In this work, the modelling, simulation and optimization of a reactive distillation column for the production of Fatty Acid Methyl Ester (FAME) has been carried out. The FAME considered was methyl palmitate, which was produced from an esterification reaction between palmitic acid and methanol. The reactive distillation column used was set up in Aspen HYSYS environment using Distillation Column Sub-Flowsheet and the fluid package employed was Wilson model. The column had 17 stages, excluding the condenser and the reboiler, and it was divided into seven sections, viz, condenser section, rectifying section, upper feed section, reaction section, lower feed section, stripping section and reboiler section. Palmitic acid (fatty acid) entered the column through the upper feed section while methanol (alcohol) was fed at the lower feed section of the column. The developed model was simulated to convergence using Sparse Continuation Solver. Furthermore, the optimizer tool of Aspen HYSYS was used to optimize the process using three different algorithms (Box, mixed and sequential quadratic programming). The good convergence obtained from the simulation carried out on the developed Aspen HYSYS model of the reactive distillation process showed that Aspen HYSYS has been able to handle this process successfully. Furthermore, the achievement of the value of the objective function given by the optimization of the process when the estimated optimum values of reflux ratio, feed ratio and reboiler duty were used to run the model revealed that the optimum values obtained were from Aspen HYSYS were theoretically valid. Therefore, it has been shown that the developed Aspen HYSYS model of this research work can be used to represent, simulate and optimize a FAME reactive distillation process successfully.
Reactive distillation process has been given special attention in the past two decades because of its potential for process intensification for certain types of chemical reactions (Popken et al., 2001; Murat et al., 2003). Reactive distillation process is a growing chemical unit operation that involves the integration of a reactor and a distillation column in one unit i.e. it merges two different unit operations in a single apparatus. In other words, reactive distillation involves simultaneous chemical reaction and multi-component distillation. The chemical reaction usually takes place in the liquid phase or at the surface of a solid catalyst in contact with the liquid phase (Seader et al., 2006). General application of reactive distillation is the separation of a close-boiling or azeotropic mixture (Terril et al., 1985).
The most interesting application involves combining chemical reactions and separation by distillation in a single distillation apparatus. The most important benefit of reactive distillation technology is a reduction in capital investment, because two unit operations can be carried out in the same device. Such integration leads to lower costs in pumps, piping and instrumentation. For exothermic reaction, the reaction heat can be used for vaporization of liquid. This leads to savings of energy costs by the reduction of reboiler duties. Reactive distillation process is also advantageous when the reactor product is a mixture of species that can form several azeotropes with each other. Reactive distillation conditions can allow the azeotropes to be “reacted away” through reaction. But the combination of reaction and distillation is only possible if the conditions of both unit operations can be combined (Taylor and Krishna, 2000).
Reactive distillation can be used with a variety of chemical reactions e.g. acetylation, aldol condensation, alkylation, amination, dehydration, esterification, etherification, hydrolysis, isomerization, oligomerization, transesterification of fatty acids etc.
Fatty acid methyl esters (FAMEs) are a type of fatty acid ester derived by trans-esterification of fats with methanol. They are used to produce detergents and biodiesel. Fatty acid esters are produced by vegetable oils and animal fats trans-esterification with short chain aliphatic alcohols. This process reduces significantly the vegetable oils viscosities without affecting its calorific power, thereby, allowing their use as fuel. Fatty acid methyl esters are typically produced by an alkali-catalyzed reaction between fats and methanol in the presence of base such as sodium hydroxide or sodium meth-oxide. The physical properties of Fatty acid esters are closer to fossil diesel fuel than pure vegetable oils, but the properties depend on the type of vegetable oil (FAME fact sheet, 2011). A mixture of different fatty acid methyl esters is commonly referred to as biodiesel, which is a renewable alternative fuel. Biodiesel is known for being a clean-burning diesel fuel with minimum negative environmental impacts and potential to greatly reduce greenhouse gas emissions. It is a biodegradable fuel with negligible sulfur content and ultra-low sulfur emissions. It has similar physical properties as fossil diesel fuel, which makes it compatible for combustion in internal combustion engines and boilers. Biodiesel can be used as a blending component or a direct replacement for diesel fuel in the diesel engines. It is defined as a mixture of monoalkyl esters of long chain fatty acids (FAME) derived from a renewable lipid feedstock, such as vegetable oil or animal fat. The scarcity of conventional fossil fuels, growing emissions of combustion- generated pollutants, and their increasing costs will make biomass sources more attractive (Sensoz et al., 2000). Petroleum-based fuels have limited reserves concentrated in certain regions of the world. These sources are on the verge of reaching their peak production. The fossil fuel resources are shortening day by day. The scarcity of known petroleum reserves will make renewable energy sources more attractive (Sheehan et al., 1998).
According to Demirbas, An alternative fuel to petro-diesel must be technically feasible, economically competitive, environmentally acceptable and easily available. Biodiesel is one of the current alternative diesel fuels, which has high heating value a little bit lower than gasoline (46 MJ/kg), petro-diesel (43 MJ/kg) or petroleum (42 MJ/kg), but higher than coal (32–37 MJ/kg). Biodiesel is also a good lubricant and can improve the lubrication properties of the diesel fuel blend (Extension, 2010). The production of biodiesel can be simulated using a software package known as Aspen HYSYS.
Aspen HYSYS is a program that offers a complete integrated solution to chemical process industries. This software package can be used in almost every aspect of process engineering from design stage to cost and profitability analysis. It has a built-in model library for distillation columns, separators, heat exchangers, reactors etc. custom models can extend its model library. Aspen HYSYS can interactively change specifications such as flow sheet configuration, operating conditions and feed compositions to run new cases and analyze process alternatives. Aspen HYSYS software allows us to perform a wide range of tasks such as estimating and regressing physical properties, generating custom graphical and tabular output results, fitting plant data to simulation models, optimizing process and interfacing results to spreadsheets.
1.1 Problem Statement
Crude oil has limited reserves and is the backbone of Nigeria’s economy. Also, it is not a renewable source and contributes to the unwanted effect to the world environment. Alternative, renewable, clean and environmentally friendly energy is sought after to support the availability of crude oil fractions.
This research project is aimed at determining the optimum parameters required for obtaining fatty acid methyl ester (FAME) of high purity with the aid of Aspen HYSYS.
The objectives of this work are:
To study, simulate and understand fatty acid methyl ester reactive distillation process using Aspen HYSYS software package.
To determine optimal parameters for high purity of the desired product (fatty acid methyl ester).