Extraction tower

The I2 formed stays in solution, exerting a certain vapor pressure, and is extracted from the brine in a countercurrent air blow-out process. The extracted brine leaves the extraction tower and is discarded or reinjected into the wells to avoid sinking of the soil. The iodine-loaded air is then submitted to a cocurrent desorption process by means of an acidic iodide solution to which SO2 is added. By this solution the iodine is reduced to iodide by the following reaction ... 

The process (Fig. 3) is a countercurrent Hquid-Hquid extraction. The feedstock is introduced near the top of an extraction tower and the Hquid propane near the bottom, using solvent-to-oil ratios from 4 1 to 10 1. The deasphalted oil—propane solution is withdrawn overhead and the asphalt from the bottom, and each is subsequently stripped of propane. 

The height equivalent to a theoretical stage (HETS) in an extraction tower is simply the height of the tower Zt divided by the number of theoretical stages achieved [Eq. (15-29)]. 

The mass-transfer coefficients depend on complex functions of diffii-sivity, viscosity, density, interfacial tension, and turbulence. Similarly, the mass-transfer area of the droplets depends on complex functions of viscosity, interfacial tension, density difference, extractor geometry, agitation intensity, agitator design, flow rates, and interfacial rag deposits. Only limited success has been achieved in correlating extractor performance with these basic principles. The lumped parameter deals directly with the ultimate design criterion, which is the height of an extraction tower. 

Kiihni Tower The extraction towers designed at Kiihni [see Mogh and Biihlmann, in Lo, Baird, and Hanson (eds.). Handbook of Solvent Extraction, Wiley-lnterscience, New York, 1983, sec. 13.5]... 

Transition metal oxides or their combinations with metal oxides from the lower row 5 a elements were found to be effective catalysts for the oxidation of propene to acrolein. Examples of commercially used catalysts are supported CuO (used in the Shell process) and Bi203/Mo03 (used in the Sohio process). In both processes, the reaction is carried out at temperature and pressure ranges of 300-360°C and 1-2 atmospheres. In the Sohio process, a mixture of propylene, air, and steam is introduced to the reactor. The hot effluent is quenched to cool the product mixture and to remove the gases. Acrylic acid, a by-product from the oxidation reaction, is separated in a stripping tower where the acrolein-acetaldehyde mixture enters as an overhead stream. Acrolein is then separated from acetaldehyde in a solvent extraction tower. Finally, acrolein is distilled and the solvent recycled. 

Figure 1. First Edeleanu Continuous Extraction Towers...

The advantages that may justify the additional costs of reflux at the bottom of a solvent extraction tower are illustrated in Figure 6. Minimum reflux is represented by the tie line from / to fe. Maximum reflux would be represented by the line / to the extract layer which would exist for infinite solvent to feed ratio. Practical operation of the equipment will fall between the limits of minimum and maximum reflux, as represented by fy. The operating point for the enriching section of the extraction column is located by an intersection between the lines erE and fy, and the reflux ratio is the ratio of the distances k e /k e (22). 

During recent years pilot scale equipment, smaller than the prototype pilot plants described and capable of operating with exceedingly high efficiency, has been designed. Such equipment as the York-Scheibel solvent extraction tower and the Podbielniak countercurrent centrifugal mixer and extractor are typical. Data from this equipment may be correlated with commercial performance. 

An important feature in the construction of continuous countercurrent extraction towers is the design of the nozzles through which the solvent and oil feed enter. Nozzles should be designed to distribute the stream evenly over the cross-sectional area of the tower in order to minimize channeling and to make maximum use of the packing near the inlet. Consideration should also be given to the use of intercoolers to enforce a temperature gradient. 

It certainly is apparent that today s countercurrent packed extraction tower is not the ultimate in contacting and separating equipment. These huge towers, 40 to 80 feet in height, are often equivalent to only 1.5 to 4 theoretical stages. The desirability of a more compact, economical, efficient device is self-evident. 

Figure 3.16. Extraction tower control, (a) Operation with heavy solvent, interface in the upper section, top liquid level on LC. (b) Same as part (a) but with overflow weir for the light phase, (c) Same as part (a) but with completely full tower and light phase out at the top. (d) Operation with interface on ILC in the lower section, removal of the light phase from the upper section by any of the methods of (a), (b), or (c).

Figure 3.17. Some other controls on extraction towers, (a) Solvent flow rate maintained in constant ratio with the feed rate, (b) Solvent flow rate reset by controlled composition of raffinate, (c) Temperature of solvent or feed reset by the temperature at a control point in the tower.

The other mixing operations of the list require individual kinds of equipment whose design in some cases is less quantified and is based largely on experience and pilot plant work. Typical equipment for such purposes will be illustrated later in this chapter. Phase mixing equipment which accomplishes primarily mass transfer between phases, such as distillation and extraction towers, also are covered elsewhere. Stirred reactors are discussed in Chapter 17. 

Application of the rules given here for sizing extraction towers without mechanical agitation is made in Example 14.10. The results probably are valid within only about 25%. The need for some pilot plant information of the particular system is essential. 

At 313 K and 8.4 MPa, the slope of extract phase solubility versus pressure is 0.06 weight fraction oil/MPa. For a 15 m tall extraction tower operated at a density of 0.5 g/cm3, the pressure at the bottom is higher than that at the top by 0.075 MPa. The solubility at the bottom will then be 0.15 wt% higher at the bottom... 

Fig. 83. Production diagram of trimethylborate 1, 3, 5 - batch boxes 2 - filter 4 -coolers 6, 9, 11, 13, 14,18, 19 - collectors 7 - rectification tower 8 - apparatus for preparing the solution 10 - synthesis tower 12 - extraction tower 15, 17-containers 16- distillation tank...

The purest caustic solution is obtained by extraction with ammonia. The initial 50 per cent caustic solution is led to the top of an extraction tower and flows countercurrently to a mixture of 75 per cent ammonia and 25 per cent water which rises from below. The two solutions are immiscible. The ammonia solution extracts almost completely the chloride and sodium chlorate from the caustic liquor so as to leave no more than 0.08 per cent NaCl (related to 100 per cent NaOH) and practically no chlorate. Ammonia regeneration equipment complements the extraction unit. If anhydrous liquid ammonia is used instead of an ammonia solution a certain degree of concentration of the caustic solution can be achieved at the same time as ammonia also extracts water from the caustic solution. 

FIGURE 14 Sieve-tray extraction tower arranged for light liquid dispersed. Reprinted from Treybal5 with permission of The McGraw-Hill Companies. 

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