Abdelhamid Ajbar

On the existence of oscillatory behavior in unstructured models of bioreactors

Continuous cultures of some microorganisms such as Saccharomyces cerevisiae and Zymomonas mobilis have long been known to exhibit oscillatory behavior under suitable operating conditions (JoK bses, Egberts, Baalen, & Roels, 1985; Parulekar, Semones, Rolf, Lievense, & Lim, 1986). The understanding and the modeling of these oscillations is important for the knowledge of pathway regulationof cell metabolism and also for the optimization and control of the microbial process. Various models with di!erent levels of complexity have been proposed in the literature to describe these spontaneous oscillations (Porro, Martegani, Ranzi, & Alberghina, 1988; StraK ssle, Sonnleitner, & Fiechter, 1988; Cazzador, 1991; Hjortso & Nielsen, 1994). The need for such complex models arises from the fact that the basic unstructured model, inwhich, the metabolic activity is described solely by growth rate and yield coe

Stability and bifurcation of an unstructured model of a bioreactor with cell recycle

cient, does not recognize any internal structure of the cell nor a diversity between cell forms, and thus fails to describe situations where the cell compositionor the morphology of the cell culture are also important variables (Nielsen & Villadsen, 1994). The inadequacy of the unstructured model manifests itself in its failure to predict transient behavior following sudden changes in operating parameters, e.g. dilution rate, and in predicting stable autonomous oscillations occurring under suitable conditions.

Mass transfer in supported liquid membranes: A rigorous model

An unstructured model of a bioreactor with cell recycle and substrate inhibition kinetics is used to investigate the bifurcation and stability characteristics of this unit. The singularity theory used for this investigation allows a global analysis of steady-states multiplicity and the different bifurcation mechanisms occurring in the system including hysteresis and pitchfork. Analytical criteria are also derived for the safe operation of the reactor and to prevent wash-out conditions. The investigation of the dynamic bifurcation, on the other hand, shows that the model cannot exhibit periodic attractors with any growth kinetics model. The inability of this widely used model to exhibit periodicity despite the experimental results that support the existence of periodic behavior in many bioreactors suggests that new approaches are to be taken for the modeling.

Model reduction and robust control of multi-stage flash (MSF) desalination plants

A rigorous steady-state model for the facilitated solute transport across ceramic-tube-supported liquid membranes is presented. The proposed permeation model accounts for the effect of the variation of the distribution coefficient, as well as nonconstant solute concentration in the strip phase and nonequilibrium interfacial reactions. The resulting convection-diffusion transport equations form a set of boundary-coupled partial differential equations. As a case system taken from hydrometallurgy, the extraction of copper using ceramic tubes impregnated with LIX-84 as a carrier is considered. It is shown that the variation of the distribution coefficient is a factor that must be considered when designing or analyzing the performance of supported liquid membrane modules. The model is also used to examine the effect of key operating conditions on the modules performance.

Stability analysis of the biodegradation of mixed wastes in a continuous bioreactor with cell recycle.

An orthogonal collocation method is used for the reduction of a large nonlinear model of a multi-stage flash (MSF) desalination plant. The real plant located in Saudi Arabia consists of 19 stages in the heat recovery section and three in the heat rejection section. It is shown that three collocation points, i.e., two for the heat recovery section and one for the heat rejection section, are sufficient for a good representation of the full-order model for both steady-state and dynamic behavior. The reduced model is then used for a robust controller design of the plant using a constrained nonlinear model predictive control (NLMPC) strategy. Numerical simulations show that the controlled system obtained when the reduced model is used for output prediction yields the same performances as with the full-order model but with a considerable reduction in the computational time and a much easier tuning of the controller.

Determination of cost-effective operating condition for CO2 capturing using 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

The stability characteristics of a continuous bioreactor with cell recycle for biodegradation of mixed wastes are investigated. The system involves a pure culture of Pseudomonas putida and media containing phenol and glucose as carbon and energy sources. The model growth kinetics for the two substitutable substrates were experimentally validated in a previous study. The stability analysis carried out using elementary principles of bifurcation theory shows rich dynamics characteristics of the reactor model, including steady-state multiplicity and hysteresis. The effect of the bioreactor operating parameters on the stability behavior of the model is discussed. Practical criteria are also derived for the safe operation of the unit and to prevent the occurrence of wash-out conditions.

Periodic and nonperiodic oscillatory behavior in a model for activated sludge reactors

Abstract1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) ionic liquid (IL) is considered for CO2 capturing in a typical absorption/stripper process. The use of ionic liquids is considered to be cost-effective because it requires less energy for solvent recovery compared to other conventional processes. A mathematical model was developed for the process based on Peng-Robinson (PR) equation of state (EoS). The model was validated with experimental data for CO2 solubility in [BMIM][BF4]. The model is utilized to study the sorbent effect and energy demand for selected operating pressure at specific CO2 capturing rates. The energy demand is expressed by the vapor-liquid equilibrium temperature necessary to remove the captured CO2 from the spent solvent in the regeneration step. It is found that low recovery temperature can be achieved at specific pressure combination for the absorber/stripper units. In fact, the temperature requirement is less than that required by the typical monoethanolamine (MEA) solvent. The effect of the CO2 loading in the sorbent stream on the process performance is also examined.

Effect of frequency on pulsed fluidized beds of ultrafine powders

A dynamic model for an activated sludge process is proposed to investigate the stability and bifurcation characteristics of this industrially important unit. The model is structured upon two processes: an intermediate participate product formation and active biomass synthesis processes. The growth kinetics expressions are based on substrate inhibition and noncompetitive inhibition of the intermediate product. The bifurcation analysis of the process model shows static as well as periodic behavior over a wide range of model parameters. The model also exhibits other interesting stability characteristics, including bistability and transition from periodic to nonperiodic behavior through period doubling and torus bifurcations. For some range of the reactor residence time the model exhibits chaotic behavior as well. Practical criteria are also derived for the effects of feed conditions and purge fraction on the dynamic characteristics of the bioreactor model.

Stabilization of chaotic behavior in a two-phase autocatalytic reactor

Deagglomeration of ultrafine powders poses an important challenge towards their efficient and effective utilization. In the present study, we investigate the effect of frequency on the hydrodynamics of pulsed fluidized beds of ultrafine powders that show strong agglomeration behavior. We have carefully selected square waves of three different frequencies: 0.05 Hz, 0.10 Hz, and 0.25 Hz. The lowest frequency used here allowed the fluidized bed to settle completely before another pulse was introduced whilst the highest frequency ensured that the bed remained in a state of continuous turbulence between occurrences of consecutive pulses. On the other hand, the intermediate frequency pulse was just sufficient to complete the process of bed collapse before the start of the next pulse. Both local and global bed dynamics in all the three cases were rigorously monitored using fast response pressure transducers. The pressure transient data during the bed collapse were processed using the bed collapse model reported in the literature to compute the effective hydrodynamic diameter of agglomerates. Though there was substantial decrease in the agglomerate size, the effect of the frequency appeared to be rather insignificant as the global pressure transients remained rather insensitive to the change of the fluidization velocity.

Biodegradation of substitutable substrates in a continuous bioreactor with cell recycle: A study of static bifurcation

Abstract The effect of feed forcing on the chaotic behavior found by Alhumaizi and Aris (Chaos, Solitons & Fractals 1994;4:1985) in the autocatalytic reaction of Gray and Scott (A+2B → k 1 3B, B → k 2 C) , taking place in a two-phase reactor, is numerically investigated. It is shown that by periodic forcing of the feed flow rate, the chaotic behavior of the reactor can be controlled for even small forcing amplitudes or frequencies. The behavior of the forced system alternates between chaotic and period behavior via period adding, quasi-periodic and intermittent bifurcations. The results of the stabilization of the chaotic behavior are summarized in an amplitude–frequency excitation diagram showing the different bifurcation mechanisms. A numerical investigation has also revealed that the reaction conversion can be improved by an appropriate selection of the forcing frequency.