anurag ppt
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SEMINAR ON BIOCHEMICAL ROUTES :ANAEROBIC DIGESTION
TRANSESTERIFICATIONPHOTOFERMENTATION
By ANURAG CHANDRA SHEKHAR(M.TECH. FIRST YEAR )ROLL No. 06
In The Expert Guidance OfDr. SONAL DIXIT
ASST. PROFESSOR(GUEST)DEPT. ENVIROMENTAL SCIENCE
Anaerobic Digestion
Anaerobic digestion is the conversion of organic material in the absence of oxygen to methane and carbon dioxide by a microbial consortia.
CXHYOZ CO2 +CH4+ anaerobic bacterial biomass
How Do We Get Methane?
Need Methanogens:Either Hydrogen (Lithotrophic methanogens)4H2 + CO2 CH4 + 2 H2O
Acetate (acetoclastic methanogens)CH2COOH + H2O CH4 + CO2
But have a limited range of substrates can also include carbon monoxide, formate, methanol.
The Anaerobic Digestion Microbial Consortia
The production of biogas is dependant on the successful interaction of interdependent microbial species.
The production of methane is a property of a (relatively) limited number of micro-organism species, the “methanogens”.
There is a limited number of substrates that the “methanogens” can use to produce methane.
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Anaerobic Digestion Process
Liquefaction
LiquefyingBacteria
Acid Production
Liquefiedsoluble organic
compounds
InsolubleCompounds
(organic,inorganic,
water)
Acid-FormingBacteria
BiogasProduction
Simpleorganic
acids
Methane-FormingBacteria
Manure
BiogasBiogas
(Methane, CO2, misc.)
Effluent
End Products
The Stages in Anaerobic Digestion
HydrolysisLong chain polymers broken down to smaller molecules.
AcidogenesisProduction of hydrogen and volatile fatty acids.
AcetogenesisAlcohols, >C2 VFAs converted to acetate and hydrogen.
MethanogenesisHydrogen and acetate converted to methane.
The Steps in Anaerobic Digestion
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
Taken from Guwy, (1996). Modified from Mosey, (1983)
Factors Effecting the Rate of Hydrolysis
Particle size of the waste, smaller is better. Accessibility of the substrate i.e. how easy is it for the
enzymes to attack the substrate. Problems with lipids . Residence time in the reactor. Chemical structure of substrate. Negative impact of lignin and hydrocarbons, Organic content of the substrate.
Acidogenesis
The sugars, amino acids and longer chain fatty acids are then fermented to acetate, propionate, butyrate, valerate, ethanol, lactate, hydrogen, CO2, ammonia and sulphide by the acidogenic bacteria.
The proportion of the organic products of the acidogenic bacteria is determined by the H2 concentration and pH.
Acetogenesis
The acetogens (obligate hydrogen producing acetogenic bacteria) convert the fermentation products which the methanogens cannot use (alcohols, >C2 VFAs, aromatic compounds) to the substrates which the methanogens can utilise.
The Gibbs free energies for the conversion of ethanol, propionic and butyric to acetate and hydrogen are energetically unfavourable i.e. positive at standard free biochemical energy levels (pH 7.0, 1 atm.).
Why are Propionic and Butyric Acids bad?
Don`t smell very nice . Wasted energy as methanogens can`t use them. Very difficult to get rid of.
CH3CH2COOH + 2H2O CH3COOH + CO2+ 3H2
Propionic acid
NOT beneficial for the microorganisms as: G0 =+ 76.1.9 KJ mol -1
Energy could be required to be put in to use the propionic acids.
Role of Interspecies Hydrogen Transfer
The H2 partial pressure must be maintained at between 10-6-10-
4 atmospheres for sufficient energy for growth to be obtained from propionic and butyric acids.
As H2 is continually being generated it must be continually be removed for methanogenesis to occur.
The removal of H2 is achieved by conversion to methane by the hydrogen utilising methanogens.
This symbiosis is known as interspecies hydrogen transfer and is crucial to the success of the anaerobic digestion process.
Graphical representation of the hydrogen-dependant thermodynamic favourability of acetogenic oxidations and inorganic respirations associated with the anaerobic degradation of waste organics. (1) Propionic oxidation to acetic acid, (2) butyric oxidation to acetic, (3) ethanol to acetic, (4) lactic
to acetic, (5) acetogenic respiration of bicarbonate, (6) methanogenic respiration of bicarbonate, (7) respiration of sulphate to sulphide, (8) respiration of sulphite to sulphide, (9) methanogenic
cleavage of acetic acid, (10) SRB-mediated cleavage of acetic acid. (From Harper and Pohland, 1985).
Methanogenic “Hydrogen” Niche
The Methanogens
A group of gram –ve bacteria. Belong to the group of bacteria termed the Archaea or
archaebacteria. Are strict anaerobes they evolved billions (3.5 billion) years
ago when the earth had very low or zero levels of oxygen. Utilise simple inorganic substrates such as H2, CO2 or simple
organic substrates such as acetate, formate and methanol.
Image source: Dr L. Hulshoff and Professor van Lier; Sub-department of Environmental Technology, Wageningen University, Netherlands
The Archaea
Scanning electron micrograph of archaea from the genus Methanosaeta. Members of the genus Methanosaeta are acetotrophic, i.e. they produce methane from acetate.
Scanning electron micrograph of a cocci-shaped archaea from the genus Methanosarcina. Members of the genus Methanosarcina can use all 3 routes to methane.
Specific Growth Conditions
These micro-organisms have been difficult to culture in vitro as in nature they are dependant on other bacteria to provide a suitable environment for their growth.
The fermentative bacteria provide a environment with :Low redox potential (-330 mV).Extremely low in O2 .Suitable range of substrates .
Analysing The Consortia Traditional microbiology
“petri” dish technology (some problems here) Enzymatic profiles
Molecular Biology Techniques PCR/RT PCR DGGE FISH Genomics and Proteomics Microarrays
Limited Range of Substrates
Hydrogen (Lithotrophic methanogens)
4H2 + CO2 CH4 + 2 H2O
Very beneficial for the microorganisms as G0 =-138.9 KJ mol -1 is gained.
But only 30% of methane produced via this route.
Cont…
Acetate (acetoclastic methanogens)
CH2COOH + H2O CH4 + CO2
Not so beneficial for the microorganisms as only G0 = - 32 KJ mol -1 is gained.
But 70% of methane produced via this route.
Other Anaerobic Processes
Unfortunately other gases are produced as well as methane and CO2.
These include hydrogen sulphide (H2S) and ammonia (NH3). Hydrogen Sulphide is produced by a competing group of
bacteria to the methanogens called the sulphate reducing bacteria.
What Effects the % and amount of Methane in the Biogas
Relative solubility of the gases. Is alkalinity being destroyed? Biodegradability of substrate. Chemical composition of biodegradable substrate.
– Buswell equation
Sulphate Reducing Bacteria (SRBs)
Are a problem for methanogenesis as? Compete with methanogens for their source of energy
(hydrogen and acetate). Can become inhibitory (cellular toxin) at dissolved levels of
50 mg/l. More of the dissolved form at lower pH.
Competitive Nature of SRBs
4H2 + SO42- H2S + 2 H2O
Very beneficial for the micro-organisms as G0 =-154 KJ mol -1 is gained.
CH2COOH + SO42- H2S + 2HCO3
-
Not so beneficial for the micro-organisms as only G0 = - 43 KJ mol -1 is gained BUT more than the methanogens
The bacterial consortium requires a number of factors to be controlled to maintain performance. These include: Temperature pH (related to buffering capacity) Essential Nutrients Avoid toxic compounds Sufficient residence time to reproduce
The Effect of Temperature Three temperature optima have been reported for the
anaerobic digestion process phsycrophilic (around 15oC), mesophilic (around 35oC) and thermophilic (around 55oC) temperatures.
Methanogenesis has been found to occur upto 75oC but the optimum temperature is thought to be 55-60oC.
The advantage of the higher temperature ranges is that the process will proceed at a faster rate than the lower temperature ranges as stated by the Arrhenius equation.
The Effect of pH Different groups in the anaerobic consortia can cope with
different pH levels. Methanogens prefer pH 6-5- 7.5 Acidogens also prefer pH 6.5-7.5 BUT can cope with pH 5.2 Therefore if pH is lowered methane production will slow and
consumption of VFA reduce while VFA production continues. Spiral of Decline.
Effects methane production but also distribution of volatile fatty acids.
Bicarbonate Alkalinity and pH
pH Scale is the log10 of the H+ concentration. Bicarbonate alkalinity is a measure of the H+ buffering
capacity of the reactor. Measured in mg/l CaCO3
If you have a alkalinity buffer then difference is between walking a tight rope and a plank when operating the digester.
If alkalinity is decreasing may be a sign of trouble before pH drops below critical level.
Nutrient Requirements
The anaerobic consortia need nutrients to grow their cells and drive their enzymes and metabolic processes.
Can be divided up into macro and micro nutrients.
Need to be “available”
Macro nutrients Relatively large quantities
C/N/P ratiosAlso need sulphur C N P S
500-1000:15-20: 5 : 3
Micronutrients (Trace Elements) Needed in much lower quantities (g per m3) Required for enzyme activity Ni, Fe, Co, Se, Mg etc.
Toxic or Inhibitory Compounds Divided into two groups:
Too much of a good thingAmmoniaSulphateCa/Na/K
Should not be here at all Cleaning compoundsAntimicrobialsSolventsHeavy metals
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Environmental Benefits
Reduces odor from land application. Protects water resources. Reduces pathogens. Weed seed reduction. Fly control after digestion. Greenhouse gas reduction. Help in bio gas generation &
WWT.
Conclusions The anaerobic digestion process depends on the effective
working of a complex interaction of microorganisms. It has been working successfully for 3.5 billion years. However there a wide number of ways which we can make it
not work very well. It can be breaking down and still seem to be working.
Transesterification
Transesterification is based on the chemical reaction of triglycerides with Methanol to form methyl esters and glycerin in the presence of an alkaline catalyst .
Methanol & Catalyst are injected into Light layer from esterification and virgin oil and reacted in the Bio-Conversion Processor (BCP).
Crude Biodiesel and Crude Glycerin are separated using Centrifuge after completion of the transesterification reaction .
Crude Biodiesel and Crude Glycerin are refined using distillation columns.
Use of Ultrasonication Conversion can speed up this process significantly.
Transesterification RXN O O || || CH2 - O - C - R1 CH3 - O - C - R1 | | O O CH2 - OH | || || | CH - O - C - R2 + 3 CH3OH => CH3 - O - C - R2 + CH - OH | (KOH) | | O O CH2 - OH | || || CH2 - O - C - R3 CH3 - O - C - R3
Triglyceride methanol mixture of fatty esters glycerin
Triglyceride Sources
Rendered animal fats: beef tallow, lard Vegetable oils: soybean, canola, palm, etc. Chicken fat Rendered greases: yellow grease (multiple sources) Recovered materials: brown grease, soapstock, etc.
Feedstock OilMethanol Catalyst
Raw Biodiesel
Fuel-Grade Biodiesel
Water
Wash Water
Glycerol (with methanol)
Glycerin
Transesterification
Drier
Reactor
Washer
Separator
Standard Recipe
100 lb oil + 21.71 lb methanol
→ 100.45 lb biodiesel + 10.40 lb glycerol + 10.86 lb XS methanol
Plus 1 lb of NaOH catalyst
Competing Reactions
Free fatty acids are a potential contaminant of oils and fats.
O || HO - C - R
Carboxylic Acid (R is a carbon chain)
O || HO - C - (CH2)7 CH=CH(CH2)7CH3
Oleic Acid
O
|| + KOH HO - C - (CH2)7 CH=CH(CH2)7CH3
Oleic Acid Potassium
Hydroxide
O ||
→ K+ -O - C - (CH2)7 CH=CH(CH2)7CH3 + H2O
Potassium oleate (soap) Water
Fatty acids react with alkali catalyst to form soap.
Water is also a problem
Water hydrolyzes fats to form free fatty acids, which then form soap.
O || CH2 - O - C - R1 CH3 - OH | | | O | O O | || | || || CH - O - C - R2 + H2O >>> CH3 - O - C - R2 + HO - C-R1 | | | O | O | || | || CH2 - O - C - R3 CH3 - O - C - R3
Triglyceride Water Diglyceride Fatty acid
Soap
Soaps can gel at ambient temperature causing the the entire product mixture to form a semi-solid mass.
Soaps can cause problems with glycerol separation and washing.
Process Issues
Feedstock requirements Reaction time Continuous vs. batch processing Processing low quality feed stocks Product quality Developing process options
Feedstock Used in Biodiesel Production
Triglygeride or fats and oils (e.g. 100 kg soybean oil) – vegetable oils, animal fats, greases, soapstock, etc.
Primary alcohol (e.g. 10 kg methanol) – methanol or ethanol (44% more ethanol is required for reaction)
Catalyst (e.g. 0.3–1.0 kg sodium hydroxide) Neutralizer (e.g. 0.25 kg sulfuric or hydrochloric acid)
Reaction time Transesterification reaction will proceed at ambient (70°F)
temperatures but needs 4-8 hours to reach completion. Reaction time can be shortened to 2-4 hours at 105°F and 1-2
hours at 140°F. Higher temperatures will decrease reaction times but require
pressure vessels because methanol boils at 148°F (65°C). High shear mixing and use of cosolvents have been proposed
to accelerate reaction.
Batch vs Continuous Flow Batch is better suited to smaller plants (<1 million gallons/yr). Batch does not require 24/7 operation. Batch provides greater flexibility to tune process to feedstock
variations. Continuous allows use of high-volume separation systems
(centrifuges) which greatly increase throughput. Hybrid systems are possible.
Hybrid Batch/Continuous Base Catalyzed Process
Ester TG
AlcoholTG
AlcoholCatalyst
Ester
Biodiesel Mixer
Glycerol Decanter
CSTR 1 CSTR 2
Glycerol
Alcohol
Alcohol Water
Water
Dryer
Wash Water
Acid
CrudeGlycerol
Processing Lower Quality Feedstock’s
Biodiesel feedstock’s vary in the amount of free fatty acids they contain:
• Refined vegetable oils < 0.05%• Crude soybean oil 0.3-0.7%• Restaurant waste grease 2-7%• Animal fat 5-30%• Trap grease 75-100%Price decreases as FFAs increase but processing demands
increase, also.Not suitable for high FFA feeds because of soap formation.
Preferred method for High FFA feeds: Acid catalysis followed by
base catalysis Use acid catalysis for conversion of FFAs to methyl esters,
until FFA < 0.5%.Acid esterification of FFA is fast (1 hour) but acid-
catalyzed transesterification is slow (2 days at 60°C).Water formation by FFA + methanol ==> methyl ester + water can be a problem.
Then, add additional methanol and base catalyst to transesterify the triglycerides.
Acid Catalyzed FFA Pretreat System
Hi-FFA TG Esters
toAcid base-
process Alcohol
Acid Reactor NeutralizeSeparate
WaterMethanol recovery
Product Quality Product quality is important – modern diesel engines are very
sensitive to fuel. It is not biodiesel until it meets ASTM D6751. Critical properties are total glycerol (completeness of reaction)
and acid value (fuel deterioration). Reaction must be >98% complete.
Developing Process Options Schemes for accelerating the reaction
Supercritical methanolHigh shear mixingCo solvents (Biox)
Solid (heterogeneous) catalystsCatalyst reuseEasier glycerol clean-up
Environmental Impacts of Biodiesel
Large Reductions in:CO2SOxParticulatesOdor
Slight Increase in:NOx
Conclusions Biodiesel is an alternative fuel for diesel engines that can be
made from virtually any oil or fat feedstock. The technology choice is a function of desired capacity,
feedstock type and quality, alcohol recovery, and catalyst recovery.
Maintaining product quality is essential for the growth of the biodiesel industry.
Photobiological This method involves using sunlight, a biological component,
catalysts and an engineered system. Specific organisms, algae and bacteria, produce hydrogen as a
byproduct of their metabolic processes. These organisms generally live in water and therefore are
biologically splitting the water into its component elements. Currently, this technology is still in the research and
development stage and the theoretical sunlight conversion efficiencies have been estimated up to 24%.
Materials Requirements for Photobiological Hydrogen Production
Biological H2 production is expected to be one of many processes that
contribute to the ultimate supply of H2.
But it is useful to consider that in an area of less than 5,000 square miles (about 0.12 % of the U.S. land area) of bioreactor footprint in the desert southwest, photobiological processes could in principle produce enough H2 to displace all of the gasoline currently used in the U.S. (236 million vehicles).
The underlying assumptions are that H2 could be produced from water at 10
% efficiency (the maximum theoretical efficiency of an algal H2 production
system is about 13 %) and that fuel cell-powered vehicles could get 60 miles/kg of H2.
DESCRIPTION OF THE PROCESS The photobiological production of H2 is a property of only three classes of organisms
photosynthetic bacteria, cyanobacteria, and green algae. These organisms use their photosynthetic apparatus to absorb sunlight and convert it into
chemical energy. Water (green algae and cyanobacteria) or an organic/inorganic acid (photosynthetic bacteria) is
the electron donor. This review will focus on oxygenic organisms such as green algae and some cyanobacteria, which
produce hydrogen directly from water, without an intermediary biomass accumulation stage.
This process is considered to have highest potential sunlight conversion efficiency to H 2, which as mentioned above, could be on the order of 10 to 13 %.
To accomplish this, green algae and cyanobacteria can utilize the normal components of photosynthesis to split water into O2, protons, and electrons, deriving the requisite energy from sunlight.
However, instead of using the protons and electrons to reduce CO2 and storing the energy as starch molecules (the normal function of photosynthesis), these organisms can recombine the protons and electrons and evolve H2 gas under anaerobic conditions (figure 5.1) using a reaction catalyzed by an induced hydrogenate.
Conti..
The enzymes responsible for H2 production are metallocatalysts that belong to the classes of [NiFe]- (in cyanobacteria) or [NiFe]- (in green algae) hydrogenases.
Although sustained and continuous photobiological H2 production has been achieved with green algae, the establishment and maintenance of culture anaerobiosis is currently a sine qua non for the process.
This requirement results from the following biological realities.
Cont… The hydrogenase genes in green algae and in some cyanobacteria are not
expressed (the genes are not turned on) in the presence of O2, and a variable
period of anaerobiosis is required to induce gene transcription. The expression and function of the genes that catalyze the assembly of the
catalytic metallocluster of the algal [FeFe]-hydrogenases require anaerobiosis, while in most cyanobacteria, the [NiFe]-hydrogenase genes are expressed and the protein assembled in the presence of O2; in this case, the proteins are
assembled in an inactive form, but can be activated quickly when exposed to anaerobic conditions.
The activity of the catalytic metallocluster of the hydrogenases is inhibited reversibly (in cyanobacteria) or irreversibly (in green algae) by the presence of O2.
Reactor Materials
Engineering design of full-scale photobioreactors and the balance of the facility for photobiological hydrogen production has not been considered beyond very general concepts.
Consequently, there has been little effort to identify construction material and establish boundaries for specifying materials performance and properties.
Major Benefits Of The Method High theoretical conversion yields. Lack of oxygen involving , which causes problem of oxygen
in activation of different biological systems. Ability to use wide spectrum of light. Ability to consume organic substrates derivable from waste
and then , for their potential to be used in association with wastewater treatment.
ConclusionsThe photo biological hydrogen production processes are mostly operated at ambient temperature and pressure, but less energy intensive. These process are not only environment friendly, but also they lead to opening of new avenues for the utilization of renewable energy resources , which are inexhaustible. They can also use various waste material , which facilitate waste recycling.