Microfluidics is an emerging field that offers numerous application possibilities in R&D, particularly for the development of processes. It is based on the control of flows and reactions at the microscale, and makes it possible to generate original properties, products or production methodologies.
Primarily implemented at IFPEN on systems operating in conditions close to ambient conditions, microfluidics can, in fact, be used to conduct experiments in more severe conditions, representative of situations for which our researchers are developing new technologies and "materials solutions".
Such is the case for experimental testing devoted to the development of catalysts, which involves to know and reproduce temperature and pressure conditions representative of the processes for which they are intended. For this field of application, IFPEN is exploring how microfluidics in controlled conditions (HP-HT) can contribute to a better understanding of the thermodynamic conditions for pilot-unit experimentation.
This avenue is the subject of a research thesis under way (Specific properties of supercritical fluids for fast and exothermic reactive systems) in partnership with the ICMCB, with the idea of preparing reaction tests in supercritical conditions, in order to eliminate gas/liquid transfer limitations and to improve diffusion and fluid/solid transfer coefficients.
When it comes to catalyst tests, one of the difficulties lies in validating the operating conditions to be implemented in order to ensure that the reaction medium reaches a given state (in this case, a supercritical state, whereby the disappearance of the gas/liquid interface is observed). However, the media in question are not well characterized on a thermodynamics point of view, since we are dealing here with mixtures that are complex (and multiple), consisting of industrial feeds (light oil cuts), in the presence of hydrogen and a solvent (methane, ethane or CO2).
The prior work required to characterize reaction systems can be carried out in high-pressure/high-temperature cells but this can be a long and tedious process, particularly due to procedures imposed by the presence of hydrogen and because of the large number of conditions to be tested.
Consequently, the field of microfluidics emerged as an interesting alternative for the objective targeted here: to determine the supercritical state transition point for all catalyst test mixtures.
In terms of thermodynamic data acquisition, its advantages were as follows:
The work was carried out in several stages, the first of which involved the design and implementation of the micro-system. The purpose of this chip micro-system (Figure 1) was to make it possible to observe the disappearance of the gas phase in the liquid. The custom-made system was created at the laboratory of the ICMCB, the thesis partner, which has a room dedicated to this type of production.
It consists of a silica support (engraved using a photo-lithographic process) and a pyrex plate (sealed by anodic bonding). The three fluids (oil cut, hydrogen and solvent) are injected at A, B and C and then move into the insulating zone, a mixing zone and finally the analysis zone in which different optical tools (high-speed camera, Raman, etc.) can be placed.
This specific and innovative design makes it possible to ensure that the fluid arrives in the analysis zone under the right pressure and temperature conditions.
In addition, it enables rapid data acquisition thanks to constant pressure conditions (whatever the increase in temperature) and to fluids that are almost static within the micro-system.
The operation of this device is described in a recent publication (Pinho & al. 2014).
The principle for obtaining the supercritical P-T coordinates is as follows. Direct visualization of the fluid, during temperature scanning, means that the bubble point (appearance of gas phase) and dew point (appearance of the liquid phase) can be easily determined. This process is then repeated for different pressures until a pressure-temperature envelope, enabling the critical pressure and critical temperature to be determined, is obtained (Figure 2).
Ultimately, this method makes it possible to obtain the critical coordinates of a given mixture, 5 times faster than when using a traditional visualization cell method. A thermodynamic model (Figure 3) can then be rapidly determined thanks to which, by interpolating the results, it is a straightforward process to obtain the critical points for the entire potential working zone of the catalyst testing facilities (Figure 3).
The use of (P-T) microfluidics made it possible to determine the operating conditions for a catalytic test, simply, with no significant investment and using real fluids. Once the system is in place, data acquisition is a rapid process and, beyond direct visualization, it is possible to consider combining various types of optical analysis (Raman, IR, etc.) with a view to optimizing the system's results (for example in situ concentration measurements).
For applications whereby absolutely precise measurements are not required, as here, microfluidics enables rapid acquisition of thermodynamic data.
The field of application for this technique, when implemented in variable and controlled (HP-HT) conditions, extends well beyond this example. As well as also making it possible to access physical properties, such as mixture viscosity or density, there is a growing interest in the use of the technique for nanoparticle synthesis.
In addition, it can be used to tackle fundamental problems in some of IFPEN's other fields of interest. For Geosciences, for example, it has been used for many years in the form of genuine "geological laboratories on chip" (also called micromodels) to study complex problems relating to reactive flows: combining a range of phenomena including wetting, dissolution-precipitation, invasion-percolation, etc.
A microfluidic approach for investigating multicomponent system thermodynamics at high pressures and temperatures
B. Pinho, S. Girardon, F. Bazer-Bachi, G. Bergeot, S. Marre, C. Aymonier
Lab on a chip, 2014, vol. 14, n° 19, p. 3843–3849. DOI: 10.1039/c4lc00505h.
A geological lab on-chip
For the past fifteen years or more, IFPEN's Geosciences Division has been exploiting glass micromodels to visualize multiphase flows in porous media, as encountered in oil production. These models, which reproduce phenomena at the pore scale, can be operated under high temperature and pressure conditions of up to 60°C and 120 bar. Designed with a view to studying the distribution of fluids within the porous space - a parameter that most influences their circulation in reservoir rocks -, the equipment is used to determine laws that will ultimately be incorporated into PNM-type modeling (Pore Network Modelling).
It should be noted that the configuration of these micromodels makes it possible to easily differentiate between the different phases present, without to the need for additives to improve optical contrast.
 Depressurization under tertiary conditions in the near-wellbore region-experiments, visualization and radial flow simulations
Les Rencontres Scientifiques d'IFP Energies nouvelles: "Microfluidics: from laboratory tools to process development"