Catalyst fins (cat fines) are often found in heavy fuel oils. They are a common cause of high piston ring and cylinder liner wear in low speed engines. If fuel containing cat fins is bunkered, they must be removed before reaches the engine. Cat fins are found by an analysis of the aluminum (Al) and silicon (Si) content of the fuel oil. An aluminum and silicon of up to 15mg/kg is tolerable after treatment at engine inlet.
EFFECTS OF CATALYST FINS:
Sudden emergence of excessive piston ring and cylinder liner wear in low speed engine on all cylinder is often caused by cat fines in the fuel oil. The presence of cat fines can be recognized by erosion on the spill valve stem of the fuel pump and by the numerous particles found embedded in the graphite flakes of the running surfaces on microscopic examination of rings and liner replicas. Cat fines primarily cause three body abrasion of ring and liner, but some are retained in the graphite flakes of the metal surface and these continue to abrade the running partners for a long time after they have entered the engine. When the running surface of an older liner is examined microscopically it is often possible to find a few cat fines trapped in the surface structure and worn down level with the surface. Cat fines are the hard abrasive particles 5 to 50 microns in size. The particles of 10 to 20 microns are the most dangerous, as they are most readily caught in the oil film and in the fine graphite structure of rings and liner. Smaller particles tend not to cause wear and larger particles are not easily held in the oil film.
The main problem caused by cat fines is ring and liner wear. But if the fuel oil is heavily contaminated with cat fines then there can be problems on all exposed surfaces:
Fuel pumps Seizure, wear (if particles size equals clearance between barrel & plunger)
Fuel injection valves Seizure, wear, on nozzle holes and needle seat
Piston rod & Excessive wear, excessive supply of piston rod stuffing box oil to piston rod stuffing boxes
A cylinder liner surface which has been roughened by cat fines is more susceptible to corrosive
wear, and the combination of cat fines and corrosive wear can be very severe. The wear profile
of a cylinder liner damaged by cat fines shows high wear and sometimes a wear maximum in the upper mid-stroke region.
ANALYSING FOR CATALYST FINES:
Standard Test Methods:
The presence of cat fines in fuel is determined by burning the fuel and roasting the ash at 550°C. The ash is then dissolved in very strong acid and the solution analysed for the two elements aluminium and silicon, which are constituents of cat fines (ISO 10478:1994). Other less common tests include measuring the quantity of centrifuge sediment and microscopic examination of the sediment.
Why Catalyst Fines are not always found:
Cat fines that are present in the fuel are sometimes not detected because of the problems of taking and analyzing representative samples:
- The fuel oil sample is not always representative of the complete fuel oil bunker. Cat fines are not uniformly distributed in the fuel. They can settle out or be resuspended.
- Aluminium and silicon content should not be measured directly, without firstly ashing the fuel oil, then roasting and dissolving the ash. .
- In the laboratory a one liter fuel oil sample is heated to 50 to 60°C and stirred for 5 minutes before a smaller sample is removed for analysis. If this preparation is neglected or insufficient, cat fines which have settled to the bottom are not found in the smaller sample.
In one case a large reduction in cat fines was observed during fuel storage, most likely because
the cat fines had settled out Another time cat fines were found after storage where before there
had been none. Cat fines, at levels that are not apparent in the fuel analysis, can accumulate in
the tanks from successive deliveries. During heavy weather they will be churned up. It is usually
not possible to take samples from the bottom of the tank
In another case a small but definite increase in liner wear was observed, when the content of cat fines in the fuel increased but stayed within the specification.
On more than one occasion when an engine was found to have suffered extreme wear due to cat fines in the fuel oil, filter blocking or an increase in automatic filter back flashing had been recorded. The filter usually had a mesh site which was larger than the average size of the cat fine particles
The Density Method:
A statistic of density plotted against aluminium plus silicon content showed that the combined
aluminium and silicon content only exceeds 10 mg/Kg when the fuel density is greater than
0.96 g/ml. High density does not indicate cat fines, but it seems that the risk of finding them is
greater. Heavy fuel oil is a mixture of various products of petroleum refining and the
components which contain cat fines tend to have a higher density.
ACTION TO TAKE WHEN CATALYST FINES ARE PRESENT:
Cat fines are more common in some areas of the world than in others. If a vessel generally
receives fuel free from cat fines, les monitoring is required. But if cat fines are likely to be
present, then it is important to analyse the fuel oil more often, even more than once per bunker,
and to regularly clean out storage and settling tanks
The bunker supplier should be informed when cat fines are found in the fuel which they supply. We do not recommend exceeding the normal cylinder lube oil feed rate much, as a measure to counteract cat lines. A certain flushing effect is possible, but it is probably not very effective and other problems can occur if the lube oil feed rate is too high. An increase of approximately 20%is tolerable.
After the engine has been damaged by cat fines in the fuel oil, piston rings with cat fines embedded in them must be replaced and liners should be honed to remove the cat fines and hard brittle layers caused by excessive wear. Otherwise damage will continue even if the fuel no longer contains cat fines. Using a microscope it is possible to examine the rings and liners to decide what action should be taken.
Cat fines are highly hydrophilic and if water is present they are incorporated into water droplets. When the water is separated the cat fines are also removed. The density of fresh water is similar to that of high density fuel at the separation temperature. If water, in particular fresh water, cannot be removed in the fuel oil separator then the cat fines will also not be removed. When the fuel oil is stirred, water is mixed in. It forms an emulsion and becomes more difficult to separate. A decanter upstream from the separator hinders cat fine removal because of the stirring effect. For the same reason the centrifuge should be as close as possible to the setting tank.
The presence in the fuel oil of used automotive lubricating oil containing strong detergents is thought to cause water to emulsify, making cat fine removal more difficult. Treating fuel containing emulsified water with a demulsifying fuel additive can improve cat fine removal. The separation efficiency of water is improved when water is removed from the fuel by evaporation or distillation. This is not beneficial for the removal of cat fines. The water in the fuel is needed in the separator to assist the removal of cat fines.
FUEL OIL TREATMENT ARRANGEMENT
Arrangement of Separators:
For the correct arrangement of separators refer to the manufacturer's instruction. A separation efficiency of minimum 80% should be achieved, ie. 80% by weight of the impurities removed. If separators with gravity discs are used, two separators must operate in parallel with approximately 15% of the nominal volume flow rate. The best efficiency would be achieved with two parallel purifiers feeding one clarifier (8%). Most of the separation is achieved in the purifiers (70 to 80%), the clarifier giving the "final polish".
Cat fines are removed by centrifuging the fuel oil. The separator is not equally efficient for all sizes of cat fine particles; one theoretical calculation gave an efficiency minimum at a diameter of 9 microns. The smallest particle that is removed in the centrifuge is a function of density difference, viscosity (temperature dependent) and flow rate. This is theoretically and also for cat fines experimentally approximately 5 microns, which means that the smallest particles are neither removed by a centrifuge nor by a 5 micron filter.
The cat fines of diameter less than 10 microns are the most difficult to remove. The fuel reaching the engine should not contain more than 15 mg/Kg aluminium plus silicon. The cat fines remaining in the fuel after fuel treatment should consist mainly of these smaller particles
All cat fines of diameter greater than 10 microns must be removed in the separator.
The effective removal of cat fines is often lower than the calculated theoretical efficiency. The theoretical calculation assumes solid spherical particles and uses an estimated density. The most dangerous cat fines resemble fractured hollow balls. The effective density depends on whether the pores are filled with water or fuel and therefore the estimated density is not always correct.
Cat fines are removed from the fuel oil in the separator and not in the filter. The standard fuel oil filter of size maximum 50 microns is not intended to protect the engine against cat fines. When the separator is functioning correctly no further protection is required. Secondary back flush filter(s) with a 10 micron mesh is/are used to protect the engine from serious damage if for some reason the separator has not removed all the cat fines from the fuel. In addition such a filter is a good indication of the separator efficiency. If there is reason to suspect that the fuel oil contains cat fines, the backflush from the filter should be transferred to the sludge tank and disposed of.
The most reliable way to avoid problems with cat fines completely is to use a 5 micron fine filter at the engine inlet (a tertiary filter). However we do not recommend using a filter this fine because they are easily blocked e.g. by asphaltene particles, particularly if the fuel stability is low. Some engine owners use fuel homogenisers in combination with a 5 micron fine filter to prevent the filter from blocking.
SIZE, SHAPE, COMPOSITION AND USES:
Use of Catalyst in Oil Refining:
Catalytic cracking is used by the majority of refineries. The high density cat bottoms or catalytic cracking fractionator bottoms are blended to residual components to reduce the viscosity. The high aromaticity of the bottoms increases the ability of heavy fuel fuel oil to dissolve asphaltenes and thus reduces the risk of incompatibility.
If the fluidized catalytic cracking (FCC) operates correctly then the catalyst is recovered. The catalyst is expensive and is not intended to be a waste product.
In recent years catalysts have been further developed. There is a trend towards harder catalysts, which is good for the refinery, but not good for the diesel engine.
Size and Shape:
The catalyst in oil refining is very hard porous round particles. The catalysis takes place on the surface of the particle. The particles must have the largest possible surface area to volume ratio, be light enough to be easily transported and have a high crush strength.
The mean particle size of fresh catalyst is 70 microns (range 20 to 150 microns) and 75% of the particles are larger than 25 microns. But the size is reduced during circulation in the refinery and by the time the catalyst particle reaches the fuel oil as cat fines the average size has dropped to less than 20 microns (range 3 to 42 microns). The largest particle size in fuel oil depends on the settling history of the fuel.
In the past catalyst was made from pure silica (SiO₂) or alumina (Al2O3). Now materials such as faujosite, which is broadly similar to porcelain, are used. These materials contain not only aluminium and silicon, but also smaller amounts of sodium, calcium, magnesium, potassium, etc. The content of cat fines in heavy fuel oil by weight is 3 to 15 times that of aluminium. A good approximation is that the amount of catalyst by weight is twice the combined content of aluminium and silicon.
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