Chemistry of Catalytic Reforming
Chemistry of Catalytic Reforming
The general categories of the desired reactions in catalytic reforming are identified in the list below, along with the catalysts used in the process. Considering that the main purpose of the process is to increase the octane number of heavy naphtha, conversion of naphthenes to aromatics and isomerization of n-paraffins to i-paraffins are the most important reactions of interest. Under the right reaction conditions, aromatics in the feed, or those produced by dehydrogenation naphthenes, should remain unchanged. The reforming reactions produce large quantities of hydrogen, and one should remember that the dehydrogenation catalysts used in reforming can also catalyze hydrogenation and hydrocracking of aromatics during catalytic reforming. It is, therefore, important to keep these side reactions to a minimum by controlling the reactor conditions such as temperature and hydrogen pressure, as discussed in more detail later in this section.
The catalysts used in reforming contains platinum (Pt), palladium (Pd), or, in some processes, bimetallic formulations of Pt with Iridium or Rhenium supported on alumina (Al2O3).
Catalytic Reforming Reactions and Catalysts
Reactions of Interest
- naphthenes → aromatics
- paraffins are isomerized
- aromatics are unchanged
Catalysts Used
Platinum catalyst on metal oxide support (platforming)
Pt/Al2O3
Bimetallic – Iridium or Rhenium
Pt-Re/Al2O3
The information above shows the ranges of composition for feedstock heavy naphtha and the reformate product (high-octane gasoline). Comparing the compositions of the feedstock and the product, one can see that the largest change in feedstock composition is a substantial increase in the aromatics content of the feedstock, with attendant decreases in naphthene and paraffin contents to constitute the product.
Catalytic Reforming Feedstock and Product
|
Feedstock: Heavy Naphtha Paraffins ⇒ 45-55% Naphthenes ⇒ 30-40% Aromatics ⇒ 5-10% |
Product: High Octane Gasoline Paraffins ⇒ 30-50% Naphthenes ⇒ 5-10% Aromatics ⇒ 45-60% |
Low severity (relatively low octane) → low paraffin conversion
High severity → high paraffin conversion
Lean naphtha → high n-paraffinic content - difficult to process
Rich naphtha → low n-paraffinic (high naphthene) content - easy to process
The information above also defines some specific terms for catalytic reforming related to the feedstock composition (lean, or rich naphtha), or to the extent of n-paraffin conversion in the process (low-, or high-severity). One could conclude from these terms that reforming of heavy naphtha that contains higher n-paraffin content requires more severe conditions in the reactor.
Desirable Chemical Reactions
Desirable Chemical Reactions
Figure 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).
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.
Undesired Reactions
Undesired Reactions
Hydrocracking is an undesired side reaction in catalytic reforming because it consumes hydrogen and decreases the reformate yield by producing gaseous hydrocarbons. Hydrocracking reactions are exothermic, but they can still be kinetically favored at high temperatures, and favored, obviously, by high hydrogen pressures. Below lists the heat of reactions for catalytic reforming reactions. Typically, reformers operate at pressures from 50 to 350 psig (345–2415 kPa), a hydrogen/feed ratio of 3–8 mol H2/mol feed, and liquid hourly space velocities of 1–3 h-1[1]. These conditions are chosen to promote the desired conversion reactions and inhibit hydrocracking while limiting coke deposition on the catalyst surfaces.
Undesired Reactions in Catalytic Reforming
Hydrocracking
n-C10+H2 → n-C6+n-C4
to inhibit this reaction, use
- high T
- high SV
- Low H2P
Catalytic reformers are normally run at low H2 pressure to inhibit hydrocracking!
Heats of Reactions:
paraffin to naphthene → 44 kJ/mol H2 - endothermic
naphthenes to aromatics → 71 kJ/mol H2 - endothermic
hydrocracking → -56 kJ/mol H2 - exothermic
Reaction Network
Reaction Network
A reaction network for catalytic reforming is shown in Figure 8.3 [2], indicating the role of metallic (M) and the acidic (A) sites on the support in catalyzing the chemical reactions. The surfaces of metals (e.g., Pt) catalyze dehydrogenation reactions, whereas the acid sites on the support (e.g., alumina) catalyze isomerization and cracking reactions. Metal and acid sites are involved in the catalysis of hydrocracking reactions. Achieving the principal objective of catalytic reforming—high yields and high quality of reformate—can be achieved, to a large extent, by controlling the activity of the catalysts and the balance between acidic and metallic sites to increase the selectivity to desirable reactions in the reaction network.
Catalytic Reforming Reaction network
KEY: R= Reversible, M = Metal Site, A=Acid Site
Lighter aromatics become alkylated aromatics (R)
-M/A (dealkylation)
Alkylated Aromatics become alkylated cyclohexanes (R)
-M (dehydrogenation)
Alkylated cyclohexanes become cyclopentanes (R)
-A
Cyclopentanes become n-paraffins (R)
-M/A (dehydrocyclization)
n-paraffins become i-paraffins (R)
-A (isomerization)
i-paraffins and n-paraffins to cracked products
-A (cracking)