Q & A > Question Details
Excuse my ignorance; I am neither a chemist, nor an engineer. My question revolves around RVP and RON properties of aklylate and refomrate.
From what I have read (obviously not enough), my logic dictates:
1) Unsaturated hydrocarbon molecules are more volatile than that saturated ones, thus will have higher RVP
2) Lighter hydrocarbon molecules are more volatile than longer chains, thus will have a higher RVP.
Questions that are driving me nuts:
1) How can a naphtha feed (with more saturated content) going into a catalytic reformation can have a higher RVP, than the resulting reformate which has more aromatics (unsaturated, cyclical = more volitile) content?
2) Why does alkylate which has more saturated molecules than reformate have lower RON and higher RVP?
3) Which is the most important contributor to RVP of a gasoline blendstock - the length of the hydrocarbon chain or it being un/saturation with hydrogen atoms? In other words, which has higher RVP - an unsaturated aromatic benzene molecule or a saturated paraffin pentane?
20/06/2012 A: Amauche Ikpeamanze, Kaduna Refining and Petrochemical Company, uchzuby2010@hotmail.com
higher RVP are always as a result of higher present of LPG (Propane, butane and lighter hydrocarbons) while presences of Aromatics increases the RON, however Unsaturates can increase the ron but the are always gummy and affects the oxidation stability of the gasolines
20/06/2012 A: keith bowers, B and B Consulting, kebowers47@gmail.com
There are several excellent technical references that would help you to understand a few of the complex properties of hydrocarbons. One must realize that potentially every molecule in a barrel of crude is different from all the others. Otherwise seemingly rational assumptions (rational to the unskilled) about hydrocarbons are often grossly incorrect.
1)incorrect . 'Saturated' and 'unsaturated' refer ONLY to the issue of whether or not there are any 'double bonds' in the molecule, NOT any other property. For example, an 'unsaturated' 3 carbon molecule could be 'proplyene' [2H-C=C-CH3)] since Carbon has 4 'bond positions'. Another could be propa-diene [2H-C=C=CH2] while propane (saturated) is 3H-C-CH2-C-H3.
A smaller 'saturate' , such as propane, with the same number of carbon atoms as the 'unsaturated' propylene, may exhibit lower volatility (measured as lower 'vapor pressure' or higher boiling point) . Unsaturated butene (4 carbon) has much lower volatility than saturated 3 carbon propane.
Note: RVP (Reid Vapor Pressure) IS NOT the true vapor pressure of hydrocarbons. RVP is the gauge pressure measured in a closed cylinder with a 4:1 volume ratio of air, saturated with water at 100F, to 32F test liquid, the resulting mixture shaken and heated to 100F in a water bath, until equilibrium is reached. It is designed to simulate potential pressure exerted by petroleum mixtures during summer
storage in tanks.
Question 1 During 'catalytic reforming' some lighter components such as pentanes, butane,propane, ethane, methane, hydrogen-are produced. These are removed from the 'reformate product' by distillation and routed to other processes in the refinery for additional processing. 'Reformate' RVP may range from 2.0 psi to as much as 7 psi, depending on how much of these 'light ends' are removed to achieve the best conditions for the particular refinery situation.
Question 2 'Alkylate' is a term used to mean the reaction products of iso-butane being chemically added to propylene and butylenes. One of these possible molecular products is 'iso-octane'. Long ago wise folks agreed that pure Iso-Octane represented an excellent high performance reference material for measuring relative tendency for 'knocking' during combustion. It was assigned an 'octane value' of 100. Normal heptane (7 carbon straight chain saturate ) was chosen to represent the worst possible knocking characteristics and assigned an 'octane value' of 0. The 'octane number' we talk about is the percentage of iso-octane in a mixture of 'ISO-octane' and 'normal heptane that is exactly matches the knocking tendency of the test fuel. If a 93% ISO-octane blend matches the knocking intensity of the test fuel, the test fuel is said the have an 'octane rating' of 93. Note there are two different test engines used to measure the 'octane'--a 'research method' and a 'motor method'. The 'research method' simulates 'cruising, light load conditions. The 'Motor Method' simulates more severe driving at high engine loads, higher temperature, higher engine speed. 'Alkylates' typically measure about the same 'octane number' on both test engines. Aromatics such as 'reformates' and unsaturates typically measure much lower 'octane' -as much as 15 'octanes' lower on the Motor method test than on the Research method engine. A good 'alkylate' may measure '100/98' while 'reformate' may measure 103/93.
Answer 2 --basically correct.
20/06/2012 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Benzene, the aromatic molecule has boiling point of 80 degree C and hexane has boiling point of about 60 degree. If you compare same carbon number molecules, aromatics are less volatile than paraffins. So reformate having more aromatics have lower RVP that way. However if reformate is not properly debutanised thee may be RVP problem.
Aromatics have higher RONC than iso paraffins. Isoparaffins may be obtained obtained by alylation.
RVP is highly dependent on boiling point.
20/06/2012 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Reid Vapor Pressure can be approximated by the volumetric weighted sum of vapor pressures at 38C/100F of the hydrocarbons present in either C5+ reformate or debutanized butylene alkylate or debutanized mixed olefin alkylate.
RVP of C5+ reformate will reflect the low vapor pressures of benzene & toluene & xylenes/EB & higher carbon number alkyl benzenes offset in part by the synthesis of pentanes and hexanes and pass through of n-hexane PLUS how much n-butane and isobutane are left in nominal C5+ reformate.
RVP of debutanized butylene alkylate is relatively low since the preferred chemistry derivatives of butylene alkylation are various iso-octanes plus some C12 isoparaffins. There is always some pass through pentanes and some isobutane and n-butane residuals in nominal C5+ alkylates. If there are substantial propylene and amylenes in the olefin feed, then there will be increased levels of hexanes and heptanes [due to polymerization of olefins followed by complex hydrogen transfer reactions]. This raises the RVP incrementally .
Octane RONC and MONC just reflect this aspect of combustion characteristics under compression in gasoline engines and octane test engines. It is necessary to explore tables showing RONC and MONC values for hydrocarbons ranging from isobutane and butylenes through C13/C14 alkyl benzenes to see how octanes both trend and sometimes vary up or down from the trends.
As another anomaly of petroleum refining, fluid catalytic crackers [source of most olefinic C4 LPG for butylene alkylation] tends to generate linear olefins, i.e. normal olefins and mono-methyl iso-olefins, peaking at C4 / C5 carbon numbers and trending downward to moderate to low values around C9's. At the same time, alkyl benzenes and alkene benzenes with short branches [methyls, ethenes, ethyls] increase. There are complex relationships among linear olefins and linear paraffins in the C3 to C7 carbon numbers for FCC/RFCC units operating to make higher octane gasolines and light olefins.