Velocity contactors

A differential countercurrent contactor operating with a dilute solution of the consolute component C and immiscible components A and B is shown in Figure 8. Under these conditions, the superficial velocities of the A-rich and B-rich streams can be assumed not to vary significantly with position in the contactor, and are taken to be and Ug, respectively. The concentration of C in the A-rich stream is and that in the B-rich stream is C-. ... 

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). 

Breakup in a highly turbulent field (L/velocity) ". This appears to be the dominant breakup process in distillation trays in the spray regime, pneumatic atomizers, and high-velocity pipehne contactors. 

The concepts of shp velocity and characteristic velocity are useful in defining the Flooding point and operational regions of different types of column contactors. The shp (or relative) velocity is given by the equation ... 

The value of may be identified with the average terminal velocity of the droplets within the contactor. Each different type of contactor will have a different and unique characteristic velocity. 

These relationships are not restricted to any type of contactor they can be used to predict either the flooding velocity at a given holdup. 

Contactor pressures have little effect on the glycol absorption process as k>ng as the pressures remain below. 3,000 psig. At a constant temperature liic water content of the inlet gas decreases with increasing pressure, thus less water must be removed if the gas is dehydrated at a higher pre.s-sure. In addition, a smaller contactor can be used at high pressure as the actual velocity of the gas is lower, which decreases the required diameicr of the contactor. 

Fig. 17.7. Fluidised bed adsorption of G3PDH from milled yeast homogenate onto zirconia-silica Cibacron Blue. The feedstock (20% w/v) was fed to the bead mill at a rate 4.05 dm3-h 1, which corresponded to a linear flow velocity of 250 cm-h 1 within the BRG contactor with a settled bed height of 21 cm. The disrupted baker s yeast homogenate from the bead mill was applied to the integrated fluidised bed directly and terminated when C/Ca = 0.65.

Fig. 17.8. Elution of L-asparaginase from CM HyperD LS in fluidised beds. The beds were washed with five settled bed volumes (SBVs) of buffer A at the loading flow velocity (BRG contactor 250 cm/h). Elution was achieved in fluidised bed mode at a linear flow velocity of 100 cm/h by a step of 1.0 M NaCl in buffer A.

Westerterp et al. (W5) measured interfacial areas in mechanically agitated gas-liquid contactors. The existence of two regions was demonstrated At agitation rates below a certain minimum value, interfacial areas are unaffected by agitation and depend only on nominal gas velocity and the type of gas distributor, whereas at higher agitation rates, the interfacial areas are... 

Gal-Or and Resnick (Gl) have developed a simplified theoretical model for the calculation of mass-transfer rates for a sparingly soluble gas in an agtitated gas-liquid contactor. The model is based on the average gas residencetime, and its use requires, among other things, knowledge of bubble diameter. In a related study (G2) a photographic technique for the determination of bubble flow patterns and of the relative velocity between bubbles and liquid is described. 

Foust et al. (F4) measured gas holdup in mechanically stirred gas-liquid contactors of various diameters (from 1 to 8 ft) and various liquid contents (from 5 to 2250 gal). The nominal gas velocity varied from 1 to 5 ft/min and the power input from 0.01 to 6.5 hp. The contact time (sec/ft) could be correlated by the following expression ... 

Kramers et al. (K21) measured gas residence-time distribution in a mechanically agitated gas-liquid contactor of 0.6-m diameter for various gas velocities and agitator speeds. In the region where agitation has an effect on the gas-liquid interfacial area (cf. the study by Westerterp et al. (W5), Section V,D,1), the residence-time distribution was found to resemble closely that of a perfect mixer. 

The axial dispersion coefficient [cf. Eq. (16-51)] lumps together all mechanisms leading to axial mixing in packed beds. Thus, the axial dispersion coefficient must account not only for molecular diffusion and convective mixing but also for nonuniformities in the fluid velocity across the packed bed. As such, the axial dispersion coefficient is best determined experimentally for each specific contactor. 

Technique 5. In Technique 5, the driving plate free-surface velocity always is measured with electrical pin contactors. Buildup data from all experiments on a given density of explosive are pooled on the assumption of a single curve buildup and are fitted by the least squares method to the empirical function,... 

A variant on the froth contact is the reverse jet contactor (Example 22), which can be considered as an upside-down distillation tray operated above the flooding velocity in cocurrent flow of gas and liquid. It is limited to one stage. 

Droplet systems rarely exceed a p value of 0.01. At this low level, D in a low-velocity countercurrent contactor can be approximated by Eq. (14-179). 

High-Velocity Pipeline Contactors High-velocity cocurrent flow can give more power input than any other approach. This is critical when extremely high rates of reaction quenching are needed. 

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