Wood chips drying pdf

Fluctuations in the relative humidity of the drying air due to 2. However, since the equilibrium was used for the drying experiment. The dimensions of the drier moisture content is not high for agricultural products, the above were 90 cm height , 60 cm depth and 60 cm wide. The drier con- Mathematical models used in the current work are shown sists of a fan, which blows air at a constant velocity of 0.

A mesh groups: theoretical, semi-theoretical and empirical. Theoretical tray was specially fabricated with thin steel wires, with its open- models are based on diffusion equation or simultaneous heat and ing size 5.

Whereas, semi-theoretical models the drying to take place from all the sides. The fabricated mesh are based on the closely estimated theoretical equation and empiri- was placed in the middle row of the dryer and the wood chip was cal depends on the experimental data.

Semi-theoretical models are placed on it. Dry forceps were [24, 25]. Empirical models describe the drying curve for the con- used while transferring wood chips from drier to weigh balance. These two model types deem only the external resist- The wood chips were weighed using an electronic balance.

The chip was weighed and then again were the empirical models used in this paper. Regression analysis placed in the tray drier. The weight of the wood was measured were performed and the models were chosen to be the best if the at regular intervals of one hour. All these weighing process was inished within 10 seconds. Error MBE values are minimum [24, 30]. Sridhar, G. Moisture content was decreased with increase in time and temperature Fig.

The drying time was reduced to attain determined moisture content as the tempera- ture is increased. This may be attributed to the increase in the water molecule energy due to an increase in the temperature and also because of a larger difference in the partial pressure of the vapor in the drying air to the vapor pressure of the moisture in the wood at higher temperatures [31, 32] which consequences in quicker evaporation of moisture from the wood chips.

Simi- lar observations may be made from Fig. Constant drying rate period was not observed and all the drying was found only in the falling rate period Fig. Absence of constant rate period may be due to the absence of free surface water, therefore no surface evaporation took place at constant rate.

Hence we can infer that mass transfer of the moisture during drying took place predominantly by liquid diffusion. Similar results have Fig. From Fig. These were itted to the experimental drying facts to ind the constants of each model.

The closeness of the relation of the model was determined by the correlation coeficient R2. MBE provides information on the long term performance of the correlations, helping for the comparisons of the actual deviation between experimental and predicted values [42, 43]. Various constants and the regression analysis data calcu- lated for eleven models at various temperatures T are shown Fig.

All the models used were found to predict drying different drying temperatures to a good extent, however modiied Henderson and Pabis, and Logarithmic model was found to predict the drying very well the moisture. This very good performance of the modiied Henderson and Pabis relation was reduced to Eq. The perfor- In order to use 8 , it is assumed that the wood chip is homogene- mance of the logarithmic model and modiied Henderson and ous, isotropic, drying occurs only in the falling rate period, mass Pabis model showed a straight line with R2 0.

By plotting Ln MR versus drying time, Deff can be of water in the falling rate period, assuming diffusivity to be the found from the slope of the line Fig. Deff increased with sole physical mechanism responsible for the transfer of water to increase in temperature from 4. In other words, during the falling rate the movement of water to the surface of the wood chip increases, period only the internal resistance regulates the mass transfer of the values are tabulated in Table 3.

Ln Deff versus reciprocal of absolute drying air temperature Ta were plotted Fig. DOI: Activation Energy was DOI: Forest Biomass Particles of Pinus radiata. Initial moisture content of the Casuarina wood chips was Constant rate [8] Tillman, D.

The results showed that dry- combustion consequences. The effective diffusion [10] Bartzanas, T. The activation Engineering. Luccio, M. Box , Campinas, S.

Water is considered to be present in three forms-free water, bound water, and vapor-which remain in local equilibrium. It is assumed that the heat and mass transport mechanisms are: capillarity of free water, diffusion of vapor due to the concentration gradient, and diffusion of bound water due to the gradient of chemical potential between the water molecules. The constants of the phenomenological coefficients were adjusted. Finally, the drying process in wood chips was simulated in a unidimensional mesh.

The results were compared with experimental data on drying kinetics obtained from the literature. Concentration profiles are shown, and the weight of each of the mechanisms present in the drying phenomenon is shown in graphic form and discussed. Keywords: Capillarity, chips, diffusion of vapor, diffusion of bound water.

Before being fed, they are dried by effluent gases from the boiler itself, so an ap- propriate drying process is essential if the goal of saving energy is pursued. The objective of this work is the develop- ment of a model for the drying process of a hygroscopic capillary porous medium focus- ing on the case of wood chips; however, the model can be extended to other cases.

The chips have a prismatic form and a thickness that is considerably smaller than the other two dimensions; the phenomena of heat and mass transfer were considered to occur preferential- ly in the shorter dimensional direction, which is normal to the planes of bigger area Fig. Simultaneous heat and mass transfer and equilibrium conditions between the phases were assumed; physical and transport proper- ties found in the literature were used.

The equa- tions system obtained was numerically solved, and the results of the simulation are presented and compared with experimental data from the literature. The moisture content and tempera- ture profiles are presented and discussed. Three states of water were considered: free water, bound water, and vapor.

It was assumed that all three phases were in thermodynamic equilibrium at the local temperature. There- Wnod and Fiber Science. Chip geometry. Each of the states of the water was submit- ted to one of the following transport mecha- nisms: 1 migration of free water by total pressure gradient flow as estimated by Darcy's law; 2 transfer of bound water by diffusion due to the chemical potential gradient; 3 migration of vapor by diffusion due to the partial pressure gradient of water vapor.

The assumptions adopted were: 4 The drying phenomenon inside the chip was considered unidimensional. Bound water and vapor are in equilibrium accord- ing to the sorption isotherms for wood, and they remain in a saturated state while free water is present. Thus there was no internal convective flux of vapor be- cause there was no total pressure gradient.

In Eq. Equilibrium correlations Equations 1 , 2 , and 3 form a partial differential equations system, in space and time, with four independent variables: Cbf, Cv, T, Revap. For this system to become determi- nate, another equation is necessary. It is given by the local equilibrium correlations for free and bound water in equilibrium with saturated vapor, and when free water is no longer pre- sent, bound water and vapor obey the sorption isotherms.

The effective diffusivities of free and bound water were obtained from the individual mechanisms of both of the forms, as will be explained below. The free water flux is based on Darcy's law for porous media, so it is proportional to the total pressure gradient of the liquid. The only mechanism adopted for heat transfer was conduction: -k dT "ax 16 Boundary conditions Each of the differential equations, 1 , 2 , and 3 , requires one initial and two boundary conditions.

The initial temperatures and vapor and bound water concentrations need to obey the thermodynamic equilibrium correlations. Therefore, if free water is present, these con- centrations are calculated by Eq. The first boundary condition for each of the differential balance equations is given by the heat and mass convection on the limit surface of the chips, and the second is given by the symmetry conditions.

For the mass transfer coefficient, the anal- ogy of Chilton-Colburn was used, according to the driving force considered in Eq. The implicit fi- nite volumes method Patankar was used. The system was solved and converged by the TDMA line by line method. The equilibrium correlations, Eqs. The basic steps of the algorithm are as fol- lows: 1 The initial values of variables and con- stants are given. The time and space incre- ments are set up.

The error limit adopted was 0. The calculation finishes when a previously fixed final mean moisture content, is reached. RESULTS The experimental results of Kayihan , for chips of Pinus virginiana, were compared with the model developed, to verify their agreement and determine the constants of the transport mechanisms adopted. In Fig. Moreover, in Fig. It remains constant for an initial period of 65 s, which corresponds to the constant drying rate period, and then begins to increase slowly until reaching air temperature at the end of drying.

Figure 3 shows the temperature profiles by chip thickness, which are constant at every in- stant during the process; this means that the thermal conductivity of the wood is high enough to permit the free transfer of heat from the surface to inside of the chip, so it does not act as a limiting factor.

Figures 4, 5, and 6 show the concentration profiles of each of the phases considered in the 0. Figure 4 shows that, for a time of 70 s, the chip surface has no more free water and an evaporation front appears, limiting the region where free water still exists.

The pathway of this front by thickness can be seen: at 70 s, it is near by the surface, at 90 s, it is at 0. The process finishes at s, when no more free water exists inside the chip. Figure 5 shows that the concentration pro- 0. This phenomenon occurs because the bound water remains in equilib- rium with the saturated vapor, and it changes slowly only as the temperature changes. The rate of transfer of bound water is important only away from the region limited by the evaporation front.

Beginning the moment this front disappears, concentration and rate of transfer of bound water between the center and the surface diminish to the end of the drying process. The profiles of vapor concentration inside the chip are presented in Fig. They follow the free water profiles, so the concentration is low and nearly constant at the beginning see the profiles for 0, 30, and 60 s. Vapor at these conditions is saturated, and its concentration inside this porous medium depends only on temperature and wood porosity.

The profiles change slightly from one thickness point to another due to the increase in effective poros- ity as the free water inside the wood decreases. From 70 s, the profiles follow the bound water profiles at the outer part of the chips, where vapor concentration is given by the sorption isotherms of wood; in the inner part, vapor concentration still depends on temper- ature, which increases slowly.

The differen- tials of concentration between inside and out- side are increasing at the same time that the evaporation front moves towards the center.

This feature continues up to s, when vapor concentration reaches its maximum value the evaporation front disappears and begins a de- crease that continues to the end of the process.

Order of magnitude of the different effects in the mass and energy conservation equations The order of magnitude of the different ef- fects present in the mass and energy conser- vation equations are discussed in this section. The aim is to validate the factors considered and disregarded in the model presented in this work. These values were obtained from the simulation proposed here for wood chips of Pinus virginiana, for which experimental data compared with the kinetic model are presented in Fig.

The val- ues are relative to the surface and level 0. For level 0. Effects of the diverse mechanisms of mass transfer in the mass conservation equation as a function of time, on the surface and at level 0.