Haber Process

[SarahLV]

Hello...

Would someone please be able to explain to me how the Haber process works? (In both simple and scientific terms?)

Thanks.

[Maz]

The Haber process is concerned with the production of ammonia, through the combining of nitrogen from the air and hydrogen derived from natural gas (methane). It is a reversible, exothermic reaction.


Le Chatelier’s Principle: When a change is imposed upon a system, the system will move in such a way to partially counter-react the imposed change.

N_{2 (g)} + 3H₂ (g)        <-->          2NH_{3 (g)}   


Conditions of production:
Catalyst: the catalyst isn’t always pure iron. Adding potassium hydroxide as a promoter will increase its efficiency.

Pressure: pressure varies from one manufacturing plant to another, but is always high
Recycling: Each time the gasses are passed through a reactor, only about 15% of nitrogen and hydrogen converts to ammonia (different from plant-to-plant). This is under the above conditions. Ammonia is cooled and liquefied at the reaction pressure, and then removed as liquid ammonium. Recycling of all unreacted hydrogen and nitrogen occurs at the reactant stage; the overall conversion is eventually about 98%.

Proportions
The mixture of hydrogen and nitrogen going into the reactor will be going in at a ratio of 1 nitrogen to 3 hydrogen’s. This is a ratio of one volume to 3. Avogadro’s law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. This means that the gases are going into the reactor in the ratio of 1 molecule nitrogen to 3 molecules hydrogen (indicated by the balanced equation.). Therefore, introducing nitrogen and hydrogen into the converter at a 1:3 stoichiometric ratio is a compromise that could help.
 Some reactions’ rate increases by adding more of a reactant. Excess is important to use up as much as possible of the other reactant. It is also important to avoid wasting reactor space and the space on the surface of the catalyst.


Temperature
In order to maximize ammonia yield, the equation needs to be shifted as far right as possible. The forward reaction is exothermic, and will be favored at lower temperatures, as explained by Le Chatelier’s principle. When heated, the system will move to counter-react that change by expelling heat. Optimum yield occurs when the temperature also optimum (400-450 C) is low. Decreasing temperature causes equilibrium position to move to the right. As temperature decreases, the equilibrium constant for the production of ammonia increases, resulting in more product being present in the equilibrium mixture. Though the temperature required is low, the normally used is around 400 degrees C. While this lowers rate, ammonia is still produced fast enough (economically) 
The lower the temperature, the slower the reaction will occur at. Temperature is a measure of average kinetic energy. As temperature decreases, average kinetic energy also decreases, slowing particle movement. It will take too long to produce the ammonia (making it too slow to be economical), for this reason, there is a compromise made between 350-550 degrees to keep reaction rate up. A catalyst is also helpful at this stage.
 

The equilibrium expression for this reaction is:       K= [NH3]²  /[N₂][H₂]³



Temperature
Temperature (Celsius)   K
25                            6.4 x 102
200                        4.4 x 10-1
300                            4.3 x 10-3
400                            1.5 x 10-4
500                        1.5 x 10 -5

As indicated by above table, increasing temperature lowers equilibrium constant, indicating there is less ammonia present at high temperatures; supporting the fact that it is exothermic.


Pressure
There are 4 molecules on the left-hand side, compared to the 2 on the right. To maximize ammonia yield, equilibrium should be shifted as far right as possible. Le Chatelier’s principle states that when a system is undergoing change, the system will move to counter-react the imposed change. Within high pressure, Ammonia yield will be maximized. For an equilibrium system, when the system is undergoing high pressure, the system will move to reduce the number of molecules. For this reason, the forward reaction is favored. In order to maximize Ammonia, a pressure as high as 200 atm (normally used is around 15-35 MPA) is required (chosen on economic grounds).
   An increase in pressure brings molecules together, increasing the likelihood that they will collide favorably, and sticking to the surface of a catalyst. However, very high pressure is expensive to produce for two reasons
1.   Have to build extremely strong sand pipes and containment vessels to withstand the very high pressure, increasing capital costs when plant is built
2.   High pressures cost a lot to maintain and produce; therefore the running fixed costs will be very high.
To build a plant that operates at higher pressure will add production costs and compressing gasses to even higher pressures will add risks to workers on the plant. Compromise is made at 200atm, after that the price of the extra ammonia won’t cover the price of having to make higher pressures




Catalyst
Catalysts reduce activation energy, hence speeding up the reaction. They are not however involved in the actual reaction and won’t cause a greater amount of ammonia to be made.
Catalysts ensure that reaction is fast enough for a dynamic equilibrium to be set up for the time that the gasses are in the reactor.  In this system, the catalyst will lower activation energy, so that the N2 bonds and H2 bonds can be more readily broken. Using a porous catalyst, based on iron oxide (Fe3O4 (Ferris oxide) / Fe3O4 (ferric oxide)). This catalyst can be fused with MgO, Al2O3 and SiO2. The large surface area of the porous catalyst maximizes the contact with the reactants and therefore increases the rate at which equilibrium is attained. At these low temperatures, the reduced activation energy (via catalyst) means more reactant molecules have sufficient energy to overcome the energy barrier to react, therefore reaction is faster.

Separating the ammonia
When gasses leave the reactor they are hot and at a very high pressure. Ammonia is cooled and liquefied under pressure. So, the temperature of the mixture is lowered enough so ammonia is removed as liquid ammonia. The nitrogen and hydrogen remain gasses and can be liquefied.

At 200 degrees Celsius and 750atm, there is almost a 100% conversion of reactants to ammonia.

I hope this helps

[SarahLV]

Wow. Thank you. That really helped.