Élimination de métaux par adsorption en réacteur à membranes à lit mobile de particules et en réacteur lit fixe continu - Approche cinétique Metal removal by adsorption on particles in a membrane moving-bed reactor and in a continuous packed bed reactor - Kinetic approach

Résumé

Ce travail propose un nouveau procédé d'élimination du Cu2+ par adsorption sur des résines échangeuses d'ions poreuses, mises en œuvre dans un réacteur à membrane à lit mobile de particules. Ce procédé a été étudié sur le plan cinétique pour des solutions synthétiques de Cu(II) dans de l'eau. L'application visée à moyen terme est l'intégration de cette opération unitaire dans une filière de traitement complète des effluents industriels liquides.

L'étude cinétique a été réalisée avec une résine cationique forte Mono C-350H en réacteur agité fermé et thermostaté. Les isothermes d'adsorption ont été établies à 21°C et les paramètres de Langmuir et de Freundlich ont été calculés. Une étude de l'influence de la vitesse d'agitation et de la température sur la vitesse réactionnelle apparente a permis de déterminer la zone opératoire correspondant à un régime cinétique. Ce régime est obtenu pour un Reynolds d'agitation supérieur à 4700 et une température inférieure à 60°C. Dans ces conditions, l'ordre de la réaction par rapport au Cu²⁺ est de un et la constante cinétique exprimée par unité de masse de particules est k_wapp=0,03721 lg_rés^-1min^-1 à 21°C. L'énergie d'activation de la réaction a également été déterminée.

Par ailleurs, en s'appuyant sur les résultats de l'étude cinétique, les premiers éléments de comparaison d'un réacteur classique à lit fixe et d'un réacteur original à membranes à lit de particules sont présentés. Pour le même débit à traiter, en fixant dans les deux cas un taux de conversion objectif de 95 %, le nouveau procédé permettant d'utiliser des particules de plus faible diamètre conduit à une réduction du volume de particules réactives d'un facteur 4, de la puissance énergétique à fournir au système d'un facteur 3 à 5. Les performances prometteuses de ce procédé restent à valider lors d'une étude pilote.

Mots-clés:

• Métaux,
• Cu(II),
• adsorption,
• cinétique,
• réacteur agité fermé,
• réacteur lit fixe continu,
• membranes d'ultrafiltration,
• réacteur à membrane,
• eaux industrielles

Mots-clés:

• Metallic compounds,
• Cu(II),
• adsorption,
• kinetic study,
• closed stirred reactor,
• continuous fixed bed reactor,
• ultrafiltration membranes,
• membrane reactor,
• industrial waste waters

Abstract

Legislation in France and in Europe is becoming more rigorous for industrial fluid wastes, which is encouraging many industries to find a proper solution to their effluent problems and to include a complete wastewater treatment plant in their process. This paper focuses on effluents containing metallic compounds, which can be toxic. For Cu(II) the maximal allowed concentration is 1 mg/l. Many unit operations can be used to remove metal compounds from industrial waste waters, but the most widely used are scaling followed by settling and/or filtration, adsorption, solvent extraction and incineration. The main drawbacks encountered with these processes are:

1. a low yield,

2. high operating and investment costs,

3. sludge production.

This paper deals with a new unit operation to be included in a complete treatment line in place of adsorption, for removing metallic compounds. This new operation consists of a membrane moving bed reactor, which uses very fine dispersed adsorbent particles, so as to increase the particles' specific area. The particles are circulated with the water and separated from the treated water by an ultrafiltration hollow fibre or tubular membrane. The potential advantages of the process are the following: use of high specific area particles, production of a permeate without particles or suspended matter, no bed fouling (in comparison with fixed bed adsorption), continuous separation of metallic compounds and particles by the membrane. Moreover, we have chosen porous ion exchange resins as adsorbent particles, thus allowing a further continuous regeneration and recycling of the particles.

The aim of this first paper on this new reactor is to introduce a comparison between the performances of a conventional fixed bed reactor and those of the moving bed membrane reactor, for the removal of Cu(II) from pure water. The first step for this study consists of a characterization of the particles/metallic compounds interactions in terms of kinetic rates and adsorption isotherms. Ion exchange is here considered with a chemical engineering approach, as a heterogeneous liquid/solid reaction.

Experiments have been carried out in a thermostatted 1 litre stirred batch reactor. The solutions are prepared with CuSO₄ . 5H₂0 in demineralised water. Cu(II) concentrations are measured by spectro-colorimetry using the regulation T90022 from AFNOR. Particles are cationic ion exchange resins referenced as Mono C-350H. Their mean diameter (315 mm) is higher than the membrane pore diameter (0.01 mm). No membrane internal fouling will then occur during particle filtration. Moreover the particle diameter is lower than the inner diameter of the fibres to prevent plugging of the hollow fibre.

The adsorption isotherms have been determined at 21°C and the relevant Langmuir and Freundlich parameters are given in Table 2. The influence of stirring velocity and temperature in the reactor on the reaction rate has been studied. For stirring velocities above 100 rpm, the apparent reaction rate is independent of the stirring velocity. That means that the kinetic regime is obtained for stirring Reynolds numbers higher than 4700. The plot of Lnkwapp against T-¹, at a constant and high enough stirring velocity to prevent external diffusion limitations, shows two linear areas :

- under 60°C, the kinetic regime is obtained, thus allowing the determination of the reaction activation energy (given in Table 3);

- above 60°C, the internal diffusion is limiting.

In the kinetic controlled area, the reaction was found to be first-order and the rate constant expressed by mass of particles is k_wapp=0.03721 lg_rés^-1 min^-1 at 21 °C.

Using the kinetic data, it is possible to design and to compare the characteristics of a fixed bed reactor and a moving bed reactor, which would have the same conversion rate (95 %), for the same effluent flow rate to treat (5 m³/h). As a first approximation, we consider operating conditions inducing a kinetic regime. We also consider that the reaction mostly takes place at the particle surface. This assumption allows one to obtain the reaction rate expressed by particle's area for any particle size. At 21 °C, the previously defined reaction rate is k_s=4.43x10^-5 m³/s.m². The expression of the conversion rate as a function of ks, the reaction volume and the particle diameter is obtained for each reactor from the mass balance written in terms of Cu(II). The pressure drop expression is also given for the reactors. In the case of the moving bed reactor the increase of dynamic viscosity induced by the presence of particles has been taken into account. For each reactor, a solid retention, particle diameter and fluid velocity corresponding to values widely used industrially have been chosen (see Table 4). For the membrane reactor, we consider hollow fibres with 0.93 mm inner diameter, 1.2 m length and a total filtration area of 50 m² per module. Calculations have been done for two limit values of the permeation flux: 50 and 100 l/h.m².

A comparison of the two reactors is given in Table 5, which demonstrates the interest of the moving-bed reactor, as it allows one to reduce the volume of adsorbent particles by a factor 4. This result is linked to the high specific surface of the fine dispersed particles. The membrane reactor also induces a high reduction of the pressure drop (by a factor 40) and of the energy consumption (from 4.7 for a one- module plant to 2.6 for a two-module plant).

These promising results are encouraging to continue the research on this process with the aim to optimize it. Further studies will include:

- comparison of different adsorbent particles and choice of the best available;

- validation of this preliminary simulation by experiments on a pilot plant, with synthetic and then industrial waters;

- integration of this unit operation in a complete treatment plant.