PROCEDURE FOR PASSIVATING METAL SURFACES ATTACKED BY OPERATING CONDITIONS AND CORROSION PROMOTION END OF MEDIA

This invention concerns a system for the passivation of metal surfaces in equipment affected by operating conditions and by agents which promote corrosion, in general of the surfaces in chemical plants involving the presence and formation of corrosive compounds especially where the environmental conditions (temperature and pressure) intensify said corrosive action.

More particularly the invention concerns a system for the passivation of the metal equipment used in chemical plants, and exposed to the intensive action of highly corrosive compounds, and what is more under temperature and pressure conditions higher than ambient conditions.

In an embodiment of great industrial interest, the invention consists in a system for the passivation of metal equipment used in urea synthesis and treatment processes.

Without affecting the generality of the invention, but only as a reference to one of the most interesting and immediate instances, in plants for the industrial production of urea through the synthesis of the reactants NH3 and CO2, with the formation of various compounds, besides urea, which are highly corrosive such as more or less concentrated aqueous solutions of ammonium carbamate, ammonium carbonate, urea itself and other kinds of ionic species.

The metal surfaces of the various parts of equipment which come into contact with the above-mentioned compounds are subjected to chemical aggression which affects their integrity and efficiency.

It is well known that the synthesis of urea is carried out at a high temperature (on average 180 DIVIDED 215 DEG C) and high pressure (on average 130 DIVIDED 400 bar). Downstream the synthesis section are found a number of decomposition stages of urea synthesis by-products which have not been transformed into urea such as ammonium carbamate which through heat is decomposed into NH3 and CO2 and separated from the synthesis elements, as gaseous NH3 and CO2, which are condensed in successive condensation stages forming aqueous solutions of ammonium carbamate and/or ammonium carbonate which are recycled in the synthesis section, while the urea is concentrated in successive stages operating at decreasing pressure until the final vacuum concentration stage is reached from which is obtained virtually pure melted urea which is then sent to the finishing stage effected with various techniques.

Various systems have been put forward for the passivation of equipment used in the stages mentioned above which is subjected to aggression by the corrosive compounds treated in it. For example Belgian Patent No. 625.397 describes the use of oxygen as passivating agent at 180 DEG C and 270 kg/cm2 for the surfaces in a urea synthesis reactor in stainless steel containing up to 19% Cr and 14% Ni; in general oxygen is replaced by other passivating agents, for example hydrogen peroxide and alkaline metal peroxide or alkaline earth peroxide.In European Patent 0096151 a passivation system is described for strippers where the effluent from the urea synthesis reactor is treated at a high temperature and pressure, between 120 and 240 kg/cm2, as a thin falling film countercurrent with NH3 or CO2; to a first passivating agent consisting of oxygen-containing gas introduced from the bottom of at least one stripper is added as second passivating agent a liquid injected from the top of the stripper and selected from hydrogen peroxide, alkaline metals persulphate or perborate, peracetic acid, organic peroxide.

Oxygen may be introduced into the plant as pure oxygen or mixed with air or with hydrogen peroxide.

The oxygen in gas form is introduced by injecting it into the CO2 before this is compressed, or into the ammonia entering the synthesis zone, or as hydrogen peroxide into the various liquid flows upstream the entrance to the equipment to be protected.

The passivating system with hydrogen peroxide requires in any case the simultaneous injection of gaseous oxygen, either as air or as pure oxygen as mentioned before.

The above-mentioned passivation systems are used to protect from corrosion the metal material usually employed in industrial plants for the production of urea (various types of stainless steel, titanium, etc.).

Besides the above-mentioned passivating agents others have been put forward (for example DE-A-1800755) such as soluble ammonium nitrite, soluble sodium nitrite, and other substances not used industrially.

Nowadays the technique universally adopted to passivate metal surfaces in contact with the solutions and vapours present in the plant's various stages is to send to the synthesis reactor air and oxygen by injecting them into the CO2; in some cases, besides this injection, hydrogen peroxide is introduced into the liquid flow upstream of the equipment to be passivated.

The oxygen content injected into the CO2 has a concentration of between 0.2% and 0.8% in volume and this causes some troubles of which just a few pointed out as follows: 1. the oxygen injected as air enriches the reactor with nitrogen, with the obvious consequence that the synthesis zone is enriched with inerts, thus creating a gas phase the consequence of which is a reduction in urea yield in the reactor; 2. since the CO2 comes from the synthesis gas decarbonation section for the production of ammonia, it contains H2, N2, CO, CH4 in such proportions that together with the oxygen they create explosive mixtures.

It has been seen that to escape the explosion field the amount of oxygen injected into CO2 must be below 0.2% vol., but under these conditions it has also been observed that the protective action over the metal surfaces is greatly reduced thus leaving them exposed to the corrosive action of both liquid and gas substances present in the various parts of equipment.

Object of the present invention is now a process which eliminates the drawbacks of the present State of the Art and allows to overcome the situation of contrast between the necessity of having oxigen in large amount suitable for the desired passivation on one side and the necessity of avoiding the risks of explosion on the other side.

Surprisingly it has been found that to escape the field of explosion, the oxygen content may be reduced below 0.3% vol. without affecting the passivation of equipment, by means of adding to the passivation air small amounts of ozone (O3).

The synergic action of the oxydization of air and O3 permits the achievement of, among others, the following advantages: 3. notable reduction of inerts in the urea synthesis reactor with the advantage of an increase in urea yield. 4. reduction of the vapour phase in the reactor thus increasing the volume of the liquid phase increasing its residence time. 5. avoidance of the field of explosion, since the O2 does not create an explosive mixture with the H2, CO, CH4 present in the various stages of the plant.

The advantage of the invention is undeniable and significant all the more since the production of O3 is achieved with amply experimented conventional methods, just as absolutely conventional is the system for eliminating O3 from the gas inerts discharged into the athmosphere where the laws for the protection of the environment forbid its being released into the atmosphere, the production of O3 and its elimination being a well-known technique.

The content of O2 in the CO2 may vary between 0.05% vol. and 0.3% vol. while the content of O3 may vary between 0.01% vol. and 0.1% vol. When a compound selected from the group consisting of hydrogen peroxide (H2O2), alkaline metals peroxides, alkaline hearts peroxides, alkaline metals persulphates or perborates, organic peroxides and acetic acid is present together with O2 and O3, the oxigen amount can be reduced below 0.05% vol., f.i. between 0.01% vol. and 0.05% vol., preferably the third compound selected from the above group is hydrogen peroxide (H2O2).

A further advantage of the invention is that the concentration of O3 may be maintained at a high level without the danger of explosions and with an appreciable increase in passivation, reducing the hydrogen (H2) content (together with the CO, N2 and CH4 content) in the CO2 obtained from the decarbonation plant of the ammonia synthesis gas, by preference through an additional pre-flash of the solution rich in CO2 to be regenerated, before in effect it enters this regeneration stage.

The above pre-flash may be simply carried out at about 3 bar abs.

EXAMPLES

The purpose of the following examples is to compare the composition of the CO2 sent to the synthesis reactor and of the inert gas discharged into the athmosphere, both according to the known technique and to the invention.

We take into consideration the case of a 1000 t/d urea plant:

Composition of the CO2

Comparison example 1.

Known technique

The amount of CO2 necessary for synthesis and its composition after the addition of air is as follows: EMI8.1

Example 2. Invention

The amount of CO2 necessary for synthesis and its composition after the addition of air enriched with O3 is as follows: EMI8.2 It can be seen that the amount of oxygen is appreciably reduced in the case of the invention as compared to the amount required for the known technique while the passivating effect is equally efficient thanks to the addition of ozone (O3).

Composition of the inert gas discharged into the air in the two cases:

Example 3 EMI9.1

The field of explosion of the two mixtures can easily be calculated by conventional method and it will be seen that the mixture in example 3 according to the invention is outside the limits of explosion.

The invention has been deliberately described and exemplified with reference to urea which is one of the most difficult cases and quite emblematic as far as the phenomenon of corrosion is concerned. Of course the invention can be applied to the majority of analogous cases, and more particularly to all chemical processes where, as for urea, it is necessary to passivate with oxygen (air)with or without a peroxide; now, according to the invention, passivation is synergically improved with the addition of O3. Application to these processes remains therefore within the scope and the spirit of this invention.