Process Stability Evaluation and Optimization for Thermoplastic Starch Production Using a Modified Extruder

The evaluation of process stability was carried out by Mitrus. Temperature changes in each extruder section were recorded in the processing of all materials with different process parameters. The stability of temperature behavior of the final product immediately after it had left the extruder die was also studied.

Temperatures in the cylinder were measured using thermocouples fixed in each section and connected to meters installed in the extruder control board. The product temperature was recorded with a digital thermometer (CIE 370 type) equipped with an appropriate measuring lance. All the data were registered by PC.

Feeding stability and mass flow through the barrel and extruder crosshead were assessed by recording evidence of extruder shutdowns, problems with material feeding, and blocking of die openings. These data are important for determining the optimal parameters for TPS production while maintaining production stability and invariant process parameters, particularly in long production cycles.

The use of a modified TS-45 extruder (Polish design) fitted with an additional cooling system allowed the process temperature to be maintained at the appropriate level within the 80-100°C range for almost all trials. Slight mean fluctuations of temperature (±5°C) during TPS extrusion were detected at each extruder section for all starch types.

Influence of screw rotation speed, mixture composition, and moisture content on temperature changes were recorded in all extruder sections. Temperature increases were observed with increasing screw rotation speed and were highest for mixtures with both the lowest moisture (15%) and glycerol (15%) contents. This effect was caused by friction heat inside the cylinder and conversion of mechanical energy into heat energy.

Temperature decreases in all extruder sections were noted with an increase in glycerol content in the mixtures up to 25%. Processing of mixtures with higher glycerol contents than this induced temperature increases throughout the length of the plastifying system.

A reverse impact on the extrusion temperature changes was noted in cases of changes in material moisture levels. Temperature increases in all extruder sections were recorded with mixture moisture level rises of up to 25%. Above this value, however, only an insignificant temperature increase was recorded.

Wheat starch produced the highest extrusion temperatures regardless of glycerol content, while potato starch produced the lowest. For potato starch blends with different amylose contents, the extrusion process was more stable with blends containing higher amylose content.

During material mixture investigations, clogging in the extruder charging hopper and difficulties with material delivery were noted, requiring manual feeding. This caused uncontrolled changes in extrusion-cooking temperature, even up to ± 20 °C. Failure or reduction of material feed resulted in a drop in extruder section temperature, followed by a drastic rise after feed restart. This caused excessive TPS expansion, formation of steam bubbles, and granulate surface destruction. Blends with glycerol content over 20% caused granulate stickiness, hindering separation. Precise and careful control of material flow and delivery to the screw is necessary to avoid these effects.

Blends with water content over 15% resulted in excessive expansion and damaged surfaces with numerous steam bubbles. The extended extruder variant TS-45, equipped with a final cylinder cooling system with variable flow of cold water, improved stabilization of the final product. A maximum water flow of 400 L/h was used for processing maize and wheat starches, as well as potato starch mixtures with 15% glycerol content. In other cases, the water flow ranged from 140 to 370 L/h.

TPS temperature at the slit die depended mainly on the screw rotation speed used. The temperature of the final product increased with increasing screw rotations, irrespective of moisture and mixture composition. TPS produced at 100 rpm was 9-24°C higher than that produced at 80 rpm. With increasing glycerol content in the mixture, drops in the temperature of products leaving the die were noted. Product temperature varied over a small range of ±5°C for screw speeds of 80 and 90 rpm, while fluctuation in product temperature reached as high as 12°C at 100 rpm.

Leave a Reply

Your email address will not be published. Required fields are marked *