DE2604054A1 - Removal of hydrogen in carbon dioxide gases for urea synthesis - by leading the impure gas with oxygen over precious metal catalyst - Google Patents

Abstract

carbon dioxide gases used as starting material in the urea synthesis from NH3 and CO2, are mixed with O2 in the presence of a catalyst of Ru,Rh,Pd and/or Pt to react the H2-gas to form H2O. The treated CO2-gas may then be re-used as starting material in the synthesis. The process removed H2 from the impure gas and obviates the danger of explosions without undue influence on the urea synthesis.

Description

SUMITOMO CHEMICAL COMPANY, LTD.,
Osaka, Japan

¹¹ Process for the production of gases containing carbon dioxide for urea synthesis "

Priority: February 4, 1975, Japan, No. 15 086/75

A gas containing carbon dioxide is hereinafter referred to as a gas which, in addition to carbon dioxide, contains small amounts of hydrogen, methane, Contains carbon monoxide, nitrogen and / or water vapor.

In the industrial production of urea, liquefied ammonia and gaseous carbon dioxide are generally used as starting materials. Under normal urea synthesis conditions (150-210 ^° C.; 150-250 kg / cm ² pressure) ammonia is reacted with carbon dioxide at elevated temperatures and pressures. Most of the ammonia and carbon dioxide are converted into urea and water. However, some of them remain unimplemented. The urea obtained is separated off as an aqueous solution which, after treatment in flash and rectification zones, has essentially no free ammonia and

609852/0603

Contains carbon dioxide. By concentrating, crystallizing, drying and optionally granulating the urea processed into the end product.

On the other hand, the unreacted ammonia and carbon dioxide, which have been separated from the aqueous urea solution are passed through absorption or rectification zones to the conditions of urea synthesis described above set and then in the urea synthesis zone together with fresh liquid ammonia and gaseous carbon dioxide fed in again.

The raw materials are generally recovered and used according to the following processes:

In one of the processes, all of the carbon dioxide in the urea synthesis zone is converted into urea and, if so Ammonia is fed into this zone in excess, the entire excess, which is theoretically above that for urea formation required amount is recovered as liquid ammonia and then again in the urea synthesis zone fed in. Another process uses the unreacted ammonia for other processes, for example for the production of ammonium sulfate or ammonium nitrate, with no recovery at least in the urea synthesis zone all or part of the unreacted ammonia he follows,

609852/0603

In both cases, ammonia and carbon dioxide are used as starting materials, at least the ammonia is practically completely implemented.

The liquid ammonia used as starting material for urea synthesis is generally made from an animoniaksynthesesystera obtained, as well as the by-product accumulating and separated gaseous carbon dioxide, which is in the urea synthesis system is fed in. This gaseous carbon dioxide generally contains small amounts of hydrogen, oxygen, nitrogen, Methane, carbon monoxide or argon as impurities. An example of the composition of the impurities of a Carbon dioxide-containing gas produced as a by-product in an ammonia synthesis system is shown in Table I. specified.

Table I.

fr * 3 it.

Component Nnr / 1 000 Nnr content,

Carbon dioxide contain-% by volume of the gas

Hydrogen 3-12 0.3-1.2

Oxygen 1-4 0.1-0.4

Nitrogen 3-12 0.3-1.2

further connections traces traces

Annotation:

* Normal conditions at 20 ^° C. and 1 atmosphere.

In the urea synthesis system, in the ammonia and carbon dioxide almost completely converted into urea, the carbon dioxide-containing gas with the above impurities

609852/0603

is used as a starting material, these impurities are excreted as a gas with an approximate composition as indicated in Table I. At least as far as ammonia is concerned, these impurities are essentially after fully recovering ammonia as an outlet end gas with a composition ratio excreted, which is close to the ratio given in Table I, even if unreacted ammonia is treated outside of the urea synthesis system. The composition given in Table I corresponds to that of an explosive Gas mixture. Therefore there is a possibility that a gas mixture of similar composition an explosive gas mixture in the urea synthesis system or in those mentioned above other systems.

The treatment of such an explosive gas mixture that is formed in the urea synthesis system is, for example, in • the JA-OS 66/1972 described. Thereby the composition of the exiting gas by diluting the gas shortly before the gas outlet, i.e. in the recovery stage of the unreacted Ammonia from the gas, outside the flammable limit range held.

As another method, a method in which an inert gas such as nitrogen is used as the starting materials has been considered or is added directly to the urea synthesis system. Included the composition of the circulating gas in the system is kept outside the ignition limit range. In order to

6098 5 2/0603 - ¹

to put the composition of the gas outside the ignition limits, In this case, the content of gaseous oxygen must be less than 5 percent by volume or the content of gaseous Hydrogen can be reduced to less than 4 percent by volume. Therefore, a large amount of an inert gas is required. the Feeding such a large amount of an inert gas into the urea synthesis system lowers the yield of the urea and causes various disadvantages such as an increase in the amount of circulating unreacted gases, Apparatus and electricity costs and, in addition, an increase in apparatus and handling costs to remove the ammonia before the escaping gas is eliminated.

Another method that can be considered is a method in which the free oxygen contained in the carbon dioxide-containing gas is removed will. However, this method loses its importance when oxygen is used to protect the urea synthesis device is fed against corrosion. In this case, the oxygen that goes into the urea synthesis system protects the urea Synthesis against corrosion protection fed in v / ird, last mixed with inert and impure components that are contained in the starting product and escapes as leaking ' Gas. The gaseous oxygen serves to reduce the concentration of the gaseous hydrogen to a certain extent, which is contained in the carbon dioxide-containing gas used. In connection with an increase in the oxygen content however, the risk of the formation of explosive gas mixtures is not excluded at all.

^L 609852/0603 ^J.

The invention is based on the object of making the formation more explosive Gas mixtures in the treatment zone of the unreacted gas to prevent.

Another object of the invention is to achieve the above object without any significant obstruction to urea induction to solve.

These objects are achieved by the invention.

The invention thus relates to that characterized in the claims Object.

When removing hydrogen from a carbon dioxide-containing gas, which is used as a starting material for the synthesis of urea, in the process according to the invention a carbon dioxide-containing gas is fed into the urea synthesis zone with a catalyst system composed of at least one metal from the platinum group, such as ruthenium, rhodium, palladium or platinum, reacted at temperatures from 0 ° to 300 ⁰ C in the presence of sufficient oxygen in contact oxidize contained in the carbon dioxide-containing gas to hydrogen to water ·.

According to the process according to the invention, this is used as the starting material used carbon dioxide-containing gas mixed with oxygen before feeding the gas into the urea synthesis zone. In the process according to the invention, the implementation is running,

^L 609852/0603 ^J.

which by the general equation

H ₂ + 1/2 O ₂ - ^ H ₂ O

is represented, essentially stoichiometrically, so that

of the added amount of oxygen depending on the amount / in the carbon dioxide Gas containing Viasserstoffs and after the desired removal percentage of the hydrogen is determined. Therefore, the reaction of the hydrogen with oxygen is carried out with an at least stoichiometric amount of oxygen.

When the oxygen in the carbon dioxide-containing gas is in stoichiometric Amount is available at the beginning, an additional supply of oxygen is not necessary. However, oxygen lies generally not available in sufficient quantities. Therefore, free oxygen must also be fed in. If free Oxygen is added to protect the urea synthesis system against corrosion, the necessary amount of free oxygen can together with that used in the method according to the invention Oxygen can be added.

Air is usually used as the source of oxygen. It can but also pure oxygen diluted with an inert gas can be used.

The carbon dioxide-containing gas with a predetermined content of oxygen is sent through a catalyst layer, which is provided with a noble metal catalyst. The reaction pressure can be freely in a range from normal pressure to the pressure

609852/0603 ^J.

which is required for urea synthesis can be selected. The process according to the invention can be used in any process stage of the compression of the carbon dioxide before the gas is fed into the urea synthesis system. The choice of temperature is also essentially free. In general, however, a temperature of from 0 to 30O ⁰ C gives a satisfactory result. In the process according to the invention, however, treatment temperatures of 0 to 160 ° C. are preferred in view of the temperature of the carbon dioxide-containing gas required for synthesis of the urea. The space velocity is usually 1,000 to 1,000,000, preferably 10,000 to 700,000 hours "" ¹ .

The noble metal catalyst used in the process according to the invention tor contains at least one metal of the platinum group, such as ruthenium, rhodium, palladium or platinum, with palladium and platinum are particularly preferred. The noble metal is on a carrier, preferably on aluminum oxide or silicon dioxide, upset. The preparation of the catalyst can be carried out by processes known per se. For example the above-mentioned carrier is impregnated with an aqueous solution of chlorides of the platinum group metals, getrock- ■ net and optionally treated with a reducing gas.

As the device containing the catalyst, any device that can be used for this purpose nowadays can be used. About the lifespan and to maintain the activity of the catalyst over a longer period of time, vapors from the carbon dioxide-containing gas are intended

609852/0603 -J

removed. However, saturated water vapor has little or no adverse effects.

According to the method according to the invention can essentially total hydrogen contained in the carbon dioxide, as a starting material for the synthesis of urea serving gas .ist, are converted into water. This can make the formation more explosive Gas mixtures without adverse effects on the synthesis of urea can be prevented.

The examples illustrate the invention.

Example i

15.0 Nm / h a carbon dioxide-containing gas containing 5500 ppm hydrogen, 1500 ppm oxygen and 3500 ppm nitrogen as Contains impurities and was produced and separated as a by-product in an ammonia synthesis plant with 0.09 Nm-ystd. Air mixed. The gas mixture is in fed into a reaction zone, which is provided with a catalyst (1.5 liters total volume), and under atmospheric

-1 pressure at a Raura speed of 10,000 hours. And at temperatures of 50, 100 and 150 ⁰ C implemented. The results are summarized in Table II. The catalyst used to convert the hydrogen with oxygen is produced in the following way:

0.5 percent by weight of a metal of the platinum group (palladium, platinum, rhodium or ruthenium), based on aluminum dioxide

^L 609852/0603 ^J.

(Carrier), are applied to the carrier and shaped into balls with a diameter of 5 mm.

The fourth in Table II relates to the percent hydrogen removal. The percentage distance is calculated from the remaining amount of hydrogen in the carbon dioxide-containing gas after passing through the catalyst layer.

Table II

I """""" - ■ —— temperature
Catalyst ■ - ■ —__ 50 ⁰ C 100 ⁰ C 150 ⁰ C palladium
platinum
Rhodium
Ruthenium 100 %
100 % 95 %
42 % 100 %
98 %

Example 2

With the carbon dioxide-containing gas used according to Example 1 and containing hydrogen, and with that described in Example 1 Mixing ratio of the gas containing carbon dioxide with air the reaction is carried out with palladium as a catalyst and a space velocity of 50,000, 100,000 and 200,000 hours, which was adjusted by using different amounts of the catalyst used in the catalyst layer. The percent hydrogen removal is summarized in Table III.

609852/0603

- 11 Table III

~~~~~~~~ --—- ^. .Temp era door
Raumges chwin ^^ ~~~~ ^ -— ~ ^ ___ ^^
di ^ ity "~ - ~ ^ ----_ 25 ⁰ C 50 ⁰ C 50,000 hours " ¹
100,000 hours " ¹
200,000 hours " ¹ 100 %
98 %
21 % 100 %
100 %
92 %

Example 3

According to Example 2, the carbon dioxide-containing gas is with a palladium catalyst ^{rail at 25 0} C and with a space velocity of 100,000'h. treated. The carbon dioxide-containing gas treated in this way is continuously in proportions of 15 Nm / h. compressed and fed into an autoclave. Furthermore, in the autoclave 35.2 kg / h. liquid ammonia and 6.6 kg / h Water that comes from other sources is fed in. The reaction in the autoclave is carried out at 195 ° C. and a pressure of 230 kg / cm. g performed. The pressure of the product emerging from the autoclave is continuously increased to 22 kg / cm. g decreased. The product is then divided into 2 parts, one containing water vapor, ammonia and carbon dioxide in the gaseous state, the other water, urea, ammonia and ammonium carbam.t in the liquid state. The hydrogen content of the two parts is measured. No hydrogen content is found in the liquid part, while only traces of hydrogen are found in the gaseous part.

6098 5 2/0603

The reaction is repeated, but no catalytic treatment of the carbon dioxide-containing gas is carried out will. No hydrogen is found in the liquid part, while 2200 ppm hydrogen is found in the gaseous part to be established ... .

6098B2 / 0603

Claims (11)

Claims 1. Process for the production of gases containing carbon dioxide as a starting material for the synthesis of urea from ammonia and carbon dioxide, characterized in that a
that one brings / gaseous hydrogen-containing kohlendioxidhalti sat gas with oxygen or free oxygen-containing gases in the presence of a ruthenium, rhodium, palladium and / or platinum-containing catalyst system and converts the hydrogen contained in the carbon dioxide-containing gas into what water. 2. The method according to claim 1, characterized in that one carries out the reaction of the hydrogen with oxygen or the gas containing free oxygen at temperatures of up to 300 ⁰ C D. 3. The method according to claim 2, characterized in that one carries out the reaction at temperatures from 0 to 160 ⁰ C. 4. The method according to claim 1, characterized in that there is a palladium and / or platinum-containing catalyst system begins. 5. The method according to claim 1, characterized in that the catalyst system used is ruthenium, rhodium, palladium and / or platinum on a support. 609852/0603 6. The method according to claim 5, characterized in that aluminum oxide or silicon dioxide is used as the carrier. 7. The method according to claim 5, characterized in that as the catalyst system palladium-on-aluminum oxide or -Silicon dioxide is used. 8. The method according to claim 5, characterized in that the catalyst system used is platinum on aluminum oxide or
-Silicon dioxide is used. 9. The method according to claim 1, characterized in that the reaction of the hydrogen with at least one
carries out stoichiometric amount of molecular oxygen. 10. Verfahrjen according to claim 1, characterized in that a gas with 0.3 to 1.2 volume percent hydrogen and 0.1 to 0.4 volume percent oxygen is used as the gas containing carbon dioxide and 0.3 to 1.2 volume percent nitrogen is used. 11. The method according to claim 1, characterized in that the carbon dioxide-containing gas before the reaction of the hydrogen freed from moisture with molecular oxygen. 609852/0603 ^J.