WO2015015463A1

WO2015015463A1 - Process and relating plant for the production of hydrogen

Info

Publication number: WO2015015463A1
Application number: PCT/IB2014/063615
Authority: WO
Inventors: Flavio MANENTI; Sauro PIERUCCI; Lucio Molinari
Original assignee: Politecnico di Milano
Current assignee: Politecnico di Milano
Priority date: 2013-08-02
Filing date: 2014-08-01
Publication date: 2015-02-05
Anticipated expiration: 2016-02-02
Also published as: ITMI20131311A1

Abstract

Process for producing hydrogen comprising an endothermal reaction of sulfur with steam, wherein the energy supply is provided by an exothermal oxidation of a portion of said sulfur to SO₂ according to the following scheme: R2: S + O2-> SO₂ and said endothermal reaction is carried out according to the overall theoretical reaction R1, which does not take into account the aforementioned reaction R2, R1 : S + 2 H₂O-> SO₂ + 2 H₂ And the amount of oxygen by volume fed to carry out the aforementioned exothermal reaction R2 is comprised between 5 and 25% over the total volume of the gaseous reaction mixture. This process can be conducted both in an independent process unit or in a complementary unit to Sulfur Recovery Units installed in petrochemical plants, refineries and natural gas production plants, or in the production of sulfuric acid or ammonia.

Description

"PROCESS AND RELATING PLANT FOR THE PRODUCTION OF

HYDROGEN".

FIELD OF THE INVENTION

The present invention concerns a process for producing hydrogen from sulfur with steam in the presence of oxygen or air.

STATE OF THE ART

Several technologies are known for producing hydrogen, like those involving water electrolysis or those passing through natural gas reforming.

The former are economically disadvantageous since the production cost of hydrogen is higher than the market price, whereas the latter are inevitably linked to the cost and availability of fossil fuels.

Moreover, alternative technologies are already known to the two above mentioned classic hydrogen production branches.

For example "Exergy analysis of hydrogen production from different sulfur-containing compounds based on H2S splitting cycle. Moniri, Armin; Mertins, Pascal; Wang, Hui. INTERNATIONAL JOURNAL OF HYDROGEN ENERGY Volume: 37 Issue: 20 Pages: 15003-15010 DOI: 10.1016/j.ijhydene.2012.07.088, OCT 2012 describes a process and a plant for producing hydrogen that is based on H₂S splitting cycle to give hydrogen and sulfur.

WO2012154041 and WO2012154043 describe a process for producing hydrogen, that substantially exploits the aforementioned H₂S splitting cycle in the sulfur production process according to Claus method, where by operating in conditions of low air or oxygen content, the pyrolysis reaction of H₂S is promoted to give S and H₂ . "Solar hydrogen production by the Hybrid Sulfur process, Corgnale, Claudio; Summers, William A. INTERNATIONAL JOURNAL OF HYDROGEN ENERGY Volume: 36 Issue: 18 Pages: 11604-11619 DOI: 10.1016/j .ijhydene.2011.05.173 Published: SEP 2011, describes the production of hydrogen with the hybrid sulfur cycle (Hys), encompassing the production of hydrogen with a thermal reaction at a temperature above 800°C for splitting sulfuric acid to give oxygen that is removed, S0₂ and water, that in an electrolysis process give hydrogen and once again sulfuric acid that is recycled at the thermal reactor. These types of alternative technologies are also very expensive.

The purpose of the present invention is to obtain a process for producing hydrogen that is cost-effective and that is energy-sustainable.

Moreover, a purpose of the present invention is to have a process that has affinity with different industrial processes of inorganic chemistry, refinery and treatment of gas and oil, substantially reducing the investment, plant revamping and operating costs for producing hydrogen, but also substantially reducing operating costs for compressing hydrogen, thanks to pressure boosts of the same conventional plants.

SUMMARY OF THE INVENTION

The Applicant has now found that it is possible to achieve the desired and other purposes with the process and with the plant or production unit for producing hydrogen, according to the present invention as claimed in the claims attached hereto.

In particular, this process for producing hydrogen comprises the endothermal reaction of sulfur with steam, wherein energy supply is provided by the exothermal oxidation of a portion of said sulfur to S0₂ according to the following scheme: R2: S + 0₂^ S0₂

said endothermal reaction being carried out according to the overall theoretical reaction Rl, not taking into account the aforementioned reaction R2,

Rl : S + 2 H₂0^ S0₂ + 2 ¾

and wherein the volume amount of fed oxygen to carry out the aforementioned exothermal reaction R2 is comprised between 5 and 25% on the total volume of the reactants gaseous mixture .

This process can be carried out in a plant or production unit that can be independent or can be integrated with production units of the conventional type such as a Claus production unit for producing sulfur, production units for producing sulfuric acid with all variants thereof, production units for purifying natural gas, for dehydrosulfurization reaction of petroleum deposits and for coal or hydrocarbons gasification.

DESCRIPTION OF THE FIGURES

Figure 1 represents a block diagram of the thermal reactor wherein the hydrogenation process object of the present invention is carried out. Figure 2 represents a block diagram of the regenerative thermal reactor wherein the process according to the present invention is carried out.

Figure 3 represents a block diagram of a production unit for carrying out the process according to the present invention in the presence of oxygen.

Figure 4 represents a block diagram of a production unit for carrying out the process according to the present invention in the presence of air.

Figure 5 represents a block diagram of a conventional Claus sulfur recovery unit.

Figure 6 represents a block diagram of a production unit for carrying out the process in the presence of air according to the present invention integrated with a Claus recovery unit. Figure 7 represents a block diagram of a production unit for carrying out the process according to the present invention in the presence of oxygen, only integrated with the catalytic section of the Claus process.

Figure 8 represents a block diagram of a unit for producing sulfuric acid of the conventional type. Figure 9 represents a block diagram of a production unit for carrying out the process according to the present invention with oxygen integrated with the plant for producing sulfuric acid.

Figure 10 represents a block diagram of a production unit for carrying out the process of the invention with oxygen applied to the plant for producing sulfuric acid, only associated with the catalytic section of Claus process.

Figure 1 1 represents a block diagram of a production unit for carrying out the process according to the present invention with oxygen only associated with a catalytic section of Claus process applied to the production of sulfuric acid according to Uhde technology.

Figure 12 represents a block diagram of a production unit for carrying out the process according to the present invention with pure oxygen only applied to the catalytic section of the Claus process, integrated with a dehydrosulfurization unit of petroleum deposits.

Figure 13 represents a block diagram of a production unit for carrying out the process in the presence of air according to the present invention integrated with a production unit for ammonia synthesis.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention by the term overall theoretical reaction scheme we mean the general stoichiometric conversion scheme of the reactants. This reaction scheme and all associated stoichiometry can vary according to the reactor methodologies, as well as operating conditions thereof.

For the purposes of the present invention, where not specifically indicated, by oxygen we mean pure oxygen, oxygen-enriched air, combustion air. For the purposes of the present invention by gaseous mixture of reactants we mean the mixture of reactants involved in the reactions Rl and R2, possibly in the presence of inert gases, preferably nitrogen.

For the purposes of the present invention by the term Claus catalytic train we mean the series of catalytic converters and condensers for sulfur recovery, located downstream the thermal section of a Claus unit.

For the purposes of the present invention by operative unit we mean a plant for carrying out the process of the invention comprising a reactor and at least one separation section capable of separating hydrogen from the other gases leaving the reactor and at least one recycling section of the unconverted reactants.

For the purposes of the present invention by independent operative unit we mean an operative unit for carrying out the process of the invention wherein "the at least one separation section" and/or "the at least one recycling section" are different from those belonging to operative units intended for other industrial processes. For the purposes of the present invention by integrated operative unit we mean those operative units in which "the at least one separation section" and/or "the at least one recycling section" belong to an operative unit intended for other industrial processes.

For the purposes of the present invention by the term "portions of S we mean the volume % of S that reacts directly with the amount of oxygen being fed according to the reaction scheme R2, said percentage volume being calculated on the total volume of the gaseous mixture of the reactants.

For the purposes of the present invention by effective regeneration of a regenerative thermal reactor we mean that the regeneration section of the reactor is able to significantly preheat fed gases up to a temperature close to that of the thermal section where the reactions Rl and R2 take place. The reaction responsible for the formation of hydrogen is

Rl : S + 2 H₂0- S0₂ + 2 ¾

This reaction and the related reactor for producing hydrogen according to the present invention represent the pivotal element of the innovative process. This reaction is endothermal. For this purpose the reactor is also fed with air, oxygen-enriched air or pure oxygen, since the reaction of a portion of the sulfur with the aforementioned gases that takes place according to the following scheme R2:

R2: S + 0₂ - S0₂ is highly exothermal. The amount of oxygen is generally comprised between 5 and 25 vol % on the total volume of the gaseous mixture fed at the thermal reactor, more preferably between 5 and 15 vol % when the regeneration is very effective so as to obtain temperatures of the reaction Rl, that allow high conversions to hydrogen, while ensuring optimal partial combustion conditions to prevent the excessive oxidation of the system (S0₂ to S0₃ and H₂ to H₂0.)

The presence of oxygen does not alter the synthesis of hydrogen. As well as the overall chemical reactions Rl and R2, in the thermal reactor other secondary reactions take place that generate by-products, including the formation of H₂S according to the following global reaction:

R3: 3 S + 2 H₂C-» S0₂ + 2 H₂S

The process according to the present invention is preferably carried out in a regenerative thermal reactor or in a thermal reactor.

The latter is a refractory PFR, wherein gaseous streams pass in a turbulent flow, and it is connected to a waste heat boiler.

According to a particularly preferred embodiment, gases leaving the thermal reactor cool down, while entering the boiler on the tubes side, whereas medium-high pressure steam is generated on the shell side. In integrated configurations, this steam can be sent to the thermal reactor as reactant. The thermal reactor can assume different configurations with reactants fed at one or more areas. For example, feed can occur in a burner where sulfur, oxygen and steam flow together or where sulfur and oxygen flow together, and steam is fed elsewhere. The operating temperature of the reactor is preferably comprised between 1000-1550 °C, more preferably at temperatures higher than 1300°C, compatibly with the activation of the reaction of sulfur with steam and with the technological limits of construction and operation of the furnace and for residence times of between 0.1 and 3 seconds.

The operating pressure can vary from conditions slightly above ambient pressure up to medium-high pressures, like for example 30-100 bar. Low pressures (preferably 1.3-1.8 absolute bar) favor higher production of hydrogen and integration with a Claus process that operates at these conditions, whereas high pressures have better affinity with high pressure processes using hydrogen (like methanol or hydrodesulfurization). Residence times are preferably comprised between 0.5 and 1.5 seconds. These parameters were obtained by carrying out a simulation that we describe herein below for illustrating but not limiting purposes.

The thermal reactor is simulated by using complex kinetic schemes (Manenti et al. Multi-scale modelling of Claus thermal furnace and waste heat boiler using detailed kinetics. Computers and Chemical Engineering, 59, 219-225, 2013), that comprise 2426 chemical reactions and 142 species. The detailed kinetic scheme was validated on more than 20 sulfur recovery plants in good agreement with the predictions of the model and the industrial data. The simulation of the invention cannot be carried out through commercial software, since the main process simulators are not equipped with complex kinetic schemes. Therefore, this simulation is carried out with the aforementioned tools, previously validated in field.

The thermal reactor is schematized in Figure 1. The data of the simulations are given in the following tables 1 and 2.

Table 1. Data for the thermal reactor simulation: case at 1300°C.

Table 2. Data for the thermal reactor simulation: case at 1500°C.

An alternative configuration of the reactor is a (energy-integrated) regenerative thermal reactor. In particular, the regenerative thermal reactor preferably comprises:

a) a regenerative section consisting of a tubular PFR reactor in which the gases pass in a turbulent flow,

b) a thermal section consisting of a refractory PFR reactor (like the thermal reactor), in which the reactions Rl and R2 take place,

said regenerative thermal reactor possibly being connected to a waste heat boiler.

An embodiment of this type of reactor is shown in figure 2. The reactor receives the stream of sulfur and steam at relatively low temperature (for example 250°C). The stream is preheated up to 700 °C and beyond in the regenerative section by exchanging heat in counter-current with the effluents of the furnace (for example at 1300°C). Once 700°C are reached, oxygen is supplied to further increase the temperature up to the desired value (1300-1550°C). The effluents finally exchange heat with inlet gas . In this way a significant heat recovery is obtained, that allows such an effective regeneration that the amount of oxygen required to sustain the process is much less than that required when the process according to the present invention is carried out in the thermal reactor. The simulation data for the integrated reactor are given in the following tables 3 and 4. Table 3. Data for the regenerative thermal reactor simulation: case at 1300°C.

Inlet Streams Sx o₂ H₂0

Inlet temperature [°C] 700 700 700

Inlet pressure [a bar] 1.5 1.5 1.5

Inlet flow rate [kmol/h] 10 1.91 16

RESULTS

4.87%

H₂ (molar fraction)

46.60%

H₂0 (molar fraction)

8.17%

H₂S (molar fraction)

13.62%

SO2 (molar fraction)

25.98%

S₂ (molar fraction) Table 4. Data for the regenerative thermal reactor simulation: case at 1500°C.

The ratio between produced hydrogen and ideally converted sulfur tends to 2 by volume according to stoichiometry of the global reaction Rl + R2, whether the process is carried out with the thermal reactor or with the regenerative thermal reactor. The effluent of the reactor is then cooled down and hydrogen is separated from the other compounds, including unreacted water and sulfur and other reaction products and byproducts (S0₂ and H₂S). For this reason, the plant for conducting the process of the present invention also comprises a conversion/ condensation/ compression section that allows the separation of the various components of the gaseous mixture leaving the reactor, said separation being able to be particularly differentiated, according to the uses of the various streams, besides hydrogen, that can be obtained, and also according to the production site wherein the process of the invention is conducted.

The separation can be realized by succeeding operations, even in a different order, or by integrated operations.

For example, the joint separation of H₂S and S0₂ can take place by catalytic converters of Claus type. It is possible to take advantage of existing technologies for the separation (e.g., condensers, phase separators, membranes, absorption/stripper columns). Hereafter some possible variants are given.

Figure 3 shows a particular preferred embodiment of the independent production unit intended only for producing hydrogen obtained with the process according to the present invention carried out by feeding the thermal reactor (indicated with "NEW" in the figures), in addition to elemental sulfur (S_x) and steam, pure oxygen.

In the reactor the partial conversion of S₂ and H₂0 with generation of H₂ and S0₂ occurs. The effluent is cooled down in a waste heat boiler, not represented in the figures but described earlier. Cooling must be quick enough to prevent effluent extensive composition rearrangements. (Manenti et al., Design of SRU thermal reactor and waste heat boiler considering recombination reactions, Procedia Engineering, 42, 414-421, 2012).

After cooling, sulfur separation is realized by condensation in the section indicated in the figures with Sep. 1. Sulfur condensate is sent to sulfur pit, indicated in the figures with SULFUR PIT, then recycled to the thermal reactor. The remaining stream contains S0₂, H₂S (formed with the aforementioned reaction R3), H₂0 and H₂.

In the second section indicated in the figures with Sep. 2, by further cooling, water is separated by condensation, separated water is recycled and sent to thermal reactor feed. Effluents coming from Sep. 2 comprise H₂, ¾S and S0₂. Since the reaction environment, in the reactor indicated with NEW and wherein the process reaction occurs, is characterized by a low oxygen content, no formation of S0₃ is observed, that would be particularly corrosive in aqueous condensates. Possible sulfurous substances dissolved therein are in any case recycled to the thermal reactor for subsequent conversion. It is then possible to separate S0₂ in the third section indicated in the Figures with Sep. 3, by solvent washing or by condensation. The last separation step of H₂ from H S indicated with Sep.4, can be carried out by amine washing. Unlike H₂S, hydrogen does not have acid behavior, therefore it is not absorbed by water-amine solution and is released at the top of the absorption tower. Absorbed H₂S at the absorption column is stripped at the top of these processes stripper column, associated with the absorption column. The operating conditions of the absorption column can be reformulated based on hydrogen compression requirements. Absorption can be promoted at higher pressures.

S0₂ and H₂S are not recycled to the reactor, but can be supplied to different chemical plants: S0₂ can be stored as liquid or sent to the sulfuric acid production cycle; alternatively H₂S and S0₂ can be fed to a Claus line for the production of elemental sulfur.

The production unit for carrying out the process according to the present invention is therefore able to produce hydrogen, and is energy-sustainable and with the possibility of zero emissions, for example if the production unit according to the present invention is associated with a hydrodesulfurization process. Moreover, recycling unconverted S₂ and H₂0 is accomplished by pumping liquids with low operating costs. The separations for hydrogen purification can take place without any additional compression cost, if compared with classical separation processes technologies (amine washing).

In case air is fed (variant of Figure 4), the production unit does not differ from that of Figure 3 with the sole difference that the product is a H₂/N₂ mixture, that can be used directly for some chemical productions (ammonia) or it needs a further separation of H₂ from N₂.

A further aspect of the present invention as shown above are the fields of application of the process of the invention in industrial processes known in inorganic chemistry and in refinery.

Hereafter some preferred embodiments of the plant for carrying out the present invention are shown integrated/applied in plants for conducting the aforementioned industrial processes.

1- Sulfur recovery unit with co-generation of hydrogen (revamp- 1 and revamp-2)

A conventional Claus recovery unit is shown in Figure 5.

The acid gas (H₂S) to be treated is fed with combustion air to a Claus furnace, indicated in the figures with CTF where partial combustion of H₂S takes place to give S0₂ according to the reaction:

R4: H₂S + l,5 0₂ -> S0₂ + H₂0 The reaction generally occurs at temperatures higher than 1000°C, in some cases higher than 1500°C. The stream, leaving Claus furnace, is quickly cooled down to about 300°C in a waste heat boiler indicated in the figures with WHB, thereby producing medium pressure steam. A condenser Cond downstream WHB refines cooling by separating elemental sulfur, which condenses and is sent to a sulfur pit, indicated in the figures with SULFUR PIT. At this point, the stream, free from sulfur, mainly consisting of H₂S, S0₂ and H₂0, enters the first Claus catalytic converter (CC1), where the Claus reaction takes place:

R5: 2 H₂S + S0₂^ 3 S + H₂0

This is an equilibrium reaction, that is required to be conducted in many stages with intermediate heat treatment and progressive removal of the product (S₂). With 2-3 catalytic stages conversion yields of over 90% are reached. The residue is usually a small percentage of unreacted H₂S (e.g. due to H₂S/S0₂ molar ratios slightly above 2) leaving the furnace. The stream is then sent to amine washing to reduce the residual H₂S, after flowing in a hydrogenation and quenching section for the transformation of the sulfur compounds into H₂S and, then, for separation of the process water. The washing process is carried out in an absorption column indicated in the figures with ABSORBER and in a stripper column indicated in the figures with STRIPPER. A heat exchanger is associated with the absorber for energy recovery, not represented in the figures. The stream to be purified is fed at the bottom of the ABSORBER and, rising up the column, meets an amine solution in counter-current that reduces the content of H₂S and of other acidic compounds (e.g. C0₂). The purified stream leaves the top of the ABSORBER. The amine solution rich in H₂S is sent, after heat exchange, to a stripper indicated in the figures with STRIPPER. The STRIPPER operates at low pressures to promote the outflow of gases absorbed in the ABSORBER. H₂S is recycled from the head of the STRIPPER and is recycled upstream, as Claus furnace feed, in addition to other acid gas sent to Claus process.

Amine washing is also used for other processes in refinery or for natural gas, to separate for example methane or light hydrocarbons from H₂S.

The process of the invention and the production units that allow it to be carried out make it possible to use at the best the existing technology of Claus process,s thereby producing hydrogen with very low costs. A possible revamp (Revamp- 1) is shown in Figure 6. In this case, since Claus process uses combustion air, it is preferably to supply also the reactor of the process of the invention, indicated with NEW in the figures, with combustion air (the outlet hydrogen would still have a fraction of N₂, corresponding to that of Claus, also in case pure oxygen is used as feed in the process of the invention). Steam and sulfur fed to the reactor derive directly from Claus: the first generated by the WHB, the second taken directly from sulfur pit also indicated in this figure with SULFUR PIT. The composition leaving the reactor is shown in tables 1-2 for the thermal reactor, whereas for the regenerative thermal reactor, reference is made to tables 3-4. The effluents of the CTF Claus furnace and the reactor NEW of the process according to the invention are both conveyed to the first condenser indicated with Cond for sulfur separation. At this point, it is possible to exert optimally both the process of the invention and the Claus process, to reach an optimal H₂S/S0₂ ratio equal to 2 entering the catalytic converters, indicated with CC1 and CC2 also in this figure. This is possible by suitably dosing the combustion air to the two thermal reactors. In this case, the invention not only makes it possible to have a closed cycle as far the use of the sulfur and hydrogen production are concerned, but it also makes it possible to manage at the best Claus catalytic conversion train, by adding a degree of freedom to the control system of the entire plant, since a specific amount of S0₂ and H₂S is provided, permitting the ratio of 2 to be reached for H₂S/S0₂ fed to the catalytic train. After the first sulfur removal, the mixture of H₂S, S0₂, H₂0 and H₂ (also including N₂ of the combustion air) is fed to the CC1 and CC2, where the Claus reaction (R5) takes place. H₂ is inert on the catalytic bed, in the condensation and intermediate removal operations. Therefore the effluents leaving the third condenser will contain H₂S in a very small fraction thanks to the better control of Claus process, by the introduction of the process according to the invention, together with H₂ and N₂. The water formed both with the process of the invention and with Claus process is previously separated.

H₂ and N₂ are then recovered at the top of the ABSORBER; the H₂S is recovered at the STRIPPER and recycled upstream. With the revamp-1 of the Claus process, the invention also takes advantage that the catalytic converters, indicated in the figures with CC1 and CC2, actually replace the separation process of S0₂ from the stream that contains hydrogen.

A second revamp (Revamp-2) of the sulfur recovery units is shown in Figure 7. It < makes it possible to produce pure hydrogen instead of N₂/H₂ mixture. In this case, the thermal section of Claus (Claus furnace and waste heat boiler) is removed. The invention can receive 0₂ as feed (no longer air), as well as steam and elemental sulfur coming from the pit. The thermal reactor of the invention generates the aforementioned hydrogen-rich stream. This stream, as in Revamp-1, is sent to the first condenser indicated in the figures with Cond for S₂ separation. It is then sent to sulfur pit indicated in the Figures with SULFUR PIT and fed back to the reactor NEW. At G st either before or after) the condenser Cond the stream leaving the reactor meets the acid gas (H₂S) and is dosed in order to reach the optimal H₂S/S0₂ ratio for Claus reaction (R5). Alternatively, if the flow rate of acid gas cannot be arranged, S₂, H₂0 and 0₂ streams fed to the reactor NEW, can be suitably controlled, also in this case providing a further degree of freedom to manage the catalytic train of Claus process. Indeed, it is worth emphasizing that the addition of oxygen entering the thermal reactor of the invention not only makes it possible to energetically sustain the production of hydrogen, but also unbalance towards S0₂ the formation of the by-product H₂S through partial oxidation thereof according to the reaction R4 (primary reaction for Claus furnace and secondary for the invention). The hydrogen produced by the invention remains inert inside the catalytic beds of the CC1-CC2, and in the thermal preheating and cooling operations typical of Claus processes. Similarly to what has already been described, finally, hydrogen is recovered at the top of the ABSORBER of the amine washing. The stripped H₂S is recycled upstream the thermal reactor of the invention.

Process for producing sulfuric acid (Revamp- 1, Revamp-2 and Revamp-3

The production of sulfuric acid encompasses the direct oxidation of sulfur compounds, like acid gas (H₂S) according to the global reaction R4 or elemental sulfur (S₂) according to the global reaction R3, proposed hereafter:

R3: S + 0₂ - S0₂ With subsequent further oxidation of S0₂:

R6: S0₂ + 0.5 0₂ ^ S0₃ and finally absorption in water of S0₃ to give sulfuric acid:

R7: S0₃ + H₂0 -=> H₂S0₄

The invention enters the production process of sulfuric acid generating hydrogen and forming in part or entirely the amount of S0₂ required to avoid the oxidation furnace for the reaction R3. The conventional process for producing sulfuric acid is shown in Figure 8; the entry of the invention in a possible revamp (Revamp- 1) is shown in Figure 9. Since the thermal furnace for direct oxidation of S₂ to S0₂ (R3) is replaced by the process of the invention, wherein S0₂ is still obtained, the net return is the entire production of hydrogen obtained with the process according to the invention. The production unit for carrying out the process according to the invention offers the possibility of unbalancing the formation of S0₂ at expense of H₂S. However if also small fractions of H₂S are wished to be recovered, it is possible to combine the production unit according to the invention with one or more Claus catalytic conversion stages and then send S0₂ to the sulfuric acid production process as shown in Figure 10. In this case, by unbalancing the reaction towards higher production of S0₂, H₂S could be converted to sulfur already in the first CC, avoiding a series of CC. As an alternative, H₂S generated as by-product can be recycled to the reactor. H₂S has, indeed, a shorter ignition time than sulfur and hydrogen itself, so that it would be the first compound that, upon coming in contact with 0₂/air, is oxidized to S0₂ and H₂0 and, by partial pyrolysis, promotes the formation of further hydrogen.

Finally, the invention (as far as the production of hydrogen is concerned) allows some technologies to be made sustainable: production of sulfuric acid with zero emissions (Revamp-3, Figure 1 1); this technology, indeed, foresees the reduction of the emissions of S0₂ to H₂S through hydrogenation:

R8: S0₂ + 3 H₂ -» H₂S + 2 H₂0 and subsequent use of H₂S in a furnace for primary oxidation of ¾S to S0₂. In this representation the corresponding reactor wherein said possible primary oxidation takes place is not represented. If the primary oxidation is not foreseen in the sulfuric acid production technology, H₂S generated by hydrogenation is conveyed to the start of the catalytic section of Claus process.

Hydrogen produced by the invention allows the production of sulfuric acid with zero emissions to be made sustainable, providing H₂ necessary for the reaction R8.

3. purification of natural gas

Natural gas is a mixture of so-called light or volatile hydrocarbon compounds, often associated with a certain percentage of H₂S varying according to the deposit. For example, one of the largest deposits of natural gas in the world, Kashagan (Kazakhstan), has a high content of H₂S (about 20%). The need is felt to purify the natural gas from H₂S and the most common technology is amine washing, already mentioned in the previous cases. Since the removal of large amounts of H₂S has the problem that this gas needs to be stored and treated, usually Claus technologies are exploited for partial combustion and production of sulfur. In this context of co-presence of washing plants and of sulfur lines, the process of the invention entry allows significant amounts of hydrogen to be produced.

4- Sustainabilitv of hydrodesulfurization processes

Hydrodesulfurization processes are particularly important for removing sulfur from petroleum deposits for the production of fuels with low sulfur content, but also for removing sulfur from the increasingly common biofuels of second or third generation. Hydrocarbon deposits containing sulfur are treated in a strongly reducing environment usually at high pressure operative conditions to promote the reaction:

R9: Ri-S- R₂ + 2 H₂ -» RjH + R₂H + H₂S wherein R] and R₂ are alkyl residues of hydrocarbon chains. The hydrogen necessary for the hydrodesulfurization reaction is provided by the process of the invention (Figure 12). In this figure, a preferred embodiment of the production unit to carry out this process is represented. The hydrogen is supplied in excess to the hydrodesulfurization reactor indicated with HYDRODESULF and the residual hydrogen, together with H₂S therein formed, are sent to a washing station. The hydrogen recovered at the top of the absorber, indicated in the figure with ABSORBER, is recycled upstream of the hydrodesulfurization reactor, whereas H₂S separated at the top of the stripper column indicated in the figures with STRIPPER, is recycled upstream the catalytic section of the Claus process. 5 - Ammonia production unit

For the production of ammonia, occurring according to the global reaction:

R10: N₂ + 3 H₂ ^ 2 NH₃ particularly high pressures and large amounts of H₂ are necessary. The invention makes it possible to partially or entirely sustain the supply of N₂ and H₂ for the production of ammonia through the use of all of the aforementioned variants encompassing the use of air (or enriched air) for the energy sustainment of the thermal reactor of the invention. Figure 13 shows a preferred embodiment and in particular one that considers the use of the plant shown in Figure 4, so that the effluents from Sep 4 are fed to the ammonia production plant. Also in this case the production of H₂ can be associated with the Claus process.

Claims

1. Process for producing hydrogen comprising an endothermal reaction of sulfur with steam, wherein energy supply is provided by an exothermal oxidation of a portion of S to S0₂ according to the following scheme: R2: S + 0₂-> S0₂

wherein said endothermal reaction is carried out according to an overall theoretical reaction Rl, not taking into account the aforementioned reaction R2,

Rl : S + 2 H₂0 -> S0₂ + 2 H₂

and the amount by volume of oxygen fed to carry out the aforementioned exothermal reaction R2 is comprised between 5 and 25% on the total volume of the gaseous mixture of reactants.

2. Process according to claim 1, wherein said amount of oxygen by volume is comprised between 5 and 15% on the total volume of the gaseous mixture of reactants.

3. Process according to any one of claims 1 or 2, carried out at a temperature comprised between 1000 and 1550°C for a time period comprised between 0.1s and 3s.

4. Process according to claim 2 wherein said process is carried out at a temperature above 1300°C for a time period comprised between 0.5s and 1.5s.

5. Process for producing hydrogen according to any one of claims 1-4, carried out in a production unit comprising a reactor (NEW) where the reaction Rl and R2 are carried out, and at least one independent or integrated separation section (Sep l-Sep.4, CC1- CC3, CC Cond, ABSORBER, STRIPPER) of the various components of the gas mixture leaving the reactor and at least one section dedicated to recycling the sulfur and the unconverted water.

6. Process according to any one of claims 1-5, wherein said reactor is selected from a regenerative reactor or a thermal reactor.

7. Process according to claim 6, wherein said thermal regenerative reactor comprises: a) a regenerative section consisting of a tubular PFR reactor in which gases pass in a turbulent flow,

b) a thermal section consisting of a refractory PFR reactor, in which the reactions Rl and R2 take place,

said thermal regenerative reactor possibly being connected to a waste heat boiler (WHB).

8. Process according to claim 6, wherein said thermal reactor is a refractory PFR reactor through which the gases pass in a turbulent flow, and is equipped downstream with a waste heat boiler (WHB)

9. Process according to claim 7 or 8, wherein said waste heat boiler receives the gases leaving the reactor that cool down on the tubes side, generating medium-high pressure steam on the shell side, said steam being possibly recycled as feed to the reactor.

10. Process according to one of claims 5-9, wherein said reactor permits feeding separated or pre-mixed reactants.

1 1. Process according to any one of claims 5-10, for producing hydrogen, wherein the reactor (NEW) is fed with pure oxygen or air, elemental sulfur and steam and wherein the section that allows the separation of the various components of the mixture comprises:

• a first separation obtained with Sep.l having the purpose of separating sulfur produced in the invention, said elemental sulfur being collected in a sulfur pit (SULFUR PIT) and then recycled and heated to be fed to the reactor (NEW),

• a second separation wherein the gaseous mixture leaving the first separation section (Sep. 1) and comprising H₂, S0₂, H₂0 and S0₂ and possibly nitrogen, in case air is used in place of pure oxygen, is cooled to allow the separation of the water by condensation,

· a third separation section (Sep. 3) of S0₂ from the gaseous mixture leaving the second separation section (Sep. 2) through suitable washing,

• finally a fourth separation section (Sep. 4), wherein only ¾S is removed from the mixture leaving Sep. 3, and H₂ and possibly N₂ are released; • a possible further separation area of H₂ from N₂.

12. Process according to any one of claims 5-10, wherein gases fed into the reactor (NEW) are sulfur, air and steam, and wherein gases leaving the same reactor are sent to a first condenser (Cond.) arranged downstream a steam generator (WHB) of a Claus unit, said condenser being intended for the separation of sulfur that is collected in a sulfur pit (SULFUR PIT) and then recycled to the thermal reactor (NEW), whereas gases leaving said first condenser are sent to a catalytic conversion section, where the reaction R5: 2H₂S+S0₂ -> 3S +2H₂0, takes place, said section comprising at least two catalytic converters (CC1 and CC2) arranged in succession and separated by at least 2 condensers (Cond) to allow the partial recovery of the sulfur formed in each converter, said sulfur being conveyed to a sulfur pit (SULFUR PIT), and gases leaving the last condenser (Cond) of said catalytic conversion section and comprising H₂, N₂ and residual amounts of sulfur products, after flowing in a hydrogenation reactor and in a quenching column, are sent to an absorption column (ABSORBER), wherein an amine washing is carried out to absorb H₂S and, at the top of said column H₂ and N₂ are recovered, whereas H₂S absorbed in said absorption column and recovered at the top of a stripper column (STRIPPER) associated with said absorption column, is subsequently sent to Claus furnace (CTF) feed.

13. Process according to any one of claims 5-10, wherein gases fed into the reactor (NEW) are sulfur, pure oxygen and steam, and wherein gases, leaving the same reactor, are sent to a catalytic section of a Claus unit, comprising at least two catalytic converters and at least two condensers to allow the partial recovery of sulfur, formed in each of said converters, and recycle thereof to the reactor (NEW), whereas gases leaving the last condenser and comprising H₂ and residual amounts of H₂S, are sent to an absorption column (ABSORBER), where at the top hydrogen is recovered, whereas absorbed H₂S recovered at the top of a stripper column (STRIPPER) associated with said absorption column, is sent to a condenser (Cond) arranged upstream of said catalytic conversion section.

14. Process according to any one of claims 12 and 13, carried out at pressures comprised between 1.3 and 1.8 bar a.

15. Process according to claim 11 , wherein gases fed to the reactor (NEW) are sulfur, pure oxygen and steam, and S0₂ coming from the third separation section Sep. 3, is sent to a first reactor (R6) together with 0₂ or air, of a production unit of sulfuric acid, where the first reaction R6: S0₂ + 0.5 0₂ - S0₃ is carried out and wherein sulfur trioxide obtained in said first reactor (R6) is conveyed to a second reactor (R7) together with steam and where the homonymous reaction R7: S0₃ + H₂0 - H₂S0₄ is carried out.

16. Process according to any one of claims 5-10, wherein gases fed to reactor (NEW) are sulfur, pure oxygen and steam, and wherein gases leaving the same reactor are sent to ta catalytic section of a Claus unit, comprising at least one catalytic converter (CC) and at least one condenser (Cond) to allow the recovery of sulfur formed in said converter, and gases leaving the last condenser and comprising H₂ and S0₂ are separated in the separation section (Sep.) by washing with solvent or by condensation of S0₂ and the latter is conveyed together with oxygen or air, in a first reactor (R6) of a production unit of sulfuric acid, in which first reactor (R6) the reaction R6: S0₂ + 0.5 0₂ -> S0₃ takes place and wherein sulfur trioxide obtained here is conveyed to a second reactor (R7) together with steam, wherein the reaction R7: S0₃ + H₂0 H₂S0₄ is carried out.

17. Process according to claim 16, wherein the production unit for producing sulfuric acid comprises a third reactor (R8) arranged downstream the second reactor (R7), in which third reactor (R8) the reaction R8: S0₂ + 3 H₂ - H₂S + 2 H₂0 is carried out and hydrogen coming from the separation section (Sep.) of the production unit is in part conveyed to said third reactor, wherein it reacts with the residual S0₂ leaving the second reactor (R7), and H₂S formed in said third reactor (R8) is conveyed upstream the first reactor (R6), if the production starts from the oxidation of H S, or, alternatively, it is conveyed upstream the catalytic train of a Claus unit.

18. Process according to claim 11 or 12, for removing H₂S from natural gas.

19. Process according to claim 12, wherein hydrogen recovered at the top of the absorption column is sent to a hydrodesulfurization reactor (HYDRODESULF) of a petroleum deposit (OIL) for conducting the reaction R9: Ri-S- R₂ + 2 H₂ - RiH + R- ₂H + H₂S wherein Ri and R₂ are residues of hydrocarbon chains, H₂S formed is sent to a washing section and the excess hydrogen, recovered at the top of the washing column (ABSORBER), is recycled to the reactor (HYDRODESULF), whereas the H₂S recovered by a stripper column (STRIPPER) of the washing section, is sent to a first condenser (Cond) arranged upstream of a Claus catalytic conversion section.

20. Process according to claim 19, carried out at pressures comprised between 30 and 100 a bar.

21. Process according to claim 1 1, wherein gases fed to the reactor (NEW) are steam, air and elemental sulfur, and wherein H₂ and N₂ leaving the fourth separation section (Sep. 4) are sent to an ammonia production unit.