Currently, biodiesel is commonly made from soybean oil in the U.S., rapeseed, sunflower or soybean oil in the EU, and palm oil in Southeast Asia. However, in the event of the food vs. fuel conflict, many are shifting into the exploration of non-edible oil feedstocks.
The conventional biodiesel production technique is transesterification by the chemical approach, which is a well-developed technology that has been commercialized worldwide. Though great efforts have been placed in the improvement of this process, it still suffers from high production costs and environmental concerns, e.g., wastewater, chemical disposal and low quality of the glycerol co-product. Because most chemical approaches are comprised of the alkaline process, the production is limited to only the transesterification reaction. It cannot handle conversion of free fatty acid (FFA) effectively and oftentimes leads to soap formation. On the other hand, the acid process is limited to only the esterification reaction, i.e., conversion of FFA into biodiesel, because the rate of transesterification is too slow to make it feasible for industrial production. Therefore, given an oil source that contains a large amount of FFA, only a combination of the acid and alkaline processes can fully utilize the feedstock oil. As a result, capital investment is doubled, as well as the operating cost, leading biodiesel to cost higher than petroleum diesel. Without subsidies, whether explicit or implicit, businesses running on this kind of technology will find it hard to survive in the long run.
Since the time biofuel was produced, its price has been higher than that of fossil fuel, leading the general public to prejudice against its feasibility. An examination of the common cost allocation for biodiesel production through the chemical approach shows that 70-90 percent is allocated for the feedstock, 20-25 percent for operating cost and 5 percent for other fees. Thus, cost allocated for the feedstock is prohibitive to the product. In order to reach a reasonable budget, this value would have to be decreased to 50 percent or less.
The enzymatic process is known to be a clean and environment friendly technique for biodiesel production. It can simultaneously convert both FFA and triglyceride into biodiesel. Through the years, it has been stereotyped into having long reaction times and high operating costs, due to the expensive lipase. Unfortunately, these problems are still widespread in academic journals and mass media.
Multiple drivers for cleaner, sustainable energy necessitate a deviation from the traditional methods of energy production. A broader and more open perspective on the possibilities brought about by a change in strategy is already beginning to shape new and better technologies. In the conventional refinery process, a thermal or catalytic cracking method is applied, whereby the raw material is broken down and reformulated to obtain the product. This kind of approach normally involves many side reactions and results to some undesired products. To obtain a product with the same properties as, say, diesel, an alternative method is to apply simple reformulation or re-synthesis that results to clearly defined pure products, such as is the case with the traditional or enzymatic biodiesel production.
The enzymatic approach for biodiesel production is the exact example of “green chemistry”. Admittedly, it is known to have disadvantages, such as high enzyme cost, low yield, short catalyst lifespan and long reaction times. The advantage of the enzymatic process is that it is environmentally friendly, meaning that there is no chemical discharge and wastewater, due to the fact that there is no need for water washing. The typical enzymatic approach can simultaneously perform esterification and transesterification for FFA and triglyceride, respectively. This is especially useful for oil sources that cannot immediately be processed, such as those in storage for a period of time. The enzymatic process is also much preferred because of its mild reaction conditions. Thus, the energy requirement to produce liquid biofuels via the enzymatic approach is lower.
Using an enzymatic approach to produce biodiesel is not a new subject as the idea has been postulated for approximately 30 years. The slow pace of progress can be attributed to the prevailing standard of using a solvent-free system. Methanol and ethanol are known to be relatively insoluble in oil and to deactivate the lipase. However, another important and overlooked matter is the deactivation of the lipase by the glycerol droplet. It means that even though a solvent is employed together with methanol or ethanol in oil, if the glycerol droplet is separated in a second phase, the lipase will still be gradually deactivated. This implies that a biodiesel production system using an enzymatic process should have the reaction carried out in a homogenous solution. The discovery, first made in early 2000’s, led to a breathrough.
A new enzymatic transesterification process (ET Process, patented), has been developed to address prevailing concerns about biodiesel production technology. The process consists of two reactors: a primary reactor and a trim reactor. The alcohol reactant, oil and inert solvent are mixed well with recycled biodiesel before it is fed into the primary reactor. The reactor can have a packed bed or CSTR design. The reactor outlet is connected to an evaporator, where the unreacted alcohol, inert solvent, and water are evaporated. The residual unreacted oil, biodiesel and glycerol are then separated into two liquid layers. The upper layer is crude biodiesel and the lower layer is crude glycerol, which contains a trace amount of solvent and alcohol.
Figure 1: Biotechnology process
Crude biodiesel is mixed with the inert solvent and the make-up alcohol reactant and forwarded to the trim reactor, where the reaction goes to completion. The output is also sent to an evaporator, where biodiesel and crude glycerol likewise separate into two layers. The biodiesel product is retrieved in pure form, without the distillation step normally required. Crude glycerol is collected and sent to an evaporator, where all residual solvent, alcohol and water are evaporated. It produces pure, colored glycerol, the quality of which can be enhanced to pharma-grade by decolorization using activated carbon.
Alcohol reactant, inert solvent and water are condensed and collected before they are forwarded to a solvent recovery unit. Here, the alcohol reactant and inert solvent are separated, purified and recycled. Trace water, which is coproduced or contained in oil and alcohol reactant, is discharged.
The reaction is typically carried out at room temperature (25-30°C). The reaction time varies with the oil source and the alcohol reactant, but typically falls within a 10-60 minute range. Higher reaction temperatures will enable higher reaction rates but at the expense of a shorter lipase life cycle.
The performance of the immobilized lipase has a profound effect on the reaction rate. In addition to the lipase source, the immobilization process is a further factor that can improve the performance of the catalyst. A suitable immobilization process may be used to produce lipase with competitive performance levels, even when starting with different lipase sources.
This enzymatic process offers much better economics than the conventional chemical method. The lipase can be utilized in the operation for a long period of time, usually about 12-18 months, while the half life of the lipase can be more than 60 months. The amount of lipase required for the reaction to work is less than 3 percent per stream day. That is, for a plant with a capacity of 100 tpd, the lipase required can be less than 3 tons. Given the current lipase cost of US$350/kg, the expense of biodiesel produced for a life cycle of 12 months will be $0.029/kg. If the life cycle extends to 18 months or the lipase cost decreases by 1.5 times its current price, then the expense will drop to $0.019/kg. There is still much room for improvement of the lipase performance and cost. For instance, lipase cost can be lowered with time through mass production. This makes the lipase expense an insignificant factor of production in the long run.
Figure 2 is a mini-unit with a capacity of 4 gal/d. The system design is the same as a commercial production unit except for the scale. The unit is designed to be an integrated and automated system. The primary reactor is a simple packed bed. Figure 3 shows the crude biodiesel and glycerol products, which have separated into distinct layers during normal operation. The scale up of this unit into a commercial plant is but a straightforward process.
Figure 2: ET Process mini unit with a capacity of 4 gal/d
Figure 3: Crude products: biodiesel and glycerol separated into distinct layers
Crude palm oil is chosen as the subject of cost analysis hereafter because its price fluctuates almost in parallel with crude oil. The price of crude palm oil with high FFA content is also higher compared to non-edible sources that have similarly high FFA content. In 2009, when crude oil was $72/barrel, the market price for crude palm oil in Malaysia was $0.882/kg, that for biodiesel obtained by the chemical approach $1.085/kg and that for diesel $0.968-0.951/kg. The cost difference between crude palm oil and biodiesel is $0.203/kg and that between biodiesel and diesel is $0.117-0.134/kg. In this case scenario, biodiesel produced by the chemical method is more costly than diesel by an average of $0.125/kg.
The price of crude palm oil with 8 percent FFA in Southeast Asia was $681.5/mt in December 2008. If this oil source is used as a feedstock for an alkaline biodiesel plant, the FFA should first be removed. In order to remove 1 percent FFA, another 1 percent of neutral oil will be lost, so the total loss in feedstock will be about 16 percent. Given this knowledge, savings of $129.8/mt in feedstock can be obtained if this problem is eliminated. Given the simultaneous conversion characteristic of the ET Process, such amount equals the savings that can be obtained using the enzymatic process. The additional profit contributed by glycerol sums up to earnings of about $75/mt biodiesel. Costs amounting to $46/mt biodiesel can also be recovered from the operation. The total savings compared to the chemical method is about $250/mt or $0.250/kg.
Alternatively, in more recent data, total savings are similar to that in 2009, although the prices are generally higher. The CPO data has an average FFA content of 4.5 percent. Thus, the actual amount of savings is equal to $111/mt. Given similar savings in glycerol and operations, the current total amount of savings is $232/mt. These data show that the amount of savings is relatively insensitive to the period of time with which it is applied to.
This means that biodiesel can be marketed for a cost less than diesel, especially taking into consideration other non-edible oil sources, e.g., [oil, (FFA%)] jatropha (10-15), yellow grease (15-20), rubber seed oil (17), castor oil (19), rice bran oil (≥40), just to name a few. All these are marketed at a cost less than crude palm oil with 8 percent FFA. Should these be used as feedstocks, the biodiesel price can decrease considerably more, and so subsidies will no longer be needed to sustain biodiesel production.
The cost of diesel (and transport in general) is a big burden to end users. This is an especially serious challenge to areas with poor infrastructure (e.g., roads, transportation systems, storage, distribution stations). For such settings, a distributed rather than a concentrated biofuel production model will be more desirable. An integrated design, starting from oil extraction, to oil degumming, to the ET Process and finally to power generation can provide basic energy needs to a community in a cost-effective and environmentally friendly manner. For oil extraction, biodiesel produced can also be used directly as an oil extraction solvent, resulting in a total economic solution for the community. In addition to oil seeds produced locally, rapid progress in algae technology will help make localized oil production a reality in the near future.
Images courtesy Sunho Biodiesel Corp.