Measuring large volume molasses presents users with several distinct problems. As well as having a viscosity that is similar to that of API 10 crude oil and a widely varying density, there is also the fact that this by-product of sugar milling is prone to fermentation.
This fermentation causes the presence of a thick layer of heavy foam (with a density that is around a quarter of that of the liquid) which forms on top of the commodity. Because the foam cannot be sold, owners prefer to measure the liquid without the foam.
Historically this measurement was undertaken using a pressure transmitter or even a manometer that was connected to the bottom of the storage tank. This approach included the weight of unsellable foam, suffered from the aforementioned problem of variable density and was no use in helping to prevent overfill.
There have been several attempts to address this problem using non-contact level transmitters over recent years and because the foam is dense and relatively conductive it can produce a good reflection when probed with either radar or sonic instruments. This does not tell the user anything about the amount of sellable liquid in the tank though, and even TDR (sometimes known as guided radar) is unable to penetrate the conductive foam and ascertain the liquid level.
Drexelbrook can address this particular measurement challenge by using a precisely selected combination of RF probe and operating frequency. The probe insulation's capacitance value is the core factor in its performance, with Drexelbrook able to produce a wide range of capacitance.
The instrument is selected to have total independence of the liquid conductivity while at the same time having minimal sensitivity to the lower conductivity of the foam. This approach requires prior knowledge of the conductivity within the specific liquid molasses being measured, so it is advised to check a sample for each processor in order to maintain the best foam rejection.
This approach measures the liquid in the tank but is unable to return the value of exactly how much the tank can hold. As the layer of foam is so thick and variable, the realistic choice is to either take a chance on a potentially messy and unsafe overflow of foam, or waste space by allowing additional margin.
Ztron III Level Switch
Drexelbrook also offers specialist level switches which are designed to prevent overflow in a huge range of tanks. Their RF probes are already in place, preventing spills in thousands of tanks ranging from sewage sludge to lime slurries and liquified gases.
The company’s Ztron III level switch is the most cost-effective model within their renowned Coteshield range. It is capable of safely showing the presence or absence of molasses foam, even when it is thickly coated with molasses. There are no moving parts which could become fouled up, and coating does not result in false high alarms like regular capacitance probes. The Ztron III simply needs to be mounted horizontally through a 3/4 NPT half coupling at the maximum height in the tank before it is connected to the integral electronics via power and signal leads.
This information has been sourced, reviewed and adapted from materials provided by Drexelbrook.
For more information on this source, please visit Drexelbrook.