Consider Europe's Most Popular Catalyst

Alkoxide catalysts-a sodium methylate or potassium methylate solution in methanol-can be an economical alternative for commercial-scale biodiesel production where annual capacity exceeds 5 million gallons. The majority of large-scale biodiesel plants in Europe can't be wrong.
By Michael Markolwitz | May 01, 2004
As biodiesel production increases worldwide, U.S. producers are becoming aware of cost-effective production choices already being used in Europe, where production efficiencies can be significant, depending on the plant capacity.

Commercial production of biodiesel in Europe grew to more than 224 million gallons in 2003, where solid catalysts, such as sodium hydroxide or potassium hydroxide, and alkoxide catalysts in solution can both be used to produce standard-specification biodiesel from rapeseed oil. Each process results in biodiesel that meets Euronorm (EN 14 214), which sets parameters for the fuel's properties in Europe. When we take into account the entire manufacturing process, including handling of raw materials and the technical expenditure for producing biodiesel and glycerol, alkoxide solutions can be an economical alternative for commercial-scale production where annual capacity exceeds 5 million gallons. In fact, alkoxide catalyst solutions are used at most large-scale biodiesel plants in Europe.

Here's why this catalyst is an economical choice.

The production process
Rapeseed oil, soybean oil, sunflower oil or yellow grease is reacted with methanol in the presence of an alkaline catalyst to produce the desired methyl ester. A byproduct of this process is glycerol, which is usually sold for use in the cosmetics, pharmaceuticals and food industries, and adds significantly to the cost-effectiveness of the production process. On a commercial scale, solid products, such as sodium hydroxide or potassium hydroxide, and solutions, such as sodium methylate in methanol, are both used as catalysts. They are neutralized with acids when the reaction is over.

Solid catalysts
Sodium hydroxide and potassium hydroxide are supplied as flakes. Because they can't be used in this form, normally they must first be dissolved in methanol. Separate plant units are required for this process. Cooling systems must also be part of the system in case they are needed to control the heat released during this process (exothermal reaction). A blanket of nitrogen is also recommended, since the temperature can exceed the boiling point of methanol (64.8 C/148.64 F).

Large quantities of solid catalysts are deliverable in bulk tanker trucks. However, mid-sized and small producers of biodiesel most often acquire these products in sacks or "Big Bags," which must be thrown away or recycled. This equals more time and effort-and ultimately more expense.

The sodium hydroxide used is more than 99 percent pure. Currently, four biodiesel producers in Europe use these catalysts in their transesterification plants. But potassium hydroxide is used more broadly, mainly in smaller plants with an annual capacity below 5.9 million gallons. Potassium hydroxide has an active content of 90 percent to 92 percent and contains about 8 percent to 10 percent crystalline water. After the reaction is over, it returns to the glycerol phase.

Because potassium hydroxide feeds water into the reaction mixture, it is preferred for processing unrefined, economical oils or used cooking oils. Since these oils normally contain free fatty acids, they (a) require more catalysts to process. If the free fatty acid content is 3 percent, roughly twice the amount of catalyst will be needed, and (b) generate more byproducts, such as soaps, which also concentrate in the glycerol phase. The raw glycerol produced in this manner is about 50 percent pure. Even though present-day separating and decanting technology offers opportunities to purify the raw glycerol, the equipment required is relatively expensive.

By contrast, using potassium hydroxide as a catalyst and subsequently neutralizing it with sulfuric acid produces potassium sulfate as a byproduct, which can be used as an agricultural fertilizer. Because of its crystalline structure, it is relatively easy to isolate from the glycerol phase.

Alkoxide catalysts
While solid catalysts are used in plants with annual capacities of up to about 8.9 million gallons, alkoxide catalyst solutions are currently used mainly in large-scale plants in Europe and the United States with annual capacities up to 29.9 million gallons. Plants with annual capacities as small as 5 million gallons may also use these alkoxide catalysts, which are marketed as solutions in methanol. Either a 30 percent sodium methylate solution in methanol or a 32 percent potassium methylate solution in methanol is most often used. The process calls for ready-to-use solutions that can be delivered directly from the storage tank to the production process and are therefore easy to handle.

Since alkoxides are hygroscopic, they can react with moisture in the air. To prevent this, they are usually blanketed with an inert gas like nitrogen during transport and storage to protect them from external influences. Tanks, pumps and pipelines must be made of stainless steel (e.g., 1.4572). Also, since alkoxide catalysts are strongly alkaline, they should be delivered for safety reasons in a closed-circuit system. Another advantage of the closed circuit is that it doesn't contaminate the air in the workplace and eliminates the need to wear respirators to protect against dust.

Due to their hygroscopic properties, alkoxide catalysts are especially suitable for water-free processes. If the oils used as raw materials are high enough quality (water and free fatty acid content less than 0.1 percent), the process will not only produce biodiesel but also high-quality raw glycerol with a purity of at least 80 percent.

This is why, on the whole, processes based on alkoxide catalysts are more economical than processes catalyzed by sodium hydroxide or potassium hydroxide. Even though alkoxide catalysts cost more than sodium hydroxide and potassium hydroxide catalysts, this is compensated by the high added value of the glycerol byproduct and the considerably lower handling costs for the catalyst.

Summary
Solid catalysts, such as sodium hydroxide or potassium hydroxide, and alkoxide solutions can both be used to produce biodiesel from soybean or rapeseed oil (with solid catalysts being preferred for used cooking oils and waste fats). The biodiesel produced using this catalyst meets EN 14 214. Despite the higher handling costs for the catalyst, solid catalysts are economically feasible for plants with a capacity of up to about 8.9 million gallons per year, since relatively inexpensive components can be used as raw materials. Recovery of the glycerol, however, is more expensive due to the undesired byproducts of the process. The byproduct potassium sulfate can be sold.

By contrast, ready-to-use solutions in the form of alkoxide catalysts-a 30 percent sodium methylate solution in methanol or a 32 percent potassium methylate solution in methanol-are more economical for processing soybean, rapeseed or sunflower oils in plants whose capacities exceed 5 million gallons. Handling is easier and, therefore, more cost-effective. Additionally, the glycerol produced as a byproduct of biodiesel production is relatively pure, which simplifies its recovery as a marketable product. This is why about two-thirds of the large-scale biodiesel plants in Europe are designed for these catalysts.

Michael Markolwitz is a business unit manager for Degussa AG, an international specialty chemicals company. For more information on Degussa's alkoxide catalysts for biodiesel, please contact Bernard Murphy, Degussa Building Blocks. E-mail: bernard.murphy@degussa.com. Additional information can be found online at www.degussa-biodiesel.com.
 
 
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