Recovering silicon poisoned catalyst
Silicon poisoning usually spells the end for hydrotreating catalysts, but a rejuvenation process can extend the effective life of silicon trap catalysts.
PATRICIA PEREZ-ROMO and LUIS M RODRIGUEZ OTAL
Mexican Petroleum Institute
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In recent years, the oil refining industry has been characterised by an increase in crude oil demand, the need to increase conversion of barrel bottoms into valuable hydrocarbon streams like naphtha, jet fuel and diesel, and tighter control of contaminants in products. Therefore an important increase in residue conversion technologies, both thermal and catalytic, such as hydrocracking, mild hydrocracking, visbreaking and coking of vacuum residue, has been observed. In the last 20 years, adoption of thermal processes has expanded significantly, particularly coking processes including delayed coking and Flexicoking.
Delayed coking is the most important source of petroleum coke with low ash content, which is necessary to manufacture the electrodes used in the aluminum and steel industry. Coke from delayed coker units is used as calcination coke in the cement industry. Coker units also produce important quantities of gases, naphtha, light coker gas oil (LCGO), and heavy coker gas oil (HCGO). Coker naphtha is used as feedstock for catalytic reforming units (CRU), while LCGO is used to produce diesel, and HCGO is fed to fluid catalytic cracking (FCC) units.
Products obtained in the coker unit are the result of decomposition and condensation reactions of the compounds contained in vacuum residue. The formation of coke takes place before maximum conversion of the liquid products is reached. In order to accomplish long operation cycles, a heater output stream is introduced to the coker chambers where the reaction processes continue in adiabatic mode to completion, maximising the liquid products and coke. Finally, the products are sent to a fractionator system. Once fractionated, they flow to downstream units where they are finished.
Origin of the problem
Due to the dynamic fluid in coke chambers, foam formation takes place. This must be avoided in order to prevent the foam being dragged to the separation system. For this purpose, an antifoaming agent such as polydimethylsiloxane (PDMS) is introduced into the coker chambers. This polymer breaks down under the chamber’s operating conditions (480-500°C) to form dimethylsiloxane compounds, where typical fragments are hexamehyltricyclosiloxane and octamethylcyclotetrasiloxane (see Figure 1). Since these compounds have boiling points of 134-245°C, they tend to be concentrated in the liquid products, mainly in the naphtha stream.
The distribution and concentration of silicon in delayed coking products depend on a polydimethylsiloxane’s properties (molecular weight, viscosity, and polydispersity). However, regardless of its properties, silicon is distributed in all the liquid products of the coker units; the highest concentration is found in the naphtha stream.
The products from the coker units have high sulphur and nitrogen levels, therefore they must be hydrotreated, especially naphtha which is fed to the CRU, and LCGO which is used to produce ultra low sulphur diesel (ULSD). But these streams also contain silicon compounds which poison hydrotreating and reforming catalysts. In this context, until today silicon has been considered a permanent poison, so poisoned catalyst cannot be regenerated or reactivated and is sent for disposal. This has a negative impact on refinery profitability while spent hydrotreating catalysts are considered a hazardous waste.
Coker products are the most important silicon sources in refinery streams, however refineries without coker units in some cases also encounter silicon poisoning in hydrotreating catalysts. This happens because siloxanes are used as surfactants in drilling muds as well as in the extraction and transport of crude oil for refineries. To be fractionated, crude oil must be heated to 330-340°C before being introduced into the distillation tower. This temperature is above the initial thermal decomposition temperature of PDMS (about 315°C), therefore the slow degradation to lighter silicon compounds begins during the crude oil fractionation. An overview of the silicon based fluids used in the oil industry is presented in Table 1.1
Silicon poisoning pathways
Coker naphtha exhibits important differences with respect to other naphtha streams. It contains the highest olefin and diolefin levels and consequently has the highest tendency to form gums, with high concentrations of nitrogen, sulphur, and silicon. Therefore, it is not common practice to hydrotreat it (alone or co-processed with other naphthas) in units traditionally used to hydrotreat straight-run naphtha because:
a) It presents high exothermicity due to its olefin and diolefin concentrations.
b) At typical HDT conditions (T<280°C), polymerisation reactions of diolefins will deposit gum on the top layer of the catalyst bed.
c) It is important to trap all silicon in order to avoid poisoning catalysts in downstream units such as naphtha catalytic reforming.
d) Harsher conditions are necessary to meet the specification for downstream units due to the high nitrogen and sulphur contents in this stream.
Units where coker naphtha is processed or co-processed with other naphtha streams have a separate design. The recommended configuration has three reactors: a diolefin saturation reactor (DIO), a silicon trap reactor (STR), and a main hydrotreating reactor (MHTR). Figure 2 shows a typical unit with three reactors.
Each one of these three reactors and the catalysts loaded in them have very specific goals in order to ensure the specifications needed for catalytic reforming units (S <0.3 wtppm, N <0.3 wtppm, no olefins, arsenic, and silicon).
The DIO reactor is designed to saturate diolefins in the feedstock in order to avoid gum formation. For this reason, the severity in this reactor is the lowest; the operating temperature is typically lower than 210°C.
The function of the STR is to remove heavy contaminants, especially silicon and arsenic. Olefin saturation, HDS, and HDN also take place. Because of the temperature of the reactions (260°C on average), only the least difficult HDS and HDN reactions can be carried out.
In the MHTR, the temperature is higher than in the previous reactor (315-330°C) and the most difficult HDS and HDN reactions are carried out. In this reactor it is also important to eliminate any mercaptans due to recombination reactions. The MHTR is also the last opportunity to trap silicon that could leak from the STR.
Catalysts used to trap silicon are characterised by their higher specific surface area. There is a direct relationship between the specific surface area of the catalyst and the amount of trapped silicon. It is also accepted that this poison is seized by OH groups on the surface of g-alumina which is used as a support in hydrotreating catalysts.
Coker naphtha also contains arsenic, which is another aggressive poison for hydrotreating and reforming catalysts and must be eliminated. Refineries generally use the same reactor and the same catalysts to eliminate silicon, arsenic, olefins, and other potential poisons as well as routine poisons such as sulphur and nitrogen; for these reasons, the most common industrial catalysts in the STR contain nickel and molybdenum. However, there are also metal-free catalysts on the market. The metal component does not participate directly in the reaction to trap silicon, but it is necessary to trap arsenic and hydrogenate olefins.
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