Residence time distributions of starch with high moisture content in a single-screw extruder
Introduction
The extrusion process has wide applications in the production of snacks, breakfast cereals, bread crumbs, pasta, animal feeds, and plastic materials. Extruders, which are used in the manufacture of those products, also function as reactors. Extrusion is a continuous process in which feed materials can experience shearing, heating, shaping, mixing, and reaction. The process time is very important for the reaction in an extruder and every element of feed material is supposed to be subjected to the similar residence time, but particles experience variations in residence time due to screw geometry and rheological effects. This results in the residence time distribution (RTD) that gives information about the degree of mixing, the residence time expectancy of fluid and the degree of uniformity of reaction the feed material undergo during the their passage through an extruder. The feed composition like moisture content and the extruder parameter like screw speed are the variables of process that can affect the mean residence time, the RTD, and the flow pattern of the feed that undergo reaction in an extruder.
The reduction of reactor size enhanced use of extruders as reactors for the polymerization, degradation, and modification of plastic materials. Extruders not only are used in the reaction of plastic materials, but also considered as reactors for phosphorylation (Salay & Ciacco, 1990), acetylation (Miladinov & Hanna, 2000), and degradation of starch (Linko, Hakulin, & Linko, 1983).
In addition to the presence of starch in cereals as food, there is a concern for the incorporation of starch into plastic products in order to reduce a solid waste problem. The extrusion of cereals at low moisture content is mainly for the expansion of extrudates, but the extrusion of starch at low moisture content increases the water solubility of starch (Gomez & Aguilera, 1984). The extrusion of starch at low moisture content may not be desirable when it is incorporated into plastic products because of a negative effect of starch solubility. Phosphorylation of starch for cross-linking reduced the water solubility of rice in a batch system (Rutledge, Islam, & James, 1974), and the starch with lower water solubility index can be obtained by cross-linking it in an extruder at higher moisture content. Moisture content not only affects product properties but also affects the mean residence time, the RTD, and the flow pattern in an extruder.
The effect of moisture content on the mean residence time was studied for the extrusion of cereals with the moisture content lower than 30% in a single-screw extruder (Davidson, Paton, Diosady, & Spratt, 1983; van Zuilichem, de Swart, & Buisman, 1973; van Zuilichem, Jager, & Stolp, 1988). Since the moisture content intervals studied results in degraded starch, the study of effect of moisture at the higher percentages on the residence time distribution provides information about the residence time of starch with lower degradation, extend and homogeneity of reaction in an extruder.
The residence time distributions can be determined by an analysis of melt flow with velocity profile. This type of analysis faces with the description of screw geometry, thermal and rheological properties of material, which are very complicated. An alternative analysis is the construction of a flow pattern with conceptual models. The most widely reported models include a combination of plug flow and perfect mixing. van Zuilichem et al. (1973) reported that increasing the moisture content of feed exaggerated the tail portion of the distribution. Davidson et al. (1983) found that the Levich model, which includes active and dead space regions with a low cross-flow of material between well mixed and stagnant region, represented the early region better than the model of Wolf and Rescnick (1963), which include inert dead volume, but the model of Wolf and Rescnick (1963) resulted in the better approximation of the tail portion of distribution in a single-screw extruder. On the other hand, Yeh and Jaw (1998) reported that a mathematical model including plug flow reactor in series with a continuous stirred tank reactor cross-flowing with a dead volume fitted the experimental data better than the model of Wolf and Rescnick (1963) in the extrusion of rice with 37% moisture content, but the fitness of model to experimental data was not determined at the higher moisture contents than at 37%.
Altomare and Ghossi (1986), Vergnes, Barres, and Tayep (1992) and Yeh and Hwang (1992) examined the effect of reverse screw elements and kneading blocks as a mixing element in twin-screw extruders on the mean residence time, and the presence of screw section without flight element increased axial mixing in a twin-screw extruder (Yeh & Hwang, 1992). On the other hand, the residence time distributions were widely studied in a single-screw extruder without a mixing element (Davidson et al., 1983; van Zuilichem et al., 1973; van Zuilichem et al., 1988), and Yeh and Jaw (1998) studied the effect of mixing element as a mixing disc on the RTD, but the effect of other type of mixing element on the RTD was not studied. The objective of this study was to determine the effect of moisture content, at the higher percentages compared to the previously reported studies, on the residence time distribution of starch extruded with sodium trimetaphosphate and sodium hydroxide at three screw speeds in a single-screw extruder with a mixing element as pins, and to determine the fitness of experimental data to the models of Wolf and Rescnick (1963) and Yeh and Jaw (1998).
Section snippets
Sample preparation
Commercial raw corn starch (25% amylose) was granulated by introducing starch and spraying water to starch at constant flow rate while tumbling it in a rotating drum to achieve the similar granular size. The granulated starch was dried at 40 °C to the moisture content of 5–10%, and the moisture content of the granulated starch was determined by an oven drying. Starch is cross-linked with NaOH and sodium trimetaphosphate in a batch system (Lim & Seib, 1993), so the volume of 0.2 M sodium
The effect of screw speed
The effect of screw speed on the mean residence time was shown in Fig. 2 for the starch with 28.5%, 35.5% and 41.2% moisture content. Increasing the screw speed from 90 to 190 rpm reduced the mean residence time (p < 0.001). The mean residence time is proportional to the filled volume of barrel and inversely proportional to volumetric flow rate. Barres, Vergnes, Tayyeb, and Della Valle (1990) stated that increasing screw speed decreased the fraction filled in the screw at constant feed rate, so
Conclusions
The mean residence time did not change as the moisture content of feed was increased from 28.5% to 41.2% with a mixing element in the single-screw extruder. The effect of feed moisture content on the spread of residence time distribution curve and flow pattern also was not significant at the screw speeds between 90 and 190 rpm. Increasing the screw speed from 90 to 190 rpm reduced the mean residence time of starch and spread of residence time distribution curve at moisture content between 28.5%
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