Fouling online measurement

Prevention and Reduction of Liquid-Side Fouling. Among the most frequently used techniques for reduction of liquid-side fouling is the online utilization of chemical inhibitors/ additives. The list of additives includes (1) dispersants to maintain particles in suspension, (2) various compounds to prevent polymerization and chemical reactions, (3) corrosion inhibitors or passivators to minimize corrosion, (4) chlorine and other biocide/germicides to prevent biofouling, and (5) softeners, acids, and poliphosphates to prevent crystallization. Finally, an efficient mechanical removal of particles can be performed by filtration. An extensive review of fouling control measures is provided in Ret 150. 

The fouling factor has to be determined from actual heat exchanger performance based on online measurements taken from a process unit test run. Heat exchanger clean performance is obtained from process flowsheet simulation software (e.g., Hysys by Aspen Tech or Unisim by Honeywell), while dirty performance from exchanger rating software (e.g., HTRI by Heat Transfer Research Institute). 

New ways to build microelectrodes have made it possible to implant electrodes inside the membrane or on both sides of it. This makes measurements using acoustics possible (see Chapter 11) [16]. The acoustic waves can give a wave pattern that differentiates a clean membrane from a fouled membrane because the fouling spots can be identified as a new resistance. In a similar way cleaning can be studied as the removal of these resistive spots. The electrodes can be placed in such a way or be part of the membrane so that also membrane pore size could be measured or (at least with the techniques available today) pinholes could be found. Measurements made with acoustics seem to be very promising online measurements. 

Electrochemical shear probe Measurements membrane surface shear rate in the presence of air sparging. Online measurement with high speed camera allows the observation of bubble shapes No visualization of fouling layer [77]... 

Performance measurement Pressure drop, flux, conductivity or turbidity Fouling assessment Online, in situ, non-invasive, foulant composition analysis Information acquirement Real time, representative, accurate, reproducible, automatic Device requirement Reliable, user friendly, robust, low cost ... 

Additionally, online monitoring methods have been developed to adapt off-line characterization methods into in situ (i.e., in-reactor) probes for determination of kinetics and monomer conversion with optical methods such as mass spectroscopy (MS), ESR, FTIR, near IR, and Raman spectroscopy. However, frequently, due to high turbidity and viscosity of the polymer reaction milieu, the optical surfaces are easily fouled, leading to frequent sensor failure. Furthermore, data acquired with these probes are model dependent the empirical and inferential calibration schemes used can be expensive and time consuming to develop and can drift and become unreliable as reactor conditions change and as sensors become fouled. Another limiting feature of these methods is that they usually measure only one characteristic of the reaction, such as monomer conversion and are not directly sensitive to polymer molecular mass and intrinsic viscosity. More detailed discussion of these techniques can be found in Chapters 6-10 of this book. 

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