Re: FCC Aeration Flows as MVs in DMC Controllers

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Comments:
: This is what I found out about why aeration flows are not normally used as MV in DMC controllers.

: My sources are
: 1. Conversation with Charlie Cutler
: 2. Information that is available on the web.

: Aeration Flows as MV
: Part of the reason why they never included aeration in DMC controllers is, aeration is a function of condition of the catalyst (how well it flows and catalyst fines), state of the aeration flows (such as plugging or not), cat circulation.  It varies with the pressure balance of the unit.  DMC has control over the pressure balance and the cat circulation.  What it does not have control over is the condition of the catalyst and plugging of the aeration lines.  
: Based on cat circulation, the air that is put in as aeration can either go up (into the regen bed) or down (with the circulating catalyst) - affects the density of the catalyst in the stand pipe thus the density of the catalyst flow.  At different cat circulation rates, the densities start varying and aeration has to be adjusted to ensure uniform density and then try to maximize the density. 
: The key thing in aeration is to keep constant density of the catalyst such that the head increases as we go down the stand pipe. The optimum is to maximize this density at each section (while keeping the density at each section constant) without getting into bubbling.  To enable that we need additional density measurements along the stand pipe.  We also need flow control valves at all the points where we add aeration to the standpipe. If the bulk density increases, (which also means there is more catalyst), then the slide valve will close to maintain the catalyst / oil ratio (in other words the riser temperature). This will give enough room on the slide valve. Then DMC can be adjusted to increase charge, or lower reactor pressure etc to take advantage of that.
: If we decide to improve aeration with DMC controllers - it will be a stand alone controller - until all the process issues are understood and addressed. The behavior has to repeatable and linear (or non linearity has to be understood and repeatable). Charlie's comments were "it is not a trivial problem to solve".
: He did mention that by adjusting aeration flows, they were able to get increase of 10% in cat circulation flow - this was not done with DMC, but outside of DMC in a pilot plant at Shell. 
: Fluidization Index (Ratio of minimum bubbling velocity to minimum fluidization velocity) also plays a significant part in stabilizing the aeration.  This is entirely dependent on the catalyst characteristics and the amount of catalyst fines that is in the system.

: I also found that the same issues are being addressed by Exxon Mobil.  Enclosed is their paper which came out in Hydrocarbon processing in April of 2003.  It talks mainly in terms of instrumentation -  which is the first step in ensuring reliable aeration flows are achieved, before the other changes can be made to achieve optimum aeration.
: I have also included a paper that I found from Engelhard website on cat circulation. See link below.

: 
: Best Regards,
: Venky


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