Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-10-29
2001-01-30
McDermott, Corrine (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06178776
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the production of pressurized oxygen gas and more particularly to the production of pressurized oxygen gas from low pressure oxygen gas.
BACKGROUND ART
The compression of gaseous oxygen to produce pressurized oxygen gas is very expensive when compared to the cost of compression of other atmospheric gases. The cost is higher in both power consumed and in initial capital cost of the compression equipment. This high cost is due to the reactive nature of gaseous oxygen. Mechanical tolerances are set much looser for an oxygen compressor than for a nitrogen or air compressor so as to reduce the risk of a rub within the machine that could cause a fire. These looser tolerances or high clearances result in significantly reduced compressor efficiencies, on the order of six to ten percent. This lower efficiency corresponds to a higher compressor power.
The problem of high cost in the production of pressurized oxygen gas is not acute when the oxygen is produced by the cryogenic separation of air because the oxygen can be recovered as high pressure gas directly from a column, or can be taken from a column as liquid, pressurized and then vaporized. However these expediencies are not available when the oxygen is produced by a non-cryogenic air separation method such as by vacuum pressure swing adsorption.
Those skilled in the art have addressed this problem by making small, incremental improvements in oxygen compressors. Incremental improvements in centrifugal compressors have been achieved, but the gains have been modest. Positive displacement machines have been used in place of centrifugal compressors, and while they have a lower initial capital cost, the life cycle cost of such machines is higher due to increased power consumption and higher maintenance cost. In summary, improvements in such machines over the years has been only incremental, not a step change.
In the operation of a non-cryogenic oxygen plant, such as a vacuum pressure swing adsorption plant, the cost of the oxygen compressor is a significant portion of both the total capital cost and the power usage of the plant. If a significant reduction in the cost of oxygen compression can be achieved, a substantial decrease in the total cost of a non-cryogenic oxygen production plant can be attained.
Accordingly it is an object of this invention to provide an improved system for the production of pressurized oxygen gas.
It is another object of this invention to provide a system for the production of pressurized oxygen gas from low pressure oxygen gas without the need for employing an oxygen compressor.
SUMMARY OF THE INVENTION
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing pressurized oxygen gas comprising:
(A) condensing low pressure oxygen gas by indirect heat exchange with vaporizing multicomponent refrigerant fluid to produce low pressure oxygen liquid and vaporized multicomponent refrigerant fluid;
(B) pumping at least some of the low pressure oxygen liquid to produce pressurized oxygen liquid, and compressing the vaporized multicomponent refrigerant fluid to produce higher pressure multicomponent refrigerant fluid; and
(C) vaporizing at least some of the pressurized oxygen liquid by indirect heat exchange with condensing higher pressure multicomponent refrigerant fluid to produce condensed higher pressure multicomponent refrigerant fluid and pressurized oxygen gas.
Another aspect of the invention is:
Apparatus for producing pressurized oxygen gas comprising:
(A) a heat exchanger, means for providing oxygen gas to the heat exchanger, a liquid pump, means for passing oxygen liquid from the heat exchanger to the liquid pump, and means for passing oxygen liquid from the liquid pump to the heat exchanger;
(B) a compressor, means for passing multicomponent refrigerant fluid from the heat exchanger to the compressor, and means for passing multicomponent refrigerant fluid from the compressor to the heat exchanger; and
(C) means for recovering product pressurized oxygen gas from the heat exchanger.
As used herein, the term “column” means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer's Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (statewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein, the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term “oxygen gas” means a gas having an oxygen concentration of at least 30 mole percent and preferably at least 90 mole percent.
As used herein, the term “oxygen liquid” means a liquid having an oxygen concentration of at least 30 mole percent and preferably at least 90 mole percent.
As used herein, the term “top condenser” means a heat exchange device that generates column downflow liquid from column vapor.
As used herein, the term “bottom reboiler” means a heat exchange device that generates column upflow vapor from column liquid.
As used herein, the term “variable load refrigerant” means a multicomponent fluid, i.e. a mixture of two or more components, in proportions such that the liquid phase of those components undergoes a continuous and increasing temperature change between the bubble point and the dew point of the mixture. The bubble point of the mixture is the temperature, at a given pressure, wherein the mixture is all in the liquid phase but addition of heat will initiate formation of a vapor phase in equilibrium with the liquid phase. The dew point of the mixture is the temperature, at a given pressure, wherein the mixture is all in the vapor phase but extraction of heat will initiate formation of a liquid phase in equilibrium with the vapor phase. Hence, the temperature region between the bubble point and the dew point of the mixture is the region wherein both liquid and vapor phases coexist in equilibrium. In the practice of this invention the temperature differences between the bubble point and the dew point for the multicomponent refrigerant fluid is at least 10° K, preferably at least 20° K and most preferably at least 50° K.
As used herein, the term “atmospheric gas” means one of the following: nitrogen (N
2
Bonaquist Dante Patrick
Mahoney Kevin William
Olszewski Walter Joseph
Drake Malik N.
Ktorides Stanley
McDermott Corrine
Praxair Technology Inc.
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