WO2009000497A1

WO2009000497A1 - Purification process for gas from hydrogen sulfide through regenerated ferric ion solutions

Info

Publication number: WO2009000497A1
Application number: PCT/EP2008/005089
Authority: WO
Inventors: Giorgio La Sala; Renato Guerriero
Original assignee: Stc S.R.L. Science Technology & Consulting
Priority date: 2007-06-28
Filing date: 2008-06-24
Publication date: 2008-12-31
Also published as: ITBR20070004A1

Abstract

The present invention relates to a continuous or discontinuous purification process for gas or gas mixtures or liquefied gas from hydrogen sulfide through ferric ion solutions electrochemically regenerated. The removal of hydrogen sulfide H2S from gas is carried out by contacting the gas with an acquous solution, eg. of iron(III) sulphate Fe2(SO4)3, in one or more reactors chosen from a filler scrubber, a tray column or a bubble column or by 'membrane contactors', in order to achieve a gas-liquid or liquid-liquid absorbtion with reaction. The iron(III) sulphate and the hydrogen sulfide react at room temperature: the ferric ions Fe3+ are converted into bivalent iron ions Fe2+ (ferrous ions) while the hydrogen sulfide is oxidated to elementary sulphur. The ferrous iron obtained in this way is oxidated by an anode into ferric iron in an unipolar or multiple bipolar electrolytic cell and is then returned to the beginning of the gas-liquid or liquid-liquid absorption phase.

Description

"PURIFICATION PROCESS FOR GAS FROM HYDROGEN SULFIDE

THROUGH REGENERATED FERRIC ION SOLUTIONS" Description The present invention regards a purification process for gas or gas mixtures or liquefied gas from hydrogen sulfide through ferric ion solutions, electrochemically regenerated. Hydrogen sulfide, H₂S, is a polluting gas present in natural gases, in biogas and in liquefied gas from the industry, but it can also be present as a by-product in gas emissions from the chemical industry, from oil-refining and from gasification of carbon. Hydrogen sulfide is also often present in refluents from industrial plants. The content of H₂S in these gas flows can slightly vary but in general it oscillates between 0,005% up to above 10% in volume. Hydrogen sulfide is a very known problem in the industry because of its toxicity as well as for corrosion problems that can occur in the plant in the case it is not removed from the gas. For this purpose the maximum admissible operative concentration of H₂S in an industrial flow is 1000 ppm while the maximum concentration of H₂S in a gas flow released in the environment is 5 ppm by law. The most commonly used techniques for the desulphurization of gas emissions are based on the Claus reaction between hydrogen sulfide and oxygen to form sulphurous anhydride and water:

(1) H₂S + 3/2O₂ → H₂O + SO₂

(2) 2H₂S + SO₂ → 2H₂O + 3S The Claus technology requires, however, great investments, it can not be directly applied to the natural gas treatment and it is usually not used with gas emissions that have sulphur quantities less than 15 tons/day. In fact, in these cases at the end of the Claus process, despite the recovery of the main part of the sulphur, the gas flow in the exit is still rich of sulphurated species: therefore, an additional gas treatment phase is necessary, constituted by the conversion of all sulphurated species in H₂S through cathalytic hydrolysis, and with a subsequent capture of the H₂S with a suitable solvent. Such plants turn out to be extremely complex and thus appropriate only for petrochemical plants with high operating costs.

Other processes have been proposed for the hydrogen sulfide removal from gas, most of them consist in contacting H₂S with an acid solution of ferric sulphate.

The overall reaction that involves the absorption with reaction of the hydrogen sulfide in which ferric sulphate is consumed obtaining elementary solid sulphur, sulphuric acid and ferrous sulphate is the following:

(3) H₂S + Fe₂ (SO₄ )₃ → S + 2FeSO₄ + H₂SO₄ After the separation of the sulphur, the solution is treated to regenerate the ferric sulphate starting from sulphuric acid and ferrous sulphate according to the reaction:

(4) 2Fe 5O₄ + H₂SO₄ + 0.5O₂ → Fe₂ (SO₄), + H₂O

The commonly adopted method to regenerate the ferric sulphate, starting from ferrous sulphate, is oxidation that can be carried out either with air or taking advantage of the cathalytic capacity of iron-oxidizing bacteria such as Thiobacilli, that are able to accelerate for circa 500.000 times, in an acid environment, the natural oxidation velocity. In the last years multiple attempts have been performed to develop separation processes for hydrogen sulfide based on following reactions:

(5) H₂S + Fe₂ (SO₄ )₃ → S + FeSO₄ + H₂SO₄ (6) 2FeSO₄ + H₂SO₄ + 0.5O₂ → Fe₂(SO₄)₃ + H₂O

Whose overall reaction is:

(7) H₂S + 0.5O₂ → S + H₂O

The use of these reactions present multiple advantages:

• The chemical reaction (5) is almost instantaneous and can be carried out with relatively high iron ion concentrations, without using complex organic agents, limiting in this way precipitation phenomenon of the reactant in hydroxide form under acid pH conditions;

• The biological reaction (6) takes place spontaneously at room temperature, does not foresee consumption of expensive products but only of microorganisms, such as T. ferroxidans, that require only CO₂ (present in air) and basic mineral salts, for its multiplication. There are no problems of sterility since the Thiobacilly are the only microorganisms capable to multiply at strongly acid pH-values, using the energy obtained from the oxidation of the ferrous ions to ferric ions;

• The separation processes of H₂S based on the reactions (5) and (6), are extremely versatile and flexible, and can be used in almost all application contexts and for whatever kind of gas composition to be treated.

Despite these advantages, the development of technologies based on the reactions (5) and (6) is obstructed by the necessity to resolve following problems:

• The removal process of crystalline sulphur, formed subsequently to reaction (5), is a critical stadium for the continuity of the unit operations, to minimize the loss of iron ions and to obtain a product of sufficient purity to be resold on the market;

• Reaction (6) by biological means presents a very delicate equilibrium on which the stability of the continuous advancement of the two main unit operations depends. One of the equilibrium critical parameter is, for example, the pH that can vary only in a very narrow range, usually between 1.4 and 2.0. Below the minimum value, the activity of the T. ferroxidans is inhibited, above the maximum value precipitation phenomenon of the ferric ions will be triggered;

• Another critical parameter consists in maintaining the correct concentration of ions ammonium, phosphate, potassium and magnesium, necessary together with CO₂, for the multiplication of T.ferroxidans to avoid precipitation of the non soluble substances under the operative conditions used in the process;

• The low efficiency of the common reactors for the oxidation of ferrous ions into ferric ions biocatalyzed by Thiobacilli. Being forced to use reactors of considerable dimensions creates economic and technical problems in the practical use of reaction (6),

In line with data in literature, many technological approaches have been proposed, finalized to use reaction (5) and (6) and resolve the practical problems mentioned above. EP220776 for example describes a chemical-biological desulphurization process that uses the reactions (5) and (6) in the bioreactor, in which the ferric sulphate is regenerated, in and the microorganisms are inserted by a hanging medium and which are then recovered from the solution containing oxidized iron using membranes for ultrafiltration. At the same time high purity sulphur is obtained.

EP 280750 describes a process, in batch and for a limited time of maximum three days in continuous, for chemical-biological desulphurization that uses the reactions (5) and (6) in which the bioreactor contains a solid medium on which the iron-oxidative bacteria have been deposited, and operates in a submerged medium. In this way it is possible to increase the efficiency of the bioreactor and at the same time limit the bacterial contamination of the produced sulphur.

EP811416 describes a chemical-biological desulphurization process that uses the reactions (5) and (6), in which the ferrous sulphate oxidation takes place in a bioreactor containing a solid carrier covered with a biofilm of iron-oxidative bacteria of the type Thiobacillus, that operates in a submerged medium (fluid bed). This patent also describes the possibility to conduct reaction (5) at temperatures higher than room temperature in order to obtain more easily separable crystalline sulphur. US4931262 describes a chemical-biological process that uses the reactions (5) and (6) that applies first an oxidation phase of the ferrous sulphate into ferric sulphate in a reactor vessel filled with a support on which an iron-oxidative bacteria has been deposited, which is followed by an absorption phase of H₂S in the ferric sulphate solution, with subsequent separation and recovery of the produced basic sulphur and a return of the ferrous sulphate solution, free from elementary sulphur, to the oxidation vessel.

US2006/0251571 describes a continuous chemical-biological process for the desulphurization of gas flows containing H₂S in which the gas flow to be purified is fed in the bottom of an absorption column together with the liquid flow, coming out from the bottom of a biological reactor, containing a ferric/ferrous sulphate solution. The gas flow, purified from H₂S, is collected in the top of the absorption column, together with another flow constituted of a ferrous/ferric sulphate solution and in which elementary sulphur is suspended that is then separated and removed by filtration. The liquid flow containing ferric/ferrous sulphate solution purified from sulphur, hydroxide and ammonium phosphate is fed in the head of a biological reactor, constituted of a fluid bed reactor containing acid- resistant carrier colonized by T. ferroxidans, while in the bottom a gas flow composed by air or air enriched with O₂/CO₂ is fed.

A process has now been found, that is based on three main operations: gas-liquid or liquid- liquid absorption with reaction realized with traditional methods, removal of sulphur, regeneration of the ferrous iron through anodic oxidation in a unipolar or multiple bipolar electrolytic cell, which allows the hydrogen sulphide removal both in continuous and in discontinuous from gas flows using solutions containing ferric ions electrochemically regenerated and obtaining in this way purified flows with a residual hydrogen sulphide content up to 1 ppm. In this way all problems connected to the conventional technologies which foresee other processes of chemical or biological oxidation are avoided, reducing investments and making in this way the whole process more versatile and economic and obtaining by-products of high added value.

The electrochemical operation is conducted using a particular type of electrolytic cell whose first application is mentioned in the patent EP0268319 describing a method to obtain simultaneously metallic manganese and manganese dioxide by electrolysis in a unipolar electrolytic cell in which the anodic and cathodic compartments are separated by an anionic membrane. In this application the anodic compartment of the cell is fed with sulphuric solution of manganese sulphate while the cathodic compartment is fed with a sulphuric solution of manganese sulphate, ammonium sulphate and SO₂: the electrolysis takes place by the passage of SO₄ ^2" ions from the cathodic compartment to the anodic one. The process, subject of the present invention, for the purification of gas or liquefied gas flows from hydrogen sulfide through aqueous solutions containing ferric ions electrochemically regenerated comprises essentially:

^■ A gas-liquid or liquid-liquid absorption with reaction of the said gas flow containing H₂S with an acid liquid flow containing predominantly ferric ions and, in a smaller quantity, ferrous ions, obtaining in this way a gas or liquefied gas flow purified from H₂S and a liquid flow containing mainly ferrous ions and, in a smaller quantity, ferric ions and in which elementary sulphur is suspended;

^■ A removal of elementary sulphur suspended by the liquid flow coming from the gas-liquid or liquid-liquid absorption; " A regeneration of the ferric ions from the ferrous ions contained in the acid liquid flow, purified from the sulphur, through anodic oxidation in an electrolytic cell. The process in agreement with the present invention allows the gas or liquefied gas flow purification in continuous with zero emission: in fact, it does not generate any type of effluents to dispose and it is autoconsistent. The ferric sulphate Fe₂(SO₄)₃ and the ferrous sulphate FeSO₄ are the preferred compounds containing ferric and ferrous ions.

The gas-liquid or liquid-liquid absorption can be carried out in one or more reactors chosen from a filler scrubber, a tray column or a bubble column or by "membrane contactors" or in other industrial equipment suitable for chemical absorption, operating at room temperature and at pressures comprised between 1 bar and 50 bar, preferably between 1 bar and 10 bar for the filler scrubber, the tray column and the bubble column. The removal of the elementary sulphur from the liquid flow can be carried out by a filtration system at one ore more stages and subsequent washing, recovering elementary sulphur with a purity up to 99.9%. The filterability can be improved by heating the solution to temperatures higher than the fusion temperature of sulphur followed by cooling. The gas or liquefied gas flow containing H₂S and the acid liquid flow containing predominantly ferric ions and, in a smaller quantity ferrous ions, in total iron ion concentrations preferably between 20 and 200 g/1, even more preferably between 100-150 g/1, and in which the ratio between the concentrations of the ferrous and ferric ions, Fe²⁺/Fe³⁺, in g/1 is comprised preferably between 1/5 and 1/12, are fed in equicurrent or countercurrent.

The electrolytic cell, that can be unipolar or multiple bipolar, is preferably divided by a permoselective anionic membrane into two parts, anodic compartment and cathodic compartment: the liquid flow removed from the sulphur , in which the ratio between the concentrations of ferrous and ferric ions, Fe²⁺/Fe³⁺, in g/1 is between 5/1 and 12/1 and in which the oxidation reaction of the ferrous ions into ferric ions occurs, is sent to the anodic compartment; in the cathodic compartment of the cell an acid solution is sent with a concentration of acid preferably such as to maintain an isotonicity relative to the anions in the two compartments and in which the reduction reaction of the ions H⁺ occurs with the development of gas hydrogen accompanied to the liberation of the anions which migrate through the permoselective anionic membrane from the cathodic compartment to the anodic compartment and where the anionic membrane furthermore inhibits any countercurrent of iron ions. hi the case of multiple bipolar cell it is possible to modulate, depending on the speed of the flow to be treated, the number of compartments present introducing suitably bipolar electrodes. The anodic compartments and the cathodic ones in a multiple bipolar cell can be fed in parallel or in series. The used electrolytic cell can have several geometries and, depending on this, the anode can be bidimensional (plate) or tridimensional (flow bed or fixed bed). The materials that can be used as anodic materials have to be chemically stable in the conditions of use, they have to present high overvoltage compared to the competing reactions, especially the one of oxygen discharge. As a non exhaustive example of materials suitable for the purpose are metals with "valve effect" (for example titanium, tantalium, zirconium, etc) covered appropriately with electrocatalyzing oxides. The lead and its alloys appear to be very good anodic materials in sulphuric solutions and in solutions are covered with their own oxides presenting in this way electrocatalyst properties relative to the reaction. The anode can moreover be realized using steel suitably alloyed, noble metals or conductors covered with noble metals, carbon and graphite. The cathode, preferably bidimensional, has to be constituted of materials preferably with low hydrogen overvoltage and resistant in highly acid solutions. All noble materials are suitable for the purpose. A material that can present acceptable characteristics both for the hydrogen overvoltage and for its chemical resistance is stainless steel. The bipolar electrodes can be constituted in order to present different chemical and physical properties between the anodic face and the cathodic one and they can be realized with conductor plates treated for example electrochemically or mechanically in order to differentiate the two surfaces.

The electrolytic cell can be designed with a hydraulic head or a ΔP, between catholyte and anolyte. Moreover, it is preferable to operate in fluid dynamic conditions of turbulent regime and in a limited concentration range of the compartments: for this purpose it can be necessary to introduce an apposite recycle ratio into the two circular circuits in the cell. The permoselective anionic membrane presents flow density values within a broad range, preferably between 200 and 400 A/ m². The process in agreement with the invention can in particular comprise following stages:

^■ contact in a reactor (RA), preferably in a filler scrubber, in order to realize a gas- liquid or liquid-liquid absorption with reaction, a gas or liquefied gas flow (1) containing H₂S, with an acid liquid flow (2) containing total concentrations of iron ions between 20 and 200 g/1, preferably between 100-150 g/1, and In which the ratio between the concentrations of ferrous and ferric ions, Fe²⁺/Fe³⁺, in g/1 preferably is between 1/5 and 1/12. The above mentioned flows (1) and (2) can be fed in equicurrent or countercurrent, operating preferably at room temperature and at pressures preferably, for the filler scrubber between 1 bar and 10 bar. In the exit from the scrubber head, a gas or liquefied gas flow (3), purified from H₂S with residual H₂S concentrations up to 1 ppm can be collected, and from the bottom a liquid flow (4) containing ferrous/ferric sulphate solution, in which the ratio between the concentrations of ferrous and ferric ions, Fe²⁺ZFe³⁺, in g/1 preferably is between 5/1 and 12/1 and with elementary sulphur suspended with a concentration between 1 and 50 g/1 can be recovered; ■ subject the liquid flow coming from the scrubber to a filtration system at one or more stages and subsequent washing (SS) in which elementary sulphur with purity up to 99,9% is recovered;

■ send the liquid flow (5), purified from sulphur coming from the filtration system (SS), to the anodic circuit of an electrolytic cell (CE), in which the oxidation reaction of the ferrous ions into ferric ions is carried out, regenerating in this way the feed flow of the reactor (RA). In the electrolytic cell (CE) there is an entering flow from the cathodic circuit, through the selective anionic membrane, of anions that are liberated at the cathode. In the other circuit of the cell is fed an acid solution with an acid concentration preferably such as to maintain an isotonicity relative to the anions in the two compartments and is separated from the anodic circuit by a permoselective anionic membrane, and in which the reduction reaction of the ions H⁺ occurs that moreover form a gaseous hydrogen flow (7) with a purity superior of 99,99%.

Making use of figure 1 a realization in agreement with the invention will now be described that is not supposed to be considered a limitation of the scope of the invention itself.

The reactor (RA) is fed in equicurrent or countercurrent with a gas or liquefied gas flow (1) - constituted, for example, of CO₂, CH₄, H₂S, other hydrocarbons and, possibly, traces of COS, CS₂, mercaptan - and a liquid flow (2) - constituted of a regenerated solution of ferric ions obtained by the anodic oxidation of the flow (5) leaving the electrolytic cell (CE).

The absorption reaction that occurs in the reactor (RA) is the following:

(absorption reaction) H₂S + 2Fe³⁺ → S + 2Fe²⁺ + 2H⁺ in which the ferric iron Fe³⁺ is converted into ferrous iron Fe²⁺ while hydrogen sulphide is oxidized to elementary sulphur. The gas or liquefied gas flow (3) that leaves the reactor (RA) has a residual H₂S content that normally is up to 1 ppm.

The liquid flow (4) coming from the absorption phase is constituted by an aqueous solution containing ferrous ions in which elementary sulphur is suspended that is formed in RA and in which the quantity of suspended solids varies preferably between 1-50 g/1. The flow (4) is sent to a filtration system at more stages (SS) from which a solid phase (6), constituted of crystalline sulphur with a purity up to 99,9 %, is recovered. The liquid flow (5) purified in this way, is fed to the anodic circuit of the unipolar or multiple bipolar electrolytic cell (CE), in which the anodic oxidation of ferrous iron occurs for the regeneration of ferric iron according to reaction: (anodic reaction) Fe²⁺ → Fe³⁺ + e^~

The ferric iron regenerated in this way is returned to the head of the gas-liquid or liquid- liquid absorption unit - flow (2).

The total iron quantity contained in such a solution can extensively vary depending also on the type of treated gas: the total iron concentrations can vary between 20 and 200 g/1, preferably between 100- 150 g/1.

The electrochemical anodic oxidation process is conducted so that small quantities Fe²⁺ are still maintained in the regenerated solution, to avoid the undesired reaction, that brings the discharge to the anode of O₂ and to obtain, therefore, a nearly unitary faradic yield. Possible ratio values between ferric iron ions and ferrous iron ions Fe²⁺/Fe³⁺ in the anodic circuit feed of the cell are preferably between 5/1 and 20/1 while at the end of the anodic circuit they are respectively between 1/5 and 1/20.

The quantity of acid present in the feeding solution of the anodic circuit of the cell is higher than the quantity formed after the absorption reaction in the (RA) and is fixed at such a value to avoid, for each type of treated gas, the precipitation of hydrated iron after the oxidation. The cathodic compartment of the electrolytic cell is fed by an aqueous solution flow of acid, for example sulphuric, with a concentration preferably such as to maintain an isotonicity relative to the anions, for example SO₄ ^2", between the anodic and the cathodic compartments. In the electrolytic cell the anodic compartment and the cathodic one are separated by an anionic permoselective membrane that inhibits possible flows of iron cations from the anodic compartment to the cathodic one while it permits the flow of anions from the cathodic compartment to the anodic one. The anionic permoselective membrane can be a polymeric membrane preferably functionalized with quaternary ammonium groups. In the cathodic circuit following reduction reaction of H⁺ ions occurs:

(cathodic reaction) 2H⁺ + 2e^~ — » H₂ with the formation of gaseous hydrogen (7), with a purity higher than 99,99%, that can be recovered. In order to better illustrate the present invention an example is now provided. Example

A flow of 100 m3/h of natural gas containing H₂S at 15% vol is sent to a filler scrubber in countercurrent with an acid solution of 815 Kg/h, constituted of H2SO4 at 7% and of a total iron concentration equal to 120 g/1 and in which the relative ratio between the ferrous ions and the ferric ions Fe²⁺/Fe³⁺ is 1/12. The weight percentage of ferrous sulphate and of ferric sulphate in the acid solution sent to the scrubber are respectively 2,1% and 33%. The hydrogen sulphide reacts with the ferric iron with a conversion degree equal to 100% and with a gas-liquid absorption efficiency equal to 99,9%, obtaining in this way in the head of the scrubber a purified gas flow in which a residual concentration of H₂S equal to 210 ppm is obtained. From the bottom of the scrubber is collected an acid liquid flow equal to 845,2 Kg/h: the flow is constituted of ferrous sulphate and ferrric sulphate and elementary sulphur in suspension that is separated by filtration and subsequently washed, obtaining in this way in the exit a solid flow equal to 21,4 Kg/h of elementary sulphur with a purity of 99,8%. The liquid flow, purified from the sulphur and with sulphuric acid concentrations of 15%, equal to 816,4 Kg/h in which the relative ratio between the ferrous iron and the ferric iron Fe²⁺/Fe³⁺ is 1/12, is fed to the anodic circuit in a unipolar electrolytic cell in which the anode is of lead, the cathode is of stainless steel and the anodic circuit and the cathodic one are separated by an anionic permoselective membrane with an ionic exchange capacity equal to 1,30 meq/g and functionalized through quaternary ammonium ions. The anodic circuit of the cell foresees furthermore a recycle ratio equal to 1/30. The cathodic compartment of the cell is fed with an aqueous solution of sulphuric acid at 39,2% in weight. In the electrolytic cell occurs an anodic oxidation of the ferrous iron to ferric iron, regenerating in this way the liquid flow, that is returned to the scrubber, while a reduction of hydrogen ions takes place at the cathode, setting in this way free a hydrogen gas flow, purity 99,98%, equal to 15 m3/h. The process has been conducted with a faradic yield nearly unitary, with an energetic consumption of 1,44 KWh/Kg converted Fe, with a cathodic flow density of 380 A/m² and an anodic flow density of 200 A/m².

Claims

1. Process for the purification of gas or liquefied gas flows from hydrogen sulfide through aqueous solutions containing ferric ions electrochemically regenerated comprising essentially: " A gas-liquid or liquid-liquid absorption with reaction of the above mentioned gas flow containing H₂S in an acid liquid flow containing predominantly ferric ions and, in a smaller quantity, ferrous ions, obtaining in this way a from H₂S purified gas flow and a liquid flow containing essentially ferrous ions, and in a smaller quantity ferric ions and in which elementary sulphur is suspended; " A removal of suspended elementary sulphur from the liquid flow leaving the gas- liquid or liquid-liquid absorption;

^■ A regeneration of ferric ions from ferrous ions contained in the acid liquid flow, purified from sulphur, through anodic oxidation in an electrolytic cell.

2. Process according to claim 1 where gas-liquid or liquid- liquid absorption is carried out in one or more reactors chosen from a filler scrubber, a tray column, a bubble column or using membrane contractors.

3. Process according to claim 1 where the removal of elementary sulphur from the liquid flow is performed through an heating and a filtration system at one or more stages and subsequent washing ricovering elementary sulphur with a purity up to 99,9%.

4. Process according to claim 2 where the chemical absorption is performed working at room temperature and at pressures comprised between 1 bar and 50 bar, preferably between 1 bar and 10 bar for the filler scrubber, the tray column and the bubble column.

5. Process according to claim 1 where the gas or liquefied gas flow containing H₂S and the acid liquid flow containing essentially ferric ions and, in a smaller quantity, ferrous ions, in total iron ion concentrations between 20 and 200 g/1, preferably between 100- 150 g/1, and in which the ration between the concentrations of ferrous ions and ferric ions, Fe²⁺/Fe³⁺, in g/1 is preferably between 1/5 and 1/12, are fed in equicurrent or in countercurrent.

6. Process according to claim 1 where the electrolytic cell is divided in two parts, anodic compartment and cathodic compartment, by an permoselective anionic membrane: the liquid flow removed from the sulphur, in which the ratio between the concentrations of ferrous and ferric ions, Fe²⁺ZFe³⁺, in g/1 is between 5/1 and 12/1 and in which the oxidation reaction of the ferrous ions to ferric ions occurs, is sent to the anodic compartment; in the cathodic compartment of the cell an acid solution is sent with a concentration of acid preferably such as to maintain an isotonicity relative to the anions in the two compartments and in which the reduction reaction of the ions H⁺ occurs with the development of gas hydrogen accompanied with the liberation of the anions that migrates through the permoselective anionic membrane from the cathodic compartment to the anodic compartment and where the anionic membrane furthermore inhibits any countercurrent of iron ions.

7. Process according to claim 1 in which the electrolytic cell is unipolar or multiple bipolar.

8. Process according to claim 7 in which the electrolytic cell is multiple bipolar and the anodic compartments are fed in parallel.

9. Process according to claim 7 in which the electrolytic cell is multiple bipolar and the anodic compartments are fed in series.

10. Process according to claim 6 in which the anode of the electrolytic cell is bidimensional or tridimensional (flow bed or a fixed bed ) and is constituted preferably of noble metals or alloys of lead, steel, carbon or graphite.

11. Process according to claim 6 in which the cathode, preferably bidimensional, is constituted of materials with low overvoltage and resistant in highly acid conditions, preferably steel or noble metals.

12. Process according to claim 6 in which the permoselective anionic membrane presents values of the flow density between 200 e 400 A/m².

13. Process according to claim 6 where the electrolytic cell foresees a hydraulic head or a ΔP between the catholyte and anolyte.

14. Process according to claim 6 where an apposite recycle ratio in the circulating circuits in the electrolytic cell is introduced, to permit fluid dynamic conditions of turbulent regime.

15. Process according to claim 6 where gaseous H₂ is developed at the cathodic compartment of the electrolytic cell with a purity higher than 99,99%.

16. Process according to at least one of the previous claims comprising following stages:

^■ contact in a reactor (RA), preferably in a filler scrubber, in order to realize a gas- liquid or liquid-liquid absorption with reaction, a gas or liquefied gas flow (1) containing H₂S, with an acid liquid flow (2) containing total concentrations of iron ions between 20 and 200 g/1, preferably between 100-150 g/1, and in which the ratio between the concentrations of ferrous and ferric ions, Fe²⁺/Fe³⁺, in g/1 preferably is between 1/5 and 1/12. The above mentioned flows (1) and (2) can be fed in equicurrent or countercurrent, operating preferably at room temperature and at pressures preferably, for the filler scrubber between 1 bar and 10 bar. In the exit from the scrubber head, a gas or liquefied gas flow (3), purified from H₂S with residual H₂S concentrations up to 1 ppm can be collected, and from the bottom a liquid flow (4) containing ferrous/ferric sulphate solution in which the ratio between the concentrations of ferrous and ferric ions, Fe²⁺ZFe³⁺, in g/1 preferably is between 5/1 and 12/1 and with elementary sulphur suspended with a concentration between 1 and 50 g/1 can be recovered;

^■ subject the liquid flow (4) from the scrubber to a filtration system at one or more stages and subsequent washing (SS) in which elementary sulphur with purity up to 99,9% is recovered;

^■ send the liquid flow (5), purified from sulphur from the filtration system (SS), to the anodic compartment of an electrolytic cell (CE), in which the oxidation reaction of the ferrous ions to ferric ions is carried out regenerating in this way the feed flow of the reactor (RA). In the electrolytic cell (CE) there is an entering flow from the cathodic compartment, through the selective anionic membrane, of anions that are liberated at the cathode. In the other circuit of the cell (CE) is fed an acid solution with an acid concentration preferably such as to maintain an isotonicity relative to the anions in the two compartments and is separated from the anodic compartment by the permoselective anionic membrane, and in which the reduction reaction of the ions H⁺ occurs that moreover form a gaseous hydrogen flow (7) with a purity superior of 99,99%.