Archive for October, 2011

THE ART OF PLATFORMING.

WHAT IS PLATFORMING?

The first time I heard about this word I thought about a platform to sit on………

Never mind me.

Consider this one of the things that happens to your fuel and you will never get a chance of knowing it ever happens. Quite strange that our refinery here in Kenya is quite behind and has a lot of catching up to do.

An upgrade of the refinery is overdue and should have been done yesterday.

History

In the 1940s, Vladimir Haensel,[1] a research chemist working for Universal Oil Products (UOP), developed a catalytic reforming process using a catalyst containing platinum. Haensel’s process was subsequently commercialized by UOP in 1949 for producing a high octane gasoline from low octane naphthas and the UOP process become known as the Platforming process.[2] The first Platforming unit was built in 1949 at the refinery of the Old Dutch Refining Company in Muskegon, Michigan.

In the years since then, many other versions of the process have been developed by some of the major oil companies and other organizations. Today, the large majority of gasoline produced worldwide is derived from the catalytic reforming process.

Very few, if any, catalytic reformers currently in operation are non-regenerative.

Catalytic reforming is a chemical process used to convert petroleum refinery naphthas, typically having low octane ratings, into high-octane liquid products called reformates which are components of high-octane gasoline (also known as petrol). Basically, the process re-arranges or re-structures the hydrocarbon molecules in the naphtha feedstocks as well as breaking some of the molecules into smaller molecules. The overall effect is that the product reformate contains hydrocarbons with more complex molecular shapes having higher octane values than the hydrocarbons in the naphtha feedstock. In so doing, the process separates hydrogen atoms from the hydrocarbon molecules and produces very significant amounts of byproduct hydrogen gas for use in a number of the other processes involved in a modern petroleum refinery. Other byproducts are small amounts of methane, ethane, propane, and butanes.

Motor gasoline (Mogas) or what we call petroleum production starts with the distillation of crude oil. One of the products out of that process is a fraction of low octane gasoline, normally referred to as naphtha, typically boiling in the range 100 – 160 0C. Other gasoline fractions are produced as a result of secondary processes like catalytic cracking, isomerisation, alkylation and platforming. Petrol is then produced by blending a variety of these gasoline components of different qualities to meet a series of product specifications.

One very important property of Mogas is the octane number, which influences “knocking” or “pinking” behavior in the engine of cars. Traditionally lead compounds have been added to petrol to improve the octane number. Over the past years, in many countries legislation has been implemented aimed at reducing the emission of lead from exhausts of motor vehicles and this, calls for other means of raising the octane number.

The role of a platformer is to pave the way for this by a process which reforms the molecules in low octane naphtha to produce a high octane gasoline component. This is achieved by employing a catalyst with platinum as its active compound; hence the name Platformer. For many refinery catalyst applications, a promoter is used, and in the platforming process, it is a chloride promoter which stimulates the ‘acidity’ of the catalyst and thereby the isomerisation reactions. Often, a bimetallic catalyst is used, i.e. in addition to the platinum, a second metal, for instance Rhenium is present on the catalyst. The main advantage is a higher stability under reforming conditions. The disadvantage is that the catalyst becomes more sensitive towards poisons, process upsets and more susceptible to non-optimum regenerations.

You can check,

http://en.wikipedia.org/wiki/File:CatReformer.png

 

Chemistry:

The main reactions of platforming process are as follows:

  • Dehydrogenation of naphthenes, yielding aromatics and hydrogen

The dehydrogenation of naphthenes to convert them into aromatics as exemplified in the conversion methylcyclohexane (a naphthene) to toluene (an aromatic), as shown below:

  • Dehydro-isomerisation of alkyl cyclopentanes to aromatic and hydrogen
  • Isomerisation of paraffins and aromatics

The isomerization of normal paraffins to isoparaffins as exemplified in the conversion of normal octane to 2,5-Dimethylhexane (an isoparaffin), as shown below:

  • Dehydrocyclisation of paraffins to aromatics and hydrogen
  • Hydrocracking of paraffins and naphthenes to ligher, saturated paraffins at the expense of hydrogen

The process literally re-shapes the molecules of the feed in a reaction in the presence of a platinum catalyst. Normally it is the hydrocarbon in the C6-C10 paraffin’s that get converted to aromatics.

The above reactions take place concurrently and to a large extent also sequentially. A majority of these reactions involve the conversion of paraffin’s and naphtenes and result in an increase in octane number and a nett production of hydrogen. Characteristic of the total effect of these reactions is the high endothermicity, which requires the continuous supply of process heat to maintain reaction temperature in the catalyst beds. That is why the process is typically done in four reactors in series with furnaces in between, in order to remain sufficiently high reactor temperatures.

The reactions takes place at the surface of the catalyst and are very much dependent, amongst other factors, on the right combination of interactions between platinum, its modifiers or activators, the halogen and the catalyst carrier. During operating life of the catalyst, the absolute and relative reaction rates are influenced negatively by disturbing factors like gradual coke deposition, poisons and deterioration of physical characteristic of the catalyst (surface area decline).

The process of platforming:

The feedstock of the platformer is drawn from the refinery’s distillation units. This is first treated by passing the feedstock together with hydrogen over a catayst, in a process called ‘hydrotreating, to convert the sulphur and nitrogen compunds to hydrogen sulphide and ammonia, in order to prevent poisoning of the expensive platformer catalyst. After hydrotreating, the reactor effluent moves on through a stabiliser column to remove the gases formed (hydrogen sulphide, ammonia and fuel gas). In a second column, the C5 and some of the C6 is removed in a separate fraction called ‘tops’. The reason to remove C5/C6 is that this component will crack in the platformer to produce fuel gas, while C6 gets converted into benzene, which can only be allowed in limited amount into the mogas because of its toxicity. From the bottom of the splitter column, the naphtha stream is produced, which is the feed for the Platforming section.

At the heart of the Platformer process are the four reactors, each linked to furnaces to sustain a suffiently high reaction temperature, about 500 0C at the inlet of the reactors.

Over time, coke will build up on the catalyst surface area, which reduces the catalyst activity. The catalyst can be easily regenerated however, by burning the coke off with air. After coke burning, the catalyst needs to be reconditioned by a combined treatment of air and HCl under high temperature. This regeneration step is called ‘oxy-chlorination’. After this step the catalyst is dried with hot nitrogen and subsequently brought in its active condition by reducing the surface with hot hydrogen. The refinery will therefore regularly have to take out one of the reactors to undergo this regeneration process. This type of process is therefore called semi-regen platforming.

During the regeneration process, the refinery will suffer production loss,. in a Continuous Catalytic Reformer, CCR. In the CCR unit, the reactors are cleverly stacked, so that the catalyst can flow under gravity. From the bottom of the reactor stack, the ‘spent’ catalyst is ‘lifted’ by nitrogen to the top of the regenerator stack. In the regenerator, the above mentioned different steps, coke burning, oxychlorination and drying are done in different sections, segregated via a complex system of valves, purge-flows and screens. From the bottom of the regenerator stack, catalyst is lifted by hydrogen to the top of the reactor stack, in a special area called the reduction zone. In the reduction zone, the catalyst passes a heat exchanger in which it is heated up against hot feed. Under hot conditions it is brought in contact with hydrogen, which performs a reduction of the catalyst surface, thereby restoring its activity. In such a continuous regeneration process, a constant catalyst activity can be maintained without unit shut down for a typical run length of 3 – 6 years. After 300 – 400 cycles of reaction/regeneration, the surface area of the catalyst will have dropped to a level (120 – 130 m2/g) that it becomes more difficult to maintain catalyst activity and at such a time normally the catalyst will be replaced by a fresh batch. The batch of ‘spent’ catalyst is then sent for platinum reclaim to recover the valuable precious metals.

Catalysts and mechanisms

Most catalytic reforming catalysts contain platinum or rhenium on a silica or silica-alumina support base, and some contain both platinum and rhenium. Fresh catalyst is chlorided (chlorinated) prior to use.

The noble metals (platinum and rhenium) are considered to be catalytic sites for the dehydrogenation reactions and the chlorinated alumina provides the acid sites needed for isomerization, cyclization and hydrocracking reactions.[11]

The activity (i.e., effectiveness) of the catalyst in a semi-regenerative catalytic reformer is reduced over time during operation by carbonaceous coke deposition and chloride loss. The activity of the catalyst can be periodically regenerated or restored by in situ high temperature oxidation of the coke followed by chlorination. As stated earlier herein, semi-regenerative catalytic reformers are regenerated about once per 6 to 24 months.

Normally, the catalyst can be regenerated perhaps 3 or 4 times before it must be returned to the manufacturer for reclamation of the valuable platinum and/or rhenium content

References

    1. ^ A Biographical Memoir of Vladimir Haensel written by Stanley Gembiki, published by the National Academy of Sciences in 2006.
    2. ^ Platforming described on UOP’s website
    3. ^ Canadian regulations on benzene in gasoline
    4. ^ United Kingdom regulations on benzene in gasoline
    5. ^ USA regulations on benzene in gasoline
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