Filippo Gorra
Praoil Srl
Milan
Lee L. Breckenridge, Wayne M. Guy
Mobil Research & Development Corp.
Paulsboro, N.J.
Robert A. Sailor
Mobil Research & Development Co.
Princeton, N.J.
A selective toluene disproportionation process based on Mobil's ZSM-5 catalyst, Mstdp, has completed a full 500-day first cycle at Enichem Anic's refinery in Gela, Italy.
After regeneration, a second cycle operated for 100 days to confirm the regenerability of the catalyst.
For Enichem, the main objective was to evaluate process yields and economics. For Mobil, the main objective was to demonstrate commercial catalyst selectivation (the establishment of high selectivity through a pretreatment step), cycle length, and regenerability.
PROCESS CHEMISTRY
Disproportionation is a chemical reaction in which a single compound serves as both oxidizing and reducing agent. In the Mobil process, called Mstdp, toluene feed disproportionates to benzene and paraxylene. Purity of paraxylene was as high as 90% of the toal xylenes in the Enichem test. The primary reaction is shown in Fig. 1.
A small amount of by-products is formed, mainly light gases, meta and orthoxylene, and C9+ aromatics.
In conventional toluene disproportionation processes offered by Mobil and others, the xylene product mix contains paraxylene at its equilibrium concentration of 24%. 1 2 Mstdp is the only available toluene conversion process that gives a highly enriched paraxylene product.
UNIT HISTORY
Enichem had operated conventional toluene disproportionation (MTDP) with Mobil catalyst from 1982 until 1988. The unit was originally constructed as a toluene dealkylation unit (Hydeal process) in 1970.
To convert the unit to Mstdp service, Enichem's engineers worked closely with Mobil engineers to plan the commercial test run. A simplified process flow diagram of the Enichem Mstdp unit is shown in Fig. 2.
Stabilized product from Mstdp goes into the main plant benzene/toluene/xylenes (BTX) fractionation system, where it combines with other streams. The combined xylene streams are shipped to another location for paraxylene recovery.
In typical Mstdp applications the para-enriched product goes directly to paraxylene recovery.
Only minor unit modifications were necessary for Enichem's demonstration of the Mstdp technology. Most of the modifications were based on the catalyst selectivation conditions, which require higher temperatures than does MTDP.
Enichem improved the reactor insulation in preparation for this high-temperature test.
The reactor is a conventional cold-wall design with internal insulation. A special thermowell was installed to measure radial temperatures in the catalyst bed. In Mstdp, a very uniform catalyst temperature is required, and radial heat loss is a concern in a reactor of this type.
A 2-in. nitrogen line was added to deliver plant nitrogen for catalyst selectivation.
CATALYST SELECTIVATION
In the Mstdp process, very high selectivity for paraxylene is obtained through the combination of a proprietary Mobil catalyst and a novel pretreatment procedure. The pretreatment step is performed at the beginning of each cycle.
The relative diffusivity of paraxylene is several orders of magnitude greater than that of the ortho and meta isomers. Mstdp takes advantage of this to obtain a xylene product with paraxylene concentration significantly greater than equilibrium.
Although the development of this technology had been thoroughly tested in Mobil's pilot plants, Enichem elected to make a practice run to ensure that the commercial equipment and controls functioned properly.
On Oct. 19, 1988, full Mstdp selectivation conditions were achieved over a catalyst fill of conventional toluene disproportionation catalyst. No equipment problems developed despite the severe test conditions.
Catalyst bed temperatures were found to be almost uniform, with only a slight temperature drop at the reactor wall. Nitrogen and hydrogen makeup gas systems functioned as designed.
After purging and unloading the catalyst from the reactor, the Mstdp catalyst was loaded Nov. 2, 1988.
The Mstdp reactor is a conventional downflow design (Fig. 3), holding 3,700 kg of catalyst with an L/D ratio of 0.85. Final equipment checks were made after catalyst loading, and the heat-up/dry-down steps began Nov. 9, 1988.
The unit contained toluene feed driers, but no recycle gas driers. Since Mstdp catalyst is sensitive to moisture, the heat-up proceeded very slowly to purge water vapor from the unit with dry makeup H2 gas.
Recycle gas driers are normally required for Mstdp. The makeup gas source was a conventional naphtha reformer, which supplied dry hydrogen.
The selectivation step began Nov. 15, 1988, with the introduction of toluene feed to the reactor. Total time of selectivation was 37 hr.
The termination point for selectivation is defined by a predetermined paraxylene (PX) concentration in the xylene product. The termination point at Enichem (76% PX) was chosen so that the process could achieve 30% toluene conversion with 82% paraxylene selectivity for the initial operating period.
Although higher paraxylene concentrations can be achieved, this value was set conservatively to attain a long cycle length.
During selectivation, the reaction products were fed to the main plant BTX fractionation facilities.
PERFORMANCE
During the first cycle the unit performance was very steady. Only two brief controlled shutdowns for mechanical repairs occurred during the entire 500-day run. Neither shutdown affected catalyst performance.
The operating conditions after lineout are:
- Weight hourly space velocity (WHSV) = 4
- Temperature, C. = 455
- H2/HC ratio, mol/mol = 3
- Toluene conversion, % = 30
- PX selectivity, % = 82.
Detailed yields at these conditions are shown in Table 1. As the catalyst aged during the run, the temperature was increased to maintain 30% toluene conversion. At 470 C., the temperature was held constant and toluene conversion was allowed to drop.
This approach prolonged the cycling time and allowed paraxylene selectivity to increase. When toluene conversion had declined to 25%, Enichem decided to regenerate the catalyst.
By this time, paraxylene selectivity had increased to 90% (Figs. 4-6).
REGENERATION
Following Cycle 1, the catalyst was unloaded and shipped off site for regeneration. The Mstdp catalyst was given a complete decoking under controlled conditions by a toll regenerator. No chemical treatment of the catalyst is required.
Enichem decided to regenerate off site because its equipment was not designed for regeneration conditions. In situ regeneration is, however, an option for grassroots plants.
CYCLE 2 PERFORMANCE
The objectives of the second cycle were:
- To verify the regenerability of the catalyst, and
- To achieve a higher concentration of paraxylene in the xylene product.
The selectivation step for Cycle 2 was identical to the first selectivation, except for a higher endpoint of 82% paraxylene. The lined out conditions of the process cycle gave 86% paraxylene at 30% toluene conversion and a temperature of 465 C.
A higher temperature was required to achieve 30% conversion in Cycle 2 because of the deeper selectivation (i.e., higher PX endpoint) and a slight decrease in catalyst activity following regeneration.
Figs. 7-9 show the performance during Cycle 2. Second cycle objectives were met, and the Enichem unit was shut down after only 100 days because of local economic conditions.
OTHER APPLICATIONS
The successful demonstration of the Mstdp process at Enichem resulted in two new process licensees, both of which are located in the U. S. The first, a partial retrofit unit, came on stream in May 1991. Since startup, this unit has met all expectations.
The second, a grassroots plant at Koch Refining Co.'s Corpus Christi, Tex., refinery, started up in May.
ACKNOWLEDGMENT
The authors acknowledge the assistance of Giorgi Scribano, Enichem's Gela refinery engineer, who was instrumental in implementing the plant modifications that preceded the Mstdp demonstration and responsible for compiling plant data obtained during Mstdp operations.
REFERENCES
- Han, S., Shihabi, D.S., Absil, R.P.L., Huang, Y.Y., Leiby, S.M., Marler, D.O., and McWilliams, J.P., "ZSM-5 catalyst developed for toluene disproportionation," OGJ, Aug. 21, 1989, p. 83.
- Menard, Kevin P., "Improved process disproportionates toluene at Fina's refinery in Port Arthur, Tex.," OGJ, Mar. 16, 1987, p. 46.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
