Hydrocracker
Also known as:
HCK, HCU, Unicracking, Isocracking
The hydrocracker upgrades heavy fractions (such as VGO) through thermal catalytic cracking while injecting hydrogen. This typically yields a high volume of high-quality diesel and kerosene product, as well as naphtha and lighter material. This is in contrast to the FCC that, using the same feed (VGO), produces more and better quality gasoline and light olefins.
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 is designed to handle heavier vacuum resid as a feed.
How it works
In a typical, multi-stage hydrocracker a mixture of hydrocarbon feed and recycled gas rich in hydrogen is heated and injected into the top of a reactor vessel.
The reactor contains multiple beds of catalyst, witch space between the beds. The mixture of hydrocarbon and gas move downward through each bed. The first bed typically contains a hydrotreating catalyst that removes impurities (sulfur and nitrogen) that would otherwise affect the performance of catalyst in subsequent beds.
As the hydrocarbon moves downward through the catalyst beds, large molecules are cracked and hydrogenated generating lighter, low sulfur, saturated hydrocarbons and H2S gas. Between each catalyst bed, cool fresh hydrogen is injected to control catalyst bed temperature.
The entire hydrocarbon/gas mixture exits the bottom of the reactor and goes through a high pressure separation drum to take off the gas. The gas stream is compressed and recycled to the front of the unit, with some gas removed to control H2S build up, and fresh hydrogen added to maintain hydrogen content.
Liquid product is sent to a fractionator column (along with liquid product from the second reactor). C4 and lighter is removed as an overhead gas stream and sent to the gas plant for separation. Light naphtha, heavy naphtha, kerosene and diesel are drawn off as side streams. Unconverted oil (UCO) is drawn off as bottoms and sent to a second reactor, or bled off as UCO for other uses.
At the second reactor, UCO from the fractionator is mixed with hydrogen rich recycled gas, heater and injected into the top. The second reactor is similar in design to the first, but without an initial hydrotreating catalyst bed. The output from the second reactor goes to a high pressure separation drum. Gas is recycled and product sent to the fractionator, just as in the first reactor.
Inputs
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.
Cycle oils - These low-quality diesel range streams can be hydrocracked to make gasoline range material. Because the feed is already highly aromatic it can yield naphtha with a high octane that makes for a good gasoline blend stock.
Atmospheric gasoil and cracked distillates - This diesel range material can be hydrocracked to increase the gasoline production by generating additional naphtha feed for the reformer
Products
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 blend stock (high cetane and low sulfur)
Hydrocracked gasoil - (also known as unconverted oil or hydrowax) - it is a low sulfur, VGO range material that can be used as feedstock for FCC units or ethylene crackers. The hydrocracking process results in a highly paraffinic and waxy material that cracks particularly well in an FCC.
Hydrocracked kerosene - This is a high quality jet fuel (or diesel) blend stock 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 blend stock with low octane but also low sulfur.
Isobutane - This is valuable in a refinery with an alkylation unit that requires isobutane as a feed stock
Operating conditions
Under normal operation conditions in the hydrocracker are optimized to manage conversion and catalyst life. Key variables are:
Pressure - Higher pressure supports higher hydrogen partial pressure, driving higher conversion and longer catalyst life. Typically pressure is kept at its maximum limit.
Hydrogen content - Higher hydrogen content supports higher hydrogen partial pressure, and thereby higher conversion and longer catalyst life. Hydrogen is maintained in excess of what is required for reaction, and fresh hydrogen is injected to offset losses due to hydrogenation and H2S formation.
Temperature - temperature, measured as the weighted average bed temperature (WABT) across the catalyst beds is used to control conversion. As the catalyst deactivates over time, temperature has to be raised to maintain conversion rate. It also results in higher energy consumption and higher yeild of lower value light gases.
Recycle - Increased recycling of unconverted oil yields higher net conversion to light product, however it also reduces effective unit throughput and can accelerate catalyst deactivation.
Economics
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, it's volume yield of naphtha is low and of low quality (low N+2A) unless cracking an already highly-aromatic feed (such as LCO).
Hydrocracker margins are highest when diesel is at a premium to gasoline, the light heavy differential is wide, and hydrogen costs are low.
While a hydrocracker competes with FCC for straight run VGO, having both an FCC and hydrocracker has advantages. The UCO from the hydrocracker performs well as FCC feed and the cycle oils from the FCC perform well as hydrocracker feed. Also, other cracked gasoils such as coker distillate, crack better in a hydrocracker than an FCC.
The flexibility 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.
Technology types and licensors
Hydrocrackers have several different design options:
Stages - Hydrocrackers can be either one or two stage. In contrast to the two stage design described above, a single stage hydrocracker would only have the first reactor. This results in lower conversion, but comes at a lower capital cost.
Severity - A standard high pressure hydrocracker is considered a full conversion unit. In contrast a mild hydrocracker will run at lower pressures and aim for only 20-30% conversion, with a primary goal of generating low sulfur UCO for feed to the FCC or making low sulfur fuel oil, and doing this at much lower capital and operating costs. A mild hydrocracker can also be used to reduce the wax content of VGO being used to make lubricants.
Feed - Feed to a typical "distillate" hydrocracker is VGO range material, or lighter if maximizing jet and naptha production. And alternative design called a "resid" hydrocracker process vacuum resid, but with a very different ebulated bed catalyst design that comes at much higher cost.
There are a number of different distillate hydrocracking licensors:
Chevron Lummus Global (CLG) - Isocracking gasoil hydrocracking; LC-fining resid hydrocracking
UOP - Unicracking and Unibon gasoil hydrocracking
ExxonMobil - GOfining gasoil hydrocracking
BP - Ultracracking gasoil hydrocracking
Shell - Shell hydrocracking gasoil hydrocracking, HYCON resid hydrocracking
Axens IFP - IFP hydrocracking, HyK gasoil hydrocracking, H-Oil resid hydrocracking