Development and design of the in-situ regeneration section of Vitrisol®, a novel, highly
selective desulphurization process
Wermink, Wouter Nicolaas
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Publication date: 2019
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Wermink, W. N. (2019). Development and design of the in-situ regeneration section of Vitrisol®, a novel, highly selective desulphurization process. Rijksuniversiteit Groningen.
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1
Chapter 1: General introduction
1.1 Hydrogen sulphide
Hydrogen sulphide (H2S) is a highly toxic, flammable and corrosive gas, frequently encountered in gas streams originating from environmental resources like a/o natural oil and gas wells, and mines. H2S is produced from the microbial anaerobic digestion of organic matter. The Threshold Limit Value (TLV) of H2S is 1 ppmv. Death might occur from concentrations of 100 ppmv; immediate unconsciousness resulting in death occurs from a concentration of 1000 ppmv. H2S burns with oxygen to form sulphur dioxide (SO2) and water. SO2, in turn, is more poisonous than H2S, with a TLV of 0.25 ppmv. H2S, dissolved in water, or present in humid gas, is highly corrosive; it can lead to extensive damage of metals by uniform corrosion, pitting and cracking.
H2S has the odour of rotten eggs at non-lethal concentrations, but becomes odourless at lethal concentrations. In the past miners used canaries to alert them to the presence of poisonous gases like a/o H2S and CO. If the canary became distressed or died, miners could escape the mine before concentrations of poisonous gases reached levels dangerous to humans.
Figure 1.1: Miner rescuer and his best friend.
Needless to say, removal of H2S is of utmost importance for safe process operations, health, environmental and economic reasons.
1.2 H
2S removal
Several regenerative and non-regenerative processes exist for the removal of H2S. These processes are applicable only for specific gas compositions and gas flow rates, because of process restrictions and/or economic reasons. For small quantities of sulphur to be removed (up to 50 kg/day of sulphur), technologies are preferred with relatively low capital costs like e.g. scavengers. For increasing H2S quantities to be removed, overall process costs can decrease with technologies that are able to reduce operational costs. The reduction in operational costs outweigh the additional costs of capital required for these technologies. E.g. RedOx processes and biological processes are used for sulphur quantities between 50 kg/day and 15 ton/day. Amine and physical processes, combined with the so-called Claus process, are used for sulphur quantities above 15 ton/day.1
Most of the regenerative processes such as THIOPAQ, LO-CAT, SulFerox, amine processes and physical processes capture CO2 to varying extents besides H2S. This results in an increase in OPEX and CAPEX. Moreover, deep removal of H2S cannot always be guaranteed.
2
1.2.1 Amine process
The conventional method of removing H2S from natural gas is with an amine process. In the amine process, a lean alkanolamine solution is used to chemically absorb acid gases in an absorber; afterwards acid gases are released in the stripper. Heat is required to release the acid gases, which is provided by the reboiler connected to the bottom section of the stripper. The gas exiting the stripper still contains H2S.2 If selective H2S removal is required for gas streams containing CO2, the regeneration costs of the amine process are substantially increased due to the coabsorption of CO2.3 For large-scale applications, H2S in the stripper gas is converted to elemental sulphur in the Claus process.
1.2.2 Physical process
Alternatively, H2S can be removed with physical solvents (e.g. Selexol, Rectisol, Purisol) and stripped by reducing the pressure. Acid gas pressures are required to be 2 MPa or higher; at lower pressures chemical processes are used. Unlike the chemical processes, acid gases are not chemically bonded, therefore little or no heat is required to release the acid gases. However, depending on stringent H2S and/or CO2 specifications, thermal regeneration is often required. Physical processes can dissolve a variety of components to varying extents. Gases requiring treatment should not contain high concentrations of C4+ hydrocarbons because of accumulation in the solvent.4,5
Selexol (licensed by UOP LLC) uses a solvent consisting of polyethylene glycol dimethyl ethers to absorb acid gases at temperatures ranging from -18 °C to 175 °C. Rectisol (licensed by Linde AG and Air Liquide) uses chilled methanol at temperatures ranging from -62 °C to -40 °C to absorb acid gases. Purisol (licensed by Air Liquide) uses N-methyl-2-pyrrolidone (NMP) to absorb acid gases either at ambient temperature or down to -15 °C. The physical solvents exhibit different solubilities of gas components. Purisol exhibits the highest selectivity of the physical solvents for H2S over CO2; the solubility of H2S in NMP is 10 times higher compared to CO2. For large-scale applications, stripped H2S is converted to elemental sulphur in the Claus process.4,5
1.2.3 Claus process
The Claus process is a large scale catalytic desulphurization process which converts H2S to elemental sulphur. The equilibrium reactions are:
(1.1)
(1.2)
For gas streams containing more CO2 than H2S, the acid gases released by the chemical or physical process will result in an inlet acid gas stream for the Claus process that is low in H2S and high in CO2 content. The inlet gas stream for the Claus furnace should contain at least 20 mol.% of H2S to be able to produce a stable flame.6,7
In the furnace H2S is burned to produce sulphur and obtain a tail gas with a H2S : SO2 molar ratio of 2 (as required for Reaction 1.2). Most of the sulphur produced in the Claus process is formed in the combustion step (about 70 % of the H2S). Sulphur is separated in a condenser and the remaining gas is reheated prior to entry in the first reactor, where H2S and SO2 react on a bauxite catalyst to form elemental sulphur and water. Products are cooled to remove liquid sulphur, and after reheating are fed to the second reactor. A flow diagram of a Claus process is shown in Figure 1.2.
Typically a Claus process consists of 2 to 4 reactors. For a well-designed Claus process consisting of three catalytic reactors, H2S conversions of 98 % up to 99.8 % can be achieved. The remaining tail gas requires further treatment in a tail gas treatment unit like e.g. the SCOT process to obtain H2S conversions of 99.9+%.8,9,10
3 Figure 1.2: Flow diagram of a Claus process.
1.2.4 Biological process
The THIOPAQ process is a biological desulphurization process which uses a caustic solution with a pH ranging from 8.2 to 9. CO2, if present in the gas, is co-absorbed. After absorption, dissolved sulfide ions are oxidized to elemental sulphur by thiobacillus bacteria in an aerated bioreactor. A make-up consisting of caustic, nutrients for the bacteria and water is added to the bioreactor. Full conversion of sulphides can be obtained with a 95 % to 98 % selectivity to elemental sulphur. A slurry containing up to 20 wt.% of sulphur can be obtained from the bioreactor. This slurry can be further processed with a decanter centrifuge to obtain a sulphur cake with up to 65 wt.% dry solids content. The circulating solution contains between 0.5 wt.% to 2.0 wt.% sulphur. The sulphur purity is approximately 95 % to 98 %, the remaining 2 % to 5 % is organic material and salts (mainly sodium bicarbonate and sodium sulphate). A flow diagram of the THIOPAQ process is shown in Figure 1.3.11
4
1.2.5 RedOx process
Processes like Sulferox and LO-CAT are RedOx desulphurization processes which absorb H2S in an alkaline ferric solution containing chelates. H2S is oxidized by the ferric ions, forming elemental sulphur with a particle size varying between 8 m to 45 m and reducing the ferric ion to ferrous ions.12 Subsequently the ferrous ions in solution are oxidized to ferric ions by introducing air, thus regenerating the solution. The solution rich in sulphur is recycled over a settler, which produces a sulphur slurry and returns the solution lean in sulphur to the oxidizer. The sulphur slurry can be further processed in a belt filter or decanter to obtain a sulphur cake containing up to 65 wt.% sulphur. Due to degradation of chelates and the removal of chemicals with the sulphur cake, a chemical make-up is required. The make-up consists of the solvent, chelates and caustic and is added to the oxidizer. A flow diagram of the LO-CAT process is shown in Figure 1.4.13
Figure 1.4: Flow diagram of the LO-CAT process.
Because of the nature of the sulphur produced and/or the chemicals present in the sulphur cake in the THIOPAQ and LO-CAT processes, it is proven difficult to obtain 99+% pure sulphur.
1.3 The Vitrisol® process
The novel Vitrisol® desulphurization process is based on the selective removal of H2S by precipitation with copper sulphate (CuSO4) in an aqueous, acidic solution, without the coabsorption of CO2. Copper sulphide (CuS) and sulphuric acid are formed in the gas treating process:14,15,16
( ) ( ) (1.3)
The current status of the Vitrisol® process is a scavenger-like application. Cu2+, the active compound in the absorption liquid, becomes depleted during H2S removal. It must be noted, however, that copper is an expensive commodity; an increase in H2S quantity increases operational costs. To reduce the operational costs for large quantities of H2S, a regeneration step is required.
5 The Vitrisol® liquid can be regenerated through oxidation of CuS by ferric sulphate (Fe2(SO4)3), an operation encountered in copper ore processing (a/o CuS).17,18 Copper sulphate, elemental sulphur (So) and ferrous sulphate (FeSO4) are produced in the process:
( ) ( ) ( ) (1.4)
Ferrous sulphate can be re-oxidized to ferric sulphate with O2, according to:
( ) (1.5)
The presence of cupric ions increases the oxidation rate of ferrous sulphate.19 Resulting in the overall net reaction for the removal of H2S:
( ) (1.6)
Subsequently sulphur can be extracted with a suitable nonpolar solvent, to prevent losses of chemicals. Solid, orthorhombic sulphur can be reclaimed from the nonpolar solvent.
1.4 This thesis
The removal of H2S from gas streams by cupric ions (Reaction 1.3) is known for over a century.20 However, to this day there have not been successful attempts in realizing a full-scale desulphurization process based on this principle because of issues with respect to process design. E.g., how can CuS be continuously regenerated without losses of chemicals, and a highly pure sulphur be obtained? These topics are addressed in the present development project and dissertation, as well as other research topics to arrive at a thorough process design for the regeneration section of the Vitrisol® process.
Reactions 1.4 and 1.5 are the main reactions to regenerate CuS to elemental sulphur and Cu2+ ions. They occur parallel in the oxidation step of the regeneration section. For design purposes, it is required to understand these reactions, and preferably obtain kinetic relationships.
Chapters 2 and 3 discuss the oxidation of ferrous ions in aqueous, acidic sulphate solutions with air at elevated pressures. Experiments were performed in a glass Büchi autoclave with a gas entrainment impeller to ensure a sufficiently large contact area between gas and liquid phase and high mass transfer coefficients. In Chapter 2 the pseudo-first order approach is used to determine orders of reaction in Fe2+, O2, H2SO4, and the components that H2SO4 dissociates into, i.e. HSO4-, H3O+ and SO42-. A power law kinetic equation was derived from initial reaction rates, valid for H2SO4 concentrations of 1 M and higher. In Chapter 3 a different approach was used to determine the kinetics of the Fe2+ oxidation reaction. The kinetic equation was determined by fitting experimentally determined Fe2+ concentration profiles. The derived kinetic equation is valid for all H2SO4 concentrations and accounts for the effect of Fe3+ on the Fe2+ oxidation rate. The two different methods of interpreting kinetics are discussed in Chapter 3.
As was explained, Cu2+ is the active compound in the Vitrisol® solution for H2S removal. From a literature survey it was concluded that Cu2+ ions increase the Fe2+ oxidation rate too. Therefore, Chapter 4 discusses the reaction behaviour of the Fe2+ oxidation reaction in the presence of Cu2+ at elevated air pressures in a high intensity gas-liquid contactor. The effects of Fe2+, Cu2+, Fe3+, H2SO4 and the partial oxygen pressure on the Fe2+ oxidation rate were investigated. A mass transfer model was created to study the effect of mass transfer of oxygen. An intrinsic kinetic equation could not be derived.
6
Chapter 5 describes the dissolution of CuS particles with Fe3+ in acidic sulphate solutions in an oxygen-free environment. CuS particles were obtained from H2S removal operations from biogas with an acidic CuSO4 solution in a Vitrisol® pilot absorber. The characteristics of the CuS particles were investigated, i.e. porosity and particle size distribution. The effects of Fe3+ and H2SO4 on the Fe3+ dissolution rate were determined. A mass transfer model was established to investigate the effects of extraparticle and intraparticle mass transfer on the CuS dissolution rate.
In Chapters 2 to 5 the design rules for the Vitrisol oxidizer are derived. Elemental sulphur, present in an aqueous, acidic solution containing metal sulphates, is the effluent from the oxidation reaction. A sulphur recovery system comparable to THIOPAQ and/or LO-CAT could be applied, but this would result in high chemical losses in the sulphur sludge or sulphur cake. As the Vitrisol® solution is particularly suited for extraction with a nonpolar solvent (unlike e.g. LO-CAT due to the presence of chelates), extraction of sulphur with a nonpolar solvent is incorporated as one of the sulphur recovery steps in the regeneration section of the Vitrisol® process.
Chapter 6 investigates the solubility of sulphur in in toluene, m-xylene, p-xylene and o-xylene at temperatures ranging from 303.15 K to 363.15 K. Interestingly, the solubility of sulphur varied among the various solvents. A hypothesis was postulated concerning the difference in solubility based on the molecular structures of orthorhombic sulphur and the solvents. The temperature dependence of the solubility equilibria of sulphur in the varying solvents was determined.
From Chapters 2 to 6, as well as additional IP developed by Procede, the design rules for the entire regeneration section of Vitrisol®, i.e. oxidation, extraction, and sulphur recovery, were obtained. Therefore, in Chapter 7 the performance of Vitrisol® is demonstrated for two typical applications in shale gas production by comparing them to a standard amine treating process. For Case 1, only H2S requires removal, because CO2 is already below pipeline specifications. In Case 2, both H2S and CO2 need to be treated to reach pipeline specifications. From the case studies it is concluded that it is advantageous to first selectively remove H2S from a gas stream containing both H2S and CO2 prior to CO2 removal to reduce OPEX. Moreover, contrary to the amine process, Vitrisol® does not require additional treatment of the off-gas stream as the H2S is directly converted to crystalline sulphur and CO2 that can be emitted to the environment.
The IP developed during the course of this PhD is patented. The patent, describing the Vitrisol® process with regeneration, is presented in Appendix A.
1.5 References
[1] Bowman, D.F. Selective H2S removal with Sulferox. GPA conference, 26September 1991.
[2] Hamborg, E.S. Carbon dioxide removal processes by alkanolamines in aqueous organic solvents. Dissertation, University of Groningen, Groningen, 2011.
[3] Wermink, W.N., Ramachandran, N. and Versteeg, G.F. Vitrisol® a 100% selective process for H2S removal in the presence of CO2. J. Nat. Gas Eng. 2017, 2 (1), 50─83.
[4] Burr, B. and Lyddon, L. A. Comparison of physical solvents for acid gas removal. 87th Annual convention of the Gas Processors Association 2008, vol. 1 of 2, Grapevine, Texas, USA, 2008. [5] http://home.agh.edu.pl/~lstepien/co2_mit/Lectures/05.pdf, visited 24-07-2018.
[6] Mcintyre, G. and Lyddon, L. Claus sulphur recovery options. Pet. Technol. Q. Spring 1997, 57─61. [7] Perry, D., Fedich R.B. and Parks, L.E. Better acid gas enrichment. Sulphur 2010, 326, 38─42. [8] Maadah, A.G. Calculated chemical reaction equilibrium for the Claus process. Dissertation,
Oklahoma State University, Stillwater, 1978.
7 [10]
https://www.shell.com/business-customers/global-solutions/gas-processing-licensing/licensed-technologies/sulphur-recovery/scot-process.html (accessed 25-07-2018).
[11] Cline, C., Hoksberg, A., Abry, R. and Janssen, A. Biological process for H2S removal from gas streams. The Shell-Paques/THIOPAQTM gas desulfurization process. LRGCC 2003 conference proceedings, 53rd Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, USA, February 23–26, 2003.
[12] http://www.digitalrefining.com/article/1001047,LO_CAT__a_flexible_hydrogen_sulphide_remov al_process.html#.W1rdzIoyWUk (accessed 27-07-2018).
[13] http://www.merichem.com/LO-CAT-Flexible-H2S-Removal-Process (accessed 27-07-2018). [14] Versteeg, G.F. and Ter Maat, H. Method and system for selective removal of contamination from
gas flows. WO patent 1998055209 A1, assigned to Procede Twente B.V., priority date June 2, 1997.
[15] Ter Maat, H., Hogendoorn, J.A. and Versteeg, G.F. The removal of hydrogen sulfide from gas streams using an aqueous metal sulfate absorbent. Part I. The absorption of hydrogen sulfide in metal sulfate solutions. Sep. Purif. Technol. 2005, 43, 183─197.
[16] Ter Maat, H., Al-Tarazi, M., Hogendoorn, J.A., Niederer, J.P.M. and Versteeg, G.F. Theoretical and experimental study of the absorption rate of H2S in CuSO4 solutions. The effect of enhancement of mass transfer by a precipitation reaction. Chem. Eng. Res. Des. 2007, 85 (1), 100─108.
[17] Peacey, J., Guo, X.-J. and Robles, E. Copper hydrometallurgy – current status, preliminary economics, future direction and positioning versus smelting. Trans. Nonferrous Met. Soc. China
2004, 14 (3), 560─568.
[18] Dutrizac, J.E. and MacDonald, R.J.C. The kinetics of dissolution of covellite in acidified ferric sulphate solutions. Can. Metall. Q. 1974, 13 (3), 423─433.
[19] Wermink, W.N., Spinu, D. and Versteeg, G.F. The oxidation of Fe(II) with Cu(II) in acidic sulphate solutions with air at elevated pressures. Chem. Eng. Commun. 2018, accepted.
