Bio-acrylic: glycerol to acrylic acid and butyl acrylate

Biodiesel glycerol (5 mol% in water, the dilute crude a transesterification plant actually sells) is vaporised and dehydrated over an acid catalyst at 553 K to acrolein, oxidised with air at 573 K to acrylic acid, quenched, dewatered in a vacuum column, and esterified with n-butanol to n-butyl acrylate: the two-step route from a renewable C3 to the acrylic monomer, written out so its water, oxygen and carbon bookkeeping can be checked end to end. Each reaction is a conversion reactor with an exact element balance: dehydration (glycerol -> acrolein + 2 H2O), the competing C-C cleavage that every gas-phase dehydration catalyst shows (glycerol -> acetaldehyde + CO + H2 + H2O), selective oxidation (acrolein + 1/2 O2 -> acrylic acid), over-oxidation (acrolein + 3/2 O2 -> acetic acid + CO2), and esterification (acrylic acid + n-butanol -> n-butyl acrylate + H2O). A knock-out flash after dehydration rejects most of the water before the oxidiser; the quench liquid goes to a 0.15 atm acrylic-acid column (vacuum because acrylic acid polymerises when hot, which is also why its inhibitor profile is the question the column-inhibition analysis answers on this column); a caustic wash takes the unreacted acids out of the ester before a 0.2 atm butyl-acrylate column.

What the flowsheet shows is where the water goes: 102 mol/s of it arrive with 5 mol/s of glycerol, the dehydration makes two more per acrolein, and nearly all of it has to be removed before the acid is concentrated, which is the energy cost that decides whether a bio-acrylic plant is viable. Solved: the vacuum column delivers a crude acid of 87 mol% acrylic acid (11% water, 2% acetic) and loses about 7% of the acid overhead with the water; the ester column delivers 94 mol% n-butyl acrylate, the balance unreacted butanol.

Honesty notes: (1) The conversions are stated screening INPUTS, not predictions: 75% of the glycerol to acrolein (with the C-C cleavage taking most of the rest), 90% of the acrolein to acid with half the remainder over-oxidised, 95% esterification. They sit inside the ranges reviewed by Katryniok et al., Green Chem. 12 (2010) 2079 and Ullmann's 'Acrylic Acid and Derivatives', and are not a fit to any one catalyst. The real byproduct slate (hydroxyacetone, propanal, allyl alcohol, formaldehyde, phenols, coke) is lumped into acetaldehyde + CO + H2: hydroxyacetone has no ideal-gas Cp in the bundled data and formaldehyde has no NRTL binaries here, so writing them in would have meant inventing properties. (2) No heats of reaction are given: the reactors are isothermal and their duties carry sensible heat only, so the oxidiser's large exotherm (and the salt-bath cooling it needs) is not in the reported duty. (3) The acid section runs on NRTL with the permanent gases as Henry's-law solutes; the esterification, wash and ester column run on Peng-Robinson, because n-butyl acrylate has no ideal-gas heat capacity the activity-model enthalpy path can use. (4) The caustic wash is a fixed-recovery separator, not a reactive extraction, and the esterification is a single pass with no water removal, so the ester column carries the unreacted butanol and water overhead rather than recycling them. (5) The acrylic-acid column's product is the crude a real plant would still finish in a separate glacial column. No acrylic polymerisation kinetics ship; the inhibitor analysis takes them as an input.