2.4. Purification processes
The reaction product was transferred to cooling water unit and then to a separator. It was separated into two phases for 100 min (a good separating time). The top phase contained excess MeOH and water formed during the reaction, while the FAME phase was at the bottom. MeOH was purified by rectification and then reused as starting material.
The FAME phase was taken off at the bottom and passed into evaporator to remove traces of methanol. The FAME still had residual FFA about 1.4 wt%, requiring further purification.
The purification was investigated by using three different methods as follows.
The first method was neutralization with sodium hydroxide in methanol (NaOH–MeOH) solution at a reaction temperature of 80 °C. NaOH (4 wt%) was dissolved in MeOH and then mixed with FAME. The mixture was heated to its reaction temperature and let the reaction to carry out for 15 min. Both neutralization and transesterification reactions took place at the same time.
The second method was carried out using 3 M of sodium hydroxide in water (NaOH–H2O) solution using the same conditions as the first one. In this case there are no transesterification reactions only the formation of soaps, which were removed by 2 wt% salt (NaCl) addition (2 g of salt/100 g of FAME phase).
The third method was neutralization with 3 M of NaOH–H2O solution at a reaction temperature of 65 °C for 15 min. The neutralized product was settled and the soap phase was then removed. After that the neutralized FAME phase was transesterified at a temperature of 80 °C under its own pressure for 15 min with 0.396 M of NaOH–MeOH solution.
The product obtained from the purification step was settled in a separator. The FAME phase was separated and washed with water at temperatures of 25–90 °C to remove impurities. The solution was settled for water separation and finally the residual water was evaporated.
2.5. Analysis of FAME
The compositions of the reaction mixture samples were determined by a thin layer chromatography equipped with flame ionization detector (TLC–FID) using an Itronscan MK-6s with Chromarods type S-III quartz rod (Mitshubishi Kagaku Iatron). One microliter of the reaction medium, diluted in hexane at appropriate dilution, was spotted onto chromarods. Firstly, chromarods were immersed in a solvent mixture of hexane:diethyl ether:formic acid (50:20:0.3 v/v) until the solvent reaching to 8 cm (approximately 15 min). Secondly, the chromarods were immersed in a solvent mixture of benzene:hexane (50:50 v/v) for about 35 min (or until the solvent reach to 10 cm). The spotted samples were developed in these two solvent mixtures. The chromarods were then dried at 105 °C for 5 min and scanned with TLC–FID. Scanning was performed using a 160 ml/min of hydrogen flow rate and 20 l/min of air flow rate to produce a chromatogram. The compositions were calculated as wt% based on the peak areas of each component.