Exergy analysis of Fluidized bed dryers

1. Exergy analysis of Fluidized bed dryers BY: Chris Tremblay

2. What is a fluidized bed dryer • Piece of equipment used to remove moisture from a wet solid by bringing the moisture into a gaseous state. • A drying medium (usually air) is utilized to fluidize and dry moist particles. • May be carried out in a batch or continuous process. • On average 12% of energy used on drying for a manufacturing process. Which could be 60-70% of the total cost.

4. Applications • Food • Pharmaceutical • Chemical industries

5. Exergy analysis of fluidized bed drying of moist particles • The results show that exergy efficiencies are less than energy efficiencies due to irreversibilities which are not taken into consideration in energy analysis, and that both energy and exergy efficiencies decrease with increasing drying time • Drying kinetics is another significant aspect in drying processes. The drying rate, N.

6. The exergy balance is derived by applying mass, energy and entropy balances to the drying column in batch fluidization • The energy efficiencies decrease exponentially as the surface moisture evaporates until the end of the drying process.

7. Exergy leaving: Outlet air • Exergy Destruction(3 irreversibilities): heat transfer, mixing, and friction • Exergy losses:exergy loss to drying air leaving the column. Heat transfer across the boundary • Exergy of material: Increase of exergy of the material • Both energy and exergy efficiencies decrease sharply with decreasing moisture content of the material.

8. Energy and exergy analysis of fluidized bed dryer based on two-fluid modeling • The effects of parameters such as: • inlet gas velocity • inlet gas temperature • particle size diameter • on the energy, exergyefficiencies and the availability of gas

9. When the inlet gas temperature is increased from 70 C to 100 C with the inlet gas velocity of 4 m/s, energy efficiency increases up to 25% and the exergy efficiency up to 21%. • However there is practical limitation due to damage of the material.

10. Article does not explain well, but larger particles have lower efficiencies due to larger distance moisture must travel to reach the outer edge of the particle and smaller surface area

11. The availability of gas to do work decreases with time due to the decreasing difference is temperature.

12. Thermodynamic modeling of fluidized bed drying of moist particles • Exergyis consumed or destroyed, due to irreversibilities in any real process. The exergy consumption during a process is proportional to the entropy created due to irreversibilities associated with the process. • Lower exergyefficiency leads in general to higher environmental impact. • Cost of energy, the availability of fuel, and an impact on the environment make efficiency of exergy important in the drying process.

13. Comparing corn vs. wheat • The moisture diffusion coefficient of wheat is mainly dependent on temperature • The moisture diffusion coefficient of corn is dependent upon both the temperature and the moisture content of particles

14. Thermodynamic efficiencies were obtained to be higher at the beginning of the process than at the final stage • inlet air temperature effect: • wheat particles where the diffusion coefficient is only a function of the temperature, the increase of the drying air temperature increases the efficiency though not in a linear way • corn particles, where the diffusion coefficient depends on the temperature and the moisture content of particles, the increase of the drying air temperature does not increase the efficiency automatically

15. Gas velocity effect: • For wheat particles, energy and exergy efficiencies increase for a reduction of about 15% in the air velocity • For corn particles, both energy and exergy efficiencies do not show any difference at the end of the drying process

16. Thermodynamic analysis of fluidized bed drying of carrot cubes • Particle size, bed depth, and inlet drying air temperature were evaluated to asses the effects on exergy and energy efficiency. • Exergyloss through outlet air and dryer frame account for large percentage, so it important to reduce these and have more exergy loss into the product being dried.

17. Exergy loss increased with increasing drying air temperature and bed depth • Exergy loss decreased with increasing cube size

18. A Comparative Study on ExergeticPerformance • A comparison of three types of dryers were conducted using exergy destruction rates and improvement potential rates. • Tray dryer, fluid bed dryer, and a heat pump dryer • Exergy destruction and improvement potential rates were lowest in the fluidized bed dryer

19. Conclusion • The exergy analysis of a fluidized bed dryer is a much more accurate way to look at the efficiency of the dryer than looking at the energy analysis.