Supercritical CO2 oilseed extraction in multi-vessel plants. 3. Effect of extraction pressure and plant size on production cost.

Abstract

This work completes a series of three studies about the estimation of the cost of extracting vegetable oil from seeds using supercritical CO2. The focus of this work was determining the effect of extraction pressure (30, 50, and 70 MPa) in the production cost of a supercritical CO2 extraction of a packed bed of 2-mm oilseeds in plants with two-extraction vessels with aspect ratio of 4.5, using a superficial velocity of 5 mm/s. When extraction pressure increased while keeping other variables constant, production cost decreased. For 2 x 1 m(3) extraction plants, increasing extraction pressure from 30 to 50 MPa decreased production cost 30.9%, but when extraction pressure further increased from 50 to 70 MPa production cost only decreased 9.9%. Results suggest that the recommended extraction pressure for oilseed extraction is 50 MPa. Also, this work compared plants operating at the three extraction pressures when annual productivity was kept constant (231, 456, or 596 ton of oil per year). The total extraction volume of the plant diminished as the extraction pressure increased. Optimum production cost in these cases was quite similar for plants operating at 30, 50, and 70 MPa, but the total volume of the plant decreased 63.6% when increasing extraction pressure from 30 to 50 MPa for the plant producing 456 ton of oil per year, and an additional 31% when increasing it from 50 to 70 MPa. The lowest production cost estimated was 5.52 USD per kg of oil (optimum extraction time of 2.75 h) in a 4 x 1 m(3) plant processing 2-mm prepressed oilseeds, and operating at 40 degrees C and 50 MPa. The manuscript includes a sensitivity analysis on production cost as a function of a variation of +/- 50% of uncertain variables cost (respect to nominal values), which were considered as constant in our previous contributions. That cost items were unitary cost of annual labor, CO2, and substrate, being the variation of the last one the more significant effect on production cost.