Experiment 3: Biosynthesis and Distillation of Ethanol

introduction

• sucrose, table sugar, is converted into ethanol and CO2 via fermentation

• reaction for fermentation to ethanol: sucrose + water → 4CO2 + 4CH3CH2OH

introduction

• 1 mole of sucrose produces 4 moles of ethanol 

• this full 4:1 yield is hard to obtain, since ethanol concentrations of 12-14% are fatal to yeast, so fermentation stops when ethanol content reaches 12%, even if some sucrose remains in the ferment 

introduction

• a 12% solution of ethanol in water is one in which 12g of solute, ethanol, is dissolved in 100 mL total solution → 12% w:v

• thus, yeast will form 4 moles of ethanol for each mole of sucrose only when the sucrose solution is rather dilute such that the concentration of ethanol remains below 12%

introduction

• in this experiment, a small amount of yeast, plus 50g of sucrose, will be mixed with water to initiate fermentation

• the mixture will ferment for one week

• we will distill it twice to obtain ethanol in as pure of a state as possible

• the main way of purification will be through fractional distillation

distillation

• distillation is used to separate components of a liquid mixture, using the differences in boiling points of the components

• by boiling a liquid mixture and and condensing the vapours obtained at different temperatures, separation of the liquids will occur  

distillation

• the distillation of a pure liquid: at the BP, the vapour and liquid will be in equilibrium, and the temp of the vapour will be a constant during the course of the distillation

• liquids that are mixtures, give a range of temperatures on boiling only pure liquids have a constant boiling temp

• the boiling temp of a pure liquid will not usually vary by more than 2-3 degrees during distillation

distillation

• the distillation of a liquid that is contaminated with some sort of impurity: usually all distillations

• example: lets say we want to distill mixture of A (lower BP) and B (higher BP)

• if the BPs of A and B are within about 20 degrees of each other, the distillation will vary over the entire range of temps which separate A and B 

distillation

• as the distillation starts, both A and B will be present in the distillate, but A will predominate since it is the lower boiling component

• when a small amount of A/B distillate has been collected, the boiling temperature will have increased a bit

• this is because the liquid which is boiling is now slightly richer in '“B” than the liquid which first boiled

• as distillation proceeds, the distillate will become poorer in A and richer in B (so will the boiling liquid), and the boiling temperatures will constantly rise

distillation

• overall, there are two differences between the distillation of a pure liquid and a mixture.

• the pure liquid, has a constant boiling temperature (vs the constantly rising temperature of a mixture)

• and the pure liquid distillation gives a pure distillate (vs the changing composition of the distillate from a mixed liquid)

fractional distillation

• differs from simple distillation because the vapour has to rise through a fractioning column before entering the distillation head

• the fractioning column has a large internal SA for re-condensation and re-distillation of vapour 

fractional distillation

• as vapour passes through the column, it condenses and re-vapourizes repeatedly on the interior of the fractioning column itself

• each of these re-vapourizations is effectively a simple distillation, and so leads to a vapour at each stage which is successively richer in the lower-boiling component

fractional distillation

• the end result is that the vapour which reaches the distillation head has been subjected to repeated distillation

• theoretical plates - may or may not need to know

vaporization and condensation

• Liquid to gas: vaporization

• Gas to liquid: condensation

procedure: first-week: preliminary

• weigh 50-55g of sucrose and pour into a large beaker.

• add 225 mL of water, and stir thoroughly to dissolve the sugar

• weight 0.3 to 0.4 grams of Na2HPO4 and pour into a 500 mL RB flask.

• add in 2g of yeast and pour in the sugar solution

• use 25 mL of water to rinse out the beaker and add the rinse to the RB flask

• limewater (saturated calcium hydroxide)

• a liquid acts as a sealant, ensuring air can escape the ferment but without lab air entering the ferment

second week - a simple distillation

• decant the liquid ferment into a large beaker

• pour as much liquid as possible and least amount of sediment as possible

• this is best done by pouring in one continuous motion, with no breaks in the pouring

second week - a simple distillation

• the presence of some solids is inevitable, causing the liquid to appear murky

• solids in the ferment will lead to 2 experimental difficulties:

• 1) the problem of foaming during distillation, - the more solid, the more foaming

• 2) they reduce the amount of recoverable ethanol, because solids increase the viscosity of the liquid, preventing some ethanol from being distilled

second week - a simple distillation

• set up the apparatus for simple distillation

• add in 2 boiling chips and 1 drop of 1-octanol before distilling

1-octanol

• 1-octanol is a light-density liquid with a high boiling point

• its immiscible with water and floats on top of the ferment

• while distillation is taking place, the ferment will boil (78-100 degrees celsius), but the octane (bp 195 degrees celsius) won’t 

• the octanol, floating inert on top of the ferment, will disrupt foam as it occurs but it itself will not boil away 

second week - a simple distillation

• gently distill the ferment, collecting all distillate up to 97 degrees celsius (ca 50 mL total) 

• when complete, ensure the distillate is room temp before continuing

• determine the density and volume of the distillate collected

second week - a simple distillation

• don’t continue distilling pointlessly.

• at 97 degrees celsius or above, the distillate is virtually alcohol-free, being almost completely water

• continued distillation simply dilutes the collected distillate, without adding to the product alcohol later collected

• store your distillate until next week in a corked flask

third week - fractional distillation

• assemble apparatus for fractional distillation

• use an RB flask which has around twice the capacity of the volume you want to distil

• add in 2 boiling chips

third week - fractional distillation

• start the fractional distillation

• heat the flask just until the boiling starts, then continue heating with close watching

• as the solution boils, you will notice the ring of condensation travel up the fractioning column slowly toward the top

• when heating is too rapid, the liquid and vapour fail to equilibrate in the column, and this condensation ring suddenly rushes to the top of the column

third week - fractional distillation

• the vapour/condensation ring, is a visible example of the ‘vaporize-condense-vaporize-condense’ cycle discussed earlier

• overheating will cause the vapour to rush up and out of the fractioning column aka overheating the RB flask reduces the number of theoretical plates

third week - fractional distillation

• collect the distillate in two portions

• 1) distillate 75-80 degrees celsius

• 2) distillate 80.1-89 degrees celsius

• record density and volume of each

• if there isn’t enough volume to measure the density with the hygrometers provided, you can add 96% ethanol until an overall density can be observed, a total volume recorded, and then the density of the original distillate calculated

third week - fractional distillation

• determine the ethanol content (%) and ethanol quantity (in g), in all of these ethanol samples

• what is the reaction mass efficiency (RME) for ethanol in the simple distillate?

• what is the RME for ethanol in the combined fractional distillates