Method and apparatus for separating air

A method and apparatus for separating air in which a compressed and purified air stream is cooled in a main heat exchanger. Thereafter, the compressed and purified air is separated in a distillation column system to produce product streams. The product streams warm within the main heat exchanger by indirectly exchanging heat with the compressed and purified air stream. One or more of the product streams is distributed to a plurality of vortex tubes at successively warmer temperatures so that warm and cold streams produced thereby become successively warmer and one or more of the warm streams has a temperature warmer than that of said compressed and purified air stream upon its introduction into said main heat exchanger. All but the warm stream(s) having the warmer temperature are recycled back to said main heat exchanger to participate in the indirect heat exchange and heat is rejected by discharging said warm stream(s) so that heat is rejected at the warmer temperature and refrigeration is produced.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus in which compressed 
and purified air is cooled in a main heat exchanger and is then separated 
in a distillation column to produce a product stream that warms within the 
main heat exchanger. More particularly, the present invention relates to 
such a method and apparatus in which one or more of the product streams is 
distributed to a plurality of vortex tubes operated at successively warmer 
temperatures so that at least one of the vortex tubes produces a warm 
stream at a temperature warmer than the compressed and purified air upon 
its introduction into the main heat exchanger to generate refrigeration. 
Air is separated by a variety of well-known cryogenic rectification 
processes. In such processes, the air is compressed and then purified to 
remove moisture, carbon dioxide and hydrocarbons. The compressed and 
purified air is then cooled within a main heat exchanger before being 
introduced into a distillation column system. The distillation column 
system can be a single column designed to produce nitrogen as a tower 
overhead or one in which the air is introduced into an intermediate 
location thereof so that an oxygen product is also produced as a column 
bottoms. Another common distillation column system is a double column 
having higher and lower pressure columns associated with one another in a 
heat transfer relationship. The higher pressure column produces nitrogen 
as a tower overhead which is in part condensed to reflux both the higher 
and lower pressure columns. The column bottoms produced in the higher 
pressure column is introduced into the lower pressure column for further 
refinement to produce an oxygen product. 
Since all of the systems discussed above operate at cryogenic temperatures, 
the main heat exchanger and distillation column system must be insulated 
from the environment by an external structure known as a cold box. 
Nevertheless, there is heat leakage through the cold box and also warm end 
losses due to the temperature of the incoming air. As a result, any low 
temperature rectification process must be refrigerated in order for the 
process to remain in heat balance. To this end, refrigeration is generated 
by expansion machines such as turboexpanders. Either part of the incoming 
air or vaporized liquid bottoms or nitrogen may be heated and then 
expanded to a low temperature. Refrigeration is generated because energy 
is dissipated from the system as shaft work. For instance, the 
turboexpander can be connected to an electrical generator or an energy 
dissipative oil brake. 
Turboexpanders and expansion machines, however, add expense and complexity 
to any air separation plant. As will be discussed, the present invention 
provides a method and apparatus for separating air does not use expansion 
machines such as turboexpanders and thus, produces a simpler and more cost 
effective plant than has been considered in the prior art. 
SUMMARY OF THE INVENTION 
The present invention provides a method of separating air in which a 
compressed and purified air stream is cooled in a main heat exchanger. The 
compressed and purified air stream is then separated in a distillation 
column system to produce product streams. The product streams are warmed 
within the main heat exchanger by indirectly exchanging heat with the 
compressed and purified air stream. At least one of the product streams is 
distributed to a plurality of vortex tubes at successively warmer 
temperatures so that the warm and cold streams produced thereby become 
successively warmer and at least one of the warm streams has a temperature 
warmer than the compressed and purified air stream upon its introduction 
into the main heat exchanger. All but the at least one of the warm streams 
are recycled back to the main heat exchanger to participate in the 
indirect heat exchange. The at least one warm stream is discharged so that 
heat contained therein is rejected at its temperature above that of the 
incoming air, thereby to produce refrigeration. 
In another aspect, the present invention provides an air separation 
apparatus including a main heat exchanger configured to cool a compressed 
and purified air stream by indirect heat exchange with product streams. A 
distillation column is connected to the main heat exchanger and is 
configured to separate air contained within the compressed and purified 
air stream, thereby to produce the product streams. A plurality of vortex 
tubes are connected to the main heat exchanger so that at least one of the 
product streams is distributed to the plurality of vortex tubes at 
successively warmer temperatures. Warm and cold streams produced thereby 
become successively warmer and at least one of the warm streams has a 
temperature warmer than compressed and purified air stream upon its 
introduction into the main heat exchanger. The plurality of the vortex 
tubes are connected to the main heat exchanger so that all but the at 
least one of the warm streams recycle back to the main heat exchanger to 
participate in the indirect heat exchange and the at least one of the warm 
streams discharges so that heat contained therein is rejected at its 
temperature, thereby to produce refrigeration. 
It is to be noted that as used herein and in the claims, the term 
"distillation column system" as used herein and in the claims can be a 
single column designed to produce a nitrogen product or alternatively a 
single column to produce an oxygen product. Additionally, the term 
"distillation column system" also encompasses a distillation column system 
including higher and lower pressure columns associated with one another in 
a heat transfer relationship. The term "vortex tube" as used herein and in 
the claims means a known device, generally in the form of a tube that 
separates an incoming gas stream through a tangential nozzle into two 
streams with different stagnation temperatures. Such a device operates in 
accordance with the known Ranque effect. As a result, in comparison to the 
incoming gas, gas leaves one end of the tube at a warm temperature and at 
the other end, at a cold temperature. 
As stated above, since a distillation process for separating air is 
conducted at a cryogenic temperature, there must be a compensation for 
heat leakage in order to keep the plant in balance. By distributing a 
product stream such as a vaporized oxygen enriched stream that has been 
used as coolant in a head condenser of a nitrogen generator, heat may be 
rejected at a higher temperatures than that of the incoming air. The net 
effect of this is to remove heat from the system and thereby to produce 
refrigeration by a device, namely a vortex tube, that is far less 
expensive than a turboexpander and has the added benefit of having no 
moving parts.

DETAILED DESCRIPTION 
With reference to the sole FIGURE, an apparatus 1 in accordance with the 
present invention is illustrated. In apparatus 1 a compressed and purified 
air stream 10 is cooled within a main heat exchanger 12 to near its dew 
point temperature and is then rectified within a distillation column 
system 14 having a single distillation column 16 coupled to a head 
condenser 18 for condensing reflux. 
Although not illustrated, compressed and purified air stream 10 is formed 
by compressing filtered air, removing the heat of compression from the 
air, and then purifying the air in a pre-purification unit to remove 
carbon dioxide, moisture and possibly also hydrocarbons. There are many 
known systems for effectuating such purification. Commonly, adsorbent beds 
operating out of phase in accordance with pressure swing adsorption and 
temperature swing adsorption cycles are used. 
Distillation column 16 has mass transfer contacting elements such as 
indicated by reference numerals 20 and 21. Such mass transfer containing 
elements may be either structured packing, random packing or trays. 
Distillation column 16 produces a nitrogen rich vapor as a tower overhead 
and an oxygen enriched liquid as a column bottoms. The nitrogen rich tower 
overhead, depending on the design of the column, can be of high purity. 
In order to reflux distillation column 16, nitrogen vapor stream 22 is 
removed and divided into a reflux stream 24 and a first product stream 26. 
Reflux stream 24 is condensed within head condenser 18 to produce a liquid 
reflux stream 27 to reflux distillation column 16. Coolant for the head 
condenser 18 is produced by a liquid oxygen enriched stream 28. After 
expansion through an expansion valve 30, liquid oxygen enriched stream 28 
is vaporized within head condenser 18 to produce a second product stream 
36. Second product stream 36 and first product stream 26 are introduced 
into main heat exchanger 12 for cooling the incoming compressed and 
purified air stream 10. 
Second product stream 36 which consists of the vaporized oxygen enriched 
liquid. As second product stream 36 warms within main heat exchanger 12, 
it is distributed to vortex tubes 38, 40, 42, 44, and 46. This is 
effectuated by providing nozzles in main heat exchanger 12 to expel 
successively warmer subsidiary streams 48, 50, 52, 54, and 56 to vortex 
tubes 38, 40, 42, 44, and 46. Vortex tubes 38 through 46 inclusive produce 
warm streams 58, 60, 62, 64, and 66, and cold streams 68, 70, 72, 74, and 
76. As can be appreciated by those skilled in the art the foregoing warm 
streams 58 through 66 and cold streams 68 through 76 become successively 
warmer as the warm end of main heat exchanger 12 is approached. Aside from 
warm stream 66, all of the remaining warm and cold streams 58 through 76 
inclusive are recycled back to main heat exchanger 12 to help cool 
compressed and purified air stream 10. Valves 78, 80, 82, 84 and 86 are 
used to balance the warm and cold flows as contained in streams 58 through 
76 inclusive. 
Warm stream 66 has a temperature warmer than that of the warm end of the 
main heat exchanger 12 or the temperature of compressed and purified air 
stream 10. It is discharged from apparatus 1 to reject heat at its 
temperature and thereby produce refrigeration. Warm stream 66 may simply 
be combined at an appropriate level with second product stream 36 as 
waste. 
Although all of the refrigeration requirements of apparatus 1 are provided 
by vortex tubes 38, 40, 42, 46, and 48, it is understood that for the 
aforementioned vortex tube arrangement might only supply part of the 
refrigeration requirements. The remainder of the refrigeration 
requirements could be provided by an external liquid source injected 
either directly to column 16 as additionally reflux or into head condenser 
18 to increase the reflux. The advantage of such an arrangement over 
conventional liquid assist plant would be to conserve on the amount of 
liquid that was introduced into the plant. Although only vortex tube 46 
produces a warm stream 66 to be discharged, a conceivable system might be 
to have first product stream 26 also connected to vortex tubes so that it 
could serve to provide refrigeration. Depending upon the particular 
system, proper functioning might dictate that multiple warm streams be 
produced by vortex tubes operating in a temperature range such that such 
streams would be discharged to reject heat and thus generate 
refrigeration. 
A valve 88 can be provided to control the flow of first product stream 26. 
Regulation of valve 88 can be used, in an inverse manner, to regulate the 
flow of second product stream 36 and therefore the amount of refrigeration 
to be generated. At one extreme, during startup, valve 88 might be set to 
cut off the flow of first product stream 26 so that there was a greater 
flow with second product stream 36 to thereby generate more refrigeration. 
The following chart is a calculated example of the present invention 
applied to a main heat exchanger of an air separation plant designed in 
accordance with apparatus 1. 
______________________________________ 
Stream No. 
Flow Composition Temperature 
Pressure 
From FIG. 
SM.sup.3 /hr 
% O.sub.2 .degree.K. 
bar (a) 
______________________________________ 
10 1000 21 300 10 
26 400 0.001 102.96 9.5 
36 600 34.93 102.1 5.5 
48 120 34.93 180.0 5.48 
50 120 34.93 200.0 5.46 
52 120 34.93 223.8 5.44 
54 120 34.93 250.4 5.42 
56 120 34.93 285.0 5.40 
58 60 34.93 200.0 1.17 
60 60 34.93 224.0 1.13 
62 60 34.93 251.5 1.12 
64 60 34.93 283.0 1.11 
66 60 34.93 323.0 1.10 
68 60 34.93 154.8 1.20 
70 60 34.93 172.0 1.18 
72 60 34.93 192.5 1.16 
74 60 34.93 215.0 1.14 
76 60 34.93 244.4 1.12 
26* 400 0.001 299.4 1.10 
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*At warm end of main heat exchanger 12 
Vortex tubes are available commercially. As one option and for the example 
presented herein, the vortex tubes can be obtained from Air Research 
Technology Company of Cincinnati, Ohio, United States or EXAIR Corporation 
also of Cincinnati, Ohio, United States. Both companies publish the same 
performance data for their vortex tubes and the heating and cooling 
performance can be scaled by the absolute temperature of the inlet gas. 
For any particular temperature level, it is understood that best 
engineering practice may dictate that multiple tubes be utilized. 
While the invention has been described with reference to a preferred 
embodiment, as will occur to those skilled in the art, numerous changes, 
additions and omissions may be made without departing from the spirit and 
scope of the present invention.