Study of single-screw extruders for continuous feeding of canola paste for supercritical fluid extraction

• Main Author: Wang, Fangzhi
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/1226

The suitability of using a single-screw extruder to feed Canola seeds continuously into a supercritical fluid extractor was studied. The experiments were conducted with a custom-built extruder (0.44 m long, 36.75 mm ID barrel) fitted with a screw having a single flight 4.9 mm high and 3 mm wide. The clearance between the top of the flight and the inside of the barrel was 0.0254 mm. Three screws with flight angles of 5.98°, 7.97° and 9.94° were used. The extruder screw was driven by two variable speed hydraulic motors operating typically at 66.5, 88.5 and 100.5 rpm. The extruder discharge port was fitted with a variable-size opening. The pressures and temperatures along the extruder barrel were measured by means of Iron-Constantan thermocouples (at 6 equispaced locations) and pressure transducers (at 4 locations), respectively. The discharge rate was determined gravimetrically. A computer program of a three-velocity model was developed to simulate the extruder performance. The model was verified by comparison with experimental results obtained with Newtonian fluids in the viscosity range of 130 Pa•s. The agreement is satisfactory. The effects of operating parameters (like screw rotational speed) and geometric parameters (like flight angle) on the discharge rate and discharge pressure have been thoroughly investigated. The slip between the Canola paste and metal surface has been considered in the model, which plays a very important role in predicting the extruder performance. Although the Canola paste has a limited yield stress which may cause some plug flow in the screw channel, it was found that the flow behavior index n, which is 0.5 for the present concentration Canola pastes, has a stronger effect on the velocity profile than the yield stress. Thus ignoring the yield stress did not likely introduce significant errors in the predictions. The reliability of the three-velocity model is satisfactory in predicting the output variables. Experiments using pre-crushed Canola seeds showed that average discharge pressures ranging from 10 to 30 MPa could be achieved at flow rates varying from approximately 9.5 to 1.7 kg/hr. The average pressure along the screw was found to increase almost linearly and the pressure, at any point on the extruder barrel, varied sinusoidally around the mean. The amplitude of the pressure variations were typically 6% to 9% of the average pressure. If desired, higher discharge pressures can be achieved by lengthening the extruder and varying the screw-barrel geometries. The apparent viscosity of Canola paste was determined at temperatures 288 to 323K, concentrations 35.3% to 95.7% (g solid/g liquid) and shear rates 0.15 to 225 s⁻¹. A modified power law equation was developed to express the apparent viscosity as a function of temperature, concentration and shear rate. The equation was found to be satisfactory in simulating the extruder performance. The extruder discharges proved to be well suited for extraction with super-critical carbon dioxide since the discharge pressure of the extruder fell within the range of optimal operation of supercritical carbon dioxide extraction. The extrusion helped to rupture the Canola cell walls and thereby released the oil, thus improved the efficiency of extraction of Canola oil with supercritical carbon dioxide.