What is the significance of catalytic reforming?
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The main product of catalytic reforming is aromatics, including benzene; however, strict limits exist on the benzene content in gasoline. So how can catalytically reformed gasoline be added to regular gasoline? Should there be further processing, or should less be added?
In catalytic reforming for the production of high-octane gasoline, the feed is a wide-range fraction with a boiling point range of generally 80–180°C; whereas for the production of aromatics, the feed is a narrow-range fraction with a boiling point range of generally 60–165°C.
The most straightforward meaning of catalytic reforming is to convert feedstocks with a lower density into products with a slightly higher density; even though the products become heavier, that’s why it is also called \"heavy reforming\".
It’s about turning light oil into heavier oil, relatively speaking!
The chemical reactions involved in the reforming process fall into two categories: favorable reactions – those that increase the octane rating and produce a large amount of hydrogen as a by-product. Try to promote the occurrence of these reactions. Adverse reactions: That is, such reactions can reduce the octane number, lower the purity of hydrogen, and decrease the yield of the product. Try to suppress the occurrence of these reactions. A Hydrogen production reactions: a) Cycloalkane dehydrogenation – Cycloalkane compounds, namely cyclohexane, methylcyclohexane, dimethylcyclohexane, and cycloalkanes up to C10, undergo dehydrogenation to produce benzene, toluene, xylene, C9 and C10 aromatic hydrocarbons, respectively; per mole of these compounds. b) Alkane dehydrogenation and cyclization – The dehydrogenation and cyclization of n-alkanes (linear) and isoparaffins (branched) take place in several steps: cyclohexane releases one mole of hydrogen and then rearranges to form a cycloalkane, after which that cycloalkane undergoes dehydrogenation. Molecular rearrangement to form cycloalkanes is the most difficult reaction to facilitate, but subsequent aromatization of the cycloalkanes can yield a high octane number. B Favorable reactions that increase octane number without producing hydrogen: a) Straight-chain alkane isomerization: The isomerization of alkanes results in a slight increase in octane number. From a kinetic perspective (Figures 1 and 2), high temperature facilitates the isomerization reaction, while hydrogen partial pressure is unimportant. These reactions are promoted by the acidic functionality of the catalyst. b) Naphthenes isomerization: An alkylcyclopentane isomerizes into an alkylcyclohexane, involving ring rearrangement; this reaction is favorable because the resulting alkylcyclohexane is subsequently dehydrogenated to form aromatics. C Adverse reactions: a) Cracking reaction. b) Hydrocracking. The occurrence of this adverse reaction is similar to that of hydrocracking; it also involves the consumption of hydrogen and the breaking of carbon chains. It includes two types of reactions: hydrocracking and hydrolysis. c) Hydrodealkylation reaction: The hydrodealkylation reaction refers to the breaking of aromatic ring side chains (-CH3 or -C2H5). d) Alkylation reaction: The alkylation reaction is a condensation reaction in which an alkene molecule is added to an aromatic ring. e) Disproportionation (alkyl asymmetry): The two toluene rings (with one CH3 side chain) can undergo disproportionation to yield one benzene ring (without side chains) and one xylene ring (with two side chains). f) Coking: The formation of coke on the catalyst is the result of a very complex chemical reaction
We still need to look at the process of the reforming unit; generally, the feedstock is naphtha, or catalytically treated stable gasoline from which dienes are removed to isolate some light gasoline. Plants that have the capability for etherification can carry out this process, while the heavier components are mixed with naphtha and fed into the reforming unit. In the reforming stage, the feedstock is generally subjected to pre-hydrogenation first to remove most of the impurities such as S and N, thereby meeting the requirements for the reforming feed. Then the production department produces products such as reformed gasoline, liquefied gas, dry gas, and hydrogen. The reformed gasoline can be sent directly to the tank area for blending. If there is a benzene extraction unit available, it is usually processed first through an octane removal tower to remove some of the heavier components with molecular weights of 8–9 or higher; this resulting product is heavy benzene gasoline, which is an excellent component for blending. The remaining C6–C7 components are subjected to benzene extraction, ultimately yielding a series of high-value products such as benzene and toluene. Therefore, the significance of reforming lies in the ability to cyclize certain hydrocarbons in gasoline products, thereby increasing their octane rating, and producing a large amount of high-value benzene-based products as a by-product.
Thank you for your reply; your explanation made things much clearer. But another problem arises: the products of reformation enter the octane removal tower, and the C8 components removed are excellent high-octane gasoline additives; however, a certain amount of benzene is inevitably carried along. There are strict limits on the benzene content in gasoline, so should this C8 fraction be further subjected to benzene removal? Or should benzene be hydrogenated? Not limited to reformed gasoline, but for automotive gasoline specifically, is there any value in researching benzene hydrogenation?
There’s another question: you said that \"if there is a benzene extraction unit downstream, the stream generally passes through an octane removal tower first to remove some of the heavier components with molecular weights above C8–C9.\" So, if there is no benzene extraction downstream, would there still be an octane removal tower? If there is neither an octane rejection tower nor benzene extraction later on, then can’t the reformate be used as gasoline? Too high a benzene content?
Thank you for your reply; I really hadn’t realized that before
If there’s nothing, then it’s not a complete restructuring, no!
It depends on the ratio used during blending; as long as the benzene content in the product is within acceptable limits, that’s sufficient. It can be blended with other benzene-free oils whose octane rating can be slightly lower.