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Use advanced catalysts to improve xylenes isomerization

02.01.2012  |  Shouquan, G. ,  Sinopec Zhenhai Refinery & Chemical Co., Zhenhai, ChinaChua, J. ,  Zeolyst International, Singapore

This refiner wanted to increase ethylbenzene conversion while limiting aromatics losses

Keywords: [catalyst] [xylenes] [paraxylene] [aromatics] [ethyl benzene] [isomerization] [petrochemicals]

Sinopec Zhenhai Refinery and Chemical Co. (ZRCC) started up a 450,000-tpy paraxylene (PX) complex in August 2003. The complex was originally designed to use a locally produced xylene isomerization catalyst. The performance of xylene isomerization catalyst during the first cycle began to deteriorate rapidly after the regeneration in March 2007. The ethylbenzene (EB) conversion rate dropped from 25 wt% before regeneration to 22 wt%, and the PX/xylene ratio in the product also dropped from 22.7% to 22% and directly led to a 0.4 wt% reduction of PX concentration in the PX adsorption unit feed. As a result, the PX production dropped by 50 tpd of PX; the C8 aromatics loss was higher than 4%, which is very high. Temperature and pressure increases in the reaction system were increasing significantly faster than before regeneration, with the rate of temperature increase jumping from 0.5°C /month to 2°C/month. As the isomerization catalyst performance deteriorated rapidly, ZRCC had planned to replace the xylene catalyst during a scheduled maintenance turnaround in 2008.

CATALYST SELECTION

After discussions with a number of catalyst technology owners (foreign and domestic) and conducting a catalyst technical evaluation among them, ZRCC selected the latest generation of a xylene isomerization catalyst. The new first-generation xylene isomerization catalyst was initially commercialized in 2001, and it has now been applied in more than 11 units outside of China. In June 2006, the Sinopec Yangzi branch applied the first-generation catalyst successfully in its aromatics plant with outstanding results. The newer generation catalyst is developed based on the concept of the first-generation catalyst with an improved manufacturing process. Processing benefits from the new catalyst system included high activity, high EB conversion rate, high PX/xylene ratio in the product, low C8 aromatics losses and long cycle life. In addition, this catalyst is very robust and can perform well in different operating conditions. It is in operation at three operating units in Taiwan and outside of China.

CATALYST PARAMETERS

This catalyst is jointly developed and uses a proprietary carrier. Upon delivery, ZRCC sampled and analyzed the catalyst. The results showed that the catalyst had a loss of ignition of 0.77 wt% at 420°C and a specific surface area of 267 m2/ g, with no particles smaller than 30 mesh.

Xylene isomerization process.

The xylene isomerization reaction of the EB reforming type catalyst is designed to isomerize aromatics present with PX in an amount often less than 1% in the reactor feed into four xylene isomers—PX, metaxylene (MX), orthoxylene (OX) and EB—close to equilibrium, at a defined temperature and pressure with the presence of a catalyst. The objective is to reduce the EB content and to increase xylene concentration of the feed for the PX adsorption unit. The higher xylene content to the adsorption unit increases the PX product yield and minimizes recycling and energy consumption.

For the isomerization reaction, higher EB conversion rate and PX concentration in the product will bring the C8 aromatics closer to equilibrium. At the same time, the C8 aromatics loss will be higher. This shows that, within a certain range, the activity and selectivity are in an inverse relationship. Therefore, careful consideration should be given to the activity and selectivity while operating with this catalyst. Table 1 lists the guaranteed values of the catalyst performance parameters.

INDUSTRIAL APPLICATION OF ZRCC PX COMPLEX

ZRCC’s PX isomerization unit has a designed throughput of 267 tons/hr. In the design, an extra C8 naphthenes recycle column was added downstream of the deheptanizer to reduce the circulation path of the C8 naphthenes. Fig. 1 is a simplified flow diagram of the ZRCC’s isomerization unit.

 

  Fig. 1.  Simplified flow diagram of
  ZRCC’s PX isomerization unit.



Catalyst loading.

ZRCC’s isomerization reactor is a radial-flow reactor. Based on the calculation, centerpipe modification in the original reactor was required to fully optimize the catalyst performance. This was done by removing the original seal and slump-catalyst layer and covering the top of the catalyst with a proprietary material that could withstand high temperatures. This top cap was followed with a layer of ceramic balls.

Pretreatment of the reactor. The spent catalyst did not undergo carbon burning before unloading. Thus, the reactor needed a pre-treatment step to remove the residual hydrocarbons. In the pre-treatment process, the carbon dioxide (CO2) level was monitored every hour. When the CO2 level in the reaction system was reduced to less than 0.2% and the water content to less than 1,800 ppm, the carbon-burning treatment was considered complete.

Reactor catalyst loading. After the carbon-burning treatment of the reactor, the catalyst was densely loaded using a proprietary dense-loading technology. The total actual catalyst loading was 84.96 tons.

Pretreatment of catalyst before feed.

Industrially produced catalysts will absorb moisture during manufacturing, transportation and loading. To ensure the activity of fresh catalyst, a drying process is done before introducing feed to the catalyst bed. The drying process requires heating the catalyst using nitrogen with an oxygen content of 1 mol% to 3 mol% at 10 barg. As new insulation materials were installed in the furnace during the downtime, to dry the insulation materials, the heating rate was very slow at first, and temperature was kept constant. When the temperature reached 200°C, after which the reactor inlet temperature was increased to 400°C at a rate of 25°C/hr and kept constant for two hours. Water was detected at 230°C and, at 260°C, the maximum water content was about 1,800 ppm, but no free water was detected at the low points of the system.

Catalyst reduction and passivation. After drying the catalyst was completed, the oxygen in the system was fully displaced by nitrogen. The pressure was then gradually raised with hydrogen to reduce the catalyst. During the reduction, four hours after the inlet temperature reached 420°C, water began discharging at low points of the system. The temperature was kept constant until the amount discharged started to decline. In total, 150.2 kg of water was collected during the reduction process; it is equivalent to 0.18 wt% of the water content of the fresh catalyst.

Catalyst sulfiding. Platinum, in a reduced state of the fresh catalyst, has a too high activity, which can lead to side reactions such as extensive cracking and/or temperature runaway in the case of direct feed to the reactor. All side reactions impact the catalyst’s long-term performance and service life. For that reason, catalyst pre-sulfiding is done before introducing feed in the catalyst bed.

A total of over 100 kg of DMDS sulfiding agent was injected into the system in two steps through a specific device onsite. It was hard to detect hydrogen sulfide (H2S) due to monitoring a large range of tubes, and no H2S was detected at the outlet of the reactor after the sulfur injection. After the sulfiding, the reactor inlet temperature was maintained at 335°C for the final preparation for feed introduction.

Catalyst feed introduction.

Prior to introducing feed to the catalyst bed/reactor, the composition of the liquid feed (raffinates and makeup hydrogen) was analyzed, and the feed met the required specification. The liquid feed was introduced into the reactor under these conditions: inlet temperature of 335°C, reactor pressure of 11 barg, recycle-gas hydrogen purity greater than 80% and makeup-hydrogen purity greater than 97 mol%. The weight hourly space velocity was 3.0 hr–1. After feed introduction, the reaction pressure decreased rapidly, and temperature rose sharply, during which the reactor outlet temperature rose to a maximum of 418°C, and the maximum reactor ∆ temperature was 50°C. After the first round of heat waves passed, the reactor ∆ temperature dropped to 35°C. This signified a successful catalyst feeding. Fig. 2 shows the reaction temperature increase and pressure changes during feed introduction.

 

  Fig. 2.  Reaction temperature increase and
  pressure changes during feed introduction.    


During the initial operating period after feeding, there were obvious increases in hydrogen consumption and in gas generation due to the formation of C8 naphthenes within the first few hours. After 24 hours, the difference between the temperatures at the reactor inlet and outlet was reduced to about 21°C, and the reactor outlet pressure was reduced from 10 barg to 6.3 barg, with the hydrogen purity up to 86%. The EB-ate and PX-ate were maintained at a relatively high level. With the catalyst further stabilized, the C8 aromatics loss was significantly reduced, thus indicating that the C8 naphthenes balance was established. After 48 hours, the PX-ate and EB-ate remained unchanged, with the C8 aromatics losses further reduced to the expected values. Thus, the feed introduction was successful.

INDUSTRIAL APPLICATION

After smooth operations for more than three months, ZRCC and the catalyst provider conducted a 72-hour catalyst performance test run. Tables 2 and 3 summarize the catalyst performance test run conditions and results, respectively. During the catalyst performance test run, all performance indicators were met, with PX-ate and EB-ate, and the C8 aromatics loss was better than the guaranteed values.

 



Performance comparison between catalysts. From the catalyst performance test run results, the new catalyst shows a great advantage in performance as compared to the original isomerization catalyst. The new catalyst system operates better in each of the indicators, especially the PX/xylene ratio, EB conversion rate and C8 aromatics loss, under less severe operating conditions (at a similar temperature but at a lower pressure of 0.7 barg). The comparison of specific performance indicators is summarized in Table 3.

From the comparison of the reactor inlet and outlet compositions, the PX concentration in the product increased by 2.2 wt% and EB concentration was reduced by 3.1 wt% using the new catalyst. Daily PX production increased by about 110 tpd, and ZRCC can achieve the operations target of increasing PX production while significantly reducing the C8 aromatics losses without any modifications to the process or to the configuration in the complex.

In 2009 and 2010, after the new catalyst was installed, ZRCC produced 500,000 tons of PX annually, in spite of some performance decline in the PX adsorption unit observed at the end of the period. The PX production was maintained at a similar level before replacing the catalyst, with the throughput remaining unchanged, and the C8 aromatics losses were reduced by 30,000 tons and 20,000 tons, respectively. The excellent range of performance offered by the new xylene isomerization catalyst allowed to compensate declining PX adsorbent separation performance by increasing operational severity and thus helped to maintain the competitiveness of the complex.

Longer operation cycle for the catalyst.

By January 2011, the xylene isomerization unit has been in operation for over two years, and the catalyst still demonstrated good isomerization activity and conversion rates while maintaining a low C8 aromatics loss. The reaction inlet temperature is now 374°C, and the rate of temperature increase is less than 0.5°C/month on average, particularly in 2010 when the temperature was only raised by 4°C for the entire year. These conditions indicate that the catalyst has excellent stability.

ZRCC’s PX plant has been in continuous operation for nearly eight years. In the second half of 2010, a decline in PX separation efficiency was observed. In order to maintain PX production throughput, ZRCC decided to reduce the EB concentration in the adsorption unit feed and to optimize the adsorption unit efficiency. ZRCC increased the operating pressure of the isomerization unit to maintain a relatively high EB conversion rate. In this case, even with a relatively high EB conversion rate and high PX/xylene ratio, the C8 aromatics loss was kept at a relatively low level, indicating that the catalyst is a very robust catalyst and can operate well in a wide range of conditions.

Observations.

The new generation xylene isomerization catalyst showed a very good performance after the startup. The performance test run data confirmed that indicators such as PX-ate, EB-ate and C8 aromatics loss are superior to the guaranteed values. The application at ZRCC is considered as a success.

With better PX/xylene ratio in the product and a higher EB conversion rate, the catalyst helped to optimize the PX adsorption unit feed while increasing PX production and lowering C8 aromatics loss. This led to a reduction in the C8 aromatics feed requirements and increased the total competitiveness of the PX complex, without any modification to the process and/or configuration of the complex. In addition, data from the long cycle operation indicate that the catalyst operated with good performance even under severe feed conditions, and it is a very robust catalyst with a good response to different operating conditions. HP

 



BIBLIOGRAPHY

Yingbin, Q., “The progress and application of catalysts for isomerization of C8 aromatics,” Engineering Science, Vol. 1, No. 1, 1999.
Zhanggui, H., “Industrial application of the SKI-400C catalysts for isomerization of C8 aromatics,” Petroleum and Petrochemical Today, Vol. 35, No. 4, 2005.
Yang, J. and D. Shi, “Industrial application of the SKI-400-type catalysts for isomerization of C8 aromatics,” Petroleum Refinery Engineering, Vol. 1, No. 1, 1999.

Notes

1 This catalyst is jointly developed by Zeolyst and Axens, using a proprietary carrier.

The authors 

Guo Shouquan joined ZRCC in 2001 in the aromatics production and technical management department. 

Jenson Chua joined Zeolyst International in 2006 as a technical consultant. 




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