Phase countercurrent

Hsieh and McNulty [210] developed a new correlation for weeping of sieve and valve trays based on experimental research and published data. For sieve trays the estimation of the weeping rate and weep point is recommended using a two-phase countercurrent flow limitation model, CCFL. 

Nonagitated three-phase countercurrent-flow reactors (spouted-bed reactors)... 

In the less turbulent flow through the straight channel of a monolith, momentum transfer from the fluid to the wall is less effective and in the case of two-phase, countercurrent annular flow, momentum transfer between gas and liquid will also be less than in the interstitial channels of a packed bed. The lower rates of momentum transfer, which is the reason for the higher permeability of monoliths, should in principle improve the possibility for achieving countercurrent flow of gas and liquid at realistic velocities. 

Billet, R., and Schultes, M. (1993), Physical model for the prediction of liquid hold-up in two-phase countercurrent columns, Chemical Engineering Technology, 16(6) 370-375. 

Flow number to characterize the two phase countercurrent flow in packed columns... 

As multistage extraction with phase countercurrent flow... 

Hochman, J. M. and E. Effron. Two-Phase Countercurrent Downflow in Packed Beds. Ind. Eng. Chemistry Fundamentals,... 

An alternative model for slugging fluidized bed reactors was proposed by Raghuraman and Potter (48). This Is an extension of their three-phase (bubbles, cloud-wake phase, emulsion phase) countercurrent backmlxlng model (21) which accounts for gas downflow In the dense-phase, said to occur for slugging beds when U > 2.5 U f Agreement with experimental results Is said (48,49) to be somewhat better for this model than for the Hovmand-... 

Most mass transfer equipments consist of gas (vapor) and liquid two-phase flow, for instance, vapor-liquid two-phase cross-current flow is undertaken in tray distillation column gas-liquid two-phase countercurrent flow is taken place in packed absorption column. Some processes may also include solid phase, such as adsorption or catalytic reaction. Thus, the fluid system may contain gas and liquid two phases, or gas, liquid single phase besides solid phase. 

We have seen in Section 8.1.1.3 that in a two-phase countercurrent flow system, where an equilibrium separation process is going on, the separation system lacks... 

Billet and Schultes [314] developed a plqrsical model for prediction of liquid holdup in two-phase countercurrent columns. The model is valid firr random and structured packinp and requires an experimental constant depending on the packing tyj and dimensions. Fru calculation of the tot liquid holdup at the flooding point Htp, the same authors [316] offered the equation ... 

R Billet, M. Schultes, Capacity Studies of Gas-Liquid Two Phase Countercurrent Flow Columns , I. Chem. E. Symp. Ser. No. 104, B255,(1987). 

Let s assume that the solute to be separated is present in an aqueous phase of 1 M HCl and that the organic phase is benzene. Because benzene has the smaller density, it is the upper phase, and 1 M HCl is the lower phase. To begin the countercurrent extraction the aqueous sample containing the solute is placed in tube 0 along with a portion of benzene. As shown in figure A6.1a, initially all the solute is present in phase Lq. After extracting (figure A6.1b), a fraction p of the solute is present in phase Uq, and a fraction q is in phase Lq. This completes step 0 of the countercurrent extraction. Thus far there is no difference between a simple liquid-liquid extraction and a countercurrent extraction. 

In a countercurrent liquid-liquid extraction the lower phase in each tube remains in place, and the upper phase moves from tube 0 to higher numbered tubes. This difference in the movement of the phases is indicated by referring to the lower phase as a stationary phase and the upper phase as a mobile phase. With each transfer some of the solute in tube r is moved to tube r -I- 1, and a portion of the solute in tube r - 1 is moved to tube r. As a result, a solute introduced at tube 0 moves with the mobile phase. The solute, however, does not move at the same rate as the mobile phase since, at each step, a portion of the solute is extracted into the stationary phase. A solute that is preferentially extracted into the stationary phase spends proportionally less time in the mobile phase and moves at a slower rate. As the number of steps increases, solutes with different values of q separate into completely different sets of extraction tubes. 

Two solutes, A and B, with distribution ratios of 9 and 4, respectively, are to be separated by a countercurrent extraction in which the volumes of the upper and lower phases are equal. After 100 steps, determine the 99% confidence interval for the location of each solute. 

Fig. 17. Effect of axial dispersion in both phases on solute distribution through countercurrent mass transfer equipment. A, piston or plug flow B, axial...

A hypothetical moving-bed system and a Hquid-phase composition profile are shown in Figure 7. The adsorbent circulates continuously as a dense bed in a closed cycle and moves up the adsorbent chamber from bottom to top. Liquid streams flow down through the bed countercurrently to the soHd. The feed is assumed to be a binary mixture of A and B, with component A being adsorbed selectively. Feed is introduced to the bed as shown. 

The heights of a transfer unit ia each phase thus contribute to the overall heights of a transfer unit. Data on values of HTU for various types of countercurrent equipment have been reviewed (1,10). In normal operating practice, the extraction factor is chosen to be not greatiy different from unity, within the range of 0.5—2. 

However, in a countercurrent column contactor as sketched in Figure 8, the holdup of the dispersed phase is considerably less than this, because the dispersed drops travel quite fast through the continuous phase and therefore have a relatively short residence time in the equipment. The holdup is related to the superficial velocities U of each phase, defined as the flow rate per unit cross section of the contactor, and to a sHp velocity U (71,72) ... 

As the throughput in a contactor represented by the superficial velocities and is increased, the holdup / increases in a nonlinear fashion. A flooding point is reached at which the countercurrent flow of the two Hquid phases cannot be maintained. The flow rates at which flooding occurs depend on system properties, in particular density difference and interfacial tension, and on the equipment design and the amount of agitation suppHed (40,65). 

Coalescence and Phase Separation. Coalescence between adjacent drops and between drops and contactor internals is important for two reasons. It usually plays a part, in combination with breakup, in determining the equiHbrium drop si2e in a dispersion, and it can therefore affect holdup and flooding in a countercurrent extraction column. Secondly, it is an essential step in the disengagement of the phases and the control of entrainment after extraction has been completed. 

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

Commercial Extractors. Extractors can be classified according to the methods appHed for interdispersing the phases and producing the countercurrent flow pattern. Eigure 11 summarizes the classification of the principal types of commercial extractors Table 3 summarizes the main characteristics. 

Commercial soy protein concentrates typically contain 70 to 72% cmde protein, ie, nitrogen x 6.25, dry wt basis. Soy protein isolates are prepared from desolventhed, defatted flakes. A three-stage aqueous countercurrent extraction at pH 8.5 is used to disperse proteins and dissolve water-soluble constituents. Centrifugation then removes the extracted flakes, and the protein is precipitated from the aqueous phase by acidifying with HCl at pH 4.5. 

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