Production Technologies of Maleic Anhydride

Production Technologies of Maleic Anhydride
Maleic anhydride is a multifunctional basic chemical compound used in various chemical processes. The article describes processes of maleic anhydride production and purification.

Properties of Maleic Anhydride

Maleic anhydride is an important chemical intermediate with wide industrial applications: from production of unsaturated polyester resins up to API synthesis. Normally MAN is colorless or white solid with rhombic crystal structure with an acrid odor. In Russia and CIS countries the main technical standard for maleic anhydride is GOST 11153-75.
There are two main methods for industrial synthesis of maleic anhydride:
vapor-phase oxidation of benzene over a vanadium-molybdenum oxide catalyst;
vapor-phase oxidation of n-butane over a vanadium-phosphorus oxide catalyst.
The first method is outdated and today it is mainly used in China.

Maleic anhydride is a highly toxic substance of the 2nd hazard class, requires special storage and transportation conditions. It is hygroscopic, long-term storage leads to a gradual change in chemical behavior of raw material and formation of fusible impurities. Typical warranty shelf life is 6 months from the date of production.

Application of maleic anhydride

Maleic anhydride is widely used in chemical industry, mainly in polymerization processes producing high-demand polymer compounds. Approximately 50-55% of world maleic anhydride output is used in production of unsaturated polyester resins, which are basic for the manufacturing of fiberglass and other polymeric construction materials.

MAN is used for the manufacture of compositions, which form a strong and plastic polymer film once they are applied to various surfaces. The technology is commonly implemented in protective coating of building sites. Maleic anhydride is used as a plasticizer in concrete, providing better viscosity and pot life.

Polymerization reactions with maleic anhydride are used for production of fibers and various additives for modification of coatings, providing increase of hardness lifetime.
Maleic anhydride is used in following synthetic processes:
synthesis of fumaric, malic, succinic, maleic acids;
maleic acid hydrazide (plant growth regulator);
defoliants (e.g. endotal);
fungicides (canton, etc.);
insecticides (kalbofos);
production of polyester and alkyd resins for construction industry;
as an additive to lubricating oils to reduce friction;
as a feedstock in the production of tetrahydrophthalic anhydride, THF and butyrolactone;
manufacturing of paints, varnishes and lacquers to obtain water-soluble polymers;
in artificial sweeteners and flavor enhancements;
for paper sizing agents, water treatment detergents, hair sprays and pharmaceuticals.

Production of maleic anhydride: benzene or n-butane?

Maleic anhydride was first commercially produced in the early 1930s by the vapor-phase oxidation of benzene. The use of benzene as a feedstock for the production of maleic anhydride was dominant in the world until the 1980s. Several processes to produce maleic anhydride from benzene were developed, the most common of them was Scientific Design.

In the 60s, Petro-Tex Chemical Corporation (USA), and Imperial Chemical Industries Ltd. (UK), developed an industrial process for the production of maleic anhydride from butylene fraction, but selectivity and stability of the catalyst were quiet low thus making the process unprofitable at that time. Soon, these units started working with benzene.

Fixed and fluid bed processes for production of maleic anhydride from butenes present in mixed C4 streams had been practiced commercially. None of these processes are running nowadays.

The first commercial production of MA from n-butane was started in 1974 at Monsanto's J.F. Queeny (USA) plant. Further, in 1983, Monsanto started up the world's largest n-Butane to maleic anhydride production facility at 59 000 t/yr capacity, incorporating an energy efficient product collection and refining system.

The obvious cost and environmental advantages of butane over benzene have led to a rapid conversion of benzene- to butane-based plants by the mid-1980s in the United States. By that time, 100% of maleic anhydride production have used butane as the feedstock. Over the years, production of MA has been gradually decreasing the share of the benzene as the reactant of choice.

In the second half of the 1980s. Mitsubishi Kasei Corporation (Japan), Sohio (USA), Du Pont (USA) and Alusuisse (Italy) introduced the technology of vapor-phase oxidation of n-butane over a fluidized bed oxide vanadium-phosphorus catalyst in their plants.

However, due to such limitations as high loss of catalyst in the gas phase, difficulties in determining the optimal fluidized bed catalyst composition, the technology using a fixed-bed catalyst remains predominant.

Today three companies offer license for fixed-bed processes: Huntsman, Pantochim (acquired by BASF), Scientific Design (a joint venture of Saudi Basic Industries Corporation (SABIC) and Sud-Chemie AG (now part of Clariant)).

BP Chemicals and ABB Lummus Global (now CB&I Lummus), in turn, are licensors of fluidized-bed processes, while Lonza (Switzerland) licenses for both types of technological processes.

Maleic Anhydride Production

Industrial processes for producing maleic anhydride:
Vapor-phase catalytic oxidation of benzene by air oxygen over a vanadium-molybdenum oxide catalyst in fixed bed reactors.
Vapor-phase oxidation of n-Butane over a vanadium-phosphorus oxide catalyst t in fixed and fluidized bed reactors.
Some MA is also isolated from byproducts in the oxidation of o-Xylene to phthalic anhydride.
Структура импорта (в натуральном выражении) по видам изоцианатов за 2017 год
Reaction of benzene and n-Butane oxidation to maleic anhydride
To produce 1 ton of maleic aldehyde 1.11 tons of benzene or 1 ton of n-butane are required, while benzene is almost 1.5 times more expensive than n-butane.

Both reactions are exothermic (n-butane > benzene). Catalysts for these processes comprise mainly vanadium pentoxide, which is characterized by the reactions of destructive oxidation of organic substances. Various promotors – oxides (TiO2, МоО3), sulfates and phosphates are added to V2O5 for increase of catalyst activity and selectivity.
The general concept of the production of maleic anhydride from any type of raw material consists of four steps:
Preparation of the initial mixture;
Catalytic oxidation of the initial mixture with air oxygen;
Extraction of maleic anhydride from reaction mixture gases;
Purification and distillation of crude maleic anhydride.
MA production by oxidation of benzene
The production of maleic anhydride from benzene is a vapor-phase oxidation reaction with a fixed bed catalyst comprising vanadium and molybdenum mixed oxides.

The best catalyst for the oxidation of benzene is a mixture of V2O5+ MoO3, which is usually supported on wide-porous Al2О3. The catalyst is often modified with phosphorus, titanium, boron oxides.

All main processes are based on the most common scheme from Scientific Design. Differences between them mainly exist only on the extraction stage.
Off Gas
Process flow diagram for the maleic anhydride production from benzene: 1 - heat exchanger; 2 - refrigerator; 3 - wasteheat exchanger; 4 - contact apparatus; 5 - separator; 6 - scrubber; 7 - dehydrator; 8 - cistern for crude maleic anhydride; 9 - distillation column
This method is currently hardly ever used in the world except China (almost 90% of maleic anhydride produced in China is made from benzene).
MA production by oxidation of n-butane
The production of maleic anhydride from n-butane is a vapor-phase oxidation process based upon use of a heterogeneous catalyst containing vanadium and phosphorus mixed oxides. The n-butane oxidation reaction producing maleic anhydride is highly exothermic. The main reaction byproducts are carbon monoxide and carbon dioxide.

At first stage, the air is compressed to moderate pressure, usually from 100 to 200 kP using a centrifugal or radial compressor and then it is mixed with superheated evaporated butane. Butane concentrations are often limited to less than 1.7 mol. % to remain below the lower flammability limit of butane.

The reaction is carried out in a multi-tubular reactor with fixed bed catalyst cooled with mixture of nitrite and nitrate salts. The heat generated in the highly exothermic reaction is removed from the salt mixture by the production of steam in an external salt cooler. The reactor temperature is in the range from 390 to 430°C. Despite the rapid salt flow circulation on the shell side of the reactor, catalyst temperature can be 40-60°C higher than the salt temperature. Exiting gas stream is cooled in heat exchangers.

The next step is isolation and purification of maleic anhydride. The gaseous mixture containing maleic anhydride contacts with water or organic solvents in the absorber. For cleaning up the off gases, catalytic afterburning is performed. Then, maleic anhydride is purified from impurities by fractional distillation in a distillation column.

The butane to maleic anhydride oxidation reaction typically reaches its maximum efficiency at about 85% butane conversion with a molar yield of MA of about 50 to 60%.

Another one fluidized bed technology for MA production from n-Butane by oxidation is the so-called ALMA process developed jointly by Lonza and ABB Lummus.
Cooling water
Heavy End Impurities
Solvent Recycling
To Incineration
Stripping Column
Crude Maleic Anhydride
Light Fractions
Separation Column
Light End Impurities
Heavy Fractions
Separation Column
Pure Maleic Anhydride
Flow diagram of the ALMA maleic anhydride production process
The process is carried out in a fluidized bed reactor. The heat of the exothermic reaction is removed by cooling coils located inside reactor and generating high-pressure steam inside the coils.

Special organic solvent absorbs maleic anhydride from the off gases in an absorber column. The cleaning process is based on continuous distillation with the separation of light and heavy by-products from the target material in result.

Treated gases containing unreacted n-Butane and carbon monoxide are sent to catalytic afterburner, where additional steam is generated to be subsequently used at other production areas.
Methods for isolation and purification of maleic anhydride
There are two possible technologies for recovery and purification of the product:
Maleic anhydride is extracted by water and forms maleic acid, azeotropic distillation (batch process) is used to convert it back to maleic anhydride.
Uncondensed maleic anhydride from the gas phase is absorbed by organic solvents that could be separated from the target product by distillation (continuous process).
Solvent-based systems have a higher recovery of maleic anhydride and are more energy efficient than water-based systems.

The Scientific Design water-based collection and purification system is widely used worldwide in butane-based and benzene-based plants. The off gas is cooled in the heat exchanger with steam generation. Then the off gas is sent to a separator where condensed maleic anhydride is separated from the reaction mixture.

Non-condensed maleic anhydride contacts with water in the absorber forming maleic acid. Maleic acid can be thermally dehydrated from an aqueous solution to maleic anhydride in a plate- or film-type evaporator or through azeotropic distillation with o-xylene. After dehydration water is returned to the absorber column.

The resulting maleic anhydride combined with crude maleic anhydride after partial condensation is sent to a distillation column for purification.

Huntsman solvent based recovery process. The reactor exit gas is cooled in two heat exchangers for energy recovery. The cooled gaseous product stream is passed to a solvent absorber where a proprietary solvent is used to almost completely absorb the maleic anhydride containing in the product stream.

The solvent stream, coming from the bottom of the absorber with a high concentration of maleic anhydride, known as rich oil, is sent to a stripper where the rich oil is heated and maleic anhydride is vacuum-stripped from the solvent. The resulting maleic anhydride usually has a purity of more than 99.8% and, if necessary, it is sent to the purification section where it is batch distilled to produce extremely pure maleic anhydride.

The UCB collection and purification technology (owned by BP Chemicals) also depends on partial condensation of maleic anhydride and scrubbing with water to recover the maleic anhydride from the reaction off-gas. The UCB process is significantly different from the Scientific Design process when maleic acid is dehydrated to maleic anhydride.

In the UCB process, water in the maleic acid solution is evaporated to concentrate the acid solution. The concentrated acid solution and the condensed crude maleic anhydride are converted to maleic anhydride by a thermal process in a specially designed reactor. Then the resulting crude maleic anhydride is purified by distillation.

In various technological methods for the production of maleic anhydride with any method of MA extraction from reaction gases, the raw anhydride has to be distilled after thermochemical processing in order to obtain product that conforms to the market and industry standards.

The resulting distilled maleic anhydride is placed into sealed containers or crystallized. Crystallization can be carried out either by flaking process or granulation by spreading of molten MA onto a cold surface.

In Russia, maleic anhydride is included in the list for import substitution of the Ministry of Industry and Trade. However, there are economic reasons that hinder the launch of MA production. It can be profitable with volumes 3-4 times higher than current domestic demand. It's necessary to work out ways for exporting MA. Read more about the situation on the global and Russian market of maleic anhydride in the next article.
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