Content – Other fuels
Another bio based diesel is produced by hydrocracking or hydrotreating (Hydrogeneation) but then it is often called “Green diesel” or “renewable diesel” and is a different product from what normally is called biodiesel, this due to the chemical composition and the production processes.
“Green diesel” or “renewable” diesel has similar properties to petroleum based diesel and does not cause the same technical issues as biodiesel.
Production of green diesel by Hydrotreating (Hydrogenation)
In this process the feedstock is reacted with hydrogen under elevated temperature and pressure to change the chemical composition of the feedstock. Hydrogen is introduced to the feedstock in the presence of a catalyst to remove other atoms such as sulphur, oxygen, nitrogen and aromatics to convert the bio-oil (triglyceride) molecules into paraffinic hydrocarbons. In addition to creating a fuel that is very similar to petrodiesel, this process creates propane as a byproduct.
The hydrogenation process is also used to convert coal to liquids (The “Bergius process”), by mixing the dry, grinded coal with heavy oil before heating (400 – 500 oC), mixing with hydrogen and a catalyst.
Production of green diesel by Hydrocracking
Hydrocracking is a catalytic cracking process that takes place in a hydrogen rich atmosphere. Unlike a hydrotreating, where hydrogen is used to remove sulphur and nitrogen, hydrocracking uses hydrogen to break carbon –carbon bonds.
The process takes place at 250 – 425 oC
Hydrocracking is a catalytic chemical refinery process, using temperature and pressure in the presence of a catalyst to convert high-boiling hydrocarbons to lower-boiling hydrocarbons by breaking down larger molecules into smaller molecules, such as those found in bio-oils or crude oil, into shorter and lighter hydrocarbon chains such as gasoline, kerosene, jet fuel and diesel oil.
The feedstock will often contain sulphur and nitrogen that to a large extent are hydrogenated during the process to form gaseous hydrogen sulfide (H2S) and ammonia (NH3), which are subsequently removed. The result is that sulphur and nitrogen impurities are removed such that the hydrocracking products consist mostly of paraffinic hydrocarbons.
Various hydrocracker designs and configurations are being used.
The most commonly used for production of green diesel are the following:
Single stage hydrocracker
The single stage hydrocracker only have one reactor, recycling is not done.
The feedstock needs to be hydrotreated to remove ammonia and hydrogen sulfide or the catalyst must be capable of both hydrotreating and hydrocracking.
Single stage hydrocracker with recycle
The Single stage hydrocracker with recycle is the most commonly used configuration. Uncracked residual hydrocarbon from the bottom of the destillation tower is recycled back into the reactor for further cracking. The feedstock needs to be hydrotreated to remove ammonia and hydrogen sulfide or the catalyst must be capable of both hydrotreating and hydrocracking.
Two stage hydrocracker
The two stage hydrocracker uses two reactors. Uncracked residual hydrocarbon from the bottom of the destillation tower is fed into the second stage reactor for further cracking. The first stage reactor accomplishes both hydrotreating and hydrocracking, therefore the second stage reactor feed is to a large extent free of ammonia and hydrogen sulphide, which permits the use of noble metal catalysts enhancing the efficiency of the reactor.