The flow sheet developed for the production of methanol at low-pressure.
Methanol synthesis by low pressure process flow sheet description:
The first unit is a steam reformer-using hydrocarbon feedstock , such as natural gas. It is continuously fed at a pressure of 25 atm in to the tubes of the reformer. High-pressure steam mixed with the natural gas at the inlet of the reformer. By indirect heating, the tube temperatures are maintained at reaction temperature. Fuel and air are passed to the burners supported on the ceramic walls of the reformer. The hot flue gas from the reformer is used to preheat the boiler water and low-pressure steam used in the process. This system is shown in the process flowsheet schematically. For this process, the methane content in the reformed gas determines the efficiency of the reformer.
Reformed synthesis gas from the reformer is compressed to 50 atm by steam turbine centrifugal compressor. Before compression, waste heat boiler recovers heat from the synthesis to produce high-pressure steam.Reformed synthesis gas is fed to the top of the catalyst bed converter, here carbon monoxide and hydrogen reacts and produce methanol. For low-pressure reaction copper based catalyst is used in the converter. The effluent gas of converter leaves at temperature between 200-300 oC and used for preheating the inlet stream. Flash separator drops the pressure and separates unreacted synthesis gas from 75% methanol.
The flashed out gases partially used as fuel and mostly mixed into the reformer feed line.Crude methanol contains 25% of water, other compounds like dimethyl ether, n- propanol, isopropanol and ethanol. The crude methanol purified in continuous distillation column up to 99.9% purity. Heavier compounds are obtained as bottom products.
Chemistry involved in methanol synthesis by low pressure process:
The first stage is the reaction between the paraffin and steam, exemplified with methane the reactions that can occur are:
- CH4 + H2O↔ CO + 3H2 ∆H +49 KCal
- CO + H2O↔ CO2 + H2 ∆H -10 KCal
- CH4 + 2H2O↔ CO2 + 4H2 ∆H -39 KCal
It is shown from equilibrium constant for these reaction that at atmospheric pressure the methane is completely converted at slightly above 800oC largely to hydrogen and carbon monoxide ( in the mole ratio of about 3.5 or 4 to 1) with a small proportion of carbon dioxide in the stream.
Effect of pressure ad temperature on methane-steam reaction equilibrium (H2O/CH4 = 2:1)
|PRESSURE||Methane contact dry basis of reformer exit gas at temperatures quoted|
|1 atm||–||700oC||–||–||800 oC|
|10 atm||800 oC||870 oC||910 oC||950 oC||1000 oC|
|20 atm||870 oC||950 oC||1000 oC||1050 oC||1000 oC|
|40atm||940 oC||1020 oC||1080 oC||1130 oC||1200 oC|
The formation of methanol for mixtures of carbon monoxide, carbon dioxide and hydrogen proceeds according to the following reactions
- CO + 2H2↔ CH3OH ∆H298K -90.77 KJ/mol
- CO2 + 3H2↔ CH2OH + H2O ∆H298K -49.52 KJ/mol
It is a common practice in industry to use the sum of equations and the reverse water gas shift reaction
- CO2 +2H2 ↔CO + H2O ∆H298K 41.25 KJ/mol