Combined rotational and capillary rheomtery to determine slip coefficients and other rheological properties of orange pulp.

Abstract

The characterization of the rheological properties of orange pulp under typical processing temperatures is needed for the design and optimization of orange pulp processing systems. The flow of orange pulp produced slip at shear rates at ∼1 to 5 s-1 . Rotational rheometry revealed that the flow behavior of orange pulp before slip occurrence followed the Power Law model for concentrations of ∼500 to 800 g/L at 4 to 80°C. The consistency coefficient (K) ranged from 33 to 234 Pa·sn and the flow behavior index n ranged from 0.18 to 0.24. Both, K and n decreased with temperature. While K fitted well an Arrhenius-like model, n best fitted a linear model. As concentration increased K increased linearly, while n was not significantly (P >0.05) affected. The flow without slip was calculated using the Power Law parameters from rotational rheometry and the wall shear stress (σw ) from capillary rheometry for the experimental flow rates. This allowed calculating the corrected slip coefficient βc and obviated the need for pipes with multiple diameters. βc decreased by one order of magnitude when temperature increased from 4 to 50 °C when σw was 0.1kPa. The effect was exacerbated with increased flow rate. Similarly, βc increased by about one order of magnitude when pulp concentration increased from ∼550 to 850 g/L at 80°C. The increase in βc with temperature indicated that the effect of temperature in the consistency of the bulk was different from its effect on the consistency of the liquid phase near the pipe wall. PRACTICAL APPLICATION: Design and optimization of processes equipment and industrial handling systems of orange pulp require detailed knowledge of their rheological (flow) properties. Citrus pulp like fruit pastes and purees produce less friction than one would anticipate when they flow because the liquid fraction acts as a lubricant. This study presents an original method for such characterization and shows that wall slip is greatly affected by temperature and concentration.