Vapor absorption/stripping

Data on the gas-liquid or vapor-liquid equilibrium for the system at hand. If absorption, stripping, and distillation operations are considered equilibrium-limited processes, which is the usual approach, these data are critical for determining the maximum possible separation. In some cases, the operations are are considerea rate-based (see Sec. 13) but require knowledge of eqmlibrium at the phase interface. Other data required include physical properties such as viscosity and density and thermodynamic properties such as enthalpy. Section 2 deals with sources of such data. 

In a perfectly-buffered solution the SO2 vapor pressure will be directly proportional to the total concentration of SO2 and bisulfite, giving a linear equilibrium relationship. In simple alkali sulfite solution without added buffer, the equilibrium relationship is highly nonlinear, because H-1" accumulates as SO2 is absorbed. Under these conditions is it not possible to carry out reversible SO2 absorption/stripping in a simple system, resulting in greater steam requirements than expected with a linear equilibrium relationship. Weak acid buffers such as sodium citrate have been proposed to "straighten" the equilibrium relationship and thereby reduce ultimate steam requirements (Jl, 2, 7). Citrate buffer is attractive because it is effective over a wide range, from pH 2.5 to pH 5.5 in concentrated solutions. 

Figure 1 shows the vapor pressure of some of the relevant compounds as a function of temperature. At -230.8°F and -184°F, the vapor pressure of propane is about 0.1 mm Hg and 1 mm Hg respectively. Absorption/stripping process is a conventionally practical process so it is decided to evaluate the separation of methane from hydrogen and carbon monoxide by this process. Since the operating pressures of the absorber and stripper are about 500 psia and 487 psia respectively, the mole fractions of propane in the outlet gas streams of the absorber and stripper are about 6.75 x 10 6 and 1 x 10 4 at -230.8°F and -184°F respectively. 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

Poddar TK and Sirkar KK. A hybrid of vapor permeation and membrane-based absorption-stripping for VOC removal and recovery from gaseous emissions. J. Mem. Sci. 1997 132 229-233. 

The graphical method assumes an idealized absorption/stripping model that is defined in terms of essentially three components or groups of components a liquid, a gas, and a distributed component or solute. The liquid is assumed not to vaporize (i.e., it has a very low A -value), and the gas is assumed not to dissolve in the liquid (i.e., it has a very high A -value). The distributed component is the key component to be absorbed by the liquid or stripped by the gas. It distributes itself between the two phases to satisfy vapor-liquid equilibrium criteria (Chapter 1). 

If only a small amount of one component is present in a mixture, changing the phase of the components in high concentrations should be avoided. In such a case, absorption, stripping, or selective adsorption best removes the minor component. Adsorption is particularly effective because of the high selectivity of adsorbents and is widely used for purification, where small amounts of a solute are removed from a liquid or vapor feed. 

The analogy is with equilibrium-stage vapor-liquid operations such as absorption, stripping, or distillation, where the liquid phase is, by definition, considered at its bubble point, that is, at saturation. In distillation, however,... 

We will now briefly touch upon plate towers for dispersive gas-liquid absorption/stripping. The treatment of the plate tower dynamics will be given in greater detail in Section 8.1.3.5 on vapor-liquid distillation. A schematic ofthe plate tower with a sieve plate for countercurrent gas-liquid absorption/stripping has already been shown in Figure 8.1.5. 

Figure 8.1.17. (a) Feed gas composition vs. fractional chemical absorbent consumption operating line and equilibrium behavior. (After Rousseau and Staton (1988).) (b) Schematic for absorption stripping process using a stripping vapor or gas. 

As practiced in gas absorption/stripping analyses (equations (8.1.54a-c), we may define for the enriching section the height of a transfer unit based on the overall vapor phase, HTUove, and the number of transfer units based on overall vapor performance, NTUove, as follows ... 

We have seen in Section 8.1.3 that multistage distillation is generally carried out in a vertical multiplate column in which each plate has crossflow. Vapor bubbles rise vertically through a liquid layer flowing horizontally in cross-flow over the plate (Section 8.1.3.5) the same two-phase flow scheme is employed in a plate for gas-liquid absorption stripping. We were introduced to the notion of stage efficiency, plate efficiency, tray efficiency, etc., in Section 8.I.3.4. We will introduce here the models used to... 

In a large number of processes, there ate unit operations related to vapot-Hquid separations distillation, absorption, extraction, stripping, flashing, and separation of Hquid and vapor stream arising from changes in temperatures and pressures. Calculations for these unit operations necessitate trial and... 

FIG. 13-7 Separation operations related to distillation, (a) Flush vaporization or partial condensation, (h) Absorption, (c) Rectifier, (d) Stripping, (e) Reboded stripping, (f ) Reboiled absorption, (g) Refluxed stripping, (h) Extractive distillation. ( ) Azeotropic distillation. 

When it is desired to compute, with rigorous methods, actual rather than equilibrium stages, Eqs. (13-69) and (13-94) can be modified to include the Murphree vapor-phase efficiency T ij, defined by Eq. (13-29). This is particularly desirable for multistage operations involving feeds containing components of a wide range ol volatility and/or concentration, in which only a rectification (absorption) or stripping action is provided and all components are not sharply separated. In those cases, the use of a different Murphree efficiency for each component and each tray may be necessary to compute recovery accurately. 

The design of a plate tower for gas-absorption or gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical plates needed to achieve a specified composition change (see Sec. 13). Distillation differs from gas absorption in that it involves the separation of components based on the distribution of the various substances between a gas phase and a hquid phase when all the components are present in Doth phases. In distillation, the new phase is generated From the original feed mixture by vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

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