Absorption physical

The thermodynamics of physical absorption are formulated in analogue to distillation. Important qnantities are the minimum demand of solvent (analogue minimum reflux) and the minimum demand of stripping gas (minimum reboil). Both quantities can be formulated via material balances. 

In the special case of a veiy high column, phase equilibrium is reached at the bottom of the column  

According to (5.3-10) the demand of solvent is directly proportional to the amount of raw gas to be treated and inversely proportional to the system pressure. As the Henry coefficient He increases with temperature, low system temperatures are advantageous for absorption. It is important to note that the minimum demand of solvent does not depend on raw gas concentration. In technical processes, absorption is effected with a surplus of the solvent  

The minimum amount of stripping gas can be found analogously. A material balance delivers (Fig. 5.3-5)  

In a very long column phase equihbrium is reached at the top of the column  

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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). 

Physica.1 Absorption. Whereas chemical absorption rehes on solvent reactions to hold acid gas components in solution, physical absorption exploits gas—hquid solubiUties. The amount of absorption for these solvents is direcdy proportional to the partial pressure of the acid gas components. Thus these processes are most appHcable in situations involving high pressure feed streams containing significant concentrations of acid gas components. To favor absorption, lower temperatures are often employed. Some processes require refrigeration. 

Hybrid Processes. A number of processes have been developed which use both chemical and physical absorption solvents to offer high purity treat gas and low energy solvent regeneration. The operation of these processes is usually similar to that of the individual chemical or physical absorption processes. The solvent composition is typically customized to meet the requirements of individual appHcations. 

The processes using physical absorption require a solvent circulation proportional to the quantity of process gas, inversely proportional to the pressure, and nearly independent of the carbon dioxide concentration. Therefore, high pressures could favor the use of these processes. The Recitsol process requires a refrigeration system and more equipment than the other processes. This process is primarily used in coal gasification for simultaneous removal of H2S, COS, and CO2. 

The drying mechanisms of desiccants may be classified as foUows Class 1 chemical reaction, which forms either a new compound or a hydrate Class 2 physical absorption with constant relative humidity or vapor pressure (solid + water + saturated solution) Class 3 physical absorption with variable relative humidity or vapor pressure (soHd or liquid + water + diluted solution) and Class 4 physical adsorption. 

There is no sharp dividing hne between pure physical absorption and absorption controlled by the rate of a chemic reaction. Most cases fall in an intermediate range in which the rate of absoration is limited both by the resistance to diffusion and by the finite velocity of the reaction. Even in these intermediate cases the equihbria between the various diffusing species involved in the reaction may affect the rate of absorption. 

In any event the value of iri the presence of a chemical reac tion normally is larger than the value found when only physical absorption occurs, 7c . This has led to the presentation of data on the effects of chemical reaction in terms of the reaction factor or enhancement factor defined as... 

It is important to understand that when chemical reactions are involved, this definition of Cl is based ou the driving force defined as the difference between the couceutratiou of un reacted solute gas at the interface and in the bulk of the liquid. A coefficient based ou the total of both uureacted and reached gas could have values. smaller than the physical-absorption mass-transfer coefficient /c . 

Caution is advised in distinguishing between systems involving pure physical absorption and those in which a chemical reaction can significantly affect design procedures. 

Introduction Many present-day commercial gas absorption processes involve systems in which chemical reactions take place in the liquid phase. These reactions generally enhance the rate of absorption and increase the capacity of the liquid solution to dissolve the solute, when compared with physical absorption systems. 

A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption, as discussed earlier in this section and in Section 5. There are a number of excellent references the subject, such as the book by Danckwerts Gas-Liquid Reactions, McGraw-Hill, New York, 1970) and Astarita et al. Gas Treating with Chemical Solvents, Wiley, New York, 1983). 

Simultaneous Absorption of Two Reacting Gases In multi-component physical absorption the presence of one gas often does not affect the rates of absorption of the other gases. When chemical reactions in which two or more gases are competing for the same hquid-phase reagent are involved, selectivity of absorption can be affected by... 

While the carbon dioxide/caiistic test method has become accepted, one should use the results with caution. The chemical reaction masks the effect of physical absorption, and the relative values in the table may not hold for other cases, especially distillation applications where much of the resistance to mass transfer is in the gas phase. Background on this combination of physical and chemical absorption may Be found earher in the present section, under Absorption with Chemical Reaction. ... 

Competing Processes Membranes are not the only way to make these separations, neither are they generally the dominant way. In many apphcations, membranes compete with ciyogenic distillation and with pressure-swing adsorption in others, physical absorption is the dominant method. The growth rate for membrane capacity is higher than that for any competitor. 

For purely physical absorption, the mass-transfer coefficients depend on trie hydrodynamics and the physical properties of the phases. Many correlations exist for example, that of Dwivedi and Upadhyay (Ind. Eng. Chem. Proc. De.s. izDev.,... 

For physical absorption, values of the mass-transfer coefficients may not vary greatly, so a mean value could be adequate and coiild be taken outside the integral sign, but for reactive absorption the variation usually is too great. 

The numerical solution of these equations is shown in Fig. 23-28. This is a plot of the enhancement fac tor E against the Hatta number, with several other parameters. The factor E represents an enhancement of the rate of transfer of A caused by the reaction compared with physical absorption with zero concentration of A in the liquid. The uppermost line on the upper right represents the pseudo-first-order reaction, for which E = P coth p. 

Gas-Film Coefficient Since the gas film is not affected by the liquid-phase reaction, one of the many available correlations for physic absorption may be apphcable. The coefficient also may be found directly after elimination of the hquid-film coefficient by employing a solution that reacts instantaneously and irreversibly with the dissolved gas, thus cancehng out any backpressure. Examples of such systems are SO2 in NaOH and NH3 in H2SO4. 

Some performance data of plants with DEA are shown in Table 23-11. Both the absorbers and strippers have trays or packing. Vessel diameters and allowable gas and liquid flow rates are estabhshed by the same correlations as for physical absorptions. The calciilation of tower heights utilizes data of equilibria and enhanced mass-transfer coeffi-... 

Physical absorption The process of collecting a gas in water or another fluid. 

There are two main schemes proposed for sequestration of carbon dioxide. The first (referred to as a chemical absorption process), suitable for use at low pressures and temperatures, is usually adopted where the CO2 is to be removed from exhaust flue gases. The second (usually referred to as a physical absorption process), for use at higher pressures, is recommended for separation of the CO2 in syngas obtained from conversion of fuel. 

Fig. 8,2, The physical absorption process (after Chiesa and Consonni [."I)).

Manfrida [4] argues that the heat demand and the substantial power loss associated with presssure-swing physical absorption makes it less attractive than chemical absorption, even for high pressure sequestration. The expansion work in the fonner is difficult to recover as. several expanders are needed. 

Cycle D5 is another variation of a CCGT plant with full oxygenation of the fuel as shown in Fig. 8.22 again it is a semi-closed cycle using pure oxygen. But now the CO2 is abstracted after compression, which may require the use of physical absorption plant. 

Chiesa, P. and Consonni. S. (1999), Shift reaction and physical absorption for low emission IGCCs, ASME J. Engng Gas Turbines Power 121(2), 295-305. 

Hikita, H. and Ishikawa, H., 1969. Physical absorption in agitated vessels with a flat gas-liquid interface. Bulletin of the UniversityOsaka Prefect, A18, 427-437. 

Physical absorption using a selective absorption solvent. 

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