Patent Application
20100290977
The present inventive embodiments provide a method and apparatus for removing impurities from a gas. In particular, this invention provides a method for removing hydrocarbons and carbon species from for example carbon dioxide.
Carbon dioxide is used in a number of industrial and domestic applications, many of which require the carbon dioxide to be free from various impurities. Unfortunately, carbon dioxide obtained from natural sources such as gas wells, chemical processes, fermentation processes or produced in industry, particularly carbon dioxide produced by the combustion of hydrocarbon products, often contains impurity levels of hydrocabons, sulfur compounds such as carbonyl sulfide (OCS, commonly written as COS) and hydrogen sulfide (H2S). When the carbon dioxide is intended for use in an application that requires the carbon dioxide to be of high purity, such as in the manufacture and cleaning of foodstuffs and beverage carbonation, medical products, and electronic and optical devices, the impurities contained in the gas stream must be removed to very low levels if not eliminated prior to use. The level of impurity removal required varies according to the application of carbon dioxide. For example, during electronic cleaning applications, the total carbon species level in the carbon dioxide (CO2), specifically those carbon species which result in formation of persistent solid and liquid non-volative residues, should be below 0.1 parts per million (ppm).
Since many end users of carbon dioxide require the carbon dioxide they use to be substantially free of hydrocarbons, and because natural sources of carbon dioxide and industrially manufactured carbon dioxide often contain such impurities, economic and efficient methods for effecting substantially complete removal of hydrocarbons from carbon dioxide gas streams, without concomitantly introducing other impurities into the carbon dioxide, are continuously sought
The present embodiments include a method of purifying a gas comprising heating a gas stream to above ambient temperature; passing the heated gas stream into an organic removal unit; cooling the heated gas stream exiting the organic removal unit to form a purified gas stream; and removing moisture and other impurities from the purified gas stream.
In an embodiment, the gas comprises carbon dioxide, and the impurities comprise carbon species.
For more complete understanding of the present embodiments, reference may be had to the description of the embodiments which follow taken in connection with the accompanying drawings, of which:
The carbon dioxide that is typically produced for industrial operations has impurities present in it. These impurities will often be a concern for many uses of the carbon dioxide. In the production of products intended for human consumption, such as carbonated beverages, and in the production of electronic components and equipment, the purity of the carbon dioxide is paramount and can influence the taste, quality, operation and legal compliance of the finished product. With respect to carbon impurities or species to be removed from the carbon dioxide, such impurities may be selected from at least one of aromatic hydrocarbons, aliphatic hydrocarbons and branched hydrocarbons.
The impure carbon dioxide which can be obtained from any available source of carbon dioxide will typically contain as impurities hydrocarbons and carbon species such as toluene, xylene, benzene, and aliphatic species, e.g. esters such as hexane. The embodiments provide for low cost methods for the removal of these impurities. The impurity removal can be used in various ways depending on whether the carbon dioxide is purified at a production plant, or at a point of use. Various point-of-use applications of carbon dioxide include a beverage filling plant, a food freezing plant, an electronics manufacturing plant and a fountain type carbon dioxide dispensing location.
For purposes of the embodiments, at least some of the impurities, such as toluene and xylene, are removed at an elevated temperature, a temperature of 50° to 150° C. In the point-of-use application, this temperature can be obtained by using a combination of heater and heat exchange means. In a production plant, this temperature may be obtained during the compression of feed carbon dioxide after the final compression stage but before the aftercooler. In a production plant, for the feed containing high levels of reactive sulfur species (>10 to several hundred ppms), removal of hydrocarbons should occur prior to compression, and the temperature for removal is obtained by heater and heat exchanger means. The impure carbon dioxide gas stream having been raised to the proper temperature is directed to an organic removal bed. This bed is typically a vessel that will contain certain catalyst and adsorbent materials which will either react with or adsorb the hydrocarbons.
Preferably the catalyst materials are those that will cause the hydrocarbons to convert to CO2 and moisture. Purification materials according to the inventive embodiments include noble-metal catalysts; metal oxides such as copper, zinc, chromium or iron oxide either alone or supported on a microporous adsorbent such as activated alumina, activated carbon or silica gel; monoliths; and metals of alumina substrates.
The stream exiting the hydrocarbon removal bed can optionally be further heated and sent to a catalytic reactor for oxidation of various hydrocarbon impurities. The stream exiting the reactor bed is cooled to close to ambient temperatures in heat exchange means.
Referring to
The impure carbon dioxide which is now essentially free of most sulfur impurities is optionally directed through line 11 to a second heat exchanger 50 where its temperature is raised to over 150° C. The impure carbon dioxide exits the second heat exchanger through line 13 and is further heated to a temperature between 150 and 450° C. in a heater not shown. The heated carbon dioxide enters a catalyst reactor 60 containing a pelleted or a monolith catalyst. Various impurities including aromatic and aliphatic hydrocarbons such as toluene, benzene and aldehydes in the feed react with oxygen in the catalytic reactor and are converted to carbon dioxide and water, i.e. such that the hydrocarbons are destroyed or are removed from the carbon dioxide.
The now essentially purified carbon dioxide gas stream leaves the catalytic reactor bed through line 15 where it returns to the second heat exchanger 50.
The purified carbon dioxide gas stream leaves the second heat exchanger through line 17 and is directed into the first heat exchanger 20 where its temperature is reduced to less than 40° C. The cooled purified carbon dioxide gas steam can be sent to downstream processing equipment 70 through line 19 where it is further purified and optionally liquefied. It can also be sent to a CO2 use process, unit 80, via line 21.
Purification of carbon dioxide in a carbon dioxide production plant is shown in
In
The stream exiting the sulfur removal unit 125 is further heated in an optional heat exchanger 130 and optional heater 135 and enters the optional catalytic reactor 140. The catalytic reactor contains supported noble metal catalysts such as palladium or platinum in pelleted or monolith forms. The catalytic reactor operates at a temperature between 150 and 450° C. depending on the impurities in the feed stream. The hydrocarbon impurities are oxidized to water and carbon dioxide in this reactor. The stream exiting reactor 140 is cooled in heat exchanger 130 and heat exchanger 110. If reactor 140 is not used, the stream exiting the sulfur bed 125 is cooled in heat exchanger 110. The stream exiting heat exchanger 110 is compressed in a compressor 145 to pressures between 10 and 20 bara and is cooled in an aftercooler 150 to a temperature close to ambient. The cooled, purified carbon dioxide gas steam can optionally be sent to downstream processing equipment 155 where it is further purified and optionally liquefied. It can also be sent to a CO2 use process unit 160. “CW” identifies cold water introduced into and exiting from the aftercooler 150.
The embodiment in
In
The stream exiting the optional sulfur removal unit 225 is further heated in an optional heat exchanger 230 and optional heater 235 and enters the optional catalytic reactor 240. The catalytic reactor contains supported noble metal catalysts such as palladium or platinum in pelleted or monolith forms. The catalytic reactor operates at a temperature between 150 and 4500C depending on the impurities in the feed stream. The hydrocarbon impurities are oxidized to water and carbon dioxide in this reactor. The stream exiting reactor 240 is cooled in heat exchanger 230 and is further cooled in an aftercooler 245 to a temperature close to ambient. The cooled, purified carbon dioxide gas steam can optionally be sent to downstream processing equipment 250 where it is further purified and optionally liquefied. It can also be sent to a CO2 use process, unit 260. “CW” identifies cold water introduced into and existing from the aftercooler 245.
A feed containing 9 ppm COS in carbon dioxide at a pressure of 14.6 bara and a temperature of 100° C. was passed through a bed containing 0.12 kgs of activated carbon containing 20 wt % potassium carbonate at a flow rate of 19.8 std liters/min. About 100 ppm of oxygen was added to the feed. An equilibrium COS capacity of 5.15 wt % was obtained at this temperature. The same feed was passed through the same bed at a temperature of 25° C. and an equilibrium COS capacity of <0.1 wt % was obtained.
The same feed now containing 50 ppm H2S in carbon dioxide at a pressure of 14.6 bara and a temperature of 100° C. was passed through a bed containing 0.154 kgs of activated carbon containing 20 wt % potassium carbonate at a flow rate of 15.6 std liters per min. About 100 ppm oxygen was added to the feed. An equilibrium H2S capacity of 18 wt % was obtained. The same feed was passed through the same bed at a temperature of 25° C. and an equilibrium H2S capacity of around 10 wt % was obtained.
Both these experiments indicate that a significant improvement in the removal capacity for COS and H2S is possible by operating at an elevated temperature.
Testing was performed using a purification skid containing 17.1 kgs of activated carbon impregnated with 20 wt % potassium carbonate. Carbon dioxide at a pressure of 17 bara, at a temperature of 85° C., and at a flow rate 109.7 std m3/hr was passed through the bed. The feed contained 25-100 ppb of ethyl and methyl mercaptans. No mercaptans were seen at the bed outlet during a test period of about one week.
Initial testing of the beverage grade CO2 using gas chromatagraph mass spectrometer (GC/MS) revealed hydrocabons heavier than C6 (chain of 6 carbon atoms in molecule) at levels in excess of 10 ppm. These hydrocarbons were indentified as toluene and benzene compounds commonly identified as belonging to the decane family. After purification with the carbon impurity removal embodiments, there was complete reduction of these compounds such that a pure CO2 spectra down to parts per trillion (ppt) resolution was observed with the GC/MS.
It will be understood that the embodiments described herein are merely exemplary, and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. It should be understood that embodiments described above are not only in the alternative, but may also be combined.
