Packed absorption tower

Use of Mass-Transfer-Rate Expression Figure 14-3 shows a section of a packed absorption tower together with the nomenclature that will be used in developing the equations which follow. In a differential section dh, we can equate the rate at which solute is lost from the gas phase to the rate at which it is transferred through the gas phase to the interface as follows ... 

FIG. 14-5 Nnmher of overall gas-phase mass-transfer units in a packed absorption tower for constant mGf /LM solution of Eq. (14-23). (From Sherwood and Pigford, Absorption and Extraction, McGraw-Hill, New York, 1952. )... 

A packed absorption tower is used to remove S02 from an air stream by absorption in a solvent. The tower is 5 ft in diameter and 60 ft high and contains 1.5 in. plastic Pall rings. The temperature and pressure in the tower are 90°F and 30 psig. The gas stream flow rate is 6500 scfm. The liquids SG is 1.25, and its viscosity is 25 cP. 

Packed absorption towers, 25 695-696 Packed bed adsorption columns,... 

Berl Saddles. P ieces of chemical porcelain, chemical stoneware or carbon in the shape of saddles, used for packing absorption tower or distn columns. They are manufd under trade name "Intalox Saddles" by the US Stoneware Co, Akron 9, Ohio... 

Schematic diagram of a packed absorption tower Figure 6.22...

A random-packed absorption tower is to be used for removing a hydrocarbon from a... 

The off-gas from a chloral production unit contains 15 vol % Cl 75 vol % HC1, and 10 vol % EtCl2. This gas is produced at a rate of 150 cfm based on 70°F and 2 psig. It has been proposed to recover part of the Cl, by absorption and reaction in a partially chlorinated alcohol (PCA). The off-gas is to pass continuously through a packed absorption tower counter-current to the PCA, where Cl, is absorbed and partially reacts with the PCA. The gas leaving the top of the tower passes through an alcohol condenser and thence to an existing HC1 recovery unit. 

Describe how one can make a rough estimate of the required diameter and height for a randomly packed absorption tower to achieve a given degree of gas cleanup without detailed information on the properties of the dirty gas, knowing only that the absorbent is water (or has properties similar to those of water) and that the pollutant has a strong affinity with the absorbent. 

Example 9.12 A packed absorption tower is designed to removed SO2 from a coke oven stack. The stack gas flow rate measured at one atmosphere and 30°C is lOmVs, and the SO2 content is 3.0%. Using an initially pure water, 90% removal is desired. The equilibrium curve of SO2 in water may be approximated by y = 30ji . Determine the water requiranent if 150% of the minimum flow rate is deemed adequate. Calculate the height of the tower. Assume KyfU =0.024 kgmols/m s mol fraction. Assume the total cross-sectional area of tower equals 11.0 m. ... 

A packed absorption tower is designed to removed SO2 from a coke oven stack. The water flow rate L = 16.64 kgmols/s and the gas flow rate G = 0.39 kgmols/s. The mole fraction of SO2 in the air at the bottom of the tower is 0.03 and its mole fraction in the absorption water at the bottom of the tower is 0.0007. If the mole fraction of SO2 in the air at a point in the tower 7 meters from the bottom is 0.016, what is the corresponding mole fraction of the SO2 in the downward flowing scrubbing water. 

All runs were conducted at 20° C., with the exception of runs 81, 82, and 83. The pH was maintained at 11.3 0.5 in order to hold this variable constant. These runs were all conducted in the bottle-type reactor, with the exception of 73 and 74, which were conducted in the packed absorption tower. 

Wet Scrubbing in Packed Towers. The packed absorption tower has been considered and used in lime and limestone scrubbing of SO2. It has demonstrated its capabilities in most of the previously listed characteristics. However, commercially available packings have difficulty maintaining a reliable operation with calcium-based wet scrubbing because of scale deposition. 

The tower absorber section is separated from the cyclonic mist eliminator by a plate containing a conical hat. This arrangement allows the flue gas to leave the cyclonic mist eliminator but prevents the slurry leaving the packed absorption tower from combining with the slurry stream leaving the FDS. Hence, most of the fly ash entering the system is isolated from the absorption tower and is conflned to the FDS slurry stream. 

Eq. (22) defines the volumetric mass transfer coefficients KyU and Kxa, which have the typical unit moles/(hrm mole fraction). Note also that the term Adz represents the differential packed tower volume. Therefore, Eq. (22) can serve as a model to determine packing height in a packed absorption tower. 

Fig. 3 illustrates a packed absorption tower with countercurrent flow where flow rates are constant. Making a differential mass balance around the differential cross section in molar units results in Eq. (23). This equation can now be integrated from the top of the column down... 

As shown in the previous section, the diameter of a packed absorption tower depends on the quantities of gas and liquid handled, their properties, and the ratio of one stream to the other. The height of the tower, and hence the total volume of packing, depends on the magnitude of the desired concentration changes and on the rate of mass transfer per unit of packed volume. Calculations of the tower height, therefore, rest on material balances, enthalpy balances, and estimates of driving force and mass-transfer coefficients. 

MATERIAL BALANCES. In a differential-contact plant such as the packed absorption tower illustrated in Fig. 22.9, there are no sudden discrete changes in composition as in a stage-contact plant. Instead the variations in composition are continuous from one end of the equipment to the other. Material balances for the portion of the column above an arbitrary section, as shown... 

Holloway (1940) obtained the following correlation for the liquid-film mass-transfer coefficient in packed absorption towers ... 

Piche, S.R., Larachi, F, and Grandjean, B.P.A. (2001c), Improving the prediction of irrigated pressure drop in packed absorption towers, The Canadian Journal of Chemical Engineering, 79(4) 584-594. 

The height of packing required to perform a specified amount of mass transfer can be related to the mass transfer coeflicient and a material balance for the absorbed component at any poim in a tower. Referring to Fig. 6.3-2, which represents a packed absorption tower, a material balance within a differential height of packing, dz, yields... 

I. Operating-line derivation. For the case of solute A diffusing through a stagnant gas and then into a stagnant fluid, an overall material balance on component A in Fig. 10.6-6 for a packed absorption tower is... 

Figure 10.6-6. Material balance for a countercurrent packed absorption tower.

Where ferric chloride is used for leaching and converted to ferrous iron, it can be regenerated by reaction of the solution (after lead chloride removal) with chlorine from the electrolytic cell. The can be done using a packed absorption tower irrigated with depleted solution in exchange with an npflow of gas from the anode compartment of the cells. 

Operation of the collection system below atmospheric pressure facilitates the purging of chlorine vessels, pipes, etc. The risk of corrosion in dry chlorine installations by moisture from the treatment system must be excluded. The most commonly used and recommended reagent is caustic soda. The effluents are treated in an absorption system, such as packed absorption towers, venturi scrubbers, etc. An example of a flow sheet for a large plant is shown in Figure 85. 

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