PROCESS FOR PURIFYING CARBON DIOXIDE

Abstract: A process for purifiying crude carbon dioxide which con­ tains methane and other hydrocarbons to obtain carbon diox­ ide having a good quality, (1) by controlling the feeding tem­ perature to reaction system, flow quantity for feeding and con­ sequently reaction temperature, and (2) by controlling the feeding of crude carbon dioxide to the above reaction system by utilizing the heat contained in the catalytic reaction pro­ ducts. Particularly in this process, control of reaction conditions is more easily performed by including two kinds of catalytic reaction steps, wherein said reaction is initiated by the cata­ lysts containing manganese dioxide and copper oxide and sub­ stantially finished by the catalysts carrying metals of the 8th group in the periodic table.

Description: PROCESS FOR PURIFYING CARBON DIOXIDE The present invention relates to a process for purifying carbon dioxide. Carbon dioxide is widely used in the form of liquefied carbonic acid gas or dry ice for preserving fishes, butter, cheese, ice cream and the like, low temperature transportation, cooling, welding, foundry, manufacture of soft drinks, fire extinguishers etc. Natural gas, fermentation gas, by-product gas from oil refinery, by-product gas from ammonia synthesizing process etc. are industrially applied as crude carbon dioxide raw materials of liquefied carbonic acid gas and dry ice. Processes for preparing carbon dioxide of high purity are already established technically. For example, crude carbon dioxide which is a by-product of ammonia production process contains methane and hydrogen as major impurities, and can be easily purified by distillation method or the like, because these impurities keep gaseous state under the liquefying conditions of carbon dioxide. On the other hand, when crude carbon dioxide obtained as a by-product of ethylene oxide manufacturing process is applied for a carbon dioxide source, methane, ethylene, acetaldéhyde and the like are contained as the impurities. Ethylene, acetaldehyde and the like are converted to liquid under the liquefying conditions of carbon dioxide, and thus they exist as the impurities of liquefied carbon dioxide, which accompanies strange odor. Such carbon dioxide cannot be used for the preservation and transportation of foods or an additive of the soft drinks, because not only of the strange odor but also of a harmful influence for human bodies. When further utilizing as dry ice, ethylene exists as residual impurities and vaporizes under the dry ice manufacturing conditions (normal pressure, -78'C), which causes an explosive destruction of compression molded products of dry ice or break in handling the products due to a severe reduction of strength. Methane, ethylene, acetaldehyde and the like are, as well known, oxidized in the presence of catalysts. Metals of the 8th group in the periodic table, metallic platinum and metallic palladium in particular, are generally known to have a strong activity among the catalysts having a high oxidation activity as perfectly oxidizing catalysts and retaining the performance for a long period. Reaction temperatures, however, greatly differ according to the object materials to be oxidized. When two or more impurities such as methane, ethylene, acetaldehyde are removed by oxidation in particular, there is a difficulty of carefully selecting and maintaining reaction temperatures. That is, when purifying crude carbon dioxide containing methane and other hydrocarbons, the present invention intends to solve the problem of separating methane by other processes and converting other hydrocarbons except methane to carbon dioxide by the oxidizing reaction. In this case, there were hardships of inhibiting methane oxidizing reaction as much as possible and completely carrying out other oxidizing reactions, as well as problems of selecting highpriced materials for reactors to avoid danger of unexpected accidents due to frequently occuring violent reaction. The object of the present invention is to provide a process for purifying crude carbon dioxide which containes methane and other hydrocarbons, such as the by-product of ethylene oxide manufacturing process, to carbon dioxide having a good quality suitable for use in liquefied carbon dioxide and dry ice. Another object of the present invention is to provide a process for purification containing a catalytic reaction step with the catalysts which are suitable for the above described purifying process. The inventors of the present invention carried out a hard research to achieve these objects and succeeded in completing this invention. The present invention is a process for purifying carbon dioxide by catalytically reacting crude carbon dioxide containing methane and other hydrocarbons with oxygen or an oxygen-containing gas in the presence of oxidizing catalysts and followed by removing methane which comprises in the catalytic reaction: : (1) feeding to the reaction system crude carbon dioxide heated above the temperature which is determined according to the variety and quantity of methane and other hydrocarbons contained in -crude carbon dioxide and wherein the hydrocarbons except methane initiate oxidizing reaction, (2) proceeding said oxidizing reaction at the temperatures wherein the hydrocarbons except methane in crude carbon dioxide are completely oxidized to carbon dioxide and methane is substantially stable, (3) controlling said reaction temperatures by the temperatures and flow quantities of crude carbon dioxide, (4) controlling the temperatures of crude carbon dioxide by the heat contained in the catalytic reaction products, and (5) feeding to the catalytic reaction system required quantity of oxygen or oxygen-containing gas for complete combustion of the hydrocarbons except methane in crude carbon dioxide. The present invention is a process for purifying carbon dioxide which comprises preferably initiating said catalytic reaction by the catalysts containing manganese dioxide and copper oxide, and substantially finishing the reaction by the catalysts carrying metals of the 8th group in the periodic table. In the process of this invention, crude carbon dioxide containing methane and other hydrocarbons firstly reacts catalytically with oxygen or the oxygen-containing gas in the presence of the oxidizing catalysts, and the hydrocarbons except methane are oxidized to carbon dioxide. In this reaction: (1) The inlet temperatures of the catalyst layer are maintained so that the oxidation of the hydrocarbons except methane can be initiated, and controlled by the temperatures of crude carbon dioxide charged. (2) The reaction temperatures measured at the outlet of the catalyst layer rise up as a result of the oxidizing reaction. In order to proceed the reaction at the temperatures wherein the hydrocarbons except methane are completely oxidized and methane remains substantially stable, said temperatures are controlled by the inlet temperatures of the catalyst layer and consequently by those of crude carbon dioxide and further, if necessary, controlled by the flow quantity of crude carbon dioxide charged into the reaction system. (3) The determination of reaction temperatures wherein the hydrocarbons except methane initiate to oxidize and methane remains stable is carried out according to the information previously collected from the variety and quantity of methane and other hydrocarbons in crude carbon dioxide as well as kind of the catalysts and the like. (4) The waste heat generated by the oxidizing reaction is transferred to a part of crude carbon dioxide which is in use for the temperiture control of crude carbon dioxide supplied to the reaction system. (5) The quantity of oxygen or the oxygen-containing gas supplied to the reaction system is enough to completely combust the hydrocarbons except methane, the amount of which is calculated from the flow quantity of crude carbon dioxide and the information based on analysis. Crude carbon dioxide which includes methane and other hydrocarbons in the process of this invention is crude carbon dioxide containing not only saturated and unsaturated hydrocarbons such as methane, ethylene, butane and the like, but also aldehydes such as acetaldehyde and fatty acids such as acetic acid which are low grade oxidation products of the hydrocarbons. Besides, as the oxidizing catalysts in the process of the present invention are used one or more than one components of oxides of metals in the 4th period of the periodic table which include titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper, or one or more than one components of so called platinum group metals in the 8th group of the 5th and 6th periods of the periodic table, or the catalysts of these components supported on carriers such as alumina and the like. More preferably, by using the following compounds as the oxidation catalysts, the oxidation reaction is carried out by the two kinds of catalytic reaction steps. One is the reaction step of using catalysts which contain manganese dioxide and copper oxide. Reduction of ignition temperature becomes possible by this method. The other is the reaction step of using the catalysts supporting metals of the 8th group in the periodic table. As the metals of the 8th group in the periodic table, palladium and platinum are suitably selected and the platinum catalyst is preferable in particular. A variety of carriers can be in use and -alumina is preferable in particular. The two reaction steps may be performed in separate reactors, and yet are preferably performed in one reactor containing two layers of the catalysts. In the process of this invention, the hydrocarbons except methane are thus removed by oxidizing to carbon dioxide through the oxidizing reaction. The resulting crude carbon dioxide is further processed with normal methods such as distillation, liquefication and the like to separate methane and led to carbon dioxide having a very high purity. The process of the present invention certainly enables to manufacture carbon dioxide having a extra high'purity and no ethylene or acetaldehyde at all, from crude carbon dioxide containing the impurities which include,for example, the hydrocarbons such as ethylene and their oxidation products such as acetaldehyde. Therefore the carbon dioxide thus obtained can safely be in use for the preservation and transportation of foods or the additive of the soft drinks etc. Dry ice having a very excellent quality without destruction can also be prepared from the resulting carbon dioxide thus obtained. In addition, when two kinds of steps, that is, (1) the reaction step of using the catalysts which contain manganese dioxide and copper oxide, and (2) the reaction step of using the catalysts which carry metals of the 8th group in the periodic table, exist in the catalytic reaction steps, the hydrocarbons except methane can completely be oxidized even at low reaction temperatures, while preventing the oxidation reaction of methane contained in crude carbon dioxide. Therefore, the violent reaction due to the oxidation of methane can be inhibited, the control of reaction conditions becomes easy, safety is secured, selection of equipment materials considering abnormal reaction becomes unnecessary, and the purification of carbon dioxide can advantageously be performed commercially. The present invention will be further illustrated in detail by way of the following Examples, and with reference to the accompanying drawings, in which: Fig. 1 illustrates a flow sheet of an example of purifying process in the present invention. Fig. 2 illustrates relationship between the reaction temperature and the percentage of removal of ethylene, acetaldehyde and methane. Fig. 3 illustrates relationship between the outlet temperature of the catalyst layer and the percentage of removal, which is an example of the effect of this invention. Fig. 4 illustrates relationship between the outlet temperature of the catalyst layer and the percentage of removal, when platinum catalyst supported on < -alumina is used alone. Example 1 An example of purification process of the present invention was illustrated in Figure 1 with a flow sheet. Crude carbon dioxide I, a by-product of ethylene oxide manufacturing process, containing 3,500 ppm of methane, 6,000 ppm of ethylene, and 500 ppm of acetaldehyde, was fed to the reaction system through the valve 2 at a flow rate of 25 Nm3/min. A part of crude carbon dioxide was heated by the waste heat of reacted gas in the heat exchanger 3, other part was by-passed through the valve 4, and transfered to the oxygen mixer 5 after thermally controlled to the necessary temperature of the reaction. On the other hand, the quantity of oxygen for completely combusting ethylene and acetaldehyde was calculated according to the flow-quantity of crude carbon dioxide and the analytical information. Oxygen 6 was fed to the oxygen mixer 5 through the valve 7 at the flow rate of O.5Nm3/min and mixed with crude carbon dioxide. A hot gas mixture 8 of crude carbon dioxide and oxygen was fed to the reactor 10 filled with the platinum catalyst XC-140-2-062 (Trade name of CCI Far East) 9 which was supported by < -alumina, and the inlet temperature of the catalyst layer was maintained at 200C by the heat of the gas mixture. The temperature of 200it was determined by the composition of crude carbon dioxide and kind of the catalysts used. Namely, in this case, by using the gas mixture of crude carobon dioxide and oxygen having the same composition as described above, and in the presence of 50 ml of the same catalysts, crude carbon dioxide having the flow rate of 820 llhr was reacted with oxygen having the flow rate of 30 l/hr. The results as illustrated in Fig. 2 had been previously obtained for the relationship between the temperatures and the percentage of removal of ethylene, acetaldehyde and methane. It was found from these results that the decomposition of ethylene initiated at 1600C and acetaldehyde started to decompose at lower temperatures. Therefore the temperature of 200'C was settled as the initiation temperature of the reaction, namely, the inlet temperature of the catalyst layer. When the reaction was performed at the temperature of 2000C measured at the inlet of catalyst layer, the reaction temperature rised up to 4OOC because of oxidizing reaction. As clearly shown in Fig. 2, acetaldehyde 1 and ethylene 2 in crude carbon dioxide were completely oxidized at this temperature, and methane was substantially stable. Therefore the reaction temperature was hereafter maintained at 400'C. The reaction temperature was maintained by controlling the inlet temperature of the catalyst layer within the temperature range wherein ethylene was completely oxidized. The temperature control was carried out at the valve 4 by varying the flow quantity ratio of crude carbon dioxide which passed through the heat-exchanger 3 and the by-pass. Further control was performed, when needed, by the flow quantity of crude carbon dioxide 1 which was fed to the reaction system through the valve 2. The resulting crude carbon dioxide 11 was transfered to the methane separator 12, after exchanging heat with a part of crude carbon dioxide in the heat-exchanger 3. The methane content of the resulting crude carbon dioxide was 3,380 ppm, and ethylene and acetaldehyde were not detected. Methane 13 was separated from the resulting crude carbon dioxide by normal procedure, for example, liquefaction to obtain purified carbon dioxide having a purity of 99.97%. Example 2 The catalysts were packed in two layers in a vertical reactor 20 L of a mixed catalyst MTC-101 (Trade name of CCI Far East) composed of manganese dioxide and copper oxide were packed in the first layer. 180 M1 of a platinum catalyst DASH-220 (Trade name of Engelhardt) which was carried on t-alumina was packed in the second layer. Fed to the reactor was a gas mixture composed of 25 Nm3/min of crude carbon dioxide containing 2,300 ppm of methane, 5,600 ppm of ethylene and 350 ppm of acetaldehyde, and 9.5 Nm3/min of oxygen. The temperature at the reactor inlet was about 150eC. The temperature of gas at the reactor outlet was about 290'C. The gas at the reactor outlet contained 2,260 ppm of methane. Ethylene and acetaldehyde were not detected. The resulting gas was liquefied by the normal method and methane was separated to obtain pure carbon dioxide having a purity of 99.97 %. Comparative Example The process of Example 2 was repeated except using 200 1 of platinum catalyst DASH-220 alone, which was carried on T -alumina. The outlet temperature of the reactor was about 340C which was about 502C higher than Example 2. The methane content of resulting gas was 2,100 ppm which was lower than Example 2. Ethylene and acetaldehyde.wcre not detected. The purity of carbon dioxide obtained was 99.97 %. Reference Example In order to analyze the reaction more clearly, the relation between the outlet temperature of catalyst layer and the percentage of removal on methane, ethylene, and acetaldehyde were examined. The results were shown in Fig. 3 and Fig. 4. Fig. 3 illustrated the percentage of removal on acetaldehyde 1, ethylene 2, and methane 3, when the catalyst mixture of manganese dioxide and copper oxide was used in the first layer and the platinum catalyst supported on d-alumina was used in the second layer. Fig. 4 showed the percentage of removal on acetaldehyde 1,: ethylene 2, and methane 3, when the platinum catalyst supported on -alumina was used alone. The catalysts used in Fig. 3 were 5 ml of MTC-101 and 45 ml of DASH-220 as in Example 2. The catalysts used in Fig. 4 were 50 ml of DASH-220. The feed gas was in all cases a mixture of 826 l/hr of crude carbon dioxide containing 2,930 ppm of methane, 6,270 ppm of ethylene, and 341 ppm of acetaldehyde, and 36.5 l/hr of oxygen. These Figures showed that, in the process of this invention, the same percentage of removal on hydrocarbons except methane could be obtained at about 50*C lower temperatures as compared with T -alumina supported platinum catalyst alone

Claims: 'CLAIMS What is claimed is 1. A process for purifying carbon dioxide by catalytically reacting crude carbon dioxide containing methane and other hydrocarbons with oxygen or oxygen-containing gas in the presence of oxidizing catalysts and followed by removing methane which comprises in the catalytic reaction: : (1) feeding to the reaction system crude carbon dioxide heated above the temperature which is determined according to the variety and quantity of methane and other hydrocarbons contained in crude carbon dioxide and wherein the hydrocarbons except methane initiate oxidizing reaction, (2) proceeding said oxidizing reaction at the temperatures wherein the hydrocarbons except methane in crude carbon dioxide are completely oxidizedto carbon dioxide and methane is substantially stable, (3) controlling said reaction temperatures by the temperatures and flow quantities of crude carbon dioxide, (4) controlling the temperatures of crude carbon dioxide by the heat contained in the catalytic reaction products, and (5) feeding to the catalytic reaction system required quantity of oxygen or oxygen-containing gas for complete combustion of the hydrocarbons except methane in crude carbon dioxide. 2. The process according to claim 1 wherein said catalytic reaction is initiated by the catalysts containing manganese dioxide and copper oxide, and substantially finished by the catalysts carrying metals of the 8th group in the periodic table.

Inventors: KAWABE, TOYOTARO, ONSE, NORIO, SONODA, MITOSHI
Applicants: MITSUI TOATSU CHEMICALS, INC.
Application year: 1985
Main classification: C01B31
IPC codes: C01B31