Physical Absorption Solvents

The Sepasolv MPE process (BASF) [667], [685] is similar and uses polyethylene glycol methyl isopropyl ether. No commercial plants are in operation so far. 

The physical solvents methanol (Rectisol process), sulfolane (Sulfinol Process) and N-rnethylpyrrolidone (Purisol) are preferentially used in the treatment of partial oxidation gases and will be described separately in the following section. 

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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. 

Procedures, General Design Concept, Choice of Solvent, Selection of Column Diameter, Physical Absorption, Solvent Absorption, Natural Gas Dehydration, Gas Drying, Sulfuric Acid Manufacture, Formaldehyde Absorption, Absorption with Chemical Reaction, Cracked Gas Scrubbing, Amine Systems, Hot Carbonate Systems, Multicomponent Absorption, Reboiled Absorbers, Example Problem, Notation, References... 

The Rectisol and Amisol processes, which were developed in Germany by Liirgi, are physical and physical-chemical absorption processes using organic solvents to remove acid gas and sulfur, respectively, from various gas streams. Both processes use methanol as the physical absorption solvent the Amisol process also uses monoethanolamine (MEA) as the chemical absorber to improve the overaU purification efficiency. The MEA used in the Amisol process is not pertinent to our topic. Only methanol used in the Retisol process is discussed here, and the discussion is also applicable to the physical absorption by methanol used in the Amisol process. 

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. 

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. 

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). 

Physical absorption using a selective absorption solvent. 

Exit gases from the shift conversion are treated to remove carbon dioxide. This may be done by absorbing carbon dioxide in a physical or chemical absorption solvent or by adsorbing it using a special type of molecular sieves. Carbon dioxide, recovered from the treatment agent as a byproduct, is mainly used with ammonia to produce urea. The product is a pure hydrogen gas containing small amounts of carbon monoxide and carbon dioxide, which are further removed by methanation. 

In this case, the reactions can be reversed at a regeneration stage in a stripping column by the input of heat in a reboiler. If the solvent is to be recovered by stripping the solute from the solvent, the chemical absorption requires more energy than physical absorption. This is because the energy input for chemical absorption must overcome the heat of reaction as well as the heat of solution. However, chemical absorption involves smaller solvent rates than physical absorption. 

Removal of sulfur from gas streams is generally removal of H2S or removal of S02n. Generally, removal of the sulfur in the form of H2S is much more straightforward than in the form of S02. H2S can be removed by absorption, as already discussed. Chemical absorption using amines is the most commonly used method. However, other solvents can be used for chemical absorption, for example, potassium carbonate. Physical absorption is also possible using solvents such as propylene carbonate and methanol. 

Physical absorption depends on properties of the gas stream and liquid solvent, such as density and viscosity, as well as specific characteristics of the contaminants in the gas and the liquid stream (e.g., diffusivity, equilibrium solubility). 

Emergency Scrubbers (Absorbers) Emergency relief effluent streams are often sent to scrubbers (also called absorbers, columns, or towers) for final disposal by absorption of the gas into a solvent. Some gases or vapors can be removed by physical absorption. Other gases or vapors can be removed by chemical absorption (reaction of the vent gas/vapor into a liquid that reacts with it). 

Emergency Scrubbers (Absorbers) Emergency relief discharges are often passed through scrubbers (also called absorbers, or absorption columns or towers) for removal of flammable, corrosive, or toxic chemicals. The removal mechanism in some scrubbers involves physical absorption in a solvent, whereas in others chemical absorption (reactive scrubbing) is required. 

Within such a plant, depending on the pressure of the syngas, the separation can be performed by chemical absorption (usually with amine solvents) under lower pressure conditions or by physical absorption (e.g., with methanol) under higher pressure conditions (see also Chapter 6). Likewise, pressure-swing absorption can be employed. With the special properties of hydrogen, membrane separation processes could also be a very promising solution for the separation task. 

The position, intensity and shape of absorption bands of a compound in solution will vary with the solvent (see Fig. 11.7). These changes reflect the physical solute-solvent interactions that modify the energy difference between the ground and excited states. Study of the displacements and of the variation of absorption band intensities with solvent can be used to determine the type of transition. 

The gas from separation and stabilization flows to contacting towers, where it is contacted counter currently with a solvent, either diethanolamine or sulfinol. Through a combination of chemical reaction and physical absorption, essentially all the hydrogen sulfide and... 

Introduction Many present-day commercial gas absorption processes involve systems in which chemical reactions take place in the liquid phase an example of the absorption of C02 by MEA has been presented earlier in this section. These reactions greatly increase the capacity of the solvent and enhance the rate of absorption when compared to physical absorption systems. In addition, the selectivity of reacting solutes is greatly increased over that of nonreacting solutes. For example, MEA nas a strong selectivity for C02 compared to chemically inert solutes such as CH4, CO, or N2. Note that the design procedures presented here are theoretically and practically related to biofiltration, which is discussed in Sec. 25 (Waste Management). 

Existing physical absorption AGR processes are relatively energy inefficient for application in coal gasification they use substantial amounts of steam or stripping gas to regenerate lean solvent and power to pump lean solvent into the AGR absorber. In the treatment of crude gas with substantial carbon dioxide content, work available by expansion of separated carbon dioxide from its partial pressure in the crude gas, typically 100-300 psia, to atmospheric pressure, is not recovered. In theory, an AGR process could recover and utilize this potential energy. 

There are two reasons why artists are particularly vulnerable to diseases caused by exposure to toxic materials. First of all, artists, by the nature of their work, may have daily, long-term contact with materials that are highly toxic. Second, art materials are used as aerosols, powders, dusts, and in solution, from which maximum physical absorption and adsorption are possible. An artist can inhale aerosols. Powders and dusts are also inhaled and, in addition, can be absorbed through the skin. Solutions and many solvents evaporate into the air for the artist to inhale over long periods of time. This chemical assault, day after day, causes a variety of illnesses. In the following section on artists illnesses, all the chemicals cited are used by artists as they draw and paint, sculpt, work with metals, or develop and print photos—in general, as they engage in any art-associated activity. 

Reactive absorption represents a process in which a selective solution of gaseous species by a liquid solvent phase is combined with chemical reactions. As compared to purely physical absorption, RA does not necessarily require elevated pressure and high solubility of absorbed components because of the chemical reaction, the equilibrium state can be shifted favorably, resulting in enhanced solution capacity (17). Most RA processes involve reactions in the liquid phase only in some of them, both liquid and gas reactions occur (18,19). 

In the physical absorption process, the CO2 is absorbed in a solvent according to Henry s Law and then regenerated using heat, pressure reduction or both heat and pressure reduction. Typical solvents are Selexol (dimethylether of polyethylene glycol) and Rectisol (cold methanol) which are applied at high pressure. At lower pressures, the chemical absorption processes are more economical. The Selexol physical solvent process is frequently specified for coal gasification applications199. 

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