The present invention relates to a process and apparatus for the separation of air by cryogenic distillation. It relates in particular to processes and apparatus for producing oxygen and/or nitrogen at elevated pressure.
Gaseous oxygen produced by air separation plants are usually at elevated pressure about 20 to 50 bar. The basic distillation scheme is usually a double column process producing oxygen at the bottom of the low-pressure column operated at 1.4 to 4 bar. The oxygen must be compressed to higher pressure either by oxygen compressor or by the liquid pumping process. Because of the safety issues associated with the oxygen compressors, most recent oxygen plants are based on the liquid pumping process. In order to vaporize liquid oxygen at elevated pressure there is a need for an additional motor-driven booster compressor to raise a portion of the feed air or nitrogen to higher pressure in the range of 40-80 bars. In essence, the booster replaces the oxygen compressor.
In the effort to reduce the complexity of an oxygen plant, it is desirable to reduce the number of motor-driven compressors. Significant cost reduction can be achieved if the booster can be eliminated without much affecting the plant performance in terms of power consumption. Furthermore, the air purification unit conceived for a traditional oxygen plant would operate at about 5-7 bar which is essentially the pressure of the high-pressure column, and it is also desirable to raise this pressure to a higher level in order to render the equipment more compact and less costly.
A cold compression process as described in U.S. Pat. No. 5,475,980 provides a technique to drive the oxygen plant with a single air compressor. In this process, air to be distilled is chilled in the main exchanger then further compressed by a booster compressor driven by an expander exhausting into the high-pressure column of a double column process. By doing so, the discharge pressure of the air compressor is in the range of 15 bar which is also quite advantageous for the purification unit. One inconvenience of this approach is the increase of the size of the main exchanger due to additional flow recycling which is typical for the cold compression plant. One can reduce the size of the exchanger by opening up the temperature approaches of the exchanger. However, this would lead to inefficient power usage and higher discharge pressure of the compressor, therefore increasing its cost. An illustration of this prior art is presented in
In
An oxygen enriched liquid stream 28 is expanded and sent from the high-pressure column to the low-pressure column. A nitrogen enriched liquid stream 29 is expanded and sent from the high-pressure column to the low-pressure column. High-pressure gaseous nitrogen 14 is removed from the top of the high-pressure column and warmed in the heat exchanger to form a product stream 24. Liquid oxygen 20 is removed from the bottom of the low pressure column 31, pressurized by a pump 21 and sent as stream 22 to the heat exchanger 5 where it vaporizes by heat exchange with the pressurized air 10 to form gaseous pressurized oxygen 23. A top nitrogen enriched gaseous stream 25 is removed from the low-pressure column 31, warmed in the heat exchanger 5 and then forms stream 26.
Some different versions of the cold compression process were also described in prior art as in U.S. Pat. No. 5,379,598, U.S. Pat. No. 5,596,885, U.S. Pat. No. 5,901,576 and U.S. Pat. No. 6,626,008.
In U.S. Pat. No. 5,379,598 a fraction of feed air is further compressed by a booster compressor followed by a cold compressor to yield a pressurized stream needed for the vaporization of oxygen. This approach still has at least two compressors and the purification unit still operates at low pressure.
In U.S. Pat. No. 5,596,885, a fraction of the feed air is further compressed in a warm booster whilst at least part of the air is further compressed in a cold booster. Air from both boosters is liquefied and part of the cold compressed air is expanded in a Claude expander.
U.S. Pat. No. 5,901,576 describes several arrangements of cold compression schemes utilizing the expansion of vaporized rich liquid of the bottom of the high-pressure column, or the expansion of high-pressure nitrogen to drive the cold compressor. In some cases, motor driven cold compressors were also used. These processes also operate with feed air at about the high-pressure column's pressure and in most cases a booster compressor is also needed.
U.S. Pat. No. 6,626,008 describes a heat pump cycle utilizing a cold compressor to improve the distillation process for the production of low purity oxygen for a double vaporizer oxygen process. Low air pressure and a booster compressor are also typical for this kind of process.
Therefore it is a purpose of this invention to resolve the inconveniences of the traditional process by providing a solution to simplify the compression train and to reduce the size of the purification unit. This can moreover be achieved with good power consumption. The overall product cost of an oxygen plant can therefore be reduced. The main improvement in power consumption is due to the reduction in the cold compressor flow by using essentially latent heat instead of specific heat.
All percentages listed are molar percentages.
According to the present invention, there is provided a process for separating air by cryogenic distillation in a column system comprising a high pressure column and a low pressure column comprising the steps of:
i) compressing all the feed air in a first compressor to a first outlet pressure
ii) sending a first part of the air at the first outlet pressure to a second compressor and compressing the air to a second outlet pressure
iii) cooling at least part of the air at the second outlet pressure in a heat exchanger to form cooled compressed air at the second outlet pressure, liquefying at least part of the air at the second outlet pressure and sending the liquefied air to at least one column of the column system
iv) cooling a second part of the air at the first outlet pressure in the heat exchanger and expanding at least part of the second part of the air in an expander from the first outlet pressure to the pressure of a column of column system and sending the expanded air to that column
v) removing liquid from a column of the column system, pressurizing the liquid and vaporizing the liquid by heat exchange in the heat exchanger
vi) at least partially vaporizing an auxiliary fluid, eventually further warming said auxiliary fluid in the heat exchanger, sending at least part of this auxiliary fluid to a third compressor to be compressed to a third outlet pressure, introducing at least part of said auxiliary fluid at said third outlet pressure in the heat exchanger, cooling said auxiliary fluid and at least partially liquefying said auxiliary fluid, removing said auxiliary stream from the heat exchanger and expanding it to a fourth pressure level before reintroducing it in the heat exchanger where it will be partially vaporized as above-mentioned.
According to optional features of the invention:
According to another aspect of the invention, there is provided an apparatus for the separation of air by cryogenic distillation comprising:
a) a column system
b) first, second and third compressors
c) an expander
d) a conduit for sending air to the first compressor to form compressed air at a first outlet pressure
e) a conduit for sending a first part of the air at the first outlet pressure to the second compressor to form air at a second outlet pressure
f) a heat exchanger, a conduit for sending at least part of the air at the second outlet pressure to the heat exchanger to form cooled compressed air at the second outlet pressure,
g) a conduit for removing liquefied air at the second outlet pressure from the heat exchanger and for sending the liquefied air to at least one column of the column system
h) a conduit for removing a second part of the air at the first outlet pressure from the heat exchanger and for sending at least part of the second part of the air to the expander
i) a conduit for sending air expanded in the expander to at least one column of column system
j) a conduit for removing liquid from a column of the column system, means for pressurizing at least part of the liquid to form pressurized liquid and a conduit for sending at least part of the pressurized liquid to the heat exchanger and
k) a refrigeration cycle comprising the third compressor and a second expander (16), a conduit for sending an auxiliary fluid from the third compressor to the heat exchanger, a conduit for sending the auxiliary fluid from the heat exchanger to the second expander, a conduit for sending the auxiliary fluid from the second expander to the heat exchanger and a conduit for sending the auxiliary fluid from the heat exchanger to the third compressor.
According to further optional aspects of the invention, the apparatus may include a further expander and means for sending nitrogen from a column of the column system or air to the further expander.
In this case, one of the second and third compressors may be coupled to the expander and the other of the second and third compressors may be coupled to the further expander.
At least one of the second and third compressors is coupled to the air expander.
Preferably the conduit for sending a first part of the air at the first outlet pressure to the second compressor is connected to an intermediate point of the heat exchanger.
Preferably the second and third compressors are connected in series.
The expander may be chosen from the group including an air expander whose outlet is connected to the high pressure column, an air expander whose outlet is connected to the low pressure column, a high pressure nitrogen expander and a low pressure nitrogen expander.
The apparatus may include a further expander chosen from the group including an air expander whose outlet is connected to the high pressure column, an air expander whose outlet is connected to the low pressure column, a high pressure nitrogen expander and a low pressure nitrogen expander.
Preferably the further expander is coupled to one of the second and third expanders.
The invention will now be described in greater detail with reference to
In the embodiment of
The above temperatures T1, T2, T3 and T4 are provided as the preferred arrangement. Above, going from the hottest temperature to the coldest the temperatures are T2, T5, T1 and T3. Depending upon the pressure of the vaporized oxygen and the pressure of the column system the order of these temperatures can be modified to optimize the performance of the process.
It is useful to note the booster brake compressors 3 is a single stage compressor and is usually provided as part of the expander-booster package and therefore its construction is much simpler and its cost structure much lower than the stand-alone or motor-driven booster compressor. However if necessary, compressor 3 may be a stand-alone or motor-driven booster compressor. Compressor 8 could be either a stand-alone or motor driven booster compressor with one to four stages depending upon the pressure of stream 4 and stream 23. It could be driven directly by expander 18 (alternately expander 13) at the same speed or through a gear to optimize the performances of the booster and expander.
The range of the process variables of the embodiment of
Stream 11 pressure: about 9 to 17 bar a
Stream 4 pressure: about 16 to 50 bar a
Stream 9 pressure: about 5 to 20 bar a in case of a mixture rich in krypton
T1: about −110° C. to −165° C.
The flow compressed by the booster brake compressor 8 can be reduced by optionally extracting some of stream 12 as liquefied air flow 33. As such, less power is required to drive the booster brake compressor 8 and some power savings can be achieved. The amount of air liquefied at the first pressure should not be more than 50% of the liquefied air sent to the column system, preferably not more than 40%, more preferably not more than 35%.
It is common practice in air separation technology to substitute the nitrogen expander with an air expander. The embodiment of
The above technique can be modified slightly as described in
In many situations where there is a need for a significant amount of nitrogen rich gas product at elevated pressure, it is no longer economical to utilize the nitrogen rich gas expander 18. Instead as shown in
The process may be modified to vaporize pumped liquid nitrogen as an additional stream or as a stream replacing the pumped oxygen stream.
The illustrated processes show double column systems but it will be readily understood that the invention applies to triple column systems.
In the case where the double or triple column systems operate at elevated pressures, some of the low pressure nitrogen may be expanded in an expander 18.
Number | Date | Country | Kind |
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05108826.8 | Sep 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2006/066601 | 9/21/2006 | WO | 00 | 3/21/2008 |