Absorption countercurrent

FIG. 5-27 Identification of concentrations at a point in a countercurrent absorption tower,... 

For dilute systems in countercurrent absorption towers in which the equilibrium curve is a straight line (i.e., yj = mXi) the differential relation of Eq. (14-60) is formulated as... 

An acetone-air mixture containing 0.015 mole fraction of acetone has the mole fraction reduced to 1 per cent of this value by countercurrent absorption with water in a packed tower. The gas flowrate G is 1 kg/m2s of air and the water flowrate entering is 1.6 kg/m2s. For this system, Henry s law holds and ye = 1.75x, where ye is the mole fraction of acetone in the vapour in equilibrium with a mole fraction v in the liquid. How many overall transfer units are required ... 

Sulphur dioxide is recovered from a smelter gas containing 3.5 per cent by volume of SO2, by scrubbing it with water in a countercurrent absorption tower. The gas is fed into the bottom of the tower, and in the exit gas from the top the SO2 exerts a partial pressure of 1.14 kN/m2. The water fed to the top of the tower is free from SO2, and the exit liquor from the base contains 0.001145 kmol SC /kmol water. The process takes place at 293 K, at which the vapour pressure of water is 2.3 kN/m2. The water flow rate is 0.43 kmol/s. 

Figure 13.15. Mechanism, nomenclature, and constructions for absorption, stripping and distillation in packed towers, (a) Two-film mechanism with equilibrium at the interface, (b) Sketch and nomenclature for countercurrent absorption or stripping in a packed tower, (c) Equilibrium and material balance lines in absorption, showing how interfacial concentrations are found, (d) Equilibrium and material balance lines in stripping, showing how interfacial concentrations are found, (e) Equilibrium and material balance lines in distillation, showing how interfacial concentrations are found.

Figure 6.18 shows the schematic diagram of a countercurrent absorption column with the molar flow rates shown symbolically at each of its N trays. 

In this chapter we have presented multistage systems with special emphasis on absorption processes. We have studied multitray countercurrent absorption towers with equilibrium trays for both cases when the equilibrium relation is linear and when it is nonlinear. This study was accompanied by MATLAB codes that can solve either of the cases numerically. We have also introduced cases where the trays are not efficient enough to be treated as equilibrium stages. Using the rate of mass transfer RMT in this case, we have shown how the equilibrium case is the limit of the nonequilibrium cases when the rate of mass transfer becomes high. Both the linear and the nonlinear equilibrium relation were used to investigate the nonequi-librium case. We have developed MATLAB programs for the nonequilibrium cases as well. 

FIGURE 1.7 The blood drainage of the villous and the countercurrent absorption resistance. 

The absorption column is required to absorb nitrous reaction gases thus producing 80% (wt.) nitric acid product (dissolved impurity-free basis). This is achieved by the countercurrent absorption of the nitrogen oxide components from the reaction gas into a water/weak-acid media. The column specification requires an operating pressure of 950 kPa and an absorption temperature in the range of 10°C to 65°C. 

CO is extracted by countercurrent absorption in a column operating at about 2.10 Pa absolute, with a feedstock inlet temperature of about 40 C and effluent exit around 65 C. This is accompanied by the physical dissolution of small amounts of other constituents, including hydrogen, which are salted out by cooling and expansion of the extract at 0.5, 10 Pa absolute Thecomple.x obtained is then preheated to 100 to 105 C and sent to a regeneration column. In this column, operating at 0.15.10 Pa absolute, CO is liberated... 

The absorption of ozone by unagitated batch cyanide solutions at atmospheric pressure involves the sequence of initially complete absorption, followed by a period when ozone appears in the exhaust gas and cyanides are still present in the batch reactor. A countercurrent absorption tower was indicated to provide more intimate contact and better mass transfer where the last traces of ozone in the carrier gas would come in contact with the fresh solution at the top of the tower, and the weakest solution leaving at the bottom of the tower would come in contact with gas of the highest ozone content entering at that point. To investigate the characteristics of such a system, an all-glass absorption tower was constructed to study the effects of gas and liquid rates, tower height, and pH. Some of the data from these tests are presented in Tables I, II, and III. 

The pressure on these absorption solutions is increased from step to step, until the synthesis pressure is regained. With this countercurrent absorption a concentrated ammonium carbamate solution is obtained containing little water, which produces equilibrium conditions favorable for urea formation. 

Among the unit operations, adsorption may be considered a prototype for all fluid-solid separation operations. When it is conducted under countercurrent conditions, the calculation methods required are entirely analogous to those for countercurrent absorption or extraction (H3). Often, however, it is most economical to conduct adsorption in a semi continuous arrangement, in which the solid phase is present as a fixed bed of granular particles. The fluid phase passes through the interstices of this bed at a constant flow rate and for an extended period of time. The concentration gradients in the fluid and solid phases display a transient or unsteady-state behavior, and their evolution depends upon the pertinent material balances, rates, and equilibria. 

When molasses is fermented to produce a liquor containing ethanol, a C02-rich vapor containing a small amount of ethanol is evolved. The alcohol will be recovered by countercurrent absorption with water in a packed-bed tower. The gas will enter the tower at a rate of 180 kmol/h, at 303 K and 110 kPa. The molar composition of the gas is 98% C02 and 2% ethanol. The required recovery of the alcohol is 97%. Pure liquid water at 303 K will enter the tower at the rate of 151.5 kmol/h, which is 50% above the minimum rate required for the specified recovery (Seader and Henley, 1998). The tower will be packed with 50-mm metal Hiflow rings and will be designed for a maximum pressure drop of 300 Pa/m of packed height. 

Ethylene and Propylene. Sulfation of the lower alkenes, intermediate to production of the alcohols, is well suited to continuous operation. Production is large (many hundred million gallons), and the reaction proceeds rapidly by countercurrent absorption in a tower operating under pressure. This process has always been operated continuously, except briefly during early development. Further details are given in the next section. 

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