Experimental Characterization of Two Phase Interfacial Mass Transfer in Novel Milli-Scale Reactors

Experimental characterization of two phase interfacial mass transfer in novel milli-scale reactors

Over the last decade, researchers are striving to make chemical process more cost efficient and safer. The ACS-GCI Pharmaceutical Round table¹ have identified process intensification as key research area for sustainable chemical industry. Moreover, demand for development of novel concept in continuous multiphase reaction system is identified which can minimize waste and energy use while maximising use of renewable raw materials. One approach to achieve this goal is based on the use of micro-scale reactors. As micro-scale reactors handle only a very small amount of substance at a time, they are capable of producing high quality goods from minimum resources with efficient management of hazardous material. However, micro-scale quantity production is not economical for the industrial scale.

The limitations of micro-structure systems can be overcome by novel milli-scale reactors based on porous micro-structured foams. The idea is to use highly porous micro-structured inserts (metal foam structures) for plug flow reactors in the millimetre range such that high throughput is provided at a comparable small pressure drop. A huge potential arises from open cell metal foams, due to their high specific surface area combined with high porosity, typically larger than 70%. The induced turbulence in the micro-structured geometry leads to an enhanced heat and mass transfer which is e.g. shown by Hutter et.al.² for single phase systems. However, the potential of metal foams to enhance interfacial transport processes in multiphase flow application is not yet addressed.

Figure 1: Representation of different micro-structured foams studied. A) Silicon scaffolds B) Metal foam by 3DFD technique C) Metal foams by SLS.

In order to optimise the performance of novel micro-structured foam reactors for multiphase systems, it is important to first gain a better understanding on the behaviour of this reactor for multiphase systems and the underlying physics of the transport processes. This can be achieved by investigating the effect of various design parameters like porosity and ligament shape, cell size etc. on interfacial mass transfer in the multiphase system. In this study, we achieve the same by evaluating a variety of porous structures (Figure 1) ranging from random silica scaffolds (manufactured by NCL, India) to custom made metal foam structures by techniques like 3-dimentional fibre deposition (3DFD, manufactured by VITO, Belgium) as well as Selective Laser Sintering (SLS, manufactured by Inspire, Switzerland). In this study, we apply a combination of PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence), where PIV is used to assess the fluid velocity field while LIF is used to measure the concentration distribution of a species. Consequently, the PIV data is used to characterize the turbulence intensity induced by the porous structure, while the LIF data is used to characterize the two phase flow distribution. The combination of both techniques facilitates a detailed study of interfacial mass transfer as well as its physical mechanism.

Moreover, the effect of design parameters on the mass transfer in liquid-liquid flows is quantified by physical and chemical methods. The physical method consists of two systems: i) Toluene- Water with acetone as transfer species and ii) 1-Butanol- Water with succinic acid as transfer species. On the micro-scale, the first system is represented by Taylor flow, while 1-butanol and water results in stratified flow due to the low interfacial tension. Moreover, these flow systems allow to study the influence of surface energy and resulting flow patterns in porous medium on mass transfer. As chemical method, we have investigated fast chemical reactions that occur at the interface of two phase systems; i) Hydrolysis of N-butyl formate (NBF) with sodium hydroxide and ii) Neutralization reaction of trichloroacetic acid with sodium hydroxide. The main aim of studying different two phase systems is to examine the effect of interfacial area and mass transfer coefficient independently. In addition, the performance of the novel porous structures is assessed by comparing the obtained results with the performance of milli-scale packed bed reactors under identical operating conditions.

We have observed that design parameters play vital role in determining interfacial transport processes in novel micro-structured foam reactors. We hypothesize that the porous structures lead to a promotion of merging and break up of slugs which has major impact on mass transfer through constant change in shape and size of the interfacial area as well as the renewed boundary layers. It is also observed that metal foam structures give enhanced mass transfer performance at negligible pressure drops compared to conventional milli-scale packed bed reactors.

The results of this study will not only help in the development of sustainable chemical process but will also increase the fundamental understanding of multiphase processes in a porous media.

(1)         Jiménez-González, C.; Poechlauer, P.; Broxterman, Q. B.; Yang, B. S.; Am Ende, D.; Baird, J.; Bertsch, C.; Hannah, R. E.; Dell’Orco, P.; Noorman, H.; Yee, S.; Reintjens, R.; Wells, A.; Massonneau, V.; Manley, J. Org. Process Res. Dev. 2011, 15, 900–911.

(2)         Hutter, C.; Allemann, C.; Kuhn, S.; von Rohr, P. R. Chem. Eng. Sci. 2010, 65, 3169–3178.