Abstract

Packed beds of spherical particles in a cylindrical vessel have a high porosity region next to the vessel wall that allows preferential fluid flow. Consequently, there are radial variations in porosity (epsilon) and superficial fluid velocity (U) that depend on the vessel-to-particle diameter ratio (D/d(p)) and the flow regime of the fluid. This work ascertained if these radial variations affected SuperCritical (SC) CO(2)extraction curves of oil from pre-pressed seeds at 40 degrees C and 28 MPa, as compared with the commonly adopted plug flow condition. It focused specifically on comparing extraction curves as a function of the controlling mass transfer mechanism (characterized by the dimensionless Biot number,Bi) andD/d(p)ratio. A predictive model was adopted to describe the SC-CO(2)extraction of oil from sheared seeds comparing plug flow with radial variations in superficial CO(2)velocity,U(r), from literature correlations. Selected independent variables included the initial oil content of the substrate (132.7 <= C-o <= 397.2 g/kg),d(p)(1 or 2 mm),U(1-4 mm/s), and vessel volume (0.038-495 L).C(o)markedly affected the effective diffusivity of the oil (0.780 <= D-e <= 6.24 x 10(-10) m(2)/s), whereasd(p)andUmoderately affected the film mass transfer coefficient (2.44 <= k(f) <= 7.40 x 10(-5) m/s). Radial variations in superficial CO(2)velocity decreased extraction rates, with differences between extraction curves when considering plug flow or adoptingU(r) diminishing asBiincreased forD/d(p) = 20, or asD/d(p)increased forBi = 18.Biincreased by increasingUandk(f), or decreasingC(o)andD(e), whereasD/d(p)increased by increasing vessel volume. The radial variations in porosity in a packed bed and associated changes in superficial CO(2)velocity may have a more pronounced negative impact in laboratory or pilot plant extraction vessels (smallD) than industrial vessels (largeD), mainly when extracting small particles and applying large superficial CO(2)velocities. A proxy for the SC-CO(2)extraction of oil from pre-pressed seeds in an industrial extraction vessel (495-L capacity,D/d(p) = 270) would be plug flow using the porosity, and superficial CO(2)velocity predicted for the axis of the extraction vessel (epsilon(o)andU(o), respectively). Literature correlations predict a value of epsilon(o)slightly less than epsilon, and value ofU(o)slightly less thanU. The remainder of the CO(2)bypassing the vessel along a high porosity region near the vessel wall, containing a small fraction of the loaded substrate.