THE EFFECT OF SUBSTRATE MASS TRANSFER LIMITATIONS IN AN IMMOBILIZED ENZYME MEMBRANE REACTOR

An unstirred ultrafiltration cell can be quite easily converted into an immobilized enzyme reactor. Indeed, if suitable amounts ofproteic solutions are fed and if the protein molecular weight exceeds the membrane cut off, gel precipitation occurs onto the active surface of the membrane because of concentration polarization phenomena. Two different gel formation procedures have been followed. Indeed, two different proteic solution have been injected, namely

1.a mixture of an inert protein and the enzyme

2.co-polymer obtained by co-cross-linking the enzyme and the inert protein.

Substrate mass transfer limitations which occur in the gel immobilized enzyme reactor have been considered. The relevance of mass transfer resistances upstream from the immobilized gel layer has been discussed together with the intrinsic enzyme kinetics.

A heterogeneous gel model has been proposed which adequately describes the experimental results. Effectiveness factor correlations of fairly general applicability have been also presented.     

A KINETIC MODEL FOR THE PRODUCTION OF LIQUIDS FROM THE FLASH PYROLYSIS OF BIOMASS

A kinetically based prediction model for the production of organic liquids from the flash pyrolysis of biomass is proposed. Wood or other biomass is assumed to be decomposed according to two parallel reactions yielding liquid tar and ( gas + char) The tar is then assumed to further react by secondary homogeneous reactions to form mainly gas as a product

The model provides a very good agreement with the experimental results obtained using a pilot plant fluidized bed pyrolysis reactor

The proposed model is shown to be able to predict the organic liquid yield as a function of the operating parameters of the process, within the optimal conditions for maximizing the tar yields, and the reaction rate constants compare reasonably well with those reported in the literature     

SELECTIVITY STUDIES IN THE PHOTOCHLORINATION OF METHANE II: EFFECT OF THE RADIATION SOURCE OUTPUT POWER AND OUTPUT SPECTRAL DISTRIBUTION

The effect of the output power and output spectral distribution of the radiation energy fed to a photoreactor upon the reaction yield and selectivities is studied both theoretically and experimentally. The study aims at the analysis of these influences in a series reaction such as the photochlorination of methane where methylene chloride and eventually chloroform are the most valuable products. The computational model employed has been verified for a group of selected conditions by means of bench-sale experiments.

It is found that the chlorine conversion shows the expected square root dependence with respect to the Local Volumetric Rate of Energy Absorption (LVREA). This dependence, in a first approximation, can also be assigned to the effect of the radiation source output power upon yield. It is also found that by manipulating the amount of radiation power fed to the reactor it is possible to ascertain the operating conditions favoring the production of predetermined intermediate products (e.g., methylene chloride or chloroform).

It is concluded that the use of customized lamps especially designed for each particular product on the basis of feasible changes in the characteristics of the existing radiation sources will favor a selective and more economical production of some of the stable intermediates, i.e., lamp design is an important feature in any optimal process design.     

ON THE BEHAVIOR OF CONSECUTIVE ENZYMATIC REACTIONS IN CYCLICALLY OPERATED REACTORS

The dynamics of a process involving consecutive enzymatic reactions were investigated for a case in which the reactions are carried out in a cyclically operated reactor. Each cycle of the operation protocol involves three phases. During phase I the reactor operates in a semi-batch mode involving input of the reactants. During the second phase, the vessel operates in a batch mode. During phase III the reactor has an output only and a fraction of its contents are emptied before another identical cycle begins.

It was found that there are regimes in the operating parameter space where the system can reach more than one limit cycle (multistability). Using computer software based on the bifurcation theory for forced systems, as well as one- and two-parameter continuation algorithms, the impact of various parameters on the dynamics of the system was investigated. The results are presented in the form of diagrams. Conditions under which formation of the intermediate product in the reaction sequence is maximized were also investigated.

Production of the intermediate product in a limit cycle was compared to that obtained in batch and semi-batch operation. Implications of the proposed operation protocol for process optimization and pollution prevention are discussed.     

PRODUCTION OF ETHANOL WITH SACCHAROMYCES CEREVISIAE IN A CONTINUOUS REACTOR Note II: tests and modelling for a packed tubular reactor with immobilized yeasts

The biochemical production of ethanol has been studied in a packed tubular reactor with Saccharomyces Cerevisiae immobilized on wooden cubes.

The kinetics of the reaction was described in a previous paper. The experimental axial profiles for the substrate and product concentrations are compared with those calculated from a reactor model obtained by introducing the biochemical kinetic expression in the design equations. A good fit between the experimental and the calculated values could only be obtained by including a biological efficiency coefficient.

The reactor was kept in operation for 28 days to assess its technological reliability. It was found to be biologically stable. Its productivity was constant and comparable to that reported in the literature for similar reactors.     

A MONTE-CARLO APPROACH TO THE INTERPRETATION OF BUBBLE PROBE SIGNALS IN FREELY BUBBLING FLUID BEDS†

The interpretation of the signals generated by a double probe may be done by the three characteristic times method:

t₁ the time duration of the pulses;

t₂ the time shift between the signals of the two probes;

t₃ the time interval between two pulses on one channel.

Each of these times is largely dispersed and the corresponding histograms may be constructed.

The present work is a trial to go over from the time histograms to physical properties of the bubbles combining a Monte-Carlo simulation and a flexible simplex optimisation procedure.

As a result, the percentage of oblique bubbles cutting just one level, the bubble size distribution, the average velocity-size relation, and the individual dispersion around it may be defined.

The procedure is finally applied to experimental results obtained with a light probe in a fluid bed of glass beads.     

CONTRIBUTION TO THE ANALYSIS OF THE SELECTIVITY OF GAS-LIQUID REACTIONS PART II: COMPARISON OF EXPERIMENTAL RESULTS WITH MODEL PREDICTIONS

The chlorination of paracresol is used in an experimental study of selectivity in gas-liquid contactors.

Experiments in a batch reactor show the influence on selectivity of the dimensionless numbers presented in Part I and involving competition between mass transfer and chemical reaction together with the hydrodynamics.

The extension of open reactor model presented in Part 1 to the batch reactor permits a comparison between theory and experiments and shows a good agreement     

INVESTIGATIONS INTO PUMPING CHARACTERISTICS OF AXIAL FLOW IMPELLERS IN AN INTERNAL LOOP REACTOR FOR ANIMAL CELL CULTURE

This investigation was intended to aid in the selection of impeller design and operating conditions, which would exhibit minimum turbulence in an internal loop reactor for axial flow velocities up to 18 cm/s.

For this purpose we measured power consumption, liquid flow velocity and mixing time with two marine propellers (pitch ratio tan alpha =1 and 0.57, respectively) and with flat-blade fan-type impellers (blade angles 10 to 90°).

The present results showed that at flow velocities between 7 and 14 cm/s a reduction in the pitch ratio (tan alpha) of the marine propeller from 1 to 0.57 increased the mechanical flow efficiency.

In addition, the fan-type impeller with a blade angle of 20° displayed flow characteristics comparable to the marine propellers; therefore, owing to its simple design, the fan-type impeller offers a practical substitute for the marine propellers.

In cell culture, draft-tubes used in impeller-driven reactors (internal loop reactors) offer two important features:

1. They simplify the design and scale-up process over that of open-blade impellers (flow patterns are more uniform);

2. They provide a mechanical support for bubblte-free membrane oxygenators;

Furthermore, on the assumption that loop reactors display lower bulk turbulence than open blade impellers for similar power consumption, this investigation was intended to aid in the selection of impeller design and operating conditions which would exhibit maximum pumping efficiency (with minimum mixing).

For this purpose, using a pH tracer method, we determined mechanical flow efficiency and dimensionless mixing time, which served as parameters for pumping efficiency to establish the following specific design characteristics:

1. Bulk mixing decreases relative to bulk flow with increasing bulk flow (mixing therefore conforms to the flow-in-pipe model) for two marine propellers (pitch tan alpha = 1 and 0.57) and for fan-type impellers (variable pitch) up to a blade angle of 30°;

2. Liquid velocity increases directly proportional to impeller speed up to 17 cm/s for (above) propellers, as well as fan-type impellers with blade angles of less than 30°;

3. The mechanical efficiency of axial flow (liquid velocity/power input) increases by decreasing the impeller pitch of marine impellers from tan alpha = 1 to tan alpha = 0.57, in the range of 7-14 cm/s;

4. A fan-type impeller (variable pitch) with a pitch angle of 20° displayed flow velocity and mechanical efficiency values, which lay between those of the (above) two marine propellers.

Fan-type impellers are considerably simpler in construction and are simpler to modify than marine propellers; for this reason this impeller type serves as a useful investigational tool. In addition we have found that at an impeller of 20° this type of impeller may be used to substitute marine propellers with a pitch ratio between 1 and 0.57.

We also suggest that square pitch marine propellers are not the most suitable for reactors in which mechanical efficiency of axial flow is critical, such as those for “shear sensitive” animal and plant cells.     

RECIRCULATING REACTOR FOR THE STUDY OF MULTIPHASE KINETICS

A new laboratory reactor was set up to measure kinetic coefficients in a solid (catalyst)-liquid-gas reacting system.

The reactor consists of two parts: an absorber, where the liquid is partially saturated by the gas reactant and a reacting zone, where the liquid alone, containing the dissolved gas, flows through a fixed bed of catalyst.

The ricircle of the liquid in the absorber maintains a high concentration of the gas reactant in the liquid also in the zone of reaction, allowing the use of a high mass of catalyst (significative from a statistical point of view) and the achievement of sufficiently high conversion.

The tested reaction is the catalysed hydrogenation of ∝-metylstyrene: in order to consider a drastic situation and to verify the results with the literature data, the experimental conditions examined corresponded to very high chemical reaction rate (instantaneous reaction) at the surface of the pellets.

The tests were carried out with the reactor working both in batchwise and in continuous operative mode (steady state); the results show the reliability of the new reactor above all when the steady state operation is considered. For the use of the reactor in batchwise condition, the accumulation of the product inside the catalyst particles must be considered for an accurate measurement of the kinetic parameters,     

ON THE INFLUENCE OF SEVERE INTERNAL DIFFUSIONAL LIMITATIONS ON OPTIMUM TEMPERATURE OPERATIONS CRITERION AND POLICIES IN DEACTIVATING FIXED-BED AND BATCH REACTORS

During the forming of glass articles by a variety of different processes, it is important to be able to accurately measure the temperature of the glass. Also, to be practical for production applications, the temperature measuring technique must not interfere with the process or disturb the product. Only infrared radiation pyrometry is capable of meeting these requirements.

Narrow-band radiation pyrometers are non-contacting sensors that (depending on the operating wavelength) either measure the surface temperature or some weighted average of the surface and internal temperatures of semi-transparent glass objects without significantly affecting the product or its heat exchange with the environment. When attempting pyrometer temperature measurements on glass, it must be recognized that the incident radiation originates not only from the surface, but also from the interior which may be at a different temperature. However, a knowledge of the directional spectral volume emissive power of the glass object can be used to determine a "best" operating wavelength for the desired results and/or to help interpret indicated temperature measurements.

A sophisticated mathematical model of coupled radiation and conduction heat transfer in glass has been formulated to calculate the apparent temperature indicated by a hypothetical narrow-band infrared pyrometer receiving radiation emitted by glass plates of specified thickness and non-uniform temperature distribution. These results are presented for a wide range of operating wavelengths, covering the three distinct regions of the transmission spectrum of a typical soda-lime-silica glass.     

ADDITION OF PORE DIFFUSION LIMITATION TO THE “UCKRON-I” KINETIC RATE OF METHANOL SYNTHESIS

Kinetic data with pore diffusion limitation on methanol synthesis were generated by extending the “UCKRON-I” kinetic rate expression. The best fit model and the extended “true” model were compared using their respective rates to simulate temperature profiles in a non-isothermal plug flow tubular reactor.

The objective of this work was to add pore diffusion resistance to the UCKRON-1 kinetic rate for methanol synthesis (Berty, et al. 1983). Kinetic modeling of the data with 5% experimental error added, showed the best model to be that developed from a previous kinetic model (Shalabi, et al. 1983) with apparent activation energy approximately one-half the activation energy at no pore diffusion.

Methods used in this work to determine and evaluate pore diffusion parameters can be utilized for other reaction systems where pore diffusion may play a role in reaction rate.

Temperature profiles estimated from reactor simulation studies showed good argeement between ideal and predicted models for temperature.     

MEASURING OZONATION RATE CONSTANTS IN GAS-LIQUID REACTORS UNDER THE KINETIC-DIFFUSIONAL TRANSITION REGIME

A lumped parameter mathematical model has been developed, which simulates the operation of a gas-liquid stirred reactor in the transition domain between kinetic and diffusional regimes of absorption with reaction.

In order to determine the rate constants and the coefficient of mass transfer, the model has been compared with experimental data for the ozonation of organic substrates in aqueous solution (maleic acid, fumaric acid and phenol), The upper working limit of the method has been determined at about Ha = 25.     

STEADY-STATE MODELLING OF A LATEX REACTOR TRAIN FOR THE PRODUCTION OF STYRENE-BUTADIENE RUBBER

A mathematical model is developed for the emulsion copolymerization of styrene and butadiene carried out in a continuous train of stirred tank reactors. The model predicts copolymer composition, conversion, molecular weight averages, and long chain branching frequencies, as well as the latex particle size distribution for all reactors in the train. It is capable of simulating closely the behaviour of industrial SBR processes.

Several simulation studies are performed. Topics investigated include: process operating modifications to improve productivity; the effect of chain transfer agent flow rate and number of reactors on the molecular weight development; the effect of process modifications on the development of the particle size distribution down the reactor train; and the effect of reactor design on particle generation rates.     

THE ROLE OF THE VOLUME-AVERAGED TEMPERATURE IN THE ANALYSIS OF NONISOTHERMAL, MULTIPHASE TRANSPORT PHENOMENA

The analysis of nonisothermal transport phenomena in multiphase systems is almost always accompanied by the use of some type of volume-averaged or spatially-smoothed temperature. In such systems one always encounters parameters, such as a reaction rate coefficient or a viscosity, that are temperature dependent. Given the point relation for some generic parameter in the β-phase, i.e., ψ_β = ψ_β(T_β), one can wonder how the volume-averaged form of this parameter depends on the volume-averaged temperature. In this paper we show that the local volume-averaged form of ψ_β is given by

$

in which A represents a second order tensor that depends on the system parameters. A somewhat more complex form is encountered for area-averaged functions of the temperature. These functions are especially prevalent in reactor design calculations, which are considered both in terms of early intuitive developments and from the perspective of the method of volume averaging.     

OPTIMAL CATALYST DISTRIBUTION AND DILUTION IN NONISOTHERMAL PACKED BED REACTORS

The problem of the optimal distribution of two different catalysts in a nonisothermal packed bed reactor is analyzed. State variable constraints, such as maximum allowable temperature, may be incorporated through a penalty function approach into the steepest ascent solution of this problem.

Loading a wall-cooled reactor with two chemically different catalysts is compared to dilution of a single catalyst with inerts for temperature moderation. A criterion is presented for estimating whether catalyst dilution may be necessary in order to keep reactor temperature below some maximum allowable value.

Conditions for optimal loading with a two-dimensional reactor model are presented and applied to catalyst dilution in a butane oxidation reactor. Suboptimal catalyst loadings are compared with the optimal loading.     

PRODUCTION OF ETHANOL WITH SACCHAROMYCES CEREVISIAE IN A CONTINUOUS REACTOR Note III: an experimental, kinetic study with suspended, recycled biomass

By way of comparison with the kinetic investigation of the continuous production of ethanol in a tubular reactor with immobilized yeasts, a study was carried out of continuous fermentation in a stirred reactor, with final separation and recycling of the microorganisms. Flocculation with potassium ferrocyanide and zinc sulphate was used to separate the biomass. The maintenance of the metabolic activity was assessed. Next, discontinuous fermentation at various substrate and biomass concentrations was performed to determine the kinetics of glucose-ethanol transformation by flocculated and reinoculated Saccharomyces Cerevisiae.

Once again, the kinetic expression was a Michaelis-Menten equation, with un-competitive inhibition of the substrate and linear inhibition of the product.

The values observed for the various parameters were not very different from those for a biomass immobilized on an inert support or for a free biomass. Finally, the productivities of several types of reactor are compared.     

SELECTIVITY STUDIES IN THE PHOTOCHLORINATION OF METHANE I: REACTOR MODEL AND KINETIC STUDIES IN A NON-ISOTHERMAL, POLYCHROMATIC ENVIRONMENT

The photochlorination of methane employing nitrogen as an inert diluent is studied in a continuous flow, tubular, non-isothermal, bench-scale, polychromatically irradiated photoreactor with particular emphasis on selectivities.

Using a mechanistic sequence of 22 steps, predictions from the mathematical model are compared with experimental results obtained for three different nominal temperatures (297.9 K, 312.8 K and 322.2 K) and three different radiation sources (40, 360 and 1200 W ) that also differ in their output emission characteristics.

An extremely high sensitivity to some of the values of the employed specific rate constants is found, particularly when an accurate prediction of selectivities is being sought. However, within the reported or attributable errors in the published values of the kinetic constants( ± 5%), the experiments show very good agreement with the computed predictions obtained from the mathematical model of the photoreactor.

Using these experimental results that show excellent reproducibility a complete and reliable set of process independent reaction kinetic constants, and a verified reactor model are obtained which can be used for photoreactor design purposes, including selectivity optimization.     

INTRAPARTICLE CONVECTION, DIFFUSION AND ZERO ORDER REACTION IN POROUS CATALYSTS

An assessment is made of the influence of convective effects in the behaviour of calalyst pellets with large pores

Concentration profiles and effectiveness factors are calculated for zero order reactions in isothermal regimes using a model which involves convective transport as well as diffusion and reaction processes in the pellet

Errors made in current reactor design are discussed. A predictive method for calculating the intraparticle fluid velocity in industrial cases is reported. Finally, the analysis is applied to the design of biological reactors.     

KINETICS OF THE HYDROLYSIS OF ACRYLONITRILE TO ACRYLAMIDE OVER RANEY COPPER

Measurements of the kinetics of the hydrolysis of acrylonitrileover Raney copper catalysts have been made in the temperature range 40 to 100°C in a tubular reactor operated differentially with and without recycle and over concentration ranges of 0-25 weight percent acrylonitrile, 0-35 weight percent acrylamide and 40-99 weight percent water.

For the concentration range 0 to 7 wt. % acrylonitrile and 0 to 7 wt. % acrylamide, data were fitted by an adsorption model of the type

A + K,CA + KCCC and at higher concentrations by the powers law expression

The activation energy for the reaction was found to be 49.2kJmol^-1. Both models showed that the product acrylamide strongly inhibits the reaction.     

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.