Operating pressure is one of the core process parameters for catalyst selection, as it affects chemical reaction equilibrium, material diffusion and structural stability of catalysts. Varying pressure conditions lead to differences in material density, adsorption characteristics and coking tendency of the system. Proper catalyst selection based on actual operating pressure can well adapt to process requirements, slow down catalyst deactivation, and improve the overall operational stability and economic efficiency of the unit.
Operating conditions under atmospheric pressure and slight positive pressure are relatively mild, imposing low requirements on the mechanical strength of catalysts. Such working conditions are commonly found in conventional environmental protection and light chemical processes including flue gas denitrification, desulfurization and VOCs treatment. Priority shall be given to catalytic activity and selectivity in catalyst selection. Conventional oxides, standard molecular sieves and dedicated catalysts for flue gas are all applicable. Excess structural strength is unnecessary, and the selection shall mainly meet the demands for routine material diffusion and catalytic reactions.
Medium and low pressure conditions range approximately from 1.0 MPa to 4.0 MPa, which are widely adopted in gas purification, organic sulfur conversion, medium and small-scale hydrofining and other processes. Elevated pressure increases the concentration of reactant molecules and reaction load, and meanwhile raises the risk of coking and pore blockage of catalysts. For this working condition, cobalt-molybdenum and nickel-molybdenum hydrotreating catalysts with excellent anti-coking performance and suitable pore structure, as well as modified molecular sieves, are preferred. Attention shall also be paid to the structural stability of carriers to mitigate pulverization and deactivation during long-term operation.
High pressure conditions above 4.0 MPa are typical of large-scale complete sets of plants such as hydrocracking, ammonia synthesis and methanol synthesis units. Continuous mechanical compression is exerted on catalysts under high pressure, and material adsorption and polymerization become more severe, which may cause carrier fragmentation, pore collapse and coverage of active sites. When selecting catalysts for high pressure service, emphasis shall be placed on mechanical strength, structural compactness and resistance to pressure fluctuation. Special hydrotreating and synthesis catalysts with high-strength sintered carriers are recommended to reduce operational risks such as rising pressure drop, catalyst damage and rapid deactivation.