Abstract
In this work, the production of biodiesel via reactive distillation process has been modelled
and simulated with the aid of ChemCAD for both steady state and dynamics. Also, the
control of the process has been carried out using MATLAB/Simulink. In order to achieve
this aim, dynamics data showing the response of biodiesel mole fraction in the column
bottoms (controlled variable) to a change in reboiler duty (manipulated variable) and reflux
ratio (selected disturbance variable) were extracted from the ChemCAD dynamic simulation
of the developed process model and used to obtain the first-order-plus-dead-time transfer
function relation between biodiesel mole fraction in the column bottoms, reboiler duty and
reflux ratio with the aid of MATLAB. The open loop simulation was done by applying steps
to the input variables (reboiler duty and reflux ratio). Furthermore, the set-point tracking
and disturbance rejection control of the system were carried out using a PID controller tuned
1.1 Background of Study
Due to an increased demand of energy by the world population and the non-renewability of
crude oil, the development of renewable energy generation techniques for future generations
has gained great importance over the century (Madhu et al., 2012). One of these renewable
energy has been identified to be biodiesel.
Biodiesel is a renewable, non-toxic, biodegradable substitute for diesel produced from crude
oil. Generally, it is produced by transesterification of vegetable oils and animal fat by short
chained aliphatic alcohols. Commercially, the production of biodiesel from vegetable oils
and fats still have various drawbacks. Both batch and continuous processes utilize almost
100% excess alcohol than the stoichiometric molar requirement (3:1) in order to drive the
transesterification reaction to completion and produce the maximum amount of biodiesel
per unit consumption of oil (Kiss et al., 2008). At the end of the process, unreacted alcohol
must be recovered by a separate distillation column. The use of a separate distillation column
for alcohol recovery increases capital as well as operating cost. Therefore, there is the need
to develop alternative means for the commercial production of biodiesel which minimizes
cost without reducing the yield and quality of biodiesel produced. Reactive distillation is
one of such alternative means.
Reactive distillation combines separation and reaction into a single vessel to minimize
operation and equipment costs (Kiss et al., 2008). In this process, the products formed are
removed as soon as they are formed. This characteristic makes it possible to overcome the
equilibrium thermodynamics of a reaction, reaching high conversion and selectivity. Thus,
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it is particularly effective for reversible reactions such as the transesterification of vegetable
oil and fats to biodiesel (He et al, 2006). However, the combination of reaction and
separation into a single unit that resulted in many complexities of the process has made its
dynamics and control study of this process a challenge to Process Engineers.
Dynamics in chemical engineering is the study of how process variables vary with time. As
all real-life process variables vary with time, it is therefore important to study the dynamics
of the biodiesel production process. Control is the external intervention needed to guarantee
the satisfaction of operational requirements such as safety, production specifications,
environmental regulations, operational constraints, economics (Stephanopoulos, 1984).
Since the structure of biodiesel reactive distillation process is complex, due the need to
maximize mass and energy raw materials, there is therefore need to develop a suitable
control system for the process.
This research project is aimed at providing an outlook at the dynamics of biodiesel
production by reactive distillation and developing a control system for the process by means
of CHEMCAD and MATLAB modelling and simulation.
1.2 Research Problem Statement
Biodiesel is a valuable renewable fuel that can supplement and replace petroleum diesel in
diesel engines. However, its cost of production by the reversible transesterification of
vegetable oil and fats with alcohol by conventional means to achieve high purity of the
product is relatively high. This high cost is a big problem that needs to be solved through
provision of an alternative, novel, route and development of a reliable control method to
make the process behave efficiently.
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1.3 Aim and Objectives
The aim of this project is to model, simulate and control a reactive distillation process used
for the production of biodiesel from the transesterification reaction between triolein and
methanol. This aim would be achieved by realizing the following objectives:
• developing and simulating the ChemCAD steady-state model of the process,
• converting the steady-state model into a dynamic type to generate dynamic data,
• using the generated dynamic data to develop the process transfer functions with the
aid of MATLAB,
• using the transfer function model of the process to obtain the tuning parameters of a
PID controller,
• applying the PID controller to make the mole fraction of the biodiesel be at the
desired set-point value.
1.4 Scope
This work is limited to employing ChemCAD and MATLAB to develop a model, simulate
the model and carry out the open-loop and closed-loop simulations of the model for a
reactive distillation used for biodiesel production from the transesterification reaction
between triolein and methanol.
1.5 Justification of Research
The successful accomplishment of this work will enlighten the process engineers on the
methods that can be utilized in handling a reactive distillation process very well to make it
behave as it is desired at any time.