Counter-current backmixing

An Initial Value Approach to the Counter-Current Backmixing Model of the Fluid Bed... 

The counter-current backmixing model of Fryer and Potter has been modified by assuming mixed flow in the emulsion phase. The terminal conversions obtained with the present model are compared with those of the original model and found to agree well except at very low values of bubble diameter. The assumption of complete mixing in the emulsion phase converts the original two-point boundary value problem into a simpler initial value problem, thereby considerably reducing the mathematical complexity. 

Jayaraman V.K., B.D. Kulkarni and L.K. Doraiswamy. An Initial Value Approach to the Counter-Current Backmixing Model of the Fluid Bed. A.C.S. Symp. Series, 168 (1981) 19-29. 

The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid-solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters. During the past decade, extensive research efforts have been made in order to improve our know-how in these areas. Chapters 6 to 8 present a unified review of the reported studies on these aspects for a variety of fixed bed columns (i.e., co-current downflow, co-current upflow, and counter-current flow). Chapter 9 presents a similar survey for three-phase fluidized columns. 

An often used gas-liquid reactor is the bubble column. The gas is usually fed from the bottom through a sparger and the liquid flows either cocurrently or counter-currently. Counter-current operation is more efficient than co-current, but for certain types of parallel reactions, cocurrent operation can give better selectivity. Bubble columns are often operated in semi-batch mode the gas bubbles through the liquid. This mode of operation is attractive in the production of fine chemicals which are produced in small quantities - especially in the case of slow reactions. The flow patterns can vary a lot in a bubble column. Generally, as a rule of thumb, the liquid phase is more back-mixed than the gas phase. The plug flow model is suitable for the gas phase whereas the liquid phase can be modelled with the backmixed, dispersion, or plug flow model. 

Continuous, (stagewise) counter-current, Cross-flow Plug flow Completely backmixed ( ) Each plate of a plate column Sectional zed bubble column... 

Columns with gas distributor (porous sintered plate, perforated plate, sieve tray, spigots, nozzles), gas flows cocurrently or counter-currently through a continous liquid phase in form of bubbles or gas jets homogenous (quasi-laminar) bubble flow, is characterized by a large interfacial area and little backmixing with equal bubbles at a superficial gas velocity Wg < 0.05 m/s. Bubble coalescence to form larger bubble, bubble splits, heterogenous (turbulent) bubble flow may be observed at Wg > 0.05 m/s. = 1 m/s gas load-... 

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