Jacques Villermaux

A new parallel competing reaction system for assessing micromixing efficiency—Experimental approach

Abstract Several kinds of methods can be used in order to study the state of mixing at molecular scale in stirred reactors. Conductometric and optical methods have not enough resolution. The aim of this work is therefore to propose a chemical reaction which acts as a molecular probe for assessing micromixing efficiency. A literature review shows that the number of available test reactions is limited. We propose a new system of parallel competing reactions A + B → R and C + νB → S whereby the state of micromixing in industrial reactors can be studied. The first reaction is a neutralization, the second one is the Dushman reaction between the iodide and iodate. We show that the formation of iodine (S) under the influence of an acid (B) in an alkaline medium (A) is a measure of segregation. This system makes it possible to study the influence of feed time, feed location and reactant concentrations on segregation. The method is validated by a study of mixing in standard stirred tanks of one and twenty litres. Scale-up rules are proposed.

A new parallel competing reaction system for assessing micromixing efficiency : Determination of micromixing time by a simple mixing model

Abstract A system of parallel-competing reactions producing iodine was developed to study partial segregation in stirred tanks. The experimental results, obtained in 1 and 20 litre stirred tanks, are interpreted by the incorporation model, relating the micromixedness ratio, α, to the ratio of the reaction time to micromixing time. The model being simple, the determination of the micromixing time can thus be used easily.

Future challenges for basic research in chemical engineering

Abstract Chemical engineering is not a simple application of classical disciplines. In the coming years, it has to take up industrial and economic challenges, and to cope with constraints imposed on processes by their natural and social environment. Engineers are faced with new classes of objects and phenomena requiring novel approaches; promising concepts are emerging which may be adapted to chemical engineering processes. The aim of this paper is to stimulate further discussion and research in these areas. Six main topics are dealt with. • Complex media, description and chemical changes. • Novel concepts and paradigms, especially in the field of non-linear dynamic processes, management of energy supply and creation of entropy, strucutre and organisation of condensed matter. • Processes operation and equipment design; innovations are awaited from new contacting and coupling modes, process integration and synthesis, novel reactors and separation devices, new energy sources and recent developments in process control. • Products and quality control, the challenge being to produce end-use properties at will. • Models and the search for “first principles” of a new kind applicable to complex systems at a higher level of integration. • Data and tools such as sensors, computers and visualization techniques. The classical analysis going from macroscopic down to microscopic scale should be completed by a systems approach integrating phenomena from microscopic up to macroscopic complex systems. In conclusion, tracks are proposed for future research about basic principles which make the core of chemical engineering.

Representation du melange imparfait de deux courants de reactifs dans un reacteur agite continu

A one parameter interaction model (IEM) between segregated regions in a fluid has been developed in a previous paper, and applied to coalescence redispersion processes in a liquid—liquid suspension. The aim of this paper is to decide whether the model is able to describe the phenomena occurring in an incompletely mixed medium. A statistically ideal macromixing is achieved in a small spherical reactor thanks to the spontaneous stirring produced by liquid jets issuing from two syringe needles, but the flow rates are such that a partial micromixing occurs. Several experiments are performed with this device: concentration fluctuations are monitored by a conductivity microprobe connected to a multichannel analyser. The influence of the poor mixing is studied on the extent and selectivity of chemical reactions: instantaneous reaction, separate feeds (phenol + sodium hydroxyde); second order reaction, separate and premixed feeds (alkaline hydrolysis of nitromethane); second order consecutive—competitive reaction, separate and premixed feeds (alkaline hydrolysis of glycol diacetate). The experimental results are interpreted with the help of the model which yields in each case an estimation of the interaction parameter. The values obtained from these different experiments show a reasonable consistency, which proves the validity of the IEM model, and its usefulness in the design of imperfectly micromixed reactors.

Extension of a chemical method for the study of micromixing process in viscous media

Abstract The use of the iodide-iodate test reaction was extended to study micromixing in glycerin-water mixtures with viscosities up to 170 mPa s. The properties of the mixtures and the kinetics of the test reaction were determined as a function of viscosity. Experiments were carried out in a stirred tank yielding the micromixedness ratio α = (1−X s ) X s as a function of stirring speed for different viscosities. A micromixing model involving two steps (incorporation of bulk fluid and exchange between smaller eddies) has been used for the interpretation of the experimental data. Further work is required to interpret experiments at higher viscosities where hydrodynamic data are still lacking.

Influence of process parameters on the precipitation of organic nanoparticles by drowning-out

Ethylcellulose (the organic substance), ethanol (the solvent) and water (the nonsolvent) are chosen as a model system for the production of uniform nanoparticles by drowning-out. Experiments carried out under various operating conditions show that many process parameters, such as the type of stirrer and the stirring speed, the temperature and the solvent–nonsolvent ratio, have practically no influence on the formation of ethylcellulose particles. On the other hand, the ethylcellulose concentration in the initial alcoholic solution seems to be the most important parameter controlling the particle size distribution. A comparison is made between experimental results and predictions of classical nucleation theories or aggregative growth models.

Mass transfer in annular cylindrical reactors in laminar flow

Abstract Radial diffusional mass transfer is studied in a fluid flowing in fully developed laminar flow in an annular cylindrical reactor in which a first order heterogeneous reaction is taking place at the wall. The asymptotic behaviour of the concentration decrease far from the reactor inlet is especially investigated. Limiting values of Sherwood numbers are numerically determined and represented by semi-empirical expressions. Additivity relationships between homogeneous and heterogeneous contributions are established. Theoretical results are found in excellent agreement with those yielded by a new experimental method based on heterogeneous decomposition of ozone. Annular reactors exhibit a mass transfer efficiency which is noticeably higher than that of empty tubes. This efficiency may be characterized by three criteria related to inner space utilization, catalytic surface utilization and/or mechanical energy degradation.

Micromixing in a static mixer and an empty tube by a chemical method

Micromixing in a static mixer and an empty tube was characterized by the product distribution of fast consecutive competing reactions. As a test reaction, the pptn. of BaSO4 complexed by EDTA in alk. medium under the effect of an acid was used. The acid was injected at a point of the reactor into an excess of basic complex. The pptd. BaSO4 is a measure of the degree of segregation which can be characterized by an index Xs, representing the amt. of macrofluid in the partially segregated fluid. Expts. were carried out in a tubular reactor and in a static mixer in the laminar flow regime (0.4 < Reynolds no. < 300) by varying the fluid viscosity (10-3 < m < 26 * 10-3 kg/ms) and the flow rate. Furthermore, the characteristic reaction time tr was varied from .apprx.1 to 75 s. Depending on the exptl. conditions, the Xs was 0.1-0.3 for the mixer and from 0.2-0.75 for the empty tube. The micromixing time, tm, was deduced from Xs and tr. The mixing tm was .apprx.10 times larger in an empty tube than in the static mixer. [on SciFinder (R)]

Optimal operation of a semi-batch reactor by self-adaptive models for temperature and feed-rate profiles

A heuristic method was developed, which is both fast and efficient to identify stoichiometric and kinetic tendency models for reaction taking place in batch or semi-batch reactors. A general control strategy for simultaneous optimization of temperature and feed rate profiles together with usual control parameters was also developed. The method is fairly robust with respect to analytical inaccuracies as only convergence to optimal conditions is looked at. The method relying on both algorithms was found very performing by several validations issued from real life industrial problems. On these occasions the fast determination of the optimal operating conditions was especially appreciated

Influence of Mixing Characteristics on the Quality and Size of Precipitated Calcium Oxalate in a Pilot Scale Reactor

Evidence is presented of mixing effects on the precipitation of calcium oxalate from aqueous solutions of calcium chloride dihydrate and sodium oxalate in both MSMPR and single-feed semi-batch precipitators. Experiments of precipitation were carried out in a pilot tank of 20 dm3 volume stirred by three different impellers. The influence of stirring speed, stirrer type and reagent feed tube location on the product size and morphology was studied. Whereas weak effects of these operating parameters were noticed in the continuous operation mode, the semi-batch single-feed mode was shown to be more sensitive to mixing effects. Experimental results on crystal mean size during single-feed semi-batch experiments could be related to the temporal variance of the concentration field of a fluorescent inert tracer injected into the same feed location and tube in the tank which was then continuously operated. These variances were measured by laser sheet visualization and image processing techniques.