Crude oil extracted from the earth is unsuitable for direct consumption; it must be refined to create value added products. The refining process also helps to remove impurities in the crude oil that could cause harmful emissions during combustion of the fuel products. This makes refining an integral part of the oil and gas value chain that delivers products to consumers.
The Basic Phases in the Refining Process
There are three basic phases in the refining process that transforms crude oil into desirable products.
The first step is the distillation process, where only heat and temperature are used to split crude oil into basic products and intermediate products such as straight run gases, naphtha and gas oils. These are known as intermediate products because they must be processed further to make them consumable again.
These intermediate gas oils are taken to the next phase of refining to make more value added products. This phase is called the conversion process, which is where the FCCU (fluid catalytic cracking unit) plays a key role in producing more value added products. In this phase, a number of different refinery process technologies are used, including processes to remove impurities such as sulfur.
The difference in this step is that in addition to heat and pressure, catalysts are used to promote chemical reactions that actually modify the hydrocarbon's molecular structure. This is the key to transforming undesirable products into desirable ones. The magic is possible because the products are changed at the molecular level. The flow of intermediate products between these units can be a very complex path. FCCUs provide absolute flexibility in the refining process, both from the perspective of processing a wide range of feedstock and also in the operating range of the conversion units. This unlimited operational flexibility produces different ranges of gasoline in the FCCU process.
The third phase is called blending, where outputs from conversion process are mixed to make more customized, valued added products per market requirements.
Evolution of the Fluid Catalytic Cracking Unit (FCCU)
FCCUs are the resultant technological upgrade from the basic catalytic process that was first developed during the 1920s by Eugene Houdry to upgrade residue. This process was commercialized later in the 1930s and was based on cyclic fixed bed. FCCU feedstock is normally light gas oil from vacuum distillation columns. Catalytic cracking cracks low value high molecular weight hydrocarbons to more value added products (of lower molecular weight) such as gasoline, liquified petroleum gas (LPG) and diesel along with important petrochemical feedstocks including propylene and C4 gases like isobutylene, Isobutane and butane.
Full-fledged commercialization of FCCUs came into existence after 1942. Since then more than 400 refineries have installed FCCU units, which are also called secondary processing units. FCCUs provides about 50% of the transportation fuel and 30% of the total gasoline pool.
FCCU: The Most Profitable Unit in a Refinery
FCCUsl remain one of the most important and most profitable processes in a refinery operation. (To learn more about refinery operations, read Refinery Configurations: Changing Dynamics.) This is mainly due to the process that converts the low value feedstock into a wide range of much more valuable products. These include yields of about 60% of high-octane gasoline, about 20% of a diesel-boiling-range product and significant amounts of lighter gases, some of which become feedstock for other downstream processes, as well as a major contributor to the refinery fuel gas system.
Another interesting feature of the FCCU is that it helps refineries gain in terms of the product volumes, i.e., the total volume of products represents an actual increase over the volume of feedstock. This is due to the addition of catalysts that substantially increase the output volumes. The process also has great flexibility to handle a wide range of feedstocks, and operating conditions can be manipulated to satisfy diverse product requirements. Thus, optimizing FCCU operation has a very significant bearing on the refinery's profitability.
As stated above, FCCU has great flexibility but is not without complexity. Hence, it is possible to vary the operating conditions to process a variety of components for better product mix and profitability. However, to achieve the maximum economic opportunities with FCCU, the unit has to be designed perfectly. Modern FCCUs maximize value added products through what is known as short contact time cracking.
Today, most refineries use riser cracking, as compared to the dense bed cracking that was used during the 1960s and 1970s. Technology has vastly improved. Some of these risers deploy short contact time cracking reactors in which the feed backed up by high velocity is injected to engage with the catalyst to speed up the contact cracking time of the feed. This helps to improve the product mix while minimizing thermal cracking possibilities. However, due to a higher velocity of injection, the reactor is hard on the catalyst and has an adverse impact on the catalyst power for cracking. Since catalysts are expensive and capital intensive components, the unit must be properly design to optimally use catalysts. A robust design must exist for effective catalyst stripping and regeneration.
Most of the world's refineries survive mainly due to the secondary processing facilities that provide much needed economic value to their operations. This is mainly through the products produced in fluid catalytic cracking units, especially the gasoline range of products with high octane numbers. Also, through their ability to process multiple feedstocks, FCCUs can provide value added olefins and aromatics rich stream for extracting benzene, toluene and xylenes by processing C3/C4 components. Indeed, these petrochemicals are high value products and are used by variety of industries such as chemicals, perfumes, fertilizers, paints and tire manufacturers.
FCCU plays a crucial techno-commercial role in a refinery. It is a very comprehensive unit that provides great flexibility in refinery operations through its ability to process a variety of feedstocks. At a macroeconomic level, it helps to balance the demand supply scenario of the gasoline range of products.