RO114589B1 - Process for producing ammonia - Google Patents

Abstract

The invention relates to a process for the production of ammonia through the primary conversion of a gas mixture natural and steam. Unconverted natural gas is submits the final conversion to a reformer usually secondary, using air with a content of 22 ... 32% oxygen. After recovery heat from the primary conversion phases and secondary conversion of carbon dioxide to carbon dioxide as well as after the phase the proper synthesis of ammonia, steam previously turbulent energy synthesis gas compressor, trains a turbine for the air compressor process, concomitantly and with the training of a generator that produces electric power for consumers in the plant. It is taken in parallel, from the compressor drive turbine air, a steam outlet, of which one side represents process steam at the conversion stage primary, and another is the fluid engine in the ejector of a generator steam, fluid obtained by heat recovery from the flue gas, the chimney, finally leading to a steam with a pressure goes 8 ... 16 at and o temperature of 300 ° C. in the case of a source additional hydrogen, outside the source hydrogen from primary and final conversion, the final conversion is done in the presence of a amounts of oxygen with normal oxygen content, at a volumetric nitrogen: hydrogen ratio of 1:

Description

The invention relates to a process for obtaining ammonia, a product used in the chemical fertilizers, amines, dyes industry.

Processes for the production of ammonia from natural gas (methane gas), other hydrocarbons are known, comprising the following technological phases:

- compression of natural gas or other hydrocarbons used at pressures between 20 and 38 at;

- preheating of the raw material due to the heat of the flue gas from the primary reforming (conversion);

- desulfurization of hydrocarbons by retention of sulfur on zinc oxide;

- making the hydrocarbon-steam mixture at a temperature of 36O ... 45O ° C and a pressure of 20 ... 38 at, in a carbon-weight ratio: steam of 1: 3.5 ... 4.0;

- the conversion of the hydrocarbon-steam mixture to the Ni catalyst, in the primary reformer tubes, at temperatures between 760 ² and 82 ° C, the thermal contribution necessary for the conversion, ensuring by burning, with air, a part of the raw material, outside the tubes reform;

- the secondary (final) conversion of the gas remaining unconverted to the primary conversion, carried out on a Ni-based catalyst, by adding compressed and heated air to

4OO ... 62O ° C, based on the heat of the flue gases from the primary reformer, in an amount that ensures the advanced conversion of the raw material and the subsequent technological operations of the synthesis gas consisting of hydrogen and nitrogen, in volumetric ratio of 3: 1; from the secondary reformer the resulting gases come out with a temperature of 940 ... 10OO ° C;

- recovery of the heat of the converted gas, by cooling it to 360 ... 38 ° C, obtaining an energy steam of 24 ... 110 at;

- conversion of CO to CO ₂ at high temperatures, 36O ... 42O ° C, on Fe-Cr catalyst;

- conversion of CO to CO ₂ at lower temperatures, 23O ... 26O ° C using copper-based catalyst;

- washing C0 ₂ of the gas converted with potassium carbonate solution, activated with substances specific to each technology;

- methanisation of the traces of CO and C0 ₂ left in the synthesis gas after washing (separation) C0 ₂ ;

- compression of the synthesis gas, resulting after methanisation from 16 ... 35 at, to 1 50 ... 300 at;

- ammonia synthesis at pressures of 160 ... 320 at and temperatures of 46O ... 52O ° C;

- separation by cooling and condensation of ammonia and its storage.

These known procedures have a number of disadvantages:

- high specific energy consumption in which the share of raw materials (natural gas, other hydrocarbons) is 80 ... 98%;

- high working temperatures at the raw material conversion phase (76O ... 82O ° C), achieved through increased fuel agent consumption;

- lower returns on the use of energy steam from installations;

- a weight ratio of steam: carbon (natural gas) of 3.5 ... 4: 1 at the entrance of the mixture of natural gas-steam in the tubes of the primary reformer (primary conversion), which leads to a higher consumption of process steam .

- a lower recovery of the physical heat of the flue gases, which escape into the atmosphere.

RO 114589 Βί

The technical problem, which the invention solves, mainly consists in reducing the specific fuel consumption, by lowering the temperature at the primary conversion, the use of air with increased oxygen content at the final conversion, and the rational use of steam energy.

The process according to the invention removes the mentioned disadvantages, in that 50 is subjected to the primary conversion at a temperature of 65O ... 76O ° C a mixture of natural gas: steam, at a weight ratio of 1: 3..3,2, achieving -is a conversion of 25 ... 60%. after which the unconverted natural gas is subjected to the final conversion into a usual secondary reformer, in the presence of air with a content of 22 ... 32% oxygen resulting from the passage of air over natural or synthetic zeolites or by additional addition of oxygen, 55 if a conversion of 97 ... 99%. at a temperature of 93O ... 1OOO ° C, and after recovering the heat from the primary and secondary conversion phases, conversion of carbon oxide to carbon dioxide and after the synthesis of ammonia, the resulting energy vapor with a pressure of up to 110 at and a temperature of 400 ... 500 ° C, after stretching in the synthesis gas compressor turbine, drives a 6o backpressure turbine for the process air compressor required for the final conversion, together with the drive and a generator that produces energy Electricity for consumers in the installation, in parallel, takes from the turbine of the air compressor a steam outlet, having a pressure of 16 ... 40 at and a temperature of 225 ... 450 ° C, of which a one part represents process steam at the primary conversion phase, and another part represents 65 the motor fluid in the ejector associated with a steam generator with a pressure of 2.5 ... 5 at by additional heat recovery from the flue gas, finally, leading to a steam with a pressure of 8 ... 16 at and a temperature of 2OO ... 3OO ° C, which is introduced into the steam having the same pressure and temperature, resulting after the trigger in the turbine driving the compressor. of air. 70 in the case of an additional source of hydrogen, outside the source of hydrogen from the primary and final conversion, the final conversion is carried out in the presence of an amount of air, with normal oxygen content, at a volumetric nitrogen: hydrogen ratio of 1: 3.

By applying the process according to the invention, the following advantages are obtained:

- performing a primary conversion in a percentage of less than 25 ... 60%. involves 75 operation of the primary reformer at lower temperatures, which leads to lower fuel consumption and more favorable operating conditions;

- the reduction of the steam ratio; natural gas at the entry in the primary reformer has as an effect a significant decrease of the process steam consumption;

- the use at the final conversion of the enriched air into oxygen allows an 8 primary conversion to be reduced, respectively 25 ... 60% of the natural gas, following that the untransformed natural gas is converted into the secondary reformer, reaching a degree of conservation final raw material of 97 ... 99%;

- improving the efficiency of the use of energy steam by holding it in a single turbine of high capacity and efficiency that drives both the compressor for the process air required at the final conversion and an electric generator that produces electricity used for part of the consumers existing in the plant;

- the use of the energy steam saved by reducing the steam ratio: natural gas for the more advanced extraction of the heat of the flue gas using a boiler 90 steam generator and an ejector whose engine fluid is this steam.

RO 114589 Bl

The following is an example of an embodiment of the process according to the invention in connection with the figure which represents the flow diagram of the technological flow of ammonia.

Natural gas (or other hydrocarbons used as feedstocks) with a pressure of

4 .. .6 at and a temperature of 35 ° C, it is compressed to a pressure of 25 ... 45 at, and then preheated to a temperature of 36O ... 39O ° C, either on account of the heat of the flue gases resulting from the reformer primary, either due to the heat resulting from the combustion of a combustible agent. At this temperature, the natural gas is desulphurized on a ZnO catalyst, retaining the sulfur compounds, after which it is mixed with the process steam from an outlet of the turbine compressor operating the air, making a mixture. natural gas-vapor in which the ratio H ₂ 0: C is 3.0 ... 3.2: 1. It enters the reformer tubes of the primary reformer, where a first conversion of methane gas takes place in the proportion of 25 ... 60%. and 100% higher hydrocarbons. Primary conversion is based on strong endothermic reactions, characterized by the following basic equilibria:

CH ₄ + H ₂ Q - CO + 3H ₂ +49.2 Kcal / mol

CO + H ₂ 0 - C0 ₂ + H ₂ -9.8 Kcal / mol

Because the first reaction is strongly endothermic, the necessary heat is provided by burning outside the tubes of the primary reformer, a combustible agent (natural gas) and the purge gases resulting from the synthesis of ammonia.

Part of the heat resulting from combustion of the fuel agent outside the tubes of the primary reformer is used for heating the raw material (natural gas) at 650., 7BO °. burns for preheating of natural gas as raw material and process air, overheating of the steam to drive the turbine as well as for obtaining an additional quantity of steam.

The gas partially converted after the primary reformer with a temperature of 65O ... 76O ° C and a pressure of 20 ... 40 has entered the secondary reformer where on a Ni catalyst the final conversion takes place in the presence of oxygen-enriched process air, respectively with an oxygen content of 22 ... 32%. preheated in the heat reformer area of the primary reformer.

If additional hydrogen is available from other sources, depending on the amount of available hydrogen, the oxygen content in the air can be reduced to normal, consuming a greater amount of air, corresponding to the stoichiometry of chemical equilibria.

The final conversion to the secondary reformer is based on strongly exothermic reactions that cause the temperature rise in the secondary reformer to 93O ... 1OOO ° C and is based on the following chemical equilibria:

2H ₂ + O ₂ - 2H ₂ 0 -63.3 Kcal / mol

CH ₄ + H ₂ 0 - CO + 3H ₂ +49.2 Kcal / mol

CO + H ₂ 0 = C0 ₂ + H ₂ -9.8 Kcal / mol

After the secondary reformer the converted gas enters a heat recovery system, cooling to a temperature of 360 ... 38 ° C, with saturated steam production of

16 .. .110 at, and then enters for the conversion of CO to C0 ₂ which takes place at the temperature of

400 .. .420 ° C, on a catalyst containing Fe-Cr, when the CO content is reduced to 3 ... 5% vol.

RO 114589 Bl

After CO conversion, the gas enters to recover the heat in the form of saturated steam of 16 ... 110 at and of hot water for the boilers, cooling to 21O ... 23O ° C, temperature with which it enters a new conversion of CO at C0 ₂ at low temperature on a Cu-based catalyst, where the CO content is reduced to 0.3 ... 0.5% vol.

further the gas enters the C0 ₂ wash with K ₂ C0g solution activated with vanadium pentaoxide and diethanolamine where the C0 ₂ retention of the gas takes place, so that the C0 ₂ content in the gas after washing is 0.1 ... 0.2 % you. Because both CO and C0 ₂ are powerful poisons for the ammonia synthesis catalyst, the converted gas containing CO 0.3 ... 0.5% vol., C0 ₂ 0.1 ... 0.2% will enter CO methanization. and C0 ₂ where on a Ni base catalyst occur at temperatures of 27O ... 38O ° C and pressures of

16.5 .. .37.5 at the following chemical reactions:

CO + 3H ₂ - CH ₄ + H ₂ 0 -49.2 Kcal / mol

C0 ₂ + 4H ₂ - CH ₄ + 2H ₂ 0 -39.4 Kcal / mol

After methanization, the synthesis gas mixture containing H ₂ and N ₂ is obtained in stoichiometric ratio, gas which is compressed from 17 ... 35 at to 150 ... 300 at, after which it enters the ammonia synthesis circuit where at the temperature of 380 ... 520 ° C and pressures of 150 ... 300 at a Fe0 + Fe ₂ 0 ₃ catalyst give the ammonia which is delivered in liquid and gaseous form. The synthesis of ammonia is based on the reaction:

3H ₂ + N ₂ - 2NH ₃ -26 Kcal / mol in the synthesis gas there are variable amounts of CH ₄ and Ar, which do not participate in the synthesis of ammonia and which accumulates in time dilutes the composition of the synthesis gas so that it is necessary to remove them , an operation that is carried out by a continuous purge that is introduced in a component separation plant resulting in the following products:

- ammonia water (25% NH ₃ ) finished product;

- technical argon;

- hydrogen that is recycled to the ammonia synthesis process:

- CH ₄ rich fuel agent that burns to the primary reformer.

The heat not consumed in the operations prior to obtaining the synthesis gas mixture as well as from the ammonia synthesis is recovered in the form of steam as follows:

- saturated steam of 16 ... 110 obtained by recovering the heat from the primary reforming, the secondary reforming, the conversion of CO to C0 ₂ at high temperature and from the synthesis of ammonia, steam that overheats at the temperature of 400 ... 480 ° C in the heat recovery system from the primary reformer, after which it extends into the drive turbine of the synthesis gas compressor, then of the compressor for the process air and the electric generator, resulting after 8 ... 16 at and

200 .. .300 ° C.

From the turbine drive of the air compressor and the electric generator, a steam intake of 16 ... 40 at and 225 ... 450 ° C is taken, most of which is used as process steam for mixing. natural gas-steam in the ratio H ₂ 0: carbon of 3.0 ... 3.2: 1, and the other part which represents the surplus created by reducing the ratio H ₂ 0: carbon from 3.5 ... 4: 1 at 3.0 ... 3.2: 1 it is used as a motor fluid at the ejector of the steam generator for further recovery of the flue gas heat.

140

145

150

155

160

165

170

175

180

RO 114589 Bl

Saturated steam of 3 ... 5 at is obtained in the steam generator boiler by the additional heat recovery from the flue gases that cool from 23O ... 25O ° C to

145 .. .180 ° C. This steam is taken over by an ejector related to the steam generating boiler whose engine fluid is part of the steam outlet of the turbine, resulting from the reduction of the steam ratio: natural gas. finally, a steam of 8 ... 16 at and is obtained

200 .. .300 ° C which mixes with the steam resulting from the superheated steam holder in the air compressor turbine having the same parameters, delivering to the network for other consumers on the platform.

It is also mentioned that for the stripping of the technological condensate of C0 ₂ , CO, NH ₃ , CH ₄ , H ₂ and other dissolved substances an inert gas (nitrogen, air or other gas available on the platform) is used, and the steam of 2.5 ... 4 used for stripping the technological condensate, is reused when regenerating the solution of K ₂ CO ₃ resulting after washing CO ₂ of the converted gas.

Claims (2)

1. Process for obtaining ammonia, by compression of natural gas, consisting, in particular, of methane gas, at pressures of 25 ... 45 at, preheating of compressed natural gas by taking over the heat of the flue gases resulting from the primary conversion phase, natural gas desulphurisation by retention of sulfur on zinc oxide, natural gas mixing, primary conversion of natural gas-steam mixture to Ni catalyst at temperatures of 760 ... 820 ° C, final conversion of unconverted gas after primary conversion, air addition compressed and heated based on the heat of the flue gases resulting from the primary conversion phase and the actual synthesis of ammonia, characterized in that it is subjected to the primary conversion at a temperature of 65O ... 76O ° C a mixture of natural gas at a ratio by weight of 1: 3..3,2, a conversion of 25 .. .60%, after which the unconverted natural gas is subjected to the final conversion into a usual secondary reformer, in the presence of air with a content of 22 ... 32% oxygen resulting from the passage of air over natural or synthetic zeolites or by addition. additional oxygen, with a conversion of 97 ... 99%. at a temperature of 93O ... 1OOO ° C, and after recovering the heat from the primary and secondary conversion phases, conversion of carbon oxide to carbon dioxide and after the synthesis of ammonia, the resulting energy vapor with a pressure of up to 110 at and a temperature of 400 ° C, 5OO ° C after it is released in the synthesis gas compressor turbine, drives a turbine with backpressure related to the process air compressor needed for the final conversion, in conjunction with the drive and a generator that produces electricity for consumers in the installation, in parallel with the air compressor drive turbine, a steam intake with a pressure of 16 ... 40 at and a temperature of 225 .. .450 ° C, of which one part represents process steam at the primary conversion phase, and another part represents the motor fluid in the ejector related to a steam generator with a pressure of 2.5 ... 5 at recovery additional heat of flue gas, finally leading to a steam with a pressure of 8 ... 16 at and a temperature of 2OO ... 3OO ° C, which is introduced into the steam having the same pressure and temperature, resulting after the air compressor drive turbine. 2. Process according to claim 1, characterized in that, in the case of an additional source of hydrogen, outside the source of hydrogen from the primary and final conversion, the final conversion is performed in the presence of an amount of air, with normal oxygen content, at a volumetric nitrogen: hydrogen ratio of 1: 3.