Team 7:Mengesteab Adera Zachary Alford James Foreman Griffin Jayne Claus Kinetics Team 7:Mengesteab Adera Zachary Alford James Foreman Griffin Jayne

Review of kinetics r = k CAn CBo CCp… Chemical reaction rates are dependent on the change in moles (which is the same as molarity w/ constant volume) r = k CAn CBo CCp… where: r: Rate of chemical reaction in units of mol/time k: Rate constant; units change depending on order of reaction and/or number of reactants Cx: Individual chemical concentration in mol/volume (typically molarity) n,o,p...: Order of the reaction

Reaction Rate Heavily dependent on temperature Dependent upon thermodynamics Equilibrium concentration limitations Endothermic vs. Exothermic and subsequent response to heat Reaction Order determines dependence on reactant concentration

Rate Constant k k = A e-Ea/RT Where: Introduces temperature dependence A: Arrhenius constant (reaction-specific) Ea: Activation energy of forward reaction (for readily reversible reactions directionality can be arbitrary) R: Gas constant (8.314 J/mol K) T: Temperature in K Introduces temperature dependence Constant with non-constant units

General Reactions(a) Specific to Claus (1) 2 H2S + 3 O2 → 2 SO2 + 2 H2O (occurs in furnace) (2) 2 H2S + SO2 → 3 S + 2 H2O (occurs in separate “Claus reactors” as well as within furnace)

Claus Process The most commonly used gas desulfurization process for recovering elemental sulfur from gaseous hydrogen sulfide Gases with a H2S content of over 25% are suitable for the recovery of sulfur in straight-through Claus plants The Claus technology can be divided into two process steps: Thermal step Catalytic step A Claus process plant source:http://chemengineering.wikispaces.com/Claus+process

Claus Process Flow Diagram(a) Source:http://chemengineering.wikispaces.com/Claus+process

Thermal step Hydrogen sulfide containing gas reacts with air at temperatures above 1000 °C The air to the acid gas ratio is controlled such that in total 1/3 of all hydrogen sulfide(H2S) is converted to SO2 2 H2S + 3 O2 → 2 SO2 + 2 H2O Unburned H2S in the acid gas reacts with SO2 to form elemental sulfur vapor 2 H2S + SO2 → 3/2 S2 + 2H2O ~67% of the conversion of H2S to elemental sulfur(S2) occurs in the thermal stage >10000C

Thermal step - Primary combustion process The composition of the main combustion rxn depends on the ratio of H2S and O2. Source: www.topsoe.com/sitecore/shell/Applications/~media/Topsoe_Catalysis/Clark.ashx

Thermal Step - Side Reactions The formation of hydrogen gas: 2 H2S → S2 + 2 H2 CH4 + 2 H2O → CO2 + 4 H2 The formation of carbonyl sulfide: H2S + CO2 → S=C=O + H2O The formation of carbon sulfide: CH4 + 2 S2 → S=C=S + 2 H2S

Thermal Step - Impurities Important function of the thermal step is to destroy the impurities that may be present in the feed acid gas stream such as ammonia (NH3) & hydrocarbons

Catalytic step(j) Claus reaction continues in the catalytic step with activated aluminum(III) or titanium(IV) oxide as a catalyst at lower temperature(200-350°C) A catalyst is needed in the second step to help the components react with reasonable speed Conversion is optimized with two or three stages, with sulfur being removed between the stages Concentration, contact time and reaction temperature are set to give the best conversions

Catalytic step continued…(j) More hydrogen sulfide (H2S) reacts with the SO2 formed during combustion in the reaction furnace, and results in gaseous elemental sulfur 2 H2S + SO2 → 3/x Sx + 2 H2O Here, x is temperature-dependent, usually x = 6, 7, 8

Catalytic step continued…(j) The catalytic recovery of sulfur consists of three substeps: heating, catalytic reaction and cooling plus condensation These three steps are normally repeated to a maximum of three times. Where an incineration or tail-gas treatment unit (TGTU) is added downstream of the Claus plant, only two catalytic stages are usually installed. The first process step in the catalytic stage is the gas heating process. It is necessary to prevent sulfur condensation in the catalyst bed, which can lead to catalyst fouling.

Catalytic step continued…(j) The required bed operating temperature in the individual catalytic stages is achieved by heating the process gas in a reheater until the desired operating bed temperature is reached. The catalyst not only increases the kinetics (i.e., the rate of reaction) of the Claus reaction equation (1), but it also hydrolyzes the carbonyl sulfide (COS) and carbon disulfide (CS2) that is formed in the reaction furnace: COS + H20 → H2S + CO2 CS2 + 2H20 → 2H2S + CO2

Catalytic step continued…(j) The catalytic conversion is maximized at lower temperatures, but care must be taken to ensure that each bed is operated above the dew point of sulfur. The operating temperatures of the subsequent catalytic stages are typically 240 °C for the second stage and 200 °C for the third stage (bottom bed temperatures) In the sulfur condenser, the process gas coming from the catalytic reactor is cooled to between 150 and 130 °C. The condensation heat is used to generate steam at the shell side of the condenser

Catalytic step continued…(j) Before storage, liquid sulfur streams from the process gas cooler, the sulfur condensers and from the final sulfur separator are routed to the degassing unit, where the gases (primarily H2S) dissolved in the sulfur are removed. The tail gas from the Claus process still containing combustible components and sulfur compounds (H2S, H2 and CO) is either burned in an incineration unit or further desulfurized in a downstream tail gas treatment unit.

Claus Converter Chemistry - Main Rxn(i)

Values of Standard Equilibrium Parameters Chemical Standard Enthalpy of Formation (kJ/mol) H2S(g) -20.1(c) SO2(g) -296.1(b) S(aq) H2O(g) -241.8(b) O2(g) Standard Gibbs Free Energy (kJ/mol) -33.0(c) -300(c) -229(c) Standard Entropy (J/mol K) 206(c) 249(c) 31.9(c) 189(c) 205(c)

Equilibrium Equations(g) KP: Equilibrium Constant with respect to partial pressures P: Partial Pressure ni: Reactant order ∆G: Gibbs Free Energy R: Gas Constant T: Temperature X: Equilibrium Parameter (Entropy, Gibbs Free Energy, or Enthalpy)

Parameters Specific to Reaction 1 Chemical Standard Enthalpy of Formation (kJ/mol) H2S(g) -20.1(c) SO2(g) -296.1(b) H2O(g) -241.8(b) O2(g) Standard Gibbs Free Energy (kJ/mol) -33.0(c) -300(c) -229(c) Standard Entropy (J/mol K) 206(c) 249(c) 189(c) 205(c) 2 H2S + 3 O2 → 2 SO2 + 2 H2O (occurs in furnace) -1058kJ/mol -1075.8 kJ/mol -66 kJ/mol -40.2 kJ/mol -992 kJ/mol -1035.6 kJ/mol

Parameters Specific to Reaction 2 Chemical Standard Enthalpy of Formation (kJ/mol) H2S(g) -20.1(c) SO2(g) -296.1(b) S(aq) H2O(g) -241.8(b) Standard Gibbs Free Energy (kJ/mol) -33.0(c) -300(c) -229(c) Standard Entropy (J/mol K) 206(c) 249(c) 31.9(c) 189(c) 2 H2S + SO2 → 3 S + 2 H2O (occurs mainly in separate “Claus reactors”) -687 kJ/mol -725.4 kJ/mol -366 kJ/mol -336.3 kJ/mol -321 kJ/mol -389.1 kJ/mol

Equilibrium Limitations ∆Grxn1 = -992 kJ/mol ∆Grxn2 = -321 kJ/mol T1: 1273 K T2: 573 K Kp,1 = 1.098 Kp,2 = 1.070

Equilibrium Limitations Continued Within the furnace:

Temperature Effects This overall equilibrium equation shows that decreasing temperature will drive up sulfur production. However, the first step of the Claus Process, the furnace, requires a temperature of at least 850°C to achieve the combustion reaction. The remaining steps require temperatures above sulfur’s dewpoint (120°C-150°C) to prevent fouling of the catalyst.

Pressure Effects As we would expect for gaseous reactions, increasing pressure will increase reaction rate. However, natural gas streams contain contaminants whose varying partial pressures will slow down reaction rates. Therefore, as with temperature, a middle ground is preferred.

Isothermal or Adiabatic? It is actually beneficial to run the Claus Process isothermally or adiabatically, as opposed to choosing neither. However, recent research has shown that isothermal operation results in superior sulfur production.

Optimization Typical industry Claus furnaces are ran at 1.5 bar gauge and 1000°C Then for the 3 catalytic steps, 320°C, 240°C, and 200°C respectively. Interesting correlation for furnace temperature at different contaminant levels:

Reactor Type Two types of reactor used in Claus Process: First step (furnace) is effectively a Plug Flow Reactor (PFR) All steps following which include the catalytic are packed bed reactors (PBRs), although research is being done into using fluidized bed reactors.

Process performance(i) Using three catalytic stages, the process will typically yield over 98% of the sulfur in the input stream The unrecovered sulfur (H2S, COS, CS2) from the last condenser is referred to as "tail gas" Tail gas clean-up units(TGCU) are usually added to increase sulfur recovery and minimize emissions

Summary The Claus process is the most commonly used gas desulfurization process for recovering elemental sulfur from gaseous hydrogen sulfide Thermal step 2 H2S + 3 O2 → 2 SO2 + 2 H2O 2 H2S + SO2 → 3/2 S2 + 2H2O Catalytic step 2 H2S + SO2 → 3/8 S8 + 2 H2O

Summary continued... Reaction kinetics are heavily dependent on thermodynamics. The Claus Process typically has 4 stages: A combustion stage followed by 3 catalytic stages. For optimal results the process should take place isothermally at the lowest practical temperatures for each stage. By using three catalytic stages, the process will typically yield over 98% of the sulfur in the input stream. Tail gas clean-up units(TGCU) are usually added to increase sulfur recovery (~99%) and minimize emissions

Sources Cited (a) http://chemengineering.wikispaces.com/Claus+process (b) http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf (c) http://boomeria.org/chemtextbook/cch20.html (d) http://www.ipcbee.com/vol23/19-CCEA2011-A20004.pdf (e) http://www.epa.gov/ttnchie1/ap42/ch08/final/c08s13.pdf (f) J. H. Dymond and E. B. Smith, The Virial Coefficients of Pure Gases and Mistures, Clarendon Press, Oxford, 1980 (g) http://www.docbrown.info/page07/delta3SGd.htm (h) http://enu.kz/repository/2010/AIAA-2010-1356.pdf (i) http://www.topsoe.com/sitecore/shell/Applications/~/media/PDF%20files/Topsoe_Catalysis_Forum/2007/Clark.ash x (j) Rouquette, Charlette, et al. Monitoring of the chemical species in a liquid-phase Claus Reaction. Energy fuels, 2009.