U.S. patent application number 09/862253 was filed with the patent office on 2002-11-28 for ethylene plant refrigeration system.
Invention is credited to Wei, Vitus Tuan.
Application Number | 20020174679 09/862253 |
Document ID | / |
Family ID | 25338048 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020174679 |
Kind Code |
A1 |
Wei, Vitus Tuan |
November 28, 2002 |
Ethylene plant refrigeration system
Abstract
The refrigeration system for an ethylene plant comprises a
tertiary refrigerant containing methane, ethylene and propylene. In
the closed loop system, a portion of the constant composition
refrigerant from the compressor is separated into a methane-rich
vapor portion and a propylene-rich liquid portion. The various
refrigerant streams are then used to cool the charge gas to
separate the C.sub.2 and heavier hydrocarbons from the hydrogen and
methane. The separated refrigerant streams are then recombined to
form the constant composition before recycle to the compressor.
Inventors: |
Wei, Vitus Tuan; (Houston,
TX) |
Correspondence
Address: |
Alix, Yale & Ristas
750 Main Street
Hartford
CT
06103-2721
US
|
Family ID: |
25338048 |
Appl. No.: |
09/862253 |
Filed: |
May 22, 2001 |
Current U.S.
Class: |
62/612 |
Current CPC
Class: |
F25J 3/0238 20130101;
F25J 2270/18 20130101; F25J 2215/60 20130101; F25J 3/0233 20130101;
F25J 2270/66 20130101; F25J 2200/38 20130101; F25J 3/0252 20130101;
F25J 2205/04 20130101; F25J 2210/12 20130101; F25J 3/0219 20130101;
F25J 2270/902 20130101; F25J 2270/12 20130101 |
Class at
Publication: |
62/612 |
International
Class: |
F25J 001/00 |
Claims
1. In a process for the production of ethylene from a charge gas
containing hydrogen, methane, ethylene and other C.sub.2 and
heavier hydrocarbons wherein said process includes a low pressure
demethanizer operating at a pressure below 2.41 MPa (350 psi) and
wherein said charge gas is cooled by a refrigeration system, a
method for cooling said charge gas by the use of a tertiary
refrigerant in said refrigeration system comprising the steps of:
(a) compressing a tertiary refrigerant comprising a mixture of
methane, ethylene and propylene having a selected composition; (b)
cooling and partially condensing said tertiary refrigerant and
forming a vapor refrigerant stream having an increased percentage
of methane and a liquid refrigerant stream having an increased
percentage of propylene; (c) progressively cooling said vapor
refrigerant stream and said liquid refrigerant stream in a series
of heat exchangers; (d) bringing said charge gas into heat exchange
contact in said series of heat exchangers with said progressively
cooled vapor refrigerant stream and said progressively cooled
liquid refrigerant stream thereby cooling said charge gas and
producing a remaining gas stream containing said hydrogen and a
portion of said methane and producing liquid demethanizer feed
streams containing another portion of said methane and concentrated
in said ethylene and other C.sub.2 and heavier hydrocarbons; (e)
feeding said liquid demethanizer feed streams to said low pressure
demethanizer and producing a demethanizer overhead stream
consisting essentially of methane and producing a demethanizer
bottoms product stream; (f) contacting said demethanizer overhead
stream with said progressively cooled refrigerant streams; and (g)
combining said vapor refrigerant stream and said liquid refrigerant
stream to form a combined refrigerant stream having said selected
composition and returning said combined refrigerant stream to said
step of compressing.
2. In a process as recited in claim 1 and further including the
steps of separating a portion of said liquid refrigerant stream,
superheating said separated portion of said liquid refrigerant
stream and combining said superheated portion of said liquid
refrigerant stream with said combined refrigerant stream for return
to said step of compressing.
3. In a process as recited in claim 1 wherein said demethanizer
overhead stream is partially condensed in said heat exchanger and
said condensed part is returned as reflux to said demethanizer.
4. In a process for the production of ethylene from a charge gas
containing hydrogen, methane, ethylene and other C.sub.2 and
heavier hydrocarbons wherein said process includes a low pressure
demethanizer and wherein said charge gas is cooled by a
refrigeration system, a method for cooling said charge gas by the
use of a tertiary refrigerant in said refrigeration system
comprising the steps of: (a) compressing a tertiary refrigerant
comprising a mixture of methane, ethylene and propylene having a
selected composition; (b) cooling and partially condensing at least
a portion of said tertiary refrigerant; (c) separating said
partially condensed tertiary refrigerant into a vapor refrigerant
stream having an increased percentage of methane and a liquid
refrigerant stream having an increased percentage of propylene; (d)
cooling said charge gas by heat contact with said vapor refrigerant
stream and said liquid refrigerant stream thereby cooling said
charge gas and producing a remaining gas stream containing hydrogen
and a portion of said methane and producing a liquid demethanizer
feed stream containing another portion of said methane and
concentrated in said ethylene and other C.sub.2 and heavier
hydrocarbons; (e) combining said vapor refrigerant stream and said
liquid refrigerant stream to form a combined refrigerant stream
having said selected composition and returning said combined
refrigerant stream to said step of compressing.
5. In a process as recited in claim 4 and further including the
steps of separating a portion of said liquid refrigerant stream,
superheating said separated portion of said liquid refrigerant
stream and combining said superheated portion of said liquid
refrigerant stream with said combined refrigerant stream for return
to said step of compressing.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention pertains to a refrigeration system to
provide the cooling requirements of an ethylene plant. More
particularly, the invention is directed to the use of a tertiary or
trinary refrigerant comprising a mixture of methane, ethylene and
propylene for cooling in an ethylene plant.
[0002] Ethylene plants require refrigeration to separate out
desired products from the cracking heater effluent. Typically, a
C.sub.3 refrigerant, usually propylene, and a C.sub.2 refrigerant,
typically ethylene, are used. Often, particularly in systems using
low pressure demethanizers where lower temperatures are required, a
separate methane refrigeration system is also employed. Thus three
separate refrigeration systems are required, cascading from lowest
temperature to highest. Three compressor and driver systems
complete with suction drums, separate exchangers, piping, etc. are
required. Also, a methane refrigeration cycle often requires
reciprocating compressors which can partially offset any capital
cost savings resulting from the use of low pressure
demethanizers.
[0003] Mixed refrigerant systems have been well known in the
industry for many decades. In these systems, multiple refrigerants
are utilized in a single refrigeration system to provide
refrigeration covering a wider range of temperatures, enabling one
mixed refrigeration system to replace multiple pure component
cascade refrigeration systems. These mixed refrigeration systems
have found widespread use in base load liquid natural gas plants.
The application of a binary mixed refrigeration system to ethylene
plant design is disclosed in U.S. Pat. No. 5,979,177 in which the
refrigerant is a mixture of methane and either ethylene or ethane.
However, such a binary refrigeration system cascades against a
separate propylene refrigeration system to provide the
refrigeration in the temperature range of -40.degree. C. and
warmer. Therefore, two separate refrigeration systems are
required.
SUMMARY OF THE INVENTION
[0004] It is an object of the present invention, therefore, to
provide a simplified, single refrigeration system for an ethylene
plant having a low pressure demethanizer utilizing a mixture of
methane, ethylene and propylene, as a tertiary refrigerant. This
system replaces the separate methane, ethylene and propylene
refrigeration systems or the binary methane/ethylene systems which
are used in conjunction with a propylene refrigeration system in
conventional plants and thereby reduces the number of compressor
systems. The invention involves the processing of the constant
composition tertiary refrigerant from the single compressor
including the separation of the refrigerant compressor effluent
into a methane-rich fraction and a propylene-rich fraction so as to
provide various temperatures and levels of refrigeration in various
heat exchange stages while maintaining the constant refrigerant
composition flowing back to the compressor. The objects,
arrangement and advantages of the refrigeration system of the
present invention will be apparent from the description which
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The drawing is a schematic flow diagram of a portion of an
ethylene plant illustrating the refrigeration system of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0006] The present invention involves an ethylene plant wherein a
pyrolysis gas is first processed to remove methane and hydrogen and
then processed in a known manner to produce and separate ethylene
as well as propylene and some other by-products. The separation of
the gases in an ethylene plant through condensation and
fractionation at cryogenic temperatures requires refrigeration over
a wide temperature range. The capital cost involved in the
refrigeration system of an ethylene plant can be a significant part
of the overall plant cost. Therefore, capital savings for the
refrigeration system will significantly affect the overall plant
cost.
[0007] Ethylene plants with high pressure demethanizers operate at
pressures higher than 2.758 MPa (400 psi) and can produce overhead
reflux by condensation against a pure component ethylene
refrigeration. Demethanizer overhead temperatures of these systems
are typically in the range of -85.degree. C. to -100.degree. C.
Ethylene refrigeration at approximately -101.degree. C. is
typically used for chilling the overhead condenser. At pressures
below 2.758 MPa, the overhead temperature is typically too low to
use ethylene refrigeration unless a vacuum suction is used. But
that is not desirable because of the capital cost increase and the
safety concern due to potential air leakage into the system.
[0008] The present invention involves the use of a low pressure
demethanizer and a tertiary refrigerant system. For purposes of the
present invention, a low pressure demethanizer is one which
operates below about 2.41 MPa (350 psi) and generally in the range
of 0.345 to 1.034 MPa (50 to 150 psi) and with overhead
temperatures in the range of -105 to -145.degree. C. The advantage
of the low pressure demethanizer is the lower total plant power
requirement and lower total plant capital cost while the
disadvantage is the lower refrigeration temperature required and,
therefore, the need heretofore of two or three separate
refrigeration systems and compressors.
[0009] The tertiary refrigerant of the present invention comprises
a mixture of methane, ethylene and propylene. The percentage of
these components can vary depending on the ethylene plant cracking
feedstock, the cracking severity and the chilling train pressure
among other considerations, but will generally be in the range of 8
to 20 percent methane, 10 to 30 percent ethylene and 50 to 82
percent propylene. A typical composition would be 10% methane, 15%
ethylene and 75% propylene. The use of the tertiary refrigerant
provides the refrigeration load and temperatures required for an
ethylene plant having a low pressure demethanizer while obviating
the need for two or three separate refrigerant systems. The
tertiary refrigerant of this invention can also be used with a high
pressure demethanizer. In that case, the tertiary system can be
designed to provide ethylene and propylene levels of refrigeration.
The methane content in the refrigerant would then be 5 to 12%.
[0010] The purpose of the present invention is to provide the
necessary refrigeration for the pyrolysis charge gas to separate
out the hydrogen and methane and provide the feed for the
demethanizer. The tertiary refrigeration system of the invention
reduces the capital cost for refrigeration and provides operational
stability. Referring to the drawing, the charge gas feed 12, which
is the pyrolysis gas conditioned as required and cooled, is
typically at a temperature of about -35 to -37.degree. C. and a
pressure of about 3.45 MPa (500 psi), and is typically already
partially liquified. The charge gas contains hydrogen, methane, and
C.sub.2 and heavier components including ethylene and
propylene.
[0011] The charge gas 12 is first cooled in the heat exchanger 14
to about -40.degree. C. and then sent to the stripper 16. A methane
free bottom stream 18 is produced which is sent directly to a
deethanizer (not shown). This substantially reduces the ethylene to
be recovered in the deep chilling sections of the refrigeration
system and in the demethanizer. The stripper reboiler is shown at
20 where it provides a portion of the condensing duty for the
compressed tertiary refrigerant as will be discussed later.
[0012] The vapor 22 from the stripper 16 is further cooled
sequentially in the heat exchanges 24, 26 and 28 with intermediate
separations at 30 and 32 and another separation at 34. The
separated liquids from the separation drums 30, 32 and 34
respectively form the lower feed 36, the middle feed 38 and the top
feed 40 to the demethanizer 42. The vapor 44 from the separation
drum 34, which is normally at about -130.degree. C., feeds the
Joules-Thomson heat exchanger 46 and separator 48 where a high
purity (about 95%) hydrogen stream 50 and a low pressure methane
stream 52 are produced. The demethanizer 42 recovers the ethylene
and heavier components in the bottom stream 54. The overhead 56,
which is at a temperature of about -135.degree. C., is partially
condensed in heat exchanger 28 and separated in drum 58 to generate
the required reflux 60 for the demethanizer 42. The refrigeration
value of the remaining vapor distillate 62 is recovered in the heat
exchangers 26, 24 and 14 and is then sent as high pressure methane,
normally as regeneration gas in the ethylene plant.
[0013] The supply of the refrigeration in the present invention is
from a single tertiary refrigeration system. The system is a closed
loop with constant refrigerant composition through each stage of
the compressor 64. The compressor effluent 66 is at a pressure in
the range of 3.0 Mpa to 5.0 Mpa and has a fixed ratio of methane,
ethylene and propylene with this ratio being selected on the basis
of the loading profile of the required refrigeration levels. The
compressor effluent 66 is partially condensed at 68 by cooling
water and further partially condensed in the reboiler 20 of the
stripper 16. The partially condensed tertiary refrigerant is sent
to the separation drum 70. The temperature in the separation drum
70 is normally in the range of 5 to 20.degree. C. depending on the
desired split of the vapor 72 and the liquid 74 to generate the
desired lighter and heavier refrigerants respectively. The
proportions of vapor and liquid can be varied by changing the level
of cooling at 68 and 20.
[0014] The tertiary refrigerant vapor 72 is rich in methane and
usually contains a portion of the ethylene. After further
condensing and subcooling in the heat exchangers 14, 24, 26 and 28,
it is flashed through the letdown valve 76 forming the stream 78
which is at a pressure approaching that of the compressor suction.
This stream 78 supplies the levels of refrigeration equivalent to
methane and ethylene refrigeration ranging from -150.degree. C. to
-45.degree. C. in the heat exchangers 28, 26 and 24. The liquid 74
from the separator 70 is rich in propylene and usually contains the
other portion of the ethylene. This supplies the ethylene and
propylene refrigerant levels warmer than -102.degree. C. This
liquid 74 is subcooled in the heat exchangers 14, 24 and 26 and
flashed through the letdown valve 80 to form the refrigerant stream
82 also at a pressure approaching that of the compressor suction.
This refrigerant stream 82 provides refrigeration to the heat
exchangers 26, 24 and 14 by the vaporization of the mixed
refrigerant at different temperatures. To supplement the
condensation and subcooling duty for the lighter refrigerant 72, a
slip-stream 84 of the subcooled liquid 74 at the exit from the heat
exchanger 14 is flashed through letdown valve 86 and passed as
stream 88 back through the heat exchanger 14 where it is
superheated.
[0015] The variables which can be used to control the process
chilling duties include the adjustment of the temperature in the
separation drum 70, the adjustment of the overall refrigerant
composition and adjustment of the compressor operating conditions.
The system design is responsive to be able to achieve the total
recovery of the ethylene which is mainly affected by the drum 34
temperature (normally about -130.degree. C.) and the ability to
generate the reflux 60 in the drum 58. Small variations in the
temperatures in drum 30 and 32 can also be used as control
features.
[0016] The superheated refrigerant vapor 88 is mixed with the
refrigerant streams 78 and 82 in the suction drum 90 before
entering the compressor 64 through line 92. Since all three
refrigerant streams 78, 82 and 88 are re-mixed in the suction drum
90, the composition of the tertiary refrigerant at the compressor
suction is identical to the composition at the compressor
discharge. Maintaining a constant composition of the refrigerant
through the compressor is similar to the operation of a
conventional single component refrigeration compressor. Therefore,
the stability and operating flexibility of the compressor is
enhanced.
[0017] Another important issue addressed by the present invention
and relating to the operating stability of the compressor is the
degree of superheating of the refrigerant at the compressor
suction. Higher superheating results in more compression power
while providing the capability of avoiding any phase separation in
the suction drum 90. A phase separation would have an immediate
impact on the refrigerant composition. Therefore, a reasonable
degree of superheating above the refrigerant dew point (normally 5
to 30.degree. C.) will not only provide operating stability during
load variations and process upsets, but also simplify the design of
the total system and provide investment cost savings. To conserve
energy and maintain the operating feasibility of the compressor, it
is normally provided with one or more intercoolers 94 to reduce the
interstage temperatures.
[0018] The closed loop tertiary refrigeration system of the present
invention provides a versatile system in which various refrigerant
compositions can be formed while maintaining a single, constant
refrigerant composition in each and every stage of the refrigerant
compressor. This provides precise temperature control in an
efficient and economical manner. There is a reduction in the number
of compressor systems needed and there is the ability to use all
centrifugal or axial compressors instead of a methane reciprocating
compressor.
* * * * *