Mass transfer and shear in an airlift bioreactor: Using a mathematical model to improve reactor design and performance

Several studies have shown a strong relationship between morphology and agitation ( [Cui et al., 1997] and [Berzins et al., 2001] ). The shear stress distribution and mass transfer are the important parameters which can improve the performance of bioreactor. In this work, a mathematical model using computational fluid dynamics (CFD) techniques is used to study the gas–liquid dispersion in an airlift reactor. Multiple rotating frame (MRF) technique is used to approximate the movement of the impeller in the stationary reactor. Population balance modeling (PBM) is used to describe the dynamics of the time and space variation of bubble sizes in the reactor. The PBM equation is solved using an approximate method known as the class method (CM) and the bubble sizes are approximated through a discrete number of size ‘bins’, including transport, and different bubble phenomena. These equations of the CM are then written as scalar transport equations and added to the multiphase fluid mechanical equations describing the dynamics of the flow. All these equations are solved using control volume formulation through the use of an open-source CFD package OpenFOAM. The model is used to analyze an existing geometry of an airlift bioreactor and validate the modification on the initial design. The new design of airlift gives a clear performance by the increase of the global and local mass transfer and the decrease of the shear stress.     

Modelling of the substrate heterogeneities experienced by a limited microbial population in scale-down and in large-scale bioreactors

A methodology based on stochastic modelling is presented to describe the influence of the bioreactor heterogeneity on the microorganisms growth and physiology. The stochastic model is composed of two sub-models: a microorganism circulation sub-model and a fluid mixing sub-model used for the characterization of the concentration gradient. The first one is expressed by a classical stochastic model (with random number generation), whereas the second one is expressed by a stochastic Markov chain. Their superimposition permits to obtain the concentration profiles experienced by the microorganisms in the bioreactor. The simulation results are expressed in the form of frequency distributions. At first, the study has been focused on the design of scale-down reactors (SDR). This kind of reactor has been reported to be an efficient tool to study at a small-scale the hydrodynamic behaviour encountered in large-scale reactor [P. Neubauer, L. Horvat, S.O. Enfors, Influence of substrate oscillations on acetate formation and growth yield in Escherichia coli glucose limited fed-batch cultivations, Biotechnol. Bioeng. 47 (1995) 139–146]. Several parameters affecting the shape of the frequency distributions have been tested. Among these, it appears that the perturbation frequency, the exposure time and the design of the non-mixed part of the SDR have a significant influence on the shape of the distributions. The respective influence of all these parameters must be taken into account in order to obtain representative results. As a general trend, the increase of the recirculation flow rate between the mixed and the non-mixed part of the SDR induce a shift of the frequency distribution for the lower relative concentrations, which suggests an attenuation of the scale-down effect. This has been validated by using the SDR in the case of the cultivation of Saccharomyces cerevisiae. However, the influence of the non-mixed part of the SDR is not quite well understood if only taking account of the frequency distribution analysis, and supplementary experiments are required to elucidate the underlying mechanism.

The aspect of the frequency distributions suggests that both the design and the operating conditions of a scale-down reactor need to be adjusted in order to match the behaviour of a given large-scale reactor. Examples of frequency distributions obtained in the case of large-scale reactors are given.     

Einsatz von Biocompounds zum Abbau chlorierter Kohlenwasserstoffe in der Grund- und Abwasserreinigung

Use of Biocompounds for Degradation of Chlorinated Hydrocarbons in Treatment of Ground Water and Waste Water . Process engineering approaches to waste water treatment using immobilised microorganisms (MO) permit greater biomass concentrations in the bioreactor and hence higher conversions per unit time. Moreover, the biofilms formed assure favourable conditions for degradation specialists. Immobilisation of microorganisms is feasible on both static and freely mobile supports. Use of the latter permits modification of the flow conditions in the reactor.     

Miniaturization of screening devices for the combinatorial development of heterogeneous catalysts

The present work is focused on the determination of the advantages, bottlenecks and challenges of miniaturized screening systems which are essential to the success of combinatorial high-throughput methodologies in heterogeneous catalysis. Two different reactor configurations with different degrees of miniaturization were developed for the parallel and fast screening of heterogeneously catalyzed gas phase reactions: a monolithic reactor system acting as a multichannel reactor and a microreaction system based on microfabrication techniques. In both cases, a scanning mass spectrometry technique was successfully applied for quantitative product analysis within 60 s per catalyst. Due to its flexibility and high spatial resolution, this three dimensional scanning MS can be used with different and highly parallel reactor arrays. Many experiments were carried out to study the efficiency and reliability of the different screening systems, with the oxidation of methane, the oxidation of CO, and the oxidative dehydrogenation of i-butane as model reactions. Moreover, chip modules in silicon–glass technology having a number of parallel microchannels were developed, each of them containing a different catalyst. Using this approach, “catalysis-on-a-chip” proved in methane oxidation was possible. Finally, a multibatch reactor consisting of a number of parallel mini autoclaves was developed and tested in the liquid-phase hydrogenation of citral in order to overcome the lack of parallel and fast screening procedures for heterogeneously catalyzed gas–liquid reactions widely spread in the chemical industry.     

Dimensional modelling of wood pyrolysis using a nodal approach

New gasification installations and techniques are being tested today but they all struggle with mainly the same drawbacks such as removal of various pollutants in the producer gas or clogging of material pathways.This work is oriented on developing a new model for the non-oxidative pyrolysis step of a gasification process as a part of a wider research conducted on the overall gasification of wood waste. A batch reactor is modelled by means of nodal modelling, a technique widely used for simple heat transfer processes. Additionally to the heat transport inside the batch reactor the model uses a simple and versatile generic chemistry and simplified mass transfer principles. Thermal data from modelling is compared with data obtained from an experimental batch pyrolysis reactor using wood sawdust and cutter shavings. Experimental and theoretical results regarding thermal phenomena are in good agreement.     

生物质热裂解实验和模型研究(英文)

The analysis on the feedstock pyrolysis characteristic and the impacts of process parameters on pyrolysis outcomes can assist in the designing,operating and optimizing pyrolysis processes.This work aims to utilize both experimental and modelling approaches to perform the analysis on three biomass feedstocks—wood sawdust,bamboo shred and Jatropha Curcas seed cake residue,and to provide insights for the design and operation of pyrolysis processes.For the experimental part,the study investigated the effect of heating rate,final pyrolysis temperature and sample size on pyrolysis using common thermal analysis techniques.For the modelling part,a transient mathematical model that integrates the feedstock characteristic from the experimental study was used to simulate the pyrolysis progress of selected biomass feedstock particles for reactor scenarios.The model composes of several sub-models that describe pyrolysis kinetic and heat flow,particle heat transfer,particle shrinking and reactor operation.With better understanding of the effects of process conditions and feedstock characteristics on pyrolysis through both experimental and modelling studies,this work discusses on the considerations of and interrelation between feedstock size,pyrolysis energy usage,processing time and product quality for the design and operation of pyrolysis processes.     

Dynamic multivariate statistical process control using partial least squares and canonical variate analysis

Multivariate statistical techniques have been shown to be useful tools for multivariate statistical process control (MSPC) and process modelling. However, these approaches have been mainly applied to static systems. In the present work, a well known system representation, the state space model, is developed to deal with dynamic situations. The states of the system are approximated using two multivariate statistical projection techniques, Partial Least Squares (PLS) and Canonical Variate Analysis (CVA). These two model representations are compared both in terms of their predictive ability and also their monitoring power using a simulation example. An application to an industrial fluidised bed reactor will be presented at the conference following company approval.     

Control of residual organic matter to reduce bacterial regrowth potential for wastewater reuse

Several problems have been reported about accumulated microorganisms in reclaimed water distribution systems. This paper presents the results of residual organic matter (OM) removal and apparent bacterial regrowth potential of treated wastewater obtained from laboratory-scale experiments using advanced biological treatments: two immobilization processes in series and a membrane bioreactor (MBR) process. Furthermore, a nanofiltration (NF) membrane process was applied to effluents of both advanced biological treatments. The immobilization process removed large molecular weight (MW) fractions >5,000 since immobilized microorganisms had sufficiently acclimated. The NF membrane was more effective in rejecting large MW fractions in the effluents of the immobilization and the MBR treatments. But it was difficult to reject small MW fractions <1,000 by NF. Neutral hydrophilic fraction of DOC was reduced by both advanced biological processes, and it can be thought that the microorganisms in the advanced processes could decompose and grow on some part of the neutral hydrophilic fraction. Quantity of attached microorganisms in the second immobilization reactor was significantly reduced compared to that in the first immobilization reactor. This suggests that apparent bacterial regrowth potential is controlled by the accumulation of effective microorganisms in the first reactor.     

Controlling nanostructure in thermal plasma processing: Moving from highly aggregated porous structure to spherical silica nanoparticles

The synthesis of SiO₂ nanoparticles in a radio frequency (RF) plasma reactor is studied. Scanning electron microscopy (SEM), nitrogen absorption (BET), and laser diffractometry techniques are used to determine the morphology, product size and aggregation level of the resulting nanopowder. Computational fluid dynamics (CFD) modelling employing Fluent 6.2.16 is used to better understand the flow and temperature fields inside the reactor and their effect on the nanoparticles. The theoretical and experimental results are later combined to describe the effects of the above mentioned parameters on the formation (nucleation and growth) of the nanoparticles by different mechanisms. It is demonstrated that the quench gas configuration and reactor geometry can now be designed to control the morphology and size of nanoparticles in these reactors. Various nanostructured products have been synthesised: i.e., highly aggregated nanostructure, partially sintered nanospheres and spherical nanoparticles with very low levels of aggregation. These nanostructures have their primary particles sized between 10 and 200 nm, while the aggregate sizes can lie in the range of between hundreds of nanometers to several micrometers. The critical parameters that should be considered for the large-scale production process are finally identified.     

Useful experimental technique for the study of heterogeneous reactions

The heterogeneous CaO/SO₂ reaction has been thoroughly investigated by developing a series of new experimental techniques including the TGA reactor, the volulmetric reactor and the entrained flow reactor. The heterogeneous system is designed in such a way that most of the gas film and pore diffusion resistances are reduced. The modelling of each step related to the reaction is discussed while the chemical reaction and product layer diffusion are emphasized as the main influences on the SO₂removal. The unchanging size shrinking core model is used to describe the reaction progress with a two stage assumption which has been confirmed in the TGA reactor: first, a very fast surface reaction, followed by a product layer diffusion controlled reaction. It was found from the experiments that the SO₂-partial pressure aat the very beginning is very important for a high removal efficiency during the initial reaction period.     

Observing Biomass Concentration in a Fixed-Bed Bioreactor

The growth of microorganisms in a biofilm was monitored, based on available substrate measurements and first principle modeling. The biofilter was described by a set of partial differential equations obtained from species mass balance. Using two interior collocation points, the model was reduced to a set of ordinary differential equations, with sufficient accuracy to describe the growth of biomass along the reactor bed. An extended Kalman filter was designed at three measurement locations, which resulted in an accurate estimation of the process. Simulation results, provided in Mathematica^R and Matlab^R environments, showed a significant decrease in the biomass error covariance: 0.0065 to 2.50E-05 and 0.0065 to 1.73E-06 for two different measurement noise levels using a normalized bioreactor length of 0.21. Similar results were obtained at the remaining collocation points. The proposed procedure may have applications for water pollution control.     

Observing Biomass Concentration in a Fixed-Bed Bioreactor

The growth of microorganisms in a biofilm was monitored, based on available substrate measurements and first principle modeling. The biofilter was described by a set of partial differential equations obtained from species mass balance. Using two interior collocation points, the model was reduced to a set of ordinary differential equations, with sufficient accuracy to describe the growth of biomass along the reactor bed. An extended Kalman filter was designed at three measurement locations, which resulted in an accurate estimation of the process. Simulation results, provided in Mathematica^R and Matlab^R environments, showed a significant decrease in the biomass error covariance: 0.0065 to 2.50E-05 and 0.0065 to 1.73E-06 for two different measurement noise levels using a normalized bioreactor length of 0.21. Similar results were obtained at the remaining collocation points. The proposed procedure may have applications for water pollution control.     

APPLICATION OF GASndashLIQUID TWO-PHASE CROSS-FLOW FILTRATION TO PILOT-SCALE METHANE FERMENTATION

As part of a national project, “Aqua-Renaissance '90,” by the MITI, a pilot-scale evaluation of membrane-enhanced anaerobic fermentation, has progressed for the wastewater from a pulp and paper mill. A novel membrane filtration system was newly proposed with the aim of saving energy. That is, a gas-liquid two-phase cross-flow filtration which was generated with liquid circulation by an air-lift pump effect, was combined in the anaerobic bioreactor. It was confirmed that the membrane filtration not only offered very stable and large permeate flux, but enhanced the processing efficiency by retaining the microorganisms in the bioreactor. Furthermore, the power consumption per unit permeate volume in the membrane system of 1.78 kWh/m³ was achieved. which was a very highperformance result from the viewpoint of saving energy, as compared with 3ndash;5 kWh/m³ of conventional liquid single-phase cross-flow filtration.     

Deactivation handling in a high-throughput kinetic study of o-xylene hydrogenation

The objective of the study is to develop an HT methodology to enable kinetic modeling of diverse catalysts. The reaction of o-xylene hydrogenation is selected as a probe reaction to evaluate the “metallic” features of a library of about 80 diverse bimetallic catalysts. A major issue to overcome is deactivation phenomena which are catalyst dependent and which cannot a priori be predicted. A prerequisite is therefore to handle correctly deactivation processes for accessing intrinsic kinetic parameters. For this purpose, an adapted screening strategy is developed, using a proprietary 16 channels multi-tubular reactor which enables to test catalysts at the same time-on-stream. Original data treatment procedures are implemented in order to correct observed data from deactivation phenomena for the calculation of kinetic parameters. Effects of metal nature, dopants and supports on deactivation rate are analyzed using a statistical approach, and a tentative classification of deactivation processes based on coke analyses performed on aged materials is provided.     

A NONLINEAR OBSERVER BASED ON HYBRID MODELLING OF CHEMICAL REACTORS

The successful design of an observer for inferring the outlet composition from a chemical reactor heavily relies on the goodness of the adopted kinetic rate model (Baratti et al., 1993). On the other hand, often, it is difficult to dispose of a simple, but, exhaustive kinetic model because of the complexity of the reaction scheme one has to deal with.

In this work, we explore the possibility to represent global (lumped) reaction rate laws by the use of neural network models. The aim is to develop a nonlinear observer (extended Kalman filter, EKF) of an heterogeneous gas-solid reactor that relies on a grey model where the “neural reaction rate” law is integrated within a first principles model. The procedure is outlined for the case of the catalytic oxidation of carbon monoxide over Pt-alumina catalyst. The results show that neural networks (NN) can be effectively used in representing lumped reaction rates since NN are able to capture the essential characteristics of the functional relationship relating the state variables.     

A high containment polymodal pilot-plant fermenter—design concepts

A 225 dm³ pilot-plant bioreactor system has been designed and constructed that is suitable for biohazardous fermentations. The design enables operation at containment levels above the requirements of good industrial large-scale practice (GILSP) without secondary containment of the whole plant. The main biosafety features of the systems include the use of steam barriers on O-ring seals, supply lines and stirrer seals, multiple O-ring seals, piping of condensate lines and pressure relief systems to a ‘kill tank’, double filtration of inlet and off gases and a mobile isolation unit that allows localised containment of sample valve and probe entry ports. The fermenter can, with minor modifications, be operated as a bottom- or top-stirred reactor for the culture of microbial or animal cells, or as an airlift reactor. The design offers considerable flexibility that could prove cost-effective for process development and production. The relevance of the various design features to enable bioreactor operations at pilot-plant scale to be carried out in compliance with current guidelines for large-scale culture of recombinant microorganisms and microbial pathogens is discussed.     

Dynamic behavior of an internal-loop airlift bioreactor for degradation of waste gas containing toluene

Experiments were performed to study the transient behavior of an internal-loop airlift bioreactor for degradation of toluene in a waste gas stream. The gas pollutant flowed into the reactor from the bottom, and it was then degraded by the microorganisms suspended in the liquid phase. Whenever the operating condition was changed, the gas phase toluene concentration initially increased sharply and the time required to reach a new steady-state concentration was short except when the dissolved oxygen decreased to below about 2K_O, where K_O was Monod constant for oxygen in the microbial kinetics. However, even though the gas phase toluene concentration had already reached a new steady state, the whole system still did not yet reach a new steady state. It took 960-1850 s for the whole system to reach a new steady-state except when the dissolved oxygen decreased to below about 2K_O in this airlift bioreactor. For latter cases, it took 4990-7065 s. Moreover, the time required to reach a new steady state for the whole system increased with increasing input gas phase toluene concentration.A mathematical model was developed to describe the dynamic behavior of toluene degradation in the internal-loop airlift bioreactor. The mathematical model took into account the gas and liquid flow patterns in various sections (e.g. riser, gas-liquid separator, downcomer and bottom), the gas-liquid mass transfer of the reactants and the microbial kinetics. The dynamic behavior of the internal-loop airlift bioreactor simulated by the proposed model showed good agreement with the experimental results.     

Numerical methods for retrieving aerosol size distributions from optical measurements of solar radiation

Retrieving aerosol size distributions from ground measurements is an ill-posed problem. Several methods have been developed to “regularize” it, giving a stable approximation of its solution. The aim of this paper is to review the techniques usually adopted and present some new approaches. The case of solutions retrieved both in their parametric and non-parametric form is considered. For all the considered methods the problem of choosing an optimal level of regularization is also discussed. Finally, numerical results comparing some inversion techniques in a real case are presented.     

Fineparticle characterization aspects of predictions affecting the efficiency of microbiological mining techniques

The various factors influencing the efficiency of microbiological mining techniques, such as the surface of a rock fragment, the structure of a crack fracture in an ore body, the progress of bacteria digesting the mineral species to be recovered by the mining process over and into the surface of the exposed mineral, and the structure of the tortuous paths existing in a permeable rock mass, are reviewed. The relevant characterization procedures are discussed. The use of fractal geometry to describe the surface structure of a fractured surface, the use of randomwalk modelling to describe the progress of bacteria digesting the required species, the relevance of tortuosity factors to the provision of nutrients to the active bacteria, and the subsequent removal of the digested mineral out of the permeable system are discussed. The use of fractal dimensions to describe the tortuous paths and randomwalks in fractal space to describe the progress of bacterial digestion of mineral species is outlined. The movement of gas molecules such as radon and randomly moving bacteria moving through a porous system is outlined. The fact that many problems encountered in the study of microbiological mining are common to those encountered in the design of monolithic time release drug delivery systems is discussed briefly.     

Classification and analysis of integrating frameworks in multiscale modelling

Chemical engineers are turning to multiscale modelling to extend traditional modelling approaches into new application areas and to achieve higher levels of detail and accuracy. There is, however, little advice available on the best strategy to use in constructing a multiscale model. This paper presents a starting point for the systematic analysis of multiscale models by defining several integrating frameworks for linking models at different scales. It briefly explores how the nature of the information flow between the models at the different scales is influenced by the choice of framework, and presents some restrictions on model—framework compatibility. The concepts are illustrated with reference to the modelling of a catalytic packed bed reactor.