Location: RER - Europe
The process “propionic acid, at plant, RER” is modelled for the production of propionic acid from ethylene in Europe. Raw materials are modelled with a stoechiometric calculation. Emissions are estimated. Energy consumptions, infrastructure and transports are calculated with standard values.
Propionic acid (CH3CH2COOH; CAS 79-09-4) is a clear, colourless liquid with a slightly pungent odour (odour threshold in air 0.03 vol %). Solid propionic acid forms monoclinic crystals and undergoes an increase in volume on melting of 12.2 %. In the liquid state and in the gas phase a considerable portion of the molecules of propionic acid are dimerized as is the case with its lighter homologues acetic and formic acids. Propionic acid is miscible with water and most organic solvents in all proportions. Propionic acid – water mixtures show a positive deviation from Raoult's law. Industrially, propionic acid is currently produced almost exclusively by three different processes:
1. Carbonylation of ethylene with carbon monoxide and water
2. Oxidation of propanal
3. Direct oxidation of hydrocarbons

Carbonylation of Ethylene (BASF Process)
In the Reppe synthesis (see Carbonylation – Monoalkenes) ethylene is reacted with carbon monoxide and water in the presence of Ni(CO)4. The reaction takes place at high pressure and is characterized by low raw material costs, high conversion, high yield, and a simple workup.

Process Description
Ethylene and carbon monoxide are compressed and continuously pumped into the high-pressure reactor together with feed solution. The crude propionic acid formed at 100 – 300 bar and 250 – 320 °C is drawn off at the head of the reactor and cooled in a heat exchanger with production of steam. Part of the cooled reaction product is recycled to the reactor for temperature regulation, the main quantity is allowed to expand and is separated into an off-gas and a crude acid stream. Nickel is recovered from the off-gas and led back into the reactor. The off-gas is incinerated with recovery of heat. The crude acid stream is subsequently dehydrated and worked up by distillation in several columns. The nickel salts thus formed are recycled into the process. The pure propionic acid is finally obtained by distillation. The product residue is channeled out of the process.

Oxidation of Propanal
The oxidation of propanal is an important route to propionic acid. This route is economically attractive although two steps (production of propanal and subsequent oxidation) are required:
Propanal is formed in large quantities as an intermediate in the production of n-propanol by hydroformylation of ethylene, and the oxidation takes place in plants in which other aliphatic carboxylic acids are also produced. A high utilization of available capacity is therefore guaranteed in both steps.

Propanal Production
The production of propanal is carried out by the hydroformylation of ethylene. It is favored because, unlike the case of the higher aldehydes, n/iso mixtures cannot be formed.
Two syntheses compete: the classical cobalt- catalyzed high-pressure carbonylation at 200 – 280 bar, and 130 – 150 °C, and the rhodium- or iridium- catalyzed low-pressure carbonylation at ca. 20 bar, and ca. 100 °C.
In the high-pressure synthesis the yield is generally impaired by partial hydrogenation of propanal to give propanol. The isolation of the aldehyde takes place, after removal of cobalt, by distillation of an azeotrope with ca. 98 % aldehyde content.
In the low-pressure synthesis, the aldehyde can be distilled directly from the reaction mixture in 99 % purity.

Oxidation
Propanal is subsequently oxidized under very mild conditions at 40 – 50 °C to propionic acid with high selectivity.
In the United States propionic acid is produced by the oxidation of propanal by Union Carbide and Eastman Kodak.

Direct Oxidation of Hydrocarbons
A large quantity of propionic acid is obtained by the direct oxidation of hydrocarbons, predominantly naphtha. In this process, which is principally used for acetic acid production, formic acid, propionic acid, and an isomeric mixture of butyric acids are formed as byproducts. Whether this process is considered economic for propionic acid production is principally a question of the market evaluation of the different products. The process has favorable raw material costs, but requires a relatively complex workup of the product mixture.

BP Chemicals Process.
Naphtha is preheated to 170 °C and oxidized with air at 40 – 45 bar in several reactors in series. The heat of reaction is used for steam generation. The cooled discharge from the reactors is separated from the reaction off-gas in a separator. Entrained liquid is recovered from the off-gas and recycled to the reactor; the off-gas is then incinerated. The liquid reactor discharge is separated into an organic phase, which contains unreacted hydrocarbons, and an aqueous phase which contains the product mixture. The unreacted hydrocarbons are recycled to the reactor. The low- and high-boilers are separated from the aqueous phase which then gives the crude acid. From this the C1- to C4-acids are obtained by extractive dehydration followed by fractional distillation.

Propionic acid is produced by direct oxidation at BP Chemicals (UK), Hoechst – Celanese (United States), and Daicel (Japan).

Use as Food and Feed Preservative
Propionic acid and its salts are broad-spectrum preservatives because of their bactericidal, fungicidal, insecticidal, and antiviral effects, and their acaricidal effect which is found at higher concentrations.

Other Uses
Zinc, cadmium, lead, and mercury propionates can be used as vulcanization regulators in rubber production.
Propionic acid is used as an intermediate, frequently in the form of its anhydride, for the production of esters. The most important esters are cellulose acetate propionate, from which thermoplastics are produced, and vinyl propionate ( Vinyl Esters) which is used as a basic monomer for dispersions
Methyl, ethyl, propyl, and butyl propionates are used as solvents for resins and paints. Because of their characteristic fruity aromas various propionate esters are also used in flavors and fragrances.
Small quantities of propionic acid are also further processed to propionyl chloride, which is used as a reactive intermediate for the introduction of the propionyl group in syntheses.
Until the end of the 1980s a considerable proportion of propionic acid was chlorinated and further processed to form herbicides. With the arrival of more potent, optically active herbicides and of products with a broader spectrum of activity, the importance of racemic herbicides based on propionic acid has decreased considerably.

Frischknecht R., Jungbluth N., Althaus H.-J., Doka G., Dones R., Heck T., Hellweg S., Hischier R., Nemecek T., Rebitzer G. and Spielmann M. (2007) Overview and Methodology. Final report ecoinvent v2.0 No. 1. Swiss Centre for Life Cycle Inventories, Dübendorf, CH, retrieved from: www.ecoinvent.org.

Gendorf (2000) Umwelterklärung 2000, Werk Gendorf. Werk Gendorf, Burgkirchen as pdf-File under: http://www.gendorf.de/pdf/umwelterklaerung2000.pdf

Ulf-Rainer Samel, Walter Kohler, Armin Otto Gamer, Ullrich Keuser: Propionic Acid and Derivatives. Published online: 2003. In: Ullmann's Encyclopedia of Industrial Chemistry, Seventh Edition, 2004 Electronic Release (ed. Fiedler E., Grossmann G., Kersebohm D., Weiss G. and Witte C.). 7 th Electronic Release Edition. Wiley InterScience, New York, Online-Version under: DOI: 10.1002/14356007.a22_223

Undefined unit processes (UPRs) are the unlinked, multi-product activity datasets that form the basis for all of the system models available in the ecoinvent database. This is the way the datasets are obtained and entered into the database by the data providers. These activity datasets are useful for investigating the environmental impacts of a specific activity (gate-to-gate), without regard to its upstream or downstream impacts.