Abstract: The downflow section of the catalyst circulation in fluid catalytic cracking (FCC) units is characterized by dense phase transport within the standpipe, exhibiting complex flow behavior. The standpipe operates under a negative pressure differential due to the pressure balance between the reactor and the regenerator, with the outlet pressure being higher than the inlet pressure. This negative pressure differential is a design parameter, and due to its fixed nature, there is limited scope for adjustment by industrial field technicians, resulting in scarce reports on related design and research. This study measured the axial pressure distribution in the negative pressure differential standpipe under various operating conditions in an industrial FCC unit and employed CPFD software to investigate the effects of standpipe pressure differential on axial pressure, particle concentration, and catalyst circulation rate. The findings revealed that the lower section of the negative pressure differential standpipe was susceptible to plug flow, leading to a reversal of axial pressure. Simulations indicated that the negative pressure differential impeded catalyst discharge in standpipe, increased particle storage, and enhanced pressure buildup within the standpipe. Conversely, a positive pressure differential resulted in less particle degassing, and as the positive pressure differential increased, particles in the standpipe tended towards a more uniform dense phase flow, leading to an increase in standpipe pressure buildup and catalyst circulation rate. These research outcomes could inform the design and operational adjustments of standpipe.

Key words: standpipe, negative pressure difference, positive pressure difference, catalyst flow state, mass flow rate