MXPA01004293A - Process and converter for the preparation of ammonia - Google Patents

Abstract

Process for the preparation of ammonia comprising steps of contacting an ammonia synthesis gas with an ammonia synthesis catalyst arranged as reaction zone in one or more catalyst tubes;cooling the reaction zone by heat conducting relationship with a cooling agent;and withdrawing an ammonia rich effluent stream from the reaction zone. The converter comprises at least one catalyst tube adapted to receive ammonia synthesis gas and to hold a reaction zone of ammonia synthesis catalyst and a cooling agent at the shell side of the catalyst tubes.

Description

PROCEDURE AND CUSTOMER FOR THE PREPARATION OF AMMONIA DESCRIPTIVE MEMORY The present invention relates to the preparation of ammonia by catalytic conversion of ammonia synthesis gas. More particularly, this invention relates to the synthesis of ammonia at high conversion rates of ammonia synthesis gas in the presence of an ammonia synthesis catalyst arranged in a tubular reaction zone cooled by a cooling agent on the side covered with ammonia. the tubular reaction zone. The synthesis of ammonia from synthesis gas of hydrogen and nitrogen is an exothermic process and the process requires cooling to obtain high conversion rates. Even if the concentration of hydrogen and nitrogen in the synthesis gas is close to the stoichiometric composition for ammonia formation, a complete reaction to ammonia can not be obtained by a simple passage of the synthesis gas through the catalyst bed. In addition, due to the exothermic nature of the ammonia synthesis, the increasing temperature during the passage through the catalytic bed displaces the equilibrium concentration towards a lower concentration of ammonia. Various methods are known for cooling the ammonia synthesis process.

The usual methods for the preparation of ammonia from synthesis gas use either an indirect or direct cooling of the synthesis gas between a number of catalytic beds, in which the synthesis of ammonia passes over an ammonia synthesis catalyst. By direct cooling, cold synthesis gas is introduced into a partially reacted synthesis gas between the beds. The disadvantage of this cooling method is the dilution of the partially reacted gas with unreacted gas resulting in a lower concentration of ammonia in the product stream of the process. By the indirect cooling method, partially reacted synthesis gas is cooled with cold gas, usually fresh synthesis gas in a heat exchanger arranged between the outlet and the inlet of two catalytic beds. It has now been observed that the conversion rate of the synthesis gas from ammonia to ammonia is greatly improved when the synthesis gas is cooled while advancing through a catalytic bed of ammonia synthesis catalyst by means of heat transfer to an ammonia synthesis agent. cooling that is in continuous thermal contact with the procedure. Accordingly, this invention provides a process for the preparation of ammonia comprising the steps of: Contacting an ammonia synthesis gas with an ammonia synthesis catalyst arranged as a reaction zone in one or more catalyst tubes; Cool the reaction zone continuously by transferring heat from the reaction zone to a cooling agent; Remove the ammonia-rich effluent stream from the reaction zone. In its most general embodiment, the above procedure is performed in a converter with one or more catalytic tubes arranged in a shell to retain the cooling agent. The synthesis gas is introduced into the upper part of the catalytic tube and passed through the reaction zone of an ammonia synthesis catalyst. The heat that is revealed during the conversion of hydrogen and nitrogen contained in the synthesis gas to ammonia is continuously transferred through the wall of the catalytic tube to the cooling medium surrounding the tube. By means of continuous cooling of the process, an increase in adiabatic temperature is substantially avoided, so that the process is carried out under substantially isothermal conditions. The isothermal conversion of the synthesis gas results in higher gas conversion rates to ammonia than in the known ammonia synthesis processes with indirect or direct cooling of partially reacted synthesis gas., wherein the cooled gas is contacted with the catalyst under adiabatic conditions. Having removed the heat of reaction from the reaction zone, the cooling medium is continuously or periodically removed from the converter and externally cooled, for example, by heat exchange with water or steam and recirculated to the converter by conventional means. In a specific embodiment of the invention, the cooling agent is retained in a space formed by an outer wall of the catalyst tube and an inner wall of a cooling tube concentrically surrounding the catalyst tube. As a favorable feature of the latter embodiment, the shell of a reactor with a number of catalytic tubes can be avoided or made of a material with considerably less mechanical force than in conventional ammonia converters. Preferably, the cooling tubes surrounding the catalyst tubes are designed with less mechanical force than the catalyst tube. In the case of rupture of a catalyst tube, the reaction gas escaping at a high pressure in the cooling tubes is vented to a space outside the cooling tube. Therefore, the synthesis gas is depressurized out of the cooling tubes and harmful reactions of the gas with the cooling agent are favorably avoided. Another object of the invention is to provide a converter for the preparation of ammonia by reaction of ammonia synthesis gas in the presence of an ammonia synthesis catalyst and by cooling the reaction while continuing its course through the synthesis catalyst, the converter comprises at least one catalyst tube adapted to receive the ammonia synthesis gas and to maintain a reaction zone with the ammonia synthesis catalyst, with at least one catalyst tube arranged in a container with a cooling agent, as schematically shown in the attached figure 1. The cooling medium useful as a cooling agent in the above process and reactor will be any solid or liquid having a melting or boiling point below the desired temperature in the reaction zone, including salt or mixture of salts, metals or liquids. that are inert under the actual conditions of the procedure. Such cooling agents include eutectic mixtures of salts such as KN03, NaN03 and NaN02 (supplied by Degussa) and eutectic mixtures of NaOH and KOH. Other mixtures of eutectic salts and cooling liquids are well known in the chemical industry. The usual temperature condition in the above procedure will be between 300 ° C and 600 ° C. The temperature of the cooling agent must be maintained at a predetermined level within the operating temperature range by external cooling of the agent as already mentioned herein. The removal of ammonia from the ammonia-rich product gas that is removed from the catalyst tubes is furthermore an embodiment of the invention obtained through adsorption in an adsorbent having a high affinity to ammonia at high pressures, said regeneration of the spent adsorbent. it is carried out by means of the depressurization of the adsorbent and recovery of the ammonia-rich gas similar to the separation of, for example, oxygen or nitrogen in the known oscillating pressure adsorption processes. In addition, the ammonia can be separated from the unconverted synthesis gas by cooling and condensing the ammonia in the ammonia-rich effluent stream of the process. The unreacted synthesis gas that is separated in the ammonia in the product gas can then be recirculated to the catalyst tube or passed to a subsequent catalyst tube for further conversion, as schematically shown in Figure 2 and Figure 3.

EXAMPLE In a specific embodiment of the present invention, a synthesis feed gas at a pressure of 13.8 MPa is preheated to 350 ° C and introduced into a reactor equipped with 600 reactor tubes with an internal diameter of 80.1 mm. The tubes were loaded with an upper portion of iron ammonia catalyst and a smaller portion of ruthenium ammonia catalyst. The synthesis gas is distributed in the tubes and reacted on the ammonia catalyst. The catalyst tubes are surrounded by a shell. In the space between the shell and the tubes, a salt melt is circulated in countercurrent with the direction of gas flow within the tubes and in a heat conducting relationship with the synthesis. The circulation of the salt melt serves to remove the heat generated by the exothermic reaction of ammonia synthesis. The salt melt is introduced at 360 ° C into the cooling space and leaves the reactor at 420 ° C. The hot melt is cooled inside the reactor at 360 ° C in a heat exchanger, in which the heat extracted from the salt melt is used to preheat the synthesis gas. The cooled salt melt is pumped back to the reactor. Having passed through the reacted synthesis gas of the catalyst, rich in ammonia, it leaves the tubes and is removed from the reactor. The gas is cooled by heat exchange with fresh synthesis gas. Next, Table 1 lists the concentrations of the components in the gas stream entering and leaving the reactor as obtained in the previous experiment.

TABLE 1 The process of the invention can be used in a direct ammonia synthesis section as well as in a conventional type ammonia synthesis loop section or in combination with other similar types of ammonia converters in more advanced ammonia synthesis loop sections, for example, that they comprise feed gas converters and / or purge gas converters. The ammonia product must be recovered from the ammonia-rich product gas in the synthesis section by cooling and condensing the ammonia in the ammonia-rich stream or by absorption. The removal of the ammonia can be carried out in one or more stages, between and / or after each of the reaction zones.

Claims (8)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the preparation of ammonia comprising the steps of contacting an ammonia synthesis gas with an ammonia synthesis catalyst arranged as a reaction zone in one or more catalytic tubes; cooling the reaction zone by a heat conducting relationship with a cooling agent; and removing an ammonia-rich effluent stream from the reaction zone; characterized in that the cooling agent is selected from salts, mixtures of salts and metals having a melting point below the temperature in the reaction zone.

2. The process according to claim 1, further characterized in that the ammonia synthesis gas is brought into contact with the ammonia synthesis gas arranged in two or more reaction zones with an intermediate removal of an effluent stream rich in ammonia. ammonia between the reaction zones.

3. The process according to claim 1, further characterized in that the ammonia-rich effluent stream is separated into an unconverted ammonia synthesis gas stream and a product stream of ammonia, the unconverted ammonia synthesis gas It is recirculated to the reaction zone.

4. The process according to claim 2 and 3, further characterized in that the separation is obtained by cooling the effluent stream and condensing the ammonia.

5. The process according to claims 2 and 3, further characterized in that the separation is obtained by absorption of the ammonia contained in the effluent stream.

6. The method according to claim 1, further characterized in that the cooling agent is circulated inside cooling tubes, each concentrically surrounding a catalyst tube.

7. A converter for the preparation of ammonia comprising at least one catalyst tube adapted to receive ammonia synthesis gas and to maintain a reaction zone of ammonia synthesis catalyst; and at least one cooling tube concentrically surrounding at least one catalyst tube and adapting to keep the cooling agent selected from salts, mixture of salts and metals with a melting point below the temperature in the reaction zone.

8. The converter according to claim 7, further characterized in that the wall of the cooling tube or tubes has a mechanical force less than that of the wall of the tube or tubes catalysts. _ L