The conversion of used cooking oils into biodiesel

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The use of biofuel as a substitute for petroleum diesel is a valuable solution to the inescapable global issues regarding energy shortage and environmental pollution.
For both economic and environmental reasons, implementing used cooking oils (UCO) for biodiesel production has gained a significant amount of consideration within the world of fuel production.
It has been determined that the price of raw materials can be held responsible for approximately 75% of the overall biodiesel production cost.
This article, by Dr Raj Shah, an adjunct professor in chemical engineering at State University of New York, Dr Vikram Mittal, assistant professor at the United States Military Academy in the Department of Systems Engineering, and Ms Eliana Matsil, a chemical engineering student at Stony Brook University highlight the growing importance of UCO.
Altering the ingredients of production from unprocessed raw materials to low-cost UCO helps neutralise the dominance that petroleum diesel holds in the industry.
These oils can be collected from food processors, restaurants, and even home kitchens, which helps reduce waste that would otherwise end up in a landfill or sewer pipes.
A 2017 study by the Environmental Protection Agency found that restaurants and hotels in the US generated 3 billion gallons of UCO per year.
This amount of UCO can produce enough biodiesel to offset 10 percent of the current domestic diesel demand.
UCO can be successfully converted into biodiesel by the means of a simple chemical reaction called transesterification, which can be performed using a variety of catalysts including basic, acidic, and enzymatic.
These catalysts can also be categorised as homogeneous or heterogeneous. Each catalyst used to convert cooking oil into biodiesel results in different levels of complexity, cost, energy consumption, and feasibility.
According to the American Society for Testing and Materials (ASTM), biodiesel is defined as a “fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats”.
During the process of transesterification, fatty acid triglycerides in vegetable oil react with an alcohol, most often methanol due to its accessibility and low cost, with the help of a suitable catalyst.
This chemical interaction results in the production of fatty acid methyl esters (FAME), which has properties similar to diesel fuel. The byproduct of this reaction is glycerol which is extracted and used for other products including soaps and cosmetics. This process is described in Figure 1.

Although UCO is cheap and accessible, it presents special challenges for biodiesel production.
Most often, the feedstock will need to be filtered prior to the chemical process in order to remove suspended particles left over from cooking.
Typically, if the oil being used contains a high free fatty acid (FFA) content (more than 2 wt.%), it will need to undergo an initial esterification process due to the formation of soap when in reaction with an alkali-catalyst.
Additionally, high FFA will often result in severe engine deposits, injector coking, and piston ring sticking [20]. UCO based on canola, corn, or sunflower oil are preferred for storage and manufacturing since they generally have lower FFA content and have lower pour points. During esterification of vegetable oil, carboxyl acid (FFA) in the oil reacts with methanol in the presence of a catalyst, most often sulfuric acid, to convert the FFA content into esters. The esters are then ready to enter the process of transesterification where they are transformed into a substance with petroleum diesel-like properties, such as a reduced viscosity and exceptional lubricity.
Catalysts used for transesterification can be classified into two groups based on their phase. Homogenous catalysts are catalysts that operate in the same phase as the reactants, while heterogeneous catalysts function in a different phase than the reactants. Regarding this specific reaction, homogenous catalysis normally takes place when both the reactants and catalysts are liquid, whereas heterogenous catalysis involves liquid reactants with solid catalysts.
Both types can be either acidic or alkali. Most commercial production of biodiesel today is done with an alkali homogenous catalyst, such as sodium or potassium hydroxide, due to homogeneous catalysts’ tendency to provide a higher FAME yield in less reaction time. However, advancement in heterogenous catalysis technology has emerged in recent years and may begin to outweigh traditional catalyst operations.
In a study done by the University of Ottawa’s Department of Chemical Engineering in 2003, four continuous biodiesel production processes were tested using a simulation software. The purpose of this experiment was to compare the commercial utilisation of homogeneous acidic vs. basic catalysts, as well as the application of virgin vs. used vegetable oil, and analyze the efficacy and costs of each.
Process I involved an alkali-catalyzed system using virgin oil, while Process II involved an alkali-catalysed system with the employment of UCO instead. Process III was comprised of an acid-catalysed system using UCO, and Process IV was an alteration of Process III with the addition of a hexane extraction process.
The study exhibits that different methods of biodiesel production entail several steps aside from the direct transesterification process such as material refining and product separation/purification.
Process I began with a transesterification reaction with a 6:1 methanol to soybean oil molar ratio at 60°C and 400 kPa and 1% sodium hydroxide catalysis. The results indicated that 95% of the inputted oil was converted into FAME, with glycerol as an expected by-product. Following transesterification, the products undergo methanol recovery, water washing, FAME purification, alkali removal, glycerine purification, and waste treatment.
Furthermore, Process II began with a preliminary esterification of the UCO due to its high FFA content of 6%, which is a common characteristic of waste oils. Under the conditions of a 6:1 molar ratio of methanol to UCO at 70°C and 400 kPa, all the FFA were successfully converted into methyl esters with the help of a sulfuric acid catalyst.
The stream was then put through glycerine washing and methanol recovery processes to remove the resulting water and catalyst, as well as recycle the methanol from esterification. The oil was now ready to encounter transesterification, in which the same conditions as Process I were implemented, and identical results were observed. The product then journeyed through the same subsequent techniques listed in Process I.
For Process III, although the use of WCO was implemented, the need for initial esterification was not present due to acidic catalysts’ insensitivity to FFA in comparison to basic catalysts. Thus, the process began with a transesterification reaction with set conditions of 80 °C, 400 kPa, a methanol to WCO ratio of 50:1, and a sulfuric acid to waste oil ratio of 1.3:1. The observed results expressed that 97% of the oil was able to be converted into FAME after a period of 4 hours, and further experimentation showed a 99% conversion to FAME when the methanol to waste oil molar ratio was increased to 245:1. The product went on to undergo methanol recovery, acid removal, water washing, and FAME purification processes. In an attempt to avoid emulsion formation during water washing operations, Process IV proposed the application of hexane as a solvent in order to separate FAME from other components in the stream.
For this process, the procedural conditions were the same as those of Process III up to the methanol recovery step after transesterification.
Following this step, the stream contained a mixture of FAME, methanol, sulfuric acid, glycerol, and unconverted oil, from which a sufficient separation was achieved by hexane with the addition of water to methanol in a volume ratio of 1:10. The stream then required subsequent hexane extraction, FAME purification, and glycerine purification treatments. Among the many factors considered when comparing the four processes, it was determined that overall, the alkali-catalysed process which used virgin oil (Process I) is most recommended.
However, if the cost of raw materials is a major concern, as well as the environmental effects of waste oils, the acid-catalyzed process which used WCO (Process III) is a promising alternative.
Altogether, each of the four processes generated a feasible, high quality product. Nevertheless, a technical assessment reveals a proper comparison between the selections. For instance, the need for a pretreatment in Process II led to an increase in complexity, as well as operating and equipment cost, whereas the remaining four processes did not hold this burden.
Despite the savings in raw material cost on account of the use of WCO, its complexity offset that reduction.
Additionally, unique operating conditions of the systems allowed the use for carbon steel as a cheaper alternative for stainless steel in several of the processing units. Since alkali-catalysed transesterification is less corrosive to equipment than acid-catalyzed, Processes I and II were able to utilise this substitution more thoroughly, thus lowering the cost of their equipment.
Moreover, it was observed that the acid-catalyzed systems were able to produce a greater yield upon a significant increase in methanol, which created the need for more processing equipment and increased costliness. Process IV in comparison with Process III was generally indistinguishable, besides the extension of hexane and methanol/water solvents which were proven to have no significant impacts besides an increase in expense.
Therefore, the two most feasible options were determined to be Processes I and III due to their simplicity: a homogenous alkali-catalysed system using virgin raw material, and a homogenous acid-catalyzed system with the employment of UCO.
Although homogeneous catalysis is more common among commercialized operations, the development of promising heterogeneous catalyst alternatives is arising within the research of biodiesel production using UCO.
A study was done on a system involving a calcium oxide nano-catalyst, which acts as an effective alkali catalyst for the production of biodiesel.
The catalyst was formed by a thermal-decomposition method, followed by calcination at 500◦C, a synthesis which is reported to be cheaper than most widely used homogenous alkali catalysts.
It was found that with a UCO to methanol molar ratio of 1:8, 1 wt. % of calcium oxide nano-catalyst, a temperature of 50◦C, and a reaction time of 90 minutes, a biodiesel yield as high as 96% was achieved, which is among exceptional results considering the reaction conditions.
Upon finishing the transesterification process, a minor increase in heating was implemented for the removal of excess methanol, and suspended solid catalysts were removed by allowing the solution to settle for two to three days.
This process did not require such post-treatments as discussed previously. However, the UCO used for this operation was only subject to an FFA content of 1.54%, so an initial esterification process was not present within this experiment.
In the case of an oil with a higher FFA content, a removal pretreatment would have been necessary to account for the use of an alkali catalyst and its sensitivity to FFA contamination. Overall, the catalyst successfully converted WCO into an acceptable biodiesel by the means of a cheap and efficient process.
A global team of researchers have established a new ultra-efficient means of recycling UCO into biodiesel using a heterogenous acid-base catalyst made up of a unique porous structure.
The multifunctional catalyst is designed to allow the molecules of feedstock to navigate through the framework in a sequential manner.
First, the feedstock enters the system through larger macropores coated with a solid acid catalyst, sulfated zirconia.
At this stage, an esterification pretreatment takes place to neutralize the FFA contamination in WCO so that it does not interfere with the subsequent reactions. The resulting mixture diffuses into smaller mesopores located within the same particle. The mesopores contain solid base magnesium oxide nanoparticles as a catalyst for the stream to undergo transesterification.
This strategic incorporation enables a one-pot process to merge two reactions into one. Accordingly, this offers a method requiring fewer unit operations, which reduces energy and solvent use, lowering the cost of the procedure considerably. This potentially resolves the main burden associated with an additional esterification reaction needed when using an alkali-catalyzed system for the transesterification of UCO with a high FFA content. Additionally, the structure is conveniently inexpensive and easy to manufacture.
This unique process demonstrates the advantages of a heterogenous catalyst, which is heightened by the fact that most catalysts of this sort are reusable and effective for several repetitions.
The development of this hierarchical porous framework based on spatial localization of functions was widely funded by the Australian Research Council, and it has the potential to unlock an extremely efficient approach to producing biodiesel.
The alternative to using acid or base catalysts for transesterification is the application of an enzyme. In the case of enzymatic transesterification, the formation of soap due to a high FFA content is not a concern. Therefore, contaminated feedstock is able to be converted into FAME in a single step, suggesting that WCO is a viable option for this type of process. The absence of an initial esterification treatment reduces process complexity and energy cost.
Additionally, enzymatic transesterification is not accompanied by the downstream processing issues faced by its acid or base counterparts.
These challenges include wastewater generation, difficulty in glycerol recovery, and other post-transesterification treatments that hinder efficiency. In contrast, the use of biocatalysts enables a reaction without the generation of overwhelming by-products, allowing for easy product recovery.
In addition, enzymatic catalysts can work under mild reaction conditions, further lowering energy use. Thus, this has been proven to be a more eco-friendly approach to biodiesel production, for it does not produce waste or consume large amounts of energy. On account of these factors, the use of enzymatic catalysts may be considered a promising alternative to traditional chemical transesterification.
However, constraints apply to this method, such as the high cost of enzymes, enzyme deactivation, and slow reaction rates. Therefore, this method is not used commercially, and has only been tested on the laboratory scale.
Nevertheless, these limitations may be overcome by immobilization technology, which provides a rigid backbone for enzymatic molecules such as lipase.
The results of this application show faster reaction rates as well as reusable enzymes. Hence, further research is being conducted on the exploration of immobilized enzymatic transesterification and its potential for commercial use.
There is another alternative means of transesterification being studied that does not require the presence of any catalyst.
This is called supercritical transesterification, and it is done by applying temperature and pressure conditions to the system that succeed the mixture’s critical point. Although reactions involving homogeneous or heterogeneous catalysts are most commonly operated, several impediments are accompanied by these selections which do not apply when supercritical conditions are imposed.
This includes high water content and FFA sensitivity, as well as the need for separation and purification processes after the reaction has taken place. In the case of WCO as feedstock, supercritical transesterification is a viable option that reduces costs associated with these constraints. However, the need for very high temperatures and pressures results in increased cost of equipment, thus neutralizing the advantages.
In one study, a comparative analysis was done on the transesterification of virgin vegetable oil vs. UCO using both catalyzed and supercritical processes.
The catalytic system was carried out under the conditions of a 1:6 oil to alcohol molar ratio at 65°C and 1 bar pressure.
However, the supercritical system required a temperature of 260°C and 65 bar pressure. It was found that a successful conversion was achieved in all four operations, with closely matching results. For the supercritical process, it was proven that UCO use is superior than virgin oil.
This is due to low cost of feedstock and the absence of inconvenient pre- and post-treatments, which would otherwise be necessary in a catalytic process using WCO. Regarding the comparison between the catalytic processes and supercritical processes, there are several factors to take into consideration. While the supercritical process does not entail extra UCO treatments, it does increase operating cost with the need for extreme conditions. Accordingly, while the need for more research on this topic is present, it is apparent that the continuous production of biodiesel from WCO by the means of supercritical transesterification holds valuable competition with traditional catalyzed processes.
Although it may seem the incorporation of WCO into the biodiesel production system is not always the most economically feasible option, there is prospect for that to change.
For instance, an increase in biodiesel production may cause the price for the raw materials used in this production to increase as well. Ethanol, which is used as a biofuel additive, is mainly derived from corn.
The expansion of ethanol use over the years has resulted in an increased price of corn by an estimation of up to 68%.
This demonstrates the impact of supply and demand of valuable raw materials on their market price.
Similarly, as biodiesel continues to increase in popularity, the price of vegetable oils may start to go up with demand. An obvious solution to this would be the use of WCO instead of unprocessed materials, for used cooking oils are estimated to be worth about half the price of virgin oil as it is.
Putting economic feasibility aside, the environmental impact of these developments holds significance in their worth.
To illustrate, used cooking oil itself can be harmful to the environment if not dealt with properly. When UCO is discarded down drains or sewers, it can potentially be damaging to aquatic life.
Additionally, if dumped directly, WCO can devastate the physical environment by coating animals and plants with oil, depleting oxygen and disabling their survival.
Hence, increased collection and recycling of WCO will have a direct impact on the natural world.
Moreover, the general use of biodiesel has environmental benefits as well. Compared to petroleum diesel, biodiesel as an alternative reduces greenhouse gas emissions by 86%, lowers hazardous particulate matter by 47%, and depletes hydrocarbon emissions by 67%. Thus, although commercial priority is most often based on economics, there are other reasons why biodiesel and use of UCO is important to consider, such as environmental sustainability. Moreover, UCO can be locally sourced rather than relying on imports from potentially unstable countries.
If large scale application is unrealistic, nothing is stopping a commoner from producing his own UCO biodiesel right at home. This gives you the opportunity to do something beneficial for the environment, as well as save yourself a few bucks. Homemade biodiesel can be used as a motor fuel additive, or even for the heating of your home. More than 78% of diesel-powered vehicles today are approved for biodiesel-petroleum blends of up to 20% biodiesel.
Pure biodiesel is called B100, but more commercially available biodiesel blends are B20 (20% biodiesel, 80% petroleum diesel) and B5 (5% biodiesel, 95% petroleum diesel). B100 is typically avoided due to issues with clogging the injectors.
However, B20 is approved for use in heavy-duty diesel trucks. The most recent models of the Ford Power Stroke, GM Duramax and Dodge Cummins diesel engines can all run B20 biodiesel without warranty concerns. Since 2011, all Ford Super Duty pickups even have “B20” boldly displayed on the Power Stroke fender badge.
As society progresses, we must continue to mindfully integrate eco-friendly options into our traditional operations. Research shows that WCO biodiesel can be a valuable contribution toward the future of environmentally sustainable and cost-effective fuel. Of course, more investigation is needed for the growth and optimization of using WCO as a raw material for the production of biodiesel.
However, with the exploration done thus far, we have developed the incredible ability to transform recycled restaurant grease and waste oil into a marketable and clean product.