The global increase in biodiesel production has led to a marked increase in world glycerin production. Glycerin is produced as an approximately 11 percent byproduct in the transesterification of triglycerides, which are the predominant feedstock material for producing biodiesel. Glycerin remains one of the most versatile and valuable known chemicals and has a wide variety of uses and applications. The added production of glycerin by the biodiesel industry has created a supply that has encouraged the development of new industrial applications for this material. For example, recent reports tout glycerin’s ability to be used as a raw material building block for the production of high-volume industrial chemicals such as propylene glycol, epichlorohydrin, acrylic acid and polyhydroxybutyrate.
Purification is required to transform crude glycerin to a usable state for existing or emerging uses. The purity requirements for the emerging applications of glycerin vary, and are often intermediate to the crude and refined grades previously established for the classical applications. The salt content in crude glycerin, stemming from the use of homogeneous alkaline catalysts, often ranges from 5 percent to 7 percent, which makes conventional techniques cost intensive. This suggests that for future glycerin markets a new low-cost purification strategy may be more cost-effective than conventional routes.
The glycerin produced in the transesterification of triglycerides reaction is a crude grade. Almost all biodiesel production today involves homogeneous alkaline catalysts such as sodium methylate. The transesterification of triglycerides with methanol generates a methyl-ester phase and a glycerin phase. Impurities such as catalyst, soap, methanol and water are preferentially concentrated in the glycerin phase. The glycerin phase is typically neutralized with acid and the cationic component of the catalyst is incorporated as a salt.
For example, sodium chloride is formed in the neutralization with hydrochloric acid of glycerin containing sodium methylate. Catalyst usage rates vary across the industry, but it is common to find crude glycerin issued from biodiesel production with a salt content of 5 percent to 7 percent. It should be noted that heterogeneous processes using enzyme and solid metal-oxide catalysts are promoted as alternatives to homogeneous alkaline catalysts. The Esterfip-H process marketed by Axens is an example of a heterogeneous transesterification process. Nevertheless, even in heterogeneous transesterification processes, impurities in the natural feedstock materials tend to accumulate in the glycerin phase and therefore, purification may still be required.
Conventional Techniques for Purifying Glycerin
Distillation is the most commonly practiced method for purifying glycerin. The advantages of the distillation process are well known. Namely, it is an established technology that produces high-purity glycerin in high yield. However, the distillation of glycerin is an energy-intensive process. Glycerin has a high heat capacity, which demands a high-energy input for vaporization.
Classical ion-exchange techniques have long been applied to glycerin purification. However, the high salt content of glycerin issued from biodiesel production makes classical ion-exchange uneconomical for this application. Specifically, the chemical regeneration cost for the resins becomes exceedingly high when salt contents approach the 5 percent- to 7 percent-range commonly found in the biodiesel industry.