Why does hydrocracking produce more C4?
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It seems that our hydrocracking unit produces a much larger amount of liquefied gas than methane and ethane; what’s the reason for the varying amounts of propane produced? When there is more propane, the proportion of n-butane correspondingly decreases. The ratio of isobutane to n-butane is also sometimes unstable. Does any expert know the reason?
Is there anyone who knows what’s going on?
Was my question not detailed enough?
Q&A on Hydrocracking Technology----------The hydrocracking of hydrocarbon-like molecules in feed oil follows a mechanism similar to that of the FCC process; both processes adhere to the carbocation reaction mechanism and the principle of chain scission at the β-position of the carbocation. The difference is that the hydrocracking process is accompanied by a hydrogenation reaction from start to finish. The normal carbocation mechanism for hydrocarbon cracking reactions: According to the β-scission rule, the primary carbocations that are formed are unstable; they undergo hydrogen transfer reactions to yield relatively stable secondary or tertiary carbocations, or they isomerize into tertiary carbocations. Larger tertiary carbocations then undergo β-scission to produce an isomeric alkene and a smaller normal carbocation. The alkene is hydrogenated to form an isomeric alkane, while the smaller normal carbocation returns hydrogen ions to the catalyst, thereby generating an alkene molecule; this alkene molecule is subsequently hydrogenated to form an alkane molecule. This characteristic of normal carbocations is the inherent reason why hydrocracking products are rich in isomeric alkanes. The hydrocracking of alkanes involves chain scission at the β-position of their carbocations, with little formation of low-molecular-weight hydrocarbons of C3 or lower; the yield of liquid products from hydrocracking is high ； Non-hydrocarbon compounds are essentially completely converted, and olefins are also largely hydrogenated and saturated. The pressure in the hydrocracking reaction is very high; the conversion rate of aromatic hydrocarbons under hydrogenation is extremely high, and the quality of the products from hydrocracking is good. Polycyclic aromatic hydrocarbons are hydrocracked through ring-by-ring hydrogenation/ring-opening, yielding small-molecule alkanes and cycloalkano-aromatics ； Cycloalkanes with two or more rings undergo ring-opening cleavage and isomerization, ultimately yielding monocyclic cycloalkanes and smaller alkane molecules ； Monocyclic aromatics and cycloalkanes are relatively stable; they are not easily hydrogenated or opened, and the main transformations that occur are the cleavage of side chains or isomerization of those side chains. They are concentrated in naphtha. Naphthenes and alkanes are more difficult to crack, so the circulating oil in cracking units contains a higher proportion of naphthenes as well as large amounts of alkane components.
Based on actual production, the main factors affecting the C4 content in liquefied gas are: a low top pressure, which leads to an increase in C4 levels； If the temperature at the bottom of the tower is too high, the C4 content in the liquefied gas will increase ； Less reflux at the tower top results in a higher tower top temperature, which in turn increases the C4 content in the liquefied gas. The feed temperature also affects the level of C4 in the liquefied gas.
Damn, I can’t understand it. Can you make it simpler?
It is related to the reaction mechanism; the yield of liquefied gas is much higher than that of C1 and C2
……Thank you. I know it has something to do with the reaction mechanism. Could you explain it in a bit more detail? I’ve forgotten all my chemistry knowledge; could you use simpler terms to explain it?
Building 4# provides a very detailed explanation; the original poster breaks it down – the first part explains why gases are generated and how molecules transform, while the second part deals with the origin of the yield of the liquid product； It’s easier to understand it by breaking it down; I’m learning this as I go too ；
You can turn to design institutes and catalyst manufacturers for the designed liquid recovery. 1. Your operations are very unstable; the four key indicators related to the fixed-bed reactors fluctuate, as does the reaction temperature. 2 refers to the specific design and the selectivity of the catalyst; an increase in the number of secondary cracking steps can be caused by various factors.
This post was last edited by uyouu1 on 2020-11-18 at 14:03. Excessive depth of cracking leads to a high yield of light components: both the amount of dry gas and liquefied gas increases； Unstable operation of the hydrogen sulfide removal tower can cause fluctuations between the volume of dry gas and the volume of liquefied gas ； Also, isn’t your liquefied gas sent to a stabilization and desulfurization unit to recover components above C5?