Stripping Towers

Volatile removal is a function of the air/Hquid ratio, and media height is shown in Figure 8. Typical stripping towers are shown in Figure 9. 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

More often than not the rate at which residual absorbed gas can be driven from the liqmd in a stripping tower is limited by the rate of a chemical reaction, in which case the liquid-phase residence time (and hence, the tower liquid holdup) becomes the most important design factor. Thus, many stripper-regenerators are designed on the basis of liquid holdup rather than on the basis of mass transfer rate. 

Approximate design equations apphcable only to the case of pure physical desorption are developed later in this sec tion for both packed and plate stripping towers. A more rigorous approach using distiUation concepts may oe found in Sec. 13. A brief discussion of desorption with chemical reac tion is given in the subsec tion Absorption with Chemical Reaction. ... 

Mass-transfer theory indicates that for trays of a given design the factors most hkely to inflnence E in absorption and stripping towers are the physical properties of the flnids and the dimensionless ratio Systems in which the mass transfer is gas-film-controlled may be expected to have plate efficiencies as high as 50 to 100 percent, whereas plate efficiencies as low as 1 percent have been reported for the absorption of gases of low sohibility (large m) into solvents of relatively high viscosity. 

The fat oil is fed to a splitter or stripping tower, where the absorbed tight constituents are separated from the oil by distillation. Usually the lean oil is the same material as the heavier part of the absorber feed, so that the bottoms from the stripper are split into lean oil, which is recycled to the absorber, and a stabilized gasoline product, which is passed on to subsequent processing operations. 

Figure 8-55. Schematic stripping tower using air to strip organics from water solution. Adapted and used by permission, Li, K. Y. and Hsiao, K. J., Chem. Eng., V. 98, No. 7 (1991) p. 114.

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. 

The objective of the primary absorber/stripping towers is to maximize recovery of C3 and heavier components while rejecting and lighter to fuel, C3 is first absorbed and then and lighter components are stripped. Although maximizing C3-C4 recovery for alkylate feed is very profitable, lower recoveries are often accepted to maximize the FCC conversion and/or feed rate. 

Integrated vapor extraction and steam vacuum stripping can simultaneously treat groundwater and soil contaminated with VOCs. The system developed by AWD Technologies consists of two basic processes a vacuum stripping tower that uses low-pressure steam to treat contaminated ground-water and a soil gas vapor extraction/reinjection process to treat contaminated soil. The two processes form a closed-loop system that provides simultaneous in situ remediation of contaminated groundwater and soil with no air emission. 

Many refineries now use vacuum pumps and surface condensers in place of barometric condensers to eliminate generation of the wastewater stream and reduce energy consumption. Reboiled side-stripping towers rather than open steam stripping can also be utilized on the atmospheric tower to reduce the quantity of sour-water condensate. 

Install stripping towers for solvent removal (recover solvents wherever possible) Conduct a program of sampling and testing solvents on wastewater flows ... 

Reforming is a relatively clean process. The volume of wastewater flow is small, and none of the wastewater streams has high concentrations of significant pollutants. The wastewater is alkaline, and the major pollutant is sulfide from the overhead accumulator on the stripping tower used to remove light hydrocarbon fractions from the reactor effluent. The overhead accumulator catches any water that may be contained in the hydrocarbon vapors. In addition to sulfides, the wastewater contains small amounts of ammonia, mercaptans, and oil. 

The costs of three sizes of a proprietary dual-phase extraction system were estimated in 1991. The costs associated with Radian International s AquaDetox/SVE system (see summary T0641) are detailed in Table 1. The system uses a moderate vacuum stripping tower and low-pressure... 

Dispersion is another reactor mixing topic that will be discussed in Chapter 6. Dispersion normally is used when cross-sectional mean concentrations and velocities are being computed. A cross-sectional mean concentration is useful for a pipe, stripping tower, river, or groundwater transport. 

EXAMPLE 6.10 Air-Stripping tower with first-order degradation, modeled as plugfiow, plug flow with dispersion, and mixed tanks-in-series reactors... 

The air-stripping tower, illustrated in Figure E6.10.1, provides the air-water contact area, either through a porous medium that is unsaturated with water, through bubbles rising through the water or both. The polluted water comes in at the top (Co), and cleaner water comes out of the bottom of the stripping tower, while clean air comes in at the bottom, and air with trichloroethylene in it comes out the top of the tower. This... 

Table E6.10.1 Time response of the air-stripping tower output after a conservative tracer pulse input att = 0...

The plug flow reactor model predicts that there will be 1 % of the trichloroethylene remaining at the bottom of the stripping tower. 

Now, we need a solution to the plug flow with dispersion model for steady-state operation of an air-stripping tower. The mass transport equation for this situation, assuming minimal trichloroethylene builds up in the bubble, is... 

Steady-State operation of a stripping tower with first-order degradation, equation (E6.5.7) gives... 

The 160°F BFW is efficiently mixed with the incoming steam, in what is effectively a small, vertical stripping tower, mounted above the large deaerator drum. The majority of the steam condenses by direct contact with the 160°F BFW and in so doing, the latent heat of condensation of the steam is used to increase the sensible-heat content of the 160°F BFW to 230°F. 

A small amount of steam is vented from the top of the stripping tower to the atmosphere. Using a gate valve, with a hole drilled through the gate, is a simple way to control the venting rate. The dissolved air in the cold BFW is vented with this steam. 

The pressure inside the deaerator started to drop, as there was not enough steam flow to keep the water in the drum at its boiling point. The reduction in the deaerator pressure increased the volume of steam flow through the bottom tray of the stripping tower. Why ... 

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