Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-05-05
2001-05-29
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S375000, C423S392000, C423S403000, C423S659000
Reexamination Certificate
active
06238634
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
“Not Applicable”.
1. Field of the Invention
This invention relates to a process for nondestructive heating and supply of ammonia or feed gases containing ammonia. The heated gases prepared according to the present invention may be used for the preparation of hydrogen cyanide or in other industrial synthesis.
2. Description of the Prior Art
Preheated ammonia is used in the production of hydrogen cyanide, hydrazine, and other nitrogen containing compounds.
Hydrogen cyanide is produced commercially by contacting ammonia, hydrocarbon gas, and an oxidizing gas with a platinum-group-metal catalyst (Andrussow Process) or by mixing low temperature ammonia gas feed with high temperature (more than 1,100° C.) methane in a chamber having ceramic walls coated with a platinum group metal catalyst (Degussa Process). In either of these processes, the various components of this feed gas may be preheated. Typically in the Andrussow Process, ammonia is preheated only to about 230° C. since it is subject to decomposition and is highly corrosive. Examples of the hydrogen cyanide process may be found in U.S. Pat. No. 1,934,838, U.S. Pat. No. 3,104,945 and British Patent No. 685,744.
Other uses of preheated ammonia are the production of nitric acid from an ammonia oxidation process, production of hydrazine, production of melamine, and ammonia reduction of nitrogen oxides (example, DENOX processes).
U.S. Pat. No. 3,455,659 describes a method of superheating ammonia to 550° C. to 750° C. using a reactor whose surface is either a nickel-chromium alloy or a nickel-chromium-molybdenum alloy. U.S. Pat. No. 2,906,604 discloses a method for producing hydrogen cyanide in which ammonia is preheated to 300-600° C. in aluminized reactor tubes. U.S. Pat. No. 5,431,894 also uses an aluminized reactor to preheat ammonia. In the patent, ammonia is heated up to 610° C. at atmospheric pressure. U.S. Pat. No. 3,401,108 preheats ammonia above 400° C., preferably 450-550° C.
Although it was known that ammonia containing gases could be heated to temperatures above 230° C., the prior art failed to teach the nondestructive heating of ammonia or ammonia containing feed gases or a way by which hot ammonia could be transferred nondestructively at an elevated temperature from a preheat zone into the reaction zone. Prior art also failed to teach a method of heating ammonia or ammonia containing feed gases in such a way that corrosion to the surfaces in direct contact with these hot gases was minimized.
Prior art preheating methods were also limited in that other gases such as methane or steam had to be excluded from the ammonia or mixed in only at very low concentrations. Thus, in the case of mixed reaction feed gases, each reactant, or in some cases mixtures of reactants excluding the ammonia, were subjected to separate preheating operations.
Preheated ammonia is highly corrosive, particularly to iron group metals and alloys, and causes high levels of metal corrosion and loss in equipment surfaces in contact with the hot gas. The corrosion not only leads to contamination of the preheated ammonia gas feed and ammonia decomposition (destruction) forming a more flammable hot gas, but also loss of operation time due to metal equipment repair or replacement.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method to preheat and supply that is transfer within a process, hot ammonia, as well as hot mixtures of ammonia with methane and/or steam.
The present invention provides a method to heat and supply these gases at temperatures up to 700° C. and at pressures of at least 1 atmosphere with minimum destruction of the ammonia and minimum corrosion of the metal surfaces in contact with the ammonia.
Feed gas streams containing ammonia preheated according to the present invention show less than 1.0% ammonia decomposition and corrosion rates of less than 20 mils (508 microns) per year at pressure. In general one can practice the present invention such that the ammonia decomposition is less than 10% and the corrosion rate is less than 50 mils per year (1,270 microns) and realize all the benefits of the invention. Such preheated feed gases are of benefit in production of hydrogen cyanide and other industrial products based on reaction of ammonia with other gaseous materials.
Thus, the present invention provides a process for the non-destructive heating and transferring of an ammonia containing gas having an oxygen content of less than 1.0% by volume comprising the steps of:
(a) heating the ammonia containing gas to a bulk gas temperature greater than 230° C. in equipment having a metal surface in contact with the hot ammonia and being characterized by a spatial surface density and a wall temperature; and
(b) transferring the heated gas into a zone of chemical reaction through equipment having a metal surface in contact with the hot ammonia and being characterized by a spatial surface density and a wall temperature;
wherein the corrosion of metal surfaces in contact with the hot ammonia is less than 1,270 microns per year and the decomposition of the ammonia is less than 10% by volume of the total volume of ammonia present in the gas; with the proviso that (i) when the metal surface in contact with the hot ammonia is selected from the group palladium, platinum and gold, the spatial surface density of said equipment is less than 15 cm
2
/cm
3
, the contact time of the hot ammonia and the metal surface is less than 10 sec, the bulk gas temperature is not more than about 700° C. and the wall temperature is not more than about 800° C. and (ii) when the metal surface in contact with the hot ammonia is selected from the group consisting of austenitic steels and alloys of nickel (such as inconel and hastelloy), the spatial surface density of said equipment is less about 5 cm
2
/cm
3
, the contact time of the hot ammonia and the metal surface is less than 3 sec, the bulk gas temperature is not more than about 500° C. and the wall temperature is not more than about 600° C.
When the metal surface in contact with the hot gas is selected from the group consisting of austenitic steels and alloys of nickel such as inconel and hastelloy, the present process supplies hot ammonia containing gas at a bulk gas temperature of from 400 up to 500° C. When the metal surface in contact with the hot gas is selected from the group consisting of palladium, platinum and gold, the hot ammonia containing gas is supplied at a bulk gas temperature as high as 700° C.
As is illustrated by the Examples below, the present invention allows hot ammonia to be produced and supplied with less than 1% decomposition and with less than 508 mils of corrosion to the metal surface in contact with the hot ammonia or hot ammonia containing feed gas.
The equipment in the present process may be heated externally so that it functions as a preheater or it may simply be tubes or pipes to transfer the hot gas to the reaction zone. For either of these purposes, the equipment may be constituted of a first metal. This first metal may then be surface coated with a second metal at a preferred thickness of from 254 to 2,032 microns (10 to 80 mils). The second metal is selected from the group consisting of palladium, platinum and gold.
The ammonia containing gas may consist essentially of ammonia or be a mixture of ammonia and steam or ammonia and hydrocarbons. Other gases may also be present. In a mixture of ammonia, the hydrocarbon preferred is natural gas or methane, and it is preferred that the natural gas or methane be present at a volume ratio of from 0.7 to 1.3 compared to the volume of the ammonia.
The present process is useful in producing and delivering the hot ammonia containing gas into a chemical reaction zone to make hydrogen cyanide.
DETAILED DESCRIPTION OF THE INVENTION
As used herein the term bulk temperature or bulk gas temperature means the temperature of the hot gas measured at the center of the pipes or tubes used to heat or transfer the hot gas.
As used herein the term wall temperature is the temperatu
Barnes John J.
Bletsos Ioannis V.
Gelblum Peter Gideon
Herron Norman
Kim Tae Hoon
Dunn Tom
E.I. Du Pont de Nemours and Company
Medina Maribel
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