Q & A > Question Details
Excuse my ignorance, I am neither a chemist nor an engineer. I am actually in finance trying to figure out product properties.
I am having hard time understanding how is it possible to have a >10psi RVP naphtha feed go into a catalytic reformer, and have a <4psi RVP reformate come out the other end?
My logic dictates the following:
1) The lighter the molecule (shorted hydrocarbon chain) the higher the RVP and lower the Octane (and vice versa)
2) Unsaturated molecules are more reactive and therefore will have higher RVP compared to saturated chains of equal carbon atoms.
The questions driving me nuts:
1) Why does alkylate which has more saturated content have higher RVP than reformate?
2) Is the reason for akly having lower RON than reformate the fact that its molecules are lighter than in reformate?
 
Answers
19/06/2012 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
First of all, there is more going on that just the reactions. Any light molecules formed in the process are rejected by stripping/distillation after the reactors in both alklation and reformation.
Alkylate is always iso-paraffinic - branched chains, and is largely C7-C9 molecules, depending on the feedstock. Reformate may be iso-paraffinic, naphthenic and aromatic. Aromatics have lower vapour pressures than the paraffin with the same carbon number.
To your logic:
(1) RVP and octane depend on more than just the moleweight and "chain length". Benzene has a far higher octane number than n-heptane, even though benzene is only C6H6 and heptane is C7H16. Molecular structure is the most important factor.
(2) unsaturated molecules of the same structure will have higher vapour pressure. see (1), Benzene is as unsaturated as can be, but has a lower vapour pressure than the comparable n-hexane which is heavier and saturated.
To your questions:
(1) Alkylate is all branched chains. Reformate contains ring structures which have lower vapour pressures.
(2) Alkylate RON is pretty closely fixed to the 224-MPentane value of 100 (it is usually a little below this) due to other molecules. Reformate can be above 100 if it contains aromatics (toluene = 111, xylene = 117).
19/06/2012 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The product from the reformer reactor section is distilled/fractionated to adjust the reformate boiling range, including RVP.
Reformate has a higher ON because the aromatics have higher ONs than the non-aromatics in the alkylate.