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.

Avoiding Rare Safety Events – Plant Shut-Downs and Accidents

Plant shut-downs and accidents are very rare events that are difficult to predict and prevent.  Over two decades, we have been experimenting with the use of near-miss alarm data to design alarms and safety systems.  We use Bayesian statistics to estimate dynamically, during process operations, the failure probabilities of these safety systems – in an approach called Dynamic Risk Analysis (DRA).  These methods are particularly effective when the possibilities of plant shut-downs and accidents are well-recognized in response to anticipated abnormal events leading to effective alarms and safety system actions.

Unfortunately, most rare plant shut-downs and accidents are triggered by rare unanticipated (un-postulated) abnormal events – that cannot be accounted for in process design and operating models.  We have been gaining success using process models with statistically-generated noise that creates dynamic rare paths from normal to unsafe/unreliable operating regions.  As in the molecular dynamics simulations of rare chemical reaction pathways, we locate highly probable rare paths leading to rare accidents and plant shut-downs.  Our latest techniques enable us to generate reliable alarms and safety systems that prevent them.  We are creating alarm rationalization techniques to improve reliability before checking their effectiveness using DRA to estimate safety system failure probabilities.

Most recently, we are applying these methods to radical-reaction polymerization systems operated at unstable steady states to avoid high-temperature accidents and low conversions at low-temperatures.  We use game-theory in process design for multi-objective optimization – to locate operating regions that maximize profitability, controllability, safety, and flexibility.

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 and Bioproducts

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 for both the extraction and recovery of bioproducts from algae. In collaboration with the Soft Materials Research and Technology (SMART) Group at Penn, we have used sonically driven microbubbles to fracture the algae cell walls in a low temperature and solventless environment using techniques in Very Large-Scale Microfluidic Integration (VLSMI). After lysis, we have pursued the avenue of froth flotation for recovery of bioproducts. Hydrophobic oils and other products will attach to microbubbles to lower free energy and be carried to the top of the solution where they can be easily recovered. Algae oil is recovered to be converted into biofuel, but other products that can be recovered at low temperatures (below 100°C), are omega-3 fatty acids (EPA and DHA), which are used in nutraceuticals and food supplements.  These high-value bioproducts yield revenues to reduce sharply the cost of biodiesel fuel.  Most recently, we have been perfecting experimental 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. Additionally, we have built a bioreactor that is used in growing various strains of algae for experiments in both lysis and product recovery.

Natural Gas for Power and CO2 Sequestration

Given that 40% of electrical power in the U.S. is generated from natural gas (typically shale gas), we have been experimenting with variations on the Allam Cycle design to operate large-scale power plants (~300 MWatt) while sequestering CO2 greenhouse gas in underground cavities at elevated pressures and high profitabilities.  This decarbonization through sequestration is potentially attractive while new regenerative energy sources are being implemented.  Our research seeks to improve the thermodynamic efficiency of the Allam Cycle with design modifications using heat and power integration.