Gas separation: processes – Liquid contacting – Defoaming or antifoaming agent
Reexamination Certificate
2001-05-15
2003-08-12
Chang, Ceila (Department: 1626)
Gas separation: processes
Liquid contacting
Defoaming or antifoaming agent
C558S319000
Reexamination Certificate
active
06605137
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a composition comprising ammonia and an anti-foaming agent, to a process for substantially suppressing foaming during the vaporization of ammonia, and to a process for using the vaporized ammonia for producing chemicals.
BACKGROUND OF THE INVENTION
Ammonia can be used in a variety of industrial applications. Typical chemical compounds that can be produced from ammonia include hydrogen cyanide, amines, nitriles and nitric acid.
Hydrogen cyanide can be produced from natural gas, ammonia, and oxygen over a platinum/rhodium gauze catalyst at a temperature greater than 1000° C. (the “Andrussow Process”). Hydrogen cyanide can also be produced from methane and ammonia passed through porous ceramic tubes lined or coated with platinum, at about 1300° C. (the “BMA” process). Still further hydrogen cyanide can be produced from propane and ammonia in the presence of an electric current, typically at temperatures greater than 1500° C. (the “Shawinigan Process”).
Nitriles and derivatives can be produced from ammonia and a hydrocarbon, for example, in the catalytic ammoxidation of propylene to form acrylonitrile. Nitriles are important industrial chemicals, especially in the plastics, surface coatings, and adhesive industries. For example, acrylonitrile and methacrylonitrile can be used to produce acrylic fiber, as an intermediate in the syntheses of antioxidants, pharmaceuticals, dyes and surface active agents.
Ammonia is generally supplied to the processes by either direct pipeline or barge transfer. Ammonia supplied to and taken from storage tanks has a high foaming potential, which may be due to a threshold concentration of one or more impurities contained therein—these impurities can be present either from concentration in the ammonia process or due to impurities received from barge transfers.
Before entering the reactors for producing hydrogen cyanide or a nitrile, for example, the ammonia feed may be flashed or heated to convert it from liquid state to vapor state. The ammonia feed can also be mixed with other feeds and heated. Heating or flashing ammonia to produce ammonia vapor, particularly at low pressures such as, for example, below 100 psig (about 700 kPa), results in foaming situations. This foam can result in various process limitations, such as, for example, low vaporizer capacity, carryover of material to downstream equipment, such as reactors, resulting in catalyst damage and high manufacturing costs, unstable flow control, inappropriate material on the inlet side of relief valves, which is a safety hazard.
It is well known in the ammonia industry that ammonia contains small quantities of oil, mostly occurring from compressor seal leakage. Water is also present in many cases to prevent corrosion. Suggested causes of foam include the oil, detergents present in the oil, water, non-condensable gases such as CO
2
, N
2
, H
2
, and non-volatile materials such as iron. Eliminating the oil via dry seals on compressors on barges and in the ammonia production unit may be considered, but is an expensive option and, since the actual cause for foaming is unknown, dry seals may not offer a real solution.
Therefore, there is an ever-increasing need to develop a process for handling ammonia to produce an ammonia that is substantially free of, or has reduced, foam potential.
SUMMARY OF THE INVENTION
According to the first embodiment of the invention a composition is provided, comprising ammonia and an anti-foaming agent wherein the composition is substantially free of water or is substantially anhydrous. This composition is especially effective when using ammonia that has concentrated levels of impurities from an ammonia synthesis process or ammonia that has been retrieved from ammonia storage tanks.
According to a second embodiment of the invention, a process that can be used for vaporizing ammonia is provided. The process comprises contacting an ammonia-containing fluid with a foam-suppressing amount of an anti-foaming agent wherein the fluid is substantially free of water or is substantially anhydrous and subsequently vaporizing the ammonia by known means.
According to a third embodiment of the invention, a process for use of ammonia vapor in a chemical process is provided. The process comprises contacting, in the presence of an ammoxidation catalyst, an oxygen-containing fluid, a hydrocarbon and an ammonia vapor composition.
DETAILED DESCRIPTION OF THE INVENTION
The term “fluid” used herein refers to liquid, gas, or combination thereof. The term “gas” includes vapor. The term “substantially” denotes “more than trivial”. The term “anti-foaming agent” refers to a chemical that, when added to a fluid, can substantially reduce the surface activity of the fluid thereby substantially preventing the fluid from foaming. The terms “anti-foaming agent” and “anti-foam” can be used interchangeably. The term “substantially free of water” means less than about 5, preferably less than about 3, and most preferably less than about 1 weight % water, based on the total weight of the composition to which the term refers.
This invention applies to any process where liquid ammonia is being vaporized or heated prior to utilization of the ammonia.
According to the first embodiment of the invention, a fluid composition comprising, consisting essentially of, or consisting of ammonia and an anti-foaming agent is provided.
The ammonia can be any ammonia, i. e., liquid ammonia, gaseous ammonia, or combinations thereof. The ammonia is advantageously ammonia that has concentrated levels of impurities from an ammonia synthesis process, referred to herein as “cold ammonia” or ammonia that has been retrieved from ammonia storage tanks, “storage ammonia”. Cold ammonia is defined herein to mean ammonia from an ammonia synthesis process wherein the pressure has been let down to near atmospheric in order to provide process cooling and to allow for storage. Storage ammonia is defined herein to mean ammonia comprising cold ammonia from ammonia synthesis as well as added ammonia from barges, tank cars, etc., which is stored in storage tanks at atmospheric pressure and temperature below −30° C., at a location accessible to subsequent chemical processing. Warm ammonia herein means ammonia from an ammonia synthesis process wherein the ammonia stream is taken off some intermediate location of the final flash stages such that the resulting pressure of the liquid stream is substantially higher than atmospheric pressure and is generally at ambient temperature conditions. Cold ammonia typically contains impurities from the ammonia process. Storage tank ammonia typically will contain the same impurities as cold ammonia in addition to water, oils, etc. due to contamination from transfers. Cold ammonia and storage ammonia have been found to develop severe foaming problems when vaporized for introducing into subsequent chemical processes.
Any anti-foaming agent that can substantially prevent an ammonia-containing fluid from foaming, especially during vaporization of the fluid, used in the invention. Generally such anti-foaming agents include, but are not limited to, silicones, siloxanes, especially polydimethylsiloxane, hydrocarbon oils such as decane, and combinations thereof. The presently preferred anti-foaming agent is polydimethylsiloxane for it is readily available and effective. For uses wherein the vaporized ammonia is subsequently used in the presence of a platinum-based catalyst, it is preferred that the anti-foaming agent is free of hydrocarbon oils.
Generally, the anti-foaming agent can be present in the composition in a foam-suppressing amount, which is the amount that can substantially reduce the surface activity of ammonia. Such foam-suppressing amount can be in the range of from about 1 to 5000, preferably about 5 to 4000, and most preferably 5 to 3000 mg/kg (ppm) of ammonia. Generally, the concentration of the anti-foaming agent can depend on the ammonia quality and the amount of blowdown taken from the vaporizer. Blowdown is defined herein to mean the amount
Shankwitz Gregory P.
Stauffer Timothy P.
Chang Ceila
E. I. du Pont de Nemours and Company
Saeed Kamal
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