Process for removing unreacted ammonia from an effluent in a...

Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component

Reexamination Certificate

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C423S375000, C423S376000

Reexamination Certificate

active

06716405

ABSTRACT:

This invention relates to a fluidized bed reactor for ammoxidation of hydrocarbons and, more particularly, to a fluidized bed reactor with internals installed in proximity to upper region of the catalyst bed, capable of enhancing the contact efficiency between gas and solid phases.
It is an important subject in petrochemical industry to produce unsatured nitrites by ammoxidation process from hydrocarbons, among which the ammoxidation of propylene and isobutene has long been commercialized for the manufacture of acrylonitrile and methyl acrylonitrile, respectively, and the ammoxidation of paraffinic hydrocarbons has also been under development. A common problem associated with these reactions is due to the fact that unsaturated nitrites are usually unstable and liable to polymerization under basic conditions. For any ammoxidations therefore, it is necessary to eliminate the unreacted ammonia from the product gases. To remove the unreacted ammonia, the prior art has employs a method by sulfuric acid quenching. This method will produce large quantities of nitrile-containing ammonium sulfate waste water, which is difficult to dispose. The strict legislation imposed on pollutant emissions in most countries has rendered the disposal of ammonium sulfate byproduct a critical issue.
Taking the ammoxidation of propylene to acrylonitrile for example, propylene, upon passing through a fluidized bed reactor together with ammonia and air, is ammonia oxidized to form a major product of acrylonitrile and a number of byproducts including acetonitrile, hydrogen cyanide, acrolein, acrylic acid, carbon monooxide and carbon dioxide, as well as small amounts of unreacted propylene and ammonia. After exiting the reactor, the gaseous effluent is cooled and then enters a neutralizing column, where the unreacted ammonia is absorbed by aqueous sulfuric acid to produce ammonium sulfate. At the same time, parts of water vapor are also condensed out, thereby causing the formation of ammonium sulfate waste water. After removing the unreacted ammonia, the gas is introduced into an absorber, where all the organic compounds are absorbed from the gas by low-temperature water. The absorption liquid is then sent to an acrylonitrile recovery and refining unit for the separation of high purity of acrylonitrile, hydrogen cyanide and acetonitrile.
In the above described process of acrylonitrile production, it is very important to remove unreacted ammonia with sulfuric acid from the effluent gas in the neutralizing column, because both acrylonitrile and hydrogen cyanide are substances liable to polymerization, especially in neutral and slightly alkaline conditions. Not only will this cause product loss of acrylonitrile and hydrogen cyanide, but contaminate the equipment and produce ammonium sulfate waste water.
The composition of ammonium sulfate waste water is complex, approximately comprising the following groups:
1. polymers: Because pH value of the circulating spray liquid in the neutralizing column is controlled within the range of 2-7, certain amount of products, such as acrylonitrile, hydrogen cyanide and acrolein, may polymerize to form high polymers. Losses caused by polymerization, calculated based on the total amount of their formation are: acrylonitrile 2-5%, hydrogen cyanide 3-8%, and acrolein up to 40-80%. Therefore the polymer content in the ammonium sulfate waste water is very high. A wider molecular weight distribution is another characteristics of polymers present in the waste water, i.e. some polymers with low molecular weight are soluble in the ammonium sulfate waste water, but other polymers with higher molecular weight will form black solid insoluble in water, thereby leading difficulty in recovering ammonium sulfate.
2. high boiling components: Since the operation temperature within the neutralizing column is about 80° C., acrylic acid will be condensed from the effluent gas and present in the ammonium sulfate waste water. Another high boiling component is cyanhydrin, which is formed by the condensation between carbonyl compounds and hydrogen cyanide.
3. low boiling components: mainly acrylonitrile, acetonitrile and hydrogen cyanide etc. dissolved in the ammonium sulfate waste water, their content being normally in the range of 500-5000 ppm, depending on the temperature of spray liquid.
4. catalyst fine particles: During the production of acrylonitrile in fluidized bed reactor, the major part of catalyst fine particles entrained from the catalyst bed by the product gas is recovered by cyclones and circulated back to the bed. However, small amount of catalyst fines will be blown out of the reactor by the effluent gas and then scrubbed down in the neutralizing column. The catalyst blow-off quantity is about 0.2.-0.7 kg per ton of acrylonitrile produced.
Accordingly, it is very difficult to recover crystalline ammonium sulfate from the ammonium sulfate waste water. Simply burning off the effluents without a prior recovery of ammonium sulfate therefrom will cause secondary pollution owing to the formation of sulfur dioxide, which is not allowed to discharge directly to the atmosphere in most countries. Another problem associated with the disposal of ammonium sulfate waste water by burning method is a combustion temperature as high as 850-1100° C. required to burn out the cyanides from the waste water, thus causing large quantities of fuel consumption. Since sulfur dioxide contained in the combustion flue gas is corrosive to steel material the use of waste heat boilers for recovering the heat energy is limited. Moreover, direct vent of high temperature flue gas will cause thermal pollution to the atmospheric environment.
In summary, the formation of ammonium sulfate in the production of acrylonitrile leads to a severe problem, which has severely limited further development of the acrylonitrile manufacture industry. Therefore development of a clean process for the production of acrylonitrile which produce no ammonium sulfate has become the concerned focus in the art worldwide. The key point of this clean process is to maximize ammonia conversion during the production in order to elate the unreacted ammonia
Elimination of unreacted ammonia can be achieved by two ways: one starts with the catalyst to increase ammonia conversion of the catalyst; the other starts with the ammoxidation reaction to enable ammoxidation of propylene and elimination of unreacted ammonia to proceed separately.
Further increasing ammonia conversion of the propylene ammoxidation catalyst can be difficult to achieve. To take account of ammoxidation only, the catalyst is required to have lower ability to decompose ammonia, i.e. to yield higher acrylonitrile yield while using a lower feed ratio of propylene to ammonia. If the catalyst has a higher decomposition ability for ammonia, the increase of ammonia consumption will render it uneconomic. Therefore these two requirements are contradictory. Since ammonia conversion of the current catalyst is very low, to increase ammonia conversion of the catalyst to certain level without increasing the ammonia consumption still deserves attention. Because certain amount of acrylic acid is also formed during propylene ammoxidation, it is not necessary to increase ammonia conversion of the catalyst to as high as 100%. For a conversion up to 97-98%, it may not need essentially to add sulfuric acid for neutralization. For example, Chinese Patent 96116456.5 is an example in an effort to increase ammonia conversion of the catalyst. From the viewpoint of extended stable operation of the plant, the inventor believes that there should be other measures to attain complete elimination of the unreacted ammonia. This is because of the fact that the ability of a catalyst to decompose ammonia is related to how long it has been used, and also influenced by the operating conditions of the reactor, which cannot be maintained unchanged for long periods of time.
To eliminate the unreacted ammonia by virtue of the secondary reaction of propylene ammoxidation is a useful method and has been disclosed in patents. U.

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