U.S. patent application number 12/595644 was filed with the patent office on 2010-05-27 for method for cooling a cryogenic exchange line.
This patent application is currently assigned to L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des Procedes Georges Claude. Invention is credited to Philippe Court, Antoine Hernandez, Christian Monereau.
Application Number | 20100126215 12/595644 |
Document ID | / |
Family ID | 38830440 |
Filed Date | 2010-05-27 |
United States Patent
Application |
20100126215 |
Kind Code |
A1 |
Court; Philippe ; et
al. |
May 27, 2010 |
Method For Cooling A Cryogenic Exchange Line
Abstract
The invention relates to a method for the cryogenic separation,
the cooling or the liquefaction of a fluid using an exchange line,
that comprises extracting from said exchange line at least one dual
phase fluid (11), separating said dual phase fluid into at least
one vapour fraction (4) and one liquid fraction (5) in a phase
separator (40), expanding at least one portion of the liquid
fraction (5) using a first expansion means (60, 90), reinjecting,
reheating and at least partially vaporising said expanded liquid
fraction in the exchange line, the first expansion means being a
valve, wherein during the cooling of said exchange line, at least a
fraction of the fluid extracted from the exchange line (2, 4 or 5)
and/or from the phase separator (40) is expanded in second
expansion means (61, 71, 81, 91) parallel to the first expansion
means (6), while during a normal operation, the second expansion
means is essentially closed.
Inventors: |
Court; Philippe; (Paris,
FR) ; Hernandez; Antoine; (Le Plessis-Trevise,
FR) ; Monereau; Christian; (Paris, FR) |
Correspondence
Address: |
AIR LIQUIDE;Intellectual Property
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Assignee: |
L'Air Liquide Societe Anonyme Pour
L'Etude Et L'Exploitation Des Procedes Georges Claude
Paris
FR
|
Family ID: |
38830440 |
Appl. No.: |
12/595644 |
Filed: |
April 2, 2008 |
PCT Filed: |
April 2, 2008 |
PCT NO: |
PCT/FR08/50575 |
371 Date: |
January 15, 2010 |
Current U.S.
Class: |
62/617 ;
62/619 |
Current CPC
Class: |
F25J 2270/04 20130101;
F25J 1/0055 20130101; F25J 3/062 20130101; F25J 2270/904 20130101;
F25J 1/0022 20130101; F25J 2210/42 20130101; F25J 2245/02 20130101;
F25J 2240/40 20130101; F25J 1/0247 20130101; F25J 2270/18 20130101;
F25J 3/064 20130101; F25J 3/0655 20130101; F25J 3/0295 20130101;
F25J 3/0635 20130101; F25J 2280/10 20130101; F25J 3/0695
20130101 |
Class at
Publication: |
62/617 ;
62/619 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2007 |
FR |
0754462 |
Claims
1-9. (canceled)
10: A process for the cryogenic separation, refrigeration or
liquefaction of a fluid by means of a heat exchange line
comprising: extracting at least one two-phase fluid from said heat
exchange line; separating said two-phase fluid into at least a
vapor fraction and a liquid fraction in a phase separator;
expanding at least one portion of said liquid fraction by means of
a first expansion means; and re-injecting, warming and at least
partially vaporizing said expanded liquid fraction in the heat
exchange line, wherein: said first expansion means is a valve; as
said heat exchange line is being cooled down, at least one fraction
of the fluid extracted from the heat exchange line and/or from the
phase separator is expanded in a second expansion means in parallel
with the first expansion means; and in normal operation, said
second expansion means is essentially closed.
11: The process of claim 10, wherein said second expansion means is
a valve.
12: The process as of claim 10, in which said first and said second
expansion means are installed in parallel.
13: The process of claim 10, in which vapor is sent from the phase
separator into a third expansion means and, as said heat exchange
line is being cooled down, at least one fraction of the vapor is
expanded in a second expansion means in parallel with the third
expansion means.
14: The process of claim 13, wherein vapor coming from the third
expansion means is sent into the liquid coming from the phase
separator.
15: The process of claim 10, wherein the HP of the second expansion
means is equal to three times the HP of the first expansion
means.
16: The process of claim 10, wherein the HP of the second expansion
means is equal to five times the HP of the first expansion
means.
17: The process of claim 14, in which the HP of the second
expansion means is equal to three times the HP of the third
expansion means.
18: The process of claim 14, in which the HP of the second
expansion means is equal to 5 times the HP of the third expansion
means.
19: The process of claim 10, in which, during the cooling-down, the
second expansion means is controlled manually or the pressure of
the feed gas is regulated.
20: The process of claim 10, in which the cryogenic separation is a
process selected from the group consisting of the separation of
hydrocarbons, the production of hydrogen, the production of
CO.sub.2, a process for eliminating nitrogen from a heavier
fraction, a process for eliminating argon from a heavier fraction,
and the liquefaction is a liquefaction of natural gas.
21: The process of claim 20, wherein said hydrogen has a purity of
90 to 98%.
22: The process of claim 20, wherein said CO.sub.2 has a purity of
greater than 95%.
23: The process of claim 22, wherein said CO.sub.2 has a purity of
greater than 98%.
Description
[0001] The present invention relates to a heat exchange line and to
a method of cooling down such a heat exchange line.
[0002] It is known to use cryogenics to fractionate a gas stream
into at least two fluids of different composition, generally into
what is called a light fluid, i.e. one essentially composed of the
more volatile constituents, and what is called a heavy fluid
essentially consisting of the more easily condensable constituents.
To accomplish this, the mixture to be fractionated is cooled in a
heat exchanger or in a number of heat exchangers, called a heat
exchange line, until a liquid/vapor two-phase mixture extracted
from said heat exchange line and separated in a liquid/vapor
separator is obtained. The vapor may be further cooled until a new
two-phase state is obtained and fractionated a second time.
[0003] To cite an example, a stream of hydrocarbons (C.sub.1,
C.sub.2, . . . , C.sub.i, C.sub.i+1, . . . C.sub.n) is fractionated
into a fluid essentially consisting of the lighter hydrocarbons
(methane C.sub.1, ethane C.sub.2, . . . , C.sub.i) and a second
fluid essentially consisting of the heavier hydrocarbons
(C.sub.i+1, . . . C.sub.n). The term "essentially" is used to
indicate that a small fraction of the lighter compounds will in
general be found in the heavy fraction, and conversely a small
portion of the heavier compounds will in general be found in the
vapor fraction.
[0004] This separation may be improved by inserting trays into the
two-phase separation system and by adding a reboiling section
and/or a stripping section in order to remove the light components
from the liquid fraction and/or a condenser and/or by increasing
the reflux in order to remove the heavy components from the vapor
fraction. These processes are known to those skilled in the art and
are not discussed in the present invention. Consequently, the
expression "liquid/vapor separator" will be used to cover all
equipment generating at least one liquid output and at least one
gaseous output from at least one two-phase feed. Such equipment may
be of the horizontal or vertical gravity separator type, whether or
not equipped with a devesiculator, of the cyclone or distillation
column type, etc.
[0005] The liquid output may contain a small amount of bubbles
entrained by the speed of the liquid, likewise the vapor output may
contain liquid aerosols or droplets, without in any way departing
from the scope of the invention.
[0006] Other applications consist in recovering a methane-rich
fluid and a methane-depleted fluid from a source rich in various
hydrocarbons. Thus, several fluids may also obtained, such as a
methane-rich fraction, an ethane- or ethylene-rich fraction and a
C.sub.3+ fraction. This type of process makes it possible in
particular to recover hydrogen with a purity of about 95% from a
mixture of hydrogen and hydrocarbons and to remove a portion of the
nitrogen contained in gases rich in hydrocarbons. It also makes it
possible to recover a fraction very rich in CO.sub.2 and a waste
gas containing lighter constituents, such an N.sub.2, argon,
O.sub.2, etc.
[0007] This fractionation may not be an objective per se, but only
a means of delivering the refrigerating power intended for
liquefying another fluid, such a natural gas. In this case, the
various separated fluids are recombined after being warmed,
recompressed and reinjected into the heat exchange line. This is
then referred to as a refrigeration cycle.
[0008] These applications have given rise to many developments both
in processes and in technology. In particular, the heat exchangers
may be of the coiled type, such as a tube/shell heat exchanger, or
preferably of the plate heat exchanger type. In the latter case,
many improvements have been named regarding heat exchange
corrugations and regarding the injection of the fluids, in
particular two-phase fluids, into these heat exchangers so as to
optimize the heat transfer.
[0009] In what follows, all the percentages mentioned are molar
percentages.
[0010] One example of these units will now be described in relation
to FIG. 1. This example relates to the production of pressurized
hydrogen with a purity of 95% from a pressurized gas mixture
containing about 70% hydrogen, 18% methane and 12% heavier
hydrocarbons.
[0011] The mixture 1 to be separated is injected at ambient
temperature and under a pressure of 40 bar absolute into the plate
heat exchanger 10 to be cooled therein via the heat exchange
passages 11. At a first temperature level dependent on the
composition of the heaviest hydrocarbons and on the pressure,
generally between -40 and -120.degree. C., the fluid 1, then a
two-phase fluid, is extracted from the heat exchanger and separated
into its vapor fraction 2 and its liquid fraction 3 in the
liquid/gas separator 30. The liquid fraction 3 is expanded via the
expansion valve 50 down to a low pressure and revaporized in the
heat exchange line via the heat exchange passages 13.
[0012] The vapor phase 2 enriched in hydrogen and in methane is
again cooled in the heat exchanger 20 via the passages 22,
partially condensed and extracted at around -160.degree. C. The
vapor fraction 4 coming from the separator 40 constitutes the
production of hydrogen with a 95 mol % content. It is then warmed
in the passages 24 then 14 of the heat exchangers 20 and 10.
[0013] The liquid fraction 5 mainly consisting of methane is
expanded down to a low pressure in the valve 60, revaporized in the
heat exchanger 20 (passages 24) and warmed in the heat exchanger 10
(passages 14).
[0014] The fluids 6 and 7 associated with heat exchangers 20 and 10
respectively may optionally be used as refrigeration top-up. They
may be external fluids, such as for example liquid nitrogen coming
from a storage tank or from a neighboring air separation unit, or a
fluid internal to the process, such as for example a fraction of
hydrogen produced, which is partially warmed, then expanded in an
expansion turbine and reinjected into the cold end of the heat
exchanger 20.
[0015] It is also possible to promote vaporization of the methane 5
by injecting a small fraction of the hydrogen production. This is
shown by the optional circuit that includes the expansion valve
70.
[0016] It should be noted that the expansion valves 50 and 60 are
used to expand liquids from a high pressure, here 40 bar abs, down
to a low pressure. They are therefore small valves.
[0017] It is common practice to use the notion of HP (horsepower)
when speaking of the size of the valves.
[0018] Many works or documents give, on the one hand, methods of
calculation and, on the other hand, the HP of commercially
available valves. In the latter case, it is conventional to
indicate the HP in the fully opened position, which makes it
possible to determine the maximum flow rate of a fluid that can
pass through the valve under given operating conditions. As an
illustration, but without wishing to go into the calculations, for
a gas feed flow rate of the order of 10 000 Nm.sup.3/h, the HP of
these valves will be less than 1.
[0019] The same applies to the optional valve 70 that serves to
expand a very small fraction (a few percent at most) of the
hydrogen produced.
[0020] Conventionally, such a separation unit is cooled down either
by free expansion of the gas to be treated or, more generally,
using an external refrigeration top-up.
[0021] What is called here the cooling-down of the heat exchange
line is the procedure for obtaining the normal operating
conditions, here a first cut-off temperature between the heat
exchangers 10 and 20, for example -80.degree. C., and a temperature
at the cold end of -160.degree. C. in order to obtain the required
purity from equipment operating at ambient of sub-ambient
temperature if the heat exchange line has not had the time to reach
the ambient temperature.
[0022] The cooling-down problem using just the free expansion of
the gas to be treated in the expansion valves 50, 60 and optionally
70 is that the total expanded flow is very small and therefore the
refrigeration power obtained is itself very low. However, this
refrigeration power is intended to cool the heat exchange line and
the ancillary equipment, such as the separators, and to compensate
for the thermal losses, such as the heat exchange with the external
medium. Such a cooling-down procedure may take several tens of
hours and may even possibly not reach the desired operating
point.
[0023] This occurs in particular if the thermal losses become, at a
certain temperature level reached at the cold end, equal to the
refrigeration power produced by free expansion. At this point, the
cooling-down stops and it is not possible to cool down further.
[0024] For this reason, it is common practice to use for example
the refrigeration top-up circuit 6 and 7 in order to hasten the
cooling-down. The passages 26 and 17 may be used permanently or
only temporarily during the cooling-down phases. As indicated
above, it is conventional to use low-pressure or preferably
medium-pressure liquid nitrogen to speed up the process for
obtaining the intended temperature levels.
[0025] However, it is apparent that no more than the simple free
expansion in the process expansion valves (here 50, 60 and possibly
70), the use of external refrigeration top-up is not a satisfactory
solution.
[0026] This is because refrigeration top-up into a heat exchange
line which is still warm, and in which in particular little fluid
flows in the normally heavily used passages, i.e. the liquid
revaporization passages (here passages 13 and 25 in particular),
causes thermal shocks and high stresses between heat exchange
passages and in the inlet/outlet boxes. These shocks and stresses
are liable to rapidly cause mechanical problems at the brazed or
welded joints between constituent components of the heat
exchanger.
[0027] This is particularly the case for brazed aluminum plate heat
exchangers--the technology used at the present time for most heat
exchange lines for cryogenic separation or gas liquefaction
units.
[0028] According to another aspect of the invention, what is
provided is a process for the cryogenic separation, refrigeration
or liquefaction of a fluid by means of a heat exchange line
comprising: [0029] the extraction of at least one two-phase fluid
from said heat exchange line; [0030] the separation of said
two-phase fluid into at least a vapor fraction and a liquid
fraction in a phase separator; [0031] the expansion of at least one
portion of said liquid fraction by means of a first expansion
means; and [0032] the reinjection, warming and at least partial
vaporization of said expanded liquid fraction in the heat exchange
line,
[0033] characterized in that: [0034] the first expansion means is a
valve; [0035] as said heat exchange line is being cooled down, at
least one fraction of the fluid extracted from the heat exchange
line and/or from the phase separator is expanded in a second
expansion means in parallel with the first expansion means; and
[0036] in normal operation, the second expansion means is
essentially closed.
[0037] Optionally: [0038] at least one of the first and second
expansion means is a valve; [0039] the first and second expansion
means are installed in parallel; [0040] vapor is sent from the
phase separator into a third expansion means and, as said heat
exchange line is being cooled down, at least one fraction of the
vapor is expanded in a second expansion means in parallel with the
third expansion means; [0041] vapor coming from the third expansion
means is sent into the liquid coming from the phase separator;
[0042] the HP of the second expansion means is equal to three times
the HP, preferably five times the HP, of the first expansion means;
[0043] the HP of the second expansion means is equal to three times
the HP, preferably five times the HP, of the third expansion means;
[0044] during the cooling-down, the second expansion means is
controlled manually or the pressure of the feed gas is regulated;
[0045] the cryogenic separation is a process for the separation of
hydrocarbons or for the production of hydrogen, preferably with a
purity of 90 to 98%, or for the production of CO.sub.2, preferably
with a purity of greater than 95% and even more preferably greater
than 98%, or a process for eliminating nitrogen or argon from a
heavier fraction, or the liquefaction is a liquefaction of natural
gas.
[0046] The solution recommended in the present invention will now
be explained with the help of FIG. 2.
[0047] This figure shows the modifications made to the cold end of
the heat exchange line described above. These modifications may
also be made at the first separator pot 30 and more generally at
each point of expansion of a liquid fraction.
[0048] The invention consists in adding, to the scheme
corresponding to the normal steady-state operation, an expansion
valve, called here a cool-down expansion valve, which is used only
(or mainly) when starting up the unit.
[0049] The purpose of this valve is twofold. Firstly, it allows a
large flow of gas to be expanded, thus considerably increasing the
refrigeration power produced by the unit itself, that is to say it
enables the cool-down time to be reduced and normally makes it
possible by itself to reach the required temperature levels.
[0050] Secondly, in the case of very rapid start-up with an
external refrigeration power top-up, such as the use of liquid
nitrogen, this valve firstly allows the equipment to be partially
cooled and correspondingly to limit the thermal shocks, but in
particular to rebalance the heat exchange line by making large
volumes flow through the revaporization passages 25 and 13.
[0051] This new valve must therefore allow a large fraction of the
high-pressure gas, here the fluid 2, to be expanded and enable this
expanded fluid to be introduced into the passages 25 normally
reserved for the liquid fraction 5.
[0052] This valve will preferably be installed as a by-pass for the
expansion valve 60 and will therefore be about 10 times
larger--this the valve 61 shown in FIG. 2.
[0053] It is also possible to add instead a valve between the fluid
2, i.e. between the outlet of the heat exchanger and the separator
pot 40, and the inlet of the passages 25--this is then the valve
81.
[0054] A fraction 4 of the stream may also be expanded via a valve
71.
[0055] In all cases, the additional expansion valves 61, 71, or 81
may pass a flow of an order of magnitude at least 10 times higher
than that which can be expanded in the valve 60 or 70.
[0056] This additional valve will be gradually closed as the
cooling-down progresses, in particular as liquid appears at the
exchanger outlet.
[0057] It will a priori be completely closed under normal
operation.
[0058] It will generally be manually controlled (HIC), but may also
be controlled by maintaining the high pressure (PIC).
[0059] In all these cases, the additional valve (61, 71 or 81) may
thus be in parallel with the expansion valve 60.
[0060] It should be noted in this regard that it is not possible
with most commercially available valves to have both a valve for
passing a large flow of gas, i.e. one having an HP when fully
opened of 10 or more, and then to regulate with an opening
corresponding to an HP of about 0.3. It is conventional to use a
valve in an opening range with a factor of 5, preferably 3, i.e.
for example with an HP of 0.1 to 0.5 or from 0.1 to 0.3, but not
beyond this. A factor of 5 (or 3) usually makes it possible to
carry out nominal operation or reduced operation (with a reduced
flow rate) without any particular regulation problem. In the case
of the example shown in FIG. 1 or FIG. 2 in normal operation, the
expansion valve 60 makes it possible to maintain the liquid level
in the separator pot 40. It therefore controls the flow of liquid
that has been expanded and revaporized in the heat exchange line.
Since this flow is the main refrigeration feed for the heat
exchanger 20, it will be understood that its regulation is
critical. It would be completely impossible with an oversized
valve, a fortiori with a valve 10 times larger than necessary.
[0061] As explained above, the additional expansion of a large flow
of gas to be treated must therefore take place via a complementary
means, which will no longer be used in normal operation or which
will be at least partially closed so as to allow the unit to
operate satisfactorily.
[0062] Finally, it should be noted that it is possible, from the
moment that an appreciable portion of the feed gas is expanded via
an additional valve in the circuit 25, to also inject into this
circuit a refrigeration top-up flow, such as a flow of liquid
nitrogen, without creating excessively large stresses. Depending on
the circumstances, this top-up flow may be eliminated or
maintained, at least partially, during normal operation whereas the
additional expansion valve will be closed or essentially
closed.
* * * * *