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RE: Asphaltene content for DCU feed


Charles Randall

Mr. Prasanna,
Like most things in the coking area few are straightforward or simple and usually involve multiple parameters both chemical and physical and they are often specific to the coker studied.  There are general parameters that fit most delayed coking operations but often there are just too many parameters to gain any universal application (number of coke drums & fractionators tied to the coking unit/operation, size (diameter & height) coke drums, drum cycle times: 9hrs to 24hrs per drum, type of coker feed and  rate (Zero recycle, 10% Naphtha, 10% coker bottoms, ect) and type of antifoam, use & rate so you see your trying use general info about horses to try and understand a  unicorn (aka your specific coker) to begin with.

Before we look at your first 2 general questions lets look at the last part: You say you understand it all depends on the chemistry at play inside the drum But your first 2 questions make me think perhaps not? 

 Before I launch into my version of an answer let me first say that there are two groups of folks you should talk to and get the best view around this topic: First are the folks that sell antifoam (Nalco/Betz/DOW/others) and those that use them (~any coking refinery & check those contacts answering questions around Antifoam use/Coker Drum cycle times like espana, Lucky, Claus, CVX Jim Blevins & many others).  The next group would be the groups selling Coker Technology License Foster Wheeler. ConocoPhillips, CBI&Lummus,ect and sometimes those offering alternate bottoms  treating technologies because they have to model the chemistry happening to the heavy residual feed inside the coker.
You asked your question in the subtopic:  Frac & Process section under  All Forums there is alternate Antifoam & Quench sub topic as well & if you go back to Home page there is listing for Old Discussions under Coker News & Refinery News in far right column. This will take you to some older conversations on this item which can be quite valuable as well since we tend change lot parameters in each 5 year coking cycle which re-introduces these topics which surface at about the midpoint of every new cycle (i.e. 1992-1998 cycle = 1996; 1999-2005 cycle = 2003; 2006-2010 = 2007/08) and its because the drums usually get larger: wider/taller, drum cycles get more frequent, and feedstock gets heavier.. about only thing that stays same is operators pay! 
<Might also get Paul or Gary @ talk you thru a topic search using Antifoam, Asphaltene, COT, ect topics>
Here are topics /dates to  look at 1) Antifoam: April 2, 2004; Antifoam use Sept 2, 2003; Antifoam & Steam injection Aug 31, 2004; Coke drum foaming Feb 21, 2003; Cycle time Oct 27, 2003; Polymerization Coker liquid Jan 23, 2004; Feedstock/Asphaltene May 16, 2007; and Sludge Injection/Antifoam Query Feb 23, 2003 and ect.

Ok now for my myopic viewpoint on this topic.  
The reason I say you may not fully understand is that we have about 3 very complicated chemical reactions taking place and most advanced chemist are not sure exactly of the components driving the reactions nor of the process taking place but frame out a solution by using the observed results from changes (aka no one understands the complex chemistry between yeast and flour dough in process making bread but it hasnt stopped mankind making bread for hundreds of years). 
 1) First is the simple chemistry of distillation where lighter fractions of crude/resids are distilled off as a gas, cooled to their product range and withdrawn as liquid at that temperature.  A lot of the physical & operating parameters of the coke drum will establish the velocity and volume of these fractions thru the liquid surface and hence the foaming tendencies.
2)  These natural and thermally cracked distillation components become the gas moving thru the drum liquid and can create foam due complex chemistry interactions at the liquid / air surface due to reactions with the surfactant and changes in the more complex coking chemistry.  The key element in foaming is the Critical Micelle Concentration (CMC) at which foam propagation begins and it is driven to a large degree by the rapid changes in Viscosity of the liquid.   If you picture a mosquito walking on the surface of water it helps to visualize whats going on at the coke drum liquid surface.  The water surface is tension is higher at the air interface because of dissolved molecules that have a surface active agent (surfactants) that have a hydrophobic head (water insoluble) at the air-liquid interface and a hydrophilic tail (water soluble) tail. The higher surface tension at the surface of water allows the mosquito to walk on the surface instead of sinking thru as it would below this layer.
 The bulk viscosity is a contributing factor to surface tension and also foam stability.  As the viscosity of liquids increase, entrained gas, becomes a bubble and can be trapped below the liquid surface.  Increasing viscosity of the system also reduces the coalescence capability of smaller bubbles merging to become larger bubbles (which become less stable as diameter increases).  When the surface tension is lowered on the bubble it will burst this is process by which defoamer interacts to disperse the foam and its bubble formation and is a physical interaction with aqueous liquid.  The Marangoni effect which is driven by osmotic pressure can become a stabilizing factor in foam, where in some cases the liquid is being pulled thru the bubbles walls creating regions of low & high surfactant concentrations which sets up a gradient along the bubble surface & pumps liquid back onto the walls (aka a surface transport).
Because of the high temperature required for the coking reaction to occur few chemicals survive long enough to act as a antifoam or defoamer .  Silicon based antifoams are one of the few chemicals that are successful in this +900F application one problems become that often any surfactant capable of defoaming, can become a foam stabilizer if used in too high a concentration. The tendency to knock foam down during filling process with slugs of antifoam agent instead of controlled low level use to prevent foam from forming can come back to haunt user in critical last hours of drum cycle where full drum has little disengaging space, the velocity gas is higher and slugs of antifoam start to stabilize the foam layer.
There are of course only two types of foamovers bad and very bad foamovers. The very bad ones are at the end of the drum cycle.
3) The coking chemistry occurs around the concentration of the resid/crude asphaltene concentration and is process by which the resins are stripped away & the heavy asphaltene molecules (known as poly methyl Chickenwire) begin cross linking to form petroleum coke.  As I mentioned previous post the asphaltenes are insoluble in the alkane products that make up most of the other products in the rest of the initial resid/crude and are only kept in solution by the aromatic or resin portion.  The resin & asphaltene in fact form a sort of micelle in much the same way that the surface active agents for a critical micelle at the drum liquid surface which propagate the foam formation. When the asphaltene go thru the furnace and the aromatics are stripped away the asphaltenes clump together and flocculate to form pre-coke particles that are often spheres when the asphaltene concentrations are very high and this becomes shot coke. The reaction is somewhat delayed until the liquid reaches the coke drum and the asphaltene linkage reaction completes which is why the process is called a delayed coking process.  Now the process of stripping off the resin and dropping out the asphaltenes has a big impact on the liquid viscosity more importantly the rapid change to lighter viscosity. This viscosity impacts directly on the liquid surface tension and the foam propagation as well.
So back to your first two questions:  Impact of Asphaltene on foam formation & does COT suppress the foam ? Answers are maybe with chance of yes.
There are so many parameters that this cannot be a simple direct correlation. The factors from the 3 chemical processes are interactive and they can be limited due to the parameters of coking unit & its operation.  Some cokers never have a foaming problem & often need only use the antifoam carrier to knock down what does foam. Other cokers always have foam issues and never have a full drum of coke (aka they dont have the asphaltene concentration to be at the cycle they are trying to operate at or their drums are too small and the velocity is too high, ect).

But in general the higher asphaltene content  is going to make coke a lot faster because there is more asphaltenes to precipitate out, hence it is going to force the cycle time lower, this changes the operation dynamics and tends to shift the coke towards the shot coke type. The higher asphaltene content also means that the change in viscosity of the coke drum liquid will be more pronounced and frequent which will move the surface dynamics towards more stable foam propagation.

Changing the COT to suppress the foam is again another maybe, but the “cure could be lot worse than the cold” in this case. Raising the COT temperature would tend to lower the viscosity of drum liquid and make the change in viscosity, as aphaltenes drop out,  less of a delta.  
BUT (and its a big one).. any gain on foaming and staying in drum longer will likely cause big difference in coker run time by increasing the number of days on furnace decoking as higher temperature increases coking/fouling rate of furnace tubes. And the increase to cycle-stress on coke drums going from ambient to 920F (increase both quench & heat up cycle deltas) in the daily cycles could shorten the total number of lifetime coker cycles (normally ~3000-5000 average) and increase tendency to form/propagate existing micro stress cracks. Also it should increase the tendency to form hot spots when the resid properties and operation are on borderline between a sponge and shot type petcoke (function of asphaltene content also).   Another aspect of higher COT is that the amount of product stripped from the coke will be much higher (ie petcoke will have lower Volatile Matter/be MUCH harder).  And again when the resid is borderline between sponge & shot it can either tip it over into shot coke (problem if unit/drum is not set up for auto-unheading & dealing with shot cokes) or worse make hard sponge coke that doubles the drill/decoking portion of drum cycle operation and ect.

Whenever you talk experienced coke operators about coker COT changes they get same as look cats get when you throw them in room full of rocking chairs… because they know not a lot good can come from it.  
So fairly certain I gave you more questions than I answered but as I mentioned at start nothing is ever simple when comes to delayed coking process parameters.
<Sorry about the edit but there were few typo’s>

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