Pour points branched paraffins

Paraffin isomerization of heavy alkane feeds is often used to alter the cloud or pour point of diesel or lube fractions. Catalysts for this reaction are almost always dual-function catalysts of Pt supported on a one-dimensional zeolite. Using a onedimensional zeolite allows control of the isomerized product to contain few branches, usually methyl branches (Table 12.4). 

The melting point and the pour point are essentially the same. The addition of branches as methyl groups decreases the melting point by at least 30°C and up to 60°C. A closer look at the table shows that it is not the number of methyl or branches that appear to impact the melting point, but the presence of at least one branch. However, there is a significant difference when the branch is located near the end of the alkyl chain, i.e. in position 2 the melting point only decreases by about 30°C or half the diminution observed for the other branching positions. It is therefore preferable to have the methyl located toward the center of the alkyl chain to minimize the pour point. The conversion of linear paraffins to isoparaffins also impacts the VI (Fig. 8.9). 

These are the most widely used pour point depressants. R in the ester has a major effect on the product and is usually represented by a normal paraffinic chain of at least 12 carbon atoms, which ensures oil solubility. The number average molecular weight of the polymer, Mn, is also very important typically, these materials are between 7000 and 10,000 amu. Commercial materials normally contain mixed alkyl chains which can be branched. 

Branching of the n-paraffin chain reduces the VI and the more substituents there are, the lower the VI. Table 3.5 shows this effect for C26 and C30 paraffin isomers. (Bear in mind that branching improves the pour point and is the sought-after objective of hydroisomerization.)... 

Hydroisomerization is the catalytic process for dewaxing waxy lubes and conversion of waxes to high VI base stocks by isomerization of n-paraffin structures to isoparaffins with one or more branches. These branches are usually methyl branches. We have already seen in Chapters 2 and 3 that iso-paraffins have lower pour points than n-paraffins and can have quite high Vis if the branches are close to the chain ends. Hydroisomerization is distinguished from catalytic dewaxing via ZSM-5-type catalysts in that the latter cracks n-paraffin structures to C3 to C8 molecules (Figure 10.18), whereas the former causes isomerization and has... 

The separation of linear and branched alkanes is also of importance in the process known as dewaxing, in which the removal of normal alkanes makes the product hydrocarbon less viscous and reduces the so-called pour point temperature. Such processes can be combined with catalytic isomerisations to optimise the value of oil fractions (Chapter 8). Linear paraffins are also separated using a zeolite-based process from kerosene fractions to give reactants for the synthesis of linear alkylbenzene sulfonate anionic surfactants, which are both cost effective and biodegradable. 

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