Christos Chatzidoukas

Microbial production of polyhydroxybutyrate with tailor-made properties: an integrated modelling approach and experimental validation.

The microbial production of polyhydroxybutyrate (PHB) is a complex process in which the final quantity and quality of the PHB depend on a large number of process operating variables. Consequently, the design and optimal dynamic operation of a microbial process for the efficient production of PHB with tailor-made molecular properties is an extremely interesting problem. The present study investigates how key process operating variables (i.e., nutritional and aeration conditions) affect the biomass production rate and the PHB accumulation in the cells and its associated molecular weight distribution. A combined metabolic/polymerization/macroscopic modelling approach, relating the process performance and product quality with the process variables, was developed and validated using an extensive series of experiments and measurements. The model predicts the dynamic evolution of the biomass growth, the polymer accumulation, the consumption of carbon and nitrogen sources and the average molecular weights of the PHB in a bioreactor, under batch and fed-batch operating conditions. The proposed integrated model was used for the model-based optimization of the production of PHB with tailor-made molecular properties in Azohydromonas lata bacteria. The process optimization led to a high intracellular PHB accumulation (up to 95% g of PHB per g of DCW) and the production of different grades (i.e., different molecular weight distributions) of PHB.

Optimal grade transition campaign scheduling in a gas-phase polyolefin FBR using mixed integer dynamic optimization

Abstract Transitions between different polymer grades seem to be a frequent operating profile for polymerization processes under the current market requirements for product with diverse specifications. A broader view of the plant operating profile without focusing only on a single changeover between two polymer grades raises the problem of the optimal sequence of transitions between a certain number of grades. An integrated approach to the optimal production scheduling in parallel with the optimal transition profiles is carried out using Mixed Integer Dynamic Optimization (MIDO) techniques. A remarkable improvement on process economics is observed in terms of the off-spec production and the overall transition time.

A combined metabolic/polymerization kinetic model on the microbial production of poly(3-hydroxybutyrate)

In the present work, an integrated dynamic metabolic/polymerization kinetic model is developed for the prediction of the intracellular accumulation profile and the molecular weight distribution of poly(3-hydroxybutyrate) (P(3HB) or PHB) produced in microbial cultures. The model integrates two different length/time scales by combining a polymerization kinetic model with a metabolic one. The bridging point between the two models is the concentration of the monomer unit (i.e. 3-hydroxybutyryl-CoA) produced during the central aerobic carbon metabolism. The predictive capabilities of the proposed model are assessed by the comparison of the calculated biopolymer concentration and number average molecular weight with available experimental data obtained from batch and fed-batch cultures of Alcaligenes eutrophus and Alcaligenes latus. The accuracy of the proposed model was found to be satisfactory, setting this model a valuable tool for the design of the process operating profile for the production of different polymer grades with desired molecular properties.

Plantwide Economical Dynamic Optimization: Application of a Borealis Borstar Process Model

Abstract A novel plantwide dynamic optimizer PathFinder has been applied to a dynamic model of a Borealis Borstar process. PathFinder optimizes dynamic paths subject to a merely economical criterion. Introduction of process constraints allows for a gradual migration from the currently used transition towards a more optimal transition. Special care has been taken to integrate the optimizer with on-line control tools. The results show a significant improvement in added value during a grade transition.

Model-based Dynamic Optimisation of Microbial Processes for the High-Yield Production of Biopolymers with Tailor-made Molecular Properties

Abstract The dynamic optimal control of microbial processes for the economically efficient production of biopolymers with tailor-made molecular properties is a complicated problem. In the present work an integrated metabolic/polymerization/macroscopic model for the production of polyhydroxybutyrate (PHB) in A. latus bacteria was developed. The model relates the process performance and product quality with operating variables of interest and predicts the dynamic evolution of the biomass growth, the polymer accumulation, the consumption of carbon and nitrogen sources and the average molecular weight of the final biopolymer. The process optimization led to a high intracellular PHB accumulation (up to 95% g of PHB per g of DCW) of different biopolymer grades with a maximum molecular weight equal to 8.08×10 5 g/mol.

Development of a Macroscopic Model for the Production of Bioethanol with High Yield and Productivity via the Fermentation of Phalaris aquatica L. Hydrolysate

This study deals with the development of a structured macroscopic model for the dynamic simulation of the bioethanol production through the fermentation of sugars derived from the hydrolysis of lignocellulosic biomass of the perennial herbaceous species Phalaris aquatica L. In the proposed model, the growth of Saccharomyces cerevisiae cultures consuming hydrolysate sugars as carbon source in parallel with the intracellular bioethanol production and excretion are quantitatively described accounting for substrate and product inhibition phenomena. A number of different feeding policies were designed and investigated on a model-base and verified experimentally in a real fermentation process. Substantial improvement on the real process performance was attained, resulting in a final ethanol concentration equal to 59.1 g/L corresponding to 2.19 g/(Lh) overall ethanol productivity.

Optimal Grade Transitions in an Industrial Slurry Phase Catalytic Olefin Polymerization Loop Reactor Series

Abstract Present market needs combined with the broad range of polyolefin applications have forced the polyolefin industry to operate under frequent grade transition policies. Consequently, under such market-driven operating schedules, the minimization of offspec polymer production and grade changeover time is prerequisite to any profitability analysis of the polyolefin production processes. In the present study, the optimal grade transition problem is examined in relation to an industrial Ziegler-Natta catalytic slurryphase ethylene-1-hexene polymerization loop-reactor series.

Dynamic optimization of molecular weight distribution using orthogonal collocation on finite elements and fixed pivot methods: An experimental and theoretical investigation

In the present study, two numerical methods, namely the orthogonal collocation on finite elements and the fixed pivot technique, are employed to calculate the MWD in an MMA free-radical batch suspension polymerization reactor operating up to very high conversions (e.g., � 95%). The theoretical MWD predictions are directly compared with experimentally measured MWDs, obtained from a pilot-scale batch MMA suspension polymerization reactor. It is shown that there is a very good agreement between model predictions and experimental measurements on both monomer conversion and MWD. Subsequently, two different time-optimal temperature trajectories are calculated to obtain a polymer having either a narrow or a bimodal MWD in minimum batch time. The calculated time optimal trajectories are then applied, as set point temperature changes, to a pilot plant batch polymerization reactor. It is shown that the measured MWDs are in very good agreement with the off-line calculated optimal MWDs.

OPTIMISATION OF GRADE TRANSITIONS IN AN INDUSTRIAL GAS-PHASE OLEFIN POLYMERIZATION FLUIDIZED BED REACTOR VIA NCO TRACKING

Abstract Gas-phase olefin polymerization fluidized-bed reactors are complex processes that are characterized by large time constants and slow transitions. Optimal grade changes can be computed off-line on the basis of a detailed process model and applied to the process in an open-loop mode. However, the presence of uncertainties in the form of process-model mismatch and disturbances make this approach clearly non-optimal. Fortunately, measurements can be used to improve the situation; this can be done either explicitly via process model refinement and repeated optimisation, or implicitly via direct input adjustment. This work considers the optimisation of grade-transition changes by enforcing on-line the necessary conditions of optimality (NCO tracking) using available measurements. The key steps are the generation of an adjustable model of the optimal solution and its adaptation using appropriate measurements or estimates of the controlled variables. A realistic simulation process model is employed to compare the performance of NCO tracking to that obtained via the open-loop application of nominal optimal trajectories.

Model-assisted operational design of bacterial PHA-production processes: the obstacle of heterogeneity inducing modules

Abstract Microbial communities generate phenotype heterogeneity to ensure their survival under rapidly changing conditions. This behaviour often represents an important obstacle to technological applications, where processes are designed on the assumption of cells having a well-defined and stable metabolic phenotype. Here, a detailed model for the fermentative PHA production is developed and segregated to include phenotypically distinct populations. Simulations show that culture dynamics and productivity can be largely influenced by the composition of cell population, thereby demonstrating the value of segregated multiscale models in PHA process modelling and control.