Design and Operation at Unstable Steady States
Many processes have been overdesigned because engineers are reluctant to design near or within regimes of complex operations, where they are often economically optimal. This is prevalent in processes with exothermic and autocatalytic reactions and where phases appear and disappear, especially in the critical region. We are developing designs that operate more economically closer to these nonlinear regimes, which are characterized by multiple steady states, periodic and even chaotic operation, often exhibiting inverse response. Improved control strategies are being developed to permit reliable operation near these regimes.
Dynamic Risk Assessment of Inherently Safe Chemical Processes
To obtain inherently safer plant designs, we are experimenting with game theory to solve the multiobjective optimization problem that involves tradeoffs between profitability, controllability, safety and/or product quality, and flexibility. Then, given more optimal designs, that are inherently safer, we are developing methods for plant-specific, dynamic risk assessment using accident precursor data; that is, data recorded when abnormal events occur. Our models estimate the failure probabilities of various critical accident scenarios, associated with a process unit after the occurrence of an abnormal event, using Bayesian analysis and copulas.
Our emphasis is on improving process and human operator models to achieve more accurate risk analysis. To accompany sparse alarm and process data associated with plant trips and accidents, we have been constructing informed prior distributions to generate low-variance posterior distributions in Bayesian analysis. To quantify model quality, which impacts prior and posterior distributions, we have been experimenting with higher-frequency alarm and process data to select the most relevant constitutive equations and assumptions.
Path-Sampling to Understand Better Rare Paths to Plant Shut-Downs and Accidents
Beginning with strategies developed to track rare paths in molecular dynamics simulations, we track rare paths that proceed from normal operation to plant shut-downs and accidents in chemical plants. First, for an exothermic reaction in a CSTR, we used transition-path sampling to track rare paths from a high-temperature, high-conversion steady state to a low-temperature, low-conversion steady state. More recently, we have used forward-flux sampling more effectively and efficiently. Our studies have shown the impact of process stability, disturbance noise, and controller gain on the probability of tracking rare paths, leading to smarter alarms that alert operators to increased shut-down and accident risks.
Bifurcation Control of High-Dimensional Nonlinear Chemical Processes
Our emphasis has been on the control of nitroxide-mediated, radical polymerization (NMRP) in a continuous-stirred-tank reactor (CSTR) to achieve reduced levels of poly-dispersity. We have been developing new optimization algorithms for washout-filter feedback control. Our algorithms adjust the eigenvalues of the model Jacobian matrix to relocate the Hopf bifurcation points, stabilizing solution branches in specified regions having high monomer conversion.
Algae to Biofuels
We were members of the National Alliance for the Advancement of Biofuels and Bioproducts (NAABB) sustainability team, which concluded in April 2013. Our efforts focused primarily upon the simulation of algae-oil transesterification processes. We collaborated with Albemarle-Catilin and took kinetic measurements of their T-300, solid-base catalyst. Using the collected data, we were able to build and simulate a complete transesterification process (including a glycerolysis pre-treatment section) using ASPEN PLUS. We carried out sizing and costing of the process equipment, which was used to conduct profitability analyses and optimizations.
During our work with NAABB, the need for further development of the algae oil extraction and transesterification processes was evident. In recent years, we have been focusing on the intensification of these processes using CO2, first using sonically-driven microbubbles to fracture the algae cell walls, followed by supercritical transesterification to renewable biodiesel. At low temperatures (below 100°C), we have been developing processes to recover omega-3 fatty acids (EPA and DHA), for nutraceuticals and food supplements. These high-value bioproducts yield revenues to reduce sharply the cost of biodiesel fuel. Most recently, we have been perfecting experimentally microbubble processes to lyse cell walls permitting the extraction of high-value bioproducts such as PHA (polyhydroxyalkanoates) from cyanobacteria (i.e., blue-green algae) for biodegradable plastics. For scale-up, we are creating small pilot-plants to measure extraction efficiencies and energy requirements.