I have three questions: 1) What reactions are source of high hydrogen purity at naphtha platforming (catalytic reforming) unit? Either naphthene's or paraffins reactions?
2) The NHT-Plaformer unit was down for couple of days. The startup was executed after 1 week. After startup it was observed that, system pressure was not meeting the set point of 350 psi. But in previous history, where ever startup was performed, at turndown capacity the system pressure of 350 psi was meet. This is its not even reaching to system pressure of 350 psi. While hydrogen purity is also decreased up to 60%. HCL is recycle gas is almost ranges between 0.5 to 1 ppm and H2S is also 1 ppm. During startup at 880F, the HCL was found in traces while H2S was found 2ppm. Platformer catalyst is UOP-R56.
3) Further more about NHT, it's not removing sulfur properly even though we have done skimming recently. Due to low hydrogen purity of platformer, the ratio is limited to 340 to 350. While ratio must be 380. The reaction temperature is 630 F (which is 5 F higher then EOR for catalyst). Still it's not removing properly, The stripper is operating and minimum pressure and maximum bottom temperature. NHT catalyst is UOP-HYT-1119 Can you tell me about this to improve sulfur removal currently?Jan-2024
Marcio Wagner da Silva, Petrobras, firstname.lastname@example.org
Regarding your questions, I have the following answers and suggestions:
1 - The main reactions of the naphtha catalytic reforming process is the naphthene dehydrogenation to aromatics, alkyl cyclopentanes dehydroisomerization to aromatics, alkanes dehydrocyclization to aromatics, n-alkanes isomerization to branched alkanes, and cracking reactions to lower carbon products. The first three reactions are responsible to produce hydrogen, the paraffin dehydrocyclization which involves the conversion of paraffin's in aromatics which contributes significantly to the octane index of the reformed naphtha. Unfortunately, these reactions are extremely slow and it is necessary to offer adequate residence time to ensure that the paraffin dehydrocyclization reactions occur.
Long chain paraffins tend to suffer hydrocracking which involves the reaction of the paraffins with hydrogen to produce methane, ethane, and propane. These side reactions can be responsible for the reduction in the octane index and hydrogen purity which is mentioned in your question, especially considering that paraffin hydrocracking is a quick reaction in comparison with paraffin dehydrocyclization.
2 - It's important to analyze how was the shutdown procedure of the processing unit, despite to processing a high quality naphtha as feed stream an inadequate shutdown procedure can lead to a coke deposition over the catalyst, considering the informed catalyst type (UOP-R56) your processing unit is a semi-regenerative reforming unit which demands even more attention with the risk of catalyst bed coking during the shutdown and stop period, it's important to check the operating manual of the processing unit to verify the licensors orientation regarding the attention points before to carry out a planned shutdown of the processing unit. For semi-regenerative processing units, it's fundamental an adequate management of water/chloride ratio aiming to ensure an adequate balance between the acidic and metal functions of the catalyst. Normally, fresh catalytic reforming catalysts presents close to 1,0 % wt of chloride, to maintain this chloride concentration it's necessary to control the water concentration aiming to allow an effective interaction between the alumina (catalyst support) and the chloride, reaching then a good performance of acidic sites of the catalyst which is responsible by the cracking reactions.
According to the literature, several factors impact the chlorides concentration in catalytic reforming catalysts. The reactor temperature and surface area of the support can directly affect the chloride concentration in the catalyst and are the most relevant factors. Still according to the literature, fixed bed catalytic reforming reactors should operate keeping the water to chloride molar ratio between 15 to 25 in the recycle gas aiming the keep the activity of the catalyst, to control this parameter it's necessary to install sample facilities or on line monitoring systems in adequate points aiming to keep this parameter according to the licensor specifications. It's possible to find in the specialized literature chlorides equilibrium curves capable of helping the refiners to control the water to chloride ratio in the catalyst under the specifications defined by the licensors. The presence of only traces of HCl in the recycle gas can indicate that the chloride concentration over the catalyst is low, reducing the acidic function leading to an unbalance between the acidic and metal functions of the catalyst. A suggestion is to carry out a regeneration procedure of the catalyst beds in order to recovery the catalytic activity as recommended to the licensor, another point to check is the paraffinicity of the feed, high paraffins concentration can lead to the reduction of hydrogen purity and RON of the reformate if the severity of the processing unit was not adjusted, as previously mentioned, the paraffin dehydrocyclization are extremely slow and it is necessary to offer adequate residence time to ensure that the paraffin dehydrocyclization reactions occur. Long chain paraffins tend to suffer hydrocracking which involves the reaction of the paraffins with hydrogen to produce methane, ethane, and propane.
3 - Considering the given information regarding the catalyst (UOP-HYT-1119), this is a Ni-Mo catalyst with high activity and high hydrogen consumption. The poor performance related to the hydrodesulphurization can be related to the low hydrogen/feed ratio caused by the low hydrogen purity, leading to an insufficient hydrogen partial pressure to the hydrotreating reactions. Another point which needs to be checked is the quality of the feed, if the final boiling point of the feed was increased, it will be observed difficulties to remove nitrogen and sulphur.