter to 0.86mm had a marked effect: the normalised drying rate at a given relative moisture content became larger as the particle size became smaller. This phenomenon is attributed to an increase in available contact area per unit volume with diminishing particle size.The thin-layer technique thus appears to be a useful and robust way of obtaining a general characteristic drying curve for a given particulate material.A review of various works ( Keey, 2) has shown that the concept ofa characteristic drying curve may be used to describe the drying kinetics of paniculate materials below 20mm in size for modest changes in process variables ( air temperature, humidity and velocity). This concept has found to be very useful to help model drying processes of a wide variety of particulates, cross-circulated slabs, heaped loaw fabric fibres, hygroscopic ceramic cylinders and discrete vermiculite particles. The drying of a single particle has been related lo the drying kinetics of a fluidized bed by the use of this ida. ( Tsotsas, 7). A grater understanding of the properties of the characteristic drying curve will provide a greater confidence in applying thir concept more generally to process design and the analysis of industrial drying equipment.The goal of this study was to examine further the experimental and theoretical foundations of the characteristic drying curve, using thin-layer methods.

ISSN:0737-3937    

DOI:10.1080/07373939508917013

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This paper relates work which has been conducted in order to obtain a control strategy for French mixed-flow dryers. Special emphasis is given to the system analysis ( static and dynamic). Potential difficulties for the dryer control are: non-linearities, long delay, high order, high disturbances. Special emphasis is also made on the importance of signal pre-processing. Then, different control slrategies are compared on a simulation basis : classical Pl. pole-placement and non-linear Pl controllers. The conclusion is experimentally based.

ISSN:0737-3937    

DOI:10.1080/07373939508917014

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This study presents me development of an efficient adaptive finite element methodology for the analysis of drying problems. The propsed technique first uses an unified error estimator to assess the accuracy and reliability of the finite element solution. It then employs an automatic and adaptive finite element refinement strategy to reduce the errors to acceptable values. A new adaptive strategy applied to transient problems is also presented. The performance of the proposed methodology is evaluated by applying it to timber and clay brick drying problems. The estimated error agreed with the expected behavior: greater in regions of steeper gradients and lower in regions of smaller gradients. In addition, the proposed new adaptive strategy avoids unnecessary remeshing, i.e., remeshing when the error is decreasing with time at a reasonable rate.

ISSN:0737-3937    

DOI:10.1080/07373939508917015

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation

ISSN:0737-3937    

DOI:10.1080/07373939508917016

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Suspensions, slurries and paste-like materials can be dried in the Mechanically Spouted Bed ( MSB) dryer with men packing. The circulation of the men particles characteristic of classical spouted beds is provided with a houseless conveyor screw mounted in the vertical axis of the bed. Radioactive isotopic tracer technique was used for measurement of the cycle time distribution ( CTD) of the spherical inert particles as a function of the operational parameters of drying. The variances ( σ2) of the CTDs and the particle velocity m the various zones of the MSB dryer were calculated. The circulation of the mert particles can be characterised by nearly plug flow. According to the physical model of drying on inert packing the heat and mass transfer coefficients were calculated. Que to the relativeiy uniform film-like, wet coating formed on the surface of the spheical inert particles, the drying process may be characterised with the constant rate of drying. A method has been elaborated for calculation of drying time, hereby the partial processes of drying on inert particles can be synchronised.

ISSN:0737-3937    

DOI:10.1080/07373939508917017

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Retention or loss of trace volatile compounds during spray drying can be vital for product quality. Examples of cases where loss or retentions of volatile substances are important include retention of balanced flavor and aroma in food products, removal of odiferous substances, and control of the release of volatile organic compounds to the atmosphere. Factors and mechanisms determining losses of these substances are reviewed. Losses can occur during atomization, from undisturbed drops and as a result of morphological development. On the basis of this insight, several avenues for controlling the retention of volatile substances are identified and analyzed. These include control of atomizer pressure or rotation speed, choice of spray angle, configuration of air input, lteration of the air temperature profile, feed concentration, the presence of an oil phase and/or suspended solids, foaming of the feed, feed composition, and steam blanketing of the atomizer.

ISSN:0737-3937    

DOI:10.1080/07373939508917018

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor