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CHAPTER # 4
CATALYST SELECTION
4.1 CATALYST:
Substance that changes the rate of reaction but does not take part in reaction.
4.2 TYPES OF CATALYSTS:
The schemes of proposing processes are analogous generally. The differences are defines by performances of usable catalysts due to their type. Main parameter which is the octane number of produced isomerate depends on process temperature. That‘s why we will dwell on the issue of thermodynamic of isomerization reaction. First of all hydrocarbons isomerization reaction is balanced reaction, and equilibrium yield of isoparaffins increases with temperature reducing, but it can be reached only after an
―infinite residence time‖ of the feed in reaction zone or an equivalent very small value for LHSV. On the other hand an increase in temperature always corresponds to an increase in reaction velocity. So that at low temperature the actual yield will be far below the equilibrium yield, because of low reaction velocity. On the contrary, at higher temperature, the equilibrium yield will be more easily reached, due to a high reaction rate. Consequently, at higher temperature the yield of isoparaffins is limited by the thermodynamic equilibrium, and at lower temperature it is limited by low reaction rate (kinetic limitation) (Figure 4.1)
Figure 4.1 Dependence of n-paraffins conversion on reaction temperature
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Paraffin-isomerization catalysts fall mainly into two principal categories: those based on Friedel-Crafts catalysts as classically typified by aluminum chloride and hydrogen chloride and dual-functional hydro isomerization catalysts.
4.2.1 First Generation Catalysts:
The Friedel-Crafts catalysts represented a first-generation system. Although they permitted operation at low temperature, and thus a more favorable isomerization equilibrium, they lost favor because these systems were uneconomical and difficult to operate. High catalyst consumption and a relatively short life resulted in high maintenance costs and a low on-stream efficiency.
4.2.2 Second Generation Catalysts:
These problems of first generation systems were solved with the development of second-generation dual-functional hydro isomerization catalysts. These catalysts included a metallic hydrogenation component in the catalyst and operated in a hydrogen environment. However, they had the drawback of requiring a higher operating temperature than the Friedel-Crafts systems.
4.2.3 Third Generation Catalysts:
The desire to operate at lower temperatures, at which the thermodynamic equilibrium is more favorable, dictated the development of third-generation catalysts. The advantage of these low-temperature [below 200°C (392°F)] catalysts contributed to the relative nonuse of the high-temperature versions. Typically, these noble-metal, fixed-bed catalysts contain a component to provide high catalytic activity. They operate in a hydrogen environment and employ a promoter. Because hydrocracking of light gases is slight, liquid yields are high.
An improved version of these third-generation catalysts is used in the Penex process.
Paraffin isomerization is most effectively catalyzed by a dual-function catalyst containing a noble metal and an acid function. The reaction is believed to proceed through an olefin intermediate that is formed by the dehydrogenation of the paraffin on the metal site.
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The isomerization catalysts employed during World War II were all of the Friedel Crafts type. Those which contained aluminum chloride only were either a hydrocarbon/aluminum chloride complex (the so-called sludge process) or they were manufactured in-situ by deposition onto a support such as alumina or bauxite. They were intended to operate at very low temperatures [49-129°C (120-265°F)] and to approach the very favorable equilibrium composition characteristic of these temperatures.
The catalyst tended to consume itself by reaction with the feedstock and/or product.
When temperature was raised a little in an effort to compensate for loss of catalyst and to speed the reaction to effect more isomerization, light fragments were formed by cracking and these, when vented caused an excessive loss of the HCI promoter.
Corrosion of downstream equipment was also commonplace, due to the solubility of aluminum chloride in hydrocarbon, to its relatively high volatility and to the difficulty of removing it from the product by caustic washing. Aluminum chloride deposition in and plugging of reboiler tubes was not uncommon.
4.3 DUAL-FUNCTIONAL HYDROISOMERIZATION CATALYSTS:
4.3.1 Hydro-isomerization catalysts [above 199°c (390°f)]:
The operational problems which had characterized the Friedel-Crafts type isomerization plants, the advent of catalytic reforming which not only made hydrogen generally available in refineries but also demonstrated the practicality of using noble metal containing catalysts on a large scale, and the octane numbed race which postwar high compression engines initiated all combined in the 1950's to spawn a spate of hydro-isomerization processes. These catalysts generally contained a noble metal and some halide, operated at temperatures between about 299°C (560°F) and temperatures approaching those characteristic of catalytic reforming, employed recycle hydrogen to prevent catalyst carbonization and utilized either no promoter or traces at most. In general, they did not require an especially dry feedstock but did benefit from a low sulfur content feedstock. Most achieved a close approach to the equilibrium characteristic of their particular operating temperature.
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Because of their high operating temperatures and their necessarily low conversions to iso-paraffins, these high temperature catalysts were quickly replaced with the advent of the ―third generation‖ low temperature catalysts.
4.3.2 Hydro-isomerization catalysts [below 199°c (390°f)]:
Low temperature is considered rather arbitrarily for catalyst classification purposes as anything below 199°C (390°F) operating temperature. Typically these are fixed bed catalysts containing a supported noble metal and a component to provide acidity in the catalytic sense. They operate in a hydrogen atmosphere and may employ a catalyst promoter whose concentration in the reactor may range from parts per million to substantially higher levels. They generally all require a dry, low sulfur feedstock;
however, they may differ importantly in their tolerance of certain types and molecular weights of hydrocarbons. Hydrocracking to light gases is generally slight, so liquid product yields are high. The type of catalyst used in the Penex unit is of this type.
The acid function is the support itself and some examples include acid zeolites, chlorided alumina and amorphous silica alumina. Noble metals have a positive effect on the activity and stability of the catalyst. However they have a low resistance to poisoning by sulfur and nitrogen compounds present in the processed cuts.
In order to prepare a suitable catalyst for hydroconversion of alkanes, good balance between the metal and acid functions must be obtained. Rapid molecular transfer between the metal and acid sites is necessary for selective conversion of alkanes into desirable products.
4.4 ALUMINA CATALYST:
Alumina or aluminum oxide (AlR2ROR3R) is a chemical compound with melting point of about 2000°C and sp. gr. of about 4.0. It is insoluble in water and organic liquids and very slightly soluble in strong acids and alkalies. Alumina occurs in two crystalline forms. Alpha alumina is composed of colorless hexagonal crystals with the properties given above; gamma alumina is composed of minute colorless cubic
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crystals with sp. gr. of about 3.6 that are transformed to the alpha form at high temperatures. Figure (4.2) shows the shape of AlR2ROR3R [Ulla, 2003].
The most common form of crystalline alumina, α-aluminum oxide, is known as corundum. If a trace of the element is present it appears red, it is known as ruby, but all other colorations fall under the designation sapphire. The primitive cell contains two formula units of aluminum oxide. The oxygen ions nearly form a hexagonal close-packed structure with aluminum ions filling two-thirds of the octahedral interstices.
Typical alumina characteristics include: