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01 Apr 2016

Theoretical Extension and Innovative Applications of Reaction Engineering Approach to Modelling Drying and Other Transport Processes

Aditya Putranto, January 2013
Curtin University of Technology

Abstract
Drying is a water removal process involving simultaneous heat and mass transfer process. Usually, it is referred to a process involving vapor removal. Study of drying is important since it is an energy-intensive process because large amount of heat needs to be supplied for evaporating water. Drying also affects significantly the product quality of materials. Optimization exercises need to be carried out to maintain the highest possible product quality of the materials during drying as well as minimizing energy consumption.

The optimization procedures often involve modeling. Hence, reliable drying model can assist in process design, process simulation and optimization. For process design, it can be used to explore new innovative designs of a dryer, to evaluate the performance of existing dryer and to assess its energy consumption. For maintaining product quality, a reliable drying model can be applied to explore new processes and to optimize the existing process to achieve high quality products. A reliable drying model should ideally be simple, accurate, able to capture the physics of drying process and require minimum sets of experiments to generate the drying parameters. The minimal number of laboratory trials required is a feature useful for industry.

The reaction engineering approach (REA) was proposed by Professor X.D. Chen in 1996 and has been used successfully to model several drying processes mainly thin layer drying and drying of small particulates of food materials. The physics of the drying process is captured by the relative activation energy which represents the level of difficulty to ‘extract’ moisture during drying in addition to evaporating free water. Initially, it may be zero near the start of drying of high moisture product and keeps on increasing during drying as drying progresses. When the low equilibrium moisture content is reached, the relative activation energy becomes one. The relative activation energy of the same materials can be used to model other drying processes with the similar initial moisture content. The REA framework allows a very effective way to obtain the necessary parameters.

Because of the efficiency of the REA framework established so far, it is worthwhile to develop further the REA in an innovative manner and to implement the REA to more complex scenarios. The REA, which was previously proposed in the lumped format, is now labeled as the lumped reaction engineering approach (L-REA) and more comprehensively, we have developed the spatial reaction engineering approach (S-REA) in the current work. In L-REA, the REA is used to model the global drying rate while in S-REA, the REA is applied to model the local evaporation rate and coupled with a system of equations of conservation to yield a spatial model.

To expand the L-REA approach, it is implemented in this study to model convective infrared-heating drying, convective drying of several centimeters thick samples, intermittent drying under time-varying temperature, humidity and infrared-heating, baking, roasting and heat treatment of wood under linearly increased temperature. In all the cases of food and natural materials, appropriate shrinkage models are required. The S-REA is developed and applied here to model convective drying, intermittent drying and heat treatment of wood under constant heating rate, where spatial energy and mass balances are resolved.

For modeling of the convective drying of other materials, the original formulation of the L-REA is implemented. Without any modification, the L-REA can model the convective drying of the mixture of polymer solutions accurately. For modeling the infrared-heating drying, a new definition of the equilibrium activation energy has to be introduced. For modeling of convective drying of several centimeters of thick sample using the L-REA, the approximation of spatial distribution of sample temperature is used. The surface temperature is also implemented in the mass and heat balances as well as the evaluation of saturated water vapor concentration. It is emphasized that the L-REA does not actually assume uniform moisture content inside the sample but the L-REA evaluates the average moisture content during drying. The results indicate that the L-REA models well the convective drying of non-food materials, infrared-heating drying and convective drying of several centimeters of thick sample.

The L-REA is applied to model the intermittent drying of food and non-food materials under time-varying humidity, temperature and infrared-heating intensity. Surprisingly, for modeling the intermittent drying, no major modification of the original formulation of the REA is necessary. In order to incorporate the effect of time-varying humidity and temperature, the equilibrium activation energy is evaluated according to the corresponding humidity and temperature in each drying period. The relative activation energy generated from convective drying of materials under constant environmental conditions can be used to model the intermittent drying. The results indicate that the L-REA can model actually the intermittent drying of food and non-food materials under slow and rapid change of ambient humidity and temperature. For modeling the intermittent drying under time-varying infrared-heating intensity using the L-REA, two schemes of definition of equilibrium activation energy is used. The first scheme employs the relationship between the infrared-heating intensity in each stage and the final product temperature in each stage should the infrared heating be prolonged to equilibrium. The second scheme uses direct relationship between the infrared-heating intensity in each stage and equilibrium activation energy. Both definitions are combined with the relative activation energy, generated from convective drying run under constant environmental conditions to yield the activation energy. It has been shown that the L-REA can also model very well the intermittent drying under time-varying infrared-heating intensity.

The L-REA is further implemented to model the simultaneous heat and mass transfer processes at high temperature namely baking of bread, roasting of barley and coffee and heat treatment of wood under constant heating rate. For modeling these processes, no modification of the original formulation of the REA is required. For modeling the heat treatment of wood under constant heating rate which is essentially a drying process under linearly increased gas temperature, the equilibrium activation energy is evaluated according to corresponding humidity and temperature during the process. The results indicate that the L-REA can model these processes well.

The use of non-equilibrium multiphase drying model is suggested as the model can offer better understanding of drying process and it can be used to assess the suitability of equilibrium multiphase drying model. However, the model requires explicit formulation of the local evaporation rate. The REA is further implemented to model the local evaporation rate and coupled with a system of equations of conservation of heat and mass transfer to yield a spatial model called the spatial reaction engineering approach (S-REA), as a non-equilibrium multiphase drying model. The S-REA consists of the spatial mass balances of liquid water and water vapor as well as the heat balance in the conventional manner. In the mass balances of liquid water and water vapor, the REA is used as the depletion and source terms, respectively.

The REA is also adopted as the local evaporation rate term in the heat balance. The relative activation energy, implemented in the L-REA and generated in one accurate drying run, is used to model the local evaporation rate for the same material but the average moisture content is now replaced by the local moisture content. In this study, the S-REA has been implemented to model the convective drying, intermittent drying and heat treatment of wood under constant heating rate. The accuracy of the S-REA to model these processes as well as the applicability of the REA to describe the local evaporation rate has been assessed.

For modeling convective drying using the S-REA, using the approach mentioned above, it has been shown that the results of modeling match well with the experimental data. The SREA is capable to model the spatial profiles of moisture content, concentration of water vapor and temperature accurately. Due to the application of the REA as the local evaporation rate, the profiles of local evaporation rate and concentration of water vapor can now be generated so that better insightful physics of drying can be gained. The S-REA has also been successfully applied to modeling of the intermittent drying and heat treatment of wood under linearly increased temperature.

Based on the extensive modeling exercises carried out in this study, it can be concluded that the REA framework is very useful in characterizing various challenging drying and other simultaneous heat and mass transfer processes. The L-REA has been proven to be accurate and effective to model these processes with simplicity being a major advantage. The REA framework has also been shown to be able to model the local evaporation/condensation rate well. The SREA is an effective non-equilibrium multiphase drying approach to provide better understanding of transport phenomena of drying and other simultaneous heat and mass transfer processes that involve water transformations. It is interesting to note that the L-REA parameters obtained in laboratory can also be used in S-REA simulations for the same material being dried. This presents an obvious practical advantage.

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