U.S. patent number RE34,038 [Application Number 07/708,948] was granted by the patent office on 1992-08-25 for separating argon/oxygen mixtures using a structured packing.
This patent grant is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to Douglas L. Bennett, Keith A. Ludwig, George S. Witmer, Charles M. Woods.
United States Patent |
RE34,038 |
Bennett , et al. |
August 25, 1992 |
Separating argon/oxygen mixtures using a structured packing
Abstract
The present invention relates to improvements to a process and
apparatus for the cryogenic distillation of mixtures, which
comprise oxygen, nitrogen and argon, e.g. air. The improvement
comprises effectuating intimate contact of the liquid and vapor
phase streams utilizing a structured packing to effectuate the
intimate contact in at least .[.those.]. .Iadd.one of the
.Iaddend.regions of the distillation column system where argon
concentration is within the range from about 0.6 to about 75 volume
percent.[.,.]. and .[.operating the process such that.]. the
densimetric superficial gas velocity .[.in those regions.]. is at
least 0.06 feet per second. .[.The present invention also relates
to a method for improving a cryogenic distillation apparatus having
at least one distillation column for the separation of mixtures
containing oxygen and argon, comprising the step of replacing the
distillation trays with a structured packing in at least those
regions of the distillation column system where the concentration
of argon is in the range from about 0.6 to about 75 volume
percent..]. .Iadd.The present invention also relates to the use of
structured packing to reduce the HETP (height of packing equivalent
to a theoretical plate) in a region of a distillation column system
separating oxygen and argon by intimately contacting a liquid phase
stream containing oxygem and argon, and a vapor phase stream
containing oxygen and argon at an argon concentration within the
range from 0.6 to 75 volume percent and a densimetric superficial
gas velocity of at least 0.06 feet per second. .Iaddend.
Inventors: |
Bennett; Douglas L. (Allentown,
PA), Ludwig; Keith A. (Emmaus, PA), Witmer; George S.
(Macungie, PA), Woods; Charles M. (Germansville, PA) |
Assignee: |
Air Products and Chemicals,
Inc. (Allentown, PA)
|
Family
ID: |
26830460 |
Appl.
No.: |
07/708,948 |
Filed: |
May 31, 1991 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
132535 |
Dec 14, 1987 |
04836836 |
Jun 6, 1989 |
|
|
Current U.S.
Class: |
62/650; 62/906;
62/924 |
Current CPC
Class: |
F25J
3/04909 (20130101); F25J 3/04309 (20130101); F25J
3/04412 (20130101); F25J 3/04678 (20130101); B01J
2219/326 (20130101); F25J 2290/10 (20130101); F25J
2250/42 (20130101); F25J 2250/58 (20130101) |
Current International
Class: |
F25J
3/04 (20060101); F25J 003/04 () |
Field of
Search: |
;62/9,11,17,18,20,22,23,24,31,32,34 ;261/94,95 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
V P. Alekseev et al., "Packed Columns for Air Rectification:
Advantages of Regular Shaped Packing", 1983. .
J. L. Bravo et al., "Mass Transfer in Gauze Packing", Hydrocarbon
Processing, Jan. 1985..
|
Primary Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Jones, II; Willard Marsh; William
F. Simmons; James C.
Claims
We claim: .[.1. In a process for the separation of mixtures, which
comprise oxygen, nitrogen, and argon, by cryogenic distillation,
wherein in certain regions of a distillation column system having
at least one column, a liquid phase stream containing oxygen, argon
and nitrogen, and a vapor phase stream containing oxygen, argon and
nitrogen, are intimately contacted thereby allowing mass transfer
which enriches the liquid phase stream with oxygen and strips argon
from the liquid phase stream, and enriches the vapor phase stream
with argon and strips oxygen from the vapor phase stream, the
improvement comprising effectuating intimate contact of the liquid
and vapor phase streams utilizing a structured packing in at least
those regions of the distillation column system where argon
concentration is within the range from about 0.6 to about 75 volume
percent, and operating the process such that the densimetric
superficial gas velocity in those regions is at least 0.06 feet per
second..]. .[.2. The process of claim 1 wherein the mixture is
air..]. .[.3. In a process for the separation of mixtures, which
comprise oxygen and argon, by cryogenic distillation, wherein in
certain regions of a distillation column system having at least one
column, a liquid phase stream containing oxygen and argon, and a
vapor phase stream containing oxygen and argon, are intimately
contacted thereby allowing mass transfer which enriches the liquid
phase stream with oxygen and strips argon from the liquid phase
stream, and enriches the vapor phase stream with argon and strips
oxygen from the vapor phase stream, the improvement comprising
effectuating intimate contact of the liquid and vapor phase streams
utilizing a structured packing in at least those regions of the
distillation column system where argon concentration is within the
range from about 0.6 to about 75 volume percent, and operating the
process such that the densimetric superficial gas velocity in those
regions is at least 0.06 feet per second..]. .[.4. A method for
improving a cryogenic distillation apparatus having at least one
distillation column for the separation of mixtures containing
oxygen and argon, comprising the step of replacing the distillation
trays with a structured packing in at least those regions of the
distillation column system where the concentration of argon is in
the
range from about 0.6 to about 75 volume percent..]. .Iadd.5. A
process for the separation of mixtures, which comprise oxygen and
argon, by cryogenic distillation, wherein, in a distillation column
system having at least one column, a liquid phase stream containing
oxygen and argon and a vapor phase stream containing oxygen and
argon are intimately contacted in at least one region of the
distillation column system where argon concentration is within the
range from 0.6 to 75 volume percent, thereby allowing mass transfer
which enriches the liquid phase stream with oxygen and strips argon
from the liquid phase stream, and enriches the vapor phase stream
with argon and strips oxygen from the vapor phase stream,
characterized in that intimate contact of said liquid and vapor
phase streams in said region is effected utilizing a structured
packing and the densimetric superficial gas velocity in said region
is at least 0.06 feet per second (1.8 cm/sec)..Iaddend. .Iadd.6. A
process as claimed in claim 5, wherein said mixture to be separated
further comprises nitrogen. .Iaddend. .Iadd.7. A process as claimed
in claim 6, wherein said mixture is air. .Iaddend. .Iadd.8. A
process as claimed in claim 7, wherein the distillation column
system comprises a high pressure column, a low pressure column and
an argon column and said region is in the low pressure
column. .Iaddend. .Iadd.9. A process as claimed in claim 7, wherein
there is more than one region having said argon concentration and
all of said regions utilize structured packing and operate within
said densimetric superficial gas velocity range. .Iaddend.
.Iadd.10. A process as claimed in claim 7, wherein the structural
packing is limited to the said region(s) in which the argon
concentration is in the range 0.6 to 75 volume percent and the
densimetric superficial gas velocity is at least
0.06 feet per second (1.8 cm/sec). .Iaddend. .Iadd.11. A process as
claimed in claim 6, wherein there is more than one region having
said argon concentration and all of said regions utilize structured
packing and operate within said densimetric superficial gas
velocity range. .Iaddend. .Iadd.12. A process as claimed in claim 6
for the separation of a mixture comprising oxygen, nitrogen, and
argon, wherein the distillation column system is an integrated
multi-column distillation system having a low pressure column and
an argon sidearm column integrally communicating with the low
pressure column and said regions are in the low pressure column and
the argon sidearm column. .Iaddend. .Iadd.13. A process as claimed
in claim 12, wherein the integrated multi-column distillation
system is a three column distillation system comprising a high
pressure column in addition to the lower pressure column and the
argon sidearm column. .Iaddend. .Iadd.14. A process as claimed in
claim 13, wherein the high pressure column is packed with a
structured packing. .Iaddend. .Iadd.15. A process as claimed in
claim 6, wherein the structural packing is limited to the said
region(s) in which the argon concentration is in the range 0.6 to
75 volume percent and the densimetric superficial gas velocity is
at
least 0.06 feet per second (1.8 cm/sec). .Iaddend. .Iadd.16. A
process as claimed in claim 5, wherein there is more than one
region having said argon concentration and all of said regions
utilize structured packing and operate within said densimetric
superficial gas velocity range. .Iaddend. .Iadd.17. A process as
claimed in claim 5, wherein the structural packing is limited to
the said region(s) in which the argon concentration is in the range
0.6 to 75 volume percent and the densimetric superficial gas
velocity is at least 0.06 feet per second (1.8 cm/sec).
.Iaddend.
.Iadd. The use of structured packing to reduce the HETP (height of
packing equivalent to a theoretical plate) in a region of a
distillation column system separating oxygen and argon by
intimately contacting a liquid phase stream containing oxygen and
argon, and a vapor phase stream containing oxygen and argon at an
argon concentration within the range from 0.6 to 75 volume percent
and a densimetric superficial gas velocity of at least 0.06 feet
per second (1.8 cm/sec). .Iaddend.
Description
TECHNICAL FIELD
The present invention relates to a process and apparatus for the
separation of mixtures comprising nitrogen, oxygen and argon by
cryogenic distillation. More specifically, the present invention
relates to the use of a structured packing in the cryogenic
distillation where argon is present in concentrations between 0.6
and 75 vol %.
BACKGROUND OF THE INVENTION
Numerous processes are known for the separation of air by cryogenic
distillation into its constituent components, representative among
these are U.S. Pat. Nos. 3,729,943; 4,533,375, 4,578,095;
4,604,116; 4,605,427 and 4,670,031.
In addition, examples of structured or ordered packings are known
in the art, representative among these are U.S. Pat. Nos.
.[.4,128,684;.]. 4,186,159; 4,296,050; 4,455,339; 4,497,751;
4,497,752 and 4,497,753.
SUMMARY OF THE INVENTION
The present invention relates an improvement to .Iadd.a process for
the separation of mixtures, which comprise oxygen and argon, by
cryogenic distillation. Wherein, in a distillation column system
having at least one column, a liquid phase stream containing oxygen
and argon and a vapor phase stream containing oxygen and argon are
intimately contacted in at least one region of the distillation
column system where argon concentration is within the range from
0.6 to 75 volume percent, thereby allowing mass transfer which
enriches the liquid phase stream with oxygen and strips argon from
the liquid phase stream, and enriches the vapor phase stream with
argon and strips oxygen from the vapor phase stream. The
improvement is characterized in that intimate contact of said
liquid and vapor phase streams in said region is effected utilizing
a structured packing and the densimetric superficial gas velocity
in said region is at least 0.06 feet per second (1.8 cm/sec).
.Iaddend..[.a process for the separation of mixtures, which
comprise oxygen, nitrogen, and argon, (e.g. air) by cryogenic
distillation, wherein in certain regions of a distillation column
system having at least one column, a liquid phase stream containing
oxygen, argon and nitrogen, are intimately contacted thereby
allowing mass transfer which enriches the liquid phase stream with
oxygen and strips argon from the liquid phase stream, and enriches
the vapor phase stream with argon and strips oxygen from the vapor
phase stream. The improvement comprises effectuating intimate
contact of the liquid and vapor phase streams utilizing a
structured packing in at least those regions of the distillation
column system where argon concentration is within the range from
about 0.6 to about 75 volume percent, and operating the process
such that the densimetric superficial gas velocity in those regions
is at least 0.06 feet per second..].
The process of the present invention is also applicable to separate
mixtures that .[.do not.]. contain nitrogen. .Iadd.The present
invention is particularly applicable to the separation of air.
.Iaddend.
.Iadd.In the process of the present invention, there can be more
than one region having said argon concentration and all of said
regions can utilize structured packing and operate within said
densimetric superficial gas velocity range. .Iaddend.
.[.The present invention also relates to a method for improving a
cryogenic distillation apparatus having at least one distillation
column for the separation of mixtures containing oxygen and argon,
comprising the step of replacing the distillation trays with a
structured packing in at least those regions of the distillation
column system where the concentration of argon is in the range from
about 0.6 to about 75 volume percent..].
.Iadd.The present invention also relates to the use of structured
packing to reduce the HETP (height of packing equivalent to a
theoretical plate) in a region of a distillation column system
separating oxygen and argon by intimately contacting a liquid phase
stream containing oxygen and argon, and a vapor phase stream
containing oxygen and argon at an argon concentration within the
range from 0.6 to 75 volume percent and a densimetric superficial
gas velocity of at least 0.06 feet per second.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic diagram of a typical three column air
separation process producing argon and oxygen products.
FIG. 2 is a plot of the measured overall gas phase mass transfer
coefficient versus the correlation predicted value.
FIG. 3 is a plot of the height equivalent to a theoretical plate
versus the densimetric superficial gas velocity for oxygen/argon
separations.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improvement to a process and
apparatus for the separation of mixtures comprising oxygen,
nitrogen and argon, e.g. air, by cryogenic distillation. The
invention is also applicable to mixtures which do not comprise
nitrogen. Essentially, the improvement of the present invention is
the use of a .[.structures.]. .Iadd.structured .Iaddend.packing in
place of distillation trays in at least those regions of the
distillation column system where the argon content will be in the
range of about 0.6 to about 75 volume percent and where the
densitmetric superficial gas velocity is at least 0.06 ft/sec.
For example, the cryogenic separation of air to produce nitrogen,
oxygen and argon products is usually carried out in a three column
distillation system. These three columns are called the high
pressure column, the low pressure column and the argon column.
Examples of air separation processes which separate argon and
oxygen and produce both as products are shown in U.S. Pat. Nos.
3,729,943; 4,533,375; 4,578,095; 4,604,116; 4,605,427 and
4,670,031, the specifications of which are incorporated herein by
reference. A flow sheet for a typical air separation process
producing oxygen and argon products is illustrated in FIG. 1.
With reference to FIG. 1, compressed air, which has been cooled to
cooling water temperature and has had removed any impurities which
may freeze at cryogenic temperatures, e.g. water and carbon
dioxide, is fed via line 10 to heat exchanger 12, wherein it is
cooled to the stream's dew point. This cooled, compressed,
impurity-free air, now in line 14, is then split into two portions.
The first portion is fed via line 16 to a lower location in high
pressure column 18. The second portion, in line 20, is further
split into two portions. The first portion is fed to argon product
vaporizer 94 via line 21 and the second portion is fed to and
condensed in product vaporizer 22 to provide boiling of liquid
oxygen in the sump surrounding product vaporizer 22, and removed
from product vaporizer 22 via line 24. The condensed liquid, in
line 24, is then separated into two portions, the first portion
which is fed as feed to an intermediate location of high pressure
column 18 via line 26 and the second portion, in line 28, which is
subcooled in heat exchanger 30 flashed in J-T valve 32 and fed into
an intermediate location of low pressure column 36 via line 34.
Overhead is removed from high pressure column 18 via line 40 and
then divided into two portions. The first portion is warmed in main
heat exchanger 12 to recover refrigeration and then removed as high
pressure nitrogen product via line 44. The second portion is fed
via line 46 to reboiler/condenser 48 located in the bottom of low
pressure column 36 wherein it is condensed and removed via line 50.
This condensed pure nitrogen stream is then split into three
portions. The first portion is fed via line 52 to the top of high
pressure column 18 to provide reflux to high pressure column 18.
The second portion is removed as liquid nitrogen product via line
54, and the third portion, removed via line 56, is subcooled in
heat exchanger 30.Iadd., .Iaddend.flashed in J-T valve 58 and fed
to the top of low pressure column 36 via line 60, to provide a pure
nitrogen reflux to the top hat portion of low pressure column
36.
Oxygen-enriched liquid bottoms from high pressure column 18 is
removed via line 62. This stream is combined with stream 100, a
condensed air stream from argon product vaporizer 94, to form
combined oxygen-enriched liquid stream 64. This combined liquid
stream is subcooled in heat exchanger 30 and then split into two
substreams. The first substream, line 66, is flashed in J-T valve
68 and fed into an upper-intermediate location of low pressure
column 36. The second substream, line 70, is flashed in J-T valve
71 and fed to the sump surrounding condenser 86 located at the top
.Iadd.of .Iaddend.argon column 72 to provide refrigeration for
condenser 86. A gaseous overhead is removed from the overhead
portion of the sump via line 74 and is combined with the liquid
removed from the sump via line 76 to form combined stream 78. This
combined stream 78 is then fed into an intermediate location of low
pressure column 36.
A side stream is removed from a lower-intermediate location of low
pressure column 36 via line 80 and fed to a lower portion of argon
column 72. The bottoms liquid from argon column 72 is returned to
low pressure column 36 at the same location as the side stream 80
draw in order to provide intermediate column reflux. Overhead argon
is removed from argon column 72 via line 84, condensed in condenser
86 and split into two portions. The first portion is returned to
the top of argon column 72 via line 90 to provide reflux to argon
column 72. The second portion is removed and fed via line 92 to
argon product vaporizer 94. Argon gas product is removed from
product vaporizer 94 via line 96 and argon liquid product is
removed from product vaporizer 94 via line 98.
A bottoms liquid stream is removed from low pressure column 36 (the
bottom sump surrounding reboiler/condenser 48) and fed to the sump
surrounding product vaporizer 22 via line 102. Gaseous oxygen
product is removed from the overhead of the sump surrounding
product vaporizer 22 via line 106, warmed to recover refrigeration
in main heat exchanger 12 and removed as gaseous oxygen product via
line 108. A liquid oxygen product is removed from a lower portion
of the sump surrounding product vaporizer 22 as liquid oxygen
product via line 104.
A liquid side stream is removed via line 110 from an intermediate
location of high pressure column 18. This impure liquid side stream
is subcooled in heat exchanger 30, reduced in pressure and fed as
reflux an upper portion of low pressure column 36 via line 112. In
addition, a gaseous side steam is removed via line 114 from a
similar location of high pressure column 18. This side stream is
warmed in main heat exchanger 12 to recover refrigeration and work
expanded in expander 116 to recover refrigeration. This expanded
stream is now in stream 118.
A gaseous side stream is removed via line 120 from an upper
location of low pressure column 36 and split into two portions. The
first portion, in line 122, is warmed in heat exchanger 12 to
recover refrigeration, used as reactivation gas and removed from
the process via line 124. Reactivation gas is necessary to
reactivate a mole sieve adsorption unit which is used to remove
water and carbon dioxide from compressed feed air. If reactivation
gas is unnecessary, then stream 124 would be vented to the
atmosphere as waste. The second portion of the side stream, line
126, is warmed in heat exchanger 30 to recover refrigeration and
combined with expanded stream 118 to form combined stream 130. This
combined stream 130 is then warmed in heat exchanger 12 to recover
any residual refrigeration and vented as waste via line 132.
Finally, an overhead from low pressure column 36 is removed via
line 134 and warmed in heat exchanger 30 to recover refrigeration.
This warmed overhead, now in line 136, is further warmed in heat
exchanger 12 to recover any residual refrigeration and removed as
low pressure nitrogen product via line 138.
The distillation columns in the above process would utilize columns
with distillation trays. Although dependent upon the selected
cycle, product makes, and relative values of power and capital,
typical theoretical tray counts for the high pressure column, low
pressure column and argon column are; 50, 70 and 40 respectively.
Typically, specially designed distillation trays have been used
within the columns to effect the separation. These distillation
trays are generally designed with a tray spacing ranging from 4 to
8 inches. For large plants, sieve trays are usually used. The hole
area is typically 5 to 15% of the tray deck. In an effort to
maximize performance for a given pressure drop, try designs which
allow multiple weirs on each tray are common. The reduction in
liquid invention due to the presence of multiple weirs, results in
a loss of point efficiency. An optimized design will typically
yield a pressure drop per theoretical stage of separation of from
1.5 to 3.0 inches of liquid per theoretical stage of separation of
from 1.5 to 3.0 inches of liquid per theoreticalstage of
separation.
A further reduction in the pressure drop per theoretical stage
could lower the required outlet pressure of the feed air
compressor. This effect is not only important in the high pressure
column, but especially important in the low pressure column. This
results from the fact that a relatively modest reduction of the
bubble point of the reboiling stream will result in a substantial
reduction in the required pressure in the top of the high pressure
column.
A distillation device which would allow separation with a pressure
drop per theoretical stage substantially below that attainable with
distillation trays would have substantial value for the cryogenic
separation of air.
In the cryogenic industry, one method to reduce the pressure drop
per theoretical stage is to increase the open area fraction on the
distillation tray. If the open area fraction is increased beyond
about 0.20, and the superficial velocity is kept sufficiently low
to prevent tray flooding at reasonable tray spacings, substantial
weeping will occur. This results in a significant degradation of
column performance.
The solution of the present invention is the use of structured or
ordered packings. By the term structured or ordered packing, it is
meant a packing which will promote liquid and/or vapor mixing in a
direction perpendicular to the primary flow direction. Examples of
ordered or structured packings are disclosed in U.S. Pat. Nos.
.Badd..[.4,128,684;.]..Baddend. 4,186,159; 4,296,050; 4,455,339;
4,497,751; 4,497,752 and 4,497,753, the specifications of which are
incorporated herein by reference. These patents disclose specific
examples of structured (ordered) packings, however, they do not
present an exhaustive list of examples. It should be noted that it
is not the intention of the present invention to prefer one type of
structured packing over another. All types of structured packings
are believed to be applicable to the present invention. It should
be pointed out that the performance of these packing elements are
reasonably well known for hydrocarbon separations, however, no
suggestions of this use appear in the art for the cryogenic
separation of air.
Since no known data is available for commercial packings for the
cryogenic separation of air, the evaluation of using random or
ordered packing in the cryogenic distillation of air requires the
use of mechanistic models for determining the mass transfer
characteristics. Examples of such mechanistic correlations can be
found in many texts, e.g. R. E. Treybal, Mass Transfer Operation,
Chapter 3.
The development of such a correlation requires experimental data to
allow regression of the required constants. Following the general
approach given in the Treybal reference, a correlation for the mass
transfer behavior of an ordered packing is given by: ##EQU1##
where: Re=Reynolds Number: dimensionless
Sc=Schmidt Number: dimensionless
Sh=Sherwood Number: dimensionless
d.sub.h =characteristic dimension of flow: ft
D=diffusivity: sq ft/sec
k=mass transfer coefficient: lb-mole/(sec)(sq ft)(.DELTA.conc)
v=superficial velocity: ft/sec
.GAMMA.=mass rate of flow per unit width: lb-mass/(ft)(sec)
.mu.=viscosity: lb-mass/(ft)(sec)
.rho.=density: lb-mass/cu ft subscript g=vapor phase subscript
1=liquid phase
and a.sub.1, a.sub.2, x.sub.1, x.sub.2, y.sub.1, and y.sub.2 are
constants and are obtained from numerical regression of raw data
for a particular system.
Values for k.sub.l and k.sub.g are substituted into an intermediate
expression for the overall gas phase mass transfer coefficient:
##EQU2## where: G=superficial vapor molar flux: lb-mole/(sq
ft)(sec)
K.sub.y =overall vapor phase mass transfer coefficient:
lb-mole/(sec)(sq ft)(mole fraction)
k.sub.y =k.sub.g (.rho..Iadd..sub.g.Iaddend. /M.sub.g):
lb-mole/(sec)(sq ft)(mole fraction)
k.sub.x =K.sub.l (.rho..sub.l /M.sub.l): lb-mole/(sec)(sq ft)(mole
fraction)
L=superficial liquid molar flux: lb-mole/(sq ft)(sec)
m=slope of the equilibrium line: dimensionless
M=molecular weight: lb-mass/lb-mole
These expressions are used to calculate the height of the overall
gas transfer unit: H.sub.tOG, where: ##EQU3## where: H.sub.tOG
=height of an overall gas transfer unit: ft
a=specific area of a fixed bed of packing: (sq ft)/(cu ft)
and finally the height of packing required to obtain a theoretical
stage of separation can be calculated:
where:
HETP=height of packing equivalent to a theoretical plate
.lambda.=(M)(G)/L
This approach should be generally valid, and allows the calculation
of performance over a wide range of operating conditions and
systems with varying properties.
In order to demonstrate the efficacy of the present invention, to
provide comparative data for other systems, and to demonstrate the
validity of typical mechanistic approaches to mass transfer in a
cryogenic separation application, mass transfer data was collected
for oxygen/nitrogen and oxygen/argon separations, where the
concentrations of the components in the two-component separation
systems were varied over a rather wide range.
This mass transfer performance data was generated using two similar
apparatuses.
The first apparatus, an eight inch diameter column, was used to
develop a portion of the oxygen/nitrogen data and all of the
oxygen/argon separation data. The eight inch diameter column is
packed with nine feet of structured packing. The apparatus is such
that liquid is fed to the top of the column through a nozzle,
distributed over the top most layer of the packing and flows
downward through the packing matrix. The liquid which drains off
the bottom of the packed bed is collected and removed from the
column. Vapor is fed to the column through another .[.nozzel.].
.Iadd.nozzle .Iaddend.and distributed by a means of a perforated
pipe. The vapor travels up through the packing counter-current to
the liquid. The vapor exits through a nozzle at the top of the
column. During operation of the apparatus (in a recycle loop mode),
overhead vapor from the column flows directly to a condenser. This
condenser consists of 5 one-inch i.d. copper tubes which are 44 ft.
long coiled inside a liquid nitrogen bath. The pressure of this
nitrogen bath controls the condensing temperature and thus the
pressure in the column. The condensed vapor from the condenser is
fed to the column as liquid. Vapor feed to the column is provided
by boiling the liquids removed from the bottom of the column in a
electrically heated thermosyphon reboiler. Varying the heat input
of the reboiler changes the rate at which vapor is produced. This
vapor rate controls the throughput of vapor and liquid within the
column.
The second apparatus was used only to generate nitrogen/oxygen
separation data. This column was five feet in diameter and is
packed with 9.33 feet of structured packing. As with the first
apparatus, the liquid enters the top of the column and is
distributed over the top most layer of the packing. Vapor enters
from directly underneath the packing through a vapor distributor.
The liquid and vapor contact each other countercurrently in the
same manner as in the first apparatus. The flows in this apparatus
were not recycled. The saturated vapor feed was supplied by an
external source. The liquid feed supply was also external to the
apparatus. These flows were controlled directly by valves in the
liquid feed piping and vapor outlet piping.
The data collection was essentially identical for both apparatuses.
First, the vapor and liquid flows were adjusted to give the desired
rates within the column. Next, the overhead and bottom vapor purity
was monitored until the equipment achieved steady state operation,
i.e. neither of these purities changed with time. Upon reaching
steady state, the overhead and bottoms vapor and liquid
compositions were analyzed and the compositions were noted. Knowing
the flow rates and composition of all the streams entering and
leaving the column, a multicomponent distillation computer
simulation was performed. The simulation determined the number of
theoretical stages within the column. This value was divided into
the total packed height to give the data value for HETP. In order
to provide accurate data analysis for the eight (8) inch column,
all experiments were run at an L/G ratio of .about.1.0.
In evaluating the data collected, FIG. 2 compares the overall gas
phase mass transfer coefficient (K.sub.g.times.a) predicted by the
correlation presented in this application to the measured data. The
data in FIG. 2 are divided into three groups: nitrogen/oxygen,
wherein the oxygen concentration ranged from 2 vppm to 50 vol %,
(illustrated by a square), very high argon/oxygen, wherein the
argon concentration ranged from 82.5 to 97 vol %, (illustrated by a
circle) and oxygen/argon, wherein argon ranged from 0.6 to 85 vol
%, (illustrated by a blackened diamond).
The nitrogen/oxygen data are a compilation of data from an 8 inch
diameter column operating at 30 psia and a 5 foot diameter column
operating at 18 psia. The compositions of these data ranged from 50
vol % oxygen in nitrogen to 2 vppm oxygen in nitrogen. Since the
development of this correlation was based on these data the
agreement between the data is, as expected, quite good. The data
and correlation generally agree to well within .+-.10%. The few
points at the upper right which exhibit additional scatter and fall
above the +10% line are operating at a very high liquid and vapor
throughput. These data are unusual for two reasons. First, the
large liquid rates were beyond the capacity of the distributor.
Thus, the liquid was not distributed properly over the column
cross-section. This has been known to cause poor performance in
packed columns. Second, the high liquid and vapor rates indicates
that operation was conducted very near the flooding point of the
column. This causes phenomena such as .[.backmixing.].
.Iadd.back-mixing .Iaddend.of the liquid, entrainment and intrinsic
maldistributions within the packing. All of these factors may have
contributed to the loss in performance. Operation of a packed
column this near its flooding point is usually not done. Therefore,
the correlation was based on the fundamental mechanisms which
dominate the normal operating range of throughput. Hence, the lack
of agreement near the flooding point indicates the validity of the
correlation rather than any problems with it. In addition, the
correlation accurately predicts the performance over a factor of
.about.2 change in pressure and a factor of 7.5 in column diameter
and for a wide range of compositions.
The argon/oxygen data were measured in the same 8 inch column that
was used to generate part of the nitrogen/oxygen data. The
composition range of these data is 82.5-97 vol % argon with the
remainder oxygen. These data were taken at 30 psia. As shown by the
parity plot in FIG. 2, these data agree very well with the
correlation. These data are coincident with nitrogen/oxygen data.
Since the correlation was not regressed with these data, the
excellent agreement indicates that the fundamental mechanisms used
in this correlation accurately describes the behavior of these
data.
The third set of data presented in FIG. 2 is the oxygen/argon data.
These data were taken over the 0.6-85 vol % argon in oxygen
concentration range. These data were taken in the same 8 inch
column used for the nitrogen/oxygen data and the high concentration
argon in oxygen discussed previously. These data clearly exhibit a
remarkable improvement in performance over all the other data. In
general, most of the data values are more than 10% better than the
expected values from the correlations. In addition, the data
exhibit a markedly different slope than all of the other data. This
is another indication that there is some additional mechanism which
enhances the mass transfer performance in this range of argon
concentrations in oxygen. This enhancement appears to increase as
the throughput in the column increases.
Returning to the correlation and the calculation of HETP's, a
regression of the data for nitrogen-oxygen system gives the
following values for the required constants.
______________________________________ .alpha..sub.1 = 0.0295
.alpha..sub.2 = 0.052 x.sub.1 = 0.893 x.sub.2 = 1.19 y.sub.1 = 0.33
y.sub.2 = 0.33 ______________________________________
The values for HETP calculated from this correlation for the
nitrogen-oxygen system range from 10 to 12 inches.
Additional data, taken with argon/oxygen mixtures with
concentrations of 82.5-97 vol % argon yield HETP values from 7.6 to
8.2 inches. Again, predictions using the correlation based on the
nitrogen/oxygen data base are from 7.6 to 8.2 inches. The agreement
of the predictions from the nitrogen/oxygen based correlation with
this high argon content data base clearly shows the general
applicability of this type of mechanistically based
correlation.
The above calculated values of HETP, in addition to the large
number of required theoretical stages clearly show that the use of
the tested structured packing would require columns with
substantial increase in column height over that possible with a
trayed column. This will result in a substantial capital penalty
when compared to columns utilizing conventional cryogenic
distillation trays.
On the other hand, tests separating oxygen from argon where the
argon content was between 85% and 0.6% have shown a remarkable and
unexpected enhancement in separation performance. For example, the
HETP can be as low as 6.8 inches, when the expected (predicted)
value is 8.5 inches. FIG. 3 more clearly shows the remarkable
enhancement of the mass transfer for the oxygen/argon data. FIG. 3
plots the HETP data for the high oxygen concentration data versus
K.sub.v. K.sub.v is the densitmetric superficial gas velocity in
the column and is calculated by:
where:
Also plotted is the expected HETP calculated from the correlation.
These calculated values correspond to what would be expected for
the HEPT based on all the other available cryogenic data. As
illustrated by the plot, the measured HETP data values are
significantly lower than the expected values at K.sub.v greater
than 0.06 ft/sec. In fact, the enhancement appears to generally
increase with increasing K.sub.v. This enhancement may be due to an
increase in interfacial area or an unexplained reduction in liquid
phase resistance for this range of concentrations.
As can be seen from the above experiments, there is an overlap in
ranges of argon concentration for the two argon/oxygen data sets.
In one data set the argon concentration is between 0.6 and 85 vol %
and in the other, between 82.5 and 97 vol %. For the first data
set, a distinct benefit of using a structured packing is seen; this
benefit is the fact that the height required for structured
packings are comparable to the height for distillation trays to
achieve the same overall separation while retaining the lower
pressure drop advantages. For the second data set, the heights
using structured packing will exceed the height required for
distillation trays for the same overall separation. When dealing
with this overlap region, it is important to note that because the
experimental apparatus did not have the capability to accurately
analyze the compositions of the liquid and vapor phases at
intermediate locations in the column and because the concentration
with height produced by a packed column is continuous rather than
stepwise as for tray columns, only overall HETP's could be
calculated with any sense of confidence. It is believed that there
is a transition point in the argon concentration where the
unexpected benefit of lower HETP's is no longer seen. This
transition is believed to be somewhere between 75 and 85 vol %
argon. Therefore, the present invention embodies the use of an
ordered (structured) packing in at least those regions of the
distillation column system where the argon concentration can be in
the range of 0.6% to 75% argon by volume. This measured, unexpected
improvement exists for values of K.sub.v exceeding about 0.06
ft/sec. At values of K.sub.v less than 0.06 ft/sec, measured HETP
values do not exceed anticipated values.
Any attempt to manipulate the constants within the correlations so
that the predictions would agree with the data causes the
correlation to severely underpredict the HETP's for the other
systems. This further substantiates the observation that there is a
significant enchancement of performance within the composition
range of 75% to 0.6% argon in oxygen/argon separation.
Another reason that correlations fail to predict mass transfer
phenomena well is that they are used for systems where the key
physical transport properties differ substantially from the systems
used to develop the correlation. The correlation fails because it
is being used in a region of extrapolation beyond the data base
used to generate the correlation. Therefore, if this were true for
oxygen/argon at low concentrations of argon, one would expect
.[.thef.]. .Iadd.the .Iaddend.basic physical properties of oxygen
to be substantially different from either nitrogen or argon. In
Table I, the physical properties of saturated vapor and liquid for
nitrogen, oxygen and argon are listed. All the values for oxygen
are comparable to the values for nitrogen and argon. Therefore, a
significant property difference causing a correlation error for
oxygen-rich compositions is not evident.
TABLE I ______________________________________ Key Transport
Properties of N.sub.2, O.sub.2, Ar N.sub.2 O.sub.2 Ar
______________________________________ Saturated Liquid Density:
lb-m/cu ft 48.4 75.6 83.0 Viscosity .times. 10.sup.4 :
lb-m/(ft)(sec) 0.7 1.07 1.1 Diffusivity .times. 10.sup.8 : sq
ft/sec 8.6 7.4 7.5 Surface Tension .times. 10.sup.4 : lb-m/ft 5.1
7.85 7.1 Saturated Vapor Density: lb-m/cu ft 0.359 0.621 0.685
Viscosity .times. 10.sup.6 : lb-m/(ft)(sec) 3.9 5.3 5.5 Diffusivity
.times. 10.sup.5 : sq ft/sec 1.15 1.34 1.31
______________________________________
Table II gives (for the subsets of data) the range of values for
the dimensionless groupings. The oxygen-rich data do not fall
outside the ranges for the nitrogen-rich and argon-rich data
subsets. Therefore, the oxygen-rich data subset clearly .[.faoos.].
.Iadd.falls .Iaddend.within the range of the correlation. This
further supports the unexpected nature of this phenomena.
TABLE II ______________________________________ Range of
Dimensionless Grouping Nitrogen-Rich Argon-Rich Oxygen-Rich
______________________________________ Sh.sub.l 24-143 51-119 27-78
Sh.sub.g 15-67 36-69 20-95 Re.sub.l 48-356 148-300 93-358 Re.sub.g
1600-6500 2900-5900 1600-5800 Sc.sub.l 23-17 18 19 Sc.sub.g 0.60
0.61 0.63 ______________________________________
Established and fundamentally sound correlation methods have
predicted values for HETP between 8.5 and 12 inches for the
cryogenic distillation of air. Since the regions where oxygen/argon
separations occur usually requires a large number of theoretical
stages, a significant capital penalty has been associated with
using ordered packing in this application. Undoubtedly, this
significant capital penalty has contributed to the lack of use of
order packing in oxygen-rich regions of distillation columns. This
new discovery allows cryogenic air separation plants to be designed
with HETP's which are comparable to distillation trays in areas
where the argon content is less than 75%. This will substantially
reduce the capital cost associated with using ordered packing and
allow the benefits of its reduced pressure drop to be fully
realized.
To demonstrate the energy savings benefit of the present invention,
an analysis has been done which calculates the improvement in the
total power consumption of a cryogenic air separation plant as the
pressure drop per theoretical stage in the column system is
reduced. For this discussion the column system can be broken down
into two parts, the high pressure column and the low pressure
column-argon column combined system. Reducing pressure drop in the
high pressure column obviously reduces the discharge pressure of
the air compressor feeding the plant. A reduction of pressure drop
in this area leads to substantial but not overwhelming power
saving. The reason is that the high pressure column, by necessity
of the cycle, operates at near 100 psia. The pressure drop of a
well designed trayed high pressure column ranges from 2 to 3 psi.
Since power is generally inversely proportion to the log of the
pressure ratio, a total elimination of the pressure drop in the
high pressure column would reduce the power by about 2.6%.
However, a reduction in the pressure drop within the low pressure
column-argon column system can result in power savings on the order
of 6% depending on which cycle is used. The reason for this is
twofold. First, there are nearly twice as many theoretical stages
in the low pressure column/argon column system as are in the high
pressure column. Therefore, a reduction in the pressure drop per
theoretical stage has a much greater impact in the low
pressure-argon column system than in the high pressure column.
Secondly, the pressure drop in the low pressure column directly
controls the pressure and thus the bubble point of the reboiling
stream. Since all the product must be discharged at or above
atmospheric pressure the pressure in the reboiling stream is:
where:
.DELTA.P.sub.out =pressure drop for overhead products leaving the
plant
.DELTA.P.sub.LPC =pressure drop within the low pressure column
P.sub.atm =ambient atmospheric pressure
P.sub.R/B =pressure of the reboiling stream
Because this stream is reboiled by condensing vapor in the high
pressure column the bubble point of this stream and the temperature
approach at the top of the heat exchanger set the .[.dewpoint.].
.Iadd.dew point .Iaddend.of the condensing stream. Therefore, the
high pressure column pressure is set by the pressure at which the
vapor at the top of the high pressure column will condense at this
specified dewpoint. The relationship between pressure and dewpoint
in the condensing stream causes approximately a tripling of any
pressure change in the reboiling stream.
Simply stated, for every 1 psi change in the pressure at the bottom
of the low pressure column the high pressure column pressure
changes by about 3 psi. Thus, reducing the pressure drop in the low
pressure column can dramatically reduce the high pressure column
pressure. This, in turn, will effect a comparable reduction in
power consumption. For an 800 TPD high purity oxygen plant, for
distillation trays the pressure drop per theoretical stage would be
.about.0.07 psi/stage. Experiments indicate that ordered packings
would use, on average, 0.008 psi/stage. This would result in a
power savings of 8%.
The present invention has been described with reference to some
specific embodiments thereof. These embodiments should not be
considered a limitation on the scope of the invention, such scope
being ascertained by the following claims.
* * * * *