Acid-Catalyzed Esterification

Springboard Biodiesel
  • Home
  • Products
    • BioPro 190 Biodiesel Processors
    • BioPro 380 Biodiesel Processors
    • Glycerin and Biodiesel Demethylation
    • CLL Biodiesel Production System
    • SpringPro T76
    • INCOSEP Accelerator
    • BioPro Processor Accessories >
      • Biodiesel Fueling Station
      • SpringFlow 250 Heat Exchanger
      • BioPro Pressurized Water Kit
      • Oxidative Stabilizer
      • WVO Transfer Pump
      • BD ZorbX Drywash Media
      • Methanol Pump
      • Oil Collection Systems
      • Fuel Nozzles
      • Screw Plug Immersion Heaters
      • DW-R10 Ion Exchange Resin
    • Return on Investment Calculator
    • Financing
    • BioPro Processor Gallery
  • Store
  • Library
    • Why a BioPro
    • Biodiesel Basics >
      • What Is Biodiesel
      • How the BP190 Works
      • Biodiesel Production
      • Technical Specs BioPro 190
      • Technical Specs for the BioPro 380
      • How the BP380 Works
      • Safety Features of BioPro Processors
      • Biodiesel Processor Basic Ingredients
      • Biodiesel Safety Information
      • Facts and Myths on Making Biodiesel
      • Biodiesel Quality
      • Benefits of BioPro Biodiesel Processors
      • Springboard Biodiesel Go/No-Go Kit
      • Biodiesel Frequently Asked Questions
    • Biodiesel Questions >
      • SVO vs Biodiesel
      • Biodiesel and Engine Life
      • Testing Small-Scale Biodiesel Quality
      • Preparing Feedstock for Biodiesel Production
      • Beef Tallow and Hydrogenated Oils
      • Methanol Vapors in BioPro Biodiesel Processors
      • Glycerin Layer in Biodiesel Production
      • Storing Biodiesel
      • Cold Weather Biodiesel Washing
      • Winter Storage and Retrieval
      • Storage for BioPro Processors
      • SpringPro T76 FAQ
    • More Information on BioPro Processors >
      • Acid-Catalyzed Esterification
      • Titration Batch Processing
      • Titration Methodologies
      • Reducing the CO2 Footprint
      • Biodiesel Mandates and Initiatives
      • ASTM
      • Reducing Carbon Footprint with Biodiesel
    • BioPro Videos
  • Markets
    • Biodiesel for Farmers
    • Biodiesel for Casinos
    • Biodiesel for Municipalities
    • Biodiesel for Schools >
      • Schools using BioPro Technology
    • Biodiesel for Restaurants
    • Biodiesel for the Military
    • Other Markets
    • Testimonials from BioPro Owners
  • About Us
    • About Springboard Biodiesel
    • Contact Springboard Biodiesel

Adding acid to reduce acidity in feedstock oils: the counterintuitive process of acid-catalyzed esterification explained

Possibly the single most common chemical obstacle encountered by producers of biodiesel is the presence of free fatty acids (FFAs) in the feedstock (vegetable or animal oil, including but not limited to used cooking oil).  In its most common form, biodiesel production utilizes a basic catalyst such as NaOH, KOH, or sodium methoxide to catalyze the transesterification reaction.  These catalysts react rapidly with free fatty acids to form soaps.  Not only does this consume the catalysts before they have a chance to catalyze the transesterification but it forms a large amount of soaps that can be nearly impossible to remove.  Upon neutralization, every 1% FFAs in the feedstock produce approximately 1.11% soaps.  Depending on the method of product purification, every 1% of soaps formed during the transesterification reaction, reduces the final product yield by at least 2%.  Considering the fact that yellow grease can have up to 15% FFA content, it is essential to reduce this concentration before starting the transesterification reaction. 

The most common way that this is performed for relatively low concentrations of FFAs is through a process known as acid-catalyzed esterification.  In this process, an alcohol (typically methanol) is added to the feedstock along with an acid catalyst (most commonly sulfuric acid).  The resulting esterification reaction produces methyl esters (biodiesel) and water with the following reaction.

Notice that while on the products side of the reaction, both species are relatively neutral, in the reactants side, we have an acidic species (the FFA).  Thus, the further the reaction progresses, the less acidic the overall mixture will be.  As can be seen though, this is a reversible reaction.  This means, that as the concentration of products (biodiesel and water) increases and the quantity of reactants (FFA and Methanol) decreases, the reaction will find and equilibrium and cease to progress forward toward the products.*

Considering these factors, it becomes apparent that it is very important to have the feedstock as dry as possible when starting the reaction so that it may progress forward as far as possible and reduce the FFA content of the feedstock as much as possible (see graph on relationship between FFA and water).  In cases where the feedstock starts out with high water content, the reaction does not progress forward at all or can even have the net effect of increasing the acidity of the feedstock.  This effect can often be seen with as little as 0.5% water content in the feedstock.

It is also apparent, that to the extent that costs allow, it is worthwhile to add a large excess of methanol in order to drive the reaction equilibrium toward the products as much as possible.

Finally, even if the feedstock is very dry to start with, and there is a large excess of methanol, the reaction will always reach equilibrium before all of the FFA are consumed.  The higher the starting FFA content of the feedstock, the more FFAs will remain after the esterification.  Furthermore, the water produced in this reaction also interferes to some degree with the subsequent transesterification reaction.  Thus, it is rare to see starting levels for FFAs above 10% pretreated in this way in the biodiesel industry.
Acidity over time during esterification in biodiesel production

Attached we have a graph of the acidity of a sample of feedstock mixed with 30% methanol (vol/vol). We see a slight increase in the average total acid number (given in mgKOH/g fluid) of the mixture during the first 30 minutes due to the acidity of the sulfuric acid used as a catalyst. Within 4 hours of commencing the reaction though, the total acidity of the mixture is it only about 1/7th of the original acidity of the feedstock. Most of this remaining acidity is due to the residual sulfuric acid catalyst. This sulfuric acid is quickly neutralized to form Na2SO4 (aka salt) at the start of the base catalyzed reaction. This salt is very easily removed from the fuel and does not significantly affect the final product yield.

*In this particular reaction it is almost exclusively water that hinders the forward progress. The concentration of the biodiesel seems to have very little effect. This is due to their differences in solubility with the methanol and H2SO4.

Learn More About Biodiesel

  • Glossary of biodiesel terms
  • Biodiesel library
  • BioPro videos​
  • Customer Testimonials

About Springboard Biodiesel

  • Who are we?
  • The Springboard Biodiesel Network
  • Call us at +1.530.894.1793
  • Home
  • Products
    • BioPro 190 Biodiesel Processors
    • BioPro 380 Biodiesel Processors
    • Glycerin and Biodiesel Demethylation
    • CLL Biodiesel Production System
    • SpringPro T76
    • INCOSEP Accelerator
    • BioPro Processor Accessories >
      • Biodiesel Fueling Station
      • SpringFlow 250 Heat Exchanger
      • BioPro Pressurized Water Kit
      • Oxidative Stabilizer
      • WVO Transfer Pump
      • BD ZorbX Drywash Media
      • Methanol Pump
      • Oil Collection Systems
      • Fuel Nozzles
      • Screw Plug Immersion Heaters
      • DW-R10 Ion Exchange Resin
    • Return on Investment Calculator
    • Financing
    • BioPro Processor Gallery
  • Store
  • Library
    • Why a BioPro
    • Biodiesel Basics >
      • What Is Biodiesel
      • How the BP190 Works
      • Biodiesel Production
      • Technical Specs BioPro 190
      • Technical Specs for the BioPro 380
      • How the BP380 Works
      • Safety Features of BioPro Processors
      • Biodiesel Processor Basic Ingredients
      • Biodiesel Safety Information
      • Facts and Myths on Making Biodiesel
      • Biodiesel Quality
      • Benefits of BioPro Biodiesel Processors
      • Springboard Biodiesel Go/No-Go Kit
      • Biodiesel Frequently Asked Questions
    • Biodiesel Questions >
      • SVO vs Biodiesel
      • Biodiesel and Engine Life
      • Testing Small-Scale Biodiesel Quality
      • Preparing Feedstock for Biodiesel Production
      • Beef Tallow and Hydrogenated Oils
      • Methanol Vapors in BioPro Biodiesel Processors
      • Glycerin Layer in Biodiesel Production
      • Storing Biodiesel
      • Cold Weather Biodiesel Washing
      • Winter Storage and Retrieval
      • Storage for BioPro Processors
      • SpringPro T76 FAQ
    • More Information on BioPro Processors >
      • Acid-Catalyzed Esterification
      • Titration Batch Processing
      • Titration Methodologies
      • Reducing the CO2 Footprint
      • Biodiesel Mandates and Initiatives
      • ASTM
      • Reducing Carbon Footprint with Biodiesel
    • BioPro Videos
  • Markets
    • Biodiesel for Farmers
    • Biodiesel for Casinos
    • Biodiesel for Municipalities
    • Biodiesel for Schools >
      • Schools using BioPro Technology
    • Biodiesel for Restaurants
    • Biodiesel for the Military
    • Other Markets
    • Testimonials from BioPro Owners
  • About Us
    • About Springboard Biodiesel
    • Contact Springboard Biodiesel