Acrylic Acid Reactor Technology
Publication Date: 2005-Jun-01
The IP.com Prior Art Database
Acrylic acid is manufactured on a commercial scale by the two stage propylene oxidation route, using a vapor phase heterogeneous catalysis system. The key to this technology was the development of highly active and very selective catalysts. In the first reaction stage, propylene is oxidized with air to acrolein and then fed directly to the second stage where the acrolein is further oxidized with air to acrylic acid. The first stage catalyst is a mixed metal oxide catalyst composed of mainly molybdenum and bismuth oxides with several other metals. The second stage catalyst is also a complex mixed metal oxide catalyst composed primarily of molybdenum and vanadium. The two different catalysts have been optimized for their individual performance. Acrylic acid yields of 80-90% from propylene have been realized for these commercial catalyst systems. Acrylic acid is produced on a commercial scale in a shell and tube type reactor with either a tandem reactor system (i.e. two different reactors in series, one for the first stage and one for the second stage) or a single reactor design. The single reactor design can be either a single tube with one long tube containing both catalysts stacked one on top of the other separated by an inert layer, or a two tube sheet design where two separate tubes for the different catalysts are separated by an interstage. Some reactors have the same tube length for the first and second stage tubes while other reactors may have different lengths. The lower section of the tube is packed with inerts and is used as a cool down zone before the gases enter the interstage. A portion of this cool down zone can be packed with second stage catalyst. This is referred to as an extended bed. Catalyst in this area can serve several functions. It helps control interstage cool flames by decreasing the acrolein and acetaldehyde content of the gases in the interstage. It also increases the total inventory of second stage catalyst in the reactor, thus increasing the lifetime of the catalyst.
Acrylic acid and the commodity acrylate esters (methyl, ethyl, butyl, and 2-ethylhexyl) comprise one of the most versatile monomer series for controlling polymer performance characteristics. These monomers all have the alpha beta unsaturated carboxyl structure and find extensive applications in surface coatings, adhesives and plastics. Furthermore, the sodium salt of polyacrylic acid is the superabsorbent polymer found in baby diapers. World production capacity for crude acrylic acid is almost eight billion pounds per year.
In 1843, Redtenbacher reported the first synthesis of acrylic acid. This was accomplished via an air oxidation of acrolein. A hundred years later the first commercial production of acrylic acid was developed using acetylene as the raw material. Since this time acrylic acid has been manufactured on a commercial scale using several different technologies and raw materials. These processes include the acrylonitrile hydrolysis route, the beta-propiolactone dehydration route, and the ethylene cyanohydrin hydrolysis/dehydration route. All of these technologies have been replaced by the two stage propylene oxidation route. This is a vapor phase heterogeneous catalysis system.
Propylene is a relatively inexpensive feed stock for the production of acrylic acid. Also the process adds cheap weight (i.e. oxygen from air) to the propylene as it is converted to acrylic acid. In fact 100 pounds of propylene has a theoretical yield of 171 pounds of acrylic acid. The key to this technology was the development of highly active and very selective catalysts. In the first stage propylene is oxidized with air to acrolein and then fed directly to the second stage where the acrolein is further oxidized with air to acrylic acid.
The feed gas to the acrylic acid reactors is typically 6 – 9 vol% propylene and 12 -15 vol % oxygen (coming from air) with a make up of either recycle gas or low pressure steam. The steam (or recycle gas) is added as a diluent to avoid forming a flammable mixture of propylene and oxygen. Typically the mixture is kept on the fuel rich side of the flammable envelope. A stoichiometric excess of oxygen is normally fed to the reactors to prevent reduction of the catalyst. The oxygen to propylene molar ratio is generally held between 1.6 and 2.0 which means that the exit gases contain oxygen.
The first stage catalyst is a mixed metal oxide catalyst composed of mainly molybdenum and bismuth oxides with several other metals. The second stage catalyst is also a complex mixed metal oxide catalyst composed primarily of molybdenum and vanadium. Several other components have been incorporated in the catalyst to optimize activity and selectivity. Both first and second stage catalysts are commercially available as either extruded pellets or supported on inert spheres. The two different catalysts have been optimized for their individual performance. Acrylic acid...