Countercurrent hydrodynamics

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

Recently, hydrodynamic aspects of multiphase applications have been studied in detail for the cocurrent and countercurrent flow regimes. Useful correlations were determined and it was found that foams combine high rates and low pressure drop, proving their high potential in multiphase applications [9, 107, 108]. 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Section 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the mass transfer capacity coefficient term, now used in the model equations, must however be a realistic value corresponding to the hydrodynamic conditions, actually existing within the column and, of course, will be substantially less than that leading to an equilibrium condition. 

In most industrial applications, multistage countercurrent contacting is required. The hydrodynamic driving force necessary to induce countercurrent flow and subsequent phase separation may be derived from the differential effects of either gravity or centrifugal force on the two phases of different densities. Essentially there are two types of design by which effective multistage operation may be obtained ... 

Bisschops et al. have investigated the hydrodynamic capacity and solids holdup in countercurrent two-phase flow in the centrifugal field, as well as the relation between the pressure drop and void fraction. Moreover, the analysis included a check as to whether the two-phase flow in the centrifugal field was homogeneous or heterogeneous. 

The gas and liquid can each flow countercurrently, cocurrently downwards or cocurrently upwards. The hydrodynamics and the mass-transfer conditions are different in each of these flow conditions. 

A major conclusion from these relatively few studies is, that it is necessary to promote the investigation of the hydrodynamics of countercurrent fixed beds and the physical properties of... 

R. Stockfleth and G. Brunner, Hydrodynamic behaviour of aqueous systems in countercurrent columns at high pressure, 279-283, GVC-Fachausschuss High Pressure Chemical Engineering, Karlsruhe, March 3-5 (1999). 

B. Benadda, K. Kafoufi, P. Monkam and M. Otterbein, Hydrodynamics and mass transfer phenomena in countercurrent packed column at elevated pressures, Chem. Engng. Science, in press. 

An inactive countercurrent test was first carried out in the G1 facility at the CEA Marcoule in a battery of eight centrifugal contactors using a synthetic nitric acid solution containing Nd(III) and Eu(III) at 2 g/L and 100 mg/L, respectively, to assess the hydrodynamic behavior of the system, which appeared to be satisfactory (206). 

Bisschops MAT, Luyben K, van der Wielen L. Hydrodynamics of countercurrent two-phase flow in a centrifugal field. AIChE J 2001 47(6) 1263—1276. 

Lebens PJM, Stork MM, Kapteijn F, Moulijn JA. Hydrodynamics and mass transfer issues in a countercurrent gas-liquid internally finned monolith reactor. Chem Eng Sci 1999 54 2381-2389. 

Lebens PJM, Meijden Rvd, Edvinsson RK, Kapteijn F, Sie ST, Moulijn JA. Hydrodynamics of gas-liquid countercurrent flow in internally finned monolithic structures. Chem Eng Sci 1997 52 3893-3899. 

Flooding Hydrodynamic instability occurring in continuous countercurrent extraction due to excessive flow rates supplied to the process. 

In monolithic catalyst carriers with wider channels, the hquid forms a film on the channel walls, whereas in the core of the channel a continuous gas phase exists. As shown by Lebens [10], countercurrent gas-liquid operation is now possible, and shows certain advantages over the countercurrent trickle bed operation. Typical channel diameters are 3-5 mm, and the geometric surface areas are between 550 and 1000 m2 m 3. Below the flooding point, almost no hydrodynamic interaction between the gas and hquid can be observed for example, the RTD is the same for both co-current and countercurrent operation. Apart from some surface waves, the film flow is completely laminar. 

The hydrodynamics control the mass transfer rate from gas to liquid and the same from liquid to the solid, often catalytic, particles. In concurrently operated columns not only the gas-continuous flow regime is used for operation as with countercurrent flow, but also the pulsing flow regime and the dispersed bubble flow regime (2). Many chemical reactors perform at the border be-... 

The countercurrent column is considered as a cascade of cells in series. The adsorption equilibrium is assumed to be reached in each cell or equilibrium stage or plate. The broadening effects, linked to the mass transfer kinetics and to the hydrodynamics, are lumped together and are quantified by the number of theoretical plates N, which can be derived from an analytical pulse injection. 

The density of the liquid phase of the system oleic acid/carbon dioxide increases with increasing pressure, whereas the density of the liquid phase of the system oleic acid/ethane decreases with increasing pressure. Therefore the hydrodynamics in a countercurrent column and mass transfer may dramatically depend on the activity of the near-critical fluid. 

In industry, many installations operate under pressure for technical and/or economic reasons. Although many hydrodynamic studies have been reported for liquid-gas contactors at atmospheric pressure, there are few concerning elevated pressure. The aim of this study is to investigate the influence of pressure on fluid flow in a packed column operating with a gas-liquid countercurrent. 

The first mode of operation described above possesses certain shortcomings. There is a tendency towards channeling and backmixing of liquid which makes true countercurrent operation impossible. Good design can minimize these channeling problems.5,36,9S An alternative to this mode of operation is a three-phase spouted-bed column. The hydrodynamics (i.e., pressure drop, bed expansion, and the liquid holdup) of this type of column have been studied by Vukovic et... 

Therefore, the hydrodynamic behavior of gas and liquid in countercurrent flow has been studied in single tubes provided with internal fins. The tubes were made of transparent polycarbonate to allow visual inspection and video recording of the phenomena observed. The tubes were typically 1 m long and were mounted in a vertical position. Liquid was fed to the top end of the tube, while gas was fed in at the bottom end. 

To obtain a better idea of the latter limits, a special measure was taken to avoid the interaction of liquid and gas at the channel outlet viz., gas was introduced through a capillary inserted in the bottom end of the channel, and the liquid was drained through a wad of mineral wool, as shown in Fig. 14. Although this solution may not be of industrial interest, it is helpful in understanding the hydrodynamic behavior of countercurrent flow in the internally finned channel in the absence of inlet and outlet effects. 

Unlike fixed bed designs where scale up data may be obtained semi-empirically, continuous countercurrent ion exchange plant requires model hydrodynamic data for both the liquid and resin phases as well as predetermined equilibrium and kinetic data for a chosen system. A continuous cycle becomes particularly attractive when required to treat more highly concentrated liquors or operate at high treatment flowrates. 

Open-column chromatography with silica gel and alumina is not applicable to the fractionation of tanins because of their strong binding to these adsorbents, which induces extensive loss of tannins. Such losses do not occur with countercurrent chromatography, as it does not use a solid stationary phase. Such molecules are very polar, so butanol-based solvent systems can be used. Centrifugal partition chromatography is more adequate in this case, as compared to hydrodynamic CCC, because of the good retention of the stationary phase of a such solvent system. 

---