What steps can we take in our mild hydrocracking operations to improve FCC conversion performance?Mar-2022
Rainer Rainer Rakoczy, Clariant Catalysts, Rainer.Rakoczy@clariant.com
Besides asphaltic species, the level viscosity of the desired FCC feed can be key to improving and optimising FCC unit operations. Clariant has various options to improve FCC feed viscosity in FCC pretreaters with Clariant’s HYDEX Series catalysts, including mild hydrocracking and beyond.
Eelko Brevoord, Catalyst Intelligence Sarl, Brevoord@catalyst-intelligence.com
To boost FCC performance, the mild hydrocracker product should have the following features:
1. Minimum nitrogen content
2. Maximum hydrogen content
In mild hydrocracking (MHC), the reactor temperature is usually maximised immediately from start of run (SOR) to obtain maximum conversion of feed to diesel. At SOR, this leads to product quality give-away, as the catalyst activity is still very high, and the reactor temperature does not need to be maximised to meet, for instance, product sulphur requirements. The conditions for the best FCC operation should be optimised over the full cycle, which does not necessarily mean that you maximise reactor temperature at SOR. This is especially valid as MHC units operate at relatively mild pressures, and at high temperatures, aromatics saturation is not favoured. Consequently, hydrogen addition is not optimal if you focus only on the conversion in the MHC unit itself. Overall, we can conclude that an operation at the highest aromatics saturation point and the lowest nitrogen content over the cycle is optimal. It is recommended to do a sophisticated modelling study to obtain the best operating strategy. Catalyst Intelligence has developed the HydroScope model, which can optimise the MHC unit with the FCC operation in mind.
Meritxell Vila, MERYT Catalysts & Innovation, email@example.com
To improve the conversion performance of our FCC, we can consider three different actions:
1. Revise the quality of the HCK bottom residue: It is important to have a detailed analysis of the HCK bottom residue: the content of aromatic carbon, naphthenic carbon, and its distribution. If we want to increase yields in the FCC, this residue should have a high naphthenic carbon content and a low triaromatic+ in the aromatic carbon distribution. Of course, these contents depend on the aromatic feed content, on the temperature of the hydrocracker unit, and on the catalyst capacity to convert aromatics into naphthenes.
We recommend performing some changing reaction temperature testing of the hydrocracker to obtain the best conditions to produce maximum HCK residue with maximum carbon naphthene. This should be tested for each VGO quality to the HCK and the catalyst in use. Pay attention to these tests results regarding the residue and the other products and the qualities you need to obtain in the hydrocracker, such as sulphur and metals.
2. Adjust the cutpoint of the HCK bottom residue: to obtain more yield (m3/h or kg/h) in FCC is to increase the feed quantity. This yield increasing, of course, depends on the products you need to obtain in your FCC.
If you want to produce more LCO or naphtha, you could reduce the HCK residue you send to the FCC, making this FCC feed lighter and decreasing the cat/oil ratio. Conversion will be adjusted with riser temperature and maximum regeneration limits. But if you want to produce more olefins, you could increase the FCC feed until maximum riser cat/oil with riser temperature, considering maximum regenerator temperature GasCon limits.
3. Make an integrated evaluation of both units: HCK and FCC. We recommend making an integrated evaluation of both units regarding the performance of all the catalysts involved and the processes and, therefore, doing an integrated economic balance. It may be that the best catalyst for the FCC is not the best when we evaluate the yields and economy integrating both units. Alternatively, the upgraded catalyst for HCK may not be the optimum when we do the global evaluation of the performance of both units.
The best way is to evaluate the impact on both units when you evaluate the catalysts with the help of process simulation models
Per Zeuthen, Haldor Topsoe, firstname.lastname@example.org
Aromatic compounds are not converted in FCC units, so steps to improve FCC conversion will be minimising feed monoaromatic and polyaromatic compounds the most.
A mild hydrocracker is typically a single-stage reactor unit loaded with various hydroprocessing catalysts, including hydrotreating and hydrocracking catalysts. To improve FCC conversion, the following should be considered:
- Increase hydrogen partial pressure the most; this will increase aromatic saturation, lower aromatic compounds in the FCC feed, and increase conversion by increasing the total pressure or the hydrogen purity in the make-up hydrogen supply.
- Increase aromatic saturation activity of the loaded catalysts and ensure that the most HDA active catalysts are used. This is important also for the conversion catalyst if this is included in the catalyst load. Topsoe has a number of catalysts (hydrotreating and hydrocracking) with improved aromatic saturation activity.
- Use the latest generation of reactor internals; this will ensure that all loaded catalyst is utilised and exposed to hydrogen — a very important step that is often overseen. Again, Topsoe holds an extensive reference list and industrial feedback for such improvements.
Finally, mild hydrocrackers are typically being operated at relatively high temperatures, particularly during the second half of the cycle. This ensures the right product sulphur and nitrogen level in the FCC feed and sulphur specks in the FCC products. However, high temperatures, higher than 390°C, at the outlet of the mild hydrocracker are unfavourable for the saturation of the polyaromatic compounds. Thus, for thermodynamic reasons, there are no catalytic ways to lower the product aromatics in the FCC feed at such conditions. A minor revamp of the unit, for example, with Topsoe’s Aroshift layout, by installing a small high LHSV without fractionation, being operated at a lower temperature with a proprietary catalyst, will lower the FCC feed polyaromatics by typically 25% and thus show a great positive impact to the FCC conversion and yield structure.