Desorption or stripping

In gas absorption a soluble vapor is absorbed by means of a liquid in which the solute gas is more or less soluble, from its mixture with an inert gas. The washing of ammonia from a mixture of ammonia and air by means of liquid water is a typical example. The solute is subsequently recovered from the liquid by distillation, and the absorbing liquid can be either discarded or reused. When a solute is transferred from the solvent liquid to the gas phase, the operation is known as desorption or stripping. In dehumidification a pure liquid is partially removed from an inert or carrier gas by condensation. Usually the carrier gas is virtually insoluble in the liquid. Removal of water vapor from air by condensation on a cold surface and the condensation of an organic vapor such as carbon tetrachloride out of a stream of nitrogen are examples of dehumidification. In humidification operations the direction of transfer is from the liquid to the gas phase. In the drying of solids, a liquid, usually water, is separated by the use of hot, dry gas (usually air) and so is coupled with the humidification of the gas phase. 

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. 

The Murphree efficiency is defined using vapor concentrations as a matter of custom, but the measured efficiencies are rarely based on analysis of the vapor phase because of the difficulty in getting reliable samples. Instead, samples are taken of the liquid on the plates, and the vapor compositions are determined from a McCabe-Thiele diagram. A plate efficiency can be defined using liquid concentrations, but this is used only occasionally for desorption or stripping calculations. 

DESORPTION OR STRIPPING. In many cases, a solute that is absorbed from a gas mixture is desorbed from the liquid to recover the solute in more concentrated form and regenerate the absorbing solution. To make conditions more favorable for desorption, the temperature may be increased or the total pressure reduced or... 

In gas absorption a soluble vapor is absorbed by means of a liquid in which the solute gas is more or less soluble, from its mixture with an inert gas. The washing of ammonia from a mixture of ammonia and air by means of liquid water is a typical example. The solute is subsequently recovered from the liquid by distillation, and the absorbing liquid can be either discarded or reused. When a solute is transferred from the solvent liquid to the gas phase, the operation is known as desorption or stripping. 

The operations described above require mass transfer of a substance from the gas stream to the liquid. When mass transfer occurs in the opposite direction (i.e., from the liquid to the gas) the operation is called desorption, or stripping. For example, the benzene and toluene are removed from the absorption oil mentioned above by contacting the liquid solution with steam, whereupon the aromatic vapors enter the gas stream and are carried away, and the absorption oil can be used again. Since the principles of both absorption and desorption are the same, we can study both operations simultaneously. 

Rate processes, on the other hand, are limited by the rate of mass transfer of individual components from one phase into another under the influence of physical shmuli. Concentrahon gradients are the most common stimuli, but temperature, pressure, or external force fields can also cause mass transfer. One mass-transfer-based process is gas absorption, a process by which a vapor is removed from its mixture with an inert gas by means of a liquid in which it is soluble. Desorption, or stripping, on the other hand, is the removal of a volatile gas from a Hquid by means of a gas in which it is soluble. Adsorption consists of the removal of a species from a fluid stream by means of a solid adsorbent with which it has a higher affinity. Ion exchange is similar to adsorption, except that the species removed from solution is replaced with a species from the solid resin matrix so that electroneutrality is maintained. Lastly, membrane separations are based upon differences in permeability (transport through the membrane) due to size and chemical selectivity for the membrane material between components of a feed stream. 

Distillation can also be combined with desorption (or stripping). An example of such a process is the fractionation of nitric acid with the entrainer sulfuric acid (Geriche 1973). Nitric acid has a maximum azeotrope at a concentration of 37 mol% HNOj. Thus, a high boiling entrainer has to be used. 

The loaded solvent is usually regenerated by reversing the process, by desorption or stripping. After the solvent is recovered, it is ready for reuse. The solvent is regenerated in a desorber linked downstream to the absorber (Fig. 3-1), and the recovered solvent is hence recycled to the absorption unit. Since gas absorption is favored under low temperature and high pressure (see Chapter 1.4.3.3), desorption is carried out under high temperature and low pressure. 

I. Introduction to absorption. As discussed briefly in Section 10.IB, absorption is a ma s-transfer process in which a vapor solute. 4 in a gas mixture is absorbed by means of a liquid in which the solute is more or less soluble. The gas mixture consists mainly of an inert gas and the solute. The liquid also is primarily immiscible in the gas phase i.e., its vaporization into the gas phase is relatively slight. A typical example is absorption of the solute ammonia from an air-ammonia mixture by water. Subsequently, the solute is recovered from the solution by distillation. In the reverse process of desorption or stripping, the same principles and equations hold. 

Mass transfer between liquids and gases depends on the vapor pressure of the components as functions of temperature. Thus appropriate selection of operating temperature and pressure allows the reverse (desorption or stripping, and dehumidification) to be performed. The purpose of absorption and stripping operations is to remove and recover the maximum amount of a particular component from a feed stream. It is most efficiently accomplished in multiple stages, as in tray or packed columns. Humidification and dehumidification arc similar in principle, but are directed toward control of an environment short of equilibrium (e.g., <100 percent humidity) for them, a single stage is ordinarily sufficient. 

Both phases may be solutions, each containing, however, only one common component (or group of components) which distributes between the phases. For example, if a mixture of ammonia and air is contacted with liquid water, a large portion of the ammonia, but essentially no air, will dissolve in the liquid and in this way the air-ammonia mixture can be separated. The operation is luown as gas absorption. On the other hand, if air is brought into contact with an ammonia-water solution, some of the ammonia leaves the liquid and enters the gas phase, an operation known as desorption or stripping. The difference is purely in the direction of solute transfer. 

The process can be integrated with existing technologies, such as thermal desorption, air stripping, or soil vapor extraction. 

The inverse operation, called stripping, desorption, or regeneration, is employed when it is desirable to remove the solutes from the solvent in order to recover the solutes or the solvent or both. 

The analysis of the evolution and/or destruction of hydrocarbons during the incineration of MSW and hazardous waste involves heat transfer, mass transfer, and reaction kinetics. The key phenomena include the flashing of liquid hydrocarbons the vaporization, desorption, and stripping of hydrocarbons the pyrolysis and charring of hydrocarbons and the oxidation of char. To a certain extent these processes occur in parallel (steps 2, 3,4, and 5) and are common to most thermal treatment processes. 

In the field ion microscope, very high fields can be applied to a surface, sufficient to cause a significant distortion to the potential energy barrier experienced by an atom at the surface, resulting in desorption or field stripping. The method is used to produce the perfect surface to be imaged in the microscope and attempts have also been made to correlate the field required for desorption with the adatom bond energy to the surface [227]. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

Stripping Equations Stripping, or desorption, involves the removal of a volatile component from the hquid stream by contact with an inert gas such as nitrogen or steam. In this case the change in concentration of the liquid stream is of prime importance, and it... 

Stripping or desorption is the transfer of gas, dissolved in a liquid, into a gas stream. The term is also applied to that section of a Fractionating column below the feed plate. 

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