Effects of O2 and SO2 on the Capture Capacity of a Primary-Amine Based Polymeric CO2 Sorbent
Alexander P. Hallenbeck and John R. Kitchin, “Effects of O2 and SO2 on the Capture Capacity of a Primary-Amine Based Polymeric CO2 Sorbent”, Industrial & Engineering Chemistry Research 52(31), 10788-10794 (2013). https://doi.org/10.1021/ie400582a
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Question
Aqueous amine solvents such as monoethanolamine carry a large regeneration energy penalty because the water must be heated, and they lose amine through volatility, oxidation and reaction with flue gas impurities. Solid amine sorbents could lower the energy penalty, but their economics depend strongly on how long they last. Building on earlier work showing that the commercial ion-exchange resin Lewatit VP OC 1065 (a primary-amine functionalized, divinylbenzene cross-linked polystyrene) is a promising CO2 sorbent, this paper asks whether the resin tolerates O2 and SO2 at flue gas concentrations, whether any poisoning is reversible, and how activity might be restored (Hallenbeck 2013).
Approach
CO2 capacity was measured in a custom packed bed reactor holding 3 to 4 g of dried resin, with the effluent CO2 tracked by an infrared analyzer and a calibrated mass spectrometer. Adsorption was done at 50 C until breakthrough, and desorption combined an N2 purge (pressure swing) followed by heating at 3 C/min (temperature swing); capacities were obtained by integrating the effluent CO2 flow relative to a baseline, with an empirical dead volume correction of 0.12 mol CO2. Cycling tests kept the simulated flue gas flowing continuously while the bed temperature swung between 50 and 127 C. One gas contained 12 vol% CO2, 4% O2 and 84% N2 (17 cycles); the other contained 12.5 vol% CO2, 4% O2, 431 ppm SO2 and balance N2 (three runs). Combustion elemental analysis tracked C, H, N, S and O, and N2 isotherms gave BET surface areas and BJH pore sizes. Regeneration was attempted thermally (N2 up to 208 C) and chemically (1.5 M NaOH for 3 days), and a sample was also aged at 120 C in air for 7 days. All gas streams were dry because of apparatus limitations (Hallenbeck 2013).
Key findings
Elemental analysis gave 7.9% N, or 5.9 mol N/kg of total amine. Saturating the resin with sulfuric acid loaded 2.7 mol H2SO4/kg, which the authors take (assuming 1:1 stoichiometry) as the accessible amine loading, so the 2.5 mol/kg capacity from pure CO2 at 50 C is near the limit. Under 4% O2 the capacity held steady over 17 cycles at an average of 1.27 mol/kg, with variations within the 0.1 mol/kg reproducibility.
SO2 was far more damaging. Capacity fell nearly linearly over the first seven cycles and settled near 0.2 mol/kg. Essentially no SO2 escaped the bed until about the sixth cycle, after which some thermally reversible SO2 peaks appeared. Sulfur uptake measured by elemental analysis was 0.98 mol/kg, while capacity dropped by 1.31 mol/kg after nine cycles. Surface area and pore structure were unchanged (34 versus 40 m2/g, pore volume 0.23 cc/g in both), so the loss was attributed to chemical reaction of SO2 with the amines. Heating the poisoned resin to 208 C released no measurable SO2 and restored no capacity. In a control, sulfuric acid poisoning dropped capacity from 1.39 to 0.08 mol/kg and NaOH fully restored it (1.51 mol/kg, all sulfur removed), but the same NaOH treatment on the SO2-poisoned resin removed only 43% of the sulfur and recovered about 35% of the capacity (0.1 to 0.48 mol/kg). The authors suggest SO2 binds through more than one mechanism, one of them partly reversible, and that water (absent here) might favor bisulfite formation and more reversible sorption.
Prolonged heating in air at 120 C for 7 days cut capacity to 0.28 mol/kg, a 79% loss, and eliminated the temperature swing component of desorption. Pore structure was not degraded, but nitrogen content fell by an amount corresponding to 1.1 mol amine/kg, matching the 1.11 mol/kg capacity loss, which the authors interpret as irreversible amine loss through oxidation (Hallenbeck 2013).
Significance
The study shows that for this primary-amine resin, O2 is tolerable over short cyclic operation but SO2 at coal flue gas levels poisons the sorbent within a handful of cycles and cannot be removed thermally, and long continuous hot air exposure also destroys amines. The authors conclude that flue gas desulfurization (or conversion of SO2 to sulfate species) ahead of an amine sorbent is needed for economic viability, and expect other primary-amine sorbents to be similarly vulnerable. They note their tests span far fewer than the roughly 1000 cycles estimated as necessary for an economical solid sorbent (Hallenbeck 2013).