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Feb-2010

Ring-shaped catalysts make the grade

Shape-optimised topping materials, together with ring-shaped hydroprocessing catalyst, provide a solution for units that are pressure drop limited

Brian Moyse
Haldor Topsøe
Viewed : 2007
Article Summary
Oil refineries in the US, Canada and other regions of the world are showing their age. Within the worldwide 
oil refining system, there are refineries in operation that are more than 100 years old. These are, indeed, the senior citizens of our industry. So, it comes as no surprise, then, that corrosion is a large and expensive issue for refiners to address on a daily basis. A refinery can end up spending millions of dollars every year dealing with corrosion issues.

Obviously, fouling affects many different aspects of refinery operations.1 However, this article will deal with only one aspect of fouling, which inevitably results from corrosion. We will deal exclusively with the subject of fouling of fixed-bed catalytic reactors used for hydroprocessing of the whole range of refinery streams.

The problem
As regards catalytic hydroprocessing units, it is desirable to maximise on-stream time and achieve the desired cycle length without outages or interruption. It is not 
unusual to see some operations halted due to unfavourable pressure drop across all or part of the 
catalytic reactor. Most often, this is caused by feedstock contaminants, which usually include a very well- known corrosion product — iron. This is shown in Figure 1, a photograph of debris recovered from the top of a gas/liquid distributor tray. Note the variation in the size 
of particles.

It should also be mentioned that, in hydroprocessing, there are other fouling sources besides iron corrosion products:
• Catalyst fines and dust (FCC-derived streams)
• Coke fines (coker-derived streams)
• Sediment (tar sands, inefficient desalter operation)
• Salts (Na, K, P, etc)
• Carbonaceous material spalled from heater (polymeric)
• Metal sulphides (Ni, V, etc).

It should also be pointed out that treating cracked feedstocks or blends containing cracked material presents an additional complication. This is because cracked stocks contain olefins and, in particular, diolefins. If such feeds are exposed to oxygen, as for example when using non-blanketed feed tanks, gums will form. The gums can cause fouling of the heater itself and the heat exchange train. Deposits can build up in the heater and will eventually slough off as fairly large-sized flakes and deposit on top of the catalyst bed.

Even if feeds containing cracked stocks are fed directly to the unit and not via a tank, there are still issues to be faced. In these cases, the olefins and diolefins react readily in the reactor inlet to form polymeric materials. These gummy reaction products provide the “glue” to cement together any foreign matter entrained in the feed and can ultimately form a crust on top of the bed. Of course, this results in high delta P, flow maldistribution and possibly hot spots in the catalyst bed.

As everyone is no doubt aware, a catalyst bed makes for a great filter. Unfortunately, this undesirable facet brings with it an increase in pressure drop as the catalyst bed fouls. Feed filtration is a good idea in such cases, because whatever material can be removed in this step means that much less is passed to the reactor. The average size of the filter element employed is 25 micron. This means that even units currently employing feed filtration are not necessarily “out of the woods”, since particles smaller than the nominal filter element size are going to pass through to the reactor. Also, it appears that for any given industrial application, the micron size of the filter is selected not for its efficacy but rather on what the refiner considers to be an acceptable replacement or backwash frequency.

Figure 2 shows an example from an actual naphtha hydrotreating unit, depicting the development of the reactor pressure drop as a function of months on stream. It can be seen that normally the reactor pressure drop is predictable and follows the variations typically encountered as a result of the usual fluctuations in feed rate. This is normal and to be expected. This plot is also typical of hundreds that we have seen from hydroprocessing units worldwide. The only difference is the actual values of psi and on-stream time. The shape of these curves is exactly the same.

Figure 2 also shows that, after a little more than a year on-line, the reactor pressure drop rises exponentially and requires that the cycle be terminated and the unit shut down for corrective action. This situation can and will occur when the bed void fraction, either overall or in a particular layer, is reduced from its start-of-run (clean bed) value down to 22%. The range is actually between 20% and 25%, so we selected 22% as representative. When this occurs, pressure drop takes off exponentially, resulting in channelling, catalyst bypassing, hot spots and so on.

Table 1 shows start-of-run, clean bed void fraction of materials that are typically loaded into a reactor, be they catalytic or inert topping/support media. The right-hand column shows the useful void or space available for deposition of contaminants before pressure drop increases exponentially. This number is derived by subtracting 22%, the point at which delta P rises significantly, from the SOR clean bed value. This is the actual maximum void space available to deposit debris in the top catalyst layers of the bed.

Spheres are the worst possible choice in cases where pressure drop is an issue. It is permissible to use spherical support media at the bottom of the bed or reactor, but they should definitely not be used as bed topping in any units experiencing pressure drop issues.

We might also mention that even units that do not have pressure drop issues could actually benefit from replacing spherical topping with a high-void type product. This would protect the unit from unfavourable pressure drop excursions in cases of plant trips, refinery upsets and power outages. Normally, if a refinery experiences an emergency trip due to a power outage, for instance, the pressure drop across the hydrotreater reactor will be higher on restart than it was prior to the event.

Early solutions
One of the early approaches to mitigate this kind of problem was the use of scale traps or “trash baskets” in the top of the reactor. It is obvious that these baskets could only hold back particles larger than the mesh size used in their construction.2 Anything of a smaller size would naturally pass through into the catalyst bed.
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