User:Milton Beychok/Sandbox

References

(CC) Diagram: Milton Beychok
Structural diagrams of the BTX aromatic hydrocarbons.

In the petroleum and petrochemical industries, the acronym BTX refers to mixtures of the aromatic hydrocarbons benzene, toluene, and the three xylene isomers. If ethylbenzene is included, the mixture is sometimes referred to BTEX. The xylene isomers are distinguished by the designations ortho – or o –, meta – or m –, and para – or p – as indicated in the adjacent diagram.

The BTX aromatics are very important petrochemical materials. The world wide demand for these hydrocarbons is steadily increasing. The demand for xylenes, particularly para – xylene, has increased in proportion to the increase in demand for polyester fibers and film. Benzene is a highly valuable product for use as a chemical raw material. Toluene is also a valuable petrochemical for use as a solvent and intermediate in chemical manufacturing processes and as a high octane gasoline component.

Properties of the BTX hydrocarbons

The table below lists some of the properties of the BTX aromatic hydrocarbons, all of which are liquids at typical room conditions:

Properties(ref name=CRC-Hdbk/)

	benzene	toluene	ethylbenzene	p - xylene	m - xylene	o - xylene
Molecular formula	C6H6	C7H8	C8H10	C8H10	C8H10	C8H10
Molecular mass, g · mol –1	78.12	92.15	106.17	106.17	106.17	106.17
Boiling point, °C	80.1	110.6	136.2	138.4	139.1	144.4
Melting point, °C	5.5	– 95.0	– 95.0	13.3	– 47.9	– 25.2

Production of BTX hydrocarbons

BTX content of Pyrolysis Gasoline and Reformate(ref name=Chem-Systems/)   Pyrolysis Reformate Standard severity Medium severity CCR SR BTX content, wt % 58 42 51 42 Benzene, wt % of BTX 48 44 17 14 Toluene, wt % of BTX 33 31 39 39 Xylenes, wt % of BTX 19 25 44 47 CCR = Continuous catalytic regenerative reformer SR = Semi-regenerative catalytic reformer

Benzene, toluene, and xylenes can be made by various processes. However, most BTX production is based on the recovery of aromatics derived from the catalytic reforming of naphtha in a petroleum refinery.

Catalytic reforming usually utilizes a feedstock naphtha that contains non-aromatic hydrocarbons with 6 to 11 or 12 carbon atoms and typically produces a reformate product containing C₆ to C₈ aromatics (benzene, toluene, xylenes) as well as paraffins and heavier aromatics containing 9 to 11 or 12 carbon atoms.

Another process for producing BTX aromatics involves the steam cracking of hydrocarbons which typically produces a cracked naphtha product commonly referred to as pyrolysis gasoline, pyrolysis gas or pygas. The pyrolysis gasoline typically consists of C₆ to C₈ aromatics, heavier aromatics containing 9 to 11 or 12 carbon atoms, and non-aromatic cyclic hydrocarbons (naphthenes) containing 6 or more carbon atoms.

The adjacent table compares the BTX content of pyrolysis gasoline produced at standard cracking severity or at medium cracking severity with the BTX content of catalytic reformate produced by a either a continuous catalytic regenerative (CCR) reformer or by a semi-regenerative catalytic reformer.

The BTX aromatics can be extracted from catalytic reformate or from pyrolysis gasoline by many different methods. Most of those methods, but not all, involve the use of a solvent either for liquid-liquid extraction or extractive distillation. Many different solvents are suitable, including sulfolane (C₄H₈O₂S), furfural (C₅H₄O₂), tetraethylene glycol (C₈H₁₈O₅), dimethylsulfoxide (C₂H₆OS), and N-methyl-2-pyrrolidone (C₅H₉NO).

Below is a schematic flow diagram of one method, involving extractive distillation, for extraction of the BTX aromatics from a catalytic reformate: