As concerns of global climate changes spark initiatives to reduce carbon dioxide emissions, its economic removal from gas streams is becoming increasingly important. Removal by absorption/stripping is a commercially promising technology, as it is well suited to sequester carbon dioxide (CO2). Such carbon dioxide emissions may be produced by a variety of different processes, such as the gas stream produced by coal-fired power plants. The removal of CO2 can be an expensive process, potentially increasing the cost of electricity by 50% or more. Therefore, technology improvements to reduce the costs associated with the removal are highly desirable.
The removal of CO2 from fuel gas and flue gas by absorption/stripping with aqueous amines is a disclosed and commercially practiced technology. A typical flowsheet for such a process is give by Kohl and Nielsen (1997) (FIG. 1). The gas at 30 to 50° C. containing CO2 and inerts such as methane, hydrogen, or nitrogen is contacted countercurrently in a trayed or packed column with lean aqueous solvent entering at 30 to 50° C. The aqueous rich solvent containing 3 to 6 molar amine is heated by cross exchange with the hot lean solvent. The approach temperature for this exchanger has historically been 10 to 30° C. with a lean solution loading of 0.01 to 0.25 moles CO2/mole amine. CO is removed from the solvent at 1.5-2 atm and 90-130° C. in a countercurrent reboiled stripper with trays or packing.
Commercially used amines that are used by themselves in water include monoethanolamine, diethanolamine, methyldiethanolamine, diglycolamine, diisopropanolamine, some hindered amines, and others (Kohl and Nielsen (1997)). These amines are soluble or miscible with water at ambient temperature at high concentrations that are used in the process to maximize capacity and reduce sensible heat requirements. Other amines, including piperazine, are used in combination with methyldiethanolamine and other primary amines.
A number of mono- and polyamines, including piperazine, are identified as potentially useful solvent components but have not been used because they are insufficiently soluble in water when used by themselves. Piperazine is a diamine that has previously been studied as a promoter for amine systems to improve kinetics. In water at 25° C., solid piperazine has a solubility less than 2 M, so it cannot be used in traditional systems at concentrations that give adequate CO2 capacity for good energy performance. BASF has disclosed the used of piperazine in combination with other amines (such as alkanolamines) or highly water soluble organics (such as triethyleneglycol) to promote the water solubility of piperazine.
It has also been claimed that number of potentially useful amines such as piperazine would be too volatile if used in high concentrations in aqueous solvents. The boiling point of piperazine (146.5° C.) is lower that that of monoethanolamine (170° C.), so the use of Raoult's law would suggest that it would have a greater volatility at the top of the absorber.
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While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments have been shown in the figures and are described in more detail below. It should be understood, however, that the description of specific example embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, this disclosure is to cover all modifications and equivalents as illustrated, in part, by the appended claims.