03_process Description

CHAPTER NO. 3

Process Description

PROCESS DESCRIPTION

The process comprises of three sections: 1) Feed preparation section 2) Production section 3) Recovery and purification section The main equipments in the current manufacturing process include:      

Tubular reactor Heat exchanger 2-Phase separator Absorber Dehydration unit Distillation column

3.1 Reactor (PFR-100): The partial oxidation of benzene is carried out in vapour phase over a promoted catalyst in the tube side of a shell and tube reactor. The reactor contains 940 tubes of 2 inch diameter and 2.5 m length. The reactor is usually made of carbon steel SA-516 Grade 70. Benzene to air ratio i-e 1:24 is first pre-heated, mixed and then sent into the rector. The reaction takes place at 363oC and 202 kPa. Molten salt mixture of V2O5 (70%) and MoO3 (30%) is pumped through the reactor shell to remove the heat of reaction and to control the temperature at about 363 oC-400 oC. The heated molten salt is then sent to the salt bath cooler where heat is recovered by steam generation. Conversion in the reactor is 92% with a yield of 73%.

3.2 Heat exchanger (E-102): The product gases passes through a series of heat exchanger. The temperature of the gases is cooled down to 76 oC using water as a cooling medium in the tube side.

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CHAPTER NO. 3

Process Description

3.3 2-Phase separator (V-100): The cooled gases coming from the heat exchanger are then separated on the basis of their volatility. Here the purpose of the separator is to recover 50% of the formed maleic anhydride from the vapour stream directly in the liquid form at a temperature of 76.3 oC. This molten crude maleic anhydride is then sent to the mixer MIX-102, from where it is pumped to the distillation column for further processing.

3.4 Absorber (T-100): The off-gas stream from the separator is then fed to the absorber section where counter current absorption with 40% solution of maleic acid removes essentially all of the maleic anhydride as maleic acid. The over head gases from the absorber containing benzene are then sent to the benzene adsorption section. Maleic acid solution accumulates in the mixer MIX-101, from where it is pumped to the refining section for further processing.

3.5 Dehydration unit (T-101): In this unit vacuum azeotropic reactive distillation of the 40% maleic acid solution takes place. The purpose of the dehydration unit is to convert the maleic acid into maleic anhydride using o-xylene as an entrainer. O-xylene forms an azeotrope with water and separate out as water-o-xylene azeotrope from the top of the dehydration column while the crude maleic anhydride is taken from the bottom of the column.

3.6 Distillation column (T-102): Mixer MIX-102 acts as a feed unit for the distillation column. Feed enters at 160 oC in the column. Maleic anhydride of 99.5% purity is obtained from the bottom of the distillation column while the remaining components are discarded from the top of the column.

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