(70d) The Netmix Reactor: Heat and Mass Transfer Modeling and Mixing Properties Assessment

The NETmix Reactor: Heat and Mass Transfer Modeling and Mixing Properties Assessment

C. M. Fonte, M. F. Costa, M. M. Dias, J. C. B. Lopes

Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Porto, Porto, Portugal

e-mail: lopes@fe.up.pt

Static mixers are currently used in industrial applications to perform continuous operations and have become standard equipment since the 1970s for the mixing of miscible or immiscible fluids, homogenization of solid particles, and for the enhancement heat and mass transfer [1, 2], as an alternative to mechanically stirred vessels. The effectiveness of static mixers to deliver good mixing, or to enhance heat and mass transfer, derives from their capacity to statically induce radial mixing and to bring fluid elements into close proximity [2]. In these devices, products are mixed only by the energy of the flow, and they do not require external power, except for the necessary energy for pumping the fluids through the mixer, And this combined with small equipment size, low equipment cost and no moving parts [2].

The NETmix reactor is a new static mixer/reactor consisting of a network of mixing chambers interconnected by transport channels [3]. Networks are generated by the repetition of units of mixing cells, where each mixing unit cell consists of one mixing chamber, two inlets and two outlet channels oriented at a 45º angle from the main flow direction. Above a critical channel Reynolds number, the device main flow pattern evolves to a self-sustained oscillatory laminar flow regime inside the mixing chambers inducing local strong laminar mixing, which results from the geometrical features of the NETmix reactor. A network model was previously developed to describe and predict the behavior and mixing performance of NETmix [4]. From the point of view of modeling, and in practical terms, it was shown that chambers behave as perfectly mixing zones and the channels as plug flow segregation zones. The NETmix reactor can therefore be conceptualized in mixing terms, as essentially a plug flow reactor with local maximum mixing. The mixing degree was defined and quantified [4, 5] showing that mixing can be controlled effectively and efficiently making it particularly suited for complex and fast kinetics reactions. Furthermore, NETmix is also a good efficient device, for heat and mass transfer, due to its high surface to volume ratio of the order of 10³ m²/m³, one or two orders of magnitude higher when compared to conventional heat exchanger/reactors [6], and conventional reactors for catalytic reactions [7, 8]. Although the surface to volume ratio is one order of magnitude smaller than for micro-structured reactors [8, 9], the heat and mass transfer coefficients are expected to be strongly enhanced by the induced laminar mixing oscillations in the NETmix chambers, above the critical Reynolds number, [10, 11], and additionally preforming lower pressure drops when compared with micro-reactors.

Computational fluid dynamics (CFD) is an increasingly effective alternative to speed up equipment design and to be used as a fundamental tool for optimizing heat and mass transfer using static mixers [1]. In this work, a new 3D CFD model is introduced that enables the computation of transport properties of a more realistic representation of the NETmix geometry, the NETmix Unit Block(NUB) model. This simulation tool can be applied to any particular geometry of the NETmix reactor and overcomes the need of experimental data each time a new configuration is designed. The NUB model was used to study heat and mass transfer in the NETmix reactor and from this work, correlations of unit cell Nusselt and Sherwood numbers were developed. The present study suggests that 3-5 times higher heat/mass transfer rates are achieved when compared to the efficient parallel plates devices. Maximum convective heat and mass transfer coefficients are achieved when the flow inside the mixing chambers evolves to a self-sustained oscillatory laminar flow regime inducing strong local laminar mixing. Finally, a phenomenological model is presented to assess the rates of interfacial area formation along the NETmix reactor and compared with VOF dynamic mixture CFD simulations with the NUB model.

[1] E.S. Szalai, F.J. Muzzio, Fundamental approach to the design and optimization of static mixers, AIChE Journal, 49 (2003) 2687-2699.

[2] R.K. Thakur, C. Vial, K.D.P. Nigam, E.B. Nauman, G. Djelveh, Static Mixers in the Process Industries—A Review, Chemical Engineering Research and Design, 81 (2003) 787-826.

[3] J.C.B. Lopes, P.E. Laranjeira, M.M. Dias, A.A. Martins, Network mixer and related mixing process. PCT/IB2005/000647, February 2005. European Patent EP172643 B1, October 2008, in, 2005.

[4] P.E. Laranjeira, A.A. Martins, J.C.B. Lopes, M.M. Dias, NETmix®, a new type of static mixer: Modeling, simulation, macromixing, and micromixing characterization, AIChE Journal, 55 (2009) 2226-2243.

[5] P.J. Gomes, C.P. Fonte, R.J. Santos, M.M. Dias, J.C.B. Lopes, Experimental and numerical characterization and quantification of mixing in a NETmix® reactor, in:  ECCE 7 - 7th European Congress of Chemical Engineering & CHISA 2010- 19th internation Congress and Process Engineering, Process Engineerig Publisher, Prahe, Prague, Czech Republic, 2010.

[6] Z. Anxionnaz, M. Cabassud, C. Gourdon, P. Tochon, Heat exchanger/reactors (HEX reactors): Concepts, technologies: State-of-the-art, Chemical Engineering and Processing: Process Intensification, 47 (2008) 2029-2050.

[7] F.J. Keil, Catalytic reactions and reactors, Chemical Engineering Science, 59 (2004) 5473-5478.

[8] L. Kiwi-Minsker, A. Renken, Microstructured reactors for catalytic reactions, Catalysis Today, 110 (2005) 2-14.

[9] M.N. Kashid, L. Kiwi-Minsker, Microstructured Reactors for Multiphase Reactions: State of the Art, Industrial & Engineering Chemistry Research, 48 (2009) 6465-6485.

[10] J.G. Khinast, A. Bauer, D. Bolz, A. Panarello, Mass-transfer enhancement by static mixers in a wall-coated catalytic reactor, Chemical Engineering Science, 58 (2003) 1063-1070.

[11] D. Li, K. Xiong, Z. Yang, C. Liu, X. Feng, X. Lu, Process intensification of heterogeneous photocatalysis with static mixer: Enhanced mass transfer of reactive species, Catalysis Today, 175 (2011) 322-327.