Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HIGH PRESSURE REVAMP OF LOW PRESSURE
DISTILLATE HYDROTREATING PROCESS UNITS
FIELD OF THE INVENTION
[00011 The present invention relates to a cost effective method for
revamping a low pressure distillate hydrotreating process unit to a high
pressure
distillate hydrotreating process unit. A high pressure hot-feed pump is added,
the furnace is retubed for higher pressures, the low pressure reactor is
replaced
with a high pressure reactor, a high pressure let-down valve is added at the
reactor outlet, and the low pressure recycle compressor is replaced with a
high
pressure recycle compressor.
BACKGROUND OF THE INVENTION
[00021 Impurities such as sulfur in diesel fuels require removal, typically by
hydrotreating, in order to comply with product specifications and to ensure
compliance with environmental regulations. For example, beginning with the
2007 model year, pollution from heavy-duty highway vehicles was required to
be reduced by more than 90 percent. Sulfur in diesel fuel was required to be
lowered to enable modern pollution-control technology to be effective on such
heavy-duty highway vehicles as trucks and buses. The United States
Environmental Protection Agency required a 97 percent reduction in the sulfur
content of highway diesel fuel from a level of 500 ppm (low sulfur diesel, or
LSD) to 15 ppm (ultra-low sulfur diesel, or ULSD). These new regulations
required engine manufactures to meet the 2007 emission standards and to have
the flexibility of meeting the new standards through a phase-in approach
between 2007 and 2010. These standards are comparable to those in most
industrialized nations.
[00031 Some of the processes presently in commercial use for producing
diesel fuels will not be capable of sufficiently reducing the sulfur content
to the
new required levels without modifications of some existing hydrotreating
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processes and equipment. Hydrotreating is an established refinery process for
improving the qualities of various petroleum streams from naphtha boiling
range
streams to heavy oil boiling range streams. Hydrotreating is used to remove
contaminants, such as sulfur, nitrogen and metals, as well as to saturate
olefins
and aromatics to produce a relatively clean product stream for downstream
product sales.
[00041 Diesel fuels are typically hydrotreated by passing the feed over a
hydrotreating catalyst at elevated temperatures and pressures in a hydrogen-
containing atmosphere. One suitable family of catalysts that has been widely
used for this service is a combination of a Group VIII metal and a Group VI
metal of the Periodic Table, such as cobalt and molybdenum, on a support such
as alumina. After hydrotreating, the resulting product stream is typically
sent to
separator to separate hydrogen sulfide and light gases from the treated
stream.
The resulting hydrotreated stream can then be sent to a stripper to produce
two
or more desired fractions, such as a diesel fuel fraction and a wild naphtha
fraction.
[00051 A substantial portion of the diesel pool must now have to comprise
ultra-low sulfur diesel. This is putting a great deal of pressure on refiners
to find
ways to meet the growing demand for such ultra low sulfur feedstocks. Low
pressure distillate hydrotreating process unit have been used for many years
for
removing sulfur from distillate feeds. Low pressure distillate hydrotreating
units
were the norm until recently because they were able to meet the sulfur
requirements at the time. As sulfur requirements became more and more
stringent, higher pressure units were needed. In many instances, grass root
high
pressure distillate hydrotreating process units were built and in other
instances
older lower pressure units were totally dismantled and replaced with new
higher
pressure units. Completely replacing a lower pressure hydrotreating process
unit
with a higher pressure unit, or building a grass roots unit, is very
expensive.
Therefore, there is a need in the art for ways to revamp existing lower
pressure
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hydrotreating units to higher pressure hydrotreating units at substantially
less
cost than completely scraping the lower pressure units and replacing it with
grass
roots high pressure units.
SUMMARY OF THE INVENTION
[00061 In accordance with the present invention, there is provided a method
for converting a low pressure distillate hydrotreating process unit to a high
pressure distillate hydrotreating process unit, which low pressure process
unit
comprises:
i) a pump for introducing a distillate feedstream to the
hydrotreating process unit;
ii) a heat exchanger comprised of a first passageway contiguous to
but not in fluid communication with a second passageway,
wherein said first passageway is in fluid communication with
said pump;
iii) a furnace containing tubes having a first end and a second end
and designed for pressures up to about 500 psig (about 3.4
MPag) and through which distillate feedstream can flow, which
tubes have an effective surface area to heat the feedstream to a
predetermined reaction temperature and wherein the first end of
said tubes is in fluid communication with said first passageway
of said heat exchanger and the second end of said tubes is in
fluid communication with the inlet of reactor of c) below;
iv) a reactor designed for operating pressures not exceeding about
500 psig (about 3.4 MPag) and which reactor has an inlet in
fluid communication with the second end of said tubes of said
furnace and an outlet for removing product, which outlet is in
fluid with said second passageway of said heat exchanger;
v) a separator vessel having an inlet in fluid communication with
said second passageway of said heat exchanger, said separator
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having a first outlet for removing vapor phase components and a
second outlet for removing a liquid phase product stream;
vi) a stripper in fluid communication with said second outlet of said
separator vessel; and
vii) a compressor having an inlet and an outlet and wherein said inlet
is in fluid communication with the first outlet of said separator
vessel and wherein said outlet of said compressor is in fluid
communication with the first end of said furnace tubes, which
compressor is capable of an outlet pressure of up to about 500
psig (about 3.4 MPag);
which method comprising:
a) installing a high pressure pump between said heat exchanger and
said furnace, which pump is capable of pumping a liquid feed to
a pressure up to about 1,500 psig (about 10.3 MPag);
b) replacing the furnace tubes with tubes that can withstand
pressures up to about 1,500 psig (about 10.3 MPag);
c) replacing said reactor with a reactor designed for pressures up to
about 1,500 psig (about 10.3 MPag);
d) installing a high pressure letdown valve at the outlet of the
reactor capable of reducing the pressure of a feedstream from a
pressure of about 1,500 psig (about 10.3 MPag) to a pressure
less than about 500 psig (about 3.4 Wag); and
e) replacing the recycle compressor with a high pressure
compressor capable of compressing a vapor stream to a pressure
up to about 1,500 psig (10.3 MPag).
BRIEF DESCRIPTION OF THE FIGURE
[00071 The Figure hereof is a schematic representation of a preferred
conventional low pressure distillate hydrotreating process unit that has been
revamped to a high pressure unit. The components shown in dashed lines are the
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components that have been replaced or added to covert the unit to a high
pressure unit. Other variants of this flow schematic are also within the scope
of
this invention, for example ones that would show an additional heat exchanger,
a
make-up hydrogen compressor, a high pressure compressor in series with a low
pressure compressor, or an additional separator or a fractionators with or
without
a reboiler.
DETAILED DESCRIPTION OF THE INVENTION
[00081 The present invention provides a method for revamping, as opposed
to completely replacing, a low pressure distillate hydrotreating process unit
to
run at higher pressures suitable for meeting ultra-low sulfur specifications.
[00091 Conventional low pressure distillate hydrotreaters are designed to
operate at pressures in the range from about 150 psig (about 1.0 MPag) to
about
500 psig (about 3.4 MPag), preferably from about 350 psig (about 2.4 MPag) to
about 500 psig (about 3.4 MPag), more preferably from about 350 psig (about
2.4 MPag) to about 450 psig (about 3.1 MPag). While such hydrotreaters have
met with commercial success before ultra-low sulfur requirements, they are
unable to meet the new stringent low sulfur levels. High pressure distillate
hydrotreaters that have operating pressures in excess of about 600 psig (about
4.1 MPag), preferably from about 600 psig (about 4.1 MPag) to about 1,500 psig
(about 10.3 MPag), more preferably from about 600 psig (about 4.1 MPag) to
about 1,200 psig (about 8.3 MPag), and most preferably from about 600 psig
(about 4.1 MPag) to 1,000 psig (about 6.9 MPag) are better able to meet the
stringent sulfur requirements.
[00101 Distillate boiling range streams, particularly diesel fuels require
additional deeper desulfurization in order to meet the stricter governmental
regulations with respect to ultra low sulfur levels. The diesel boiling range
feedstreams are generally described as high boiling hydrocarbon streams of
petroleum origin. Such feedstreams will typically have a boiling point from
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about 350 F to about 750 F (about 175 C to about 400 C), preferably about
400 F to about 700 F (about 205 C to about 370 C). Non-limiting examples of
such streams include gas oils; catalytic cracking cycle oils, including light
cat
cycle oil (LCCO) and heavy cat cycle oil (HCCO); clarified slurry oil (CSO);
as
well as other thermally and catalytically cracked products, such as coker
light
gas oil, are potential sources of feeds for distillate hydrotreating. If used,
it is
preferred that cycle oils make up a minor component of the feed. Cycle oils
from catalytic and thermal cracking processes typically have a boiling range
of
about 400 F to 750 F (about 205 C to 400 C), although light cycle oils may
have
a lower end point, e.g. 600 F or 650 F (about 315 C or 345 C). Because of the
high content of aromatics found in such cycle oils, as well as undesirable
amounts of nitrogen and sulfur, they require more severe process conditions.
Lighter feeds may also be used, e.g. those in the boiling range of about 250 F
to
about 400 F (about 120 C to about 205 C). The use of lighter feeds will result
in
the production of higher value, lighter distillate products, such as kerosene.
[00111 Distillate boiling range feedstreams that can be used in the practice
of the present invention can contain a substantial amount of nitrogen, e.g.
from
about 10 wppm to about 1000 wppm nitrogen in the form of organic nitrogen
compounds. The feedstreams can also contain a significant sulfur content,
ranging from about 0.1 wt% to 3 wt%, and higher.
[00121 The main components of a low pressure conventional distillate
hydrotreating process unit are shown in the Figure hereof. These main
components are: feed pump P, heat exchanger HE, furnace F, reactor R,
separator S, stripper STR, recycle compressor C and optionally an acid gas
scrubber AGS. In accordance with the present invention, a conventional low
pressure distillate hydrotreating process unit is revamped to a high pressure
unit
by: a) installing a high pressure pump HPP between heat exchanger HE and
furnace F, which pump is capable of pumping a liquid feed to a pressure up to
about 1,500 psig (about 10.3 MPag); b) replacing the furnace tubes with tubes
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that can withstand pressures up to about 1,500 psig (about 10.3 MPag); c)
replacing said reactor with a reactor designed for pressures up to about 1,500
psig (about 10.3 MPag); d) installing a high pressure letdown valve LDV at the
outlet of the reactor, which valve is capable of reducing the pressure of the
treated feedstream from a pressure of about 1,500 psig (about 10.3 MPag) to
less
than about 500 psig (about 3.4 MPag); and e) replacing the recycle compressor
with a high pressure compressor C, or adding a second higher pressure
compressor in series with the lower pressure compressor, so that the vapor
stream can be compressed to a pressure up to about 1,500 psig (about 10.3
MPag). These revamping modifications to an existing low pressure distillate
hydrotreating unit are shown by dashed lines in the Figure hereof.
[00131 During service, a distillate feed is introduced into the system via
line
and feed pump P where it is passed through heat exchanger HE that can be
any suitable heat exchanger for this purpose. The heat exchanger will
preferably
be a "shell and tube" type of heat exchanger that is well known in the art.
Shell
and tube heat exchangers are typically comprised of a series of tubes
positioned
within a shell. A set of these tubes contains the fluid that must be either
heated
or cooled, in this case the distillate feedstream that will be preheated. The
second
fluid, the hot product stream from reactor R is introduced in the shell and
passes
over the tubes and transfers heat to preheat the feedstream. A set of tubes is
called the tube bundle and can be made up of several types of tubes: plain,
longitudinally finned, etc. For purposes of this disclosure, the "passageway"
can
be used to describe both the tube bundle or interior of the shell for a shell
and
tube type of heat exchanger as well as for the other types of heat exchangers.
The preheated feedstream is passed via line 12 to furnace F where it flows
though furnace tubes FT of sufficient surface area to provide the desired
heating
of the feedstream before it is passed to reactor R via line 14. One element of
the
revamp of the present invention is to include a high pressure pump HPP
between heat exchanger HE and furnace F. This high pressure pump is able to
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withstand pressures up to 1,500 psig (about 10.3 MPag), preferably up to about
1,200 psig (about 8.3 MPag). Furnaces for heating feedstreams to a desired
reaction temperature range are well known in the art and any suitable furnace
can be used as long as it can heat the distillate feedstream to temperatures
of the
operating conditions of the reactor, which will typically be from about 260 C
to
about 425 C, preferably from about 300 C to about 400 C, more preferably from
about 345 C to about 385 C. Since the furnace tubes of low pressure
hydrotreating process units are typically designed for pressure of no more
than
about 500 psig (about 3.4 MPag), the furnace tubes will be replaced with
furnace
tubes able to withstand the high revamp pressures as previously mentioned.
[00141 The heated feed will be conducted from furnace F to reactor R,
which for purposes of this invention will be replaced with a reactor that is
capable of operating at pressures up to about 1,500 psig (about 10.3 MPag),
preferably up to about 1,200 psig (about 8.3 MPag). Reactors used for
distillate
hydrotreating typically contain one or more fixed beds of catalysts CB.
Suitable
hydrotreating catalysts for use in the present invention are any conventional
hydrodesulfurization catalyst and includes those that are comprised of at
least
one Group VIII metal, preferably Fe, Co or Ni, more preferably Co and/or Ni,
and most preferably Co; and at least one Group VI metal, preferably Mo or W,
more preferably Mo, on a relatively high surface area support material,
preferably alumina. Other suitable hydrotreating catalyst supports include
zeolites, amorphous silica-alumina, and titania-alumina. Noble metal catalysts
can also be employed, preferably when the noble metal is selected from Pd and
Pt. It is within the scope of the present invention that more than one type of
hydrodesulfurization catalyst be used in the same reaction vessel. The Group
VIII metal is typically present in an amount ranging from about 2 to 20 wt%,
preferably from about 4 to 12 wt%. The Group VI metal will typically be
present
in an amount ranging from about 5 to 50 wt%, preferably from about 10 to 40
wt%, and more preferably from about 20 to 30 wt%. All metals weight percents
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are on support. By "on support" we mean that the percents are based on the
weight of the support. For example, if the support were to weigh 100 grams,
then 20 wt% Group VIII metal would mean that 20 grams of Group VIII metal
was on the support.
[00151 Returning now to the Figure, hot reaction products from reactor R
are partially cooled by flowing via line 16 through high pressure let-down
valve
LDV wherein the pressure of the product stream is let-down to the pressure of
conventional low pressure distillate hydrotreater pressures of about 500 psig
(about 3.4 MPag) or less, preferably from about 150 psig (about 1.0 MPag) to
about 450 psig (about 3.1 MPag). Conventional low pressure hydrotreating
process units typically do not need pressure let-down valves, thus as part of
the
revamp of the present invention a suitable pressure let-down valve is
installed.
High pressure let-down valves are well known in the art and no additional
description is needed for purposes of this disclosure. The product stream, now
at
the lower pressure is conducted through heat exchanger HE where it passes
through second passageway to preheat the feedstream passing through the first
passageway of heat exchanger HE. The product stream is then sent to separator
S via line 18 where a light vapor fraction comprised primarily of unused
hydrogen, hydrogen sulfide and other gases are removed overhead via line 20
and a substantially sulfur-free distillate product stream is recovered via
line 22.
The substantially sulfur-free distillate product stream can be sent to
stripper STR
where a stripping gas, preferably steam, is used to strip the product stream
into
predetermined boiling point cuts, preferably a vapor cut, a wild naphtha cut
and
a distillate product cut. The vapor cut will be comprised of gases that were
carried over from the separator as dissolved gases and include gaseous
components such as H2S and light ends. It is within the scope of this
invention
that a fractionator (not shown) be used to separate the various desired
product
fractions with or without a reboiler.
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[00161 The light vapor fraction exits separator S via line 20 and can be
passed to acid gas scrubber AGS which, although optional is preferred, to
remove acid gases, primarily H2S. Any suitable acid gas treating technology
can
be used in the practice of the present invention. Also, any suitable scrubbing
agent, preferably a basic solution can be used in the acid gas scrubbing zone
AGS that will adsorb the desired level of acid gases (H2S) from the vapor
stream. One suitable acid gas scrubbing technology is the use of an amine
scrubber. Non-limiting examples of such basic solutions are amines, preferably
diethanol amine, mono-ethanol amine, and the like. More preferred is diethanol
amine. Another preferred acid gas scrubbing technology is the so-called
"RectisolTM Wash" which uses an organic solvent, typically methanol, at
subzero
temperatures. The scrubbed stream can also be passed through one or more
guard beds (not shown) to remove any trace amounts of catalyst poisoning
impurities such as sulfur, halides etc. Amine scrubbing is preferred and a
lean
amine stream is introduced into acid gas scrubber AGS via line 24 and a rich
amine stream is removed from the scrubber via line 26. The rich amine stream
will contain absorbed sour gases which can be sent to a hydrogen recovery unit
(not shown). After purging a portion to maintain hydrogen purity, a hydrogen-
rich gas is passed through high pressure compressor C via line 28 along with
make-up hydrogen via line 30 to bring the stream up to the designed pressure
of
the hydrotreating process unit. The compressed stream is then sent to furnace
F
via line 32.
