Chemical Technology and Industrial Processes

Petrochemical reforming and cracking reactions

Reforming and cracking are the two main families of reactions that convert petroleum fractions into petrochemical intermediates. Cracking breaks carbon–carbon bonds to give smaller, mostly unsaturated molecules, whereas reforming rearranges and dehydrogenates naphthenes and paraffins into aromatics and branched isomers. Each reaction type is recognised by how the carbon skeleton and the hydrogen count change, so the basic types should be learned together with their balanced equations:

• Dehydrogenation — loss of H_2 with the carbon skeleton unchanged, e.g. \mathrm{C_6H_{12}} \rightarrow \mathrm{C_6H_6} + 3\mathrm{H_2} (naphthene to aromatic).
• Dehydrocyclization — ring formation from a straight-chain paraffin with loss of H_2, e.g. \mathrm{C_6H_{14}} \rightarrow \mathrm{C_6H_6} + 4\mathrm{H_2}.
• Isomerization — rearrangement at constant formula, e.g. n-pentane to isopentane, which raises octane because branched chains burn more smoothly.
• Cracking — C–C scission into smaller molecules, e.g. \mathrm{C_{12}H_{26}} \rightarrow \mathrm{C_6H_{12}} + \mathrm{C_6H_{14}}, the route used to make light olefins.

A ring expansion of a five-membered naphthene into a six-membered one is also classed as isomerization, and it is the step that prepares six-membered rings for aromatization. Hydrocracking is the variant of cracking carried out under high hydrogen pressure. Because it converts paraffins into light gases such as methane, ethane and LPG while consuming hydrogen, it lowers the liquid yield and is treated as an undesirable side reaction in naphtha reforming. Catalytic reforming of straight-run gasoline and naphtha over a platinum-based catalyst produces reformate, a high-octane gasoline blending component rich in aromatics.

Industrial chemical manufacturing processes

Most industrial routes are known by the named process or catalyst that made them commercial, so each product should be linked to its characteristic reaction and catalyst.

• Ostwald process — catalytic oxidation of ammonia to nitric oxide over a platinum–rhodium gauze, followed by absorption; it is the standard route to nitric acid.
• Contact process — oxidation of SO_2 over a vanadium catalyst, the standard route to sulphuric acid.
• Wacker process — oxidation of ethylene to acetaldehyde using a palladium and copper chloride catalyst system.
• Twitchell process — fat splitting of oils with a sulphonated acid reagent to give fatty acids and glycerol, the first stage of stearic acid manufacture.
• Electric furnace process — reduction of phosphate rock with coke and silica to give elemental phosphorus.
• Sulphate (kraft) pulping — cooking of wood with NaOH and Na_2S to dissolve lignin.

The same logic applies to catalysts. Iron–molybdenum mixed oxide converts methanol to formaldehyde, vanadium pentoxide oxidises naphthalene to phthalic anhydride, silver catalyses the epoxidation of ethylene, and anhydrous ferric chloride, a Lewis acid, catalyses ring chlorination of benzene.

Common names of fertilizers and minerals

Trade and textbook names of minerals and fertilisers are often used in examinations in place of formulae, so each common name should be memorised together with its composition.

• Gypsum — hydrated calcium sulphate, CaSO_4·2H_2O, used as a retarder in cement and as a soil conditioner.
• Dolomite — double carbonate of calcium and magnesium, CaCO_3·MgCO_3; it must not be confused with calcite, which is CaCO_3 alone.
• Triple superphosphate — calcium dihydrogen phosphate, Ca(H_2PO_4)_2, a high-analysis phosphate fertiliser that contains no calcium sulphate, unlike ordinary superphosphate.
• Glauber's salt — sodium sulphate decahydrate, Na_2SO_4·10H_2O; sodium sulphate is the make-up chemical in the chemical recovery cycle of kraft pulping.

Do not confuse these with similar-looking salts. Washing soda is Na_2CO_3·10H_2O, and disodium hydrogen phosphate is Na_2HPO_4·2H_2O, so each formula corresponds to a different common name.

Sulfuric acid and oleum production

Sulphuric acid is made by the contact process, in which SO_2 is oxidised over a vanadium(V) oxide catalyst at a moderate temperature: 2\mathrm{SO_2} + \mathrm{O_2} \rightleftharpoons 2\mathrm{SO_3}. The reaction is exothermic and reversible. A moderate temperature gives an acceptable rate, while excess oxygen and elevated pressure favour the formation of SO_3 at equilibrium. The SO_3 is absorbed in concentrated sulphuric acid rather than in water, because direct contact with water produces a fog of acid mist that is hard to collect.

• Oleum — fuming sulphuric acid, a solution of free sulphur trioxide in sulphuric acid. The SO_3 combines with H_2SO_4 to form pyrosulphuric acid, H_2S_2O_7, and the free SO_3 escapes as fumes in moist air.
• Strength adjustment — oleum is diluted with water to obtain sulphuric acid of any desired strength, including 100% acid.

Pulp and paper production

Paper is made from cellulose fibres, and in wood these fibres are held together by lignin, which acts as a binding matrix. Wood is therefore the raw material, cellulose fibres are the product of interest, and lignin is the substance removed during pulping. Chemical pulping dissolves lignin while leaving the cellulose largely intact, so that the fibres can be separated. In the kraft (sulphate) process, logs are first debarked and reduced to chips, the chips are digested with white liquor (NaOH and Na_2S) at high temperature, the cooked pulp is washed to separate the spent black liquor, and the washed pulp is finally bleached. The correct sequence is debarking, chipping, digestion, washing and bleaching.

• Digestion — the cooking stage in which chips are treated with liquor at elevated temperature and pressure to remove lignin.
• Black liquor — spent cooking liquor containing dissolved lignin and sodium salts, which is recovered for its chemicals.
• Bleaching — the final whitening step, applied to washed pulp.

Soap manufacturing by saponification

Soap is made from fats and oils, which are triglycerides, that is, esters of glycerol with three long-chain fatty acids. Hydrolysis splits each ester bond, so that one triglyceride yields three fatty acid molecules and one glycerol molecule. Saponification is the alkaline hydrolysis of a triglyceride that gives soap and glycerol directly, for example \mathrm{(C_{17}H_{35}COO)_3C_3H_5} + 3\mathrm{NaOH} \rightarrow 3\mathrm{C_{17}H_{35}COONa} + \mathrm{C_3H_5(OH)_3}. The overall chemistry is often described as hydrolysis to fatty acids followed by neutralisation with alkali to form the soap salt.