In the first refineries of the United States, Pennsylvania crude was easily distilled to produce kerosene for lamps (which burned cleanly without producing much smoke because of the paraffinic composition of Penn crude) and lubricating oil for steam engines. Lighter fractions obtained from distillation such as naphtha, propane, and butane were largely considered a nuisance and were flared for disposal because of their high vapor pressure and low flash points that would cause difficulties with storage. Figure 11.1 shows a typical batch distillation process in early refineries of the 1850s used primarily to produce kerosene, labeled as “product” from the separator in the diagram. For the distillation process, crude oil feed was filled into a kettle heated by burning gas, or other fuels in a fire box and the residual tar was removed after the distillation was over. The Dephlegmator Tower worked as a distillation column and as the crude boiled in the still, the vapor fraction from the dephlegmator was condensed and sent to the separator [2]. The separated kerosene fraction often went through a second distillation process to control the flash point for the safe use of fuel in lamps and reduce odor. The residue fraction was also distilled using vacuum to produce lubricating oil and grease for the steam engines and wax for making candles.

Charging the kettle with crude oil and emptying the residue left over from distillation took a lot of time and effort, making the batch distillation highly inefficient. In most cases, the recovered overhead fraction was fed back to the same still to drive off more hydrogen sulfide and lighter fractions to control the flammability (flash point) of kerosene. Driving off hydrogen sulfide and other light sulfur species also reduced the odor of kerosene and of the products obtained from burning kerosene in gas lamps. As the demand increased for kerosene, the refiners began to use two stills, one for the first fractionation of a kerosene cut and the second one to redistill the kerosene for purification. Using two stills in series marked the beginning of the continuous stills [2].

Figure 11.1. An early batch distillation process to produce primarily kerosene as product [2].

 

Continuous fraction with multiple stills replaced batch operations in the refineries, enabling increased throughputs and the production of multiple distillate fractions as products from a refinery [2]. As shown in Figure 11.2, a series of stills could operate continuously by taking an overhead fraction from the crude oil in the first still by flashing and moving the remaining liquid to a still drum while continuously introducing more fresh feed to the first still. The second still operates at a higher temperature to produce a higher boiling distillate. The reflux to the column with bubble trays was adjusted from the color of the overhead stream, utilizing a “look box,” shown in the diagram in Figure 11.2, to improve separation.

The demand for kerosene as a source of light declined with the invention of the electric light bulb in 1879. However, the first powered airplane flight in 1903 and mass production of an automobile (Model T) in 1908 ushered in a large demand for gasoline that cannot be met by simple distillation. Thermal cracking provided means to increase gasoline supply. This was the beginning of a new era in petroleum refining, incorporating a conversion process with separation processes.

Figure 11.2. Continuous fractionation process using two stills in series.

Incorporating thermal cracking of gas oil into the refinery increased the yield of light and middle distillates, i.e., gasoline, kerosene, and diesel fuel, from crude oil. Although the electric light made the kerosene lamps obsolete, there was still continued demand for kerosene in rural regions because of slow electrification outside the urban areas. The evolution of the refinery in the three decades between 1910 and 1940 was driven largely by the development of thermal cracking processes, although finishing (or chemical treating) processes also started to become important in this era to stabilize and purify the products of thermal cracking.

Figure 11.3 shows a simple schematic diagram of the Burton-Clark batch thermal cracking process. The process employed tubular heating similar to those used in a steam boiler. The series of tubes in the firebox circulate the hot gas oil back to the drum by thermal convection for more uniform heating. The hot gases from the coal-fired furnace are directed up over the high end of the tubes and down over the low end of the slanted bundle. Feed is introduced in the low end of the tubes and tar is withdrawn from the bottom set of tubes. The products of thermal cracking are fractionated in the Bubble Tray Tower and in the high-pressure and low-pressure separators. In the high-pressure separator are the gaseous products hydrogen, methane, and ethane, and in the low-pressure separator are the gases ethane and propane and the liquid products gasoline and kerosene.

Figure 11.3. Burton-Clark Batch Thermal Cracking Process to increase distillate yields [3].

 

Figure 11.4 shows the configuration of different processes in the thermal refinery. As different from the thermal processes in a current refinery, the thermal refinery includes processes such as Thermal Polymerization, making gasoline from the light olefins propene and butene; Thermal Reforming, to make relatively high octane number gasoline from straight-run naphtha; and gasification of heavy gas oil with steam, to produce town gas (CO+H2) which predates the use of natural gas in cities for domestic heating and cooking.

Figure 11.4. The configuration of the thermal refinery [3].

Click here for a text alternative to the figure above

Atmospheric Column (Pipe Still of Series of Shell Stills)

-Light ends & Other gas sources

Gas Plant

Methane and ethane (fuel gas)

Propane and Butane

Thermal Polymerization

Gas

Tar Residue

Gasoline (joins gasoline treating process)

-Naphtha

Thermal Reforming (light cracking)

Gas

Tar

Gasoline

Gasoline Treating

Gasoline

-Kerosene (Distillate)

Kerosene Treating

Kerosene

-Gas Oil

Thermal Cracking (heavy cracking)

Gas

Tar

Kerosene (Joins kerosene treating process)

Gasoline (joins gasoline treating process)

Gasification using the shift (uses Steam)

“town gas” (hydrogen and carbon monoxide)

-Lube Oil

Lube treating

Lube Oils

-Asphalt Resid (Tar)

Treating asphalt resid (uses steam and air)

Asphalt

-Other Sources of Tar

Fuel Oil

 

The essential driver of the Thermal Refinery was the shift in demand to gasoline from kerosene because of the introduction of the automobile, the airplane, and electricity. The demand for gasoline rapidly increased when the U.S. declared war on Germany in 1917 and became a party in World War I. Thermal refinery processes, thermal cracking, thermal reforming, and thermal polymerization enabled the expansion of gasoline supply [3]. With the introduction of tetraethyl lead (TEL) as an octane number boosting additive in 1923, a growing interest was directed to production of high-performance gasoline which would be defined later as a high-octane number-gasoline after the introduction of a test method to measure the octane number of gasoline as an anti-knock property in 1931. Because of the toxicity of lead, TEL concentration was limited to 3 milliliters per gallon of finished gasoline (approximately 800 ppm by volume). The addition of lead to motor gasoline continued until the 1970s in the United States when the mandate for adding catalytic converters to automobiles took effect in accordance with the Clean Air Act to reduce tailpipe emissions, and the unleaded gasoline was introduced. Lead is still added to aviation gasoline used in turboprop aircraft in quantities 0.3-0.56 g/L in a range of avgas grades, and efforts are underway to remove lead from the aviation gasoline as well in the near future.

Up through 1924, even with the rapid introduction of various thermal cracking processes, only 20% of the gasoline produced in the U.S. came from thermal processes. But after the introduction of TEL, the contribution of gasoline produced by thermal cracking has steadily increased to reach over 50% of the gasoline pool by the end of the age of the Thermal Refinery in 1940. For reasons discussed in Lessons 6 and 7, the Catalytic Refinery arrived in the scene of brutal competition of making high-performance gasoline and other petroleum fuels in the period leading to and during World War II.

As discussed in Lessons 6 and 7, the development of catalytic processes has changed the chemistry of petroleum refining from free radical to ionic reactions. World War II provided the stimulus to urgently develop catalytic technologies that were being investigated in the late thirties. The catalytic age of refining, which could be bracketed between1940 and 1970 also brought the advent of the petrochemical industry.

Figure 11.5 shows a configuration of the catalytic refinery which resembles, to a large extent, the current day refineries focused on making high yields of gasoline. The introduction of catalytic cracking, reforming, alkylation, polymerization has revolutionized the ways of making high octane number gasoline. Development of hydrotreatment processes was also an important asset of the catalytic refinery. Hydrotreatment was essential to protect the platinum catalyst used in reforming from sulfur and as a versatile finishing process to replace the chemical treatments used in the thermal refinery to finish fuels.

Figure 11.5. The configuration of the catalytic refinery [4].

 

One should note in Figure 11.5 that the catalytic refinery also incorporated new thermal processes such as delayed coking and visbreaking, and separation processes, such as deasphalting. Principles of chemical engineering have found great applications in the development of the catalytic refinery with particular emphasis on designing different catalytic process configurations (remember Fixed-Bed, Moving-Bed, and Fluid-Bed Catalytic Cracking), catalyst development, thermal efficiency (e.g., FCC) and product yield and selectivity. The catalytic refinery produced large quantities of LPG (for reasons discussed in Lesson 7) and witnessed the increasing demand for kerosene, now as jet fuel. The time-line for the development of refining processes shown in Table 11.1 shows the intense activity of process development, particularly during World War II.

The age of catalytic refining may be considered to have ended in the 1970s, not because new chemistry was introduced, as it happened in the transition from thermal to catalytic refinery or the development of new process concepts. The oil crises of the 1970s highlighted the significance of refinery flexibility with respect to the diversity of crude oil slates. Further, the concerns for environmental pollution by the combustion of petroleum fuels have brought emphasis on more effective finishing processes. These factors lead to the development of the modern refinery focused on processing the heavy ends of petroleum and making cleaner fuels.

Oil crises of 1973 and 1979, which created crude oil price shocks, contributed to an increasing emphasis on energy efficiency and independence. These events along with the stricter environmental regulations set up the evolution of the End of the Century Refinery (or Heavy Ends Conversion Refinery) which has focused on efficient processing of heavy crudes as well as the heavy ends of crude oils to produce higher yields of distillate fuels. Producing cleaner fuels and cleaner operation of refining processes have been mandated by environmental regulations.

Figure 11.6 shows the configuration of the Heavy Ends Conversion refinery, with emphasis on processes marked with the red rectangle in Figure 11.6. These processes include hydrotreating of heavy gas oils before catalytic cracking to remove sulfur that contaminates downstream catalysts and to saturate aromatic C-C bonds to produce higher yields of fuel products from catalytic cracking. With better protection from sulfur contamination, FCC units can use modified catalysts to increase gasoline yield and reduce coke yield.

HECR offers four primary heavy oil or residue processing technologies as options for processing heavy crudes or “bottom-of-the barrel” processing. These processes have become more important as the world crude slate becomes heavier and more contaminated with sulfur and other heteroatom species, as sulfur limits become more stringent with environmental regulations.

Hydrocracking of heavy oils removes downstream catalyst contaminants and saturate aromatic compounds to produce higher yields of fuel products. Hydrocracking offers flexibility to choose between gasoline, jet fuel, and diesel fuel by coordinating the operations of fluid catalytic cracking and hydrocracking.

Other options for residue processing include coking, visbreaking, and deasphalting. Coking followed by catalytic upgrading of coking products (naphtha and gas oils) by hydrogenation, or hydrocracking generates high-quality distillates from residua that are not suitable for catalytic processes due to large concentrations of asphaltenes and heteroatom compounds (sulfur, nitrogen, oxygen, metals). Visbreaking and deasphalting removes the highly reactive compounds and asphaltenes from residua making the visbroken and deasphalted oils attractive feedstocks for catalytic hydroprocessing to produce distillate fuels.

The End of the 20th Century Refinery (the present-day refinery) is much more complex and versatile than its predecessor, The Catalytic Refinery. In addition, there are differences that are not so visible in conventional descriptions and flow sheets, such as computerized control of the operations and on-line measurements of product composition and properties.

Figure 11.6. The heavy ends conversion refinery (or the End of the 20th Century Refinery) [5]

 

Figure 11.7 lists four factors that may influence the future of petroleum refining, including product demand, crude supply, environmental regulations, and new technology development. It is expected that in the near future, the demand for distillate fuels will keep increasing, while the conventional crude oil slate will become heavier and more contaminated. This conflict between the trends in supply and demand that is aggravated by stricter environmental regulations on the purity of fuels can be mitigated by new and more effective technologies (processes and catalysts). Although the conventional crude oils are becoming heavier (Figure 11.8), non-conventional liquids such as synthetic crude oil from oil sands in Canada and shale gas liquid by-products are lighter than the conventional crude oils and could be used as blend components to dilute the heavy crudes. Natural gas liquids and coal-derived liquids may also be used as alternative feedstocks for refining. The diversity in crude oil supply calls out the need to plan/operate a more flexible and versatile refinery.

Figure 11.7. Some predictions on future trends in petroleum refining.

Click here for a text alternative to the figure above

Future Trends in Petroleum Refining

Forces and Factors

-Demand for products

Product slate: gasoline/diesel

Overall energy demand structure

-Feedstock supply

Geopolitics & degrading quality of conv. Crude oil

Alternative feeds: shale gas liquids, oil sand, natural gas, coal

-Environmental Regulations

More strict regulations on: petroleum fuels & refinery operations

-Technology Development

New refining technologies: processes & catalysts

Other technology: fuel cell

Need for more flexible and versatile Refinery!

 

Figure 11.8. Gravity and sulfur contents of the U.S. crudes [1].

 

Figure 11.9 recaps some considerations for the future refinery, as discussed above, and lists the future challenges that include the need to process heavier and more contaminated crude oils to produce cleaner products than before. Production of diesel from highly aromatic by-product from FCC (LCO) remains a concern for the cost and quality of diesel fuel produced in the U.S. refineries. Also, extensive hydrotreatment to comply with the limits on heteroatom levels in fuels would negatively affect the octane numbers of gasoline because during hydrotreatment, olefins and aromatic compounds may be hydrogenated, thus reducing the octane number. No such conflict exists for diesel fuel, because cetane number of diesel actually increases with hydrogenation. It is clear that the hydrogen demand for the future refinery will keep increasing, and refineries will build or expand the existing hydrogen production capacity.

Figure 11.9. Considerations for future refineries.

Click here for a text description of the figure above

Future Refinery

-Increasingly heavier and more sour crude supply

-Demand for cleaner and higher-performance transportation fuels

-Challenges for future refineries

Heavy residue conversion

Reduction of heteroatoms and aromatics in fuels

Diesel problem in the US

Attainment of the required performance characteristics

Octane number (Conflict!)

Centane number

-Increasingly higher demand for hydrogen

 

 

Exercise 10 Instructions

1. Draw a block flow diagram for a petroleum refinery that includes three different separation processes, five different conversion processes along with one or more finishing processes, and one supporting process to produce altogether nine commercial fuels and materials from a sour crude oil. Clearly label all the feedstocks, processes, and products on your diagram including the type of process (separation, conversion, finishing, and support) and the intermediate products. Make sure that all processes are correctly connected. Write down the principal objectives of this refinery. 60 pts
2. Draw a block flow diagram for a petroleum refinery to maximize the diesel fuel yield from a paraffinic crude oil. In your flow diagram, include only the processes that are absolutely necessary for maximum diesel yield and make sure that the whole crude is converted completely to commercial (sellable) products in this refinery. 40 pts

Instructions for Submitting Response:

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Quiz 4. Will cover material in Lessons 10-11. Check the Syllabus, or Course Calendar for Quiz 4 schedule.