Liquid flash between expanders in gas separation

In the separation of low boiling gases such as ethane and heavier from natural gas utilizing two expanders in series, liquid condensed before expansion is not passed to the fractionator but is flashed and the resulting vapor combined with the expanded vapor from the first expander. This results in both increased work output from the second expander and simplified design of the downstream fractionator column since it reduces the amount of lighter materials introduced into the lower section thereof.

BACKGROUND OF THE INVENTION 
This invention relates to the separation of higher molecular weight 
components from lower molecular weight components in a fluid stream. In a 
specific embodiment, it relates to the separation of the ethane and higher 
molecular weight components from a natural gas stream containing methane. 
Natural gas as it comes from the ground generally is not suitable for use 
directly without some processing. The basic processing operations carried 
out in a natural gas plant are to first remove acid gases such as CO.sub.2 
and H.sub.2 S and then to pass the gas through a dehydration means to 
remove water. The resulting product can then be used as a fuel. However, 
such streams generally contain a substantial amount of higher molecular 
weight components such as ethane and to a lesser extent, propane, butanes, 
and higher components. The ethane and heavier components are of greater 
value as chemical feedstocks than they are as a fuel. 
It has long been known to separate ethane and higher components from 
methane by the use of an expander wherein a natural gas feedstream is 
passed to a high pressure separator and the vapor taken off and passed to 
an expander with the resulting vapor going to the upper portion of a 
demethanizer and the liquid from the separator going to the lower portion 
of a demethanizer. Such a system is not particularly efficient, however. 
Accordingly, attempts have been made to improve the efficiency simply by 
utilizing two or more expanders in series. However, even with multiple 
expanders in series, such separations are still difficult. For one thing, 
the subsequent demethanizer, must be rather large. Also, sufficient work 
may not be extracted from the system by means of the expanders even with 
the two or more in series to be sufficient to handle all of the 
compression requirements and to supply all the refrigeration needs of the 
overall plant. 
SUMMARY OF THE INVENTION 
It is an object of this invention to increase the total horsepower output 
of the expanders in a gas processing plant; it is a further object of this 
invention to increase the amount of refrigeration produced by the process 
stream; it is a still further object of this invention to simplify the 
design of the demethanizer column of a natural gas processing plant; and 
it is still yet a further object of this invention to provide improved 
separation of ethane from methane in a natural gas processing plant. 
In accordance with this invention, liquid from a high pressure separator 
upstream from the first of at least two expanders in series is passed to a 
feed separator and flashed with the resulting vapor being combined with 
the expanded vapor from the first expander.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The principle of this invention wherein the liquid product of a high 
pressure separator feeding vapor to the first of a series of expanders is 
flashed and the resulting flashed vapor combined with the expander vapor 
from the expander is broadly applicable to any separation of higher and 
lower molecular weight gaseous components (for instance, separating butane 
and higher from ethane, and the like). However, it will be described 
hereinafter in terms of the preferred embodiment wherein ethane and higher 
components are separated from the methane in a dehydrated natural gas 
stream. 
Referring now to the Figure, line 2 carries feed which is a natural gas 
stream which has been subjected to conventional processes to remove acid 
gases such as CO.sub.2 and H.sub.2 S and which has been subjected to 
conventional dehydration processes to remove water. This natural gas vapor 
feed line stream is then divided and the first portion passes via gas line 
4 to gas-gas residue exchanger 6 for the purpose of recovering 
refrigeration from the residual gas which is primarily methane. The 
proportion of the feed passing via line 4 is adjusted by means of a valve 
5 so as to efficiently utilize the refrigeration available in the residual 
gas contained in line 90. The second portion of the feed passes via gas 
line 8 to product heat exchanger 10 and thence via line 9 to demethanizer 
bottom reboiler 12 and thence via line 20 to chiller 18, and thence via 
line 21 through second side reboiler 22 (the first side reboiler will be 
described hereinafter). The thus cooled feed in line 23 is combined with 
the cooled feed in line 14 to form combined stream 24 which passes to high 
pressure separator (first expander inlet separation zone) 26. In the high 
pressure separator, the liquid is drawn off the bottom via liquid line 32, 
and the vapor drawn off the top via vapor line 27 and passed to the first 
expander (expansion zone) 28. Expander 28 drives compressor 30 to produce 
external work. Of course, expander 28 can drive any mechanical means such 
as a generator, and the like, if desired. Also, expander 28 and the 
subsequent expanders can be connected by a common shaft to a single 
compressor or generator means, if desired, or to separate means as shown 
herein. The thus cooled expanded fluid stream from first expander 28 is 
drawn off via line 38. 
Liquid drawn off from high pressure separator 26 via line 32 passes through 
first liquid level control and expansion valve 16 and thence to a flash 
separation zone 34 operating preferably at essentially the discharge 
pressure of expander 28. This is the heart of the invention. Instead of 
passing the liquid directly to a middle or lower portion of demethanizer 
column (fractionation zone) 48, it has been found that substantial 
advantages are obtained if it is passed through an expansion valve to a 
feed separator with the flashed vapor being taken off as shown via line 36 
and combined with the expansion vapor from the first expander carried by 
line 38. This puts more volume through the second expander (to be 
described hereinbelow) thus giving a gain in horsepower output that would 
otherwise be lost in the flash down to column pressure. Also, the 
demethanizer column operates more efficiently with this vapor being 
removed and actually can be constructed with a smaller diameter as a 
result thereof. The liquid from feed separator 34 passes via line 43 
through second liquid level control and expansion valve 44 and thence via 
line 46 to demethanizer column 48. The combined flashed vapor and 
expansion vapor stream 40 which may contain some liquid is split and the 
first portion passes via cold exchange gas line 41 to cold gas exchanger 
42, which serves to both recover refrigeration from the very cold gas from 
the top of the demethanizer and to cool stream 41. The second portion of 
stream 40 passes via line 50 to first side reboiler 52. The fluids from 
exchanger 42 and reboiler 52 are withdrawn by lines 53 and 51, 
respectively, and passed via combined stream line 54 to low pressure 
separator (second expander inlet separation zone) 56. Low pressure 
separator 56 operates as an expander inlet separator for the second 
expander in the same manner that high pressure separator 26 operates as 
the expander inlet separator for the first expander 28. The vapor from 
separator 56 passes via vapor line 58 to second expander 60 which drives 
compressor 62. The vapor (which may contain some liquid) from expander 60 
is withdrawn via line 64 and passed to a demethanizer 48. The liquid is 
withdrawn from separator 56 via line 68, passed through third liquid level 
control and expansion valve 66, and thence to demethanizer 48 via line 69. 
Generally this entry point is below the entry of line 64 although lines 64 
and 69 can be combined. 
Liquid is withdrawn from demethanizer 48 via line 70 and passed to first 
side reboiler 52 where it picks up sufficient heat to heat this portion of 
the demethanizer column 48 on being returned thereto via lines 72 and 46. 
A second liquid stream is withdrawn from demethanizer 48 via line 74 and 
passed to second side reboiler 22 where it picks up sufficient heat to 
heat the lower intermediate portion of demethanizer column 48 on being 
passed back thereto via line 76. A third liquid stream is withdrawn from 
demethanizer 48 via line 78 and passed to demethanizer bottom reboiler 12 
where it picks up sufficient heat to heat the bottom of demethanizer 48 on 
being returned thereto via line 80. 
Finally, the bottom product from demethanizer 48 which is predominantly 
ethane is withdrawn via line 82 and passed by pump 84 and line 85 to 
product heat exchanger 10 where it is heated to essentially ambient 
temperature and discharged via line 86 as product of the process. 
The residue gas from the top of the demethanizer 48 is withdrawn via line 
88. This residue gas is primarily methane and nitrogen and is passed 
through cold gas exchanger 42 and gas-gas residue exchanger 6 where it is 
heated to the desired temperature for discharge. Residue stream 90 
generally is compressed by means of compressors 30 and 62 and used in this 
form as a fuel source, i.e., natural gas for firing furnaces, and the 
like. 
The chiller 18 is cooled generally by some external source, such as propane 
refrigerant. Except for this and pump 84 which may be powered by a 
relatively small electric motor, most of the energy for this operation 
comes from the potential energy stored in the feed gas as a result of it 
being under compression. 
The initial pressures for feed line 2 are in the neighborhood of 730 to 750 
psia (5.03 to 5.17 MPa) and are reduced to pressures in the neighborhood 
of 480 to 490 psia (3.31 to 3.38 MPa) after passing through the first 
expander and to 200 psi (1.38 MPa) after passing through the second 
expander. The invention is applicable to systems, however, having initial 
pressure in the range of 400 to 1,000 psia (2.76 to 6.89 MPa), preferably 
500 to 875 psia (3.45 to 6.03 MPa). The demethanizer pressures can vary 
from 50 to 450 psia (0.34 to 3.1 MPa), preferably from 100 to 350 psia 
(0.689 to 2.4 MPa). Generally, the pressure after the first expander will 
be controlled such that: (1) there is the same drop in pressure after each 
expander; or (2) the same horsepower is obtained from each expander; or 
(3) a relatively constant ratio of expansion is obtained. As shown in the 
following example, a constant drop in pressure is used (about 275 psia). 
Feed pressures are frequently about 5 MPa, fractionator pressures about 1.4 
MPa, and the pressure between the two expanders about 3 MPa. 
The invention can be utilized with more than two expanders in a series, 
either with a feed separator after all but the last one or after only one 
of the initial expanders. 
The following example is based on calculations which have been found to 
agree closely with typical operating conditions in actual operation. 
EXAMPLE 
A natural gas stream is passed through a conventional process for removing 
acid gases and, thence, through a conventional process for dehydration and 
then to a plant as shown in the drawing. The pressures and temperatures of 
the various streams are as shown in Table I and the material balance in 
moles per day are shown in the Table II. 
Table I 
______________________________________ 
Temperature Pressure 
Stream No. .degree. F. 
.degree. C. 
Psia MPa 
______________________________________ 
2 90 32 750 5.17 
14 -57 -49 730 5.03 
9 56 13 745 5.14 
20 25 -4 740 5.10 
21 -23 -31 735 5.06 
23 -48 -44 730 5.03 
24 -51 -46 730 5.03 
27 -51 -46 730 5.03 
32 -51 -46 730 5.03 
38 -81 -63 485 3.34 
36 -70 -57 490 3.38 
43 -70 -57 490 3.38 
40 -80 -61 485 3.34 
53 -119 -84 480 3.31 
51 -117 -83 480 3.31 
54 -119 -84 480 3.31 
58 -119 -84 480 3.31 
68 -119 -84 480 3.31 
64 -168 -111 200 1.38 
70 -134 -92 200 1.38 
72 -92 -69 200 1.38 
46 -99 -73 200 1.38 
74 -71 -57 200 1.38 
76 -32 -36 200 1.38 
78 -2 -19 200 1.38 
80 21 -6 200 1.38 
82 21 -6 200 1.38 
86 80 27 456 3.14 
______________________________________ 
Table II 
__________________________________________________________________________ 
MATERIAL BALANCE, KG MOLS/DAY 
__________________________________________________________________________ 
Stream No. 
2 4 8 27 32 36 43 41 
Liquid 
Gas To 
Gas To 
Gas To 
To Cold 
Residue 
Product 
First Flash Flash 
Flash 
Exchanger 
Component 
Feed 
% Exchanger 
Heater 
Expander 
Separation 
Vapor 
Liquid 
Gas 
__________________________________________________________________________ 
Nitrogen 
1,857 
2 557 1,300 
1,593 264 182 82 1,278 
Methane 
78,349 
71 
23,505 
54,844 
52,873 
25,476 9,210 
16,266 
44,712 
Ethane 
16,675 
15 
5,002 11,673 
4,503 12,172 697 11,475 
3,744 
Propane 
9,255 
8 2,777 6,478 
881 8,374 99 8,274 
706 
i-Butane 
1,139 
1 342 797 46 1,093 4 1,089 
36 
N-Butane 
2,590 
2 777 1,813 
77 2,513 6 2,507 
60 
C.sub.5.sup.+ 
903 1 270 633 8 895 -- 895 6 
Totals 
110,768 
33,230 
77,538 
59,981 
50,787 10,198 
40,588 
50,542 
__________________________________________________________________________ 
Stream No. 
50 58 68 70 74 78 82 88 
Low De-C.sub.1 
De-C.sub.1 
De-C.sub.1 
Gas Gas To 
Pressure 
Liquid to 
Liquid To 
Liquid 
To Side 
Second 
Separator 
1st. Side 
2nd. Side 
To Demethanized 
Residue 
Component 
Reboiler #1 
Expander 
Liquid 
Reboiler 
Reboiler 
Reboiler 
Product Gas 
__________________________________________________________________________ 
Nitrogen 
497 1,599 
176 3 -- -- -- 1,857 
Methane 
17,371 45,197 
16,886 
5,255 
6,319 1,503 
477 77,872 
Ethane 
1,456 1,208 
3,992 6,604 
19,974 
21,558 
15,891 784 
Propane 
274 56 925 1,121 
9,669 10,123 
9,251 4 
i-Butane 
14 1 49 53 1,154 1,180 
1,139 -- 
N-Butane 
23 1 82 88 2,613 2,655 
2,590 -- 
C.sub.5.sup.+ 
2 -- 8 9 904 910 903 -- 
Totals 
19,637 48,062 
22,118 
13,133 
40,633 
37,929 
30,251 80,517 
__________________________________________________________________________ 
While this invention has been described in detail for the purpose of 
illustration, it is not to be construed as limited thereby but it is 
intended to cover all changes and modifications within the spirit and 
scope thereof.