The Fifth International Renewable Energy Congress
14-BME-7-O
Biodiesel Production from Algal Oil- A Simulation Study Nabil Abdel Jabbar*, Ahmad Aidan, Heba Razouk, Nasser Chihadih, Shabnam Faraghat, Youssef El-Tal Department of Chemical Engineering American University of Sharjah Sharjah, UAE
[email protected] Abstract— Process simulation using ASPEN Plus is carried out to model a two-stage alkali catalyzed transesterification reaction for converting micro algal oil to biodiesel. A 6:1 methanol to algal oil feed ratio is assumed using NaOH as a catalyst at 60 oC reaction temperature, which results in almost 99 % biodiesel yield for the trans-esterification reaction. Product quality is assessed and compared with market bio-diesel and petro-diesel. It is shown that this project is technically feasible, which makes algal oil a much more competitive substitute to food-based plant oils. Keywords— Transesterification; Aspen Plus; ; biofuel; biomass
I.
INTRODUCTION
Bio-diesel is simply diesel with biological characteristics that distinguish it from common diesel [1]. For decades people used diesel as the main fuel to run engines that don’t run on typical petrol. Being a very cheap source was also one of the reasons behind it being so popular. However, as years passed governments and environmentalists became more aware of the danger that diesel imposes on the environment, especially due to its high sulfur content. This awareness, lead to an increases interest in alternative fuels, especially biodiesel. The reason being, that biodiesel, if processed well, can give the same energy value that diesel gives but with much less environmental damage. Processes by which bio-diesel is produced vary depending on the raw material used. In general the most widely used process is called base catalyzed trans-esterification process. The name refers to the esterification process that the bio-diesel production involves. Oil or fat (feedstock) is mainly composed of triglycerides which react with alcohol to produce an ester. Along with alcohol, a catalyst is used that is usually a base [2]. Recently, there has been a great resistance in using foodbased oil for biodiesel production. This prompts the need to seek other feedstock alternatives. The objective of this work is to design a plant that produces what is called a 3rd generation biodiesel from algal oil [3]. The plant was simulated using Aspen Plus [4]. The simulation results demonstrate promising results in terms of recovery, yield, product quality, and process economics.
II. PROCESS DESCRIPTION This biodiesel process includes the following units: 1) Mixing of alcohol and catalyst 1 wt% NaOH by weight is mixed with methanol before it is heated to 60 oC to be fed to the reactor. It should be noted that a pump was used before the heat exchanger to account for the pressure drop in the heat exchanger. 2) First Reaction The alcohol/catalyst mix is then fed to a continuous stirredtank reactor (CSTR) and the algal oil is added again at the same temperature as the alcohol/oil mixture. This system is totally closed from the atmosphere to prevent the loss of alcohol. The reaction mixture is kept just below the boiling point of methanol (60oC) so that the reaction takes place. Furthermore, the recommended reaction time varies from 1 to 8 hours. The excess methanol is normally used to ensure that the algal oil is totally or highly converted to the methyl esters (biodiesel). Basically, about 76.5% conversion was achieved, which permits the usage of a second reactor. It is important to trace the amount of free-water and free fatty acids in the algal oil being fed. Otherwise, soap formation would hinder the separation of biodiesel from glycerin in any of the decanters. 3) First Separation After the completion of the reaction, glycerin and biodiesel are produced. Each has a substantial amount of the excess methanol that was used in the reaction. Then this complex mixture of glycerin, biodiesel, un-reacted excess feed (algal oil, methanol, NaOH and of course biodiesel) is separated into two layers. Un-reacted excess feed layer and the glycerin layer. It should be noted that both layers contain methanol but the excess methanol is included in the un-reacted excess feed layer. 4) Second Reaction Same as reactor 1, the un-reacted excess feed is injected into another reactor to further produce biodiesel at 60 oC and 1 atm. This reaction produces more biodiesel and glycerin.
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5) Second Separation Again, biodiesel and glycerin are separated from each other based on their densities. And also, both of them contain methanol. After separation, glycerin phase from the second decanter is fed along with the glycerin from the first decanter. 6) Alcohol Removal Now, methanol is separated from both the biodiesel glycerin phase layers using distillation columns 1 and 2. yield of methanol recovered was 100% from glycerin 99.3% from biodiesel. The biodiesel yield achieved 99.48%.
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7) Gylecrin Purification Further treating of glycerin that is the removal of salts (soaps) and water, produces 99% or more pure glycerin which could be sold to the cosmetic and pharmaceutical markets. 8) Methyl Ester (Biodiesel) Washing After separation from the methanol, the biodiesel is sometimes purified by gently washing with warm water to remove remaining soaps or catalyst, dried, and then sent to be stored. In some cases this step is unnecessary; like in our process that is washing with water decreased the yield and purity of biodiesel so this step was eliminated. III. ASPEN-PLUS SIMULATION APPROACH A. Algal Oil Composition: The complex composition of algal oil that constitutes of a certain proportional fractions of triglycerides and fatty acid is challenging to obtain. Algal oil contains pseudo components that should be identified according to their molecular structures, and ultimately from that the physical and chemical properties of algal oil are used to proceed in the transesterification reaction. The composition of the algal oil was defined for this project using ISIS which is to be entered into AspenPlus. That is why the algal oil feed was defined for this project before-hand [5, 6]. The following assumptions are made to define algal oil: -
The algal oil is 99.95% triglycerides and 0.05% fatty acids. - Only triglycerides are included (no mono- or diglycerides). - Each triglyceride is made of the same three fatty acid chains. The problem with processing algal oil with high free fatty acid (FFA) content is that FFA could not be transformed to fatty acid methyl esters (FAME or biodiesel) using an alkaline catalyst like NaOH due to the production of the “undesired” fatty acid salts better known as soap. This soap would hinder the separation of the methyl ester (biodiesel) layer from the glycerin layer in the decanter because biodiesel will be dissolute in glycerin. So if the FFA content is greater than 1%, then an acid-catalyzed method is needed to esterify this excess FFA. Also if a high FFA content is present, it could produce
water due to the reaction with methanol. Of course water is not a desirable product to be found in this process because water hinders the main transesterification reaction of the triglycerides to biodiesel, to have a competing reaction to form soap. Based on the assumption for FFA (
