Dry washing

Introduction

An alternative to water washing is dry washing, so called because no water is used in the removal of soaps from the biodiesel.

This is considered a "final polish" stage, after the majority of soap has been allowed to settle out naturally following Demething.

Dry washing involves passing the biodiesel through an ion exchange resin or hardwood filter medium.

Ion Exchange

Because the ASTM D6751 standard for biodiesel has a greater tolerance for Free Fatty Acid content than soap, an ion exchange resin can be employed to convert any remaining soap in the fuel into Free Fatty Acids.

This is, however, to the detriment of the acid number of the final product.

Hardwood

If the biodiesel is passed through a tower packed with hardwood shavings, the soaps becomes trapped within the course structure of the shavings and sawdust. Any trace Methanol is also absorbed by the medium.

Typically, a 10 litre tower should have biodiesel pumped or gravity fed through it, bottom to top, at a rate of 3 litres per minute. If the fuel is well settled before filtering, 10 litres of medium should be capable of filtering 1000 litres of biodiesel.

• Example towers
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  A tower made out of 110mm drain pipe and stepped solvent weld tails

• 

  Another tower with tank connectors for inlet and outlet

• 

  Using a stocking to contain the drywash medium