U.S. patent application number 11/196405 was filed with the patent office on 2007-02-08 for cryogenic air separation main condenser system with enhanced boiling and condensing surfaces.
Invention is credited to Vijayaraghavan Srinivasan Chakravarthy, Michael James Lockett.
Application Number | 20070028649 11/196405 |
Document ID | / |
Family ID | 37716399 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070028649 |
Kind Code |
A1 |
Chakravarthy; Vijayaraghavan
Srinivasan ; et al. |
February 8, 2007 |
Cryogenic air separation main condenser system with enhanced
boiling and condensing surfaces
Abstract
A cryogenic air separation system having a main condenser
comprising a plurality of tubes having fluted external condensing
surfaces upon which downflowing nitrogen vapor condenses, and
having structured internal boiling surfaces upon which downflowing
oxygen liquid vaporizes.
Inventors: |
Chakravarthy; Vijayaraghavan
Srinivasan; (Williamsville, NY) ; Lockett; Michael
James; (Grand Island, NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
37716399 |
Appl. No.: |
11/196405 |
Filed: |
August 4, 2005 |
Current U.S.
Class: |
62/643 |
Current CPC
Class: |
F25J 2250/04 20130101;
F25J 5/005 20130101; F28F 1/42 20130101; F28F 13/187 20130101; F25J
2245/50 20130101; F25J 2235/50 20130101; F25J 3/04412 20130101;
F25J 2290/44 20130101; F28F 1/422 20130101; F25J 5/002
20130101 |
Class at
Publication: |
062/643 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A method for operating a cryogenic air separation plant having a
higher pressure column, a lower pressure column, and a main
condenser having a plurality of tubes wherein each of said tubes
has a fluted external surface and a structured internal surface,
said method comprising passing nitrogen vapor from the higher
pressure column to the upper portion of the main condenser, flowing
oxygen liquid from the separation section of the lower pressure
column to the upper portion of the tubes of the main condenser,
passing the nitrogen vapor down the main condenser in contact with
the external surfaces of the tubes, passing the oxygen liquid down
the tubes of the main condenser in heat exchange relation with the
downflowing nitrogen vapor wherein at least some but not all of the
downflowing oxygen liquid is vaporized, and withdrawing both oxygen
vapor and oxygen liquid from the main condenser in a liquid to
vapor mass flowrate ratio within the range of from 0.05 to 10.
2. The method of claim 1 wherein the liquid to vapor mass flowrate
ratio is within the range of from 0.2 to 2.0.
3. The method of claim 1 wherein the main condenser is a
shell-and-tube heat exchanger.
4. The method of claim 1 wherein the main condenser is a brazed
aluminum heat exchanger.
5. The method of claim 1 wherein the main condenser comprises a
plurality of condenser modules.
6. A downflow condenser, particularly suitable as the main
condenser of a double column cryogenic air separation plant, having
a plurality of tubes wherein each of said tubes has a fluted
external surface and a structured internal surface.
7. The condenser of claim 6 having from 300 to 800 tubes.
8. The condenser of claim 6 wherein the structured internal surface
has cavities with a depth within the range of from 0.5 to 2.0
millimeters.
9. The condenser of claim 6 wherein the tubes have an internal
diameter within the range of from 15 to 25 millimeters.
Description
TECHNICAL FIELD
[0001] This invention relates generally to cryogenic air separation
and, more particularly, to cryogenic air separation employing a
double column.
BACKGROUND ART
[0002] Cryogenic air separation systems employing downflow main
condensers with high flux tubes, while operating effectively, may
exhibit one or more disadvantages. One disadvantage is the high
cost associated with the porous coating. In addition, any
non-uniformity of the porous coating could result in variations in
the boiling performance.
SUMMARY Of THE INVENTION
[0003] One Aspect of the Present Invention is:
[0004] A method for operating a cryogenic air separation plant
having a higher pressure column, a lower pressure column, and a
main condenser having a plurality of tubes wherein each of said
tubes has a fluted external surface and a structured internal
surface, said method comprising passing nitrogen vapor from the
higher pressure column to the upper portion of the main condenser,
flowing oxygen liquid from the separation section of the lower
pressure column to the upper portion of the tubes of the main
condenser, passing the nitrogen vapor down the main condenser in
contact with the external surfaces of the tubes, passing the oxygen
liquid down the tubes of the main condenser in heat exchange
relation with the downflowing nitrogen vapor wherein at least some
but not all of the downflowing oxygen liquid is vaporized, and
withdrawing both oxygen vapor and oxygen liquid from the main
condenser in a liquid to vapor mass flowrate ratio within the range
of from 0.05 to 10.
[0005] Another Aspect of the Present Invention is:
[0006] A downflow condenser, particularly suitable as the main
condenser of a double column cryogenic air separation plant, having
a plurality of tubes wherein each of said tubes has a fluted
external surface and a structured internal surface.
[0007] As used herein, the term "separation section" means a
section of a column containing trays and/or packing and situated
above the main condenser.
[0008] As used herein, the term "enhanced surface" means a special
surface geometry that provides higher heat transfer per unit
surface area than does a plain surface.
[0009] As used herein, the term "column" means a distillation or
fractionation column or zone, i.e. a contacting column or zone,
wherein liquid and vapor phases are countercurrently contacted to
effect separation of a fluid mixture, as for example, by contacting
of the vapor and liquid phases on a series of vertically spaced
trays or plates mounted within the column and/or on packing
elements such as structured or random packing. The term, double
column is used to mean a higher pressure column having its upper
end in heat exchange relation with the lower end of a lower
pressure column.
[0010] Vapor and liquid contacting separation processes depend on
the difference in vapor pressures for the components. The high
vapor pressure (or more volatile or low boiling) component will
tend to concentrate in the vapor phase whereas the low vapor
pressure (or less volatile or high boiling) component will tend to
concentrate in the liquid phase. Partial condensation is the
separation process whereby cooling of a vapor mixture can be used
to concentrate the volatile component(s) in the vapor phase and
thereby the less volatile component(s) in the liquid phase.
Rectification, or continuous distillation, is the separation
process that combines successive partial vaporizations and
condensations as obtained by a countercurrent treatment of the
vapor and liquid phases. The countercurrent contacting of the vapor
and liquid phases is generally adiabatic and can include integral
(stagewise) or differential (continuous) contact between the
phases. Separation process arrangements that utilize the principles
of rectification to separate mixtures are often interchangeably
termed rectification columns, distillation columns, or
fractionation columns. Cryogenic rectification is a rectification
process carried out at least in part at temperatures at or below
150 degrees Kelvin (K).
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a simplified representational schematic diagram of
one preferred embodiment of the cryogenic air separation system of
this invention.
[0012] FIG. 2 is a partial cut away view of one embodiment of a
doubly enhanced tube which may be used in the practice of this
invention.
[0013] FIGS. 3 and 4 each illustrate preferred structured boiling
surfaces, in highly magnified side views, which may be employed as
the internal surface of the tubes of the downflow main condenser in
the practice of this invention. Presently, these surfaces are
commercially manufactured on the external surfaces of tubes.
DETAILED DESCRIPTION
[0014] The invention comprises a novel heat exchanger having
defined doubly enhanced tube surfaces, and its use as the main
condenser in a double column cryogenic air separation plant. The
tubes are characterized by having both enhanced boiling surfaces
and enhanced condensing surfaces. The enhanced condensing surfaces
comprise flutes for at least a portion of the length of the tubes.
The enhanced boiling surfaces are structured surfaces. A structured
boiling surface is an enhanced boiling surface formed by metal
working to form nucleation sites on the heat transfer surface
characterized by a plurality of cavities that trap vapor and
initiate boiling at low wall superheats.
[0015] The invention will be described more fully with reference to
the Drawings. Referring now to FIG. 1, there is shown a partial
schematic of a double column cryogenic air separation plant, having
a higher pressure column 30 and a lower pressure column 31, and
showing the placement of main condensers 32, also referred to as
condenser/reboilers, inside the lower pressure column. The main
condenser(s) of this invention may be of the shell and tube type or
of the brazed aluminum heat exchanger type. The main
condenser/reboilers thermally link the higher pressure and lower
pressure columns. Nitrogen vapor, at a pressure generally within
the range of from 45 to 300 pounds per square inch absolute (psia),
is passed in line 10 from higher pressure column 30 to the upper
portion of the main condenser or condensers wherein the nitrogen
vapor exchanges heat with oxygen liquid within the tubes as both
fluids flow down through the main condenser(s). The oxygen liquid,
which is at a pressure generally within the range of from 1 to 100
pounds per square inch gauge (psig) is partially vaporized and the
resulting oxygen vapor and remaining oxygen liquid are withdrawn
from the main condensers(s) as shown by flow arrows 34 and 33
respectively. The nitrogen vapor is completely condensed by the
downflow passage through the main condenser and the resulting
nitrogen liquid is withdrawn from the main condenser in line 11 and
passed in lines 35 and 36 respectively as reflux into the higher
pressure and lower pressure columns.
[0016] In the lower pressure column 31, oxygen liquid descending
the column through packing 12 or trays (not shown) is collected in
collector/distributor 13. Open risers 14 extend up from the floor
of the collector box for the oxygen vapor generated in the main
condenser to flow up through the column. Oxygen liquid from the
collector flows through distributor pipe 15 and collects in the
distributor section 16 of the individual modules. The oxygen liquid
from the flow distributor section flows through the individual
tubes where it is partially vaporized. These passages have enhanced
boiling surfaces, i.e. structured internal surfaces, which
significantly increases the ability of the liquid to wet the
surface of the boiling side and reduces the amount of liquid flow
needed to achieve wetting. The unvaporized liquid 17 collects at
the bottom of the column and is withdrawn from the column as a
product in line 38. The product boiler pump 18 is used to raise the
pressure of oxygen to the required product pressure. If desired, a
portion 40 of stream 38 may be passed through valve 41 and
recirculated to the main condenser(s). The ratio of liquid to vapor
mass flowrate (L/V) at the exit of the main condenser tubes or
vaporizing passages ranges from 0.05 to 10, and is preferably
within the range of from 0.2 to 2.0.
[0017] Each condenser/reboiler 32 comprises a plurality of
longitudinally oriented tubes that are attached, usually by
welding, to a top tubesheet and a bottom tubesheet in the case
where the main condenser is a shell and tube heat exchanger. The
tubesheets are not shown in FIG. 1. Each tube has an internal
surface and an outer surface. The external surface of each tube is
fluted, i.e. it has a plurality of flutes running along the length,
preferably the entire length, of the tube to enhance the
condensation heat transfer. The nitrogen vapor flows downwardly
over the tubes and is condensed, preferably completely condensed,
by the time it traverses the length of the tubes. The resulting
condensate, i.e. nitrogen liquid, is withdrawn from the bottom
exits of the shell side. The nitrogen liquid is passed out of the
condenser/boilers 32 in conduit 11 and is passed into the upper
portion of the higher pressure column and also into the upper
portion of the lower pressure column as reflux liquid for carrying
out the cryogenic rectification. For simplicity, only one of the
connections from the condenser/reboilers 32 to line 11 is shown in
FIG. 1. If desired, a portion of the nitrogen liquid may be
recovered as product nitrogen.
[0018] The inner surface of each tube has an enhanced or structured
boiling surface characterized by a plurality of cavities or
depressions which have a depth generally within the range of from
0.5 to 2.0 millimeters. Two examples of such cavities are shown in
cross-section in FIGS. 3 and 4. The enhanced boiling surface with
the re-entrant cavities operates by trapping vapor within the
cavities for initiating boiling at low tube wall superheats, which
is defined as the temperature difference between the tube wall
surface and the saturation temperature of the fluid to be
vaporized. The oxygen liquid flows downwardly along the inner
surfaces of the tubes in cocurrent indirect heat exchange with the
previously described downflowing condensing nitrogen vapor. As the
oxygen liquid flows down along the enhanced boiling inner surfaces
of the tubes, a portion of the downflowing oxygen liquid is boiled
off or vaporized, as shown by arrows G in FIG. 1, while the
remaining liquid, shown by arrows L in FIG. 1, is collected in the
sump of the upper column as shown by liquid pool 17. The oxygen
vapor boiled off the inner surfaces of the tubes passes up through
the upper column as vapor upflow for the cryogenic rectification.
If desired a portion of the oxygen vapor may be recovered as
product gaseous oxygen. If desired a portion of the remaining
oxygen liquid 17 may be recovered as product liquid oxygen.
Alternatively, the remaining oxygen liquid 17 is recirculated to
the tubes in order to ensure that the boiling surfaces of the tubes
remain wet, thereby avoiding a boiling to dryness condition which
is inefficient and, when the liquid comprises liquid oxygen, is
also dangerous. For the recirculation flow, oxygen liquid 17 is
withdrawn from the upper column 31 and pumped by liquid
recirculation pump 18.
[0019] Typically in the practice of this invention the tubes will
have an internal diameter within the range of from 15 to 25
millimeters. FIG. 2 illustrates a doubly enhanced tube for use with
this invention which has a fluted external surface 45 and a
structured internal surface 46. The condenser will typically
contain from about 300 to 800 such tubes. It may also contain one
or more other tubes which are not characterized by having such
doubly enhanced surfaces. FIGS. 3 and 4 show side views of two
typical embodiments of structured boiling surfaces for use with
this invention. The surface shown in FIG. 3 is commercially known
as GEWA surface. The shape of the cavity is called double reentrant
cavity and traps vapor very effectively. The surface shown in FIG.
4 is typical of Wolverine Turbo-B.RTM. tubes. Fins are first formed
on the surface and they are modified by cold working to give the
desired shape of the cavities.
[0020] Although the invention has been described in detail with
reference to certain preferred embodiments, those skilled in the
art will recognize that there are other embodiments of the
invention within the spirit and the scope of the claims.
* * * * *