QUESTIONS AND ANSWERS

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Q-44

What is the correct and standard method of reporting FCC conversion?

1. 100 - (LCO vol% + DCO vol%) several literature use this method to report FCC volumetric conversion. Here LCO and DCO volume yields are calculated as volumetric flow of LCO/DCO divided by volumetric flow of Feed, and not the total volume of the products. Where as, the sum of the volume yields is normally greater than 100%, why 100 - (LCO vol% + DCO vol%) is used to determine the conversion?

2. 100-(LCO wt% + DCO wt%) in some literatures this is used to report FCC conversion in terms of mass percent. To me, this method looks reasonable

.3. Ideally we call FCC conversion as 430 deg F conversion. Meaning the volume % of products boiling below 430 deg F. Still I have noticed in some literature that coke is also considered into it.Please suggest me the best and most practiced method across the global refining industry.

A-44:

Your questions on conversion are intuitive. Most do not think or understand there is a dilemma. There are actually TWO conversion definitions; apparent and actual. But first you need to understand that conversion is a relative term to measure reaction system performance. LCO and DCO have traditionally been the lowest value FCC products. The kinetic dynamics of FCC have been to produce gasoline and LPG. That is not necessarily true today but it is the historical reference. As such, all liquid products that are not gasoline and LPG are lumped together as "unconverted" for the sake of having a relative indication of performance. There is nothing magical about the number; it just is the generally accepted number indicating value which may or may not apply to your particular operation or processing goal or profit margin.

Apparent conversion is the one you mention. It is simply the proportion of the total liquid yield that is not LCO or DCO. It does not consider coke or gas yield, since one of the primary advantages of FCC is producing more liquid products than feed. This is a good relative indication of day-to-day performance but not necessarily to profitability or margin. The way we balance this issue is to also look at selectivity; how much gasoline or LPG as a ratio to liquid conversion or ratio of  LCO/DCO
.
Actual conversion is defined as the disappearance of 650F from the feed, which is the traditional endpoint of LCO. The difference is obviously one is adjusted for distillation and one is not. This actual conversion has much fallen out of favor in the industry and rarely even referred to anymore. It suffers from the same issues in defining performance. It just represents gasoline and lighter and does not take coke or gas yield into account. I am not aware of any ratios or indicators that relate coke or dry gas to liquid conversion. Normally, there is no issue in this regard, although it is possible to have high apparent or actual liquid conversion with no selectivity. I personally have operated an FCC at 96% conversion, but the products were 25 wt% coke, 71 wt% gas, and 4 wt% slurry.

This said, either value is still relevant. What you need to consider is adjusting your value or targeting another value to reflect what is important to your operation and margin and what you are trying to accomplish. For example, if you are interested in LCO yield, then you need to look at LCO/DCO vol. ratio as well as LCO as a percent of feed. Remember these are relative values to indicate better or worse performance. They are not absolute and require the kind of thought process you are going through to understand what these values actually represent.