Ronald G. McClung
Engelhard Corp.
Edison, N.J.
Reduction of sulfur in catalytic reforming feeds below that typically obtainable with naphtha hydrotreating improves the reformer operation significantly.
In evaluations of pilot-plant and commercial operations by Engelhard Corp., high-rhenium content reforming catalysts have been used with low-sulfur naphtha feeds and have been shown to dramatically affect catalyst cycle length, gasoline yield, and stability.
The consequences of insufficient removal of sulfur include shorter cycle length, more rapid reactor temperature rise to maintain octane, and as a result of that increase, more rapid catalyst deactivation.
PLANT DATA
Commercial plant data show that a high-rhenium reforming catalyst, where the rhenium-to-platinum (Re/Pt) ratio equals 2.8, operating on naphtha feed with essentially no sulfur, has a relative stability benefit of approximately 200%, compared to a balanced platinum and rhenium catalyst (Re:Pt equals 1) operating on a naphtha feed with similar sulfur content. (The tests for sulfur were conducted in accordance with ASTM D-4045-87.)
Tests conducted under this standard cover the determination of sulfur in petroleum products within the range 0.02-10.0 mg/kg sulfur. But the method may be extended to higher concentrations by dilution.
The performance of high-rhenium catalysts displays a much more adverse effect with operation on high-sulfur naphtha than balanced, bimetallic catalysts when operated on the same feed sulfur level (Fig. 1). Correspondingly, high-rhenium catalyst performance shows a substantially more positive effect from reduction of sulfur in the feed.
The reason for this most likely relates to rhenium's high affinity for sulfur. For catalysts with Re:Pt greater than 1.0, feed sulfur specifications are recommended to be less than 0.5 wt ppm in order to realize the advantage imparted by high rhenium compared to balanced, bimetallic catalyst operations.
For Re:Pt equal to 1.0 or less, most vendors recommend that the feed sulfur level be less than 1.0 wt ppm.
ADDITIONAL FEED PRETREATMENT
Maintaining the reformer feed sulfur levels consistently below 0.5 wt ppm requires close attention to normal pretreatment procedures, such as hydrotreating and proper reboil stripping. When these methods prove inadequate, feed sulfur can be further reduced using sulfur sorption technology.
Called Sulfur Guard, the vapor-phase technology has been installed on more than 14 operating reformers. The operating units treat a wide variety of feeds to reformers of many varying operating pressures and severities.
A joint development of Engelhard and Atlantic Richfield Co. (ARCO), the process consists of an adsorbent bed that removes the last traces of sulfur without adding substantial complexity to the reforming operations.
The process is a vapor-phase process with the reactor inserted into the reformer train at a point downstream of where the feed is combined with recycle hydrogen (Fig. 2).
By inserting the unit at this point, both the feed and recycle are processed through the adsorbent bed.
This procedure minimizes the total sulfur in the system because if any trace sulfur escapes during the first pass through the unit, it will be removed from the recycle gas on the second pass after being converted to hydrogen sulfide over the reforming catalyst.
Commercial run data using the sulfur guard unit in the reforming process show that there is essentially no sulfur in the reformer stabilizer overhead (Fig. 3).
Fig. 3 shows hydrogen sulfide content of the stabilizer overhead because it is difficult to measure sulfur in the vapor phase from the sulfur guard reactor.
The outlet of the unit is about 800 F., and two phases occur when the sample is cooled to ambient. Therefore, to measure the sulfur level at the exit of the sulfur guard reactor, the hydrogen sulfide concentration at the exit of the reformer stabilizer is measured.
If there is zero or only trace hydrogen sulfide concentration at the stabilizer exit, the sulfur content at the exit of the sulfur guard reactor must be zero.
The unit does not require plant Utilities and requires little operating attention. The material used for the adsorbent in the unit is selective to sulfur only; therefore, it is unaffected by most feed contaminants.
The unit can be used to lower system pressure to obtain greater gasoline and hydrogen yields while maintaining the same cycle length and gasoline octane.
It can also be used to protect the reformer from naphtha hydrotreater sulfur upsets and substantially improve cycle length, activity, and yields.
Finally, the unit can allow the use of the latest high rhenium-to-platinum catalysts to increase reformer severity with higher-sulfur-content feeds.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.