DE ANGELIS ALBERTO (IT)
POLLESEL PAOLO (IT)
MILLI MARCELLO (IT)
BELLUSSI GIUSEPPE (IT)
DE ANGELIS ALBERTO (IT)
POLLESEL PAOLO (IT)
MILLI MARCELLO (IT)
| WO2005075351A2 | 2005-08-18 |
| US5093094A | 1992-03-03 |
CLAIMS
1. A process for the reoxidation, without the formation of nitrogen oxides, of ferrous solutions used in removal processes in continuous of hydrogen sulfide from gaseous streams containing them, which comprises: a) putting a gas containing H 2 S in contact with an oxidizing acid aqueous solution of ferric nitrate and containing a heteropolyacid having redox properties, as such or partially salified with an alkaline metal or with ammonium, selected from those having general formula (I) :
H n XV 7 M 112 -7 ) O 40 , ( D wherein n is an integer ranging from 3 to 6, X is selected from P, Si, As, B, Ge, y an integer ranging from 1 to 3 and M consists of Mo or W; b) filtrating and separating the sulfur produced due to the oxidizing effect of the ferric nitrate which is reduced to ferrous nitrate; c) reoxidizing the ferrous nitrate to ferric nitrate with a stream consisting of air enriched in oxygen wherein the partial oxygen pressure with respect to the total ranges from 0.5 to 30 atmospheres; d) recycling the solution containing ferric nitrate and heteropolyacid to the oxidation step (a) ; e) discharging the air impoverished in oxygen.
2. The process according to claim 1, wherein the hetero- polyacid is used in a solid form insoluble in water, selected from:
- partial or complete salification with metals, whose salts are insoluble, selected from cesium, ammonium, potassium, silver and thallium (I);
- supporting and immobilization on silica;
- supporting and immobilization on mesoporous molecular sieves, such as HMS and MCM-41; - supporting an immobilization on activated carbon.
3. The process according to claim 1 or 2, wherein the gaseous stream consists of natural gas or associated gas.
4. The process according to any of the previous claims, wherein the ferric nitrate is present in the oxidizing so- lution in concentrations ranging from 0.01 to 3 moles/1.
5. The process according to any of the previous claims, wherein the heteropolyacid compound (I) is present in concentrations ranging from 0.01 to 0.5 moles/1.
6. The process according to any of the previous claims, wherein the molar ratio heteropolyacid compound (I) /ferric nitrate ranges from 1/1 to 1/100.
7. The process according to any of the previous claims, wherein the acid aqueous solution has a pH ranging from 0 to 6. 8. The process according to any of the previous claims, wherein the hydrogen sulfide is present in the gas fed in a concentration ranging from 0.01 to 100% by volume.
9. The process according to any of the previous claims, wherein the reoxidation step takes place at a temperature ranging from 20 to 100 0 C at a pressure higher than atmospheric value, with a relative pressure ranging from 0.5 to 50 atm.
10. The process according to any of the previous claims, wherein the enriched air pressure ranges from 1 to 50 atm. 11. The process according to claim 10, wherein the partial oxygen pressure ranges from 0.5 to 30 atm.
12. The process according to claim 3, wherein the oxidation of hydrogen sulfide (H 2 S) takes place at a pressure ranging from 1 atm to the pressure of the production well . |
PROCESS FOR THE REOXIDATION, WITHOUT THE FORMATION OF NITROGEN OXIDES, OF FERROUS SOLUTIONS COMING FROM THE REMOVAL PROCESS IN CONTINUOUS OF HYDROGEN SULFIDE FROM GASEOUS STREAMS BY MEANS OF THE REDOX METHOD
The present invention relates to a process for the re- oxidation, without the formation of nitrogen oxides, of ferrous solutions coming from the removal process in continuous of hydrogen sulfide from gaseous streams by means of the redox method.
More specifically, the present invention relates to a process for the reoxidation of ferrous solutions coming from the removal process in continuous of hydrogen sulfide and/or mercaptans contained in natural gas or in gas asso- ciated with oil wells by means of the redox method, avoiding the formation of secondary products such as nitrogen oxides (NO x ) .
As is known, the hydrogen sulfide normally contained in many production wells or natural gas fields or in gas associated with oil wells, is disposed of, if the Claus
process cannot be used either due to the relatively low quantities of H 2 S or to the fact that the concentration of hydrogen sulfide is lower than a certain quantity, by transforming it into sulfur by means of a chemical-redox process.
The process which, at present, has improved characteristics and is most widely used in the world is the Lo-CAT process which, however, has the disadvantage of producing unmarketable sulfur, requiring the use and consumption of considerably costly chemicals (chelating agents) and necessitating the use of extremely diluted iron solutions (max 1.5 g iron/1) .
By using acid solutions of ferric salts in the pres-' ence of heteropolyacids having general formula (I) : H n XVyM (12 . y) O 40 , (I) wherein n ranges from 3 to 6, X is selected from P, Si, As, B, Ge, y ranges from 1 to 3 and M consists of Mo or W, the oxidation of hydrogen sulfide to sulfur is obtained with the simultaneous reduction of the ferric salts to ferrous salts.
The subsequent reoxidation of the ferrous salts to ferric salts can be easily obtained by treatment with air at bland temperatures (20-80 0 C) . The reoxidation of the solution of bivalent iron in an acid environment, in the ab- sence of a heteropolyacid, on the contrary, does not occur
even after several days, not even when the solution is heated to a temperature close to boiling point to accelerate its kinetics. The oxidation can take place in an alkaline environment, in this case, however, it is necessary, as in the above-mentioned Locat process, to add chelating products which prevent the precipitation of iron sulfide. The addition of these products, however, involves high additional costs and the presence of impurities in the sulfur produced sulfur. Detailed information on the removal of hydrogen sulfide from natural or associated gas using acid solutions of ferric salts in the presence of heteropolyacids can be found in international patent application PCT/EP2005/ 000669. The process previously described can be carried out with different ferric anions, provided that both the ferric salt and the corresponding ferrous salt are soluble in acid aqueous solutions, such as for example nitrate, sulfate, citrate, acetate, perchlorate and chlorate. Among these anions, the nitrate anion is particularly preferred since through its use the reoxidation kinetics of the solution of the corresponding ferrous nitrates is particularly rapid.
By using aqueous solutions of ferric nitrate, however, it has been observed that during the reoxidation phase of bivalent iron to trivalent iron, there is also occasionally
the formation of a secondary reduction reaction of the nitrate ion with the contemporaneous generation of nitrogen oxides (NO x ) . The nitrogen oxides which are therefore present in the air at the outlet of the reoxidation reactor could be easily and effectively separated with a normal washing with a solution of NaOH, which transforms them into the corresponding nitrites and nitrates . By operating in this way, there is an effective separation of the nitrogen oxides but also the contemporaneous loss of a chemical rea- gent.
This system also requires the necessity of handling alkaline waste products, which burdens the process both quantitatively and economically.
The Applicants have now found, as is better described in the enclosed claims, that the formation of nitrous vapours containing nitrogen oxides NO x can be eliminated by effecting the reoxidation of bivalent iron with normal air or air enriched in oxygen, at a pressure higher than the atmospheric value and preferably having a partial oxygen pressure in the air fed at least equal to 0.5 atmospheres. Operating in this way, it has in fact been found that there is not only a more rapid reoxidation of the ferrous salts, but the formation of nitrogen oxides (NO x ) is surprisingly totally inhibited. For the reoxidation of the reduced ferrous solution,
different reactor configurations can be advantageously- used, such as for example, a plate or filled absorption column or a bubble column.
A plant solution proposed for illustrative and non- limiting purposes is shown in Figure 1 enclosed.
In Figure 1, A, R, L and F respectively illustrate: A the absorber in which the hydrogen sulfate contained in the natural or associated gas is absorbed and oxidized to elemental sulfur by means of the redox system used in the process object of the present invention;
R the reoxidation reactor of the reduced solution into which the ferrous nitrate enters and is converted to ferric nitrate;
L the filtration and washing section of the sulfur produced sulfur;
F is the separation apparatus (or flash) of the incondensable or volatile compounds from the liquid stream of ferrous solution, such as, for example, methane, ethane, CO 2 , by the decompression action of the solution itself. Said apparatus can be advantageously used if the operating pressure of the absorber is higher than the operating value of the rest of the plant.
The natural or associated gas (1) to be purified is fed to the base of the reactor A into which the recycled oxidizing solution (6) , containing the ferric nitrate and
heteropolyacid having general formula (I) in solution, is fed. The softened gas (2) substantially free of H 2 S is discharged from the head of the reactor A, and can be introduced into the system or fed to other forms of treatment, whereas the stream (3) is fed to the flash apparatus from which the reduced solution of ferrous nitrate and heteropolyacid containing elemental sulfur in dispersion is recovered, by means of (4) , whereas the incondensable compounds (9) are vented. Filtration devices (L) allow the sulfur to be separated from the solution, which can be sent to the reoxidation unit (5) . In the sulfur separation unit L, the stream (10) consists of water destined for washing the sulfur, whereas the stream (11) consists of the clean sulfur destined for drying and storage and the stream (12) consists of the waste water of the sulfur washing destined for treatment.
The reoxidation unit of the ferrous solution (5) consists of the oxidation reactor R.
In particular, the reduced solution (5) is fed to the head of the oxidation reactor R to the base of which the oxidizing gas consisting of air enriched in oxygen (7) , is fed, at atmospheric pressure or a higher value, so as to obtain the complete reoxidation of the ferrous nitrate to ferric nitrate which can therefore be recycled, as a stream (6) , to the oxidation reactor of the hydrogen sulfide A.
The gas phase (8) leaving the reactor R, impoverished in oxygen, can be discharged into the environment as it does not contain nitrogen oxides NO x which could have been formed as secondary products during the reoxidation phase, but whose formation has been unexpectedly inhibited by the excess oxygen present in the feeding air (7) .
The process for the reoxidation, without the formation of nitrogen oxides, of ferrous solutions coming from the removal process in continuous of hydrogen sulfide from gaseous streams, object of the present invention, can be better understood by referring to the following examples which represent an illustrative and non- limiting embodiment . Reference Example 1 Oxidation of H 2 S with a 0.1 molar solution of Fe (NO 3 ) 3 /H 6 PW 9 V 3 O 40 and the subsequent reoxidation of the re- duced solution
40.4 g of Fe (NO 3 ) 3 .9H 2 O (0.1 moles) and 24.84 g of H 6 PW 9 V 3 O 40 (0.01 moles), are dissolved in 1,000 ml of dis- tilled water, obtaining a limpid solution with a pH of about 1. This solution is introduced into a stirred reactor, thermostat-regulated at 25 0 C into which H 2 S at 10% in nitrogen is passed at a flow-rate of 5 Nl/h for a total of 3 hours obtaining the complete oxidation of the H 2 S to sul- fur and the reduction of 70% of ferric iron to ferrous
iron .
The reduced solution is filtered thus separating the sulfur produced by oxidation of the hydrogen sulfide.
The reduced solution is heated in a reactor to 8O 0 C, in a flow of air at atmospheric pressure, with a partial oxygen pressure equal to 0.2 atmospheres, obtaining the complete reoxidation of bivalent iron to trivalent iron. During the reaction, nitrogen oxides are formed, which are absorbed in two successive traps each containing 500 ml of NaOH at 10% by weight in water. The concentration of the nitrites and nitrates in the first soda trap amounts to 257 ppm. The second trap does not contain nitrites and nitrates, thus showing the complete separation of the NO x produced ' . Example 2
Oxidation of H 2 S with a 0.1 molar solution of Fe(NOa) 3 ZH 6 PW 9 V 3 O 40 and the subsequent reoxidation of the reduced solution with enriched air
40.4 g of Fe (NO 3 ) 3 .9H 2 O (0.1 moles) and 24.84 g of H 6 PW 9 V 3 O 40 (0.01 moles), are dissolved in 1,000 ml of distilled water, obtaining a limpid solution with a pH of about l. This solution is introduced into a stirred reactor, thermostat-regulated at 25 0 C into which H 2 S at 10% in nitrogen is passed at a flow-rate of 5 Nl/h for a total of 3 hours obtaining the complete oxidation of the H 2 S to sul-
fur and the reduction of 70% of trivalent iron to bivalent iron.
The reduced solution is filtered thus separating the sulfur produced by oxidation of the hydrogen sulfide. The reduced solution is heated in a reactor to 80 0 C, in a flow of enriched air at atmospheric pressure, with a content of O 2 equal to 36% by volume and therefore with a partial oxygen pressure equal to 0.36 atmospheres, obtaining the complete reoxidation of bivalent iron to trivalent iron. During the reaction, nitrogen oxides are formed, which are absorbed in a trap containing 500 ml of NaOH at 10% by weight in water. The concentration of the nitrites and nitrates in the soda trap proves to be lower than 100 ppm. Example 3
Oxidation of H 2 S with a 0.1 molar solution of Fe (NO 3 ) 3 /H 6 PW 9 V 3 O 40 and the subsequent reoxidation of the reduced solution with pressurized enriched air
40.4 g of Fe (NO 3 ) 3 .9H 2 O (0.1 moles) and 24.84 g of H 6 PW 9 V 3 O 40 (0.01 moles), are dissolved in 1,000 ml of distilled water, obtaining a limpid solution with a pH of about 1. This solution is introduced into a stirred reactor, thermostat-regulated at 25°C into which H 2 S at 10% in nitrogen is passed at a flow-rate of 5 Nl/h for a total of 3 hours obtaining the complete oxidation of the H 2 S to sul-
fur and the reduction of 70% of trivalent iron to bivalent iron.
The reduced solution is filtered thus separating the sulfur produced by oxidation of the hydrogen sulfide. The reduced solution is heated in a reactor to 8O 0 C, in a flow of enriched air, with a content of O 2 equal to 36% by volume, at a pressure of 2.5 atmospheres, with a partial oxygen pressure equal to 0.9 atmospheres, obtaining the complete reoxidation of bivalent iron to trivalent iron. During the reaction, there is no formation of nitrogen oxides. The air leaving the reoxidation reactor is absorbed in two successive traps each containing 500 ml of NaOH at 10% by weight in water. The concentration of the nitrites and nitrates in the soda trap proves to be lower than the minimum concentration analytically determinable (20 ppm) .
Comparative Example 4
Oxidation of H 2 S with a 0.3 molar solution of Fe (NO 3 ) 3 /H 6 PW 9 V 3 O 40 and the subsequent reoxidation of the re- duced solution with normal air at atmospheric pressure
121.2 g of Fe (NO 3 ) 3 .9H 2 O (0.3 moles) and 74.67 g of H 6 PW 9 V 3 O 40 (0.03 moles), are dissolved in 1,000 ml of distilled water, obtaining a limpid solution with a pH of about 1. This solution is introduced into a stirred reac- tor, thermostat-regulated at 25°C into which H 2 S at 10% in
nitrogen is passed at a flow-rate of 10 Nl/h for a total of 4 hours and 30' obtaining the complete oxidation of the H 2 S to sulfur and the reduction of 70% of trivalent iron to bivalent iron. The reduced solution is filtered thus separating the sulfur produced by oxidation of the hydrogen sulfide.
The reduced solution is heated in a reactor to 80 0 C, in a flow of normal air at atmospheric pressure, with a partial oxygen pressure equal to 0.2 atmospheres, obtaining the complete reoxidation of bivalent iron to trivalent iron. During the reaction, there is a substantial formation of nitrogen oxides, which are absorbed by passing the gases leaving the reoxidation reactor into two successive traps each containing 500 ml of NaOH at 10% by weight in water. The concentration of the nitrites and nitrates in the soda traps proves to be equal to 1,578 ppm. Example 5
Oxidation of H 2 S with a 0.3 molar solution of Fe (NO 3 ) 3 /H 6 PW 9 V 3 O 40 and the subsequent reoxidation of the re- duced solution with enriched air (36% oxygen) at atmos- pheric pressure
121.2 g of Fe (NO 3 ) 3 .9H 2 O (0.3 moles) and 74.67 g of H 6 PW 9 V 3 O 40 (0.03 moles), are dissolved in 1,000 ml of distilled water, obtaining a limpid solution with a pH of about 1. This solution is introduced into a stirred reac-
tor, thermostat-regulated at 25 0 C into which H 2 S at 10% in nitrogen is passed at a flow-rate of 10 Nl/h for a total of 4 hours and 30' obtaining the complete oxidation of the H 2 S to sulfur and the reduction of 70% of trivalent iron to bi- valent iron.
The reduced solution is filtered thus separating the sulfur produced by oxidation of the hydrogen sulfide.
The reduced solution is heated in a reactor to 80 0 C, in a flow of air enriched in oxygen (36% O 2 ) at atmospheric pressure, with a partial oxygen pressure equal to 0.36 atmospheres, obtaining the complete reoxidation of bivalent iron to trivalent iron. During the reaction, there is the formation of nitrogen oxides, which are absorbed by passing the gases leaving the reoxidation reactor into two succes- sive traps each containing 500 ml of NaOH at 10% by weight in water. The concentration of the nitrites and nitrates in the soda traps proves to be equal to 755 ppm (about 50% of that obtained in Comparative Example 4 with normal air) . Example 6 Oxidation of H 2 S with a 0.3 molar solution of Fe(NO 3 ) S yH 6 PW 9 V 3 O 40 and the subsequent reoxidation of the reduced solution with enriched air under pressure
121.2 g of Fe (NO 3 ) 3 .9H 2 O (0.3 moles) and 74.67 g of H 6 PW 9 V 3 O 40 (0.03 moles), are dissolved in 1,000 ml of dis- tilled water, obtaining a limpid solution with a pH of
about 1. This solution is introduced into a stirred reactor, thermostat-regulated at 25 0 C into which H 2 S at 10% in nitrogen is passed at a flow-rate of 10 Nl/h for a total of 4 hours and 30' obtaining the complete oxidation of the H 2 S to sulfur and the reduction of 70% of trivalent iron to bivalent iron.
The reduced solution is filtered thus separating the sulfur produced by oxidation of the hydrogen sulfide.
The reduced solution is heated in a reactor to 80 0 C, in a flow of enriched air, with an O 2 content equal to 36% by volume, at a pressure of 5 atmospheres, therefore having a partial oxygen pressure equal to 1.8 atmospheres, obtaining the complete reoxidation of bivalent iron to trivalent iron. During the reaction, there is absolutely no formation of nitrogen oxides. The air leaving the reoxidation reactor is absorbed in two successive traps each containing 500 ml of NaOH at 10% by weight in water. The concentration of the nitrites and nitrates in the soda traps proves to be lower than the minimum concentration analytically determinable (20 ppm) .
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