Desirable Chemical Reactions
Desirable Chemical ReactionsFigure 8.2 illustrates more specifically the desirable chemical reactions of catalytic reforming, including:
- dehydrogenation of naphthenes to aromatics,
- dehydroisomerization of alkyl-C5-naphthenes,
- dehydrocyclization of n-paraffins to aromatics, and
- isomerization of n-alkanes to i-alkanes.
All of these reactions significantly increase the octane number (research octane number [RON] from 75 to 110 in Reaction 1, from 91 through 83 [cyclohexane] to 100 in Reaction 2, from 0 to 110 in Reaction 3, and from –19 to 90 in Reaction 4).
Text description of Figure 8.2.
This diagram illustrates four key reaction pathways in catalytic reforming, showing how different hydrocarbon structures are converted into higher-octane aromatic compounds and branched isomers, often with hydrogen (H₂) produced as a byproduct.
At a high level, the figure presents four labeled reactions—“1. Dehydrogenation,” “2. Dehydroisomerization,” “3. Dehydrocyclization,” and “4. Isomerization”—each with simplified line structures representing hydrocarbons and arrows indicating chemical transformation steps.
1. Dehydrogenation:
- A ring-shaped hydrocarbon (a six-membered ring with a short side chain) is shown on the left.
- An arrow points to a similar ring structure on the right that now contains a circle inside, indicating an aromatic ring.
- To the right of the product, the label “+ 3H₂” indicates that three molecules of hydrogen are produced.
- This reaction shows conversion of a cycloalkane to an aromatic compound with hydrogen release.
2. Dehydroisomerization:
- A five-membered ring with a short branch is shown on the left.
- An arrow points to a six-membered ring, indicating rearrangement of the structure.
- A second arrow leads to a six-membered aromatic ring (with a circle inside).
- To the right, the label “+ 3H₂” indicates hydrogen production.
- This sequence shows ring rearrangement followed by dehydrogenation to form an aromatic compound.
3. Dehydrocyclization:
- A straight-chain hydrocarbon (zig-zag line) is shown on the left.
- An arrow points to a six-membered ring, indicating cyclization.
- A second arrow leads to an aromatic ring (with a circle inside).
- To the right, the label “+ 4H₂” indicates four hydrogen molecules are produced.
- This reaction shows a paraffin converting into an aromatic compound through ring formation and dehydrogenation.
4. Isomerization:
- A straight-chain hydrocarbon (zig-zag line) is shown on the left.
- An arrow points to a branched hydrocarbon structure on the right.
- No hydrogen is shown as a product.
- This reaction represents rearrangement into a branched isomer to improve fuel properties.
Operating conditions and notes (bottom text):
- A statement reads: “to promote these reactions, use:” followed by four conditions:
- “high T”
- “low P”
- “low SV”
- “low H₂/HC ratio”
Reaction conditions that promote the desirable reactions are also listed in Figure 8.2. As can be seen in Figure 8.2, aromatic compounds and large quantities of by-product H2 are produced in the highly endothermic Reactions 1–3. High temperatures, low hydrogen pressures, low space velocity (SV), and low H2/HC ratio strongly promote the conversion in Reaction 1-3. Although maintaining a low hydrogen pressure is needed for promoting equilibrium conversion in Reactions 1-3, it is, however, necessary to maintain a sufficiently high hydrogen pressure in the reactors to inhibit coke deposition on the catalyst surfaces.