Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including specific material of construction
Patent
1997-03-21
2000-01-04
McMahon, Timothy
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including specific material of construction
228184, 228189, 228594, 228599, 228603, 228636, B01J 1902
Patent
active
060106697
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for restoring the functionality of equipment subjected to heavy corrosion in a plant for the production of urea.
More specifically, the present invention relates to a method for repairing and restoring the functionality of metallic parts or equipment subjected to erosion and/or corrosion by contact, under conditions of high temperature and pressure, with fluids comprising water mixed with ammonia, urea and/or ammonium carbamate, typically present in a plant for the industrial production of urea.
2. Discussion of the Background
It is well-known that urea is obtained with industrial processes which require operating conditions of high temperature and pressure at least in some parts of the plant.
In these processes, ammonia, which is generally in excess, and carbon dioxide are reacted in one or more reactors, at pressures usually of between 100 and 250 bar and temperatures of between 150 and 240.degree. C., obtaining as end-product an aqueous solution containing urea, ammonium carbamate not transformed into urea and the excess ammmonia used in the synthesis. The above aqueous solution is purified of the ammonium carbamate contained therein by its decomposition in decomposers operating, successively, at decreasing pressures. In most of the present processes, the first of these decomposers operates at pressures which are basically the same as the synthesis pressure or slightly lower, and generally uses stripping agents to decompose the ammonium carbamate and at the same time remove the decomposition products. Stripping agents can be inert gases, or ammonia or carbon dioxide, or mixtures of inert gases with ammonia and/or carbon dioxide, and the stripping can possibly be carried out by also using the excess ammonia dissolved in the mixture coming from the reactor (self-stripping), without requiring therefore any external agent.
The decomposition products of ammonium carbamate (NH.sub.3 and CO.sub.2), together with the possible stripping agents, excluding the inert gases, are normally condensed in suitable condensers obtaining a liquid mixture comprising water, ammonia and ammonium carbamate, which is recycled to the synthesis reactor. In plants which are technologically more advanced, at least one condensation step is carried out at pressures more or less the same as those of the reactor or slightly lower.
As a reference, among the many in existence, patents U.S. Pat. No. 3,886,210, U.S. Pat. No. 4,314,077, U.S. Pat. No. 4,137,262 and European patent application publication No. 504,966, can be cited, which describe processes for the production of urea with the above characteristics. A wide range of processes mainly used for the production of urea is described in "Encyclopedia of Chemical Technology", 3rd Edition (1983), Vol. 23, pages 548-574, John Wiley & Sons Ed.
The most critical steps of the process are those in which the ammonium carbamate is at its highest concentration and temperature, and therefore, in the above processes, these steps coincide with the reactor and subsequent equipment for the decomposition (or stripping) and condensation of the ammonium carbamate operating in similar or almost similar conditions to those of the reactor. The problem to be solved in this equipment is that of corrosion and/or erosion caused by the ammonium carbamate, ammonia and carbon dioxide which behave as highly corrosive agents, especially in the presence of water, at the high temperatures and pressures necessary for the synthesis of urea.
This problem of corrosion has been faced with different solutions in existing industrial plants, and others have been proposed in literature. There are, in fact, numerous metals and alloys capable of resisting for sufficiently long periods to the potentially corrosive conditions which are created inside a reactor for the synthesis of urea. Among these lead, titanium, zirconium and several stainless steels such as, for example, AISI 316L steel (urea grade), steel INOX 25/22/2 Cr/Ni/Mo, aus
REFERENCES:
patent: 3986243 (1976-10-01), Schussler et al.
patent: 4600139 (1986-07-01), Murase
Patent Abstract of Japan, vol. 12, No. 149 (M-694) May 10, 1988 of JP-62-270894.
Granelli Franco
Miola Cesare
McMahon Timothy
Snamprogetti S.p.A.
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