Lesson 8 Overview

Hello. Welcome to lesson eight. We will talk about more catalytic conversion processes. We'll talk about in this class about alkylation, about polymerization, catalytic reforming, and isomerization. All of these processes are catalytic processes to produce high octane number gasoline.

Remember, for high performance, high power, we needed to produce high octane number gasoline. FCC obviously is the principal process in a refinery to produce high octane gasoline. Catalytic reforming, developed also during the Second World War, was really popular, very popular process.

Now the feed stock for catalytic cracking comes from the light ends unit. You'll remember the naphtha fractionater in the light ends unit. The heaviest product from the light ends unit is the heavy naphtha. The reason it's heavy-- it has a lot of naphthenes, or cycloalkanes in its composition.

So what happens in catalytic reforming, essentially converting these naphthenes or cycloalkanes into aromatics. Aromatics have very large octane numbers. Benzene, for example, has octane number of 100. So that it's highly, highly desirable high octane number component in the refinery blending scheme, if you will.

So dehydrogenation of naphthenes using a precious metal catalyst like platinum is pretty straightforward if you do have a clean heavy naphtha. If you have sulphur, associated with clean, or with naphtha feed, you need to hydro treat it to remove, because platinum is very susceptible to poisoning by sulphur. So you need a pre hydro treatment before catalytic reforming.

Now, up until 1990s, cat reforming was one of the most popular processes in the refinery as far as producing high octane number gasoline. But with the introduction of 1990 Clean Air Act amendments, the amount of gasoline or benzene and aromatics in gasoline were limited because of environmental issues or reasons of toxicity.

So all of a sudden now, catalytic reforming that produces a high aromatic content wasn't so desirable. But the refiners could not give up catalytic reforming. Why? Because there is a byproduct from cat reforming that is very, very valuable for the refinery. It has become, of course, increasingly valuable in the recent times. And that is hydrogen.

If you do dehydrogenate naphthenes, the byproduct is hydrogen, in addition to making aromatics. And hydrogen is needed in hydro treating processes and finishing processes that we will be talking about in the next lesson. So that is the cheapest source of hydrogen.

Obviously, you can make hydrogen from natural gas by reforming natural gas. And that is the done in refineries as well to produce additional hydrogen. But the cheapest source of hydrogen in a refinery comes from catalytic reforming. So it's still used in US refineries to make gasoline that is a reformate and, of course, the byproduct hydrogen.

The second process we will talk about this alkylation. Alkylation is, in a sense, the opposite of cracking, where you have a larger molecule, you crack it into smaller molecules. In Alkylation, we do the opposite. We take the smaller molecules and combine them into larger molecules that would fall in the boiling range for gasoline.

So the feed stocks for alkylation are three to four carbon atom alkanes, isoalkanes. Isobutane is the principal feed stock that comes from FCC. And also, olefins, three to four carbon atom olefins. That is propene and butene.

So combining isobutane with propene or butene will put you in the gasoline boiling range with seven to eight-carbon atoms. And the resulting product will be an isoalkane with a high octane number. That would be essentially making up the alkylate.

So alkylation is an alternative to catalytic reforming to make high octane gasoline without the aromatic. So that looks like really a nice alternative. But there is one problem. And the problem is for alkylation, you would need as catalyst highly concentrated acids, like highly concentrated sulfuric acid or highly concentrated hydrofluoric acid. These are, of course, not easy to our work with or transport or to have around, because of the risks involved with using this highly acidic materials.

Another process that generates larger hydrocarbons from smaller fragments are called polymerization. The difference between alkylation and polymerization is that we use just olefins in polymerization. No isobutane here.

So we use three to four carbon atom olefins-- typically again come from FCC process-- combine them using a milder acid as a catalyst this time-- so phosphoric acid. So the problems with handling phosphoric acids are not as severe as those with highly concentrated sulphuric or hydrogen fluoride used in alkylation. So polymerization produces branched olefins, which also have respectable octane numbers.

The last process we will talk about is isomerization. That's actually adding branching to straight-run paraffins. This is essentially the light naphtha that comes from the light end unit, as opposed to heavy naphtha that is naphthenic like naphtha is paraffinic. But it has only straight chain paraffins with low octane numbers. So isomerization add branching to these straight chains to increase the octane number. So all these processes just make high octane gasoline, which is, of course, the most important fuel in US refineries.