Catalytic Reforming

Catalytic reforming (from Latin reformare = to transform) is a refinery process in which alkanes and cycloalkanes from naphtha ( raw gasoline ) of various origins are converted into aromatic compounds and branched alkanes with the aim of increasing the octane number . The main liquid product, the so-called reformate, consists mainly of benzene , toluene , xylenes , C ₉ and C ₁₀ / C ₁₁ aromatics as well as branched and linear alkane compounds.

An important by-product is hydrogen , which is required in desulphurisation and hydrocracking processes, among other things . By cracking , the gaseous hydrocarbons are methane , ethane , propane and butane produced.

history

The first catalytic reforming process was developed in the 1930s by the Kellog company, seven of which were built and operated from 1939. The process used a molybdenum-containing catalyst which , however, had poor selectivity and service life.

In the 1940s, Vladimir Haensel at Universal Oil Products (UOP) developed a more stable and activating reforming catalyst based on platinum on aluminum oxide . The first commercial platformer , a composition of platinum and reformer created by UOP , was built as early as the 1940s . The process replaced the Kellog process due to the longer service life of the catalyst.

In the following period, the catalyst developed by Haensel was further developed. In particular, the use of tin (Sn) and rhenium (Re) as co-catalysts increased the service life and also the activity. The focus of the process engineering development was in the field of catalyst regeneration. The type of process installed until the early 1970s is known as the Semi Regenerative Catalytic Reformer ( SRR ). Here three reactors with the respective heaters are connected in series (see Fig. 1).

The process variant widespread today is the so-called Continuous Catalyst Regenerative Reformer ( CCR ), in which four moving bed reactors are connected in series. The catalyst runs through all four reactors and is then continuously regenerated.

raw materials

A reformer's feedstock can come from a variety of sources. The main source is the crude oil distillation, which supplies so-called "straight-run" naphtha, an unstabilized hydrocarbon mixture, i.e. containing propane and butane with a boiling range of approx. 25 to 135/180 ° C. The end of boiling is selected depending on the process conditions for the reformer. This raw gasoline is also contaminated with nitrogen and sulfur compounds.

Hydrotreating

Sulfur and nitrogen compounds are catalyst poisons that must be removed. A common procedure is therefore to desulfurize or denitrify the raw naphtha directly using a hydrotreater . The hydrogen sulfide (H ₂ S) formed under the process conditions of the reformer would react with the metals on the reformer catalyst and thereby deactivate the catalyst. A typical reformer feed requires a sulfur content of around 0.5 ppm. Ammonia (NH ₃ ) formed in the reformer reacts with the Cl ^- to form ammonium chloride (NH ₄ Cl) and destroys the acidic function. Therefore nitrogen compounds must also be largely removed.

stabilizer

After the hydrotreater, the naphtha is stabilized, that is, the butane, propane, ethane, methane, the hydrogen sulfide formed and the water present in the product and the remaining hydrogen are distilled off in a stabilizer. Propane and butane are produced as liquid gas (liquefied petroleum gas, LPG) that still contains H ₂ S. The lighter components are released into the refinery gas that has not yet been desulfurized . Water collects in a water pocket in the head reflux container. Water is also a catalyst poison because it washes out the chloride.

The end of boiling of this stabilized, desulphurized naphtha and thus the proportion of heavy reformate components (C ₉ / C ₁₀ / C ₁₁ ) is already determined in the crude oil distillation, but should not be above about 180 ° C, otherwise the end of boiling of the reformate is significantly above 210 ° C rises and therefore compliance with the petrol boiling end specification is not guaranteed. Hydrogenating the LPG and light petrol of the crude oil at this point avoids the installation of additional desulphurisation systems for LPG or light petrol.

Reformer feed pre-fractionation

The start of boiling of the stabilized naphtha has to be set by another distillation after stabilization, since pentanes (C ₅ ) in naphtha do not contribute to the production of aromatics or hydrogen. Although n- pentane would largely be isomerized to i- pentane, it only makes a small contribution to the increase in octane number. In addition, pentanes lead to increased coke formation through cracking. For these reasons, the pentanes are removed from the reformer feed.

Hexanes (C ₆ ), especially n- hexane , methylcyclopentane and cyclohexane, are mainly converted into benzene in the reformer (so-called benzene precursor). Together with the benzene already present in straight-run naphtha, the reformate would have benzene contents of about 8% by weight to 12% by weight. Refineries therefore usually also distill off hexanes in order to achieve benzene contents of around 2% to 4%. Refineries that a benzene extraction possess pursue a benzene maximization strategy. However, unsuitable iso- hexanes are also distilled off.

Heptanes (C ₇ ) generate sufficient hydrogen. If you do not necessarily have to rely on C7 proportions in the reformate for reasons of blending ( gasoline ), want to produce more xylenes and have enough reformer feed available, then you also cut the heptanes from the reformer insert.

There are three basic types of reformer feed:

• Pentane / i- hexane-reduced naphtha (benzene production desirable, boiling range: 70–135 / 180 ° C)
• C ₆ -reduced naphtha (standard feed, boiling range: 85–135 / 180 ° C)
• C ₇ -reduced naphtha (xylene production preferred, boiling range: 115-135 / 180 ° C).

The distilled desulphurised light petrol can be used as a petrol component or steam cracker feed, depending on the quality .

Other sources of naphtha

In addition to the straight-run naphtha from crude oil distillation, there are other naphtha sources in the refinery: the non-straight-run naphtha. These include hydrocracker heavy naphtha, which may have to be desulfurized, the Coker heavy naphtha or FCC heavy naphtha. These have to be hydrogenated to remove olefins and sulfur. If necessary, the start and end of boiling are already set in the corresponding systems. The hydrogenation can, under certain circumstances, be carried out together with the straight-run naphtha.

The quality of the reformer feed is based on the so-called PIONA (analysis P araffine, I somere, O lefine, N aphthene, A romaten) determined. This is a gas chromatographic determination of the individual components in mass fraction (% by weight).

Procedure

Scheme of the catalytic reforming process

Reforming takes place at temperatures of around 500 ° C and, depending on the type of process, at pressures of 30 bar for the semi-regenerative process or 3.5 to around 8 bar for processes with continuous catalyst regeneration. Bifunctional catalysts such as platinum-tin or platinum-rhenium on chlorinated aluminum oxide or zeolites are used .

The reactions that take place are endothermic . The reactor outlet temperature can drop by 100 K as a result. Therefore, several reactors with intermediate heaters, mostly in the form of gas-fired ovens, are installed to compensate for the temperature loss and to achieve a sufficient degree of conversion .

By dehydrogenation - and polymerization reactions is produced on the catalyst coke , which by physical blockage of the active sites affects the activity of the catalyst. In the semi-regenerative process, the coke is burned off at intervals of 6 to 24 months, with the catalyst remaining in the reactor. This is followed by what is known as oxychlorination, during which the acidic functionality is restored. The reactor pressure of a semi-regenerative reformer is around 30 bar. The yields are poorer than those of a continuous regenerative reformer, since the high partial pressure of hydrogen leads to re-hydrogenation of the aromatics.

In the process with continuous regeneration, coke formation takes place much faster due to the lower hydrogen partial pressures. For regeneration, the coke-laden catalyst is discharged at the bottom of the last reactor and transported to the top of a so-called regenerator of the moving bed reactor type via a gas lift system. There, the coke is first gently burned off the catalyst in various zones and then it is oxychlorinated. Then they are transported back by a gas lift system from the bottom of the regenerator to the top of the first reactor.

When UOP system are the four reactors stacked ( stacked ). As a result, the catalyst runs from the top of the first reactor to the bottom of the fourth reactor solely by gravity. Axens uses a horizontal arrangement, similar to the semi-regenerative process. The catalyst is transported between the reactors by a lift system from the bottom of the respective reactor to the top of the next.

A low hydrogen partial pressure increases the formation of coke, but improves the yield and quality of the reformate through intensified dehydrogenation reactions with increased hydrogen production and a higher aromatic yield. The quality of the hydrogen gas also improves as it contains fewer cracked products such as methane or ethane. To maintain the acid function, small amounts of a chlorinated hydrocarbon are metered into the insert, which are immediately hydrogenated to HCl and the corresponding hydrocarbon under the prevailing process conditions . Chloride losses, caused for example by nitrogen and water input, are compensated for.

The typical apparatus of a catalytic reformer are

• Feedstock pump
• Feeding of the cycle gas
• Heat exchanger (against the reactor outlet of the last reactor), a so-called feed-effluent heat exchanger.
• Furnace / reactor combination (3 to 4 times), then again heat exchanger
• Product cooler
• High pressure separator (SRR), overhead recycle and hydrogen gas
  • Hydrogen-rich gas is fed into the refinery's hydrogen network after being cleaned again
  • Recycle gas is transported back to use with the circulating gas compressor
  • unstabilized reformate runs to the stabilizer (pressure distillation at ~ 8–15 bar)
• Stabilizer column, overhead LPG and refinery gas, "stabilized" reformate in the sump.

Products

The following components can be distinguished as products of the catalytic reformer

• hydrogen-rich gas (depending on the process variant: 20% by weight to 45% by weight H ₂ in the gas, the rest is methane , ethane , possibly propane and small amounts of butane )
• Refinery gas (sulfur-free)
• LPG (sulfur free)
• Reformat: depending on the process: boiling range ~ 25–155 / 210 ° C, d. H. C ₅ -C ₁₁ compounds of the types: n - and i -alkanes, aromatics [C ₆ ⁺  !], About 0.1% to 0.2% olefins, about 1% to 2% cycloalkanes.

In the reformer process, cracking and dealkylation also result in groups of substances that were originally not included in the reform feed or were only included to a very minor extent. Even if the feed only from C ₈ consists compounds, the reformate comprises a not inconsiderable proportion of n - and i -pentane to continue benzene and toluene, as well as hexanes and heptanes.

Reformate fractionation

The direct blending of “full-range reformate” into motor gasoline is not common in Europe, since a benzene content of 2% to 4% in the reformate quickly leads to the benzene specification being exceeded. The reformate is usually worked up by distillation in order to remove the benzene. A sensible solution is a so-called 3-cut splitter (distillation column with top and bottom product, as well as a side draw). Similar to the reformer feed distillation, a section with pentane and i- hexane compounds as well as small amounts of benzene (approx. 1 vol .-% to 1.5 vol .-% in the product) is distilled off overhead, so-called light reformate, that is used in petrol or in the steam cracker.

In the side take-off, a so-called heartcut is drawn off, which contains the majority of benzene, as well as hexanes and lighter C ₇ alkanes. The heartcut is being sold. The bottom product (C ₇ -C ₁₁ ) also contains only a little benzene and can be used directly as a gasoline component. Some refineries process the bottom product further and distill C ₇ - (toluene and C ₇ non-aromatics), C ₈ - (xylenes, very few C ₈ non-aromatics) and C ₉ / C ₁₀ / C ₁₁ compounds (almost exclusively aromatics) apart. C ₇ and C ₉ / C ₁₀ / C ₁₁ run into motor gasoline, the xylene mixture is sold or sent to a xylene separation system (e.g. PAREX ).

mechanism

The hydrogenation / dehydrogenation reactions preferably take place at the metal centers of the catalyst, while the acid centers catalyze isomerization and ring closure reactions. An undesirable side reaction is the cracking of alkanes into low-chain products and dealkylation reactions.

Typical reactions to reforming are

Cyclization

Cyclization of n- heptane to methylcyclohexane or 1,2-dimethylcyclopentane (reacts further, see isomerization )

Dehydration

also as a subsequent reaction of the cyclization,

here dehydrogenation of methylcyclohexane to toluene (also called dehydrocyclization )

Isomerization

Isomerization of n - in iso alkanes, e.g. B. n- octane in 2,5-methylhexane

Isomerization of 1,2-dimethylcyclopentane in methylcyclohexane

Cracking

Cracking of alkanes, e.g. B. n- octane in n -butane and n -butene . The double bond is then immediately hydrogenated

Cracking, animated illustration

Dealkylation

Dealkylation of aromatics, for example toluene in benzene and methane

Process variants

AROMAX

Aromax is a special reformer for processing a C ₆ / C ₇ naphtha cut. The high conversion rates of C ₆ and C ₇ alkanes result in high yields of benzene and toluene, and the H ₂ generation rate is also excellent.

Fully regenerative reformers

Works like an SRR, but there is a fourth, so-called swing reactor available, which cyclically takes over the function of another when it is regenerated off-line. This means that this so-called Powerformer (Exxon name) does not have to be run down every six to 24 months, but can be operated for five years without stopping. However, this process control requires a complicated system of motorized valves with appropriate switching logic.

outlook

The reduction of the aromatic content in petrol from 2005 through the European Auto / Oil Program (Auto Oil 2, AOP II for short) from 42% by volume to 35% by volume has already posed a challenge for operating a reformer in the past A process that produces a product with an aromatic content of approx. 65% no longer fits into the plant portfolio.

On the other hand, hydrogen production is indispensable for many refineries. Many producers then accepted “negative” reformer margins, produced more gasoline and exported the surpluses overseas (USA). However, with falling gasoline demand in Europe and the USA, this “loophole” is no longer available. At the same time, the demand for hydrogen continues to rise, for example by reducing the sulfur content in the EL heating oil from 1000 to 50 ppm.

The producers reacted / react with 3 strategies

• Installation of an expensive hydrogen production ( steam reforming ), shutdown of the reformer or throughput reduction, sale of the naphtha (steam cracker feed).

• Removal of aromatics from the gasoline pool,
  • through the sale of pure toluene or xylene,
  • Generation of benzene and xylenes from toluene ( transalkylation )

• Closing down entire refineries, especially FCC refineries that make a lot of gasoline.

literature

• Manfred Baerns , Arno Behr , Axel Brehm: Technical Chemistry . 757 pages, Wiley-VCH, 2006, ISBN 3-527-31000-2 .
• George J. Antos, Abdullah M. Aitani: Catalytic Naphtha Reforming , Marcel Dekker Inc., 2004, ISBN 0-8247-5058-6

Individual evidence

1. ^ History of Reforming; doi : 10.1021 / ie51396a028
2. ↑ Lourdes Ramos: Comprehensive two dimensional gas chromatography . Elsevier, 2009, ISBN 0-08-093269-X , pp. 164 ( limited preview in Google Book search).
3. ^ Reformer pressure .
4. ^ UOP CCR ​​Platforming ™ Process. (No longer available online.) Archived from the original on July 5, 2013 ; Retrieved June 27, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
5. ↑ Chevron AROMAX .
6. ↑ AROMAX process .