A Complementary Study of Ultra-Fast Magnetic Resonance Imaging and Electrical Capacitance Tomography for the Scale-up of Gas-Solid Particulate Systems

Lead Research Organisation: University of Cambridge
Department Name: Chemical Engineering and Biotechnology

Abstract

The behaviour of multiphase particulate or granular systems (e.g. in fluidised beds and pneumatic conveyors) presents severe experimental problems because they are opaque, largely preventing the use of optical techniques. Also, inserting physical probes inevitably disturbs the system under investigation. Thus, it has been difficult to develop reliable scale up criteria or validate numerical simulations of these systems. We aim to validate and explore the limitations of measurement using two complementary, non-intrusive experimental techniques: Electrical Capacitance Tomography (ECT) and Magnetic Resonance Imaging (MRI), using the combined expertise of Ohio State University (ECT) and Cambridge (MRI). Particular regard will be paid to the applicability of these techniques in the validation of the predictions of Discrete Element Modelling (DEM) and in the development of scale-up criteria in gas-fluidised beds. This is timely, given recent developments in all three of these areas, particularly in the potential that ECT could have in the design of truly industrial-scale fluidised beds, provided it is properly validated.The experimental techniques considered here (MRI and ECT) are complementary in that their strengths lie in measuring different features of multi-phase granular systems. MR enables the bulk solids motion to be visualised, as well as the particle velocity profiles, in both the dense solids phase and the lean (bubble, jet, void) phase . Furthermore, it is possible to determine voidage profiles. We also propose to extend MR to be able to image gas directly for the first time in a multiphase system. ECT has a major advantage in that it does not have any serious restrictions regarding size, although equipment must be non-metallic. As with MRI, it is possible to determine the velocity of voids, i.e. bubbles and slugs, and voidage profiles. The velocity of the bulk solids cannot be determined, however. Importantly, there is an overlap in the variables which can be measured by either technique, the most important ones being voidage profiles and the rise velocity of voids. These measurements will be used for cross-validation of the two techniques.
 
Description Studying multiphase, particulate systems (e.g. gas-solid fluidised beds and pneumatic conveyors) presents substantial experimental problems because they are opaque, largely preventing the use of optical techniques. Also, inserting physical probes inevitably disturbs the system under investigation. Thus, it has been difficult to develop reliable scale up-criteria or validate numerical simulations of these systems. Our previous research has successfully shown, firstly, the utility of Magnetic Resonance Imaging (MRI) for quantifying fundamental parameters in fluidisation (under EP/C547195/1).



For scale-up, the first comparison of MRI with Electrical Capacitance Tomography (ECT) has been undertaken (with Prof. Fan, Ohio) on fast fluidisation using this EPSRC Award (EP/F041772/1). In ECT, the permittivity distribution is estimated from measurements of the capacitance between pairs of electrodes. Unlike MRI, it has relatively few restrictions on the type and size of system. Using ECT, hydrodynamic phenomena have been studied in gas-fluidised beds of between 50 mm and 300 mm diameter, including the coalescence and splitting of bubbles, the discharge of bubbles at an orifice, the shape of bubbles, and their rise velocities. However, the reconstruction of an image from the capacitance measurements poses two major challenges: (1) low sensitivity leading to large electrodes and significant axial averaging, and (2) it is an inherently ill-posed problem leading to complex non-linear reconstruction and a poorly defined spatial resolution. Even with recent advances in image reconstruction, the resolution of ECT measurements is still limited and difficult to quantify. Further, the non-linearity of the measurement means that quantitative estimates of the resolution cannot be performed using phantom studies for which the permittivity distribution may not match the system of interest. A comparison of MRI and ECT measurements of the same systems found that the resolution of the ECT in a fluidised bed is typically about 20% of the diameter of the column being studied. However, ECT is sufficiently promising that, in a related, separate proposal, Dr Holland is developing a new image reconstruction approach that will build on the ideas of Compressed Sensing to improve the spatial resolution of ECT imaging. Furthermore, through a combination of funds from the Department of Chemical Engineering and two competitive grants from the University of Cambridge, Dr Holland has purchased an ECT measurement system (value £70 000) which is now installed in Cambridge.
Exploitation Route ECT has a multitude of uses in industrial practice, especially if our ideas on improving resolution can be applied to it. Please see above for follow-on grants to expand our use of ECT for studying granular flows. These will have practical utility for making measurements at a process plant scale and therefore will be of direct relevance to the process industries.



We intend to couple MRI, ECT and X-ray tomography in a future proposal for funding, with X-ray imaging being important for studying metal vessels of process size. The X-ray activity will be collaborative with UCL (Dr Lettieri).
Sectors Chemicals

 
Description Isaac Newton Trust
Amount £51,823 (GBP)
Funding ID 11.35(i) 
Organisation University of Cambridge 
Department Isaac Newton Trust
Sector Academic/University
Country United Kingdom
Start 04/2011 
End 04/2012
 
Description University of Cambridge
Amount £26,195 (GBP)
Funding ID Departmental Grant (Chemical Engineering & Biotechnolog 
Organisation University of Cambridge 
Sector Academic/University
Country United Kingdom
Start 06/2012 
End 05/2013
 
Description University of Cambridge
Amount £49,828 (GBP)
Funding ID RG64929 and RG64927 
Organisation University of Cambridge 
Sector Academic/University
Country United Kingdom
Start 04/2011 
End 05/2012