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Fouling in Microstructured Devices: a Review ChemComm View Article Online FEATURE ARTICLE View Journal | View Issue Fouling in microstructured devices: a review a b a a Cite this: Chem. Commun., 2015, M. Schoenitz, L. Grundemann, W. Augustin* and S. Scholl 51,8213 Microstructured devices are widely used for manufacturing products that benefit from process intensification, with pharmaceutical products or specialties of the chemical industry being prime examples. These devices are ideally used for processing pure fluids. Where particulate or non-pure flows are involved, processes are treated with utmost caution since related fouling and blocking issues present the greatest barrier to operating microstructured devices effectively. Micro process engineering is a relatively new research field and there is limited understanding of fouling in these dimensions and its underlying processes and phenomena. A comprehensive review on fouling in microstructured devices would be helpful in this regard, but is currently lacking. This paper attempts to review recent developments of fouling in micro dimensions for all fouling categories (crystallization, particulate, chemical reaction, corrosion and biological growth fouling) and the Received 4th October 2014, sequential events involved (initiation, transport, attachment, removal and aging). Compared to fouling in macro Accepted 24th February 2015 dimensions, an additional sixth category is suggested: clogging by gas bubbles. Most of the reviewed papers Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DOI: 10.1039/c4cc07849g present very specific fouling investigations making it difficult to derive general rules and parameter dependencies, and comparative or critical considerations of the studies were difficult. We therefore used www.rsc.org/chemcomm a statistical approach to evaluate the research in the field of fouling in microchannels. Introduction includes hybrid unit operations and miniaturization, as well as widely applied process intensification technologies such as This article is licensed under a Process intensification is important for the chemical industry structured catalysts and packings. because it can help increase efficiency in both ecological and An intensifying process employing a microstructure device is economic terms. Stankiewicz et al.1 emphasized the significance associated with at least one process that occurs at a dimension of process intensification by describing it as a ‘‘paradigm shift’’ below 1 mm, and consequently results in very high surface-to- Open Access Article. Published on 24 February 2015. Downloaded 9/24/2021 3:54:20 PM. for process and equipment design. Micro process engineering volume ratios. Laminar flow typically results from the combi- nation of characteristic dimensions in the m-scale and flow velocities that are frequently limited by an acceptable pressure a Institute for Chemical and Thermal Process Engineering, drop. This means low shear stresses are exerted on the fluid. Technische Universita¨t Braunschweig, Langer Kamp 7, Braunschweig, Germany. E-mail: [email protected] Applied miniaturization also leads to short transport lengths b Creavis, Evonik Industries AG, Marl, Germany for heat and mass transfer as well as low residence times that Martin Schoenitz is a doctoral Laura Grundemann is a project researcher at the Institute for manager in the unit ‘Sustainable Chemical and Thermal Process Businesses’ at Creavis, the strategic Engineering, Technische Universita¨t innovation unit of Evonik located in Braunschweig, Germany. In 2010 he Marl, Germany. She received her received his Dipl.-Ing. degree from Dr.-Ing. from the Technische Univer- Technische Universita¨t Braun- sita¨t Braunschweig. Her current schweig. He is currently working work focuses on managing and on the transfer of pharmaceutical controlling innovation projects macro-batch processes to conti- while taking economic, environ- nuous processes in microstructured mental as well as social aspects devices with a focus on fouling into account. M. Schoenitz phenomena. L. Grundemann This journal is © The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51, 8213--8228 | 8213 View Article Online Feature Article ChemComm can positively affect a wide range of applications. In particular, A comprehensive overview on fouling was first described by reactions with fast kinetics, reactions initiated by mixing events Epstein14 in 1983 using a 5 Â 5 matrix which was based on the or highly temperature sensitive reactions are especially likely to five primary fouling categories and the five successive events benefit.2 This applies to existing processes for which the during the formation of fouling layers. 26 years later, Wilson advantages are being investigated by companies like BASF updated the matrix and his analysis indicated that progress had and DuPont.3 Likewise, micro process engineering may act as a been made for all research topics.15 ‘‘process enabler’’ in situations where the creation of completely Conversely, fouling in microstructured devices and the under- new processing routes in micro structures can render produc- lying phenomena in the dimensions of r1000 mmhaveonly tion economically viable, having formerly been not feasible in been investigated sparsely so far. Although fouling had already macroscale equipment.2,4 been identified by Hessel et al.16 in 2004 as a major barrier to the The benefits are, however, at the expense of lower capacities successful implementation of microstructured devices, only a few compared to macro equipment. Microstructured devices are there- research projects have focused on the topic in the subsequent fore suited for products with maximum capacities of 100–1000 t/a: years. These projects led to some very basic design rules and rules that is, specialties or products of the pharmaceutical industry.5 of thumb to prevent fouling, such as the design of channels ‘‘as Particulate flows are another challenge for microstructured devices small as necessary and beneficial, not as small as possible’’.17 since the ratio of the characteristic dimension to the particle Wiles and Watts18 refined that rule for particulate systems and diameterdecreasessignificantly.Thisisoneofthereasonswhy from three examples concluded that plugging occurs when the fouling – i.e., the unwanted deposition on surfaces – and plugging particle size is larger than 10% of the smallest dimension in the occur regularly in microstructured devices. The consequences for system. Despite all efforts, Hartman stated in 2012 that plugging process performance can be dramatic: fouling layers lead to local was still the greatest challenge to the design of continuous constrictions that change flow velocities and increase pressure processes using micro structured devices.4 This work aims to drops. In microstructured devices with internal numbering-up, i.e., comprehensively review and assess fouling and plugging in Creative Commons Attribution-NonCommercial 3.0 Unported Licence. multi-channel apparatuses, maldistributions of the flow can result in microstructured devices for better understanding of the under- spatial and time distribution of relevant process parameters. In the lying phenomena, and to enable more efficient equipment design. worst case, complete blockages of the microchannels can occur. For structuring purposes, the 5 Â 5matrixinventedbyEpstein14 Fouling in general is a well-known problem, and one that for macro-scale processes is applied for the microstructured has been encountered ever since our ancestors domesticated systems. Membrane fouling is, however, excluded from this review fire.6 Fouled layers cause high financial losses in all industry for it differs significantly from microchannels due to the porous sectors due to increased transport and installation costs, capital nature of the fouling surface combined with the influence of costs for antifouling equipment,fuelcosts,maintenancecosts transmembrane flow on the fouling process.19 and due to production losses.7 InGermany,thelossisequivalent This article is licensed under a to 0.25% of the gross domestic product, or about six billion h/a. Fouling can also lead to high capital expenditure. For Classification of fouling example, heat exchangers have been reportedly oversized by 30% on average to compensate for fouling.7 A desire to over- Fouling phenomena in macro-scale devices were divided into Open Access Article. Published on 24 February 2015. Downloaded 9/24/2021 3:54:20 PM. come such costs have therefore led to increased research efforts five different categories by Epstein14 according to the cause of to understand macro-scale fouling phenomena.8–13 fouling and the type of matter that consequentially attaches to Wolfgang Augustin is a senior Stephan Scholl is professor of researcher at the Institute for Chemical and Thermal Process Chemical and Thermal Process Engineering and head of the Engineering, Technische Universita¨t respective Institute at the Braunschweig, Germany. He Technische Universita¨tBraun- received his Dr.-Ing. in 1992 from schweig, Germany. He received Technische Universita¨t Braun- his doctorate degree in 1991 schweig. His main research from the Technical University of interests are heat and mass Munich. After eleven years with transfer, fouling, viscous fluid flow, BASF AG, Ludwigshafen, Germany, and surface interactions on the he joined TU Braunschweig in micro scale level. 2002. His main research areas are W. Augustin S. Scholl sustainable production concepts, innovative equipment technologies, fouling and cleaning, reaction and separation
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