# From building blocks to plastics, fibres and detergents: Building blocks and their family trees Source page: https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/p1/ [Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 6: From building blocks to plastics, fibres and detergents](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/) Part 1 of 5 ## Building blocks and their family trees A handful of molecules carry most of the tonnage of the polymer industry. Ethylene, propylene, the C4 stream and the aromatics benzene, toluene and xylenes come out of steam crackers and refineries, and each feeds a short list of large derivatives. Knowing these trees, and the yield of each step along them, explains most of what drives cost and demand further downstream. ### 1.1 Ethylene goes mostly into polyethylene, with three other big branches Ethylene is the largest building block by tonnage, and almost all of it is used for chemicals rather than fuel. Wittcoff sorts its chemistry into a few big jobs. It is polymerised directly into the three polyethylenes. It is oxidised to ethylene oxide (EO). It is chlorinated to ethylene dichloride (EDC) on the way to vinyl chloride and PVC. And it is joined to benzene to make ethylbenzene, the precursor of styrene. In the United States in 2000 the polyethylenes took about 53% of ethylene, EO 12%, EDC 11% and ethylbenzene 6%; together with alpha-olefins and vinyl acetate the listed uses took about 90%. Brennan puts polyethylene at around 60% of world ethylene demand. Ethylene oxide is the branch where chemistry and money meet most clearly. Ethylene and oxygen pass over a silver catalyst at about 250 C and 15 bar. The same catalyst can also burn ethylene, and the EO itself, to carbon dioxide and water, a far more exothermic reaction. Selectivity (the share of reacted ethylene that becomes the wanted product) was 65-70% with early catalysts and averaged about 82% by 2004, with the newest catalysts above 90% at the start of a run. In 2002 about 61% of US EO was hydrated to ethylene glycol. A huge excess of water, 18-24 moles per mole of EO, stops the glycol reacting on to di- and triethylene glycol, yet the three still come out at about 91:8.6:0.4 by weight. About 68% of glycol goes into polyester; most of the rest is antifreeze. The remaining EO becomes ethoxylate surfactants, ethanolamines and polyols, which return in Part 5. Value chain From ethylene (United States, 2000) - **Ethylene** about 25 Mt used in the US in 2000 - **Polyethylenes** about 53% - **HDPE** 25% - **LLDPE** 14% - **LDPE** 14% - **Ethylene oxide** 12% - **Monoethylene glycol** 61% of EO; mostly to PET - **Ethoxylate surfactants, ethanolamines, polyols** rest of EO - **Ethylene dichloride** 11% - **Vinyl chloride monomer** - **PVC** - **Ethylbenzene (with benzene)** 6% - **Styrene** - **Polystyrene, SBR, ABS** - **Alpha-olefins** 6%; LLDPE comonomers, detergent alcohols - **Vinyl acetate** 2% Source: Wittcoff (2004), Ch 3, Table 3.3, pdf p. 164 Worked example #### What a point of selectivity is worth Wittcoff notes that about 75% of the cost of EO is ethylene. A tonne of EO contains 28/44 = 0.636 t of ethylene. At 70% selectivity a plant needs 0.636/0.70 = 0.91 t of ethylene per tonne of EO; at 82% it needs 0.78 t; at 90%, 0.71 t. Going from 70% to 82% saves 0.13 t of ethylene per tonne. At the US contract price of $496/t in Wittcoff's 2002 price table, that is about $65 per tonne of EO. - Wittcoff (2004), Ch 3 Chemicals and polymers from ethylene · pdf pp. 126, 152-154, 164-165 - Wittcoff (2004), Ch 2, Table 2.8 · pdf p. 99 - Brennan (2020), Ch 2 Market evaluation · pdf p. 55 ### 1.2 Propylene's reactive methyl group gives it a wider spread of chemistry For many years propylene was the by-product nobody wanted, sold at a little above fuel value or simply burned (Wittcoff). In the United States in 2001 about 49% came from steam crackers and 51% from refinery catalytic cracking, and a large share went into gasoline rather than chemicals. How much a cracker makes depends on its feed. Ethane gives almost none: the propylene to ethylene weight ratio is 0.019. Naphtha gives 0.43-0.57 depending on how hard it is cracked. As polypropylene grew, producers added on-purpose routes: propane dehydrogenation over platinum or chromia catalysts at 530-700 C, metathesis of ethylene with 2-butene, and deep catalytic cracking of heavy gas oil. The chemistry differs from ethylene because of the methyl group next to the double bond. Its hydrogens are allylic, meaning easy to pull off. That lets propylene be oxidised at the methyl end to acrolein and acrylic acid, or ammoxidised with ammonia and air to acrylonitrile. The same feature spoils free-radical polymerisation, which is why useful polypropylene had to wait for Ziegler-Natta catalysts (Part 2). In the United States in 2001 polypropylene took 51% of propylene, acrylonitrile 12%, propylene oxide 10.5% and cumene 10%. Acrylonitrile is made in a fluidised bed because the reaction releases so much heat; one catalyst gives full propylene conversion with 86.9% selectivity at 320 C, plus hydrogen cyanide as a by-product. Propylene oxide shows the co-product problem. The old chlorohydrin route wastes chlorine, while the hydroperoxide routes make about 2.2 moles of tert-butanol, or 2.2-2.5 lb of styrene, per unit of propylene oxide, so profits hinge on selling the second product. Value chain From propylene (United States, 2001) - **Propylene** about 14-15 Mt used for chemicals in the US in 2001 - **Polypropylene** 51% - **Acrylonitrile** 12% - **Acrylic fibre** - **ABS and SAN** - **Adiponitrile, acrylamide** - **Propylene oxide** 10.5% - **Polyether polyols for polyurethanes** largest use - **Propylene glycol** about one third - **Cumene (with benzene)** 10% - **Phenol and acetone** 0.6 t acetone per t phenol - **Bisphenol A, nylon 6, methyl methacrylate** - **Oxo alcohols (butanol, 2-ethylhexanol)** about 5% - **Acrylic acid** 4% - **Isopropanol** 4% Source: Wittcoff (2004), Ch 4, Table 4.3, pdf p. 236 Case #### A catalyst that doubled a cumene plant Older cumene units alkylated benzene with propylene over phosphoric acid or aluminium chloride at 200-350 C, feeding 8-10 moles of benzene per mole of propylene. Zeolite catalysts, introduced in the early 1990s, were more selective, so the ratio could fall to 4-5:1. Because the reactor no longer had to carry so much excess benzene, the capacity of an existing reactor roughly doubled with almost no new capital, although the separation train had to grow. Overall yield rose to 98%. - Wittcoff (2004), Ch 4 Chemicals and polymers from propylene · pdf pp. 193-197, 207-213, 221-224, 236-237 ### 1.3 Butadiene and the aromatics feed rubber, nylon and polyester The C4 cut from naphtha cracking is roughly 47% butadiene, 23% isobutene and 25% n-butenes. Butadiene is recovered by extractive distillation: a polar solvent such as acetonitrile, N-methylpyrrolidone or dimethylformamide holds back the other C4s so that butadiene can be distilled out. About 70% of butadiene goes into synthetic rubber. In the United States in 2001 SBR took 27%, polybutadiene 25%, styrene-butadiene latex 11%, and nitrile and chloroprene rubbers 3% each. Another 14% went to hexamethylenediamine for nylon 66, made by adding hydrogen cyanide to butadiene over a nickel phosphite catalyst, the cheapest of several routes. ABS took 5%. Benzene comes mainly from catalytic reforming in the United States and from pyrolysis gasoline (a liquid by-product of steam cracking) in Europe. Reformate is only 10-15% benzene but 50-55% toluene and 30-35% xylenes. In 2001 US benzene went 47% to ethylbenzene for styrene, 27% to cumene, 13% to cyclohexane for nylon and 8% to nitrobenzene for aniline and MDI. Toluene, the least wanted, is mostly converted: dealkylated with hydrogen to benzene, or disproportionated, two toluenes to benzene plus xylenes. A shape-selective zeolite can make that xylene 95-99% para-xylene. Xylenes are a separation puzzle, since the four C8 aromatics boil within 9 C of each other. Para-xylene is taken out by low-temperature crystallisation or by adsorption on molecular sieves in a simulated moving bed (UOP's Parex process). The unwanted isomers are isomerised back to an equilibrium mixture over ZSM-5 and recycled until little is left. Almost all para-xylene is oxidised to terephthalic acid for PET. Value chain From the aromatics (United States, 2001) - **BTX from reformers and pyrolysis gasoline** - **Benzene** - **Ethylbenzene to styrene** 47% - **Cumene to phenol and acetone** 27% - **Cyclohexane to adipic acid and caprolactam** 13%; nylon 66 and nylon 6 - **Nitrobenzene to aniline and MDI** 8% - **Alkylbenzenes (detergent LAB)** 2% - **Toluene** about 70% of US reformer toluene converted to benzene and xylenes (2002) - **Toluene diisocyanate** polyurethane foams - **Solvents** - **Xylenes** - **Paraxylene to purified terephthalic acid** to PET fibre, bottles, film - **Ortho-xylene to phthalic anhydride** plasticisers, resins - **Meta-xylene to isophthalic acid** Source: Wittcoff (2004), Ch 7 Fig 7.2, Ch 8, Ch 9, pdf pp. 296, 338, 348 In the data #### Why feedstock shapes the downstream industry Brennan compares feeds per 100 t of ethylene. Ethane needs 125 t of feed and gives only about 3 t of propylene. Naphtha needs 330 t of feed but also gives about 52 t of propylene, 14 t of butadiene and 22 t of benzene. A region built on cheap ethane therefore gets ethylene almost alone and must buy or make its propylene, butadiene and aromatics separately. A naphtha-based region gets a full basket of co-products, but needs bigger, costlier crackers and must find markets for all of them. - Wittcoff (2004), Ch 5 Chemicals and polymers from the C4 stream · pdf pp. 249-260 - Wittcoff (2004), Ch 7 Benzene, Ch 8 Toluene, Ch 9 Xylenes · pdf pp. 293-296, 338-340, 346-348 - Brennan (2020), Ch 2 Market evaluation · pdf p. 55 ### See it in the table [H₂O Water](https://www.materialinsights.org/commodities/water/)[Al Aluminium](https://www.materialinsights.org/commodities/aluminum/)[Ni Nickel](https://www.materialinsights.org/commodities/nickel/)[Ag Silver](https://www.materialinsights.org/commodities/silver/)[Pt Platinum](https://www.materialinsights.org/commodities/platinum/)[H3PO4 Phosphoric acid](https://www.materialinsights.org/chemicals/phosphoric_acid/)[Cl2 Chlorine](https://www.materialinsights.org/chemicals/chlorine/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[CO2 Carbon dioxide](https://www.materialinsights.org/chemicals/carbon_dioxide/)[NH3 Ammonia](https://www.materialinsights.org/chemicals/ammonia/)[C2H4 Ethylene](https://www.materialinsights.org/chemicals/ethylene/)[C3H6 Propylene](https://www.materialinsights.org/chemicals/propylene/)[BD Butadiene](https://www.materialinsights.org/chemicals/butadiene/) ### Check your understanding Why does a 10-point gain in ethylene oxide selectivity matter so much to producers? Wittcoff puts ethylene at about 75% of EO cost. Higher selectivity means less ethylene is burned to CO2 for each tonne of EO, which feeds straight into unit cost. What makes propylene chemically different from ethylene in ways industry exploits? The allylic hydrogens let propylene be converted to acrolein, acrylic acid and acrylonitrile. They also spoil free-radical polymerisation, which is why polypropylene needed Ziegler-Natta catalysts. A country cracks mainly ethane. Which statement best follows from Brennan's figures? 125 t of ethane gives 100 t of ethylene and only about 3 t of propylene, with little C4 or aromatics, so these must come from refineries, on-purpose plants or imports. [← Module contents](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/)[Mark read · Part 2: How polymers are made →](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/p2/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/index.md) - [How polymers are made](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/p2/index.md) - [The big bulk plastics](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/p3/index.md) - [Synthetic fibres](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/p4/index.md) - [Detergents: from benzene, paraffins and ethylene oxide](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/building-blocks-to-polymers/review/index.md)