U.S. patent application number 14/397890 was filed with the patent office on 2015-05-07 for process for reliquefying a methane-rich fraction.
This patent application is currently assigned to LINDE AKTIENGESELLSCHAFT. The applicant listed for this patent is LINDE AKTIENGESELLSCHAFT. Invention is credited to Heinz Bauer, Andreas Bub, Hubert Franke.
Application Number | 20150121953 14/397890 |
Document ID | / |
Family ID | 48227143 |
Filed Date | 2015-05-07 |
United States Patent
Application |
20150121953 |
Kind Code |
A1 |
Bauer; Heinz ; et
al. |
May 7, 2015 |
PROCESS FOR RELIQUEFYING A METHANE-RICH FRACTION
Abstract
A process for reliquefying a methane-rich fraction, in
particular boil-off gas, is described. In this process, a) a
methane-rich fraction is compressed to a pressure at least 20%
above the critical pressure of the fraction to be compressed, b)
liquefied and supercooled, c) depressurized to a pressure in the
range from 5 to 20 bar, d) separated into a gaseous nitrogen-rich
fraction and a liquid nitrogen-depleted fraction, e) the
nitrogen-depleted fraction is depressurized to a pressure in the
range from 1.1 to 2.0 bar, f) the gaseous fraction obtained,
without being warmed and compressed, is mixed into the methane-rich
fraction, and g) the liquid product fraction obtained in the
depressurization of the low-nitrogen fraction has a nitrogen
content of .ltoreq.1.5 mol %.
Inventors: |
Bauer; Heinz; (Ebenhausen,
DE) ; Franke; Hubert; (Pullach, DE) ; Bub;
Andreas; (Munchen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LINDE AKTIENGESELLSCHAFT |
MUNCHEN |
|
DE |
|
|
Assignee: |
LINDE AKTIENGESELLSCHAFT
MUNCHEN
DE
|
Family ID: |
48227143 |
Appl. No.: |
14/397890 |
Filed: |
April 18, 2013 |
PCT Filed: |
April 18, 2013 |
PCT NO: |
PCT/EP2013/001157 |
371 Date: |
October 30, 2014 |
Current U.S.
Class: |
62/619 |
Current CPC
Class: |
F25J 1/0052 20130101;
F25J 2200/02 20130101; F25J 2270/66 20130101; C10L 2290/46
20130101; F25J 3/064 20130101; F25J 2210/90 20130101; F25J 2220/62
20130101; F25J 1/0219 20130101; C10L 2290/06 20130101; F25J 1/0295
20130101; F25J 2270/12 20130101; C10L 3/105 20130101; F25J 3/0635
20130101; F25J 2215/04 20130101; F25J 3/0615 20130101; F25J 1/003
20130101; F25J 3/066 20130101; F25J 3/0233 20130101; F25J 2230/30
20130101; C10L 2290/48 20130101; F25J 3/061 20130101; F25J 1/0254
20130101; F25J 1/004 20130101; F25J 3/0209 20130101; F25J 1/0291
20130101; F25J 3/0257 20130101; F25J 2230/32 20130101; F25J 2210/06
20130101; F25J 1/0025 20130101; F25J 2200/40 20130101 |
Class at
Publication: |
62/619 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 3/06 20060101 F25J003/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 3, 2012 |
DE |
102012008961.9 |
Claims
1. Process for reliquefying a methane-rich fraction, in particular
boil-off gas, wherein a) the methane-rich fraction (1) is
compressed (C1) to a pressure which is at least 20% above the
critical pressure of the fraction to be compressed, b) liquefied
and supercooled (E2), c) depressurized (V1) to a pressure in the
range from 5 to 20 bar and d) separated into a gaseous
nitrogen-rich fraction (4) and a liquid nitrogen-depleted fraction
(7) and e) the nitrogen-depleted fraction (7) is depressurized (V2)
to a pressure in the range from 1.1 to 2.0 bar, f) where the
gaseous fraction (8) obtained is, without being warmed and
compressed, mixed into the methane-rich fraction (1) and g) the
liquid product fraction (9) obtained in the depressurization of the
low-nitrogen fraction has a nitrogen content of .ltoreq.1.5 mol
%.
2. Process according to claim 1, where the liquefaction and
supercooling (E2) of the methane-rich fraction (1) are carried out
against at least one refrigerant circuit and/or at least one
refrigerant mixture circuit and this/these has/have at least one
circuit compressor (C3), characterized in that the pressure to
which the methane-rich fraction (1) is compressed (C1), the
pressure to which the liquefied and supercooled methane-rich
fraction (3) is depressurized (V1) and the temperature to which the
methane-rich fraction (2) is cooled are selected or varied in such
a way that the drive power of the compressor (C1) used for
compressing the methane-rich fraction (1) and the drive power of
the circuit compressor(s) (C3) are shifted relative to one another
without the total power changing by more than .+-.5% or the drive
power of the compressor (C1) used for compressing the methane-rich
fraction (1) and the drive power of the circuit compressor(s) (C3)
are shifted relative to one another in such a way that a division
of the total power in the range from 30/70 to 70/30 is
achieved.
3. Process according to claim 1, characterized in that the
methane-rich fraction (1) is compressed (C1) to a pressure which is
at least 30% above the critical pressure of the fraction to be
compressed.
4. Process according to claim 1, characterized in that the
liquefied and supercooled methane-rich fraction (3) is
depressurized (V1) to a pressure in the range from 7 to 15 bar.
5. Process according to claim 1, characterized in that the
nitrogen-depleted fraction (6) is depressurized (V2) to a pressure
in the range from 1.2 to 1.8 bar.
Description
[0001] The invention relates to a process for reliquefying a
methane-rich fraction, in particular boil-off gas.
[0002] In the following, the term "boil-off gas" refers both to
boil-off gases and also to gas mixtures which have a similar
composition; displacement gases which arise, for example, in the
loading of LNG into transport tanks on ships or goods vehicles may
be mentioned merely by way of example.
[0003] In the liquefaction of methane-rich gases or boil-off gases,
appropriate measures for discharging a nitrogen-rich fraction are
required above a certain nitrogen content in order to limit the
nitrogen content of the liquefied natural gas (LNG) to usually 1
mol %.
[0004] U.S. Pat. No. 5,036,671 discloses a method of discharging a
nitrogen-rich fraction, in which gas streams which have a
significantly increased content of nitrogen compared to the crude
gas are taken off at the cold end of the liquefaction process via
one or more separators. These gas streams are generally compressed,
optionally partly recirculated to the crude gas and usually used as
fuel gas. In the liquefaction process described in U.S. Pat. No.
5,036,671, the boil-off gas flowing from the LNG tank located
downstream of the liquefaction process is warmed and compressed at
approximately ambient temperature.
[0005] Since the operating pressure in such LNG tanks is normally
only slightly, typically 50 mbar, above ambient pressure, there is
a high probability of generating subatmospheric pressure in the
compressor in the warm-intake compression of the boil-off gas. This
can lead to entry of air and thus oxygen and thus represent a
safety risk.
[0006] It is an object of the present invention to propose a
process of the type in question for reliquefying a methane-rich
fraction, which avoids the abovementioned disadvantages.
[0007] To achieve this object, a process of the type in question
for reliquefying a methane-rich fraction, in which [0008] a) the
methane-rich fraction is compressed to a pressure which is at least
20% above the critical pressure of the fraction to be compressed,
[0009] b) liquefied and supercooled, [0010] c) depressurized to a
pressure in the range from 5 to 20 bar and [0011] d) separated into
a gaseous nitrogen-rich fraction and a liquid nitrogen-depleted
fraction and [0012] e) the nitrogen-depleted fraction is
depressurized to a pressure in the range from 1.1 to 2.0 bar,
[0013] f) where the gaseous fraction obtained is, without being
warmed and compressed, mixed into the methane-rich fraction and
[0014] g) the liquid product fraction obtained in the
depressurization of the low-nitrogen fraction has a nitrogen
content of .ltoreq.1.5 mol %, is proposed.
[0015] If the liquefaction and supercooling of the methane-rich
fraction are carried out against at least one refrigerant circuit
and/or at least one refrigerant mixture circuit and this/these
has/have at least one circuit compressor the pressure to which the
methane-rich fraction is compressed, the pressure to which the
liquefied and supercooled methane-rich fraction is depressurized
and the temperature to which the methane-rich fraction is cooled
are selected or varied according to the invention in such a way
that [0016] the drive power of the compressor used for compressing
the methane-rich fraction and the drive power of the circuit
compressor(s) are shifted relative to one another without the total
power changing by more than .+-.5% or [0017] the drive power of the
compressor used for compressing the methane-rich fraction and the
drive power of the circuit compressor(s) are shifted relative to
one another in such a way that a division of the total power in the
range from 30/70 to 70/30 is achieved.
[0018] Further advantageous embodiments of the process of the
invention for reliquefying a methane-rich fraction, which are
subject matter of the dependent claims, are characterized in that
[0019] the methane-rich fraction is compressed to a pressure which
is at least 30% above the critical pressure of the fraction to be
compressed, [0020] the liquefied and supercooled methane-rich
fraction is depressurized to a pressure in the range from 7 to 15
bar and/or [0021] the nitrogen-depleted fraction is depressurized
to a pressure in the range from 1.2 to 1.8 bar.
[0022] The process of the invention for reliquefying a methane-rich
faction and also further advantageous embodiments of this process
will be illustrated below with the aid of the example shown in FIG.
1.
[0023] The methane-rich fraction 1 to be reliquefied is compressed
in the single-stage or multistage compressor unit C1 to a pressure
which is at least 20%, preferably at least 30%, above the critical
pressure of the methane-rich fraction 1 to be reliquefied. In this
way, two-phase streams of the methane-rich fraction 1 to be
reliquefied are avoided in the heat exchanger(s) of the subsequent
liquefaction stage.
[0024] According to the invention, the methane-rich fraction 1 to
be reliquefied is not warmed before being compressed in C1. Owing
to the compression in C1, the methane-rich fraction to be
reliquefied is heated to a temperature above that of the
surroundings, and it is therefore cooled to approximately ambient
temperature against cooling water or air in the heat exchanger
E1.
[0025] In the heat exchanger E2, the compressed methane-rich
fraction 2 is cooled to a temperature in the range from -100 to
-140.degree. C., preferably from -110 to -130.degree. C., and
thereby liquefied and supercooled.
[0026] The cooling of the compressed methane-rich fraction can in
principle be carried out against any refrigerant circuit or
refrigerant mixture circuit or combinations of these. The
refrigerant mixture circuit shown in FIG. 1 is merely one of the
many possible variants. The heat exchanger E2 shown in FIG. 1 can
in reality be formed by a plurality of separate heat exchangers
and/or heat exchanger sections. It is preferably configured as
helically coiled heat exchanger having two bundles or as soldered
plate exchanger.
[0027] After liquefaction and supercooling, the methane-rich
fraction 3 taken off from the heat exchanger E2 is depressurized in
the valve V1 to a pressure in the range from 5 to 20 bar,
preferably from 7 to 15 bar. The gaseous, nitrogen-rich fraction 4
obtained here is taken off at the top of the separator D1 located
downstream of the valve V1, warmed in the heat exchanger E2 against
the methane-rich fraction 2 to be cooled, with this warming being
optional. The warmed nitrogen-rich fraction 5 is, if desired,
subsequently compressed in one or more stages C2 and passed via
line 6 to further use, for example as fuel. This nitrogen-rich gas
5 preferably has a pressure in the range from 5 to 20 bar, in
particular from 7 to 15 bar. It is thus, for example, directly
suitable for firing steam-generating boilers. When used as fuel gas
in gas turbines, the outlay for compression is significantly
reduced compared to the prior art in which the initial pressure is
a lower tank pressure.
[0028] The liquid nitrogen-depleted fraction 7 obtained in the
separator D1 after depressurization is depressurized in the valve
V2 to a pressure in the range from 1.1 to 2.0 bar, preferably from
1.2 to 1.8 bar. The gaseous fraction obtained in this
depressurization is taken off via line 8 from the top of the
separator D2 and, without warming, mixed into the methane-rich
fraction 1 to be compressed. The liquid fraction obtained at the
bottom of the separator D2 represents the liquefied natural gas
product (LNG); this has a nitrogen content of .ltoreq.1.5 mol
%.
[0029] Owing to the cold intake of the fractions or gas mixtures 1
and 8 to be compressed in the compressor stage C1, the safety risk
mentioned at the outset which exists in the case of warm-intake
compression of boil-off gases can be effectively prevented.
Undesirable and dangerous entry of air and thus oxygen into the
compressor C1 is thus ruled out.
[0030] Owing to the recirculation of the gaseous fraction 8
obtained after the second decompression V2 to the methane-rich
fraction 1 to be compressed, the amount of LNG product can be
increased, which is advantageous in terms of costs, and the total
energy consumption can be reduced.
[0031] A process alternative which is not shown in FIG. 1 is to
replace the separator D1 by a stripper. In this, the methane-rich
fraction 3 which has been depressurized in the valve V1 is stripped
of nitrogen from below by a substream of the methane-rich fraction
2 to be cooled over suitable internals such as packing and/or
trays. As the required stripping gas, a substream of the
methane-rich fraction 2 to be cooled is drawn in either between the
heat exchangers E1 and E2 or, in an embodiment as helically coiled
heat exchanger having two bundles, between the bundles.
[0032] As mentioned above, cooling and liquefaction of the
methane-rich fraction 2 is effected in the heat exchanger E2
against a refrigerant mixture circuit shown merely by way of
example. The refrigerant mixture is, after warming and vaporization
in the heat exchanger E2 against the methane-rich fraction 2 to be
cooled, conveyed via line 10 into a separator D3 located upstream
of a two-stage compressor unit C3. This is in the interests of the
safety of the compressor unit C3, since liquid particles entrained
in the refrigerant mixture precipitate therein.
[0033] The refrigerant mixture to be compressed is conveyed from
the top of the separator D3 via line 11 to the compressor unit C3
and compressed in the first stage thereof to an intermediate
pressure. After cooling in the intermediate cooler E3, the
refrigerant mixture which has been compressed to the intermediate
pressure is fed via line 12 to a second separator D4. The
relatively low-boiling refrigerant mixture fraction taken off from
the top of the latter is fed via line 13 to the second compressor
stage of the compressor unit C3 and compressed in this to the
desired final pressure. This refrigerant mixture fraction is
subsequently cooled in the after-cooler E4 and fed via line 15 to a
third separator D5.
[0034] The liquid fraction obtained in this separator D5 is
recirculated via line 16 and valve V3 to a point upstream of the
second separator D4. The relatively low-boiling refrigerant mixture
fraction taken off from the top of the third separator D5 via line
17 is, after mixing with the liquid relatively high-boiling
refrigerant mixture fraction 14 taken off from the bottom of the
second separator D4, conveyed via line 18 through the heat
exchanger E2. In order to be able to "bridge" the pressure
differences in the lines 14 and 17, a pump P is provided in the
line 14.
[0035] The refrigerant mixture 18 which has been cooled, liquefied
and supercooled against itself in the heat exchanger E2 is, after
having been taken off from the heat exchanger E2, depressurized in
the valve V4 so as to generate cold and subsequently conveyed via
line 19 through the heat exchanger E2 again in countercurrent to
the methane-rich fraction 2 to be liquefied.
[0036] In the process of the invention for reliquefying a
methane-rich fraction, the powers of the feed gas compressor C1 and
of the refrigeration circuit compressor C3 can be shifted relative
to one another by suitable choice of the pressures downstream of
the compressor unit C1 and the valve V1 and of the temperature of
the cooled methane-rich fraction 3 before depressurization in the
valve V1, without the total power being appreciably, meaning an
increase or decrease of .+-.5%, changed.
[0037] It is possible, advantageously, to adapt the required powers
of the drives A and B of the compressors/compressor units C1 and C3
to such an extent that drives (gas turbines, steam turbines and/or
electric motors) having the same power can be used. This
simplification is of great economic advantage. Such a
redistribution of the drive powers of the feed gas compressor C1
and the refrigeration circuit compressor C3 is neither known from
the prior art nor rendered obvious thereby.
[0038] The amount of gas taken off at the top of the separator D1
can be kept constant by varying the pressure in the separator D1.
This results in a variable amount of gaseous fraction 8 which is
recirculated from the separator D2 to the suction side of the feed
gas compressor C1.
[0039] As mentioned, a preferred redistribution between the
compressor/compressor units C1 and C3 leads to equal drive powers.
Instead of this 50/50 solution, any other distribution in the range
from 30/70 to 70/30 can be achieved. The solution preferred in each
case depends, for example, on the power steps of customary drives
(gas turbines).
* * * * *