L9 Flashcards
Saccharification
The process of breaking a complex carbohydrate (such as starch or cellulose into its monosaccharide components
Fermentation Steps
- ) Converting feedstock to fermentable sugars
- ) Converting glucose to ethanol through fermentation
Challenge is to create yeast that converts other sugars that is not glucose into ethanol
Ethanol fermentation definition
- A biological process which converts sugars such as glucose, fructose, and sucrose to ethanol and CO2 as a side product
- Yeasts used in the absence of oxygen -> anaerobic fermentation
- C6H12O6 + Zymase -> 2C2H5OH + 2CO2
- Zymase is an enzyme complex that catalyses the fermentation. Occurs naturally in yeast
Yeast
- Yeast is reproduced by budding
- 75% water and 25% dry matter
- Growth requirements
- — Water
- — Carbon source as energy source
- — Oxygen/lipids
- — Amino acids and peptides as nitrogen source
- — Vitamins
- — Inorganic ions
Frequently used yeast
Accharomyces cerevisiae
- robust and high ethanol yield
- only ferments C-6 sugars
Feedstock for fermentation to ethanol
Lignocellulosic biomass for fuel ethanol
- Wood
- Crop residues
- Purpose grown biomass
Food grains and crops for beverage alcohol and fuel
- Sugar-based: sugarcane and sugar beet
- Starched based: corn, sorghum, rice, barley
Food processing residues
- Whey: a by-product from cheese manufacturing
Pre-treatment of starch feedstock
- Separation of fermentable sugars
Step 1
- Break down granular structure of starch with endoenzyme in process
Step 2
- Liquefaction or hydrolysis of starch to dextrins by alpha-amylase enzyme.
Step 3
- Saccharifcation to release glucose molecules from dextrins
Routes of lignocellulosic biomass to ethanol
Route 1.) Enzymatic hydrolysis and fermentation
Route 2.) Simultaneous saccharification and fermentation
Route 3.) Direct acid hydrolysis for fermentation
Lignocellulosic biomass pretreatment in route 1
Purpose
- to liberate cellulose and hemicellulose from the lignin seal and its crystalline structure so as to make polysaccharides accessible for a subsequent hydrolysis step
- Lignin in removed
Methods
- Physical: milling, grinding
- Thermal: steam explosion, hot water
- Chemical: Alkali, acids
- Biological: selected white-rot fungi
Lignocellulosic biomass hydrolysis of saccharifcation in route 1
Purpose
- Cellulose and hemicellulose depolymerized to glucose and xylosa
Method
- Enzymatic hydrolysis
- A series of different enzymes degrade molecules into fermentable sugars
- Techno-economic bottleneck
Lignocellulosic biomass in route 2
Simultaneous saccharification and fermentation after pretreatment
- ) using a mixture of biomass saccharfication microorganisms and fermentation microorganisms
- ) using genetically engineered microorganisms that can do both saccharification and fermentation
Advantages
- higher ethanol yields due to removal of end product inhibition
- only one reactor for both steps
Disadvantages
- difficult to optimize due to microorganism optimal temperature differences
- genetically engineered organisms in development stage
Lignocellulosic biomass direct acid hydrolysis in route 3
Purpose
- Release sugars in lignocellulosic biomass in one step
Method
- Concentrated acid hydrolysis
- Dilute acid hydrolysis, higher temperature and longer exposure time
- Ionic liquid-mediated hydrolysis
- Solid acid hydrolysis using zeolite catalysts
Challenge: inhibitory substances in lignocellulosic hydrolysis
In pretreatment, extractive components and organic or sugar acids also undergo hydrolysis. This produces compounds that inhibit sugar fermentation.
Fermentation configurations considerations
- Type of microorganisms used
- Their fermentation kinetics
- Type of lignocellulosic biomass used
- process economics aspects
- If there should be immobilization or recirculation of cells
Batch fermentation
Rapid batch fermentation
- Done in a cylindrical-conical fermenter
- The slope of the bottom should be sufficient for the mash to run out when discharging
Batch process: advantages and disadvantages
Advantages
- Simple and easy to control, especially control of contamination
- More flexible and capable of handling high ethanol concentrations
Disadvantages
- Fermenter cleaning is accomplished with clean-in-place equipment
- Cleaning/sterilization takes 50 mins
- Lower fermenter usage and higher cleaning costs
Continuous fermentation
Internal volume of medium remains constant, continuous flow of nutrients into fermenter and constant flow of product out
Continuous stirred rank reactor
- Fermentation broth is continuously stirred by agitators
- Composition in reactor is homogenous
Plug flow reactor
- Uniform velocity profile across radius
- Reaction proceeds as the reagents travel through the reactor
- Yeasts have to be constantly fed into the reactor
Cascade continuous fermentation
- 3-5 fermenters linked in series
- In balance growth, no need for adding yeast and nutrients
Advantages of continuous fermentation
- Fermentation conditions do not change with time
- higher fermentation efficiency and productivity
- Reduced vessel downtime for cleaning
- Flexibility to run in batch mode when necessary
Contamination in a fermentation plant
- All equipment can be infected
- Lactobacillus bacteria and wild yeasts will compete with sacharomyces yeast for nutrients and produce acids as their end products
- Stressful condition for yeast growth: High ethanol, high temperature and low pH
Contamination control and challenges
To reduce number of microbes in the feed sugar solution such that the levels of bacterial end products are minima;
- Low pH
- Add antimicrobials or antibiotics
Challenges in fermentation of biomass
- Low conversion of efficiency (loss approx 40% feedstock)
- Low energy efficiency (pre-treatment, final distillation)
Challenges in fermentation of starch and sugar feed stocks
- Low energy efficiency (final distillation)
- competing with food supply
