Transcript Oxidative coupling of methane - A. James Clark School of

Oxidative coupling of Methane
&
other reactions for treating Sulfur
CHBE 446 – Gp5
Stephan Donfack
Benjamin Harbor
Nguyen Huynh
Cyndi Mbaguim
AGENDA
• Introduction
 Source of Methane and Sulfates in marine environment
• Anaerobic oxidation of Methane
 Mechanism & Syntrophy
• Treatments of Sulfur derivative
 Hydrogen peroxide
 Metal Salts & Potassium Permanganate
INTRODUCTION
• Sea floor and microbial inhabitants have an important role
in the biogeochemical cycling of elements.
•
Different gases found in marine environment: Methane 5070%, Carbon dioxide 30-50%, trace gases (nitrogen,
ammonia, and hydrogen sulfide)
• Carbon mineralization process through anaerobic oxidation
of Methane which is generally coupled to sulfate reduction
leads to sulfur derivatives
-SO42-+ CH4 (organic matter)
HS2-+ H2O + HCO3ΔG°’= –16.6 kJ.mol–1
• Hydrogen Sulfide is a notorious gas among sulfur derivatives:
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Corrosion to concrete and metal pipes
Odorous (recognition at 4.5 ppb)
Health concerns (10 ppmV and death at 600 ppmV)
Combustion and acid rains
Ozone depletion
Total sulfide at 90-250mg-S/L, inhibits methanogenesis
• The removal of H2S is immediately recommended to protect
downstream equipments.
• This removal is done through different processes or reactions.
Anaerobic Methane Oxidation(AMO)
• Little methane escapes from anoxic sediment to the water column
and atmosphere
• This postulates that methane is somehow consumed by microbes
• Recent studies reveals that these micro-organisms undergo
anaerobic oxidation, as they consume methane
• Methanotrophs are aerobic bacteria:
𝐶𝐻4 + 2𝑂2 → 𝐶𝑂2 + 2𝐻2 𝑂
• So what favors the anaerobic oxidation?  SULFATES
EVIDENCE
• In cold seep areas, thermodynamic and kinetic conditions
favor sulfate reduction over methanogenesis
 Sulfate reduction rate are extremely high
 Sulfate concentrations decrease with sediments depths
matched by upward depletion of methane
• This provides confirmation that there is a link between
methane and sulfate turnover
MECHANISM
1.
Reverse Methanogenesis :
Energetically favorable only if CO2 , H2O are rapidly removed.
C𝐻4 + 2H2 O → 4H2 + CO2
𝑪𝑯𝟒 + 𝟐𝑯𝟐 𝑶 → 𝟒𝑯𝟐 + 𝑪𝑶𝟐
2.
Sulfate-reducing bacteria convert sulfate to
sulfite in the presence of H2 product from “1”
−
𝑯𝟒 +𝟒𝑯𝟐 + 𝑺𝑶𝟐−
𝟒 → 𝑯𝑺 + 𝟒𝑯𝟐 𝑶
3.
Overall “syntrophy” reaction
−
3𝑪𝑯𝟒 +𝑺𝑶𝟐−
𝟒 → 𝑯𝑺 + 𝑯𝟐 𝑶 + HCO
*** 𝑯𝑺− converts to sulfites, sulfides, sulfates
derivatives
𝐻4 +4𝐻2 + 𝑆𝑂42− → 𝐻𝑆 − + 4𝐻2 𝑂
Chemicals Which Have Been Tried to Control
Hydrogen Sulfide
• Hydrogen Peroxide
• Chlorine
• Metal Salts
• Potassium Permanganate
Hydrogen Peroxide
• Hydrogen peroxide reacts with hydrogen sulfide under acid, neutral and
alkaline conditions.
• The reaction is accelerated by increasing temperature and/or the addition of
catalysts such as iron.
• Stoichiometry is also affected by pH.
• Under acidic or neutral conditions the reaction with hydrogen peroxide
produces sulfur and water:
H2S + H2O2 → S + 2 H2O
Since waste streams often contain other reactive materials, it may be
necessary to add more than the stoichiometric amount of hydrogen peroxide.
• In alkaline solution (> pH 8), the dominant reaction is:
Na2S + 4 H2O2 → Na2SO4 + 4 H2O
The 4/1 wt. ratio of H2O2/Sulfide can be lowered by using a combination of
air and hydrogen peroxide.
Chlorine
• Chlorine has been applied to wastewater in a dose that is at least
three to nine times the concentration of sulfide to be oxidized.
Chlorine combines with water to form hypochlorous and hydrochloric
acids.
Disadvantages: Chlorine kills the natural (and Clear-Flo®) wastedegrading bacteria. Chlorine combines with the urine in the waste
stream to form chloramines, which are difficult to remove. Toxic or
carcinogenic chlorinated hydrocarbons may form during treatment.
Chlorine is a hazardous material, requiring special safety precautions.
Metal Salts
• By reacting certain metal salts, such as ferrous sulfate, with hydrogen
sulfide, an insoluble metallic sulfide will be formed. The dose is 4.5
grams of ferrous sulfate for each gram of sulfide to be oxidized. This is
less expensive than peroxide or chlorine.
Disadvantages: These products may contain a high free acid content
which causes detrimental changes in the pH and alkalinity of the
stream, which can interfere with biodegradation of the waste.
Potassium Permanganate
• Potassium permanganate is a strong oxidizing agent that can react with
hydrogen sulfide in a variety of ways.
• In acidic conditions, the following reaction takes place:
Hydrogen sulfide + Potassium permanganate
Element sulfur + Water + Potassium hydroxide + manganous oxide.
3H2S + 2KMnO4
3S + 2H2O + 2KOH + 2MnO2
Potassium Permanganate (cont’d)
Under alkaline conditions the following takes place:
Hydrogen sulfide + Potassium Permanganate
Potassium sulfate + manganous oxide + potassium hydroxide + water.
3H2S + 8KMnO4
3K2SO4 + 8MnO2 + 2KOH + 2H2O.
Under conditions that are in between acidic and alkaline pHs, a
variety of reactions occur, yielding elemental sulfur, sulfate, thionates,
dithionates and manganese sulfide end products.
Potassium Permanganate (cont’d)
Disadvantages:
Dosages are difficult to predict and control in most liquid applications.
The high cost and high dose, 6 or 7 parts of potassium permanganate
are needed for each part of hydrogen sulfide, are discouraging.
Safety precautions are required for handling and storage.
Summary
REFERENCES
1. V.A. Edwards, C.P. Velasco, K.J. Edwards, "Hydrogen Sulfide-The
Relationship of Bacteria to its Formation, Prevention, and
Elimination." Alken Murray Corp. Web Feb 4. 2014
2. http://alrlab.pdx.edu/courses/Anaerobic%20Methane%20Oxidation
%20Presentation.pdf