Methanol solvent is available cheaper than other organic solvents; it is even produced in abundance than other chemical processing solvents. The demand for hydrogen fuel leads to methanol splitting technologies, as per mole balance equation one molecule of methanol contains four hydrogen atoms. With respect to cost, safety and fuel handling, hydrogen can be generated from methanol is a small station to supply it for fuel cell cars and power generator in towns.
An idea of small process plants enhances the network of hydrogen production units of small capacity that utilizing liquid methanol as raw material. However to accomplish it the process equipment should be small enough to produce the required capacity and should be maintenance free and support frequent start-up and shutdown as well.
Some of the established technologies for hydrogen generation from methanol
- Steam reforming: Thermally autonomous micro-channel reactor
- Membrane reactor technique (Inorganic and organic membranes dual bed models)
- Fuel cell technique
- Partial oxidation
The most important catalyst used for reforming of methanol
Side products like dimethyl ether formed during the reaction affect the thermodynamic equilibrium of methanol and water composition. By the experimental data, 220-250oC is optimal operating temperature for the process at the pressure ranging from 5 to 25 atm. Main compounds that affect the conversion are methane and carbon soot.
Scientific conclusion on the rate of reaction of hydrogen generation from methanol:
We need a process technology that has the capability of producing higher H2. Based on steam reforming, autothermal and partial oxidation technology main reactions that occur in a reactor are:
- Dimethyl ether formation/ Dehydration of methanol to Dimethyl ether
- Steam reforming
- Waster gas shit reaction
- Methanol splitting
Kinetics of methanol decomposition is very important to design the reactor to obtain optimum conversion. Based on the choice of the catalyst one of the four reactions proceeds further than others. The output of the reactor depends on the selection of catalyst, temperature and pressure of the reaction. The rate constant depends on reactor operating temperature and on activation energy. Dimethyl ether formation and water gas shift reactions have positive activation energy whereas methanol decomposition and steam reforming of methane reactions have negative values.
The first said two reactions are exothermic and next two are endothermic. The four reactions should be balanced to operate with less energy supply to make the process as green and sustainable technology.
One of the tricks is that to initiate first two reactions to generate heat energy. Then use that energy to start and run next two endothermic reactions in the same reactor. This method becomes automatic and demands less energy from external means. The reactor works on a combination of autothermal partial oxidation and steam reforming technology. Reactions occurring without the requirement of energy supply are always good to prevent greenhouse gas producing fossil fuel consumption. If methanol produced from biogas is used as raw material for this technology that the hydrogen produced is a green fuel.
Watch how the Aspen Plus process simulation software, reactor model workout methanol decomposition reaction.
Check the results of hydrogen generation from methanol in RGibbs reactor model. For a feed of 0.5 kmol/hr of methanol 1.425 kmol/hr hydrogen is produced at 510K and 2 atm.
|Mole Flow kmol/hr|
|Total Flow kmol/hr||1||1.998537|
|Total Flow kg/hr||25.02872||25.02872|
|Total Flow l/min||338.8457||697.2253|
|Liq Vol 60F l/min||0.4865383||1.740365|
Sensitivity analysis performed fixing the reactor pressure, feed rate constant. Reactor temperature is varied from 278K to 700K to the H2 production rate. The analysis indicates the high production occurs in the range of 450 to 500K.