Thermally Stable Silicone Solvents for the Selective Absorption of CO

Jun 16, 2016 - LumiShield Technologies, 1816 Parkway View Drive, Pittsburgh, Pennsylvania 15205, United States. ABSTRACT: Polydimethylsiloxane ...
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Thermally Stable Silicone Solvents for the Selective Absorption of CO2 from Warm Gas Streams That Also Contain H2 and H2O P. Koronaios,† C. Stevenson,† S. Warman,† R. Enick,*,†,‡ and D. Luebke§ †

Department of Chemical and Petroleum Engineering, University of Pittsburgh, 940 Benedum Hall, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States ‡ National Energy Technology Laboratory, 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh, Pennsylvania 15236-0940, United States § LumiShield Technologies, 1816 Parkway View Drive, Pittsburgh, Pennsylvania 15205, United States ABSTRACT: Polydimethylsiloxane (PDMS) is a promising hydrophobic, CO2-selective solvent for the absorption of CO2 from a hot or warm water-rich, H2-rich, postwater−gas shift reactor (WGSR) stream in an integrated gasification combined cycle (IGCC) power plant. In this work, there are three hydrophobic silicones that are more thermally stable than PDMS, including an iron-stabilized PDMS (PDMS-Fe), poly(dimethyl-co-methylphenyl)siloxane (PDMMPS), and poly(dimethyl-co-diphenyl)siloxane (PDMDPS). PDMDPS is an extremely poor CO2 solvent, which is undesirable for the proposed separation, and a poor hydrogen solvent, which is a desirable trait. PDMDPS and PDMMPS absorb about the same amount of H2. Although PDMMPS is a much better CO2 solvent than PDMDPS, PDMMPS is a slightly poorer CO2 solvent than PDMS or PDMS-Fe. PDMS-Fe and PDMS are comparable hydrophobic solvents that exhibit the greatest solvent strength for CO2; however, PDMS-Fe and PDMS absorb slightly more hydrogen than PDMDPS and PDMMPS. If the absorption/regeneration process is designed such that the solvent is exposed to temperatures no greater than 230 °C, PDMS is recommended due to its low cost. For higher temperatures, the extremely low solubility of CO2 in PDMDPS precludes its use as a CO2-selective solvent. The ferrosilicone additive in PDMS-Fe is designed to inhibit polymer degradation in an oxidizing environment, but it offers no additional stability in the oxygen-free closed system associated with the IGCC. PDMMPS absorbs less H2 than PDMS or PDMS-Fe but is a slightly poorer CO2 solvent than PDMS or PDMS-Fe. However, PDMMPS is thermally stable in closed systems to 300 °C. Therefore, PDMMPS is recommended for prolonged high temperature use as the precombustion carbon capture absorber solvent at absorption/regeneration temperatures above 230 °C. Although these hydrophobic silicones exhibit promising attributes for a warm or hot precombustion carbon capture process, the diminishing CO2 solubility and increasing CO2 solubility that occur with increasing temperature will challenge the economic viability of this proposed CO2-selective absorption process.

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Small volatile compounds such as methanol and acetone3 and polymers have been considered as physical CO2 solvents.4 The foremost advantage of the polymeric solvents is their low vapor pressure. The most common polymeric CO2 solvent is PEGDME, which is completely miscible with water. For example, the solvent used in the Selexol process5,6 is a proprietary formulation that is rich in PEGDME. Given that the CO2 solvent strength of PEGDME increases with decreasing temperature and increasing pressure, it is not surprising that in most integrated gasification combined cycle (IGCC) plant designs, the CO2 absorption is typically conducted at high pressure and low temperature (∼40 °C), as shown in Figure 1. At these conditions most of the water vapor in the post-WGSR stream has been condensed and separated from the CO2-rich, H2-rich gas stream that is fed to the absorption column. Therefore, the complete miscibility of PEGDME and water is not problematic, because there is very little water vapor (on a mass basis) in the gas entering the absorption column even though the gas is saturated with water.

INTRODUCTION

Chemical and physical absorption methods are typically used for the postcombustion and precombustion capture of CO2, respectively.1 At low CO2 partial pressures of ∼0.010−0.015 MPa associated with postcombustion capture of CO2 from flue gas, amine solutions are favored because they will react with dilute concentrations of CO2 to attain significant loadings. A 30 wt % solution of monoethanolamine (MEA) in water, for example, will bind CO2 as a water-soluble ammonium carbamate at a 2:1 molar ratio of MEA:CO2, enabling this solution to absorb about 11 wt % CO2. CO2 release and solvent regeneration is accomplished via heating.1,2 Precombustion capture of CO2 is typically accomplished with physical solvents, however, because the high CO2 partial pressure in the post water−gas shift reactor (WGSR) gas stream is sufficient to cause the dissolution of significant amounts of CO2 into the solvent. For example, at 25 °C, if the partial pressure of CO2 is about 1 MPa, about 5 wt % CO2 will dissolve in polyethylene glycol dimethyl ether (PEGDME)-rich solvents that are employed for low temperature CO 2 absorption. Regeneration of the solvent and release of the CO2 can be accomplished with temperature increase and pressure reduction. © XXXX American Chemical Society

Received: January 20, 2016 Revised: June 15, 2016

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Figure 1. Simplified process flow diagram for the conventional concept of low temperature (40 °C) absorption of CO2 (from a low temperature stream containing hydrogen and saturated with a low concentration of water) in an IGCC power plant. The solvent is based on polyethylene glycol dimethyl ether.

Figure 2. Simplified process flow diagram for the proposed higher temperature absorption of CO2 (from a gas stream also containing water and hydrogen) in an IGCC power plant. In this example the absorber is operating at the highest possible temperature of 250 °C. The postwater gas shift reactor stream can also be cooled to a temperature less than 250 °C but greater than 40 °C. The solvent is based on a silicone oil.

undetectable. The stream could be cooled isobarically to its dew point of about 180 °C before its gas phase composition would change as water begins to condense. There are obvious concerns over high temperature gas absorption with a CO2selective solvent, however, especially the high flow rates of solvent that would be required in large absorbers and

The energetic and capital costs associated with cooling the fuel gas stream to 40 °C in order to use PPGDME as a solvent are substantial, however. Consider a typical IGCC fuel gas stream (31 mol % CO2, 43% H2, 23% H2O, and 3% of other gases such as CO, COS, H2S) leaving the WGSR at 250 °C and 5.5 MPa.7 Oxygen levels in the post-WGSR stream are B

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Figure 3. Structures of PDMS and three thermally stable silicone oils, PDMS-Fe, PDMPDS, and PDMDPS. The recommended long-term operating temperatures for each fluid in the presence of air/closed systems are also provided. The 230 °C closed system value for PDMS-Fe is equal to that for PDMS 5970, because the ferrosilicone additive is designed for enhanced stability in open (i.e., oxygen-containing) atmospheres; it provides no enhancement of closed system thermal stability.

regenerators due to the low loading of CO2 at 80−250 °C. Process modeling at the US DOE NETL7 has indicated that if CO2 can be selectively removed from within the WGSR or from the post-WGSR stream with little or no cooling, as shown in Figure 2, and if the remaining H2−H2O gas mixture is combusted to generate the hot, high pressure gas stream that is expanded in the gas turbine, then IGCC plant thermal efficiency could increase by 2−3 percentage points.7 In addition to selectively absorbing CO2, the solvent must be capable of being used for prolonged periods of time at elevated temperature in the presence of a reducing atmosphere composed primarily of CO2, H2O, and H2, along with low levels of contaminants such as SO2, ammonia, HCl, and mercury.7−9 Although not illustrated in Figure 2, it is also possible to cool the post-WGSR stream from the “hot” temperature of ∼250 °C to a “warm temperature” of ∼80 to ∼240 °C if such partial cooling is required to retain reasonable solvent loading and selectivity. Previously, we proposed several hydrophobic polymeric solvents, polypropylene glycol dimethyl ether (PPGDME) and polydimethylsiloxane (PDMS), for high-temperature selective absorption of CO2 from the post-WGSR stream.10 PPGDME is much more hydrophobic than PEGDME,10 and it is available in two forms: one based on a corn-derived linear monomer (PPGDMEl; poly(1,3-propanediol) dimethyl ether) and the other is based on the branched petroleum-derived methylated monomer (PPGDMEb; poly(1,2-propane diol)dimethyl ether). Trimethylsilyl-terminated PDMS is an extremely hydrophobic solvent that is commercially available over an incredibly wide range of molecular weight. The prior study provided a comparison of the solubility of CO2 and H2 in the solvents as a function of temperature and pressure. PPGDMEl, PPGDMEb, and PDMS absorbed comparable amounts of CO2. PPGDMEl absorbed the least amount of hydrogen due to the linear nature of the polymer, while PDMS

absorbed the most hydrogen due to the greater degree of free volume associated with the two methyl groups in each monomeric unit. The recommended upper temperature limit for prolonged use of PPGDME is about 100 °C, however. Although PPGDME can be handled safely in a chemical process where the temperatures can go as high as 250 °C in an inert atmosphere especially if antioxidants and secondary antioxidants are added, the recommended duration of such usage is only a few hours. Further, PPGDME is not commercially available, absorbs small amounts of water, and can form solid polymer−water crystals when left in the presence of a small amount of excess water at low temperature.10 PDMS absorbs no detectable amounts of water, even at 125 °C and 69 MPa.4 Further, PDMS is a low cost, readily available, thermally stable, environmentally benign, low viscosity, safe liquid solvent that is available in large quantities over a wide molecular weight range. In light of the solubility of CO2, H2O, and H2 in PEGDME and PDMS, the thermal stability of these solvents, and the practical aspects of acquiring and using large volumes of these liquids, PDMS is a far more promising candidate for a CO2-selective precombustion carbon capture solvent. The objective of the current work is to assess other types of silicone polymers for the selective absorption of CO2 from the post-WGSR stream. In particular, we have focused on silicone oil variants shown in Figure 3 that are touted as having greater thermal stability than PDMS. Although PDMS is a relatively stable compound, both thermal and chemical degradation is possible.11 The earliest studies of the thermal decomposition of PDMS in an inert atmosphere at temperatures to 600 °C12,13 provided decomposition kinetics and suggested that production impurities may hasten the decomposition. Several subsequent reports3,4,14 found that both oxygen and irradiation increased the rate of decomposition, while the presence of both sped up the rate of decomposition faster than the presence of either oxygen or irradiation alone.14 Other researchers15−17 later C

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Energy & Fuels determined the rate of PDMS decomposition atmospheres of N2 and air and confirmed that the presence of an oxidizing atmosphere enhanced the rate of PDMS decomposition. In light of these PDMS stability issues, it is not surprising that several silicone polymers have been designed with the express intent of exhibiting greater thermal stability. Iron-based additives have long been used as oxygen scavengers to retard the degradation of PDMS in an oxidizing environment. For example, iron siloxanes were first prepared by Schmidbaur et al.,18 though these compounds were solids. Adrianov et al.19 prepared compounds where the iron was covalently bound to the siloxane chain via oxygen atoms, though the percentage of iron is not listed. A patent20 discloses ferrosiloxane stabilizers such as Fe3+((OSiR1R2R3)2(OCOR4), where R1, R2, R3, and R4 are alkyl, alkaryl, alkenyl, or aryl group having from 1 to 25 carbon atoms; while R2 could also be hydrogen or a siloxy group and wherein R3 could also be a siloxy or substituted siloxy group. Later papers have been published by Moran et al.,21 who constructed compounds with iron carbonyls, by Walavalkar et al.22 and Races et al.23 One study23 refers to the thermal stability of the iron siloxane, though the more complete studies have been obtained from the compound thermal data.24 Other metals apart from iron have also been studied,25 but iron remains the most commonly used thermal stabilizer for prolonging the life of PDMS when exposed to a gas stream that contains oxygen. The other strategy for generating thermally stable silicones has been to incorporate methyl and phenyl groups into the polymer itself, rather than only methyl groups (PDMS only contains methyl groups). In an oxidizing environment it is thought that because the phenyl group is larger than a methyl group, the phenyl group provides a steric barrier to attacking species. The phenyl group itself is also less susceptible to oxidative attack because oxidative degradation starts with attack of oxygen on the pendant organic group and the phenyl groups are more stable because of resonance stabilization.26 Further, the inclusion of the phenyl functionality also promotes the thermal stability of the silicone oil in a closed system because the depolymerization of Si−O−Si bond is hindered by the π−π interactions between phenyl groups.26 It has been previously demonstrated27 that both poly(dimethyl-co-methylphenyl)siloxane (PDMMPS) and poly(dimethyl-co-diphenyl)siloxane (PDMDPS) are more thermally stable than PDMS in closed systems. Poly(dimethyl-co-methylphenyl)siloxane, PDMMPS, has been studied extensively over the last 50 years,28 with other studies on electrical use29 and thermal stability.30,31 PDMMPS is more stable than PDMS as indicated in commercial product data.24 Poly(dimethyl-co-diphenyl)siloxane, PDMDPS, was reported30,32 as being more stable than PDMS. It has also been shown to be still more stable than PDMMPS.30 The lifetimes of these silicone polymer products when exposed to air at high temperature24 are shown in Table 1, along with lifetimes of Krytox perfluoropolyether (PFPE) oil33 and PEGDME 250.34 At high temperatures (∼250 °C) only PEGDME 250 had a lower thermal stability than PDMS. Of the three thermally stable silicones considered in this study, PDMSFe exhibits the greatest thermal stability in an oxidizing atmosphere. It has been previously determined that silicone oils are more stable at elevated temperature when exposed to nitrogen atmospheres rather than oxidizing environments.15−17 It is therefore likely that the lifetimes of the oils in a reducing atmosphere such as the post-WGSR gas mixture (CO2, H2,

Table 1. Lifetime Stability of Oils Exposed to Air (i.e. Oxidizing Environment) at High Temperatures parameter

value

PFPE PDMS-Fe PDMMPS PDMDPS PDMS 5970 PEGDME 250

>5000 h at 200 °C, stable at 350 °C >5000 h at 250 °C (forms a gel) 1200 h at 250 °C (forms a gel) 1500 h at 250 °C (forms a gel) 50 h at 250 °C (forms a gel) 0.1 h at 220 °C

H2O) would have greater values than those listed in Table 1. For example, Clearco documentation24 indicates that the recommended long-term upper operating temperatures in the presence of air for PDMS-Fe, PDMMPS, and PDMDPS are 220, 230, and 250 °C, respectively. In systems closed to the atmosphere, however, the recommended long-term upper operating temperatures for PDMS-Fe, PDMMPS, and PDMDPS are 230, 300, and 300 °C, respectively. The primary objective of this study was to determine the solubility of CO2, H2, and H2O in each solvent, keeping in mind that for the proposed carbon capture application, high solubility of CO2 and low solubilities of H2 and H2O in the oil are desirable. Our intent was to use this data to determine which of the thermally stable silicone oil, if any, exhibited comparable solvent strength to PDMS. Because PDMS-Fe differs least from PDMS due to the presence of a dilute concentration of a ferrosilicone compound (whose precise structure the manufacturer was unwilling to specify), it was expected that these two solvents would exhibit comparable solvent strength. Both PDMDPS and PDMMPS have different polymeric structures than PDMS, and so it was expected that more significant differences would be evidenced by these candidates. The solubility of CO2 in each liquid was determined at 25 °C. If the solubility of CO2 in the thermally stable silicone oil was dramatically lower than the solubility of CO2 in PDMS, the silicone oil was no longer considered. If the solubility values of CO2 in the thermally stable oil and in PDMS were roughly comparable, then CO2 solubility values were determined at temperatures of 40, 80, and 120 °C. Therefore, for these promising candidates, the solubility of H2 was also determined over the 25−120 °C temperature range. Unfortunately, our attempts to attain accurate H2 solubility at temperatures as high as the post-WGSR temperature of 250 °C (Figure 2) were inhibited by the upper temperature limit of the phase behavior cell (180 °C), difficulties in preventing H2 leaks at temperatures above 150 °C, and our desire to repeat our prior experiences with severe hydrogen corrosion damage to the cell at elevated temperature and pressure. However, models of the experimental results were used to estimate solubility of CO2 and H2 in these liquids at 240 °C. Finally the solubility of liquid water in each silicone oil was determined at ambient pressure and temperature.

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EXPERIMENTAL PROCEDURES

Gases. Carbon dioxide (Matheson Gas, 99.999%) and hydrogen (Matheson Gas, 99.999%) were both used without further purification. Solvents. PDMS 5970 (5970 is the average molecular weight, 100 cSt at 25 °C), PDMS-Fe (PDMS 5970 with a ferrosilicone additive, marketed as HT-110, 100 cSt at 25 °C), PDMMPS (poly(dimethyl-comethylphenyl)siloxane, marketed as PM-125, 125 cSt at 25 °C), and PDMDPS (poly(dimethyl-co-diphenyl)siloxane, marketed as DPDM400, 400 cSt at 25 °C) were obtained from Clearco Products, USA and D

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and the phase behavior cell were checked daily for small leaks prior to each experiment in an effort to enhance the safety of the experimentalists and to improve the quality of the data.) The gas and liquid are not mixed during this process in order to reduce the dissolution of H2 in the solvent during this process. The flow of hydrogen is then stopped, and the sample volume, which contains the solvent and H2 at atmospheric pressure, is isolated. The volume of H2 is determined as the product of the height of the sample volume above the liquid (as measured with a cathetometer) and the circular crosssectional area of the tube. The mass of H2 in the sample volume was calculated as the product of gas volume and H2 density.35 The amount of H2 that may have dissolved in the solvent at 0.1 MPa during the introduction of H2 is assumed to be negligible. The H2-solvent mixture was then compressed and stirred until a single phase was attained. After the single phase was attained, the measuring of the bubble point pressure was measured in the same way as the bubble points were measured in the case of CO2 bubble points.

used as obtained. The purity of these silicone products is 99.9%. PDMS 5970 serves as the base oil for the PDMS-Fe. The other three compounds are marketed by their viscosity, but the molecular mass is not listed. Solubility of Water in Solvents. Small amounts of water are added gravimetrically from a syringe to a specified mass of solvent in a vial. The mixture is then stirred for 20 min with a small magnetic stir bar at 25 °C. If the mixture formed a single transparent phase, more water was added until the solution appeared cloudy. The highest water concentration at which a single phase was attained was reported as the solubility of water in the silicone oil. CO2 and H2 Solubility. All phase behavior measurements of each binary system are carried out in an invertible, high pressure, variablevolume, windowed, agitated, view cell (Schlumberger) using the exact same procedure that was described for our prior study involving poly(dimethylsiloxane) and other polymeric solvents.10 A brief summary of this technique follows. Although the temperature of the post-WGSR inlet stream to the absorber can reach a temperature as high as 250 °C and the associated regenerator can operate at even higher temperatures, CO2 and H2 solubility data were collected only to 120 °C. The solubility of CO2 and H2 in the solvents is determined using standard nonsampling techniques for bubble point detection.3,4,10 In all cases, a single-phase mixture of known overall composition that is retained in a windowed, variable-volume cell (Schlumberger, 10−100 mL) is very slowly expanded until a single, persistent bubble is observed in equilibrium with the liquid phase. The bubble point data is determined at 25, 40, 80, and 120 °C at CO2 mass fractions ranging between 0.04 and 0.25 or H2 mass fractions ranging between 0.00005 and 0.00025 (50 to 250 ppm). The high pressure phase behavior cell is housed in an air bath (Cincinnati Sub Zero Products Inc.) capable of controlling the temperature between 20 and 180 °C, as measured with a type K thermocouple to an accuracy of ±0.2 K. In a typical experiment for a CO2 bubble point, 30 g of solvent is loaded gravimetrically from a syringe onto the floating piston at the bottom of the sample volume. A computer-controlled positive displacement (PD) pump (Schlumberger) is used to displace the overburden fluid into the bottom of the view cell below the piston, thereby displacing the piston and the solvent upward as the air above the solvent is vented from the cell. The cell is then isolated by closing a valve on top of the cell, and the solvent is then compressed to ∼10 MPa using the overburden fluid PD pump. Liquid CO2 at ambient temperature is then compressed to the same pressure in a second PD pump. The isolation valve is then opened and CO2 is then pumped into the sample volume at the same rate that the overburden fluid is removed, allowing for the well-controlled addition of CO2 into the sample volume. When the desired volume of CO2 has been introduced to the sample volume, both pumps are turned off, and the isolation valve on top of the cell is closed. By recording the initial and final volume of CO2 in the finely calibrated PD pump (0.01 mL), and the initial and final temperature and pressure of the CO2, an equation of state35 can be used to determine the amount of CO2 displaced into the sample volume. The cell can then be heated to the desired temperature, and the sample volume, which contains known amounts of solvent and CO2, is then stirred and compressed via the slow addition of overburden fluid to the bottom of the cell until a single, clear, homogeneous liquid phase (L) or fluid phase (F) is achieved. The sample volume is then very slowly expanded at constant temperature until a bubble point is observed. Pressure is determined from a certified Heise gage (14 MPa ± 0.03 MPa), which measures the pressure of the overburden fluid. Bubble point measurements are repeated three times, and an average value is reported as the bubble point. Because the mass solubility of hydrogen in these solvents is significantly less than of CO2 in the pressure range of interest, it is not possible to accurately add very small amounts of dense H2 to the cell using the same technique. Therefore, the method of charging H2 involves loading the solvent into the cell and then flushing the space above the liquid solvent with low pressure H2 in order to displace the air from the sample volume. (The hydrogen lines, valves, and fittings

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RESULTS Miscibility of the Silicone Oils with Water. Even the addition of 1 ppm of liquid water to the silicone oil yielded two liquid phases, as evidenced by cloudiness. Therefore, PDMS 5970, PDMS-Fe, PDMMPS, and PDMDPS can be considered essentially immiscible with water at 25 °C. Although we previously demonstrated that PDMS retains immiscibility with water to 120 °C and 69 MPa,4 additional high temperature, high pressure water solubility measurements were not conducted in this study. Solubility CO2 in Solvents. Bubble point pressures for mixtures of CO2 with various solvents are illustrated in Figures 4−7. Although some of the isotherm data would be better fit

Figure 4. Solubility of CO2 in the four silicone solvents at 25 °C.

using a polynomial function, in all cases best-fit straight lines through the origin are illustrated for each isotherm in order to illustrate the slopes of the lines used to determine Henry’s law constants. The bubble point pressure values for mixtures of CO2 with each of the silicone oils at 25 °C are shown in Figure 4. The bubble point pressures for PDMS 5970 and PDMS-Fe are approximately the same. This is not surprising in that the base oil for PDMS-Fe is PDMS 5970, and the concentration of the ferrosilicone additive is very low. These bubble point loci are found at lower pressures than either PDMDPS or PDMMPS, which indicates that PDMS and PDMS-Fe are the better CO2 solvents. Mixtures of CO2 and PDMMPS exhibit bubble point values that are approximately 30% higher than those of PDMS 5970 E

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Energy & Fuels or PDMS-Fe. The bubble point loci for mixtures of CO2 and PDMDPS is much steeper than the bubble point loci associated with the three other silicones, leading to very high bubble point pressures at low CO2 concentrations. PDMDPS is very poor CO2 solvent compared to the other silicones. Therefore, no further CO2 solubility measurements were carried using PDMDPS; it was considered to be unsuitable for the proposed precombustion carbon capture application. The effect of elevated temperature on the solubility of CO2 in PDMS, PDMS-Fe, and PMPDMS is illustrated in Figures 5,

Figure 7. Bubble point pressures for mixtures of CO2 and PDMMPS at 25, 40, 80, and 120 °C. Predicted bubble point pressures for mixtures of CO2 and PDMMPS at 240 °C (black dashed line).

Figure 5. Bubble point pressures for mixtures of CO2 and PDMS 5970 at 25, 40, 80, and 120 °C. Predicted bubble point pressures for mixtures of CO2 and PDMS 5970 at 240 °C (black dashed line).

Figure 8. Bubble point pressures for a mixture of 10 wt % CO2 and 90 wt % PEGDME 250, PDMS-Fe, PDMS 5970, or PDMMPS at 25, 40, 80, and 120 °C.

recognized as the best polymeric solvent for the low temperature absorption of CO2 from a stream with a low water content, PEGDME is completely miscible with water at all temperatures and therefore unsuitable for the proposed application for the absorption of water from a warm or hot gas stream with a high water vapor content. It is possible to determine Henry’s law constant values, KH, from this data. Further one can use the change in KH as a function of temperature to estimate the enthalpy of dissolution, ΔH. KH is given by eq 137

Figure 6. Bubble point pressures for mixtures of CO2 and PDMS-Fe at 25, 40, 80, and 120 °C. Predicted bubble point pressures for mixtures of CO2 and PDMS-Fe at 240 °C (black dashed line).

KH = P /w

(1)

where KH is the Henry’s law constant (MPa), P is the CO2 pressure (MPa), and w is the dimensionless mass fraction. CO2 concentrations up to and including the 0.15 mass fraction data are used because the data was linear through the origin and therefore could be assumed to reflect the slope of the slope of the bubble point curve at infinite dilution.38 Table 2 lists the Henry’s law constants for each CO2-solvent (PDMS 5970, PDMS-Fe, PMPDMS) system at 25, 40, 80, and 120 °C, based on a best linear fit of the bubble point data shown in Figures 5, 6, and 7 up to 15 wt % CO2. Henry’s law constants obtained from the literature are also provided for PEGDME and Selexol solvent.39,40 Note that the Henry’s law constants reported in the literature for PEGDME and Selexol were expressed in terms of mole fraction of CO2; these KH

6, and 7, respectively. In all cases, the solubility of CO2 decreases with increasing temperature, reflecting the diminishing strength of the favorable Lewis acid: Lewis base interactions between CO2 and the silicone polymer with increasing temperature.36 In order to compare the performance of these three solvents over a wide range of temperature, Figure 8 presents bubble point pressure values for all of the solvents only at a CO2 concentration of 10 wt %. Bubble point data for mixtures of CO2 with PEGDME 250 are also provided.10 With respect to the absorption of CO2, the solvent may be ranked (from best to worst) as PEGDME 250 > PDMS-Fe ∼ PDMS 5970 > PMPDMS ≫ PDMDPS. Although PEGDME is generally F

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Energy & Fuels Table 2. Henry’s Law Constants, KH,CO2 in MPa, for CO2Silicone Oil Systems at 25, 40, 80, and 120 °C Based on Solubility Data to 15 wt % CO2b 40

PEGDME Selexol39 PDMS-Fe PDMS 5970 PDMMPS

25 °C

40 °C

80 °C

120 °C

240 °Ca

15.6 16.8 25.5 28.8 39.7

30.2 34.8 41.0

49.0 52.5 85.3

68.5 73.1 128.1

144.1 144.6 323.4

temperature post-WGSR stream, the results of Figure 9 were extrapolated to 240 °C (1/T = 0.00195), and the predicted Henry’s law constants are provided in Table 5 while the predicted H2 solubility results are shown as the black dashed lines in Figures 5, 6, and 7. Although firm targets for high temperature CO2 loading have not yet been established by the NETL, the diminishing solubility with increasing temperature may prove challenging for high temperature operations. Solubility of H2 in Solvents. The solubility of hydrogen in PDMS 5970, PDMS-Fe, PDMMPS, and PDMDPS is provided in Figures 10−13. In all cases best-fit straight lines through the

Note that the values at 240 °C are estimates based on extrapolations of the PDMMPS, PDMS 5970, and PDMS-Fe van’t Hoff plot results shown in Figure 9. bLiterature values are also provided for to PEGDME-based solvents. a

values were converted to values expressed in mass fractions to facilitate comparison with our results for the silicone oils. The enthalpy of dissolution ΔH can be determined from the slope of the natural logarithm of ΔH of the Henry’s law constant against the inverse absolute temperature, according to the van’t Hoff equation37 δ ln KH ΔH = R δ1/T

(2)

The results, illustrated in Figure 9, are used to determine the enthalpy of dissolution values provided in Table 3. Figure 10. Bubble point pressures for mixtures of H2 and PDMS 5970, PDMS-Fe, PDMMPS, and PDMDPS at 25 °C.

origin are illustrated for each isotherm. The solubility of H2 in each solvent at high pressure is significantly less than the solubility of CO2 over the temperature range studied. Unlike CO2, the solubility of H2 in the solvents increases with increasing temperature. The increase in hydrogen solubility with temperature over wide temperature ranges has been observed previously for solutions of H2 in many solvents,41 including creosote,42 water,43 methanol,44 toluene,45 ionic liquids,46 and PDMS.10 In general it is thought that that the primary factors responsible for this behavior are small intermolecular forces between hydrogen and the solvent, and the decrease in solvent density with increasing temperature may increase the solubility. CO2 and silicone oils exhibit multiple, multipoint, favorable, Lewis acid:Lewis base interactions that enhance solubility but diminish with increasing temperature,47,48 but there are no analogous favorable thermodynamic interactions between H2 and silicones. Rather, the solubility of hydrogen in the solvents is primarily attributed to the free volume of the solvent. Because the density of these liquids decreases with increasing temperature, as shown in Table 4, a higher concentration of hydrogen can dissolve in the liquid as temperature increases. Figure 10 illustrates that PDMS-Fe absorbs H2 slightly more readily that PDMS 5970, as evidenced by a lower bubble point

Figure 9. van’t Hoff plots for CO2-silicone oil solvent systems based on solubility values up to 0.15 mass fraction CO2.

Table 3. ΔH Values for the CO2-Silicone Solvent Systems Based on the Slopes of the Data Shown in Figure 9 parameter PDMS 5970 PDMS-Fe PDMMPS

value −9.5 kJ/mol CO2 −10.3 kJ/mol CO2 −12.9 kJ/mol CO2

The ΔH values associated with PDMS 5970 and PDMS-Fe are comparable, as expected, which reflects a comparable increase with bubble point pressure with increasing temperature as shown in Figure 8 for 10 wt % CO2-90 wt % silicone oil systems. The greater ΔH value associated with PDMMPS reflects the more rapid rise in bubble point pressure (i.e., dimished solubility) associated with PDMMPS, as shown in Figure 8. In order to estimate the solubility of CO2 in these siliconebased solvents at a temperature commensurate with the highest

Table 4. Density of the Silicone Solvents at 25 and 120 °C, Determined Gravimetrically with a Pychnometer

25 °C 120 °C G

PDMS 5970, g/mL

PDMS-Fe, g/mL

PDMDPS, g/mL

PDMMPS, g/mL

0.951 0.870

0.949 0.871

1.048 0.986

1.057 0.992

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Energy & Fuels locus at 25 °C. Both PDMS 5970 and PDMS-Fe are better solvents for H2 (which is undesirable for the proposed precombustion carbon capture application) than PDMMPS. PDMDPS is the poorest solvent for H2; although this is desirable for the proposed application, PDMDPS is by far the worst CO2 solvent as illustrated in Figure 4. Because of this extremely low solubility of CO2 in PDMDPS, it was not considered to be a realistic candidate for the proposed application, and further determinations of the solubility of H2 in PDMDPS were not conducted. Figures 11, 12, and 13 show that the solubility of hydrogen in PDMS 5970, PDMS-Fe, and PDMMPS, respectively, at 25, 40,

Figure 13. Bubble point pressures for mixtures of CO2 and PDMMPS at 25, 40, 80, and 120 °C. Predicted bubble point pressures for mixtures of H2 and PDMMPS at 240 °C (black dashed line).

Table 5. Henry’s Law Constants, KH,H2 in MPa, for H2Solvent Systems at 25, 40, 80, and 120 °C Based on Solubility Data to 250 ppm of H2 PDMS5970 PDMS-Fe PDMMPS

25 °C

40 °C

80 °C

120 °C

240 °Ca

12000 9650 13800

11900 9160 14000

10500 8910 14000

9020 8330 13300

7560 7600 13200

Note that the values at 240 °C are estimates based on extrapolations of the PDMMPS, PDMS 5970, and PDMS-Fe van’t Hoff plot results shown in Figure 14.

a

Figure 11. Bubble point pressures for mixtures of H2 and PDMS 5970 at 25, 40, 80, and 120 °C. Predicted bubble point pressures for mixtures of H2 and PDMS 5970 at 240 °C (black dashed line).

Figure 14. van’t Hoff plot for the H2-solvent binary systems on solubility values up to 250 ppm of H2. Figure 12. Bubble point pressures for mixtures of CO2 and PDMS-Fe at 25, 40, 80, and 120 °C. Predicted bubble point pressures for mixtures of H2 and PDMS-Fe at 240 °C (black dashed line).

Table 6. ΔH Values for the H2-Silicone Solvent Systems Based on the Slopes of the Data Shown in Figure 14 parameter PDMS 5970 PDMS-Fe PDMMPS

80, and 120 °C. The solubility of H2 increases slightly with increasing temperature for each silicone. The method of determining ΔH was the same as used for the dissolution of CO2. Table 5 provides the Henry’s law constants for H2. The van’t Hoff plot for H2 solubility results is shown in Figure 14. The ΔH values for dissolution of H2 were determined using eq 2. Note that these ΔH values shown in Table 6 are relatively small and positive, which reflects the unusual slight increase of H2 solubility with increasing temperature.

value +3.0 kJ/mol +1.4 kJ/mol +0.4 kJ/mol

In order to estimate the solubility of H2 in these siliconebased solvents at a temperature commensurate with the highest temperature post-WGSR stream, the results of Figure 14 were extrapolated to 240 °C (1/T = 0.00195), and the predicted solubility results are shown as the black dashed lines in Figures 11, 12, and 13. H

DOI: 10.1021/acs.energyfuels.6b00140 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

proposed application. The performances of PDMS and PDMSFe were comparable. PDMS-Fe absorbed slightly more CO2 than PDMMPS, but it also absorbed slightly more H2. Therefore, neither PDMS-Fe nor PDMMPS was obviously superior based solely on gas solubility data. However, PDMMPS is thermally stable in a closed system (such as the proposed post-WGSR absorber/regenerator) to temperatures as high as 300 °C, a value much greater than the 230 °C value associated with PDMS-Fe in a closed system. Therefore, PDMMPS appears to be the best candidate for the proposed application if the temperature in the absorber or regenerator exceeds 230 °C. Because bulk quantities of PDMMPS are approximately 10 times as expensive as PDMS 5970, at process temperatures below 230 °C PDMS 5970 would be the solvent of choice. These silicones are, to the best of our knowledge, more promising than any other type of polymer for the selective absorption of CO2 from warm or hot post water gas shift reactor effluent streams. Nonetheless, high temperature CO2 capture with silicone solvents has not yet been demonstrated to be economically viable. The foremost challenges to the proposed high temperature absorption are that the solubility of H2 increases with increasing temperature, while the solubility of CO2 decreases. As a result, each of the solvents loses CO2 selectivity and CO2 loading as temperature increases within the absorber. Continued process modeling7 that includes the solubility results of this work is therefore recommended to determine the viability of this process now that CO2 and H2 solubility data with the incorporation of the solubility data are generated within this study.

In the proposed precombustion carbon capture application, it is desirable that the solubility of CO2 in the thermally stable solvent is high, but that the solubility of H2 is low. However, neither PDMS-Fe nor PDMMPS exhibited both the highest CO2 solubility and the lowest H2 solubility; PDMS-Fe was a slightly better CO2 solvent but a slightly better H2 solvent. Therefore, in order to facilitate the comparison of the solvents, the ratios of KH,H2/KH,CO2 at 25, 40, 80, and 120 °C based on the results in Tables 2 and 5 were determined for PDMS-Fe and PDMMPS. Higher values of this ratio are preferable because they reflect increased selectivity for CO2 over H2. The results based on experimental data at 25, 40, 80, and 120 °C, along with the extrapolated values at 240 °C, are shown in Table 7. Although firm targets for these ratios have not yet Table 7. KH,H2/KH,CO2 at 25, 40, 80, and 120 °Cb PDMS-Fe PDMS 5970 PDMMPS

25 °C

40 °C

80 °C

120 °C

240 °Ca

378 417 348

303 342 341

182 200 164

122 123 103

53 52 41

Note that the values at 240 °C are estimates based on extrapolations of the PDMMPS, PDMS 5970, and PDMS-Fe van’t Hoff plot results shown in Figures 9 and 14. bThe ratio reflects how much more soluble CO2 is in a solvent than H2 (mass basis) at the same temperature and partial pressure. a

been established by the NETL, the diminishing selectivity with increasing temperature may prove problematic for high temperature operations. PDMS-Fe has slightly higher ratios of KH,H2/KH,CO2, except for the results at 40 °C. These results for PDMS-Fe and PDMMPS can be considered comparable. However, PDMS-Fe is designed to be thermally stable in open systems in which the solvent is exposed to oxygen, while PDMMPS exhibits thermal stability in a closed system such as the proposed absorption application. Therefore, it appears that PDMMPS is the best solvent for the proposed selective absorption of CO2 from a warm or hot gas stream that also contains H2 and H2O. However, the diminishing CO2 loading and decreased selectivity for CO2 over H2 that occurs with increasing temperature are likely to be challenges for the economic viability of this process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

■

REFERENCES

As part of the National Energy Technology Laboratory’s Regional University Alliance (NETL-RUA), a collaboration initiative of the NETL, this technical effort was performed under the RES contract DE-FE0004000. We would like to thank Christopher Cleary of Clearco Products Co., Inc. for his helpful discussions concerning the silicone oils. We are appreciative of the helpful discussions with Dr. Robert J. Perry from GE Global Research on the stability of silicone polymers. We would also like to thank Jared Ciferno of the US DOE NETL for helpful discussions concerning absorption of CO2 at warm and hot temperatures using hydrophobic solvents.

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CONCLUSIONS The selective removal of CO2 from a warm or hot post-WGSR stream of an IGCC plant could increase the overall thermal efficiency of the power plant. However, for this separation to be accomplished via CO2-absorption, the solvent must be thermally stable and hydrophobic and absorb little H2. Therefore, the solubility of CO2 and H2 in four silicone solvents that are immiscible with water, PDMS 5970, PDMSFe, PDPDMS, and PDMMPS, were compared in this study. PDMS 5970 is stable to 170 °C in an oxidizing environment and 230 °C in a reducing environment. PDMS-Fe contains an iron siloxane antioxidant that enhances thermal stability in open systems, while PDPDMS and PDMMPS contain phenyl functionalities that enhance thermal stability in oxidizing or reducing (i.e., open or closed) systems. Although PDMDPS absorbed slightly less H2 than any of the other solvents, the solubility of CO2 in this fluid was extremely low, and it was therefore considered to be not viable for the

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DOI: 10.1021/acs.energyfuels.6b00140 Energy Fuels XXXX, XXX, XXX−XXX