Gas??????solids mixing in the inlet zone of a dilute circulating fluidized bed

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• 作者：De Wilde ; Juray ; Van Engelandt ; Gorik ; Heynderickx ; Geraldine J. ; Marin ; Guy B.
• 关键词：Fluidization ; Gas– ; solid flow ; Inlet effects ; Outlet effects ; Simulation ; Experimental validation ; Riser
• 刊名：Powder Technology
• 出版年：2005
• 出版时间：March 1, 2005
• 年：2005
• 卷：151
• 期：1-3
• 页码：96-116
• 全文大小：1.40 M

文摘

The influence of the inlet configuration on the flow pattern in dilute circulating fluidized beds is investigated. The solid flow model used is based on the kinetic theory of granular flow (KTGF). Interactions between the fluctuating motion of the gas and the solid phase are accounted for. A line-implicit simultaneous solution algorithm based on dual-time stepping is used for the numerical integration. The discretization of the inviscid fluxes is based on a low-Mach reformulation of the multi-phase preconditioned advection upstream splitting method (MP-AUSMP).

Simulation results for a side solids inlet configuration are qualitatively verified using 3D LDA data of a cold-flow circulating fluidized bed pilot unit. The pilot riser has a diameter of 0.1 m and a height of 8.765 m. The side inlet used for solids feeding is positioned 0.58 m above the gas inlet and makes a 35° angle with the riser axis. The cold-flow pilot unit is operated in the dilute regime with a superficial gas velocity of 5.31 m s⁻¹ and a solids flux of 3 kg m⁻² s⁻¹. Experimental and simulation results show that bypassing of solids by the gas occurs, but that the bypassing effects are quickly dissipated by the viscous terms. The gas–solid flow model is capable of describing the experimentally observed mixing behaviour.

In simulations of dilute industrial scale risers, the gas is fed over the entire bottom section, whereas two kinds of solids inlet configurations are used: (1) the solids are fed through a central inlet tube at the bottom of the riser; (2) the solids are fed through a side inlet at a certain distance from the bottom of the riser. The outlet is of the abrupt T-type.

With the first inlet configuration, the radial mixing of the solids is seen to be hindered, due to bypassing of the gas via the outer ring of the reactor tube. The bypassing effects are not quickly dissipated by the viscous terms. The impact of the latter is less important in large than in small diameter risers. Bypassing of the gas also plays an important role with the second inlet configuration. At the height of the solids inlet, the gas is seen to flow preferentially aside of the solids inlet and at the opposite site of the riser. This results in vortex formation, responsible for the primary mixing of gas and solids. More downstream, the maximum solids flux is calculated at the side of the solids inlet. Radial mixing is seen to be related to the granular temperature and the gas phase turbulence.

The dilute industrial scale riser simulations show that inlet and outlet configuration effects are not independent, but interact. This has an important impact on radial mixing. Inlet and outlet effects can oppose each other or cooperate, resulting in complex riser behaviour.