Ammonia production system

Abstract

An ammonia production system is disclosed. The system of the present invention involves a one through process, which avoids the use of a recycling compressor. An absorption step with water for ammonia product recovery is used, avoiding the use of costly mechanical refrigeration compression cycles. The system of the present invention is more economical in terms of operating expenses and capital investment than the conventional Haber Bosch synthesis process.

Description

DESCRIPTION“AMMONIA PRODUCTION SYSTEM”

[0001] The present invention relates to a system for production of ammonia in a new process scheme, avoiding the use of recycling compressors and mechanical refrigerationcompression chilling to recover the ammonia, using a single pass reaction system with a heat recovery system integrated. The system of the present invention provides lower capital investment and lower operating cost than conventional ammonia synthesis loop while resulting in a more efficient ammonia production method.TECHNICAL FIELD OF THE INVENTION[2] The field of the invention relates to ammonia production, from a stream rich in nitrogen and hydrogen.STATE OF THE ART AND PROBLEMS TO BE SOLVED[3] Known methods to produce ammonia comprise combining nitrogen from air with hydrogen in a proportion of 1 part of nitrogen to 3 parts of hydrogen (molar basis). This is known as the Haber Bosch process and the chemical reaction is reversible and exothermic. The Haber Bosch process is conducted within a temperature range of 350 °C to 500 °C (inlet and outlet temperature, respectively, of the catalytic bed as the reaction is very exothermic) and under pressure at 150-250 barg in a catalytic bed reactor. Commonly the catalysts used for the Haber Bosch reaction are iron based with an inorganic compound such as potassium hydroxide to promote the catalytic activity. These catalysts are normally poisoned by water or by oxygen.[4] The chemical reaction is:N2+ 3 H2<-> 2 NH3[5] The reaction is thermodynamically limited by the chemical equilibrium as it is an exothermic equilibrium reaction. According to the Le Chatelier principle, the higher the pressure the better the conversion of reactants and ammonia production. Also, as the reaction is exothermic, lower operating temperatures will provide better conditions for increasing the equilibrium conversion. The main problem is that operating at lowtemperature will also affect the reaction kinetics, therefore a compromise in pressure and temperature conditions is normally used.[6] Due to this thermodynamic limitation, the conversion per pass in the reactor is constrained to near 15%.[7] The alternatives to maximize the conversion and overcome this thermodynamic restriction, hence increase the production of ammonia, are listed below:[8] a) Lowering the temperature of reaction. Although this is positive for the thermodynamics, there is a limit from which the reaction kinetics is affected, so the catalyst should be very active or the reactor too big, due to the residence time, so the cost of inversion would increase too much.[9] b) Recycling unreacted reagents (H2 and N2). The global conversion to ammonia increases but the problem is that the cost of investment and operation also increases, hence a big quantity of unreacted reagents require bigger equipment and pipelines due to the volumetric flowrate increase (about 3 to 5 times with respect to the net inlet).

[0010] Despite the disadvantages mentioned above, lowering the temperature of reaction, and recycling unreacted reagents are the standard alternatives used to optimize and increase the yield in the conventional Haber Bosch process.

[0011] From an operational point of view, the conventional process of ammonia synthesis consists of a reaction loop with a complex reactor. Within the reactor are located two or three adiabatic catalytic beds with intermediate refrigeration between them. The reactor has integrated inside heat exchangers, located between the different catalytic beds in which the outlet of a reaction bed is refrigerated with the reactor inlet stream or quenched with a cold inlet to allow for the reaction to take place in the following reaction bed. This results in a complex reactor design. The reaction products exit the reactor and are conducted to a heat exchanger where they are cooled down. Further cooling of the gas is performed in a chiller, typically with mechanical-compression refrigeration cycles, where the ammonia is recovered in liquid state at a temperature of 0 to 10 °C. The liquid ammonia product and the unreacted gas are divided in a separator. The unconverted reactants (hydrogen and nitrogen) are separated of ammonia and are recycled back to the reactor, using a recycle compressor, creating a reaction loop. With this recycle, the overall process conversion reaches levels close to 98-100% but the flow in the reaction loop is multiplied by 3-5 times the net input flow, requiring bigger equipment. A purge is normally necessary to avoid possible inert gas accumulation in the reaction loop.

[0012] EP19218461 discloses a system for methanol synthesis from a synthesis gas rich in hydrogen and CO2 / CO, the system comprising a first adiabatic reactor with a structure comprising an inlet stream, a first catalytic bed, one Venturi type mixing element, a firstdivergent nozzle, a second catalytic bed and one outlet stream, all of which are connected sequentially to each other. A first heat exchanger connected to the outlet stream downstream the reactor; a condenser connected to the heat exchanger downstream of the heat exchanger; a separator connected to the condenser; a first cold gas stream joining the separator to both the first heat exchanger and the first Venturi type mixing element; and a first outlet stream joining the heat exchanger to a second adiabatic reactor like the first adiabatic reactor.

[0013] The methanol synthesis catalyst is resistant to water, since water is a reaction product, and the separation of methanol by condensation is done directly against cooling water, as a temperature in the range of 50-60 °C is enough for this purpose. A final distillation step is required to separate methanol from water, DME (secondary product) and dissolved CO2.

[0014] The present invention adopts some of the principles of the system disclosed in EP19218461. Although methanol synthesis shares some of the thermodynamic constraints as ammonia synthesis, there are notable differences between the disclosures of EP19218461 and the present invention. To start, the product synthetized is different: while EP19218461 discloses a methanol production method from hydrogen and CO / CO2 mixtures, the present invention refers to the production of ammonia from nitrogen and hydrogen. Operating conditions are very different, resulting in higher temperatures (200- 250 °C for methanol production and below 300 °C in the inlet up to 500 °C in the outlet for ammonia). The catalyst used is also very different: in methanol synthesis a Cu-Zn over AI2O5 is normally used, which must be used below 300 °C as it is sintered and loses activity; in NH3 synthesis process an Iron or Cobalt based derived catalyst is used which is resistant in the temperature range of 300°C to 500°C.

[0015] Furthermore, the method used for product separation represents a big difference as instead of using a condensation operation against cooling water, or even through a chiller with mechanical-compression refrigeration cycle, the system of the present invention includes a water absorption step with further distillation resulting in a complete ammonia recovery. The present invention does not require a chiller and a mechanical refrigeration cycle operating at lower temperatures 0 to 10 °C to condensate ammonia. The cooling gas sent to the venturis does not have product ammonia, increasing the efficiency in the reactor beds. Due to this fact, the system of the present invention comprises only one reactor system instead of several reactors disposed in series as disclosed in EP19218461.

[0016] Furthermore, the method of the present invention allows for distillation of water-ammonia mixture from the absorption recovery in an auto-thermal mode. As the reaction of ammonia is more exothermic and occurs at higher temperatures than methanol’sreaction, a heat recovery, producing steam is provided in the method. This allows to use this steam for distillation reboiler system, not requiring external heat sources. The system described in EP19218461 does not produce steam and does not involve distillation of the final product which requires additional heat demand, while the system of the present invention includes a steam generation that can be used as heat source in the ammonia- water distillation operation, resulting in a more economical approach and in a thermal sustainable process.OBJECT OF THE INVENTION

[0017] The object of the present invention is to provide a new ammonia production system, avoiding the use of recycling compressor and separating ammonia without using a mechanical compression refrigeration cycle for chilling.DESCRIPTION OF THE DRAWINGSDRAWINGS

[0018] For clarity and understanding of the object of the present invention, a process description is presented in Figure 1.LIST OF REFERENCED NUMERALS

[0019] 1 : Inlet mixture of hydrogen and nitrogen

[0020] 2: First feed-effluent heat exchanger

[0021] 3: Reactor

[0022] 4: First catalytic bed

[0023] 5: First throttle section - venturi type mixing element

[0024] 6: First unreacted gas recycle

[0025] 7: Second unreacted gas recycle

[0026] 8: Reactor outlet gas

[0027] 9: Steam generator

[0028] 10: Second feed-effluent heat exchanger

[0029] 11 : Cooler

[0030] 12: Water absorption column

[0031] 13: Cleaned recycle gas

[0032] 14: Purge

[0033] 15: Water and ammonia bottom stream

[0034] 16: Recycle cold water stream

[0035] 17 : Absorption water cooler

[0036] 18: Distillation column feed-effluent heat exchanger

[0037] 19: Absorption water circulation pump

[0038] 20: Water make up

[0039] 21 : Distillation column

[0040] 22: Column condenser

[0041] 23: Reflux accumulator

[0042] 24: Incondensable gases purge

[0043] 25: Reflux pump

[0044] 26: Liquid ammonia product

[0045] 27: Column reboiler

[0046] 28: Steam supply

[0047] 29: Condensate return

[0048] 30: Cylinder envelope

[0049] 31 : Reactor inlet

[0050] 32: First diverging nozzle

[0051] 33: Second catalytic bed

[0052] 34: Second throttle section - venturi type mixing element

[0053] 35: Second diverging nozzle

[0054] 36: Third catalytic bed

[0055] 37: Reaction gases

[0056] 38: Gas inlet to absorption columnSUMMARY OF THE INVENTION

[0057] The present invention solves the problems of the state of the art by:

[0058] a) Avoiding the recycle of unreacted products in a recycle compressor. This will reduce the overall equipment size, as there is not a recycle over all the process at 3-5 times the net inlet flow. On the other hand, this will avoid a recycle compressor which is an expensive equipment and all the associated electrical consumptions. This will reduce investment and operating cost.

[0059] b) Avoiding the use of a mechanical refrigeration-compression cycle to provide coolling in a chiller to recover the ammonia product. This will reduce drastically the power consumption.

[0060] c) Using a simpler design for the main reactor.

[0061] The present invention provides a system for ammonia synthesis from a synthesis gas rich in hydrogen and nitrogen, the system comprising:

[0062] an adiabatic reactor (3) arranged vertically in a cylindrical envelope (30), with a structure comprising an inlet (31) stream joined to a first catalytic bed (4), one Venturi type mixing element (5) next to and connected to the first catalytic bed (4), a first divergent nozzle (32) next to and connected to the first Venturi type mixing element (5) which is arranged to receive a mixture of reactants and product from the first catalytic bed (4), quench it with cold recycled reactants (6) and feed to a second catalytic bed (33) located next to and connected to the first divergent nozzle (32) and one outlet stream leaving from the second catalytic bed (33), a second Venturi type mixing element (34), connected to the second catalytic bed (33), a second divergent nozzle (35) next to and connected to the second Venturi type mixing element (34), quenching the outlet gases from the second catalytic bed (33) with cold recycled reactants (7) and feed to a third catalytic bed (36) located next to and connected to the second divergent nozzle (35) and one outlet (8) stream leaving from the third catalytic bed (36) and reactor (3);

[0063] a heat integration system;

[0064] an absorption column (12) to recover the ammonia product using water as absorption product; and

[0065] a distillation section where water and ammonia are separated.

[0066] In a preferred embodiment, the heat integration system of the present invention comprises:

[0067] a first feed effluent heat exchanger (2) connected to the inlet (31) of the reactor and the outlet (8) of said reactor;

[0068] a steam generator (9) that will generate steam for further ammonia separation in a distillation operation;

[0069] a second feed effluent heat exchanger (10) that will heat the recycle gases to the quenches of the reactor; and

[0070] a gas cooler (11).

[0071] In another embodiment of the present invention, the first divergent nozzle (32) forms an angle ranging from 10° to 30°.

[0072] In another embodiment of the present invention, the second divergent nozzle (35) forms an angle ranging from 10° to 30°.

[0073] In another embodiment of the present invention, the catalyst is metal-based, preferably iron-based or cobalt-based resistant to small amounts of water and active towards ammonia synthesis reaction.

[0074] The system disclosed in the present invention provides the following advantages:

[0075] - Lower cost of equipment: because only a fraction of unreacted reagents is recycled and strategically distributed in several points of the reactor embodiment (quenches), the size of reactor and pipes is significantly reduced (diameter), compared to the conventional process, in which the total amount of unreacted reagents is recycled to the feed, that increases its flowrate (flowrate in conventional reactors is about 3-5 times that of invention). Finally, no compressor is required to recycle the reagents, as explained below in ‘less electricity consumption’.

[0076] - The invention brings further economic advantages for small and medium scale production when compared to previous methods because no recycle compressor is used. For small units the cost of adding a compressor (even a small one) is higher than in large scale plants.

[0077] - More reliable (less complexity): the unit proposed is simpler that the conventional process because it has no rotative equipment inside (compressors) as explained below in "less cost of electricity". This translates in less probability of failures and loss of production.

[0078] - Less electricity consumption: the quenches of unreacted reagents over the reactor are performed by venturi effect, in the transition throats between catalytic beds. The quenches would flow naturally without need of compressors, so the consumption of electricity for compression gets reduced compared to a conventional unit.

[0079] - No need of mechanical compression refrigeration in chiller: This system is replaced by a water absorption and further distillation. The distillation uses the steam generated in the process and does not require additional heat, resulting in an auto-thermal process. This results in less electricity consumption and less operating cost.

[0080] - The possibility to reduce the working temperature (under 300 °C in the inlet of the catalytic bed) and pressure (less than 100 barg) will result also in less electricity consumption in the syngas compressor, located upstream the synthesis process, that normally works between 150 and 250 barg.In another aspect, the invention concerns to a method for ammonia production from a synthesis gas rich in hydrogen and nitrogen in a system according to the invention, the method comprising the following steps:(i) heat a mixture of hydrogen and nitrogen in a first Feed Effluent heat exchanger (2) until 300 °C(ii) introduce the heated mixture of step (i) in a first catalytic bed (4) of an adiabatic reactor (3) via an inlet (31) of the reactor(iii) recycle part of the stream of unreacted gas (13) via an inlet (6) to the adiabatic reactor (3)(iv) recirculate the unreacted gas from step (iii) and introduce it in a second catalytic bed (33) of the adiabatic reactor (3),(v) recycle the other part of the stream of unreacted gas (13) via an inlet (7) to the adiabatic reactor (3)(vi) recirculate the unreacted gas from step (v) and introduce it in a third catalytic bed (36) of the adiabatic reactor (3),(vii) separate the produced ammonia and unreacted gases in a water absorption tower (12).DETAILED DESCRIPTION OF THE INVENTION

[0081] The present invention is related to a system for ammonia production from a synthesis gas rich in hydrogen and nitrogen removing the reaction product via water absorption. Once removed, the unreacted gases are recycled via venturi mixers / quenches, avoiding the use of a recycle compressor. The reactor design is very simple, as no integral heat exchangers are needed, resulting in an envelope with three catalytic and adiabatic beds with venturis mixers between them.

[0082] The system for ammonia synthesis from a synthesis gas rich in hydrogen and nitrogen comprises:

[0083] an adiabatic reactor (3) arranged vertically in a cylindrical envelope (30), with a structure comprising an inlet (31) stream joined to a first catalytic bed (4), one Venturi type mixing element (5) next to and connected to the first catalytic bed (4), a first divergent nozzle (32) next to and connected to the first Venturi type mixing element (5) which is arranged to receive a mixture of reactants and product from the first catalytic bed (4), quench it with cold recycled reactants (6) and feed to a second catalytic bed (33) located next to and connected to the first divergent nozzle (32) and one outlet stream leaving from the second catalytic bed (33), a second Venturi type mixing element (34), connected to the second catalytic bed (33), a second divergent nozzle (35) next to and connected to the second Venturi type mixing element (34), quenching the outlet gases from the second catalytic bed (33) with cold recycled reactants (7) and feed to a third catalytic bed (36) located next to and connected to the second divergent nozzle (35) and one outlet (8) stream leaving from the third catalytic bed (36) and reactor (3);

[0084] a heat integration system;

[0085] an absorption column (12) to recover the ammonia product using water as absorption product; and

[0086] a distillation section where water and ammonia are separated.

[0087] The recovered water is recycled back to the absorption column. The heat required for such distillation operation is provided by steam generated in the steam generator (9), in an auto-thermal process.

[0088] In a preferred embodiment according to the present invention, the heat integration system comprises:

[0089] a first feed-effluent heat exchanger (2) connected to the inlet (31 ) of the reactor and the outlet (8) of said reactor;

[0090] a steam generator (9), that will generate the steam needed for further ammonia separation in a distillation operation;

[0091] a second feed-effluent heat exchanger (10) that will heat the recycle gases to the quenches of the reactor; and

[0092] a gas cooler (11).

[0093] The ammonia synthesis is catalyzed by a catalyst that is resistant to small amounts of water, as the quenched gas is saturated with water.

[0094] The catalyst is active at a temperature equal or below 300 °C, preferably between 250 °C and 300 °C, and thus also reducing the working pressure to less than 100 barg.

[0095] A catalyst suitable for the purpose of the invention is any metal-based catalyst known in the art, preferably an iron-based or cobalt-based catalyst.

[0096] In the system of the invention the first divergent nozzle (32) and / or the second divergent nozzle (35) form an angle ranging from 10° to 30°.EXEMPLARY EMBODIMENT

[0097] DESCRIPTION OF AMMONIA SYNTHESIS

[0098] Ammonia is synthetized by the reaction of hydrogen and nitrogen over a catalytic bed at high pressure (less than 100 barg) and temperature (between 250 °C and 300 °C).

[0099] A mixture of hydrogen and nitrogen (1) in stochiometric conditions (3 parts of hydrogen to 1 part of nitrogen) at high pressure (less than 100 barg) is feed to the system. It is heated in the first Feed Effluent heat exchanger (2) so that the mixture reaches a temperature high enough to initiate reaction over the catalyst (under 300 °C). This heat exchanger is using the outlet of reactor (8) as hot fluid. The hot mixture is introduced in the reactor (3). This is an adiabatic reactor made of 3 separate catalytic beds (4), (33) and (36).

[0100] In the first catalytic bed (4), the exothermic reaction of ammonia formation takes place, so that reacting gases are heated. After the first catalytic bed (4), a recycled “cold” streamof unreacted gas (6) is reintroduced so that the temperature is reduced, and the unreacted gas can be reprocessed. Recirculation of unreacted gas is possible due to a first throttle section (5) where pressure is decreased due to the velocity increase, using a venturi type mixer.

[0101] The resulting mixture is introduced in the second catalytic bed (33), where again the exothermic ammonia synthesis heats up the gases. After the second catalytic bed (33), a recycled stream of unreacted (7) gas is reintroduced so that the temperature is reduced, and the unreacted gas can be reprocessed. Again, recirculation is possible due to the second throttle section (34) that creates suction effect. The resulting mixture is introduced in the third catalytic bed (36), where again the exothermic ammonia synthesis heats up the gases. After the third catalytic bed (36), the outlet gas (8) containing the produced ammonia and unreacted syngas is sent to the first feed-effluent heat exchanger (2) for energy recovery.

[0102] From there, the gas is sent to heat exchanger (9) where the heat is used for steam generation. After that, the gas exchanges heat in the second feed-effluent heat exchanger (10), where the heat is transferred to the recycled syngas (6) and (7) that is reintroduced after the first and second catalytic beds (4, 33) respectively.

[0103] Ammonia formation in the first, second and third catalytic beds (4, 33, 36) is influenced by the chemical equilibrium. To produce more ammonia with the recycled syngas (6) and (7), it is important to separate ammonia.

[0104] First, reaction gases are cooled in Cooler (11), using cooling water as cold fluid. Ammonia is not condensed nor separated from the syngas using cooling water as no sufficient cold temperature is reached. To separate the product, it is introduced in a water absorption tower (12). The gas flows upwards in the tower, where it is cleaned with water (16) flowing downwards so that ammonia is transferred to the liquid aqueous phase. Water used in the absorption and ammonia are accumulated in the bottom of the tower (12).

[0105] The bottom stream (15) from the tower (12), is sent to distillation section. The cleaned gas from the top part of (12) is recycled to the process (13). A small fraction of gas is purged (14) to avoid accumulation of inert gases and pressure build up.

[0106] The next section describes a typical distillation operation.

[0107] The water and ammonia mixture (15) is heated in the distillation column feed-effluent heat exchanger (18), where the energy of the bottom stream of distillation is recovered. The resulting hot stream is introduced in the distillation tower (21). Distillation tower (21) will separate components, so that water will be concentrated in the bottom trays and ammonia will be concentrated in top trays.

[0108] The bottom liquid product (mainly water) will be heated and evaporated in the Column reboiler (27), that is a kettle type heat exchanger. Vapor will be reintroduced in the column (21), and a fraction of liquid will be extracted from the kettle. This liquid stream will be used in (18) to recover its energy. After (18), liquid stream of water will be cooled down in Absorption water cooler (17), using cooling water as cold fluid. Cold water (16) is pumped by Absorption water circulation pump (19) and sent to (12) to be used as absorption water. A small fraction of water (20) is added to the process to cover the small losses of water.

[0109] The top gases of (21) (mainly ammonia) are cooled down in Column condenser (22) and the condensed liquid store in Reflux accumulator (23). Incondensable gases from (23) are purged (24) to avoid pressure build up. Liquid from (23) is pumped by Reflux pump (25). Part of the liquid is sent as reflux back to the tower (21), and another fraction is extracted as liquid ammonia product (26).

[0110] The first embodiment (Figure 1) describes a system for ammonia synthesis from a synthesis gas rich in hydrogen and nitrogen, the hydrogen and nitrogen present in a proportion of 3 parts of hydrogen to 1 part of nitrogen (molar basis), the system for ammonia synthesis comprising the following elements:

[0111] - An adiabatic reactor (3) designed as an adiabatic reactor with a cylindrical envelope (30) disposed in a vertical arrangement. The gas flow pattern is through the cylinder envelope (30) going downwards. The adiabatic reactor (3) is constructed in a metal alloy material, with a structure comprising an inlet stream (31) joined to a first catalytic bed (4), one Venturi type mixing element (5) next to and connected to the first catalytic bed (4), a first divergent nozzle (32) with an angle ranging from 10° to 30° next to and connected to the Venturi type mixing element (5) which is arranged to receive a mixture of reactants and product from the first catalytic bed (4), quench it with cold recycled reactants (6) and feed to a second catalytic bed (33) located next to and connected to the first divergent nozzle (32), a second Venturi type mixing element (34), connected to the second catalytic bed (33), a second divergent nozzle (35) with an angle ranging from 10° to 30° next to and connected to the second Venturi type mixing element (34), quenching the outlet gases from the second catalytic bed with cold recycled reactants (7) and feed to a third catalytic bed (36) located next to and connected to the second divergent nozzle (35) and one outlet stream (8) leaving from the third catalytic bed (36) and reactor (3);

[0112] - A first feed-effluent heat exchanger (2) connecting the inlet of the process (1) and the inlet (31) of the reactor (3) in one side with the outlet of said reactor (8) and (37) in the other side;

[0113] - A steam generator (9) connected downstream the outlet (37) of the first feed-effluent heat exchanger (2), that will generate the steam needed for further ammonia separation;

[0114] - A second feed-effluent heat exchanger (10) that will heat the recycle gases to the quenches of the reactors (6) and (7);

[0115] - A gas cooler (11) which uses cooling water as refrigeration media or air (air cooler) connected to the second feed-effluent heat exchanger (10) and the inlet stream (38) to the ammonia absorption recovery;

[0116] - An absorption column (12) to recover the ammonia product with water (15). This has an inlet (38) with syngas and ammonia, an inlet with water (16), a gaseous stream (13) with virtually no ammonia and a water-ammonia liquid stream (15);

[0117] - A purge (14), that controls the over-pressure in the system in case of inert gas accumulation;

[0118] - A distillation section (21) where water and ammonia are separated. The recovered water (16) is used again in the absorption column (12). The heat required for such distillation operation is provided by steam generated in the steam generator (9) in an autothermal process;

[0119] - A catalyst active to the ammonia synthesis and resistant to small amounts of water, as the quenched gas will be saturated with water. This catalyst will preferably be active working at a temperature equal or below 300 °C, preferably between 250 °C and 300 °C, and thus also reducing the working pressure to less than 100 barg.

[0120] The temperature in the catalytic bed is monitored to be maintained between 250 °C and 300 °C.

[0121] The Venturi type mixing elements (5) and (34) which act as a quench element where the first and second cold gas streams (6) and (7) respectively free of ammonia are taken and mixed with the outlet gas from the catalytic beds (5) and (33). In order to avoid a recycle compressor, theses quenches are done in a venturi throat located between each catalytic beds. These quenches are performed without compressor to the lowest pressure zone of the Venturi type mixing elements (5) and (34) located in the throat. It has two beneficial effects: one of them is to increase the reagents concentration in order to misbalance the equilibrium towards products; other is to cool down the outlet from the catalytic beds (5) and (33) to a temperature where the equilibrium is misbalanced (between 300 °C and 350 °C).

[0122] The specially designed venturi with diverging nozzles at determined angles allows for gas recycling because of lower pressure located in the throat produced by gas acceleration to higher velocities. The angle of the diverging nozzles (32) and (35) determines most of the total pressure of the Venturi type mixing elements (5) and (34).Angles bigger than 45 ° are not suitable since the pressure of the outlet is even lower than the pressure in the throat, therefore is not possible to recycle the gas.

[0123] It was observed that when the angle of the divergent nozzles (32) and (35) is approximately 25°, the outlet pressure is higher than the throat pressure, this is because the pressure drop in diverging nozzle is reduced and the Bernoulli Effect of decelerating the gas from the throat high velocity to the diverging nozzle outlet diameter low velocity has more weight.

[0124] The venturi throat gradually increases the diameter of the diverging nozzles (32) and (35) to recover the pressure and keep pressure losses in the venturi to a minimum. In the present invention, the diverging nozzles (32) and (35) have an angle smooth enough to recover great part of the pressure loss. The angle of said diverging nozzles (32, 35) is between 10° and 25°, more preferably between 20 ° and 10 °, to keep pressure losses to a minimum and in order to allow recycling the cold gas to the throat, acting as a quench and as a ‘low-cost’ recycle, avoiding the use of a recycle compressor.

Claims

CLAIMS1. Ammonia production system from a synthesis gas rich in hydrogen and nitrogen characterized in that the system comprises:- an adiabatic reactor (3) arranged vertically in a cylindrical envelope (30), with a structure comprising an inlet (31) stream joined to a first catalytic bed (4), one Venturi type mixing element (5) next to and connected to the first catalytic bed (4), a first divergent nozzle (32) next to and connected to the first Venturi type mixing element (5) which is arranged to receive a mixture of reactants and product from the first catalytic bed (4), quench it with cold recycled reactants (6) and feed to a second catalytic bed (33) located next to and connected to the first divergent nozzle (32) and one outlet stream leaving from the second catalytic bed (33), a second Venturi type mixing element (34), connected to the second catalytic bed (33), a second divergent nozzle (35) next to and connected to the second Venturi type mixing element (34), quenching the outlet gases from the second catalytic bed (33) with cold recycled reactants (7) and feed to a third catalytic bed (36) located next to and connected to the second divergent nozzle (35) and one outlet (8) stream leaving from the third catalytic bed (36) and reactor (3);- a heat integration system;- an absorption column (12) to recover the ammonia product using water as absorption product; and- a distillation section where water and ammonia are separated.

2. The ammonia production system from a synthesis gas rich in hydrogen and nitrogen according to claim 1 , characterized in that the heat integration system comprises- a first feed effluent heat exchanger (2) connected to the inlet (31) of the reactor and the outlet (8) of said reactor;- a steam generator (9) that will generate steam for further ammonia separation in a distillation operation;- a second feed effluent heat exchanger (10) that will heat the recycle gases to the quenches of the reactor; and- a gas cooler (11).

3. The ammonia production system from a synthesis gas rich in hydrogen and nitrogen according to any of the claims 1 or 2, characterized in that the first divergent nozzle (32) forms an angle ranging from 10° to 30°.

4. The ammonia production system from a synthesis gas rich in hydrogen and nitrogen according to any of the claims 1 to 3, characterized in that the second divergent nozzle (35) forms an angle ranging from 10° to 30°.

5. The ammonia production system from a synthesis gas rich in hydrogen and nitrogen according to any of the claims 1 to 4, characterized in that the catalyst is metal-based, preferably iron-based or cobalt-based resistant to small amounts of water and active towards ammonia synthesis reaction.

6. Method for ammonia production from a synthesis gas rich in hydrogen and nitrogen in a system as described in claims 1 to 5 characterized in that the method comprises the following steps:(i) heat a mixture of hydrogen and nitrogen in a first Feed Effluent heat exchanger (2) until 300 °C(ii) introduce the heated mixture of step (i) in a first catalytic bed (4) of an adiabatic reactor (3) via an inlet (31) of the reactor(iii) recycle part of the stream of unreacted gas (13) via an inlet (6) to the adiabatic reactor (3)(iv) recirculate the unreacted gas from step (iii) and introduce it in a second catalytic bed (33) of the adiabatic reactor (3),(v) recycle the other part of the stream of unreacted gas (13) via an inlet (7) to the adiabatic reactor (3)(vi) recirculate the unreacted gas from step (v) and introduce it in a third catalytic bed (36) of the adiabatic reactor (3),(vii) separate the produced ammonia and unreacted gases in a water absorption tower

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