CHAPTER ONE
1.0 INTRODUCTION
Modelling and simulation may enhance the insight, clarify dependencies, predict
behaviour, explore the system boundaries; however, they will not reveal knowledge that is
unknown. A model is a reflection of the experiments that have been performed and a good
trade-off between realism and simplicity (Diran, 1999)
Process engineering offers the knowledge about an application. Understanding a process is
always the basis of modelling and control. A rigorous dynamic process model should be
developed to increase the understanding about the operation fundamentals and to test the
control hypothesis. Experimental model verification is essential to be aware of all
uncertainties and peculiarities of the process (Luyben, 1996)
Generally, a model intended for a simulation study can be a type developed with the help
of simulation software. Mathematical model classifications include deterministic (input and
output variables are fixed values) or stochastic (at least one of the input or output variables
is probabilistic), static (time is not taken into account) or dynamic (time-varying
interactions among variables are taken into account). The solutions of modelling are often
referred to as simulations, that is, they simulate or reproduce the behaviour of physical
systems and processes. Typically, simulation models are stochastic and dynamic (Maria,
1997)
The art of foretelling and predicting the future with the use of computers has become
increasingly popular, as the speed and memory of the machines have increased. In addition,
the desire to understand what happens in systems in which measurements are impossible
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or impractical has brought about the development of many computational models.
Regardless of the aims of these computer models, they all suffer the same drawback:
uncertainty (Ekberg, 1999)
To further increase the thoroughness of the investigation, a computer-simulated model is
subjected to different conditions of process parameters. The response and reaction of the
model to these parameters reveal parameters upon/to which the model is independent,
unresponsive or insensitive, and those to which it is easily affected or reactive, that is,
sensitivity analysis. Attunement of the computer model to these parameters in itself is an
experiment, which helps to manifest the permissive of operating conditions applicable to
the real life version of the model
The recent shortcomings of conventional petroleum have increased the research for
alternative energy sources, which offer a lot of promise economically and otherwise.
Biodiesel is a prominent subject in this area of research, hence the reason this project
studies. Biodiesel is considered as a “direct-pour” alternative fuel to petroleum diesel, as it
requires almost no modification to most modern diesel engines. It can be produced locally
and, therefore, reduces foreign oil dependence. It has been reported that biodiesel
combustion can result in less air pollutant emissions, such as carbon monoxide, sulphur di-
oxide, particulate matter, hydrocarbons, but with slightly higher nitrogen oxides. Since the
feedstock of biodiesel is mostly renewable, it significantly reduces carbon dioxide emission
during its whole life cycle
Fatty acid methyl esters (FAME), valuable oleo-chemicals and main constituent of
biodiesel, can be manufactured in a continuous process using reactive distillation. (Dimian,
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2007)
Reactive distillation (RD) is the process in which chemical reaction and separation are
carried out simultaneously within a fractional distillation apparatus. It may be advantageous
for liquid-phase reaction systems when the reaction must be carried out with a large excess
of one or more of the reactants, when a reaction can be driven to completion by removal of
one or more of the products as they are formed, or when the product recovery or by-product
recycle scheme is complicated or made infeasible by azeotrope formation (Perry et al
1997).
With regards to fatty acid ester production and purification, and more specifically to large
scale production of biodiesel, it would appear that reactive distillation could provide an
efficient and integrated approach to obtain the desired fatty acid esters. Biodiesel is a
renewable, clean-burning diesel replacement that is reducing U.S. dependence on foreign
petroleum, creating jobs and improving the environment. Technically, biodiesel is defined
as a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable
oils or animal fats, designated B100, and meeting the requirements of ASTM D 6751.
Computer simulations have become increasingly popular in many different areas over the
years, owing mainly to more effective and cheaper machines. In many cases, the trend
seems to be that computer simulations are replacing experiments, at least in areas in which
experiments are very difficult, expensive or impossible. One such area is that of attempting
to foresee what will happen in the future (Ekberg, 1999)
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1.1 Problem Statement
Industrially, some operators do not operate at optimal process conditions because they are
unaware of the dependency of the process outputs upon certain parameters. To gain insight
into the favourable conditions and to make performance predictions of industrial processes
of the subject matter to different operating conditions, the sensitivity of a simulated model
process needs to be analysed.
1.2 Aim
The aim of this research is to ascertain the behaviour of an ASPEN PLUS simulated model
of a fatty acid methyl ester reactive distillation process, when subjected to different
operating parameter conditions.
1.3 Objectives Of Study
The objectives intended to be achieved in this work include:
1. Developing the model of the process in Aspen PLUS environment,
2. Subjecting the model to different operating conditions of deciding variables, and
3. Examining, discovering and interpreting the functional response of a reactive
distillation process of a fatty acid methyl ester to these variables.
1.4 Significance of Study
The findings of this study will reveal the behaviour of the purity of a fatty acid methyl ester
towards variations in some certain operating parameters involved in reactive distillation
process. These findings will help in guiding the plant operators on how to choose the values
of the operating parameters for efficient production.
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1.6 Scope of Study
This work borders on the use of ASPEN PLUS to investigate the behaviour and
functionality of a fatty acid methyl ester reactive distillation process when product mole
fraction is the selected output variable.
