Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-07-01
2001-03-27
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S620000, C062S624000
Reexamination Certificate
active
06205813
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to cryogenic rectification and, more particularly, to cryogenic rectification in the purification of natural gas.
BACKGROUND ART
In the production of natural gas it is sometimes necessary to subject the raw natural gas stream to a purification process in order to produce natural gas with a sufficient combustibles content so that it may be efficiently used within a natural gas distribution network such as a pipeline system. A number of such natural gas purification systems are known and practiced commercially.
Methane, the main component of natural gas, is widely used as a chemical synthesis feedstock. As such, the methane must be of a high purity to ensure effective downstream synthesis. It would be highly desirable to produce high purity methane, suitable for use in subsequent chemical synthesis reactions, in conjunction with natural gas purification.
Accordingly, it is an object of this invention to provide a system which can process a raw natural gas feed stream and produce both fuel and high purity methane.
Conventional natural gas purification systems which process a nitrogen-containing raw natural gas stream typically produce pipeline quality natural gas having a significant nitrogen content such as up to five mole percent or more. While this is acceptable for most uses of the natural gas, such a nitrogen content cannot be tolerated in some applications, such as in certain metallurgical processes where nitrogen can cause detriment to the metal. Accordingly, it is another object of this invention to provide a system which can process a raw natural gas feed stream and produce both fuel and high purity natural gas.
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 process for the production of fuel and high purity hydrocarbon product comprising:
(A) providing a feed comprising nitrogen, methane and carbon dioxide wherein carbon dioxide comprises from 1 to 40 volume percent of the feed;
(B) removing carbon dioxide from the feed to produce a carbon dioxide depleted feed;
(C) cooling the carbon dioxide depleted feed and passing the cooled carbon dioxide depleted feed into a cryogenic rectification column;
(D) separating the carbon dioxide depleted feed by cryogenic rectification within the cryogenic rectification column into fuel and high purity hydrocarbon product; and
(E) recovering fuel from the upper portion of the cryogenic rectification column, and recovering high purity hydrocarbon product from the lower portion of the cryogenic rectification column.
Another aspect of the invention is:
Apparatus for the production of fuel and high purity hydrocarbon product comprising:
(A) a carbon dioxide removal system and means for providing a feed comprising nitrogen, methane and carbon dioxide to the carbon dioxide removal system;
(B) heat exchange means, and means for passing carbon dioxide depleted feed from the carbon dioxide removal system to the heat exchange means;
(C) a cryogenic rectification column and means for passing carbon dioxide depleted feed from the heat exchange means to the cryogenic rectification column;
(D) means for recovering fuel from the upper portion of the cryogenic rectification column; and
(E) means for recovering high purity hydrocarbon product from the lower portion of the cryogenic rectification column.
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 as countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting or 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 adiabatic and can include integral or differential 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 terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the mid point of the column.
As used herein, the term “fuel” means a fluid containing from 15 to 40 volume percent methane.
As used herein, the term “high purity methane” means a fluid containing at least 99 volume percent methane, less than 0.5 mole percent nitrogen and less than 0.5 mole percent heavier hydrocarbons.
As used herein, the term “high purity natural gas” means a fluid comprised essentially of hydrocarbons, such as methane and heavier hydrocarbons, and containing no more than 200 ppm nitrogen, preferably no more than 50 ppm nitrogen.
As used herein, the term “high purity hydrocarbon product” means either high purity methane or high purity natural gas.
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.
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Drnevich Raymond Francis
Maloney James Joseph
Shah Minish Mahendra
Black Donald T.
Doerrler William
Praxair Technology Inc.
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