From crude oil and natural gas to the C1-C4 building blocks: Propylene, the C4 stream and the aromatics

3.1 Propylene has four sources, and on-purpose routes fill the gap ethane crackers leave

Wittcoff reports that in 2001 the US made about 42 billion lb of propylene. About 49% was a co-product of steam cracking and 51% a by-product of catalytic cracking and other refinery processes. In 2002 refineries supplied 52% of the US chemical industry's propylene. Europe was the reverse. It has fewer catalytic crackers because it uses less gasoline, so FCC supplied only about 20% of its 13.4 Mt of chemical propylene in 2001. Refinery propylene is cheaper to make but comes in dilute streams mixed with propane. Separating them needs a "propane splitter" column.

Two on-purpose routes exist. Propane dehydrogenation (PDH) pulls two hydrogen atoms off propane: C₃H₈ → C₃H₆ + H2. The chemistry is simple but limited by equilibrium. It must run above 600 C for acceptable conversion per pass, yet above 700 C cracking takes over. The licensed processes in the table run at 530-700 C, near or below atmospheric pressure, over platinum or chromia catalysts that coke and must be regenerated. Metathesis swaps the ends of two olefins. Ethylene + 2-butene → 2 propylene. ABB Lummus runs it over tungsten oxide at 300-375 C, with ethylene recycled to push the equilibrium. Two-thirds of the propylene then comes from cheap 2-butene. Deep catalytic cracking, a Chinese FCC variant, gives up to 20% propylene from vacuum gas oil.

The IEA (2018) links these routes to the shale boom. Cheap ethane favours ethylene-only crackers, so propylene must come from elsewhere. PDH plants have spread in the US and China, and China imports LPG to feed them.

• Wittcoff (2004), Ch 2 sections 2.2.7, 2.2.9, 2.3.1 and Ch 4 sections 4.1-4.4 · pdf pp. 102, 112-114, 118-119, 193-197
• IEA (2018), Ch 2 and Box 4.2 · pdf pp. 33, 76

3.2 The C4 stream is a crowded mixture that must be taken apart in a fixed order

Cracking naphtha gives a large C4 fraction. Cracking ethane gives almost none. Wittcoff's typical naphtha-cracker C4 cut is about 47% butadiene, 23% isobutene, 14% 1-butene and 11% 2-butene, with a few per cent butanes. These molecules boil within a few degrees of each other, so plain distillation will not separate them. The plant uses chemistry and solvents in a set sequence.

First, butadiene is removed by extractive distillation. A polar solvent such as acetonitrile, N-methylpyrrolidone or dimethylformamide holds back the other components so butadiene can be drawn off. Butadiene is the prize. About 70% of it goes into synthetic rubbers, led by styrene-butadiene rubber for tyres. What is left is traded as Raffinate I: the two n-butenes plus isobutene.

Isobutene is the most reactive of these, so it is removed next by reacting it. With water and an acid catalyst it gives tert-butanol. With methanol it gives MTBE (methyl tert-butyl ether), at about 97% conversion and very high selectivity. Either product can be cracked back to pure isobutene if needed. The remainder, Raffinate II, holds 1-butene and 2-butene. Pure 1-butene can be recovered as a comonomer for linear low-density polyethylene. Leftover 2-butenes go to refinery alkylation or to metathesis for propylene.

The value of each C4 depends on geography. The US cracks mostly gas, so it has long been short of butadiene and has imported it from naphtha-cracking Europe and Japan. In Europe the C4 surplus was large enough that some producers hydrogenated butadiene back to butenes or butanes. Wittcoff calls this an ironic reversal.

• Wittcoff (2004), Ch 5 Chemicals and Polymers from the C4 Stream · pdf pp. 249-253
• Wittcoff (2004), Ch 2 section 2.3.2 · pdf p. 120

3.3 Aromatics come from reformers and pyrolysis gasoline, and nature's mix never matches demand

Benzene, toluene and xylenes (BTX) have two main petroleum sources. In the US the main one is catalytic reforming of naphtha. In Western Europe and Japan it is pyrolysis gasoline from naphtha and gas oil crackers. Wittcoff puts pyrolysis gasoline at about three-quarters of Europe's benzene and toluene. Coal tar now supplies only a few per cent. The IEA (2018) estimates that about 80% of BTX worldwide is sourced from refineries.

In a reformer, ring-shaped naphthenes are dehydrogenated to aromatics. Cyclohexane becomes benzene and methylcyclohexane becomes toluene. The catalyst has two functions: an acidic support isomerises molecules into the right ring shape, and the metal strips hydrogen. Because the second step follows the first at once, equilibrium never builds up at the intermediate stage. Recovering the aromatics is a separate job. The reformer liquid is washed with a solvent, such as sulfolane, that dissolves aromatics selectively. Sulfolane gives an aromatic stream with under 1% non-aromatics. Three distillation columns then split benzene, toluene and mixed xylenes.

The awkward part is the mismatch shown in the table. Reformate is 55% toluene, but chemical demand is mostly for benzene and xylenes. Within the xylenes, the C8 fraction holds only about 17-22% para-xylene, yet para-xylene is 74% of xylene demand because it makes terephthalic acid for polyester. Refiners therefore convert surplus molecules. Toluene is dealkylated to benzene using reformer hydrogen. Ortho- and meta-xylene are isomerised back to an equilibrium mix over a ZSM-5 zeolite, and the para-xylene is removed again. Each step adds cost and energy.

• Wittcoff (2004), Ch 2 sections 2.2.1.1, 2.2.3, 2.3.2 · pdf pp. 96-97, 100, 105-108, 119-120
• Wittcoff (2004), Ch 12 · pdf p. 428
• IEA (2018), Ch 2 · pdf p. 39

See it in the table

Check your understanding

Why must propane dehydrogenation run hot (above about 600 C) but not too hot (below about 700 C)?

In the C4 separation sequence, what is removed first and how?

Reformate is 55% toluene, but chemical demand is led by benzene and para-xylene. What do producers do?