One of the challenges in managing filtration and separation processes is to know what’s going on inside the filter or separator. Industrial Tomography highlights the benefits of scanning in achieving this
While there are many models and empirical data to estimate optimal performance, there is still a high level of variability from batch to batch. This can be caused by different mechanisms from process steps upstream which can change the materials being filtered or separated to the actual conditions in the process vessel changing over time.
However with the correct real time information from the actual process unit it will be possible to improve consistency of filters and identify end points in different separation processes. This would significantly improve efficiency and yield. The technology of process tomography based on CAT scanning in medicine provides a tool for getting this sort of information. Industrial Tomography Systems has pioneered the commercialisation of this technology and leads the world in its application.
The technology works by providing a map of the electrical properties across a wide volume of process material through using either a probe or circular arrangement of sensors. In most processes the electrical properties of the different phases varies considerably so detection of interfaces and end points is clear from the instrumentation. In process control, the mapped information is consolidated to provide the time and position of a transition point as a 4-20mA output.
Wide application
Key applications of process tomography in the field of filtration and separation have been in filter/driers and phase separation.
In the case of a filter/drier, a sensor array is mounted at the base of the filter plate to enable engineers to see the rate at which a filter cake dries out and the extent to which drying is even across the cake. An example of this was carried out in a 36m3 filter drier (Fig 1) which went through two successive acid washes followed by two water washes. It was straight-forward to see that the wettest part of the cake was above the feed nozzle and to determine the rate at which good and bad batches dried.
The graph (Fig 2) shows the average readings of two batches and their variability in performance – from 56 hours standard operating time to over 80 hours in the alternate batch. In this case the reasons behind the performance were attributed to the slower batch having smaller average particle size. This lead to them settling more slowly (as can be seen at the start of the process) and subsequently slowing filtration and drying by restricting flow through the filter pores.
Different strategies such as ploughing or reconstituting the filter cake can be when drying is taking too long.
As well as drying water-based filtration systems, ITS technology has been used to monitor pure organic and azeotropic (organic/aqueous) systems.
A second separation process is solvent extraction which is widely used across many process sectors including petrochemicals and pharmaceuticals. Based on mixing two immiscible liquids and then allowing them to separate, purifying the desired product and allowing impurities to be taken off in the second phase once the organic and aqueous phases separate.
The point of separation is important to the yield and efficiency of the process. ITS sensors identify in real time the point of aqueous/organic and water continuous emulsion phases. Other sectors where the technology has been applied include froth flotation (identifying bed height) and hyrdocyclone separation (identifying air core diameter).
Both are widely used in the mining sector. In the biotech sector, ITS sensors have been used to determine how conditions change as flow progresses through a packed column.
Simple sensors
One of the attractive aspects of the technology is that sensors are very simple – usually an array of electrodes supported on a polymer substrate. Low currents are applied to the sensors, allowing them to be placed in highly volatile atmospheres. The data collected from these simple sensors is analysed quickly through complex algorithms to provide the spatial information that engineers can use.
A range of proprietary designs for ATEX Ex(i) rated processes in environments ranging from challenging pharmaceuticals and nuclear waste processing to the more benign such as food and drink. In addition sensors can operate at high pressures and across wide temperature ranges.
Over the past 10 years a small number of blue chip process companies have taken this technology from experimental platforms to pilot plants to installation on production plants around the world. Over the next decade we can expect to see its wider use to improve process efficiencies in filtration and separation processes.
