PROCESS DESCRIPTION

PROCESS DESCRIPTION

Process for the formation of acetic acid by methanol and carbon monoxide involves carbonylation process. Carbonylation is the addition of carbon monoxide to a molecule. The reaction is given as follows

 

27 atmospheres, 190 oC

CH3OH      +      CO                         CH3COOH

[Ir (CO) 2 I2]  + Ru (CO) 4I2

 

Figure 1 is a schematic diagram illustrating the process flow. Carbon monoxide is bubbled through a liquid reaction medium in a continuous stirred tank reactor maintained at 190 oC and 27 atmospheres. The reaction medium consists of methanol, acetic acid, iodomethane, methyl acetate, water, and iridium iodocarbonyl catalyst complex and ruthenium iodocarbonyl as a promoter (Ru (CO) 4I2).

 

Side reactions producing byproducts are as follows:

 

 

27 atm, 190 oC

  1. CH3OH + CH3COOH CH3COOCH3 + H2O
[Ir (CO) 2 I2]  + Ru (CO) 4I2

 

 

 

                                                27 atm, 190 oC

  1. CO +   H2O                               CO2 + H2

                               [Ir (CO) 2 I2]  + Ru (CO) 4I2

 

 

 

                                                

                                          

                                              27 atm, 190 oC

  1. 4CO + 2H2O 3CO2 + CH4

                                [Ir (CO) 2 I2] + Ru (CO) 4I2

 

 

Carbon monoxide is the excess reactant and methanol is the limiting reactant.

 

As shown in Figure 1 stream A consists of carbon monoxide entering at pressure of 1 atm and 25 oC into compressor. The Compressor compresses the gas to 28 atm and temperature after the after cooler is 190 oC (stream B).

 

Methanol enters into pump through stream C at 25 oC and 1 atm. The pump increases the pressure to 28 atm and temperature to 28 oC (stream D).

 

All the reactions described above occur at 190 oC and 27 atm. The overall reaction is endothermic. Dowtherm Q is being used as a heating medium in the reactor jacket. Dowtherm Q enters into reactor jacket through stream 1 and leaves through stream 2.

 

Some of the gases are dissolved or entrained in the liquid and leave with the stream of liquid going out of the reactor (stream E).While the rest of gases are used to build up pressure in the reactor and leave from top (stream F) through a control valve.

 

Stream E is than send to the blowdown drum where the entrained gasses are removed by impingement and escape from top of blowdown drum (stream G). These gases combine with the gases from the top of reactor (stream F) and go to the utility section where there heat is utilized. After giving there heat these gases are scrubbed before venting them to the atmosphere.

 

Liquid stream from the blow down drum (stream H) at temperature of 190 oC and 27 atmospheres is passed through the heat exchanger, where it is cooled down from 190 oC to 110 oC using Dowtherm as cooling media. Dowtherm enters the heat exchanger (stream 3) at 30 oC and leaves (stream 4) at 80 oC. Dowtherm comes to the plant through parallel streams from the utility section at different temperatures as required.

 

The cooled liquid stream at 110 oC and 26.4 atmospheres escapes from the heat exchanger

(Stream I) and enters the throttling valve. The throttling valve reduces the pressure and produces a vapor liquid mixture (stream J) which is send to the flash tank operating at 1.4 atmospheres and 110 oC.

 

Catalyst and promoter are highly non volatile and along with some liquid proportion consisting of acetic acid, methyl acetate, water, iodomethane,

(Co-catalyst) iridium iodocarbony catalyst and ruthenium promoter are recycled back to the reactor (stream K) at the temperature of 101 oC and 1.4 atmospheres (this stream is first pumped to the pressure of 28 atmospheres before entering it to the reactor).

 

The vapor stream from the top of flash tank at the temperature of 119 oC through stream L are send to the distillation column operating at 1 atmospheres. Stream L consists of acetic acid, methyl acetate, water and iodomethane (Co-catalyst).

 

Acetic acid is the required product and the heaviest among all other components with the boiling point of about 118 oC. All the other components except water have boiling point very much less than acetic acid.

 

99 % acetic acid and 1 % water escape from bottom of distillation column through stream M at temperature of 118 oC and 1 atmosphere. This stream is first to be cooled this is done by passing it through a water cooler. Water enters the cooler at 30 oC (Stream 11) and leaves a 55oC (Stream 12). Finally Product is obtained from stream M’ at 35 oC.

 

Steam is used in reboiler of distillation column and enters through stream 5 and leaves through stream 6.

 

And from the top of distillation column methyl acetate, water, iodomethane and little amount of acetic acid is obtained ( stream N ) at the temperature of  71 oC and 1 atmospheres. Cooling water is used as a coolant in the condenser of distillation column. It enters the condenser through stream 7 and leaves through stream 8.

 

Now we have to obtain our co-catalyst (iodomethane) from the mixture obtained from the top of distillation column and recycle it back to the reactor.

 

Stream N is than send to water cooler. This reduces its temperature to 35 oC. Cooling water enters the cooler at 30 oC (stream 9) and leaves at 50 oC

(stream 10).

 

As there is a large density difference between iodomethane and other components methyl acetate, water and little amount of acetic acid which have very similar density. Iodomethane is heaviest in terms of density as compared to others. Therefore the stream from top of distillation column is send to decanter separator operating at 1 atmosphere.

 

As iodomethane has high density so it moves to the bottom of decanter and is obtained in stream O which is recycled back to the reactor.

 

A mixture of methyl acetate, water and very little amount of acetic acid moves to the top of the decanter and is obtained in stream P.

 

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