DOWNCOMER MIXING AND ITS EFFECT ON DISTILLATION TRAY EFFICIENCY

In this paper, the measurement of liquid mixing in a downcomer of segmental type of distillation column is presented. The extent of liquid mixing is calculated by means of a mixing pool model. The results indicate that liquid mixing in a downcomer is actually incomplete. It is a significant correction to the assumption of complete downcomer mixing or no downcomer mixing which is generally adopted in many distillation calculations. Besides, the present results are used in a two dimensional eddy diffusion model to calculate the distillation tray efficiency. It is shown that the assumption of complete downcomer mixing is closer to the actual situation than that of no downcomer mixing.

LIQUID BACKMIXING AND CIRCULATION IN RECTANGULAR DOWNCOMER

1 INTRODUCTIONDowncomers,employed in tray columns,are widely used nowadays in industrialseparation processes.They function as channels for the liquid phase to fall fluentlyfrom one tray onto its neighbouring places for the held-up bubble to separate fromthe liquid phase.It is well known that liquid flow patterns and mixing characteristicsin downcomers are closely related to the initial liquid distribution on the tray,andhence to tray efficiency.For a long period of time,little attention has been paid

An Experimental Study on Distribution of Vertical Stress in a Silo with a Central Inner Downcomer

The distribution of vertical stress for both active and passive state in the silo with a central innerdowncomer is reported in this paper. Experimental measurement of the axial distribution of vertical stress for bothactive and passive state in the silo are in good agreement with that predicted by theoretical analysis. The meanaxial stress is reduced due to the presence of the inner downcomer in the silo.

Numerical simulation of counter-current flow field in the downcomer of a liquid-solid circulating fluidized bed

A comprehensive study on the hydrodynamics in the downcomer of a liquid-solid circulating fluidized bed （LSCFB） is crucial in the control and optimization of the extraction process using an ion exchange LSCFB. A computational fluid dynamics model is proposed in this study to simulate the counter-current two-phase flow in the downcomer of the LSCFB. The model is based on the Eulerian-Eulerian approach incorporating the kinetic theory of granular flow. The predicted results agree well with our earlier experimental data. Furthermore, it is shown that the bed expansion of the particles in the downcomer is directly affected by the superfcial liquid velocity in downcomer and solids circulation rate. The model also predicts the residence time of solid particles in the downcomer using a pulse technique. It is demonstrated that the increase in the superficial liquid velocity decreases the solids dispersion in the downcomer of the LSCFB,

Hydrodynamics in commercial-scale internally circulating fluidized beds with different central downcomer outlets

Standpipes,or downcomers,are commonly used in fluidized beds to transport particles.The outlet structure of the downcomer greatly affects the performance of flow from it and even overall reactor performance.In this study,the hydrodynamics in commercial-scale internally circulating fluidized beds(ICFBs)with central downcomers having different outlet structures was investigated using computational fluid dynamics simulations with an energy minimization multi-scale drag model.The predicted results closely agreed with experimental data.Results showed that in an ICFB with a downcomer outlet directly open to the bed(model A),nearly 12.7%to 5.4%of the gas in the draft tube bypasses into the downcomer.In the ICFB models B and C with a conic baffle below the downcomer,the gas bypass is significantly weakened or even eliminated when the diameter of the conic baffle is 1.1 times that of the downcomer(model C).In addition,the solids circulation mass flux in ICFBs increased by about 62.5%,from 126.8 kg/(m2 s)in model A to 206 kg/(m2 s)in model C.