Patent Application
20130244305
The present disclosure relates to the use of catalytically enhanced solvents for CO2 capture from flue gas, thus avoiding the needs for promoters or higher enthalpy of reaction solvents.
For flue gas applications, the process conditions (dilute CO2 concentrations, low partial pressures, low heat capacity of the flue gas) are such that the absorption process is limited either by low absorption rates or by excessive increase of the temperature in the absorber during the corresponding exothermic reactions.
In the past, these two issues have been addressed by the use of solvents with higher enthalpy of absorption. The higher enthalpy of absorption is generally associated with the stronger alkaline properties of the solvent (higher pKa) and therefore, increased rate of reaction as well as higher solubility of CO2 in the solvent. In particular, some prominent work in CO2 capture from flue gas with amine-based solvent recommends higher enthalpy of reaction solvents for flue gas application [Rochelle].
Unfortunately, higher enthalpy of reaction solvents have a drawback, in they participate to the increase in the energy demand for regeneration of the solvent. The improved affinity of the CO2 solvent in the absorber becomes a disadvantage when it comes to reverse the reaction in the regenerator. Therefore, there is a trade-off with which to deal.
The present invention involves the efficient use of a catalyst, an enzyme for example, to reduce the constraints associated with the trade-off described above, thus providing suitable real cyclic capacity to a solvent otherwise limited by its ability to absorb and maintain a high concentration of CO2 captured from flue gas. This invention can apply to non-promoted as well as promoted solvents and to solvents with a broad range of enthalpy of reaction.
Regenerated solvent is passed to a reboiler 106 via line 104. In the reboiler, located at the bottom of the regenerator, the regenerated solvent is boiled to generate vapor 107, which is returned to the regenerator to drive the separation of CO2 from solvent. In addition, reboiling may provide for further CO2 removal from the regenerated solvent.
Following reboiling, the reboiled and thus heated solvent is passed via line 108 to a heat exchanger 109 for heat-exchanging with the used solvent from the absorber. Heat exchanging allows for heat transfer between the solutions, resulting in a cooled reboiled solvent and a heated used solvent. The reboiled and heat-exchanged solvent is thereafter passed to the next round of absorption in the absorber. Before being fed to the absorber, the solvent 110 may be cooled to a temperature suitable for absorption. Accordingly, a cooler may be arranged near the absorber solvent inlet (not shown).
Examples of conventional amine-based solvents include, for example, amine compounds such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA) and aminoethoxyethanol (diglycolamine) (DGA). The most commonly used amines compounds in industrial plants are the alkanolamines MEA, DEA, MDEA and some blends of conventional amines with promoters (e.g., piperazine) and/or inhibitors.
A typical amine-based solvent for flue gas applications absorbs CO2 at temperatures around 100-140° F. Below this lower temperature, the kinetics of absorption are limited or slower, above this upper temperature, the solubility of CO2 in the solvent is rapidly diminished. The temperature of the solvent inside the absorber can be higher than its inlet or outlet temperatures due the exothermic nature of the reaction of absorption. This can lead to an internal thermodynamic pinch and poor utilization of the absorber column for mass transfer.
This invention targets solvents with relatively high theoretical cyclic capacities (based on thermodynamic CO2 loading capacities), for example cyclic capacities greater than about 1 mole/liter, but with limited ability to absorb CO2 under real process conditions (slow absorption rate and/or temperature-altered solubility due to exothermic reaction in the absorber), therefore not achieving a significant percentage of the theoretical cyclic capacity. For example,
By using a catalyst that enhances the kinetics of CO2 absorption at lower temperatures, the process conditions in the absorber can be optimized to increase the real cyclic capacity of the solvent to a higher percentage of the theoretical cyclic capacity (as defined by thermodynamics). Such catalysts may include, for example biocatalysts such as carbonic anhydrase or its analogs. There is no limitation to how low the temperature should be, at which the catalyst should enhance the kinetics, however, from a practical perspective, the following temperature range can be recommended. The catalyst should allow achieving increased CO2 loadings compared to a non-catalyzed solvent at temperatures in the range of 80-140° F. In particular, for any solvent, a catalyst that allows reaching the same or higher absorption rate but at lower temperature is beneficial.
With a catalytically-enhanced solvent, optimization of the process for higher cyclic capacities can be achieved by:
In this example a catalytically enhanced MDEA is selected and compared it to MDEA-Pz, where Pz plays the role of a promoter. This is for illustration only, the invention can apply to MDEA, MDEA-Pz, and, in general, to any solvent that show high enough theoretical cyclic capacity for a specified degree of CO2 separation from flue gas.
Below the theoretical cyclic capacity of MDEA and MDEA-Pz are compared at a specific process temperature and flue gas composition:
Therefore, at all temperatures (95-125° F.), MDEA-Pz can theoretically accomplish the separation, while MDEA can only achieve the separation at 95° F. The liquid to gas ratio for MDEA solvent can be increased to achieve the capture rate with a cyclic capacity of less than 0.32 mol/mol but this entails a higher liquid to gas ratio and a corresponding increased energy penalty. The corresponding energy penalties are reported in Table 1 and Table 2.
From these two Tables, it is seen that a catalyst providing to MDEA a cyclic capacity equivalent to the theoretical cyclic capacity allows for a reduced energy penalty as compared to a promoted solvent with a higher enthalpy of reaction. In this specific case, catalyzed MDEA is expected to have an enthalpy of reaction of 42 kJ/mol CO2 versus ≈70-80 kJ/mol CO2 for MDEA-Pz solvent. One can also notice that a catalyst that enhances the kinetics enough to reach the theoretical cyclic capacity at low temperatures (95° F. in this case) offers improved energy numbers at the same solvent circulation rate (liquid to gas ratio) as the promoted solvent. However, if the temperature at which the catalyst performs is increased, the separation can only be achieved at the cost of a higher liquid to gas ratio and a corresponding reduction in the energy saving as compared to a promoted catalyst (in this case 15% reduction in energy demand at 95° F. versus only 6% reduction in energy demand at 125° F.).
In a real application, it is not expected that the theoretical cyclic capacity can be reached. Due to volume and contact time limitation, the real cyclic capacity will only be a percentage of the theoretical cyclic capacity. In Table 3 and 4, it is demonstrated how a catalyst, by impacting the achievable approach to the thermodynamic equilibrium loading at the absorber bottom column, can improve the energy performance of the solvent. The process conditions remain identical as the one listed earlier.
For a representative approach to equilibrium of 70-80%, the reduction in energy demand at 95° F. is between 18 and 21% when using the catalytically enhanced MDEA as compared with the Pz promoted MDEA.
At a higher temperature than 95° F. (not shown here), the same trends are expected, however, the benefit in energy reduction is expected to be less due to the need for a higher solvent circulation rate associated with the lower cyclic capacity of the solvent.
In the above example, it is demonstrated that a catalytically enhanced solvent such as MDEA can perform better than a chemically promoted solvent (such as MDEA-Pz). An energy penalty reduction of 20% or above is achievable if the catalytic enhancement occurs at low enough temperature. At a higher temperature, the benefit is also seen but with an expected energy reduction as the solvent circulation rate needs to be increased to achieve a specified degree of CO2 separation (e.g. 90%). This invention can apply to any amine-based solvent, promoted. This invention is most suitable to solvents with a lower enthalpy of reaction.
While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
This application is a continuation of U.S. patent application Ser. No. 13/195,056, filed Aug. 1, 2011, which claims priority of U.S. provisional application 61/383,046, filed Sep. 15, 2012, also claiming priority of U.S. provisional application 61/637,595 filed Apr. 24, 2012, and U.S. provisional 61/782,250, filed Mar. 14, 2013, all of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61383046 | Sep 2010 | US | |
| 61637595 | Apr 2012 | US | |
| 61782250 | Mar 2013 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13195056 | Aug 2011 | US |
| Child | 13868714 | US |
