Selecting catalysts for marine fuels production

Accelerated activity and stability testing of hydrotreating catalysts.

ED OUWERKERK, Catalyst Intelligence

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Article Summary

Catalyst activity and stability are important factors in boosting the refining margin of units like reformers, hydrocrackers, and hydrotreating units. A refinery has little influence on feedstock and product prices, but can increase its margin by selecting the best catalyst for the unit configuration and by wisely planning its activity over the cycle.

A western European refiner wanted to select a catalyst best suited to its 100 m3 single bed hydrodesulphurisation (HDS) reactor operating at medium pressure with the aim of producing marine fuels. The aim is to produce fuels in three separate modes using three different Urals feed cuts. The main product components using three different feed cuts are: 50 ppm sulphur industrial gasoil (IGO), 800 ppm sulphur marine gasoil (MGO), and 4500 ppm sulphur low sulphur fuel oil (LSFO). In view of the size and duration of the catalyst supply contract, the refinery decided to perform catalyst testing and not base its catalyst selection on vendor claims alone. Q8 Research together with Catalyst Intelligence designed an experimental protocol for catalyst testing which guided the selection of the best performing catalyst. The catalyst testing was performed at Q8 Research under actual process conditions with actual feedstock used in the refinery.

A maximum cycle length for the catalyst is highly desirable. Moreover, it is beneficial for the refinery to have lower hydrogen consumption and at the same time meet all product specifications. The catalysts used for hydrotreating have a lifetime ranging from one to six years, depending on the composition of the feed (gasoil, vacuum gasoil, atmospheric resid, and so on) and process conditions (pressure, hydrogen/oil ratio, temperature, LHSV, hydrogen purity) used in the reactor. Deactivation of hydrotreating catalysts occurs typically due to coke formation, metals deposition, and sintering of active sites.1 In the case of processing middle distillates (kerosene or gasoil), the metal content of the feed is low and catalyst deactivation occurs most likely due to coke deposition over its lifetime.2,3 In the case of heavier feedstocks (atmospheric residue, vacuum resid), catalyst deactivation occurs most likely by both metal deposition on catalysts and coke formation. Moreover, hydroprocessing catalyst deactivation is possible for all feeds due to the sintering of active sites over its lifetime. Sintering of active sites is directly related to temperature and can be enhanced when the reactors are operated at higher temperatures near to the end of life of the catalyst, or at high temperatures during operations, caused by upsets in the commercial reactor.

To evaluate hydroprocessing catalysts’ stability, it is essential to perform an accelerated aging test, as it is not practically feasible to run a test for the exact lifetime of the catalyst. However, the parameters used to design the accelerated test must be within the operating window of the actual process conditions used in the refinery.

The term accelerated deactivation has no strict definition for a catalyst and can apply to different sets of process conditions used in a pilot unit, depending on the type of test. Many times, accelerated tests are designed with process variables which are never experienced in a real process and can result in different deactivation mechanisms that may not be relevant for actual stability comparison between the catalysts.4 The term completely loses its purpose when it is referred to without indicating the actual process conditions (pressure, temperature, hydrogen/oil, LHSV) at which the respective catalysts were operated.1,5 The experimental set-up in our case is designed in such a way that performance feedback at start of run (SOR) conditions is obtained from separate parallel reactors, and accelerated deactivation feedback is obtained from another set of parallel reactors. The conditions selected for accelerated deactivation are within the operating range of a commercial hydrotreatment reactor. Moreover, the design of the experiment allows a comparison of differently operated reactors under uniform hydrotreatment conditions at the end of the test.

A 16-flow parallel reactor unit was used to perform catalyst testing. It has four different heating blocks (see Figure 1). Each heating block contains four reactors which can be set at different temperature to allow testing of multiple process conditions simultaneously. All of the experiments were performed at the same pressure, hydrogen purity, liquid hourly space velocity (LHSV), and hydrogen/oil ratio as is used in an actual reactor in the refinery.

In order to have equal internal mass transfer influence on catalyst performance from different vendors, the catalyst extrudates as received were sorted in a length range of 2-4 mm. A customised robot is used at Q8 Research to perform catalyst sorting and size distribution analysis. This is an important step, as smaller catalyst particles allow more surface area and hence higher activity. Uniform size distribution is critical to provide precise comparison of catalyst activity, especially when using small scale reactors (<20 ml) for testing commercial catalysts from different vendors. Higher amounts of poisons (metals) and other foulants were absent from the feedstocks (see Table 1), so no hydrodemetallisation catalyst beds were loaded into the reactors.

Hydrotreatment catalysts from two different vendors (CatY and CatZ) were compared to evaluate their performance. The catalysts were loaded in 8 mm ID reactors and packed with inert silicon carbide to ensure that plug flow criteria are maintained (no bypassing and limited axial dispersion). Three different feeds (see Table 1) were used to compare the performance of the catalysts under typical refinery process conditions. A common wetting and activation/sulphiding procedure was implemented and agreed with all the catalyst vendors. Dimethyl disulphide (DMDS) was added (3 wt%) to straight run gasoil for catalyst sulphidation/activation. Catalyst activation was followed by a SOR temperature of 360°C using straight run gasoil (SRGO) feed.  
Design of experiment

The experiment was designed in such a way that the activity as well as the stability of the catalysts could be compared within a relatively short period (30 days). For comparison of activity, heating blocks 1 and 2 were run close to the SOR temperature for different feeds. For insight into catalyst deactivation, conditions were repeated using LVGO feed (see Table 2). During commercial operation, catalyst deactivation is measured by an increase in the required weighted average bed temperature (WABT) to meet the required sulphur specification. Typical deactivation rates in middle distillate hydrotreatment is around a 0.5-2°C increase in WABT per month. To measure this deactivation with some accuracy, a normal test programme would typically require three months or longer. In parallel, heating blocks 3 and 4 were run close to EOR temperatures (395-405°C) to accelerate deactivation. The hydrogen to oil ratio, hydrogen purity and total operating pressure remained the same for all test conditions. In addition, as was the case with heating blocks 1 and 2, catalysts were finally tested by a repeat condition using LVGO feed to evaluate deactivation (see Table 2). In this repeat condition, the catalyst from each vendor can now be assessed at both normal as well as accelerated deactivation conditions.

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