Filtration from Feedstock to Fuel Tank

By Clay Cravens | March 17, 2008
According to the National Biodiesel Board, the estimated sales volume of biodiesel in the United States has grown from 2 million gallons in 2000 to 450 million gallons in 2007. The demand for biodiesel continues to grow. The Energy Independence and Security Act of 2007 calls for more biodiesel usage, starting with 500 million gallons in 2009 and climbing to 1 billion gallons by 2012.

A variety of filtration issues and requirements will need to be addressed in order to meet the expected demand for biodiesel. These include compatibility issues, storage and usage requirements, pre-filtration, filter and coalescer requirements, and variability in feedstocks.

While the surge in biodiesel supply and demand will create challenges for producers, it also provides opportunities to ensure purer feedstocks and ever-cleaner fuel for motorists.

While much of the consumer focus is on post-production biodiesel filtration, the process truly starts before the feedstock even enters the plant. Biodiesel production begins with pre-filters and the filtration of feedstock.

Filtration, Coalescing from Feedstock to Biodiesel
Feedstocks used by biodiesel producers in the United States are most commonly filtered through a 35- to 40-micrometer strainer. However, a high solids contamination level of particles exists at less than 35 micrometers in size. Feedstock contaminant levels can occur when particles are 100- to 2,000-parts per million by weight. Many biodiesel companies filter the incoming feedstock to reduce solids contaminants by using a combination of strainers, bag filters and micron-rated filter cartridges. Coalescing- and cartridge-style coalescers have also worked well for these applications.

High-capacity filters are proven technology for feedstock and methyl ester purification. Coalescers reduce energy costs associated with centrifuges and dryers/desiccants. Both filters and coalescers work well in many applications. However, both have significant application obstacles.

Many of these obstacles have been overcome by new media technologies. However, some process issues still need to be resolved on a case-by-case basis. Biodiesel filters, separators and coalescers have come a long way in the past two years. The performance of the separation process is dependent on the consistency of the feedstock and the transesterification process. Chemflo recommends that testing be performed on new processes because of the variance in feedstocks and processes involved in biodiesel production.

Chemflo has developed high-performance, all-synthetic filter media for biodiesel. It is important to use the correct polymers with methyl esters because of difficult chemical resistance issues.

Some customers filter methyl ester at the production facility to meet customer specifications or to remove excess solids unique to their production process. The most common method of filtering at the process level is with a strainer. Many biodiesel producers filter to a higher efficiency, either in process or at load out.

Compatibility of Filters, Coalescers
Cartridge filters are excellent for these higher efficiency separations but traditionally they have difficulty with chemical compatibility. The two most common problems are

1) pleated papers not compatible with alcohols, water and methyl esters, and

2) polypropylene media swelling as they absorb hydrocarbons, causing the cartridge life to be shortened.

The best solutions are:
1) polyester microfiber bag filters for small producers, and

2) pleated synthetic media, high-capacity cartridge filters for larger producers. Nylon should be used if carried-over caustic causes the pH to be over eight.

Metal components of filters are recommended to be stainless steel because the nickel or tin plating used on most carbon steel can react with methyl esters to form unwanted salts.

Viton fluoroelastome and Polytetrafluoroethylene are the most common gasket and o-ring materials. The most common materials for filter housings are carbon steel and stainless steel. Stainless is often preferred because of water present in the process.

Glycerin, Water, Alcohol, Caustic Removal
For larger producers, centrifuges are used most of the time to remove glycerin. However, centrifuges are expensive to operate and maintain. Coalescers and mechanical separators can reduce these operating costs by acting as an effective pre-coalescer in front of a centrifuge. Because of the wide differences in specific gravities involved, coalescers can work very well to separate methyl esters.

The most common application for coalescing in biodiesel production is water removal. These coalescers pre-coalesce in front of a dryer or desiccant system due to dissolved water issues. Because coalescers only remove free, dispersed water and do not remove dissolved water, a dryer, tower or desiccant is used after the coalescer. Coalescers can significantly reduce the operating costs of water/alcohol separations.

Caustic reduces interfacial tensions and causes emulsions that are difficult to separate. Soaps can gum up media in filters and coalescers. Adjusting the pH can help resolve this issue.

Variability of Feedstocks, Processing Biodiesel
A significant issue in separations for biodiesel producers is feedstock variability and the variability in the transesterification process itself. In Asia, where palm oil is the primary feedstock, and in Europe, where rapeseed oil is predominant, coalescers and filters tend to be used more successfully and consistently during processing than in the United States.

This is due primarily to the wide variety of feedstocks and biodiesel production processes used here. It's recommended that testing be conducted when working with a new feedstock in order to determine how it affects the separation process.

Most producers would like the filter or coalescer manufacturer to be able to tell them "this is what works," but due to the variability in feedstocks and transesterification reaction processes, the separation process must be modeled and scaled up to a specific plant's production process.

Another common issue heard recently has been that production processes have changed. Many U.S. biodiesel companies have changed suppliers or at least considered it because of impacts in feedstock costs. Several cases have occurred where proven separation methods develop inconsistent results when the feedstock changes.

Compatibility Issues
Similar to methyl esters during production, there have been significant material compatibility issues with finished biodiesel product, particularly with B100.

With prolonged exposure, B100 can degrade, soften or seep through filter media, hoses, gaskets, seals, elastomers, adhesives and plastics. Binders used in cellulose (paper) filters, polypropylene, Buna and Nitrile rubber materials are particularly vulnerable to B100. Materials such as Teflon, Viton, fluorinated plastics and Nylon are compatible with B100.

Acceptable filter vessel materials include steel, fluorinated polyethylene, fluorinated polypropylene, Teflon and epoxy binder microfiberglass.

Brass, bronze, copper, lead, tin and zinc may accelerate the oxidation of diesel and biodiesel fuels and potentially create fuel insolubles (sediments), or gels and salts when combined with some fuel components. Lead solders and zinc linings should be avoided, as should copper pipes, brass regulators and copper fittings. The fuel or the fittings will tend to change color and insolubles may plug fuel filters. Affected equipment should be replaced with stainless steel.

Handling, Storage and Pipelines
Because of B100 water content, filter vessels should be insulated. Biodiesel must be stored and handled using procedures that do not allow the temperature of the biodiesel or blend to drop below its cloud point. The fuel's cloud point, biodiesel temperature, ambient temperature, and the time the fuel is in transport or in pipelines are all factors that should be taken into consideration.

The current industry recommendation is that biodiesel be used within six months, or it should be re-analyzed after six months to ensure the fuel still meets ASTM D 6751 specifications. A longer shelf life is possible depending on the fuel composition and the use of storage-enhancing additives, but these additives can affect fuel separation.

Biodiesel is most commonly filtered first using 10- to 30-micrometer efficiency filter elements prior to load out, such as at terminals. Filters are also heated to approximately 80 to 90 degrees Fahrenheit.

Coalescers, separators or gravity settling at load out is recommended in most cases. Biodiesel filtration equipment and processes are vital to make sure that the product is very clean and reliable.

Clay Cravens is the president of Houston-based Chemflo/Process Filtration LLC. Reach him at clay@chemflo.com or (713) 691-2800.
 
 
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