Biodiesel Production

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Biodiesel Production
Objective: Kinetic Estimation of Biodiesel synthesis reactions
Here, we estimate the kinetics of biodiesel synthesis based on batch reactor data. Biodiesel (BD) is a
substitute for petroleum based diesel and can be made from either virgin or used vegetable oil. Basecatalyzed transesterification is used to react lipids with alcohol to produce biodiesel and glycerol. A
review of kinetic models for biodiesel synthesis is provided by Turner [1]. In this example, we develop
a simplified kinetic model and use experimental data from [1] to estimate the kinetic parameters. You
may download the REX file that contains the complete model here.
Features Illustrated
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Parameter Estimation in Variable Volume Batch Reactor
Reaction Model
Vegetable oil is a mixture of triglycerides (TG) and free fatty acids; the overall transesterification
reaction to biodiesel (BD) can be simplified to:
TG + 3 Alcohol → 3 BD + Glycerol
where a tri-ester (TG) is converted to three individual esters.
Using Methanol, the above reaction actually proceeds in three steps:
R1: TG + MeOH ⇄ DG + BD
R2: DG + MeOH ⇄ MG + BD
R3: MG + MeOH ⇄ G + BD
where DG, MG and G represents diglycerides, monoglycerides and glycerol respectively.
This base catalyzed reaction is performed with the addition of either KOH or NaOH which provides
the OH- active ions. They react with alcohol producing the respective alkoxide by the following
equilibrated reaction:
R-starter : MeOH + OH- ⇔ H2O + Alkoxide
Reactions R1 to R3 are first order with respect to the alkoxide concentration in both forward and
reverse directions. For example, Rate(R1forward) = kR1-f [TG] [MeOH] [Alkoxide]. They also have their
forward and reverse kinetic parameters constrained to their equilibrium constant: Keq(Rn) = kRn-f / kRn-b
OH- ions are also a reactant in competing saponification reactions, where the catalyst OH- and the
BD, TG, DG and MG are consumed:
Sap1: BD + OH- → MeOH + Soap
Sap2: TG + OH- → DG + Soap
Sap3: DG + OH- → MG + Soap
Sap4: MG + OH- → G + Soap
All these reactions take place in a batch reactor, where the liquid phase volume is calculated using
compound density values provided in the Compounds→ Properties node. You may review the entry
of the compounds and reactions in the provided rex file. Note that the reaction R-starter is marked as
a quasi-equilibrium reaction in the Reactions node.
Experimental Data
The available measurements are for a liquid phase batch reactor, where measurements are taken
along reaction time for a single experimental set. All data are at the same temperature of 55C, thus
only pre-exponentials are to be estimated from this data, while activation energies will stay fixed to
zero. You may view the data in the Measurements→Set1 node of REX, which is partially shown
below:
Parameter Estimation
The reactions to be estimated in REX are R1, R2, R3 and R-starter as marked in the Estimation
node. The saponification reactions have their parameter fixed to the values in [1]. In the Weights
node, we specify the compounds whose predicted values are to be reconciled with their data:
The weights for these measurements are generated with the hybrid method. After executing the run,
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we can view the solution in the Results tree. The following charts show the comparison between
measured and calculated values.
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The calculated reactor volume shows little variation along the reaction. Thus, the model may be
simplified by using constant volume without much compromise on the accuracy.
Further studies
You may move the project to optimization mode to evaluate the effect of MeOH and catalyst ratios on
the biodiesel yield and selectivity. Increased Catalyst (OH) improves conversion but also increases
the undesirable saponification reactions, so the optimum values can be determined through REX
optimization. You may use this as a template to add additional data to develop more detailed kinetic
models.
References
1. Turner, T.L., 2005, Modeling and Simulation of Reaction Kinetics for Biodiesel Production, M.S
thesis, North Carolina State University.
Webpage: http://repository.lib.ncsu.edu/ir/bitstream/1840.16/1037/1/etd.pdf
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