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

Abstract

The residence‐time distribution (RTD) studies are needed to characterize mixing conditions, flow patterns, and the extent of conversions and reactions of the biopolymers in any plasticating or cooking extruder. For the supercritical fluid extrusion process (SCFX), mixing of the fluid and starch‐based biopolymers is an important step affecting cell nucleation and growth. RTD was studied in the mixing zone of the extruder, where CO2 and gelatinized starch are brought together. Three levels of each of the process variables (screw speed and CO2 injection pressure) were investigated, using specially designed apparatus for color‐dye injection, and a die‐restrictor to change the pressure profile and hold up volume in the extruder. Control RTD plots were for no CO2 injection conditions at the same pressure, and the RTD of the full extruder section. Experimental RTDs were evaluated by: (1) the Wolf‐White model, (2) a cascade of continuously stirred reactors (CSTR) model and (3) a model based on plug‐flow in series with cascade of CSTRs. The last approach was most successful in capturing all aspects of the observed RTD curves including their tails. Injection of CO2 at the highest pressure (110.3 bar) increased the RTD compared to no CO2‐injection conditions. There were significant differences in the plug flow component of the RTD as well as differences in average residence time as a function of both screw speed and CO2 injection pressure conditions.

Controlling the structure of a porous polymer by coupling supercritical CO2 and single screw extrusion process

“…Finally, we determine the maximum total time required for complete solubilization of CO 2 into the fluid/melt phases and comparing with the residence time inside the extruder previously determined 14. The total mixing time required for complete homogenization of the CO 2 /polymer melts divides in two parts: the distributive mixing time t m and the diffusion time t d 21, 22 defined by eqs. (2) and (4).…”

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

“…Obviously, with everything else held constant, when screw speed is increased, the residence time is reduced. This has been validated by De Ruyck (1997), Singh and Rizvi (1998a), Ainsworth and others (1997), Yeh and Jaw (1998), Ollett and others (1989), Yeh and others (1992), Fichtali and others (1995), Van Zuilichem and others (1973), Lee and McCarthy (1996), Mange and others (1987), Olkku and others (1984), and Gogoi and Yam (1994).…”

“…These results, coupled with the knowledge of the operating variables such as temperature, screw speed, screw configuration, and moisture content, provide necessary information to predict what fraction of the material will undergo specific reactions. RTD studies are needed to characterize mixing conditions, flow patterns, and the extent of conversions and reactions of the biopolymers in any plasticating or cooking extruder (Singh and Rizvi 1998a).…”

Dispersion coefficient of coffee berries in vibrated bed dryer

“…where C i is the tracer particles concentration at the dryer outlet (g/g); t i is the average time of sampling (min); Dt is the time interval of sampling (Gautam & Choudhury, 1999;Iwe, Van Zuilichem, Ngoddy, & Ariahu, 2001;Levenspiel, 1972;Ramaswamy, Abdelrahim, Simpson, & Smith, 1995;Renaud, Thibault, & Alvarez, 2001;Singh & Rizvi, 1998). Table 4 presents results obtained during the drying and carried out the RTD for Experiments 1, 2, and 3, where v is obtained by dividing the trajectory effected by coffee berries inside the dryer (L = 3.04 m) by the average residence time (t m ).…”