ANALYSIS OF PHYSICAL PROPERTIES AND EXPERIMENTAL BEHAVIOUR OF HIGH TURBULENT FLOW IN CROSS-FLOW MICROFILTRATION OF ACAI JUICE

This paper presents an experimental investigation of the cross-flow microfiltration process applied to the clarifying of acai (Euterpe oleracea Mart.) juice. Acai juice is a complex fluid, similar to a suspension of particles (fibers and cellulose) mixed in water, which contains ions of iron, zinc, maganese and pigments, as anthocyanins. In this study, a commercial membrane of alpha-alumina (Al2O3) in the form of a tube with 1.2 mu m of average pore size was utilized to investigate the clarifying of juice. This pore size of the ceramic structure was utilized in an attempt to reduce the polarization phenomenon and improve the permeate flux without utilizing the usual enzymatic treatment made in the microfiltration processes. The rheological behaviour of the suspension was investigated in a cone/plate rheometer (model, DVIII-Ultra) and a cylindrical rheometer (model, DVIII+), both by Brookfield/USA, as the shear stress (tau) in function of shear rate (gamma) was fitted and analyzed with the power-law and Herschel-Bulkley's models. All the mixtures showed flow behaviour index values (n) near to one, characterizing Newtonian fluids (pseudo-plastic). The particle size distribution (PSD) of the samples of suspension and permeate were analyzed by APS100 (ultrasound spectroscopy) by Matec/USA. The analysis of the suspension showed the presence of particles of size equal 0.16micra, while the permeate did not present particles. The experiments were performed in a turbulent range higher than 2400 until 57500 and with variation to values of transmembrane pressure from 1 to 4bar; the usual and direct correlation between transmembrane flux and transmembrane pressure was not observed in the experiments and a new correlation to the dimensionless of TMP (trans-membrane pressure) and Reynolds (Re) was presented.