http://www.biodieselmagazine.com/articles/76504/the-chemical-kinetics-of-glycerolysis
[multiple images in on-line article]
The Chemical Kinetics of Glycerolysis
Glycerolysis on high-FFA, low-grade feedstock at varying temperatures
will result in the same end product, but higher heat will triple throughput
By Erik Anderson | May 15, 2014
The question of what oil pretreatment method is the best is an ongoing
conversation among biodiesel producers and engineers. Most low-grade
feed oils contain high levels of free fatty acids (FFA), which can cause
soap formation in traditional biodiesel processes. Several different
pretreatment methods are used commercially to assimilate or remove FFA:
acid esterification, vacuum steam stripping, caustic washing and
glycerolysis; we consider enzymatics to be at the precommercial stage.
The most prevalent pretreatment method has traditionally been acid
esterification, since it can be done at relatively low process
temperatures. The goal of acid esterification is the direct conversion
of FFA into methyl esters (biodiesel) using sulfuric acid as a catalyst,
with an excess of methanol. During acid esterification, each mole of
fatty acid converted to methyl esters produces one mole of water. The
resulting wet methanol must then be decanted, neutralized and dried via
fractional distillation with high reflux rates, before it can be reused.
Methanol drying columns can cost millions of dollars and are the biggest
users of plant energy. By not having to dry wet methanol after acid
esterification, biodiesel plants can cut their thermal energy
consumption in half.
Alternatively, glycerolysis reduces the amount of FFA in low-grade oils
without use of acid or methanol, and enables them to be converted into
final product, rather than removing them and reducing product yield. The
resulting glycerides formed during glycerolysis are then converted
directly to biodiesel via base-catalyzed transesterification. Also,
glycerolysis is done at high enough temperatures to completely dry the
feed oil before the transesterification process, thus avoiding the
formation of excess soaps and the decanting problems that can result.
Over the past decade, glycerolysis has continued to grow in popularity
among those companies successful in the industry. For example, recent
articles in several industry periodicals have noted that some biodiesel
producers have been using glycerolysis successfully for several years
(e.g., Renewable Energy Group’s Seneca, Ill., plant).
The rate of the glycerolysis reaction is determined by two variables:
the initial concentration of FFA and temperature. Many biodiesel plants
run their processes using steam heating systems, and are limited to
operating temperatures of 350 degrees Fahrenheit or less. Although
glycerolysis can be run at these lower temperatures, reaction kinetics
are vastly improved when run at temperatures at or above 450 F. However,
operating temperatures approaching 500 F are not recommended due to
possible glycerin decomposition, forming acrolein.
Some biodiesel producers may not be familiar or comfortable with
high-temperature processes, and therefore tend to shy away from the use
of thermal oil heating systems needed for glycerolysis operating
temperatures. This concern over the use of hot oil systems is due to a
lack of industrial experience, particularly with oleochemicals.
Another benefit from glycerolysis is its simplicity. The only reagent
needed for successful glycerolysis is glycerin, the byproduct of
transesterification. In plants using glycerolysis, the glycerin produced
during transesterification can be recycled back into the process, and
the excess glycerin can be refined for sale as a valuable byproduct.
Research at Superior Process Technologies was done to compare
glycerolysis at various operational temperatures. Multiple laboratory
batch-wise glycerolysis reactions were performed on brown grease at 350
F and 460 F, representing steam-heated and thermal-oil-heated systems.
The lab work and data analysis was performed by Chris Sorensen with SPT.
Samples were taken over the course of the reaction and run on a Gas
Chromatograph-Flame Ionization Detector to determine compositional
makeup versus reaction time. The initial brown grease was determined to
have an acid number of 100 (with 50 percent FFA) via wet chemistry
titration prior to glycerolysis. Each batch was brought up to their
respective temperatures under inert conditions using a nitrogen purge
before charging glycerin. Eight batches were run in total at varying
temperatures and FFA concentrations. By graphing the average acid
numbers versus time of each reaction, the difference in the rate of FFA
reduction can be seen in Figure 1.
At 460 F, the FFA concentration is lowered rapidly in the first hour,
and well below 1 percent FFA within several hours. In comparison, the
reaction at 350 F did lower the FFA below 2
