ITMI20011553A1 - PROCESS FOR THE SUMMARY OF AMMONIA - Google Patents

Description

TITLE: "PROCESS FOR THE SYNTHESIS OF AMMONIA"

Description

The present invention refers to a process for the synthesis of ammonia starting from the mixture of hydrogen and nitrogen which are reacted in the presence of the appropriate catalytic system, characterized in that said gas mixture is obtained through a partial oxidation process low contact time.

The mixture of H2 and N2 for the synthesis of ammonia is produced by the chemical processes of non-catalytic partial oxidation (OP) of heavy hydrocarbons (Falsetti, J.S, Hydrocarbon Technology International, 1993, p. 57) or by combined steam reforming processes (SR ) and autothermal reforming (ATR).

The latter are preferred when hydrocarbon fillers consisting of natural gas or naphtha are used (Rostrup-Nielsen, J.R. "Catalytic Steam Reforming", in Catalysis Science and Technology, J.R. Anderson, M. Boudart Eds. Vol. 5, Springer, Berlin 1988 , p. 1 and T.S. Christensen LI. Primdahl, Hydrocarbon Processing, March, 1994, p. 39).

The OP takes place in a combustion chamber where hydrocarbons and oxygen are introduced through a burner. The mixing takes place at a temperature in which flame reactions are triggered which lead to the formation of H, CO, CO2, HO and by-products consisting of carbonaceous compounds. These are formed by decomposition of the hydrocarbon charge and are separated from the synthesis gas and in some cases reintroduced into the burner.

OP's technology prefers the use of an oxidizing current of pure oxygen or containing low percentages of nitrogen to reduce the formation of carbonaceous compounds.

The complexity of the synthesis gas washing operations and the relatively low efficiency of the thermal recovery operations (the current leaving the reactor is at a temperature of about 1400 ° C), constitute the main weak points of the OP which is the only one capable of using heavy hydrocarbon charges to generate hydrogen.

The use of the OP in ammonia synthesis processes involves the following unitary operations:

a) separation of air into oxygen and nitrogen, b) non-catalytic partial oxidation of hydrocarbons with oxygen, c) removal of carbon residues and heat recovery from the product mixture, d) removal of sulfur compounds, e) conversion of monoxide carbon into hydrogen by means of the water gas shift reaction (WGS), f) removal of CO2, g) washing of residual CO with liquid nitrogen, h) mixing of hydrogen and nitrogen and sending the mixture to the synthesis section of ammonia.

Much more widespread than the OP is the use of combined SR and ATR processes.

As already mentioned, this solution is preferably adopted when a hydrocarbon feed consisting of natural gas or naphtha is available and these conditions occur in most ammonia production plants.

The SR and ATR processes are catalytic, allow for considerable energy efficiency and do not require the use of air separation units.

More specifically, the SR exploits a highly endothermic reaction between hydrocarbons and steam which in the case of methane can be represented by the equation

CH4 + H20 = CO 3 H2 (DH ° - 206 kJ / mole)

The heat necessary for the reaction is produced in an oven in which the reforming tubes filled with catalyst are inserted and heated by irradiation. The convective section of the reforming furnace is used to provide the heat necessary for the generation of steam and for the pre-heating of some streams of the ammonia synthesis process.

the ATR treats the output current from the SR stage and completes the methane conversion by carrying out combustion reactions with air followed by SR and WGS reactions. More specifically, at the entrance to the ATR there is a burner and a combustion chamber in which reactions take place in the gaseous homogeneous phase between the SR effluents and the oxygen in the air. Subsequently to the combustion chamber the mixture of products passes through a catalytic bed in which further reactions of SR and WGS take place.

The reactions that occur in the ATR can be represented in the case of methane by the following equations:

CH4 + 3/202 = CO 2 H20 (DH ° = - 519 kJ / mole)

which takes place in the combustion chamber

CH4 + H20 = CO 3 H20 (DH ° = 206 kJ / mole)

H20 CO = C02 + H2 (DH ° = -41 kJ / mole)

which take place in the catalytic zone located after the combustion zone.

The presence of the burner and the combustion chamber imposes restrictions on the composition of the reactant streams; in particular, restrictions on vapor / carbon ratios and on the presence of nitrogen in the oxidizing mixture.

At low vapor / carbon ratios or in cases where air or air enriched with significant quantities of N2 is used, the formation of carbon residues that cannot be tolerated by the ATR technology occurs.

The presence of carbon residues would cause the deactivation of the catalyst underlying the combustion zone and would require a synthesis gas purification system similar to that used by the OP technology; which in turn would lead to a decrease in the energy efficiency of the process.

For this reason, the ATR does not directly treat an oxidizing stream of hydrocarbons and enriched air or air but is used as a secondary "reformer" after the SR unit.

In this case, the restrictions on the presence of nitrogen and steam in the feed are overcome as the mixture of reactants contains considerable quantities of H2, CO, H2O and CO2 and small quantities of light hydrocarbons, mostly consisting of unreacted CH4. .

In the case of use of the SR and ATR processes, the sequence of the ammonia synthesis operations includes:

a) a primary SR from the desulfurized hydrocarbon feed, b) the secondary ATR of the stream leaving the primary reforming using air as an oxidizing mixture, c) the catalytic conversion of carbon monoxide into hydrogen by the WGS reaction, d) the removal of C02, e) methanation to eliminate the last traces of CO and C02, f) sending the mixture of H2 and N2 to the ammonia synthesis section.

SR + ATR technology is widely used in ammonia production.

The invention according to the present application refers to an ammonia production process in which the mixture of hydrogen and nitrogen is obtained using a low contact time partial catalytic oxidation (OPC) process, the implementation of which will be exemplified in the following. from some particular sequences of operative stages, from which however the invention itself should not be considered limited.

Significant differences with respect to the known art are the possibility of carrying out the synthesis of ammonia avoiding the recycling of nitrogen, or rather the realization of partial catalytic oxidation in combination with an ATR stage.

The OPC process is carried out using pre-mixed streams of air or enriched air and hydrocarbons and does not require the presence of an SR unit.

The OPC process is entirely catalytic and is carried out in a reactor in which there is neither a burner nor a combustion chamber.

The OPC process occurs at high spatial speeds, between 10,000 and 1,000,000 h <-1> and preferably between 50,000 and 500,000 h <-1> and mainly leads to the direct production of CO and 3⁄4 according to the reaction [1] without intermediate formation of CO2 and H2O.

CH4 1/02 = CO 2 H2 DH ° = - 35.6 kJ / mole [1]

CH4 202 = C02 2 H20 DH ° = - 801.5 kJ / mole [2]

In this way, the temperature peaks associated with the reactions of total combustion of hydrocarbons are avoided and the conditions that in the OP and ATR technologies cause the formation of carbon residues or precursors of carbon residues are avoided.

Furthermore, restrictions on the composition of the reagent mixture concerning the presence of vapor and the need to use pure oxygen or strongly enriched air as oxidant are avoided.

High space velocity synthesis gas production processes are the subject of numerous scientific publications and patents.

The positive influence of the low contact time conditions in the synthesis gas production reactions were initially highlighted by D.A. Hickmann and L.D. Schmidt (Science 1993, 259, 343) and by V.R Choudari, A.S. Mamman, S.D. Sansare (Angew. Chem. Int. Ed. Engl. 1992, 31, 1189) and more recently by L. Basini, K. Aasberg Petersen, A. Guarinoni, M. Ostberg (Catalysis Today 2001, 64, 9).

In the patent field it is noted that US 5,856,585 describes a synthesis gas production process with a low contact time integrated with Fischer Tropsch synthesis, methanol and dimethyl ether synthesis processes. The patent also claims process conditions that directly lead to the simultaneous formation of formaldehyde and synthesis gas.

WO 97/37929 describes a reactor for carrying out reactions at low contact time for the production of synthesis gas from hydrocarbons and oxygen or air with a conical geometry suitable for preventing the propagation of heterogeneous reactions in the stream of pre-mixed reactants under conditions of high pressure.

US 5,648,582 claims a low contact time catalytic partial oxidation process carried out using catalysts consisting of ceramic monolithics on which noble metals or nickel are deposited.

US 5,654,491 extends the reactivity characteristics claimed in the previous patents to metal catalysts with a transparency of at least 40%, typically the metal catalysts mentioned are constituted by networks of Pt -Rh.

WO 00/00425 and WO 00/00426 claim an OPC process in which two layers of catalysts are present. The first layer consists of catalysts containing Rh, the second layer with catalysts containing Ir, Os or Pt.

WO 01/32556 is a patent centered on the geometric characteristics of the catalyst particles and claims an OPC process in which the support has a surface / volume ratio between 15 and 230 cm <-1> and a diameter between 200 and 2000 microns .

This document refers to low contact time OPC processes strongly integrated with the ammonia synthesis section and is based on the experimental discovery that these OPC processes allow the use of air or air enriched but containing relevant percentages of nitrogen, without producing carbon residues even in the absence of steam and with 02 / C feed ratios ranging from 0.15 to 0.75 v / v.

These characteristics of the synthesis gas and hydrogen production processes by OPC allow the definition of the process diagrams represented in Figures 1-4, which as anticipated, have the purpose of exemplifying the process according to the present invention, which, due to its nature , should not be considered limited to them.

In a first diagram, shown in Figure 1, the gas consisting of H2 and N2 necessary for the synthesis of ammonia is produced from air and hydrocarbons and steam in a single OPC reactor with a low contact time. The H2 / N2 ratio used in ammonia synthesis is achieved by separating the unreacted H2 from the mixture of reaction products. This separation can be achieved with a hollow fiber separation system or by PSA or separations using cryogenic methods or other systems downstream of the ammonium synthesis reactor and recycling it in the reactant stream downstream of the WGS reactors.

More specifically, according to the diagram of Figure 1, an air flow 1 is compressed into 2 at pressures between 10 and 80 kg / cm <2> and preferably at pressures between 20 and 50 kg / cm <2> and mixed in the inlet zones to the reactor of OPC 3 with a stream of natural gas 4 de-sulphured and at the same pressure. The two streams are pre-heated in 5 and 6 at temperatures between 25 and 400 ° C, preferably between 100 and 300 ° C. The supply O2 / C ratios are comprised between 0.5 and 0.75 and preferably between 0.60 and 0.75 v / v. The inlet stream of the OPC 10 reactor may also contain steam 8 added to 9 even if its presence is not necessary. The synthesis gas stream produced by the OPC 10 reactor has a temperature between 700 and 1200 ° C and preferably between 800 and 1100 ° C and is cooled to temperatures between 200 and 500 ° C and preferably between 300 and 400 ° C in exchangers 5, 6 and 7 and mixed with a steam stream 12 in a mixer 11. After mixing, the synthesis gas stream containing a steam / carbon ratio between 1.5 and 4, preferably between 2 and 3.5 v / v, is introduced into a first reactor of WGS 13. The temperature of the gas flow leaving the reactor of WGS 13 is cooled in 14 to values between 50 and 300 ° C and preferably between 100 and 250 ° C and is introduced in a second reactor of WGS 15 which allows to further reduce the percentage of CO to values typically below 0.3% v / v. The stream leaving the WGS reactor 15 is then cooled in 16, and after condensation and separation of the steam in 17 it is sent to a C02 separation section (decarbonation) 18 and subsequently to a methanation stage 19 which further reduces the content of CO at values below 2 ppm. The gas now contains H2, N2 in ratios between 0.75 and 1.2 v / v and percentages of Ar below 0.8% and is mixed with H2 in 20. The H2 comes from the synthesis section 22 and the separation section 23 of ammonia and is separated in from N2, CH4 and Ar in 24. After mixing the gas which has an H2 / N2 ratio between 1.5 and 3.5 and preferably between 2 and 3 is compressed in 21 at pressures between 50 and 500 kg / cm and preferably between 100 and 400 kg / cm <2> and sent to the ammonia synthesis section represented by square 22. The high pressure mixture of N2, CH4 and Ar coming from the separation system 24 is heated in the scabiator 25 and expanded in 26 at pressures between 20 and 40 ATMs recovering energy. If the stream contains significant methane residues, it can be combusted in 27 with compressed air in 28 and further expanded in 29, recovering energy.

In a second scheme shown in Figure 2, pre-mixed streams of hydrocarbons and enriched air are used.

Air enrichment can be carried out either by mixing pure oxygen with enriched air or by using air enrichment systems such as those described in EP 0636845A1, and in US 6,247,333 Bl.

In this second process scheme, the H2 / N2 ratio in feeding to the ammonia synthesis section is defined by the O2 / N2 ratio of the enriched air stream, O2 / C and S / C entering the OPC reactor.

More specifically, referring to Figure 2. A flow of enriched air with oxygen percentages between 21 and 95% and preferably between 30 and 80% is compressed in 2 at pressures between 10 and 100 kg / cm <2> and preferably at pressures ranging from 20 to 80 kg / cm <2> and mixed in the OPC 3 reactor inlet zone with a stream of natural gas 4 de-sulphured and at the same pressure. The two streams are pre-heated in 5 and 6 at temperatures between 25 and 400 ° C, preferably between 100 and 300 ° C. The supply O2 / C ratios are comprised between 0.5 and 0.75 and preferably between 0.65 and 0.75 v / v. The inlet stream of the OPC 10 reactor may also contain steam 8 added to 9 even if its presence is not necessary. The synthesis gas stream produced by the OPC 10 reactor has a temperature between 700 and 1200 ° C and preferably between 800 and 1100 ° C and is cooled to temperatures between 200 and 500 ° C and preferably between 300 and 400 ° C in exchangers 5, 6 and 7 and mixed with a steam stream 12 in a mixer 11. After mixing, the synthesis gas stream containing a steam / carbon ratio between 1.5 and 4, preferably between 2 and 3.5 v / v, is introduced into a first reactor of WGS 13. The temperature of the gas flow leaving the reactor of WGS 13 is cooled in 14 to values between 50 and 300 ° C and preferably between 100 and 250 ° C and is introduced in a second reactor of WGS 15 which allows to further reduce the percentage of CO to values typically below 0.3% v / v. The stream leaving the WGS reactor 15 is then cooled in 16, and after condensation and separation of the steam in 17 from the gas the CO2 is eliminated in 18. The stream is then sent to a methanator 19 which further reduces the CO content to values below 2 ppm. 13 gas now contains H2, N2 in ratios between 0.75 and 3.5 v / v and percentages of Ar below 0.8% is possibly mixed with N2 in 20, is compressed in 21 at pressures between 50 and 500 kg / cm <2> and preferably between 100 and 400 kg / cm <2> and sent to the ammonia synthesis section represented by square 22 and to the ammonia separation section 23.

In a third scheme shown in Figure 3, two OPC reactors are used in series or a primary OPC reactor and a secondary ATR reactor. In this case an oxidizing stream of air is used in one of the two reactors and in the second an oxidizing stream consisting of enriched air. This process scheme is made possible by the ability of the OPC reactor to operate without producing caibonous residues even at O2 / C ratios <0.5 v / v using air or enriched air.

More specifically, referring to the diagram of Figure 3, an air flow 1 is compressed into 2 at pressures between 10 and 80 kg / cm <2> and preferably at pressures between 20 and 50 kg / cm <2> and mixed in 3 in the inlet zones to the reacor of OPC 10 with a stream of natural gas 4 de-sulphured and at the same pressure. The two streams are pre-heated in 5 and 6 at temperatures between 25 and 400 ° C, preferably between 100 and 300 ° C. The 02 / C ratios in feeding are comprised between 0.15 and 0.50 and preferably between 0.20 and 0.40 v / v. The inlet stream of the OPC 10 reactor may also contain vapor 8 added to 9 even if its presence is not necessary. The synthesis gas stream produced by the OPC 10 reactor has a temperature between 400 and 950 ° C and preferably between 500 and 850 ° C and is sent after mixing with oxygen or strongly enriched air in 11 and introduced into the reacor 12 which it can be an additional OPC reactor or an ATR. The amount of oxygen added in 11 is defined in such a way as to reduce the% of CH4 leaving the reactor 12 below 0.8% and preferably below 0.5% v / v. The gas flow leaving the reactor 12 is cooled in the exchangers 5-7, 24 and mixed in 14 with steam and introduced into a first reactor of WGS 15. The gas flow leaving the reactor of WGS 15 is cooled in 16 at values between 50 and 300 ° C and preferably between 100 and 250 ° C and is introduced into a second reactor of WGS 17 which allows to further reduce the percentage of CO to values typically below 0.3% v / v. The stream leaving the reactor of WGS 17 is then cooled in 18, and after condensation and separation of the steam in 19 and sent to a CO2 removal stage20 and subsequently to a methanator 21 which further reduces the CO content to values below below 2 ppm. The gas now contains H2, N2 in ratios between 0.75 and 3.5 v / v and percentages of Ar below 0.8% is compressed in 22 at pressures between 50 and 500 kg / cm and preferably between 100 and 400 kg / cm and sent to the ammonia synthesis and separation section represented by square 23 and square 24 respectively.

A fourth scheme described in Figure 4, provides for an inversion of the oxidizing currents of air and enriched air with respect to the scheme of Figure 3. More specifically, the scheme provides for a first stage of OPC in which the reaction between a stream of pure oxygen or of enriched air with an O2 / C ratio between 0.15 and 0.5 v / v and more preferably between 0.2 and 0.45 v / v and with steam / caibonium ratios between 0 and 1 and a subsequent stage carried out in a further OPC reactor or in a reactor ATR in which the partial oxidation of hydrocarbons with air is completed.

Compared to known technological solutions, the process diagrams described above have the following advantages:

a) possibility of using a large number of hydrocarbon fillers; in practice, gaseous or liquid hydrocarbon fillers can be used even in the absence of a desulphurization step prior to the OPC reaction b) simplicity of construction and operation (in the reactors there are no burners or combustion chambers, no carbon residues are formed)

c) there is no need to use furnaces to pre-heat the reagent streams and therefore there are no CO2 emissions associated with the pre-heating operations.

d) great flexibility of the plant capacity that can be adapted to changes in production needs and the characteristics of the supply currents

e) low investment costs as the reactors are small and contain modest quantities of catalyst

f) low energy consumption resulting from the possibility of entering the synthesis gas production reactors at low temperature and effectively recovering the heat coming out of them.

Claims (4)

CLAIMS 1) Process for the synthesis of ammonia starting from a mixture of H2 and N2 placed to react in the presence of a catalyst in which the gas mixture is obtained through a process of partial catalytic oxidation with a low contact time. 2) Process for the synthesis of ammonia according to the preceding claim in which the recycling of nitrogen is avoided. 3) Process for the synthesis of ammonia according to the first claim in which the catalytic partial oxidation is carried out in combination with an autothermal reforming stage. 4) Process for the synthesis of ammonia according to claim 3) characterized in that the OPC stage has an O2 / C ratio lower than 0.5.