Glycerol

by John

Glycerol Basics
Chemistry
Glycerol Recovery from Biodiesel Production
Simple Calculation
Crude Titration

Glycerol Basics
Glycerol, or glycerine, is a clear, colorless, odorless liquid with a thick, syrupy texture. It is oily to the touch and very sweet to the taste. It is an alcohol with three hydroxyl (OH) groups. Other synonyms for glycerol are 1,2,3-Trihydroxypropane or 1,2,3-propanetriol. It is soluble in water and alcohol but insoluble in ether, benzene, or oils.

Chemistry
Glycerol C3H8O3

Melting Point	20 ° C
Boiling Point	290 ° C
Flash Point	177 ° C
Specific Gravity	1.26 (20/20 ° C)

Glycerol is chemically stable under normal storage and handling conditions. It has no self-reactivity, spontaneous combustibility, or explosivity properties. However, it may become explosive when in contact with strong oxidizing agents such as potassium chlorate or potassium permanganate. (see Wikipedia and Sakamoto Yakuhin Kogyo Co. for more information)

Glycerol Recovery from Biodiesel Production
Glycerol is produced as a byproduct of the transesterification reaction conducted during production of biodiesel. This glycerol byproduct is in a crude form and is mixed with varying amounts of soap, alcohol, catalyst, and water. This mixture can be in a liquid or solid form, and range in color from transparent and light yellow to opaque and dark brown. From a single stage base catalyzed reaction using waste fryer oil feedstock, the amount of actual glycerol in the mixture is typically around 40% with the majority of the rest of it being soap. Although some soap will always be produced during the biodiesel process, the amount of soap produced depends heavily on the quantity of free fatty acids (FFAs) and water present in the feedstock. Biodiesel made from low FFA, low water feedstock using minimum amounts of catalyst will produce less soap in the byproduct than biodiesel made from feedstock with high water and FFA content using lots of catalyst. Soap production is the primary factor that determines the appearance and texture of the mixture. The more soap produced, the darker and thicker the byproduct, although soap produced from a KOH-catalyzed reaction will not actually solidify, whereas soap from a NaOH-catalyzed reaction can. There are techniques to reduce the water and FFA content, the most common of which that is applicable at small scales is a two stage, acid-base method that uses sulfuric acid during the pretreatment stage.

Regardless of the method used during biodiesel production, most of the glycerol can be recovered from the byproduct by using a technique called acidulation. This process uses phosphoric acid (H3PO4) to separate the soap into free fatty acids and salts. This method results in separation of the byproduct into three distinct layers: an upper layer of FFAs, a middle layer of glycerol, and a bottom layer of salts. According to Addison and Swearingen this method can recover glycerol to 95% purity, and should be performed prior to methanol removal from the byproduct, as the alcohol present in the mixture lowers the viscosity of the mixture and facilitates the separation of the salts and FFAs. However, Alovert suggests performing this technique only after alcohol has been removed from the mixture, and that a purity of 80% can be expected for the glycerol.

The soap formed during biodiesel production is a result of the catalyst reacting with fatty acids. This results in molecules that consist of sodium or potassium (depending on whether NaOH or KOH catalyst was used) bonded with a fatty acid. During acidulation, the phosphoric acid reacts with the sodium or potassium to create sodium phosphate or potassium phosphate. The H+ ion bonds with the fatty acid to form a free fatty acid.

For potassium phosphate (KOH catalyst) where R is the fatty acid:

H3PO4	+	KR	-->	HR	+	KH2PO4
Phosphoric Acid	+	Soap	-->	FFA	+	Potassium Phosphate

The process for this method is rather straightforward. Simply add the appropriate amount of phosphoric acid, mix thoroughly, allow to settle into three layers, then separate the components. The FFAs and the salts can be composted and used as fertilizer. The FFAs could possibly be burned as a fuel, but this most likely needs more experimentation to determine the appropriate applications it is suitable for. The glycerol will be heavily acidified by this process, and will also contain colorants, water, and alcohol (unless the alcohol was removed prior to this process). The acids can be neutralized easily with diluted caustic soda or baking soda, resulting in a crude, industrial-grade glycerol. The glycerol can be distilled to 100% pure, but this is a very costly and difficult process.

The most difficult part of the acidulation process is determining the appropriate amount of phosphoric acid to use. Here are a few different techniques to try, but it may be useful to experiment with other techniques as well.
The following techniques are taken from Journey to Forever and Girl Mark's Biodiesel Homebrewer's Guide

Simple Calculation
For 85% strength phosphoric acid, use 1.5 to 1.7 ml per gram NaOH used (a different amount would be used for KOH, but there was no suggested amount for it). Although traces of catalyst remain in the biodiesel, the majority ends up in the glycerol mixture, so during the calculation use the full amount of catalyst used during biodiesel production.

Crude Titration
This method might work better when working with glycerol accumulated from multiple batches, since feedstocks vary greatly from batch to batch, and therefore the quantity of soap left in the glycerol layer.

Take a 250 ml sample of glycerol layer and put it into a jar. Add 10 ml of 85% phosphoric acid solution. Shake the jar vigorously. The phosphates should begin to form as a white substance. Continue to add phosphoric acid in 5 ml increments until the phosphates form fine particulates and begin to precipitate out. This occurs when an excess of acid is added. This is not a problem and will cause the separation to occur faster, but is a waste of phosphoric acid and will require more caustic or baking soda to neutralize the finished product. To determine the minimum amount of phosphoric acid necessary, perform several more titrations with slightly less amounts of phosphoric acid in each, lowering the amount of acid successively in 2 ml increments. Then observe the samples after 24 hours and determine the point at which the samples begin to separate. Use this to calculate the amount required for the entire batch of glycerol layer to be processed.