(663h) Cloudy-Zone Modeling of a Gas-Solid Bubbling Fluidized Bed with Liquid Spray

Fluidized bed reactors with liquid spray are widely used in industrial processes especially in the process of olefin polymerization, owing to the excellent heat removal capability and high performance of the polymer products. Liquid spraying from the side wall of a gas-solid fluidized bed results in a gas-particle-droplet interaction zone, or named the ‘cloudy zone’, in which the homogeneity of temperature and contact between particles and gas are broken and the hydrodynamics and heat conduction are affected by the conduction and evaporation of liquid. However, such effects are different with the existence forms of liquid. Specifically speaking, the liquid sprayed into the bed usually has two existence formations, droplets and liquid films on particles. The evaporation of droplets absorbs the heat of hot gas whose temperature decreases. However, the heat of gas and particles is absorbed by the evaporation of liquid films. So the temperature of catalyst active center is decreased by liquid films which also provide comonomer. Therefore, droplets and liquid films have different effect on the reaction in the process of olefin polymerization. As a result, the temperature of cloudy zone decreases and there will be a significant dissimilarity between the cloudy zone and non-cloudy (gas-solid) zone. Therefore, it is essential to characterize the ratio of droplets and liquid films and the influence of liquid spray on the fluidized bed.

In this work a mathematical model is setted up to describe cloudy zone and gas-solid zone in the gas-solid bubbling fluidized bed with liquid spray. The model is composed of hydrodynamic equations and heat transfer equations, such as the equations for gas mass balance, bubble phase velocity, bubble phase force balance, emulsion phase heat balance and bubble phase heat balance, etc. The volume ratio of droplets to liquid films is calculated to account for the relative contribution of different liquid existence forms in the bed. For the closure of the equations, an objective function is proposed, containing the energy consumption from the change of surface of liquid phase, the mass-specific suspension energy consumption rates of emulsion and interphases, and the energy consumption from the expansion of bubbles. The model without the consideration of liquid spray is firstly verified through the experimental voidage data from literature ^[1]. Then a self-designed fluidized bed with back heating and side-wall liquid spray is applied for the verification of predicted voidage and liquid hold-up. According to the modeling results, the volume ratio of droplets to liquid films increases with increasing the velocity of gas because of the increase of voidage and the decrease of the collision frequency between the liquid droplets and particles. The model expresses the effect of liquid spray on the gas-solid fluidized beds and it is of great significance in describing the cloudy zone and the whole reactor more accurately.

Reference

[1] Taghipour F, Ellis N, and Wong C. Experimental and computational study of gas–solid fluidized bed hydrodynamics. Chemical Engineering Science, 2005. 60(24), 6857-6867.