BLENDING
GASOLINE BLENDING
Finally you get to gasoline, the subject everybody knows a little about. Gasoline is the product almost everybody buys and
occasionally spills on their shoes. They know what yesterday’s gasoline price was because it’s posted on every other corner. They all understand that higher octane is better than lower octane, but they probably don’t know why. That’s all okay for most people because a competitive marketplace delivers gasoline grades that will work just fine in any car. Most people don’t need to understand much more—except you.
It used to be that understanding how to make gasoline was a fairly simple matter. However, since the 1980s, governments and institutions have learned how bad «old fashioned» gasoline can be for your health. Governments, especially in America, have successively put more restrictions on the content of gasoline, giving progressively bigger headaches to refiners. The best way to cover the accumulated complexities is with a historical tour, starting with what refiners think of as «the good old days» and environmentalists consider the «black past.» So this article will start with gasoline in the 1960s and work its way to the 21st century, covering these questions:
What happens when gasoline is burned in a car engine?
What is octane, and the equally important property, vapor pressure?
Whatever happened to tetraethyl lead, and what did that do?
How does gasoline effect the environment?
How do refiners blend gasoline now to not affect the environment?
What effect does gasoline blending have on the way to run a refinery?
Gasoline Engines
The essential parts of a gasoline engine, at least for this discussion, are the gas tank, the fuel pump, fuel injection, the cylinders, the pistons, and the spark plugs. Engines without the last item (spark plugs) will be discussed later because they’re diesels.
You might say the whole motive process in a car starts at the gas tank when you fill it at a gas station. As you start the engine, the fuel pump sucks gasoline out of the tank and sends it to the fuel injection apparatus.
The purpose of fuel injection is to get the gasoline in the right state and in the right place to burn it, releasing the chemical energy. Gasoline, like other hydrocarbons, doesn’t burn in its liquid state. It has to be vaporized and mixed with oxygen to ignite. For example, when you throw charcoal lighter on the coals in your barbecue grill and toss in a match, the vapors ignite, not the liquid. That’s why some dummies can get away with squirting charcoal lighter directly on a lit fire without barbecuing themselves.
The liquid stream doesn’t ignite. Unless the charcoal lighter has been sitting in the sun or next to the grill and is warm, the stream doesn’t vaporize in the air, only when it hits the hot coals.
The fuel injection apparatus mixes the gasoline with air, pumps it up to a high pressure, then squirts it into the cylinder. The sudden release of pressure is enough to vaporize the mixture, filling the cylinder with a gasoline/air mixture, ripe for ignition. The nitrogen that’s also in the air just passes through the process, more or less unaffected. (Less is bad, as you’ll read below).
The sequence of happenings illustrated in Figure 12-1 shows one full cycle of a four cycle engine. Keep in mind you may have four, six, or eight cylinders doing this in your car, cycling at 2-3,000 times per minute.
The downward movement of the piston works together with the fuel injection apparatus to suck the gasoline/air mixture into the cylinder. At the bottom of the stroke, the space in the cylinder reaches its maximum and is filled with the fuel. The fuel injection apparatus closes and in the next step, compression, the piston moves up the cylinder, compressing the vapor. When the piston reaches the top of the stroke at the point of ignition, the spark plug gives off a powerful spark, igniting the gasoline vapor. The gasoline burns rapidly, just short of an explosion. The gases expand and put huge pressure on the piston, forcing it down the cylinder in the power stroke. If the gasoline is formulated correctly, different molecules will burn at different times so that the combustion takes place over the whole length of the power stroke, smoothing out the motion. Power is transmitted to the crankshaft as the piston is forced down the cylinder in the power stroke. At the bottom of the power stroke, the exhaust valve at he top of the cylinder opens. Burnt fuel is pushed out in the exhaust stroke as the piston moves up the cylinder. At thetop of the stroke, the outlet valve closes, the injector opens up again to squirt the fuel in, and me process is ready to be repeated.
Note that each cycle requires two trips of the piston up and down the cylinder, which is not the reason why it’s called a four cycle engine. The four cycles are intake, compression, power, and exhaust, which aren’t really cycles, but nobody ever said automotive engineers were all that articulate.
Some small two cycle engines (lawnmowers, outboards, etc.) use gasoline. The big two cycle engines run on diesel fuel or the heavier residual fuel.
Vapor Pressure
One of the crucial steps in the engine cycle is the vaporization of the gasoline. When the engine is warm, there is no problem—the engine heat assures that 100% of the gasoline will enter the cylinder in vapor form. When the engine is being started from cold, the injector may spit some droplets instead of vapor into the cylinder, making ignition difficult at worst, less smooth at best.
The trick in handling cold starts is to have enough volatile hydrocarbon in the gasoline to get a vapor-air mixture that will ignite. The measure of volatility is vapor pressure, and more specifically Reid vapor pressure(RVP), immortalizing the man who designed the test apparatus.
Definition. Vapor pressure is a measure of the surface pressure it takes to keep a liquid from vaporizing. A light hydrocarbon like propane will have a very high vapor pressure since it is very volatile. A heavier hydrocarbon like gas oil will have nearly zero vapor pressure, since it will vaporize very slowly—at normal temperatures. If you think for a moment you’ll realize that any vapor pressure is a function of temperature.
RVP is measured at 100°F, an arbitrary number Reid chose.
Engine conditions. Definitions out of the way, go back to the vaporization problem. The RVP of gasoline must meet two extreme conditions. On cold starts, enough gasoline must vaporize to provide an ignitable mixture. Once ignition occurs, the rest of the gasoline will vaporize and burn too. The other extreme happens when the engine is running while completely warmed or even more extreme, is being restarted when it’s hot. At that point, the gasoline vapor must not expand in the injection apparatus so much that no air can be mixed with it. Again, the mixture must be ignitable.
Refiners have found that there is a direct correlation between a gasoline’s ability to meet those conditions and the RVP. Furthermore, they have found that the ideal RVP for gasoline varies with the seasons. In the dead of winter in a place like Bemidji, Minnesota, cold starts need a gasoline with a 12 psi RVP. During the dog days of August in Brownsville, Texas, cars won’t restart if the gasoline has a higher RVP than 8.5 psi.
Vapor lock. One other constraint on vapor pressure is worth mentioning— vapor lock. Car engines can shut down if they encounter an unexpected high altitude or high temperature. At high altitudes, the atmospheric pressure is lower, and high RVP gasoline will tend to vaporize anywhere in the system. High temperatures aggravate the problem. The fuel pump tries to pump a combination of vapor and liquid when it is designed to handle only liquid. Consequently, the whole system will starve, and the engine will quit and won’t start again until the temperature of the gasoline is lowered. That could take hours. To avoid vapor lock, gasoline RVP is localized to accommodate the temperature and pressure conditions in the area, including seasonal temperature and barometric swings.
Blending gasoline to meet RVP requirements. So much for what cars need. How do refiners get there? If you look at the list of gasoline blending components in Table 12-1, you’ll see that all but three have RVPs below the limits mentioned above. The answer might jump right out at you—as the industry evolved, butane became the pressuring agent of choice.
If you sat down to design the scheme for the refining industry to blend gasoline, you’d probably reject the thought that there would be enough butane around to be the marginal component used for all vapor pressure control. But, for many years and places that’s the way it has turned out. Butanes are made in refineries as a by-product of conversion processes. They also come into the refinery mixed with some crudes and are recovered from natural gas in gas processing plants. Somehow these three rather inelastic supplies end up providing all the butane that’s needed for gasoline blending.
Getting down to nuts and bolts, the procedure for calculating the amount of butane needed to pressure up the gasoline involves only algebra and weighted averages. Vapor pressure calculations are not exactly related to the volumetric weighted averages, but for the purposes of this drill, they’re close enough. Suppose the RVP specification is 10 psi and you have a blend of five components. Table 12-2 shows how much normal butane is needed.
So the calculation is pretty simple, but some of the implications might be mentioned. Since the specification for RVP is higher in the winter than in the summer, the capacity to produce gasoline is higher in the winter. The higher the RVP spec the more butane refiners can blend in and the more total volume of gasoline they make. Unfortunately, in most markets, except those like Miami Beach, Horida or Vail, Colorado, the demand for gasoline is lower in the winter than the summer. The additional winter gasoline blending capacity does, however, give some flexibility to increase distillate fuels production without having to under-manufacture gasoline.
Normal versus isobutane. Why use normal butane instead of isobutane to pressure gasoline? There are several good reasons. First of all, the RVP of normal is 19 psi less than iso (71 versus 52), which means that more normal butane can be blended in. Second, normal butane is more «normal» and plentiful in nature. Third, isobutane has another welcome home, alkylation. Very often refineries don’t even have enough isobutane to satisfy the appetite of the alky plant, and they have to feed some normal butane to a butane isomerization plant to make isobutane. All this results in normal butane prices normally lower than isobutane prices and that gives an incentive to blend as much normal butane as allowed.
Do you recall back in the «good old days» watching a car gas tank as you filled it and seeing a wavy-looking vapor appear around the gas cap. That was butane escaping from the gasoline blend. If you recall correctly, you probably saw more vapors in the winter than in the summer. That’s the result of the higher vapor pressure spec and more butane in the winter versus the summer. Since those times, the escaping butane and other hydrocarbons have been identified as major contributors to air pollution and lower RVPs have been mandated to restrict those emissions. Many places have clumsy vapor recovery systems on the nozzle to capture and recycle the vapors.
Octane Number
Everyone knows that bigger and more powerful cars need higher octane, and they accept that higher octane gasoline should cost more.
This section will give you a clue why that is, in fact, true.
Octane numbers are measures of whether a gasoline will knock in an engine. That’s a fine definition, but it requires an explanation of another almost universally obscure phenomenon—knocking.
Knocking. You’ll find it helpful to refer back to Figure 12-1, the diagram of the engine cycle. After the gasoline/air vapor is injected into the cylinder, the piston moves up to compress it. As the vapor is compressed, it heats up. (Ever feel the bottom of a bicycle pump after you’ve pumped up a tire? If s hot. Same effect as in an engine cylinder.) If the gasoline/air vapor is compressed enough, it will get hot and start to self-ignite, without the aid of a spark plug. If this happens before the piston reaches the top of the stroke, the engine will knock and push the piston in the wrong direction, against the crankshaft instead of with it. Typically, knocking occurs only momentarily before the spark plug flashes, so knocking is usually perceived as a thud, ping, or…well, knock coming from the engine. Obviously knocking is something to be avoided since it not only works against the engine’s motive power, but it’s also tough on the mechanical parts.
At the very early stages of engine development, refiners discovered that the various types of gasoline blending components had different knock behavior. A consistent way to characterize when a blending component would knock turned out to be the compression ratio. In Figure 12-2 the compression ratio is shown as the cylinder’s volume at the bottom of the stroke divided by the volume at the top of the stroke. The compression ratio at which self-ignition first takes place at the top of the stroke determines a blending component’s octane number.
To make life easier, refiners devised a series of guide numbers to measure the compression ratio at which any gasoline component knocked. They defined iso-octane, C8H18, as 100 octane gasoline and normal heptane, C7H16, which knocks at a much lower compression ratio, as zero octane gasoline. By using a test engine, any gasoline component can be matched with blends of iso-octane and normal heptane.
Definition. The octane number of any gasoline blend or blending component equals the percent of iso-octane in the iso-octane/normal heptane blend that knocks at the same compression ratio as the gasoline or component being evaluated.
Testing for knock. An explanation of the test procedure might help. In its most basic form a test engine is used that has a top that can be screwed up or down to vary the compression ratio. The gasoline whose octane number is to be measured is fed into the engine while the head is being turned down. At some point, knocking will occur. Originally testers sat next to the engine with their heads cocked and listened for the knock by ear. Now they have a little more interesting life because they can use electronic detonation meters. After noting the compression ratio, the cylinder head is backed off. Two blends of iso-octane and normal heptane are concocted, one that the tester guesses will knock at a higher and one at a lower compression ratio. The octane numbers of these blends are known by definition (the % iso-octane). The tester runs each through the same test procedures noting the compression ratio when knocking occurs. By plotting the three data points, the octane number of the gasoline component can be read off a graph like that in Figure 12-3.
In that example, on a test engine a gasoline component knocked at a compression ratio of 8.1:1. Two test blends are made up, one with 88% iso-octane (88 octane) the other with 96% iso-octane (96 octane). In the test engine, they knock at 7.2:1 and 8.4:1, respectively. From the chart, the octane number of the gasoline component must be 94.0 octane.
Octane requirements. Now you know what octane numbers measure. Why are they important? The design of an engine demands that the fuel behave in a certain way. The compression ratio of an engine determines the amount of power it can deliver. The higher the compression ratio, the longer the power stroke, the more powerful the engine. Different size cars have different engine designs, therefore different requirements for gasolines of different octane numbers. Put more simply, you don’t get to vary the compression ratio of your car by turning the head up or down. So you have to buy the quality of gasoline that accommodates the car and compression ratio you have.
Types of octane numbers.You need to know two more sets of nomenclature about octane numbers—the different kinds and their uses. First of all, the tests for octane numbers are run under two sets of conditions. The research octane number (RON) test simulates driving under mild, cruising conditions; the motor octane number(MON) test is run under more severe conditions and simulates operations under load or at high speeds. The two measures, RON and MON, give an indication of a gasoline’s performance under the full range of conditions.
In the late 1960s there was a controversy among the U.S. Federal Trade Commission (FTC), US refiners, and the car manufacturers about posting octane numbers on gasoline pumps. The FTC wanted octane numbers posted. The refiners had been advertising «100 octane gasoline» as their premium product, which referred of course to the RON. They wanted to post only the RON. They didn’t want to confuse the public (or embarrass themselves) by putting the lower MON on the pump. The car manufacturers favored the MON as a better measure of how their products performed, and they were happy to sell cars that needed the «lower» MONs. The FTC considered posting RONs and MONs. «Too confusing,» said the refiners and car manufacturers. The FTC finally arrived at a compromise by ordering the following to be posted on gasoline pumps:
That measure didn’t have any particular meaning other than the fact the controversy was over. (R+M)/2 remains the nominal industry standard.
The 87,89, 91, or slightly higher and lower numbers you see on the gas pumps are the result.
The second piece of information about octane numbers relates to how they behave. When two gasoline components are mixed together, the octane numbers do not blend linearly. That is, the resulting octane is not the simple, volume-averaged octane. However, most fortunately, there is such a thing as a blending octane number for the RON and MON of every component that does blend linearly. The blending octane number is related
to the true (test engine) number in a constant way, and is developed by experience. When references are made to RONs or MONs of components, they can mean either the true or blending octane number. To make things simple here, all references to octane numbers will mean the blending
octane number, not the true number.
Blending for Octane Number. An example may help pull these ideas together. Take the blend of gasoline in the previous example where butane was added to achieve the vapor pressure spec. Calculate the RON and MON of the blend using the typical octane numbers in Table 12-3.
Now calculate how much alkylate must be added to meet a minimum specification of 84 MON and 90 RON, which would be 87 (R+M)/2. Alkylate has octane numbers of 95.9 MON and 97.3 RON.
In order to bring the 22,131 bbl blend with an MON of 78.1 up to an MON spec of 84, the following alkylate with MON of 95.9 must be added:
Meeting the MON spec doesn’t mean meeting the RON spec. The same, but separate calculation is done to determine how much alkylate to add to meet the RON spec instead:
More barrels are needed to meet the MON spec; that decides the volume, because both the MON and the RON specs are minimums. The extra RON of the resulting blend, which turns out to be 90.7, is something refiners call octane giveaway.
You may have noticed a subtle problem undermines this example. If you add 10,973 bbl of alkylate to the blend to meet the minimum octane specifications, you no longer meet the RVP specification. By the use of two equations and two unknowns, i.e., the volume of butane and the volume of alkylate, you could figure out the correct blend to satisfy both octane and vapor pressure specs. Maybe that’s more algebra than you can stand right now.
Driveability. The composition of gasoline has another effect on how smoothly a car will run on a gasoline blend. As the gasoline ignites in an engine’s cylinder, not all the molecules do or should burn at once. The piston needs a continuous push down the cylinder. A gasoline blend with a full spectrum of molecules usually provides the best chance of gradual, if you can use that word for something that happens during a thousandth of a second, burning. That gives great value to oil-based gasolines, which, as you saw in the chapter on distilling, are mixtures of all types of compounds.
It also makes refiners cautious when they add very much chemical blending stock like ethanol which has only a single boiling point. It gives a flat spot on the distillation curve.
Leaded Gasoline
Until the 1970s refiners added lead to gasoline as a simple and economical way to increase the octane number. Lead, in the form of tetraethyl lead (TEL), increases the octane number of gasoline without effecting any other performance characteristics, including vapor pressure.
Adding a small amount of lead, say about 3 grams of lead per gallon, could increase the octane number of a gasoline blend by as much as 5 RON. You can see that in Figure 12-4 from the effects on the various blending components.
Unfortunately, TEL is terribly toxic and in low concentration in vapor form can cause memory loss, blindness, or death. Because of this hazard, as late as the 1960s the Surgeon General of the United States set the maximum amount of TEL allowed in gasoline sold in the US at 4 grams per gallon. The US Environmental Protection Agency (EPA) succeeded to the authoritative position of the Surgeon General and when the pressure to do something about the poor air quality in many US cities became unbearable, the EPA mandated the use of catalytic mufflers in all new cars to improve the exhaust characteristics. It turns out that lead poisons the catalyst in these mufflers. To deal with both the health effects of airborne lead and the detrimental impact on the catalytic mufflers, the EPA then ordered a gradual phase out of the lead content in US gasoline, starting in 1975. The use of lead in gasoline in America and most countries around the world is now just a memory, at least for those who never had a personal encounter with lead vapor.
Petrochemical Blending Components
With the advent of lead phase down, refiners began a desperate search for ways to maintain the octane level of their gasoline pool. They took the most obvious and readily available step—crank up the severity of the cat reformer, making higher octane reformate. Of course that reduced the volume of gasoline that could be produced on cat reforming. They faced the dilemma of building more refining capacity or looking for other ways to expand volume and octane at the same time. Oil company connections to the petrochemicals industry led them to experiment in the 1980s with a handful of readily available commodities: methanol, ethanol, TBA, MTBE, ETBE, TAME, and the totally unpronounceable THxME, THpME, THpEE and THxEE. Only some of them made the final cut.
Methanol. One of the oldest industrial chemicals around is methanol, CH3OH, also known as methyl alcohol or sometimes wood alcohol cause of the historic practice of making it by chemically treating fresh-cut lumber from hardwood trees. Since 1923, chemical companies have used a more efficient process that starts with natural gas, or at least with dominant constituent, methane. Some companies, in some parts of the world, use naphtha as a starter.
With a casual look at the formulas for methane and methanol, CH4 and CH3OH, you might think the route from one to the other would involve something like a catalyst and some water, maybe a little pressure and temperature. Unfortunately chemists have been searching to no avail for the silver bullet that would do the catalysis. They have found some, but none that have proved commercial. Instead, the route from methane to methanol involves a complicated, intermediate step, the creation of synthesis gas, a mixture of CO and hydrogen (H2):
The formulas are deceptively simple. The process and hardware are complicated and pricey, and they involve expensive catalysts, and temperatures of 500-800°F and pressures of 4,000-5,000 psi. For that reason, almost all refiners chose to buy their methanol from chemical companies that specialized in the commodity and just blended it in.
Methanol has some properties that eventually undermined the enthusiasm for continuing to use it. First of all, it is corrosive and toxic and really bad if it gets on you. Second, it has an affinity for water in the same way your favorite bourbon or scotch whiskey does. Unlike oil/water mixtures, it won’t settle out and has to be separated by distillation, which is not at all practical if water and methanol mix anywhere but in a refinery or chemical plant. To make matters worse, if methanol as a blendstock in gasoline picks up enough water, the methanol/water mixture will phase separate. It will remove itself from the rest of the gasoline blend. That makes handling difficult enough in refineries but especially in pipelines and terminals, which are not meant to be water free. (They didn’t need to be because the water is removed at the terminals by drawing it off the bottoms of the tanks as BS&W, bottom sediment and water). Worst of all, if a methanol blend leaks out of an underground storage tank, which almost all gas stations have, the hydrophilic (water-loving) methanol can get into the underground water supply more easily than the hydrocarbon.
In the 1980s oil companies tried to commercialize a methanol blend in the US called M-85, made of 85% oil gasoline and 15% methanol. During the process they discovered more bad news about methanol. When it is burned in an internal combustion engine, it emits a scary carcinogen, formaldehyde, in the exhaust. Eventually the health hazards and water affinity caused M-85 and all other blends of methanol and gasoline to fail under their own weight.
Ethanol. To the surprise of traditional oil refiners came a strong contender, both politically and commercially, for winning the octane enhancer race in the 1980s and 1990s. Ethanol, or ethyl alcohol, comes from the natural fermentation of vegetables. For milleniums when farmers fermented grapes they got wine, potatoes they got vodka, and grain or corn they got whiskey. Ethyl alcohol, CH3CH2OH, is the operative ingredient in all these beverages. During the 1970s and 1980s, farm lobbies around the world successfully convinced their national or local governments that growing and fermenting corn, sugar cane, and some other crops and using it for automotive fuel would be good for their energy independence and their economies. Government subsidies brought on large amounts of natural ethanol capacity and created a consumer market for gasohol, a blend of 90% conventional motor gasoline and 10% ethanol. Petrochemical companies also make ethanol from ethylene by reacting it with water in the presence of a phosphoric acid catalyst. The low conversion rate of 4-6% requires a lot of ethylene recycling through the reactor to get the job done. Consequently, ethanol from the petrochemical industry doesn’t compete with ethanol from the agricultural sector, especially when government subsidies are in place.
Ethanol’s octane number of 114 gives a real boost to the gasoline pool, but the RVP of 19 means that other lighter hydrocarbons have to be backed out of the pool to meet the vapor pressure limitations. That displaces all the butane from ethanol-enhanced gasoline and even some of the C5′s in the various blending components. Refiners have to run their fractionation columns differently and look for a home for the rejects. The pentanes are sometimes sent to the reformer to be converted to cyclic compounds or sold to chemical companies for ethylene plant feeds. The C olefins can be made into the next group of exotics.
TBA. At about the same time the chemical companies started promoting another chemical, tertiary butyl alcohol (TBA). TBA originated as a by-product from making propylene oxide. It can also be made on purpose by reacting butane with water or isobutylene and propylene. Refiners were attracted to TBA because it worked in sympathy with methanol as a cosolvent. TBA dissolved easily in both methanol and conventional gasoline blending components. So it helped methanol stay in solution in the gasoline blend, even as it picked up water. With the demise of M-85, most of the hype around TBA vaporized too.
MTBE. Methyl tertiary butyl ether (with a name like that, no wonder it’s called MTBE) became an expensive but more user friendly successor to methanol. Like methanol, MTBE has affinity for water. But it is an ether, not an alcohol and it does not phase separate like methanol. Like methanol, it gives a big octane boost at 109 (R+M)/2 to the gasoline pool and has an RVP of only 8. (Both octane and RVP depend on what other components are blended in the gasoline.) Unlike methanol, it will not make you any sicker than the rest of the gasoline pool. On balance, refiners found MTBE attractive and started building their own MTBE plants.
The route to MTBE starts with iso-butylene and, ironically, methanol. Refiners still had an aversion to building methanol plants and continued to buy the commodity from chemical companies. Isobutylene was available from cat cracking and other cracking processes, but sometimes in limited supply. That created a market for iso-butylene and an incentive to build plants that would create iso-butylene from isobutane or even normal butane.
ETBE. For a while, refiners bought some ethyl tertiary butyl ether from chemical companies. Some have built ETBE plants, or used modified MTBE plants. ETBE has a higher octane number (109) and lower RVP (4) than MTBE, about the same water affinity and toxicity, but it is more expensive to make and not a big item on refiners’ to-do lists.
TAME, THxME, THpME, THxEE, and THpEE. This group of tongue strangling chemicals emerged when refiners needed to remove C5, C6, and C7 olefins from the gasoline pools to meet the eventual specifications you’ll read in a few paragraphs. Besides the olefins, the primary ingredient in these chemicals is either methanol or ethanol. TAME is tertiary amyl methyl ether (amyl is a synonym for a C5, as in amylene, C5.) THxME is tertiary hexyl methyl ether; THpME is tertiary heptyl methyl ether; THxEE is tertiary hexyl ethyl ether; and THpEE is tertiary heptyl ethyl ether. All these chemicals have high octane numbers, low RVPs, and no other bad stuff like sulfur in them. Refiners are loath to build these expensive plants unless regulators back them into a corner.
Combating Smog and Ozone
As governments developed more science about air pollution, they found more bad actors. They connected CO, oxides of nitrogen, including NO, N02, and N03, which they termed NOx, and the hydrocarbons floating around in the air, to the smothering ozone and thick layers of brownish smog that hung over the big cities. In the 1990s governments started mandating that gasoline must have a minimum oxygen content of several
percent. Since hydrocarbons from crude oil have no oxygen content, that meant non-traditional blending components. Refiners had, by that time, already been blending the oxygenates, ethanol, MTBE, and some of the other ethers, all of which have an oxygen atom in each molecule. Now they had to accelerate the program to meet the government proscriptions.
The theory behind the mandate involved introducing oxygen to the combustion process to assure more complete burning of the hydrocarbons to CO2 and H2O, with virtually no CO. It also reduced the amount of unburned hydrocarbons coming out of the engines. The change was intended to enhance the operation of the catalytic mufflers (that were already supposed to be doing the same thing) and improve those cars that had old-fashioned mufflers.
TOX, NOx , VOCs, and SOx
This unsavory group, which sounds like the name of a law firm formed by the children of itinerant circus performers, was the next show that came into town. Extensive, invasive engineering research found these culprits spewing out the tailpipes of cars, vaporizing out the gas tank fill pipes, and generally leaking throughout the distribution systems. Governments, oil companies, and environmentalists adopted a shorthand clarion call, TOX, NOx, VOCs, and SOx, which stand for toxic compounds, nitrogen oxides, volatile organic compounds, and sulfur oxides.
The conventional wisdom deemed a number of givens. The heaviest parts of the gasoline blending components, those at the high end points, contributed to unburned hydrocarbon getting through the combustion system. The lightest of the hydrocarbons such as butane were evaporating or leaking out of the gas tanks, the engine seals, and the gas pumps before they could even get to the combustion chamber. Benzene in small concentrations was positively identified as a carcinogenic threat and therefore one of the toxic compounds. Other aromatics compounds could result in unburned emissions, including benzene. NOx and VOCs reacted with sunlight to create smog and ozone. SOx caused the catalyst in mufflers not to work so well, emitting more VOCs and CO. And on and on and on.
The consequence of all this good news/bad news was a succession of rules for the content and behavior of gasoline under the banner ofreformulated gasoline for fuel sold in densely populated areas where ambient conditions don’t disperse pollutants very effectively.
The object of the regulations is to prevent air pollution, but TOX, NOx, VOCs, and SOx coming out of cars contribute to different forms of pollution in mysterious, non-linear ways. The emissions from burning gasoline cannot exceed various combinations of the four emitants (a term emerging from the pollution-speak community) in panel two. The control mechanism sets limits on combinations of the four.
At the same time the generation of the four emitants is connected in complicated ways to the gasoline characteristics in the third panel. T50and T90 (the temperatures at which 50 and 90% of the gasoline boil off) and RVP measure volatility and shape of the distillation curve. The others relate to the composition. Because of their impact on the four emitants, all these properties have explicit limits, some by statute, at least in the US.
Finally, in order for gasoline to work well in car engines, refiners must make gasoline that meets their own performance specifications, shown in the fourth panel. (Distillation curve continuity is the property refiners watch to assure the gasoline blend has a full range of compounds so it ignites smoothly over the whole power thrust of the piston. That of course limits the addition of single boiling temperature blend stocks like the alcohols and ethers).
Gasoline Blending and its Impact on Operations
Do refiners have a clue how to deal with all these constraints? How do governments know if they are complying? There is no pretending that optimizing the blending of gasoline is simple. Consider for a moment the ever-increasing levels of complexity as refiners set the operating conditions in the processing units:
• Given the demands for three grades of gasoline and the availability of components, blend up the requirements—with no leftovers
• Now consider varying the operating conditions of some of the process units. Change the severity on the reformers to adjust yields versus octane numbers, but be mindful of the benzene and aromatics content; increase the temperature in the cat cracker to generate more olefins and ultimately more alkylate, etc
• Finally, consider diverting streams in and out of units. Send cat cracked light gas oil to be blended to furnace oil rather than hydrocracked; remove the light ends from straight run gasoline to reduce RVP; cut the benzene precursors out of reformer feed; cut the bottoms off the straight-run naphtha (reformer feed) to make more kerosene /turbine fuel, and so on.
Refiners have put in place predictive computer models of increasing complexity and are developing others to help cope with these challenges. The models involve the old techniques, linear programming to simulate refinery operations. The intakes, outturns, capacities, and costs of each operation, from distilling to blending, are described using equations and numerical values. The crude oil availabilities and costs, and the product demands and prices are specified. The linear programming technique will find the solution to the equations (there are usually many) that makes the most profit.
The computer is necessary because of the thousands of calculations that are necessary to find the optimal solution. Even this is only an approximation, for several reasons:
• The data fed into the models are estimates of the process unit yields. Depending on any number of things (time since the last shutdown, catalyst activity, air temperature, cooling water temperature, etc.) the yields and octane numbers could vary
• The crude composition could vary
• The demands and prices could vary
Further, the inevitable unscheduled shutdowns in some part of the refinery will interrupt orderly flow. Nonetheless, as an analytical technique to develop a model or a plan, the linear program is an invaluable tool.
To get from panel three to four in Figure 12-5, refiners are now adding other mathematical techniques like regression analysis, followed by lots of testing to see if the theoretical results came out correctly.
Conclusion
The subject of gasoline blending brings into focus most of the operations in a refinery. The rudiments of meeting octane and RVP specs are simple enough, but optimizing gasoline blending implies optimizing the entire refinery.
As a gesture of conciliation to you after reading this exhausting litany of refiners’ problems, no TOX, NOx, VOCs, and SOx blending problem will be included in the exercises.