Amanda Lee
Louisiana State University <!doctype html public
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Manufacturing
Maleic Anhydride
Introduction
Maleic anhydride has numerous industrial uses and is of significant commercial interest worldwide. The primary use of maleic anhydride is in the manufacture of polyester and alkyd resins. These resins are added to fiberglass reinforced plastics to make a strong, lightweight, and corrosion resistant material that is found in boats, autos, trucks, pipelines and electrical goods.
In a secondary capacity, maleic anhydride (MA) is employed in the manufacture of lacquers, lube-oil additives, and agricultural products. The addition of MA to drying oils decreases the required drying time and improves the coating quality of lacquers; dispersants derived from MA prolong oil change intervals and improve the efficiency of automotive engines. Agricultural products made from MA include herbicides, pesticides, and plant growth regulators. Furthermore, fumaric and maleic acid are important MA derivatives used in paper sizing resins and as food and beverage acidulants.
Maleic anhydride was first synthesized in the 1830’s, but was not manufactured commercially until about 1930. Prior to 1930, MA was formed only in small quantities as a by-product of the phthalic anhydride process. The advent of patents for the catalytic oxidation of benzene coupled with improvements in the vanadium oxide catalysts was integral to the creation of a commercially viable process for MA. Today, nearly three billion pounds of MA are made each year throughout the world using variations of this process.
Initially, maleic anhydride was produced by the partial oxidation of benzene using a vanadium oxide catalyst. In brief, the process involves the oxidation of a low concentration of benzene in air followed by a separation step to recover the MA from the reactor effluent. Because benzene is a hazardous chemical strictly regulated by the EPA and OSHA, efforts to find a suitable replacement have been pursued. Over the years, both n-butane and butylene have been utilized with increasingly successful results and are gradually overtaking benzene as the reactant of choice. Following is a discussion of the traditional benzene process along with the newer n-butane process, and the catalysts employed by each.
Benzene Oxidation Process
Reaction Details
The traditional process begins by mixing benzene with an excess of air to give concentrations from 1-1.4 mole %. A low benzene concentration must be utilized in order not to exceed the flammability limit of the mixture. The reaction gas mixture then passes over the catalyst in a multitubular fixed-bed reactor at an optimum pressure range of 0.15-0.25 MPa. The desired reaction, as given below,
+ 9/2 O2 + 2 CO2 + 2 H2O
is highly exothermic, causing "hot spots" of 340-500° C to occur in the catalyst. Approximately 27MJ of heat are generated per ton of benzene reacted. Molten eutectic salts that circulate around the reactor tubes are primarily responsible for dissipating the heat. Steam is generated when the molten salts are cooled and can be used to drive air compressors or in other plant operations. View the process flow diagram here.
Recovery Steps
Upon exiting the reactor, the vapor mixture is cooled to 150-160° C by heat exchangers. Partial condensers further cool the gas to 55° C, the melting point of maleic anhydride, to recover between 40-60% of the maleic anhydride as liquid. The condensed maleic anhydride must be removed as soon as possible to avoid prolonged contact with the water in the reaction gas. Exposure of liquid maleic anhydride to water results in the undesired formation of maleic acid that further limits recovery if allowed to continue for an extended period of time. The maleic anhydride that cannot be recovered is eventually washed out with water as maleic acid. Water scrubbing and subsequent dehydration of the maleic acid stream is required to purify and reform the remaining maleic anhydride.
Catalyst
As a rule, commercial catalysts for benzene oxidation are supported by an alumina or silica carrier and have a surface area of 1-2 m2/g. A typical catalyst is comprised of V2O5, MoO3, and, occasionally, a small amount of Na2O. The catalyst may be modified with a promoter to increase the conversion, yield, and selectivity. Molybdenum is the most popular modifying element, but phosphorus, alkali and alkaline earth metals, tin, boron, and silver, among others, are also used.
Benzene is passed through the catalyst at 60-130 g benzene/L catalyst/h. In operation, 97-98% conversion is routinely achieved with an initial selectivity of over 74%. The lifetime of the catalyst can be as long as 4 years depending mainly on the reactor operating temperature and the purity of the starting materials.
Butane Oxidation Process
Recent processes for the manufacture of maleic anhydride employ C4 hydrocarbons, such as n-butane and n-butylene, as feedstocks. Even existing processes originally using benzene as a reactant are being converted to the C4 hydrocarbons. A major advantage of C4 HC over benzene is that no carbon is lost in the reaction to form MA. For a theoretically achievable conversion of 100%, the yield from butane is a third greater than that from benzene. From a raw material viewpoint, the relatively low purchase price of C4 is much more attractive than the expense of benzene.
Other influential factors that favor C4s are safety, health, and the environment. Benzene is a known carcinogen and one of the chemicals most stringently regulated by the government. The flammability limits for C4 HC are also lower than those for benzene, which is an additional safety advantage of the process. For all of these reasons, the fixed-bed process with n-butane has been the only MA route used commercially since 1985 in the United States.
Reaction Details
The
commercially predominant butane oxidation process employs a fixed-bed, but
processes using fluidized beds and transport beds are also being developed. In
the fixed-bed process, a low concentration of butane is passed over the
catalyst in tubular reactors similar to the benzene process. The selective
reaction is given below:
n-C4H8 + 3
O2 + 3 H2O
The reactor is operated at temperatures from 400-480° C, and the pressure is held at 0.3-0.4 MPa in order to force the exit gases downstream for scrubbing and purification. Unlike the benzene process, only a small amount of the MA can be condensed from the reactor effluent due to the increased formation of water. The remaining product is washed out as maleic acid in a scrubber and then dehydrated to reform MA. An alternative recovery method absorbs the MA from the reaction gas by organic solvents. The MA is then distilled from the high boiling solvent in a fractional distillation column.
Catalyst
Unsupported vanadium phosphorus oxide (VPO) catalysts are preferred for the fixed bed reactors. Promoters such as lithium, zinc, and molybdenum are commonly used. Recent research has shown that Mg, Ca, and Ba ions are also effective promoters, generating higher conversion, yield, and selectivity than the unmodified VPO catalyst.
The ratio of phosphorus to vanadium determines the activity of the catalyst, and, in turn, the life. Catalyst activity is greater at high phosphorus-vanadium ratios, but the catalyst life is sacrificed as the activity increases. A phosphorus-vanadium ratio of 1.2 in the catalyst appears to provide the optimal balance between activity and catalyst life.
Preparation of the catalyst begins by mixing the VP with nearly anhydrous phosphoric acid and an organic solvent. Next, the mixture is heated and the organic solvent is removed by volatilization. The product is dried and calcined to yield the catalyst precursor, which is then pelletized or formed into spheres. Finally, the catalyst is loaded into the reactor where it is activated under carefully controlled conditions.
Links
These are chemical vendor sites that sell the catalyst components:
Here are some more
interesting links:
Chemical Engineering Home Page
References
Elvers, Barbara, Stephen Hawkins, and Gail Schulz. Ullman’s Encyclopedia of Chemical Industry. Fifth ed. “Maleic Anhydride.” Vol A16, 1990: 54-62.
Kirk-Othmer, Encyclopedia of Chemical Technology. Third ed. “Maleic Anhydride, Maleic Acid, and Fumaric Acid.” Vol 14, 1981: 770-771, 780-788.
Satterfield, Charles N. Heterogeneous Catalysis in Industrial Practice. Second ed. Malabar, FL: Kreiger Publishing Co., 1991.
Greiner, Elvira.“CEH Abstract: Maleic Anhydride.”
http://ceh.sric.sri.com/Public/Reports/672.5000/
Brutovsky, Milan, et al. “Vanadium-Phosphorus Catalysts Modified with Magnesium, Calcium, and Barium”
http://cccc.uochb.cas.cz/Vol/62/No03/19970392.html