Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
2001-09-14
2002-09-10
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S612000
Reexamination Certificate
active
06446465
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a liquefaction process and apparatus.
In the liquefaction of natural gas with a refrigerant it is known to try to match the natural gas cooling curve with the refrigerant warning curve by splitting the refrigerant into two streams which are cooled to different temperatures. This is described, for example, in ore WO-A-9527179.
In our WO-A-9713108 there is disclosed a compact LNG plant for use in the offshore liquefaction of natural gas.
FIG. 1
of the attached drawings illustrates a natural gas liquefaction apparatus of the general type disclosed in WO-A-9713108, although there are differences between FIG.
1
and the disclosure of WO-A-9713108.
In
FIG. 1
pretreated natural gas is fed via a conduit
101
to a heat exchanger
166
at a pressure of about 8.3 MPa. In one example, the natural gas in conduit
101
would have the following composition: 4.2 mol% nitrogen; 85.1 mol% methane; 8.2 mol% ethane; and 2.5 mol% propane. The natural gas in the conduit
101
is cooled to a temperature in the range about 5° C. to 10° C. by heat exchange with chilled water, and is discharged into a conduit
102
.
The natural gas exiting the heat exchanger
166
is fed to the warm end of a CWHE (coil wound heat exchanger)
150
via the conduit
102
. The CWHB
150
comprises a single shell, which houses two separate heat exchanger bundles
151
and
152
. The natural gas is cooled in the CWHE
150
by countercurrent heat exchange with a nitrogen refrigerant. The cooled natural gas leaves the CWHE
150
at a temperature around −90° C., and is fed to a farther heat exchanger
153
via a conduit
104
. the heat exchanger
153
may be an aluminum PFHE (platefin heat exchanger). The natural gas is cooled to a temperature of about −150° C. in the heat exchanger
153
, and exits the cool end of the exchanger
153
into a conduit
106
.
The natural gas in conduit
106
is fed to the warm end of a heat exchanger
154
, in which it is cooled to a temperature of about −160° C., and it exits the cool end of the exchanger
154
into a conduit
107
. The natural gas in conduit
107
is fed to the top of a nitrogen stripper column
157
. The column
157
is needed when the nitrogen content of the feed gas is high and the required composition of the LNG product cannot be achieved using one or two stages of flash separation drums. The stripping process is assisted by using the exchanger
154
to provide reboil heat transferred from the natural gas in conduit
106
. LNG is fed from the column
157
to a conduit
167
, tom where the LNG is fed to the cool end of the exchanger
154
. The exchanger
154
warms the LNG to a temperature of about −160° C.; the LNG exits the warm end of the exchanger
154
into a conduit
168
, through which it is fed back to the column
157
.
LNG is fed from the bottom of the column
157
to a conduit
111
and then to a transfer pump
158
. The pump
158
pumps the LNG into a conduit
112
and on to a LNG storage tank
186
.
The flash gas, which contains methane and a high proportion of nitrogen, exits from the top end of the column
157
to a conduit
109
. The flash gas in conduit
109
, which is at a temperature of about −167° C., is fed to the cool end of a heat exchanger
155
, in which the gas is warmed to a temperature of about −40° C. The warmed gas is fed from the warm end of the exchanger
155
to a conduit
110
, from which it is fed to a multistage fuel gas compressor
180
. The compressor
180
has at least four stages of compression with intercooling between each stage using cooling water. The flash gas is compressed in the compressor
180
from just above atmospheric pressure to a pressure which is typically in the range 2.7 to 5.5 MPa, and is then fed to a turbine
173
of a refrigerant compressor
159
, as described in more detail below. High fuel gas pressures are required when the turbine is an aeroderivative turbine, owing to the high compression ratios used in such turbines. The fuel gas compressor
180
thus has a significant power requirement, owing to the high discharge pressure and high nitroen content of the gas, such that a gas turbine drive is usually used from economic considerations, rather than an electric motor drive. As described below, the flash gas fed through the conduit
110
is used to provide the bulk of the fuel gas requirements of the liquefaction plant.
The nitrogen refrigeration cycle which cools the natural gas to a temperature at which it can liqueur will now be described. Nitrogen refrigerant is discharged from the warm end of the CWHE
150
into a conduit
132
at a temperature of about 5° C. The nitrogen is fed to a multistage compressor unit
159
, which comprises at least two compressor stages
169
and
170
, with at least one intercooler
171
, and an aftercooler
172
. The compressor stages
169
and
170
are driven by a gas turbine
173
. The operation of the compressor unit
159
consumes almost all of the power required by the nitrogen refrigeration cycle. The gas turbine
173
is driven by the fuel gas derived from conduit
110
.
The compressed nitrogen is discharged from the compressor unit
159
into a conduit
133
at a pressure of about 5.1 MPa. The conduit
133
leads to two conduits
134
and
135
between which the nitrogen from the conduit
133
is split according to the power absorbed by the compressor. The nitrogen in the conduit
134
is fed to a compressor
162
in which it is compressed to a pressure of about 8.5 MPa, and is then fed from the compressor
162
to a conduit
136
. The nitrogen in the conduit
135
is fed to a compressor
163
in which it is compressed to a pressure of about 8.5. MPa, and is then fed from the compressor
163
to a conduit
137
. The nitrogen in both the conduits
136
and
137
is fed to a conduit
138
and then to a heat exchanger
164
, where it is cooled to ambient temperatures. The nitrogen is fed from the heat exchanger
164
through a conduit
139
to a heat exchanger
165
in which it is cooled to a temperature of 5° C. to 10° C. by chilled water. The cooled nitrogen is fed from the exchanger
165
to a conduit
140
, which leads to two conduits
120
and
141
. The nitrogen flowing through the conduit
140
is split between the conduits
120
and
141
: about 2% of the nitrogen in conduit
140
flows through the conduit
141
. art The nitrogen flowing through the conduit
141
is fed to the warm end of the heat exchanger
155
, where it is cooled to a temperature of about −123° C. by countercurrent heat exchange with the flash gas from the column
157
. The cooled nitroen is discharged from the cool end of the exchanger
155
to a conduit
142
.
The conduit
120
is connected to the warm end of the CWHE
150
, whereby the nitrogen is fed to the warm end of the heat exchanger bundle
151
. The nitrogen from conduit
120
is pre-cooled to about 13° C. in the heat exchanger bundle
151
. A majority of the nitrogen refrigerant is withdrawn from the CWHE
150
, after passing through the bundle
151
, via a conduit
122
. The remainder of the nitrogen refrigerant passes through the bundle
152
, is cooled to a temperature of about −90° C., and is discharged from the CWHE
150
into a conduit
124
.
The nitrogen in the conduit
122
is fed to a turbo expander
160
, in which it is work expanded to a pressure of about 1.9 MPa and a temperature of about −95° C. The expanded nitrogen is discharged from the expander
160
into a conduit
128
. The nitrogen in the conduit
124
is mixed with the nitrogen in the conduit
142
, and is then fed to a turbo expander
161
in which it is work expanded to a pressure of about 1.9 MPa and a coolest nitrogen temperature of about −151° C. The expanded nitrogen is discharged from the expander
161
into a conduit
126
. The turbo expander
160
is arranged to drive the compressor
162
, and the turbo expander
161
is arranged to drive the compressor
163
. In this way the majority of the work produced by the expanders
160
and
161
c
BHP Petroleum PTY, Ltd.
Doerrler William C.
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