An example of the value of relatively
minor debottlenecking was studied for another LSR Stabilizer.
This Stabilizer was
in the same service as the one discussed previously. Although
the refinery configuration was slightly different, the same
products and desired separations applied. The Stabilizer under
discussion was undersized for the service as the upstream
equipment had been debottlenecked allowing higher charge
rates. The undersized stabilizer led to high C4-
concentrations in the bottoms and high C5+ losses
in the overhead.
Case I shows that typical operation
allowing almost 14 LV% of C5+ material in the
overhead product, the lowest attainable C4-
concentration in the bottoms was 3.9 LV%. This high
C4- content resulted in high RVP’s of 12 and
greater. The high C4- concentration in the
Stabilized LSR was causing high RVP’s and significant RVP
blending problems. A study was initiated to reduce the RVP
with only minor equipment modifications desired.
Since the bottom portion of the tower was
limiting, the study concentrated on increasing the feed
preheat temperature to the tower. The tower had a feed/bottoms
exchanger, but the feed still entered the column significantly
subcooled. The subcooling of the feed contributed to the
bottom tray flooding limit. The subcooled feed had to be
heated up to its bubble point temperature by using the heat
supplied by the reboiler. Using the reboiler heat increased
the traffic in the loaded, bottom section of the tower.
Alternatively, additional feed preheat could be added
externally to the tower which would not increase the tower
loadings. The higher preheat temperature could be achieved
easily with another small feed/bottom exchanger.
Cases II and
III illustrate the significant
debottlenecking effect of additional feed preheat. Raising the
feed preheat to 200° F cut the amount of butanes in the
bottoms by more than half. The RVP dropped accordingly from
12.1 in Case I to 11.1 in Case II. The graph of bottoms RVP vs.
feed preheat temperature illustrates how much higher feed
preheat temperatures reduce the bottoms' RVP. While the column
separation was still not good even at the higher preheat
levels, the bottom RVP reduction was significant.
Additional feed preheat is not a panacea
for all column debottlenecking. Preheat usually increases
capacity in columns only with subcooled feeds that are
reboiler limited or limited by tray flooding below the feed.
Additional feed preheat also reduces the energy efficiency of
the tower. For a given amount of feed and reboiler duty, a
better column separation is achieved if more duty is applied
to the reboiler. However, if additional preheat is available
through feed/bottoms exchange, then the energy debit may
become moot if the bottom heat was to be rejected against a
utility. As with any debottlenecking effort, all of the
ancillary equipment must be checked. Additional feed preheat
increases the loads upon all of the equipment above the feed
tray. Finally, there may be other debottlenecking options such
as high capacity trays or packing which may be more suitable
depending on the individual circumstance.
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