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Hot High Pressure Separator Letdown
Application DiscussionAD125
June 15, 2003
As environmental regulations tighten relating to sulfur, heavy
metals and aromatichydrocarbons, hydroprocessing has become a much
more utilized process in the refining
industry. Hydroprocessing enables the processing of many
different feedstocks to produceproducts that are economically
favorable at a given time while removing entrainedcontaminants.
Hydroprocessing includes processes such as hydrotreating and
hydrocracking. Hydrotreatingprocesses are used to remove
undesirable materials from a feedstock by selective reactionswith
hydrogen in a heated catalyst bed. This removes sulfur, nitrogen
and certain metalcontaminants. This process is often used to remove
catalyst poisons from a feedstock beforedownstream processing. In
this process, olefins and aromatics are converted to
saturatedhydrocarbons.
The hydrocracking process converts (cracks) heavy feedstocks
into lighter components byselective reactions with hydrogen in
multiple heater catalyst beds. This process is mostcommonly used to
create gasoline or diesel product streams.
Most hydroprocessing today utilizes a single stage fixed-bed
catalytic process. The fresh feedis mixed with makeup hydrogen and
recycle gas (rich in hydrogen content) and sent through aheater to
the first reactor. If the feed has not been hydrotreated, there is
a guard reactor beforethe hydrocracking reactor. The catalyst in
the guard reactor converts organic sulfur andnitrogen compounds to
hydrogen sulfide (H2S), ammonia (NH3) and additional hydrocarbons
toprotect the precious metal catalysts in the other reactors. The
hydrocracking reactor is operatedat a sufficiently high temperature
to convert 40 50 percent (volume) of the reactor effluent
tomaterial boiling below 400 degrees. The reactor effluent goes
through heat exchangers to thehot high pressure separator (HHPS)
where the hydrogen-rich gases flashed off overhead. Thehydrogen
rich gases are then sent to the cold high pressure separator (CHPS)
for additionalseparation from which the hydrogen rich gases are
recycled to the first stage of the process formixing with
additional hydrogen and the fresh feed. The liquid effluent from
both the HHPS andCHPS is sent to a fractionalization column where
the butane and lighter gases are taken offoverhead and the light
and heavy naphtha, jet fuel and diesel fuel are removed as liquid
sidestreams. Figure 1 shows the process flow diagram of a
hydrocracking operation.
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Figure 1: Generic Hydrocracking Process Flow Diagram
The separator letdown valves control the liquid level in the
HHPS and CHPS. In someprocesses, these valves may dump the liquid
effluent from the HHPS and CHPS to a low
pressure separator before flowing to the fractionalization
tower. Utilizing a low pressureseparator allows additional removal
of hydrogen and light hydrocarbons.
In order to control the level in the HHPS, two valves (typically
angle style) are normally used tocontrol flow to the
fractionalization column or low pressure separator. With pressures
in theHHPS ranging between 1800 and 3500 psig and pressures in the
fractionalization tower rangingfrom 100 to 300 psig, concerns arise
due to flashing damage and vibration. High temperature(400 850 deg
F) is another issue that must be dealt with.
Not only will the flashing fluid be erosive it can also be
corrosive. This is because the entrainedH2S and NH3 can attack the
trim and body materials. Because there may be some
entrainedcatalyst from the reactor, the valve also must be able to
pass the flowing particulate without
plugging the flow passages and without causing erosion damage.
The most common materialused for the trim components is 316 SST
with an Alloy 6 overlay. Valve bodies can be eitherWCC (heat
treated for NACE) or 316 SST.
For this application, Fisher recommends the use of the Dirty
Service Trim (DST) or the Type461 Sweep-Flo control valve. The 461
Sweep-Flo is a rugged valve designed for high pressuredrops where
erosion, flowing particulate or flashing can cause severe valve
damage. The anglevalve design protects the valve body from erosion
concerns due to flashing and flowingparticulate. DST utilizes a
staged pressure reduction to eliminate the formation of
damagingcavitation and also compensates for volume expansion of
flashing fluids via expanded areastaging. DST is also designed to
pass particulate up to in diameter ensuring the pluggingdue to
catalyst fines will not occur.
