FUEL SCIENCE PROBLEM SET #3. Q&A

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FUEL SCIENCE PROBLEM SET #3 The usual hints, instructions, terms and conditions apply.

1. In 2004 (the last year for which complete data are yet available) there was a price difference of $10.55 per barrel between petroleum with API ≥ 45° and that with API ≤ 20°. (a) Why would refiners pay a price premium based on API gravity? In particular, why can a 45° API crude command a higher price than, say, a 20° crude? (b) Of these two oils, which is the more likely to give problems with coke deposition in thermal or catalytic processes? Why? (c) Which would more likely benefit from being processed through a visbreaker before being sent further through the refinery? Why? (d) And, in going through distillation, which is likely to give the lower yield of resid? Why? a) The higher API the higher amount of gasoline can be produced. b) Due to its high concentration of heavies in the crude, the 20° is more likely to give problems with coke deposition. c) If the 20° was placed through a visbreaker it would increase the amount of gasoline obtained from the refining process. d) The 45° because of its low concentration of small hydrocarbon molecules 2. Suppose that you have the job assignment of selecting catalyst for an FCC unit.

Suppose further that you can select from a variety of available catalysts that have different Si/Al ratios: Catalyst A has Si > Al, catalyst B has Si ≈ Al, and catalyst C has Si < Al. (a) Which catalyst would you specify, and why, if the FCC feedstock consists almost entirely of alkanes? (b) Answer the same questions if there were abundant alkenes in the feedstock. a) Catalyst C would be the best in this case because of the high concentration of alkanes in the feedstock. b) Catalyst A 3. Classify each of the following as a Lewis acid or a Brønsted acid: (a) HClO4, (b) BF3, (c) AlH3, (d) H2S, (e) HCO3–, (f) SO3, (g) HF, (h) H+ (think!), (i) ZnCl2, (j) H3PO4.

A

F

Lewis

G

Bronsted

H

Lewis

I

Lewis

Bronsted B Lewis C Lewis D Bronsted

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E

J

Bronsted

Bronsted 4. (a) Why is coking of the catalyst inevitable in an FCC unit? Explain in terms of the “inverted V” diagram we first used to discuss kerogen maturation. (b) If process control is not adequate, it is possible sometime to “over-crack” the feed, and get high yields of gases instead of gasoline. If this happens, will catalyst coking be worse or better than during normal operation? Why? a) Coke is produced from carbon rich molecules. As hydrogen is transferred from and to the crude oil, molecules become rich in hydrogen and carbon. This transfer of Hydrogen molecules is what causes coking. b) It would be better than normal operation, because over-cracking decreases the amount of residue left over from the process.

5. Show the disproportionation reactions for the following radicals reacting with R•: (a) CH2CH•CH3, (b) CH3CH2CH2CH2CH2CH2CH2•, (c) (CH3)2CHCH•CH3, and (d) (CH3)3C•.

6. Consider the carbocation CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2+. (a) Did this ion likely form on a Brønsted acid site on a catalyst, from -bond scission of an even bigger carbocation, or on a Lewis acid site? Explain your reasoning. Show the reactions of this ion in (b) -bond scission (think about the structure of the product), (c) rearrangement without  scission, (d) a 1,5-hydride shift, and (e) hydride abstraction from CH2CH2CH(CH3)2.

7. Shown below is the first introductory paragraph from a manuscript submitted to a leading professional journal in the energy and fuels field. The Saudi Arabia accounts for 26% of the world’s known oil reserves.

The Jurassic carbonate, Cretaceous, and Paleozoic reserves of Saudi Arabia comprise billions of barrels of sweet light crude oil. Middle Eastern sweet crude oil remains a high-value commodity due to its low

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heteroatom and impurity content. In contrast, heavy crude oils are high in heteroatom content and less desirable. Heavy crude oils require extensive downstream processing to remove heteroatomic species that contribute to fouling in hydrocarbon streams, corrosion, and NOx and SOx emissions during combustion. Sole reliance on Middle Eastern sweet crude oil will result in a steady increase in petroleum exportation cost to its neighbors.

As the supply of light sweet Middle Eastern crude oils is exhausted, reliance will shift to highly acidic heavy oils high in heteroatoms from the United States, China, Eastern Europe, and Venezuela. Consequently, catalytic hydrotreatment becomes increasingly essential. (a) What is the likely API gravity (or at least lower limit) of Saudi crude? (b) What is the likely sulfur content (or upper limit) for this oil? (c) In addition to nitrogen and sulfur, what other kind of “impurity” might require a hydroprocessing operation? (d) Compounds containing what heteroatom are likely to be corrosive? (e) Why can the Saudis get a price premium for selling sweet, light crude? (f) Is the last sentence (Consequently….) true? Why or why not?

8. Show the rearrangements of the carbon skeleton in the following carbocations. Note that in some cases no rearrangement occurs. As usual,  is the symbol for an aromatic ring. Cp is the symbol for a cyclopentane ring. (a) (CH3)3C+, (b) CH3CH2CH2CH2+, (c) CH3CH2CH+CH3, (d) CH2+, (e) CH3CpCH2+ (f) CH3CH2CH2+, (g) + + CH3CH CH2CH(CH3)2, (h) CH3CH CH2CH2.

9. In a propane de-asphalting unit, much of the propane can be recovered and recycled by evaporating or flashing off from the two product streams. However, inevitably some make-up propane would be needed, to compensate for losses in the process. Where, in a refinery, would you get propane?

10. Imagine that you are one of the engineers in charge of running a catalytic reactor that uses an iron catalyst on a mullite support (mullite is a solid solution, typically 3 Al 2O3 • 2.5 SiO2). The catalyst activity seems to be dropping, and the catalyst particles are turning black, suggesting coke deposition on the catalyst. It is important that this unit be out of service for only the minimum length of time, so the night-shift engineer, a graduate of Purdue, finds out that mullite is stable up to 1850°C, and decides to regenerate the catalyst very quickly by burning off the coke in a stream of oxygen. The next morning, you discover that the catalyst activity is lower than ever, even before the coke built up. (a) What likely happened? (b) Why?

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