U.S. patent application number 13/412723 was filed with the patent office on 2013-09-12 for structured packing.
This patent application is currently assigned to AIR PRODUCTS AND CHEMICALS, INC.. The applicant listed for this patent is Patrick Alan Houghton, Swaminathan Sunder, Jonathan Wilson. Invention is credited to Patrick Alan Houghton, Swaminathan Sunder, Jonathan Wilson.
Application Number | 20130233016 13/412723 |
Document ID | / |
Family ID | 55304552 |
Filed Date | 2013-09-12 |
United States Patent
Application |
20130233016 |
Kind Code |
A1 |
Wilson; Jonathan ; et
al. |
September 12, 2013 |
Structured Packing
Abstract
An apparatus for a heat transfer or mass transfer process,
comprising a column or divided column having at least one pair of
converging walls or wall portions and, within at least a region of
the column or divided column bounded by at least one pair of
converging walls or wall portions, a structured packing having a
corrugation angle of at least about 50.degree.; a method of heat
and/or mass transfer applicable to the apparatus; and a method of
installation of structured packing into a relevant apparatus.
Inventors: |
Wilson; Jonathan; (Sale,
GB) ; Sunder; Swaminathan; (Allentown, PA) ;
Houghton; Patrick Alan; (Emmaus, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wilson; Jonathan
Sunder; Swaminathan
Houghton; Patrick Alan |
Sale
Allentown
Emmaus |
PA
PA |
GB
US
US |
|
|
Assignee: |
AIR PRODUCTS AND CHEMICALS,
INC.
Allentown
PA
|
Family ID: |
55304552 |
Appl. No.: |
13/412723 |
Filed: |
March 6, 2012 |
Current U.S.
Class: |
62/643 ; 261/158;
29/890.03 |
Current CPC
Class: |
B01J 2219/32244
20130101; B01J 2219/32483 20130101; B01J 2219/32213 20130101; B01J
2219/32408 20130101; B01J 19/32 20130101; B01J 2219/32268 20130101;
B01J 2219/3313 20130101; F25J 3/04412 20130101; F25J 3/04909
20130101; B01J 2219/32255 20130101; B01J 2219/32237 20130101; F25J
2290/12 20130101; B01J 2219/32275 20130101; B01D 3/32 20130101;
B01J 2219/32272 20130101; B01J 2219/32227 20130101; B01J 2219/3221
20130101; B01J 2219/3222 20130101; F25J 3/04678 20130101; F25J
3/04939 20130101; B01J 2219/3306 20130101; Y10T 29/4935
20150115 |
Class at
Publication: |
62/643 ;
29/890.03; 261/158 |
International
Class: |
F25J 3/00 20060101
F25J003/00; F28C 3/06 20060101 F28C003/06; B21D 53/02 20060101
B21D053/02 |
Claims
1. An apparatus for a heat transfer or mass transfer process,
comprising a column or divided column having at least one pair of
converging walls or wall portions and, within at least a region of
the column or divided column bounded by at least one pair of
converging walls or wall portions, a structured packing having a
corrugation angle of at least about 50.degree..
2. The apparatus according to claim 1, in which the column
cross-section or divided column cross-section comprises at least
one corner.
3. The apparatus according to claim 2, wherein the at least one
corner or angle is an internal angle or less than or equal to about
120.degree..
4. The apparatus according to claim 1, in which the corrugation
angle of the structured packing is from about 55.degree. to about
65.degree..
5. The apparatus according to claim 1, wherein the corrugation
angle of the structured packing is about 60.degree..
6. The apparatus according to claim 1, in which the column or
divided column is a column division formed within a column having a
circular cross-section by providing at least one dividing wall
within the column which divides the cross-section of the column
into at least two divisions such that the dividing wall converges
with the wall of the column to form at least one corner.
7. The apparatus according to claim 1, wherein the column size is
greater than about 0.5 m in diameter, where the column is of
circular cross-section, or of greater than the equivalent
cross-sectional area where the cross-section is of another
shape.
8. The apparatus according to claim 1, wherein the mass transfer
process is a cryogenic separation process in which air is separated
into nitrogen-, oxygen- and argon-enriched streams.
9. A method of heat and/or mass transfer, comprising supplying one
or more fluids to a column or column division having at least one
pair of converging walls or wall portions and which, within at
least a region of the column or column division bounded by at least
one pair of converging walls or wall portions, contains a
structured packing having a corrugation angle of at least about
50.degree. such that the one or more fluids contact the structured
packing in order to effect heat transfer and/or mass transfer.
10. The method according to claim 9, wherein the column
cross-section or column division cross-section comprises at least
one corner.
11. The method according to claim 9, wherein the at least one
corner or angle is an internal angle of less than or equal to about
120.degree..
12. The method according to claim 9, wherein the corrugation angle
of the structured packing is from about 55.degree. to about
65.degree..
13. The method according to claim 9, wherein the corrugation angle
of the structured packing is about 60.degree..
14. The method according to claim 9, in which the column or column
division is a column division formed within a column having a
circular cross-section by providing at least one dividing wall
within the column which divides the cross-section of the column
into at least two divisions such that the dividing wall converges
with the wall of the column to form at least one corner.
15. The method according to claim 9, wherein the column size is
greater than about 0.5 m in diameter, where the column is of
circular cross-section, or of greater than the equivalent
cross-sectional area where the cross-section is of another
shape.
16. The method according to claim 9, wherein the mass transfer
process is a cryogenic separation process in which air is separated
into nitrogen-, oxygen- and argon-enriched streams.
17. A method of installation of structured packing into an
apparatus for a heat and/or mass transfer process, which apparatus
comprises a column or column division having at least one pair of
converging walls or wall portions, comprising the steps of:
providing a structured packing having a corrugation angle of at
least about 50.degree., and installing the said structured packing
into at least a region of the column or column division bounded by
at least one pair of converging walls or wall portions.
18. The method according to claim 17, wherein the corrugation angle
of the structured packing is from about 55.degree. to about
65.degree..
19. The method according to claim 18, wherein the corrugation angle
of the structured packing is 60.degree..
20. The method according to claim 17, wherein the at least one pair
of converging walls or wall portions converges towards an internal
angle of less than or equal to about 120.degree..
21. The method according to claim 17, in which the column or column
division is a column division formed within a column having a
circular cross-section by providing at least one dividing wall
within the column which divides the cross-section of the column
into at least two divisions such that the dividing wall converges
with the wall of the column to form at least one corner.
22. The method according to claim 17, wherein the column size is
greater than about 0.5 m in diameter, where the column is of
circular cross-section, or of greater than the equivalent
cross-sectional area where the cross-section is of another
shape.
23. The method according to claim 17, wherein the mass transfer
process is a cryogenic separation process in which air is separated
into nitrogen-, oxygen- and argon-enriched streams.
Description
FIELD
[0001] The present invention generally relates to structured
packing. Structured packing has particular application in heat
and/or mass exchange columns, especially in cryogenic air
separation processes, although it may be used in other
applications, such as heat exchangers, for example.
BACKGROUND
[0002] The term "column" as used herein means a distillation or
fractionation column or zone, i.e. a column or zone wherein liquid
and vapour phases are countercurrently contacted to effect
separation of a fluid mixture, such as by contacting of the vapour
and liquid phases on packing elements or on a series of
vertically-spaced trays or plates mounted within the column.
[0003] A divided wall column is a system of thermally coupled
distillation columns. In divided wall columns, at least one
dividing wall is located in the interior space of the column. The
dividing wall generally is vertical. Two different mass transfer
separations may occur on either side of the dividing wall, for
example.
[0004] The term "packing" means solid or hollow bodies of
predetermined size, shape and configuration used as column
internals to provide surface area for the liquid to allow heat
and/or mass transfer at the liquid-vapour interface during
countercurrent flow of two phases. Two broad classes of packings
are "random" and "structured".
[0005] "Random packing" means packing wherein individual members
have no specific orientation relative to each other or to the
column axis. Random packings are traditionally small, hollow
structures with large surface area per unit volume that are loaded
at random into a column.
[0006] "Structured packing" means packing wherein individual
members have specific orientation relative to each other and to the
column axis. Structured packings usually are made of thin metal
foils stacked in layers.
[0007] In processes such as distillation, it is advantageous to use
structured packing to promote heat and/or mass transfer between
counterflowing liquid and vapour streams. Structured packing, when
compared with random packing or trays, offers the benefits of
higher efficiency for heat and/or mass transfer with lower pressure
drop. It also has more predictable performance than random
packing.
[0008] The separation performance of structured packing is often
given in terms of height equivalent to a theoretical plate (HETP),
which is the height of packing over which a composition change is
achieved that is equivalent to the composition change achieved by a
theoretical plate. The term "theoretical plate" means a contact
process between gaseous and liquid phases such that the existing
gaseous and liquid streams are in equilibrium. The smaller the HETP
of a particular packing for a particular separation, the more
efficient the packing, because the height of the packing bed being
used decreases with HETP.
[0009] Cryogenic separation of air is carried out by passing liquid
and vapour in countercurrent contact through a distillation column.
A vapour phase of the mixture ascends with an ever increasing
concentration of the more volatile components (e.g., nitrogen)
while a liquid phase of the mixture descends with an ever
increasing concentration of the less volatile components (e.g.,
oxygen). Various packings or trays may be used to bring the liquid
and gaseous phases of the mixture into contact to accomplish mass
transfer between the phases.
[0010] There are many processes for the separation of air by
cryogenic distillation into its components (i.e. nitrogen, oxygen,
argon, etc.). A typical cryogenic air separation unit 10 is shown
schematically in FIG. 1. High (or higher) pressure feed air 1,
typically at a pressure of from 2 to 10 bar (200 to 1000 kPa), is
fed into the base of a high (or higher) pressure distillation
column 2. Within the high pressure column 2, the air is separated
into nitrogen-enriched overhead vapour and oxygen-enriched bottoms
liquid. The oxygen-enriched bottoms liquid stream 3 is fed from the
high pressure distillation column 2, after suitable pressure
reduction (not shown), typically to a pressure of from 1.1 to 2
bara (110 to 200 kPa absolute), into a low (or lower) pressure
distillation column 4. Nitrogen-enriched vapour stream 5 is passed
into a condenser 6 where it is condensed to provide reboil to the
low pressure column 4. The nitrogen-enriched liquid stream 7 is
partially returned via stream 8 as reflux to the top of high
pressure column 2, and is partially fed via stream 9 into the top
of low pressure column 4 as liquid reflux.
[0011] Low pressure column 4 consists of a lower section 11, in
which is placed structured packing 20, and upper narrower section
12 in which is placed structured packing 21. A separate low
pressure column 13, also known as an auxiliary or sidearm column,
comprising structured packing 22 is provided for production of an
argon-enriched stream 14.
[0012] In the low pressure column 4, the streams 3 and 9 are
separated by cryogenic distillation into oxygen-rich and
nitrogen-rich components. Structured packings 21 and 20 may be used
to bring into contact the liquid and gaseous phases of the oxygen
and nitrogen to be separated. The nitrogen-rich overhead component
is removed as a vapour stream 16. The oxygen-rich bottoms component
is removed as a liquid stream 17. Alternatively the oxygen-rich
component can be removed from a location in the sump surrounding
reboiler/condenser 6 as a vapour. A waste stream 15 also is removed
from the low pressure distillation column 4.
[0013] Feed stream 18 is removed from an intermediate point between
lower section 11 and upper section 12 of low pressure column 4 and
is passed to column 13. A condenser 25 is provided in the upper
portion of column 13 to generate a reflux from the feed stream 18.
Passage of this feed stream in countercurrent flow with reflux from
condenser 25 through structured packing 22 creates an
argon-enriched overhead vapour stream 14, and oxygen-enriched
bottoms liquid stream 19 which is returned to the low pressure
column 4 above structured packing 20 and below structured packing
21 as reflux.
[0014] FIG. 2 shows an alternative arrangement for cryogenic
distillation of air to provide nitrogen, oxygen and argon, in which
a divided low pressure column is used in place of the separate
column 13 for argon production in FIG. 1. Such an arrangement is
described in, for example, U.S. Pat. No. 6,240,744 (Agrawal et
al.). Features in common with the arrangement in FIG. 1 have the
same reference numbers. In this arrangement, the vapour stream
leaving the top of structured packing 20 at the lower end of low
pressure column 4 is divided into two portions, of which the first
rises to the structured packing 21 and the second rises to the
structured packing 22 which is divided from the structured packing
21 by dividing wall 23 and from the upper part of low pressure
column 4 by end wall 24. In the Figure, the dividing wall is shown
as a flat wall centrally mounted in the low pressure column 4, such
as to divide the column 4 along its diameter into two equally sized
sections of semi-circular cross section; however, as explained in
U.S. Pat. No. 6,240,744, many other arrangements for the separation
of structured packing 22 from structured packing 21 are possible.
The vapour portion entering structured packing 21 is separated into
a nitrogen-rich overhead vapour stream and an oxygen-rich bottoms
liquid stream as described for FIG. 1. The vapour portion entering
structured packing 22 is separated into an argon-enriched overhead
vapour stream 14 and an oxygen-enriched bottoms liquid, as
described for column 13 in FIG. 1, but without requiring the
additional expense and complication of providing the second column
13. Further, with the arrangement of FIG. 2 it is not necessary to
adapt low pressure column 4 by the narrowing depicted in FIG. 1 to
compensate for the withdrawal of vapour from column 4 to pass to
column 13 in order that the mass transfer performance of column 4
is maintained despite the reduced vapour flow in the upper section
12 compared with lower section 11.
[0015] US2010/0096249 (Kovak) describes a divided exchange column
into which trays or structured packing are placed. The document
discloses division of the column into two sections by a chord wall
(both equal and unequal divisions are contemplated) and also
division of the column into three sections by means of radial walls
intersecting at the centre of the cylindrical column.
[0016] Other prior art relating to the structure of columns used in
cryogenic distillation of air includes:
[0017] U.S. Pat. No. 5,339,648 (Lockett et al.), U.S. Pat. No.
5,946,942 (Wong et al.), EP1162423 (Messer AGS GmbH),
US2006/0005574 (Glatthaar et al.), U.S. Pat. No. 7,357,378 (Zone et
al.), U.S. Pat. No. 6,250,106 (Agrawal et al.), US2006/0260926
(Kovak), and U.S. Pat. No. 5,669,236 (Billingham et al.).
[0018] None of the prior art known to the inventors considers in
detail the nature of the structured packing required to obtain an
optimal result in columns that are not circular in cross-section
but have a cross-section having at least one pair of converging
walls or wall portions, such as those including at least one corner
or angle in their cross-section, such as divided columns. The prior
art instead simply discusses the generic use of any structured or
random packing and/or trays in partitioned or divided wall
columns.
[0019] Structured packing is defined in the present invention as a
thin metal or plastic foil that has been perforated, fluted and
corrugated to meet specific requirements for its intended
application. A representation of a typical structured packing is
shown in FIG. 3, in which is shown a foil 40 which is corrugated by
folding along fold lines 45, and which has a pattern of fluting,
that is, depressions and/or elevated areas 50 in the form of
horizontal striations, formed for example by embossing the foil 40,
and a pattern of perforations, or through holes, 55. The
perforations 55 and the texture formed by elevated/depressed areas
50 aid liquid/vapour spreading on the surface of foil 40, thus
improving the heat and mass transfer efficiency of the packing.
Typically, the surface area of the foil occupied by the
perforations is from about 5% to about 20%. Typically, the fluting
may be in the form of horizontal striations, or a bidirectional
surface texture in the form of fine grooves in crisscrossing
relation.
[0020] Within a layer of structured packing in a column, multiple
foils are oriented vertically (that is to say, with the plane of
the foil substantially parallel to the axis of the column), with
adjacent foils having their corrugations oriented transversely
(that is to say, if a first foil has its corrugations running from
bottom left to top right, an adjacent foil will be oriented such
that its corrugations run from bottom right to top left). Such an
arrangement is depicted in FIG. 3 of U.S. Pat. No. 4,296,050
(Meier). It is conventional to rotate successive layers of
structured packing, typically by an angle of about 90.degree. about
the column axis with respect to the underlying layer, in order to
improve the flow characteristics. Such an arrangement is shown in
FIG. 4 of U.S. Pat. No. 4,296,050 (Meier). However, each rotation
increases the pressure drop through the column comprised of the
packing.
[0021] EP1036590 (Sunder et al.) describes optimum ranges of
several packing parameters, e.g. a surface area density of from
about 350 to about 800 m.sup.2/m.sup.3, a corrugation angle (i.e.
the angle between the horizontal and the longitudinal axis of the
corrugation when the packing element is vertical in the column) of
from about 35 to about 65.degree., and open area of perforations of
from about 5 to about 20%. There is no discussion in this document
of the use of divided wall columns or non-cylindrical columns.
[0022] U.S. Pat. No. 5,876,638 (Sunder et al.) and U.S. Pat. No.
5,901,575 (Sunder) also discuss developments in structured
packing.
SUMMARY
[0023] It is an aim of the present invention to provide a
structured packing that is optimised for use in a column whose
cross-section is not wholly rounded, that is, a column whose
cross-section has at least one pair of converging walls or wall
portions, such as a divided wall column in which the division
creates at least one corner or angle within the column. In
particular, it is an aim of the present invention to provide a
structured packing that is optimised for use in such a column in a
cryogenic distillation apparatus, in particular one used in the
separation of components of air.
[0024] Accordingly, in a first aspect, the present invention
provides an apparatus for a heat transfer or mass transfer process,
comprising a column or divided column having at least one pair of
converging walls or wall portions and, within at least a region of
the column or divided column bounded by, or lying between, at least
one pair of converging walls or wall portions, a structured packing
having a corrugation angle of at least about 50.degree..
[0025] In a second aspect, the present invention provides a method
of heat and/or mass transfer, comprising supplying one or more
fluids to a column or column division having at least one pair of
converging walls or wall portions and which, within at least a
region of the column or divided column bounded by, or lying
between, at least one pair of converging walls or wall portions,
contains a structured packing having a corrugation angle of at
least about 50.degree. such that the one or more fluids contact the
structured packing in order to effect heat transfer and/or mass
transfer.
[0026] In the context of the present invention, a column division
is a part of a column physically separated from the remainder of
the column by at least one dividing wall arranged substantially to
co-extend with the longitudinal axis of the column. That is, where
the column has its longitudinal axis positioned vertically, as is
usual in use, the or each column division is created by the
presence of a substantially vertical wall within the column that
physically segregates a part of the column volume from the
remainder, such as to prevent the mixing of fluid present in the
column division with fluid present in the remainder of the column
over the vertical distance over which the dividing wall
extends.
[0027] It is believed by the present inventors that the presence of
at least one pair of converging walls or wall portions, and in
particular the presence of an angle or corner, in the cross-section
of a column or column division restricts the mixing of fluid within
the column in an edge zone close to the column or column division
wall, resulting in reduced efficiency of mass transfer and/or heat
transfer within the column where a structured packing optimised for
use in a cylindrical column is used. The present invention provides
benefit in terms of cost savings and increased efficiency of mass
transfer by use of a structured packing optimised for use in a
column or column division having at least one pair of converging
walls or wall portions, which optimisation has not previously been
considered necessary.
[0028] In a third aspect, the present invention provides a method
of upgrading an apparatus for a heat transfer or mass transfer
process, which apparatus comprises a column or column division
whose cross-section comprises at least one pair of converging walls
or wall portions and which contains a structured packing having a
corrugation angle of less than about 50.degree., comprising the
steps of:
removing the structured packing having a corrugation angle of less
than about 50.degree. from at least a region of the column or
column division bounded by, or lying between, the at least one pair
of converging walls or wall portions, and replacing the structured
packing having a corrugation angle of less than about 50.degree.
with a structured packing having a corrugation angle of at least
about 50.degree..
[0029] Preferably, the structured packing having a corrugation
angle of about 50.degree. or more has a corrugation angle of about
55.degree. or more.
[0030] In a fourth aspect, the present invention provides a method
of installation of structured packing into an apparatus for a heat
and/or mass transfer process, which apparatus comprises a column or
column division having at least one pair of converging walls or
wall portions, comprising the steps of:
providing a structured packing having a corrugation angle of at
least about 50.degree., and installing the said structured packing
into at least a region of the column or column division bounded by,
or lying between, at least one pair of converging walls or wall
portions.
[0031] The following preferred features apply to all aspects of the
invention, where appropriate, and may be combined.
[0032] The term "at least one pair of converging walls or wall
portions" describes the situation wherein column walls, or parts of
column walls, approach each other increasingly closely. The walls
or parts of walls need not intersect or contact one another as a
result of their convergence, but may intersect or contact one
another to form an angle or corner in the cross-section of the
column.
[0033] Suitably, the structured packing is used across the whole of
the cross-sectional area of the said column or column division, and
not only in a region bounded by, or lying between, at least one
pair of converging walls or wall portions.
[0034] Preferably, the corrugation angle of the structured packing
used in the present invention is between about 50.degree. and about
70.degree., more preferably between about 55.degree. and about
65.degree., and is most preferably about 60.degree..
[0035] Preferably, the column or column division comprises at least
one internal angle of less than or equal to about 120.degree. in
its cross-section, more preferably less than or equal to about
100.degree., such as less than or equal to about 90.degree.. It is
believed that the disruption to mixing within the edge zone
increases with the acuteness of the angle or angles present in the
cross-section of the column or column division, and so greater
benefit is obtained for the present invention where the angle or
angles are more acute.
[0036] Suitably, the column or column division cross-section may be
an irregular cross-section which includes a corner or angle, or may
be an irregular or regular polygon, or may be a figure formed by
the intersection of a chord with a circle or other rounded shape
resulting in one or more angles or corners. For example, the column
or column division may be of hexagonal, pentagonal, square,
rectangular, triangular, semicircular, part-circular, or
quarter-circular cross-section. Again, it is expected that the
benefits of the invention will be greater the more acute the angle
or angles present in the cross-section.
[0037] The column or column division may comprise one, two, three,
four, five, six or more angles or corners. It is expected that the
benefit of the invention will increase with the number of angles
present that are able to disrupt mixing. Preferably, at least one,
and more preferably all, of the angles are about 120.degree. or
less, such as about 100.degree. or less, more preferably about
90.degree. or less, such as about 70.degree. or less.
[0038] Preferably, the invention is applied to a column division
formed by providing at least one dividing wall within the column
which is in contact with the outer wall of the column in at least
one place. Preferably, the column which is to be divided has a
circular cross-section, and at least one of the divisions of the
column thus formed has a non-circular cross-section which comprises
at least one angle or corner. Suitably, the column may be divided
into more than two divisions, such as three, four, five, six, ten
or twenty divisions, by an appropriate number of dividing walls,
which may intersect each other and/or the column wall to form the
required number of divisions. The dividing walls may be the same as
one another or may take different forms, and may individually form
a straight line or a curved line within the cross-section of the
column to be divided. The dividing walls may be the same lengths or
different lengths, and the divisions formed may be regular or
irregular shapes or polygons, and may be of the same or different
cross-section and/or cross-sectional area as one another. These
parameters can be selected depending on the intended use of the
divided column.
[0039] Preferably, however, the column is divided into two
divisions by a single dividing wall. Where the column which has
been divided has a circular cross-section, preferably the dividing
wall is a chord wall. Where the column to be divided is not
circular in cross-section, the column is preferably divided by a
dividing wall that extends across the cross-section of the column
such that each end of the dividing wall intersects the column wall
in different places. In either case, the cross-sectional areas of
the column divisions may be selected according to the required flow
of fluid through each column division. Suitably, where the flow
through each division is to be equal, the column divisions are of
equal cross-sectional area, and, in this case, where the column
which has been divided has a circular cross-section, the column
divisions are of semi-circular cross-section.
[0040] Preferably, the structured packing has a surface density of
from about 350 to about 800 m.sup.2/m.sup.3. Preferably, the
fluting of the structured packing is in the form of horizontal
striations. Preferably, the open area of the perforations is in the
range of from about 5 to about 20%.
[0041] Preferably, the column size is greater than about 0.5 m in
diameter, such as greater than or equal to about 0.9 m in diameter,
more preferably greater than or equal to about 1 m in diameter,
where the column is of circular cross-section, or is of greater
than the equivalent cross-sectional areas (that is, greater than
about 0.196 m.sup.2, greater than or equal to about 0.64 m.sup.2,
and greater than or equal to about 0.79 m.sup.2 respectively) where
the cross-section is of another shape.
[0042] Preferably, the maximum column diameter is about 15 m, such
as about 10 m, about 9 m, about 8 m, about 7 m, about 6 m, about 5
m, or about 4 m, for a column of circular cross-section. Again,
where the column cross-section is of another shape, the maximum
column size is of the corresponding maximum cross-sectional area,
that is, about 177 m.sup.2, about 78.5 m.sup.2, about 63.6 m.sup.2,
about 50.3 m.sup.2, about 38.5 m.sup.2, about 28.3 m.sup.2, about
19.6 m.sup.2, or about 12.6 m.sup.2 respectively.
[0043] Preferably, the invention is applied in a cryogenic
separation process, such as a cryogenic air separation (cryogenic
distillation) process, which includes, but is not limited to, the
separation of air into nitrogen-enriched, oxygen-enriched and
argon-enriched streams. Thus, it may be applied to the cryogenic
separation of air into nitrogen- and oxygen-enriched streams.
Suitably, the invention is applied to the cryogenic separation of
air into oxygen- and argon-enriched streams.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] The foregoing summary, as well as the following detailed
description of exemplary embodiments, is better understood when
read in conjunction with the appended drawings. For the purposes of
illustrating embodiments of the invention, there is shown in the
drawings exemplary embodiments of the invention; however, the
invention is not limited to the specific methods and instruments
disclosed. In the drawings:
[0045] FIG. 1 shows a schematic representation of a known
arrangement for the cryogenic distillation of air;
[0046] FIG. 2 shows a schematic representation of a known
arrangement for the cryogenic distillation of air, in which a
divided wall (partitioned) column is used;
[0047] FIG. 3 shows a schematic representation of structured
packing;
[0048] FIG. 4 shows a schematic representation of the longitudinal
axes of corrugation in structured packing when placed in a
column;
[0049] FIG. 5 shows in plan view the appearance of structured
packing in (a) a column having a circular cross-section, and (b) a
column having a semi-circular cross-section;
[0050] FIGS. 6 to 15 illustrate plan views of examples of column
arrangements to which the present invention is applicable;
[0051] FIG. 16 shows the results obtained for a prior art
structured packing when used in a column having a circular
cross-section and a column having a semi-circular cross-section in
terms of HETP;
[0052] FIG. 17 shows the results obtained for a prior art
structured packing when used in a column having a circular
cross-section and a column having a semi-circular cross-section in
terms of pressure drop;
[0053] FIG. 18 shows the results obtained for a structured packing
according to the present invention when used in a column having a
circular cross-section and a column having a semi-circular
cross-section; and
[0054] FIG. 19 shows the results obtained in terms of pressure drop
for a structured packing according to the present invention when
used in a column having a circular cross-section and a column
having a semi-circular cross-section.
DETAILED DESCRIPTION
[0055] The present invention is applicable to columns in which, in
the plan view of the column, there are at least one pair of
converging walls or wall portions, such as where one or more
dividing walls create angles or corners, or such as where the
cross-section is not wholly rounded but instead has at least one
angle or corner. It is believed by the present inventors that the
advantages of the present invention in terms of separation
efficiency are obtained for all such columns.
[0056] When designing an optimum structured packing for a
particular column, the skilled person is aware that a number of
"trade-offs" are used in determining the best overall parameters.
For example, the mass transfer efficiency and pressure drop are
found to be higher for a corrugation angle of 45.degree. than for a
corrugation angle of 60.degree., whereas the operating capacity is
lower for a 45.degree. than 60.degree. corrugation angle; these
effects must be balanced in order that the chosen packing exhibits
acceptable mass transfer efficiency, pressure drop and operating
capacity for a particular column.
[0057] When passing through structured packing in a column, fluid
flows mainly along the channels formed by the corrugations in the
foil. Part of this fluid flowing in these channels mixes with the
fluid flowing in the adjacent criss-crossing channels which are in
a transverse diagonal direction as explained above. Also due to the
presence of apertures in the foil, some of the fluid mixes with
fluid flowing through adjacent channels. Fluid mixing in these ways
is important to correct any composition imbalance that may develop
within a cross-section of a distillation column, and is a
significant factor in the separation efficiency of the column. It
can be seen from FIG. 4 that a 45.degree. angle provides a longer
lateral distance compared with a 60.degree. angle within the column
for the fluid to travel in each layer of structured packing, and
thus this provides a larger area for mixing of fluid from separate
channels to take place within each layer of structured packing.
[0058] EP1036590 teaches an optimum corrugation angle of
35.degree.-65.degree. for cylindrical columns. Within this range it
is a common industrial practice to use packings with a corrugation
angle of about 45.degree. to provide the trade off of parameters
such as mass transfer efficiency, pressure drop and operating
capacity as described above for a particular cylindrical
column.
[0059] The present inventors are aware of no prior art in which is
discussed the optimisation of structured packing for either
non-cylindrical columns or divided columns in which there is at
least one pair of converging walls or wall portions, or in which it
is disclosed or suggested that the optimal parameters for
structured packing for use in either non-cylindrical columns or
divided columns in which there is at least one pair of converging
walls or wall portions are different from those for conventional
cylindrical columns. However, the present inventors have
surprisingly discovered that the optimum packing for a divided
column or a column in which there is at least one pair of
converging walls or wall portions is different from that for a
column of circular cross-section. It is believed that this
difference is due to the difference in mixing behaviour of fluids
in the column at the "edge zone", explained below, for the two
types of column.
[0060] It has surprisingly been found by the present inventors that
the use of a corrugation angle of about 60.degree. in a column or
column division in which the cross-section has at least one pair of
converging walls or wall portions provides the same separation
efficiency as the use of the same packing in a cylindrical column.
However, use of packing having a corrugation angle of about
45.degree. in a column or column division in which the
cross-section has at least one pair of converging walls or wall
portions results in significant degradation of the separation
efficiency compared with the same packing used in a cylindrical
column.
[0061] In a cylindrical column, fluid can flow freely within the
annular edge zone close to the column wall. The edge zone is the
lateral distance from the column wall in which a corrugation
channel in the structured packing will end at the wall rather than
at the structured packing in the layer above or the layer below. It
is calculated as (layer height of the structured packing)/(tan
[corrugation angle]). A typical layer height for such structured
packing is about 200 mm, and with a corrugation angle of 45.degree.
an annular edge zone of about 200 mm would be present within which
mixing may take place. However, in a column or division of a column
where at least one pair of converging walls or wall portions is
present, fluid instead tends to accumulate in the region in which
the walls or wall portions converge, and so thorough mixing and
composition balancing of the fluids in these regions does not take
place. As a result, column performance in terms of separation
efficiency is impaired compared with an equivalent cylindrical
column.
[0062] Without wishing to be bound by theory, one possible
explanation by the present inventors is that the maintenance of the
separation efficiency for the 60.degree. corrugation angle packing
is as a result of a smaller edge zone close to the wall of the
column or column division formed when using structured packing
having a corrugation angle of about 60.degree. compared with that
observed for a structured packing having a corrugation angle of
about 45.degree.. As a result, the expected increase in composition
imbalance due to poor mixing of fluids in the region of the column
in which the walls or wall portions converge is significantly
reduced or avoided completely, and so separation efficiency is
maintained compared with use of the same packing in a cylindrical
column.
[0063] Accordingly, the optimum structured packing corrugation
angle for use in a column or column division in which the
cross-section has at least one pair of converging walls or wall
portions is different from the angle in the prior art for
cylindrical columns. None of the prior art of which the inventors
are aware discusses any possible difference in the performance of
structured packing in columns of different cross-section, despite
the widespread use of divided columns in the distillation industry
for over 50 years.
[0064] The finding that separation efficiency does not degrade at a
corrugation angle of about 60.degree. for a column or divided
column having at least one pair of converging walls or wall
portions relative to a more commonly used angle of about 45.degree.
permits advantage to be taken of the higher operating capacity of
60.degree. corrugation angle packing--i.e. the "trade-off" for this
column unexpectedly shifts in favour of a corrugation angle of
about 60.degree.. This is of particular benefit in a system such as
that depicted in FIG. 2, in which a divided column is used, as the
higher operating capacity and cost benefits of such a system can be
obtained with the present invention without requiring a 30-50%
addition to the height of the divided column to compensate for the
poorer mass transfer characteristics of that column when used in
conjunction with a conventional 45.degree. corrugation angle
structured packing.
[0065] Examples of columns to which the present invention is
applicable are shown in FIGS. 6-15. It will be appreciated by the
skilled person that these are merely examples, and that other
column cross-sections or divided wall column arrangements are
possible to which the invention will equally apply.
[0066] FIG. 6 shows a column in plan view that has been divided by
a plurality of intersecting dividing walls into a number of column
zones each with a square cross-section. FIG. 7 shows a column in
plan view that has been divided by a plurality of intersecting
dividing walls into a number of column zones each with a hexagonal
cross-section. It will be appreciated that other arrangements of
tessellating polygonal column zone cross-sections may be used.
Equally, it will be appreciated that the benefits of the present
invention will be obtained with the use of a single column having a
polygonal cross-section. It is expected that the benefits of the
invention will be greater for the use of square or rectangular
cross-section columns, such as those shown in FIG. 6, than for
hexagonal columns such as those shown in FIG. 7, as the angles
formed between the column walls are more acute for the square or
rectangular columns, and so the effect of the angles in creating
edge zones in which fluid mixing is reduced is expected to be
greater. The closer to circular is the cross-section of the column
or column division, the less the benefit of the present invention
will be obtained.
[0067] FIG. 8 shows a column in plan view in which two dividing
walls each span the radius of the circular cross-section of the
column in order to divide one-quarter of the area of the
cross-section from the remaining three-quarters. An angle or corner
of 90.degree. is formed at the middle of the cross-section of the
column, and at the intersections of the dividing walls with the
circumference of the column the dividing walls are perpendicular to
a tangent to the column wall at the point of intersection. It is
anticipated that the benefit of the invention will be obtained in
both of the column divisions thus formed. In the smaller division,
the angles between the two dividing walls and between each dividing
wall and the circular column wall are expected to restrict fluid
mixing significantly. Similarly, the angles formed between each
dividing wall and the circular column wall in the larger division
are expected significantly to reduce column mixing; the reflex
angle at the junction of the two dividing walls at the centre of
the column may also affect fluid mixing, but this is expected to be
to a much lesser extent than that observed at the acute angles at
the circular column wall.
[0068] Again, it will be appreciated that other arrangements of two
dividing walls are possible in which a larger or smaller angle is
formed between the two dividing walls.
[0069] FIG. 9 shows a column in which a dividing wall which is a
chord of the circular cross-section of the column splits the column
cross-sectional area into two unequal parts. An angle or corner is
formed at each end of the dividing wall where it intersects the
circumference of the column. It is expected that the advantages of
the present invention will be obtained in both of the divisions of
the column, but that a greater degree of advantage will be obtained
for the smaller division, as the more acute angles formed between
the circular column wall and the dividing wall are expected to more
significantly retard fluid mixing close to these corners.
[0070] FIG. 10 shows a column having two dividing walls that are
chords of the circular cross-section of the column, which in this
case are placed parallel to one another to define three regions
within the column: two sectors and an area between the sectors
crossing the centre of the column which is close to rectangular in
cross-section. Again, angles or corners are formed where the chords
intersect the circumference of the column. The benefit of the
present invention is expected to be greater for the two sector
divisions, having more acute angles formed at the junctions between
the dividing wall and the circular column wall, than the centre
division, although all three areas are expected to derive some
benefit from the invention.
[0071] FIG. 11 shows a column having three dividing walls that are
chords of the circular cross-section of the column, and which each
intersect an adjacent dividing wall at the point where they
intersect the circumference of the column such that they define a
column region of triangular cross-section in the centre of the
column. Angles or corners are formed at each point of intersection.
It will be appreciated that alternative arrangements are possible
in which the walls need not intersect one another when intersecting
the circumference (that is, a number of sectors may be formed whose
dividing walls do not intersect with one another), and that more
than three dividing walls may be provided that are chords to the
circumference of the column. It is expected that the benefit of the
present invention would be obtained for all of the divisions of
this column.
[0072] FIG. 12 shows a column in which the dividing walls do not
intersect the column wall but define a region of triangular
cross-section within the column. Similarly, FIG. 13 shows an
arrangement where the dividing walls enclose a region of square
cross-section within the column without any of the dividing walls
intersecting with or contacting the column wall. It will be
appreciated that more walls may be provided to enclose regions
having different polygonal cross-sections. Also, one or more of the
dividing walls may intersect with the circumference of the column,
and the walls may be of different lengths, thus resulting in a
central region of irregular polygonal cross-section. It is expected
that the benefit of the present invention will be obtained in both
areas formed by the column division, as fluid mixing will be
restricted both by the converging walls in the outer division and
the angles formed by the intersecting walls of the inner division,
but that the benefit will be greater for the inner division than
the outer division. The benefit obtained by the triangular central
area in FIG. 12 is expected to be greater than that obtained for
the square inner area in FIG. 13 due to the more acute angles of
the column in the former case.
[0073] FIG. 14 shows a column in which a dividing wall encloses a
region having circular cross-section, which dividing wall contacts
the circumference of the column. Thus, the dividing wall and the
column circumference converge to form angles at the point of
contact between the dividing wall and the column circumference. It
will be appreciated that the dividing wall need not enclose a
circular area but may form any generally rounded shape. It will
also be appreciated that the dividing wall need not contact the
column wall, but must be arranged such that at least a part of the
dividing wall and at least a part of the column wall converge. It
is expected in this case that the benefit of the present invention
will be obtained only in the outer column division, as the inner
column division is of circular cross section, whereas the acute
angles formed by the contact between the column wall and the
dividing wall will have a restrictive effect on fluid mixing in the
outer column section.
[0074] FIG. 15 shows a column in which three dividing walls extend
radially from the centre of the column to the circular column wall,
thus dividing the cylindrical column into three equal segments. The
benefit of the present invention is expected to be obtained for all
of the segments due to the restricted fluid mixing caused in the
corners of each segment.
EXAMPLES
[0075] A comparison of the performance of structured packing
according to the prior art with structured packing according to the
invention was conducted in a cryogenic distillation apparatus
including either a column with a D-shaped cross-section or a column
with a circular cross section for the separation of argon from
oxygen. For the column with a circular cross section, approximately
20 layers of packing, where each layer of packing is approximately
210 mm in height and 900 mm in diameter, are stacked on top of each
other at 90.degree. orientations inside a cryogenic distillation
column. For the column with a D-shaped cross-section, approximately
20 layers of packing, where each layer of packing is approximately
210 mm in height and has a 900.times.450 mm semi-circular area, are
stacked on top of each other at 90.degree. orientations inside a
cryogenic distillation column. All the comparisons were conducted
under total internal reflux at a column pressure of 0.4 barg (40
kPa gauge). The separation of binary mixtures of Argon/Oxygen were
studied by measuring the composition of the liquid and vapour
streams entering and leaving the column to ascertain the mass
transfer efficiency and pressure drop. Both structured packings
used conform to the general type shown in FIG. 3, with horizontal
striations, open area of perforations of about 10%, and each has a
surface area of approximately 500 m.sup.2/m.sup.3. The structured
packings differ in their corrugation angle, which is 45.degree. in
the case of the prior art packing (Packing A) and 60.degree. in the
case of the packing according to the invention (Packing B). The
performance is presented in terms of the HETP and the measured
dynamic pressure drop through both packings, both of which are
presented as functions of K.sub.v, the density corrected
superficial gas velocity, which is defined as:
K.sub.v=U[(.rho..sub.v/.rho..sub.L-.rho..sub.v).sup.0.5],
wherein U=superficial velocity of the vapour phase in the column in
m/s; .rho..sub.v=density of the vapour phase in the column in
kg/m.sup.3; and .rho..sub.L=density of the liquid phase in the
column in kg/m.sup.3. The values of HETP, K.sub.v and pressure drop
have been normalized in order to compare the performance of the
packings in the two different forms of columns used.
[0076] The results obtained for the prior art structured packing A
in a column of circular cross-section and a column of semi-circular
cross-section are shown in FIGS. 16 and 17. Two datasets are shown
for circular cross-section columns (circular and triangular
datapoints) and one for semi-circular cross-section columns
(rhomboid datapoints). It can be seen from FIG. 16 that the HETP
value is higher by around 30-50% (depending on K.sub.v), and thus
the separation efficiency is lower, for this packing in a
semi-circular cross-section column compared with a column of
circular cross-section. The curves begin to converge at a
normalized K.sub.v of around 1.35, at which point both curves begin
to show a steep increase of HETP with increasing K.sub.v, which
latter observation implies that the operating capacity is similar
for both the columns used. FIG. 17 shows that the pressure drop of
packing A is similar in both column types used. In addition, the
loading point of both columns, which in this case is defined as the
K.sub.v at which the pressure drop increase becomes more rapid with
further incremental increases in K.sub.v, are similar at a K.sub.v
of 1.05.
[0077] The results obtained for the structured packing B according
to the present invention are shown in FIGS. 18 and 19. Again, two
datasets are shown for circular cross-section columns (in this
case, circular and rhomboid datapoints) and one for semi-circular
cross-section columns (triangular datapoints). It can be seen from
FIG. 18 that a structured packing B with a corrugation angle of
60.degree. (compared to 45.degree. for the prior art packing A)
unexpectedly does not show a significant increase in HETP for the
semi-circular cross-section column compared with the circular
cross-section column (compare with FIG. 16). In fact, the
datapoints for the semi-circular column in general lie between the
two sets of datapoints for the circular cross-section column. As
for FIG. 16, the HETP for all datasets starts to increase rapidly
on increase of K.sub.v at a similar value, so again it can be
inferred that the operating capacity for the packing is similar for
both column types used. FIG. 19 shows, similarly to FIG. 17, that
the pressure drop of the packing and the loading point of the
packing is similar in both column types used.
[0078] An overall comparison of the performance of the two packings
in the two column types is presented in Table 1 in terms of
relative HETP and relative operating capacity. The relative HETPs
are representative of the HETPs along the flatter parts of the
curves in FIGS. 16 and 18, prior to the rapid increase in HETP at
higher K. Relative operating capacities are evaluated at the
K.sub.v at which the HETPs start to converge.
TABLE-US-00001 TABLE 1 Relative HETP Relative operating Corrugation
(semicircular/ capacity (semicircular/ Angle (.degree.) circular)
circular) Packing A 45 1.3-1.5 1.0 Packing B 60 1.0 1.0
[0079] Thus, it can be seen that the operating capacity of Packing
A is similar in both the semi-circular and circular cross-section
column. The operating capacity of Packing B is also similar in both
the semi-circular and circular cross-section column, although
higher than Packing A. The higher corrugation angle structured
packing B also provides a similar separation efficiency in both the
semi-circular and circular cross-section column, whereas Packing A
loses separation efficiency in the semi-circular cross-section
column. Thus, use of packing B in semi-circular columns permits use
of a lower column height than would otherwise be expected if the
relative HETP was the same as Packing A, and thus use of Packing B
provides a more cost-effective trade-off of height versus operating
capacity than use of packing A.
[0080] Whilst the invention has been described with reference to a
preferred embodiment, it will be appreciated that various
modifications are possible within the scope of the invention.
[0081] In this specification, unless expressly otherwise indicated,
the word `or` is used in the sense of an operator that returns a
true value when either or both of the stated conditions is met, as
opposed to the operator `exclusive or` which requires that only one
of the conditions is met. The word `comprising` is used in the
sense of `including` rather than in to mean `consisting of`. All
prior teachings acknowledged above are hereby incorporated by
reference.
* * * * *