Batch 17: Acid Value, Water and Winter Blending
A farmers' cooperative makes biodiesel from used cooking oil by transesterification with methanol and NaOH catalyst. Deliveries vary in quality, so each batch is checked before reaction. Batch 17: a 1.000 g oil sample dissolved in propan-2-ol was titrated with 0.100 mol/L KOH to phenolphthalein, mean titre 2.45 mL. Karl Fischer titration gave water 0.8% (m/m). The co-op's rule is that oil with an acid value above 5.0 mg KOH/g, or water above 0.10%, must be pre-treated. Batch 17 oil is converted to biodiesel with a cloud point of +3 °C. Local diesel is winter-grade with a cloud point of -12 °C, and the co-op sells a 20% v/v biodiesel blend (B20) that must remain clear at -4 °C. Assume the blend cloud point varies linearly with volume fraction.
1. Calculate the acid value of Batch 17 and decide whether it must be pre-treated. Show the calculation.
Read the worked answer
Calculation
- n(KOH) = 0.100 × 2.45 × 10^-3 = 2.45 × 10^-4 mol
- m(KOH) = 2.45 × 10^-4 × 56.1 = 1.37 × 10^-2 g = 13.7 mg
- Acid value = 13.7 mg KOH per 1.000 g oil = 13.7 mg KOH/g
Decision
The limit is 5.0 mg KOH/g, so this oil is nearly three times over. Water is 0.8% against a 0.10% limit, eight times over. Batch 17 fails both tests and must be pre-treated regardless of what was paid for it.
The acid value is also a quantitative warning about soap: 2.45 × 10^-4 mol of free fatty acid per gram will consume the same amount of NaOH - about 9.8 g NaOH per kg of oil - before any transesterification happens. That is catalyst consumed to make waste.
2. Explain, with equations, why free fatty acids and water cause problems in a base-catalysed transesterification, and describe the treatment you would specify.
Read the worked answer
Chemistry of the problem
- RCOOH + NaOH → RCOONa + H2O: every free fatty acid is converted to soap, consuming catalyst and producing more water.
- C3H5(OOCR)3 + 3NaOH → C3H5(OH)3 + 3RCOONa: water also drives alkaline hydrolysis of the triglyceride to soap and glycerol.
Why soap matters
Soap is a surfactant. It stabilises an emulsion between the FAME phase and the glycerol/methanol phase, so glycerol cannot be settled or centrifuged out. The result is a hazy product that fails the soap and water specification, a lower FAME yield, and extra washing that creates more wastewater. The co-op's rule exists because it is far cheaper to prevent soap than to remove it.
Treatment
Acid-catalysed pre-esterification: FFA + methanol, H2SO4 catalyst → FAME + water. This converts the free acids into product instead of wasting them. Dry the oil afterwards (vacuum or molecular sieve) to below 0.10% water, then run the normal base-catalysed transesterification.
Judgement
Take oleic acid as a representative FFA (M ≈ 280): 2.45 × 10^-4 mol/g × 280 g/mol ≈ 6.9% by mass, i.e. about 13.7 kg of FFA in a 200 kg batch, which pre-esterification turns into roughly 13.7 kg of FAME and saves about 2 kg of NaOH. The extra methanol, acid and time are worth it on this batch, but not on oil that is already within spec.
3. Using the blend data, decide whether the co-op can sell B20 through a winter where -4 °C must be met, and state the assumptions and limits of your answer.
Read the worked answer
Calculation
Linear blending: CP(B20) ≈ 0.20 × (+3) + 0.80 × (-12) = 0.6 - 9.6 = -9.0 °C
-9.0 °C is about 5 °C below the -4 °C requirement, so B20 from this oil should stay clear in the co-op's coldest weather. B100 at +3 °C would have gelled and blocked filters.
Decision
The co-op can sell B20 through winter, and should not sell higher blends from this batch.
Limits of the answer
- Linear blending is an approximation. Cloud point often sits closer to the higher-melting component than linear mixing predicts, so the margin may be less than 5 °C. A measured cloud point and cold filter plugging point on a real blend are needed before the first cold snap.
- Residual soap and water raise the cloud point, so a poorly washed batch will behave worse than the calculation.
- At B30 the same arithmetic gives about -7.5 °C, leaving almost no margin, and the CFPP is typically a few degrees above the cloud point.
Judgement
Sell B20 in winter with a measured CFPP on each batch, keep the blend tank heated as backup, and drop to B10 in the coldest month if a measurement comes in above -4 °C.