CATALYTIC ELIMINATION OF ACETYLENE IN AIR DECOMPOSITION
DESCRIPTION OF THE INVENTION: The invention relates to a process for the decomposition of air at a very low temperature, where the compressed air is cooled in at least one switchable heat exchanger and then decomposed in at least one distillation column. To remove or eliminate water vapor and carbon dioxide from the air, on the one hand, air decomposition devices are used with switchable heat exchangers, regenerators or Revex, on the other hand molecular filter adsorbers. In appliances in which the amounts obtained of oxygen and nitrogen is less than 50% of the amount of air used, the use of heat exchangers has the advantage that the annoying parts such as water vapor or carbon dioxide are removed of the heat exchanger between the supply of hot air and cold remaining gas. Such apparatuses, however, present the disadvantage that hydrocarbons, especially acetylene, are only imperfectly removed from the air and can be enriched in liquid oxygen. If the acetylene concentration is exceeded as far as a solubility limit is concerned, then it crystallizes and forms a
REF: 26578 easily ignitable explosive mixture. Due to low oxygen solubility, the acetylene concentration should not exceed 1 ppm. In devices with switchable heat exchangers, so-called gaseous phase adsorbers are used in combination with switchable adsorbers or pressure column adsorbers in combination with switchable adsorbers, which at very low temperatures absorb acetylene widely. These adsobedores are arranged in pairs, since after a certain time of operation must be switched to regenerate. The expense for the operation of such adsorbers is considerable, so that after the arrival of the molecular filter, the adsorbers are installed in the hot zone of the air decomposition apparatus, that is before the heat exchanger. These molecular filters not only absorb acetylene but also very well water vapor and carbon dioxide. Therefore it is no longer necessary to use switchable or changeable heat exchangers for liquids cooled at very low temperatures. The adsorbent effect of molecular filters depends strongly on temperature. For economical reasons, therefore, they are operated at a temperature between 5 ° and 25 ° C, so that, as a general rule, a pre-cooling of the air to be decomposed and leaving the compressor is necessary. A disadvantage is also the pressure loss that occurs on the filter, which especially increases the energy expenditure of the device during the period of heating of the regenerated gas. In addition, the molecular filters are essentially larger than the adsorbers of an apparatus with exchangeable heat exchangers, which results in higher investment costs. Frequent sources of danger in cryogenic air-decomposing devices is acetylene which dissolves poorly in oxygen. The other hydrocarbons such as propane and ethylene, which also in the molecular filter adsorbers are only stopped in a limited way, as well as methane and ethane, which in the molecular filter adsorbers are almost not stopped, have a good solubility in the Oxigen. An enrichment of these hydrocarbons to a permitted extent is avoided as a rule by removing a certain amount of fluid from the main condenser, from the main enrichment position, and by subsequent evaporation. In the article "SAFE DESIGN AND OPERATION OF LOW TEMPERATUR AIR SEPARATION PLANTS" Chem. Eng. Prog. Vol no. 52.11 pages. 441 to 447 (1956) is proposed in the apparatus or plants of air decomposition, which are in operation, where the air is strongly contaminated with acetylene (10 to 30 ppm), acetylene remove it catalytically. For this the hot air after the compressor is conducted through a special catalyst, in which the acetylene content is lowered to less than 1 ppm. A sharp decrease in acetylene concentration to few ppb is not possible in any case. In addition, the catalyst only works at sufficiently high temperatures. It is object of the present invention to catalytically oxidize acetylene before the air enters the heat exchanger at a temperature between 70 ° and 130 ° C, preferably between 80 ° and 110 ° C, and water and carbon dioxide remove them essentially of the air in the heat exchanger. According to the invention, the removal of acetylene is carried out before the removal of water and carbon dioxide. Acetylene is removed from the air even before the air enters the heat exchanger, that is, in the hot part of the air decomposition apparatus, while water and carbon dioxide are removed in the heat exchangers, interchangeable regenerators or revex of the air. With this, the air before entering the heat exchanger (s) is neither heated nor cooled by additional installations, so that the process is carried out easily and at a good cost. In air decomposition apparatuses at very low temperature the air is first compressed in a multi-stage compressor, where the air is cooled intermediate between each compression stage. According to the invention, acetylene is removed from the air by means of catalytic oxidation. For this, the air is conducted at a temperature between 70 and 130 ° C, preferably between 80 and 110 ° C, to the catalyst to remove the acetylene. For this the catalyst can be arranged between the last compression stage and the next compressor cooler. The concentration of acetylene is after catalytic oxidation less than 1 ppb. The acetylene free air is then passed through a cooler to the exchangeable or switchable heat exchanger, where the water and carbon dioxide are separated. Advantageously, a noble metal is used as a catalyst in a carrier, since acetylene can be removed from the reactor with a high degree of efficiency. As particularly suitable platinum and palladium have been presented in a carrier as for example A1203. Also the application of metal oxides, especially of Mn02,, Hopcalite, C0304, Fe203 and NiO or the application of mixtures of metal oxides as catalysts is advantageous, since with the temperatures that exist after the last compressor a good elimination of acetylene is achieved from the air that has to decompose. The process according to the invention clearly has advantages compared to the current state of the art. The removal of acetylene in the hot part of the apparatus for the decomposition of air is easier to perform technically than in the cold part. The removal can be carried out without additional heating and / or additional cooling of the air, whereby for example an additional heat exchanger can disappear. This clearly presents savings in both investment costs and energy expenditure. The removal of carbon dioxide and water from the air is carried out in payable heat exchangers. The catalyst provided for the removal of acetylene can thereby have a relatively small dimension. The invention is especially suitable for very large plants, and for plants in which only oxygen is obtained. In the compression of the air used it must be observed that the air due to the heat of the compression does not get too hot, otherwise the compression work increases excessively. For this reason, compression is performed, as a rule in several stages with interconnected coolers, which remove the heat from the compression. By the selection according to the invention of the field of temperature between 70 and 130 °, a preheating of the air can be dispensed before the catalyst and the compression of the air can be carried out at low temperatures, thereby saving energy compared to the processes until now usual. Next, and in relation to the schematic drawing, the invention will be explained exemplarily by a preferred embodiment. The only figure shows an apparatus of decomposition or separation of air at very low temperature with Revex, and a catalyst for the removal of acetylene in the hot part of the apparatus to decompose the air. The air to be decomposed is passed through an air filter 1 to an air compressor 2. The canpressor 2 is of several stages, where between each compression stage an intermediate air cooling takes place. The air leaving the compressor 2, has before the subsequent cooling, a temperature between 80 and 110 ° C, the acetylene is about 10 ppb, but can also be up to some pbb. The air is conducted without further heating directly to a catalyst 3, where acetylene is catalytically oxidized. As a catalyst material, Pd is used on an Al203 carrier. After the catalytic reaction, the air contains less than 1 ppb of acetylene. In this way, an enrichment of the acetylene in the liquid oxygen in dangerous concentrations is avoided, without requiring other devices to eliminate the acetylene. After the catalyst 3, the acetylene free air is cooled in the usual rear cooler 4, of the compressor 2, and then it is brought to the pair of Revex heat exchangers 5, 6, in these switchable heat exchangers 5, 6, it is cooled the air to the dew point, whereby carbon dioxide and water are removed from the air. Revex 5 and 6, is connected to a conventional distillation column, which has a pressure column 8, a main condenser 9, and a low pressure column 10, where oxygen is obtained in a known manner, the total part that is at a very low temperature of the air decomposition apparatus is in a cold box (Coldbox) shirt 11.