What are the main differences between delayed coking and fluid coking processes in terms of feedstock suitability and product yields?

Delayed coking and fluid coking are both thermal cracking processes used in refineries to upgrade heavy, low-value residua into lighter, more valuable products and coke. However, they differ significantly in terms of feedstock suitability and product yields. Delayed coking is better suited for processing a wider range of residua, including those with moderate to high metals content and moderate asphaltene content. It can handle feedstocks that are not easily processed in fluid coking units. The process involves heating the residuum to high temperatures in a furnace and then transferring it to large coke drums, where it undergoes thermal cracking over a period of hours. The coke formed in the drums is then removed mechanically. Fluid coking is typically used for processing heavier, more contaminated residua, particularly those with very high metals content and high asphaltene content. The process involves spraying the hot residuum onto hot coke particles in a reactor, where it undergoes rapid thermal cracking. The coke particles are fluidized by steam or other gases, providing good contact between the residuum and the coke. The coke formed in the reactor is continuously withdrawn and partially burned in a burner to provide heat for the process. In terms of product yields, delayed coking generally produces a higher yield of liquid products, such as gas oil and naphtha, and a lower yield of coke compared to fluid coking. The coke produced in delayed coking is typically a higher-quality, more crystalline coke that can be used as a fuel or in the production of anodes for aluminum smelting. Fluid coking produces a lower yield of liquid products and a higher yield of coke. The coke produced in fluid coking is typically a lower-quality, more amorphous coke that is primarily used as a fuel. Fluid coking also produces a significant amount of heavy gas oil, which can be further processed in other refinery units. The operating conditions, such as temperature and pressure, also differ between the two processes. Delayed coking typically operates at lower temperatures and pressures than fluid coking. This results in a longer residence time for the residuum in the reactor, leading to more complete cracking and a higher yield of liquid products. For example, processing a vacuum residue with moderate metals content would likely be better suited for delayed coking to maximize liquid product yields, whereas processing a heavy oil sands bitumen with very high metals content would be more appropriate for fluid coking despite its lower liquid yields, because the fluid coker is better equipped to handle that particular feedstock. Therefore, the choice between delayed coking and fluid coking depends on the specific characteristics of the feedstock and the desired product yields.