| WO2017105245A2 | 2017-06-22 |
| GB1397256A | 1975-06-11 | |||
| CN106345232A | 2017-01-25 | |||
| US20070029264A1 | 2007-02-08 | |||
| US20130267614A1 | 2013-10-10 |
| Patent Claims Claim 1: A process for removal of H2S and other sulfur-compounds and siloxanes in biogas and other gases comprising CH4 and C02 without removing CH4 and C02 in the gas comprising the subsequent process steps of: a) compression of the gas to above 3 bar abs. in one or several steps with intercooling and condensation of H20 in the gas at 0-50 °C; b) contacting the gas in an absorption tower with recirculating concentrated sulfuric acid, with a concentration of 80-99 % at a temperature in the range of 30 - 130 °C measured in the gas exiting the absorption tower, said acid comprising H2S208 generated by electrolysis of H2S04 in an electrolyzer inserted in said recirculation of sulfuric acid or by adding H202 to the circulating acid; and c) controlling the strength of the recirculating acid to a desired value between 80 and 99 % H2S04, by adding to the circulating sulfuric acid a stream of water controlled by continuous measurement of the acid strength. Claim 2: A process according to Claim 1, further comprising the subsequent steps of: (al) Adding an amount of oxygen corresponding to 0,1 - 1 vol % 02 remaining in the gas after calculated complete oxidation of H2S and other S-compounds to in the biogas to S02; and (a2) Passing the gas through a reactor with catalyst for oxidation of H2S and other S- compounds to S02 at a temperatures in the range of 180 to 400 °C. between step a) and b). |
Description of the process of the invention
The present invention relates to a novel Process for up to 99.9 % removal of H 2 S and other S-compounds and up to 99 % removal of siloxanes in biogas extracted from landfills or produced by anaerobic digestion of organic materials such as manure, food and household waste. The content of C0 2 , CH 4 , N 2 and 0 2 in the biogas pass unaffected through the Process. The biogas purified in the Process may be utililized directly as fuel gas for gas turbines or gas engines, or it may be passed to a plant for catalytic conversion of the content of C0 2 in the gas to CH 4 , higher hydrocarbons or methanol by catalytic reduction with appropriate catalysts and H 2 supplied from an external source and from the electrolysis of H 2 S0 4 in the Process.
The Process can as well be used for purification of other gases comprising CH 4 , C0 2 , 0 2 and N 2 .
Catalysts used for C0 2 -reduction processes such as methanation, methanol synthesis or Fischer-Tropsch synthesis are extremely sensitive to sulfur poisoning, requiring sulfur to be removed down to 10-20 ppb for stable long-term operation. This final ultra-purification, which is not part of the present invention, is typically accomplished by passing the gas though an active carbon filter bed followed by a bed of hot zink- oxide pellets upstream of the C0 2 -reduction reactor.
The Process uses only biogas and electricity as input and generates no secondary output apart from a stream of condensed water, comprising also the content of NH 3 and basic compounds in the biogas, and a useful stream of concentrated sulfuric acid and H 2 from the H 2 S0 4 -electrolyzer equivalent to the amount of sulfur in the biogas. The Process in which the H 2 S is absorbed directly in the sorption tower, as seen in fig 1 and 3, generates 4 mol H 2 per mol of H 2 S while the Process in which H 2 S is oxidized to S0 2 upstream of the sorption tower generates 1 mol H 2 per mol H 2 S in the inlet gas. The electrolyzer may be operated with additional H 2 -production and 0 2 generated at the cathode in place of H 2 S 2 0 8 .
A wide range of processes for removal of H 2 S or other combustible sulfur-compounds are known but no one mention use of peroxides dissolved in concentrate sulfuric acid for desulfurization of the gas.
Detailed description of the Process of the invention.
The diagram in fig 1 shows Process principles without catalytic oxidation of H 2 S to S0 2 upstream of the sorption tower. Figs 2 and 3 show examples of embodiment with and without said oxidation of H 2 S to S0 2 , respectively. The numbers in cursive refer to the numbers seen on the figures.
The raw biogas in line 3 from anaerobic digestion of organic materials in the digester 2 is typically saturated with H 2 0 at 30-35 °C at slightly above atmospheric pressure and contains on dry basis typically about 60% CH 4 40% C0 2 , 0,05 - 1 % H 2 S and up to 100 mg siloxanes per Nm 3 . The digestor should be operated without admission of air and at conditions giving highest possible yield of CH 4 and H 2 S. The biogas is then compressed to about 12 bar abs in two compressors 4 and 7 with condensation of water in the intercooler 5. NH 3 and basic compounds in the gas are extracted as carbonates in the condensate.
The Process (fig 2) with oxidation of H 2 S to S0 2 upstream of the absorption tower. Here, the biogas is heated in the heat exchanger 9 to a temperature in the range of 180 - 300 °C and passed through an oxidation reactor 11 loaded with a catalyst by which H 2 S and other combustible S-compounds are oxidized selectively to S0 2 according to the reaction:
(1) H 2 S + 1½ 0 2 => S0 2 + H 2 0 + 530 kJ/mol
The amount of 0 2 needed for complete oxidation to S0 2 of H 2 S and other S-compounds is added to the gas upstream of the oxidation reactor in an amount corresponding to 0.2 - 0.8 % excess 0 2 remaining in the gas after complete oxidation of H 2 S and other combustible S-compounds to S0 2 . Known catalysts comprising oxides of V and Ti (ref 5) are very active for oxidation of H 2 S down to below 200 °C and do not affect CH 4 or C0 2 at temperatures up to 350 - 400 °C at the actual operating conditions.
Most of the remaining H 2 0 is then condensed by cooling the gas to typically 25-30 °C in 13 before the gas is passed to the S0 2 absorption tower 15 in which the S0 2 is absorbed by H 2 S 2 0 4 (peroxy disulfuric acid) dissolved in 100 0 C hot circulating, 95-99 % concentrated sulfuric acid, according to the reaction:
(2) S0 2 + H 2 S 2 0 8 + 2H 2 0 => 3H 2 S0 4 + 150 kJ/mol
The H 2 S 2 0 4 is generated continuously according to reaction
(3) 2H 2 S0 4 + power (approx. 0.1 kWh/mol H2) => H 2 S 2 0 8 + H 2 + heat (approx. 90 kJ/mol) by high current density electrolysis of strong and hot sulfuric acid in a special electrolyzer 36 inserted in the sulfuric acid circulating loop, as seen in the flow schemes in fig 2 and 3.
(An unknown fraction of the H 2 S 2 0 8 in concentrated sulfuric acid is actually present as H 2 S0 5 (Cam ' s acid) formed by the reaction H 2 S 2 0 8 = H 2 S0 5 + H 2 S0 4 . In the present script both peroxy acids are calculated together as H 2 S 2 0 8 .)
Generation of H 2 S 2 0 8 by high current density electrolysis of hot, highly concentrated sulfuric acid is well known and proven technology (ref 8 and 9) which could easily be adapted to the process of the invention in cooperation with established suppliers of such electrolyzers.
Hence, the over-all reaction (1) + (2) + (3) is
(4) H 2 S + 1 ½ 0 2 + H 2 0 => H 2 S0 4 + H 2
H 2 S 2 0 4 may instead be generated by adding H 2 0 2 in aqueous solution to the circulating acid. Then the Process would produce excess H20 and make the maintenance of a high acid strength impossible and necessitate operation with more dilute acid down to 80 % H2S04.
I calculate that the rate of absorption of S0 2 in the circulating acid in the sorption tower 15 is liquid film controlled which means that the rate increases with increasing solubility of S0 2 in sulfuric acid. This seems confirmed by the experience in ref 7 with removal of small concentrations of S0 2 in tail gas from sulfuric acid plants by scrubbing the tail gas at atmospheric pressure with hot concentrated sulfuric acid containing a small concentration of H 2 S 2 0 8 generated by electrolysis of H 2 S0 4 or by adding H 2 0 2 to the sulfuric acid. The solubility of S0 2 in sulfuric acid, and thereby also the rate of absorption, increases strongly with gas pressure and with acid strength above 85% H 2 S0 4 in the acid (ref 8). Furthermore, I have observed that siloxanes, which are insoluble in water and dilute sulfuric acid, are soluble in sulfuric acid stronger than 50% H 2 S0 4 , the solubility increasing infinitely with acid strength. Acid of 90-98% strength dissolves at least 20 % siloxanes which may separate out again, apparently when the solution is diluted with a lot of water.
I see no signs that the siloxanes dissolved in concentrated sulfuric acid are oxidized by H 2 S 2 0 8 dissolved in the acid under formation of Si0 2 or other precipitates, even not after weeks of residence time of siloxanes dissolved in 96% H 2 S0 4 with 1-2 % H 2 S 2 0 8 .
Maintenance of minimum 90 % strength, preferably 98-99% strength of the circulating sulfuric acid is crucial for optimal removal of S0 2 as well as siloxanes in the sorption tower.
The strength of the circulating acid in line 32 is controlled by controlling the flow of make-up water added in line 40 to the circulating acid in order to make-up for the deficit of w kg/h of H 2 0 in the H 2 0 balance calculated by the over-all reaction of sorption of S0 2 and the formation of H 2 S 2 0 8 in the electrolyzerr:
(2) + (3) S0 2 + 2 H 2 0 H 2 S0 4 + H 2 .
Thus the flow w (kg water/h) of make-up water in line 40 being added in order to maintain the strength of the circulating acid at a desired constant level is calculated from the H 2 0-balance:
(5) w kgH20/h = (a) H 2 0 consumed by reaction (2)+(3) - (b) H 2 0 in the tower exit gas in line 17
+ (c)H 2 0 in the tower exit gas in line 17 + (d)the content of H 2 0 in the products stream in line 50.
In order to maintain the acid strength in the circulation loop at a desired set-point, w is controlled by the control valve 42 (or a volumetric pump) controlled by the continuous acid strength analyzer 41 being connected to recirculating acid flowing in line 32.
For calculations of water balances, I use H 2 0 vapor pressure (and H 2 S0 4 vapor pressure) in equilibrium with sulfuric acid at various values of temperature and acid strength seen in table 1. H 2 S0 4 vapor pressure becomes significant at 10 bar with above 98.5% acid strength at 100 °C acid temperature. H 2 S0 4 mist is removed in the mist filter 16.
w should be at least 0,5-1 kg H 2 0/h per 1000 Nm3/h In order to have robust control of acid strength. The absorption process should be operated with 95 - 99 % (minimum 90%) acid strength and about 100 °C temperature (max 120 0 C) of the Tower exit gas in line 17. Maintenance of high strength of the acid in the circulating loop is easier, the higher the S0 2 -content in the gas because 2 mol of H 2 0 is removed per mol of S0 2 absorbed in the absorption process (2). This means that at below 600 ppm S0 2 in the gas, minimum 90- 92% strength of the circulating acid can only be maintained at above 40 bar total pressure (requiring 3 compressors in series), condensation of H 2 0 at 10 0 C in the gas inlet cooler 13, and heating of the circulating acid to 110 0 C in 33. However, with 0.6 % S0 2 , 95-98% acid strength is easily maintained even at 10 bar pressure and condensation at 30 -40 °C.
At 11-12 bar pressure, 100 0 C and above 95 % acid strength, up to 99,9 % removal of S0 2 and 99 % removal of siloxanes can be achieved with sufficient size of the absorption tower, as S0 2 is absorbed without equilibrium limitations. Higher pressure will increase absorption efficiency and improve the H 2 0-balace on the expense of higher investment costs. An active carbon guard filter downstream of the absorption tower may insure desired removal efficiencies. Final ultra-purification to 10-20 ppb S may by a bed of ZnO adsorbent upstream of C02 hydrogenation reactor. It is not important to minimize the consumption of H 2 S 2 0 8 insidenummere the process according to the invention, or to minimize parallel formation in the electrolyzer of 0 2 (by electrolysis of H 2 0) in addition to the formation of H 2 S 2 0 8 , because all formation of H 2 S 2 0 8 and 0 2 is accompanied with formation of the equivalent amount of useful H2.
The stream of condensate in line 52 may be mixed with the stream in line 53 of concentrated sulfuric acid generated by the oxidation of the H 2 S in the biogas and added to the digested slurry from the digester. Remaining trace of H 2 S 2 0 8 will immediately be reduced to H 2 S0 4 in contact with the slurry.
The only input for the process is electricity and biogas.
The process (figs 1 and 3) without the step of oxidation of H 2 S to S0 2 upstream of the sorption tower Here, H 2 S is adsorbed directly in the sorption tower according to the reaction
(6) H 2 S + 4H 2 S 2 0 8 + 4H 2 0 => 9H 2 S0 4 + approx. 7200 kJ/mol,
as is seen in fig 1 and 3. The system for generation of peroxy sulfuric acid and control of acid strength is identical to that seen in fig 2.
This simplified version of the process is clearly superior to the version with oxidation of H 2 S to S0 2 and would be preferred immediately if there were similar experience with absorption of H 2 S as with absorption of S0 2 by H 2 S 2 0 8 in strong sulfuric acid. The control of acid strength is more robust as the absorption of 1 mol H 2 S consumes 4 mol of H 2 0 including the electrolysis; the 4 times higher power consumption per mol H 2 S is compensated by the production of 4 mol H 2 per mol of H 2 S.
Furthermore, the risk of deactivation of the oxidation catalyst by siloxanes in the gas is avoided.
Ref (6) reports 95% removal of H 2 S by absorption in dilute H 2 0 2 dissolved in water at pH above 3-4, but the absorption stops when the liquid becomes more acidic. The solubility of H 2 S in water is about 8 times lower than that of S0 2 .
No references are found in literature to experiments with absorption of H 2 S by H 2 S 2 0 8 or H 2 0 2 dissolved in concentrated sulfuric acid. However, one single reference ("H 2 S solubility in sulfuric acid" by Alexendrova et al in J. Applied Chem (USSR) 1978, 57, 1221-23) indicate that the solubility of H 2 S in concentrated sulfuric acid increases strongly with acid strength, like that of S0 2 in concentrated sulfuric acid, indicating that absorption of H 2 S by H 2 S 2 0 8 in hot circulating sulfuric acid could be an attractive alternative at sufficiently high total pressure.
However, arguments for the feasibility of absorption and oxidation of H 2 S to H 2 S0 4 in H 2 S 2 0 8 dissolved in hot, high strength sulfuric acid according to (6) are very convincing from another angle of approach: It is known that H 2 S cannot be dried by bubbling it through 95-98 % H 2 S0 4 because even at room temperature, some of the H 2 S is oxidized to sulfur and S0 2 by the acid according to the reaction
(7) H 2 S + H 2 S0 4 => 2S0 2 + S + 2 H 2 0.
Use of this reaction for industrial production of sulfur from H 2 S reacting with H 2 S0 4 is known (ref 8).
A simple, indicative laboratory experiment at atmospheric pressure with bubbling dilute H 2 S-gas through a flask A with 96% H2S04 and another flask B with NH 4 HS 2 0 8 dissolved in 96% H 2 S0 4 , both at 80-100 °C, was very convincing: In flask A, there was a lot of precipitation of sulfur and smell of S0 2 from the flask. In flask B, the solution remained clear with no smell of S0 2 or H 2 S and with significant heating up of the liquid during the experiment.
The rate of absorption of H 2 S in hot, concentrated acid with H 2 S 2 0 8 may be higher than that of S0 2 because very fast oxidation of H 2 S by H 2 S 2 0 8 in the liquid with no equilibrium limitation will decrease the liquid film restriction of the absorption of H 2 S.
The biggest advantage of direct absorption of H 2 S is that the cost and complications of the step of oxidation of H 2 S to S0 2 and the risk of deactivation of the oxidation catalyst are avoided. Furthermore, it is easier to maintain high concentration of H 2 S0 4 in the acid loop at low concentrations of H 2 S in the gas, because one mol of H 2 S takes out double as much H 2 0 as one mol of S0 2
It is no disadvantage that the absorption of H 2 S consumes 4 times more H 2 S 2 0 8 than absorption of S0 2 , as it also generates 4 times more useful H 2 per mol of H 2 S in the inlet gas.
It shall be noticed that the absorption of S0 2 and H 2 S in concentrated sulfuric acid containing peroxy (di)sulfuric acid according to Claim 1 a), b) and c) may as well be applied to desulfurization of any gas with H 2 S, S0 2 or other S-compounds in gas streams comprising C0 2 , H 2 0, 0 2 , N 2 and CH 4 and, possibly, also H 2 and lower aliphatic hydrocarbons which are not absorbed in the circulating acid.
Prior Art
In extensive search in Google and in recent, exhaustive surveys (ref 1,2,3) of technology for purification of biogas or gases with H 2 S and combustible S-compounds comprised in gases comprising C0 2 , 0 2 , CH 4 , I have found no relevant prior art to the Process of my invention in literature, apart from partly relevant prior art in refs 7, Hand 12.
It is relevant prior art to remove S0 2 in off gas from sulfuric acid plants by scrubbing it with concentrated sulfuric acid containing peroxy (di)sulfuric acid generated either by electrolysis of hot, concentrated sulfuric acid or by adding H 2 0 2 to the acid (ref 7, Hamond/DuPont US 3760061 A, filed 02.03.1999).
It is not relevant prior art to remove H 2 S from biogas by oxidizing H 2 S with H 2 0 2 to sulfates in alkaline or neutral aqueous solutions at above pH 3-4 (ref 6).
The most relevant prior art found in patent literature are:
Ref 9, Bowe, US 2007/0029264 filed 08 february 2008, claims 1, 3 and 4. Describes a process for purifying biogas comprising C0 2 and CH 4 for use in a reformer generating feed gas for a subsequent Fischer-Tropsch process. The biogas is desulfurized by liquid scrubbing absorption without mentioning use of peroxides or concentrated sulfuric acid.
Ref 10, US 2013/00267614 Al(Corey et al.) 10 October 2013. describes a process for converting biogas to liquid fuels comprising the steps of removing moisture from the gas by compression and cooling, removing siloxanes and other contaminants, such as H 2 S, by filtration with activated carbon before feeding the biogas to a syngas reactor.
Ref 11, US 2013/034616 (Iyer) 26 december 2013 and (12) CN 106345232 A (Changzhou Vocational Inst. Eng.) 25 Januar 2017, WPI Abstract AN-2017-093073, describe removal of siloxanes in biogas by scrubbing with concentrated sulfuric acid but do not mention use of peroxides for removal of H 2 S or other S- compounds in the gas.
Ref 12, Chanzhou Voc. Inst. Eng, CN 106345232 filed 25.1.2017, WPI Abstract AN-2017-093073. None of the latter 4 references can be seen as prior art of my invention
References:
(1) "Biogas upgrading technologies - developments and innovations", Petersson and Wellnger, I EA report from 2009, wwwJnfotek-biomasse.ch//175 2009 IEA.
(2) "Biogas Upgrading and Global Markets", Susan Haft, bcc report February 2014 (337 pages)
(3) "Biogas upgrading. Evaluation of methods for H2S removal" Danish Technological Institute, 2014 (31 pages)
(4) "F0reningar I biogas: Validering av analys H 2 S0 4 metodik for siloxaner", Svensk Gasteknologisk Center, November 2011 (25 pages)
(5) Catalysts for low temperature oxydation of H2S. HTAS patentapplication WO 2015/082352 Al.
(6) "Process for Removing H2S from Gas". Patent US200901130008 Al. Michael N. Funk. Priority
19.11.2007.
(7) "High-strength acid containing H202 to scrub S02", Hamond/DuPont, US 3760061 A, filed 02.03.1999
(8) "Reactions between H 2 S and sulfuric acid: A Novel Pocess for sulfur removal and recovery", Qjnglin Zang et al, Ind. Eng. Chem. Res., 2000, 39, 39(7), pp 2505-2509.
(9) US 2007/0029264 (Bowe) 08 february 2008.
(10) US 2013/00267614 Al(Corey et al.) 10 October 2013.
(11) US 2013/034616 (Iyer) 26 december 2013.
(12) CN 106345232 A (Changzhou Vocational Inst. Eng.) 25 Januar 2017, WPI Abstract AN-2017-093073.
Figures and table.
Fig 1. Process principles without oxidation of H 2 S to S0 2 upstream sorption tower
Fig 2. Process FS at 11-12 bar abs with catalytic oxidation of H 2 S to S0 2 upstream sorption tower.
Fig 3. Process FS at 11-12 bar abs with no oxidation of H 2 S upstream sorption tower.
Table 1. H 2 0 and H 2 S0 4 vapor pressures over sulfuric acid at various temperatures and acid strength
(2) heating the gas to typically 200-250 °C and oxidizing its content of S-compounds selectively to S0 2 by a catalyst comprising oxides of V and Ti with a small excess of 0 2 added to the gas in step (1),
(3) cooling and condensing H 2 0 at 0 -50 °C, typically at 20 - 40 °C
(4) oxidizing the S0 2 and H 2 S in the biogas to H 2 S0 4 by contacting the gas at about 100 °C in an absorption tower with recirculating hot concentrated sulfuric acid comprising H 2 S 2 0 8 generated preferably by electrolysis of H 2 S0 4 in an electrolyzer inserted in said recirculation of concentrated sulfuric acid. The