BTX

(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. The xylene isomers are distinguished by the designations ortho – or o –, meta – or m –, and para – or p – as indicated in the adjacent diagram. If ethylbenzene is included, the mixture is sometimes referred to as BTEX.

The BTX aromatics are very important petrochemical materials. Global consumption of benzene, estimated at more than 40,000,000 tons in 2010, showed an unprecedented growth of more than 3,000,000 tons from the level seen in 2009. Likewise, the para-xylene consumption showed unprecedented growth in 2010, growing by 2,800,000 tons, a full ten percent growth from 2009.

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

	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   Pyrolysis Gasoline 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. About 70 percent of the global production of benzene is by extraction from either reformate or pyrolysis gasoline.

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:

(CC) Diagram: Milton Beychok
Schematic flow diagram for the extraction of BTX aromatics from a catalytic reformate.

Petrochemicals produced from BTX

The BTX aromatics produced by catalytic reforming are used to raise the octane number in unleaded gasolines.

There are a very large number of petrochemicals produced from the BTX aromatics. The following diagram shows the chains leading from the BTX components to some of the petrochemicals that can be produced from those components:

(CC) Diagram: Milton Beychok
Diagram of the chain of petrochemicals derived from the BTX aromatics.

Polyethylene terephthalate (PET) is a reasonably economical plastic with excellent material properties and is recyclable. PET is produced in large quantities from terephthalic acid (or dimethyl terephthalate), which in turn is produced by oxidizing p-xylene. This demand makes p-xylene the most valuable of the xylene isomers, with o-xylene second in demand, and m-xylene the least in demand, although it is still used. The xylenes from BTX come in the 3-isomer mixture, and a couple of processes can be used to separate the xylene isomers, and re-isomerize the xylene stream least in demand for re-separation. Good separation of the xylenes by distillation is difficult because the boiling points are close, especially the para and meta isomers.