(445c) 2-Acetoxy Propionic Acid Using Reactive Distillation – An Intermediate in Synthesis of Bio-Acrylic Acid From Lactic Acid

Acrylic acid, its esters, and acrylate polymers are high volume chemicals and critical chemical building blocks used in a wide gamut of applications including superabsorbents, hard plastics, paints, detergents, and adhesives. The global acrylic acid market is estimated at 4.5 million tons and $12.4 billion annually in 2012, with a growth rate of about 4% per year. 

Acrylic acid synthesized from biobased lactic acid is a drop-in replacement for the petroleum-based product synthesized from propylene.  Lactic acid is a mature biobased feed stock with a proven track record of high yield (>95%) from glucose at a low cost. 

Synthesis of biobased acrylic acid from lactic acid involves a two-step process.  In the first step, lactic acid is reacted with acetic acid to form 2-acetoxy propionic acid (2-APA).  In the second step of the process, 2-APA is cracked under high temperature to form acrylic acid.  The first step reaction is thermodynamically unfavorable, with an equilibrium constant of 0.25, and results in poor conversions if carried out in a batch reactor even with high mole ratios of acetic acid to lactic acid.

In this work, the selective formation of 2-APA by reaction of lactic acid with acetic acid has been carried out using continuous reactive distillation.  The reaction takes place in a six-meter tall, 50 mm ID Pyrex column operated at atmospheric pressure, and is catalyzed by Amberlyst-15 cation exchange resin supported inside Sulzer Katapak-SP11 structured packing.  Lactic acid feed stocks from 20 wt% to 85 wt% in water were used in experiments along with glacial acetic acid.  High conversion of lactic acid and its oligomers were observed (>98%).  When 20% lactic acid was used, a selectivity of 96% to 2-APA was obtained. 

An unusual observation in the experimental work was the enhancement  of the acetic acid – water separation by the presence of lactic acid.  Because of this enhancement, it is possible to obtain a distillate stream containing 99% water from the reactive distillation column while retaining acetic acid within the column where reaction occurs.Ternary VLE experiments were performed to obtain the NRTL parameters describing this system.

Experimental studies are complemented by process analysis using AspenPlus simulation software.  By implementing a pseudo-homogeneous kinetic model generated in our laboratories from batch experimental kinetic data, and descsribing vapor-liquid equilibria based on experimental data, we are able to predict the experimentally observed pilot-scale column behavior. 

It can be concluded that 2-APA can be synthesized very efficiently from aqueous lactic acid solutions by reaction with acetic acid using a continuous reactive distillation column.