Structural Anomalies in Stirred Submerged Bioreactors Relevant to Immersed Membrane Use

Abstract

Separation of value-added food additives is often practiced by micro and ultrafiltration membranes in integrated submerged membrane bioreactors (sMBR) and the flow conditions are of major importance for their performance. The immersed membranes affect fluid circulation and may cause operational difficulties. Such malfunction termed flow structural deterioration of integrated vessels is addressed in this study, based on the effect of the (1) non-Newtonian component presence, and the (2) gas flow rate. Ranges of input parameters referring to power law non-Newtonian fluids with consistency coefficients of 0.02 to 0.1Pa.sn (flow index n<1) and gas flow rate 1 - 2vvm are studied. Computational fluid dynamics (CFD) simulation of a dual impeller bioreactor equipped with a mono-tubular membrane module and alternatively flat-blade or curved-blade impellers was carried out. 3-D “k-ε” turbulent flow model is used and 2-D contour plots are worked out to illustrate cases of restricted fluid mobility in the vicinity of membrane walls. The corresponding performance parameters, gas volume fraction and fluid surface velocity are discussed. Flow structural anomalies referring to zones at the immersed membranes of extremely low membrane surface velocity (<1mm.s-1) and velocity gradients (<10s-1) that are risky for membrane fouling are uncovered.Practical applications. Fermentation of food ingredients in stirred vessels combined with recovery of the value-added product is the target application. Examples are the production and recovery of peptides, gums’ (gelatin, pectin) concentration, production of fructoolligosaccharides, galactoglucomannan, enzymatic hydrolysis combined with selective ultrafiltration in processing of vegetable proteins, production of antioxidants. Viscous dispersions of food ingredients such as starch or xanthan and operating variables - impeller speed, rate of gassing - at various level may cause undesirable effects in the bioreactor flow uniformity leading to decrease of membrane separation efficiency. The cases engaging process fluids of high consistency such as exopolysaccharide dispersions are specific in this category. Restricted fluid mobility in the vicinity of the membrane module reduces the rate of fusion across the membrane surface and blocks up the product recovery. The resulting membrane fouling and the flow structural anomalies adhere to the problem of fouling control and to submerged membrane bioreactor applications