NO137010B - USE OF FERRITIC NICKEL-FREE STEEL IN THE PROCESSING OF CARBAMATE-CONTAINING READINGS AT HIGHER TEMPERATURE - Google Patents

Description

The present invention relates to the use of ferritic nickel-free steels for processing ammonium carbamate-containing solutions at a higher temperature. Such solutions are formed e.g.

in the production of urea from ammonia and carbon dioxide as well as in the production of melamine from urea, whereby the gas that remains after separation of melamine from the reaction mixture is processed in a wet way.

With the modern methods for urea synthesis, ammonia is produced

and carbon dioxide added to an autoclave under a pressure of 100 - 300 kg/cm^ and at a temperature between 160 and 25o°C, whereby first the intermediate product ammonium carbarnate is formed, after which urea is obtained from this product by splitting off water.

The conversion of ammonium carbamate into urea does not proceed completely, so that the solution, which flows out of the autoclave in addition to urea and water, still contains an amount of ammonium carbamate as well as the ammonia added in excess. From this synthesis solution, urea must be separated, which usually happens by splitting ammonium carbamate with the help of heat in a number of expansion steps, whereby ammonia and carbon dioxide are released. The ammonia formed is separated and the ultimately remaining urea solution is evaporated or crystallized. The reaction components separated in the gas phase can be separately processed together with the corresponding equilibrium quantity of hydrogen. For the most part, however, they are condensed and returned to the autoclave as ammonium carbamate solution. Another often used option is heating the synthesis solution under high pressure, e.g. synthesis pressure, with simultaneous supply of a stripping gas, such as e.g. the fresh carbon dioxide, ammonia or an inert gas used during the synthesis, whereby a cleavage of the ammonium carbamate also takes place, whereby at the same time the cleavage products are driven off from the solution. In a low-pressure step, the cleavage of the remaining ammonium carbarnate and separation of the cleavage products then follow. The driven-off gas mixture is likewise condensed under high pressure, and the resulting ammonium carbamate solution is fed to the autoclave together with the diluted ammonium carbamate solution, which is obtained by means of the gas mixture separated by condensation in the low-pressure stage.

A frequently used method for the production of melamine is the one where urea is converted by pyrolysis and with the help of a catalyst in an ammonia atmosphere into a gas mixture, which, in addition to melamine, contains ammonia, carbon dioxide and by-products. From this mixture, melamine is separated by dressing with e.g. a liquid medium, while carbon dioxide and part of the ammonia are removed from the residual gas by means of absorption in water or an aqueous solution. The remaining part of the ammonia is fed back into the reactor. The ammonium carbamate solution formed is usually processed in a conventional urea plant. The solution is then concentrated first by desorption and absorption under higher pressure.

It is known that liquid ammonium carbarnate, concentrated solutions thereof in water as well as aqueous ammonium carbamate-containing urea solutions in large quantities are corrosive, and above all at higher temperatures. The choice of construction materials and adaptation of the technology of the process also represent problems, which have been dealt with since the production of urea on a technical scale began. Lead-clad reactors have been used. However, the use of lead makes it necessary to completely remove any oxygen present from the fresh carbon dioxide because oxygen attacks lead. Silver, titanium and zirconium are materials which, with regard to corrosion resistance in ammonium carbamate-containing media, also come into consideration. However, the acquisition costs and problems with processing these materials are the reason why they are not used that often. Furthermore, it has already been proposed to use chrome-nickel steel using an inhibitor that is added to one of the reaction partners, e.g. a polyvalent metal or a salt thereof, or a substance which in a urea environment transforms into negatively charged colloid particles. in this case, a foreign substance is added to the synthesis solvent, which later at least partially occurs as contamination in the final product, and which can have an unfavorable influence on the color, which is particularly a disadvantage when so-called technical urea is processed into synthetic resins.

For this reason, chrome-nickel steel is often used as a construction material in connection with a small amount of oxygen, which

is added to the fresh carbon dioxide. It is known to use austenitic chrome-nickel steel with at least 16% chromium and at least 8% nickel in connection with amounts of oxygen, which preferably corresponds to 0.1 - 3% by volume of the amount of freshly added carbon dioxide. The type that contains approx. 18% Cr, Approx. 12% Nine and approx. 2 1/2% Mo. Furthermore, for this purpose, corrosion-resistant steel grades with a composition within the following limits have been proposed: 16 - 27% Cr, 0 - 7% Ni, 0 - 7% Mo, 0 - 16% Mn, o - 1.5% N, the rest iron , carbon and impurities, whereby the content of Mn + N or Ni + Mo or Ni + Mn + N is such that the material exhibits a stable and completely austenitic structure. Also when using these materials, the presence of a smaller amount of oxygen in the medium is necessary. Another series of tests has shown that also steel types with 25% Cr, 1 - 6% Ni and 1 - 3% Mo, which have a ferrite-austenitic structure, can be used as construction material in plants for urea production.

In all the materials mentioned above, a content of austenite-forming elements, such as nickel and nitrogen, is necessary to maintain a complete austenitic structure or a ferrite-austenitic double structure. For such structures, the impact strength at room temperature is usually sufficient to be able to use the steel and mechanically process it. If these austenite-forming elements do not occur in sufficient quantity, then the structure is completely ferritic, which means that the material is so brittle at room temperature that shaping is almost only possible by stopping. Such nickel-free materials practically cannot be welded either. However, nickel is expensive and, moreover, a so-called strategic material, so that the supply and thus also the price of nickel-containing alloys fluctuates greatly. The offer may expire, whereby this material cannot or is very difficult to obtain. A further difficulty is that nickel makes the alloy sensitive to media that contain some sulfur compounds, whereby nickel forms sulphide.

The addition of oxygen to the medium is unconditional in the method described above, except for the method where steel with 25% Cr, 1 - 6% Ni and 1 - 3% Mo is used. With this method, this precaution becomes optional, but still recommended as it results in a noticeable reduction in corrosion. The addition of oxygen is usually done by mixing a certain amount of air into the carbon dioxide before or in the intermediate stage of the CO2~compression. This requires an increase in the compression capacity. Only a portion of the oxygen is used to passivate the construction material. The remaining part is found together with the rest of the air as a gas in the synthesis solution formed in the synthesis reactor, and these inert components finally accumulate with the other and with the reaction components co-fortified inert components, such as hydrogen and nitrogen, in the head of the reactor, from where they are continuously released out.

The presence of these inert gases limits the effective reactor volume, so that the reactor must be constructed larger. The washing column, in which the ammonia and carbon dioxide supplied by the inert gases are recovered, must likewise have a larger capacity than in the case where air is not added. Furthermore, when washing this gas mixture, which therefore mainly contains nitrogen, hydrogen, oxygen, ammonia and carbon dioxide, precautions must be taken to avoid explosion.

The inventor has now found that nickel-free and completely ferritic steels with a high chromium content can be used as a construction material in devices for the production of urea, provided that the carbon and nitrogen content is very low.

Ferritic chromium steels are previously known from the US patents Nos. 2,183,715 and 2,624,671. Thus, 18-8-2 CrNiMo steel has been used in the production of urea, and when using these steels, some oxygen is added to the fresh carbon dioxide.

Absence of Ni precludes the formation of Ni sulphides. One can therefore ignore the sensitivity to sulfur compounds. Despite the steel's completely ferritic structure, and in contrast to the usual chrome steels of commercial quality, which exhibit a low impact toughness, these steels exhibit a high impact toughness that enables mechanical processing and shaping. At the very low carbon and nitrogen contents, the transition temperature, i.e. the temperature at which the material changes from being tough to brittle, drops to values below 0°C, so that there are no difficulties when using and processing the material. And so welding these materials takes place without difficulty. The coefficient of expansion is significantly smaller than that of the usual materials CrNiMo 18-12-2 1/2, and the thermal conductivity is significantly greater.

According to the invention, ferritic, nickel-free steels containing 25-29% chromium, 0-3% molybdenum, and where the sum of the carbon and nitrogen content is a maximum of 0.035, are now used as construction material in devices for processing carbamate-containing solutions at a higher temperature. Nickel-free alloys are here understood to mean alloys where nickel, when this metal is present, is only present as a contaminant.

In the construction materials used, molybdenum can occur in an amount of 0 - 3%. However, it has been shown that molybdenum in chromium-iron alloys with a Cr content of 29% or more, which are exposed to solutions containing ammonium carbamate, does not give any noticeable increase in corrosion resistance.

In what follows, the invention shall be illustrated by means of results from investigations carried out by the inventor.

A number of materials have been tested, the analyzes of which appear in Table I.

Attempt 1

Sample pieces of the materials listed in Table I are exposed to a medium with the following composition in a so-called batch-filled autoclave, which has a capacity of 1 litre:

446 g of urea

129 g of H 2 O

131 g of NH3

30.5 g of C02

The gross molar ratio NH^/CX^ is therefore 2.78. This composition corresponds to what occurs in a urea synthesis solution, and which is used in urea plants that work according to the stripping method.

The temperature is 185°c and the pressure 120 ato. The agitator and . the inside of the autoclave is lined with polytetrafluoroethylene

(Teflon) to avoid corrosion products from the material for the stirrers and autoclave affecting the experiments.

The experiments were carried out both in the absence and in the presence of oxygen.

In the latter case, 7 g of air was introduced into the autoclave, corresponding to 0.57 volume-% oxygen calculated on the total amount,

i.e. both free and bound CO^.

The results and trial duration are shown in Table II.

From this experiment it appears that materials with a high Cr content and extremely low content of Ni, C and N show no measurable corrosion both with and without oxygen, while the other tested materials can only be kept unaffected by corrosion in the presence of oxygen•

Attempt 2

In a flow-through autoclave, a number of test pieces of the materials 1, 3 and 8 are exposed at a temperature of 165 C for 168 hours to the same composition as indicated in trial 1, whereby the composition was constantly renewed. An analysis showed a contamination of 20 ppm in C02, which corresponds to a content of 3 ppm in the solution.

In addition to the corrosion rates, the potential of these materials was also measured. Ag/AgSO^ electrode was used as a comparison electrode. The results appear in table III.

The results from this trial show that the materials with help

of their measured potentials in a urea synthesis solution can be divided into two groups, namely those with a low absolute potential

(of -70 to -250 mV) and which are passive and which do not or almost do not corrode, and those with high absolute potential (of -660 to -700 mV), and which are active and which. corrodes.

Attempt 3

In the same flow-through autoclave and in a medium whose composition is the same as stated in the previous experiment, a test is carried out at a temperature of 185°c to see if the materials tested in test 2 can be transferred from one state to another by that for a shorter time (approx. 1 minute) an external voltage is applied. The applied voltage was 2 V. It has thus proved possible in this way to transfer CrNiMo 18-12-2 1/2 (material no. 1) from the passive state to the active one. However, it has not been successful to passivate the material by reversing the external voltage. In the attached figure, this is shown schematically and graphically. Curve 1 shows the progress of the potential of the mentioned material in the subsequent stages. In the initial state A^, the material has a potential that lies in the passive potential range I,- and which agrees with the low absolute potentials found in experiment 2. By applying a negative voltage of -2 V, the potential momentarily drops to . After removing the voltage, the potential rises to the value C-^, which lies

in the active potential range II.. This range corresponds to the group of high absolute potentials, and which was measured in experiment 2. By finally applying a positive voltage of +2 V, the potential of D1 rises above the passive potential range I.

After removing this voltage, the potential assumes the value E1, which lies in the active potential range. After applying a positive as well as a negative external voltage, the material thus assumes an active potential.

CrNiMo 25-25-2 (material No. 3) can be activated if desired

or passivated in this way (see curve 2). After temporarily applying a negative voltage, the material can be activated:

point C^. If a voltage with the opposite sign is applied then

the material assumes a passive potential E^ for a certain time after the tension has been removed.

CrMo 28-2 (material No. 8), on the other hand, cannot be continuously added in the active state (curve 3): after removing the applied negative voltage, the material spontaneously again assumes a passive potential (E^).

This experiment shows that CrNiMo 18-12-2 1/2 under the specified conditions exhibits a single stable potential in a urea synthesis solution, and this potential lies in the active potential range. CrNiMo 25-25-2 thus has two stable potentials: one in the active potential range and one in the passive potential range, and CrMo 28-2 a single stable potential, namely that in the passive potential range.

Attempt 4

In the same flow-through autoclave, which was used in tests 2 and 3, some test pieces of materials 3, 4 and 8 are exposed for 40 hours to a medium which has the same composition as that which was; used in the previous experiments. However, the temperature is higher, namely 195°C. CX^ contains only 7 ppm O^ as a contaminant. After the test, the corrosion rates specified in Table IV are measured.

The experiments described show that the fully ferritic chromium steels investigated and with very low carbon and nitrogen content are highly corrosion resistant in urea synthesis solutions also at higher temperatures and in the presence of extremely small amounts of oxygen.

Claims (1)

■ Use of ferritic nickel-free steels containing 25-29% chromium, 0-3% molybdenum, and where the sum of the carbon and nitrogen content is a maximum of 0.035%, as construction material in devices for processing carbamate-containing readings at a higher temperature.