Metal fusion bonding – Process – With shaping
Reexamination Certificate
1998-10-13
2002-07-02
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
With shaping
C228S119000, C228S184000, C422S241000
Reexamination Certificate
active
06412684
ABSTRACT:
The present invention relates to a protective lining for pressure equipment which can be used in processes for the synthesis of urea.
More specifically, the present invention relates to a lining for equipment suitable for tolerating pressures of up to 100 MPa, capable of providing adequate protection of the relative pressure-resistant body, normally made of carbon steel, from the aggressive action of typical process fluids in industrial plants for the production of urea, particularly with reference to equipment included in the synthesis cycle.
The construction technique of high pressure chemical equipment, whether it be reactors, separators, boilers, etc., normally comprises the preparation of a compact body capable of tolerating the operating pressures, guaranteeing maximum safety and time duration of the mechanical specifications, equipped with necessary passages for external communication and the inlet and outlet of process fluids. The most widely used material for this construction is steel, owing to its excellent combination of high mechanical properties, its relatively low cost and commercial availability.
Processes for the production of urea normally used in industry comprise at least one section which operates at high temperatures and pressures (synthesis loop), at which the process fluids, i.e. water, ammonia and especially saline solutions, become particularly aggressive. It has long been known that normal carbon steel is not capable of resisting the corrosion of these fluids at a high temperature and when in contact with them, undergoes a progressive deterioration which weakens the structure causing external losses and even explosions.
In these processes, ammonia, generally in excess, and carbon dioxide are reacted in one or more reactors, at pressures normally ranging from 10 to 30 MPa and temperatures between 150 and 240° C., obtaining an aqueous solution containing urea, the non-transformed ammonium carbamate residue and the excess ammonia used in the synthesis. This aqueous solution is purified of the ammonium carbamate contained therein by its decomposition in decomposers operating, in succession, at gradually decreasing pressures. In most of the existing processes, the first of these decomposers operates at pressures which are substantially equal to the synthesis pressure or slightly lower, and basically consists of an evaporator-decomposer (more widely known as “stripper”, used hereafter) in which the aqueous solution of urea is heated with external vapor in the presence of a vapor phase in countercurrent which favours the decomposition of the carbamate and at the same time acts as entrainment fluid of 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; the stripping can also possibly be carried out by using the excess ammonia dissolved in the mixture coming from the reactor (autostripping), consequently without introducing another external agent.
The decomposition products of ammonium carbamate (NH
3
and CO
2
), together with the possible stripping agents, inert gases included, are normally condensed in a suitable condenser obtaining a liquid mixture comprising water, ammonia and ammonium carbamate, which is recycled to the synthesis reactor. In technologically more advanced plants, this condensation step is carried out at pressures substantially equal to those of the reactor or slightly lower.
As reference, among the many existing patents, U.S. Pat. Nos. 3,886,210, 4,314,077, 4,137,262 and published European patent application 504,966, can be mentioned, 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 provided in “Encyclopedia of Chemical Technology”, 3rd Edition (1983), Vol. 23, pages 548-574, John Wiley & Sons Ed.
The most critical steps in carrying out the process are those in which the ammonium carbamate is at its highest concentration and highest temperature and consequently, in the processes mentioned above, these steps coincide with the equipment of the synthesis cycle, such as the reactor, the stripper and ammonium carbamate condenser, to mention the most important, all operating under analogous or similar conditions to those of the reactor. The problem to be solved in this equipment is that of corrosion and/or erosion particularly caused by contact with solutions of ammonium carbamate at the high temperatures and pressures necessary for the synthesis of urea.
This problem of corrosion has been confronted with various solutions in existing industrial plants and others have been proposed in literature. There are in fact numerous metals and alloys capable of withstanding for sufficiently long periods the potentially corrosive conditions arising inside a synthesis reactor of urea. Among these, lead, titanium, zirconium and several stainless steels such as, for example, AISI 316L (urea grade)steel. INOX 25/22/2 Cr/Ni/Mo steel, special austenitic-ferritic steels, etc. can be mentioned. For economic reasons however, equipment of the above type cannont be entirely constructed with these corrosion-resistant alloys or metals. Usually containers or columns are used, made of normal carbon steel, possibly multilayered, with a thickness varying from 40 to 350 mm, depending on the geometry and pressure to be tolerated (pressure-resistant body), whose surface in contact with the corrosive or erosive fluids is uniformly covered with an anticorrosive metal lining from 2 to 30 mm thick.
In particular, the reactor normally consists of a vertical container with an inlet of the reagents from below and discharge of the reaction mixture from above. The pressure-resistant body usually comprises a cylinder from 0.5 to 4 m in diameter made with a multilayer or solid wall technique, of which the two ends are closed by caps adequately welded to it. Inside the reactor, an anticorrosive lining is applied to all the walls subject to corrosion, which can consist of, for example, titanium, lead, zirconium, or preferably, stainless steels (urea grade) of the type mentioned above.
The subsequent carbamate stripper, especially if operating at the same pressure as the reactor, consists of a tube-bundle exchanger. Also in this case the pressure-resistant body is made of normal carbon steel, whereas titanium or urea-grade stainless steels are preferably used for the lining. In particular zones of the stripper there are conditions of extreme aggressivity of the fluids. This can be attributed to the high temperature, but also to the geometry of the equipment which does not allow a uniform distribution of the passivating agents, such as air, possibly combined with hydrogen peroxide, normally introduced in small quantities mixed with the process fluids.
Moreover, the injection of passivating air in the high pressure section of a urea plant can raise a risk of explosion, besides the advantage of improving the corrosion resistance of the linings most frequently used. In fact, most part of the oxygen introduced with the injected air is not consumed in the plant and is purged, mixed with the inert gas, usually from either the carbamate condenser or the top of the reactor. This gas stream contains also ammonia and hydrogen in such an amount as to produce an explosive mixture with the oxygen at the pressure and temperature conditions of the urea process, which may have catastrophic consequences in industry.
The gases leaving the stripper are usually recondensed in a carbamate condenser which is therefore in contact with a mixture similar to that of the decomposer (except for urea) and therefore extremely corrosive. Also in this case the internal lining preferably consists of the above special urea-grade stainless steels.
In the above equipment or plant units, the anticorrosive lining is obtained by the assembly of numerous elements having adequate resistance to corrosion, so as to form, at the end, a hermetically sealed structure at the high operating pressure. For the various junctions and
Elve M. Alexandra
Johnson Jonathan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Snamprogetti S.p.A.
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