Thermal drying technologies: new developments and future r&d potential
Summary (3 min read)
INTRODUCTION
- Almost all industrial sectors involve thermal dehydration during one or several phases of their manufacturing processes.
- This massive energy consumption mainly using fossil fuels directly impacts CO2 emission levels which are being curtailed actively under the Kyoto Protocol.
- To make matters worse, industrial dryers, most of which were designed during the era of cheap and abundant energy, typically operate at thermal efficiencies in the range 20 to 60 percent.
- Clearly there is tremendous scope for improvement of industrial drying technologies- a task that must logically begin with R&D to enhance their knowledge base on drying as well as dryers.
- The latter is a coupled process involving transport phenomena and material science.
MATHEMATICAL MODELS OF DRYING AND DRYERS
- The authors cannot provide a general overview of the wide assortment of modelling approaches, their relative merits and limitations, within the limited scope of this presentation.
- The type of model clearly depends on the object of modelling, type of product and drying equipment.
- Modelling of drying is a microscopic level approach that ideally is independent of the material.
- Huang (2005) developed a CFD model to predict flow patterns and overall drying performance of different spray dryer designs [5]: the conventional cylinder-on-cone one; the dryer with two pressure nozzles, rotating disk atomizer and ultrasonic nozzle; the dryer with conical, hour-glass shape and lantern shape chambers.
- The typical flow pattern of the spouted bed is shown in Figure2 (a), where a stable spout, fountain and annulus regions were observed clearly.
Thermo-Mechanical Models of Drying
- When water is removed during drying processes, pressure imbalance is produced between inner pressure of the material and the external pressure, generating contracting stresses that can lead to material shrinkage or collapse, changes in shape.
- When rapid drying rate conditions are used and intense moisture gradient through the materials are observed, shrinkage is not uniform resulting in the formation of unbalanced stress.
- Surface cracking may occur leading to permanent deformation or even failure of the material.
- To avoid such undesired effects, simulation tools that can handle simultaneously the drying problem and the coupled mechanical problem are necessary.
- The thermo-mechanical models often comprise of simplified drying models based on the diffusion equation with different coefficients and boundary conditions for the first and second periods of drying.
Distribution of moisture content
- The distributions of moisture content are often evaluated based on the diffusion model equations.
- When temporal moisture content distribution is obtained, the shrinkage of materials and stresses can be then determined.
Drying- induced stresses
- The author assumed that wood beyond the fibre saturation point (FSP) does not shrink and has the lowest mechanical strength despite its moisture content.
- The shrinkage is then regarded as proportional primarily to the change of moisture content; the volumetric strain due to shrinkage is θε )(3 Xk= .
- Substituting equation (5) and (7) into the physical relation (9-10), the authors can obtain the differential equation for determining stresses.
- In summary, the thermo-mechanical model can be used to estimate shrinkage, capillary forces and thermal tensions cause stresses in drying bodies.
- Table 2 lists the applications of thermo-mechanical models reported in the literature for various materials and its geometries, including organic, inorganic, biological materials.
Multiscale Models for Drying
- Perre (2002) has discussed the characteristics and needs for multi-scale models in drying of such materials as wood and paper [22].
- When the overall drying behaviour is influenced by both what happens at the micro scale and what happens at the macro scale, the authors need multi-scale models for proper representation of the drying behaviour.
- On assuming isothermal conditions and constant total pressure, the microscopic vapour flux can be expressed with the gradient of the boundary-water content as the driving force by equation 12.
- Vapour diffusion at macro scale will change the state variable (density, moisture content, temperature, etc) of the wood.
- If the scale levels can not be considered as independent, other strategies have been proposed through which several scales can be considered simultaneously, such as the parallel flow model, mesocopic model [24], distributed microstructure models [25], etc.
R&D NEEDS IN DRYING
- As is evident from the previous discussion, thermal drying of solids is a complex phenomenon that has defied development of a universal theory.
- Unlike heat exchangers, design and analysis of the heat and mass exchangers (viz. dryers) is not amenable to use of appropriate software.
- Kudra and Mujumdar (2001) have discussed a multitude of industrial drying technologies which deserve R&D for optimization [29].
- Use of bio-gas, bio-diesel instead of oil or natural gas will become more common as petroleum products escalate in price.
- Changes in technologies in one field will have an impact on technology in a related field.
CONCLUDING REMARKS
- This paper has focused on a small area of R&D needs in the vast field of thermal drying.
- As an energy consuming operation responsible for 10-25% of national industrial use in the developed countries, and an increasing fraction in the emerging economies as well, there is strong incentive to understand this operation better.
- Aside from attempts to develop workable models for drying and dryers, it is also essential to carry out careful experiments on different size equipment to test models for scale-up.
- Aside from models for heat and mass transfer it is also necessary to develop predictive models for the quality parameters of the dried products.
- With potential legislations limiting carbon emissions and imposition of carbon tax, it will be essential in the next few decades to come up with ultraefficient, compact drying systems to remain globally competitive.
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References
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...If the scale levels can not be considered as independent, other strategies have been proposed through which several scales can be considered simultaneously, such as the parallel flow model, mesocopic model [24], distributed microstructure models [25], etc....
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...fundamental models for shrinkages were summarized in Reference [19]....
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...Other types of fundamental models for shrinkages were summarized in Reference [19]....
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"Thermal drying technologies: new de..." refers background in this paper
...Kudra and Mujumdar (2001) have discussed a multitude of industrial drying technologies which deserve R&D for optimization [29]....
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...The correlation can be linear: Lozano et al (1980) used the equation XccDR 21 += to model shrinkage of the cylindered apple [16]....
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Frequently Asked Questions (15)
Q2. What are the main reasons for the growth of dryers?
More efficient combustion technologies which also provide thrust for gas movement in dryer, e.g. pulse combustion, will become more common.
Q3. What is the effect of low rank coal on the environment?
Use of low rank coal which contains large volume of water (similar to peat) will increase along with the need to dewater and dry coal costeffectively possibly using superheated steam.
Q4. What is the role of the carbon tax in the development of drying systems?
With potential legislations limiting carbon emissions and imposition of carbon tax, it will be essential in the next few decades to come up with ultraefficient, compact drying systems to remain globally competitive.
Q5. What is the thermo-mechanical model for drying?
The thermo-mechanical models often comprise of simplified drying models based on the diffusion equation with different coefficients and boundary conditions for the first and second periods of drying.
Q6. What is the CFD model for a cylindrical spouted bed dryer?
The CFD model was also used to evaluate the influence of operation parameters such as particle size, density, and pulsating jet, etc on flow pattern of the bed.
Q7. Why did the energy crisis drive drying R&D?
As energy costs dropped and levelled off drying R&Dcontinued but driven by need to reduce costs, improve quality and meet global competition.
Q8. What are the advantages of solar dryers?
Solar dryers utilizing photovoltaics and possibly wind energy to boost the drying rates and efficiency will become more popular even in developed countries.
Q9. What are the main challenges of drying?
Even in conventional areas like drying of paper, ceramics, wood etc there is scope to make the processes more energy-efficient, environmentally friendly and safe.
Q10. What is the potential of the multiscale model in drying?
the multiscale model shows high application potential in drying, especially of porous materials such as wood, ceramics, etc.
Q11. What are the advantages of superheated steam?
In the coming decade one should see more industrial technologies utilizing superheated steam as the drying medium because of its numerous inherent advantages.
Q12. What is the use of the CFD model in the dryer?
the CFD model was used toevaluate an innovative one-stage and two-stage, two dimensional horizontal spray dryer (HSD) concept.
Q13. What are the main problems of drying?
Compared to what the authors find in the literatures on porous media, the corresponding drying problem is complicated by the fact that the porous material can undergo physical deformation and even chemical reactions, change of moisture transport mechanisms during drying, can involve glass transitions, precipitation of solids, micro-crystallization etc.
Q14. What are the different scales that can be considered as independent?
If the scale levels can not be considered as independent, other strategies have been proposed through which several scales can be considered simultaneously, such as the parallel flow model, mesocopic model [24], distributed microstructure models [25], etc.
Q15. What is the thermo-mechanical model used to estimate shrinkage, capillary forces and thermal?
In summary, the thermo-mechanical model can be used to estimate shrinkage, capillary forces and thermal tensions cause stresses in drying bodies.