Also known as: HCK, HCU, unicracker, VGO hydrocracker
In a refinery, the hydrocracker upgrades VGO through cracking while injecting hydrogen. This yields a high volume of high-quality diesel and kerosene product. This is in contrast to the FCC, which uses the same feed (VGO) but produces more and better-quality gasoline.
The hydrocracker is particularly valuable in a refinery that is trying to maximize diesel production and reduce residual fuel oil. The hydrocracker yields a high volume of kerosene and light gasoil (distillate) of good quality (high cetane and low sulfur). However, its volume yield of naphtha is low and of low quality (low N+A). Markets that have very low sulfur limits for diesel also favor use of hydrocrackers, as the diesel product does not need subsequent hydrotreating.
Often, to achieve a high yield of light products with a balance between gasoline and diesel, a refinery will have both an FCC and a hydrocracker. In this case, the two units can be highly complimentary, with the FCC taking the unconverted feed from the hydrocracker and the hydrocracker taking the heavier cracked products (LCO or HCO) from the FCC.
The flexiblity in the design and operation of hydrocrackers allows a wide range of feeds and of product yields. However this comes at very high capital and operating costs.
A resid hydrocracker is a variant on the typical VGO hydrocracker. It is a similar unit yielding a similar range and quality of products, but it is designed to handle heavier vacuum resid as a feed.
How it works
In a two-stage hydrocracker, a mixture of hydrocarbon feed and hydrogen is heated and injected into a reactor vessel containing a hydrotreating catalyst. This catalyst accelerates the reactions that remove sulfur and nitrogen from the hydrocarbon and open up and saturate aromatics rings. The entire output from this reactor is then injected into a second reactor containing a hydrocracking catalyst, which helps the reactions that crack apart the hydrocarbons while saturating them with hydrogen. The resulting mix of converted and unconverted hydrocarbon is then separated. Unconverted hydrocarbon can then be recycled to the hydrocracking step for further conversion, sent to a second hydrocracking vessel, or sent to another conversion unit as feed (e.g., an FCC). Diesel range material can also be drawn off at the separation steps to maximize diesel production, or it can be processed further (through recycling or second-step hydrocracking) to maximize naphtha production. Some hydrocrackers are single-stage units with just one reactor that is usually filled with hydrocracking catalyst, but the rest of the process is the same.
Hydrocrackers can take a wide variety of feeds depending upon the desired products. The most common are:
- VGO - This lighter fraction from the vacuum distillation unit is the most common feed for most hydrocrackers. It is a desirable feed when the refiner is attempting to maximize overall diesel production
- Coker gasoil - This VGO-range product from the coker is well suited to a hydrocracker, which is better able to handle its unsaturated components than an FCC unit is
- Cycle oils and cracked distillates - These low-quality diesel-range streams can be hydrocracked to make jet fuel and gasoline-range material
- Atmospheric gasoil - This straight run diesel-range material can be hydrocracked to increase gasoline production by generating additional naphtha feed for the reformer
A hydrocracker can produce a wide range of products depending upon what feed it processes and how it is designed and operated. Typical products are:
- Hydrocracked distillate - This is a high-quality diesel blendstock (high cetane and low sulfur)
- Hydrocracked gasoil (also known as unconverted oil) - It is a low-sulfur, VGO-range material that can be used as feedstock for FCC units or steam crackers
- Hydrocracked kerosene - This is a high-quality jet fuel (or diesel) blendstock with low sulfur and high smoke point
- Heavy hydrocrackate - This is a good-quality reformer feed, with moderate N+A and low sulfur
- Light hydrocrackate- This is a low-quality gasoline blendstock with low octane but also low sulfur
- Isobutane - This is valuable in a refinery with an alkylation unit that requires isobutane as a feedstock