Date of Award
Master of Science
Agricultural and Biosystems Engineering
Agricultural and Biosystems Engineering
Dirk E Maier
The importance of biosecurity is ever increasing within the animal feed industry. Salmonella is a major microbial hazard that can persist within animal feed and cause human illness. The risk of Salmonella contamination can be reduced by killing pathogens through heat treatment as a function of processing temperature and time. A retentioner is a device used to extend the time that animal feed is exposed to desired treatment temperature resulting from steam conditioning. The overall objective of this research was to improve biosecurity practices in feed manufacturing by ensuring effective steam conditioning and retentioning treatment, and preventing recontamination during the drying, cooling and transfer process. This objective was achieved by (1) conducting pilot plant trials measuring the radial and longitudinal temperature distribution of the feed during retentioning, (2) developing, validating, and applying a transient thermal model of conduction heating within a retentioner, (3) analyzing steam usage for conditioning, and (4) designing a pneumatic transfer system to simultaneously transport, dry, and cool feed after processing to prevent contamination.
Two sets of experimental trials measuring the longitudinal and radial temperature within a pilot scale retentioner were conducted. Three retention times (90, 180, and 240 seconds) were tested with two heating mat settings (on/off). Retentioning was found to be the most effective when a high steam flow rate was used for conditioning and the heating mats were turned on. For both experimental trial sets, the use of the heating mats resulted in a more stable radial temperature profile of the feed mash throughout the retentioner. A transient thermal model of the retentioner was created and the temperature data from the experimental trials was used to validate the model. The validated model was then applied to the largest commercial scale retentioner and inferences about scale up were analyzed. The largest retentioner model showed a similar trend to the pilot scale model as temperatures varied (by up to 8%) only within 30 mm from the retentioner wall. The model was successfully applied and can be useful in predicting conditions during scale up to larger retentioner sizes. Steam conditioning greatly influences temperature and moisture homogeneity of feed mash ahead of the retentioner and/or pellet press but its influence on feed quality and safety is often not well understood. A mass and energy balance of steam conditioning from two sets of experimental trials was conducted. An average steam utilization of 70% was determined with 90% agreement between the steam energy exchange and the increase in feed mash temperature moisture content. A spreadsheet-based tool was created to help feed mill operations professionals to analyze steam energy utilization and optimize feed mash conditioning, retentioning and pelleting. Industry guidance for pellet mill capacity versus steam requirement were revised and updated to reflect larger pellet mill capacities and improve understanding of steam utilization. "What-if" scenarios illustrated that when controlling both steam flow rate and pressure drop between the boiler and the conditioner, the lowest energy costs occur when the initial product temperature is high, the specific heat is low, and the steam enthalpy is high. The lowest amount of steam needed for the "what-if" scenarios occurred when the inlet product temperature was high, the specific heat was low, and the enthalpy of the steam was high (low pressure drop).
A design for a pneumatic conveying system has been proposed to dry and cool feed mash or pellets and maintain their safety after heat treatment as an alternative to a conventional counterflow cooler and bucket elevator system proposed for the new Iowa State University feed mill. The 5 t/h dilute-phase, positive pressure system can dry feed from 17 to 13.5% moisture and cool feed from 185°F (85°C) to near ambient temperature. A techno-economic analysis compared the designs and found that the pneumatic system was 148% more costly with respect to fixed capital (434%) and variable (72%) costs. However, the proposed system would reduce the risk for recontamination of feed after hydrothermal processing, and enhance the potential for biosecurity and human safety at the new Iowa State University feed mill.
Michelle Ann Friedmann
Friedmann, Michelle Ann, "Increased biosecurity in feed manufacturing through effective hydrothermal treatment and recontamination prevention during drying, cooling and handling" (2020). Graduate Theses and Dissertations. 18045.