Laminate zeolite structure prepared using papermaking techniques for carbon dioxide capture: synthesis, characterisation and performance
2017-02-23T01:41:57Z (GMT) by
Carbon capture and storage technology has become a possible solution to address the problem of global warming due to the increasing use of fossil fuels. The application of this technology by many carbon dioxide emitting industrial units is still hindered by the process and equipment cost associated with the capture of carbon dioxide. Zeolites have become a major player in this field due to their high adsorption capacity for carbon dioxide. Traditional use of zeolites in the form of pellets or beads has issues relating to high mass transfer resistance and increased energy consumption required to overcome high pressure drop for such systems. Novel structured adsorbents have been developed to manage these problems but they require high precision and cost for their manufacture. Although they are superior to beads/pellets in terms of breakthrough characteristics, they have very low loading of adsorbent. To overcome the limitations of high cost and low loading of non-particulate structures and the disadvantages of the conventional adsorbent structures, new preparatory techniques for adsorbent structures are being researched. For more practical applications, a zeolite structured adsorbent needs to be created that has high adsorption capacity, low pressure drop, high mass transfer and provides overall high system efficiency. In addition, the very large scale of the flue gas capture application demands that the adsorbent structure should be made using low cost materials and processes. Motivated by the low cost of papermaking technology, we have studied and adapted this technique to create a highly loaded, large surface area sheet structure. Zeolites have already found use in the paper industry as fillers and retention aids and can therefore be easily adapted to current paper mills to create structured laminate adsorbents. This project looks towards creating zeolite laminate structured adsorbent using papermaking techniques for carbon dioxide capture. The selection of materials for the preparation of laminate structures is initially done to create structures that are strong and have high porosity with high loading of zeolite. An easy method of preparing laminate zeolite sheet structures is also outlined. This leads to the next part of the project which is characterisation of the zeolite laminate sheet structures. Although few materials were used in preparation of the sheets, the many variables involved in sheet forming would make it very time consuming to create each unique sheet structure and characterise the properties. Hence, to speed up the process, a partial factorial design method was used to characterise the laminate sheets. 16 experiments were completed. SEM and mercury porosity methods were used to characterise the porosity of the sheet structures. These results show that porosity of the structure is mainly affected by the amount of silica present in the structure. Nitrogen adsorption and carbon dioxide adsorption measurements were studied to determine the loading in the laminate sheet structures, since the adsorption capacity is completely dependent on the zeolite. The strength of the laminate structures were measured using a novel testing method as it was difficult to apply the routine strength measurement methods to some of the weak laminate samples. Considering the application of the laminate structures, a vibratory sieve was used to determine the comparative strength of the different structures from the partial factorial design. It was found that laminate structures with either 40-55wt% micron zeolite or 30wt% nano zeolite had adequate strength for breakthrough testing. Structures with higher loading of zeolite fell apart very easily. The kinetics of the laminate samples were also measured using the Rate of Adsorption software in ASAP 2010. These results show that the presence of nano zeolite increases the adsorption kinetics of the laminate structures and on the whole the laminate structure had higher kinetics when compared to powder or beads due to overcoming macropore diffusion resistance. The structures which had the best characteristics from the partial factorial design were used for breakthrough testing. Testing for different porosities, it was found that structures with high porosity i.e. samples having 40wt% zeolite had higher effective diffusivities and lower pressure drop when compared to structure with low porosity i.e. samples having 20wt% zeolite. Other methods of increasing spacing between the folds of the laminate structures were also tested and using a wire to create spacing was found to be the best method to lower pressure drop. In some instances, the effective diffusivities were reduced while in other instances they were the same. But the method of winding to produce laminate structures for breakthrough testing produced highly variable results. An idea of printing hydrophobic channels on the paper samples was considered to improve breakthrough characteristics of the laminate structures. Since printing is easy, cheap and can be adapted to a large industrial setting, this method of creating channels for gas flow in the laminate structures has many advantages. An initial assessment of this method showed that using AKD to create hydrophobic patterns on the sheet was successful as different levels of depth were observed between the AKD treated and untreated sections in the final laminate structure. To create distinguishable regions of AKD treated and untreated sections, the amount of the zeolite loading and ceramic loading were also fixed at 8g zeolite and 6.5g ceramic fibres. A method of printing parallel hydrophobic channels was established and the sheets were coated with two different weight % of colloidal silica (40wt% and 20wt%). These differing amounts of colloidal silica create laminate structures with different depths of channels as seen from SEM imaging. Breakthrough assessment of the printed laminate structures show that the structures with 20wt% silica have higher effective diffusivities and lower pressure drop when compared to structures with 40wt% silica or structures without hydrophobic printing, showing that laminate structures with printed channels is a very attractive adsorbent structure for CO2 capture.