Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
Rectified Reflux Deethanizer
The present invention relates to the fractionation of
light hydrocarbons. The present invention especially
relates to deethanization.
BACKGROUND OF THE INVENTION
The present invention relates to deethanization and
ethylene/ethane splitting fractionation steps of cracked
gases for olefin recovery. In order to properly
appreciate the technological field of the fractionation
trains used for separation of olefins from other
components in cracked gas, the article "Ethylene from NGL
feedstocks - Part 3 Flow Scheme Comparison" (K. Ng et al,
Hydrocarbon Processing, Dec. 1983, pp. 99-103) is referred
to herein to describe the three most typical choices for
the first fractionation step in the fractionation train.
A front-end demethanizer, deethanizer and depropanizer are
evaluated for their advantages in fractionating cracked
gas from NGL feeds. The front-end deethanizer was found,
under the assumptions made at the time of the article, to
be the most preferable of the fractionation trains.
In the article "Ethylene from NGL feedstocks - Part'4
Low Pressure C2 Splitter" (H.Z. Kister et al, Hydrocarbon
Processing, Jan. 1984) describes an optimized fractiona-
tion step required in olefins separation of cracked gas.
The low pressure ethylene/ethane splitter ("C2 splitter")
is preferred for the potential for heat pumping the column
and therein provide an open ethylene refrigeration loop
for other refrigeration needs in the fractionation train.
The C2 splitter has been the focus of much study to reduce
the relatively expensive utilities required for separation
of ethylene and ethane, relatively close boiling
components.
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Other concepts in the prior art that relate to the
present invention are described below.
U.S. Patent 1,735,558 describes a multiple sidedraw
column crude oil fractionation column. The vapor from
three sidedraws from a first column is partly condensed
and is condensed and rectified in a second column. The
liquid of the second column is turned to the first column
for stripping.
U.S. Patent 1,954,839 describes a distillate
rectification in which the feed is partly vaporized and
the vapor and liquid phases separated three times to
provide for multilevel feeds to a fractionation column.
The liquid separated from the last of partial fractiona-
tion stages is recovered as the distillate product.
U.S. Patent 1,957,818 describes rectification of light
hydrocarbons and mentions ethylene and ethane as among
those. In a series of refluxed and stripped columns, the
patent describes using a condensed, rectified overhead
stream as feed to a next column. A stripped bottoms
stream of the next column is fed to the rectification
section of the first column.
U.S. Patent 2,327,643 describes a two column,:dual
pressure fractionation method wherein the column pressures
are chosen to accommodate vaporization of the condensed
overhead stream of a second column acting to indirectly
supply part of the condensing duty for the overhead stream
of a first column. The vaporized second column vapor is
recompressed and is fed to the bottom of the first column
to supply reboiling duty for the first column.
U.S. Patent 4,285,708 describes a two column method
for a single deethanization using a split feed concept.
The gaseous feed stream to the deethanization is split and
a portion is condensed and stripped in a stripping column.
The overhead vapor from the stripping column is partially
condensed and fed to the rectification section of the
deethanizer column. The stripping duty of the deethanizer
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column and the rectification section diameter are substan-
tially reduced by use of an upstream stripping column.
U.S. Patent 4,436,540 describes a full fractionation
train for olefin recovery from cracked gas using only low
pressure rectification columns for gaseous portions of the
pyrolysis furnace effluent. Liquid portions of the recti-
fication columns are further fractionated in high pressure
refluxed and stripped columns-to complete the separation.
Partial intercondensation by pumparounds and liquid
streams from the high pressure columns provide rectifi-
cation duty to the rectification columns.
U.S. Patent 4,720,293 describes a method of feed
conditioning to a demethanizer for an olefins fractiona-
tion train. The fractionation train's first separation
column is the demethanizer, and the feed to it is treated
in a dephlegmator to recover ethylene. Column 100
describes a pasteurizing section accommodating removal of
residual hydrogen from an overhead ethylene product.
U.S. Patent 4,900,347 describes a system of multiple
dephlegmations integrated into a demethanization of an
olefins recovery stream. The multiple rectifications in
three dephlegmators produce three liquid bottoms streams
that are fed to two refluxed demethanization columns. A
dephlegmated portion of the feed gas is fed to a second
demethanizer column. The overhead product of a first
demethanizer is also fed to the second demethanizer. The
bottom product of the second demethanizer is a relatively
pure stream of ethylene.
U.S. Patent 5,035,732 describes a system similar to
that of U.S. Patent 4,900,347, although the second
demethanizer is operated at low pressure.
U.S. Patent 5,253,479 describes forming a product
specification liquid stream of ethylene as a bottom
product of a demethanator column, wherein a portion of the
ethylene stream is used as an absorbing, lean liquid in an
absorber column. The gas feed to the bottom of the
absorber column is the gaseous portion of a partially
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condensed cracked gas stream comprising at least hydrogen,
methane, ethylene and ethane. The absorbing-liquid ethylene and its captured
components are fed to a
deethylenization column, from which the overhead vapor =
stream is fed entirely to the demethanator column. It is
an apparent disadvantage of the patent process wherein the
ethylene condensed at considerable cost in utility must be
vaporized in the deethylenizer and re-condensed in the
demethanator.
In the article "Temperature-Heat Diagrams for Complex
Columns, 2. Underwood's Method for Side Strippers and
Enrichers" (N.A. Carlberg et al, Ind. Eng. Chem. Res.,
vol. 28, pp. 1379-1386, 1989), complex columns are
described as having benefits and disadvantages. On page
1385, the authors state, "The question to ask is how do
complex columns compare against simple column sequences in
terms of utility consumption. The answer is that complex
columns are more energy efficient but have larger tempera-
ture ranges than simple column sequences. Basically,
complex columns are more favorable with respect to first-
law effects and less favorable with respect to second-law
effects. Thus, if there is an adequate temperature
driving force, complex columns will be favored; if not,
simple columns are more favorable from a utility point of
view." A method is presented in the article for evalua-
ting minimum reflux for complex column, i.e. those with
one or more side strippers or enrichers. In the article,
the operational definition of a side stripper or enricher
is a device that withdraws from a column a sidestream
vapor or liquid and returns to the same stage a stream
comprising liquid or vapor generated in a second column. =
Side stripping or enriching necessarily returns to the
fractionation column a portion of the withdrawn stream
which has been enriched or stripped of its original
components.
It will be apparent from the above that a simplified
and relatively inexpensive method for reducing the
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combined condensation duties of a demethanizer
deethanizer - C2 splitter combination has not been
previously developed. It is an object of the present
invention to make such an improvement.
5 SUMMARY OF THE INVENTION
The present invention is directed to at least a
portion of fractionation train wherein ethylene is
separated from cracked gas or from a combination with
light hydrocarbons from other sources, such as from the
gaseous products of fluid catalytic cracking of hydro-
carbons. More specifically, the present invention is
directed to processing a demethanized stream of cracked
gas in a deethanizer, wherein a deethanizer overhead
product stream is obtained comprising an ethylene purity
of at least product specification, although a lesser
degree of ethylene purity may be obtained in that overhead
stream depending on the desired processing requirements.
It is a requirement of the present invention to add
theoretical stages to the rectification section of a prior
art deethanizer wherein stages were originally designed to
effect only a minimum separation of ethane and ethylene
from heavier components. Conceptually, the present
invention is applicable to a combination of a deethanizer
and an ethylene/ethane fractionation column, where an
2S overhead product stream is "split" or fractionated down-
stream of a first column to obtain the desired product.
The art of fractionation train design has developed in a
similar sense for obtaining propylene and propane from
heavier components, as well as for butylene and butane
from a stream with heavier products. The present inven-
tion is thus applicable in concept to separations made
wherein (1) an overhead product stream comprising at least
two product components is separated as an overhead product
in a first column and (2) at least two product components
of the overhead product stream are separated in a second
column to form two product streams.
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It has been found, quite surprisingly, (1) that a
deethanizer separating ethylene and ethane from heavier
components produces an overhead product stream such that
ethylene purity and/or recovery may be.varied over a broad =
range for that stream and (2) that the cold utilities of
the deethanizer overhead condenser are substantially the
same regardless of the ethylene, purity or degree of
recovery of ethylene in the overhead product stream. To
accomplish this surprising result, a sidedraw must be
withdrawn from the rectification section of the deetha-
nizer comprising a significant part of the column feed
components in the rectification section. No portion of
the sidedraw stream is returned to the deethanizer,
although processes with such partial return of sidedraw
streams are well established and defined in the prior art.
Such partial or total return of sidedraw streams are shown
and described for side enrichers in the Carlberg et al
article above or are well known in the art of partial or
total intercondensing for rectification sections.
Although for the specific example below, it is preferable
to withdraw with the sidedraw of the present invention
substantially all the ethane in the deethanizer feed and
about 70 percent of the ethylene to obtain a polymer grade
ethylene product from the deethanizer overhead product
stream, such a description is not a limitation of the
present invention regarding recovery of polymer grade
ethylene from the overhead product stream of a deetha-
nizer. Subsequent fractionation in the C2 splitter of a
sidedraw stream from the deethanizer of the present
invention requires a column of smaller diameter and
significantly less condensing duty in the overhead
condenser. Thus, the overall condensation duty for the
overhead condensers for the deethanizer and C2 splitter is
also significantly reduced.
The rectification stages added to the deethanizer to
accomplish the objects of the present invention will
control the purity of the ethylene in the overhead product
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of the deethanizer. It will be shown below graphically
that any desired purity of ethylene (99.9 mole percent or
less) may be obtained in the overhead product stream of
the deethanizer, while varying the number of stages in the
deethanizer or the amount of ethylene recovered in the
overhead product to achieve optimum cost savings depending
on equipment and utilities costs. Although a specific
example of the present invention is presented in the
description below, other very advantageous modes of the
present invention will become apparent to the skilled
person with that description when cost savings are
optimized from comparison of costs of equipment and
utilities in certain circumstances.
The sidedraw is subjected to additional fractionation,
preferably in a low pressure, heat-pumped C2 splitter to
separate the ethylene from the ethane in the sidedraw
stream, although any prior art ethylene/ethane fractiona-
tion system is advantageously improved through lower
capital and cold utilities costs with incorporation of the
present invention. It is further preferable to process
cracked gases derived from feeds such as propane, butane
or naphthas. When the withdrawal rate of ethylene in the
sidedraw is as high as 70 mole percent of the ethylene in
the column feed, it has been found that about 43 actual
trays or about 30 theoretical stages should be added to
the rectification section above the withdrawal stage of
the sidedraw to generate from the column overhead a stream
of polymer grade ethylene, about 99.9 mole percent
ethylene. The stages between the withdrawal stage for the
sidedraw of the present invention in the deethanizer
rectification section and the deethanizer overhead
condenser shall hereafter be referred to as the
"additional rectification section" of the deethanizer.
As described above, the additional rectification
section preferably produces an overhead stream comprising
polymer grade (or lesser quality as required) ethylene
product equal to or less than about one third of the
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ethylene in the feed to the deethanizer, although any
portion of said feed ethylene may be recovered thereby.
The deethanizer reflux condenser duty in the present
invention is relatively constant over the range of =
operation described below. For a specific example
described below, an overhead product stream may be
obtained containing less than about 60 percent of the
ethylene in the deethanizer feed at a purity of about 98
mole percent ethylene. For the purpose of fractionation
analysis herein, the efficiency of rectification section
trays described herein is about 70 percent, such that
reference to stages will mean theoretical stages and
reference to trays will mean actual sieve trays.
It will be clear to the skilled person with disclosure
of the present invention that the total refrigeration
utilities for deethanization with an additional rectifi-
cation section in combination with an ethylene/ethane
fractionation (C2 splitting) are reduced over prior art
combinations of deethanization and C2 splitting. Combined
condenser duties for deethanization and C2 splitting can
be reduced by up to about 24 percent for the preferred
embodiment described below wherein polymer grade ethylene
is produced as an overhead stream from the deethanizer.
Greater savings in equipment cost and cold utilities will
be obtained depending on further optimization of the
present invention.
In summary, the method of withdrawing a sidedraw from
a deethanizer without return of any portion of the side-
draw stream to the deethanizer creates, for components in
the sidedraw stream, a refluxing stream comprising the
liquid stream from the stage above the withdrawal stage.
The refluxing is accomplished by rectifying the vapor
stream leaving the withdrawal stage and returning it to
the withdrawal stage having removed some substantial
portion of the ethylene from the vapor leaving the with-
drawal stage. Thus, the method of the present invention
is also hereafter referred to as a "rectified reflux
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deethanizer". The "rectified reflux" of the present
invention is that reflux needed to achieve a desired
ethylene purity in the deethanizer overhead product (the
separation of ethylene and ethane) as well as to obtain the
desired ethylene/ethane recovery desired from the
deethanizer feed (the separation of ethylene/ethane from
propylene and heavier components). It will be appreciated
that this combined refluxing, the rectified refluxing,
achieves a surprising result with no further increase in
cold utilities in the overhead condenser in the deethanizer
over the prior art operation wherein no relatively pure
ethylene stream is obtained.
In a further embodiment of the present invention,
a prior art, existing deethanizer is converted ("revamped"
or "retrofitted") to a rectified reflux deethanizer
according to the present invention. The conversion or
initial design of a deethanizer according to the present
invention includes adding rectification section stages (the
additional rectification section) to a column or providing a
separate column in which the rectified reflux method is
performed to obtain a high purity ethylene product or an
ethylene product of lesser purity as desired.
An embodiment of the present invention provides a
process for separating ethylene and ethane from light
hydrocarbons, which comprises:
(a) providing a deethanizer feed comprising
substantial amounts of ethylene and ethane and essentially
no other components with lower boiling temperatures than
ethylene;
(b) providing a deethanizer with a rectification
section above a feed stage, a stripping section below the
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feed stage and an overhead condenser above a top stage of
the rectification section providing refluxing duty to the
deethanizer;
(c) feeding the deethanizer feed to the feed stage
and performing separation of ethylene and ethane from
heavier feed components between the feed stage and a
sidedraw withdrawal stage, wherein the sidedraw withdrawal
stage is located between the feed stage and the top stage of
the deethanizer; and
(d) withdrawing at least 5 percent of the ethane
in the feed in a sidedraw stream of vapor or liquid at the
sidedraw withdrawal stage such that no portion of the
sidedraw stream is returned to the deethanizer.
Another embodiment of the present invention
provides a process for separating a first column overhead
product comprising two product hydrocarbon components with
higher and lower boiling temperatures from other
hydrocarbons, which comprises:
(a) providing a first column feed comprising at
least two product hydrocarbon components, wherein
substantially no components of the first column feed have
lower boiling temperatures than any of the product
hydrocarbon components;
(b) providing a first column with a rectification
section above a feed stage, a stripping section below the
feed stage and an overhead condenser above a top stage of
the rectification section providing refluxing duty to the
first column;
(c) feeding the first column feed to the feed
stage and separating the product hydrocarbon components from
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heavier feed components between the feed stage and a
sidedraw withdrawal stage in the rectification section,
wherein the sidedraw withdrawal stage is located between the
feed stage and the top stage of the deethanizer;
(d) withdrawing at least 5 percent of higher
boiling product component in a sidedraw stream of vapor or
liquid at the sidedraw withdrawal stage such that no portion
of the sidedraw stream is returned to the first column;
(e) obtaining an overhead product stream with more
than about 90 mole percent of a lower boiling product
component; and
(f) fractionating the sidedraw stream in a second
column to separate the higher and lower boiling components.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a prior art deethanizer combined with
a heat pumped C2 splitter.
Figure 2 is the present invention showing the
additional rectification section added to the deethanizer of
Figure 1 combined with the heat pumped C2 splitter shown in
Figure 1.
Figure 3 is a graph of deethanizer-recovered
ethylene compared with the number of actual trays required
for three levels of ethylene purity desired in the overhead
product of the rectified reflux deethanizer of the present
invention.
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Figure 4 is a plot of the components ethylene and
ethane (Key number 1 on the plot or."C2's") and propylene,
propane and butane (Key number 2 of the plot or "C3's+").
The relative vapor compositions of=the two keys are
5 plotted against actual tray number for operation of a
deethanizer according to the prior art deethanizer
described in Figure 1. A "Condenser" notation indicates
the top of the column on the x-axis of the plot. A
"Reboiler" notation indicates the bottom of the column on
10 the axis of the plot.
Figure 5 is a plot of the components ethylene and
ethane (Key number 1 on the plot or "C2's") and propylene,
propane and butane (Key number 2 of the plot or "C3's+").
The relative vapor compositions- of the two keys are
plotted against actual tray number for operation of a
deethanizer according to the prior art deethanizer
described in Figure 2. A "Condenser" notation indicates
the top of the column on the x-axis of the plot. A
"Reboiler" notation indicates the bottom of the column on
the x-axis of the plot.
DETAILED DESCRIPTION OF THE INVENTION
For the section of the deethanizer from the feed stage
to a sidedraw withdrawal stage, the present invention
operates similarly to a prior art deethanizer, wherein a
sidedraw stream is withdrawn and further fractionated in
a C2 splitter to recover ethylene from ethane and:a reflux
stream is provided to the sidedraw withdrawal stage from
the stage above it. The present invention creates above
the sidedraw withdrawal stage an additional rectification
section at substantially the same pressure as the rest the
deethanizer wherein rectification of ethylene from ethane
occurs without significant increase in cold utilities in
the overhead condenser compared to a deethanizer without
the additional rectification section. A comparison of a
prior art deethanizer with a:low pressure, heat-pumped C2
splitter is compared below with a deethanizer according to
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the present invention with a low pressure, heat-pumped C2
splitter.
. Prior Art Deethanizer
Figure 1 is the prior art process comprising columns
C100 (a deethanizer with a rectification and stripping
section) and C101 (a low pressure, heat pumped C2
splitter). Other equipment noted in Figure 1 are
exchangers E100 (deethanizer reboiler, preferably heated
with quench water), E10l (deethanizer overhead condenser,
preferably cooled with propylene refrigerant), E102
(deethanizer overhead product partial vaporizer, prefer-
ably recovered to demethanizer feed chilling), E103 (C2
splitter heat pump reboiler), E104 (C2 splitter heat
recovery reboiler, preferably chilling demethanizer feed),
E105 (coldbox exchanger, preferably recovering process
stream chilling), E106 (ethylene refrigeration load,
preferably demethanizer feed chilling), E107 (ethane
recycle vaporizer), E108 (ethylene refrigerant condenser,
propylene refrigerant) and E109 (ethylene refrigerant
cooler, propylene refrigerant), and stages S100/S101 (open
ethylene refrigerant loop compressor stages, wherein S10l
represents two compression stages) . Stage S102 (not shown
in Figure 1) is described herein for the purpose of
describing a comparative savings in compressor horsepower
in the propylene refrigerant compressor, which supplies
chilling in exchangers E108 and E109 to the ethylene
refrigeration loop.
The process streams of Figure 1 are streams 100 (upper
and lower, i.e. vapor and liquid, streams of demethanized
cracked gas), 101 (deethanizer bottoms stream), 102
(deethanizer overhead product), 103 (C2 splitter bottoms
product), 104 (top stage vapor stream from the C2
splitter), 105 (lowest pressure stage drum vapor from the
open ethylene refrigeration loop), 106 (heat-pumped C2
splitter reflux condensed in C2 splitter reboiler), 107
(highest pressure stage drum vapor from the open ethylene
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refrigeration loop), 108 (subcooled ethylene refrigerant
loop condensate for C2 splitter reflux), 109 (ethane
recycle, i.e. net bottoms product of the C2 splitter) and
110 (net ethylene product from the C2 splitter). Table 1 =
indicates the stream compositions, rates and conditions
for this example.
The upper, vapor and lower, liquid streams, streams
100, from the demethanizer feed a feed stage section in
column C100, defined by the upper and lower stages to
which they are fed. Herein, the feed section will be
referred to as a feed stage. Column C100 comprises 28
actual trays wherein streams 100 enter on trays 11 and 12
(the top tray of column C100 is tray) . For purposes of
column analysis for the detailed examples herein, tray
efficiency in the rectification sections about 70 percent
and in the stripping sections the tray efficiency is about
60 percent.
The condenser exchanger E101 and reboiler exchanger
E100 provide cold, refluxing and hot, reboiling utilities
to column C100 respectively. The relative amounts of C3's
in the overhead product stream, stream 102, and the rela-
tive amounts of C2's in the bottom product stream, stream
101, indicate a commercially desirable level of separation
of those components. This degree of separation will
generally be repeated for the example with the present
invention for purposes of comparison and is not a specific
limitation of the present invention. The relative amounts
of light hydrocarbons as stream components can vary widely
depending on the source of the feed generating cracked
gas.
The duty of exchanger E101 is about 50.9 MMBtu/hr for
column C100 operating at about 240 psia. Stream 101, as
indicated on Figure 1, is preferably further fractionated
in a C3 splitter (not shown).
Stream 102 is partially vaporized in exchanger E102
and fed to column C101 operating at about 60 psia, whose
overhead stream, stream 104, enters the open refrigeration
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loop low pressure drum, combines with vaporized ethylene
refrigerant to form stream 105, and wherein stream 105
feeds the first stage of the open refrigerant loop, stage
S100. The compressed vapor from stage S100 is split, and
one portion flows to the second stage of the compressor,
stage 101, and the rest, stream 106, is condensed in the
C2 splitter reboiler, exchanger E103, and the condensed
stream is fed to the top st=age of the C2 splitter as
reflux. The compressed vapor from stage S102 is condensed
in exchangers E107, E108 and E109. A portion of the
condensed vapor from stage S102 is withdrawn as a net
ethylene product, stream 110, while another portion is
subcooled in exchanger E105 for use as column C101 reflux
and the last portion of the stream is used as ethylene
refrigerant, ultimately flowing to exchanger E106.
The net bottoms product of column C101, stream 109, is
relatively pure ethane. Stream 103 contains stream 109,
wherein a portion of stream 103 is used for demethanizer
feed chilling. The refrigeration resulting from vapori-
zing the net ethane bottoms product of the C2 splitter is
recovered to the ethylene refrigeration loop in exchanger
E107. The conceptual operation of this low pressure,
heat-pumped C2 splitter is substantially the same for this
example and the next describing the present invention.
Thus, the operation of the C2 splitter and the open
ethylene refrigeration loop will not be discussed for the
example of the present invention other than to point out
significant differences between the prior art operation
and that of the present invention shown in Figure 2.
Present Invention Deethanizer
Figure 2 is the present invention comprising columns
C200 (a deethanizer with a stripping section and a recti-
fication section, wherein the rectification section
comprises stages between a feed section and a sidedraw
withdrawal stage and an additional rectification section,
C200A, comprising stages between the sidedraw withdrawal
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stage and an overhead condenser) and C201 (a low pressure,
heat pumped C2 splitter). Other equipment noted in Figure
2 are exchangers E200 (deethanizer reboiler, preferably
heated with quench water), E201 (deethanizer overhead
condenser, preferably cooled with propylene refrigerant),
E202 (deethanizer sidedraw stream partial vaporize,
preferably recovered to demethanizer feed chilling), E203
(C2 splitter heat pump reboiler), E204 (C2 splitter heat
recovery reboiler, preferably chilling demethanizer feed),
E205 (coldbox exchanger, preferably recovering process
stream chilling), E206 (ethylene refrigeration load,
preferably demethanizer feed chilling), E207 (ethane
recycle vaporizer), E208 (ethylene refrigerant condenser)
and E209 (ethylene refrigerant cooler), and stages S200
and S201 (open ethylene refrigerant loop compressor
stages, wherein S201 represents two compression stages).
Stage S202 (not shown in Figure 2) is described herein for
the purpose of describing a comparative savings in
compressor horsepower in the propylene refrigerant
compressor, which supplies chilling in exchangers E208 and
E209 to the ethylene refrigeration loop.
The process streams of Figure 2 are streams 200 (upper
and lower, i.e. vapor and liquid, streams of demethanized
cracked gas), 201 (deethanizer bottoms stream), 202 (side-
draw stream), 203 (deethanizer overhead ethylene product
stream), 203A (relatively impure deethanizer overhead
ethylene product stream fed to the C2 splitter), 204 (C2
splitter bottoms product), 205 (top stage vapor stream
from the C2 splitter), 206 (lowest pressure stage drum
vapor from the open ethylene refrigeration loop), 207
(heat-pumped C2 splitter reflux condensed in C2 splitter
reboiler), 208 (highest pressure stage drum vapor from the
open ethylene refrigeration loop), 209 (subcooled ethylene
refrigerant loop condensate for C2 splitter reflux), 210
(ethane recycle, i.e. net bottoms product of the C2
splitter) and 211 (net ethylene product from the C2
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splitter) Table 2 indicates the stream compositions,
rates and conditions for this example.
Table 3 is a comparative listing of the duties of the
important heat exchangers for the processes shown in
5 Figures 1 and 2. Table 4 is a comparative listing of the
horsepower of the compression stages described for the
embodiments of the prior art example shown in Figure 1 and
the present invention shown in Figure 2. The horsepower
for the propylene refrigeration compressor is shown as
10 stage S102 for the process of Figure 1 and as stage S202
for the process of Figure 2. Stage S102 horsepower is
represented as the word "BASE", as the total horsepower
for the propylene compressor comprises many refrigeration
loads other than those for C2 splitting. Stage S202
15 horsepower is represented as the word "BASE-996",
indicating a savings of 996 horsepower over the BASE
amount for the prior art embodiment of Figure 1.
The deethanizer, column C200, comprises an additional
rectification section, column C200A, wherein the vapor
from the sidedraw withdrawal stage enters from the bottom
and is rectified to form stream 203 or 203A. Column C200,
operating at about 240 psia, comprises about 57 actual
trays, wherein feed streams 100 enter on trays 41 and 42
(the top tray of column C200 is the number 1 tray). The
sidedraw withdrawal stage is at tray number 30. Stream
203 is the sidedraw stream in Figure 2 and is directed to
a number "1", indicating its continuance on the other side
of the figure at the other number "1" and inclusion of the
overhead product stream of the deethanizer in the ethylene
product drum. The duty required in the deethanizer
overhead condenser, exchanger E201, for the degree of
separation between streams 201, 202 and 203, as shown in
Table 2 for all streams in this example, is about 50.7
MMBtu/hr.
The inclusion of stream 203A in Figure 2 indicates a
mode of operation wherein product specification ethylene
in the overhead product stream of column C200 is not
CA 02207431 1997-06-10
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16
desired or cannot be achieved with the deethanizer. All
or part of the overhead product stream is then directed as
stream 203A to a stage higher in column C201 than stream
202, and the remaining portion of the overhead product
stream, stream 203, if any, is directed to the ethylene
product drum and recovered as stream 211, as indicated in
Figure 2. Alternatively, stream 203 may simply be used as
a lower grade ethylene product than that obtained from the
operation of the C2 splitting. For the present example,
stream 203 achieves a very high ethylene purity at the
cost of about 29 additional actual trays to the deetha-
nizer, column C200. In another embodiment of the present
invention, significant equipment cost savings will be made
wherein the function of exchanger E201 and its associated
drum are incorporated into exchanger E208 and its
associated product drum. For such an embodiment, the
vapor stream from the top stage of column C200A is mixed
with the process stream of the C2 splitter between
exchangers E207 and E208, thereby eliminating an exchanger
and a drum. Deethanizer reflux is obtained by pumping
liquid ethylene from the drum associated with E208 to the
top stage of column C200A.
The operation of column C200 and the associated heat-
pumped, open refrigeration loop has been substantially
described above. When the overall condensing duties of
the deethanizer and the C2 splitter are compared for the
processes shown in Figures 1 and 2, the savings in cold
utilities equals about 24 percent for the present
invention over the prior art design. Table 3 permits
comparison of the duties for those duties. This utilities
reduction is a benefit in addition to a substantial
reduction in vapor and liquid traffic in the rectification
section of the C2 splitter, indicating that reduction in
column diameter would be recommended. The associated
reduction in condensing duty in the C2 splitter indicates
that a simple overhead condenser or the associated
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17
equipment for the open refrigeration loop also be reduced
in size and cost.
In addition, it is known by the present invention that
the purity of stream 203 or 203A can be such that it's
purity is about the same specification obtained in the
overhead product of the downstream fractionator.
Alternatively, the purity of stream 203 or 203A can be
obtained at any other desired.purity and can be recovered
either as product or sent to a downstream fractionation
column for further fractionation. If a liquid product is
obtained from the overhead stream of the deethanizer,
column C200, the balance of condensed liquid from the
overhead condenser is sent back as reflux to section
C200A. It has been found that the liquid flow rate of
"reflux" to the sidedraw withdrawal stage in column C200
from the stage above it is approximately equal to the
liquid flow rate of the reflux to the top stage of column
C100 in Figure 1.
Figure 3 is a graphical representation of the extent
of range of effective operation of the present invention
for the type of feed described in Table 2 for the upper
and lower streams 100. As an explanation of the features
of Figure 3, the actual trays in the additional rectifi-
cation section, section C200A, are shown as the axis
labeled "No. of Trays". The axis labeled "o Ethylene
Recovered from Deethanizer" describes the percentage of
the ethylene in upper and lower streams 200 in Figure 2
and recovered in stream 203 of Figure 2. The lines labeled
"980", "99= s" and "99.95%-" indicate the purity of the
ethylene obtained in stream 203 by operation of section
C200A according to the present invention from
fractionation of a cracked gas stream derived from
propane. It will be apparent to the skilled person that
a further extension of the plot shown in Figure 3 will
permit accurate evaluation of the stages necessary for
higher recovery of ethylene to the overhead product stream
of the deethanizer.
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18
The present invention may be advantageously used with
C2 Splitters of any configuration. The rectification of
a vapor from the sidedraw withdrawal stage in an
additional rectification section is critical to the =
practice of the present invention. As demonstrated by the
comprehensive results shown in Figure 3, the present
invention has wide ranging application for fractionation
of cracked gases with substantial savings in equipment and
refrigeration utilities costs.
In addition, Figures 4 and 5 are included to indicate
the change in composition of the vapor streams in columns
100 and 200. The pattern of separation of keys 1 and 2
are similar in Figures 4 and 5, although it is evident
that the same degree of separation takes place with fewer
actual trays for the deethanizer of the present invention,
whose operation is shown in Figure 5, when compared to the
number of actual trays required for the same separation in
the prior art deethanizer, whose operation is shown in
Figure 4.
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19
Table 1
Stream No. 100 (u r 100 (Iwr 101 102 104 109 110
Tem .=F -11 61 108 -32 -105 90 -30
Press. psia 240 240 242 232 59 102 241
Va or frac 1.00 0.00 0.00 0.00 1.00 1.00 0.00
Components
Lb-moVhr F
Methane 1 0.3 0.3 0.6 1.7 0.6
Ethvlene 1 3425.9 2938.3 6364.2 17796.1 3.5 6360.7
Ethane 1 320.6 496.9 0.3 817.2 1 10.9 1 809.4 7.8
MAPD 1 0.7 17.3 18.0
Proa lene 1 143.7 1365.7 1503.1 6.3 I 6.3
Prooane 66.1 1 847.2 1 912.6 0.7 1 0.7
C4's 0.1 9.4 9.5 I
Table 2
Stream No. 1 200 (uorl 200 (Iwr) 201 202 203 205 i 210 1 211
Temo. F -11 1 61 112 1 -26 -33 1-105 I 90 -30
Press. osia 1 240 1 240 1 253 1 232 1 240 1 59 1 102 1 241
Vaoor frac 1.00 1 0.00 1 0.00 1 0.00 I 0.00 1.00 1 1.00 1 0.00 20 Components
Lb-mol/hr
Methane 1 0.3 1 0.3 0.1 I 0.5 1 0.18 1 0.1
Ethvfene 1 3425.9 1 2938.3 1 4310.0 1 2054.2 1 12077.6 1 3.5 1 2050.7
Ethane 1 320.6 1 496.9 1 0.3 1 816.4 0.8 7.41 813.2 1 3.2
MAPD 1 0.7 I 17.3 1 18.0 1 3.2
Propvlene 143.7 I 1365.7 1504.8 14.6 I I
Proaane 1 66.1 847.2 ( 912.9 1 0.4
C4's 1 0.1 ! 9.4 9.5
Table 3
N cn. E100 E101 E102 E003~ E105 E107 E108
Duty, 29.4 50.9 16.6 69.4 3.1 4.2 40.66
MMBtu/
I 35 Exch. { E200 E201 E202 E203 &{ E205 E207 E208
No. I E204 I
Duty. 29.6 50.7 13.7 44.4 4.0 4.2 35.4
MMBtu/
hr
Table 4
Stage S100 S101 S102
No. I E
HP 792 9657 1 BASE
Stage S200 S201 8202
No.
HP 792 7307 BASE-996
