The petrochemistry of paraxylene

Understanding the basics of the paraxylene manufacturing process.

Joseph C Gentry
GTC Technology

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Article Summary

The demand for para-di-methyl benzene, more commonly known as paraxylene (PX), is continuing to grow due to its use as a raw material in the polyester chain. Since 2009, PX demand has risen by 6% per annum, with predictions of increases by 5.1% per year from 2014 to 2018.

Key factors surrounding the production of paraxylene are:
•  Economics of scale. There are multiple process units and an extensive infrastructure required to produce paraxylene.  The scale of process facilities has increased to gain production economy.

• Raw materials shortage. Because of the high demand for paraxylene, the traditional sources of feedstock are insufficient to produce the large quantity of product needed, especially on a regional basis.

• Transparency. Persons in the industry often confuse technology brand names with process functionality, overlooking the opportunity to optimise the operation with ‘black boxes’.

It is important to understand the basics of paraxylene production, so that engineers can avail themselves of the latest technology and improvements available to use for PX production in the most cost effective manner.

To guide understanding, the elements of production are broken down into the following steps:
• Aromatics generation
• Aromatics interconversion.
• Paraxylene recovery and purification.

Aromatics Generation

The PX molecule is a benzene ring with two methyl groups arranged at opposite ends, and is typically created by catalytic reforming or thermal cracking of naphtha. The most common source of feed is heavy naphtha from refinery streams or condensate from gas fields, which is reformed into a product called catalytic reformate. Pyrolysis gasoline by-product from steam cracker petrochemical plants, and coke oven light oil (COLO) from steel production are used to a lesser extent.

Traditional feed sources
In some regions of the world, there is not enough heavy naphtha to reform into aromatics, creating an incentive to use non-traditional sources of feed. In actuality, some of the non-traditional feeds are more economical to use in any case, making these an important part of a competitive operation.

Among the non-traditional sources of xylenes are those contained within fluid catalytic cracking (FCC or RFCC) gasoline fractions. These aromatics already exist, but happen to be comingled with sulphur and olefins, which are difficult to remove by conventional techniques.

New process technology2 is now available to purify these aromatics by direct extraction from the raw FCC gasoline. This is in contrast to the practice used by several major producers, which recycles the FCC gasoline back through the naphtha hydrotreater and reforming units as an indirect means of purification and recovery. In such cases, the utility of the NHT/reformer is wasted.

The trend in industry is to operate the FCCU at higher severity to yield more propylene. The higher severity operation also increases the aromatics yield, making this source of raw material especially valuable. A beneficial side effect of aromatics extraction from the raw gasoline is that the aromatics content of the gasoline is diminished, as well as eliminating the problem of benzene in gasoline. Sulphur is removed to the specifications for Euro 5 or US Tier 3 grades of gasoline without any loss of octane value, as the unsaturated components never go to the hydrotreater units.

Another non-traditional source of aromatics derives from the olefinic fraction of FCC gasoline. It is convenient to create aromatics by aromatisation3 of the olefins from the C4-C8 boiling range, as these components are more reactive than paraffins from naphtha or condensate. The C4-C5 olefins can be obtained by direct fractionation out of the FCC unit, while the C6-C8 fraction comes from the raffinate of the extraction unit. Both of these can be combined to the aromatisation unit, which coincidently reduces gasoline production from the FCC unit.

A third non-conventional source of aromatic raw material comes from methanol via the methanol to aromatics (MTA) process. Methanol can also supplement aromatics production via the toluene methylation process. Naphtha is a product of crude oil, while methanol is produced from natural gas. There are long-term pricing trends which favour gas-based chemicals over oil-based chemicals, so that these methods can become a major contributor to the pool of xylenes.

Aromatics Interconversion
The next important step in the chain of technologies to produce paraxylene is to convert all of the aromatics from the above stated processes, into paraxylene. The materials are first converted into a mixture of xylenes by an aromatics interconversion process, and then paraxylene is extracted.

It is helpful to consider that dimethyl benzene, commonly called xylene, contains eight carbons, consisting of a stable ring with six carbons, and two methyl groups of one carbon each. If the feedstock generation operations create molecules with a six carbon ring and only one or no methyl groups, then one must add methyl groups to adjust the molecule to the eight carbons required. If there are too many methyl groups (or higher alkyl groups) from C9+ aromatics, then these need to be removed to get back to eight carbons. Even for the 8 carbon aromatic molecules with two methyl groups, there is still the need to rearrange the position of the methyl groups on the aromatic rings to produce PX, which is 1, 4-di methyl benzene.

These are the relevant unit operations to transform aromatic species into other types by shifting alkyl groups.
•  Hydrodealkylation (HDA) removes all alkyl groups from the benzene ring. This process makes on-purpose benzene instead of xylenes.
•  Transalkylation (TA) is the broad category of processes which move methyl groups to a ring with a different number of carbon molecules. The distribution of methyl groups among aromatic rings tends toward an equilibrium based on the ratio of phenyl : methyl groups in the starting feeds.
• Toluene disproportionation (TDP) or selective toluene disproportionation (STDP) are processes which use only toluene as feedstock and produce benzene plus xylenes. The selective version makes xylenes with above equilibrium concentration of paraxylene. This makes it easier to produce the high purity PX, though other drawbacks mitigate this advantage.
• Toluene alkylation (TolAlk or TM) adds methyl groups, typically using methanol as the source of the CH3. A selective version makes a concentration of PX among the isomers of dimethyl benzene along with other unwanted by-products and higher catalyst coking.

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