System and method for introducing low pressure reflux to a...

Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture

Reexamination Certificate

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C062S651000

Reexamination Certificate

active

06568208

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to air separation and more specifically to cryogenic air separation and nitrogen and/or oxygen production.
Frequently, in a column system for air separation, there is a need to introduce low-pressure nitrogen liquid to a high pressure column as reflux. Examples of column systems where this occurs include:
Side-by-side column arrangements for the production of nitrogen and/or oxygen;
Column systems for the production of nitrogen and/or oxygen with dual reboiler and nitrogen expansion; and
Column systems for the production of high pressure nitrogen with nitrogen liquid reflux pumped from the low pressure column to the high pressure column.
These column arrangements are described in detail below.
In a typical air separation unit, for example the configuration shown in
FIG. 1
, there are at least two distillation columns: a high pressure column
110
, and a low pressure column
120
. These columns are heat integrated through reboiler-condenser
130
and the low pressure column is usually built on top of the high pressure column.
With the increasing trend toward higher efficiency distillation and higher purity of products, the height of the distillation column in such a configuration increases. The height of the combined high pressure-low pressure column system ultimately becomes so tall that the design of the entire system is prohibitively expensive. Stacking the columns is also not typically desired for larger plants, where the diameters of the columns are large and the columns are heavy.
To avoid these problems, conventional high pressure and low pressure columns can be built side-by-side. The reboiler-condenser can be located on top of the high pressure column (such as the configuration shown in
FIG. 2
) or in the bottom of the low pressure column (shown in FIG.
3
). In both of these cases a pump is necessary. According to U.S. Pat. No. 6,148,637, and as shown in
FIG. 2
, liquid oxygen in stream
240
is pumped, using pump
250
, from the bottom of low pressure column
220
to reboiler
230
located on top of high pressure column
210
.
U.S. Pat. No. 6,148,637 discloses a three component system, comprised of a lower pressure column, a higher pressure column, and a heat exchanger. Included in this system is a pump for transporting liquid from the bottom of the lower pressure column to a vaporizer-condenser at the top of the higher pressure column.
As illustrated in
FIG. 3
, nitrogen liquid in stream
360
is pumped, using pump
350
, from reboiler-condenser
330
located in the bottom of low pressure column
320
back to the top of high pressure column
310
as reflux. Usually two pumps instead of one are installed for the same service—a working pump and an idle nitrogen liquid pump that serves as a spare. Cryogenic liquid pumps are expensive, require periodic maintenance and, because they contain moving parts, are more likely to fail than stationary equipment.
A column system for the production of nitrogen and/or oxygen with a dual reboiler and nitrogen expansion has been described in U.S. Pat. No. 4,796,431, and is shown in FIG.
4
. In this arrangement, air is introduced to high pressure column
410
and product nitrogen and oxygen are withdrawn from low pressure column
420
. The columns are heat integrated by lower reboiler-condenser
430
and upper reboiler-condenser
440
. Nitrogen in stream
432
from the top of high pressure column
410
is divided into streams
434
and
436
. Stream
436
is expanded in expander
438
, thus creating necessary refrigeration. The output of expander
438
is then condensed in upper reboiler-condenser
440
and the resulting stream
442
is divided into streams
444
and
446
. Stream
444
is then fed to the top of the low pressure column
420
as reflux. Stream
446
is directed to the high pressure column as additional reflux. Because its pressure was reduced by prior expansion, however, stream
446
needs to be pumped using pump
450
.
A column system for the production of high pressure nitrogen with nitrogen liquid reflux pumped from the low pressure column to the high pressure column has been described in U.S. Pat. No. 5,098,457, and it is shown in FIG.
5
. In this arrangement, air is introduced to high pressure column
510
and high pressure nitrogen product is withdrawn from the top of this column as stream
515
. High pressure column
510
is heat integrated with low pressure column
520
through reboiler-condenser
530
. Nitrogen overhead from the top of low pressure column
520
is condensed in condenser
540
and a part of this condensate in stream
545
is pumped using pump
550
back to high pressure column
510
as additional reflux
560
, thus increasing the recovery of high pressure nitrogen in stream
515
.
BRIEF SUMMARY OF THE INVENTION
A goal of the present invention is to introduce low pressure nitrogen reflux into a distillation column that operates at a higher pressure without using pumps. More specifically, the present invention includes a method of separating air to produce nitrogen and/or oxygen in a system having at least one high pressure column, at least one low pressure column, and a reboiler-condenser, the method comprising the steps of generating a high pressure nitrogen stream from the high pressure column, and using energy from the high pressure nitrogen stream to provide nitrogen reflux to the high pressure column.
The method according to the present invention, more specifically, comprises generating a high pressure nitrogen vapor stream and condensing a portion of the high pressure nitrogen vapor stream to form a high pressure nitrogen liquid stream which is reduced in pressure by transferring it to a reflux vessel, where it is collected, then using a portion of the high pressure vapor stream not condensed to pressurize the reflux vessel to a pressure equal to the high pressure column and cause the nitrogen liquid collected therein to flow into the high pressure column under force of pressurization coupled with the static head of the nitrogen liquid. In an alternative embodiment, the high pressure nitrogen vapor stream may optionally be expanded prior to condensing it to form the low pressure nitrogen liquid stream. This stream, after expansion and condensing, may then be further reduced in pressure and transferred to the reflux vessel.
In an alternate embodiment, the method of the present invention comprises generating a high pressure nitrogen vapor stream, dividing the high pressure nitrogen vapor stream into two streams, one of which is condensed to form a nitrogen liquid stream, and the other of which is combined with the thus formed nitrogen liquid stream to form a two-phase mixture which is passed to a raised reflux vessel wherein the nitrogen liquid is collected and refluxed back to the high pressure column.
The present invention also provides a method of separating air to produce nitrogen and/or oxygen in a system having at least one high pressure column, at least one low pressure column, and a reboiler-condenser located in the bottom of the low pressure column, comprising the steps of generating a first high pressure nitrogen vapor stream and a second high pressure nitrogen vapor stream from the high pressure column, condensing the first high pressure nitrogen stream to form a high pressure nitrogen liquid stream, dividing the high pressure nitrogen liquid stream into a low pressure column liquid reflux stream and a high pressure column liquid reflux stream, and using the second high pressure nitrogen vapor stream to supply energy to cause the passage of the high pressure column liquid reflux stream to the high pressure column.
Another embodiment of the present invention comprises the steps of generating a high pressure nitrogen vapor stream from the high pressure column, dividing the high pressure nitrogen vapor stream from the generating step into a first high pressure nitrogen vapor stream and a second high pressure nitrogen vapor stream, condensing the first high pressure nitrogen vapor stream in a reboiler-condenser at the

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