Carbon dioxide recovery from an oxygen containing mixture

ES 2 301 460 T3

DESCRIPTION

Recovery of carbon dioxide from a mixture containing oxygen.

Technical field

This invention relates generally to the recovery of carbon dioxide and, more particularly, to the recovery of carbon dioxide from a feed mixture that also contains oxygen.

Background of the technique

Carbon dioxide has a large number of applications. For example, carbon dioxide is used in carbonated beverages, to refrigerate, freeze, and package seafood, meat, poultry, baked goods, fruits, and vegetables, and to extend the time dairy products can be displayed for sale. It is an important environmental component in industrial waste and in the treatment of process water as a substitute for sulfuric acid to control pH levels. Other applications include drinking water treatment, an environmentally friendly pesticide, and an atmosphere additive in greenhouses to enhance vegetable growth.

Carbon dioxide is generally produced by scrubbing a waste stream, which is a by-product of an organic or inorganic chemical process. The waste stream, which comprises a high concentration of carbon dioxide, is condensed and scrubbed in multiple stages and then distilled to produce product grade carbon dioxide.

As the demand for carbon dioxide continues to increase, alternative sources of carbon dioxide are being used to supply the raw carbon dioxide feed to the scrubber system. Such alternative feeds have a much lower concentration of carbon dioxide and therefore need to improve their quality, that is, the concentration of carbon dioxide must be increased, before product quality carbon dioxide can be produced. effectively. These alternative feeds with much lower carbon dioxide concentrations are called lean feeds. An example of such a lean feed is flue gas from, for example, a combustion source such as a boiler, an internal combustion engine, a gas turbine or a lime kiln.

Quality improvement of the carbon dioxide concentration in a feed can be accomplished in several ways. A particularly preferred method is the chemical absorption of carbon dioxide from the raw carbon dioxide feed in an alkanolamine-based absorbent. The resulting carbon dioxide loaded absorbent then undergoes separation into a carbon dioxide recovery product and an alkanolamine containing absorbent, which can be recycled for reuse within the recovery system.

Raw carbon dioxide feed often contains significant levels of oxygen, which can cause degradation of alkanolamines reducing their usefulness in the recovery system and also causing corrosion problems in the system. Those skilled in the art have approached this problem in one of two possible ways. In one method, chemical inhibitors are added to the absorbent fluid to protect it against degradation by inhibiting the oxidation of alkanolamines. Such a method is disclosed in GB-A-2 156 327, which proposes a process for removing carbon dioxide from a gas containing CO₂ me₂, where the gas is brought into contact with an aqueous solution containing an alkanolamine absorbent-reactant, a Cu solution²⁺ and a further inhibitor selected from the group of dihydroxyethylglycine, an alkali metal carbonate, an alkali metal permanganate, ammonium permanganate, nickel oxide, a bismuth oxide, an alkali metal thiocyanate and ammonium thiocyanate.

Furthermore, EP-A-0 776 687 discloses a process for the transfer of carbon dioxide from gases, the method of which comprises a carbon dioxide absorption step of contacting liquid gas with a gas, having a carbon dioxide partial pressure in the range of 0.3 to 50 atmospheres, with an absorbent fluid comprising an aqueous solution containing a specified amine compound, in order to produce a treatment gas having a reduced carbon dioxide content and an absorbent fluid rich in carbon dioxide, and further comprising a regeneration step of releasing carbon dioxide from the absorbent fluid rich in carbon dioxide by flash process at approximately atmospheric pressure and / or vapor distillation, so that the carbon dioxide-poor absorbent fluid is regenerated and recirculated for use in the carbon dioxide absorption step.

In another method, a fuel is added to the raw carbon dioxide feed for combustion with oxygen in a catalytic combustion reaction. Although both methods are effective, both are characterized by high capital costs and, on the other hand, are complicated to operate.

Accordingly, it is an object of this invention to provide a system that can more efficiently recover carbon dioxide or other absorbate from an oxygen-containing feed using an alkanolamine-based absorbent to improve the quality of the feed.

ES 2 301 460 T3

Compendium of the invention

The above object and others, which will become apparent to those skilled in the art on a reading of this description, are achieved by the present invention, which is: a method for recovering absorbate as defined in claim 1, a a method for recovering carbon dioxide as defined in claim 2 and an apparatus for recovering absorbate from an oxygen-containing feed mixture as defined in claim 7.

As used herein, the term "absorption column" means a mass transfer device that enables a suitable solvent, ie, absorbent, to selectively absorb absorbate from a fluid containing one or more other components.

As used herein, the term "extraction column" means a mass transfer device in which a component such as an absorbate is separated from an absorbent, generally through the application of energy.

As used herein, the term "inhibitor" means a chemical or a mixture of chemicals that inhibits or slows down a reaction. For example, copper carbonate in combination with one or more of dihydroxyethylglycine, alkali metal permanganate, alkali metal thiocyanate, nickel or bismuth oxides with or without alkali metal carbonate inhibits the oxidative degradation of an alkanolamine.

As used herein, the term "oxygen scavenging gas" means a gas that has an oxygen concentration of less than 2 mole percent, preferably less than 0.5 mole percent, and which It can be used to extract dissolved oxygen from a liquid.

As used herein, the terms "top" and "bottom" mean those sections of a column respectively above and below the midpoint of the column.

As used herein, the term "indirect heat exchange" means putting two fluids in a heat exchange relationship without any physical contact or intermixing of one fluid with another.

Brief description of the drawings

The only Figure is a schematic representation of a particularly preferred embodiment of the invention, wherein the oxygen separator comprises an oxygen extraction column.

Detailed description

The invention will be described in more detail with reference to the Drawing. Referring now to the Figure, the feed gas mixture 1, which has typically been cooled and treated for the reduction of suspended particles and other impurities such as sulfur oxides (SOx) and nitrogen oxides (NOx ), it is passed to compressor or blower 2, where it is compressed to a pressure generally within the range of 1.0 to 2.1 bar (14.7 to 30 pounds per square inch absolute (psia)). Feed gas mixture 1 generally contains 2 to 50 mole percent carbon dioxide as absorbate, and typically has a carbon dioxide concentration in the range of 3 to 25 mole percent. The feed gas mixture 1 also contains oxygen in a concentration generally within the range of less than 1 to about 18 mole percent. The feed gas mixture 1 can also contain one or more other components such as traces of hydrocarbons, nitrogen, carbon monoxide, water vapor, sulfur oxides, nitrogen oxides and suspended particles.

The compressed feed gas mixture 3 is passed from blower 2 to the bottom of absorption column 4, which operates at a temperature generally within the range of 40 to 45 ° C at the top of the column and at a temperature generally within the range of 50 to 60 ° C at the bottom of the column. The absorbent 6 is passed to the top of the absorption column 4. The absorbent 6 comprises at least one alkanolamine species. Monoethanolamine, diethanolamine, diisopropanolamine, methyldiethanolamine, and triethanolamine are examples of alkanolamines that can be employed in absorbent fluid 6 in the practice of this invention. The alkanolamines are generally used as an aqueous solution. The concentration of the alkanolamine (s) in absorbent 6 will be within the range of 5 to 80 percent by weight, and preferably 10 to 50 percent by weight. A preferred primary alkanolamine for use in the absorbent fluid in the practice of this invention is monoethanolamine, preferably at a concentration within the range of 5 to 25 percent by weight, more preferably at a concentration within the range of 10 to 15 percent by weight. weight. Preferred secondary alkanolamines for use in the absorbent fluid in the practice of this invention are diethanolamine and diisopropanolamine.

Within the absorption column 4 the feed gas mixture rises in countercurrent flow against the downflow absorbent. The absorption column 4 contains column internals or mass transfer elements such as dishes or random or structured packing. As the feed gas rises, most of the carbon dioxide within the feed gas, oxygen, and small amounts of other species such as nitrogen are absorbed into the down-flow absorbent liquid, resulting in overhead vapor. carbon dioxide depleted at the head of column 4, and carbon dioxide laden absorbent

ES 2 301 460 T3 carbon containing oxygen dissolved at the bottom of column 4. The overhead vapor is drawn from the top of column 4 into stream 5 and the carbon dioxide loaded absorbent is drawn from the bottom from column 4 into stream 7.

Dissolved oxygen causes degradation of alkanolamines over time, leading to corrosion and other operating problems. The level of dissolved oxygen in the carbon dioxide laden absorbent is reduced as is by contacting the absorbent with oxygen scavenging gas in a mass transfer device such as the oxygen extraction column illustrated in Fig. Figure.

The carbon dioxide loaded absorbent containing dissolved oxygen in stream 7 is passed from the bottom of the absorption column 4 to the top of the additional extraction column 151. An important aspect of this invention is that the fluid that comprising stream 7 does not undergo any heating from its separation from the absorption column 4 until its passage to the oxygen extraction column 151. The oxygen-scavenging gas is passed to the bottom of the stripping column 151 in stream 152. A source of oxygen-scavenging gas is an oxygen-free carbon dioxide stream. Examples of such a stream include carbon dioxide rich vapor stream 16, shown in Figure as stream 71, carbon dioxide from a storage tank, or carbon dioxide from an additional downstream process. Other oxygen-free gases such as nitrogen can also be used.

Within stripping column 151 the oxygen scavenging gas rises in countercurrent flow against the downward circulating carbon dioxide laden absorbent. The extraction column 151 contains column internals or mass transfer elements such as dishes or random or structured packing. When the oxygen-scavenging gas rises, the oxygen within the absorbent is drawn from the downstream absorbent into the upward-scavenging gas, resulting in oxygen-containing scavenging gas at the bottom. head of extraction column 151, and absorbent loaded with oxygen-depleted carbon dioxide at the bottom of extraction column 151. The oxygen-containing scavenging gas is drawn from the top of column 151 in stream 150. Stream 150 will typically contain some carbon dioxide in addition to oxygen and other species. This stream can be discharged to the atmosphere, used as is, or mixed with the final product carbon dioxide in stream 16, as shown in the Figure as stream 72. The oxygen-depleted carbon dioxide loaded absorbent, typically containing less than 2 ppm oxygen and preferably less than 0.5 ppm, is removed from the bottom of column 151 in stream 153, passed to a liquid pump. 8 and thence in stream 9 to and through a heat exchanger 10, where it is heated by indirect heat exchange to a temperature generally within the range of 90 to 120 ° C, preferably 100 to 110 ° C.

The heated carbon dioxide loaded absorbent is passed from heat exchanger 10 in stream 11 to the top of the second extraction column or main extraction column 12, which operates at a temperature typically in the range of 100 to 110 ° C at the top of the column and at a temperature typically within the range of 119 to 125 ° C at the bottom of the column. When the heated carbon dioxide loaded absorbent flows down through extraction column 12 over mass transfer elements, which can be random or structured dishes or packing, the carbon dioxide within the absorbent is drawn from the absorbent. to upstream steam, which is generally water vapor, to produce carbon dioxide-rich overhead steam and remaining absorbent. The carbon dioxide rich fluid is drawn from the top of extraction column 12 into overhead vapor stream 13 and passed through a reflux condenser 47 where it is partially condensed. The resulting two phase stream 14 is passed to a reflux cylinder or phase separator 15, where it is separated into carbon dioxide rich gas and condensate. The carbon dioxide rich gas is withdrawn from phase separator 15 in stream 16 and recovered as a carbon dioxide product fluid having a carbon dioxide concentration generally within the range of 95 to 99.9 percent in moles on a dry basis. By "recovered" as used herein is meant recovered as a final product or separated for any reason such as disposal, further use, further processing or set aside. The condensate, which mainly comprises water and alkanolamines, is taken out of phase separator 15 in stream 17, passed through liquid pump 18 and as stream 19 passed to the top of extraction column 12.

The remaining alkanolamine-containing absorbent, which also contains water, is removed from the bottom of extraction column 12 in stream 20 and passed to a kettle 21, where it is heated by indirect heat exchange to a temperature typically within the range of 119 to 125 ° C. In the embodiment of the invention illustrated in the Figure, the boiler 21 is operated by saturated steam 48 at a pressure of 2.94 bar (28 pounds per square inch gauge (psig)) or greater, which is removed from the boiler. 21 in stream 49. Heating of the alkanolamine-containing absorbent in boiler 21 causes some water to pass as steam in stream 22 from boiler 21 to the bottom of extraction column 12 where it serves as circulating steam. ascending, above mentioned. The resulting alkanolamine-containing absorbent is removed from kettle 21 into liquid stream 23. A portion 24 of stream 23 is fed to recuperator 25 where this liquid is vaporized. The addition of calcined soda or caustic soda to the recuperator facilitates the precipitation of the secondary degradation products and heat stable amine salts. Stream 27 represents removal of heat stable amine salts and degradation by-products. The vaporized amine solution 26 can be reintroduced into extraction column 12 as shown in the Figure. It can also be cooled and mixed directly with stream 6 that enters the top of absorption column 4. Also, instead of the

ES 2 301 460 T3 reclaimer 25 shown in the Figure, other purification methods such as ion exchange or electrodialysis may be employed.

The remaining portion 148 of the heated alkanolamine-containing absorbent 23 is passed to the solvent pump 35 and from there into stream 29 to and through the heat exchanger 10 where it serves to carry out the above-mentioned heating of the dioxide-laden absorbent. carbon and from which it comes out as an absorbent containing cooled alkanolamine 34.

Stream 34 is cooled by passing through cooler 37 to a temperature of about 40 ° C to form cooled absorbent 38. A portion 40 of stream 38 is passed through mechanical filter 41, from there, as stream 42 , through the carbon bed filter 43, and from there, as stream 44, through the mechanical filter 45 for the removal of impurities, solids, degradation by-products and heat stable amine salts. The resulting scrubbed stream 46 is remixed with stream 39, which is the remainder of stream 38, to form stream 55. Storage tank 30 contains additional alkanolamine to compensate for losses. The alkanolamine absorbent is removed from storage tank 30 in stream 31 and pumped by liquid pump 32 as stream 33 into stream 55. Storage tank 50 contains make-up water. Water is drawn from storage tank 50 into stream 51 and pumped by liquid pump 52 as stream 53 into stream 55. Streams 33 and 53 together with stream 55 form combined absorbent stream 6 for passage to the top of the absorption column 4 as previously described.

Although the invention has been described in detail in relation to a certain particularly preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the claims. For example, the invention can be used to separate compounds other than or in addition to carbon dioxide, such as hydrogen sulfide. A rigorous definition of such a generalized recovery process is:

A method to recover absorbate comprising:

(A) passage of a feed mixture comprising oxygen and absorbate in countercurrent mass transfer contact with an absorbent comprising at least one alkanolamine, and passage of oxygen and absorbate from the feed mixture to the absorbent to obtain absorbent loaded with absorbate containing dissolved oxygen;

(B) separating oxygen from the absorbate loaded absorbent to obtain oxygen depleted absorbate loaded absorbent;

(C) heating the oxygen-depleted absorbate-laden absorbent to obtain heated absorbate-laden absorbent fluid; and (D) separating absorbate from the absorbent fluid to obtain an absorbate-rich fluid.