Development of a semi-automated ZLC system for rapid screening of adsorbents for carbon capture

Abstract

In this dissertation a novel ZLC setup has been developed as part of a DOE-funded grant in collaboration with UOP, to provide rapid screening of novel adsorbent materials for carbon capture (CC). The key features of the new apparatus that was developed are: the use of 5-15 mg of sample and a dual detector system – a thermal conductivity detector (TCD) for single component measurements and a mass spectrometer for studying the influence of water and other impurities. Improvements over previous ZLC apparatuses include: 1. Extension to lower flowrates, i.e. < 3 cc/min, thereby reducing consumption of gases and allowing to run the system under equilibrium control conditions; 2. A new gas dosing system that allows the use of vapours without a chilled bath and bubbler system; 3. A new switching valve system, which prevents leakages; 4. Automated series of experiments, which are implemented using Labview. The new ZLC technique was first applied to provide rapid screening capacity ranking of more than 15 MOF materials from the open literature and three typical zeolites for carbon capture. At the point of interest for flue gas application (38°C, 0.1 bar CO2 partial pressure), Mg/DOBDC was found to outperform significantly all other MOFs and benchmark zeolites at the point of interest in low pressure physisorption of CO2. The ZLC was also used to investigate steaming on Ni/DOBDC as well as see the effect of forming powders into pellets. The new ZLC system also enables one to measure micropore and macropore diffusivity. Experiments were carried out on both powders and pellets of typical MOFs and zeolites. For Co/DOBDC crystals, since the system is close to equilibrium control even at the highest flow rate, a low limit of diffusivity can be estimated. For all the formed samples of Ni/DOBDC and 13X pellets, the results indicate that mass transfer is controlled by macropore diffusion. The ZLC technique can also estimate realistic void fraction and tortuosity values for the pellets. The new ZLC technique was applied to study the stability on the MOF M/DOBDC series. The preliminary water tests showed that all M/DOBDC samples are highly hydrophilic. Therefore in a process design using these MOFs, we conclude that there is a needs to use a guard bed layer to adsorb water or use a gas drying unit before the CO2 capture section of the plant. The ZLC system appears to be extremely useful to accelerate the deactivation of samples due to SOX and NOX impurities. The key advantages are based on the fact that the treatment can be repeated in situ, in a relatively simple way using a very small sample. The results show that in the presence of impurities and water the candidate MOFs undergo significant deactivation. The Ni based material shows the best resistance to degradation. This result indicates further that there would be a need for a drying unit prior to the carbon capture adsorption process.