achievable when you have a unit operating below 800 lb.
Once you get above an operating pressure of 800 lb, the depressurization rate is increased above 100 psi/min to fall
within the recommended practice of reducing pressure to
50% in 15 min.
For a hydrocracker, we specify two emergency depressurization systems: a low-rate and a high-rate system. These
systems are designed to depressure at rates of 100 psi/min
and 300 psi/min, respectively.
The low-rate depressurization system opens on a loss of
recycle gas. Since the recycle gas flow provides the means for
temperature control in the reactors, Haldor Topsoe’s philosophy requires that depressurization be initiated at a low rate
upon stoppage of the recycle gas compressor. This moves
the reactor in a safe direction, but you can still stop the depressurization if the compressor is restarted and the catalyst
temperatures are normal.
The high-rate depressurization system opens up at the onset of a temperature runaway. A temperature runaway is classified as any reactor skin temperature exceeding the reactor
design temperature. If the temperature rise is 55° F. above the
normal operating temperature level, then the bed is severely
upset, and the unit should be fully depressured. A rate of 300
psi/min will usually exceed the 50% reduction requirement.
Part of the question was about endpoint or cold-flow
improvement. Refiners are often considering adding these
catalysts into their hydrotreaters. The operating procedures
must be revised to reflect the differences in operation, and
the operators should be trained for this new operation. At a
minimum, Haldor Topsoe recommends that the unit meets
API RP 521 of reducing the pressure 50% in 15 min. This is
lower than what we would specify for a grassroots design.
Haldor Topsoe believes, however, that this is a reasonable
compromise for a revamp if:
1. A careful analysis is made of the new operation.
2. The operating procedures are reviewed and properly
modified.
3. The operators are properly trained for the new operation.
4. Confirmation is made that the unit instrumentation is
adequate to identify the scenarios requiring depressurization.
Again, when you are using catalysts which contain zeolite, a review of the unit’s control and shutdown systems
is imperative. When considering any of these catalysts for
use in your hydrotreater, consult your experienced licensor
or catalyst vendor, both of whom should be able to provide
guidelines.
Kathy, can you define what you meant by decaying
average temperature?
Basically, it just means looking at individual TIs and
comparing their rates of change per minute. The damped
decaying average value for a set of radial thermocouples is
are clear, the operator can continue or discontinue depres-
surization at his or her option. It is also possible to discon-
tinue depressurization with the system pressure above 50%
of design as long as all other permissives are clear.
Derek had an extensive answer on this question, so I
will just add a few points regarding what we do at Shell. All
of Shell’s hydrotreating units are equipped with both low-rate and high-rate depressuring systems, and both systems
are manually activated. The low-rate depressuring systems
are sized so they depressure from a normal operating pressure to around 100 lb over 1 hr. The low-rate depression
valve closes when the high-rate depression valve opens.
The high-rate depression valve is normally used only
when there is a major unit upset or an emergency situation,
such as a big fire or a large leak. The valve is normally sized
to depressure from a normal operating pressure to 100 lb
in 15 min. When the high-rate depression valve is opened,
it normally trips the feed pumps, charge heaters, makeup
gas, recycled gas compressors, and wash oil or washwater
pumps. The unit is essentially shut down.
For the hydrocracking unit, the depression system is designed slightly differently compared with the hydrotreating
unit. For the hydrocracking unit, the low-rate depression
valve can be either manually or automatically activated, and
it can be tripped losing the recycle gas compressor. Also for
the cracking bed, if there is a 50° F. above-grade change in
the per-minute average temperature for the two TIs in the
bottom bed, the low-rate depression valve will be automatically opened. It can also be tripped on the top bed TIs if
the temperature is anywhere from 15-25° F. above the rate
of temperature change per minute. The low-rate depression
valve can also trip any single TI exceeding the designed temperature.
The high-rate depression system in the hydrocracker is
designed very similarly to the hydrotreaters. It is manually activated when there is both a major emergency on the
unit and when temperature discretion occurs, which is often
in the hydrocracker and not the hydrotreaters. At Shell, if
the zeolite catalyst is utilized in the hydrotreating unit, we
would require the hydrotreater to follow the same depressuring system as the hydrocrackers.
Derek and Kathy covered a lot, so I will now give
Haldor Topsoe’s view on this scenario. Just recognize that
any zeolite-containing material is going to possess an inherent ability to crack hydrocarbons; therefore, you will have
the potential for a temperature excursion to take place, especially in the loss of recycle gas flow.
When Haldor Topsoe designs a hydrotreater, we specify
a fast-depressurization system based on the American Petroleum Institute (API) Recommended Practice (RP) 521.
We usually set that at 100 psi/min. The goal is to reduce
the pressure to 50% in a matter of 15 min. That is usually
