Q&A

(2011) Question 76: What can cause exothermic reactions in propylene driers and guard beds and how can these reactions be prevented?

As introduction, Propylene Recovery Units downstream of FCC units typically consist of a C3/C4 Splitter, a Deethanizer and a C3 Splitter in series.

(2011) Question 77: How prevalent is the use of low slide valve dP override control in modern FCCU DCS systems and can the over-ride be considered an "independent protection layer" (IPL) when conducting a layer of protection analysis (LOPA) to protect against a pressure reversal scenario?

The panel concluded that 100% of the FCC units in their respective companies used the low slide valve dP over-ride to protect against reversals.

(2011) Question 78: What are the locations, formation mechanisms, mitigations and removal methods for coke deposition in the riser, reactor, reactor vapor line and main column inlet nozzle?

There are two ways that coke forms in FCC reactors. These are incomplete vaporization in the feed injection zone of the riser and HC vapor condensation of high boiling point species on cooler surfaces. In both cases the hydrocarbon liquid wetted on a surface has essentially infinite residence time at elevated temperature, resulting in thermal degradation reactions that create solid coke deposits.

(2011) Question 79: What procedures do you use to overcome chloride fouling problems in FCC fractionators? What is the contribution of chloride from the catalyst?

Overcoming or preventing chloride fouling problems in FCC fractionators is a primary responsibility of the unit engineer. There are several tools that can be used to monitor an operating unit.

(2011) Question 80: Does silica entering with the FCC feed have a negative impact on the performance of the catalyst or the product slate? Does the silica source matter? Does it matter if originated from crude oil production, antifoam injection in coker units, or from the crude itself? What proportion of the silica entering the FCC leaves with the product?

I did not find any internally reported issues involving silica in FCC feed as a catalyst poison or as having negatively impacted product qualities. As a result, there have not been any attempts within BP to track silica in FCC feedstocks and in FCC products therefore I don’t have a basis for responding to the portions of this question requiring this kind of analysis.

(2011) Question 81: What are the potential impacts on FCC LPG and FCC gasoline properties from a) processing coker off-gas into the FCC gas plant and b) processing coker naphtha in the FCC riser?

Coker products are known to contain more sulfur, H2S, di-olefins and mercaptans than FCC products. Care should be taken not to overburden the LGP sulfur extraction unit or send an excessive amount of diolefins to the Alky. One of our refineries experienced excessive mercaptan sulfur in the Light Cat Naphtha when a coker light ends stream was routed to the FCC gas plant.

(2011) Question 82 What are your preferred process and catalyst options to maximize LCO yield? Among the options, please discuss the ramifications of lowering the riser outlet temperature by 40-50 F on the heat balance, including suggestions on how to best utilize any excess air blower capacity at the lower riser temperature.

One way to increase refinery distillate yield if there is not an O2 or regenerator temperature limit is to increase the upstream distillation efficiency to minimize the amount of 700 F minus boiling point material to the FCC. Reducing lighter material in the VGO reduces the crackability of the feed. The typical results of this are lowering the API gravity and the aniline point while slightly increasing the VGO tail, sulfur and Conradson carbon number.

(2011) Question 83: What are your recommended practices and categories for benchmarking FCC units? Include as appropriate, process performance, reliability, capital efficiency and operations.

Valero participates in the Solomon survey that benchmarks the refinery process units for energy and reliability. Independent to that commercial survey, we have benchmarked our FCC’s on a yield basis using a constant pricing model, accounting for the feed quality differences. The results of the yield benchmarking are used to identify underperforming assets for potential capital improvements.

(2011) Question 84: What is the typical flash point for your slurry oil product? Can a flash point of 200 F or higher be achieved with steam stripping the main fractionator bottoms? What are your storage temperature guidelines? What lower explosion limit (LEL) and H2S levels are found in the tank vapor space?

This topic of slurry oil flash point and control has been a recent discussion topic at our FCC units. Collective inputs indicate the range of flash points temperatures are from 140degF to 220degF.

(2011) Question 85: How often do you clean the FCC slurry exchangers?

For us, as I'm sure for it is for others, minimizing FCC slurry exchanger cleaning has been a continuous improvement effort for quite some time. As a result we have implemented best practices in the areas of slurry circuit and fractionator operations and design. Through these efforts we have some units that only need to clean during cycle ending turnarounds.

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