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  Cryogenic air separation process with excess turbine refrigerationRelated Patent Categories: Refrigeration, Cryogenic Treatment Of Gas Or Gas Mixture, Separation Of Gas Mixture, Air, DistillationThe Patent Description & Claims data below is from USPTO Patent Application 20070095100. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation to produce oxygen product. BACKGROUND ART [0002] The separation of air into its constituent components by distillation occurs at cryogenic temperatures, and requires some amount of refrigeration. This refrigeration is typically generated by the expansion of a process gas across a turbine. When designing air separation processes, the amount of refrigeration generated by expansion is typically kept at a minimum, as all forms of refrigeration generation are penal to the process, either by degrading the efficiency of the separation or by requiring more compression energy than is minimally required by the needs of the plant's distillation columns. The efficiency of refrigeration usage for a plant is reflected by the temperature difference between the streams entering and leaving the plant. This temperature difference is referred to as the aggregate warm end temperature difference (WEDT). At the extreme minimum, a WEDT of OK indicates that only the refrigeration required to drive the air separation was generated. [0003] In liquid oxygen pumped cryogenic air separation plants, product oxygen is removed as a liquid from the bottom of a low pressure distillation column, whereupon it is pumped to an elevated pressure, boiled in the primary heat exchanger or a product boiler against a condensing air stream, and the resulting vapor is superheated in the primary heat exchanger to form the gaseous oxygen product. If the liquid oxygen is pumped to its final delivery pressure, the gaseous oxygen product is sent directly to the end user; otherwise it requires further compression. The boiling of this oxygen against the condensing air gives rise to an internal pinch temperature difference. In other words, it gives rise to the minimum aggregate temperature difference between the cooling and warming streams in the primary heat exchanger (PHX). The magnitude of the PHX internal pinch is dictated by the available heat exchanger surface area. The larger the PHX, the tighter the pinch. Typically, in liquid oxygen pumped air separation plants, the PHX pinch DT is approximately 1-2K. [0004] The condensing air stream has to be compressed to a higher pressure than that of the main air feed to the plant prior to entering the PHX. This compression is typically accomplished with a separate booster air compressor. The pressure of the condensing air stream is typically higher than that of the boiling oxygen stream. As such, when higher pressure oxygen is required as a product, the booster air compressor consumes a large amount of energy. Because of the rising energy costs, the need exists for improved cryogenic air separation processes that use less total energy. It is a goal of this invention to reduce total power consumption by reducing the compression requirements of the condensing air stream. SUMMARY OF THE INVENTION [0005] In a process for the cryogenic separation of feed air wherein feed air is cooled in a primary heat exchanger, is separated by cryogenic rectification in at least one column to produce oxygen-rich liquid and nitrogen-rich vapor, oxygen-rich liquid is increased in pressure, and the pressurized oxygen-rich liquid is vaporized by indirect heat exchange with at least some of the feed air to produce product oxygen, the improvement comprising generating sufficient excess refrigeration beyond that required to carry out the cryogenic rectification such that the aggregate warm end temperature difference of the process exceeds the minimum internal temperature difference of the primary heat exchanger by at least 2K. [0006] As used herein, the term "aggregate warm end temperature difference" means the difference between the aggregate temperatures of those streams entering the primary heat exchanger and of those streams leaving the primary heat exchanger. [0007] As used herein, the term "minimum internal temperature difference of the primary heat exchanger" means the smallest difference between the aggregate temperatures of the warming and cooling streams inside the primary heat exchanger. [0008] 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. A double column comprises a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. [0009] Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The higher vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the lower 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 (stagewise) 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). [0010] 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. [0011] As used herein, the term "feed air" means a mixture comprising primarily oxygen and nitrogen, such as ambient air. [0012] As used herein, the terms "upper portion" and "lower portion" of a column mean those sections of the column respectively above and below the mid point of the column. [0013] As used herein, the terms "turboexpansion" and "turboexpander" mean respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid, thereby generating refrigeration. [0014] As used herein, the term "cryogenic air separation plant" means the column or columns wherein feed air is separated by cryogenic rectification to produce nitrogen, oxygen and/or argon, as well as interconnecting piping, valves, heat exchangers and the like. [0015] As used herein, the term "compressor" means a machine that increases the pressure of a gas by the application of work. BRIEF DESCRIPTION OF THE DRAWINGS [0016] FIG. 1 is a schematic representation of one cryogenic air separation process which may be used with, and which can benefit by the application of, the process of this invention. [0017] FIG. 2 is a graphical representation of the temperature difference between the composite warm and cold streams in the primary heat exchanger of the process illustrated in FIG. 1 as a function of heat exchanger duty when the process is carried out with conventional practice. [0018] FIG. 3 is a graphical representation of the temperature difference between the composite warm and cold streams in the primary heat exchanger of the plant and process illustrated in FIG. 1 as a function of heat exchanger duty when the process is carried out with the practice of this invention. DETAILED DESCRIPTION [0019] In general the liquid oxygen pumped cryogenic air separation method of this invention is characterized by an aggregate warm end temperature difference (WEDT) that is at least 2K more than the primary heat exchanger's minimum internal temperature difference (PHX pinch DT). More preferably, the difference between the WEDT and the PHX pinch DT will be greater than 3K, and most preferably it is greater than 4K. The extra refrigeration required for this invention is generated by the expansion of a process gas across a turbine. In many cases the savings that will be realized by reducing the compression energy of the condensing air stream will more than offset the penalties associated with extra refrigeration production. This is particularly the case at higher oxygen boiling pressures. Continue reading... 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