Highly CO2-Philic Liquid Oligomers and Phase Change-Solvents for the Absorption of CO2

摘要

Integrated gasification combined cycle (IGCC) power plants are ¡§the power plants of the future¡¨ due to their increased thermal efficiency compared to the current fleet of pulverized coal (PC) power plants employed throughout the US. An additional advantage they have is the range of possible fuels that can be used in their employ including, coal, biomass, recycled plastics, etc. Although there are no commercial scale IGCC plants currently in use in the US today, the increase in energy demand in the US compounded with the decommissioning of current PC plants each year will result in their construction soon. As with all fossil fuel using processes the IGCC plant will give off CO2 as a major waste stream that today is currently vented to the atmosphere. With the rising levels of atmospheric CO2 and the concern of global climate change, and the contribution from CO2, technology has been developed to capture CO2 from this IGCC fuel stream. This capture process is done via physical absorbents because of the inherent high pressure driving force present in this fuel stream. The overall objective of this work is to identify the most CO2-philic compounds from three classes of compounds made up of C, N, O, and H intended to be used in the carbon capture process associated with the IGCC plant. The three classes of compounds in question are low volatility CO2-philic oligomers, volatile organic solvents, and solid CO2-philic compounds that are capable of melting in the presence of CO2. Phase behavior experiments have been carried out in order to construct phase diagrams for each solvent and CO2. These diagrams quantify the miscibility of CO2 in each solvent which helps determine the best possible solvent for absorbing CO2 from a mixed gas high pressure stream in a typical counter-current absorption column. The higher the miscibility of CO2 in the absorbent, the lower the pressure of phase separation will be throughout the phase behavior diagram.Several solvents classified as low volatile CO2-philic oligomers were tested with CO2. A mixture of low volatility CO2-philic oligomers known as poly(ethylene glycol) di-methyl ether, PEGDME, is the current solvent of choice in the IGCC capture process. Poly(dimethylsiloxane), PDMS, and poly(propylene glycol) di-methyl ether, PPGDME, are potentially better solvents, compared to PEGDME, in this process due to their limited miscibility or immiscibility with water, a constituent in the fuel stream, and their low viscosity, an important property for gas transport in and out of the liquid phase solvent. Volatile organic solvents, while not prevalent in the IGCC capture process, are very widely used as solvents for a range of separation applications and are used extensively in CO2 capture primarily in the sweetening process of natural gas. Commercial scale sorbents including methanol and propylene carbonate have been in use for years under the proprietary names of RectisolTM and the Fluor process. Several organic solvents were examined in this study in binary mixtures with CO2. It was determined that acetone is the best solvent on a weight basis due to its small spherical size and shape and the CO2-philic ketone functionality. It cannot be used commercially however due to its high vapor pressure that would cause significant evaporative losses in practice. The best solvents compared on a molar basis include 2-(2-butoxyethoxy)ethyl acetate, 2-methoxyethyl acetate, both discovered in this work, and methyl acetate. Overall the best solvents on a weight or molar basis are those that are highly oxygenated compounds, rich in carbonyl and/or ether groups that favor Lewis acid:Lewis base interactions with CO2. CO2-philic solids are from the last group of potential solvents examined with CO2 and were found in the past by our group and two others. Originally investigated to be valuable as sand binders, these solids¡¦ unique ability to melt and then mix with CO2 has great potential value in energy savings and initial capital equipment cost savings. This potential stems from these solvents¡¦ ability to release all CO2 absorbed at a moderate pressure, approximately 5 MPa as opposed to a liquid solvent that releases CO2 at 0.1 MPa. The solids were chosen from two classes known as sugar acetates and tert-butylated aromatics and were tested in a binary mixture with pure CO2 and also a ternary mixture with an equimolar mixed gas CO2/H2. Four compounds, sucrose octaacetate, 1,3,5-tri-tert-butylbenzene, 2,4-di-tert-butylbenzene, and 1,3,5-trioxane, were determined to be viable candidates for the selective absorption of CO2 from a CO2/H2 mixture that are capable of melting and selectively absorbing CO2. Lastly, higher molar mass PDMS solvents were examined and compared to PEGDME (molar mass = 310) at elevated temperatures. These PDMS solvents are all substantially larger than the PDMS hexamer tested in conjunction with the other hexamers and oligomers tested. The major benefit these higher molar mass solvents have is that they allow the capture step to be carried out at higher temperatures. Additionally these PDMS solvents are completely immiscible with water up to 68.95 MPa and 393 K. This change in the capture process allows for the elimination of heat exchangers needed to lower the temperature of the fuel gas stream, and also eliminates a condenser step that is typically needed to eliminate much of the water out of the fuel stream for the hydrophilic PEGDME solvent. Each PDMS solvent, PDMS10 (viscosity, ƒÝ, equals 10 cSt at 298.15 K, and average molar mass, (MW) ̅ equals 1,250 g/mol), PDMS 20 (ƒÝ = 20 cSt at 298.15 K and (MW) ̅ = 2,000 g/mol), and PDMS50 (ƒÝ = 50 cSt at 298.15 K and (MW) ̅ = 3,780 g/mol), was examined in a binary mixture with CO2 at 353 K, 373 K, and 393 K, respectively, at PDMS weight fractions between 0.60 and 0.95. Each PDMS solvent displayed comparable CO2 miscibility compared with PEGDME at each temperature. Additionally each PDMS solvent was mixed with H2 at the same temperatures, and was able to mix and form a single homogeneous liquid phase however, only at substantially higher PDMS weight fractions, 0.995 to 0.999. While it is not clear which solvent has the highest miscibility with H2, the comparison of H2 miscibility to CO2 miscibility in each solvent illustrates the difference in selectivity that these solvents have for CO2 over H2.