Method for sub-cooling a normally gaseous hydrocarbon mixture

A method for sub-cooling normally gaseous hydrocarbon mixtures produced in a cryogenic process unit wherein the mixture is introduced to a gas/liquid separator, which may be a storage vessel, and vapor containing at least two components of the mixture is recovered as refrigerant, employed in an open cycle refrigeration system to sub-cool the hydrocarbon mixture, and returned to the separator. The system is particularly useful for sub-cooling a hydrocarbon product stream while, at the same time, recovering boil-off vapor from a cryogenic storage vessel.

This invention relates to a method for sub-cooling normally gaseous 
hydrocarbon mixtures such as liquefied petroleum gas (LPG), natural gas 
liquids (NGL), and liquefied natural gas (LNG) associated with small 
amounts of nitrogen. The invention is particularly useful in recovery of 
boil-off vapors from cryogenic storage tanks which receive the sub-cooled 
hydrocarbon mixtures as product streams. 
In customary practice, LPG, NGL, and LNG are purified and liquefied in 
cryogenic, pressure let-down processes employing various chilling media 
such as single component refrigerant, cascade refrigerant, mixed 
refrigerant, isentropic expansion, and combinations of these. The 
resulting product streams are usually sub-cooled below their bubble point 
in order to reduce boil-off vent losses which result from heat 
assimilation in storage. 
Typically, the storage vessels are located at some distance from the 
cryogenic process facility. Despite adequate insulation and product 
sub-cooling, boil-off of lighter components of the stored hydrocarbon 
mixture invariably occurs to some degree. Loss of boil-off vapor is 
usually not desired or tolerated. Boil-off vapor is, therefore, typically 
recovered as a liquid through use of independent, closed cycle systems 
employing a single component refrigerant and returned to the storage 
vessel. Regrettably, boil-off rates are not constant because of loading 
and unloading operations as well as climatic changes. Accordingly, 
refrigeration systems employed for recovery of boil-off vapor are 
customarily sized for maximum requirements with the result that a large 
amount of refrigeration capacity is idle much of the time. The 
independent, closed cycle refrigerant system has the further disadvantage 
of a fixed refrigeration temperature. In a propane system, for example, 
the lowest available refrigerant temperature may be -40.degree. C. which 
is suitable for recovery of boil-off components expected at the time of 
plant design. However, changing feedstock or processing conditions may 
result in the boil-off vapor having an unforeseen higher content of light 
components which cannot be recovered at the fixed temperature of the 
refrigerant. 
It is therefore an object of this invention to provide a method for 
sub-cooling normally gaseous hydrocarbon mixtures such as a cryogenic 
hydrocarbon product stream by utilization of refrigeration that is also 
employed for recovery of boil-off vapor in a self-balancing system that 
will accommodate variable boil-off vapor mixtures. 
According to the invention, a multi-component, normally gaseous, 
hydrocarbon process stream is introduced to an adiabatic gas/liquid 
separation zone from which liquid product is recovered for sale, storage, 
or further processing and from which vapor is recovered. The vapor is 
recovered as a gaseous refrigerant containing at least two of the lightest 
components from the hydrocarbon process stream introduced. The gaseous 
refrigerant is compressed, condensed, sub-cooled, expanded, vaporized in 
indirect heat exchange with the incoming stream, and, finally, returned to 
the gas/liquid separation zone for intermingling with the incoming process 
stream. Because the refrigerant is used in an open cycle system which 
opens into the low-pressure end of the principal cryogenic process at the 
gas/liquid separation zone, the gaseous refrigerant will always contain 
the lightest components of the incoming stream and, therefore, the 
refrigeration temperature available for liquefaction of boil-off vapor 
will rise and fall according to composition of the boil-off gas or vapor 
flash from the incoming process stream.

The adiabatic gas/liquid separation zone may be a flash drum separator or a 
cryogenic storage vessel or a combination of the two, as shown in FIG. 4, 
according to the specific hydrocarbon mixtures being processed and 
physical arrangement of the facility. If the storage vessel is proximate 
to the main cryogenic process facility, it may function as the gas/liquid 
separator, however, use of a separate flash drum upstream of the storage 
tank is preferred in order to provide faster system response to changes in 
the hydrocarbon mixture. The gas/liquid separation zone is adiabatic in 
contrast to a reboiled fractionator or rectification column 
notwithstanding the fact that a cryogenic storage tank will have some 
normal atmospheric heat assimilation. The adiabatic gas/liquid separation 
zone may be operated at from 0.8 to 2.0 bar but will preferably be 
operated at slightly above atmospheric pressure (above 0.987 bar). 
In order to achieve the low refrigerant temperature desired to sub-cool the 
incoming hydrocarbon process stream to cryogenic storage temperature, it 
is essential to sub-cool the condensed refrigerant stream as well. 
Refrigerant may be sub-cooled with an external stream, for example, a 
refrigerant stream from the main cryogenic process unit as shown in FIG. 1 
but is preferably sub-cooled as shown in FIG. 2 by heat exchange with, 
after expansion, itself in the classic "bootstrap" cooling technique 
whereby refrigeration from expansion of a stream is utilized to cool the 
higher pressure predecessor of the expanded stream. Available 
refrigeration is, of course, also used to sub-cool the incoming process 
stream. When the incoming stream is principally methane and also contains 
a minor amount of nitrogen as is usually the circumstance in LNG units, 
the gaseous refrigerant is compressed to between 14 and 35 bar, condensed, 
and then sub-cooled to a temperature between -140.degree. and -170.degree. 
C. prior to expansion for recovery of refrigeration. When the incoming 
stream is principally ethane and also contains smaller amounts of methane, 
the gaseous refrigerant is compressed to between 7 and 31 bar, condensed, 
and sub-cooled to between -70.degree. and -110.degree. C. When the 
incoming stream is principally propane or butane or, typically, 
predominantly a propane/butane mixture including some lighter gases, the 
gaseous refrigerant is compressed to between 3 and 25 bar, condensed, and 
sub-cooled to between 10.degree. and -60.degree. C. 
The sub-cooled refrigerant is expanded to the low pressure of the adiabatic 
gas/liquid separation zone, preferably, through a Joule-Thompson valve and 
refrigeration then recovered from the resulting expanded stream without 
intervening separation of vapor and liquid. Typically, the expanded stream 
will be a two phase mixture but may be entirely liquid phase if the stream 
has been sub-cooled to a very low temperature. Recovery of refrigeration 
by indirect heat exchange with the incoming hydrocarbon process stream 
and, preferably, also with its higher pressure predecessor stream will, of 
course, revaporize the refrigerant to predominantly vapor phase for return 
to the adiabatic gas/liquid separation zone. This return stream is 
preferably introduced to the physical separator or storage tank, as the 
case may be, separately from the incoming, liquid phase, sub-cooled, 
multi-component, hydrocarbon stream expanded into, usually, the same 
vessel. The point of introduction of the return revaporized stream should 
be above the point of introduction of the sub-cooled liquid stream to 
facilitate gas/liquid separation of both streams and recovery of a 
normally gaseous, liquid phase, hydrocarbon product stream from the vessel 
or vessels employed in the gas/liquid separation zone. 
Preferably, the condensed refrigerant is sub-cooled in two indirect heat 
exchange stages as shown in FIG. 3 in order to closely match refrigeration 
duties with the two temperature level refrigerant streams thereby made 
available. In this embodiment, the entire refrigerant liquid stream is, 
therefore, initially sub-cooled and a portion of the sub-cooled stream 
expanded to an intermediate pressure between 2 and 15 bar to provide 
refrigeration required by the initial sub-cooling. The resulting 
revaporized refrigerant is then returned to an intermediate pressure point 
in the gaseous refrigerant compression step, for example, between the 
stages of a two stage compressor. The balance of the initially sub-cooled 
refrigerant liquid is then passed to a second stage of heat exchange as 
described above for final sub-cooling prior to expansion as previously 
described. 
Referring to the drawings and the descriptions thereof, the following 
nomenclature has been used for functional identification of process 
streams and treatments: 
1. multi-component, normally gaseous, hydrocarbon process stream 
1a. liquid phase, sub-cooled, multi-component, normally gaseous, 
hydrocarbon stream 
2. heat exchanger 
3. heat exchanger 
4. low-pressure, adiabatic gas/liquid separation zone 
5. normally gaseous, liquid phase, hydrocarbon product stream 
6. LPG storage tank 
7. LPG product 
8. gaseous refrigerant stream 
9. compressor 
10. heat exchanger (condenser) 
11. accumulator vessel 
12. high-pressure refrigerant liquid 
12a. initially sub-cooled high-pressure refrigerant liquid 
13. heat exchanger 
14. heat exchanger 
15. first, cold refrigerant liquid 
16. second, cold refrigerant liquid 
17. expansion valve 
18. first, intermediate pressure refrigerant 
19. first, intermediate pressure revaporized refrigerant 
20. expansion valve 
21. butane stream 
22. second, intermediate pressure revaporized refrigerant 
23. combined, intermediate pressure revaporized refrigerant 
24. knock-out drum 
25. expansion valve 
26. expansion valve 
27. first, low-pressure refrigerant 
28. second, low-pressure refrigerant 
29. first, low-pressure revaporized refrigerant 
30. second, low-pressure revaporized refrigerant 
31. combined, low-pressure revaporized refrigerant 
32. expansion valve 
It is noted that suitable heat exchangers for use in the process of the 
invention may be of the shell and tube type or the plate-fin type which 
permits heat exchange among several streams. While separate heat exchange 
zones are shown in the drawings for illustrative purpose, these zones may 
be combined into one or more multiple stream exchangers in accordance with 
the parameters of specific process designs. 
Referring now to FIG. 1, an incoming multi-component, normally gaseous, 
hydrocarbon process steam which will usually be a liquid phase stream 
under elevated cryogenic process pressure is sub-cooled in heat exchanger 
3 and the resulting sub-cooled stream 1a expanded into the low-pressure, 
adiabatic gas/liquid separation zone indicated by flash separator 4. A 
normally gaseous, liquid phase hydrocarbon product stream is withdrawn 
from the bottom of the separator through line 5 and a vapor stream, which 
constitutes the gaeous refrigerant stream is withdrawn through line 8. The 
flash separator 4 is preferably operated at or near atmospheric pressure 
in order to avoid undesirable vacuum conditions at the inlet side of 
compressor 9. Following compression of the gaseous refrigerant to an 
elevated pressure, the refrigerant is condensed in heat exchanger 10, 
typically against water, and accumulated in vessel 11. High-pressure 
refrigerant liquid is withdrawn from the accumulator on demand through 
line 12 and sub-cooled in heat exchanger 14 by an external refrigerant 
stream which may, for example, be available from the principal cryogenic 
process. This sub-cooling yields a first, cold refrigerant stream 15 which 
is then expanded through valve 25 and revaporized by heat exchange in 3 
with the incoming process stream. The resulting first, low-pressure 
revaporized refrigerant in line 29 is then returned to flash separator 4. 
FIG. 2 shows a process of the invention that is substantially the same as 
that of FIG. 1 except that an external refrigerant is not needed since the 
high-pressure refrigerant liquid stream 12 is sub-cooled also in heat 
exchanger 3 by the first, low-temperature refrigerant stream 27. 
In FIG. 3, two stage sub-cooling of high-pressure refrigerant liquid stream 
12 is shown in which initial sub-cooling is performed in heat exchanger 13 
and a second, cold refrigerant liquid stream 16 is divided out from the 
initially sub-cooled refrigerant. In this embodiment, the second, cold 
refrigerant stream has a temperature above that of the first, cold 
refrigerant stream 15 and is expanded across valve 17 to form a first, 
intermediate pressure refrigerant which is recovered in heat exchanger 13 
to form a first, intermediate pressure revaporized stream 19. Vapor stream 
19 is then returned to an interstage point of, now, two stage compressor 9 
where it is combined with the gaseous refrigerant stream 8 undergoing 
compression. Knockout drum 24 is employed to remove any liquid that may be 
present in stream 19 in order to protect the compressor. 
In production of a liquid phase, hydrocarbon product such as that recovered 
in line 5 of the drawings, it may be appreciated that an increasing 
concentration of lighter components in the incoming process stream 1 will 
tend to boil off in storage at an undesirably high rate unless their 
storage temperature is lowered. From the preceding descriptions, it is 
apparent that the processes of the invention can achieve production of a 
lower temperature product stream 5 by virtue of their self-balancing, open 
cycle characteristic since gaseous refrigerant stream 8 will necessarily 
contain a higher concentration of lighter components as they are flashed 
from the incoming stream. The resulting lighter gaseous refrigerant having 
a correspondingly lower bubble point can therefore achieve lower 
refrigeration temperatures in heat exchanger 3 and thereby provide lower 
temperature sub-cooling of the incoming hydrocarbon process stream 1 
without use of sub-atmospheric pressures in the system. 
Referring now to FIG. 4 which, as previously noted, illustrates a flow 
scheme of the invention suitable for sub-cooling an LPG stream having the 
following composition: 
______________________________________ 
C.sub.2 
= 2.1 weight % 
C.sub.3 
= 95.4 weight % 
C.sub.4 
= 2.5 weight % 
100.0 weight % 
______________________________________ 
The LPG process stream 1 is introduced to heat exchanger 2 at a pressure of 
17.8 bar and initially sub-cooled to -23.degree. C. The stream is further 
sub-cooled to -46.degree. C. in heat exchanger 3 and expanded to low 
pressure into flash separator 4 which is operated at slightly above 1 bar. 
A normally gaseous, liquid phase, hydrocarbon product stream 5 having 
substantially the same composition as stream 1 is recovered from the 
bottom of separator 4 for storage in cryogenic tank 6 from which LPG 
product is withdrawn through line 7 for sale or further processing. 
Boil-off vapor from the LPG storage tank 6 comprised of most of the ethane 
from product stream is combined with other vapors in separator 4 to form 
gaseous refrigerant stream 8 having the following composition: 
______________________________________ 
C.sub.2 
= 13.9 weight % 
C.sub.3 
= 86.1 weight % 
C.sub.4 
= trace 
100.0 weight % 
______________________________________ 
The gaseous refrigerant is compressed in two stage compressor 9 to an 
intermediate pressure of 2.7 bar and then to an elevated pressure of 19.5 
bar. High-pressure gaseous refrigerant is then condensed against water in 
heat exchanger 10 and accumulated in vessel 11. High-pressure refrigerant 
liquid is withdrawn from the accumulator through line 12 and initially 
sub-cooled in heat exchanger 13 to -24.degree. C. A portion of the 
initially sub-cooled refrigerant is further sub-cooled to -46.degree. C. 
in heat exchanger 14 and withdrawn through line 15 as the first, cold 
refrigerant liquid. Another portion of the initially sub-cooled 
refrigerant, still at -24.degree. C., is branched off through line 16 and 
a portion expanded through valve 17 to form the first, intermediate 
pressure refrigerant 18 at 3 bar which provides initial sub-cooling of the 
high-pressure refrigerant liquid in heat exchanger 13 and is thereby 
vaporized to become the first, intermediate pressure revaporized 
refrigerant in line 19. 
A parallel stream from line 16 is similarly expanded through valve 20 to 
provide initial sub-cooling for LPG process stream 1 in heat exchanger 2 
as well as sub-cooling for a separate butane stream 21 and is thereby 
vaporized to become the second, intermediate pressure revaporized 
refrigerant in line 22. The first and second, intermediate pressure 
revaporized refrigerants are combined in line 23 and returned via 
knock-out drum 24 to the second stage inlet of compressor 9 at a pressure 
of 2.7 bar. 
Referring back to heat exchanger 14, the first cold refrigerant in line 15 
is divided and expanded through valves 25 and 26 to 1.3 bar to form 
respectively the first, low-pressure refrigerant in line 27 and the 
second, low-pressure refrigerant in line 28. These streams provide final 
sub-cooling for the LPG process stream in heat exchanger 3 and the 
high-pressure refrigerant liquid in heat exchanger 14 and are thereby 
vaporized to form the first, low-pressure revaporized refrigerant in line 
29 and the second, low-pressure revaporized refrigerant in line 30. The 
revaporized low-pressure streams are combined in line 31 and returned at a 
temperature of -32.degree. C. to flash separator 4. If refrigeration 
available in stream 15 is in excess of the sub-cooling requirements in 
heat exchangers 3 and 14, the excess may be expanded through valve 32 to 
further sub-cool the LPG product stream by direct heat exchange. In the 
event that a significant excess of refrigeration is available, it may be 
utilized in one or more exchangers (not shown) in parallel with heat 
exchangers 3 and 14.