Maximizing Para-Xylene Production by Optimization of the Methyl Group

Maximizing Para-Xylene Production by Optimization of the Methyl Group

Charlie Chou
Manager, Petrochemicals/PTA
GTC Technology US, LLC

Worldwide demand for pX consumption is predicted to grow 3-5% each year over the next 10-15 years. Some regions will demonstrate a growth rate as high as 6-8%.

By using traditional, established processes, the existing pX production complexes are producing significant amounts of benzene and heavy aromatics from the feedstock (wide-accepted reformate and pygas) rather than the virgin portion.

GTC Technology has developed a suite of advanced process technologies which aim to maximize the pX production from available aromatics-rich feedstocks. To configure and integrate individual technologies is to target these objectives:

1. Maximize the mixed xylene production
2. Maximize the pX concentration in the mixed xylene
3. Maximize utilization of toluene to generate pX molecule
4. Minimize the additional benzene generation
5. Minimize the additional heavy aromatics generation

To reach the above objectives in a cost-effective way, GTC has pioneered the way to optimize the methyl/phenyl ratio in the feed through commercialization of:

1. Aromatics Transalkylation (GT-TransAlk)
2. Toluene Methylation (GT-TolAlk)

Looking at pX molecule structure, the methyl group over phenyl group ratio is 2 to 1, therefore if the methyl/phenyl group ratio in the feed is also 2 to 1, then the generation of xylenes molecule (and consequently the pX molecule) should be maximized compared with other methyl/phenyl group ratio.

So the challenge becomes to how to intentionally make in the feed the methyl/phenyl group ratio to 2 to 1 or closest.

Like in GT-TransAlk, feed, mixture of one mole of toluene molecule and one mole of tri-methyl-benzene molecule will deliver the methyl/phenyl group ratio as 2 to 1, or mixture of four moles of toluene molecule, 2 moles of tri-methyl-benzene molecule and one mole of tetra-methyl-benzene will deliver the methyl/phenyl group ratio as 2 to 1. So optimizing the mixing ratio between toluene and C9/C10 heavy aromatics will be a challenge to maximize the xylene generation from GT-TransAlk unit. Of course, the activity, selectivity and stability of the catalyst employed also play a critical role.

C9/C10 Aromatics Sources includes:

• Reforming (heavy reformate)
• Aromatization (heavy aromatizate)
• Cat Cracking (heavy FCC gasoline)
• Ethylene Steam Cracking (heavy pygas)

Integration between refinery and pX complex is also a way for optimization the methyl/phenyl group ratio.

However, in reality, C9/C10 Aromatic stream also contains other molecules than tri-methyl-benzene and tetra-methyl-benzene, 75wt% C9/C10 aromatics and 25wt% toluene (which present approximately 2:1 methyl over phenyl mole ratio). This feed combination delivers the maximum xylenes yield through transalkylation reactions.

Toluene direct alkylation with a methyl group (GT-TolAlk) is an innovative way to utilize surplus toluene to maximize the pX molecule generation. Here the donor of the methyl group is methanol. Mixture of 1 toluene molecule and 1 methanol molecule delivers methyl group over phenyl group ratio: 2 to 1, most desirable for xylenes production. The catalyst employed in GT-TolAlk is very selective toward pX in the mixed xylene generated, which has demonstrated as high as 90% pX concentration.