U.S. patent application number 13/057368 was filed with the patent office on 2011-06-23 for method of cooling using extended binary refrigeration system.
This patent application is currently assigned to LUMMUS TECHNOLOGY INC.. Invention is credited to Charles Sumner.
Application Number | 20110146342 13/057368 |
Document ID | / |
Family ID | 41664118 |
Filed Date | 2011-06-23 |
United States Patent
Application |
20110146342 |
Kind Code |
A1 |
Sumner; Charles |
June 23, 2011 |
METHOD OF COOLING USING EXTENDED BINARY REFRIGERATION SYSTEM
Abstract
A method of cooling using an extended binary refrigerant system
containing methane and a C3 hydrocarbon such as propylene and/or
propane is disclosed. The extended binary refrigerant from a
compressor final discharge is separated into a methane-rich vapor
fraction and at least one C3 rich liquid fraction so as to provide
various temperatures and levels of refrigeration in various heat
exchange stages. The method and corresponding refrigeration system
can be utilized in plants utilizing low pressure or high pressure
demethanizers.
Inventors: |
Sumner; Charles; (Tucson,
AZ) |
Assignee: |
LUMMUS TECHNOLOGY INC.
Bloomfield
NJ
|
Family ID: |
41664118 |
Appl. No.: |
13/057368 |
Filed: |
August 6, 2008 |
PCT Filed: |
August 6, 2008 |
PCT NO: |
PCT/US08/09418 |
371 Date: |
March 10, 2011 |
Current U.S.
Class: |
62/619 ;
62/117 |
Current CPC
Class: |
C09K 5/042 20130101;
F25J 2270/18 20130101; C09K 2205/12 20130101; F25J 3/0219 20130101;
F25J 3/0238 20130101; F25J 3/0233 20130101; F25J 2270/66 20130101;
F25J 3/0252 20130101; F25J 2270/902 20130101; F25J 2270/12
20130101; F25J 3/061 20130101; F25J 2210/12 20130101; F25J 2205/04
20130101; F25J 2215/62 20130101 |
Class at
Publication: |
62/619 ;
62/117 |
International
Class: |
F25J 3/00 20060101
F25J003/00; F25B 1/00 20060101 F25B001/00 |
Claims
1. A method comprising cooling a feed gas using an extended binary
refrigerant containing a mixture of methane and a C3 hydrocarbon
selected from the group consisting of propylene and propane, said
extended binary refrigerant being compressed in a multistage
compressor having a first stage and a last stage with a last stage
discharge containing at least 60 mol % C3 hydrocarbon, and being
divided into a liquid coolant stream and a gaseous coolant stream
after compression.
2. A method according to claim 1, wherein said C3 hydrocarbon is
propylene.
3. A method according to claim 2, wherein at least 90 wt % of said
C3 hydrocarbon is propylene.
4. A method according to claim 1, wherein said last stage discharge
contains 10-40 mol % methane and 60-90 mol % C3 hydrocarbon.
5. A method according to claim 2, wherein said last stage discharge
contains 15-25 mol % methane and 75-85 mol % propylene.
6. A method according to claim 2, wherein said extended binary
refrigerant contains 18-22 mol % methane and 78-82 mol %
propylene.
7. A method according to claim 1, wherein the extended binary
refrigerant can provide refrigeration within the temperature range
of about -136 Deg. C. to about 25 Deg. C.
8. A method according to claim 1, wherein said feed gas contains an
olefin and the method comprises removing said olefin from said
cooled feed gas by fractionation.
9. A method according to claim 8, wherein said olefin is
ethylene.
10. A method according to claim 8, wherein said feed gas further
includes hydrogen and methane.
11. A method according to claim 8, wherein said last stage
discharge contains 10-40 mol % methane and 60-90 mol % C3
hydrocarbon.
12. A method for cooling with an extended binary refrigerant
comprising: (a) compressing a mixture comprising 10-40 mol %
methane and 60-90 mol % of at least one C3 hydrocarbon selected
from the group consisting of propylene and propane in a multistage
compressor to produce an extended binary refrigerant vapor, (b)
cooling said extended binary refrigerant vapor to condense a
portion thereof and provide a vaporous methane-rich binary
refrigerant stream and a liquid C3 hydrocarbon-rich binary
refrigerant stream, (c) employing said methane-rich binary
refrigerant stream and said C3 hydrocarbon-rich binary refrigerant
stream to effect cooling, and (d) returning said methane-rich
binary refrigerant stream and said C3 hydrocarbon-rich binary
refrigerant stream to said multistage compressor.
13. A method according to claim 12, wherein said methane-rich
binary refrigerant stream and said C3 hydrocarbon-rich binary
refrigerant stream cool a charge gas comprising hydrogen, methane
and ethylene.
14. A method according to claim 13, wherein said charge gas is used
to produce ethylene.
15. A method according to claim 12, wherein the extended binary
refrigerant can provide refrigeration within the temperature range
of about -136 Deg. C. to about 25 Deg. C.
16. In a method for the production of olefins from a charge gas
containing hydrogen, methane and ethylene wherein said charge gas
is cooled by a refrigeration system having a series of heat
exchangers, a method for cooling said charge gas comprising: (a)
compressing an extended binary refrigerant vapor containing methane
and a C3 hydrocarbon selected from the group consisting of
propylene and propane, in a multistage compressor having a first
stage and a last stage with a last stage discharge; (b) cooling to
condense a portion of said extended binary refrigerant vapor from
said last stage discharge to form a remaining extended binary
refrigerant vapor and a heavy refrigerant stream having a greater
percentage of propylene than said selected mixture; (c) separating
said heavy refrigerant stream from said remaining extended binary
refrigerant vapor in a separator; (d) cooling to condense at least
a portion of said remaining extended binary refrigerant vapor from
said separator to form a light refrigerant stream; (e) bringing
said heavy refrigerant stream and said light refrigerant stream
into heat exchange contact with themselves and each other and with
said charge gas in said series of heat exchangers whereby said
charge gas is cooled, said heavy refrigerant stream is cooled and
then heated and vaporized, and said light refrigerant stream is
first cooled and at least partially condensed and then vaporized;
and (f) returning said light refrigerant stream and said heavy
refrigerant stream to said compressor.
17. A method according to claim 16, wherein said cooling to
condense at least a portion of said binary refrigerant vapor
comprises cooling with cooling water.
18. A method according to claim 16, wherein a portion of said heavy
refrigerant stream obtained in (c) is used for cooling in (b).
19. A method according to claim 16, wherein said light refrigerant
stream is partially condensed by said heavy refrigerant stream and
is further condensed through self-refrigeration.
20. A method according to claim 16, wherein the compressor further
includes an intermediate stage.
21. A method according to claim 20, further including using a
portion of said heavy refrigerant stream for cooling at least one
of an ethylene fractionator reboiler, a deethanizer condenser and a
depropanizer condenser.
22. A method according to claim 16, wherein the content of
propylene in said last stage discharge is greater than 50 mol
%.
23. A method according to claim 16, wherein the content of
propylene in said liquid refrigerant stream is greater than 80 mol
%.
24. A method according to claim 16, wherein the extended binary
refrigerant can provide refrigeration within the temperature range
of about -136 Deg. C. to about 25 Deg. C.
25. A method according to claim 16, wherein said method for cooling
is utilized to increase the total refrigeration capacity of an
existing olefins plant employing a cascade refrigeration system
having separate propylene and ethylene refrigeration systems.
26. A method according to claim 16, further including using the
liquid stream formed in (f) to provide additional refrigeration
duty.
27. A method according to claim 16, wherein the light refrigerant
vapor, at low pressure, is injected into the liquid refrigerant
stream entering an ethylene fractionator condenser downstream of a
valve regulating flow of said liquid refrigerant stream into said
ethylene fractionator condenser.
28. A method according to claim 16, wherein the direct
refrigeration duty of said extended binary refrigerant is decreased
by heat exchange between an ethane recycle to a cracking heater
produced at the bottom of said fractionating tower, after pressure
letdown, and a gross overhead vapor flowing from the top of said
fractionating tower.
29. A method according to claim 16, where the direct refrigeration
duty of said extended binary refrigerant is decreased by heat
exchange between an ethane recycle to a cracking heater that is
produced at the bottom of said fractionating tower, after pressure
letdown, and said refrigerant flowing to the ethylene fractionator
condenser.
30. A method according to claim 16, wherein the first light
hydrocarbon separation tower is a depropanizer comprising two
towers at different pressures, and the higher pressure tower
overhead vapor is at least partially condensed in an interreboiler
to provide reflux for the higher pressure tower.
31. A method according to claim 16, further comprising the step of
deethanizing said charge gas wherein no deethanizer condenser is
provided and reflux for said deethanizer is withdrawn proximate a
feed tray where a deethanizer gross overhead enters said
fractionating tower.
32. A method according to claim 16, wherein said extended binary
refrigerant vapor leaving said compressor comprises no more than 1
mol % hydrogen.
33. A method according to claim 16, wherein said extended binary
refrigerant vapor leaving said compressor has no more than 10 mol %
propane.
34. A method according to claim 20, where said extended binary
refrigerant is configured to increase the total refrigeration
capacity of an existing olefins plant by replacing a cascading
refrigeration system having separate propylene and ethylene
refrigeration systems.
35. An extended binary refrigerant comprising a mixture of methane
and a C3 hydrocarbon selected from the group consisting of
propylene and propane, the extended binary refrigerant containing
10-40 mol % methane, 60-90 mol % of at least one C3 hydrocarbon
selected from the group consisting of propylene and propane, and
not more than about 2 mol % hydrogen, the total mol % of methane,
C3 hydrocarbon and hydrogen being 100%.
36. The extended binary refrigerant of claim 35, wherein the C3
hydrocarbon is propylene.
37. The extended binary refrigerant of claim 35, wherein the
refrigerant can provide refrigeration within the temperature range
of about -136 Deg. C. to about 25 Deg. C.
38. The extended binary refrigerant of claim 35, wherein said
refrigerant contains 15-25 mol % methane and 75-85 mol % propylene.
Description
BACKGROUND
[0001] The embodiments disclosed herein pertain to refrigeration
systems and refrigeration methods.
[0002] Ethylene plants require refrigeration to separate out
desired products from the cracking heater effluent. Typically,
separate propylene and ethylene refrigeration systems 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
used.
[0003] Mixed refrigerant systems are known. 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 systems are
characterized by mixtures of components with sequential carbon
numbers. These mixed refrigeration systems have found widespread
use in base load liquid natural gas plants.
[0004] 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 (carbon number 1) and
either ethylene or ethane (carbon number 2). A binary refrigeration
system usually operates with a fixed composition at given
temperature and pressure conditions. However, such a binary
refrigeration system is limited in the temperature range over which
the system can be operated and therefore it must be cascaded
against a separate propylene refrigeration system that provides the
refrigeration in the temperature range of -40.degree. C. and
warmer. Therefore, two separate refrigeration systems are
required.
[0005] U.S. Pat. No. 6,637,237 teaches the use of a single
refrigeration system for all of the refrigeration requirements of
an ethylene plant. The refrigerant used is a mixture of methane,
ethylene and propylene (carbon numbers 1, 2 and 3). The system is
capable of supplying refrigeration at temperature levels from below
-140 C to near ambient temperature level.
[0006] U.S. Pat. No. 6,705,113 teaches the use of a single
refrigeration system, again with three components, utilizing a
different process configuration than U.S. Pat. No. 6,637,237.
However, both U.S. Pat. No. 6,637,237 and U.S. Pat. No. 6,705,113
require three components to supply the required refrigeration
duties at the required temperature levels. Using three components
means that the composition at any given system temperature and
pressure does not correspond to a unique composition but rather to
a range of compositions. Thus, the composition of the refrigerant
mixture at any given point in the system can vary. While this
variation is small and does not appreciably detract from the
performance of the refrigeration system, it may be preferable to
operate a system that has no refrigerant composition variation at a
given temperature and pressure to more easily adjust to changes in
ethylene plant operating conditions.
SUMMARY
[0007] It is an object of the embodiments disclosed herein to
provide a simplified, single refrigeration system utilizing a
mixture of methane with a C3 hydrocarbon as a binary refrigerant.
As this binary system is not a continuous mixture of carbon number
hydrocarbons but rather has only C1 and C3 hydrocarbon components,
this system is referred to as an extended binary refrigerant. This
extended binary system can be used to replace the separate
propylene, ethylene and methane refrigeration systems associated
with a recovery process using a low pressure demethanizer, or the
binary or tertiary refrigeration systems utilizing hydrocarbons
with consecutive carbon numbers. One use for the system is in an
olefins plant.
[0008] One embodiment is a method comprising cooling a feed gas
using an extended binary refrigerant containing a mixture of
methane and a C3 hydrocarbon selected from the group consisting of
propylene and propane, the extended binary refrigerant being
compressed in a multistage compressor having a first stage and a
last stage with a last stage discharge containing at least 60 mol %
of the C3 hydrocarbon, and being divided into a liquid coolant
stream and a gaseous coolant stream after compression.
[0009] The C3 hydrocarbon preferably is propylene. The last stage
discharge typically contains at least 70 mol % propylene and
sometimes at least 75 mol % propylene. In some cases, the last
stage discharge contains 10-40 mol % methane and 60-90 mol % C3
hydrocarbon.
[0010] Often, at least 90 wt % of the C3 hydrocarbon is propylene
with the remainder being propane. Typically, the extended binary
refrigerant contains 15-25 mol % methane and 75-85 mol % propylene,
and sometimes 18-22 mol % methane and 78-82 mol % propylene. In
many cases, the feed gas contains an olefin and the method
comprises removing the olefin from the cooled feed gas by
fractionation.
[0011] A further embodiment is a method for cooling with an
extended binary refrigerant comprising: (a) compressing a mixture
comprising 10-40 mol % methane and 60-90 mol % of at least one C3
hydrocarbon selected from the group consisting of propylene and
propane in a compressor to produce an extended binary refrigerant
vapor, (b) cooling the extended binary refrigerant vapor to
condense a portion thereof and provide a vaporous methane-rich
binary refrigerant stream and a liquid C3 hydrocarbon-rich binary
refrigerant stream, (c) employing the methane-rich binary
refrigerant stream and the C3 hydrocarbon-rich binary refrigerant
stream to effect cooling, and (d) returning the methane-rich binary
refrigerant stream and the C3 hydrocarbon-rich binary refrigerant
stream to the compressor.
[0012] Sometimes, in (c), the methane-rich binary refrigerant
stream and the C3 hydrocarbon-rich binary refrigerant stream cool a
charge gas comprising hydrogen, methane and ethylene. The charge
gas typically is used to produce ethylene.
[0013] Yet another embodiment is a method to be used in the
production of olefins from a charge gas containing hydrogen,
methane, ethylene and optionally including other C2 and heavier
hydrocarbons, wherein the charge gas is cooled by a refrigeration
system having a series of heat exchangers. The charge gas is cooled
in a method comprising: (a) compressing an extended binary
refrigerant vapor comprising a mixture of methane and a C3
hydrocarbon selected from the group consisting of propylene and
propane in a multistage compressor having a first stage and a last
stage with a last stage discharge; (b) cooling to condense a
portion of the extended binary refrigerant vapor from the last
stage discharge to form a remaining extended binary refrigerant
vapor and a heavy refrigerant stream having a greater percentage of
propylene than the selected mixture; (c) separating the heavy
refrigerant stream from the remaining extended binary refrigerant
vapor in a separator; (d) cooling to condense at least a portion of
the remaining extended binary refrigerant vapor from the separator
to form a light refrigerant stream; (e) bringing the heavy
refrigerant stream and the light refrigerant stream into heat
exchange contact with themselves and each other and with the charge
gas in the series of heat exchangers whereby the charge gas is
cooled, the heavy refrigerant stream is cooled and then heated and
vaporized, and the light refrigerant stream is first cooled and at
least partially condensed and then vaporized; and (f) returning the
light refrigerant stream and the heavy refrigerant stream to the
compressor.
[0014] A portion of the heavy refrigerant stream obtained in (c)
preferably is used for cooling in (b). The light refrigerant stream
can be partially condensed by the heavy refrigerant stream and
further condensed through self-refrigeration. In some cases, the
compressor includes an intermediate stage. The method sometimes
further includes using the heavy refrigerant stream for cooling at
least one of an ethylene fractionator, a deethanizer condenser and
a depropanizer condenser.
[0015] The composition of propylene in the last compressor stage
discharge often is greater than 50 mol percent. The composition of
propylene in the heavy liquid refrigerant stream sometimes is
greater than 80 mol percent. In some cases, the method for cooling
is utilized to increase the total refrigeration capacity of an
existing olefins plant employing a cascade refrigeration system
having separate propylene and ethylene refrigeration systems.
[0016] In some cases, the method includes using the light liquid
stream to provide additional refrigeration duty. In certain cases,
the light refrigerant vapor, at low pressure, is injected into the
liquid refrigerant stream entering an ethylene fractionator
condenser, downstream of a valve regulating flow of the liquid
refrigerant stream into the ethylene fractionator condenser.
Optionally, the direct refrigeration duty of the extended binary
refrigerant is decreased by heat exchange between an ethane recycle
to a cracking heater that is produced at the bottom of the
fractionating tower, after pressure letdown, and a gross overhead
vapor flowing from the top of the fractionating tower. In another
embodiment, the direct refrigeration duty of the extended binary
refrigerant is decreased by heat exchange between an ethane recycle
to a cracking heater that is produced at the bottom of the
fractionating tower, after pressure letdown, and the refrigerant
flowing to the ethylene fractionator condenser.
[0017] In some cases, the first light hydrocarbon separation tower
is a depropanizer, and the depropanization is split into two
towers, with one at a higher pressure. Sometimes the higher
pressure tower overhead vapor is at least partially condensed in an
interreboiler to provide reflux for the high pressure tower.
[0018] In one embodiment, the method further comprises the step of
deethanizing the charge gas, wherein no deethanizer condenser is
provided and reflux for the deethanizer is withdrawn proximate a
feed tray where a deethanizer gross overhead enters the
fractionating tower. The charge gas typically is derived from
refinery offgases, the catalytic hydrogenation of propane, or steam
cracking of hydrocarbons.
[0019] A further embodiment is an extended binary refrigerant
comprising a mixture of methane and a C3 hydrocarbon selected from
the group consisting of propylene and propane, the extended binary
refrigerant containing 10-40 mol % methane, 60-90 mol % of at least
one C3 hydrocarbon selected from the group consisting of propylene
and propane. The refrigerant contains no more than about 2 mol %
hydrogen, with the total mol % of methane, C3 hydrocarbon and
hydrogen being 100%. Usually, the extended binary refrigerant can
provide refrigeration within the temperature range of about -136
Deg. C. to about 25 Deg. C.
[0020] The objects, arrangement and advantages of the refrigeration
system and methods will be apparent from the description which
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic flow diagram of a first embodiment of
a refrigeration system that is particularly applicable when
additional refrigeration capability is required due to an expansion
of olefin plant production capacity.
[0022] FIG. 2 is a schematic flow diagram of a general application
of the extended binary refrigeration concept that is applicable to
a new olefins plant design or to a major expansion of an olefins
plant requiring a significant refrigeration supply increase.
[0023] FIG. 3 is a schematic flow diagram, which shows some of the
heat exchange concepts utilized in the embodiment of FIG. 2.
[0024] FIG. 4 is a schematic flow diagram, which illustrates
additional heat exchange concepts, which can be used as an
alternative to certain concepts shown in FIGS. 2 and 3.
[0025] FIG. 5 depicts another embodiment that is particularly
applicable when additional refrigeration capability is required due
to an expansion of olefin plant production.
[0026] FIG. 6 is a schematic flow diagram showing another
embodiment of an extended binary refrigeration system which can be
used in a new olefins plant or a major expansion of an existing
olefins plant.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The disclosed embodiments involve the separation of the
extended binary refrigerant from the discharge of the final stage
of a compressor into at least one vapor enriched in methane content
and at least one liquid enriched in propylene concentration so as
to provide varying temperatures and levels of refrigeration in
various heat exchange stages. It can be advantageous to cool and
partially condense the vapor separated at the compressor discharge
to provide a second vapor further enriched in methane
concentration. The extended binary system not only can be used in
an ethylene plant employing a low pressure demethanizer, but also
can be applied to an ethylene plant with a high pressure
demethanizer, in which case the binary system would typically only
supply refrigeration at propylene and ethylene temperature levels.
That is, the system would supply refrigeration temperature levels
from just below ambient to approximately -100 C with no
refrigeration directly supplied at lower temperatures by the
refrigeration system. The extended binary refrigeration system also
can be used to supply refrigeration to recover ethylene from
refinery offgases. Furthermore, the system can be used to supply
refrigeration to recover propylene and byproduct ethylene produced
by catalytic dehydrogenation of propane to form mostly propylene
product. The system is applicable to any process where the range of
temperatures for refrigeration duties is from ambient to about -136
Deg. C.
[0028] The method will be described in connection with an olefins
plant which is primarily for the production of ethylene and
propylene. In some cases, a pyrolysis gas is first processed to
remove methane and hydrogen and then further processed to produce
and separate selected olefins. 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 can
significantly reduce the overall plant cost.
[0029] As used herein, "extended binary refrigerant" refers to a
refrigerant system that includes two hydrocarbons with carbon
numbers that differ by at least 2 and does not also include the
intermediate hydrocarbon. For example, an extended binary
refrigerant containing methane (a C1 hydrocarbon) and propylene
and/or propane (C3 hydrocarbons) does not contain ethylene or
ethane (C2 hydrocarbons).
[0030] Ethylene plants with high pressure demethanizers operate at
pressures higher than 2.76 MPa (400 psi) with an overhead
temperature typically in the range of -85.degree. C. to
-100.degree. C. Ethylene refrigeration at approximately
-100.degree. C. to -102.degree. C. is typically used to chill the
gross demethanizer overhead vapors to produce reflux. An ethylene
plant designed with a low pressure demethanizer 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 -110.degree. C. to -140.degree. C. requires methane
temperature levels of refrigeration to generate reflux. The
advantages of the low pressure demethanizer are the lower total
plant power requirement and the lower total plant capital cost
while the disadvantage is the lower refrigeration temperature
required and, therefore, the need for a methane refrigeration
system in addition to the ethylene and propylene refrigeration
systems. The separation of C3's requires temperatures ranging from
ambient to -30 C. This represents the upper range of the extended
binary temperature range.
[0031] The extended binary refrigerant comprises a mixture of
methane with propylene and/or propane. Depending upon the source of
the methane charge and makeup, a small quantity of hydrogen can
also be present. The concentration of methane and propylene 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 10 to 30 mol
percent methane and 70 to 90 mol percent propylene as measured at
the compressor discharge. A typical composition for an olefins
plant with a low pressure demethanizer would be about 20 mol %
methane and about 80 mol % propylene. The use of the single
extended binary refrigerant provides all the refrigeration loads
and temperatures required for an ethylene plant while obviating the
need for two or three separate refrigerant systems. The refrigerant
usually circulates in a closed loop system.
[0032] The purpose of the disclosed embodiments is to provide the
necessary refrigeration to separate hydrogen and methane from the
charge gas and provide the feed for the demethanizer. Optionally,
the methods provide some or all of the other refrigeration
requirements of the entire plant.
[0033] Before describing the extended binary refrigeration system,
the flow of the charge gas through the system will be described
with examples of specific temperatures being included for purposes
of illustration only.
[0034] FIG. 1 depicts an enhanced binary refrigeration system
design useful for adding incremental refrigeration capability to an
existing olefins production plant that is being increased in
capacity. The chilling train feed 16 or charge gas, which is the
pyrolysis charge gas conditioned as required and cooled, is
typically at a temperature of about 15.degree. C. to 20.degree. C.
and a pressure of about 2 to 3.5 MPa (approximately 280 to 500
psi), and is typically a vapor stream. In designs that employ both
vapor and liquid hydrocarbon dryers, the feed to the chilling train
is a combination vapor-liquid mixture. This stream contains
hydrogen, methane, and C.sub.2 and heavier components including
ethylene and propylene. The chilling train feed 16 is progressively
cooled by the refrigeration system in the heat exchangers 18, 19,
20 and 21 with appropriate separations being made to produce
demethanizer feeds.
[0035] In addition to the chilling train feed 16 and the binary
refrigerant streams, there are various ethylene plant streams at
various temperatures which also pass through the heat exchangers
for recuperation of cold, such as stream 13.
[0036] The system shown in FIG. 1 utilizes two stages of
compression to produce refrigeration from ambient temperature down
to about -136 deg. C. This design is complementary to an existing
refrigeration system, either a classical cascade of propylene and
ethylene refrigeration, a classical cascade of propylene, ethylene
and methane refrigeration systems, or a mixed refrigeration system.
The design as depicted provides only refrigeration for charge
chilling and demethanization and does not provide for fractionation
refrigeration for C2 and C3 separations.
[0037] The chilling train feed 16, is chilled in exchangers 18 and
19, and partially condensed and then separated into a vapor stream
22 and a liquid stream 24 in a separator 26. Liquid stream 24 flows
to the demethanizer. Vapor stream 22 flows to heat exchanger 20
where it is cooled and partially condensed and then separated into
a vapor stream 28 and a liquid stream 30 in a separator 32. Liquid
stream 30 then flows to the demethanizer as the intermediate
temperature liquid stream. Vapor stream 28 then flows to heat
exchanger 21 where it is cooled and partially condensed and
separated into a vapor stream 34 and liquid stream 36 in separator
38. The liquid stream 36 is the coldest demethanizer feed. Vapor
stream 34 is further chilled (not shown on FIG. 1) and then
separated into a hydrogen rich stream 40 and a methane rich stream
42 in a separator 44 and then returned to the binary refrigeration
system heat exchanger network of exchangers 21, 20, 19, 18 and 17,
respectively, for recovery of cold.
[0038] The extended binary compressor discharge stream 1, leaving
the extended binary refrigeration compressor 50, usually contains
at least 50 mol % C3 hydrocarbon, with the remainder being methane.
In some cases, stream 1 contains 60-90 mol % propylene (or propane)
and 10-40 mol % methane composition. As the methane makeup to the
compressor system is withdrawn from the demethanizer net overhead
vapor product without further processing, some hydrogen is
contained and the methane content is reduced by a small amount, for
example about 0.5 mol %, in the compressor discharge with a 0.5 mol
% hydrogen content. Note that it is possible, and sometimes
desirable to reduce the hydrogen content, for example by controlled
venting in the refrigeration system.
[0039] Stream 1 is cooled by water or air, preferably cooling
water, in heat exchanger 52 and partially condensed and then
separated in a separator drum 54 into a vapor stream 2, and a
liquid stream 3. The vapor stream 2, enriched in methane, is
withdrawn from separator drum 54 and then cooled and eventually
totally or mostly condensed in exchangers 17 through 21 and
separated, if necessary, in separator drum 56. The condensed light
refrigerant stream 4, is then let down across a control valve 81
and optionally any uncondensed vapor 5 is cooled by expansion to a
low pressure using an optional control valve 82. Temperature
controller 91 adjusts the pressure outlet from control valve 81.
Valve 82 provides the pressure drop necessary to blend stream 5
with the discharge vapor from control valve 81. By raising or
lowering the discharge pressure from control valve 81, the
temperature entering exchanger 21 is raised or lowered in order to
provide the required cooling for stream 28 and insure coldest
liquid feed flow for the demethanizer via stream 36. In one
embodiment, the temperature of stream 6, which is a combination of
steams 4 and 5, after pressure letdown is about -136 deg. C.
[0040] Similarly, a temperature controller 92 on the vapor stream
28 leaving drum 32 is used to let down a portion of the vapor
stream 8 across a control valve 83 to be mixed into the combined
partially reheated light refrigerant stream 6 to provide chilling
to condense hydrocarbons to form the intermediate temperature
demethanizer feed stream 30.
[0041] A temperature controller 93 on the vapor stream 22 leaving
separator 26 similarly lets down some vapor stream 10 to be mixed
with the partially reheated stream 9 across a control valve 84 to
provide the chilling to form the warmest demethanizer feed, liquid
stream 24.
[0042] Streams 8 and 10 are formed by the cooling of stream 2
through interchange in exchangers 19 and 20. The composition of
these streams is unchanged as stream 2 cools to form streams 8, 10
and 15. Both the vapor and liquid from separator 56 are recombined
and become part of streams 6, 9, and 13.
[0043] The heavy liquid stream 3, leaving separator drum 54
provides some of the refrigeration for chilling of the chilling
train feed 16 in exchanger 18 and also chills and partially
condenses light refrigerant stream 2 in exchangers 17 and 18.
Stream 3 leaving separator drum 54 is chilled in heat exchanger 17
by reheat of process streams and by letting down a portion of the
chilled heavy refrigerant of stream 3 to a lower pressure as stream
11. A temperature controller 94 on the remaining portion of stream
13, i.e. stream 12, lets down stream 11 across a control valve 85.
Stream 11 is vaporized in exchanger 17, is routed to drum 77, and
then flows to the second stage suction of compressor 50. The
balance of the heavy refrigerant stream, liquid stream 12, is
further cooled in exchanger 18. It is then let down to the lowest
refrigerant pressure across control valve 86 providing additional
cooling and then mixed with light refrigerant stream 13 to form
stream 14. Light refrigerant stream 13 is the total light
refrigerant and composed of streams 6, 8 and 10. The mixing of
stream 12 with stream 13 lowers the temperature of stream 12 beyond
that achieved by reducing the pressure. Stream 14 is then reheated
in exchanger 18 and flows to first stage compression suction drum
76 and then to the compressor 50 as the first stage suction
flow.
[0044] The process shown in FIG. 1 utilizes a single compressor and
a refrigerant composed essentially of methane and propylene to
provide refrigeration from just below ambient temperature down to a
temperature of about -136 deg. C. This is beyond the range of any
binary system utilizing consecutive carbon numbers. As indicated
above, FIG. 1 depicts a system for the expansion of an olefins
plant. Only a portion of the refrigeration demands of the olefins
plant is met with this configuration.
[0045] FIG. 2 depicts an extended binary refrigeration system for a
complete olefins plant. All of the refrigeration requirements of
the process associated with an olefins plant separation system
design, whether a completely new plant or an expansion can be
supplied by this configuration. FIG. 2 describes an olefins plant
design utilizing a front end depropanizer as the first light
hydrocarbon separation distillation system. However, the extended
binary refrigeration concept can also be utilized to supply the
refrigeration requirements of an olefins plant with a demethanizer
or deethanizer as the first light hydrocarbon separation tower
system, as well as other distillation or fractionation concepts
that will separate and purify the products of a steam cracker
charging a hydrocarbon feed and producing cracked products
therefrom.
[0046] Vapor stream 101, leaving compressor 150 is cooled by an
ambient temperature cooling medium such as cooling water or air,
preferably cooling water, in a heat exchanger 152 to near ambient
temperature. The mixed vapor and liquid stream then flows to
separator drum 154 where it is separated into vapor stream 102 and
liquid stream 103. In one embodiment, vapor stream 101 has a
composition of about 10-40 mol % methane, or about 20-30 mol %
methane, and about 60-90 mol % propylene, or about 70-80 mol %
propylene. The composition depends upon the feedstock to the steam
cracking heaters and the conditions under which the hydrocarbon is
cracked, and also upon whether any additional olefin-containing
streams are flowing to the olefins plant for recovery of products.
In addition, although the refrigerant is mainly a binary mixture of
methane and propylene, it is recognized that some hydrogen can be
contained as a consequence of the source of the methane charge and
makeup and some propane can be contained as a consequence of the
source of the propylene charge and makeup to the enhanced binary
refrigeration system.
[0047] In some cases, the vapor stream 102 leaving separator 154
has a nominal methane concentration of 52-55 mol % for a typical
naphtha cracker at 0.50 to 0.55 weight propylene to ethylene ratios
leaving the cracking heater. For lighter hydrocarbon feedstocks and
higher cracking heater cracking severities, methane concentration
can increase up to 65 mol %. For heavier feedstocks cracked at
lower severities methane concentration sometimes can decrease to 45
mol %.
[0048] The liquid stream 103 leaving separator 154 sometimes has a
nominal propylene concentration of 80 to 90 mol %. With varying
process conditions, propylene concentration can be as low as 70-75
mol % and as high as 92 mol %.
[0049] A portion of liquid stream 103, stream 104, is chilled in
heat exchanger 117, typically of a core or brazed aluminum
exchanger design. Chilling is provided by process reheats as
available including process stream 106 and by letting down a
portion of the chilled stream 104 as stream 111, across control
valve 181, with subsequent reduction in pressure and temperature,
and using this stream to provide chilling for streams 102 and 104
and for chilling other process loads, as required, including stream
123. This vaporized stream 111 then flows to a compressor second
stage discharge drum 177.
[0050] The balance of stream 104, after withdrawal of stream 111,
is stream 112 and this stream is chilled in exchanger 118. Exiting
exchanger 118, the chilled stream 119 is let down through valve
182, resulting in a reduction in pressure and temperature and then
flows back to exchanger 118 to be vaporized and reheated before
flowing to a compressor second stage suction drum 176. Chilling is
also provided by process reheats as available, such as stream 125.
The refrigeration provided is utilized to chill process streams, as
required, such as stream 127, and to chill stream 102.
[0051] Vapor stream 102 having been chilled and partially condensed
in exchangers 117 and 118 is further chilled and totally or mostly
condensed by consecutive chilling in exchangers 119, 120, and 121.
The chilling in heat exchanger 119 is provided by process reheats
as available, such as in stream 129, and by letting down a first
portion of chilled stream 102, as stream 110, across valve 183,
with pressure reduction to near the suction pressure of the
extended binary refrigeration compressor 150. The remaining portion
of stream 102 is stream 107, and after passing through heat
exchanger 120, a portion of this stream is let down as stream 108
across control valve 184. The remaining portion of stream 107 is
stream 115, and this stream is chilled in heat exchanger 121.
Stream 115 after exiting exchanger 121 is let down across valve
185, reducing both pressure and temperature. This stream now
represents the coldest temperature of the system. As with the
system in FIG. 1, it is used to generate the coldest demethanizer
feed by cooling stream 128 to form liquid stream 136. This
partially reheated stream 115 is then combined with stream 108 to
form stream 109. Not shown in FIG. 2 are the temperature
controllers on steams 122, 128 and 134 that adjust the letdown
valves 183, 184 and 185 to insure the formation of demethanizer
feeds 130 and 124.
[0052] Stream 109 is partially reheated in heat exchanger 120 using
process reheats, as available, including stream 133, and combined
with stream 110, with the combined stream 113 being reheated in
heat exchanger 119. The refrigeration provided is utilized to cool
and partially condense the chilling train feed 116 which then exits
exchanger 119 and flows to separator drum 126. A liquid stream 124
is separated and flows to the demethanizer. The vapor stream 122
leaving separator 126 flows to heat exchanger 120 where it is
chilled and partially condensed and then flows to separator drum
132. In drum 132, stream 122 is separated into a liquid stream 130
and a vapor stream 128. Liquid stream 130 flows to the demethanizer
as the intermediate temperature feed.
[0053] Vapor stream 128 exits separator drum 132 and flows to heat
exchanger 121 where it is further chilled and partially condensed.
Exiting heat exchanger 121 the stream is separated into a liquid
stream 136 and a vapor stream 134 in a separator drum 138. Liquid
stream 136 flows to the demethanizer as the coldest feed. Vapor
stream 134 is further processed for separation into hydrogen-rich
and methane rich streams. Chilling of stream 128 is provided by
process reheats as available, including stream 131, and by pressure
let down of stream 115 across valve 185 and reheating and partially
vaporizing in exchanger 121.
[0054] In this design, no demethanizer overhead condenser is
provided as stream 136 is chilled to a sufficiently low temperature
such that reflux is not required. Thus, the demethanizer is a
stripping column. This embodiment can also be configured to enable
cooling of the demethanizer gross overhead vapor to provide reflux
as an alternate process design approach.
[0055] Liquid stream 105 is withdrawn from stream 103 and is let
down in pressure across control valve 186 to provide chilling for
the low pressure depropanizer condenser 142. After vaporization,
stream 105 is then combined with vaporized stream 112 as stream
114. This stream is combined with stream 168 as stream 169 and
flows to an extended binary refrigeration compressor second stage
suction drum 176.
[0056] Second stage discharge vapor 153 from the compressor 150 is
chilled by cooling water in exchanger 140 and then combined with
vaporized stream 111 as stream 154 before flowing to the extended
binary refrigeration 2.sup.nd stage discharge drum 177. Stream 156
exits drum 177, and a portion of this stream, stream 158, returns
to the extended binary refrigeration compressor 150 as the 3.sup.rd
stage suction flow. The remaining portion 160 flows first to the
ethylene fractionator reboiler 162 and then to the ethylene
fractionator side reboiler 164 where it is mostly but not totally
condensed before flowing to separator drum 166.
[0057] The vapor stream 168 leaving separator drum 166 is combined
with reheated stream 114 and flows to the extended binary
refrigeration compressor second stage suction drum 176. The vapor
stream 170 leaving the drum 176 is the second stage suction flow to
the compressor 150.
[0058] The liquid stream 172 leaving drum 166 is heated in heat
exchanger 175 and is then let down across valve 187 to near the
suction pressure of the extended binary refrigeration compressor
first stage suction. The expanded stream is then combined with
stream 113 to form stream 174, which is a mixed vapor and liquid
stream. The higher methane concentration reheated stream 113, when
injected into expanded stream 172 further lowers the temperature of
the combined expanded stream 174. Stream 174 then flows to the
ethylene fractionator overhead condenser 178 and is vaporized
providing chilling to condense reflux to enable the separation of
polymer grade ethylene from ethane. Because of the injection of
stream 113 into stream 172, the pressure of stream 174 can be
higher while maintaining an economical temperature differential in
the exchanger 175. As the vaporized stream 174 flows to drum 179
and then subsequently becomes the suction flow to the extended
binary refrigeration compressor 150, a higher pressure decreases
the power required by the refrigeration compressor 150.
[0059] The process design depicted in FIG. 2 utilizes some heat
exchange concepts that facilitate the balancing of vapor and liquid
flows in the extended binary refrigeration compressor. The ethane
recycle from the bottom of the ethylene fractionator is let down to
near the inlet pressure of the cracking heaters and is then
vaporized in an exchanger 175, located in the overhead of the
ethylene fractionator, reducing the duty of exchanger 178 and
chilling the refrigerant 172 flowing to exchanger 178. Use of a
vapor withdrawal 160 from the second stage of the extended binary
refrigeration compressor 150 allows for additional process heat
flexibility thus allowing the system to have a wider range of
duties for providing refrigeration for a new olefins unit or major
expansion as compared to a smaller expansion as in FIG. 2.
[0060] In FIG. 3, the C2 fractionator system separates gross feed
202 from a deethanizer 204, into an ethylene product 206, an ethane
recycle 208 and a reflux stream 210. Ethane recycle stream 208 is
passed across valve 281, letdown to near cracking heater inlet
pressure, and then passes through heat exchanger 212. In heat
exchanger 212, stream 172' from drum 166 (FIG. 2) is chilled prior
to passing across valve 214 (187') and to heat exchanger 216 (178'
FIG. 2). The subcooling that occurs in heat exchanger 212 reduces
the required flow rate of stream 172' required for the C2
fractionator condenser duty in 216 (178').
[0061] FIG. 3 also depicts the relationship of the deethanizer 204
to the C2 fractionator 218. Significantly, there is no overhead
condenser for the overhead stream 222 from the deethanizer. Reflux
for the deethanizer 204 is supplied by stream 210 drawn off from
the C2 fractionator at a point at or just below point that the
gross feed from the deethanizer 204, stream 222, enters the C2
fractionator 218. Stream 219 is cooled in heat exchanger 216 and
sent to drum 232. A portion of stream 219 from drum 232 returns to
the top of the fractionator as stream 230 and the rest is removed
as ethylene product in stream 206. The integration of the
deethanizer 204 and C2 fractionator 218 in this manner can
facilitate the design and operation of the extended binary
refrigeration system.
[0062] FIG. 3 also depicts an interreboiler 220. The feed to this
interreboiler is drawn off from the C2 fractionator 218 at a point
in the tower below the entry point of feed stream 222 and above the
drawoff of stream 225 to the bottom reboiler, 224. Gross overhead
stream 226 from the depropanizer is cooled and partially condensed
in the inter-reboiler 220. By integrating with the C2 fractionator
in this manner, the load for exchanger 142 (FIG. 2) is reduced.
This provides flexibility for the system and reduces the flow of
stream 105 withdrawn from heavy liquid refrigerant stream 103 and
recycled to the second compression stage. This in turn reduces the
power requirements for compressor 150. Stream 228 from the
inter-reboiler 220 is a mixture of vapor and liquid and is sent to
a drum for separation of vapor product from reflux. Stream 228 may
additionally be cooled in other exchangers before this vapor-liquid
separation either to satisfy normal operation or startup
requirements or both.
[0063] FIG. 4 depicts a variation of the FIG. 3 process
configuration. In this configuration, stream 308, the ethane
recycle withdrawn from the bottom of the C2 fractionator 318, is
let down across a valve 330 to near cracking heater inlet pressure
and is then passed to a heat exchanger 316 where the C2 recycle is
vaporized by partially condensing stream 319 leaving the overhead
of the tower 318. The flow of stream 172'' from drum 366 is reduced
as the duty required of exchanger 312 is reduced by the duty of
exchanger 316. As can be seen in FIG. 2, reducing this flow (172 on
FIG. 2) reduces the net vapor withdrawn from the second stage
discharge as stream 153. This reduces the overall flow of binary
refrigerant through the system and the power required for
compressor 150.
[0064] Net refrigeration loads and power consumption are reduced
using the heat exchange concepts in FIGS. 3 and 4. The concepts
have the further advantage of facilitating the process design and
the operation of the extended binary refrigeration compressor by
decreasing the gross flows of the extended binary refrigeration
system and thus decreasing the requirements of balancing vapor and
liquid loads on the system and specifically on the compressor.
[0065] FIG. 5 depicts an alternate to the FIG. 1 scheme. In the
FIG. 5 process scheme, vapor stream 402 and liquid stream 403 leave
the separator drum 454. The final compressor discharge 401 can be
cooled further by heat exchange by withdrawing a portion of heavy
liquid stream 403 as stream 406, reducing the pressure of this
withdrawn liquid across valve 481, and heat exchanging with
partially cooled stream 401 in heat exchanger 408, which is
downstream from cooling water exchanger 452. As a result of this
configuration, the temperature of the cooled stream 401 entering
drum 454 is lower than the temperature of stream 401 when it leaves
cooling water exchanger 452. Vaporized stream 406 leaving exchanger
408 combined with stream 411 to form stream 412 and is routed to
second stage suction drum 476 and enters the compressor 450 at the
second suction. The remaining liquid stream 404 from the separator
454 is conveyed through heat exchanger 417. The advantage of this
alternate configuration is that the total flow leaving compressor
450 is higher and the pressure is lower than in the process shown
in FIG. 1 to achieve the same degree of process refrigeration from
the combined system. Volume flow is thus increased greatly and this
can be beneficial when the compressor discharge volumetric flow is
relatively low and would result in an inefficient centrifugal
compressor design.
[0066] FIG. 6 depicts an alternate to the FIG. 2 process scheme
where the final discharge stream 501 from the compressor 550 is
cooled, separated in drum 554 into liquid stream 503 and vapor
stream 502. Additional cooling of stream 501 is effected by
withdrawing a portion of heavy liquid stream 503 as stream 506,
reducing the pressure of this withdrawn liquid across valve 581,
and heat exchanging with stream 501 (after it has left cooling
water exchanger 552) in exchanger 508, such that the temperature of
the cooled stream 501 entering drum 554 is lower than the
temperature of stream 501 leaving cooling water exchanger 552.
Stream 506 is then combined with stream 511 to form stream 507,
which is routed to second stage discharge drum 577 as part of
stream 554, and a portion of this enters the compressor 550 in
stream 513 at the third stage suction. The rest of stream 554
exiting from drum 577 forms stream 514, which combines with streams
505 and 512 to form the feed to the second stage of the compressor
550 in stream 570 after providing process duties for the ethylene
fractionator reboiler and ethylene fractionator side reboiler. The
remaining liquid stream 504 from the separator 554 is conveyed
through heat exchanger 517. The advantage of this alternate
configuration is that the total flow leaving compressor 550 is
higher and the pressure is lower than in the FIG. 2 scheme. Volume
flow is thus increased greatly and this can be beneficial when the
compressor discharge volumetric flow is otherwise too small to
provide for an efficient compressor design. This higher flow is at
a pressure higher than for the process shown in FIG. 5 that
required two compression stages instead of three as in FIG. 2. The
recycle stream 506 is ultimately routed to the second stage
discharge drum (third stage suction).
Example
[0067] A computerized process simulation was conducted in which an
extended binary refrigerant formed from 19 mol % methane and 80.5
mol % propylene (also containing 0.5 mol % hydrogen gas) was used
as the refrigerant in the process depicted in FIG. 1. The pressures
and compositions of the light refrigerant stream 2, heavy
refrigerant stream 3, and the suction stages of the compressor are
shown on Table 1 below. The extended binary refrigerant
successfully provided for refrigeration in an ethylene plant having
a low pressure demethanizer, and also can be used in a plant
employing a high pressure demethanizer.
TABLE-US-00001 TABLE 1 Com- Compressor pressor Light Heavy First
Second Discharge Refrigerant Refrigerant Stage Stage stream Stream
Stream Suction Suction (1) (2) (3) (14) (11) mol % mol % mol % mol
% mol % H.sub.2 0.5 1.4 <500 ppm 1.2 <500 ppm CH.sub.4 19.0
41.2 7.4 36.7 7.4 C.sub.3H.sub.6 80.5 57.4 92.6 62.1 92.6 MW 36.9
30.8 40.1 32.1 40.1 Pressure 410 405 405 42 149 (psia)
* * * * *