Chemical Engineering April 2022 - 54
based on the number and sizes of
flanges in the system, and it appears
that the construction crew
did too good a job of tightening all
the flanges. The absence of inerts
enhanced condensation, causing
the pressure to fall. To keep up the
pressure, the plant cut back the
WTW flowrate, but this raised the
WTW outlet temperature, which
reached the maximum allowable
to prevent WTW boiling. This was
not sufficient to bring up the tower
pressure, and it ended about 6 psi
below design, which limited tower
hydraulic capacity.
To compensate for the lack of air
ingress, a metered nitrogen purge
was added to the tower. This did
not work as intended, and the tower
pressure experienced oscillations
of ±1.5 psi, causing temperature
FIGURE 15. A steam leak caused pressure-control
problems in this vacuum tower system
and level oscillations, probably because
of the delicate balance required
between the control valve
characteristics and the nitrogen
addition rate.
The problem was eventually solved
by changing the pressure control
philosophy, and adding a WTW outlet
temperature control, keeping the
WTW valve as open as possible, and
allowing a smaller margin between
the WTW outlet temperature and its
boiling point. The improved control
arrangement is spelled out in detail
in Ref. 17.
Case 24.5 in Ref. 15 describes an
analogous problem, except that the
condenser used cooling water, with
the condensate temperature 250ºF.
At low rates, the cooling-water valve
closed, inducing high outlet temperatures
accompanied by severe
tube corrosion and even water boiling.
Fouling was not a problem because
the condenser was cleaned
every few weeks. In this case, adding
a nitrogen purge successfully
reduced condenser heat transfer,
which permitted larger opening of
the cooling water valve, preventing
excessive temperatures.
For details visit adlinks.chemengonline.com/82580-21
54
Another example is the vacuum
tower in Figure 15. This tower
had a differential pressure controller
(dPC) manipulating the addition
of non-condensable gas
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Chemical Engineering April 2022
Table of Contents for the Digital Edition of Chemical Engineering April 2022
Chemical Engineering April 2022 - Cover1
Chemical Engineering April 2022 - Cover2
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