CHEMISTRY OF BIOFUELS AND BIOFUEL ADDITIVES FROM BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 552

Example 18.1 E-factor calculation for the production of FAME

Beside (the theoretical) atom efficiency, the E-factor is a more practical parameter
for determining the greenness of a process. It refers to the kg of waste produced per
kg of product. For the process depicted in Figure 18.2 for the production of FAME,
make a reasonable assumption of the E-factor, and argue which streams should be
denoted as waste. Also, make an estimation of the use of solvents or aqueous
streams and additives required during the production (in kg).

Solution
Before we can start making an estimate, we need some details about the process.
The cheapest way to make biodiesel is to mix methanol (usually molar excess of
about two is sufficient to drive the reaction and obtain good mixing) with KOH as
base (in situ formation of KOCH 3 ) and mix this with the heated plant oil. Two
layers will be formed, with the FAME as one layer and glycerol/salt/CH 3 OH as
the other layer. The basic salt solution cannot be recycled and needs to be neutra-
lized with an equimolar amount of acid (HCl) before disposal. The actual amount
of waste produced in this step per kg of FAME (3.4 mol) is thus 0.25 kg KCl (3.4
mol). In practice, excess base is used, so probably, a factor of about 5% needs to be
added, but as we do not know these details, we will stick to a 1:1 ratio. Washing
and drying will be required to clean the biodiesel. The water required for this will
go to the wastewater treatment.
Per kg of biodiesel product (in this case 3.4 mol), we can make assumptions
about the amounts of waste produced:

1. Glycerol: 1.15 mol (0.10 kg). Officially, this is a (by-)product, not waste, so
we can leave it out of the calculation.

2. Excess methanol that cannot be recycled: The extra methanol that is pro-
duced as byproduct is 1 mol mol −1 biodiesel 3.4 mol methanol = 0.1 kg.

3. Salt resulting fromneutralizationof the “ catalyst ” used: 3.4 mol KCl (0.25 kg).

4. Wastewater: This will make up most of the waste but is also most difficult to
estimate. Moreover, water is usually excluded in determining E-factors.

Thus, in total, a minimum of 0.35 kg waste per kg of biodiesel is produced. The
E-factor is, therefore, 0.35. In practice, this number will be much higher due to
additional washing steps and downstream handling units. This number is therefore
the minimum value.
From the aforementioned example, we learn that for every kg of FAME, at least
0.35 kg of waste is produced. In practice, the real factor will be much higher
because the downstream processing of the product will require additional washings
and handling. There are many variations on the synthesis, using, e.g., an enzymatic
catalyst (Fjerbaek et al., 2009) or ethanol instead of methanol as the alcohol, but
the principle stays the same. The main drawback is the need for inexpensive and
sustainable feedstocks. Normally, palm, sunflower, rapeseed, and soybean oils are