Method for purification of carbon dioxide using liquid carbon dioxide

Abstract

The present invention relates to a method for removing at least one contaminant from a gaseous stream substantially comprising carbon dioxide. More specifically said method includes the step of subjecting the gaseous stream to an absorption step in which the absorbent is liquid carbon dioxide.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]The instant application is a continuation of and claims priority to co-pending U.S. patent application Ser. No. 13 / 054,453, filed Feb. 17, 2011, which is a National Stage Application of and claims priority benefit to PCT / DK2009 / 050159, filed Jul. 3, 2009, which claims priority to Denmark Patent Application No. PA 2008 01006, filed on Jul. 16, 2008. The aforementioned patent applications are incorporated by reference in their entireties herein.FIELD[0002]The present invention relates to a method for removing at least one contaminant from a gaseous stream substantially comprising carbon dioxide. More specifically, said method comprises the step of subjecting the gaseous stream to an absorption step in which the absorbent is liquid carbon dioxide.BACKGROUND OF THE INVENTION[0003]Carbon dioxide recovery plants are widely used to clean and / or recover carbon dioxide released e.g. from combustion of hydrocarbons, fermentation and gas processing....

AI Technical Summary

Benefits of technology

[0022]A further advantage of the present invention is that if any NOx's are present in the gaseous stream, NO2 will also be absorbed in the liquid carbon dioxide, whereby the gas phase equilibrium ½O2+NO<->NO2 is forced towards right i.e. towards NO2. Consequently, O2, NO and therefore NO2 is substantially removed from the gas phase also. According to the present invention, a single operating step is thus disclosed which is capable of removing several contaminants present in a carbon dioxide stream, e.g. from a flue gas, which are otherwise difficult to remove almost completely, while at the same time maintaining a high carbon dioxide yield.

[0023]A further object of the present invention is to increase the yield of carbon dioxide; therefore the effect of the absorption process should be improved. First of all, the amount of waste carbon dioxide is minimized when the gaseous stream fed to the column is at a temperature above the dew point temperature of carbon dioxide at the prevailing conditions. The higher temperature of the gaseous carbon dioxide causes the bottom part of the column to function as a stripper section and the top part of the column to function as an absorption section. When the temperature of the gaseous feed stream is higher than the dew point temperature, the excess heat used for reaching the dew point is used to evaporate the incoming liquid absorbent carbon dioxide, so that the amount of carbon dioxide in the contaminant rich liquid stream leaving the scrubber is as small as possible. In other words, the liquid stream denoted L2 (in both FIGS. 1 and 2) is minimized when the temperature of the gaseous feed stream is higher than the dew point temperature of carbon dioxide.

[0024]The pressure in the column is normally between 10 and 40 bar, however, other pressures are contemplated, for example if the temperature of the liquid absorbent carbon dioxide is higher than the freezing temperature of water under the prevailing pressure, the carbon dioxide would also be able to remove water from the stream. In the above set up, a preferred temperature range of the gaseous feed stream is 5 to 25° C., more preferred 5 to 15° C., such as 10° C., although temperatures in the range of −40 to 40° C. are contemplated if operating at another pressure. The dew point temperature of carbon dioxide in the above mentioned pressure range is −40 to +5.5° C.; it is within the skill of the art to determine the dew point temperature of carbon dioxide at any given pressure.

[0025]Furthermore, the improvement of the absorption process will be a compromise between sufficiently high removal of contaminants and minimizing the spent carbon dioxide absorbent. Operating plants seek at the same time to increase purity and carbon dioxide yields. As the temperature of the liquid absorbent carbon dioxide is essentially constant in the absorption column of a given process, the flow of the liquid absorbent carbon dioxide can be varied for improved results.

[0026]A suitable flow is determined by various factors that may result in the same desired degree of purification and yield. Examples of factors that influence the process are the heat transfer capacity of the streams and the temperature of the streams entering the absorber. As the aim is to obtain a high yield of pure carbon dioxide it is desired that the flow of the absorbent liquid carbon dioxide, is at such a rate that not more than 5% (by weight) contaminant rich carbon dioxide is discarded from the bottom of the absorber as compared to the carbon dioxide content of the gaseous feed stream fed to the absorption column; the upper limit of 5% is set out of an economical point of view. Technically, higher percentages are also contemplated, however, in practice if operating at higher rates, there should be provisions for recovering the “waste” contaminant rich carbon dioxide stream again, such as the use of a reboiler. A reboiler can be integrated in the absorption column or connected to or near the bottom section of the absorption column. In this embodiment, the “waste” stream of liquid carbon dioxide comprising absorbed impurities, i.e. the contaminant rich stream, is either re-circulated, e.g. to a heat exchanger, and the now gaseous stream may re-enter the absorber for purification again, or collected in a reservoir for recovery by batch distillation, or if there is a high continuous flow, by distillation of the “waste” / contaminant rich stream.

[0027]When the contaminant rich liquid carbon dioxide is re-evaporated some of the impurities will remain in the liquid phase, consequently, the re-evaporation may be considered as a further means for reducing the amount of liquid waste generated.

Problems solved by technology

The drawbacks of using a water scrubber is the large amounts of clean water used and wastewater formed.

However, if the gas comprises impurities, which are heavily dissolved in carbon dioxide, i.e., primarily non-polar organic compounds and compounds having a boiling point higher than the boiling point of carbon dioxide under the prevailing conditions, these will not be effectively removed from the stream using a water scrubber.

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