Reactive filtration

The term reactive filtration may be used in a variety of applications. A simple search of the internet provides results such as reactive filter paper [1], adsorption filters for removing heavy metals from water [2], solid matrices used in organic synthesis [3], membranes for wastewater treatment, or even dialysis machines, filters for deep-frying pans and devices for the dechlorination of shower water by reaction with vitamin C. Most of the applications termed reactive filtration would be named heterogeneous catalysis or adsorption from a chemical engineer s point of view. 

In the context of integrated chemical processes however, reactive filtration should be specified as the combination of the solids handling unit operation filtration device and the unit operation chemical reactor . This comprises the separation of solids or aerosols from a fluid stream and the chemical conversion of undesired compounds carried by the stream in one instead of two unit operations. The chemical reaction can proceed either continuously during the separation or stepwise after a certain amount of solids or aerosols has been deposited. Two general cases for the application of a reactive filtration unit may be distinguished  

The first case covers for example flue-gas treatment, which requires the filtration of fly-ash and the reduction of NOx, or gasification processes, where particulates and high-boiling tars have to be removed. An example of the second case is that of combustion processes, where incomplete combustion leads to the emission of carbonaceous particulates. The most relevant topic in this category is the reduction of diesel particulate emissions ( diesel soot ) by catalytic filtration. A more exotic example is the reaction cyclone for the thermal conversion of biomass, which also combines chemical reactions and separation in one apparatus, though its separation mechanism is not filtration. 

The chemical conversion in the reactive filter can be achieved either by simply altering the reaction environment through the deposition of material, or with the help of catalysts. The alteration of the reaction environment may lead to higher concentrations of reactive species or to different heat transfer conditions. The reactions 

Integrated Chemical Processes. Edited by K. Sundmacher, A. Kienle and A. Seidel-Morgenstern Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30831-8 

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In the following sections, the different types of reactive filters, together with their typical applications, will be presented. The separation and catalytic combustion of diesel soot is the most important and furthest investigated application. Consequently, this type of reactive filtration, with its practical and theoretical aspects, will form the main focus of this chapter. 

Reactive filters for the removal of reactive solids (i.e., diesel soot) have already passed through the test rig state. For the reactive filtration of diesel soot, industrially manufactured devices are available, but improved reaction kinetics and models predicting accurately the loading and regeneration cycles are yet to be produced. 

Filtration and chemical reactions can be usefully integrated in order to separate diesel soot particles efficiently from motor exhaust gases, and this is illustrated by Rieckmann and Volker in Chapter 15, together with a series of other examples of reactive filtration processes which are realized in the chemical industries. 

Tripp JA, Stein JA, Svec F, Frechet JMJ, Reactive filtration Use of functionalized porous polymer monoliths as scavengers in solution-phase synthesis, Org. Lett., 2 195-198, 2000. 

As a variation of the process, Frechet and coworkers introduced reactive filtration [6]. In place of beads, they used discs of cross-linked polystyrene that were cut from a rod of the material. On the surface of these discs, they grafted 2,2-dimethylazlactone. These discs were then used in a filtration to efficiently scavenge excess amines from a reaction mixmre. Subsequently, Frechet and coworkers also used such discs in a flow through acylation reaction [7]. 

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