Absorption operations

The absorption operation is assumed to be isothermal with the following equilibrium relation (King, 1980) ... 

Vaporization and diffusion of flammable or toxic liquids or gases is a primary consideration with distillation, evaporation, extraction, and absorption operations. The basic principle of safety for tliese unit operations is contaimnent of the materials witliin the system. These operations should be conducted outdoors whenever possible. In tliis way, any accidental release of flammable or... 

The rich gas from the absorption operation is usually stripped of the desirable components and recycled back to the absorber (Figure 8-57). The stripping medium may be steam or a dry or inert gas (methane, nitrogen, carbon oxides—hydrogen, etc.). This depends upon the process application of the various components. 

Mass transfer during formation of drops or bubbles at an orifice can be a very significant fraction of the total mass transfer in industrial extraction or absorption operations. Transfer tends to be particularly favorable because of the exposure of fresh surface and because of vigorous internal circulation during the formation period. In discussing mass transfer in extraction, it has become conventional (H12) to distinguish four steps (1) formation, (2) release, (3) free rise or fall, (4) coalescence. Free rise or fall has been treated in previous chapters. Steps 1 and 2 are considered here. 

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

ABSORPTION (Process). Absorption is commonly used in the process industries for separahng materials, notably a specific gas from a mixture of gases and in the production of solutions such as hydrochloric and sulfuric adds. Absorption operations are very important to many air pollution abatement systems where it is desired to remove a noxious gas, such as sulfur dioxide or hydrogen sulfide, from an effluent gas prior to releasing the material to the atmosphere. The absorption medium is a liquid in which (1) the gas to be removed, i.e., absorbed is soluble ill the liquid, or (2) a chemical reaction takes place between the gas and the absoibing liquid. In some instances a chemical reagent is added to the absorbing liquid to increase the ability of the solvent to absorb. 

The main purposes of absorption processes are the removal of one or more components from the gas phase, production of particular substances in the liquid phase, and gas mixture separation (3). Industrial absorption operations are usually realized by combining absorption and desorption units. 

Most absorption operations are carried out in counter-current flow contactors in which the gas phase is introduced at the bottom of the column and the liquid solvent is introduced at the top of the column. 

As previously mentioned, one of the main degrees of freedom in absorption operations is the amount of solvent which is required to achieve the required absorption. This amount depends on how well the undesired gas component (the solute) is absorbed into the solvent. The vapour liquid equilibrium relationship for absorption of a gas component into a liquid solvent is expressed as ... 

Packed columns are commonly used in reactive absorption operations [10, 11], They represent cylindrical or rectangular columns up to several meters in diameter... 

Despite highly developed computer technologies and numerical methods, the application of new-generation rate-based models requires a high computational effort, which is often related to numerical difficulties. This is a reason for the relatively limited application of modeling methods described above to industrial problems. Therefore, a further study in this field - as well as in the area of model parameter estimation - is required in order to bridge a gap and to provide process engineers with reliable, consistent, robust and user-friendly simulation tools for reactive absorption operations. 

Here the matrix elements Hi , = (e Hul e ) are operators with respect to nuclear = wave functions in the ground and excited electronic states and At, g = e dE g, and so forth. The two-and three-photon absorption operators D and T are defined by the r bove, identities. 

Figure 9.7a shows the formation of boundary layers in absorption operations, and Figure 9.7b shows the formation of boundary layers in stripping operations. The right-hand side in each of these figures represents the liquid phase as in the hquid phase of a droplet and the left-hand side represents the gas phase as in the gas phase of the air. 

Consider the absorption operation. Imagine the two phases being far apart initially. As the phases approach each other, a point of touching will eventually be reached. This point then determines a surface being a surface, its thickness is equal to zero. This surface is identified as the interface in the figure. This figure shows the section cut across of the interface surface. The line representing the interface must have a zero thickness. 

In absorption operations, the concentration in the gas phase is larger in comparison to the concentration in the liquid phase. Thus, the flow of mass transfer is from gas to liquid. The reverse is true in the case of stripping, and the direction of mass transfer is from liquid to gas. In other words, the liquid phase is said to be stripped of its solute component, decreasing the concentration of the solute in the liquid phase and increasing the concentration of the solute in the gas phase. In absorption, the solute is absorbed from the gas into the liquid, increasing the concentration of the solute in the liquid phase and, of course, decreasing the concentration of the solute in the gas phase. 

The plot between the concentration of the solute in the liquid phase and that in the gas phase is called the operating line. Consider an absorption operation in a tower and let G be the mole flow rate of solute-free gas phase (carrier gas) carrying solute at a concentration [F] mole units per unit mole of the gas phase solute-free carrier gas. The corresponding quantities for the liquid phase are L and [X], where L is the mole flow rate of solute-free liquid phase (carrier liquid) and [X] is the mole of solute per unit mole of the solute-free liquid carrier. 

This same technique was applied to three whole shale oil products from our controlled-state retort which has been described previously (13). The polars were removed on Florisil, and the hydroboration/oxidation and acid absorption operations were carried out on samples from 1 to 2 g which were large enough to allow for gravimetric determination of material lost on treatment. The methods usually give results which agree within 2% for each material determined directly. Results of analysis of the three oils are presented in Table II. Oils A, B, and C were produced in the retort, each at a different heating rate 0.04°F, 2°F, and 20°F min respectively. 

In gas absorption operations the equilibrium of interest is that between a relatively nonvolatile absorbing liquid (solvent) and a solute gas (usually the pollutant). As described earlier, the solute is ordinarily removed from a relatively large amount of a carrier gas that does not dissolve in the absorbing liquid. Temperature, pressure, and the concentration of solute in one phase are independently variable. The equilibrium relationship of importance is a plot (or data) of x, the mole fraction of solute in the liquid, against y, the mole fraction in the vapor in equilibrium with x. For cases that follow Henry s law, Henry s law constant m, can be defined by the equation... 

Consider transport across the phase boundary shown in Figure 11.3. We shall denote the two bulk phases by L and V and the interface by I. Though the analysis below is developed for liquid-vapor interphase transport the formalism is generally valid for all two-phase systems. Therefore, what follows applies equally to distillation, stripping, and absorption operations. With a few modifications (to be described later), the analysis below may be used in the determination of rates of condensation, evaporation, vaporization, and boiling. 

The nonequilibrium model is not limited to simulating distillation operations with no fundamental change, it can be applied to absorption operations as well. A design case study is presented here by way of illustrating the model. This problem is adapted from an application discussed by Krishnamurthy and Taylor (1986). 

As noted above, nonequilibrium models can be used for modeling absorption operations. Krishnamurthy and Taylor (1986) present results for the absorption of ammonia in water. 

Taylor, R., Kooijman, H. A., and Woodman, M. R., Industrial Applications of a Nonequilibrium Model of Distillation and Absorption Operations, The Institution of Chemical Engineers Symposium Series No. 128, Distillation and Absorption 1992, MI5-M21 (1992). 

A column is to be designed with a number of trays equivalent to 8 theoretical stages for an absorption operation to remove at least 60% of the butane in a hydrocarbon stream. 

Based on preliminary studies, it was determined that a 10-tray column is appropriate for an absorption operation. The overall tray efficiency is estimated at 60%. For the indicated feed gas and absorbent, determine the expected flow rates and compositions of the products using the Kremser method. 

Following [14], we introduce a photon absorption operator at the point r at time t as follows ... 

Considering into account that (159) is simply the positive-frequency part of the vector potential (21), we can introduce the multipole photon absorption operator as follows... 

If ( ) > 1, absorption occurs between regimes 2 and 3, and the specific rate is given in Table 11.18. Under these conditions, the impeller speed (and hence kj) is reduced so that ( ) becomes greater than 3 and the absorption operation becomes fast, i.e., reaction-controlled (regime 3). The other condition <6< ( ) (Table 11.18), is also checked. The rate of absorption per unit liquid volume is given by... 

Absorption of NO gases is an important step in the manufacture of nitric acid. It is one of the most complex of all absorption operations, because of the following. (1) The NO gas contains NO, NO2, N2O3, N2O4, and so on, and the absorption of NO gases in water results in both nitric acid and nitrous acids. (2) Several reversible and irreversible reactions occur in both the gas and hquid phases. 

To get the number of stages needed for the desired absorption, operating variables such as pressure, temperature, and heights of submergence and empty sections were varied over a wide range. The capacity of the absorber was 750 tons/day (100% basis), and 30% excess oxygen was used. Values of the parameters are listed in Table 5 in the original reference. 

Tbe operatina of absorption is considered to include cases where the liquid phase contains suspended particles of teacliva solid. An example is the removal or suliiir dioxide from flna gas with an aqueous slurry of lime or limestone. However, the transfer of a component from a gas phsee to a dry solid is nor included as an absorption operation. Normally, snch a process is referred to as adsorptiou when die gas is held on [he surface of the solid and chemisorption when the gas combines chemienUy with die dry solid material. 

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