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.


The process block diagram illustrates the process steps for the purification of crude glycerin from biodiesel.
SOURCE: ROHM AND HAAS

A New Approach
In order to provide a more cost-effective and flexible process for purifying crude glycerin issued from biodiesel production, Rohm and Haas, a provider of functional polymers, ion-exchange and catalyst technologies, has teamed up with Novasep Process, a provider of purification solutions including chromatography, ion-exchange, membranes, crystallization and evaporation. Together, the companies have developed a patent-pending technology for glycerin purification that is specifically designed with the biodiesel producer in mind.

The Ambersep BD50 glycerin purification process can be scaled to any required production volume, although it is recommended that the minimum nameplate capacity be 5,000 metric tons of pure glycerin per year. The process options, as described below, ensure that the purification system is tuned to meet the quality specifications. The system maximizes the recycling of all process water, minimizes energy consumption, and treats all of the process raffinate streams. Raffinate is the portion of an original liquid that remains after other components have been dissolved by a solvent.

The Ambersep BD50 process is commercialized as a complete technology package, including process technology, engineering, media and equipment, tailored service options, and technical support from industry experts.


Process Steps
The process block diagram on the previous page illustrates the different process steps for the purification of crude glycerin from biodiesel by the Ambersep BD50 process.

Crude glycerin from storage is first heated to 194 degrees Fahrenheit on a plate heat exchanger, using energy recovered from the purified glycerin stream, plus live steam. After a safety filtration to protect the downstream processing steps from fouling by suspended materials, the hot and clean crude glycerin is carefully degassed before entering a chromatographic separator.

The chromatographic separator combines the sequential simulated moving bed technology with Ambersep BD50, a high-performance chromatographic separation resin from Rohm and Haas, to purify crude glycerin with a high salt composition.

Such a system has a high productivity and consumes small quantities of water for the separation of the salt fraction from glycerin.

Due to the systematic recycling of the condensates produced during the reconcentration of the purified glycerin fraction, no source of fresh water is necessary for the operation of the chromatographic separator during normal operations.

The Ambersep BD50 glycerin purification system includes the option to concentrate and crystallize the salt fraction coming out of the separator. The raffinate stream contains the salts, other organic impurities (color and free fatty acids), and the minor fraction of glycerin not separated by the sequential simulated moving bed chromatography unit. The raffinate is processed in an evaporator/crystallizer unit, affording the recovery of the salts in a crystalline form and a "secondary glycerin" having a similar composition to the crude glycerin input. This solution avoids the production of effluents in the glycerin purification plant.

Depending on the required purity of the final glycerin, it is possible to produce a purified glycerin product with 99.5 percent purity or to add a polishing step employing an ion-exchange demineralization unit which enables the end user to produce a high quality glycerin product with 5 parts per million to 10 parts per million salt content.

The degree of polishing of the refined glycerin can be adjusted depending on the final application. One of the features of this process compared to the conventional distillation route is its low energy requirement. The Ambersep BD50 system does not require the vaporization of glycerin. Water is used for the chromatographic separation and therefore the energy consumption is essentially limited to the removal of water from the purified glycerin after purification.

Xavier Lancrenon is the commercial director for Novasep Process. Reach him at +1 33 6 0851 15976. Jon Fedders is with business development for Rohm and Haas. Reach him at (215) 592-2503. For more information, visit http://www.amberlyst.com/glycerol.htm