Debottlenecking
Crude Units
The first unit for which we will discuss
different debottlenecking options is the first unit in the
refinery- the crude unit. Depending upon the unit
configuration, operating conditions, and crude slate, the
crude unit can have a variety of bottlenecks. A typical
bottleneck is crude charge pressure drop through the preheat
train. Sometimes, booster pumps are required to overcome the
preheat train pressure drop. Before additional pumps are
installed, other options should be considered. Oftentimes,
pressure drop can be alleviated inexpensively by depassing the
crude side exchanger bundles. Depending on the exchanger
design, this may require relatively minor exchanger
modifications, but with large payouts. Depassing a 4 tube pass
exchanger to 2 passes reduces the pressure drop by
approximately a factor of 8. Of course, suitable exchanger
velocities must be maintained to prevent excessive exchanger
fouling.
Another common limit is the exchanger
train design pressure. Bigger impellers cannot be installed in
the charge pumps to increase the charge rate because the
increased pressure will exceed the exchanger design pressure.
Downstream booster pumps can alleviate this problem. An even
better solution is to move or install a flash drum upstream in
the exchanger train. Some exchanger trains do not have a flash
drum, or it is located at the end of the train. Moving or
adding a flash drum earlier in the exchanger train has several
advantages. First, vapor generated does not have to flow
through the rest of the exchanger train reducing the
significant two phase pressure drop. Second, moving the flash
drum from the end of the exchanger train to the beginning
allows the exchanger train to run at the lowest possible
pressure above the crude tower pressure. The lower pressure
minimizes exchanger design pressure limitations. Sometimes a
high pressure is maintained on the flash drum at the end of
the exchanger train to minimize flashing, but this is not
required. Third, a flash drum reduces the vapor traffic load
on the crude tower. For even more debottlenecking, a preflash
tower can be installed.
Debottlenecking Saturates Gas Plants
Typically part of the crude unit or just
downstream is the sats gas plant. As the crude unit is
expanded, the sats gas plant capacity may need to be
increased. Depending on the unit configuration and
bottlenecks, there are many potential bottlenecks and
expansion options. One typical limit is flooding in the bottom
of the debutanizer. Packing is an option, but may not
sufficiently debottleneck the tower since high pressure
applications may not show significant capacity increases with
packing.
One option that may not be obvious, but
can have significant benefits is additional feed preheat.
Additional feed preheat reduces the fractionating load on the
bottom section of the tower. Also, feed preheat debottlenecks
the reboiler. Increasing the feed preheat can increase the
tower capacity by over 25% in some cases. This significant
increase is much cheaper than building a new tower.
Another Saturates Gas Plant example is in
propane production. Propane typically has a vapor pressure
specification which limits the allowable ethane content.
Plants often run their saturated and unsaturated deethanizers
conservatively so that little or no ethane remains in the
propane. However, to maximize profitability, the amount of
ethane in the propane should be maximized since this allows
ethane to be sold at liquid fuel propane price rather than
ethane gas price. The effect can be even more dramatic since
it unloads the deethanizers. The reduced load upon the
deethanizers may allow higher charge rates. At the very least,
it reduces energy consumption while maximizing higher valued
liquid product sales.
Debottlenecking
FCCU's and Alky's
FCCU’s are another unit which are
typically debottlenecked significantly beyond their original
design capacities. Advances in riser technology have allowed
higher reactor charge rates and conversion. The main
fractionator or the other fractionating towers can often
become limiting as reactor capacity is increased.
Debottlenecking options in the gas plant have kept
pace.
Packing is a typical solution to
increasing the fractionating capacity. Packing is a proven
application in FCCU main fractionator and other towers. Even
if only the pumparound zones are packed, the main fractionator
capacity typically can be increased by approximately 15%.
Also, the pumparound loadings can be modified to significantly
increase column capacity. For example, increasing the slurry
pumparound duty can greatly unload the rest of the main
fractionator. As with all debottlenecking revamps, the
ramifications of the changed unit operation upon the equipment
and the product specifications need to be rigorously
evaluated.
Most plants with a FCCU have a downstream
Alkylation unit. Often the reaction section is designed
conservatively, and the charge rate can be increased with
little impact upon the alkylate octane. Sometimes the
fractionating towers become the limit. Sometimes in tall
towers with narrow boiling range feedstocks such as the Alky
deisobutanizer or debutanizer, the optimum feed location has a
greater effect than normal. For example, optimizing the
deisobutanizer feed location can improve tower capacity by up
to 10%. Also, high capacity trays have increased Alky
deisobutanizer capacity by over 15%.
Debottlenecking
Catalytic Reformers
Catalytic reformers are another unit which
can be debottlenecked beyond their design capacity.
Improvements in catalyst activity and dense loading technology
have allowed the reactors to handle higher charge rates. The
downstream fractionation equipment’s capacity must be
increased to keep pace. One such tower is the reformate
stabilizer. Improvements in fractionation capacity can be made
by installing packing as discussed for FCCU Main
fractionators. Fractionation capacity gains can also be
achieved by installing high capacity trays. Fractionation
capacity increase may then cause the reboiler or overhead
condenser to be the limiting item. If so, replacing the
exchanger bundle with an extended surface bundle can lead to
attractive payouts. In one such example, the steam reboiler
was flux limited. Replacing the reboiler bundle with an
extended surface bundle increased the reboiler area. Since the
shell side had the limiting heat transfer coefficient, the
results were dramatic. The tower capacity increased by over
25% for just the cost of a $15,000 exchanger
bundle.
Extended surface tubes can increase many
other services such as overhead condensers. Extended surface
tubes are especially applicable to services in which the shell
side has the limiting heat transfer coefficient since only the
shell side area is increased. Remember to account for the
smaller tube ID’s and hence higher pressure drops.
Debottlenecking
Hydrotreaters
Hydrotreaters also can be debottlecked.
Higher capacity catalyst, activated support balls, dense
loading of the reactor can all increase the reactor capacity.
Once a unit is built, it often is most profitable to maximize
the throughput up to the charge pump hydraulic limits.
Oftentimes, the charge pump spare can be turned on to increase
the oil charge flow. Larger control valves may reduce the
charge circuit pressure drop.
There is usually an incentive to run
hydrotreaters at the highest possible pressure up to the
equipment design pressures. There must be an acceptable safety
margin between the operating pressure and the relief valve set
pressure to handle the minor unit pressure swings so that the
relief valves do not lift. Furthermore, if conventional relief
valves are installed, the operating pressure must be
maintained below approximately 90% of the relief valve set
pressure so that the relief valves do not leak. Pilot operated
relief valves allow operation up to 95% of the relief valve
set pressure.
Debottlenecking
Low Pressure Fractionating Towers
Ancillary units also have room for
debottlenecking. For this example, we will use an aromatics
recovery unit although the discussion is applicable to any low
pressure tower. The extraction section can often be pushed
beyond the normal design rates by running at a lower solvent
to feed ratio. The downstream towers may then become limiting.
In any low pressure tower, the raising the tower pressure may
significantly unload the trays and overhead condenser. The
reboiler load will increase, but may not become a limit,
especially if it is a steam reboiler. Raising the tower
pressure can increase the tower capacity in the range of
5-10%.
Debottlenecking
Hydraulic Limits
One example of hydraulic debottlenecking
an aromatics extraction unit was particularly dramatic. A
market supply disruption raised the price of benzene
dramatically. All efforts were made to increase the unit
charge rate and maximize benzene production. However, the
benzene rundown line became the unit-limiting item. Operations
then decided to turn on the spare pump. This increased the
capacity by approximately 10%. There was room for more,
however. Operations typically ran down a single rundown line
into a day tank to minimize the amount of offspec produced by
an upset. Another set of day tanks had a separate day tank. So
operations began using the second rundown line in parallel.
This greatly debottlenecked the system. At the higher flow
rates, the control valve pressure drop became limiting so the
trim in the control valve was replaced. The combination of all
of these items increased the maximum benzene production rate
by over 35%. And the only money spent was for the new control
valve trim. The payout for replacing the control valve trim
would be measured in minutes !