From the statistical analysis it can be concluded that, within the experimental range, initial catalyst concentration is the most important factor on the transesterification process. It has a positive influence on the response; that is, ester yield increases with increasing catalyst concentration.
Figure 2 shows a schematic diagram of the processes involved in biodiesel production from feed stocks containing low levels of free fatty acids (FFA). This includes soybean oil, Canola (rapeseed) oil and the higher grades of waste restaurant oils. Alcohol, catalyst, and oil are combined in a reactor and agitated for approximately one hour at 60°C. Smaller plants often Use batch reactorsbut larger plants (> 4million litres/yr) use continuous flow processes Involving continuous stirred-tank reactors (CSTR) or plug flow reactors. The reaction is sometimes done in two steps where approximately 80% of the alcohol and catalyst is added to the oil in a first stage CSTR.
Then, the product stream from this reactor goes through a glycerol removal step before entering a second CSTR. The remaining 20% of the alcohol and catalyst are added in this second reactor. This system provides a very complete reaction with the potential of using less alcohol than single step systems.
Following the reaction, the glycerol is removed from the methyl esters. Due to the low solubility of glycerol in the esters, this separation generally occurs quickly and may be accomplished with either a settling tank or a centrifuge. The excess methanol tends to act as a solubilise and can slow the separation. However, this excess methanol is usually not removed from the reaction stream until after the glycerol and methyl esters are separated due to concern about reversing the trans-esterification reaction. Water may be added to the reaction mixture after the trans-esterification is complete to improve the separation of glycerol.
Saka and Kusiana, Dasari et al., and Diasakou et al. claim that it is possible to react the oil and methanol without a catalyst, which eliminates the need for the water washing step. However, high temperatures and large excesses of methanol are required. Dasari et al. noted the difficulty of reproducing the reaction kinetics results of other researchers and attributed it to catalytic effects at the surfaces of the reaction vessels and noted these effects would be exacerbated at higher temperatures.
Not including the effect of surface reactions could cause difficulties when scaling up reactors due to the decrease in the ratio of reactor surface area to volume. Kreutzerhas described how higher pressures and temperatures (90 bar, 240°C) can Trans esterifies the fats without prior removal or conversion of the free fatty acids. However most biodiesel plants use lower temperatures, near atmospheric pressure, and longer reaction times to reduce equipment costs.
Boocock et al. have developed a novel technique for accelerating the trans-esterification reaction rate. During its early stages, the trans-esterification reaction is limited by the low solubility of the alcohol, especially methanol, in the oil. Boocock proposes the addition of a co solvent to create a single phase and this greatly accelerates the reaction so that it reaches substantial completion in a few minutes. The technique is applicable for use with other alcohols and for acid-catalyzed pre-treatment of high free fatty acid feed stocks.
The primary concerns with this method are the additional complexity of recovering and recycling the co solvent although this can be simplified by choosing a co solvent with a boiling point near that of the alcohol being used. Additional concerns have been raised about the hazard level associated with the co solvents most commonly proposed, tetra hydro furan and methyl tertiary butyl ether.
Returning to Figure 2, after separation from the glycerol, the methyl esters enter a neutralization step and then pass through a methanol stripper, usually a vacuum flash process or a falling film evaporator, before water washing. Acid is added to the biodiesel to neutralize any residual catalyst and to split any soap that may have formed during the reaction. Soaps will react with the acid to form water soluble salts and free fatty acids as shown in the following reaction.
The salts will be removed during the water washing step and the free fatty acids will stay in the biodiesel. The water washing step is intended to remove any remaining catalyst, soap, salts, methanol, or free glycerol from the biodiesel. Neutralization before washing reduces the water required and minimizes the potential for emulsions to form when the wash water is added to the biodiesel. Following the wash process, any remaining water is removed from the biodiesel by a vacuum flash process.
The glycerol stream leaving the separator is only about 50% glycerol. It contains some of the excess methanol and most of the catalyst and soap. In this form, the glycerol has little value and disposal may be difficult. The methanol content requires the glycerol to be treated as hazardous waste. The first step in refining the glycerol is usually to add acid to split the soaps into free fatty acids and salts. The free fatty acids are not soluble in the glycerol and will rise to the top where they can be removed and recycled. The salts remain with the glycerol although depending on the chemical compounds present, some may precipitate out. Mittelbach and Koncar describe a process for esterifying these free fatty acids and then returning them to the trans-esterification reaction stream. One frequently touted option is to use potassium hydroxide as the reaction catalyst and phosphoric acid for neutralization so that the salt formed is potassium phosphate, which can be used for fertilizer.
After acidulation and separation of the free fatty acids, the methanol in the glycerol is removed by a vacuum flash process, or another type of evaporator. At this point, the glycerol should have a purity of approximately 85% and is typically sold to a glycerol refiner. The glycerol refining process takes the purity up to 99.5 to 99.7% using vacuum Distillation or ion exchange processes. The methanol that is removed from the methyl ester and glycerol streams will tend to collect any water that may have entered
the process. This water should be removed in a distillation column before the methanol is returned to the process. This step is more difficult I fan alcohol such as ethanol or isopropanol is used that forms an azoetrope with water. Then, a molecular sieve is used to remove the water. For an alkali catalyst, either sodium hydroxide (NAOH) or potassium hydroxide (KOH) should be used with methanol or ethanol as well as any kind of oils, refine, crude or frying. In this process it is better to produce the Alcoxy before the reaction to obtain a better global efficiency.