Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-05-25
2001-03-20
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S924000
Reexamination Certificate
active
06202441
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention applies in particular to the separation of air by cryogenic distillation. Over the years numerous efforts have been devoted to the improvement of this production technique to lower the oxygen cost which consists mainly of the power consumption and the equipment cost.
It has been known that an elevated pressure distillation system is advantageous for cost reduction and when the pressurized nitrogen can be utilized the power consumption of the system is also very competitive. It is useful to note that an elevated pressure system is characterized by the fact that the pressure of the lower pressure column being above 2 bar absolute. The conventional or low pressure process meanwhile has its lower pressure column operating at slightly above atmospheric pressure.
The higher the pressure of the lower pressure column, the higher is the air pressure feeding the high pressure column and the more compact is the equipment for both warm and cold portions of the plant resulting in significant cost reduction. However, the higher the pressure, the more difficult is the distillation process since the volatilities of the components present in the air (oxygen, argon, nitrogen etc) become closer to each other such that it would be more power intensive to perform the separation by distillation. Therefore the elevated pressure process is well suited for the production of low purity oxygen (<98% purity) wherein the separation is performed between the easier oxygen-nitrogen key components instead of the much more difficult oxygen-argon key components. The volatility of oxygen and argon is so close such that even at atmospheric pressure it would require high number of distillation stages and high reboil and reflux rates to conduct such separation. The elevated pressure process in the current configuration of today's state-of-the-art process cycles is not suitable nor economical for high purity oxygen production (>98% purity). Since the main impurity in oxygen is argon, the low purity oxygen production implies no argon production since over 50% of argon contained in the feed air is lost in oxygen and nitrogen products.
Therefore it is advantageous to come up with an elevated pressure process capable of high purity oxygen production and also in certain cases argon production.
The new invention described below utilizes the basic triple-column process developed for the production of low purity oxygen and adds an argon column to further separate the low purity oxygen into higher purity oxygen along with the argon by-product. By adding the argon column one can produce high purity oxygen (typically in the 99.5% purity by volume) required for many industrial gas applications and at the same time produce argon which is a valuable product of air separation plants.
The elevated pressure double-column process is described in U.S. Pat. No. 5,224,045.
The triple-column process is described in U.S. Pat. No. 5,231,837 and also in the following publications:
U.S. Pat. Nos. 5,257,504, 5,438,835, 5,341,646, EP 636845A1, EP 684438A1, U.S. Pat. Nos. 5,513,497, 5,692,395, 5,682,764, 5,678,426, 5,666,823, 5,675,977, 5,868,007, EP 833118 A1.
U.S. Pat. No. 5,245,832 discloses a process wherein a double-column system at elevated pressure is used in conjunction with a third column to produce oxygen, nitrogen and argon. In order to perform the distillation at elevated pressure a nitrogen heat pump cycle is used to provide the needed reboil and reflux for the system. In addition to the power required for the separation of argon and oxygen in the third column the heat pump cycle must also provide sufficient reflux and reboil for the second column as well such that the resulting recycle flow and power consumption would be high.
U.S. Pat. No. 5,331,818 discloses a triple column process at elevated pressure wherein the lower pressure columns are arranged in cascade and receive liquid nitrogen reflux at the top. The second column exchanges heat at the bottom with the top of the high pressure column. The third column exchanges heat at the bottom with the top of the second column. This process allows to optimize the cycle efficiency in function of the ratio of low pressure to high pressure nitrogen produced.
None of the above processes can be used economically and efficiently to produce high purity oxygen or argon.
U.S. Pat. No. 4,433,989 discloses an air separation unit using a high pressure column, an intermediate pressure column and a low pressure column, the bottom reboilers of the low and intermediate pressure columns being heated by gas from the high pressure column. Gas from the low pressure column feeds an argon column whose top condenser is cooled using liquid from the bottom of the intermediate pressure column. In this case the intermediate pressure column has no top condenser and all the nitrogen from that column is expanded to produce refrigeration.
U.S. Pat. No. 5,868,007 discloses a triple column system using an argon column operating at approximately the same pressure as the low pressure column. Gas from the bottom of the argon column is used to reboil the intermediate pressure column.
According to the invention, there is provided a process for separating air by cryogenic distillation comprising the steps of
feeding compressed, cooled and purified air to a high pressure column where it is separated into a first nitrogen enriched stream at the top and a first oxygen enriched stream at the bottom,
feeding at least a portion of the first oxygen enriched stream to an intermediate pressure column to yield a second nitrogen enriched stream at the top and a second oxygen enriched stream at the bottom, sending at least a portion of the second nitrogen enriched stream to a low pressure column or to a top condenser of the argon column, sending at least a portion of the second oxygen enriched stream to the low pressure column,
separating a third oxygen enriched stream at the bottom and a third nitrogen enriched stream at the top of the low pressure column,
sending a heating gas to a bottom reboiler of the low pressure column,
removing at least a portion of the third oxygen enriched stream at a removal point,
removing a first argon enriched stream containing between 3 and 20% argon from the low pressure column,
sending the first argon enriched stream to an argon column having a top condenser, recovering a second argon enriched stream, richer in argon than the first argon enriched stream, at the top of the argon column and removing a fourth oxygen enriched stream at the bottom of the argon column wherein the argon column operates at a pressure at least 0.5 bar lower than the low pressure column.
It is useful to note that when a stream is defined as a feed to a column, its feed point location, if not specified, can be anywhere in the mass transfer and heat transfer zones of this column wherever there is direct contact between this stream and an internal fluid stream of the column. The bottom reboiler or top condenser are therefore considered as part of the column. As an example, a liquid feed to a bottom reboiler of the column is considered as a feed to this column.
According to further optional aspects of the invention:
the process comprises sending at least a portion of the second nitrogen enriched liquid stream to the low pressure column, at least partially vaporizing a portion of the second oxygen enriched liquid stream in the top condenser of the intermediate column, sending at least a portion of the at least partially vaporized second oxygen enriched stream and a portion of the second oxygen enriched liquid to the low pressure column,
the argon column has a bottom reboiler heated by a gas stream,
that gas stream contains at least 90% nitrogen,
the gas stream heating the bottom reboiler of the argon column is at least a portion of one of the first, second and third nitrogen enriched streams,
the process comprises compressing at least a portion of the nitrogen enriched gas stream and sending it as heating gas to the bottom reboiler of the argon column,
the process comprises sen
Air Liquide Process and Construction, Inc.
Doerrler William
Young & Thompson
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