Siemens Industry, Inc. - Process Analytics

Measurement of oxygen during the purification of glycerin – Case Study


Courtesy of Siemens Industry, Inc. - Process Analytics

Background information

Glycerin is a basic chemical which is used in creams and ointments, in antifreeze and brake fluid for vehicles, as a plasticizer, and as a base for paints and lacquers. Glycerin is produced as a by-product during the manufacture of soap. This was the main source of glycerin for many years. However, the production of biodiesel has increased greatly in the last decade, and glycerin is also a by-product of this process. All vegetable oils contain glycerin, a heavy trivalent alcohol. During the industrial production of biofuels, this must be replaced by a light alcohol in order to improve the viscosity of the oil and the lowtemperature stability.

Vegetable oil, usually rapeseed, is heated to 30 to 50 °C in the presence of methanol and a catalyst. After about two hours, the methyl alcohol has displaced the glycerin, and biodiesel has been produced. The mixture of biodiesel and glycerin obtained in this manner can be separated easily since the lighter biodiesel floats on top. This is purified from residues of vegetable oil and methanol, and can be marketed as a high-grade fuel. The heavier phase at the bottom - also called substandard glycerin - contains the glycerin. Since glycerin is polar in contrast to the biofuel (in other words miscible with water and water-soluble materials), all polar materials produced during the reaction cluster in the glycerin. These are mainly soap, methanol, fatty acids and salts. This glycerin produced as a by-product is therefore highly contaminated, and has to be purified prior to further use. These impurities are separated in a multi-stage process, and the glycerin is thus purified.

The production of one ton of biodiesel results in more than 100 kg glycerin which is contaminated by approx. 100 kg of further by-products.

Description of the process

The purification of the resulting glycerin is usually carried out directly in the biodiesel production plant; in the case described here, an independent company is responsible for the purification. The raw materials are delivered in road tankers from within Germany and adjacent countries, and initially stored in intermediate tanks. The actual purification takes place in several steps. The raw glycerin is neutralized using sulfuric acid as the first step in order to separate methanol. This is followed by acidification with sulfuric acid, resulting in free fatty acids, potassium sulfate and a glycerin/water mixture. These components are isolated by a tricanter, a horizontal decanter centrifuge. In the third step, the glycerin is separated by rectification.

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