Monthly Archives: June 2014

The History of Thermal Cracking

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Petroleum conversion processFig. 1: “Petroleum conversion process” by Fred Koch (Source)

Now that we have exhausted the virtually endless range of separation processes, we concentrate on the set of thermal processes that engrained the oil and gas industry in transportation. Unlike the separation byproduct, asphalt, which is one of many pavement options, the products of thermal conversion processes are inimitable in today’s American market (unless, of course, you are an electric motor fanatic like me!). Before delving into the invaluable products, we review a brief history lesson on thermal processes in refineries.

Chemical engineer, Fred Koch, might not be the father of thermal processes, but he certainly is credited with “spreading the wealth”. Fig. 1 (above) references his patent that enabled smaller entities to profit in the spoils of big oil and gasoline production. Ironically, despite building his chemicals empire in the Soviet Union, the esteemed MIT graduate took a strong political stance against communism. But, I digress!

Fred KochFig. 2: Fred Koch (Source)

The use of automobiles increased the demand for gasoline beyond the rate of straight-run gasoline distillation. Despite a sharp increase in drilling activity, the refining industry required more light gas oil through the thermal processing of the heavy gas oil and vacuum distillate residue (VDR). Conveniently, the first applications fueled WWI and subsequent wars. Thermal cracking chiefly produces in light gas oil, gasoline, residual fuel oil, and petroleum coke. While the demand for these products has only increased since its inception, catalytic cracking has usurped the attention of industry and academia alike.

Recent concerns of aging refineries call for enhanced control systems. Perhaps, the most hazardous process takes place in the coking drums, where three phase system could lead to unexpected pressure changes. Flowserve provides an example of this complex system. An intriguing mechatronic advancement involves the use of automated decoking systems. As long as thermal cracking provides ethylene production for the petrochemical industry, engineers will continue to develop best practices for safe operation of these units.

The Uses for Thermal Cracking in Past and Present Refineries

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Thermal cracking is a useful step in the petroleum refining which allows for the “seemingly useless” vacuum distillation residue (VDR) to be converted into distillate fuels and coke. In today’s petroleum refinery, thermal cracking is primarily used for the production of coke. The two processes used to create this coke are delayed coking and fluid coking. These processes are operated at relatively low pressures (just slightly above atmospheric) and at a temperatures just above 900 degrees Fahrenheit. Depending on the duration of these processes, petroleum coke can be made into fuel-grade coke or, after further processing, anode-grade coke which can be used in batteries. In 2012, coking exports accounted for 19% of our nation’s petroleum exports. [1]

However, thermal cracking of petroleum fuels wasn’t always primarily used for the production of coke. In 1913, thermal cracking was used for means of distillate fuel production. [2] Since the “gasoline boom” was occurring during this time, refineries had to find ways to compensate for the increasing demand of fuel. This thermal cracking process also utilized low pressures and high temperature to break apart heavy fuel, otherwise known as visbreaking, in order to make the smaller chained gasoline molecules. The problem facing thermal cracking in gasoline production is the resulting low octane number. In 1930, thermal cracking was replaced by catalytic cracking because of its higher gasoline yield and higher resulting octane numbers. [2]

Sources:
1. U.S. Energy Information Administration – EIA – Independent Statistics and Analysis. (n.d.). Coking is a refinery process that produces 19% of finished petroleum product exports. Retrieved June 28, 2014, from http://www.eia.gov/todayinenergy/detail.cfm?id=9731
2. Petroleum Refining Process. (n.d.). Petroleum Refining Process. Retrieved June 28, 2014, from http://www.ilo.org/oshenc/part-xii/oil-and-natural-gas/item/384-petroleum-refining-process

Thermal cracking Pathway to Other Processes

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Thermal cracking is a process by which heavy residues under severe thermal conditions are cracks.1 This process allows for the formation of heavy fractions such as coke, once properly treated and finished.1 The first refinery opened in 1861, however the first thermal cracking process was not developed until years later in 1913.2 The first thermal cracker took heavy fuels and subjected them to both pressure and high heat, physically breaking the molecules into smaller ones, producing additional gasoline and distillate products.2 With this additional process added, the yield of products per gallon of gasoline was increased, making petroleum refining more profitable. In the 1930s this process was even further improved to produce more desirable, valuable products.2 Until the 1936 thermal cracking remained the method of choice.3 As technology developed, thermal cracking started to phase out because Catalytic cracking became more popular, as the costs for the process were being reduced. Catalytic cracking utilizes carbocation chemistry, utilizing a carbonium ion intermediate.3 Thermal cracking which was advance at its time, then became less utilized since the process produced random cuts in the hydrocarbon chains, yielding random length carbon chains.3 Catalytic cracking was the solution for this problem, whose produced in an organized manner, cutting chains near the middle.3 Thermal cracking remains an important process in petroleum refining today, however improved combined processes of thermal cracking have been developed for various purposes to improve yield and quality of products.3

  1. Mohamed A. Fahim, Taher A. Alsahhaf and Amal Elkilani, Fundamentals of Petroleum Refining
  2. http://www.ilo.org/oshenc/part-xii/oil-and-natural-gas/item/384-petroleum-refining-process
  3. Jennifer Clemons , Brian Senger, Nicholas Filippelli, Fluidized Catalytic Cracking

The History and Use of Thermal Cracking

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The first thermal cracking processes were developed in 1913 with the purpose of heating atmospheric tower residues and heavy gas oils until the molecules cracked and broke apart.1 The reason for conducting this process was to break up the less desirable petroleum products and form them into highly valuable light middle distillates such as naphtha, gasoline, and diesel fuel among others2. Thermal cracking became incredibly important with the invention of the automobile which uses an internal combustion engine fired by diesel or gasoline. With more people driving, the demand for fuel skyrocketed and the cracking process made it possible to produce more gasoline and diesel than was produced from crude oil as a straight run product3. For twenty to thirty years it was the pinnacle of petroleum refining processes, however by the start of World War II thermal cracking could no longer generate the quantity or quality that was demanded. At this time automobiles and planes required higher octane fuels that simply are not capable of being produced from the simple brute force cracking process1. Instead, a new process called catalytic cracking was introduced which was capable of yielding larger amounts of higher quality fuel. While limited in use, thermal cracking is still used today with its primary roles being the production of diesel fuel and ethylene2.

  1. “Petroleum Refining Process.” Petroleum Refining Process. N.p., n.d. Web. 26 June 2014. <http://www.ilo.org/oshenc/part-xii/oil-and-natural-gas/item/384-petroleum-refining-process>.
  2. Semih, Eser. “Lesson 6: Thermal Conversion Processes.” FSC 432: Petroleum Processing. N.p., n.d. Web. 28 June 2014. <https://www.e-education.psu.edu/fsc432/content/lesson-6-thermal-conversion-processes>.
  3. Solomon, Lee. “Visbreaking, thermal cracking, and coking.” Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 28 June 2014. <http://www.britannica.com/EBchecked/topic/454440/petroleum-refining/81801/Visbreaking-thermal-cracking-and-coking>.

Thermal Cracking, Russell Hedrick

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Thermal cracking was the first commercially used conversion process to process crude oils. It was initially implemented to produce more gasoline as well as higher octane gasoline. The higher octane gasoline was mainly used for fuels for aircrafts. The main reason thermal cracking came along was to produce light to middle distillates from heavier fractions of the crude oil. Thermal cracking became obsolete for gasoline production in modern refineries when catalytic cracking was initiated. Catalytic cracking came around in the 1930’s and could produce more gasoline at a higher octane number, then thermal cracking. Thermal cracking is still used in some modern refineries with visbreaking and coking. Visbreaking is a thermal cracking process that uses VDR as a feedstock to produce fuel oil and light products to increase distillation output of a refinery. The main goal of visbreaking is to reduce viscosity in a feedstock, but could also be used to produce lighter distillates from fuel oil. The second thermal cracking process that is used in refineries is called coking. Coking is the most severe thermal cracking process used in a refinery. It cracks the heaviest of the crude oil fractions, such as vacuum residue. Three coking processes are used to maximize the yield of distillation products. Thermal cracking is no longer the main process of a refinery but it is still plays a small role in increasing the yields of distillates.

Solvent Fractionation and Non-polar Solvents Power

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Blog 4

Write a post explaining how solvent fractionation works and review the parameters to describe the solvent power for non-polar solvents.

 

Through the process of deasphalting, a solvent is used to fractionate various feedstocks. Deasphalting performs its fractionations based upon the components of solubility and insolubility of feedstocks where distillation uses the boiling point temperatures to make fractionations. Vacuum distillation residue, known as VDR, is completely dissolved in aromatic solvents such as toluene and benzene.  VDR is typically in the form of a solid at room temperature so then the aromatic solvents are used to create a liquid mixture where a light paraffin solvent is mixed with the feedstock mixture to precipitate the VDR asphaltene. Depending upon solubility the asphaltene is then separated from the mixture.The VDR component that is soluble in this light paraffin is referred to as a maltene and is considered a one phase material solution. Through the gradient solubility model it is explained that asphaltene molecules can dissolve and give a single phase solution. Through solvent extraction and VDR the asphaltene can be removed from the solution.

The VDR compounds solubility, which effects the extraction, depends on the strength of the solvent which is measured for non-polar solvents by the Hildebrand Solubility Parameters, also known as HSP. There are two different Hildebrand solubility parameters that affect this solubility. The first parameter measures the relationship between surface tension and the cube root of molar volume. These happen to have an inverse relationship where surface tension increases with decreasing molar volume. Solubility also increases with surface tension. The second parameter measures solubility based upon the relationship between the heat energy required for vaporization and the molar volume. This is typically calculated under constant volume. Solubility increases in this case with an increase in the amount of energy for vaporization.

The Two Dewaxing Processes

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Blog 5

Write a post comparing the solvent dewaxing and catalytic dewaxing processes.

 

Dewaxing is a process used to remove waxes from oil refinery feed stocks. Once the wax is removed it can be sold as a bi-product for things such as candles and other forms of waxes. The feedstock after being dewaxed can be used as various lubricating oils and other distillate fuels such as gasoline. Dewaxing is performed on a feedstock through two different processes. One is a physical process that uses a solvent and is known as solvent dewaxing. The other is a chemical process and uses catalytic cracking and is known as catalytic dewaxing.

In solvent dewaxing a solvent is added to the feedstock and the mixture is then chilled to a desirable temperature and then proceeds through a rotary filter which separates the solid wax from the feedstock. This is because of the components varying freezing temperatures which allows for seperation to occur. Solvent dewaxing primarily uses two types of solvents being propane and methyl ethyl ketone, also known as MEK. MEK is more commonly used as a solvent due to its minor variance in filtration and pour point temperatures along with its chilling rate characteristics.

Catalytic dewaxing involves the breaking and creation of bonds. It is known as a conversion process of n-paraffins.  This form of dewaxing is able to break apart and actually remove long chain n-paraffins. Catalytic dewaxing uses sieve catalysts to filter with a pore opening size very small so that i-paraffins can be captured and filtered out and only n-paraffins shall be able to pass through. This can also help to lower the feedstock’s pour point value.  Catalytic dewaxing will produce a lube base stock with a lower pour point and a higher yield than that of a feedstock that underwent solvent dewaxing. Catalytic dewaxing is a less expensive form of dewaxing, but both processes have their pros and cons.

Solvent and Catalytic Dewaxing

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Dewaxing is the required to remove the hydrocarbons that solidify as temperatures decrease. Removing these hydrocarbons lowers the pour point which is a desired characteristic of a fuel because it can continue to function as lower temperatures. Dewaxing can be done in one of two methods. Solvent dewaxing is a physical process of freezing and removing the waxes. Catalytic dewaxing is a chemical process which removes wax by reaction of long chain n-alkanes or wax.

Solvent dewaxing is done by refrigeration of the feedstock after it is mixed with a solvent. The temperature of the refrigeration process depends on the desired pour point of the product. If the desired pour point is very low then the refrigeration will be very low. The wax crystals are separated by a cloth filter. These wax crystals are called slack wax and can be used for making candles, cosmetics and petroleum jelly. The solvents used in this process are methyl ethyl ketone (MEK) and propane.

Catalytic dewaxing utilizes a process to crack the n-paraffins (wax). This method of selective cracking takes place in the zeiolite catalyst. The small pore size of this catalyst wont allow i-paraffins to react. Increasing the concentration of i-paraffins in the fuel lowers the pour point because there are less n-paraffins to freeze at higher temperatures. Advantages of this method over solvent dewaxing include product stability, lower capital investment, and flexibility to produce both lube oil stock and light distillates.

Even More at the Bottom of the Barrel: Dewaxing Processes

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Just when you thought separation processes could not be more drawn out, dewaxing appears! Undoubtedly another slightly misleading process title, dewaxing is chiefly concerning the lubricating oil base stock product more so than the wax byproduct. The lube oil base stock market price depends on its volatility, viscosity, viscosity index, and thermal stability. Depending on the process employed, the resultant lube oil base stock may not require as many additives to enhance engine performance.

Solvent dewaxing requires a tremendous amount of energy and solvent to remove wax from the lube oil base stock. We discussed the use of solvents such as methyl ethyl ketone and propane along with the need for refrigeration units and steam-stripping to remove wax. Bechtel Corp. displays a general layout of its own solvent dewaxing process. Notably, Bechtel uses inert gas instead of energy-intensive steam for stripping. Regardless, the operating costs illustrate how significant the dewaxing footprint is within the refinery. No wonder those oil leaks cost me a fortune!

You may question the utility of wax on the open market, like I do. Never fear, catalytic dewaxing is here!

Shell Global Solutions’ website illustrates the company’s catalytic dewaxing technology for the work to study (and purchase). This separation process actually involves catalytic cracking of n-paraffins using a selective catalyst. To mitigate fouling of the catalyst, hydrogen is also applied. This process enables the operator to produce even more distillates and lube oil base stock. This is tremendously lucrative since it requires a lower capital investment; the selection of this process relies on the robustness and effectiveness of the catalyst. Perhaps, I should invest in the catalyst industry…

Shell Global Solution catalystFig. 1: Shell Global Solutions’ proprietary catalyst (Source)

Bottom of the Barrel: Solvent Fractionation and Solvent Strength

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In the oil and gas industry, the bottom line is the most essential metric of success. We have discussed the complexity of crude oil and the general refinery path which it takes. Certainly, the most energy intensive portion of refining, separation processes, dictates the success or failure of an oil refinery. Along with the increase in light distillates, we observe an increasing need for heavy crude deasphalting capacity. As distillation columns use pressure and temperature gradients to fractionate distillates and bottoms, solvent fractionation is a “carbon rejection” process that uses a “chemical gradient” to separate asphaltenes and resins from deasphalted oil (DAO) in vacuum distillation residue. Additionally, solvent dewaxing involves solvents and temperature gradient to produce wax and lube oil. Ultimately, solvents may enhance the overall refinery profitability while adding product flexibility and utilizing the entire barrel of crude oil!

In order to understand solvent fractionation, we must understand the gradient solubility model. For engineers to able to characterize the seemingly fruitless vacuum distillation residue. Resins in the crude oil dissolve asphaltene molecules in a solution, preventing precipitation. Miscibility, or the “mixabilty” property, must be altered through the use of a solvent. Paul J. Flory, a Standard Oil Development Co. scientist, correlated the difference in molecule sizes (solvent versus VDR) with system entropy; as the difference in molecule size increased, the entropy (or disorder) increased, causing large deviations from ideal miscible behavior. With this understanding, we can attempt to quantify the strength of various solvents to compare.

Paul J. FloryFig. 1: Paul J. Flory, 1974 Recipient of Nobel Prize in Chemistry (Source – Nobelprize.org)

At this point in the separation stages, large hydrocarbons such as asphaltenes and waxes can precipitate out of the crude oil solution given a specified paraffinic solvent. The strength of these non-polar solvents are defined by Hildebrand Solubility parameters. While Wikipedia only displays the Hildebrand parameter as a function of vaporization enthalpy, temperature, and molar volume, a similar value may be approximated using surface tension and molar volume. As surface tension increases (or the enthalpy of vaporization increase), the solvent’s strength increases.