关键词: 催化裂化, 汽提器, 挡板结构, 数值模拟, 双流体模型, 气固两相流, 鼓泡床

Abstract: A stripper is an important part of a fluid catalytic cracking (FCC) plant, affecting products yields, energy consumption and long-term run of the entire unit greatly. A high-efficiency stripper increases the light oil yield and improves the product distribution, moreover, it also reduces the coke-burning load of regenerator and decreases the hydrothermal deactivation of catalyst. Strippers widely used in fluid catalytic cracking units can be categorized into the packing stripper and the baffle stripper. The former owns the advantages of high stripping efficiency and better space utilization, but is not suitable for residue fluid catalytic cracking (RFCC) units, because the grid is easily blocked up with coke and difficultly cleaned. Therefore, the latter is widely used in RFCC units, benefited from simple structure, high stripping efficiency and long-term operation. Hydrodynamic characteristics of gas phase and solid phase in a commercial stripper with perforated baffle is rarely studied, subject to severe experiment condition and poor measurement methods. In this work, hydrodynamic behavior of the steam and particles in a commercial-scale fluid catalytic cracking stripper is investigated numerically, by using two-fluid model with a drag model which divides the flow region into four parts: dese, sub-dense, dilute and ultra-dilute. The mean bed density from the simulation is in good agreement with the value registered in a commercial unit. Calculation results demonstrate that strict S-shape flow of steam does not exist in stripper. Steam short circuit between the neighboring annular baffles or conical baffles occurs in the region of 0.85<r/R<1 or 0.35<r/R<0.5, respectively. Radial distribution of catalyst concentration above the baffle is uneven. The steam flux of holes on baffles reduces with the decrease in the vertical position of holes, explaining why the phenomena of the gas short circuit appears. Based on the above findings, a new baffle was proposed and adopted in order to distribute steam reasonably. Simulation results show that the steam short circuit is significantly weakened, leading to a smooth catalysts flow above the baffle and an excellent gas and solid contact.

Key words: fluid catalytic cracking, stripper, baffle structure, numerical simulation, two-fluid model, gas-solid two-phase flow, bubbling fluidized bed