1. Process Introduction
1.1 Background
C8 aromatics isomerization technology (C8AIT) is used in the aromatics complex to additionally produce the desired xylene isomer. According to the conversion pattern of ethyl benzene (EB), C8AIT is classified into two categories: ethyl benzene dealkylation of C8 aromatics isomerization technology (AIDT), in which EB is converted to a valuable benzene co-product, and ethyl benzene isomerization of C8 aromatics isomerization technology (AIIT), in which EB is converted to xylenes. A C8AIT unit is often integrated with an adsorptive separation unit or a crystallization separation unit, and a xylene splitter (called as PX loop) in the aromatics complex.
1.2 Process Description
C8AIT developed by SINOPEC is used to convert a mixture of C8 aromatic compounds depleted in one isomer to an equilibrium mixed xylene stream. In the case of para-xylene (PX) production, the liquid feed is from PX recovery section (PX adsorptive separation unit or crystallization separation unit), and the reactor effluent is sent to the xylene splitter. The obtained equilibrium mixture of C8 aromatic isomers is recycled back to PX recovery section to produce the additional PX. The process diagram is followed as fig 1.
Figure 1 C8AIT Process Flow Diagram
The raffinate of C8 aromatics from adsorptive separation is first mixed with hydrogen-rich recycle gas, preheated and vaporized through heat exchange with the reactor effluent, and heated to operating temperature, then charged into the reactor to pass through the catalyst bed. The reactor effluent is cooled by heat exchange and then sent to a gas-liquid separator where hydrogen-rich gas is taken off from the top of the separator and recycled back to the reactor. The liquid product from the separator bottom is charged into the deheptanizer. The overhead fraction is sent to upstream section for recovery of benzene. The deheptanizer bottom liquid is combined with fresh C8 aromatics feed from CCR & TDT units and pass through a clay treater and the xylene splitter, then recycled back to the adsorptive separation unit.
1.3 Process Operating Conditions
Process operating conditions of two categories of C8AIT
| Items | AIDT | AIIT |
| Reaction temperature/℃ | 360~410 | 360~420 |
| Reaction pressure/MPa | 0.5~1.20 | 0.5~1.60 |
| WHSV/h-1 | 6.0~12.0 | 3.0~5.0 |
| H2/HC mole ratio | 0.6~5.0 | 2.5~5.0 |
2. Technology Features
2.1 Operating flexibility
The catalysts used in C8AIT are suitable for the severe conditions such as high WHSV, low hydrogen to hydrocarbon ratio, and high EB content in feed, etc, and can offer high xylenes isomerization activity, high selectivity and excellent stability. By adopting C8AIT in an aromatics complex, it will be easy to achieve high xylene product yield and low energy consumption.
Case Conditions
| Items | SKI series in AIDT | RIC series in AIIT |
| WHSV/h-1 | 6.0~12.0 | 3.0~5.0 |
| H2/HC mole ratio | 0.6~4.0 | 2.5~5.0 |
| Cycle H2 purity/mol.% | may be less than 60 | may be less than 70 |
| High EB feed | EBC more than 75% | EBATE* more than 70% |
*EBATE：EBATE=( EB/C8A in feed – EB/C8A in effluent) /（EB/C8A in feed – EB/C8A of Equilibrium)×100%
Catalytic Performance
| Items | SKI series | RIC series |
| PXATE/% | 98~103 | >97 |
| EB conversion ability | EBC of 65%~75% | EBATE of 50%~70% |
| C8NA/C8Amole ratio in effluent | may be less than 0.003 | may be less than0.06 |
| Single cycle processing capacity/(t feed/kg Cat.) | Min.650 | Min.120 |
2.2 Process Flexibility
C8AIT can be used to increase the production of PX, or to increase the production of MX and OX, or to achieve co-produce benzene production with utilizing AIDT catalyst. The utilization of aromatics source and product distribution could be optimized by changing different type of catalyst as necessary.
2.3 Technology Economics
The cost in battery limit for a C8AIT unit with PX production capacity of 600 KMTA for the aromatics complex is about 0.30 billion RMB based on the construction practice in China.
Low Energy Consumption
High EB conversion ability to guarantee low EB flow rate inside the PX loop
Low C8 non-aromatics flow rate inside the PX loop
High PX concentration and PX/EB ratio to enhance the processing capacity of the adsorption unit and decrease energy consumption
Low hydrogen to hydrocarbon ratio to make low energy consumption of the compressor of isomerization unit
2.4 Feed Adaptability
C8AIT can process fresh C8 aromatics feedstock with wide range of EB content.
4. Catalysts
C8 aromatics isomerization catalyst is a kind of acid-metal bifunctional catalyst with noble metal supported on a zeolite and an inert material. Their physical properties are listed as follows.
Physical Properties of Catalysts
| No. | Items | SKI series | RIC series |
| 1 | Shape | Cylinder | Cylinder |
| 2 | Size/mm | D 1.6 × L 2~10 | D 1.6 × L 2~10 |
| 3 | Catalyst state | Reduced | Reduced |
| 4 | Pt content/w% | ~0.03 | 0.28~0.30 |
| 5 | Crush strength/N·cm-1 | Min. 100 | Min. 120 |
| 6 | Bulk density/kg·m-3 | 700±50 | 670±30 |
5. Commercial Experience
The catalysts SKI-210 （6 plants）and RIC-270, have been commercially used in 8 plants. Upgraded SKI-320（2 plants） and RIC-300 had been developed and produced. In 2012, a commercial plant for the production of Para-diethyl benzene from a mixture of diethyl benzene isomers adopting C8AIT integrated with the adsorptive separation process was put on stream. A 600 kt/a PX capacity aromatics complex using C8AIT process and RIC-270 catalyst was built in 2013, another 1Mt/a PX complex was put into operation in 2019.