Projeto de sistema recuperador de calor a partir de sólido particulado quente em leito fluidizado raso

Abstract

Heat recovery of heated gases or solids particles is one of the viable applications of fluidized systems, and being properly applied, the utilization of this energy can bring many benefits to industrial processes. In this sense, the present paper a design of a system of heat recovery of particulate solids in fluidized bed by applying a design methodology from mathematical models for a heat exchanger with shallow fluidized bed with immersed tube was prepared. A system formed by a gas heater with bubbling fluidized bed, a heat exchanger shallow fluidized bed and pneumatic transport system, to promote the return of solids to the heater, was designed. The cooling of the particles was accomplished by water flow inside a tube immersed in the bed, countercurrent arrangement, with a stream of solids with inlet and outlet temperatures of 400°C and 100°C respectively. The solid test material was sand with a mean diameter of 254 µm fluidized with air in heat exchanger 450 mm long and 75 mm wide. The system was designed to operate in permanent regime for discharges of solids changing between 13 and 17 kg/h. The fixed bed has height of 60 mm and the relationship between superficial gas velocity and minimum fluidization velocity (uo/umf) remains in the range of 1 to 6. The heating of the particulate solids is promoted by heating the LPG to reach to 400°C. After the unloading of the solid heat exchanger, it is taken back inside the heater by means of pneumatic transport, aiming at a higher energy efficiency and ease of operation of the system. Mass and energy balances between the three streams (water, air and solid) were performed, aiming to achieve high rates of heat transfer to the lower area of heat exchange. The project was evaluated in order to determine the influence of the change in mass flow rates of the material streams, in relationship between superficial gas velocity and minimum fluidization velocity, in diameter and density particle, while maintaining the dimensions fixed of the equipment. In this purpose, the thermal behavior of the system when 9 subjected to different operating conditions were raised. The results showed that the system behaves in an opposite way with respect to the value of the overall heat transfer coefficient. With increasing mass flow rates, the overall heat transfer coefficient presents an increasing trend, but when the heat exchanger is subjected to larger diameter particles, the value of the overall heat transfer coefficient tends to diminish. Therefore, the study and understanding of these parameters proved to be important for the understanding and design of equipment of this nature.