摘要:

A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally obtained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.

ISSN:0737-3937    

DOI:10.1080/07373939608917203

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The transfer of moisture from a wooden surface and the corresponding mass transfer coefficient in relation to the heat transfer coefficient are considered. Experimental data are reviewed that show that the mass transfer coefficient often is one order of magnitude smaller than what could be expected from the analogy between heat and mass transfer. The nature of this deviation and its influence on wood drying models is discussed.It has been suggested by Wadsö.that moisture flow inside the wood exhibits non–Fickian behaviour and that this may be due to a slow desorptiodabsorption by the wood fibre. A model is thus developed where the normal assumption of internal local equilibrium between the vapour and bound phases is replaced by a mass transfer resistance. The model is applied to one–dimensional moisture flow in a body of finite thickness and solved analytically.Numerical values calculated with the model are analysed in order to establish whether “apparent” non–Fickian behaviour of the kind observed could be explained by such a model. It is also analysed whether the model could explain an apparent decreased mass transfer coefficient at the surface.

ISSN:0737-3937    

DOI:10.1080/07373939608917204

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A review of the recently reported moisture diffusivity experimental data in food materials is presented in this work. Values are classified and analysed statistically to reveal the influences of temperature and moisture content. Empirical models relating moisture diffusivity with the above variables, along with relevant graphs for various materials, are included. Other factors affecting moisture diffusivity prediction are also discussed.

ISSN:0737-3937    

DOI:10.1080/07373939608917205

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A mathematical model of heat and mass transfer for fixed beds was developed according to the modern theory of process simulation and standard laws of thermodynamics and transportphenomena. The mass transfer grain–air was predicted with simplified diffusional expressions together with an equation for the static equilibrium moisture content. Four differential equations were obtained for a grain layer and they were integrated along the bed depth and time with second

ISSN:0737-3937    

DOI:10.1080/07373939608917206

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

An analysis of desorption isotherms for potato shows the following probable qualities in the water-potato system. In the natural state the water is bound rather strongly due to loss of rotational degrees of freedom in the sorbed molecules, which is caused by small molecular voids in the cellulose structure. This structure therefore is formed as a uniformly cross bound state of the OH-groups. This state prevails up to a certain temperature, above which a reaction sets in, where OH bonds are successively broken, giving possibility for a partial rotation of sorbed molecules. In the range of lower moisture content the cellulose structure is changed into a crystalline state as in wood, although the degree of crystallinity is lower, which is the probable reason why the statistical fluctuations in the isotherms are greater for potato than for wood.

ISSN:0737-3937    

DOI:10.1080/07373939608917207

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Only a few kinetic studies have been reported in the literature on the evolution of comercial quality of maize during drying and to the best of our knowledge no model allows to predict the dynamic coupling of drying and product quality evolution. The aim of this work is to present new information on the effects of the operating conditions (harvest year, weight of maize to be dried, initial moisture content of the grain and air temperature) on the evolution of maize saline-soluble protein denaturation and the wet-milling quality during drying of maize in a batch floatation fluidized bed dryer. Also, kinetic laws are proposed for the quality criteria that will be combined with drying model already derived [1, 2].

ISSN:0737-3937    

DOI:10.1080/07373939608917208

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

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.

ISSN:0737-3937    

DOI:10.1080/07373939608917209

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Cassava, a crop originated from central and south america, is now being cultivated in all tropical countries of the world. This was possible because of its adaptative capacity to all types of soil and its resistance to dry weather.The most widely used method for the transformation of fresh cassava roots is solar drying. We have studied the evolution of the cyanogenic compounds of cassava during solar drying begining at 9 h or 18 h. Three cassava varieties were dried (MBRA 195, CG 165-7 and CG 915-1) using loads of 12.5 kg/m2of chips having a dimension of 3x3x5-10 mm. Results showed that The loss of total and bound cyanides observed varied between 84-89% and 80-100% respectively, with respect to the initial content of cyanogenic compounds. In all cases, the concentration of cyanide residue was less than 40 ppm (dry basis). The rate of cyagenic compounds elimination was better in the experiments begining at 18 h than those began at 9 h. Our results also show that water diffuaion into the interior of the chips has a great influence on the elimination of cyanogenic compounds of cassava.

ISSN:0737-3937    

DOI:10.1080/07373939608917210

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The characteristic drying curves of cocoa beans are determined by using a tunnel drier where conditioned air passes a single cocoa berm suspended from an electronic balance in the test section. Weight loss, and temperatures of air, testa and nib of the cocoa bean is monitored on personal computers. The nornmalised drying rate versus the normalised moisture content is regressed by least square method to fit a new polynomial model for the penetration falling rate period and a linear model for the regular regime falling rate period. It can be concluded that there are three drying periods for cocoa beans namely the constant drying rate period, the penetration falling rate period and the regular regime falling rate period. The polynomial model estimates the penetration period quite well whereas the linear model estimates the regular regime quite well as well. There is no observable influence of relative humidity and air temperature on the characteristic drying curve of cocoa beans. However, the air velocity seems to have some influence on the curve.

ISSN:0737-3937    

DOI:10.1080/07373939608917211

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A cross-flow fluidized bed paddy dryer with a capacily of 200 kgh was designed, fabricated and tested. Experimental results showed that final moisture content of paddy should not be lower than 23 % dry-basis if quality is to be maintained. Drying air temperature was keot constant at 115°C according to the recommendation of previous work. Results obtained from the mathematical model developed in this study indicatedthat optimum operating parameters should be as follows : air speed of 2.3 m/s, bed thickness of 10 cm and fraction of air recycled of 80 %. At this condition, energy consumption was close to the minimum while drying capacity was near the maximum. A prototype fluidized bed dryer with a capacity of 1 t/h was designed, fabricated and installed with the collaboration of a private company. The unit has been used for almost the whole past harvesting season in 1994 at a paddy merchant sile with preference compared to conventional column continuous dryers. More than 300 tons of paddy were dried without any problems.

ISSN:0737-3937    

DOI:10.1080/07373939608917212

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor