2.
2 Processes Description
The process steps for the continuous base catalyzed reaction is given as;
1. Extraction of Triglycerides from Wastewater Sludge
2. Recovery of n-Hexane
3. Reaction of Triglycerides with methanol in a CSTR
4. Separation of biodiesel and glycerol phases
5. Water washing of biodiesel to purify
6. Neutralization of glycerol
Extraction of Triglycerides from Wastewater Sludge:
For the extraction of triglycerides from wastewater sludge n-Hexane is used
as solvent. n-Hexane is an excellent solvent because of high solubility in the organic liquids and
low boiling points and volatility. n-hexane is used in 1:2 w/w to wastewater sludge. The mixture
is then sent to the centrifuge separator for the separation of triglycerides and hexane from sludge
cake.
Recovery of n-Hexane:
When triglycerides and hexane are separated from the sludge, the solution is sent to the
evaporator for the separation of hexane from triglycerides. The separated hexane is sent to the
condenser where it changes its state and temperature for the reuse purposes. The remaining
portion is sent to the reactor for the production of biodiesel.
Reaction Phase:
Triglycerides are reacted with methanol, which is used in 6:1 molar ratio to triglycerides. The
more the methanol is used the more the reaction is in forward direction. The catalyst is used is
used 2% by volume of entering triglycerides. The reaction temperature is 60 oC and the pressure
is 1 bar.
TG + CH 3OH + NaOH DG + NaOH + BD
DG + CH3OH + NaOH MG + NaOH + BD
MG + CH3OH + NaOH Glycerol + NaOH + BD
Overall Reaction:
TG + 3CH 3OH + Catalyst 3Biodiesel + Glycerol Separation of
Biodiesel and Glycerol Phases:
The biodiesel and glycerol produced during the reaction are separated by a phase separator and
they are sent to separate sections for the purification.
Biodiesel is then water-washed with the help of 20% by weight of water to entering biodiesel.
Water will separate all of the impurities by dissolving them. This water is then removed by an
evaporator. The water washing techniques that are common in use are:
1. Mist Washing
2. Bubble Washing
3. Pump Washing
After evaporation of water, further purification of biodiesel is done by a distillation column,
which separates the triglycerides and remaining water and making biodiesel up to 99.6% pure.
The glycerol phase is also sent to the distillation column for the recovery of methanol for reuse.
The remaining mixture is sent to the neutralizer where Phosphoric acid is used as a reactant for
the neutralization reaction. During this reaction phosphoric acid is produced which is used as a
fertilizer. This makes the biodiesel production more profitable.
2.3 Raw Materials
1. Wastewater Sludge
2. n-Hexane
3. Methanol
4. Sodium Hydroxide (NaOH) Wastewater Sludge:
Sewage sludge refers to the residual, semi-solid material that is produced as a byproduct during
sewage treatment of industrial or municipal wastewater. The dewatered sludge from a
wastewater treatment plant is utilized as the raw material because it is sent to landfills as a
waste. As municipal sewage sludge is an inevitable waste, generated in large quantities during
treatment of wastewater, the cost of biomass production is eliminated. Therefore, we can say
that the sewage sludge is a non-cost and is abundantly available in large quantities and it can
make the production of biodiesel very profitable.
The composition of dry and treated sludge from a waste water treatment plant is given below:
Figure 2 1 Percentage Composition of Wastewater Sludge
n-Hexane:
Hexane is an organic solvent which is used to extract the triglycerides from treated wastewater
sludge. Hexane gives maximum extraction than any other organic solvent like acetone, ether,
chloroform etc.
Methanol
Methanol, also known as methyl alcohol. The other names for methanol are Wood alcohol,
wood naphtha or wood spirits with formula CH 3OH (often abbreviated MeOH). It is the
simplest form of alcohol, and is a light, volatile, colorless, flammable, liquid with a distinctive
odor that is very similar to but slightly sweeter than ethyl alcohol (drinking alcohol). At room
temperature it is a polar liquid and is used as antifreezing agent, solvent, fuel, and as a
denaturant for ethanol. It is also used for making biodiesel via trans –esterification reaction of
triglycerides.
Sodium Hydroxide (NaOH)
Sodium hydroxide (NaOH), also known as lye and caustic soda, is a caustic metallic base. It is
used in many industries as raw material, mostly as a strong chemical base in the manufacture of
pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner. Worldwide
production in 2018 was approximately 70 million tons, while demand was 66 million tons.
Pure sodium hydroxide is a white solid available in pellets, flakes, granules and as a 50%
saturated solution. It is hygroscopic and absorbs water from the air, so it should be stored in an
airtight container. It is very soluble in water with application of heat.
2.4 Capacity Selection
Capacity means how much quantity of desired product should be produce in a specific period of
time so as to easily design and operate the plant efficiency. It depends on the following factors;
1. Raw material availability
2. Need as per National/International Level
Data from different plants in the world
