by Franklin L. Foster, Ph.D.
The refining of heavy oil requires exacting and thorough chemical, engineering, and computing processes. When first brought to the refinery, heavy crude oil is comparable to "molasses in January". The re-creation of this thick, tarry raw material into products of value to end users (you and I) is what we'll discuss here.
To refine heavy oil with excellence requires more than a flashtower -- it also incorporates the best in computer technology. What's more, the whole process rests in the hands of a willing and able staff -- the people who truly make the oil flow smoothly. The computer serves many purposes at a refining site. A single example, among many, is found in the fractation process. This process requires exacting measurements of temperatures, pressures, and volumes. Essential, computerized adjustments to these features must be made to maximize the return of refined products from the feed composition and the processing itself. But setting the goals and meeting them rests with the people. And nobody does the job like the refining staff: the most valuable natural resource -- the people who provide the innovation, the creativity, the inventiveness, and the responsibility that makes the refinery a winner. Their task: to understand and accept the new challenges -- their goal: to answer the questions that assure success.
Before the actual refining begins, the stored heavy crude oil is cleaned of contaminants such as sand and water. Once completed, this clean crude feedstock is heated first by piping it close to, but separated from, the much hotter already-refined product (this is a "heat exchange"); then secondly by piping through a furnace which heats the crude oil even more. This furnace is a natural gas-fired furnace which heats the oil once it is injected into the flashtower.
At this point, the highly heated crude oil is sent into the air-free environment of the flashtower. These flashtowers are specially pre-designed and constructed to be the most effective height for the fractions desired and for the efficient application of heat. Once there, extreme heat produces the chemical splitting of the crude oil into combustion gas (furnace fuel gas), liquid products, and residuum (solid, complex hydrocarbons that often end up as asphalt). Refiners take much care to avoid the overlap of gaseous products with lower boiling points into higher boiling ranges in the tower structure, because this can risk ignition and explosion (at the fraction’s "flashpoint"). The taller the tower, and the more controlled and exacting the heating, the safer the refining process.
All the produced substances are removed from the flashtower: the combustion gas has completed its refining time there; the liquids are sent to the fractionating tower; while the residuum moves to the vacuum tower. First, we will trace the liquids.
In the fractionating tower, the flashtower liquids are subjected to further heating that causes them to "fractionate". In the refining process, in general, the longer the hydro-carbon molecule, the higher the temperature it takes to boil it out of crude oil. The temperature needed to boil out gasoline might be only 40 Celsius while a temperature of over 400 Celsius might be needed for heavy gas oil. Thus, the following substances (lightest to heaviest or from the top of the tower to the bottom) are produced: off gas, straight run gasoline (composed of molecules with 5 to 10 carbons in length), kerosene distillate (with molecules of 11 to 12 carbons in length), light gas oil (13 to 17 carbons), and heavy gas oil (18 to 25 carbons), used for lubricating oils. Collecting trays located at intervals up the tower collect products according to their density, with the least dense products such as off gas and straight run gasoline being trapped and siphoned off closer to the top of the fractionating tower, with the heavier materials such as the gas oils being taken off closer to the tower's bottom. These products can then be shipped.
The large amount of heavy residuum (26 to over 60 carbons) which is characteristic of heavy oil, however, is subjected to even more refining. Originating in the flashtower, the residuum receives more heating in a vacuum tower, where light vacuum gas oil and heavy vacuum gas oil are extracted from it. The remaining tarry solids left near the base of the tower are then sent through another heat exchange and furnace and on to the hydrocracking tower or "cracker" or they can be claimed as asphalt. There, the residuum is subjected to intense pressure, heat, catalysts, and hydrogen gas (which assists in breaking down the extremely complex hydrocarbon bonds in the residuum). Gases, liquids, and a portion of residuum are produced. The gases include: hydrogen sulfide from which sulfur can be extracted; carbon dioxide that is let off into the atmosphere; carbon monoxide which is sent into a furnace and also let off as carbon dioxide; and gas that can be collected for use as furnace fuel. The collected liquids are re-directed back to the fractation unit. Meanwhile, the still unconverted residuum moves on to another processing stage: coking
The method for coking uses two towers on an alternating basis. After again furnace-heating the residuum it is sent to one of the coking drums where heat and high pressure are applied. Off gases are collected and re-distributed to the fractionating tower and on to other processes. Only the heavy hydrocarbon "coke" is left in the coker, and is washed out with a jet stream of water; at the same time, the coking process begins in the other drum. The coke can then be shipped for common production use in the steel industry.
The production of synthetic crude products at the Husky Refinery and the Upgrader is far more involved than what has been outlined very briefly here. Nevertheless, it is hoped that this discussion has provided some basic insight into the complex and highly integrated refining process.
Once all these tasks have been completed, and the various fractions produced, usually according to market needs, the actual refining process is complete.
Refineries also produce pure chemicals, called feedstocks, from crude oil. These feedstocks are sold to the petrochemical industries, where a great variety of products are made.
|Methane (Natural Gas)||Fertilizers||Polyethylene|
Some of the technical aspects of refining which are explained more fully at this website include those in the table below: (short explanatory articles on these topics would be welcome)
|Alkylation and Polymerization||Gasification||Separation Processes|
|Catalytic Re-Forming||Hydro-treating||Sulfur Extraction|
|Dehydrogenation||Visbreaking and Coking|