RESIDENCE TIME DISTRIBUTION (RTD) AND GOODNESS OF MIXING (GM) DURING CO₂‐INJECTION IN TWIN‐SCREW EXTRUSION PART II: GM STUDIES

Abstract

Experimental residence‐time distribution (RTD) characteristics of an extrusion process may be used to estimate the extent of mixing experienced in the extruder. Using earlier theories of laminar mixing and striation thickness reduction, a new approach to estimate efficacy of mixing of two phases in the mixing zone of the extruder is proposed. Predicting the time required to achieve complete mixing and comparing it with the minimum time (plug flow time) that the extrudate spends in the mixing zone gives a ratio that can be used to evaluate the adequacy of extruder operating conditions for good mixing of gas/fluid injection and extrudate. Twin‐screw extrusion of cornmeal with carbon dioxide injection were used to investigate the effect of CO2 injection pressure and extruder screw speeds on mixing. Lower CO2 injection pressures (25 bars) and lower screw speeds (150 rpm) were theoretically shown to enhance mixing. Scanning electron micrographs (SEMs) of experimental samples taken showed smaller and more uniform cell structures for those conditions which were predicted to favor better mixing.

New challenges in polymer foaming: A review of extrusion processes assisted by supercritical carbon dioxide

“…Rizvi and co-workers have developed several processes using a co-rotating twin screw extruder with a 52 mm screw diameter (Wenger TX52) [25,26,[45][46][47][48][49][50][51][52][53][54][55][56][57]. The screw configuration and L/D ratio were varied according to the application.…”

“…During extrusion of breakfast cereals, Rizvi et al were mainly interested in the characterization of the mixing between this foodstuff and sc-CO 2 . First, they investigated the residence time distribution (RTD) of a dye injected by means of CO 2 [50], and then porosity was examined by image analysis [51]. The RTD of a reactor is a probability distribution function that describes the amount of time a fluid element spends inside the reactor [67].…”

“…Using mixing theories, the same authors were able to estimate the minimum time required to solubilize CO 2 in the extruded foodstuff [51]. For the lowest pressures (3-7 MPa), this minimum time is lower than the characteristic time of the plug flow, which seems to be favourable to obtain a good mixing.…”

A Review on Residence Time Distribution (RTD) in Food Extruders and Study on the Potential of Neural Networks in RTD Modeling

“…Ollett and others (1989) showed that MRT decreased from about 80 s at 75 rpm to 40 to 50 s at 300 rpm. Singh and Rizvi (1998b) showed that average residence time decreased sharply from 119.2 s to 80.8 s when screw speed was increased from 150 to 200 rpm, but decreased only moderately from 80.8 s to 74.8 s when the screw speed was further increased from 200 rpm to 250 rpm.…”

“…With this model a fairly close simulation of the RTD could be made. Singh and Rizvi (1998b) modeled exit age distributions, E-curves using the approach of Wolf-White model, a cascade of CSTRs. and a plug-flow in series with CSTR model.…”