Introduction - University of Edinburgh



Testing the Stability of Novel Adsorbents for Carbon Capture Applications using the Zero Length Column TechniqueXiayi Hua,c, Stefano Brandania*, Annabelle I. Beninb and Richard R. Willisba. Institute for Materials and Processes, School of Engineering, the University of Edinburgh, UKb. New Materials Research, UOP LLC, a Honeywell Company, Des Plaines, IL, USAc. College?of?Chemical?Engineering,?Xiangtan?University,?Hunan?Province,?P.?R.?China ABSTRACTIn this paper, a semi-automated ZLC technique was used to study the stability of novel adsorbents in the presence of water, SOx and NOx impurities present in coal-fired power plant flue gas. The tests were carried out at 38°C and 0.1bar pressure on the most promising materials of the M/DOBDC ((M = Co, Ni, Mg)) MOF series, as well as on commercial 13X zeolite pellets. The experimental results indicated that even if the DOBDC family shows high CO2 adsorption capacity at low partial pressure at ambient conditions, impurities have a strong effect on their stability. The ZLC system provides quantitative information on the deactivation of samples due to SOx and NOx in a relatively short time and using less than 15 mg of sample. The fact that the treatment can be repeated in situ in the apparatus used also for measuring the CO2 capacity of the sample has shown that the ZLC can be a valuable tool in screening novel adsorbents for carbon capture applications.Keywords: Zero length column (ZLC), adsorption, MOFs, CO2 capture, stability, flue gas, SOx, NOx.*Corresponding author.Email address: s.brandani@ed.ac.uk IntroductionCoal-fired power plants produce about one third of CO2 emissions; therefore, the capture of CO2 from the flue exhaust of power plants may play a significant role in mitigating the effects of CO2 on climate (Willis et al., 2010), since fossil fuels will still be a dominant source of energy supply in the foreseeable future in China and the USA. Typically the flue gases produced by coal fired power plants contain 12 to 16% CO2 by volume at ambient conditions, and impurities including water, O2, HCl, hydrocarbons, CO, SOx and NOx (Granite and Pennline, 2002). Adsorption in porous materials is known to be a potential technology for post-combustion capture because of the lower energy requirement of physisorption, but traditional zeolites may be limited by their high regeneration temperature (Harlick and Tezel, 2004), and the presence of water in flue gas can significantly affect their CO2 capacities (Brandani et al., 2004). Metal organic frameworks (MOFs) are known for their large surface areas, controllable pore structures, and versatile chemical compositions (Férey, 2008), some of them show extremely high CO2 capacity and very desirable isotherm shapes, and have therefore drawn considerable attention as potential materials for CO2 capture (Dybtsev et al., 2003; Llewellyn et al., 2006; Natesakhawat et al., 2006). Given the large number of possible novel MOF materials which can be investigated, it is important to develop rapid screening techniques which can assess experimentally the potential of these novel materials. In previous contributions the use of a purposely designed Zero Length Column (ZLC) system has been shown to provide a valuable tool to determine both adsorption capacities (Hu et al., 2015a) and mass transfer kinetics (Hu et al., 2015b) using less than 15 mg of sample. The methodology was applied to the recently developed M/DOBDC (M = Zn, Ni, Co, Mg) MOFs, which are considered to be potential candidates for carbon dioxide capture because of their extra high CO2 uptake, especially at low CO2 partial pressures (Millward and Yaghi, 2005; Mofarahi et al., 2008; Yazayd?n et al., 2009). For MOFs to be useful in an industrial application, it is important to understand how stable these materials will be when in contact with impurities present in the flue gas. Liu et al. (2010) investigated the effect of water in the adsorption of CO2 on Ni-MOF-74 and HKUST-1 at 0.1 atm and 25 ?C. Ni-MOF-74 exhibited a higher degradation resistance, maintaining most of its CO2 capacity after several cycles of water exposure and thermal regenerations. Similarly, Kizzie et al. (2011) found that despite the higher CO2 capacity, the Mg-MOF-74 sample exhibited the highest loss of capacity after exposure to water, while Co- and Ni-MOF-74 resulted the most stable samples maintaining 85% and 60% of the original capacity, respectively, after exposure to a mixture with a relative humidity of 70%. More recently experiments on MOFs stability under flue gas exposure have been presented by (Han et al. (2012); Sang et al. (2010)). The experiments were carried in a high throughput apparatus consisting of 36 sample cells each of them provided with a pressure sensor. Small changes in the capacity were observed for most of the samples after the short term exposure to humid air and acid gases, while a significant loss in the CO2 adsorption was registered after the long term exposure. Mangano et al. (2016) used the ZLC to clarify the effect of impurities and humid flue gas, on the CO2 adsorption of Mg- and Ni-CPO-27 MOF crystals by exposing the samples to a synthetic flue gas mixture containing water. The experimental results showed that Mg-MOFs exhibited the rapid deactivation simply with water present, while Ni-CPO-27 exhibited an initial decrease followed by a more stable performance through several cycles.In this contribution, the ZLC is used to study the effect of water and SOX and NOX impurities on the stability of M/DOBDC and commercial zeolites. The key advantages of this approach are the use of small samples and the ability to determine the amount of impurities that are adsorbed by the solid.Materials and Methods2.1. Adsorbent SynthesisMost MOF materials are synthesized through solvothermal reactions. The synthesis procedures for the seven MOF samples used in our research are modified from the literature (Hu et al., 2015a).Mg/DOBDC (DOBDC = dioxybenzenedicarboxylate) was synthesized by the Matzger group at the University of Michigan. Linker precursor 2, 5-dihydroxyterephthalic acid (2.98 g) was dissolved in 180 mL 1-methyl-2-pyrrolidinone (NMP) + 20 mL deionized water using sonication. Mg (OAc) 2. 4H2O (6.44 g) was added to a 0.5 L jar. The 2, 5-dihydroxyterephthalic acid solution was filtered into the jar. The solution was sonicated to dissolve completely the magnesium acetate and the jar was placed in a 120 °C oven for 20 hours. The mother liquor was decanted while still hot and the yellow solid was washed with methanol (3 × 200 mL). The methanol was decanted and replaced with fresh methanol (200 mL) three times over the course of three days. The yellow solid was evacuated at 250 °C for 16 hours and then transferred to a glove box for storage. One batch produced ~3 g of Mg/DOBDC.Ni/DOBDC, nickel(II) acetate (18.7 g, 94.0 mmol, Aldrich) and 2,5-dihydroxyterephthalic acid (DOBDC, 37.3 g, 150 mmol, Aldrich) were placed in 1 L of mixed solvent consisting of equal parts tetrahydrofuran (THF) and deionized water. The mixture was then put into a 2 L static Parr reactor and heated at 110 °C for three days. The as-synthesized sample was filtered and washed with water. Then the sample was dried in air, and the solvent remaining inside the sample was exchanged with ethanol six times over eight days. Finally, the sample was activated at 150°C under vacuum with nitrogen DOBDC, the reaction of cobalt (II) acetate and 2, 5-dihydroxyterephthalic acid (C8H6O6) in a mixture of water and tetrahydrofuran (molar ratio 2:1:556:165) under autogenous pressure at 110°C in a Teflon-lined autoclave (50 percent filling level). This yielded a pink-red, needle-shaped crystalline substance [Co2(C8H2O6)(H2O)2]8H2O. After the hypothetical removal of the noncoordinating solvent molecules in the channels, the empty channels occupy 49?percent of the total volume of the unit cell, and the average cross-sectional channel dimensions are 11.08×11.08 ?2. The empty volume increases to 60 percent if the coordinating water is also removed. The water molecules excluding and including the coordinating water account for 29.2% and 36.6% of the mass, respectively.The 13X pellets are commercial materials. 13X-APG adsorbent is the sodium form of X zeolite possessing an aluminosilicate binder. 2.2. ZLC Experimental Setup The ZLC apparatus is described in detail in our previous contribution (Hu et al., 2015a). REF _Ref431999625 \h \* MERGEFORMAT Figure 1 shows a schematic diagram of the new semi-automated ZLC apparatus used in our experiments to test the stability of novel adsorbents. Figure SEQ Figure \* ARABIC 1. Schematic diagram of the experimental system showing details of the developed semi-automated zero length column system.For the stability experiments the key feature is the gas dosing system which is designed to control the amount of water in the mixture. The dosing system includes four stainless steel cylinders (1L each) inside an oven (Sanyo, Japan) which allows keeping the gas mixtures at a temperature sufficiently high to avoid any condensation of water. Water is added to the dosing volumes using a detachable capsule similar to that described by Zielinski et al. (2007). Initially the dosing volume is evacuated and filled to approximately 0.5 bar with either the carrier gas or the gas mixture and the valve that connects the capsule to the dosing cylinders is opened slowly. The amount of water dosed is checked by a mass balance with the equilibrium pressure in the system and by detaching the capsule and recording the mass change of the capsule using a Mettler-Toledo balance (Model No.XS205). Mixtures with known concentration of water vapour can be prepared accurately up to 1% v/v. This limit is imposed in order to avoid condensation of water in the mass flow controllers. Given the high affinity to water of all the materials studied, this concentration allows to reach near saturation in the adsorbed phase, ie a higher partial pressure of water would not change the adsorbed phase water concentration.In the flow control part, a low flow rate of 1-5 cc/min was applied to the measurement, and all the lines in the system were heated to 90 C to avoid water condensation. Four different mixtures were prepared in the dosing volume (i) 16% v/v CO2 in nitrogen, which is for the CO2 capacity measurement on the adsorbent; (ii) 1% v/v water in nitrogen, which is for the initial water stability measurements, (iii) 1% v/v water and 16% v/v CO2 in nitrogen, for a further stability measurement; (iv) 16% v/v CO2, 100 ppm SO2 and 10 ppm NO, with balanced of N2, supplied by BOC, being utilized. Water could not be included in the mixture supplied by BOC, since at high pressure (200 bar) it would have condensed in the cylinder leading to safety and corrosion issues. Since the ZLC system contains an independent dosing volume, a known concentration of water was added to the flue gas. Oxygen was not included since it is incompatible with SO2, but this would likely not have a major effect, since adsorption of O2 and N2 on MOFs are similar. The characteristics of the synthetic wet flue gas are shown in Table 1.Table 1. Key wet flue gas characteristics for ZLC testsFlue gasCO2H2ONOSO2N2Concentration16%1%10ppm100ppmBalanceThe ZLC cell is made of a Swagelok 1/8?in. fitting and the 10-15mg of sample is sandwich packed by two porous sinter discs in the fitting and weighted in a Mettler-Toledo XS205 dual-range balance. Prior to the experiment, the ZLC column was thermally regenerated with a ramping rate of 1°C/min to the required temperature, as described by Hu et al. (2015a), and held at this temperature for 12h at a very low flow rate of the helium purge gas, 1 cc/min. After regeneration, the oven temperature was reduced to the desired temperature (38 °C) automatically. A typical ZLC experiment was then carried out on a fresh sample, which was first equilibrated with nitrogen stream containing 16% of CO2 that was prepared in the dosing volume. At time zero, the flow was switched to a pure nitrogen purge stream at the same volumetric flow rate. For each sample, the ZLC experiment runs at 2 different flow rates performed in the range of 1-3?cc/min and from the desorption curve the CO2 adsorption capacity was determined (Brandani et al., 2003; Hu et al., 2015a; Wang et al., 2011). Thereafter, the sample was exposed to the relevant gas mixture for 24 hours, followed by intermediate regeneration (same procedure as fresh sample). The ZLC runs were carried out on the adsorbent to evaluate the residual CO2 capacity. If the sample was still active, a further 24 hours exposure was applied until the sample became inactive or until it maintained a constant capacity. TheoryZLC measurementIn a ZLC experiment, the column is short enough to be treated as a well-mixed cell with negligible external mass and heat transfer resistance. In a desorption experiment the differential mass balance is therefore given by: (1)where Vg is the volume of gas; Vs represents the volume of solid; F is the volumetric flowrate; is the average adsorbed phase concentration and c is the sorbate concentration in the gas phase. During the experiment the gas phase concentration is measured and the carrier flowrate is assumed constant.With the assumption that equilibrium is maintained between the gas and the solid phase; ideal gas behaviour; isothermal system; and a constant carrier flowrate, ie calculating the total flowrate as F = Fc/(1y), Eq. (1) is integrated to obtain: (2)Where is the flowrate of the carrier gas, y is the mole fraction and C is the total concentration which can be calculated from the ideal gas law, . While the flowrate of the carrier gas is not strictly constant, Eq. 2 is normally valid up to mole fractions of the adsorbate of 0.4-0.5. Integration of Eq. 2 allows to calculate either the total capacity or the full adsorption isotherm.Analysis as breakthrough curvesThe ZLC can also be seen as a breakthrough experiment, especially for strong adsorbed components, i.e. SO2, based on the mass balance, the total accumulation in the column can be written as: (3) With the assumption that gas and solid phase are at equilibrium; ideal gas behaviour; and an isothermal system, the eq. (3) can be rewritten in terms of the carrier gas flowrate and the adsorbate mole fraction as (4) Where represents the slope of the secant of the adsorption isotherm between the two end points. The retention volume for a packed ZLC column, which is easily determined from the first moment of the response at a known flow rate is directly related to the dimensionless Henry constant, Assuming a constant carrier flowrate, eq. (4) is integrated to obtain: (5) Eq. (5) is only suitable for small step changes and the carrier flowrate can be assumed to be constant. 4. Results and Discussion4.1 Water stability REF _Ref379813307 \h \* MERGEFORMAT Figure 2 shows the ZLC response curves at 38 °C for Co/DOBDC after exposure to mixtures of 1% v/v water in nitrogen. Since the area between the desorption curve and the blank response is proportional to the CO2 capacity, it is possible to see clearly the effect of water on the stability of this material. REF _Ref379813316 \h \* MERGEFORMAT Figure 3 shows the results for Ni/DOBDC which is very stable in the presence of water. Figure 4 shows the results for Mg/DOBDC which is better than the cobalt sample, but loses half of its CO2 capacity after the first 24 hours of exposure. Thereafter, the rate of loss in capacity decreases. REF _Ref379813499 \h \* MERGEFORMAT Figure 5 shows the relative CO2 capacity obtained from the mass balance of the ZLC (Brandani et al., 2003; Brandani, 1998; Hu et al., 2015a) versus specific gas volume passed over the sample, i.e. the gas flowrate times the exposure time divided by the sample mass. The figure shows the quantitative comparison between the three MOFs. Although these three MOFs are synthesized via solvothermal reactions, Co/DOBDC and Mg/DOBDC clearly lack stability to moisture. These two materials are therefore very unlikely candidates for carbon capture applications.Figure SEQ Figure \* ARABIC 2. Co/DOBDC stability results after exposure to humidity (1% v/v water in nitrogen) gas. Ft plot gives a direct CO2 capacity comparison with different exposure work and also blanks.Figure SEQ Figure \* ARABIC 3. Ni/DOBDC stability results after exposure to humidity (1% v/v water in nitrogen) gas.Figure SEQ Figure \* ARABIC 4. Mg/DOBDC stability results after exposure to humidity (1% v/v water in nitrogen) gas.Figure SEQ Figure \* ARABIC 5. Comparison of the stability tests with humidity gas (1% v/v water in nitrogen).4.2 Water and CO2 stabilityThe three MOFs were then exposed to 1% water, and 16% CO2 in nitrogen. The results which can be seen in Figures 6-8 show a similar trend observed for the water stability, ie the Ni/DOBDC sample is the most stable. What is important to note is that in the presence of water the resulting acidic environment does have an impact and accelerates the deactivation for all the samples, including the Ni/DOBDC. This effect was not observed in previous studies.A quantitative comparison of the three MOFs is shown in REF _Ref379814024 \h \* MERGEFORMAT Figure 9. Figure SEQ Figure \* ARABIC 6. Co/DOBDC stability results after exposure to humidity (1% v/v water, 16% v/v CO2 in nitrogen) gas.Figure SEQ Figure \* ARABIC 7. Ni/DOBDC stability results after exposure to humidity (1% v/v water, 16% v/v CO2 in nitrogen) gas.Figure SEQ Figure \* ARABIC 8. Mg/DOBDC stability results after exposure to humidity (1% v/v water, 16% v/v CO2 in nitrogen) gas.Figure SEQ Figure \* ARABIC 9. Comparison of the stability tests with humidity gas (1% v/v water, 16% v/v CO2 in nitrogen).4.3 Wet flue gas stabilityAn important feature of the ZLC system is the fact that the gas at the outlet can be monitored continuously. This allows to ensure that during the exposure the sample reaches equilibrium, i.e. that enough impurities are adsorbed. When using very dilute impurities one must ensure that the sample saturates over the time of exposure. REF _Ref419643841 \h \* MERGEFORMAT Figure 10 shows a typical MS signal (CO2, H2O, SO2 and NO) for the Co/DOBDC sample during the first 24 hours exposure. It is clear that CO2 and NO reach equilibrium in a few minutes. For the more strongly adsorbed species the ZLC is behaving like a normal breakthrough apparatus and more than 8 hours are needed to reach equilibrium for H2O and SO2. Similar curves were obtained for the different samples. Figure SEQ Figure \* ARABIC 10. Monitored MS signal (CO2, H2O, NO, SO2) of first 24 hours wet (1% v/v water) flue gas exposure for Co/DOBDC.The results of wet flue gas stability on M/DOBDC MOFs and zeolite 13X are shown in Figures11-14. From REF _Ref379814051 \h \* MERGEFORMAT Figure 11, one can see that the behaviour of the Co/DOBDC sample is significantly different from the other two stability tests. What is interesting in this case is the fact that some residual capacity is observed and the shape of the isotherm has changed. The effect of the wet flue gas seems to result in stronger adsorption sites, so the isotherm, after the first treatment, shifts to a non-linear one. Figure SEQ Figure \* ARABIC 11. Co/DOBDC stability results after exposure to wet (1% v/v water) flue gas.The Ni/DOBDC sample showed excellent stability in water, with full regeneration at 150 °C (Figure 11). When the samples were tested with wet flue gas there was a progressive loss in capacity. What is interesting to note from the stability results is that the capacity loss seems to be uniform, i.e. at each extended exposure more material is deactivated but the general shape of the isotherm and of the ZLC response remains the same. Figure SEQ Figure \* ARABIC 12. Ni/DOBDC stability results after exposure to wet (1% v/v water) flue gas. REF _Ref379814054 \h \* MERGEFORMAT Figure 13 shows the results for the Mg/DOBDC sample. There is an additional effect due to the flue gas, but it is clear that in this case most of the deactivation is due to the effect of water. For this sample, the shape of the ZLC curve does not change qualitatively. REF _Ref379814057 \h \* MERGEFORMAT Figure 14 shows the results for zeolite 13X plotted in semi-logarithmic form because the effect of purities in wet flue gas on this zeolite was very small. There is some loss in the first 24 hours of exposure, followed by a slower decay.Figure SEQ Figure \* ARABIC 13. Mg/DOBDC stability results after exposure to wet (1% v/v water) flue gas.Figure SEQ Figure \* ARABIC 14. semi-logarithmic plot of 13X stability results after exposure to wet (1% v/v water) flue gas.Since the ZLC is behaving like a normal breakthrough apparatus in this case, the first moments (mean residence time) analysis was applied to the SO2 breakthrough curve (i.e. REF _Ref419643841 \* MERGEFORMAT Figure 10) for the three M/DOBDC MOF and 13X samples, which is related the adsorption capacity. For Co/DOBDC, it is clear in REF _Ref432094948 \* MERGEFORMAT Figure 15 that there is a very sharp reduction after the first exposure. However, in the case of Ni/DOBDC ( REF _Ref432094968 \* MERGEFORMAT Figure 16), the SO2 capacity follows a similar trend as the CO2 capacity. In Mg/DOBDC ( REF _Ref432094969 \* MERGEFORMAT Figure 17), a clear reduction of SO2 capacity is shown as well. For zeolite 13X ( REF _Ref432094971 \* MERGEFORMAT Figure 18), the SO2 capacity also shows a slower decay as CO2 capacity measured after exposure.Figure SEQ Figure \* ARABIC 15. Moment analysis on SO2 breakthrough curves for Co/DOBDC during long time wet flue gas exposure.Figure SEQ Figure \* ARABIC 16. Moment analysis on SO2 breakthrough curves for Ni/DOBDC during long time wet flue gas exposure.Figure SEQ Figure \* ARABIC 17. Moment analysis on SO2 breakthrough curves for Mg/DOBDC during long time wet flue gas exposure.Figure SEQ Figure \* ARABIC 18. Moment analysis on SO2 breakthrough curves for 13X during long time wet flue gas exposure.To confirm that the samples had degraded, the ZLC columns treated with the wet flue gas were sealed and sent for further characterisation X-Ray Diffraction (XRD) to compare the crystallinity of the freshly activated sample as well as the effect of flue gas.The typical XRD patterns shown in REF _Ref419642894 \h \* MERGEFORMAT Figure 19 give the comparison of fresh activated Mg/DOBDC and the sample after being exposed to wet flue gas for a long time. The significant reduction in peak intensity for samples exposed to the flue gas compared to the fresh activated sample indicates the transition to an amorphous structure. This confirms the reason of the CO2 capacity reduction after the exposure experiment compared to the original sample.Figure SEQ Figure \* ARABIC 19. XRD spectra for Mg/DOBDC original sample (up), exposed to 1% wet flue gas at 38?C, 3cc/min for 72h. REF _Ref379814151 \h \* MERGEFORMAT Figure 20 shows the quantitative comparison of effect of the wet flue gas on CO2 adsorption for M/DOBDC powders and 13X zeolite pellets. It is obvious that the flue gas does not affect CO2 adsorption in 13X as much as for all the M/DOBDC MOFs. Zeolite 13X only showed some loss of capacity after the first exposure, followed by a slow secondary decay. Figure SEQ Figure \* ARABIC 20. Comparison of the stability tests with wet (1% v/v water) flue gas. ConclusionsThe ZLC apparatus has been shown to provide an effective system to test the stability of novel M/DOBDC MOF materials subject to various mixtures related to flue gases of coal-fired power plants. The use of very small samples results also in a much reduced consumption of gases for the testing procedures in comparison to traditional breakthrough experiments. The fact that the exposure to impurities and water can be repeated in situ in a relatively simple way has shown that the technique is an important tool in the screening of novel adsorbents for carbon capture applications. By monitoring directly also the impurities, the ZLC allows to determine the time needed to saturate the samples with impurities, pointing to an important factor to consider when comparing different studies on the stability of novel materials.All the DOBDC MOFs were found to be strongly affected by either water (Mg and Co forms) or wet flue gas conditions (Ni/DOBDC). The lack of water stability tends to exclude Mg/DOBDC and Co/DOBDC as potential materials in practical applications. The progressive deactivation of Ni/DOBDC under acidic conditions emphasises the need to include guard beds in the design of a carbon capture system based on this material. AcknowledgementsThis project was supported by the U.S. Department of Energy through the National Energy Technology Laboratory under Award No. DE-FC26-07NT43092, National Natural Science Foundation of China (21506179) and Research?Foundation?of? Education Bureau?of Hunan?Province, China(17B255). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. ReferencesBrandani, F., Rouse, A., Brandani, S., Ruthven, D.M., 2004. Adsorption Kinetics and Dynamic Behavior of a Carbon Monolith. Adsorption 10, 99-109.Brandani, F., Ruthven, D., Coe., C.G., 2003. Measurement of Adsorption Equilibrium by the Zero Length Column (ZLC) Technique Part 1:? Single-Component Systems. Ind. Eng. Chem. Res 42, 1451-1461.Brandani, S., 1998. Effects of nonlinear equilibrium on zero length column experiments. Chem. Eng. Sci 53, 2791-2798.Dybtsev, D.N., Chun, H., Yoon, S.H., Kim, D., Kim, K., 2003. 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