Hypotheses on the C4 alkylation reaction of liquefied petroleum gas
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This post was last edited by Benpeng Guo Jibo45 on 2017-5-24 00:36. The pre-treated C4 feedstock enters the reactor to produce alkylated oil. Starting from the carbocation reaction mechanism, the yield of alkylated oil is at most equal to the content of C4 olefins multiplied by 2; however, in actual production the yield of alkylated oil is much higher than this theoretical value. Why is that? It is speculated that in addition to isobutane reacting in a 1:1 ratio with the olefin, some isobutane and n-butane also react directly; all the products formed (including the alkylated oil, the overhead product from the isobutane removal column, and the overhead product from the n-butane removal column) are alkanes. Assuming that isobutane and n-butane participate directly in the reaction, there is a paradox here: the hydrogen atoms are not conserved. The general formula for alkenes is CnH2n, so the hydrogen-to-carbon ratio is a fixed value of 2. The general formula for alkanes is CnH2n+2; therefore, the hydrogen-to-carbon ratio in alkanes decreases as the carbon chain lengthens, gradually approaching 2. Examples are as follows: the carbon-to-hydrogen ratio for C1H4 is 4, for C2H6 it is 3, for C3H8 it is 2.67, for C4H10 it is 2.5, for C5H12 it is 2.4, for C6H14 it is 2.33, for C7H16 it is 2.29, for C8H18 it is 2.25, for C10H22 it is 2.2… The carbon-to-hydrogen ratio for isobutane and n-butane is 2.5, while the smallest carbon chain among the liquid products formed under normal temperature and pressure has a carbon-to-hydrogen ratio of 2.4, corresponding to C5H12. As well as alkanes with smaller hydrogen-to-carbon ratios such as C6H14, C7H16, and so on. Where did all this hydrogen go? Refer to the test results of isobutane, n-butane, feed C4, etc., and the yield of alkylation oil. Through hypothetical calculations (excluding the hydrocarbons discharged with the waste acid and those discharged with the alkali wash water and washing water), in addition to isobutane and olefins in a 1:1 ratio participating in the reaction, approximately 8% butane and 10% isobutane also participated in the reaction (this takes into account scenarios where isobutane was added or not, depending on the level of C4 olefins in the feed) to produce alkylated oil. Based on the test results and mass balance calculations, the excess hydrogen exists in the form of propane (about 0.15%) and ethane (about 0.03%). Therefore, the light components in the refrigerant gradually accumulate and need to be removed in a timely manner; since no difficult-to-condense or non-condensable gases such as hydrogen and methane are generated, the reactor pressure does not fluctuate significantly. In summary, the reaction between n-butane and isobutane can be satisfied in the simulation calculations. However, theoretically, there is no support for the reaction of alkanes with each other; I seek guidance from experts and those with more experience.
I recommend that you read Albright’s articles from Purdue University; the mechanism of sulfate-based alkylation was essentially proposed by him.
The article you mentioned could not be found; could you share relevant information on the forum?
Then search for a book by Geng Yingjie, titled \"Alkylation Production Processes and Technologies\"; it’s basically a translation of Albright’s articles, and it might be interesting to read as well~
Well, thank you. I’ve read this book before. :handshake
Don’t just read it by yourself; share it: call::handshake
I saw this share on Haichuan Forum before; I’ll go look for it when I get back. If I can’t find it, I’ll share it.
Yield? The concept is unclear; what you might be referring to is the product quality per unit of time. Indeed, the theoretical maximum value for the product quality per unit of time of alkylated oil is 2.0357 times the mass of olefins in the raw material per unit of time. But do you analyze the components of your alkylated oils? The so-called alkylated oil products contain approximately 90 wt% of alkylates, denoted as C6+, and no specific component analysis is generally carried out. What you mentioned is much higher than the theoretical value; the excess amount is C5 that was not separated in the n-butane distillation column. Furthermore, in an alkylation reaction environment, alkanes reacting with each other – I’ve never heard of that. :L
Moderator, have you ever worked on an alkylation unit? Thankfully, what Hai Chuan posts isn’t illegal; otherwise you would have to lock him up! Hehe