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anmol singh Grade: 11
        about the reaction

7 years ago

Answers : (1)

askiitian.expert- chandra sekhar
10 Points
										

Hi,


First, the methane is cleaned, mainly to remove sulfur  impurities that would poison the catalysts.


The clean methane is then reacted with steam over a catalyst of nickel oxide. This is called steam reforming:


CH4 + H2O → CO + 3 H2

Secondary reforming then takes place with the addition of air to convert the methane that did not react during steam reforming.


2 CH4 + O2 → 2 CO + 4 H2
CH4 + 2 O2 → CO2 + 2 H2O

Then the water gas shift reaction yields more hydrogen from CO and steam.


CO + H2O → CO2 + H2

The gas mixture is now passed into a methanator, which converts most of the remaining CO into methane for recycling:


CO + 3 H2 → CH4 + H2O

This last step is necessary as carbon monoxide poisons the catalyst. The overall reaction so far turns methane and steam into carbon dioxide, steam, and hydrogen.


Ammonia synthesis – Haber process


The final stage, which is the actual Haber process, is the synthesis of ammonia using a form of magnetite, iron oxide, as the catalyst:


N2 (g) + 3 H2 (g) is in equilibrium with 2 NH3 (g) (ΔH = −92.4 kJ·mol−1)

This is done at 15–25 Mpa and between 300 and 550 °C, passing the gases over four beds of catalyst, with cooling between each pass to maintain a reasonable equilibrium constant. On each pass only about 15% conversion occurs, but any unreacted gases are recycled, so that eventually an overall conversion of 98% can be achieved.


The steam reforming, shift conversion, carbon dioxide removal, and methanation steps each operate at absolute pressures of about 2.5–3.5 MPa (25–35 bar), and the ammonia synthesis loop operates at absolute pressures ranging from 6–18 MPa (60–180 bar), depending upon which proprietary design is used.




OR




The Bosch process, also known as pulsed or time-multiplexed etching, alternates repeatedly between two modes to achieve nearly vertical structures.



  1. A standard, nearly isotropic plasma etch. The plasma contains some ions, which attack the wafer from a nearly vertical direction. (For Si, this often uses SF6.)

  2. Deposition of a chemically inert passivation layer. (For instance, C4F8 source gas yields a substance similar to teflon.)


Each phase lasts for several seconds. The passivation layer protects the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, the directional ions that bombard the substrate attack the passivation layer at the bottom of the trench (but not along the sides). They collide with it and sputter it off, exposing the substrate to the chemical etchant.


These etch/deposit steps are repeated many times over resulting in a large number of very small isotropic etch steps taking place only at the bottom of the etched pits. To etch through a 0.5 mm silicon wafer, for example, 100–1000 etch/deposit steps are needed. The two-phase process causes the sidewalls to undulate with an amplitude of about 100–500 nm. The cycle time can be adjusted: short cycles yield smoother walls, and long cycles yield a higher etch rate.


 

7 years ago
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