Advances in xylene isomerisation (TIA)

Sinopec Zhenhai Refinery and Chemical Company (ZRCC) started up its 450000 t/y paraxylene (PX) integrated aromatics complex in 2003.

Amir Taiyabi

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Article Summary

This was the first aromatics complex in mainland China to utilise IFP Eluxyl Technology. Since its successful start-up in 2003, ZRCC has applied catalysts with leading performance and improved the aromatic complex process to consistently and steadily increase xylene isomerisation efficiency and PX capacity production. As of September 2015, PX production capacity has reached 650000 t/y with good operating margin. In 2008, ZRCC selected the OparisPlus xylene isomerisation catalyst and achieved close to six years of stable operation. During the scheduled turnaround in 2014, and after multiple rounds of catalyst comparisons, ZRCC became the first user of the improved xylene isomerisation catalyst, OparisMax.

Ethylbenzene isomerisation
In a paraxylene depleted mixed xylene stream, which contains ethylbenzene, the ethylbenzene can be isomerised to an equilibrium mixture of xylene isomers (paraxylene, metaxylene, and orthoxylene) using a bifunctional catalyst. Typical feeds are mixed xylene streams depleted of paraxylene and, at times, orthoxylene. This stream is usually one of the products from an adsorption based paraxylene recovery unit or crystalliser paraxylene recovery unit.

When an Oparis series catalyst is applied, high concentrations of ethylbenzene in the fresh feed to the complex can be utilised. This is because the Oparis catalyst’s high ethylbenzene conversion limits the build-up of ethylbenzene in the recycle loop. This gives the user the ability to blend in large amounts of mixed xylenes extracted from pygas streams that are high in ethylbenzene. The appropriate process conditions can achieve very high ethylbenzene conversion levels while the approach to equilibrium (-ate) for paraxylene is almost at the thermodynamic limit of 100%.

Industrial application of OparisMax
Xylene isomerisation unit

ZRCC’s xylene isomerisation unit has a design throughput of 300 t/h. In the design of the isomerisation unit’s configuration, a naphthenes recycle column was added downstream of the deheptaniser to reduce the circulation path of the naphthenes. A simplified flow chart of ZRCC’s xylene isomerisation unit is shown in Figure 1.

Catalyst loading
The OparisMax catalyst can be either dense loaded or sock loaded, depending on the application. For the ZRCC application, OparisMax was dense loaded using Axens’ proprietary Catapac dense loading technology and the catalyst was sealed with Axens’ Texicap technology.

Catalyst feed introduction
Reactor feed was introduced under the following conditions: reactor inlet temperature of 320-350°C and a reactor pressure of 10-15 barg. After feed introduction, an initial reactor bed exotherm was observed, which is typical for this application. The reactor bed exotherm then gradually stabilised (see Figure 2).

Industrial application results Catalyst performance
After feed-in, Zeolyst and ZRCC optimised the xylene isomerisation reactor’s operating conditions and adjustments in upstream units. Table 1 shows catalyst performance after the initial adjustment.

After smooth operation for three months, ZRCC and Zeolyst conducted a catalyst performance test run. During the test run, PX-ate, EB-ate and C8 aromatics loss exceeded the guaranteed values. The test run conditions and results are shown in Table 2.

Performance comparison
From the operating data of OparisMax and OparisPlus at ZRCC, OparisMax has an advantage in performance in each of the performance indicators. Weighted hourly space velocity (WHSV) was increased by 16% and reactor inlet temperature was reduced by approximately 10°C. At slightly better PX-ate, OparisMax EB-ate had a 10% increase and C8 aromatics loss reduced by 20%. The comparison of specific performance indicators can be seen in Table 3.

Long term operating data
ZRCC’s first year operating data shows that OparisMax will exceed the cycle length of OparisPlus. The catalyst showed very good stability with ~40% lower temperature deactivation compared with OparisPlus.

OparisMax is a third generation catalyst in the Oparis family. The OparisMax operating data at ZRCC shows improved performance with regards to increased WHSV and EB-ate, and lower aromatics loss, while maintaining excellent PX-ate. These performance characteristics will enable further optimisation of the paraxylene adsorption unit feed and increase paraxylene production without any modification to the process or configuration of the complex. The high selectivity of OparisMax led to a 20% reduction in C8 aromatics loss, which will reduce the C8 aromatic feed requirement and increase the overall competitiveness of the paraxylene complex.

The long term operating data from ZRCC shows that OparisMax is able to operate with lower severity and catalyst deactivation even under higher WHSV, indicating that it is a robust catalyst with an expected long cycle life.

This short case study originally appeared in PTQ's Technology In Action feature - Q1 2016 issue.

For more information: Amir.Taiyabi@cri-criterion.com

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