CHE 2119

LECTURE 7

nitrogen

1. comprises 78% of the atmosphere

2. cannot be utilized by plants in atmospheric form

3. essential for the synthesis of amino acids, proteins, enzymes • a limiting nutrient

nitrogen cycle

the circulation or cyclic movement of nitrogen from the atmosphere to soil and back into the atmosphere

atmosphere

Nitrogen falls from the ______ to the earth by precipitation (e.g. rain or snow).

nitrogen fixation

1. conversion of atmospheric nitrogen into ammonia

2. carried out by leguminous plants and some bacteria (e.g. azotobacter, clostridium, rhizobium)

nitrification

1. carried out by nitrifying bacteria (e.g. nitrococcus, nitrosomonas)

2. ammonia is first converted to nitrites

3. then nitrites are converted to nitrates (by nitrobacter)

Nitrogen assimilation

1. the process of absorbing nitrates and ammonia into organic nitrogen

2. Organic nitrogen is transferred into animals when they consume plants.

ammonification

1. the process of converting organic nitrogen into ammonia when plants and animals die

2. carried out by saprophytes like fungi and bacteria

3. Ammonia is also produced from volcanic eruptions and excretory products of animals.

denitrification

1. Nitrates are converted into molecular nitrogen through nitric oxide.

2. carried out by denitrifying bacteria (e.g. thiobacillus denitrificans)

fixed nitrogen

1. manures

2. ammonium sulfates (by-product from coking of coal)

3. ammonia (recovered from coke)

4. Chile saltpeter (sodium nitrate)

• dry distillation of nitrogenous biomaterial waste

• decomposition of ammonium salts

Haber (Haber - bosch) process

1. Fritz Haber and Carl Bosch, 20th century

2. practical and large-scale synthetic ammonia process

3. use of abundant and inexpensive elemental H2 and N2 gas

synthetic ammonia

1. fertilizers (ammonium sulphate, DAP, urea)

2. nitric acid

3. explosives

4. fibers, synthetic rubbers, plastics (nylons and other polyamides)

5. refrigerants

6. pharmaceuticals (sulfonamide, vitamins, etc.)

7. pulp and paper

8. extractive metallurgy

9. soda ash

10. cleaning solutions

air

water

hydrocarbons

raw materials for synthetic ammonia: haber - bosh process

equilibrium

return to ______ with increase in [N2]

pressure

return to ______ upon compression

very slowly

hydrogen and nitrogen react ___________

iron

catalyst: ___ → promoted by the addition of small amounts of oxides of aluminum and potassium

space velocity

_________ → cu. ft. of exit gases in STP (0oC and 760 mmHg) that pass over 1 cu. ft. of catalyst space per hour (usually 20,000 – 40,000)

doubly promoted

13 - 14% ammonia

singly promoted

8 - 9% ammonia

pure iron

3 - 5% ammonia

catalyst promotion

feedstock desulfurization

• removal of sulfur oxide and hydrogen sulfide

  • R-SH + H2 → H2S + RH

  • H2S + ZnO → ZnS + H2

• the sulfur is removed to < 0.1 ppm S in the gas feed

• Zinc sulfide remains in the adsorption bed.

manufacture of hydrogen

• steam-water gas process

• steam-hydrogen (natural gas) process

• coke-oven gas process

• electrolysis of water

primary (steam) reforming

• reformer consists of nickel - based catalyst

secondary reforming

• only 30-40% of the hydrocarbon react

• addition of air to convert the unreacted methane molecules

shift conversion

• The process gas from the secondary reformer contains 12 – 15% CO.

• The gas is passed through a bed of iron oxide/chromium catalyst around 400 – 500 degrees Celsius

Purification

CO2 Removal

• scrubbing with water, amine solutions, or hot potassium carbonate solutions

methanation

CO and Further CO2 Removal

• small amounts of CO and CO2 are removed by conversion to CH4 using Ni catalyst

ammonia conversion

• Modern NH3 plants use centrifugal compressors for synthesis gas compression.

• The synthesis of ammonia takes place on iron catalysts.

• reaction is highly exothermic

• Only 20 – 30% is reacted per pass in the converter due to unfavorable equilibrium conditions

ammonia

by mechanical refrigeration or adsorption/distillation

ammonia storage

• urea (46% nitrogen)

• ammonium nitrate (34% nitrogen)

nitric acid

• used for the manufacture of ammonium nitrates (fertilizers)

• intermediate in polymer industry (polyamides and polyurethanes)

• oxidizing acid for the parting of gold and silver, pickling of brass, and photoengraving

nitric acid

Manufacturing Procedure: _____

• evaporation of anhydrous ammonia

• oxidation of ammonia gas with air (platinum gauze at 920 deg C)

• cooling of nitric oxide

• oxidation and hydration of nitric oxide

• collection of 61 – 65% nitric acid using acid trap

• waste gas treatment

evaporation of anhydrous ammonia

• anhydrous ammonia is evaporated using steam

Oxidation of Ammonia Gas with Air

NH3 gas is oxidized with air to nitric oxide by passing through a platinum gauze at 920oC

cooling of nitric oxide

Nitric oxide with excess air is cooled in two heat exchangers and a water cooler

Oxidation and Hydration of Nitric Oxide

Successive oxidations and hydrations of the nitric oxide are carried out with continuous water cooling in a stainless-steel absorption tower

2NO(g) + O2(g) → 2NO2(g)

3NO2(g) + H2O(ℓ) → 2HNO3(aq) + NO(g)

Collection of Nitric Acid using Acid Trap

61 – 65% nitric acid

water and nitric acid form an azeotrope

Waste gas treatment

The waste gas from the top of the absorption tower is heated and expanded before being exhausted to the atmosphere