Introduction
Addressing climate change requires significant efforts to curtail greenhouse gas emissions, in particular anthropogenic CO2 as well as acidic SO2 and NO, to keep the average increase in global temperature below 2 °C and to achieve negative emissions by the year 2100. Although a significant amount of recent research has developed innovative technologies for SO2, NO, and CO2 capture from large-point sources, such as affluent post-combustion streams,1–3 the amine scrubbing method, which is plagued by inherent drawbacks including poor recyclability, deterioration of equipment, energy requirements as high as 4.2 GJ/tonne CO2, and poor performance in the presence of SO2 or NO,4–7 remains the benchmark technology. In that regard, developing cost-effective and more efficient capture strategies is essential to addressing the impending issue of climate change.
In recent years, adsorption, a technique wherein target gases are physically (physisorption) or chemically (chemisorption) adhered to a selective medium, has cemented itself as an alternative to amine scrubbing technology for CO2capture.8,9 To bring adsorptive CO2capture technology one step closer to large-scale implementation, adsorbent materials are required to be shaped into practical contactors that not only demonstrate low mass and heat transfer resistances but also address the operational challenges such as attrition and pressure drop. Honeycomb monoliths with uniform channels and thin walls are considered suitable configurations for this purpose. To date various CO2 adsorbents including zeolite 13X, activated carbon, and metal-organic frameworks (MOFs) have been shaped into monolithic structures and their CO2 capture performance has been investigated.10–12
While much is known about the performance of CO2adsorbents under binary feed conditions (dry CO2/N2 feed), the multicomponent data is quite scarce, especially under realistic feed conditions, where trace contaminants like SOx and NOx are present. To date, most studies have focused solely on examining the zeolite stability in the presence of these species, or have focused only on equilibrium adsorption of each species, but have not compared clean and contaminated feeds to one another under dynamic conditions.13,14 For example, Deng et al.13 showed that zeolite 13X has a high affinity towards SO2 on account of its dipole moment, and predicted that this would lead to reduced CO2 adsorption in multicomponent streams, however, it has yet to be studied how the presence of multiple species impacts CO2 adsorption on zeolite 13X. Moreover, these effects are likely to be further complicated by the presence of water, which is known to co-adsorb alongside CO2 under clean-feed conditions and reduce the adsorption capacity. For example, our recent study12demonstrated that pre-humidifying zeolite 13X monoliths before exposure to CO2 leads to competitive adsorption between the two gases and a reduction in CO2 uptake of 60-80%. Other studies have demonstrated similar effects of humidity on CO2 adsorption in zeolite 13X materials,15,16 however, no study yet exists which isolates the combined influences of SOx, NOx, and H2O on CO2 adsorption in zeolite 13X.
Aiming at addressing this question, we embarked on a study to systematically investigate the CO2 adsorptive performance of zeolite 13X in both honeycomb and bead forms under various simulated flue gas conditions. We primarily addressed relative CO2 adsorption changes imparted by competitive/cooperative adsorption of flue gas impurities. Two self-standing commercial zeolite 13X monoliths, with 600 and 800 cells per square inch (cpsi) cell density and beads with particle size of 1.6 mm were used in this work. Multicomponent breakthrough runs were conducted with a simulated flue gas consisting of 10%CO2/1000 ppm SO2/1000 ppm NO/balance with N2 under dry and humid conditions and the effect of feed conditions on CO2 capture performance of 13X samples were investigated accordingly.
Experimental Section
Materials Synthesis and Characterization
All ultra-high pure and mixed gases were purchased from Airgas. The crystallinity of zeolite samples was investigated by X-ray diffraction (XRD) measurements on a PANanalytical X’Pert multipurpose X-ray diffractometer with a scan step size of 0.02°/step at the rate of 147.4 s/step. The textural properties were evaluated by N2physisorption isotherms at 77 K on a Micromeritics (3Flex) instrument. Prior to the measurements, the adsorbents were degassed under vacuum for 6 h at 350 ºC on a Micromeritics Smart VacPrep system. The Brunauer-Emmet-Teller (BET) and non-local density functional theory (NLDFT) methods were used to estimate the surface area and pore size distribution (PSD), respectively. Thermogravimetric analysis (TGA) experiments were carried out to determine the amount of zeolite in each sample. Therein, the temperature was ramped at 15 °C/min from 25 °C-700 °C in 60 mL/min air.
CO2 and N2 Adsorption Isotherm Measurements
Pure gas adsorption isotherms for CO2 and N2 were obtained on a volumetric gas analyzer (3Flex) at 25 ºC. Prior to measurement, the samples were degassed using the conditions from N2 physisorption. The isotherms were then used to estimate the ideal selectivity values using ideal-adsorbed solution theory (IAST). Additionally, TGA was also used to measure the CO2 uptake capacity of fresh samples and used samples (after breakthrough tests) using a 10% CO2/N2 gas mixture at 25 ºC and flow rate of 60 mL/min. Before the uptake runs, samples were degassed at 350 ºC under N2 with a flow rate of 40 mL/min for 1 hr.
Multicomponent Breakthrough Tests
The breakthrough experiments were performed using the setup described in our recent work. 12 The multicomponent adsorption tests were carried out at 1 bar and 25°C in a stainless-steel breakthrough column with an inner diameter of 3.2 cm and a length of 15 cm. Four experimental conditions were investigated: dry-clean, humid-clean, dry-contaminated, humid-contaminated. Under dry conditions, 10% CO2/N2 (dry-clean) or 10%CO2/1000 ppm SO2/1000 ppm NO/ N2 (dry-contaminated) gas mixtures were sent through the bed with a flow rate of 100 mL/min whereas, under humid conditions, the sample was first pre-humidified by sending a water–saturated He flow at 100 mL/min to the bed for 30 min. The He stream was humidified by passing the gas through a bubbler before entering the column. Before the breakthrough runs, the adsorbent was regenerated in-situ at 350 °C for 3-6 h under He flow at 100 mL/min and then cooled down to 25 °C to start the test. The concentration of gas mixture in the outlet stream was measured using a mass spectrometer (BELMass). Table 1summarizes the geometric characteristics of the adsorbents for the three samples. Notably, the weights were not held constant across sample runs. This was necessary to completely pack the bed. Because the samples contained different infill percentages, the amount necessary to achieve 7.5 cm of bed packing changed between samples.
Table 1. Characteristics of the adsorption column for three samples.