Sunday, 13 November 2011

Chemistry notes: Industrial process

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Industrial process
1)       Ammonia (nitrogenous plant)
Nitrogen acts as an important role in plants like producing amino acid. N2 in air can't be directly absorbed by plants while ammonia or nitrate has to be used. Artificial fertilizers are necessary to support intensive farming. The two fertilizers straight N (only nitrogen compounds are useful) and NPK (nitrogen, phosphorus and potassium containing in different ratio) can be used in different crops. Common fertilizers include NH4NO3, (NH4)2SO4, NH4PO4, H2NCONH2 (urea). Different solubility gives them different application in planting different crops.
P content is prepared from NH3 + H3PO4, while phosphoric acid can be produced from raw phosphorus and sulphuric acid. K content is prepared from KCl.
Nitrogen fixation process (which converts N2 in air into useful chemicals)
-          Drastic environment for reaction to occur, like N2 + O2 2NO in lightening and car engine
-          Action of bacteria: including nitrifying bacteria in root nodules converting nitrogen in atmosphere into nitrate ions
-          Haber process
Preparation of H2 and N2:
N2: by fractional distillation from air.
H2: by reduction of water by methane: CH4 + H2O 3H2 + CO, CO + H2O H2 + CO2, note that the proportion between methane and water must be controlled carefully to prevent production of side products. Also, water steam is used, NOT in liquid form.
Reaction flow:
I)                     Purifying and drying (remove CO, CO2 from poisoning catalyst), then compress the gas
II)                   Pump into heat exchanger and absorb heat and react in catalytic convertor
III)                  Haber process is exothermic, so excess heat is stored in heat converter
IV)                Ammonia is condense and stored, unused H2, N2 is pumped back into heat exchanger
Condition: 400~450 degree Celsius, 200 atm with finely powdered Fe catalyst.
Cost of process is reduced by recycling unused reactant and use hot product to warm up reactants.
Nitric acid HNO3 is prepared by NH3 by:
I)                     4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) in 900 degree Celsius, 10atm and Pt-Rh catalyst. In lab we can blow O2 into conc. NH3 with red-hot Pt-Rh catalyst hung in the beaker. The exothermic reaction gives enough heat for the catalyst to maintain high temperature. Beware that the reaction is explosive (enthalpy extremely negative)
II)                   2NO + O2 2NO2 N2O4 in 40 degree Celsius (or r.t.), 7-12 atm.
III)                  3N2O4(g) + 2H2O(l) 4HNO3(aq) + 2NO(g), note that cold water is used, while NO can be reused.
Industrial consideration
-          Storage: cost of tanks (chemical reactivity of chemicals like flammability, temperature and pressure needed), land cost, import cost, etc.
-          Transportation: cost of pipes (pressure, temperature, continuous VS patch)
-          Effect of heat on rate of reaction; to process purification, to dry the chemicals, to process water treatment
-          Separation and process control
Environmental problem from excess usage of fertilizers:
-          Nitrates are very soluble in water. Through underground water, water with high nitrate content went into water streams and river, causing eutrophication where algae disastrously grow, which releases toxics, uses up oxygen in water and destroy the whole environment.
-          When human drinks water with high nitrate contents, nitrate bond with hemoglobin and causing suffocation especially in baby. Since the bonded hemoglobin is blue, it's called blue baby syndrome.
2)       Chloro-alkali industry
By the electrolysis of sea water, H2, Cl2 and NaOH is produced, but they must be separated during electrolysis, otherwise H2, Cl2 may react directly to give HCl; Cl2 and NaOH may react as well.
Mercury electrolytic cell
Brine passes through between carbon anodes (Cl2 released at the top), Na+ preferentially dissolved in mercury cathode to form Na/Hg amalgam, my adding water, H2 and NaOH is collected separately. Mercury is then pumped up back to dissolve Na+.
It's very efficient and gives chemicals at high purity, but Hg is highly toxic, so most old plants uses this set-up, but it has been replaced by other set-up.
Diaphragm cell
Diaphragm only allows solutions to pass through, blocking gas and separating anode and cathode. As a result Cl2 is discharged in anode side while H2 is discharged in cathode side while a mixture of NaCl and NaOH is collected. Note that Ti electrode is used in anode while steel electrode is used in cathode. Also, the NaOH collected has low purity.
Membrane cell
Membrane blocking anion and allowing cation to pass through has been developed in a few countries. On the anode side there're bring input and output. Since chloride ion can't pass through the membrane it's discharged in anode side; at cathode side, hydrogen is discharged, water input and (quite pure) NaOH is given out. It's more environmental friendly and high purity.
3)       Methanol
Methanol has many application like the raw material of methanoic acid, methanal, hydrogen, ethene, propene, diethyl ether, MTBE (CH3OC(CH3)3, methyl tert-butyl ether as oxygenate in petrol.) Methanol can be derived from natural gas by three steps:
I)                     Syngas generation: CH4 + H2O 3H2 + CO in 730 degree Celsius, 30atm and nickel oxide catalyst.
II)                   Methanol synthesis: CO + 2H2 CH3OH in Cu, ZnO catalyst, 300 degree Celsius, 50-100 atm. The proportion is controlled carefully to prevent by-products, and the excess hydrogen is recycled.
"Green feedstock" in producing methanol
1)       Biomass
When biomass undergo gasification (organic matter heated strongly without much oxygen), they break up into H2, CO2, CH4, CO, etc. Since CO2 is in excess in the syngas preparation, 61% of it is removed. After adding suitable amount of H2 to fit the proportion of syngas, methanol is produced, then under crud methanol purification, methanol is produced.
However, oxygen and hydrogen used in this process come from electrolysis, that is, the process may not be as "green" as in ideal.
2)       Flue gas
Methanol is produced by CO2 + 3H2 CH3OH + H2O (water vapour), and hydrogen used is again from the electrolysis of water, so it's not "green" at all.
Consideration in building chemical plant
1)       Easy availability of raw materials – to lower the transportation cost
2)       Reliable supply of electricity of fuel – to ensure stable input, and hence stable output
3)       Reliable supply of water – as an important medium in industrial process like solvent, cooling, cleaning, etc.
4)       Good transport system – again lowering the transportation cost
5)       Good supply of labour – higher availability of labour lower the wage rate
6)       Cheaper land resources – lowering the fixed cost
7)       Least environmental impact on both human and environment

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