Absorption separation method

An enrichment is defined as a separation process that results in the increase in concentration of one or mote species in one product stream and the depletion of the same species in the other product stream. Neither high purity not high recovery of any components is achieved. Gas enrichment can be accompHshed with a wide variety of separation methods including, for example, physical absorption, molecular sieve adsorption, equiHbrium adsorption, cryogenic distillation, condensation, and membrane permeation. 

A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

The special case involving the removal of a low (2—3 mol %) mole fraction impurity at high (>99 mol%) recovery is called purification separation. Purification separation typically results in one product of very high purity. It may or may not be desirable to recover the impurity in the other product. The separation methods appHcable to purification separation include equiUbrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this Hst as this method typically caimot achieve extremely high purities. Table 8 presents a Hst of the gas—vapor separation methods with their corresponding characteristic properties. The considerations for gas—vapor methods are as follows (26—44). 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

Robin57 59 studied this system up to 1000 atm. He measured the concentration of phenanthrene from its light absorption—a method that can only be used where a careful study has been made of the direct effect of pressure on the spectrum. He studied the direct effect in separate experiments with unsaturated solutions. 

We have been studying the novel process for CO2 separation named membrane/absorption hybrid method. The advantages of this process are that high gas permeance and selectivity were obtained. The concept of this process is shown in Fig. 1. Both feed gas and absorbent solution are supplied to the inside of hollow fibers. While Ae liquid flows upward inside the hollow fibers, absorbent solution absorbs CO2 selectively and it becomes a rich solution. Most of rich solution permeates the membrane to the permeate side maintained at reduced pressure, where it liberated CO2 to become a lean solution. Compared to a conventional gas absorption... 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. Liquid flowrate, temperature and pressure are important variables to be set, but no attempt should be made to carry out any optimization at this stage. Other commonly used separation methods are adsorption and membranes. 

The principal analytical methods for complex samples are those that separate the mixture by differential migration and then detect the separated components. The separation methods are chromatography, electrophoresis, and field flow fractionation the detection methods—which need not be selective but must be sensitive—include absorption, laser-induced fluorescence, electrochemistry, and mass... 

A further method separates the extracted substances by absorption. Basic for this method is that there should be a high solubility of extracted substances in the absorption material, and that the solubility of absorption substance in the circulation solvent should be as low as possible. Further, the absorption material must not influence the extract in a negative way and a simple separation of extract and absorption material has to be available. An ideal absorption material is therefore a substance which is present in the raw material. Most plant-materials contain water, which can act as a very successful absorption material. An ideal example is the separation of caffeine for the decaffeination of coffee and tea. On the one hand, water has a low solubility in CO2, and on the other, water-saturated CO2 is necessary for the process. The extracted caffeine is dissolved into water in the separator and caffeine can be produced from this water-caffeine mixture by crystallization. One advantage of this separation method is that the whole process runs under nearly isobaric conditions. 

The separation in the isobaric decaffeination processes is executed with absorption of caffeine, that means, the caffeine dissolved in CO2 is carried over into water by means of a packed washing column, or by adsorption with activated charcoal, but without recovery therefrom. Other separation methods under investigation are the use of membranes, since the difference in molecular weight between extract and solvent is high enough, or by the addition of substances of low solvent power. It is questionable whether the advantage of the possible isobaric process can compensate for the investment for the additional process steps required. 

Attempts were first made in this study to separate dimers by normal-phase liquid chromatography. Silica gel absorption chromatography was first used as a separation method. Hexane, as the mobile phase, gave very long retention times for the various dimers, and no separation was... 

Figure 2. Flow diagram for methane separation by absorption/stripping method.

Techniques can be classified into two main categories those that detect total metal concentrations and those that detect some operationally defined fraction of the total. Methods which detect total concentrations such as inductively coupled plasma spectrometry, neutron activation analysis, atomic absorption spectrometry and atomic emission spectrometry have no inherent speciation capabilities and must be combined with some other separation technique(s) to allow different species to be detected (approach A in Fig. 8.2). Such separation methods normally fractionate a sample on the basis of size, e.g. filtration/ultrafiltration, gel filtration, or a combination of size and charge, e.g. dialysis, ion exchange and solvent extraction (De Vitre et al., 1987 Badey, 1989b Berggren, 1989 1990 Buffle et al., 1992). In all instances the complexes studied must be relatively inert so that their concentrations are not appreciably modified during the fractionation procedure. 

No separation methods have been available but only an estimation based on measurements at 342 nm and 360 nm of an extract, and use of a simultaneous equation. However, piperttine which, according to differential absorption measurements, makes up 5 to 15 of the piperine content (35). has not been referred to in any of the many... 

The selection of a separation method is based on the identification of a suitable characteristic property, whose variation should be important for the component(s) to be separated. Table 3.2 presents characteristic properties for gas separations. A first group of methods relies on physical properties, such as boiling point, relative volatility, solubility, etc., which generates separation techniques such as condensation, distillation, physical absorption, etc. The second category exploits the reactivity of some functional groups, as in chemical absorption, catalytic oxidation, catalytic hydrogenation and chemical treatment. 

Purification deals with the removal of impurities with the goal of achieving very high concentration of the dominant component The initial concentration of impurity in the mixture should be lower than 2000 ppm, while the final concentration of impurity in the product should be less than 100 ppm. Suitable separation methods are equilibrium adsorption, molecular-sieve adsorption, chemical absorption and catalytic conversion. 

For split generation we make use of heuristics, as given in Table 3.1. The removal of troublesome impurities is suggested in the first place, here H2S, benzene and chloro-ethane. Then the split is placed in an appropriate selector, in this case of type purification . Table 3.3 indicates that six separation methods could be applied to perform this task chemical absorption, molecular-sieve adsorption, physical adsorption, catalytic oxidation, catalytic hydrogenation and chemical treatment. 

Next, characteristic properties of components are listed to select appropriate separation method (Table 3.7). Because the trace components belong to different chemical families, we eliminate gas-phase catalytic oxidation or hydrogenation. More specific chemical-based techniques remain. A first one is reversible chemical absorption. As solvents we may enumerate liquid redox systems (chelated iron), caustic washing solutions, amines or special formulations, as Selexol . Since H2S and C02 both have an add character, we may expect that a certain amount of C02 will pass in the off-gas stream. Dry chemical treatment could also be used, as reaction of H2S with iron-sponge or impregnated wood chips. 

Chapter 11 Acrylonitrile by Ammoxidation of Propene illustrates the synthesis of a flowsheet in which a difficult separation problem dominates. In addition, large energy consumption of both low- and high-temperature utilities is required. Various separation methods are involved from simple flash and gas absorption to extractive distillation for splitting azeotropic mixtures. The problem is tackled by an accurate thermodynamic analysis. Important energy saving can be detected. 

Affinity Absorption A method of separation by affinity chromatography. It may be used, for example, to remove unwanted antibodies from an antibody preparation. The preparation is passed through a column matrix containing antigens against which the unwanted antibodies are directed. Thus, the unwanted antibodies remain bound to the column. The antibody solution leaving the column contains only the desired antibodies, purified by affinity absorption. 

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