Mechanisms for extraction of oil from oilseed flakes
Soybean oil was extracted from soybean flakes and porous alumina spheres by hexane and isopropanol. Extraction of oil from alumina spheres was normal in terms of appropriate PDE solutions of Fick's second law of diffusion. Extraction of oil from flakes agreed with Fick's second law of diffusion for short extraction times, but deviated significantly from the standard diffusion behavior at long times.
In spite of its apparent complexity, the solvent extraction of oil from flaked oilseeds appears to be a linear diffusion process and the diffusivity appears to be independent of oil concentration. This is supported by the following experimental results for soybean flakes: (a) Normalized extraction curves were independent of both oil content in the solid and initial concentration of oil in the extracting solvent; (b) Infusion and extraction occurred at the same rate; (c) Normalized extraction curves for unextracted and previously extracted flakes reinfused with refined oil were virtually identical; and (d) Extraction times for different solvents were inversely proportional to liquid diffusivities of the oil-solvent pairs.
Analytical solutions of Fick's second law are presented for batch and differential extraction of slabs and spheres, which account for both internal and external mass transfer resistances. These solutions correlate alumina sphere extraction data well.
Soybean flake extraction data were well correlated by two mathematical models. One of these treats the extraction curve as the sum of two or more independent diffusive contributions. The other is a one-parameter empirical model. For short extraction times, experimental data are well correlated by available short-time solutions of the diffusion equation, subject to appropriate initial and boundary conditions.
Experimental results for multistage batch extraction of soybean flakes agreed very well with those predicted from superposition of single-stage extraction data by means of Duhamel's theorem.
Structural non-uniformity appeared to be the most probable reason for the observed extraction behavior of flaked oilseeds. Other hypotheses have been ruled out based on experimental evidence, including: decreased solubility of the last portions of extracted oil; slow dissolution of oil in the extracting solvent; interaction of oil or solvent with the solid; and concentration-dependent diffusivity.
0542: Chemical engineering
0539: Agricultural engineering