Equilibrium line, absorption

In many practical situations involving nearly complete cleanup of the gas, an approximate result can be obtained from the equations just presented even when solutions are concentrated or when absorption heat effects are present. In such cases the driving forces in the upper part of the tower are very much smaller than those at the bottom, and the value of mGM/LM used in the eqiiations should be the ratio of the slopes of the equilibrium line m and the operating line Lm/Gm iu the low-concentration range near the top of the tower. 

Infrared spectroscopy 100-1500 Intensity 1 of rotational lines of light molecules Boltzmann factor for rotational levels related to I Also Doppler line broadening useful, principal applications to plasmas and astrophysical observations, proper sampling, lack of equilibrium, atmospheric absorption often problems... 

By absorbing excited radiation the electrons are raised from the ground state to the excited state. These transitions take place so rapidly that no displacement of the atomic nuclei occurs (Franck-Condon principle). The space coordinate thus remains unchanged and the transitions can be represented by vertical lines. Because the excited system is not immediately in a state of equilibrium after absorption of energy, it first moves towards the lowest vibrational level with loss of energy to the lattice... 

The simplest possible case occurs when (1) both the operating and equilibrium lines are straight (i.e., the solutions are dilute) (2) Henry s law is valid (y"/x = yjx, = m) and (3) absorption heat effects are negligible. Under these conditions, the integral term in Eq. (14-21) may be computed by Colburn s equation [Tran.s. Am. Inst. Chem. Eng., 35,211(1939)] ... 

For the special case of absorption from lean gases with relatively large amounts of solvent, the equilibrium lines are defined for each component by the relation... 

A typical diagram for the complete absorption of pentane and heavier components is shown in Fig. 14-11. The oil used as solvent is assumed to be solute-free (i.e., X2 = 0), and the key component, butane, was identified as that component absorbed in appreciable amounts whose equilibrium line is most nearly parallel to the operating lines (i.e., the K value for butane is approximately equal to... 

The criterion for a reaction being effectively irreversible with respect to the absorption of a gas is that the concentration of the unreacted gas in the solution [e.g., NH3(a )] is so small that its partial pressure at the interface is much less than the partial pressure of the absorbing gas [e.g., NH3(g)] in the gas phase. When this criterion is satisfied, the interfacial mole fraction of the absorbing gas y, can be considered to be zero. This obviates the need for an equilibrium line or an x-y operating diagram. 

Logarithmic-Mean Driving Force. As noted earlier, linear operating lines occur if all concentrations involved stay low. Where it is possible to assume that the equilibrium line is linear, it can be shown that use of the logarithmic mean of the terminal driving forces is theoretically correct. When the overall gas-film coefficient is used to express the rate of absorption, the calculation reduces to solution of the equation... 

Fig. 9. Simple model of adiabatic gas absorption. A, nomsotliemial equilibrium line for overall gas-phase driving force y ...

Fig. 11. j-x diagram for adiabatic absorption of acetone into water. A, isothermal equilibrium line at TL2 B, equilibrium line for simple model of adiabatic gas absorption, gas-film driving force C, equilibrium line for simple model of adiabatic gas absorption, overall gas-phase driving force C, equilibrium line for simple model of adiabatic gas absorption, overall gas-phase driving force D, rigorously computed equilibrium line, gas-film driving force (43). 

This is the equation of the equilibrium line for ammonia stripping or absorption. It may be expressed in terms of the respective mole fractions [y and [x ]. Performing the operation and solving for [y f] produces... 

With a straight operating line and a straight equilibrium line (Henry s Law for example), where the entering absorption liquid is pure, i.e., xao = 10, Nqg as shown in Eq. (48), it now becomes... 

The graphical solution to absorption is identical to that for the rectifying section of a distillation column, except that the operating line is now above the equilibrium line. This is because the solute is now being transferred from the gas to the liquid instead of from the liquid to the gas. The stepping off stages procedure is as follows. 

Analogous to the minimum reflux ratio in distillation, there exists a minimum L/G ratio in absorption. Figure 6.3 compares the operating line to the equilibrium line for absorption. Because the slope of the operating line is L/G, reducing the liquid mass-separating agent flowrate decreases the slope of the line. Compare lines aA and ab for a separation in which the gas enters with concentration of solute which must be removed to Ya and the liquid... 

For distillations, the vapor resistance dominates, whereas for the stripping or absorption of slightly soluble materials, the liquid resistance dominates. In Equation (12.75), m is related to the VLE ratio [Equation (12.19)], and for the binary distillation of an i-j mixture is defined as the slope of the equilibrium line on a y-x diagram ... 

Figure 13.5 compares the slopes of the equilibrium curves at a low concentration of ammonia in the liquid and at a higher concentration. If the concentration of ammonia is less than about 0.1 mole fraction, the equilibrium line is fairly straight. However, heat effects associated with the absorption of ammonia can move the equilibrium curve to a higher slope. Problem 13.3 ° shows how even a low concentration case may require correction. 

In the general case where the equilibrium line is curved, the mass transfer rate for gas absorption per differential packed height dZ and unit cross-sectional area of the column is given as ... 

When one component is to be transferred from the V phase to the L phase, as in the absorption of soluble material from an inert gas, the operating line must lie above the equilibrium line as in Fig. 17.5A. The driving force for mass transfer is now — y, or the difference between the actual vapor composition and the vapor composition in equilibrium with the liquid for that position in the column. 

The reverse of gas absorption is called desorption or stripping, an operation cafried out to recover valuable solute from the absorbing solution and regenerate the solution. The operating line must then lie below the equilibrium line, as in Fig. 17.5c. Usually the temperature or pressure is changed to make the equilibrium curve much steeper than for the absorption process. 

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