Catalytic Hydrocracking

Catalytic Hydrocracking sxr133 Thu, 06/27/2013 - 14:25

Catalytic hydrocracking is one of the latest additions to petroleum refining processes, with the first modern commercial unit started up by Chevron in 1958. The interest in hydrocracking has been attributed to the increasing demand for light and middle distillates, the availability of byproduct hydrogen in large quantities from catalytic reforming, and the environmental regulations limiting sulfur and aromatic hydrocarbons in motor fuels [5]. The advantages of hydrocracking include its ability to handle a wide range of feedstocks that may be difficult to process by catalytic cracking and its flexibility in selectivity between light and middle distillates. The principal objective of hydrocracking is to decrease the molecular weight and boiling point of heavy oils to produce saturated hydrocarbons (diesel and jet fuel) from highly aromatic feedstocks (e.g., LCO from FCC) and distillation residua.

The hydrocracking process has two dimensions: Hydrogenation of aromatic rings and cracking of aliphatic compounds, as shown in Figure 7.10, using naphthalene as an example for an aromatic ring system. One should note that the aromatic rings cannot be cracked before they are saturated with hydrogen. With hydrocracking, it is possible to convert an aromatic compound to a paraffinic compound without any loss of carbon, as shown in Figure 7.10. As a hydrogen-addition process, hydrocracking provides high yields of valuable distillates without producing low-grade byproducts (e.g., heavy oils, gas, or coke) as experienced in carbon rejection processes such as coking.

Shows naphthalene being hydrogenated in2 tetralin & decalin. Tetralin is cracked in2 toluene + C3H8. And Decalin in2 many other chemicals

Figure 7.10. Reactions of hydrocracking.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Reaction Systems

Reaction Systems azs2 Wed, 05/07/2014 - 14:26

The two different reaction systems in hydrocracking, hydrogenation and cracking, are supported by bifunctional catalyst formulations, as illustrated in Figure 7.11. Hydrogenation reactions are promoted by the metal component of the catalysts (e.g., Ni, Co, Mo), and the cracking takes place on catalyst support consisting of silica/alumina. Highly active noble metals (e.g., Pt, and Pd) can be used for hydrogenation of hydrocarbons with extremely low sulfur contents as the noble metals are susceptible to sulfur poisoning.

Functions of hydrocracking catalysts. See text alternative below

Figure 7.11. Functions of hydrocracking catalysts.

Click here for a text alternative for the image above

Hydrocracking

Hydrocracking has a dual function (bifunctional) catalyst
- Metals + acid supports to perform two functions

Hydrocracking Catalysts

Cracking (acid Support
- SiO₂ – Al₂O₃
- Al₂O₃
- Low zeolite SiO₂ – Al₂O₃
- High zeolite SiO₂ – Al₂O₃
Hydrogenation
- Noble – Pt, Pd
  - Susceptible to S poisoning
- Non-noble – Ni, Mo, Co, W
  - Sulfided forms (M_xS_y) for high sulfur feeds

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Hydrocracking processes most commonly include two reaction stages: Hydrotreating to remove heteroatom (S, N, O) species and Hydrocracking to increase the H/C ratio of the hydrocarbons in the feeds by hydrogenation and to decrease their molecular weight by cracking. In most cases, the hydrotreating reactor (HT) packed with cobalt-molybdenum catalysts precedes the hydrocracking (HC) reactor typically packed with nickel-tungsten catalysts (for hydrogenation) supported on alumina/silica (for cracking). Figure 7.12 shows different configurations of hydrocracking processes, depending on the heteroatom content of the feeds. For feeds with very low heteroatom contents, hydrocracking without hydrotreating may be applied, but this is very rare. Other process configurations include single stage with dual catalysts, two-stage dual and triple catalysts, as shown in Figure 7.11. The hydrocracking reactions are performed at 300–400°C and 8–15 MPa of hydrogen pressure.

Uses of Hydrocracking

Uses of Hydrocracking azs2 Wed, 05/07/2014 - 14:30

In a refinery, hydrocracking complements catalytic cracking by taking on the more aromatic feedstocks that resist cracking, including the byproducts of FCC, such as light cycle oil (LCO). Hydrocracking can also be used to upgrade residual fractions using different reactor configurations and catalysts depending on the complexity of the upgrading tasks, as shown in Figure 7.12. As shown in Figure 7.13, for hydrocracking a relatively light feedstock (e.g., atmospheric residue), a fixed-bed configuration and relatively large-size catalyst particles can be used. In extreme cases with very heavy vacuum residue, an expanded bed configuration is used, in which very fine catalyst particles are entrained in the feed at high hydrogen pressures (high hydrogen/oil ratio). These extreme reaction conditions are necessary to prevent extensive coking on catalysts that could shut down the process. For intermediate cases, an ebullated (fluidized) bed configuration can be used, as shown in Figure 7.13.

In the United States, hydrocracking of LCO (from FCC) provides a large proportion of the diesel fuel production because straight-run LGO is a preferred stock for FCC to produce gasoline as the principal product. The major licensors of hydrocracking processes include Chevron, UOP, ExxonMobil Research and Engineering, BP, Shell, and BASF-IFP.

Configuration of hydrocracking process with respect to feed properties. See text alternative below

Figure 7.12. Configuration of hydrocracking process with respect to feed properties.

Click here for a text alternative to the figure above

Hydrocracking Process

Single Stage
- Hydrocracking only
  - single catalyst
  - low S, N feeds
  - Hydrotreatment, Hydrocracking
- dual catalyst
  - high S&N
  - can be done with or without recycling
Two-Stage (separation of the products between stages 1 & 2)
- HT & HC
  - dual catalyst
  - very high S + N feeds
- HT & HC
  - triple catalyst
  - extremely high S, N, metal feed

Hydrocracking of heavy residue. See text alternative below

Figure 7.13. Hydrocracking of heavy residua.

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Residue Hydrocracking

increasing H/C for upgrading
atmospheric or vacuum distillation reside

Hydrocracking of heavy residua
Bed Type	Cat. Size	Particles/cm³
Fixed	1.5 - 3mm	120
Ebullated	0.8 - 3mm	250
Expanded	0.002mm	2.4x10⁹

Hydrocracking vs. Catalytic Cracking

Hydrocracking Vs. Catalytic Cracking
Catalytic Cracking - FCC	Hydrocracking
carbon rejection endothermic acid catalyst more gas more coke	hydrogen addition exothermic metal catalyst on acid support less gas less coke costly process ($$$)

Hydrocracking involves C-C bond cleavage to produce lighter HC's
More liquid yield with HYDRCRC
More hydrogenated products
- Cracking is less severe
- Secondary cracking reactions are minimized by stabilization of active species by hydrogen
Hydrocracking is more flexible w.r.t. the feedstock
- Refractory feedstocks (i.e. aromatics) can be processed
- C-C bond breaking after saturation with hydrogen

Above, we compare catalytic cracking (FCC - a carbon rejection process) with hydrocracking (HYDRCRC) with respect to the major attributes of both projects. Clearly, in a flexible refinery with a wide range of crude oil feedstocks, both processes are needed for the optimum conversion of the crude oil into desirable refinery products.