At present, the industrial production of biodiesel is mainly based on transesterification, namely synthesizing the Fatty Acid Methyl Ester with relatively low molecular mass ( biodiesel) and crude glycerin by transesterification reaction of alcohols with relatively low molecular weight such as methanol and the fatty acid glyceride in raw oil (various natural animal and vegetable oils and used cooking oil, etc.)
Due to different raw materials and methods, the impurities in glycerol obtained by transesterification are different in nature and quantity. Besides the major by-product glycerol, there’re a large number of other substances, such as methanol, a small amount of biodiesel and unreacted fatty acids and catalysts.
If impurities were not removed through pretreatment, it will seriously affect the subsequent refining of crude glycerol. if refined by ion exchange resin directly, the resin will soon reach saturation and deactivation, and a large amount of acid and alkali wastewater will be generated in regeneration because of the large exchange capacity. If used reduced pressure distillation, this won’t cause the glycerol to gasify due to the inclusion of the by-products biodiesel and fatty acid soap. Therefore, it is necessary to pre-treat the lower by-products of biodiesel before refining crude glycerol.
1.1 Attenuation
By-product of biodiesel is a brown and viscous liquid, with high viscosity, which is not conducive to separate. In order to improve the separation effect, it’s necessary to add solvent as a diluent to reduce the viscosity of crude glycerol. Glycerol yields varies from different solvents. The common diluents are methanol and water, and the glycerol yield is higher either use methanol or water diluents. As a part of methanol is contained in the by-product of biodiesel, methanol has good solubility for glycerol, fatty acid soap and ester. At the same time, methanol has a low boiling point and is easy to be recovered. However, methanol is volatile during operation and harmful to human health. Qian Junfeng et al. believed that when the amount of diluent methanol was increased, the glycerol yield would increase first and then decrease. When the mass ratio of methanol to glycerol was about 1:10, the glycerol yield was the highest.
The reason may be that when the amount of methanol is low, the separation effect is not good because of the high viscosity of crude glycerol. However, when the amount of methanol is high, the separation loss is large. Therefore, there is an optimal value of methanol addition.
1.2 Desalinated by Neutralization for Crude Glycerin
The pH value of by-products from biodiesel preparation is an important factor affecting the yield of crude glycerol. Desalinated by neutralization means using by acid or alkali to neutralize the crude glycerol to reach an appropriate pH value, and then, remove the salt generated during the neutralization reaction by centrifugal separation. Yang Kaihua et al. found that when the pH value is larger than 7, that is, when the mixture is alkaline, the glycerol yield is significantly lower. When the mixture is neutral or acidic, the difference in glycerol yield is small. In particular, when using alkali as catalyst, methyl ester, methanol, alkaline catalyst, soap and other impurities are easily mixed to form colloids under alkaline conditions. To separate them, it is necessary to neutralize them with acid to an appropriate pH value to generate salt and fatty acid from alkaline catalyst and soap. Fatty acid and glycerol are not mutually soluble and can be separated by centrifugation. Studies have suggested that the glycerol yield is the highest when the pH value of underlying solution is adjusted to be weakly acidic. Because soap can be decomposed into fatty acids, diacylglycerol, or glycerides to form glycerol under acidic conditions. However, if the acidity was too strong, glycerol would be esterified with the acid. Moreover, it’d be easy to polymerize for glycerol, during the refining by vacuum distillation, and decrease the glycerol yield accordingly. At the same time, too strong acidity will increase the corrosion of separation equipment. Therefore, the reaction underlying solution should be carried out under weak acidic conditions. Qian Junfeng et al. studied the purification of by-products obtained from the biodiesel preparation under the condition of alkali-catalysis, using hydrochloric acid, sulfuric acid and phosphoric acid to adjust and control the pH value of the solution. It turns out that the yield of glycerol remains constant as long as the pH value of glycerol is constant. However, the effects on the ash residual in glycerol are different with different acid regulation. The ash residual was the smallest when the middle glycerol solution neutralized by sulfuric acid was treated under the same conditions. The residual ash content was the highest while treated with hydrochloric acid. Therefore, it is better to use sulfuric acid to neutralize glycerin Solution, considering the quality and post-treatment process of glycerol. Yang Yuncai et al. investigated the influence of methanol dosage and pH value of solution on the desalination rate, purity and yield of glycerol, through the pretreatment method of acid desalination with sulfuric acid and methanol solution. The results showed that when the amount of methanol was 0.5 times as the volume of the lower by-product and the pH value of the solution was 5, the glycerol product with a purity of 90% could be obtained. And the desalination rate is up to 90% and the yield of glycerol is above 95%. For crude glycerol, the main factors affecting the yield of semi-finished glycerol are reaction temperature, reaction time and centrifugal speed, during the process of neutralization to desalinate.
1.3 Remove the Impurities and Methanol for Crude Glycerol
In order to obtain higher purity glycerol, impurities must be further removed when necessary, because of the complex composition of crude glycerol. Ao Hongwei etc treated the crude g by adding soluble sodium oxalate to statically mixture, by filtering to remove the impurities. And They’ve got the optimum conditions, namely mass fraction of sodium oxalate 0.03%, reaction temperature 80 ℃, mixing time 30 min.
There’re also much unreacted methanol in the crude glycerol, a by-product of biodiesel. Methanol should be recovered by distillation after removing impurities, which will not only improve the quality of glycerol refining, but also save the reaction materials and reduces the production cost.
