FR2480280A1 - IMPROVEMENTS TO PROCESSES FOR SYNTHESIS OF UREA - Google Patents

Abstract

SINCE THE IMPLEMENTATION OF A PROCESS FOR THE SYNTHESIS OF UREA CAUSES EXCESSIVE CORROSION OF THE EQUIPMENT BY TREATMENT FLUIDS, IN ACCORDANCE WITH THE INVENTION, EQUIPMENT COATED WITH MATERIALS OF THE TREATMENT IS USED. SILICON CARBIDE TYPE, HIGHLY RESISTANT TO CORROSION AND EROSION CAUSED BY CAVITATION PHENOMENA, IN LOCATIONS WHERE CAVITATION PRECISELY OCCURS, SUCH AS BETWEEN THE INTERIOR SURFACE 6 OF THE RING SEAT 5 AND THE SHUTTER POINT 4 OF A PRESSURE REGULATOR OR CIRCULATES A FLUID AT HIGH SPEED, AT HIGH PRESSURE AND AT A HIGH TEMPERATURE.

Description

"Improvements to processes for the synthesis of urea".

  The invention relates to improvements to the methods for implementing which equipment is used.

  comprising materials of the silicon carbide type.

  Corrosion of building materials in a urea synthesis facility is a serious problem

  since the implementation of a process for the syn-

  urea thesis involves the manipulation of a highly corrosive treatment fluid which is brought to a

  high temperature and high pressure. In particular

  bind, a pressure regulator requires the use of a special material, the valve being located between a synthesis tower (into which a treatment fluid is introduced under

  a pressure of 150 to 250 atmospheres for the synthesis of a

  ) and a decomposition vessel in which the ammonia and carbon dioxide which have not been separated and recovered

  not reacted. The reason why it is necessary to use

  reading a special material is that you have to bring in the app-

  similar a treatment fluid at high temperature and under high pressure, and highly corrosive, the pressure

  said fluid being lowered by the pressure regulator and out

  so much then at great speed. Excessive damage, due to chemical and physical causes, which is

  due to corrosion and erosion caused by cavita-

  tion, therefore appear in portions of the pressure regulator, which makes the useful life of such an arrangement very short when it is constructed using ordinary material. In a conventional manner, an alloy of the type commonly

  referred to as "stainless steel", or zirconium or an alloy

  zirconium, were the materials used for

  establish the equipment used to implement a process

dice for urea synthesis. -

  For example, an austenitic stainless steel, when we

  uses it as a material to build processing equipment

  raises a problem because its insufficient resistance

  corrosion health causes material to dissolve as a result of corrosion during use; it results

  premature pressure regulator failure.

  On the other hand, attempts have been made to use zirconium and a zirconium alloy to effectively carry out the synthesis of urea because these materials have a high resistance to

  corrosion in the processing fluids for this synthesis

  se. But the hardness of zirconium and a zirconia alloy

  nium is weak; for example, the hardness of zirconium at a degree of purity of 99.5% is only 150 degrees Brinell,

  i.e. roughly the same as that of mild steel.

  Zircoriuii and an El13iage of zirconium are therefore vulnerable to

  bles to a physical attack and, for example, in an ins-

  tallation for the synthesis of urea where is arranged a

  zirconium pressure regulator, this device is quickly

  damaged by erosion due to cavitation during the circulation of ordinary water or ammoniacal water before the production of urea, which results in an increasingly defective operation of said pressure regulator. Recently, in order to resolve this problem, recourse was first made to

  a pressure regulator made of a physically resistant material

  as such as stainless steel during the operation of circulating ordinary water or ammoniacal water,

  after which we replace this device with another pressure gauge

  made from a material highly resistant to corrosion

  ion, such as zirconium, before switching on the o

  peration of urea production. As mentioned above, the technique currently used consists in using a pressure regulator made of a first material and which is replaced by a pressure regulator constructed of another material chosen according to the type of fluid flowing at

  across the device, but this way of operating makes it natural

  The operation of an installation for the synthesis of urea is really inconvenient and ineffective and, at the same time, reveals the possibility of unexpected damage caused by the variation in operating conditions as well as a safety problem of an installation. What to do

  operate at high temperature and under high pressure

if we.

  In summary, the object of the invention is to achieve a pro-

  yielded for the synthesis of urea which makes it possible to prolong long-

  time the synthesis operation with high stability whatever

  whatever the operating conditions, by solving the above-mentioned problems of erosion (due to cavitation) and

  corrosion observed during the implementation of a pro-

  yielded from urea synthesis using silicon carbide in portions of the apparatus liable to undergo

damage.

  The single figure of the attached drawing shows a diagram

  tically, in section, a device usable during mid-

  use of a method according to the invention.

  An embodiment is described below in detail.

of a method according to the invention.

  As mentioned above, there is a strong tendency, when implementing a process for the synthesis of urea, to erosion (due to cavitation) and to

  excessive corrosion in equipment such as mano

  regulator. To prevent such damaging phenomena

  They intervene, it is necessary to use materials

  Rials with high corrosion resistance and high wear resistance in equipment susceptible to such corrosion and erosion as a result of cavitation, as mentioned above. Silicon carbide is a material that meets these requirements. Materials of the silicon carbide type comprise either a material consisting solely of silicon carbide (simple silicon carbide), or a coated material in which a base substance (chosen from metals,

  ceramic or graphite) is coated with car-

  silicon bure by using a technique of forma-

  tion of coating such as spraying and spraying into a plasma, the chemical formation of a deposit from a vapor, or the physical formation of a deposit from a vapor. A material of the silicon carbide type having a high density (specific gravity not less than 2.9) is

  preferable to other materials of the silicon carbide type.

  An embodiment of the present invention is described below, with reference to the accompanying drawing. In this embodiment, a material of the carbide type is used in the pressure regulator. The single figure of the drawing schematically shows, in section, a pressure regulator comprising a valve body 1, a fluid inlet orifice 2, a needle type shutter 3, and an annular seat 5. A

  treatment fluid at high temperature and under high pressure

  sion enters the velve body 1 through the inlet 2 and flows through the space between the

  tip 4 of the shutter 3 and the upper edge of the surface

  this interior of the annular seat 5 of the valve, the pressure of the treatment fluid then being lowered. The rate of pressure reduction is controlled and regulated by a

  vertical displacement, in one direction or the other, of the ob-

  valve actuator 3 serving as a pressure regulator. In operation

  effective operation, the tip 4 of the shutter and the interior surface 6 are exposed to the treatment fluid circulating at high speed and are therefore subject to

  excessive corrosion and erosion due to

  vitation. It is therefore necessary to use, for consti-

  kill these portions of the pressure regulator, materials with high corrosion resistance and high resistance

  to wear. In the embodiments of the present in-

  Please note, we use a material such as silicon carbide.

cium.

  Although, in the example above, only one is described

  pressure regulator, you should not lose sight of the fact that

  rials of the silicon carbide type can also be used in any portion of equipment of the kind in question which

  is exposed to a highly corrosive fluid in installations

  for the synthesis of urea, and that such materials

  rials are especially effective in areas where

  circulates a fluid at high speed. For example, the use of

  tion of materials such as silicon carbide is very effective

  cient and interesting in valves arranged in cana-

recycling rations.

  As regards the present invention, methods in which materials of the silicon carbide type are used are the conventional methods for the synthesis of urea. Thanks to the implementation of the present invention, the use of silicon carbide highly resistant to corrosion and resistant to wear in installations where the synthesis of urea is carried out has made it possible to eliminate the need for replace valves that are damaged due to corrosion, or erosion caused by cavitation, during operation, or the need for

  change valves to replace them with standard valves

  blies in different materials depending on the type of treatment fluid used, these necessities being imposed due to the lack of suitable materials. The use of silicon carbide has also made it possible to avoid damage caused by variations in operating conditions, while improving safety and stability during the operation of a facility where urea synthesis takes place. high temperatures and high pressures, the efficiency of

  the installation being improved at the same time.

Claims (4)

  1. Process for the synthesis of urea during the implementation of which materials of the silicon carbide type are used to constitute the equipment, which process is characterized in that said materials of the silicon carbide type in parts of the equipment susceptible to corrosion and erosion caused by cavitation phenomena.   2. Method according to claim 1, characterized in that one uses a material only consisting of silicon carbide, that is to say in the form of a simple substance. 3. Method according to claim 1, characterized in that silicon carbide is used in the form of a coating material.   4. Method according to any one of the claims   above, characterized in that said materials of the silicon carbide type are used in arrangements of the pressure regulator type.