مدل دو بعدی انتقال حرارت در گردش تخت سیال. بخش دوم: انتقال حرارت در یک CFB با چگالی بالا و تجزیه و تحلیل حساسیت

Experiments were conducted in a 76 mm diameter jacketed riser of a dual-loop high-density circulating fluidized bed facility with FCC particles of 65 μm Sauter mean diameter as bed material. The suspension temperature and the average and local suspension-to-wall heat transfer coefficients were measured. After superimposing the heat transfer results when the suspension near the wall is allowed to move intermittently downwards and upwards, the model proposed in Part I predicts the experimental results well. The model is used to investigate the effects of various operating parameters on the heat transfer process.

Circulating fluidized bed risers have been investigated extensively for the past two decades because of their practical applications, as well as their intrinsic interest. However, the overwhelming majority of such work has been conducted at net solids fluxes, Gs, less than 100 kg/m2 s, and at superficial gas velocities, Ug, between about 2 and 8 m/s. For these conditions, the overall volumetric solids concentrations, c, is less than about 0.1 [1]. While these conditions are relevant to circulating fluidized bed (CFB) combustion, much higher solids fluxes and holdups are encountered in CFB risers used for solid catalyzed reactions like fluid catalytic cracking and production of maleic anhydride. In such cases, Gs is commonly 300–1200 kg/m2 s, with corresponding c values ranging from 0.07 to 0.25. Grace et al. [2] defined the dense suspension upflow regime as having Gs>200 kg/m2 s, c>0.07 and solids upflow on average throughout the entire riser. Published results demonstrate that such operations differ in several important respects from low-density circulating fluidized bed systems. While numerous experiments have been carried out to investigate heat transfer in circulating fluidized beds, almost none of these applies to the high-density conditions defined above. CFB bed-to-wall heat transfer is strongly influenced by the flow pattern in the riser, especially the particle motion in the vicinity of the wall. Experimental work is needed to elucidate the heat transfer behavior in the high-density flow regime and to modify the model which was developed in Part I for low-density operating conditions.

Experiments carried out with FCC particles in the 76 mm diameter riser of a dual-loop HDCFB facility show that particles move both upwards and downwards in the vicinity of the wall. The direction of this motion is indicated by the suspension temperature distribution above and below a heat exchange section. Average suspension-to-wall heat transfer coefficients are strongly influenced by suspension density. The local heat transfer coefficient profiles are strongly influenced by the direction of particle motion. Periodic reversal of direction leads to higher heat transfer coefficients at both ends of the heat exchanger. The heat transfer model developed in Part I is extended to cover both high-density and low-density operating conditions considering the actual particle motion in the vicinity of wall by introducing the factor fd, defined as the fraction of time that particles spend traveling downwards, as estimated from the suspension temperature distribution. The resulting model predictions compare well with the experimental data. The sensitivity of the heat transfer process to changes of various parameters is investigated. The predicted influences of different parameters on the heat flux are also consistent with experimental trends where these are known. The current model for heat transfer in dense suspension upflow assumes that the steady-state results obtained for two independent layers moving upwards and downwards can simply be superimposed. In reality, particles in the vicinity of the wall alternate between upward and downward motions, yielding an unsteady problem for which the current approach may be insufficient. The model should be further extended to provide a more realistic representation of particle motion near the wall. Heat extraction in CFB combustors is usually accomplished by membrane walls. The current model does not consider the geometry of the membrane walls. It is essential to thoroughly understand the mechanisms of heat transfer between the gas-solid suspension and the membrane wall surface, to consider the heat conduction in membrane walls, and to develop an appropriate model to predict the rate of heat transfer. Suspension-to-wall heat transfer is strongly influenced by the system hydrodynamics, especially the particle and gas motion in the vicinity of the wall. However, hydrodynamic studies on high-density circulating fluidized beds are limited. Both experimental and modeling studies are needed to extend understanding of particle and gas motion in high-density flow regimes.