Typical Applications - Multiphase Flows

Multi-component and multiphase fluid flows exist in many industrial processes. The phase distribution and interfaces in an aqueous-based process or multiphase flow carries significant information about the processes. Due to the correlation of the multiphase and independence of each phase, the correct measuring of multiphase flow, in terms of concentration, disperse interface, local velocity and mass flow rate, are extremely challenging tasks. In 1990s, Electrical capacitance tomography was originally developed for oil-water two-phase flow, then electrical impedance tomography was introduced from medical research to process engineering. Significant progress has been made during the past decade and now the technology has been proved as a powerful tool for mapping the concentration and velocity distributions of the second phase in two-phase flows, where electrical impedance differences between the two-phase fluids exist. Previous and current works on process optimisation using electrical resistance tomography at the University of Leeds include mixing processing, sedimentation and sediments, bubble column, oscillated baffle reactors, air vortex detection, precipitation, heap leaching, in-line mixer, air-water flows, oil-water flows, solid-water flows, slurry swirling flows, open channel flows, etc. [1]

Control of bubble column

The gas hold-up and gas interfacial area are two of the most important parameters for a gas–liquid bubble column reactor, since they determine, to a large extent, the reactor’s throughput and reaction efficiency. In general, the interface area is a function of the column’s geometry, operating conditions and physiochemical properties of the gas/liquid phases involved.

In the case of a slow or instantaneous reaction, the surface area of uniform bubbles with the Sauter bubble diameter is proportional to the gas hold-up [2]. As the superficial gas velocity increases, the churn-turbulent flow regime starts to develop, and the gas hold-up reaches a maximum, followed by a slight decrease, before it rises again.

The methodology of using an ERT image for control of reactor operation was developed [3] to automatically reach and maintain the flow condition deemed to correspond to the maximum interfacial area, since such a condition is desirable for many chemical reactors and mass transfer operations using a bubble column, This is believed to be the first instance of using ERT images for real time control (Figs. 1 and 2). The results demonstrated that the method has a strong capability against the effects of the background conductivity variation during the column operation.

(a)

(b)

Fig. 1. ERT images and photographs of (a) a homogeneous flow and (b) a heterogeneous flow in a bubble column.

Fig. 2. The control of a bubble column’s optimum interfacial area. (a) The open-loop performance of the controlled variable (mean concentration obtained from tomography imaging). (b) The set point. (c) The controller’s output during the first 100 control points. Each point was obtained by averaging over 50 images.

3.3. Visualisation of swirling flows

Efficient slurry transportation is vital to many industries. It has been proposed that helically formed pipes, which can be used to keep particulate solids in suspension, should be applied to enhance the distribution of solids in piped slurries. The use of a new in situ measurement method based on electrical impedance tomography is proposed to assist in understanding the effect of particle suspension as well as the effect on the wear of pipes by solid particle impingement due to the application of such a swirl-inducing pipe [4]. In the absence of accurate predictive models for such complex flows, it is demonstrated that this method enables direct visualisation of the solids’ concentration profiles in the pipes. Through the application of an advanced impedance image reconstruction algorithm and other analysis software, the asymmetric solids’ concentration distribution in horizontal swirling flows can be quantified (Fig. 3).


Fig. 3. At a fixed burden volumetric concentration of 8.6%: (a) Solid volume fraction distribution at downstream positions of L/D = 3.0, 7.4, 17.7 and 23 for water flow velocities of 1, 1.5, 2.0 and 2.5 m/s (L/D: the length to the pipe diameter ratio). (b) Mean volume fraction distribution via rings of images shown in (a) (Ring number 1 denotes the outermost ring and 12 the central disk).

References

[1] Wang, M. (2005) Impedance mapping of particulate multiphase flows, Flow Measurement and Instrumentation, Vol. 16
[2] W.D. Deckwer, Bubble Column Reactors, John Wiley and Sons, Chichester, 1992.
[3] M. Wang, X. Jia, M. Bennett, R.A. Williams, Flow regime identification and optimum interfacial area control of bubble columns using electrical impedance imaging, in: Proceedings of the 2nd World Congress on Industrial Process Tomography, VCIPT, 2001, pp. 726–734.
[4] M. Wang, T.F. Jones, R.A. Williams, Visualisation of asymmetric solids distribution in horizontal swirling flows using electrical resistance tomography, Chem. Eng. Res. Des. 81 (A8) (2003) 854–861.