Piping exists to send a feed to the FCC fractionator either “hot direct” from an
upstream process or to route it through a cooler and tank, creating a “cold feed” stream.
The feed is used to remove heat from a pumparound circuit on the fractionator, and
there has to be an adequate temperature difference between the pumparound stream
and the feed stream to remove the pumparound heat. In order to ensure that the
temperature difference was large enough to meet the pumparound duty, the feed was
routinely routed through the cooler and the tank, and supplied as cold feed (see
Figure 8.1).
As part of an energy efficiency study, several projects were scoped to recover the
heat from the feed and minimize the heat rejection in the cooler. One option was
selected and progressed to detailed engineering. At this point, the existing pumparound-
to-feed heat exchanger came under scrutiny. Design specifications were
reviewed and compared with current operating data, and it became apparent that
the required pumparound duty could be achieved with the existing heat exchanger
even if the feed was supplied “hot direct”—that is, without passing through the cooler
and tank. Plant trials confirmed this conclusion. As a result, the operating procedures
were changed, and “hot direct” routing is now considered the normal operating procedure (Figure 8.2). This change increases the temperature of the feed as it enters
the fractionator. This in turn reduces the reboiler duty, saving about $600,000/year,
with no investment required by the facility.
This example highlights the fact that operating practices tend to become ingrained
over time. Operators may religiously follow procedures that were developed when the
plant first started up, even though the requirements of the plant change over time due to
new feedstocks, different product slates, changes in throughput, or a host of other factors.
Another way that operating practices can become skewed is through responses to specific
problems. For example, a valve position may be changed during an upset or unusual
weather condition, and it may be left in the new position long after the abnormal situation
has passed. The modified valve position becomes the new normal, and it can remain
unchanged for years, until someone questions it. Further examples of this kind are
discussed in Chapter 25. When we become aware that operating conditions are suboptimal, our natural
response is to adjust the process (e.g., change the valve position in the example above) in
order to optimize the operation. While this may be a good first step, it is only a short-term
fix. The next operating shift will very likely reverse the change and restore the status quo.
Moreover, the optimum valve position today may not be optimum tomorrow, when plant
throughput, feed purity, product slate, and ambient conditions may all be different from their current values. Thus, we need more than just a one-time change to ensure that we get
the most from our existing facilities.
The following are a few additional steps:
i. Modify operating procedure documentation
ii. Carry out additional operator training
iii. Add control valves
iv. Implement real-time optimization systems
v. Implement performance monitoring systems and key performance indicators
(KPIs)
Items (iv) and (v) on this list are specific technologies that are discussed in more
detail in Chapters 19 and 27, respectively.