Method for producing carbon dioxide

Abstract: A distillation system for producing carbon dioxide, particularly from a feed which contains significant levels of light components, wherein the feed is at least partially condensed upstream of the distillation by indirect heat exchange with a multicomponent heat exchange fluid recirculating in a closed refrigeration loop, and/or the overhead from the distillation is condensed for reflux by indirect heat exchange with a multicomponent heat exchange fluid recirculating in a closed refrigeration loop.

Description: Technical Field This invention relates to a method for the production of carbon dioxide. Background Art Carbon dioxide has a large number of uses. For example, carbon dioxide is used to carbonate beverages, to chill, freeze and package seafood, meat, poultry, baked goods, fruits and vegetables, and to extend the shelf-life of dairy products. It is an important environmental component in industrial waste and process water treatment as a replacement for sulfuric acid to control pH levels. Other uses included drinking water treatment, an environmentally-friendly pesticide and an atmosphere additive in greenhouses to improve the growth of vegetables. Generally carbon dioxide is produced by purifying a waste stream which is a by-product of an organic or inorganic chemical process. The waste stream comprising the carbon dioxide is condensed and then processed in a distillation column to produce the product grade carbon dioxide. US-A-4 704 146 discloses a method for producing carbon dioxide comprising: (A) partially condensing a feed comprising carbon dioxide, methane and methanol by indirect heat exchange with a heat exchange fluid, phase separating the partially condensed feed into a condensed feed portion and a vapor feed portion, discharging the condensed feed portion and passing the vapor feed portion into a column; (B) separating the feed within the column into light overhead and carbon dioxide product; (C) partially condensing light overhead by indirect heat exchange with said heat exchange fluid to produce reflux liquid and remaining vapor; (D) passing reflux liquid down the column; (E) recovering carbon dioxide product from the lower portion of the column; and (F) subcooling the carbon dioxide product by indirect heat exchange with said heat exchange fluid. As the demand for carbon dioxide continues to increase, more marginal sources of carbon dioxide are being used to supply the crude carbon dioxide feed to the purification system. Such marginal feeds may include significant amounts of light contaminants and thus require significantly more energy to carry out the requisite liquefaction prior to the distillation into product. Accordingly it is an object of this invention to provide a system which can effectively process a crude carbon dioxide feed stream which contains light contaminants in a more energy efficient manner than that possible with conventional carbon dioxide processing systems. Summary Of The Invention The above object is attained by the present invention, one aspect of which is a method for producing carbon dioxide as defined in claim 1. 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 (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. 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 "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 "condenser" means an indirect heat exchanger wherein a feed comprising carbon dioxide and light contaminants is at least partially condensed. As used herein, the term "light contaminants" means one or more species having a vapor pressure greater than that of carbon dioxide. Examples of light contaminants include nitrogen, oxygen, argon, hydrogen and carbon monoxide. As used herein, the term "multicomponent heat exchange fluid" means a fluid having two or more components which possess different saturation curves (boiling points). A multicomponent heat exchange fluid is a heat transfer fluid having at least two components. Streams which partially condense over a range of temperatures are more efficiently condensed using certain multicomponent heat exchange fluids which are designed to closely follow the condensation heat transfer curve (Q vs T) by proper selection of components, composition and operating pressure. The proper multicomponent heat exchange fluid minimizes process lost work by reducing the temperature difference between the cooling condensing stream relative to the vaporizing multicomponent heat exchange fluid through the partial condensation heat exchanger. As used herein, the term "subcooling" means cooling a liquid to be at a temperature lower than that liquid's saturation temperature for the existing pressure. As used herein, the terms "turboexpansion" and "turboexpander" mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration. Brief Description Of The Drawing Figure 1 is a schematic representation of a preferred embodiment of the invention. Detailed Description The invention comprises, in general, the use of a multicomponent heat exchange fluid recirculating in a heat exchange circuit, to partially liquefy overhead from the distillation column or to liquefy at least some, preferably all, of the feed to the column for the separation of carbon dioxide from light contaminants. The multicomponent heat exchange fluid enables the liquefaction to be carried out with less energy than would be needed if turboexpansion were used to generate refrigeration, or if a conventional pure component refrigerant were used to absorb the heat of condensation. The invention will be described in detail with reference to the Drawing . Referring now to Figure 1, feed stream 50 comprising carbon dioxide, light contaminants and water vapor, typically at about ambient pressure, is passed to compressor 1 wherein it is compressed to a pressure generally within the range of from 413.7 to 620.5 kPa (60 to 90 pounds per square inch absolute (psia)). Feed stream 50 is typically taken from the waste effluent of an organic or inorganic chemical production system such as one which produces ethanol and/or other alcohols. The concentration of carbon dioxide in the feed is generally within the range of from 25 to 98 mole percent on a dry basis. The invention will have particular utility for processing a feed wherein the light contaminants comprise at least 15 mole percent on a dry basis. Compressed feed 51 is cooled by passage through water or air driven cooler 2 and condensed moisture is separated in phase separator 3. The feed is then further compressed by passage through compressor 4 to a pressure generally within the range of from 1930 to 2241 kPa (280 to 325 psia) The further compressed feed 52 is then further cooled by passage through coolers 5 and 6. Condensed moisture is removed in phase separator 7 and the feed is further dried by passage through adsorbent bed 8. Cooled, dried feed stream 53 is then cooled to near its dewpoint by passage through column reboiler 9 and then is provided by conduit to condenser 10 wherein it is at least partially condensed, preferably substantially totally condensed by indirect heat exchange with recirculating pure component heat exchange fluid. Preferably the pure component heat exchange fluid is ammonia. Other pure component heat exchange fluids which may be used in the practice of this invention include propane and halogenated hydrocarbon refrigerants. Resulting condensed feed 54 is flashed through valve 11 and passed into column 12, preferably, as illustrated in Figure 1, at the top of column 12. Referring back now to Figure 1, gaseous heat exchange fluid 55 is compressed to a moderate pressure by passage through compressor 15 and then passed into direct contact aftercooler 16. Gaseous moderate pressure heat exchange fluid is withdrawn from aftercooler 16 in stream 56 and compressed to a high pressure in compressor 17. The high pressure fluid is then substantially totally condensed by passage through heat exchanger 18 by indirect heat exchange with, for example, water or air, and then passed through valve 19 and into aftercooler 16 as the cooling medium. Moderate pressure chilled liquid heat exchange fluid is withdrawn from aftercooler 16 in stream 57. If desired, as illustrated in Figure 1, a portion 58 of stream 57 is passed through cooler 6 to cool the feed and then returned to aftercooler 16. The remaining portion of stream 57 is passed through valve 20 and is then divided into streams 59 and 60. Stream 59 is provided by conduit to condenser 10 wherein it is vaporized by indirect heat exchange with the aforesaid condensing feed, and resulting vaporized heat exchange fluid is withdrawn from condenser 10 as stream 61. Stream 60 is passed into subcooler 13 wherein it is vaporized by indirect heat exchange with subcooling product. Resulting vaporized stream 62 is combined with stream 61 to form gaseous stream 55 and the closed loop refrigeration cycle is repeated. Column 12 is operating at a pressure generally within the range of from 1724 to 2206 kPa (250 to 320 psia). Within column 12 the feed is separated by distillation into light overhead and carbon dioxide product. The feed liquid flows down column 12 against upflowing vapor and light contaminants are stripped out of the downflowing liquid into the upflowing vapor to form the light overhead at the top of column 12 and the carbon dioxide product at the bottom of column 12. Carbon dioxide product liquid, generally having a carbon dioxide concentration of at least 99.9 mole percent, is withdrawn from the lower portion of column 12 in stream 63. A portion 64 is vaporized by passage through reboiler 9 in indirect heat exchange with cooling feed, and resulting vaporized portion 65 is passed back into column 12 to serve as the upflowing vapor. Another portion 66 of stream 63 is subcooled by passage through subcooler 13, passed through valve 14 and recovered as product carbon dioxide in stream 67. Light overhead vapor 68, containing an increased fraction of light contaminants from that in the feed, and also containing some carbon dioxide, is withdrawn from the upper portion of column 12, partially condensed by passage through heat exchanger 71, and passed into phase separator 31. Uncondensed remaining contaminant vapor is passed out of phase separator 31 in vapor stream 69 and warmed by passage through heat exchanger 71 prior to withdrawal from the system. Resulting reflux liquid is passed in stream 70 from phase separator 31 into the upper portion of column 12 and then down column 12. Heat exchanger 71 is driven by a recirculating multicomponent heat exchange fluid which preferably is comprised of the refrigerant used to drive heat exchanger 10, e.g. ammonia, and a second colder boiling species, e.g. nitrogen. Other colder boiling species which may be used include argon, methane, ethane and refrigerants having the designations R14, R23, R32 and R125. Multicomponent heat exchange fluid 72 is compressed in compressor 73 and then cooled and preferably partially or totally condensed by passage through heat exchanger 74 by indirect heat exchange with a suitable cooling fluid such as water or forced air. Resulting fluid 75 is further condensed and/or subcooled by passage through heat exchanger 76 and then divided into two portions. The first portion 77: is reduced in pressure by passage through valve 78 and then passed through heat exchanger 71 to partially condense the light overhead vapor. The second portion 79 is reduced in pressure through valve 80 and then passed through heat exchanger 76 to condense and/or subcool multicomponent heat exchange fluid 75. Streams 77 and 79 are recombined to form stream 72 and the closed loop refrigeration cycle is repeated. Now with the use of this invention one can provide efficiently and effectively product grade carbon dioxide by cryogenic rectification from a feed containing significant levels of light contaminants.

Claims: A method for producing carbon dioxide comprising: (A) condensing a feed (50) comprising carbon dioxide and light contaminants by indirect heat exchange with recirculating pure component heat exchange fluid (55) and passing the condensed feed into a column (12); (B) separating the feed within the column into light overhead (68) and carbon dioxide product (63); (C) partially condensing light overhead (68) by indirect heat exchange with multicomponent heat exchange fluid (72) to produce reflux liquid (70) and remaining contaminant vapor (69); (D) separating reflux liquid (70) from remaining contaminant vapor (69) and passing reflux liquid (70) down the column (12); (E) recovering carbon dioxide product (63) from the lower portion of the column (12); and (F) subcooling the carbon dioxide product (66) by indirect heat exchange with the recirculating pure component heat exchange fluid (55). The method of claim 1 wherein the multicomponent heat exchange fluid (72) comprises ammonia and nitrogen.

Inventors: HOWARD HENRY EDWARD, HOWARD, HENRY EDWARD
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 2000
Main classification: F25J3
IPC codes: F25J3, C01B31