Biofilm in heat exchanger

Johan Guldbj?rnsen johanag at online.no
Thu Apr 29 06:54:37 EST 2004


Hello, I am a student working in a project at Telemark University
College where we wish to find the reason for increased pressure drop
in plate heat exchengers installed in a biolog cleaning facility at
Norske Skog Union Norway (pulp and paper).

Over time the pressure drop is increased, and after a week or so it
has to be cleaned when the pressure drop is too high. It seems like
it`s a form of biofilm that is growing inside the heat exchanger. Some
og us believes it is severe biofouling that is the case. The amount of
slug removed can be up to 7 wheelbarrows...

The water entering the facility (12000 m3/day) contains about 500 mg/l
fine elements and some fibers. Most of the fibers are removed in the
pre sedimentation pool. Before the water enters the heat exchanger,
approx 0,2 g/l Ca(OH)2 is added and  approx 200L 75% H3PO4 i added.

In the heat exchangers the process water is cooled from 40-48 C to
30-34 C, the cooling water has a temperature approx 4C in the winter
increasing to approx 20 C in the summer.

The process water velocity through the heat exchanger is approx 0,2
m/s, which is very low.

Particulate fouling doesn`t seem to be case because of the increased
shear forces when the fouling builds up, how can it stay attached?

Biofouling seems to explain everything. I have taken som samples of
the sludge and viewed it in the microspoy, and there are a lot of
bacterias, even ciliates and rotatories (not sure this is the correct
english term). Some of the sludge has has a pulpy consistence and some
is like rubber. I have pictures of this is anyone is interested.
Futhermore the process water contains all the nutrients needed for the
biofilm; starch, ligning, etc from the pulp/paper facility, even Ca2+
and orto phosphates from H3PO4. The temperatures are also perfect for
biofilm growth.

Based on this information have you any arguments for or against that
severe bio fouling is the case?

When it comes to dealing with the problem:
Increase the velocity through the heat exchanger
Move the dosing location of Ca(OH)2 and H3PO4 to after the heat
exchangers
Biocides: Possible if the right amount is used, otherwise the culture
in the biologic reacotrs (HCR) can be destroyed.

Are there any other solutions?

Best regards
Johan Guldbj=F8rnsen
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