Gas separation is important in many industries for removing undesirable contaminants from a gas stream and for achieving a desired gas composition. For example, natural gas from many gas fields can contain significant levels of H2O, SO2, H2S, CO2, N2, mercaptans, and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases (e.g., H2S and CO2) be removed from natural gas as possible to leave methane as the recovered component. Small increases in recovery of methane can result in significant improvements in process economics and also serve to prevent unwanted resource loss. It is desirable to recover more than 80 vol %, particularly more than 90 vol %, of the methane when detrimental impurities are removed.
Additionally, synthesis gas (syngas) typically requires removal and separation of various components before it can be used in fuel, chemical and power applications because all of these applications have a specification of the exact composition of the syngas required for the process. As produced, syngas can contain at least CO and H2. Other molecular components in syngas can be CH4, CO2, H2S, H2O, N2, and combinations thereof. Minority (or trace) components in the gas can include hydrocarbons, NH3, NOx, and the like, and combinations thereof. In almost all applications, most of the H2S should typically be removed from the syngas before it can be used, and, in many applications, it can be desirable to remove much of the CO2.
Adsorptive gas separation techniques are common in various industries using solid sorbent materials such as activated charcoal or a porous solid oxide such as alumina, silica-alumina, silica, or a crystalline zeolite. Adsorptive separation may be achieved by equilibrium or kinetic mechanisms. A large majority of processes operate through the equilibrium adsorption of the gas mixture where the adsorptive selectivity is primarily based upon differential equilibrium uptake of one or more species based on parameters such as pore size of the adsorbent. Kinetically based separation involves differences in the diffusion rates of different components of the gas mixture and allows different species to be separated regardless of similar equilibrium adsorption parameters.
Kinetically based separation processes may be operated as pressure swing adsorption (PSA), temperature swing adsorption (TSA), partial pressure swing or displacement purge adsorption (PPSA) or as hybrid processes comprised of components of several of these processes. These swing adsorption processes can be conducted with rapid cycles, in which case they are referred to as rapid cycle thermal swing adsorption (RCTSA), rapid cycle pressure swing adsorption (RCPSA), and rapid cycle partial pressure swing or displacement purge adsorption (RCPPSA) technologies, with the term “swing adsorption” taken to include all of these processes and combinations of them.
Traditionally, adsorptive separation processes use packed beds of adsorbent particulates. However, the traditional packed beds are not likely to meet the very stringent requirements for natural gas cleanup. Alternatively, a structured adsorbent bed can be utilized to adsorb certain gas species. The structured adsorbent bed can be a monolith, either in the form of one single block or in the form of extrudates with multiple channels or cells, such as a honeycomb structured monolith. The use of adsorbent monoliths provides one approach to designing an adsorbent bed that has low pressure drop, good flow distribution, and low dispersion. Monoliths have very low flow tortuosity and can also be engineered for almost any user specified void volume to meet a specified pressure drop. Other monolith advantages include avoidance of bed fluidization or lifting. In addition to gas separation processes, these types of monoliths have historically been employed as catalyst supports in automobile catalytic converters, catalytic combustion, electrochemical reactors and biochemical reactors.
In order to prepare the monoliths for use in gas separation processes or as catalyst supports, the cells are washcoated with layers of catalytic or adsorbent coatings. The cell density of the monolith and the size of the particles in the coating have a significant effect on the ability to successfully coat the cells in the monolith to provide a structured adsorbent bed. It is known that coating difficulty increases as the cell density of the monolith increases (i.e., the channel size of the monolith decreases), as the size of the particles in the coating increases over 2 μm, as the number of coatings increase and as substrate porosity decreases toward zero porosity. For example, Agrafiotis, C. et al. report that the size of the suspended particles affects the adhesion of the washcoat on the substrate, namely particles with a diameter of less than 2 μm have increased adhesion to a monolith with a cell density of 400 cells per square inch (cpsi) than larger diameter particles. J. Mater. Sci. Lett., 18:1421-1424 (1999). Thus, typically the monoliths used in practice have lower cell densities (e.g., 300-900 cpsi), the coatings contain small particles (e.g., diameter less than 2 μm) and/or the coating is applied in very thin layers (e.g., 1 μm to 10 μm). For example, while U.S. Pat. No. 6,936,561 reports a coating layer thickness above 300 μm on a ceramic honeycomb monolith, the monolith has a low cell density of about 45 cpsi. Similarly, U.S. Pat. No. 7,560,154 reports a method of manufacturing a honeycomb structure with a coating particle size of 15 to 75 μm, but the cell density of the structure is 260 cpsi.
However, kinetic separation processes, specifically rapid cycle kinetic separation processes require structured adsorbent beds with ultra high cell density (i.e., greater than 1000 cpsi) and thicker coating layers. Furthermore, larger particle sizes in the coating are desirable because further milling to reduce the particle size can be avoided, thereby avoiding potential fracturing of the particles which can result in diminished capacity and activity. Therefore, there is a need to provide structured adsorbent beds with ultra high cell density as well as thicker coating layers and larger particles sizes in the coating.