Jul-2008

Improving hydrotreater reactor performance

Case study of a kerosene hydrotreater shows that combining new and old technologies - a unique inlet diffuser with Raschig rings - can significantly improve liquid distribution and catalyst utilisation when producing ultra-low-sulphur products in a trayless reactor.

Garry E Jacobs, Fluor Enterprises Inc
Gerianne D’Angelo, Advanced Refining Technologies

Article Summary

The performance of hydro-processing reactor internals has improved significantly over the past decade, driven in part by ultra-low-sulphur product specifications.1,2 Yet, some reactors continue to operate with internals that are inadequate for the operating conditions. This inadequacy may be caused by the introduction of heavier feedstocks or by constraints that preclude the installation of the appropriate hardware.

The following case study of a kerosene hydrotreater that failed to meet product sulphur targets, subsequent to a catalyst changeout, exemplifies the inadequacies observed with reactor internals. The reactor operates with a mixed-phase feed, but is lacking a liquid distribution tray.

Troubleshooting work included pilot plant testing of the catalyst retains, 
leak testing of the feed/effluent exchangers, and radioactive tracer tests and gamma scans of the reactor. A charge rate test was also performed, which suggested significant under-utilisation of the catalyst. Evidence of liquid maldistribution was also obtained by visual inspection of the catalyst 
bed surface and analysis of the spent catalyst samples.

Using computational fluid dynamic (CFD) modelling, a new inlet diffuser was designed to improve liquid distribution to the top of the catalyst bed and was installed during a catalyst changeout. In addition, high-void fraction random packing was loaded at the top of the bed to promote radial liquid dispersion. After a restart of the unit, the reactor was able to meet the refiner’s 10 ppmw sulphur target for ultra-low-sulphur kerosene (ULSK) product.

Background
Subsequent to a catalyst changeout in the spring of 2007, a kerosene hydrotreater failed to meet ULSK specifications. The unit feed consists of a mixture of straight-run kerosene and coker naphthas. During the previous catalyst cycle, the reactor initially treated only fully vapourised naphtha. A couple of years into that eight-year cycle, straight-run kerosene was introduced, creating a mixed-phase feed. During the final year of that cycle, the reactor was able to produce ULSK. The catalyst changeout was dictated by concerns of silicon breakthrough into the reformer feed.

The reactor did not have a liquid distribution tray. The distribution hardware consisted solely of a basic inlet diffuser, as shown in Figure 1. The 
inlet diffuser is inserted in the reactor inlet nozzle and helps dissipate the momentum of the incoming feed. The addition of a tray was considered by refinery personnel, but could not 
be justified based on acceptable performance during the catalyst cycle leading up to the 2007 turnaround.

Troubleshooting activities
A simplified comparison of the operating conditions and performance for the 
two catalyst cycles is presented in 
Table 1. Of particular note was the approximately 1.5 psi reactor pressure drop through the post-2007 T/A catalyst load. With uniform vapour/liquid distribution, the pressure drop should be at least twice as high. This was 
the first evidence of significant 
liquid maldistribution. The ensuing troubleshooting activities further explored this and other possible causes of the higher-than-expected product sulphur levels.

Feed/effluent exchangers
When producing ultra-low-sulphur distillates, extremely small and previously unnoticeable leaks in feed/effluent exchangers can result in off-spec product. Several techniques were used by the refiner to check for such leakage, including feed and product sulphur speciation and tracer tests. If raw feed is leaking into the reactor effluent, easily hydrotreated sulphur compounds (eg, mercaptans and alkylthiophenes) will be present in the product. The speciation results confirmed the presence of mercaptans, but not propyl- and lighter thiophenes that were abundantly present in the feed. The absence of these thiophenic compounds, which cannot be produced by recombination reactions, provided strong evidence that the feed/effluent exchanger leakage was not the cause of high sulphur levels in the kerosene product. This result also indicates that the mercaptans were formed by recombination reactions, as discussed subsequently.

Two tracer tests were also performed on the feed/effluent exchangers. In separate tests, a high-volatility sulphiding agent and a radioactive noble gas tracer were injected on the feed side of the exchangers. The effluent side of the exchangers was then monitored for the presence of tracer. Both tests indicated no leakage across the exchangers within detection limits. Implicitly, the absence of the sulphiding agent in the reactor effluent was also an indication of liquid-phase maldistribution, as the fully vapourised sulphiding agent was completely converted to hydrogen sulphide in the reactor.

Catalyst activity
Since the reactor does not have a distribution tray, liquid-phase sulphiding could not ensure proper activation of the entire catalyst load. For this reason, vapour-phase sulphiding was performed using dimethyl disulphide (DMDS). The sulphiding step was uneventful, with the low- and high-temperature hydrogen sulphide breakthroughs occurring as expected.

The catalyst Certificate of Analysis (COA) was reviewed and found in compliance with the Process and Quality Assurance Specifications. Catalyst retains from the commercial reactor were loaded into a pilot plant reactor and tested using a representative blend of the refiner’s feedstocks. For comparison, a standard sample of the same catalyst type was also tested in the pilot plant. The results confirmed that the activity of the retained sample was essentially equal to that of the standard sample within pilot plant testing tolerances. Furthermore, the temperature required to meet the ULSK sulphur target in the pilot plant was actually 10°F lower than the SOR projection for the commercial reactor.

The pilot plant results indicated an activity disparity of approximately 60°F compared with commercial unit performance. This is roughly equivalent to a 16-fold difference in space velocity. At first glance, this disparity may seem too dramatic to be explained by liquid maldistribution. This point will be addressed subsequently with the benefit of further observations.

Vapour/liquid distribution within the reactor
To explore the possibility of liquid maldistribution within the reactor, the refiner utilised a process diagnostics services company to perform two separate tests.