Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-05-01
2001-11-27
Capossela, Ronald (Department: 3743)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S654000
Reexamination Certificate
active
06321566
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing oxygen gas which includes compressing liquid oxygen obtained by cryogenic separation and then evaporating the liquid oxygen by heating to prepare higher pressure gaseous oxygen.
2. Description of the Related Art
A large amount of higher pressure gaseous oxygen is used in oxidizing refining steps in steel-producing converters in the steel industry, synthetic steps of ethylene oxide by oxidation of ethylene in the chemical industry, and partial oxidation steps of fuel, such as coal and petroleum residues, in fuel fired power plants. The demand for such oxygen has tended to increase in recent years.
A typical method for producing oxygen on an industrial scale is cryogenic separation, which includes rectification of raw air at low temperatures to separate out oxygen. In the cryogenic separation, nitrogen and oxygen are separated out from raw air by means of a difference in boiling point. That is, liquefied air is supplied to a rectifier, and nitrogen, having higher volatility than that of oxygen, is evaporated in the rectifier to yield a high concentration of liquid oxygen.
In a method for producing higher pressure gaseous oxygen in the cryogenic separation, liquid oxygen extracted from the rectifier is compressed using a pump and is then heated in a heat exchanger to evaporate the liquid oxygen. As an advantage of this method, compression costs can be significantly reduced compared to compression of gaseous oxygen.
Raw air contains trace amounts of impurities, such as hydrocarbons, e.g., methane, ethane, ethylene, acetylene, propane, propylene, butane, butene, and pentane; carbon dioxide; and nitrogen oxides, in addition to major components, such as nitrogen, oxygen, and argon. Since such impurities have higher boiling points than those of nitrogen and oxygen and are less volatile, these are called heavy impurities. These heavy impurities are dissolved in liquid oxygen having lower volatility than that of nitrogen. Since the heavy impurities have higher boiling points and are less volatile compared to oxygen, these are concentrated in the liquid oxygen as evaporation of the liquid oxygen proceeds in the heat exchanger, and is precipitated in an oxygen channel in the heat exchanger as a solid phase or a liquid phase when the concentration exceeds the solubility to liquid oxygen. The precipitated heavy impurities will readily react with oxygen in the heat exchanger and clogs the oxygen channel. As a result, performance of the heat exchanger and thus overall performance of the apparatus will be deteriorated.
The following conventional means are disclosed for solving such problems.
Japanese Unexamined Patent Application Publication No. 7-174460 discloses extraction of a major fraction of liquid oxygen from a liquid phase having a relatively low heavy-impurity concentration at a second-bottom stage right above the lowermost stage in a lower pressure distillation tower. Moreover, a small fraction of liquid oxygen is extracted from the lowermost stage containing the largest amount of impurities. The extracted liquid oxygen is compressed to a pressure, which is equal to or higher than the final supply pressure, to raise the boiling point of oxygen, and is fed into a heat exchanger to raise the vapor pressure of heavy impurities contained in the liquid oxygen. Evaporation of the heavy impurities is thereby facilitated in the heat exchanger and the heavy impurities are not accumulated in the heat exchanger.
Japanese Unexamined Patent Application Publication No. 8-61843 discloses a method including a recycle flow for removing heavy impurities. The recycle flow means the following gas flow. A liquid having an enriched oxygen content of approximately 40% and containing concentrated heavy impurities is extracted from the bottom of a higher pressure rectifier and is sufficiently compressed so that the heavy impurities are evaporated in a heat exchanger. The pressure of the residual air is reduced and then the air is allowed to converge to raw air. The converged air flow is supplied to a preliminary purification unit to remove the heavy impurities.
These methods, however, still have the following problems. In the former method, the liquid oxygen extracted from the second-bottom stage contains a low concentration of heavy impurities. Thus, this method is not a basic countermeasure to precipitation of heavy impurities. When the system is continuously operated for long periods, for example, a year, the heavy impurities will be significantly precipitated in the heat exchanger. Since the system has two oxygen channels, the facility and operational costs are increased due to use of expensive apparatuses, such as liquid oxygen pumps, and a complicated overall processes.
The latter method also requires additional apparatuses such as liquid oxygen pumps for the cycle reflow. Thus, this method requires high facility and operational costs due to a complicated system and a complicated operation. Accordingly, this method is also not a basic countermeasure.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for producing gaseous oxygen by cryogenic separation at low costs which does not cause precipitation of heavy impurities in an oxygen channel of a heat exchanger.
In the production of gaseous oxygen which includes compressing liquid oxygen separated by rectifying raw air to a predetermined pressure and evaporating the liquid oxygen in a heat exchanger, the present inventors have performed experiments under various conditions and have discovered that the above problems are overcome when the linear velocity of the gaseous oxygen in the oxygen channel of the heat exchanger is increased so as to satisfy the following parameters. As a result, the present invention has been completed.
A method for producing gaseous oxygen in accordance with the present invention includes compressing liquid oxygen separated by rectifying raw air to a predetermined supply pressure and evaporating the liquid oxygen in a heat exchanger, wherein gaseous oxygen in an oxygen channel of the heat exchanger flows upwards at a linear velocity which is equal to or larger than the terminal velocity, calculated depending on the supply pressure, of an oxygen droplet having a predetermined diameter.
The method for producing gaseous oxygen from raw air comprises the steps of compressing liquid oxygen separated by rectifying the raw air to a predetermined supply pressure, supplying the compressed liquid oxygen to a heat exchanger under a predetermined supply pressure, and evaporating and gasifying the liquid oxygen in the heat exchanger, wherein the gaseous oxygen flows upwards at a linear velocity which is equal to or higher than the terminal velocity u of a liquid oxygen droplet having a diameter of 200 &mgr;m calculated by equation (1):
u
=
(
4
g
2
(
ρ
L
-
ρ
G
)
2
D
p
3
225
μ
ρ
G
)
1
/
3
(
1
)
wherein u: terminal velocity of liquid oxygen droplets,
g: acceleration due to gravity,
&rgr;
L
: density of saturated liquid oxygen at the supply pressure,
&rgr;
G
: density of saturated gaseous oxygen at the supply pressure,
&mgr;: viscosity of saturated gaseous oxygen at the supply pressure, and
D
P
: diameter of the liquid oxygen droplet.
Equation (1) determines the terminal velocity of a microdroplet which follows Aren's resistance law which covers the range 2<Re<500 where in R is the Reynolds number.
Preferably, the gaseous oxygen flows upwards at a linear velocity which is equal to or higher than the terminal velocity u of a liquid oxygen droplet having a diameter of 500 &mgr;m calculated by equation (2):
u
=
(
3.03
g
(
ρ
L
-
ρ
G
)
D
P
ρ
G
)
1
/
2
(
2
)
wherein u: terminal velocity of liquid oxygen droplets,
g: acceleration due to gravity,
&rgr;
L
: density of saturated liquid oxygen at the supply pressure,
&rgr;
G
: density of saturated gaseous oxygen at the supply pressure,
&mgr;: viscosi
Asahara Kazuhiko
Tanaka Masayuki
Yamamoto Seiichi
Capossela Ronald
Kabushiki Kaisha Kobe Seiko Sho.
Reed Smith Hazel & Thomas LLP
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