Fermentation of Molasses to Ethanol

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Fermentation of Molasses to Ethanol

• Information for Parts 2 and 3
• Assignment 1 CHNG 3803

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Tips for Project Reports

• Number the questions.
• Assumptions clearly stated.
• Calculations well set out.
• Clear well drawn flowsheets.
• For questions looking at other feedstocks or
  waste treatment some appreciation of economics,
  amounts required and the environmental effects.
• 10-15 pages.

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Course Weightings

• Projects worth 50.
• Final Exam worth 50.
• This project is worth 25 of the project
  component of the course.

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Flow sheet Molasses to Ethanol
Flash vessel not always used in ethanol
fermentations. For more information on Azeotropic
distillation consult The Chemical Engineers
Handbook, Perry.
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Fermentation of Molasses to Ethanol

• Yeasts such as Saccharomyces cerevisiae and
  Saccharomyces carlsbergensis are commonly used.
• Other yeasts and bacteria can be used

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Fermentation of Molasses to Ethanol

• The word Ferment comes from a Latin word meaning
  to boil
• The term was used due to the rapid evolution of
  gas (Carbon Dioxide) during alcohol
  fermentations.
• The yeast converts sugar into ethanol and carbon
  dioxide.
• The conversion of sugar to ethanol creates heat.

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Complications

• To achieve a high rate of ethanol production
  requires a large number of cells.
• Growing large numbers of yeast consumes sugar
  which then can not be used for ethanol formation.
• High sugar concentrations reduce the rate at
  which the yeast grow (Substrate inhibition)
• High ethanol concentrations reduce the rate at
  which yeast grow and the rate at which they
  produce ethanol.

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Additional Nutrients

• Molasses contains many of the nutrients necessary
  for yeast to grow and produce ethanol.
• Ammonium Sulfate is commonly added as a nitrogen
  source for the yeast.
• Yeast from previous fermentations can also be
  used to provide nutrients.

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Yeast Desirable Traits

• Low pH - reduce competition from other organisms.
• High temperature tolerance reduce cooling
  costs, most ethanol plants are located in
  tropical areas.
• High ethanol tolerance increase fermenter
  ethanol concentration.

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Types of fermentation

• Batch
• Melle Boinot
• Continuous
• Biostil

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Fermenters

• Similar to CSTR at small scale, but large
  fermenters would require very large impellers!
• How to mix and remove heat?
• Pump out of the fermenter through a heat
  exchanger and recycle into the fermenter.

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Batch Fermentation
Final Ethanol Concentration 79 g/L
First part of fermentation is not very productive
Final Yeast Concentration 10 g/L
Batch Length 32 hrs
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Melle Boinot
Final Ethanol Concentration 79 g/L
Initial Yeast Concentration 8 g/L
Final Yeast Concentration 10 g/L
Batch Length 8 hrs
Yeast is recycled from the previous batch, this
increases the reaction rate Which in turn
shortens the fermentation time
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Continuous Fermentations

• A large number of different continuous
  fermentations exist.
• One fermenter and a yeast recycle.
• High yeast concentrations result in high rates of
  ethanol formation.
• A series of two or more fermenters.
• In the first fermenter ethanol concentrations are
  low so there is little inhibition of the yeast,
  and the substrate conversion is rapid.
• In subsequent fermenters the ethanol
  concentration is higher, however there is less
  sugar remaining to ferment.

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Biostil Fermentation Process

• Fermentation and Distillation are coupled.
• The fermentation beer is continually recycled.
• Yeast is removed by centrifugation.
• Ethanol is removed by a rectifying column.
• Provides a degree of sterilisation as bacteria
  are killed in the column.
• Less water is consumed and the stillage is more
  concentrated, i.e. less waste water.
• Used by CSR at Sarina Queensland.

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Biostil
Concentrated Ethanol Vapour
CO2
Ethanol Vapouriser
Feed
Backset
Stillage Stripper
Fermenter
centrifuge
Yeast Recycle
Yeast Bleed
Concentrated Stillage
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Stillage Recycling

• Stillage is the name given to the stream from the
  bottom of the ethanol concentrator column.
• It consists of yeast cells which have been killed
  by the distillation process and other components
  from the molasses feed and the fermentation.
• Recycling the stillage is called Backset.
• Industrially 10-20 of the stillage is recycled
  for molasses fermentations.
• Recycling Stillage can reduce nutrient and fresh
  water requirements.

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Energy Requirements

• The separation of ethanol from water requires a
  considerable amount of energy.
• Estimates range from 2.5 to 5 kg of steam per
  litre of anhydrous ethanol produced.
• If the ethanol refinery is located next to the
  sugar mill this heat steam could be raised by the
  combustion of bagasse.
• Does the production of ethanol from molasses
  produce energy?

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Wastes

• The production of ethanol produces a lot of waste
  materials.
• Particularly CO2 and waste water.
• Can the amount of waste water be reduced?
• Can other products be made from these wastes?

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Other Feedstocks

• Fermentation requires sugar.
• Sugar can be formed by breaking down starches and
  other organic materials.
• What extra equipment is required to make sugar
  from such sources?

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Part 2

• A Sugar milling company has asked for you advice
  on the construction of an ethanol plant. The
  plant is to produce 199 proof ethanol for
  blending with petrol, by fermenting the
  by-product molasses into ethanol.
• Draw the flowsheet for the fermentation of
  molasses and the purification of ethanol. Draw a
  separate flowsheet for the fermentation battery.
• What Volume is required for the fermenters?
• Conduct a Mass Balance (Excel or other program)
• Suggest methods of reducing wastes or uses for
  waste materials.

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Part 3

• During the design process you are asked to
  determine the best method of fermentation.
• Compare the size of fermenters required for the
  different fermentation technologies and the
  ethanol concentrations which can be achieved.
• Using your flow sheet from Part 2, and the
  conditions you calculated in Question 7,
  determine what effects this change would have on
  the rest of the plant, i.e. waste and the volume
  of fermenters required.
• What effect does recycling some of the thin
  stillage have on waste water production
• Ethanol purification is estimated to require 3kg
  of steam per litre of anhydrous ethanol, how much
  Bagasse would be required to raise this steam?
  How does this energy input compare to the energy
  output in the liquid ethanol?

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Further References

• Books
• Kirk Othmer Encyclopedia of Chemical Technology.
• Perry The Chemical Engineers Handbook.
• Comprehensive Biotechnology Chapter 43.
• Internet
• ABARE Viability Study - Fuel Ethanol, from Oct
  2001, good study but many things have changed
  since then.
• CSR Biostil Sarina.
• Remember Project is Due March 24th at 5pm