Evaluation of a Primary Amine-Functionalized Ion-Exchange Resin for CO2 Capture
W. Richard Alesi Jr. and John R. Kitchin, “Evaluation of a Primary Amine-Functionalized Ion-Exchange Resin for CO2 Capture”, Industrial & Engineering Chemistry Research 51(19), 6907-6915 (2012). https://doi.org/10.1021/ie300452c
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Question
Aqueous amine solvents such as monoethanolamine carry a large regeneration penalty because the water in the solution must be heated. Solid sorbents with low heat capacity could reduce this cost, but impregnated amines (for example PEI) suffer from diffusion limits and leaching, and amines grafted to silica through Si-O-C bonds can hydrolyze in steam. Ion-exchange resins carry amines bonded to a polymer backbone through C-C bonds. This paper asks whether a commercial primary-amine resin, Lewatit VP OC 1065 (Lanxess), a divinylbenzene cross-linked polystyrene bead with benzyl amine groups and a heat capacity of 1.5 J/(g K), is a viable postcombustion CO2 sorbent, what mechanisms bind the CO2, and what working capacities (delta-loadings) are available for different regeneration schemes (Alesi 2012).
Approach
The resin (0.3 to 1.2 mm beads, received with 50 wt% water) was dried at 110 C in N2 and characterized by N2 adsorption (BET and BJH), SEM and energy-dispersive X-ray spectroscopy. Water uptake and capacity in pure CO2 were measured by thermogravimetric analysis. CO2 capacities were measured in a custom packed bed at 200 mL/min of 10 vol% CO2 in N2 (gas residence time 2.7 s) at 30 to 70 C, with effluent CO2 followed by an analyzer and a calibrated mass spectrometer. Regeneration was split into a 20 min N2 purge (the pressure swing part) followed by heating at 3 C/min to 120 C (the temperature swing part), with an empirical void space correction. In situ DRIFTS between 20 and 130 C identified the bound CO2 species (Alesi 2012).
Key findings
Drying for 2 h at 120 C under vacuum gave a BET area of 26.2 m2/g and BJH pore volume of 0.26 cm3/g, but 24 h of drying dropped these to 11.9 m2/g and 0.08 cm3/g, showing that complete dehydration collapses the pores. EDS gave about 10.7 wt% N, close to the monomer value, implying a theoretical amine loading near 6.7 mol N/kg after allowing for cross-linking, roughly twice the manufacturer’s titrated minimum of 3.5 equivalents/kg, so some amines are likely buried in the polymer.
Water uptake at 50 C in 9.1 vol% H2O was only 2.7 wt% (1.5 mol H2O/kg), lower than the values the authors cite for PEI and silica sorbents and far below an amine on activated carbon (over 40 wt%). Adding CO2 to the humid stream raised capacity by roughly 30% over the dry resin, to an estimated 2.3 mol CO2/kg, though water was not needed for capture.
In the packed bed, capacity was independent of sample mass (0.6 to 5.8 g) and fell from 1.85 to 1.15 mol CO2/kg as adsorption temperature rose from 30 to 70 C, consistent with exothermic, equilibrium-limited uptake. Adsorption and desorption capacities agreed within about 0.1 mol/kg, and 18 cycles between 50 and 120 C showed no capacity loss. As temperature increased, a larger fraction of the CO2 came off in the pressure swing, which the authors assign to CO2 dissolved in the polystyrene, with diffusion likely limiting this solubility component at low temperature. The temperature swing component was assigned by DRIFTS to amine-CO2 reaction products: carbamate bands (for example 1320, 1431, 1482 and 1565 cm-1) faded on heating and the 1320 cm-1 band vanished by 120 C, while bicarbonate bands also appeared, probably from ambient humidity and residual water, so carbamic acid and carbamate could not be distinguished.
In pure CO2 the resin reached 2.5 mol CO2/kg at 50 C, about 74% of the theoretical maximum, and full regeneration in 1 atm CO2 occurred by 200 C. From isobaric loading curves, capture at 50 C in 10 vol% CO2 followed by regeneration in hot CO2 at 150 C gives a working capacity of about 1.2 mol/kg, only about 20% below the 1.5 mol/kg obtained with inert regeneration near 115 C (Alesi 2012).
Significance
The study establishes OC 1065 as a moisture-tolerant, cyclically stable solid amine sorbent with capacities of roughly 1 to 2.5 mol CO2/kg depending on temperature and CO2 partial pressure, and separates its uptake into physical solubility and carbamate chemistry. Its most distinctive result is that the resin can be regenerated under hot CO2, which could avoid steam, drying and some compression costs at the expense of a higher regeneration temperature. The authors flag that urea formation and other stability issues under hot CO2, and reactive flue gas species, were outside the scope of this work (Alesi 2012).