RICERCA ITALIANA     

Research program

Multiscale modelling and development of process reactors for polymeric nanoparticle precipitation

University Co-ordinator

Università degli Studi di BOLOGNA - INGEGNERIA CHIMICA, MINERARIA E DELLE TECNOLOGIE AMBIENTALI - ()

Research Unit Leader

Alessandro Paglianti

Description

The activities of Bologna Research Unit will have the objective to study the nanoparticles/microparticles production process by experiments and numerical simulations. The experimental data produced during the program advances will be analysed and adopted for developing a simulation methodology to be employed for the design of the industrial apparatuses. In particular, the Bologna Research Unit will focus the attention on two possible reactors typology: a stirred tank reactor and a reactor provided with static mixers. The comparison of the experimental results obtained in the two reactors will allow to identify the reactor that is most suitable for nanoparticles/microparticles production.
The first step of the experimental part of the investigation will be carried out with water and organic phase, that are typical fluids adopted for nanoparticles/microparticles production, with the main aim to pursue the fluid dynamics optimization of the mixing of the solvent in water. Therefore, the experimental runs will be aimed at identifying the optimal operating conditions for both the stirred tank and the static mixer; in particular optical and conductimetric methods will be adopted in order to determine the local concentration distribution of the liquids and the time required for the system homogenization. In the study of the reactor provided with static mixers, the superficial velocity of the two phases will be determined for identifying the dimension of the reactor, once the plan productivity is known. For the stirred tank reactor, various geometrical configurations and operating conditions will be tested (e.g. the reactor dimensions, the type and the number of impellers, as well as their dimensions and location, the shaft rotational speed, the pipe inlet, the operation mode of the stirred tank, continuous, batch, semibatch ) in order to identify the design criteria for the process optimization.
During experimental investigation of the stirred tank reactor, the mixing time for obtaining the complete mixing for the dispersion of the organic solvent in the aqueous phase will be determined. Several techniques have been already developed for the evaluation of such condition. In the present research program, conductimetric probes, that have been adopted in the recent past for the characterization of flow behaviour in gas-liquid reactors (Paglianti et al., 2000), will be used. In particular, the attention will be focused on the identification of the condition, for which the two liquids are homogeneous mixed on the whole reactor volume.
Optical techniques, that have been developed in the past by the Bologna Unit for the local measurement of the concentration of the dispersed phase in solid-liquid systems (Montante et al., 2002) will be adopted for determining the two liquids local concentration distribution. Also, data will be collected with a technique that allows to evaluate the mean hold-up of the dispersed phase on a reactor cross section and that has been already successful adopted by the authors for the hydrodynamic characterization of gas-liquid reactors (Paglianti & Pintus, 2001). A few experimental data are already available in the relevant literature, and most of them have been obtained in tanks stirred by radial impellers such as Rushton turbines, while insufficient information exist for different kind of impellers. In the present research program, both radial and axial impeller types will be adopted in order to verify if the commonly used criteria for evaluating the drops size can be successful adopted for any impeller type and for evaluating the lumps size. Indeed, this criterion as been derived from data obtained with Rushton impellers type (McManamey, 1978). In order to perform industrial scale-up of the apparatuses, the influence of Macroinstabilities inside stirred tank reactors seems to be worth of special attention (Galletti et al., 2004; Paglianti et al., 2006). Therefore, for the nanoparticles/microparticles production it is necessary to evaluate if the adoption of axial type impellers pumping up, that have been shown to reduce the Macroinstabilities effects and mixing time (Nienow & Bujalski, 2004), is to prefer to the adoption of impellers types promoting the Macroinstabilities.
Ones the homogeneous mixing conditions will be identified, particular attention will be devoted to the two-phase flow field that will be measured by Particle Image Velocimetry (PIV). This technique allows to determine the three components of the mean velocity vectors and of the turbulent characteristics of the flow field by using a pulsed laser sheet and two cameras that have to be properly synchronised with the laser emission. The acquired images analysis could allow not only the determination of the flow field in the reactor, but also the measurements of the dispersed phase concentration through the development of a suitable software.

The experimental analysis of tubular reactors provided with static mixers will partly follow the activity planned for the stirred tank reactors. For the reactor optimization the following geometrical parameters will be considered: the dimension of the mixing elements and their number, the distance between two subsequent elements. As already described for mechanically stirred tank reactors, also for the tubular reactors particular attention will be devoted to the flow field measurements. Also for this kind of reactors the PIV technique will be adopted and a purposely designed reactor section will be made in order to avoid errors caused by the curvature of the tube external surface that would produce the laser sheet deviation.
In the last part of the experimental program the precipitation of the Barium Sulphate in aqueous solutions will be investigated and the results obtained in the two reactor types will be compared with those obtained by the Torino Unit (confined impinging jet reactor) and by the Palermo Unit (Couette type reactor). The Barium Sulphate particles will be analysed by Palermo Unit with the SEM apparatus.
The experimental data relevant to the flow field will be adopted for identifying a computational procedure able to accurately predict the two reactors behaviour; in this way a reliable tool will be available for transferring the knowledge acquired on lab-scale to the industrial scale-up step. To this end, commercial Computational Fluid Dynamics codes will be adopted, based on the numerical solution of the Reynolds averaged Navier-Stokes equations (RANS). Currently, CFD simulations can be confidently applied to the predictions of the mean flow characteristics in single phase systems, while, due to the complexity of the phenomena occurring in the nanoparticles/microparticles production, further development of subgrid mathematical models and of numerical solution techniques is still required for the realistic simulation of condensation and precipitation. In this project, the system experimentally investigated in the two reactors types will be modelled using an Eulerian scheme, thus determining the mean and the turbulent flow field. This approach has been successful adopted for the prediction of fluid-dynamic behaviour of single-phase and solid-liquid stirred tanks (Montante et al., 2001; Montante & Magelli, 2005). Both the stirred tank reactors and the static mixers will be modelled with fully predictive techniques. Particular attention will be devoted to the turbulent model to adopt. The precipitation of nanoparticles/microparticles will be simulated taking into account mass transfer phenomena using population balance equations that will be provided by the Torino research unit and subgrid mathematical model that will be provided by Udine research unit.

The work plan can be divided in two workpackages that can be further separated in different activities. The two workpackages will be the following:
W.1. Experimental and numerical analysis of nanoparticles/microparticles production process in stirred tank reactors.
W.2. Experimental and numerical analysis of nanoparticles/microparticles production process in tubular reactors provided with static mixers.

All the activities relevant to the experimental analysis and the numerical simulations of the process in mechanically stirred tanks, that will be contained in the first workpackage, can be detailed in the following activities:
a.1.1 Fluid-dynamic analysis of mixing of the solvent in aqueous phase.
In this activity, the identification of the optimization of the working conditions for the nanoparticles/microparticles production process varying the impeller type and geometrical characteristics of the rector is foreseen. Once such operating conditions will be identified, the dispersed phase hold-up on different reactor cross sections and in particular positions (e.g. close to the baffles) will be evaluated by local probes. Moreover, the flow field in the reactor will be measured with the PIV technique. Finally, some experimental Barium sulphate precipitation runs will be carried out for validating the fluid dynamic optimization previously performed.
a.1.2 Numerical analysis of the solvent mixing in aqueous phase.
This activity will include the analysis of the experimental data collected in the a.1.1 step and the numerical simulation techniques development to implement in commercial CFD codes with particular attention to the turbulent model to be adopted. The predictions of the conditions for a good mixing, the prediction of the mean and turbulent flow field and of the dimensional distribution of the nanoparticles/microparticles based on population balance in collaboration with Torino unit will be the goals of this step.

In the second workpackage the activities relevant to the experimental analysis and the numerical simulations in tubular reactors provided with static mixers will be contained.
In particular, the following activities can be identified:
a.2.1 Design and construction of the experimental rig.
In this step, an experimental rig for the investigation of the nanoparticles/microparticles production process in tubular reactors will be designed and constructed. The rig will be provided with two different pumps, for treating the organic phase and water separately. The flow rates of the two phases will be measured by rotameters, the pipes will be provided with an appropriate measuring section for determining the flow field.
a.2.2 Fluid-dynamic analysis of the solvent and aqueous phase mixing.
In this activity, the identification of the good operating conditions and the dispersed phase local hold-up on different reactor cross sections will be evaluated. Moreover, the flow field in the reactor will be measured with the PIV technique. Finally, some experimental Barium Sulphate precipitation runs will be carried out for validating the fluid dynamic optimization previously performed.
a.2.3 Numerical analysis of the mixing of solvent and aqueous phase.
This activity will consist in the analysis of the experimental data collected in the a.2.2 step and, as for the activity a.1.2, in the development of numerical simulation techniques. The main goals will be the predictions of the flow field, of the local dispersed phase hold-up and of the nanoparticles/microparticles dimensional distribution based on population balance in collaboration with Torino unit and using other subgrid models provided by Udine research unit.