Reliquefaction of boil-off from liquefied natural gas

The present invention relates to an improved process for the reliquefaction of boil-off gas containing up to 10% nitrogen resulting from the evaporation of liquefied natural gas (LNG) contained in a storage vessel. In the process, a closed-loop nitrogen refrigeration cycle is utilized wherein the nitrogen is isenthalpically expanded under conditions for generating a liquid and vapor with the liquid being pressurized by pumping and warmed against an initially cooled boil-off stream. The boil-off LNG stream is initially cooled by indirect heat exchange with an isentropically expanded refrigerant stream.

TECHNICAL FIELD 
The present invention relates to a process for recovering liquefied natural 
gas (LNG) boil-off from a storage vessel. 
BACKGROUND OF THE INVENTION 
In ocean tankers carrying cargoes of liquid natural gas (LNG), as well as 
land based storage tanks, a portion of the liquid, normally amounting to 
approximately 0.1 to 0.25% per day in the case of LNG, is lost through 
evaporation as a result of heat leak through the insulation surrounding 
the LNG storage receptacle. Moreover, heat leakage into LNG storage 
containers on both land and sea causes some of the liquid phase to 
vaporize thereby increasing the container pressure. 
Shipboard LNG storage tank boil-off has typically been used as an auxiliary 
fuel source to power the ship's boilers and generators. However, recent 
LNG tanker designs have incorporated the use of diesel engines rather than 
steam driven engines thereby eliminating the need for supplemental energy 
supplied by LNG boil-off. 
Recently enacted legislation prohibiting tanker disposal of 
hydrocarbon-containing streams by venting or flaring within the vicinity 
of metropolitan areas coupled with an increased desire to conserve energy 
costs have led to incorporation of reliquefiers into the design of new 
tankers for recovering LNG boil-off. 
Attempts have been made to recover nitrogen-containing natural gas boil-off 
vaporized from a storage tank. Typically, these systems employ a 
closed-loop refrigeration system wherein cycle gas is compressed, cooled 
and expanded to produce refrigeration prior to return to the compressor. 
The following patent is representative: 
U.S. Pat. No. 3,874,185 discloses a reliquefaction process utilizing a 
closed-loop nitrogen refrigeration cycle wherein the lowest level or 
coldest level or refrigeration for condensation of LNG is provided by an 
isentropically expanded stream while the remaining refrigeration is 
provided by isenthalpic expansion of the residual second fraction of 
refrigerant. In one embodiment, the residual fraction of the 
isenthalpically expanded stream is subjected to a phase separation wherein 
liquid and vapor fractions are separated. During periods of low 
refrigeration requirements a portion of the liquid fraction is stored, 
and, during periods of higher refrigeration requirements, a portion of the 
stored liquid fraction is recycled into the refrigeration system. 
SUMMARY OF THE INVENTION 
The present invention provides a flexible and highly efficient process for 
reliquefaction of boil-off gas containing from 0 to about 10% nitrogen. 
Prior art processes are typically unable to efficiently reliquefy boil-off 
where the nitrogen content varies over such a wide range. They are 
designed to operate optimally within a narrow concentration range. As the 
concentration of contaminants moves away from design criteria, the 
reliquefiers become less efficient. Embodiments of the present invention 
eliminate this deficiency. 
The present invention is an improvement in a process for reliquefying LNG 
boil-off resulitng from the evapaoration of liquefied natural gas within a 
storage receptacle utilizing a closed-loop nitrogen refrigeration cycle. 
In the process for reliquefying boil-off gas, the closed-loop 
refrigeration system comprises the steps: 
compressing nitrogen as a working fluid in a multi-stage compressor system 
having an initial and final stage to form a compressed working fluid; 
splitting the compressed working fluid into a first and second stream; 
isenthalpically expanding the first stream to produce a cooled first stream 
and then warming against boil-off gas and warming against recycle 
compressed working fluid; 
isentropically expanding the second stream to form a cooled expanded stream 
and then warming against boil-off gas and warming against the working 
fluid; and finally 
returning the resulting warmed isenthalpically expanded and isentropically 
expanded streams to the multi-stage compressor system. 
The improvement for reliquefying LNG boil-off gas containing from about 0 
to 10% nitrogen by volume in a closed loop refrigeration process 
comprises: 
(a) effecting isenthalpic expansion of said first stream under conditions 
such that at least a liquid fraction is generated. 
(b) separating the vapor fraction, if generated, from the liquid fraction; 
(c) warming the vapor fraction against boil-off gas and recycle compressed 
working fluid; 
(d) pressuring at least a portion of the liquid fraction formed in step (a) 
e.g. to a pressure intermediate the initial and final stage of the 
multi-stage compressor system; 
(e) warming the resultant pressurized liquid fraction first against 
boil-off gas and then in parallel with the warming of said isentropically 
expanded second stream; and 
(f) returning the resultant warmed pressurized liquid fraction to a stage 
of the multi-stage compressor system. 
Several advantages are achieved by the present invention. They are: 
(a) an ability to obtain a closer match between the warming curve of the 
refrigerant cycle gases and the cooling curve of the LNG boil-off stream 
thereby reducing energy requirements to achieve liquefaction; and 
(b) an ability to obtain greater efficiency permitting reduction of the 
heat exchanger surface area required to achieve liquefaction.

DETAILED DESCRIPTION OF THE INVENTION 
The improvement in this process for reliquefying boil-off gases resulting 
from the vaporization of liquefied natural gas contained in a storage 
vessel is achieved through the modification of a closed-loop refrigeration 
system. Conventionally, the closed loop refrigeration systems use nitrogen 
as a refrigerant or working fluid, and in the conventional process, the 
nitrogen is compressed through a series of multi-stage compressors, having 
initial and final stage, and usually in combination with aftercoolers, to 
a preselected pressure. This compressed nitrogen stream is split with one 
fraction being isenthalpically expanded an the other being isentropically 
expanded. Typically, the work from the isentropic expansion is used to 
drive the final stage of compression. Refrigeration is achieved through 
such isenthalpic and isentropic expansion and that refrigeration is used 
to reliquefy the boil-off gas. The objective is to match the cooling 
curves with the warming curves and avoid significant separations between 
such curves. Separations are evidence of lost refrigeration value. 
To facilitate an understanding of the invention, reference is made to FIG. 
1. In accordance with the embodiment referred to as the Pumped JT process 
as shown in FIG. 1, natural gas (methane) to be reliquefied is withdrawn 
from a storage tank (not shown) via conduit 1 and compressed in a boil-off 
compressor 100 to a pressure sufficient for processing during 
reliquefaction. 
Refrigeration requirements for reliquefying the LNG boil-off are provided 
through a closed-loop refrigeration system using nitrogen as the working 
fluid or cycle gas. In this refrigeration system, nitrogen is compressed 
from ambient pressure through a series of multi-stage compressors having 
aftercoolers 102 to a sufficient pressure, e.g., 500-1000 psia. 
Thermodynamic efficiency is enhanced by using large pressure differences 
in the nitrogen cycle. 
In the reliquefaction process, a first stream 10 is cooled in heat 
exchanger 104 and then via line 11 in heat exchange 106. The cooled first 
stream at a temperature from about -185.degree. F. to -85.degree. F. is 
withdrawn through line 13 and expanded in JT valve 108 under conditions 
sufficient to generate a liquid e.g., to a pressure from about 25 to 125 
psia. Separator 109 is provided after the isenthalpic expansion to permit 
storage of liquid for subsequent use in the event of flowrate or 
composition change and to permit the separation of vapor, if generated by 
the expansion, from the liquid. Any vapor fraction is withdrawn from 
separator 109 and removed via line 22 and warmed against boil-off gas and 
against the first stream prior to its isenthalpic expansion via lines 23 
and 24 prior to return to multi-stage compressor system 102. The liquid is 
removed from separator 109 via line 15 and the liquid is pressurized in 
pump 111 to a pressure from about 150 to 250 psia. From there it is 
conducted via line 16 through heat exchanger 110. In heat exchanger 110, 
the boil-off gas is condensed and cooled to its lowest temperature level 
e.g., -290.degree. F. to -300.degree. F. against the pressurized liquid 
refrigerant. The pressurized liquid is then conveyed via lines 18, 19 and 
20, and warmed to a vapor state through heat exchangers 106 and 104, to a 
stage usually intermediate to the initial and final stage of the 
multi-stage compressor system 102. The use of pressure permits a closer 
match of the cooling and warming curves, particularly at the higher 
nitrogen levels than achieved with other processes, and the return of a 
recycle stream at the higher pressure. 
The remaining refrigeration is supplied by the isentropic expansion of 
second stream 30. Second stream 30 is cooled in heat exchange 104 and then 
via line 31 in heat exchanger 106 to a temperature from about -75.degree. 
to -150.degree. F. and then conveyed via line 32 to expander 112. It is 
then isentropically expanded to a pressure of about 25 to 125 psia which 
is usually at the same pressure as that of the isenthalpic expansion of 
the first stream, although it may be intermediate to that of the 
isenthalpically expanded stream and pumped stream. The isentropically 
expanded stream is conveyed via line 33 to heat exchanger 106 then via 
line 36 through heat exchangers 104 and then via line 37 to compressor 
system 102. Thus, the coldest level of refrigeration for the boil-off is 
supplied through the isenthalpic expansion of the working fluid in 
contrast to systems which have used isentropically expanded working fluids 
as the coldest level of refrigeration. 
Liquefaction of boil-off is achieved in the following manner: The boil-off 
gas is removed from the storage vessel via line 1 and compressed in 
boil-off gas compressor 100 and then passed via lines 2, 3 and 4 through 
heat exchangers 106 and 110 for liquefaction. On exiting heat exchanger 
110, the liquefied LNG is removed via line 4 and pressurized in pump 114 
where it is transferred via line 5 to the storage vessel. 
The following examples are provided to illustrate various embodiments of 
the invention and are not intended to restrict the scope thereof. 
EXAMPLE 1 
Pumped JT Process 
A recovery system for LNG boil-off was carried out in accordance with the 
process scheme as set forth in FIG. 1. Nitrogen concentrations varied from 
0% to about 10% by volume of the boil-off gas. Table 1 provides stream 
properties and rates in 1b moles/hr corresponding to the numbers 
designated in FIG. 1 for a boil-off gas containing 0% LNG. 
Table 2 provides field properties corresponding to numbers designated in 
FIG. 1 or for a boil-off gas containing approximately 10% nitrogen by 
volume. 
Table 3 provides stream properties corresponding to a prior art process 
scheme described in U.S. Pat. No. 3,874,185 where the nitrogen 
concentration in the boil-off gas is 0%. 
Table 4 provides stream properties for liquefaction of a prior art process 
scheme described in U.S. Pat. No. 3,874,185 for a boil-off gas containing 
10% nitrogen. 
TABLE 1 
______________________________________ 
FIG. 1 - Pumped JT - 0% N.sub.2 
Stream 
N.sub. 2 CH.sub.4 Press. 
No. lb Moles/hr 
Moles/hr T .degree. F. 
Psia Phase 
______________________________________ 
1 -- 292 -138 14.9 VAP 
2 -- 292 -98 20 VAP 
3 -- 292 -254 18 VAP 
4 -- 292 -275 17 LIQ 
5 -- 292 -275 35 LIQ 
10 762 -- 95 800 VAP 
11 762 -- -98 796 VAP 
13 762 -- -254 788 VAP 
14 762 -- -248 315 LIQ 
15 581 -- -283 96 LIQ 
16 581 -- -279 240 LIQ 
18 581 -- -258 238 VAP 
19 581 -- -128 234 VAP 
20 581 -- 89 232 VAP 
22 180 -- -283 96 VAP 
23 180 -- -128 92 VAP 
24 180 -- 89 90 VAP 
30 1720 -- 95 800 VAP 
31 1720 -- -98 796 VAP 
32 1720 -- -112 794 VAP 
33 1720 -- -261 96 VAP 
36 1720 -- -128 92 VAP 
37 1720 -- 89 90 VAP 
38 1901 -- 89 90 VAP 
______________________________________ 
TABLE 2 
______________________________________ 
FIG. 1 - PUMPED JT - 10% N.sub.2 
Stream 
N.sub.2 CH.sub.4 Press. 
No. lb Moles/hr 
Moles/hr T .degree.F. 
Psia Phase 
______________________________________ 
1 32 289 -202 15.5 VAP 
2 32 289 -175 20 VAP 
3 32 289 -256 18 VAP 
4 32 289 -296 16 LIQ 
10 739 -- 99 800 VAP 
11 739 -- -122 796 VAP 
13 739 -- -246 788 LIQ 
14 739 -- -300 45 VAP 
15 492 -- -304 36 LIQ 
16 492 -- -301 164 LIQ 
17 492 -- -260 162 VAP 
18 739 -- -304 43 VAP 
19 492 -- 94 156 VAP 
20 492 -- 98 156 VAP 
26 1736 -- 94 88 VAP 
30 1736 -- 99 800 VAP 
32 1736 -- - 122 792 VAP 
33 1736 -- -267 96 VAP 
36 1736 -- -159 92 VAP 
37 1736 -- 95 90 VAP 
______________________________________ 
TABLE 3 
______________________________________ 
PRIOR ART - FIG. 2 - U.S. Pat. No. 3,874,185 - 0% N.sub.2 
Phase or 
Stream 
N.sub.2 CH.sub.4 Press. 
Dew Point 
No. lb Moles/hr 
Moles/hr T .degree. F. 
Psia .degree.C. 
______________________________________ 
1 -- 292 -138 14.9 
VAP 
2 -- 292 -38 30 VAP 
3 -- 292 -243 28 V + L 
4 -- 292 -276 27 LIQ 
45 2368 -- 95 653 VAP 
46 2368 -- -150 647 VAP 
47 2368 -- -278 91.1 
VAP 
48 2368 -- -245 88.1 
VAP 
60 2368 -- 90 85 VAP 
52 415 -- 95 653 VAP 
54 415 -- -243 641 LIQ 
55 415 -- -247 348 LIQ 
56 415 -- -126 343 VAP 
58 415 -- 90 337 VAP 
______________________________________ 
TABLE 4 
______________________________________ 
PRIOR ART - FIG. 2 - U.S. Pat. No. 3,874,185 - 10% N.sub.2 
Stream 
N.sub.2 CH.sub.4 
No. lb Moles/hr 
Moles/hr T .degree. F. 
Press. Psia 
Phase 
______________________________________ 
1 32 289 -202 15.5 VAP 
2 32 289 -125 30 VAP 
3 32 289 -260 28 V + L 
4 32 289 -296 27 LIQ 
5 32 289 -295 60 LIQ 
45 2056 -- 99 653 VAP 
46 2056 -- -164 480 VAP 
47 2056 -- 298 48 VAP 
48 2056 -- -263 45 VAP 
60 2056 -- 94 42 VAP 
52 391 -- 99 653 VAP 
54 391 -- -260 641 VAP 
55 391 -- -263 202 V + L 
56 391 -- -150 197 VAP 
58 391 -- 94 191 VAP 
______________________________________ 
Calculations were made determining the heat exchanger requirements 
expressed as U times A where U is the heat transfer coefficient and A is 
the area of heat exchanger surface for the processes set forth in Tables 
1-4. Compressor power requirements are also given. These values are set 
forth in Table 5. 
TABLE 5 
______________________________________ 
Heat Exchanger 
Process 
Boil-off N.sub.2 % 
UA (BTU/Hr .degree.F.) 
Power HP 
______________________________________ 
Table 1 
0 792,244 2,724 
Table 2 
10 713,445 3,050 
Table 3 
0 797,110 2,801 
Table 4 
10 702,094 3,550 
______________________________________ 
From these results, it can be seen the Pumped JT system (Tables 1&2) is 
superior to the FIG. 2 prior art system at a 0% N.sub.2 and 10% N.sub.2 
level in the feed.