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

Source page: https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p3/

[Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 5: From crude oil and natural gas to the C1-C4 building blocks](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/) Part 3 of 5

## Propylene, the C4 stream and the aromatics

Ethylene comes almost entirely from steam crackers. Its sister products do not. Propylene comes from crackers, refinery catalytic crackers and purpose-built plants. Butadiene and isobutene must be pulled out of a crowded C4 mixture. Benzene, toluene and xylenes come from reformers and from cracker pyrolysis gasoline. How each is sourced explains regional shortages and gluts.

### 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.

Data Propane dehydrogenation processes by licensor

| Process (licensor) | Catalyst | Pressure, bar | Temperature, C | Regeneration |
|---|---|---|---|---|
| Oleflex (UOP) | Platinum on alumina | 1.38 at last reactor | 600-700 | Continuous catalyst regeneration |
| Catofin (Lummus) | Chromia-alumina | 0.13-0.68 (vacuum) | 540-700 | Cyclic, coke burn-off |
| PDH (Linde-BASF) | Chromia-alumina | 1.4-2.1 | 540 | Cyclic, in situ |
| FBD (Snamprogetti-Yarsintez) | Chromia-alumina, alkaline promoter | 1.2-1.5 | 530-600 | Fluid bed |
| STAR (Krupp Uhde) | Platinum + tin on zinc aluminate | 2.1-4.2 | 565-595 | Cyclic, in situ; steam dilution |

All run hot and at low pressure because dehydrogenation makes more moles of gas than it consumes.
 Source: Wittcoff (2004), Ch 4, Table 4.2, pdf p. 196  Case

#### Lyondell turns ethylene into propylene

Metathesis was discovered at Phillips in 1964 to turn cheap propylene into ethylene and butene. In 1984 Lyondell built a one billion lb a year unit that ran the reaction backwards. At first it even made its 2-butene by dimerising ethylene, so all of its propylene came from ethylene. It later switched to 2-butene from cracker C4 streams. Wittcoff calls this exactly the opposite of the use that motivated the invention.

- 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.

Value chain Taking apart the cracker C4 stream

- **Crude C4 from naphtha cracking** butadiene 47%, isobutene 23%, 1-butene 14%, 2-butene 11%, butanes 4%

  - **Extractive distillation** acetonitrile, NMP or DMF solvent

    - **1,3-Butadiene** about 70% to synthetic rubbers (SBR, polybutadiene); also ABS, nylon 6,6 intermediates

  - **Raffinate I** n-butenes + isobutene

    - **Isobutene reacted out** with methanol → MTBE (about 97% conversion); with water → tert-butanol; or polymerised
    - **Raffinate II** 1-butene + 2-butenes

      - **1-Butene** comonomer for LLDPE
      - **2-Butenes (Raffinate III)** to alkylate for gasoline, or metathesis with ethylene to propylene

 Source: Wittcoff (2004), Ch 5, Table 5.1 and Fig 5.2, pdf pp. 249-253  Case

#### MTBE: from by-product to boom and back

When US clean-air rules required oxygen-containing additives in gasoline, MTBE became, in Wittcoff's words, the fastest-growing chemical ever. US production rose from 1.2 billion lb in 1981 to about 20 billion lb a year in 1999-2002. There was not enough by-product isobutene, so n-butane was isomerised to isobutane and dehydrogenated to isobutene. Then MTBE was found leaking from underground tanks into groundwater, and California moved to phase it out.

- 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.

Data Reformer output versus chemical demand

| Aromatic | Share of catalytic reformate,% | Share of US chemical demand,% |
|---|---|---|
| Benzene | 11 | 45 |
| Toluene | 55 | 22.7 |
| Xylenes | 34 | 32.3 |
| Within C8: para-xylene | 17-22 of C8 fraction | 74 of xylene end use |
| Within C8: meta-xylene | 40-45 of C8 fraction | 5 of xylene end use |
| Within C8: ortho-xylene | 16-20 of C8 fraction | 21 of xylene end use |
| Within C8: ethylbenzene | 17-22 of C8 fraction | 0 |

Circa 2000. The gaps are closed by toluene dealkylation and disproportionation and by xylene isomerisation.
 Source: Wittcoff (2004), Ch 2, Tables 2.10 and 2.11, pdf p. 107  In the data

#### Rules on gasoline move benzene supply

Wittcoff describes how the US Clean Air Act of 1990 forced gasoline benzene down from about 3% to 1%. Some areas also had to cut total aromatics from 36% to 25% by 1995. Refiners began stripping the C5-C6 fraction out of reformer feed to avoid making benzene. That pushed light naphtha towards isomerisation and cracking. Extracting benzene from European gasoline could have added about 450,000 t of supply.

- 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
[H₂O Water](https://www.materialinsights.org/commodities/water/)[W Tungsten](https://www.materialinsights.org/commodities/tungsten/)[Pt Platinum](https://www.materialinsights.org/commodities/platinum/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[C2H4 Ethylene](https://www.materialinsights.org/chemicals/ethylene/)[C3H6 Propylene](https://www.materialinsights.org/chemicals/propylene/)[BD Butadiene](https://www.materialinsights.org/chemicals/butadiene/)[Bz Benzene](https://www.materialinsights.org/chemicals/benzene/)[Tol Toluene](https://www.materialinsights.org/chemicals/toluene/)[SM Styrene](https://www.materialinsights.org/chemicals/styrene/)[MeOH Methanol](https://www.materialinsights.org/chemicals/methanol/)[MTBE MTBE](https://www.materialinsights.org/chemicals/mtbe/)[SBR Styrene-butadiene rubber](https://www.materialinsights.org/chemicals/sbr/)

### Check your understanding

Why must propane dehydrogenation run hot (above about 600 C) but not too hot (below about 700 C)?
 Wittcoff explains that thermodynamics requires temperatures above 600 C for acceptable conversion per pass. Above 700 C, cracking to ethylene and methane takes over.

In the C4 separation sequence, what is removed first and how?
 Butadiene comes out first, by extractive distillation with acetonitrile, NMP or DMF. That leaves Raffinate I, from which isobutene is reacted out to give Raffinate II.

Reformate is 55% toluene, but chemical demand is led by benzene and para-xylene. What do producers do?
 Wittcoff notes that little can be done to change reformer yields because they depend on the feed. The mismatch is fixed downstream by dealkylating toluene and isomerising xylenes.

[← Part 2: Steam cracking: the engine of the petrochemical industry](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p2/)[Mark read · Part 4: C1 chemistry: synthesis gas, methanol, ammonia and coal →](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p4/)

## Other sections

- [Module overview](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/index.md)
- [The raw materials: crude oil, natural gas and the refinery](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p1/index.md)
- [Steam cracking: the engine of the petrochemical industry](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p2/index.md)
- [C1 chemistry: synthesis gas, methanol, ammonia and coal](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p4/index.md)
- [The economics: feedstock, scale, integration and carbon](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p5/index.md)
- [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/review/index.md)