Transesterification process can be carried out in two ways, chemically or biocatalytically catalyzed. Chemical catalysis has other two alternatives, alkali and acid catalysis. Industrial biodiesel production (chemical transesterification), conventionally, triglycerides, such as vegetable oils and animal fats are mixed with methanol in a reactor. Sodium hydroxide or potassium hydroxide is added as catalyst and the mixture is agitated and heated to the boiling temperature of methanol [30-32]. Fig. 2.3 represents a schematic overview of transesterification and biodiesel production process for the BPD plant. (For more information on the process flow and the plant production system it can be referred in the Index case study).
Oil + Fat KOH MetOH E-12 Biodiesel Glycerol Biodiesel Distillation
Figure 2.3. Production process in stocks del vallles (with the permission of the company)
After transesterification, the resulting two phase of methyl ester/glycerin have to be separated and further processed. The upper layer is methyl ester of fatty acids which is biodiesel and the lower layer, is crude glycerol. Excess alcohol can be recovered from the transesterified mixture or from each phase after separation. The separated glycerol contains residual alcohol, traces of catalyst and water, insolubles, unreacted solid substances present in the raw materials and some
esters. Generally, WVO and WAF will contain different amounts of proteins, ketones and aldehydes, sulphur compounds etc. which end up in the glycerol phase.
Glycerol, depending on the production plant size and the economy, can be sold as crude or refined onsite. The glycerol produced in the transesterification is of crude grade and thus, it has low value. There are different approaches to its utilization. Small producers usually limit the glycerol treatment to dehydration and either sell it to the refiners or burn it onsite for steam production. On the other hand, a refined glycerol can constitute an important economic variable for the production plant. Therefore, most big production plants refine glycerol, at least to a technical grade [33].
The crude glycerol phase generally contains almost 75% glycerol [30]. However, glycerol content in biodiesel manufacturing waste may vary for different manufacturing plants. (More detail is presented in the section of the crude glycerol composition).
Over all, the types of glycerol produced currently differ significantly in the content of water, fatty acid residues, esters, and other organic wastes. These differences are more likely due to the use of diverse feedstocks for biodiesel production. Although, most of the first use oils lead to not big differences in the glycerol layer, a completely different behavior was observed for the glycerol obtained from WVO represented by low concentration of glycerol and methanol with a high content of fats.
2.2.4 Glycerol market, production and its oversupply problem
The availability of crude glycerol has almost double since 2003, due to the increased in production of biodiesel. However, its demand has remained almost unchanged [34]. Annually nearly 160000 tons of glycerol is used for technical applications and it is expected to grow at a rate of 2.8% every year [34, 35].
Thus, this combined effect of supply excess and limited demand of raw glycerol led to low sale prices.
Although pure glycerol is an important feedstock in many industrial sectors, large-scale producers must refine raw glycerol. In order to remove impurities such as fatty acids, alcohol and catalyst, it needs to use a separation processes (filtration, chemical additions, and fractional vacuum distillation). Generally these processes are expensive and economically unfeasible for small and medium scale plants [36].
Since 2006, the glycerol oversupply forced biodiesel producers to set sales prices of 2 cents per pound or even lower for the raw product. On mid-2007, however, the price reached between 6
and 10 cents per pound [17]. On the other hand, depending on the quality and purity of the glycerol, its price showed similar trend, which was as low as 20-30 cents per pound [17,35]. As a consequence, the raw glycerol market will remain weak while a large amount of this raw component is available. Therefore glycerol is nowadays a key problem in biodiesel production. Its low sale price could convert this by-product in a residue which, then the biodiesel producers must be should find alternative uses to avoid the continue falling on the glycerol price.
2.2.4.1 Prices of crude glycerol
As biodiesel production skyrockets, the market is being flooded with crude glycerol. In US crude glycerol prices have dropped from 25 cents/lb in 2004 to 2.5-5 cents/lb in 2006 [37,38] because the current demand for glycerol is not large enough. This shows that new uses for this byproduct are clearly needed.
Until very recently, purified glycerol was considered as a high-value chemical with prices as high as $2/kg. Because 1 kg of glycerol is produced for every 10 kg of biodiesel produced. As a result, this has created a glut in the glycerol market causing sharp decrease in the price of glycerol which is now estimated to be around $0.1/kg. In addition, biodiesel production units are facing increasing production cost due to the fact that glycerol, a major income generator, has become a waste stream. This is due to the fact that the glycerol produced by a biodiesel production facility contains significant amounts of salts, heavy metals, and water. Besides, the cost of purification is way expensive than the current price of glycerol [39].
Once considered a desirable co-product that could contribute to the economic viability of biodiesel production, many now regard crude glycerol as a ‘waste stream’ with a disposal cost associated to it. For example, an analysis of the feedstock and processing costs in the production of biodiesel from soybean oil yields a gross processing margin about $0.079 per gallon of biodiesel (including a glycerol credit of $0.021, but excluding any interest expense, tax credits or fixed costs) [40]. Clearly, the development of processes to convert crude glycerol into higher value products is both an urgent need and a ‘target of opportunity’ for the development of biorefineries. Such technologies could be readily integrated into existing biodiesel facilities, thus, establishing true biorefineries and revolutionizing the biodiesel industry by improving its economics. Moreover, waste streams containing high levels of glycerol are generated in almost every industry that uses animal fats or vegetable oils as starting feed stocks (please refer Table 6.1 in chapter 6). For example, the oleochemical industry generates waste streams containing 55–90% glycerol [41]. Such glycerol surplus will not only result in a further reduction in prices, but the disposal of these streams will become a major issue [5].