Process for the generation of a low dew-point, oxygen-free protective atmosphere for the performance of thermal treatments

Abstract: A process for the generation of a protective nitrogen-based atmosphere for the performance of heat treatments of metal articles in three phases, including an initial phase in which a gaseous hydrocarbon feed and an oxidant containing oxygen react on a first catalyst to form a reaction product, a second phase in which the reaction product is added to nitrogen contaminated by the presence of oxygen and a third phase in which the reaction product is conveyed to a second catalyst to form a low dew-point gaseous mixture as a protective atmosphere.

Description: The present invention relates to a process for the generation of a protective nitrogen-based atmosphere for the performance of heat treatments of metal articles, such as annealing, tempering, pre-temper heating and the like. Conventionally, the nitrogen utilized for such purposes was obtained by cryogenic means at considerable cost. More recently, therefore, attempts were made to utilize nitrogen produced by methods more economical than the cryogenic process, for example, by the passage through diaphragm membranes or by pressure-swing adsorption (PSA). Nevertheless, the nitrogen so obtained presents the drawback of impurity, containing as it does small fractions, between 0.1% and up to about 5% of oxygen, with decisively deleterious effects on the pieces submitted to such heat treatment. Therefore, numerous procedures have already been proposed to reduce and/or eliminate the content in oxygen or oxidant derivative substances, such as water and carbon dioxide, in nitrogen produced by non-cryogenic methods, so as to purify the latter and if need be combine it with reducing additives, such as carbon monoxide and hydrogen, which exert a beneficial effect on the heat treatment process. As an example, WO-A-93 21 350 describes an endothermal catalytic process, wherein hydrocarbons are made to react to oxygen contained in the nitrogen impurities, in a reactor chamber containing conventional nickel oxide catalysts, or catalysts based on noble metals, essentially resulting in the formation of carbon monoxide and hydrogen, in preference to undesirable oxidizing compounds. Notwithstanding the presence in heat treatment furnaces of heat exchangers designed to preheat the gas intended to react in such a reactor, it is nevertheless necessary to supply heat from the outside, in order to activate the partial oxidation reaction of hydrocarbons with oxygen. On the whole, therefore, the economics of the process are adversely affected by the need to provide pre-heating exchangers and supply large quantities of outside heat. EP-A-0 603 799 describes a process for the catalytic conversion of oxygen included in non-cryogenic nitrogen, by means of hydrocarbons, so as to determine - in view of the low temperature of a suitable conversion reactor - the formation of fully oxidized water and carbon dioxide. These are then converted into reducing compounds by re-forming reactions with excess hydrocarbons present in the heat treatment furnace. Nevertheless, the kinetics of the reforming reactions is decisively slow at typical operating temperatures of such furnaces, so much so that to arrive at desirable compositions, it is necessary to provide extended dwelling times, forced gas recycling systems and the like, thus limiting the practical applicability of the process. EP-A-0 692 545 describes a catalytic system based on noble metals, in which impure nitrogen produced by non-cryogenic means is made to react directly with hydrocarbons. To secure preferential formation of reducing agents, it is necessary to work at high temperatures, requiring outside heat input, which again has a negative effect on the economics of the process. With a view to overcoming the drawbacks of known technology, the present invention envisages a process consisting of: Phase One, in which a gaseous hydrocarbon feed and an oxygen-containing oxidant are made to react with a first catalyst chosen from the group consisting of noble metals, oxides and mixtures thereof, at a temperature in the range of about 750°C to about 900°C and a space velocity of at least 10,000 h -1 , thus forming a reaction product comprising carbon monoxide, hydrogen and hydrocarbons, along with lesser quantities of water and carbon dioxide. - Phase Two, in which the reaction product is added to nitrogen contaminated by the presence of oxygen, reacting in its totality with a portion of the said hydrogen and carbon monoxide, forming additional water and carbon dioxide, and Phase Three, in which the product obtained in Phase Two is fed over a second catalyst, chosen from a group containing noble metals, at a temperature ranging from about 400°C to about 750°C, forming a gaseous low dew-point mixture, consisting essentially of nitrogen, hydrogen and carbon monoxide, such mixture being suitable for use as a protective atmosphere in heat treatments. The thermal efficiency of the invented process is distinctly superior to known processes which involve a direct reaction between oxygen present in the impure nitrogen and hydrocarbons, notably methane or natural gas. To permit formation of the desired reducing compounds with acceptable kinetics, it is in fact necessary in this latter case to work at temperatures on the order of at least 750°C, calling for the input of substantial amounts of outside heat. Conversely, according to the invented process, the above mentioned direct reaction is avoided, with its deleterious kinetic and thermodynamic drawbacks, and instead an indirect reaction is pursued by way of the three reaction stages previously described, with a limited input of outside heat. More specifically, Phase One leads to the formation of hydrogen and carbon monoxide, which in Phase Two react very quickly and easily with oxygen contained as an impurity in nitrogen. Hence, it is in that phase that oxygen is completely . eliminated, concurrently with the formation of carbon dioxide and water, whose reforming into hydrogen and carbon monoxide is facilitated in Phase Three. It should moreover be noted that the catalysts utilized in Phase One, notably those of the oxide type, promote the formation of unsaturated hydrocarbon molecules, for example ethylene and propylene, which in turn promote thermodynamic equilibrium and the kinetics of Third-Phase reforming. The reaction leading to the formation of unsaturated hydrocarbons starting from oxygen and saturated hydrocarbons, particularly methane, is referred to as the 'oxidative coupling'. An article by O.V. Krylov, published under the title of Catalytic Reactions of Partial Methane Oxidation', in Catalysis Today, Vol. 18 p. 209-302, 1993, contains a comprehensive review of processes followed to achieve oxidative coupling reactions. So far, the unsaturated hydrocarbons produced in this manner have not proved adapted for use on an industrial scale in the production of the corresponding polymers. Still, in the course of the Third-Phase reforming reaction envisaged in this invention they play a role extremely beneficial to the formation of desirable reducing compounds, as demonstrated in experimental tests (cf. Example 3 below). In the invented process, the hydrocarbon infeed is preferentially made up of methane, propane or natural gas, whereas the oxygen-containing oxidant preferentially utilized is air. Depending on the desired quantity of reduction agents in the final gaseous mixture, it is a matter of convenience to adjust the rate of flow of different raw materials used in the process. In particular, the ratio of air to hydrocarbon infeed may range between 2.3 and 0.5, preferably 2 and 0.8, whereas the ratio between the input of impure nitrogen and the reaction product in Phase One may range between 10 and 1, preferably 6 and 1. Both the first and the second catalyst may utilize a ceramic substrate, being in this case chosen from a group composed of ruthenium, rhodium, palladium, osmium, platinum and mixtures thereof. Again by way of an example, the ceramic substrate may be chosen from a group consisting of alumina, magnesium oxide, silica, zirconium oxide, titanium oxide and mixtures thereof. As previously mentioned, if the intent is to enhance the unsaturated hydrocarbon content in the gaseous products present in Phase One, it is preferable to use an initial oxide-type catalyst, chosen for example from a group consisting of Li/MgO, Li/SM 2 O 3 , Sr/La 2 O 3 and mixtures thereof. The invention will now be described in greater detail based on the following examples and the single drawing illustrating schematically the plant needed for its implementation. The examples and the figure are merely illustrative and the invention is not limited thereto. EXAMPLE 1. A mixture of air 10 and natural gas 12 in an air-to-methane gas ratio of 1.8, is fed to an oxidative coupling reactor 14 (Fig. 1) containing as catalyst 1% by weight of platinum on an alumina substrate. The space velocity meaning the flow rate of gas so produced per unit of volume of the catalyst is 50,000 h -1 and the temperature of the gas at outlet 16 is 750ºC. The gas composition is as follows: CO = 17.9% H 2 = 36.2% CO 2 = 1.0% CH 4 = 9.5% N 2 = Remainder to 100% The gases 16 are then added to impure nitrogen 18 containing 1% oxygen obtained by membrane separation. The ratio between the impure nitrogen 18 and the gas 16 equals 3. The oxygen contained in nitrogen 18 reacts immediately with a portion of the carbon monoxide and hydrogen contained in gases 16, to form water and carbon dioxide. The gas mixture 20 so obtained is fed to a reforming reactor 22 containing as catalyst 1% by weight of platinum, on an alumina substrate. The space velocity is 25,000 h -1 and the mean temperature is 652ºC. The composition of the gases 24 exiting from reactor 22 is as follows: H 2 = 11.4% CO = 6.7% CO 2 = 0.24% N 2 = Remainder to 100% The dew-point of gases 24 is - 34ºC. Next, the gases 24 are channeled to a heat exchanger 26 so as to preheat the impure nitrogen 18, and may be utilized directly as protective atmosphere for thermal treatments, containing as they do wholly negligible quantities of oxidants. Comparative EXAMPLE 2. Impure nitrogen containing 3% oxygen with methane in a ratio of impure nitrogen-to-methane of 16, is made to react directly with a catalyst identical to the one described in Example 1, at a temperature of 699ºC. The composition of the gases obtained in this manner is the following: H 2 = 10.3% CO = 4.2% CO 2 = 0.6% N 2 = Remainder to 100% Their dew-point of -9ºC is distinctly higher to the value of -34ºC of the gases obtained according to the invented process (Example 1). To obtain gases with a dew-point of -34ºC by the process described in Example 2, the reaction temperature would have to be raised to 728ºC. Hence, to obtain gases with the identical dew-point, the invented process allows reforming to take place at a temperature 76°C lower than the process utilized in Example 2. A reduction of even a few dozen degrees of reforming temperature is a decisive advantage, inasmuch as it reduces the degree of sintering of the catalyst and, by the same token, its loss of activity, while enhancing the thermal efficiency of the process and reducing the need for outside heat input. EXAMPLE 3 A mixture of air 10 and natural gas 12 in an air-togas ratio of 1.5 is fed to an oxidative coupling reactor 14 (Fig. 1), containing as catalyst samarium oxide. The gas at the outlet contains C 2 H 4 = 4% CH 4 = 4% in addition to CO, H 2 and N 2 and minute quantities of H 2 O and CO 2 . Next, the gases 16 are added to impure nitrogen 18 containing 1% of oxygen, obtained by membrane separation. The ratio of impure nitrogen 18 to the gases 16 is 3. The oxygen contained in nitrogen 18 reacts immediately with a portion of the carbon monoxide and oxygen contained in the gases 16, forming water and carbon dioxide. The gaseous mixture 20 so obtained is fed to a reforming reactor 22 containing as catalyst 1% by weight of platinum on an alumina substrate. The space velocity is 25,000 h -1 and the mean temperature is 550ºC. The composition of the gases 24 at the output of reactor 22 is as follows: H 2 = 11.6% Co = 5.8% N 2 = Remainder to 100% CO 2 ) negligible CH 4 ) quantities The dew-point of gases 24 is -35ºC, nearly equal to the gases produced in Example 1, but obtained at a decisively lower reforming temperature (550ºC vs. 652ºC), thanks to the presence of discrete quantities of ethylene. The gases 24 are fed to a heat exchanger 26, so as to preheat impure nitrogen 18, and may then be utilized directly as protective atmosphere for thermal treatments, containing as they do wholly negligible quantities of oxidants.

Claims: Process for the generation of a protective atmosphere for the execution of thermal treatments, such process comprising: an initial phase, wherein a gaseous hydrocarbon feed (12) and an oxidant containing oxygen (10) are made to react on a first catalyst chosen from a group consisting of noble metals, oxides and mixtures thereof, at a temperature comprised between approx. 750°C and approx. 900°C, at a space velocity of at least 10,000 h -1 , forming a reaction product (16) comprising carbon monoxide, hydrogen and hydrocarbons and smaller quantities of water and carbon dioxide, a second phase, wherein such reaction product (16) is added to nitrogen contaminated by the presence of oxygen (18) which reacts in its totality with a portion of such hydrogen and carbon monoxide, forming additional quantities of water and carbon dioxide, and a third phase wherein the product (20) obtained in the second phase is conveyed to a second catalyst chosen from a group consisting of noble metals at a temperature ranging between approx. 400°C and approx. 750°C, forming a low dew-point gaseous mixture (24) consisting essentially of nitrogen, hydrogen and carbon monoxide, such mixture (24) being suitable to act as a protective atmosphere for the execution of thermal treatments. Process according to Claim 1, wherein the said hydrocarbon feed (12) is formed of methane, propane or natural gas and the said oxidant (10) is air. Process according to any one of the preceding claims, wherein the ratio of the flow of air (10) to the hydracarbon feed (12) ranges between 2.3 and 0.5, preferably 2 and 0.8. Process according to any one of the preceding claims, wherein the ratio of impure nitrogen (18) and the reaction product (16) of the initial phase is comprised between 10 and 1, preferably between 6 and 1. Process according to any one of the preceding claims, wherein the first and/or the second catalyst is carried by a ceramic substrate and is chosen from a group consisting of ruthenium, rhodium, palladium, osmium and platinum, and mixtures thereof. Process according to Claim 5, wherein such ceramic substrate is chosen from a group consisting of alumina, magnesium oxide, silica, zirconium oxide, titanium oxide and mixtures thereof. Process according to any one of the preceding claims 1 to 4, wherein the said first oxide-type catalyst is chosen from a group consisting of Li/MgO, Li/SM 2 O 3 , Sr/La 2 O 3 and mixtures thereof.

Inventors: PORTO GIANLUCA, VAN DEN SYPE JAAK STEFAAN, PORTO, GIANLUCA, VAN DEN SYPE, JAAK STEFAAN
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1998
Main classification: C21D1
IPC codes: C21D1