Absorption regeneration, desorption

Arsenite Solutions. Addition of essentially stoichiometric proportions of arsenic trioxide to aqueous sodiiun or potassium carbonate solutions results in a marked increase in the rate of absorption and desorption of carbon dioxide, as compared with conventional carbonate solutions. Figure 5-31 illustrates this phenomenon by comparing, qualitatively, the rate of absorption of carbon dioxide at 1 atm partial pressure and room temperature in 40% potassium carbonate and in a typical solution used in the Giarrunarco-Vetrocoke process (Riesen-feld and Mullowney, 1959). The effects of the more rapid absorption and desorption are appreciable savings in regeneration heat, reduction in equipment size, and production of treated gas of higher purity than is possible with ordinary hot carbonate solutions. 

Figure 261 shows the absorption and the regeneration process schematically. During Absorption the concentrated salt solution is distributed over an exchange surface, which is in contact with an air stream. The air will be dehumidified and the salt solution will be diluted by the absorbed water vapour. During regeneration the diluted solution becomes concentrated again by desorption from a hot air stream. 

Too many industries currently produce varying concentrations of heavy metal laden waste streams with undesired consequences for the environment. Increasing emphasis has been placed on environmental impact minimization. This has led to the search for natural, inexpensive materials able to remove metals from factory effluents. Heavy metal absorption capacities of modified cellulose materials are found to be significant and their levels of uptake are comparable, in many instances, to other naturally occurring absorbent materials and to commercial ion exchange type resins. Many of the modified cellulose adsorbents proved to be regenerable and reusable over a number of absorption/desorption cycles. This allows easy recovery of the absorbed heavy metals in a concentrated form [22]. 

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. 

Regeneration of a chemically reactive solvent is a process of chemical desorption the chemical reaction which has taken pla ce in the absorption step takes place in the reverse direction in the regeneration step, and the absorbed gaseous components are desorbed back to a stripping gas phase, which is usually steam. 

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. 

Absorption is favored at raised pressure and low temperature. Therefore, the reverse process, desorption, favors low pressure and high temperature. With desorption the absorbed component is removed from the absorbent the solvent is degassed and regenerated before reuse. On the whole, desorption is carried out in four ways, which can either be applied individually or in combination ... 

The membrane contactor can be operated in two different ways. The principle of a gas-liquid membrane contactor is given in Figure 1. A gaseous feed, containing CO2 can be fed to the membrane module and the CO2 is selectively removed by absorption in the liquid phase. This is referred to as Membrane Gas Absorption (MGA). It is also possible to use the membrane contactor to regenerate the CO2 rich absorption liquid. In this case the loaded absorption liquid is fed to the membrane module and the CO2 desorbs from the absorption liquid because of a pressure difference across the membrane. This situation is referred to as Membrane Gas Desorption (MGD). 

An experimental, lab-scale, setup is available that allows for several configurations of gases and liquids, varying temperatures, and varying pressures (feed pressure and pressure difference across the membrane). In the experimental setup both the membrane gas absorption (MGA) step, for the transfer of CO2 from the gas phase to the liquid phase, and the membrane gas desorption (MGD) step, for the regeneration of the absorption liquid loaded with CO2, have been studied. It is possible to study the permeation behavior across the membrane of both pure CO2 and of binary gas mixtures (CO2-H2 and CO2-CH4). Two mass flow eontrollers are available to prepare a mixed gas stream with an arbitrary ratio of the two gases. 

The following example is a higly idealized example of absorption with solvent regeneration. Gas mixture of components C, and Cj is separated by absorption of C2 in a solvent where Cj is insoluble, and which can be regenerated by desorption of C2 under reduced pressure and heat supply. The solvent itself is of negligible equilibrium vapour pressure (nonvolatile) at the desorption temperature. The gas mixture (C, C2) is ideal. 

Carbon absorption utilizes activated carbon to physically absorb bioisoprene from the fermentor ofF-gas. As moisture would affect the absorption capacity of activated carbon, a dehumidifier unit is needed before the contact of the steam with activated carbon (Figure 16.5). The method is useful to recover the low concentration of bioisoprene at the laboratory level. Pioneering studies have shown that the method could absorb more than 80% of bioisoprene from the ofF-steam of 14-1 scale fermentation [52]. A preUminary study in our laboratory also confirmed that the absorption unit (filled with activated carbon fiber cloth) could effectively absorb gas-phase bioisoprene of low concentrations (1000-10 000 ppm, Zou et al., unpublished data). After the absorption step, an offline desorption/condensation step is needed to recover isoprene from activated carbon. Steam is utilized in the regeneration of the activated carbon and desorption of the isoprene. Then a series of condensers and cold traps follow to recover the liquid-phase isoprene. 

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