Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CRYOGENIC GAS PLANT
The present invention relates to a cryogenic gas plant and
particularly to a gas plant which is designed to separate natural
gas liquids (NGL), which contain ethane and higher boiling hydro-
carbons from a natural gas feedstream.
The evolution of cryogenic gas plants is reviewed in a paper
presented at the 1977 Gas Processing Association Convention
entitled "Evolution in Design" by R.L~ McKee. This paper describes
the use of turbo-expanders to increase the thermodynamic efficiency
of a gas plant and thus improve its overall economics. A similar
type of cryogenic system for recovering ethane and higher boiling
hydrocarbons from a natural gas stream is described in U.S. Patent
Specification No. 3,292,380. ~ more recent description of cryogenic
gas processin~ plants appears in the July 14, 1980 edition of the
"Oil and Gas Journal" at page 76 et. seq. All of these references
describe the use of a turbo-expander for supplying the feedstock
to a demethanizing column where the methane is separated from the
ethane and the higher boiling hydrocarbons.
While these systems have been satisfactory, they do nGt
recover all the ethane and higher boiling point liquids. The ethanP
and hlgher boiling point liquids are used as chemical feedstocks
which have more value in today's markets as chemical feedstocks
than as natural gas. The lack of complete recovery of the ethane
partially is due to the relativ2 volatility of methane to eehane
and to the fact that a considerable amount of the ethane is
~5 contained as a vapour in the methane when it enters the
demethanizing column. The portion of the ethane contained as
vapour remains in the gas phase and is discharged from the top
of the column as pipeline gas.
A further problem arises when attempts are made to operate
plants near their capacity or beyond. When operating plants at or
above their capacity, flooding of the demethanizing column with
lt~
liquid occurs. When this occurs, additional ethane and higher
boiling point liquids are lost to the natural gas stream instead
of being condensed and removed as liquid from the bottom of the
column.
The object of the present invention is to solve the above
problems.
The process for separating ethane and heavier components
from a natural gas feedstream utilizing a cryogenic plant having
A co:ld feedstream thereto comprises according to the invention
introducing the feedstream into a rectifier column to recover
ethane from the feedstream and ~ithdra~ing methane as a overhead
stream,
passlng a portlon of the overhead stream back to the top of
said rectifier column as a reflux stream, the pressure of said
reflux stream being increased above the pressure of said overhead
stream;
introducing the boetom stream of said rectifier column as a
feed to a demethanizer column; and
returning the top stream of said demethanizer colu~n as a
bottom feed to said rectifier column.
According to the invention the aforementioned problems are
solved by placing a rectifying column ahead of the demethanizing
column. The rectifying column is provided with a reflux system
while the liquefied bottom stream from the rectifying column is
used as a feed for the demethanizing column. The combination of
the rectifying column and the demethanizing column can provide
essentially 100% recovery of the ethane and higher boiling point
liquids from a natural gas stream. The reflux of the rectifying
column uses a small portion of the overhead stream from the
rectifying column, about 10% to 17%. The reflux stream is produced
by a compressor and a condenser which is cooled by the overhe2d
stream from the rectifying column. The use of a liquid reflux
stream increases recovery of NGL9 the ethane and higher boiling
point liquids, while the use of the demethanizing column insures
that sufficient methane will be removed from the NGL product~ It
is to be noted that the rectifying column also can be placed on
top of the demethani~ing stripper column, both being one common
vessel, in order to reduce equipment costs and complexity,
including elimination of the need to pump liquid from the bottom
of the rectifying column to the top of the demethanizing co umn.
The present invention will be more easily understood from
the following detailed desc~iption of a preferred embodiment
when taken in con~unction with the attached drawing showing
schematically a gas processing system constructed accordlng to
the present lnvention,
Referring to the attached drawlng, there ls shown a
rectlfying column 12 that has been added ahead of a demethanizlng
stripper column 22. In known gas plants the demethanizing column
is supplied with a partially liquefied feedstream 10 from a
turbo-expander not shown in the drawing. Such a known plant
wlthout a rectifying column will recover approximately 76% of the
ethane while the present invention provides a plant enabling the
recovery of approximately 93~D of the ethane and essentially all
of the higher boiling liquids.
Apart from a turbo-expander for supplying the feedstream 10
other systems could be used to supply the cold vapour liquid
feedstream. For example, r~frigeration systems could be used to
supply the feedstream. The feedstream should have a temperature
in the range of -73 C to -115 C and a pres~ure in the range of
1720 kPa to 3100 kPa. In an existing plane the feedstream from
the turbo-expander has a pressure of 2654 kPa and a temperature
o~ -93 C.
Rectifying column 12 is supplied with the feedstream 10,
formerly supplied in the demethanizing column 22, having a
temperature of approximately minus 94C and comprising essentially
methane gas and higher boiling point liquid hydrocarbons. Th
ethane and higher boiling point liquids are stripped from the
feedstream and removed from the bottom of the rectifying
column while the methane is withdrawn as an overhead stream 11.
A small portion of the overhead stream of the column 12 is taken
off as a reflux stream 13 and compressed by compressor 14.
As explained above, the reflux should comprise a small portion
of the total flow, approximately 10% to 17%, of the overhead
stream. The compressed reflux stream is passed through a
condenser 15 where :It is condensed by the overhead stream 11
with the flow of the overhead stream through the condenser
being controlled by a bypass valve 17. A valve 16 controls the
condensing pressure and the flow of reflux to the rectifying
column 12. The liquid reflux supplied to the rectifying column
efects recovery of most of the ethane and essentially all the
higher boiling point hydrocarbon from the feedstream 10. The
liquid bottom stream 20, including a substantial quantity of
methane, is pumpe.d to the demethanizing column 22 by a pump 21.
The top stream 23 from the demethanizing column, consisting
of methane and a smaller amount,of ethane and higher boiling
liquids, flows into the bottom part of the rectifying column 12.
The rectifying column 12 and the demethanizing column 22 are
reboiled by warmer portions of the gas stream upstream of the
~urbo-expander, not sho~m in the drawing.
NGL product is withdrawn from the bottom of demethanizing
column 22 through a line 30 and supplied by pump 31 as a chemical
feedstock to other processing units not shown. The bottom
reboiler 32 and side reboilers 24 and 25 ensure that sufficient
methane is removed from the NGL product.
From the above description, it is seen that only a small
portion of the overhead stream of the rectifying column is
used as reflux. Further, a compressor can be used to compress
this fluid to higher pressure which permits its condensation
to a liquid using the overhead stream from the column as the
cooling medium. Thus3 a liquid reflux is obtalned with minimum
expenditure of energy in contrast to the use of turbo-expanders in
- ~ s -
prior systems. The use of a liquid reflux in the rectifying
column ensures that more than 90% of the ethane and essentially
100% of the hlgher boiling point liquids are removed from
the feedstream 10. By removing some of the methane from the
liquid in the rectifying column, the load on the demethanizing
column 22 is reduced with the net result that the ehroughput
of the plant can be increased if one so desires.
