Structured Functionalized Active Carbon Sorbents for the Purification of Gas Streams

Abstract

Energy demand is constantly increasing as the world population and rapid industrial development grow fast. The main source of energy is represented by fossil fuels, responsible for greenhouse gas emissions into the atmosphere, global warming and climate change. In order to foster sustainable development, renewable sources are gaining great interest. However, the use of both fossil fuels and renewable sources requires purification processes of the gas streams from the energy production plants. Hydrogen sulphide (H2S), from natural gas processing, oil refining, biogas production and coal gasification, is a highly toxic compound for humans, it represents a poison for many catalysts and downstream fuel treatment devices and it is responsible for acid rains. A valid solution in industrial practice for the abatement of this pollutant is represented by the adsorption technique which can be considered an economic process combined with versatility, simplicity and high efficiency. Among the various adsorbent materials, activated carbons are widely used to remove hydrogen sulphide, as they offer a high surface area, a high pore volume and a variety of organic groups on its surface. The addition of metal oxides dispersed on the activated carbons facilitates the removal of H2S due to its high chemical affinity with metal cations. The use of activated carbons in structured form such as monoliths or foams is required in practical application when high pressure drop must be avoided. Unfortunately, most of carbon materials are available as powders since activated carbon in structured form is difficult to obtain due to the poor adhesion properties that require the use of binders which, despite giving good mechanical properties, reduce the adsorption capacity of the activated carbon monolith compared to that of the starting carbon powder due to the partial blocking of porosity. This thesis focus on structured activated carbon as sorbents for gas purification. In the first part of the work, the promoting effect of dispersed metals on commercial activated carbon monoliths was studied in order to improve the absorption properties of this material which are lower than the granular activated carbon samples which do not contain a binder. The reactive adsorption of H2S on copper and/or magnesium oxides dispersed onto activated carbon monolith was investigated in the co-presence of O2 and H2O in the gas stream at room temperature and in a lab-scale fixed-bed reactor. H2S capture rate and capacity of sorbents and the nature of sulphur species formed upon adsorption were analyzed using different techniques. Adsorption performance changes significantly depending on the metal although the adsorption is reactive in both cases. Two types of mechanisms were identified on the Cu-modified monolith: a faster mechanism associated to the formation of sulphates promoted by copper oxide and a slower mechanism involving the oxidation of H2S to elemental sulphur. Otherwise, a single adsorption mechanism is activated by magnesium, occurring through the dissociation of H2S into HS- and H+ promoted by the basic character of MgO. Moreover, a slow transformation of elemental sulphur into additional sulphate species was identified in the presence of O2 and water for saturated Cu-containing sorbents. Thermal regeneration of the saturated AC monoliths was evaluated and it was found that for both copper and magnesium the porous structure of the AC monoliths was completely restored due to the decomposition of sulphate species at a lower temperature, especially for Cu-modified sorbents, and the evaporation of elemental sulphur at a higher temperature, prevailing for Mg-modified sorbents. No significant loss of capture capacity was detected for sorbents after the thermal treatment which can then be proposed as effective and regenerable materials for gas purification In order to overcome concerns related to the addition of a binder, in the second part of the work, a new methodology was developed for the production of activated carbon as a three-dimensional microporous foam without any binder that does not involve the common complex multi-step procedure for obtaining foamy carbon with an open porosity including the template synthesis using a replica technique. ZnCl2 or CuCl2 were used as Lewis acid activators for the polymerization of furfuryl alcohol, chosen as carbon precursor, directly providing a polymeric foam due to the rapid evaporation of water formed in the reaction. Various synthesis parameters as temperature of polymerization and the composition of pyrolysis gas were explored in order to produce activated carbons with different porosity and metal load. The temperature of polymerization was selected according to the Lewis acidity strength of the two metal chlorides: lower for CuCl2 and higher for ZnCl2. The following pyrolysis of the polymer was investigated in the absence and in the presence of O2 traces at 600 or 850 °C in order to produce activated carbons with specific textural features and different metal content. Carbons with larger surface areas, also related to the presence of some mesopores, were obtained using ZnCl2 to activate the polymerization whereas CuCl2 promoted the formation of narrower micropores. Furthermore, copper was mostly preserved even at high pyrolysis temperature in contrast to zinc which was almost totally lost at 850 °C due to the much lower evaporation temperature with respect to metallic copper. The study provided a methodology to produce materials with different features for the adsorption of different molecules by suitably tuning the process parameters