Chemistry Reference
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conveyed in a pipeline. There must have been a compelling economic incentive for this
change. The picture is complex, but it includes maintenance, energy, spillage andmanpower.
Another PI-based gain that has generally not seen action is the end to the practice of
'piped-in spare pumps'. This seems trivial, but there are considerable sums of money
attached to it. Fifty years ago it was taught in chemical engineering that pumps should
always be doubled as they were unreliable and broke down. The pump norm from API says
that pumps shall be designed and built for 25 000 hours' maintenance-free operation. It
would seem that there is a case for reconsidering this practice. Colleagues in operations that
the authors have spoken to seem to confirm this notion.
In the oil industry, the phase separator that processes the stream from the well into gas,
oil and a water phase does not look the same today as it did 30-40 years ago. The
development has been evolutionary, and the term 'PI' has not been used, although it might
be applied if we were to look at the change over a long period of time.
Another example of PI is in the fertilizer industry. Here melted fertilizer is sprayed into
the top of a tall tower, where it falls as droplets countercurrently with rising air, which cools
and solidifies the drops. This process is referred to as 'prilling'. Such a tower can be more
than 100 m tall. An alternative is granulation, in which the melt is sprayed on to a rotating,
cooled surface. This involves a dramatically smaller process unit. Settling and centrifuga-
tion are both practised. Obviously the economics do not dictate that the PI solution is
superior, but the two options co-exist.
13.3 Process Case Study
It is useful to discuss the concept of PI and its implications in the context of a specific
process. The absorption-desorption process for the capture of CO
2
from a gas stream has
been chosen for a case study, specifically its application to flue gas treatment. This process
is reasonably simple to grasp and an overview is easy to obtain such that the key points are
not lost in a multistep process train description (Figure 13.2).
The flue gas is transferred to the capture plant through a long gas channel with a large
cross-sectional area. The gas enters the process at a temperature higher than that desired
for CO
2
capture. It is first cooled in a direct-contact cooler by a circulating water flow,
which also washes out some impurities. A fan is needed to overcome the process pressure
drop. The gas is then contacted with aqueous monoethanolamine (MEA) in the absorber,
where 85-90% of the CO
2
is typically removed. The gas desorbs a little MEA from the
absorbent, which is essentially removed in a closed-loop water wash in a separate section at
the top the absorber. The CO
2
-rich absorbent is then pumped through a heat exchanger,
where heat is recovered from the regenerated absorbent before it enters the desorption
column. CO
2
is desorbed in the desorption column, aided by water vapour generated in the
reboiler. The water vapour has two functions: one is to provide desorption heat along the
column and the other is to lower the partial pressure of CO
2
to aid the desorption. Water
vapour is recovered from the overhead stream, rendering moist CO
2
. The recovered water
is pumped back to the desorber. The CO
2
lean absorbent leaving the reboiler is pumped
back to the absorber via the heat recovery exchanger and a cooler.
The CO
2
extracted from the flue gas is compressed and dried for transport and storage
(a compressor train is shown in Figure 13.3). A drying plant is typically introduced at a
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