Page 22 - 6_TOC in daily practice

SCA-130-611

The automatic dilution function enables the ex- ■ Installation site
tension of the analyser's measuring range, as Generally, the distance between the sampled
well as the reduction of sample matrix. This re- material stream and the TOC analyser should
duces maintenance effort and as such the run- be kept as short as possible to prevent longer
ning costs for the operator. response times by larger transport time. In re-
fineries however, explosion-hazard areas can
■ Sampling technique limit possible installation sites for a combus-
Taking a representative sample is the first and tion analyser.
most important step for accurate TOC analy-
sis. Oil and fat in particular can be present in
water in very different forms: as oil film, emul-
sion, particles or whole chunks, or generally
speaking as a multi-phase mixture.
Phase separation before sampling should be
avoided so that the sum of all organic contam-
inants can be detected. Ideally, the sample is
taken from a section of turbulent flow where
good blending predominates. High pressures
and flow rates in the sample line favour the
stability of the mixture.
Under these conditions, the use of a counter-
flow type sampling mechanism, in which the
sample is taken directly from the flowing me-
dium without filtration, is recommended. Fig.: TOC analyser in pressurized cabinet

If the sample is a multi- For this purpose, the TOC-4200 can optionally
phase liquid, but not be equipped with a pressured enclosure (Ex p)
available in turbulence for zone 1 or 2. Versatile communication inter-
and at high pressure, it is faces also allow monitoring and control of the
recommended to create analyser without opening the Ex housing.
an emulsion. The addi-
tion of emulsifiers, or
more specifically surfac-
Fig.: Mechanically tants, is not a suitable
created emulsion method here however.
Surfactants are detected as TOC and lead to ■ Recommended analyser / configuration

false-high readings. TOC-4200
Mechanical emulsification of the sample by Individual sampling system
high-speed stirring is recommended instead. Optional enclosure for zone 1 or 2
By this method oil and fat droplets can be
crushed and dispersed in water. For TOC de-
termination, this only temporarily stable mix-
ture is injected immediately after its creation.
Longer dwell times that could lead to the
breaking of the emulsion, with the effect of
false-low readings, are thus avoided.