Minimising FCC slurry exchanger fouling
A case study shows when to expect reductions in slurry / FCC preheat exchanger duty from a variety of factors, including organic and inorganic-based fouling
David Hunt, Bill Minyard and Jeff Koebel, Grace Davison
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Slurry exchanger fouling is often considered the worst fouling service in the FCC process.1 The primary problem that results from slurry exchanger fouling is a reduced heat exchanger duty in the slurry/FCC feed preheat exchanger or the steam generators. The reduction in the feed preheat temperature that can result from just mild fouling of the FCC slurry/feedstock exchangers can lead to a reduced unit feed rate or conversion (for those FCC units that do not have a fired heater and are air blower limited). Additionally, an excessive pressure drop or inability to cool the slurry to the necessary rundown temperature can require a reduction in the feed rate. Excessive slurry exchanger fouling can certainly be costly to the refinery in terms of lost feed rate, conversion and maintenance expenses.
Figure 1 illustrates a typical FCC main fractionator slurry circuit. Superheated FCC product vapour is quenched as it enters the main fractionator using the reflux from the slurry pumparound circuit. Slurry exchangers, which recover this energy by heating the feedstock and generating steam, are often subject to fouling through a number of mechanisms. When slurry exchangers foul, the feed rate or reactor temperature must often be reduced.
Figure 2 shows how quickly a slurry/feed exchanger heat-transfer coefficient can deteriorate. Over a ten-week period, this refiner needed to clean their FCC slurry exchanger three times. They were forced to significantly reduce the feed rate during each cleaning cycle.
Potential causes of FCC slurry exchanger fouling are shown in Figure 3. Fouling can be generally classified as either organic or inorganic. Several organic or inorganic slurry-fouling mechanisms are possible. Each of these sources will be discussed and suggestions to prevent or reduce each type will be presented.
The most common type of fouling is organic. Potential causes are very broad. It is therefore helpful to classify this type of fouling into two general sub-types: hard and soft coke fouling.
Hard coke fouling
An example of hard coke fouling is solid coke fragments circulating through the slurry exchanger tubes and restricting flow through the exchanger. This type of fouling results in an excessive pressure drop and loss of heat-transfer duty. These shiny coke fragments generally accumulate on the exchanger tube sheets at the inlet to the tubes. Figure 4 shows how pieces of coke can block the exchanger tubes, increasing the exchanger pressure drop.
These coke fragments can originate in the reactor overhead line or the main fractionator. The coke often becomes dislodged following an FCC shutdown because of the thermal cycling of the surface that the coke adheres to. If the coke fragments are small enough to pass through the suction strainers on the slurry pumparound pumps, they can eventually foul the first slurry exchanger in the pumparound loop. Another scenario that can result is excessive hard coke accumulation in the bottom head of the main column, which restricts the suction of the circulating slurry pumps.
Smaller coke fragments that pass through the exchanger tubes can still be problematic. These are either smaller coke particles from the main fractionator or are formed by polymerisation reactions in the slurry pumparound circuit at high main fractionator bottoms temperatures. Small coke fragments can settle on the tube surface and further polymerise, resulting in a barrier to heat transfer and slurry flow.
In some cases, the formation of a thin, hard layer of deposit has been observed on the tube walls. This type of deposit has a hard, shiny appearance, similar to varnish. These deposits are formed by polymerisation reactions on the tube surface. This type of fouling can reduce the heat-transfer coefficient.
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