Open-loop unstable systems with time-delays are often encountered in process industry, which are often more difficult to control than stable processes. In this paper, the stabilization by PID controller of second-order unstable processes, which can be represented as second-order deadtime with an unstable pole (SODUP) and second-order deadtime with two unstable poles (SODTUP), is performed via the necessary and sufficient criteria of Routh-Hurwitz stability analysis. The stability analysis provides improved understanding on the existence of a stabilizing range of each PID parameter. Three simple PID tuning algorithms are proposed to provide desired closed-loop performance-robustness within the stable regions of controller parameters obtained via the stability analysis. The proposed PID controllers show improved performance over those derived via some existing methods.
With growing environmental concerns over plastic pollution, polyhydroxyalkanoates (PHAs) have recently gained significant attention as promising biodegradable polymers to substitute petroleum-based plastics. In this work, non-edible Cerbera odollam oil was employed as a renewable carbon source for PHA production to improve the economic competitiveness and environmental sustainability of the process. The optimization and mechanism of PHA production from C. odollam oil using Pseudomonas resinovorans DSM 21078 were presented. Through response surface methodology, the optimal condition for PHA production was 0.3 g/L urea concentration, 17.52 g/L oil concentration, and 10.46% (v/v) inoculum size. Results showed that a maximum PHA concentration of 0.50 g/L (with a polymer content of 26.0%) was attained at this optimal condition. The product was composed of 1.3% 3-hydroxybutyrate (3HB), 9.2% 3-hydroxyhexanoate (3HHx), 43.3% 3-hydroxyoctanoate (3HO), 32.0% 3-hydroxydecanoate (3HD), 11.9% 3-hydroxydodecanoate (3HDD), and 2.2% 3-hydroxytetradecanoate (3HTD). The PHA polymers exhibited adhesive, soft, and amorphous properties at room temperature, with high thermal stability, making them desirable for polymer processing. From the mechanism proposed, it was inferred that P. resinovorans DSM 21078 produces longer-chain PHA monomers mainly through the direct β-oxidation of long-chain fatty acids and shorter-chain monomers via the de novo fatty acid synthesis pathway when oil-based substrates are utilized. The findings from this work could pave the way for new paradigms that significantly enhance future research in the development of highly efficient oil resource valorization technologies to produce PHAs with intriguing properties, thereby contributing to the commercial success of sustainable bioplastics as an effective environmental management solution.