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Calculating Yield of Ethanol from Sucrose and its Process Intermediate Products

This Article explains about calculation of ethanol yield from various feedstocks (Intermediate products) in the sugar manufacturing process like sugarcane juice, secondary juice, syrup, A-molasses, B- heavy molasses and final molasses .. etc.

Calculating theoretical yield of ethanol from sucrose

The chemical equation from sucrose to ethanol can be written as a stoichiometric Equation

C₁₂ H₂₂O₁₁ + H₂O $\overset{\text{Invertage (or) } H^+}{\rightarrow}$ C₆ H₁₂ O₆ + C₆ H₁₂ O₆ $\overset{\text{Zymase}}{\rightarrow}$ 4 C₂H₅ OH + 4 CO₂

Sucrose → Glucose + Fructose → Ethanol (Ethyl Alcohol)

- Molecular weight of sucrose – 342.30 gm/mol or kg/kmol
- Molecular weight Glucose/Fractrose – 180.16 gm/mol or kg/kmol
- Ethyl Alcohol Molecular weight – 46.07 gm/mol or kg/kmol
- Molecular weight of carbon dioxide – 44.01 gm/mol or kg/kmol

Molar mass balance (kmol x kg/kmol)

1 x 342.30 → 4 x 46.07 + 4 x 44.01

The mass balance percentage of the above equation

100% of Sucrose → 53.84% ethanol + 51.43 % carbon dioxide

The density of anhydrous ethanol (100%) is 0.78934 Kg/L at 20°C

So Theoretical ethanol production from sucrose (100 kg) = 53.84 / 0.78934 = 68.2 Liters

Calculating the theoretical yield of ethanol from glucose/Fructose (Reducing Sugars)

The chemical equation from reducing sugars to ethanol can be written as a stoichiometric Equation )

C₆ H₁₂ O₆ $\overset{\text{Zymase}}{\rightarrow}$ 2 C₂H₅ OH + 2 CO₂

Glucose/Fructose → Ethanol (Ethyl Alcohol)

Molecular weight Glucose/Fractrose – 180.16 gm/mol or kg/kmol
Ethyl Alcohol Molecular weight – 46.07 gm/mol or kg/kmol
Molecular weight of carbon dioxide – 44.01 gm/mol or kg/kmol

Molar mass balance (kmol x kg/kmol)

1 x 180.16 → 2 x 46.07 + 2 x 44.01

Mass balance percentage of the above equation

100% of Glucose → 51.14% ethanol + 48.86 % carbon dioxide

The density of anhydrous ethanol (100%) is 0.78934 Kg/L at 20°C

So Theoretical ethanol production from sucrose (100 kg) =51.14 / 0.78934 = 65 Liters

Fermentation Efficiency:

Fermentation is bio chemical process in which organic compounds converted in to simple organic compound by enzymes which is secreted by micro-organism.

The ratio between Practical yield to Theoretical yield

Theoretical Yield = (Molasses consumed x Fermentable sugar % * 650) / 100

Hence One ton Fermentable sugar will give 650 Liters of Absolute alcohol.

Practical Yield = (Total wash production x Alcohol percentage in wash) / 100

$\text{Fermentation Efficiency} = \frac{\text{Practical Yield}}{\text{Theoretical Yield}} \times 100$

Distillation efficiency :

The distillation efficiency is the ratio of the mass of ethanol in the final product to the mass of ethanol in the feed (wash) to the distillery.

Distillation Efficiency is the ratio of Actual Alcohol production to Alcohol in wash distilled

$\text{Actual Alcohol Production} = \frac{\text{Alcohol Production} \times \text{Alcohol Proof}}{175}$

Hence 175 proof is equal to 100 percent alcohol (Absolute Alcohol)

$\text{Alcohol in Wash Distilled} = \frac{\text{Wash Distilled} \times \text{Alcohol Percentage in Wash}}{100}$

$\text{Distillation Efficiency} = \frac{\text{Actual Alcohol Production}}{\text{Alcohol in Wash Distilled}} \times 100$

Overall Efficiency

$\text{Overall Efficiency} = \frac{\text{Fermentation Efficiency} \times \text{Distillation Efficiency}}{100}$

Various feedstocks in sugar process for ethanol production

The yields of ethanol from various feed-stocks available in a cane sugar process

- Primary Juice / Mixed juice / Secondary juice
- Clarified juice/ Concentrated clear juice (Syrup)
- B Heavy Molasses / Final Molasses

Ethanol production from sugarcane juice (Mixed juice / Clarified Juice/ Secondary juice )

Let’s take one example:

S.NO	Particulars	UOM	Mixed Juice	Remarks
1	Brix of the Material	^oC	14.5	13 to 16
2	Purity of the material	%	81	78 to 84
3	Reducing Sugars (RS)	%	0.6	0.5 to 0.7
4	Un Fermentable Sugars (UFS)	%	0.1	0.1 to 0.2
5	Fermentation Efficiency (FE)	%	90	88 to 91
6	Distillation Efficiency (DF)	%	98.5	98 to 99%
7	Sucrose % (Pol)	%	11.745	(Brix) x (Purity ) / 100
8	Total Reducing sugars (TRS)	%	12.345	Sucrose % + Reducing Sugars%
9	Fermentable sugars (FS )	%	12.245	TRS – UFS
10	Yield of Ethanol	Ltrs/MT	71	FS x 0.65 x FE x DF x 10)

Ethanol production from sugarcane molasses (B Heavy and Final Molasses)

Let’s take one example of B-Heavy molasses

S.NO	Particulars	UOM	B – Heavy Molasses	Remarks
1	Brix of the Material	^oC	83	78 to 85
2	Purity of the material	%	49	45 to 55
3	Reducing Sugars percent (RS)	%	12	10 to 14%
4	Un Fermentable Sugars (UFS)	%	1.2	1 to 1.5%
5	Fermentation Efficiency (FE)	%	90	88 to 91
6	Distillation Efficiency (DF)	%	98.5	98 to 99%
7	Sucrose % (Pol)	%	40.67	(Brix) x (Purity ) / 100
8	Total Reducing Sugars (TRS)	%	52.67	Sucrose % + Reducing Sugars%
9	Fermentable Sugars (FS )	%	51.47	TRS – UFS
10	Yield of Ethanol	Ltrs/MT	297	FS x 0.65 x FE x DF x 10)

Approximate values of Various Feed Stocks in sugar process

Name of Feed Stock	Purity	Brix%	Pol% (sucrose)	Reducing sugars % by weight	Unfermentable sugars by Weight	Approx.Sp. Gravity
Primary juice	80 to 86	16 to 22	12 to 18	0.5 to 0.8	0.2 to 0.5	1.071
Secondary juice	78 to 86	10 to 14	7 to 12	0.2 to 0.5	0.2 to 0.4	1.043
Mixed juice	78 to 86	12 to 15	9 to 13	0.4 to 0.8	0.2 to 0.4	1.056
Clarified juice	78 to 86	12 to 15	9 to 13	0.4 to 0.8	0.2 to 0.4	1.061
B Heavy Molasses	48 to 55	78 to 84	36 to 47	10 to 14	1 to 1.5	1.4
Final Molasses	28 to 34	88 to 92	25 to 32	12 to 18	1.2 to 2	1.4

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Advantages of using Clear juice as a feedstock of Ethanol production:

1. Clean substrate as compared to molasses therefore juice fermentation gets good fermentation rate & efficiency.

2. Biochemical Oxygen Demand (BOD) & Chemical Oxygen Demand (COD) of the effluent will be much less as compared to molasses; therefore effluent treatment cost is very less.

3. The quality of the alcohol from clear juice is better than the alcohol drawn from molasses.

4. Cane does not have a high content of dissolved solids and therefore there is no scaling and blocking in the distillation column as it forms with the use of molasses.

5. Fermentation time less than the fermentation time taken by the molasses.

6. Colour of the effluent is less compared to spent wash from molasses based alcohol production.

Some Disadvantages of using Juice as a feedstock of the production of Alcohol :

1 . The cane juice cannot be stored in bulk for any considerable length of time as it is highly contaminated with bacteria & yeast starts to ferment spontaneously very rapidly.

2. This means the distillery unit must be located in close proximity to the cane mill

3. Cane juice normally becomes quickly infected with bacteria and yeast in the crushing process. So Mixed juice not preferable to direct fermentation. It is better to go first effect or second-effect juice

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Ethanol production from various feed-stocks in sugar process | Yield of Ethanol

Calculating Yield of Ethanol from Sucrose and its Process Intermediate Products

Calculating theoretical yield of ethanol from sucrose