Gas-liquid absorption

Figure 12.18 illustrates the conditions that occur during the steady operation of a countercurrent gas-liquid absorption tower. It is convenient to express the concentration of the streams in terms of moles of solute gas per mole of inert gas in the gas phase, and as moles of solute gas per mole of solute free liquid in the liquid phase. The actual area of interface between the two phases is not known, and the term a is introduced as the interfacial area per unit volume of the column. On this basis the general equation, 12.13,... 

Burning of coal Roasting of ores Attack of solids by acids Gas-liquid absorption with reaction Reduction of iron ore to iron and steel... 

The first two assumptions as well as the fourth are used by Levenspiel for gas-liquid absorption operations. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

Since distillation is the method used most widely for separating mixtures of liquids in the chemical-and hydrocarbon processing industries, we will examine this unit operation particularly and make some critical observations for the gas-liquid absorption. 

The process features of carbon dioxide triple-point crystallization and slurry absorption of carbon dioxide have been demonstrated with the first generation bench-scale apparatus. Current efforts are focused on the design and construction of an improved version of the carbon dioxide triple-point crystallizer in cooperation with the U S Department of Energy. Future efforts are planned to design and construct absorption units to study multi-stage slurry absorption of carbon dioxide, and the more conventional gas-liquid absorption of sulfuruous compounds with liquid carbon dioxide. 

A.E. Jansen, P.H.M. Feron, J.H. Hanemaaijer and P. Huisjes, Apparatus and Method for Performing Membrane Gas/Liquid Absorption at Elevated Pressure, US Patent 6,355,092 (March 2002). 

Product separation and catalyst recovery at the end of the homogeneous catalyzed reactions, as explained, are in most cases carried out by crystallization, filtration, distillation liquid-liquid extraction, or gas-liquid absorption. These unit operations can be performed in batch or continuous mode. The salient features of these operations are described in the following. 

In this operation a soluble gas is absorbed from a gas mixture with a liquid. Generally the gas mixture and the liquid are brought in contact with each other in a packed column. Here the liquid flows from the top and the gas mixture rises from the bottom (countercurrent). The absorbed gas or vapor can be recovered later by desorption. The packing used in a gas-liquid absorption column is the same as the packing used in packed distillation columns and aids in good gas-liquid contact. This operation is also used to strip toxic vapors from exit gases. Such absorption columns are known as scrubbers. 

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