Absorption mass transfer driving force

Countercurrent operation is the most widely used absorption equipment arrangement. As the gas flow increases at constant liquid flow, liquid holdup must increase. The maximum gas flow is limited by the pressure drop and the liquid holdup that will build up to flooding. Contact time is controlled by the bed depth and the gas velocity. In countercurrent flow mass transfer driving force is maximum at the gas entrance and liquid exit. Cocurrent operation can be carried out at high gas velocities because there is no flooding limit. In fact, liquid holdup decreases as velocity increases. However, the mass transfer driving force is smaller than in countercurrent operation. 

Because Intalox structured packing 2T has an absorption efficiency greater than 1-in. metal Pall rings, this packing will be evaluated. At a fixed liquid rate, the mass transfer coefficient will increase at the 0.75 power of the gas rate for this gas-film-controlled absorption (see Chapter 3). At 10,800 CFM air flow, the mass transfer coefficient for Intalox structured packing 2T will be more than sufficient to handle the absorption of an additional 50% of acetic acid vapor with the same mass transfer driving force and packed depth. The inlet air has a density of 0.0728 Ib/ft, while the inlet liquid has a density of 62.2 Ib/ft and a viscosity of 0.81 cps. Because only 232 Ib/h of acetic acid vapor for three trains must be removed by the scrubber, the physical properties of the gas and liquid streams do not change from top to bottom of this tower. The flow parameter at the bottom of the scrubber will be ... 

Mass transfer driving forces are intermediate between vertical concurrent scrubbers and countercurrent scrubbers [1]. If the absorbed solute obeys Henry s Law in the liquid phase, the mass transfer driving force will limit maximum solute removal efficiency to about 90% of that obtained in a countercurrent scrubber for typical chemical fumes—assuming scrubbing water flow is limited. However, if the absorption of solute is followed by a rapid chemical reaction in the liquid phase, so that there is no appreciable vapor pressure of solute above the solution, the mass transfer driving force will be the same as for a countercurrent scrubber. 

It is important to understand that when chemical reactions are involved, this definition of Cl is based ou the driving force defined as the difference between the couceutratiou of un reacted solute gas at the interface and in the bulk of the liquid. A coefficient based ou the total of both uureacted and reached gas could have values. smaller than the physical-absorption mass-transfer coefficient /c . 

According to this analysis one can see that for gas-absorption problems, which often exhibit unidirectional diffusion, the most appropriate driving-force expression is of the form y — y tyBM,. ud the most appropriate mass-transfer coefficient is therefore kc- This concept is to he found in all the key equations for the design of mass-transfer equipment. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

If the reaction is slow, there is a small effect on the overall mass transfer coefficient. The driving force for mass transfer will be greater than that for physical absorption alone, as a result of the dissolving gas reacting and not building up in the bulk liquid to the same extent as with pure physical absorption. 

The CNG process removes sulfurous compounds, trace contaminants, and carbon dioxide from medium to high pressure gas streams containing substantial amounts of carbon dioxide. Process features include 1) absorption of sulfurous compounds and trace contaminants with pure liquid carbon dioxide, 2) regeneration of pure carbon dioxide with simultaneous concentration of hydrogen sulfide and trace contaminants by triple-point crystallization, and 3) absorption of carbon dioxide with a slurry of organic liquid containing solid carbon dioxide. These process features utilize unique properties of carbon dioxide, and enable small driving forces for heat and mass transfer, small absorbent flows, and relatively small process equipment. 

Equilibrium calculations are useful in the design or operation of a flue gas desulfurization (FGD) facility and provide the necessary foundation for complex process simulation (e.g., absorber modeling) (3). Since S02 absorption into FGD slurries is a mass transfer process which is primarily limited by liquid phase resistance for most commercial applications, the solution composition, in terms of alkaline species, is very critical to the performance of the system. Accurate prediction of solution composition via equilibrium models is essential to establishing driving forces for mass transfer, and ultimately in predicting system performance. 

The absorption phenomena observed, depend largely on the extent of depletion of the amine in the mass transfer zone and can be classified into three regimes negligible interaction, 2 medium interaction and 3 extreme interaction between H2S and CO2 absorption. In the latter regime, desorption of one of the gaseous components is observed although, based on its overall driving force, absorption would be expected. 

The actual value of k a was measured by absorption of carbondiox-ide from air into a buffer solution of potassium-carbonate and bicarbonate. Care was taken that the mass transfer coefficient itself was not enhanced by the chemical reaction, although the composition of the buffers used guaranteed a substantial driving force for mass transfer over the whole length of the column. Literature about the subject is abundant and here referred to (11, 12, 13). 

---