Packed towers example

Packed Tower Example. Equation (17-17) will be used to estimate the H.E.T.P. values for the atmospheric distillation of a benzene-toluene mixture in a packed tower 5 ft. in diameter. The liquid and vapor rates are 480 and 400 lb. mols per hr., respectively. The calculation will be made for the section near the bottom of the column where the liquid and vapor are essentially toluene. 

Types of air strippers include packed towers, tray towers, and spray towers. Packed towers are packed or filled with small forms made of polyethylene [9002-88-4] stainless steel, poly(vinyl chloride) (PVC) [9002-86-2] or ceramic that provide large surface area to volume ratios which increase transfer rates into the air stream. Packed towers operate in countercurrent mode, that is, the aqueous stream enters at the top of the tower while air is blown in from the bottom. An example of this type of unit is shown in Figure 1. Channeling or short circuiting of the aqueous stream is minimized by... 

When it is known that Hqg varies appreciably within the tower, this term must be placed inside the integr in Eqs. (5-277) and (5-278) for accurate calculations of hf. For example, the packed-tower design equation in terms of the overall gas-phase mass-transfer coefficient for absorption would be expressed as follows ... 

The stagnant-film model discussed previously assumes a steady state in which the local flux across each element of area is constant i.e., there is no accumulation of the diffusing species within the film. Higbie [Trans. Am. Jn.st. Chem. Eng., 31,365 (1935)] pointed out that industrial contactors often operate with repeated brief contacts between phases in which the contact times are too short for the steady state to be achieved. For example, Higbie advanced the theory that in a packed tower the liquid flows across each packing piece in laminar flow and is remixed at the points of discontinuity between the packing elements. Thus, a fresh liquid surface is formed at the top of each piece, and as it moves downward, it absorbs gas at a decreasing rate until it is mixed at the next discontinuity. This is the basis of penetration theoiy. 

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

For example, the packed-tower design equation for a dilute system in which gas-phase reaciant A is being absorbed and reacted with liquid-phase reagent B is... 

Example 8-32 Strippii Dissolved Organics from Water in a Packed Tower Using Method of Li and Hsiao [143]... 

Figure 9-29. LHC plant stripper using packed tower for Example 9-1.

Example 9-7 Koch-Sulzer Packing Tower Sizing (used by permission, Bulletin KS-1, Koch Engineering Co. Inc.)... 

Example 9-13. Desdgn a Packed Tower Usii Caustic to Remove Carbon Dioxide from a Vent Stream... 

For purely physical absorption, the mass transfer coefficients depend on the hydrodynamics and the physical properties of the phases. Many correlations exist, for example that of Dwivedi Upadhyay (IEC Proc Des Dev 16 157, 1977) for packed towers,... 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

This equilibrium-stage concept based on a transfer unit has been proposed for such continuous-contacting devices as, for example, packed towers. 

To determine the mass-transfer rate, one needs the interfacial area in addition to the mass-transfer coefficients. The literature on tower packings, for example, normally reports kGpa values measured at very low inlet-gas concentrations, so that yBM — 1, and at a total pressure close to 1 atmosphere. Thus, the correct rate coefficient for use in packed-tower design involving the use of the driving force (y-y /yBM is obtained by multiplying the reported kGpa values by the value of Pj employed in the actual test unit (here 1 atm = 101.3 kPa) and not the... 

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

A packed tower with 3 in. metal pall rings will be analyzed for the system of Example 13.15. The packing factor is F = 15 sqft/cuft. 

An example of industrial relevance is the removal of sulfur dioxide (S02) from vent gases by absorption into water or a lime slurry (48). In the water absorption process, both gas-film and liquid-film resistance to mass transfer occurs. As a result the overall mass transfer rate is proportional to gas-flow rate and acceleration but inversely proportional to liquid-flow rate. Due to the fast reaction of S02 with lime, this system is only gas-film diffusion limited. The overall mass transfer rate is largely unaffected by gas- or liquid-flow rate and is proportional to acceleration, but to a lesser extent than the water absorption process. In both cases the overall mass transfer rate is reportedly much higher than the corresponding conventional packed towers. 

Figure 8.16 (Continued) Typical efficiency characteristics of packed towers, (c) Typical efficiency characteristics for wire-mesh structured packings (d) example of efficiency characteristics, measured for randem packing, that deviate from those in parts a and c. (Fart d from J. S. Eckert and L. F. Walter, Hydrocarb. Proc. Pet. Ref, February 1964. Reprinted courtesy of Hydrocarbon Processing.)...

Example S.2 For the depropanizer in Examples 2.4 (Sec. 2.3.1), 3.4 (Sec. 3.2.5). and 6.1 (Sec. 6.5.2), would a packed tower be better than a tray tower Proeeas loads and physical properties are the same as those in Table 6.10. The service ia nonfouling, the streams have a negligible zolid content, the corrosive tendency is low. and preeeure surges are unlikely. 

Example 9.2 asked (Sec. 9.4.2) whether a packed tower would be better than trays for the depropanizer. The design with trays is in Sec. 6.5.11. For the comparison, it is assumed that filter evaluation of the alternate design (with third-generation packing Sec. 9.4,9) it was found to be sound and not too tight. 

Comment. In this example, a tray tower Is slightly shorter, and otherwise much the same size as a packed tower. In such a case, a tray tower will be preferred both because of the height saving and due to the smaller cost of trays compared to packings plus distributors. Also, the use of a trayad tower in this example will reduce the degree of uncertainty regarding performance prediction. 

Example 6 Estimation of pressure drop in packed tower. A column 2 ft in... 

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