Monoethanolamine solutions stripping

A comprehensive study or the stripping of carbon dioxide from monoethanolamine solutions in a packed coluntu is described by Weiland et at-40 These investigntors developed a design approach that uses only fundamental physicochemical data and tested the approach against 173 experiments on the mass transfer performance or a pilot scale stripping column. Predicted mass transfer coefficients agreed with observed values within 23%. 

Figure 2-81. Calculated plate diagram for stripping of CO2 from aqueous monoethanolamine solutions.

The volatile hydrogen sulfide (b.p. —60.7°C) is then separated as an overhead gas stream. The monoethanolamine (b.p. 170°C) and diethylene glycol (b.p. 245°C) emerge as a regenerated solution, hot, from the bottom of the stripping column. To conserve heat, the hot, H2S-lean monoethanolamine stream from the stripper is heat exchanged with the cooler H2S-rich stream from the base of the absorber, sometimes with additional cooling with process water before it enters the absorber. 

A really protective gas, or even reducing gas, can be obtained, if, after drying, the gas is stripped of CO2. This can be achieved in several ways. High-pressure water scrubbing will remove some CO2 but the process is not widely used. More usual are methods of absorbing CO2 either chemically, using monoethanolamine (MEA) aqueous solution, or physically, using a molecular sieve. 

Absorption is a process that relies on a solvent s chemical affinity with a solute to dissolve preferably one species into another. It is widely proposed for CO2 separation where a solvent, generally, monoethanolamine (MEA) or a solid absorbent like lithium zirconate is used to dissolve CO2, but not the other components of a flue gas stream. C02-rich solution is typically pumped to a regeneration column, where CO2 is stripped out from the solution and the solvent recycled for a new batch of flue gas. The absorption equipment should be placed after the flue gas desulfurization-step and before the stack. Optimal conditions for absorption are low temperature and high pressure, making this the best location for absorption to occur. In addition, most solvents are easily degraded by compounds such as fly ash, other particulates, 80, (SO2, SO3) and (NO, NO2), so the absorption step must take place after electrostatic precipitation and desulfurization. In a typical absorption process, the C02-lean flue gas is either emitted to the atmosphere or possibly used in other applications e.g. chemical production). 

One of the most widely used commercial absorption processes is the removal of CO2 or HgS from a gas stream by contacting it with a solution of monoethanolamine or diethanolamine. Both of these solutions are alkaline and combine chemically with 14 mol of acid gas per mol of amine. Although a chemical combination exists, still even at equilibrium, the acid gas exhibits a vapor pressure above the solution. Because this vapor pressure of the absorbed acid gas increases rapidly with temperature increase, it is possible to regenerate the rich amine solution by stripping the heated solvent. 

Attributing corrosion to simple acid gas attack explains several observed corrosion phenomena. For example, primary amines, such as monoethanolamine (MEA) and Diglyco-lamine (DGA), are more corrosive than secondary and tertiary amines because in amine systems employing primary amines, which are difficult to strip, high concentrations of amine-acid gas salts are present in the hottest areas of the process. Conversely, methyldiethanolamine (MDEA), a tertiary amine, is easily stripped of both CO2 and H2S. Therefore, it is less corrosive because the bulk of the acid gas is evolved fiom solution at a lower temperature. 

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