Patent Application
20100098603
The present invention relates to a reactor for making hydrogen cyanide and sodium cyanide. The present reactor is using a process that is an improvement over the previously known Andrussow process for making hydrogen cyanide (HCN) and sodium cyanide (NaCN). In particular, the reactor according to the invention is using a process relating to improvement of the efficiency of the new process (hereafter called the new process) compared to the Andrussow process.
The invention thus in one embodiment relates to a reactor for converting methane, ammonia and oxygen and alkaline or alkaline earth hydroxides into alkaline or alkaline earth cyanides by two-stage reactions;
1—a catalytic reaction process over a catalyst material between ammonia, methane and oxygen wherein hydrogen cyanide, carbon monoxide/dioxide and water are formed, wherein the reaction gases are mixed, and
2—the gases being cooled prior to being absorbed with an alkaline or alikaline earth hydroxide for producing a corresponding cyanide, wherein the reactor comprises a first stage with a gas inlet, wherein the first stage is formed by a cone with distribution plates providing an even gas distribution over the catalyst material, wherein the distribution plates are located between the gas inlet of the reactor and a catalyser basket within the reactor located beneath the distribution plates, the distribution plates and being perforated with a number of holes with a diameter less than 20 mm and with a pitch larger than 1 diameter, with the distribution plates spaced from each other in the flow direction of the gas, the first distribution plate(s) functioning mainly to distribute the gas whereas the last distribution plate works as a heat radiation shield and as a distribution plate facing the catalyst gauze, and wherein the catalyst gauze is present in the form of catalyst gauze(s) fixed by catalyst weights.
Hydrogen cyanide (HCN) is one of the smaller volume industrial chemicals, which nevertheless is quite important in the chemical industry. In particular, HCN is used for the manufacture of cyanuric chloride, methyl methacrylate, adiponitrile (for nylon-6,6), sodium cyanide, ferrocyanides and chelating agents. Sodium Cyanide is mainly used for the heap leaching of Gold (Au) and Silver (Ag).
At present, almost all of the world's production of HCN/NaCN is made by one of three processes:
(1) The Andrussow process in which ammonia, methane and oxygen are reacted over an oxidation catalyst to form HCN, CO, water and H2;
(2) The “methane-ammonia direct process” or Degussa process in which ammonia and methane are reacted in the absence of air in externally heated tubes containing platinum/rhodium catalyst to form HCN and hydrogen; and
(3) The Shawinigan process in which ammonia and propane are passed between spaced electrodes within a fluidized bed of coke. In addition to the above processes for making HCN directly, it is also made as a by-product in the manufacture of acrylonitrile by reaction of propylene and ammonia over an oxidation catalyst.
To produce NaCN, all different processes must absorb HCN with sodium hydroxide (NaOH).
Though each of these processes is used commercially, by far the most widely used is the Andrussow process. In the Andrussow process, a vapour phase mixture of oxygen-containing gas (usually air), ammonia (NH3) and methane is contacted with platinum metal catalyst at a temperature of about 1200° C. by which part of the methane is burned to furnish heat to the methane-ammonia reaction, which is endothermic. The overall reaction of the Andrussow process is as follows:
CH4+NH3→HCN+3H2
CH4+NH3+3/2O2→HCN+3H2O
2H2+O2→2H2O
HCN+NaOH→NaCN+H2O
The overall reactions in the new process used in the reactor according to the present invention also include:
CH4+O2→CO+H2O+H2
CH4+2O2→CO2+2H2O
NH3+O2→NO+H2O+H2
NO+CO+3/2H2→HCNO+H2O
NO+CO+3/2H2→HNCO+H2O
HNCO+NaOH→NaCN+H2O+½O2
HCNO+NaOH→NaCN+H2O+½O2
The above reactions used in the reactor according to the present invention is by way of example shown through the basic reaction of sodium hydroxide with HCN, HNCO and HCNO, but any alkaline or alkaline earth metal such as potassium, calcium, magnesium etc. may be used equally well.
One characteristic of the Andrussow process is that the catalyst becomes less active with use. Though the cause for such deactivation is not precisely known, it is believed to be in part due to the formation of carbon on the catalyst, which results in a blocking of part of the active sites on the catalyst surface with a thin layer of carbon. This deactivation of the active sites of the catalyst lowers the conversion of ammonia and methane to an average of around 65%.
Because of the higher cost of methane and ammonia, it is essential that available methane and ammonia sources be utilized in the most effective manner.
It has now been discovered that, in a process for the synthesis of HCN and NaCN by the vapour-phase reaction of ammonia, methane and oxygen followed by the absorption of the product gas by sodium hydroxide to produce sodium cyanide, the yield of HCN and NaCN can be increased to an ammonia and methane conversion of more than 75%. Typically the conversion of ammonia and methane according to the invention is more than 90%. Such an increase is assisted by an improved mechanical construction of the reactor system, as explained infra.
Considerable work has been devoted to the supplementation of HCN process feeds. For example, in U.S. Pat. No. 2,006,981, Andrussow discloses the replacement of part of the hydrocarbon feed to the process by oxygenated hydrocarbon derivatives such as methanol, and in Italian Pat. No. 845,992, assigned to Montecatini Edison, S.p.A., the supplementation of methane with acetonitrile is disclosed. Each of these supplemental feeds is also disclosed by the same references to be capable of functioning as a primary feed for the manufacture of HCN as well.
Furthermore, inert diluent materials have also been added to the reactants in various processes for making HCN. For example, in U.S. Pat. No. 2,688,531, the author Eck mentions the addition of nitrogen to the reactants in a non-catalytic process for making HCN by reaction of CH4 with NH3 at a temperature above 1425° C. Similarly, Bellringer et al in U.S. Pat. Nos. 2,746,843 and 3,149,914 disclose the addition of steam or nitrogen in the reaction of methanol with NH3 and O2 over an antimony-tin catalyst to make HCN. In the reaction of CH4 with NH3 and O2 over a platinum group metal catalyst, Gross et al in U.S. Pat. No. 3,033,658 indicates that it is preferred to dilute the reactants with inert gases such as N2 to reduce the reaction temperature. On the other hand, Sennewald et al in U.S. Pat. No. 3,254,110 gives no reason for their indicated preference for diluting their reactants with steam, CO2 or N2 when making HCN by reaction of propylene with NH3 and O2 over a molybdenum or phosphomolybdate catalyst at 300°-405° C. Likewise, Brown et al in U.S. Pat. No. 3,577,218 do not elaborate on their preference for adding N2 as a diluent for an HCN process in which NH3 and CH4 are reacted in the absence of air over a platinum-on-alumina catalyst at 1000° C. However, in U.S. Pat. No. 3,667,907, Rushmere adds steam to the reactants for the express purpose of improving NH3 conversion in the Andrussow process. Of related interest is German Pat. No. 2,421,166 which discloses using CO2 in the absence of reactants to reduce the carbon build-up in the catalyst tubes of an ammonia-methane direct process and thus reduce pressure drop when the process is in operation.
In the Andrussow process for making HCN, a mixture of ammonia, methane and oxygen-containing gas is fed to a reactor containing a fixed bed of platinum metal catalyst. Because the process is endothermic, it is necessary to provide methane in sufficient excess of the stoichiometric amount required to form the HCN to maintain the reaction temperature at 1000°-1200° C. The reaction gases contain mostly HCN, N2, CO, H2, H2O, NH3 and small amounts of CH4, CO2 and, if air is used as the oxygen-containing gas, argon. In many commercial operations, the hot reaction gases are used to generate steam and the heating values are otherwise recovered. Upon cooling to about 75° C., the reaction gases are passed through an absorber in which the NH3 is removed by absorption into an aqueous solution of monoammonium phosphate to form diammonium phosphate. The diammonium phosphate is then steam stripped to separate the ammonia, which is recycled to the process, and thus monoammonium phosphate is regenerated and recycled to the cold off gas absorber. The ammonia-free reactor off gas is passed to a cold water absorber in which the HCN is removed and the HCN-free off gas, which has a heating value of about 90 BTU, is used as fuel. The pure HCN is then mixed with sodium hydroxide to produce sodium cyanide.
The Andrussow process is generally run at a catalyst temperature of from about 1000° to 1200° C. and preferably within the range of 1100° to 1200° C. The proportions of the reactants—CH4, NH3 and O2—will ordinarily be as near stoichiometric as possible consistent with safety, the amount of O2 and CH4 being, of course, sufficient to provide the necessary reaction temperature. Excess quantities of NH3 act mainly as a diluent and pass through the reaction system unconverted. Because of the substantial cost of treating the reaction mixture to separate the HCN product and to remove such unconverted materials, it is, of course, preferred to minimize the leakage of both unconverted NH3 and unconverted CH4 through the system.
These problems are reduced when sodium cyanide is produced by absorbing the reacted gases directly with sodium cyanide.
As described above, the direct production of NaCN (sodium cyanide) normally follows the Andrussow process from incoming air, ammonia and methane until the absorption of HCN (hydrocyanic acid) with NaOH (sodium cyanide). The Andrussow process may also be used following the indirect route where HCN first is isolated before it is mixed with NaOH and sometimes dried and briquetted.
The direct route, where NaCN is produced by the absorption of HCN in NaOH provides the possibility to use a new and novel process of production, i.e. the new process as shown in
The new process used in a reactor according to the present invention is divided into two different process parts. One where methane is oxidized to CO under lean conditions, and one where NO is produced.
Any burner may be used to produce CO; it may also be the side product from power plants. Any CO source may be used.
The production of NO may be by the oxidation of ammonia (NH3) over a Pt/Rh catalyst (or other catalysts producing NO) as in a nitric acid plant, but any source of NO may be used.
After combustion/oxidation of ammonia and methane, the reacted gases are mixed and quenched with water by direct injection or by cooling. The gas is cooled further before it is absorbed with NaOH to form NaCN.
The reactions that occur may be described by the following equations:
CH4+NH3→HCN+3H2
2H2+O2→2H2O
CH4+3/2O2→CO+2H2O
CH4+2O2→CO2+2H2O
CH4+NH3+3/2O2→HCN+3H2O
NH3+1¼O2→NO+3/2H2O
NO+CO+3/2H2→HCNO+H2O
NO+CO+3/2H2→HNCO+H2O
HNCO+NaOH→NaCN+H2O+½O2
HCNO+NaOH→NaCN+H2O+½O2
HCN+NaOH→NaCN+H2O
The overall reaction:
NH3+CH4+1½O2+NaOH═NaCN+4H2O
Production of NaCN according to the Andrussow process burn ammonia and methane in air with conversion efficiency to HCN of 50-68%. The new process used in a reactor according to the present invention will have a conversion of 75-95% as described by
The above reactions according to the present invention is by way of example shown through the basic reaction of sodium hydroxide with HCN, HNCO and HCNO, but any alkaline or alkaline earth metal such as potassium, calcium, magnesium etc. as well as sodium may be used equally well.
The reactions of the above reaction equations in relation to the present invention may be carried out in a reactor system preferably designed as shown in the attached
a shows a possible design and constitution of a reactor according to the invention,
b shows in a cut-out from
a, 3b and 3c show the catalyser basket of the reactor of
Other advantages with the new process used in a reactor according to the present invention as compared to the Andrussow-process are:
Advantages with the new reactor system shown in the
With reference to
Further, with reference to
Further, with reference to
Other advantages with the new invention are:
The novelty with the new process used in a reactor according to the present invention is that HCN/NaCN is produced by the ammonia oxidation reaction and by the methane oxidation reaction executed separately or simultaneously.
The pressure of the new process used in a reactor according to the present invention is not critical and it may be carried out at either increased or reduced pressure e.g. in the range of 1-30 bar in accordance with the engineering economics of the particular plant being considered. Most Andrussow-type operations are conducted at slightly above atmospheric pressure, e.g., 5-10 psig (1.4-1.8 bar)
Quite a large variety of oxidative catalytic materials may be used in the practice of the new process used in a reactor according to the invention e.g. catalytic materials such as is indicated in U.S. Pat. No. 1,934,838. By far the most widely used catalysts for this process are the noble metal catalysts, including platinum, iridium, rhodium, palladium, osmium, gold and silver and alloys thereof. However, oxide catalysts have also been used such as Fe2O3/Bi2O3/MoO3/P2O5/SiO2 which is disclosed in U.S. Pat. No. 3,516,789 to Sennewald, mixed antimony and stannic oxides which are taught by Bellringer in U.S. Pat. No. 3,149,914, molybdenum oxide as described in U.S. Pat. No. 2,746,843 and the rear earth metals which are also referred to in the above-cited patent to Andrussow U.S. Pat. No. 1,934,838. In U.S. Pat. No. 3,254,110, Sennewald discloses that combinations of transition metal oxides with molybdenum oxide are good catalysts for preparing nitriles. However, of all these, platinum/rhodium is used most extensively.
Though there appears to be no reason why the invention would not be operable in other than fixed bed operation, nevertheless, the process used in a reactor according to the present invention is normally carried out over a fixed bed of the catalyst in foraminous form such as pellets, spheres, chips, net, screen or gauze. When in particulate form, the catalyst will usually be supported on an inert carrier having an average dimension of 0.16 to 1.0 cm. The catalyst is quite often in the form of several layers of fine mesh gauze through which the reactant gases are passed downwardly.
Though not essential to obtaining the benefits of the invention, the economics of the process of the invention are improved when heat recovery for the reaction is maximized, e.g., by use of the reactor modifications described and claimed in U.S. Pat. No. 2,782,107 to Inman and U.S. Pat. No. 3,215,495 to Jenks.
Below are two examples related to the performance of cyanide producing reactor. The first example is with the traditional reactor and catalyst, while the second is the reactor according to the invention.
The traditional reactor has no distribution or radiation protection and the catalyst support is simple with no expansion possibilities for the catalyst support. Further the heat is removed by direct quench of water to the hot reacted gases. The running of the catalyst was done with 12 vol % ammonia, 13 vol % natural gas, 75 vol % air, pressure of 4 bar, specific catalyst load of 25 tN/m2d, 16 catalyst gauzes 90/10 Pt/Rh at 1024 meshes with a wire diameter of 0.076 mm. The measured temperature in the catalyst was 1050° C.
The campaign length was 70 days with an efficiency of 50-55% (conversion of ammonia to HCN). The campaign had to be aborted due to cracks in the catalyst.
The reactor was designed according to the invention with distribution and radiation protection plates at the upper part of the reactor. The catalyst support was as described by the invention. The reactor was run at the same conditions as described in example 1. The campaign length was 100 days with an efficiency of 70-75% and there were no cracks in the catalyst due to the improved mechanical design of the catalyst support. The distribution and radiation protection enabled less heat loss and improved distribution over the whole catalyst surface enabling better reaction condition for the reactants.
The improvements are clearly defined by the examples described above and operational costs are saved both by the reduced expenses from the catalyst and by use of less raw materials. In addition the emission to the atmosphere of green house gases are reduced dramatically.
Mellor, Treatise on Inorganic and Theoretical Chemistry, vol. 5, Longmans, Green, N.Y., (1924), pp. 817, 818.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20071153 | Mar 2007 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO2008/000077 | 3/3/2008 | WO | 00 | 10/7/2009 |
