Absorption combined with stripping

The acid-rich potassium carbonate solution from the bottom of the absorber is flashed to a flash drum, where much of the acid gas is removed. The solution then proceeds to the stripping column, which operates at approximately 245 °F and near-atmospheric pressure. The low pressure, combined with a small amount of heat input, drives off the remaining acid gases. The lean potassium carbonate from the stripper is pumped back to the absorber. The lean solution may or may not be cooled slightly before entering the absorber. The heat of reaction from the absorption of the acid gases causes a slight temperature rise in the absorber. 

Lee [524] described a method for the determination of nanogram or sub-nan ogram amounts of nickel in seawater. Dissolved nickel is reduced by sodium borohydride to its elemental form, which combines with carbon monoxide to form nickel carbonyl. The nickel carbonyl is stripped from solution by a helium-carbon monoxide mixed gas stream, collected in a liquid nitrogen trap, and atomised in a quartz tube burner of an atomic absorption spectrophotometer. The sensitivity of the method is 0.05 ng of nickel. The precision for 3 ng nickel is about 4%. No interference by other elements is encountered in this technique. 

The chlorine value can be converted into hypochlorite bleach by absorption into lime or caustic soda. It can also be used to produce hydrogen chloride by combining with hydrogen in a burner. The latter technique is particularly useful when the acid can be consumed on site in brine acidification or ion-exchange resin regeneration. Another approach that has found some favour in the past is the absorption of the chlorine into a solvent from which it could then be stripped and returned to the liquefaction plant. 

H.-J. Weigmann, J. Lademann, H. Meffert, H. Schaefer, and W. Sterry. Determination of the horny layer profile by tape stripping in combination with optical spectroscopy in the visible range as a prerequisite to quantify percutaneous absorption. Skin Pharmacol. Appl. Skin. Physiol. 12 34-45 (1999). 

The combination of preconcentration by electrodeposition with stripping by voltammetry is probably the most sensitive electroanalytical method in common use today. Consequently, this popular technique is discussed in more detail in Chapter 24. Preconcentration of material by an electrode reaction has been used in sample preparation for atomic absorption, neutron activation, x-ray fluorescence, microprobe, and several other spectroscopic techniques. 

Many absorbers and strippers deal with dilute gas mixtures and liquid solutions, and it is satisfactory in these cases to assume that the operation is isothermal. But actual absorption operations are usually exothermic, and when large quantities of solute gas are absorbed to form concentrated solutions, the thermal effects cannot be ignored. If by absorption the temperature of the liquid is raised to a considerable extent, the equilibrium solubility of the solute will be appreciably reduced and the capacity of the absorber decreased (or else much larger flow rates of liquid will be required). For stripping, an endothermic process, the temperature of the liquid tends to fall. To take into account thermal effects during absorption and stripping, energy balances must be combined with the material balances presented in Chapter 3. 

Liquid membrane technology Is Introduced and Is Identified as a subset of membrane science. A tutorial section discusses configurations, transport mechanisms, experimental techniques, and a survey of basic theoretical approaches. The concepts of reactive liquid membranes which combine traditional unit operations such as extraction or absorption with stripping are discussed. The chapters to follow in this volume are summarized and the subject of each la placed In perspective to the field of liquid membrane technology. 

Among the most important examples of RS processes are reactive distillation, reactive absorption, reactive stripping and reactive extraction. For instance, in reactive distillation, reaction and distillation take place within the same zone of a distillation column. Reactants are converted to products with simultaneous separation of the products and recycle of unused reactants. The reactive distillation process can be both efficient in size and cost of capital equipment and in energy used to achieve a complete conversion of reactants. Since reactor costs are often less than 10% of the capital investment, the combination of a relatively cheap reactor with a distillation column offers great potential for overall savings. Among suitable reactive distillation processes are etherifications, nitrations, esterifications, transesterifications, condensations and alcylations (Doherty and Buzad, 1992). 

Olsen et al. (48, 20) have described an interesting method for the determination of lead in polluted seawater using FIA and flame atomic absorption spectroscopy. The system incorporates a Chelex-100 column for on-line preconcentration of the sample. The preconcentration and elution step improves the detection limit for lead by a factor of four (50 nM). Further increases in sensitivity are easily possible. The combination of this preconcentration step with a more sensitive detector, such as anodic stripping voltammetry, may make possible the determination of trace metals in seawater on a routine basis. 

This chapter deals with the diffusional transfer of mass to and across a phase boundary. In particular, gas-liquid, gas-solid, and liquid-liquid phase combinations have been considered. Process applications include absorption, stripping, distillation, extraction, adsorption, and the diffusional aspects of chemical reactions on a solid surface. For steady-state transfer operations, the rates of mass transfer can be correlated by variations of Pick s first law, which states that the rate is directly proportional to the concentration driving force and the extent of interfacial area, and inversely proportional to the distance of movement of the mass to the interface. 

The stages are determined by combining the McCabe-Thiele graphical construction, for a suitable region in the middle, with the Kremser equations for the low and/or high ends, where absorption and stripping factors are almost constant. 

Today, RD is discussed as one part of the broader area of reactive separation, which comprises any combination of chemical reaction with separation such as distillation, stripping, absorption, extraction, adsorption, crystallization, and membrane separation. In the next decade, unifying approaches to reactive separators should be developed allowing the rigorous selection of the most suitable type of separation to be integrated into a chemical reactor. 

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