Optimization for structure of internal loop fluidized bed reactor based on computational fluid dynamics

Abstract: Internal loop fluidized bed reactor is a kind of advanced bioreactor in the field of biological sewage treatment, its fluidization working principle speeds up biochemical reactions in the reactor, it has great significance on the efficient cleaning of aquaculture wastewater. However, the reactor performance under working conditions is closely related to its internal phase fluid mechanics behavior, the complex relationship exists among fluid mechanics behavior of phase, reactor structure parameters and operating parameters, aquaculture water disposal requirements are different to other relevant fields, which make reactor design and optimization difficult. Based on the Eulerian-Eulerian model, this article selected the diameter ratio of draft tube and the reactor, ratio of draft tube height and reactor diameter, height of the gas-liquid mixture region, height of the gas-liquid separation region as structure parameters, select gas velocity of gas inlet velocity as operation parameters, and built CFD mathematical model to describe gas-liquid two phase flow in reactor with different structures and operation parameters. It analyzed the change rule of the liquid flow field, liquid circulation flow, gas holdup in the reactor with different structures and operation parameters. The conclusion is: for the reactor with inner diameter 200 mm, gas distribution plate with holes distributed uniformly at the bottom, diameter ratio of draft tube and the reactor is the pivotal parameter for determining liquid mechanics behavior of phase, when diameter ratio of draft tube and the reactor was 0.3 or 0.4, the liquid flow field is close to best circulation state; when diameter ratio of draft tube and the reactor was 0.5, the liquid flow field can achieve best circulation state; liquid circulation flow and gas holdup increase first and then decrease as the diameter ratio of draft tube and the reactor increase, when diameter ratio of draft tube and the reactor was 0.5, liquid circulation flow and gas holdup can get maximum value. Liquid circulation flow increases as the ratio of draft tube height and reactor diameter increase but gas holdup have no change. Liquid circulation flow increase as the height of the gas-liquid mixture region increase but transmission flow velocity change conversely. Liquid circulation flow increase first and then decrease as the height of the gas-liquid separation region increase, height of the gas-liquid mixture region and height of the gas-liquid separation region have no obvious effect on the gas holdup. The optimal structure parameters values when gas velocity of gas inlet velocity was 0.5-2.5 cm/s are diameter ratio of draft tube and the reactor was 0.5, ratio of draft tube height and reactor diameter was 6, height of the gas-liquid mixture region = (diameter of the reactor - diameter of draft tube)/2 mm, height of the gas-liquid separation region was 100 mm. When gas velocity of gas inlet velocity is 2.5 cm/s, liquid circulation flow and gas holdup can take a maximum value 919.2 cm3/s and 7.81%. For liquid circulation flow, it uses a relevant mathematical model to obtain the theoretical values, and compares them with the experimental values. Through the analysis of the deviation, it explores the limitation of the mathematical model application: mathematical model is a reasoning formula which regard mass conservation as premise, but reactor with special flow phenomenon, mass flow change leads to this limitation. For the operating parameters, it uses the MATLAB least square method fitting the relationship among gas velocity, liquid circulation flow, gas holdup when the reactor under the optimal structure parameters: Liquid circulation flow and gas holdup increase in direct proportion as gas velocity increase, coefficient of proportionality are 0.9908 and 0.9913. It verified the accuracy of the fitting formula, relative deviation range is ?0.3%-6%, providing theoretical and technical reference for the design and application of internal loop fluidized bed reactor in the field of aquaculture.