C&EN Special
T
he often harried executives of many oil companies will be meeting in New York City next week for the annual get-together of the American Petroleum Institute. The executives* hustling pace on New York's west side between the New York Hilton and the Americana Hotel may well reflect their pace back at their own companies, for they face problems un thought of just 10 years ago:
• Dwindling sources of raw materials and possible changes in the oil import program. • Vociferous demands for low-lead and unleaded gasoline. •Pollution controls and demands that affect decisions on raw materials, processing equipment, and marketing. The traditional problem of profitability is being pushed hard today by a closely ranked second one—sources of raw materials. Tied to this concern about raw materials are concerns about future prices of domestic crude and natural gas, changes in demand for fuels because of air pollution problems and expanding energy needs, availability of petroleum from Alaska's North Slope, costs of shipping foreign oil to the U.S., and dependability of foreign supplies of oil and gas. The most pressing immediate problem, though, is the demand for nonpolluting gasoline. The snowballing antipollution legislation further complicates the situation. According to many chemical and oil executives, the jumble of legislation so far allows only general plans to be made for future hydrocarbon supplies going to chemicals and gasolines. Some order, however, does appear on the legislative front. Last month a joint U.S. Senate and House conference committee agreed on a bill that would require auto makers by Jan. 1, 1975, to build cars with polluting emissions 90% lower than current federal standards call for. A one-year period of grace is included in this legislation as well as a monitoring of technological progress on control of auto exhaust pollution by the National Academy of Sciences. How to do it? Proposed ways to reduce or end pollution from automobiles run from abolishing autos to eliminating the use of lead alkyls as antiknock additives in gasolines. Elimination of lead alkyls would permit using catalytic reactors in exhaust systems to oxidize carbon monoxide and unburned hy52 C&EN NOV. 9, 1970
GASOLINE Antipollution forces bring marked changes to petroleum refining industry
Bruce F. Greek, Houston Bureau Head drocarbons and to reduce formation of oxides of nitrogen (leaded gasoline can't be used with most catalytic reactors because it deactivates the catalysts). Another purpose is to reduce pollution from lead particulates—if traps for particulates aren't used. Other ways to reduce auto exhaust pollutants are the thermal reactor system undergoing development at Du Pont and the lead reactor system developed by Ethyl. Both of these systems would be usable with available leaded gasolines and still meet emission standards. Still other methods to reduce auto pollution include a host of mechanical modifications to engines. Lower compression-ratio engines operate at lower temperatures, with accompanying loss of efficiency, thereby reducing the amount of oxides of nitrogen formed. Combustion chambers shaped to cause "swirling" during burning of fuels appear to reduce undesirable emissions. Fuel injection, which gives more uniform mixtures in cylinders, also improves combustion efficiency. Legislation designed to help some of the mechanical modifications along has been passed or is now pending. Such legislation differs from that concerned with reduction of lead alkyls. An example of this type of legislation was signed into law in California by Gov. Reagan this fall. The bill will prevent sales of 1972 model autos in California that require gasolines of greater than 91 research octane number (RON) to operate. A compression-ratio law and need for high-octane gasoline of any kind might be moot points if auto makers continue to make cars with engines with ratios of about 8.5:1. The current list of specifications for 1971 U.S. model cars abounds with 8.5:1 compression ratios.
ratio brings enough strident complaints, auto makers could take a relatively simple expedient of increased engine displacement to gain back any lost horsepower. The result of that, however, would be to increase exhaust volume, and very likely the amount of pollutants. Some people think a better way would be to use higher compression ratios and higher-octane gasolines plus other changes such as lean air-fuel ratios to get lower emissions. Currently, the least expensive way to obtain higher RON's is to add lead alkyls to gasoline. Lead alkyls add less than 0.6 to 0.7 cent to the average cost of a gallon of gasoline now
sold containing an average of nearly 3 g of lead alkyls per gallon. At the refinery loading rack, gasoline costs between 10 and 13 cents a gallon depending on quality. Delivery to the auto's gas tank equals this cost; taxes also add an equal amount. So far most studies indicate the internal combustion engine fueled with gasoline quite similar to that in use today will figure largely in transportation power for many years. The questions to which most people in industry and government address themselves now concern how to minimize, or possibly eliminate, pollution from internai combustion engines.
Sales of gasoline in the U.S. have grown faster recently Sales, billions of gallons. . . 100 Total 3 Road vehicles
40
20
1970 b
1960 a Includes sales for uses other than road vehicles, b Preliminary figures for 1969.
Source: American Petroleum Institute
Octane number increases as tèîraethylead concentration increases although at a declining raie as this example indicates Research octane number 100
H H M
Public dislike Feedback from buyers of 1971 model cars obviously remains small as yet. Hence, it is too early to tell about predictions that the octane race will not end, even with stringent pollution control devices and restrictions that affect performance of autos. Theory on the continued octane race seems to be based on public disapproval of the effects on auto performance because of lower compression ratios, leaner mixtures, higher maintenance costs, and the like. For example, if the apparent poorer acceleration of a car because of its lower horsepower due to lower compression
95
90 0
1
2
3
Tetraethvlead. α Deraalion
NOV. 9, 1970 C&EN 53
Octane values vary widely with hydrocarbon branching, unsaturation, and size Hydrocarbon
Unleaded research octane number
n-Octane n-Heptane n-Hexane n-Propylcyclopentane Octene-2, cis isomer n-Pentane Isopropylcyclohexane 2,4-Dîmethy! hexane Octene-4, trans isomer Isopropylcyclopentane
—19* 0.0b 24.8 31.2 56.2 61.7
62.8 65.2
73.3
81.1
Cyclohexane Pente ne-1 Hexene-2, trans isomer n-Buta ne
83.0 90.9
Propane Butene-1 2,2,4-Trimethylpentane (isooctane)
97.1 97.4
Cyclopentane Propylene 2,2,4-Trl· methylpentene-1 Benzene, technical grade 1,4-DiethyIbenzene
92.7 93.8
100.0b 101.3* 102.5a
102.5*
105.8* 106.0*
Ethylbenzene
107.4*
o-Xylene Isopropylbenzene (cumene) p-Xylene m-Xylene Toluene, technical grade
107. 4*
Toluene, chemically pure
113.1* 116.4* 117.5*
117.8*
120.1*
a Blending value; number is estimated by empirical relationships which compare octane value with amount of tetraethyllead that must be added to isooctane to prevent knocking, b Assigned standard octane number. Sources: Many and varied, including "API Technical Data Book," "Doss's Hydrocarbons," and ASTM-STP-225, "Knocking Characteristics of Hydrocarbons"
54 C&EN NOV. 9, 1970
Although many of the score cards are still out on possible use of leaded gasolines, a general opinion holds that lead alkyls will be phased out during the next 10 years, probably sooner. Emotion appears to carry more strength than does technology on this question. Systems that can handle polluting emissions from gasoline engines (leaded gas) will have to show beyond a doubt that they work. Without lead alkyls, which provide up to 10 RON's in motor gasolines, petroleum refiners must obtain more high-octane components for gasoline if they are to keep satisfying the horsepower-compression ratio-octane demands. To obtain these components refiners would have to make more of them—using processes similar to, but much more sophisticated than, ones they have used in the past. Over the years, complex petroleum refining processes, many of them chemical, to make hydrocarbons with high RON's were developed. Many of the processes—cracking, reforming, alkylation, isomerization—appear to be simple, but actually are complex because they handle very complex mixtures of hydrocarbons. As process units are more completely integrated with each other complexity increases. Gasoline, a variable Refiners make subtle and not-sosubtle changes in gasolines, depending on the weather—temperature and humidity—where they expect the gasoline to be sold, on the altitude, and on marketing competition. In addition, needs for other refinery products can influence makeup of gasolines. In winter, refiners add more light hydrocarbons—butane—to gasoline to increase its vapor pressure. A higher vapor pressure makes engine starting easier at lower ambient temperatures. (During cold starting, less of the gasoline actually burns than during normal operation. The unbumed portion passing out in the exhaust provides a severe pollution problem. It can also dilute engine oil, wash oil off cylinder walls, and lead to mechanical damage to the engine.) In both winter and summer, refiners must balance starting ease with resistance to vapor lock (boiling of gasoline in fuel lines around the hot engine that then reduces the supply of fuel to the engine) and with possible losses by evaporation. Such compromises lead to constant shifting of hydrocarbon mixtures to produce the best gasoline performance. Possible limits on volatility to reduce pollution could mean less butane used in gasoline. As a substitute, an alkylate of ethylene and isobutane, which has high RON and acceptable
volatility, may be used more, for example. If this happens, a new large use for ethylene could develop even though current costs of alkylating ethylene are high relative to other alkylates. Besides butane, which now seldom exceeds 10 vol % of gasoline, other paraffins of all kinds now account for 60 to 65 vol % of gasoline. Aromatics can be as high as 40 vol %, although 25 vol % is more common. Olefins aren't used in some gasoline; in others the volume is small. Excluding additives, other compounds in gasoline that contain oxygen, nitrogen, or other atoms are infinitesimal. Refiners add to gasoline chemicals such as tetraethyl- and tetramethyllead and other less well-known antiknock materials, detergents to help keep carburetors clean, inhibitors to minimize formation of gums or polymers, and others to prevent ice forming in carburetor throats on cool, damp days or to modify deposits in engine combustion chambers and to prevent deposits from building up on spark plugs. Each of these materials costs refiners a small amount, thus lower-grade gasolines generally have only some of the additives. More costly ingredients in gasoline will surely come if lead alkyls are limited or eliminated. Even in the large volumes of gasoline made—nearly 6 million barrels daily (42-gallon barrels; at 6 lb per gallon that's about 0.5 trillion lb a year)—economics of scale are insufficient to allow refiners to absorb costs of making higher-octane, higher-value materials. Calculating the costs to make various kinds of gasoline, with special emphasis on the octane value, has become almost a science in itself. Much engineering time is being spent now on such calculations comparing the different possible processing arrangements that refineries might use to make satisfactory gasolines without lead alkyls. The competition of chemicals for gasoline ingredients influences these calculations, especially if chemical volumes are of significant size compared to gasoline volumes. This competition involves the alternate value concept—that is, the cost of chemicals must at least equal what the cost will be to replace chemicals taken from gasoline components with other materials (or more of the same) to make the required volume of gasoline. These cost calculations are complicated by lack of a firm timetable for changes in gasoline. If legislation passes limiting use of lead alkyls and eliminating them on certain fixed dates, refiners then could lock up plans for gasoline composition and specifications to match these dates. If insufficient time is allowed for needed addi-
tions to refineries, though, a serious construction logjam could result. A further complication to calculating costs is the fact that no two refineries are the same nor are products from the same processing units always the same. Crude oils differ in composition. Marketing strategies vary with refinery location, ratio of premium to regular gasoline sales, and with product needs such as those for lubricating oil base stocks. Operating philosophies and maintenance for refineries vary, thus leading to varying optimum product mixes. All these things also affect the changes that could be made in processing. Clearly, the problems and variables are many and clear-cut solutions are elusive. Octane barrels In attempts to simplify the calculations concerned with refining economics, engineers developed the concept of octane-number-barrel ( ONB ). Costs are interrelated by cents per ONB. Sometimes octane gallons are used, but the term octane barrels has become almost universal since it fits with the unit—a barrel—of other refinery streams. In cents per ONB, the denominator reflects the measure of improvement. A refinery or a process can lose ONB's of gasoline yet show improved quality. In an oversimplified example, 100 barrels of 90-RON gasoline has 9000 research-octane-number-barrels (RONB). If 20 barrels of straight-run gasoline with a RON of 70 are removed from this gasoline, 1400 RONB's have been lost on a material basis. However, the RON of the remaining 80 barrels of gasoline may be 95, an improvement of 5 RON over the original blend. The net improvement is 80 (number of barrels left) X 5 RON (improvement in RON of remaining gasoline) which equals 400 RONB. If the cost of separating the 20 barrels is known, dividing that cost by 400 gives the cost in cents per RONB. Even with the ONB concept many qualifications are needed when comparing costs of one octane-improving method with others. Most important of these qualifications are the octane numbers of the gasoline blend or component before and after a change. For example, the cost of raising the U.S. gasoline pool—average octane number of all gasolines—from its current unleaded value of about 88.5 RON to its current leaded value of 96.5 RON is about 3 cents per RONB using lead alkyls. Costs of improving the gasoline pool by making gasoline components vary widely, varying from a little less than 10 cents per RONB to more than 25 cents per RONB. These costs, however, aren't directly com-
parable to costs of improvement using lead alkyls because the basis for the costs is different. Chemicals made in refinery units have an ONB value either as the single hydrocarbon such as benzene or as its value in making another hydrocarbon such as a propylene's value as an alkylation feedstock. As a first approximation of the cost of a chemical from a refinery, the ONB reflects the value it has to a refiner in making gasoline. In effect, the cost of replacing a particular chemical by other materials in gasoline can be directly evaluated. To replace hydrocarbons taken from refineries and sold for chemical uses, only three routes are open to refiners. They can buy components, increase output of the unit that makes the hydrocarbon being sold, or they can use another process to make components that have equivalent ONB's. The second choice also involves increased output. The bind begins when capacity is insufficient and an economic expansion cannot be justified. The choice of what to do requires that the whole refining operation, including plants, transportation, and the like, be looked at unless the quantity removed is small. The effect of removing any material reflects back to the crude oil distillation unit. Much more than a barrel of crude oil must be distilled in the crude unit to produce a barrel of product benzene. If gasolines with octane numbers greater than unleaded pool octane numbers must be sold and lead alkyls can't be used to increase the RON, refiners likely will have to remove some of the low-octane components of gasoline and replace them with higheroctane components. This will place an added burden on refinery capacity. The components that will have to be replaced are the normal and slightly branched paraffins from crude distillation known as straight-run gasoline and the paraffins unreacted in reforming that are separated by distillation or left after extraction of aromatics from reformate (the raffinate). These materials have been acceptable in gasolines, mainly regular grades, because they have high lead susceptibility, that is, their RON increases substantially with addition of lead alkyls. Some of these streams will respond with more than a 25% increase in RON—from about 72 to 92 R O N - b y the addition of 3 g of tetraethyllead per gallon. Part of these low-octane materials can be isomerized to higher-RON hydrocarbons and another part—heavy hydrocarbons above the gasoline boiling range—can be put into cracking units to be converted to olefins. The quantity potentially available now, however, appeal's so large that re-
Octane number requirements of automobile engines increase more slowly as compression ratio increases Engine compression ratio
4: 5: 6: 7: 8:
9: 10: 11: 12:
Typical RON required for knock-free operation
60 73 81 87 91 95 98 100 102
Octane ratings are measured by comparing pure hydrocarbons and hydrocarbon mixtures, with or without antiknock additives, with blends of standard high-purity normal heptane and isooctane (2,2,4-trimethyl pentane). Heptane has been assigned a standard octane number of 0.0 and isooctane 100.0. The comparisons are made using one-cylinder engines, known as CFR (for Cooperative Fuel Research) engines, which have a variable compression ratio. The tests follow a procedure specified by a committee of the American Society for Testing and Materials. Two methods are used and each gives different octane ratings—research method and motor method. The values found by the research method tend to correlate with city driving where low speeds and frequent accelerations are common. Motor octane number relates to potential highway performance. In this special report all values are given as research octane number.
finers may sell some at low prices; the result could be lower naphtha prices and more refiners making ethylene for merchant sale. Depending on the makeup of their steam cracker's downstream processing facilities, ethylene producers could find some bargain raw materials. Some refining people view availability of these hydrocarbons as speeding up the trend to manufacture of olefins from materials heavier than the liquefied petroleum gases—hydrocarbons through butane. Replacing the low-RON hydrocarbons must be done with a minimum loss of product volume, refiners say, for best economics. This situation requires the use of aromatics, the highest RON replacement material available. However, in all refineries a compromise would be worked out to permit the best possible economics in blending of gasoline components in relation to qualities of the components and capacities of units. NOV. 9, 1970 C&EN 55
Increasing the fraction of aromatics in gasoline would cost liquid volume because existing reformers would have to be operated under more severe processing conditions, which increases output of light hydrocarbons such as propane and butane. More severe processing (more catalyst contact per unit of time) also increases energy requirements. Other added costs are also incurred for increased cooling water and shorter catalyst life. The pollution aspects of an increased fraction of aromatics in gasoline fall into a grey area of limited scientific data, inadequate measurement tech nology, and emotion. Aromatics
outlook
The C 0 to C 0 aromatics are by far the most economically available hydro carbons with high RON. Some, such as toluene, have RON's up to 120; others may be even higher but above
Lead removal:
about 120 RON's are difficult to measure. Above 100 RON, oc tane values are usually called blending values and are considered estimates because of the variety of interpolation methods used. (A few very branched paraffins have RON's greater than 100; most are rare and expensive to make.) Catalytic reforming accounts for most of the benzene, toluene, and xylenes (BTX) produced in the U.S. About 80r/r of the benzene produced, for example, including that from dealkylation of toluene, currently comes from reforming. Chemical uses of BTX, including ethylbenzene in the xylenes fraction, however, require only about 13% of the BTX produced by catalytic re forming. Other sources of BTX used in chemicals are coal tar, produced from coking of coal, and pyrolysis gas olines, a coproduct of making ethylene and other olefins by steam cracking a predominately paraffin hydrocarbon
Few companies committed so far
T o lead or not to lead, that is the question—and it • is a profound question for the oil industry since the overall cost of removing lead from gasoline will run into the billions of dollars. It is a small wonder then that few companies are ready to commit them selves to either course before they know which way the winds of change in government and the auto industry will blow. At the heart of the problem is the fact that, in a closed space—such as the combustion chamber of a car's engine—gasoline doesn't bum completely. Some of the hydrocarbons are only partially oxi dized to carbon monoxide; others escape the chamber intact Moreover, at the high tempera tures found in the chamber, nitrogen and oxygen combine to form the lower nitrogen oxides. Escap ing into the atmosphere, these materials may com bine in the sunlight to produce photochemical smog. Early research indicated that the hydrocarbon pollutants could be removed easily—in the labora tory, at least—by passing the exhaust gases over a catalyst that would promote complete oxidation. However, the lead that is currently added to the gasoline (to increase the octane and prevent pre mature detonation in high-compression engines) deactivates the catalyst, so that the catalyst has to be replaced every few hundred gallons. Frequent replacement, even with base metal catalysts, is an expensive proposition, so the solution appeared obvious: Get the lead out. Proponents of this solution produce other good arguments: Particulate emissions from cars would be reduced, lead in the atmosphere is prob ably inherently harmful, and lead fouls spark plugs and promotes muffler deterioration, leading to in creased maintenance costs. The producers of tetraalkyl lead, however, have a multimillion-dollar investment to protect. Oil com panies calculate that upgrading their refineries to produce high-octane lead-free gas would cost bil lions. Furthermore, lead has been found to have a cushioning effect on the valves of a car's engine: With no lead in the gas, miniature spot welds, in
Company American Arco Chevron Gulf Humble Murphy Phillips Shell Sun Texaco Union
Lead Oc content» tane
Price per galionb
None 100+ None 91 None 91 94 0.5 None 91 91 0.5 96-97 0.5 None 91 Under 0.5 93.5 None 91 0.5 91 None 91 93.5 0.5 93.5 0.5
Ιφ over premium Η over regular 3φ over regular lf£ over regular Ζφ over regular 1-2φ under regular 3^ over regular Same as regular Same as regular Ιφ over regular Same as regular Ζφ over regular Ιφ over regular Iff over regular
a Grams per gallon. b Manufacturer's suggested price.
effect, can form between the valve and its seat at times of high stress, destroying the seal and causing a loss of compression. Companies in the affected industries began lining up behind either viewpoint, and the stage was set for a battle of the first magnitude. The battle lines within the oil industry are not yet sharply drawn, however. At press time, three of the country's top 12 gasoline marketers are selling a low (91) octane, and one is selling both high- and low-octane lead-free gasoline. Five are tem porizing by selling low-octane, low-lead gas, two are selling both low-lead and lead-free gas, and one (Mobil) hasn't disclosed its plans for lead-free gas. Together, these companies account for more than 55 billion of the 86 billion gallons of gasoline sold in the U.S. last year. The low-octane gasolines can be used by 50 to 75%x>f older cars that operate on regular gasoline; the 100-f octane product sold by American can be used in nearly all cars. Also, more than 95% of 1971 cars have been designed to run on 91-
feedstock. The aromatics in coal tar and pyrolysis gasoline are not com pletely used in chemicals; some are burned and never isolated. Depend ing on a producer's economic situation relative to fuel needs, subsequent processing, marketing needs, and the like, the aromatics produced may never be separated. Chemical demand for BTX now is supplied about 84% from catalytic re forming, 4% from pyrolysis gasoline, and 12% from coal tar. Forecasts for the sources in 1980 indicate little change in the fraction of the total coming from reforming—about 86%. They do show a jump in the pyrolysis gasoline source to 9% and a decline in coal tar BTX source to 5%. In 1980, chemical use of BTX from reforming will probably reach some 230,000 bar rels per day, about 15% of the 1.5 million barrels produced per day by reforming. These forecasts are based on as-
octane fuel. All the manufacturers also sell at least two grades of a highly leaded (3 g/gallon) fuel. Meanwhile, most oil companies are busy in their labs trying to find alternative methods of pollution control or more economoical ways to increase octane without lead. The activities at Texaco are indicative of the dilemma many companies find themselves facing. Texaco was the first in the new wave to market a lead-free gasoline—Standard Oil (Ind.) has been marketing lead-free Amoco on the East Coast for many years. The gas is essentially the premium grade with the lead removed, and consequently has a price tag 1.5 cents per gallon above premium (due to marketing costs). Sales to the pollution con scious were brisk, according to Texaco, until other companies brought out lead-free gasolines priced much closer to regular. While selling the lead-free gas, Texaco was do ing a considerable amount of work at its auto re search laboratory in Beacon, N.Y., to find new ways to control pollution without removing lead. In recent months, the lab has unveiled a modified transistorized ignition system that enables a car to operate efficiently on much leaner fuel-air ratios (to give more complete combustion), and a filter for the muffler to remove lead particulates and other solids from the exhaust. All the devices will be given to appropriate companies for licensing and development ~h • Other companies are financing the same types of ventures. The Inter Industry Emission Control Program, a group of 11 auto and oil companies headed by Mobil, is doing similar research, and claims to have developed a catalyst to control hy drocarbon emissions that is compatible with as much as 0.5 g of lead per gallon. The Esso Research division of Standard Oil (NJ.) recently disclosed the discovery of a catalyst that will control emissions of oxides of nitrogen—although the catalyst system is not compatible with lead and still requires a considerable amount of work More new discoveries are certain to be an nounced as time passes» The stakes are high and the competition is fierce. Whether there will be a clearcut winner, however, only time will tell.
sumptions that reforming capacity in the U.S. will grow at an average of 5% a year during this decade, and that reforming severity will increase from about 90 RON in 1970 to about 96 RON in 1980. The RON severity indicates the RON of the unleaded product from the reforming unit. The growth forecasts are also based on an assumption that adequate supplies of naphtha will be available for both increased capacity and for greater throughputs to replace losses caused by more severe processing. Aromatics
processing
Although many reactions occur during catalytic reforming, the reactions most desired by refiners are dehydrogenation of cyclic paraffins (naphthenes) and dehydrocyclization of normal paraffins to aromatics. Low pressures thermodynamically favor this reaction. Unfortunately, lower pressures increase catalyst aging rates, which usually mean a higher investment for a reforming unit because of more costly regeneration needs. New catalysts, the platinum-rhenium and other bimetallic materials, are helping the trend to lower operating pressures—350 psig (down from 750 psig or higher). With the new catalysts and lower pressures, liquid volume yield, often called the C 5 + reformate, declines more slowly with increasing severity. The loss of liquid volume yield in reforming comes from hydrocracking reactions that occur simultaneously with dehydrocyclization. Hydrocracking makes lighter paraffins from the C 6 to C 9 paraffins, which predominate in the feedstock. These losses to light hydrocarbons go up geometrically as severity is increased. Some refiners believe losses become economically unacceptable at unleaded product RON's of greater than about 102. Not only does the severity of operation of a reformer affect its yield and composition, but so does the feedstock. The most desirable naphtha feedstock obviously is one that has the highest concentration of CG to C 9 naphthenes, or cyclic paraffins. The least desirable feedstocks are the long-chain normal paraffins, which tend to be hydrocracked to shorter paraffins of propane and butane size. Long-chain normal paraffins are also converted to cyclics and subsequently dehydrogenated. The competing reactions decrease yields from these starting materials. An example of the relationship of product yield and unleaded product RON from reforming in the U.S. shows the penalty accompanying high octane values : at 85 unleaded product RON, yield of C 5 +reformate could be 87
vol % of the feed volume; at 90 unleaded product RON, yield drops to 84 vol %; at 95, yield is 80 vol %; at 100, 74 vol %; at 105, 67 vol %. Not all of the yield is aromatics. A reformate of 95 unleaded RON, which accounts for 80% of the feed, contains about 60 vol % aromatics; less than half of the feedstock has become aromatics. Composition of the aromatics fraction, if the feedstock is similar to the light naphthas fed to U.S. reformers, could be 10 vol % benzene, 47 vol % toluene, 7 vol % ethylbenzene, 7 vol % p-xylene, 16 vol % m-xylene,
8 vol % o-xylene, and 5 vol % C 9 and heavier aromatics. Often the reformate is blended into gasoline without further processing other than fractionation, or stabilization, to remove light hydrocarbons. As need for higher RON components develops as lead antiknocks are limited or not allowed, more extraction of an aromatics concentrate, sometimes called aromatics extract, would be done. Good quality aromatics concentrates have unleaded RON's of 110 or greater. Adding 3 g of lead alkyls adds no more than 5 RON's to aroNOV. 9, 1970 C&EN
57
Chemical refining processes
C
hemical refining processes can be classified into three main categories: • Breakdown processes—cracking of all types. • Buildup processes—alkylation and polymerization. • Change processes—cyclization, dehydrogenation, isomerization, and reforming of all types. Often, in refining, more than one of these processes is used simultaneously. Such multiple activity is generally intended by refiners to take advantage of the more than one efficient reaction that can occur under the same conditions. (Physical proc-
Break-down processes Catalytic and thermal cracking processes convert heavy hydrocarbons into lighter paraffins and olefins. Catalytic cracking generally does not break aromatic rings, although side chains may be broken or removed from the rings. Thermal cracking is now used in the U.S. mostly on specialized fractions, such as very heavy residuums, to make petroleum coke and to reduce viscosity of the residuums. Hydrocracking differs from thermal and catalytic cracking in thatfew, if any, olefins a re produced. Catalytic cracking units contain moving beds of catalysts, now generally fluidized in units operating in the U.S. The catalysts are mostly zeolites. Depending on size, investment in a catalytic cracking unit runs between $250 and $800 a barrel of daily feed capacity. A small catalytic cracking unit—about 10,000 barrel-per-day capacity—costs about $7.5 million. Operating costs are between 30 and 60 cents a barrel of feed. Name-plate capacity of catalytic cracking units installed in U.S. refineries is nearly 6 million barrels of feed a day. Reactors, regenerators, blowers, and flow controllers are the major components of fluidized bed catalytic cracking units. In a typical unit, gas oil is mixed with regenerated catalyst as both move into the reactor vessel. Cracked products are taken off the top of the reactor. The spent catalyst falls into a stripping section where steam removes much of the absorbed hydrocarbons. Heated air moves the spent catalyst to the regenerator where coke and hydrocarbons burn off. Cyclone separators take out the catalyst particles from the stack gases. Regenerated catalyst falls over a weir to return to the reactor. Thermal cracking today has its main use in the manufacture of petroleum coke. In coking, heated residuum is fractionated, further heated, and pumped to large insulated vessels called drums. Coke forms during the holding time, and vaporized gas, gasoline, and gas oil fractions pass out the top of the drum. Multiple drums are operated in cycles of filling, steaming, and dumping. iVisbreaking, a less severe form of thermal cracking, is used to reduce the viscosity of heavy feedstocks—residuums frequently—to produce a fuel oil of acceptable viscosity and with little gas or gasoline fractions. Generally visbreaking involves heating the feed, allowing the molecules to crack during holding in a soaker (an insulated short-term storage vessel), and fractionating the product, often under vacuum. Hydrocracking came into wide use during the last half of the sixties as probably the most important commercial refining process development of the decade. The process basically is a combination of catalytic cracking and hydrogénation. A wide range of feeds can be hydrocracked because conditions used in the various processes vary so widely— 450 to 800 °F, and 100 to 2000 psig, and 100 to 2000 ft3 of hydrogen feed per barrel of feedstock.
esses of refining—distillation, solvent extraction, crystallization, and others, though important, are outside the scope of this section.) Equipment and know-how for these processes is available from a number of firms—Esso Research and Engineering (Powerforming), Houdry Process and Chemical division of Air Products (Houdriflow; H-G Hydrocracking; Houdriforming), M. W. Kellogg division of Pullman, UOP Process division of Universal Oil Products (Platforming), Chevron Research (Gas Oil Isomax; Rheniforming), Phillips Petroleum, and Stratford Engineering, to mention a few.
Light and heavy gas oils, often undesulfurized, account for more than three quarters of the feeds to hydrocrackers. Because of the hydrogénation, products have negligible sulfur, nitrogen, and olefin content. In contrast to catalytic cracking units, most hydrocracking units have fixed catalyst beds. Catalysts little affected by sulfur, nitrogen, and metals such as vanadium were the key to development of the hydrocracking processes. Hydrocracking units require an investment of $800 to $1600 a barrel of daily feed capacity, or $10 million for a 10,000 barrel-a-day unit. Operating costs are between 40 and 90 cents a barrel of feed. Current U.S. operating capacity for hydrocracking exceeds 500,000 barrels a day. Reactions occurring during hydrocracking are similar to those of catalytic cracking except that no olefins are produced. Unless the operating conditions are severe, aromatic rings are not hydrogenated, but the side chains are removed. Hydrogen sulfide and ammonia can be removed from the gas product stream in conventional gas scrubbing units. The range of reactions in hydrocracking varies more than in catalytic cracking because of the greater variety of process configurations, conditions, and feedstocks that can be designed into hydrocracking processes.
Buildup processes Two important processes that build molecules valuable for gasoline are alkylation and polymerization. Alkylation will make a product only slightly less desirable in gasoline than reformate, considering research octane number only. Polymerization has been largely phased out of refining because polymer gasolines have relatively low octane ratings or high sensitivity, and their volumetric yields are about 60% of alkylate yields. Alkylation processes use three catalysts—sulfuric acid, hydrofluoric acid, or aluminum chloride, promoted with hydrochloric acid. These account for nearly all of the alkylate produced in the U.S. Slip streams of the acid catalysts are taken off for regeneration. Alkylation capacity costs between $500 and $1500 per daily barrel of feed. A complete 10,000 barrelper-day alkylation unit runs about $6 million. Operating costs are between 30 and 75 cents a barrel of feed; if sulfuric acid is used, recovery of the acid can reduce costs significantly. U.S. alkylation capacity, rising fast, approaches 700,000 barrels of product per day. In a typical alkylation process, olefins, isobutane, and an acid catalyst are fed to a stirred reactor. Cooling is used to remove some of the heat of reaction. The alkylate and unreacted feedstocks are separated initially from the catalyst by settling. If H F is the catalyst, last traces of it are stripped from the hydrocarbon mixture. The mixture is
then fractionated to produce alkylate and feed olefins and isobutane to be recycled.
Change processes Reforming is by far the most important change process used in refining. The reforming process consists of several other processes that alter hydrocarbon molecules during refining—cyclization, dehydrogenation, isomerization, and even cracking to the extent of breaking or removing side chains from substituted ring compounds. Of these processes occurring during reforming, refiners use only isomerization as a special and separate process operated alone. Reforming in the U.S. is aided by catalysts containing small quantities per unit weight of platinum, although the total weight and value of platinum in some reformers is very large. Relatively recently, rhenium and a few other metals have been combined with platinum in the catalyst, usually to give a more specialized reforming process that yields reformate with a somewhat higher octane value, and with higher concentrations of aromatics than other reformates. Investment in catalytic reforming varies more than for most processes; it falls between $300 and $1300 per barrel of daily feed capacity, depending on auxiliaries. A 10,000 barrel-per-day reformer costs about $10 million, including pretreatment facilities and catalyst charge. Costs of operation of reformers are between 40 and 65 cents per barrel of feed. U.S. reforming capacity is about 40% of the total capacity to make gasoline, or more than 2.5 million barrels per day. Following any necessary pretreatment, the feed to a reformer is mixed with recycle hydrogen, heated, and sent to one or more reactors. After coming out of the reactor, the reformed hydrocarbon-hydrogen mixture is flashed to separate the hydrogen and distilled. • Often an extraction process is used to take aromatics from the reformate. The benzene, toluene, and xylenes are then separated from the aromatics stream and purified. Isomerization has declined in importance during the past decade but may be getting a new lease on life as demands for higher-octane gasoline components increase. Technically, the conversion of a normal paraffin into a naphthene is isomerization. However, refiners reserve the term isomerization for conversion of normal paraffins to iso- or multiple-branched paraffins. Refiners are now looking closely at adapting butane "isom" units to convert normal pentane and hexane to their isomers with a substantialalmost 50%—gain in the octane values. Isomerization catalysts are usually platinum (with proprietary promotors) or aluminum chloride, although some newer catalysts may have other active ingredients. Most isomerization units operate with treated feedstocks so catalysts are not subjected to many poisons.
matics concentrates, an indication of their relatively low lead susceptibility. Currently large volumes of aromat ics are extracted from reformate for further purification before sale or use as chemical raw materials, as solvents for many uses, and as a component for gasoline. Before separation the aro matics are worth about 15 cents a gallon based on their value in gasoline. Both refiners and chemical produc ers separate the aromatics into indi vidual components this way. Ben zene, toluene, and xylenes, including ethylbenzene, are separated and puri fied by distillation into individual streams. In a few refineries, some of the C 9 aromatics are separated from the remainder. Some of the benzene is sold by re finers to be converted by chemical pro ducers into products such as styrene. Another part of the benzene is con verted to cyclohexane or cumene (isopropylene benzene) by refiners and sold to chemical companies. Toluene is dealkylated to benzene if the price of benzene is 5 to 7 cents a gallon more than that of toluene. If the price spread is not there, then most toluene goes back to the gasoline pool, excluding toluene that is dealkylated to benzene. About 257c of U.S. tolu ene production is sold as solvent and as a raw material to make various chemicals such as trinitrotoluene, tol uene di-isocyanate, and phenol; the remaining 757c goes into gasoline or, if benzene price is high enough, it is hydrodealkylated to benzene. Again depending on the refiner's processing scheme, the CiS aromatics are handled in different ways. Ethylbenzene may be fractionated out to be made into styrene, although most sty rene is made from ethylbenzene pro duced by alkylating benzene with ethylene. Then p-xylene is removed by fractional crystallization, o-xylene distilled, and m -xylene isomerized to more para and ortho. Sometimes only p-xylene is separated and the remainder of the xylene cut is put into gasoline or sold for solvents. The raffinate left after the aromatics concentrate is taken from reformate will present a bigger problem to re finers than it now does if unleaded gasoline, in particular, or even lowleaded gasoline must be sold. This raffinate, high in normal paraffins, has low octane values. It can be a serious drawback to the gasoline pool. And, it is less valuable as a reformer feed stock than is virgin naphtha because virgin naphtha has a high fraction of naphthenes, which give much better yields of aromatics. Most refiners say they would prefer to run increased amounts of crude to get more virgin naphtha than to recycle raffinate. If the raffinate is unacceptable in the
Value of lead alkyls in improving octane numbers varies widely with the types of hydrocarbons in the mixture Research method octane number»
Component Major type of hydrocarbon present
Name
Source
Unleaded
3g Pb/gal
Straight-run naphtha
Crude oil distilla tion
C5 and C6 straight, branched, cyclic paraffins
55-72
79-88
Cat naphtha
Catalytic cracking
93
98
Hydrocrackate Alkylate Reformate
Hydrocracking
C4 to C9 paraffins and olefins C 5 and C6 paraffins, no unsaturates C4 to C9 paraffins C6 to C9 aromatics; cyclic paraffins C6 and Ce iso- and cyclic paraffins Ce to C9 aromatics
86
98
93 90-95
104 96-101
82
98
110
114
Isomerate Extract
Alkylation Catalytic reforming Catalytic isomerization Solvent extraction of reformate
a Octane number of leaded pump gasoline (a blend of some or all of above components) is about 94 for regular and 100 for premium.
Chemical processes in refining grow rapidly to meet needs for high-octane gasoline components Process capacities, U.S., including Puerto Rico Year»
Catalytic cracking
Catalytic reforming
Hydrocracking
ylation
Thousands of barrels per calendar day of output
ij -ν':;;
;?ii
1960 1961 1962 1963 1964, 1965 1966 1967 1968 1969
1586 1587 1646 1691 1779 1767 1783 1847 1992 2109
1514 1540 1606 1615 1668 1644 1688 1796 1890 2030ο
1970
2214
2124°
b
b
b
b
b
b
b
b
b
b
b
b
b
106 188 307 394 444
556 623 644 681
'
a'Asbf Jan. 1. b·Not broken out or not reported.
