Method of recovering substantially pure CO2 from fermentation gas
This invention relates to a method of recovering substantially pure CO 2  from a fermentation gas polluted with organic compounds and with sulphur compounds. 
     In various processes, gas streams are obtained which contain substantial amounts of CO 2 . In particular when brewing beer, fermenting grapes, and in distilleries, large amounts of CO 2  -containing gas are developed which is polluted with organic compounds and with sulphur compounds. The value of this gas, however, is considerable because it is obtained by a natural process and can be properly applied in the beverage industry, e.g. for making carbonated soft drinks or beer. 
     Since the gas also contains substantial amounts of impurities in the form of organic compounds such as ethanol, and sulphur compounds such as H 2  S and DMS (dimethyl sulphide), it is necessary to purify the gas before it is used. The gas also contains non-condensable gases such as oxygen and nitrogen, which gases must also be removed at least partially. 
     The presence of the sulphur compounds in the gas gives an unpleasant smell and/or taste, e.g. in mineral water if this is provided with unpurified or insufficiently purified CO 2 . 
     Up to the present this fermentation gas has been purified nearly exclusively by washing with water, compression, and cooling to remove the major part of the water, removing the organic and sulphur impurities by adsorption at an activated carbon filter, further drying the gas and condensating the CO 2  to reduce the content of non-condensable gases. 
     U.S. Pat. No. 4,699,642 discloses a method of preparing substantially pure, liquid CO 2  for use in the beer brewing process. According to this patent, purification is also effected by adsorption. 
     This system has the drawback that the risk of a breakthrough of impurities is rather great. In practice, it has also been found that additional measures must be taken regularly to avoid this risk. A breakthrough of impurities is not desirable because of the smell and/or taste problems when CO 2  is used in beverages. 
     The object of this invention is to provide a method of recovering substantially pure CO 2  from a fermentation gas polluted with organic compounds and with sulphur compounds. 
     According to the invention this method comprises washing the gas with water or an aqueous solution under such conditions that the gas contains no more than 2.5, preferably no more than 1.25 ppm organic impurities, oxidizing the oxidizable impurities, removing the major part of the water, and drying the CO 2  to the desired water content. 
     Surprisingly, it has been found that this simple method gives a purified CO 2  having a content of sulphur-containing impurities of not more than 2 ppb calculated as H 2  S. 
     The amounts of organic impurities and sulphur impurities have been calculated on the basis of parts by volume per volume. 
     Crucial points of the method according to the invention are, inter alia, the very thorough washing of the gas, followed by the oxidation with an aqueous solution of an oxidator. As compared with the known methods based on the adsorption of the impurities at activated carbon, the method according to the invention is very simple. Since no adsorption stage is present, the method can be carried out continuously with fewer appliances in a simpler process. 
     Surprisingly, it is possible to eliminate the concentrations of impurities, very low as they are, by oxidation, despite a low dosage of active chlorine of 0.5×10 -4  to 3.0×10 -4  wt. % in the solution. In the method according to the invention it has also been found that substantially no trihalomethane and/or ethyl acetate is formed, as can be expected of a combination of wet washing and oxidation with chlorine compounds. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The accompanying FIGURE diagrammatically shows an installation for carrying out the method according to the invention. 
     Gas streams that can be purified according to the invention are, inter alia, the CO 2  -containing gas streams from the brewing of beer, fermentation of grapes and other fruits, preparation of distilled beverages and the like. In general, the invention is applicable to all gas streams from a fermentation, i.e. it is also possible to thus purify gas from sewage works. Such gas streams consist for the greater part of CO 2 , water vapour, non-condensable gases such as oxygen and nitrogen, and for the rest of the above organic and sulphur impurities. 
     The CO 2  content of the gas streams to be treated will generally be at least 80 vol. %, in particular more than 95, or more than 99 vol. %. 
     The gas is preferably washed in a scrubber provided with a packed bed on the basis of a loose packing or of a structured packing. Preferred is a structured packing because this enables the height and, consequently, the volume of the packed bed to be considerably reduced. 
     The removal efficiency of the organic impurities obtained in the scrubber is more than 99.5%, preferably more than 99.9%. This is sufficient to meet the requirement of a content of impurities of not more than 1.0 ppm, preferably not more than 0.5 ppm, in particular about 0.1 ppm. 
     In case of inadequate removal of the organic impurities, ethyl acetate and other undesirable compounds are formed. Especially the formation of ethyl acetate is not desired when the gas is used for beverages such as soft drinks. 
     After washing, the gas stream will preferably consist of at least 80 vol. %, preferably more than 95 vol. %, in particular more than 99 vol. % CO 2 . For the rest, the gas stream consists of the above organic and sulphur impurities, water vapour and non-condensable gases. Non-condensable gases as referred to herein are gases that do not condense upon liquefaction of CO 2 . Examples thereof are oxygen and nitrogen. 
     The purified gas is then fed into an oxidation column. This column may also be a packed bed or another system in which an intensive gas-liquid contact is obtained. In this oxidation the sulphur compounds are removed in a substantial degree, e.g. to a content of less than 5 ppb, i.e. to a content below the smell and taste limit. The oxidation can be carried out with any suitable aqueous solution of an oxidator. Preferably, however, potassium permanganate or sodium hypochlorite is used because they give the best results. Particularly preferred is sodium hypochlorite because it is easily used, gives little pollution and involves few operational problems. 
     After the oxidation, the CO 2  mainly contains water and non-condensable gases. First of all, the major part of the water is removed, after which the gas is dried. In theory, this might be done in a single stage, but in practice this is done in two stages. Preferably, the gas is first compressed and cooled, whereby a large part of the water present condenses out. Subsequently, the remaining water is substantially removed by applying a conventional drying process. 
     If desired, any breakthrough of impurities can be avoided by incorporating a police filter in the system, such as an impregnated or a non-impregnated activated carbon filter. This filter is suitably placed before the gas drying stage. 
     The purified and dried gas is finally liquefied. This, too, can be done in a conventional manner. The content of non-condensable gases is then also reduced. There is thus obtained a very pure, liquid CO 2  which meets the requirements imposed with respect to its suitability for use in the food and beverage industry. 
     It is to be observed that the suitability of CO 2  is determined by bubbling CO 2  through mineral water and determining the smell and taste thereof by means of a panel test. The CO 2  is suitable if there are no noticeable smell and taste deviations. 
     The invention will now be illustrated with reference to the accompanying FIGURE which diagrammatically shows an installation for carrying out the method according to the invention. 
     Via line 1 a fermentation gas, e.g. from a brewery, is fed into a wash column 2 at a pressure ranging from 0.5 to 5 bar and a temperature ranging from 15° to 50° C., which wash column is provided with a packing for obtaining a good gas/liquid contact. The washing liquid is supplied via line 3 and discharged via line 4. The gas stream from which the major part of the organic impurities is removed is supplied via line 5 to oxidation tower 6. Via line 7 a solution of an oxidizing agent is supplied to this oxidation tower 6, while via line 8 the oxidized sulphur compounds are discharged. The oxidation tower can be filled with a packing material for promoting the gas/liquid contact. 
     The purified gas stream is supplied via line 9 to a compression/cooling unit 10 in which the pressure of the gas is increased to a value ranging from 15 to 25 bar, and the temperature of the gas is adjusted to a value ranging from 10° to 30° C. The major part of the moisture thereby condenses and is discharged via line 11. 
     The gas is discharged from unit 10 via line 12 and supplied to police filter 13. This filter need not be present. The filter is filled with a material adsorbent to sulphur compounds for protection against a possible breakthrough of these compounds from the oxidation tower. Subsequently, the gas is supplied via line 14 to drier 15 in which the gas is dried to the desired water content. The water is discharged via line 16. The dried gas is finally supplied via line 17 to unit 18 for liquefaction thereof. There is thus obtained, on the one hand, liquid CO 2 , which is passed to a storage or transfer tank, not shown, and, on the other hand, non-condensable gases such as oxygen and nitrogen, which are discharged via line 20. 
    
    
     EXAMPLE 
     Into an installation as shown in the FIGURE a gas stream of 280 m 3  /h and having a temperature of 20° C. was supplied to wash column 2, which was provided with a Sulzer Mellapak packing. Supplied via line 3 were 150 l/h wash water. The gas leaving the wash column had a content of organic impurities of 0.25 ppm, a CO 2  content of 99.8 vol. %, and contained for the rest non-condensable gases and sulphur impurities. This gas was then supplied to the oxidation tower filled with a Sulzer BX packing. Supplied to the tower were 800 l/h of a solution of 2×10 -4  wt. % active chlorine in water. After leaving the tower the gas had a content of sulphur compounds of about 2 ppb, and the gas was free from smell and taste when bubbling through mineral water. 
     Subsequently, the gas was compressed to a pressure of 19 bar and cooled to a temperature of 20° C. The major part of the water present in the gas thereby condensed. The gas was then dried to a water content of 4 ppm. The gas thus dried was finally liquefied at a temperature of 23° C. and a pressure of 18 bar.