Column hydrodynamics

Under changing flow conditions, it can be important to include some consideration of the hydrodynamic changes within the column (Fig. 3.53), as manifested by changes in the fractional dispersed phase holdup, h , and the phase flow rates, Ln and G . which, under dynamic conditions, can vary from stage to stage. Such variations can have a considerable effect on the overall dynamic characteristics of an extraction column, since variations in hn also 

A dynamic balance for the dispersed phase holdup in stage n gives 

The fractional dispersed phase holdup, h, is normally correlated on the basis of a characteristic velocity equation, which is based on the concept of a slip velocity for the drops, VgUp, which then can be related to the free rise velocity of single drops, using some correctional functional dependence on holdup, f(h). 

The normal method of correlating dispersed phase holdup is normally of the form 

Under normal circumstances, the use of a characteristic velocity equation of the type shown above can cause difficulties in computation, owing to the existence of an implicit algebraic loop, which must be solved, at every integration step length. In this the appropriate value of L or G satisfying the value of h generated in the differential mass balance equation, must be found as shown in the information flow diagram of Fig. 3.54. 

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SEC, size exclusion chromatography OTHdC, open tubular hydrodynamic chromatography PCHdC, packed column hydrodynamic chromatography ThFFF, thermal field flow fractionation. 

The above modelling approach to the column hydrodynamics is illustrated by the simulation example HOLDUP. 

Cohen-Coon controller settings 103, 508 Coils or jackets 132 Column hydrodynamics 195... 

If necessary, the implicit nature of the calculation may, however, be avoided by a reformulation of the holdup relationship into an explicit form. The resulting calculation procedure then becomes much more straightforward and the variation of holdup in the column may be combined into a fuller extraction column model in which the inclusion of the hydrodynamics now provides additional flexibility. The above modelling approach to the column hydrodynamics, using an explicit form of holdup relationship, is illustrated by the simulation example HOLDUP. 

Because of their multicomponent nature, RSPs are affected by a complex thermodynamic and difihisional coupling, which, in turn, is accompanied by simultaneous chemical reactions (57-59). To describe such phenomena adequately, specially developed mathematical models capable of taking into consideration column hydrodynamics, mass transfer resistances, and reaction kinetics are required. 

As is the case for reactors with two or more mobile phases, a variety of flow regimes exist depending primarily on the gas superficial velocity (the driver for bubble column hydrodynamics) and column diameter. A qualitative flow regime map is shown in Fig. 19-38. 

Comparison with CBD operation Using different batch times (equal to the pass time of case 1 and case 4 of Table 11.2) the dynamic optimisation problem is solved and the results are presented in Table 11.3. In all cases single reflux ratio is used for the whole operation which is optimised. Column holdup is assumed negligible (steady state model does not include column hydrodynamics and therefore holdup does not appear in the model equations, Rose (1985)) and the number of stages is kept same as those used for SPSS operation for a direct comparison of the results. 

Bubble columns have been widely used in chemical and petroleum industries. This paper presents a brief overview on the present state of art of vertically sparged bubble columns. Hydrodynamics, mixing and transport characteristics of the bubble column are briefly evaluated. Recommendations for the future experimental work are also made. 

Note that the universal calibration relations apply to polymeric solutes in very dilute solutions. The component species of whole polymers do indeed elute effectively at zero concentration but sharp distribution fractions will be diluted much less as they move through the GPC columns. Hydrodynamic volumes of solvated polymers are inversely related to concentration and thus elution volumes may depend on the concentration as well as on the molecular weights of the calibration samples. To avoid this problem, the calibration curve can be set up in terms of hydrodynamic volumes rather than molecular weights. A general relation [20] is... 

As bubble column hydrodynamics depends on the physicochemical properties of the gas-liquid system, a generalized correlation for ki, and a is not attempted. However, small-scale experiments with the system of interest will allow scale-up on the basis of equal superficial velocity of the gas. So the data in Fig. 23 or of specific experiments can be used, noting that kao, kiO, and a vary as (K2, D9). 

Magaud, F., Souhar, M., WUd, G., and Boisson, N. (2001), Experimental study of bubble column hydrodynamics, Chemical Engineering Science, 56(15) 4597-4607. 

The internals of the bubble column reactor may have a dramatic impact on the flow patterns of the bubbles and the liquid. Companies have not divulged details about the internals to date. Some details of the US DOE pilot plant (22.5 inch 0.57 m diameter) have been published [ 106]. In this report the dimensions of the cooling tubes, their location, and their number are provided. These cooling coils occupied about 10% of the total volume of their commercial reactors slurry volume. The gas holdup and bubble characteristics as well as their radial profiles were determined in a column that was about the size of the US DOE reactor [107-109]. Dense internals were found to increase the overall gas holdup and to alter the radial gas profile at various superficial gas velocities. The tube bundle in the column increased the liquid recirculation and eliminated the rise of bubbles in the wall region of the column. These results indicate that further studies of bubble column hydrodynamics are directed toward larger scale units equipped with heat exchange tubes. 

In the opinion of this author it is unlikely that the fundamental resolution and distribution capabilities of PCS measurements will be dramatically increased, though it has been proposed that simultaneous data reduction of multiangle ACFs will do just that. To date, multiangle measurements have yielded incremental advantages in special situations without pushing the resolution of the technique into the realm of its competitors disc centrimgation, column hydrodynamic fractionation, sedimentation field flow fractionation, and electron microscopy. 

Van den Akker HEA Liquid-liquid spray columns hydrodynamic stability and reduction of axial mixing, PhD thesis. The Netherlands, 1978, Eindhoven University of Technology. 

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