3 · Gasoline Refining and Testing

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Refining

Raw Material

As it comes out of the ground, petroleum crude can be as thin and light-colored as apple cider or as thick and black as melted tar. Thin crudes have high American Petroleum Institute (API) gravities and, therefore, are called high-gravity crudes; conversely, thick crudes are low-gravity crudes. High-gravity crudes contain more natural gasoline and their lower sulfur and nitrogen contents make them easier to refine. But modern refining processes are capable of turning low-gravity crudes into high-value products like gasoline. Refining low-gravity crudes requires more complex and expensive processing equipment, more processing steps, more energy and, therefore, costs more. The price difference between high-gravity and low-gravity crudes reflects the refining cost difference.

Processes

Today’s refinery is a complex combination of interdependent processing units, the result of a fascinating intertwining of advances in chemistry, engineering and metallurgy. It all started with the simple physical separation process called distillation.

Distillation In the late 1800s, crude was separated into different products by boiling. Distillation works because crudes are composed of hydrocarbons with a broad range of molecular weights and, therefore, a broad range of boiling points. Each product was assigned a temperature range and the product was obtained by condensing the vapor that boiled off in this range at atmospheric pressure (atmospheric distillation). The earliest crude stills were simple pot stills consisting of a container where crude was heated and a condenser to condense the vapor. Later, distillation became a continuous process with a pump to provide crude flow, a furnace to heat the crude, and a distillation column to separate the different boiling cuts.

fig 3-4 

Initially, atmospheric bottoms were used for paving and sealing. Later it was found that they could yield higher-value products like lubricating oil and paraffin wax when they were distilled in a vacuum. Vacuum distillation requires sturdier stills to withstand the pressure differential and more sophisticated control systems.

The limits of distillation as the sole refining process were quickly recognized: Because the yield of each product is determined by the quantity of the hydrocarbons in its boiling range in the crude, distillation couldn't produce enough gasoline to meet the demand. And it was producing higher-boiling material for which there was no market.

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Cracking The discovery that hydrocarbons with higher boiling points (the larger ones left in the distillation bottoms) could be broken down (cracked) into lower-boiling hydrocarbons by subjecting them to very high temperatures offered a way to correct the mismatch between supply and demand. This process — thermal cracking — was used to increase gasoline production starting in 1913. It is the nature of thermal cracking to make a lot of olefins, which have higher octane numbers but may cause engine deposits. By today's standards, the quality and performance of this early cracked gasoline was low, but it was sufficient for the engines of the day.

Two common examples of thermal cracking still in use today are visbreaking and delayed coking. Visbreaking heats a heavy petroleum fraction just enough to break down large molecules but not enough to carbonize it. It is more prevalent in Europe than in the U.S. In delayed coking, petroleum residua are heated enough to cause carbonization. Both thermal processes produce highly olefinic, high-sulfur, high-nitrogen products in the gasoline range and must be further processed to be acceptable motor gasoline blend components.

Eventually heat was supplemented by a catalyst, transforming thermal cracking into catalytic cracking. A catalyst is a material that speeds up or otherwise facilitates a chemical reaction without undergoing a permanent chemical change itself. Catalytic cracking produces gasoline of higher quality than thermal cracking. There are many variations on catalytic cracking, but fluid catalytic cracking (FCC) is the heart of gasoline production in most modern refineries. The term comes from the practice of fluidizing the solid catalyst so that it can be continuously cycled from the reaction section of the cracker to the catalyst regeneration section and back again. The FCC process also produces building blocks for other essential refinery processes, like alkylation.

Hydrocracking is similar to catalytic cracking in that it uses a catalyst, but the catalyst is in a hydrogen atmosphere. Hydrocracking can break down hydrocarbons that are resistant to catalytic cracking alone. It more commonly is used to produce diesel fuel than gasoline.

The next group of processes increases a refinery's octane number pool. Although these processes predate the regulation of antiknock additives, they became more important as lead was phased out of gasoline. Without antiknock additives, the only way to produce high-octane-number gasolines is to use inherently high-octane hydrocarbons or to use oxygenates, which also have high-octane-number values (see Figure 3-3). (A manganese-based antiknock additive is not widely used in the United States at this time (2003), see Chapter 3, Gasoline Refining and Testing)

Reforming The reforming process literally re-forms the feed molecules, converting straight-chain paraffins and naphthenes into aromatics. For example, reforming cyclizes normal heptane (RON = 0) and then abstracts hydrogen to produce toluene (RON = 120). The hydrogen by-product is almost as important as the octane number upgrade. Hydrogen is an essential ingredient for processes like hydrocracking and hydrofining. Refineries often have a hydrogen deficit, which has to be made up by making hydrogen from natural gas (methane) or other hydrogen-rich feeds.

Alkylation The alkylation process combines small, gaseous hydrocarbons with boiling points too low for use in gasoline to form liquid hydrocarbons. The feed, which primarily comes from the FCC unit, includes C₄ hydrocarbons like isobutane and butylenes and sometimes C₃ and C₅ paraffins and olefins. The principal products are high-octane-number isomers of trimethylpentane like isooctane (RON = 100). Alkylation is a key process for producing reformulated gasolines because the contents of the other classes of high-octane-number hydrocarbons — olefins and aromatics — are limited by regulation.

Polymerization Another combination process is the polymerization of olefins, typically the C3 olefin, propylene, into a series of larger olefins differing in molecular weight by three carbon atoms — C₆, C₉, C₁₂, etc. Polymerization is a less-favored process than alkylation because the products are also olefins, which may have to be converted to paraffins before they are blended into gasoline.

Isomerization Isomerization increases a refinery's octane number pool by converting straight-chain (typically C₅ and C₆) paraffins into their branched isomers. For a given carbon number, branched isomers have higher-octane-number values than the corresponding straight-chain isomer.

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Hydrotreating Hydrotreating is a generic term for a range of processes that use hydrogen with an appropriate catalyst to remove impurities from a refinery stream. The processes run the gamut from mild, selective hydrotreating to remove highly reactive olefins to heavy hydrotreating to convert aromatics to naphthenes.

Desulfurization Sulfur removal, or desulfurization, traditionally has been a specialized example of a hydrotreating process. The lower sulfur limits for reformulated gasoline may require the desul-furization of a significant proportion of FCC gasoline. There are also processing reasons to desul-furize refinery streams. In reforming, excess sulfur in the feed deactivates the catalyst. In FCC, excess sulfur in the feed results in high levels of sulfur in the FCC gasoline and greater production of sulfur dioxide during catalyst regeneration.

The lower sulfur limits for reformulated gasoline (and diesel) have prompted the commercialization of several non-hydrotreating processes for sulfur removal. One option is adsorption, where an appropriate adsorbent will remove selected sulfur species directly from the FCC gasoline. One variation requires chemically changing existing sulfur species to more easily adsorbed species. Another process reacts sulfur species with olefins in the FCC gasoline, producing heavier sulfur species that may be removed by simple distillation. Another variation oxidizes many sulfur species so that they may be removed by water washing.

Etherification of olefins High-octane oxygenates called ethers may be produced at the refinery by reacting suitable alcohols such as methanol and ethanol with branched olefins from the FCC, such as isobutene and isopentene, under the influence of acid catalysts. In the mid-1990s MTBE, made by etherification of isobutene with methanol, became the predominant oxygenate used to meet EPA and CARB reformulation requirements for adding oxygen to mitigate emissions from gaso-line- powered vehicles. By 2002,some 38 etherification plants were in place at refineries in the U.S. (See Chapter 2, Gasoline and Air Quality and in Chapter 4, Oxygenated Gasoline for more detail pro and con.)

Isobutene Dimerization Very recently, environmental issues with MTBE have made it more desirable to dimerize isobutene from the FCC unit, rather than etherify it. Fortunately, isobutene dimerization may be done with minimal modifications to existing MTBE plants and process conditions, using the same acidic catalysts. Where olefin levels are not restricted, the extra blending octane boost of the di-isobutylene can be retained. Where olefin levels are restricted, the di-isobutylene can be hydrotreated to produce a relatively pure isooctane stream that can supplement alkylate for reducing olefins and aromatics in reformulated gasolines.

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