The resulting by-products are useful
too, as it contains a mixture of lime and
organic residues, which are commonly
re-used in agriculture as a soil additive and
conditioner, rich in organic matter. Most
of the sugar-processing plants have their
own lime kilns, and they require good, high
purity limestone to burn. This ‘high calcium’
limestone is converted into quicklime in lime
kilns, at a temperature of 900°C. To obtain
milk of lime, the ‘calcined’ lime is mixed into
water and it is during this process that an
accurate, reliable level measurement ensures
consistent production.
Originally a differential pressure transmitter
was used with a modified back-pressure
bubbler system, to try to maintain a clear
level measurement. However, maintenance
was unpleasant and running costs were
still high, from the use of compressed air,
accuracy compromised by tube blockages,
build up and, of course, the changing
density as the lime was added into the
water. The exothermic reaction of mixing
the alkali and water also produces heavy
condensation inside the tank.The process
area requires necessarily high level of safety
as the lime is harmful and an irritant to both
skin and eyes in particular. Tank overflows
and any consequential clean-up are
unpleasant and hazardous.
So, any form of level control that can
increase reliability and reduce exposure
of maintenance personnel in this area of
the plant is of interest, and British Sugar
were keen to look for a new measurement
solution.
Less than perfect position
As a result, British Sugar has installed a
new 80 GHz radar from VEGA, which is
successfully working and following the
liquid level, despite the turbulent surface
and steamy environment producing high
levels of condensation; as well as build up
and splashing on the antenna from the milk
of lime solution.
All outer surfaces of the device are either
plastic or stainless steel in construction with
a fully encapsulated PTFE antenna system,
which means minimal corrosion and a
longer operating life for the sensor in this
highly alkaline environment.
Another challenge was the ideal mounting
position: to minimise splashing and build up
on