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Refineries Produce Blendstock, Not Gasoline
CBOB: Conventional Blendstock for Oxygenate Blending
Matt Anderson
August 09, 2023
Refineries don’t produce gasoline. They produce gasoline components that are mixed to make blendstock. Ethanol is added to the blendstock to make finished gasoline.
The most common gasoline blendstock in the United States is CBOB, Conventional Blendstock for Oxygenate Blending. “Oxygenate” is the name of a family of chemicals, which includes ethanol.
Ethanol is mixed with blendstock at terminals prior to loading trucks. Ethanol, and gasoline that contains ethanol, is rarely transported by pipeline because of its affinity for water. [1]
90% of ethanol is transported by train or truck. 10% is transported by barge and minimal amounts are transported by pipeline (per the Department of Agriculture). [2]
Therefore, if a refinery wants to transport finished products by pipeline, it must do so with blendstock before ethanol blending. Transportation by pipeline is much more economical than trucks for large volumes.
Gasoline has specifications that must be met to be certified and sold. Does this mean that each truck must be sampled after the blendstock and ethanol are mixed at the required composition (such as 10% ethanol)? Fortunately, this is not the case.
The typical process is for the refinery to certify a large batch of blendstock. The refinery or third-party lab will take samples and add ethanol prior to completing the certification tests. This process simulates when ethanol will be added to the blendstock at terminals.
The exact specifications and test methods vary by both state and season. One typical adjustment is that gasoline specs require lower vapor pressure in the Summer than the Winter.
Ethanol has an octane number of 100, but when it is blended with gasoline it “performs as if its octane rating is 112” [3]. The blendstock octane target is set lower so that AFTER ethanol is added, the finished gasoline octane meets specification.
About: I write to provide snapshots of oil refinery operations and technology. My goal is to write in plain English, and to provide an entry-point to the downstream oil industry. Comments and questions are much appreciated.
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