Interpretation of Equilibrium Catalyst Data Sheets
The equilibrium catalyst data sheet provides regularly updated information that can be used to assess the operation of the FCC unit. The analyses included on the sheet allow the refiner and catalyst supplier to formulate a catalyst management policy consistent with the goals of the refiner.
A sample data sheet is shown in Figure 1.
A separate catalyst data sheet is used to report the data from each FCC unit. Information included in the top section of the sheet includes the refinery name and address, as well as the FCC unit type and feedstock capacity. Listed in the lower section are the results of extensive tests performed on each ecat sample. Typically one sample of equilibrium FCC catalyst per week is submitted by the refinery.
Catalyst samples are ordered so that the most recent sample appears at the bottom of the sheet. Each sample has a sample number, date received, and date taken. The analyses for each sample are grouped into categories titled MAT, Physical Properties, Chemical Properties, and Miscellaneous Tests. The name of the current catalyst is also shown under the catalyst category.
A description of the analyses contained in these areas follows.
M.A.T. (or Microactivity Test)
The reported equilibrium catalyst (ecat) activity is the weight percent conversion obtained for the catalyst sample in a standard microactivity test run with a standard feedstock. Since the in-unit conversion is a function of the FCC process conditions, feedstock properties and catalyst properties, the MAT activity provides a separate evaluation of the catalyst contribution to the unit conversion.
The microactivity test is widely used to characterize the performance of FCC catalysts. The MAT procedure has been defined by ASTM procedure D3907, but many modified procedures are in use. BASF utilizes a modified ASTM procedure in order to improve reliability and to provide additional information. For example, the BASF method utilizes a lower contact time which is closer to a commercial unit. A comparison of the two procedures is shown in Table 1.
An in-depth treatment of the MAT procedure can be found in The Catalyst Report No. Tl-825 entitled "Microactivity Testing of FCC Catalysts". The standard BASF feed is a full range mid-continent oil (28.4 API, 0.17 CCR, 0.73 S, 900 ppm N, 5 ppm V, 1 ppm Ni). Each equilibrium catalyst sample is decoked and screened before being charged to the MAT reactor.
The MAT activity is the carbon-free catalyst activity. The in-unit activity will typically be lower because of carbon on the regenerated catalyst. For each additional 0.1 wt % carbon on the catalyst, activity drops by about three numbers.
MAT activity results from BASF have a 95 % confidence limit of about +/- 3 numbers. This means that at least three consecutive sample results are required to establish a trend in activity. Because of the counterbalancing effects of the FCC heat balance, a 5 number change in MAT activity will normally result in a change of 2-4 numbers of unit conversion.
When fresh catalyst is added to an FCC unit, both the zeolitic and matrix portions deactivate significantly within the first day. After that, deactivation continues at a slower rate. The deactivation rate for each unit is a function of variables such as regenerator temperature, steam partial pressure, catalyst residence time, and the levels of contaminants (such as vanadium and nickel) on equilibrium catalyst. The equilibrium catalyst activity decreases with increases in regenerator temperature, steam partial pressure, catalyst residence time, and contaminant levels. Equilibrium activity increases with fresh catalyst make up rate.
This column provides a measure of the coke selectivity of the equilibrium catalyst compared to a steam deactivated reference catalyst at the same conversion. Increases in this value can show up as increases in regenerator temperature and delta coke. Catalyst type can impact the coke factor, but major changes are frequently a result of changing levels of contaminant metals on the ecat. The 95 % confidence level for this analysis is +/- 0.30.
Since commercial regenerator temperatures are also influenced by feed preheat temperature, reactor temperature and feedstock CCR level, the coke factor helps the refiner isolate the effect of the catalyst on the regenerator temperature. It must be remembered that the ecat metals level (which strongly influences the coke factor) also depends on the feedstock quality and the fresh catalyst make up rate.
Particle Size Distribution
The ecat particle size distribution provides important information on the circulation ability and attrition characteristics of the catalyst, as well as on the performance of the FCC cyclones.
The FCC unit needs a catalyst with both larger (>100 microns) and smaller (< 40 microns) particles to circulate the catalyst effectively. Each unit has a different requirement for fines content in the ecat. Some units may require over 10 % of the catalyst under 40 microns, while others may circulate catalyst well with less than 5 % under 40 microns. It is good practice for a refinery to document the physical properties of the catalyst and the operational variables on the unit when circulation problems become evident.
A trend of decreasing fines in the ecat is a cause for concern when accompanied by a loss problem on the unit. In this case, the amount and particle size distribution of fines in the slurry circuit, flue gas system and stack should be checked to determine where the catalyst is being lost. Two possible interim measures are to increase the fines content in the fresh catalyst and to recycle fines back to the unit. Remember that a finer catalyst can increase unit losses and increase opacity at the stack. A lower ecat fines content together with an increase in average particle size are classic symptoms of damaged cyclones.
Either an increase in catalyst losses with no change in particle size distribution, or a trend of increasing 0-20 and 0-40 micron material in the ecat can indicate an increase in catalyst attrition. In either case, the particle size distribution of the fresh catalyst should be checked. Several factors on the unit can increase catalyst attrition, such as a problem in the regenerator air grid or large amounts of steam to the regenerator or regenerated catalyst line. Excessive velocities (>300 FPS) in the feed nozzles can also cause significant catalyst attrition. Further information on catalyst losses can be found in The Catalyst Report No. Tl-830, "Troubleshoot FCC Catalyst Losses".
BASF determines the catalyst particle size distribution with a Microtrac analyzer, which measures the weight percent of particles in the range of 2.8-176 microns. The 95 % confidence limit of the 0-40 micron range is less than +/- 2 numbers. The average particle size reported is the 50 % point (by weight) of the sample. The 95 % confidence limit of the average particle size is +/- 4 microns.
Surface area is an attribute used by both catalyst manufacturers and users to monitor the activity and the stability of the catalyst. Surface area is determined by nitrogen adsorption of a decoked catalyst sample. Total catalyst surface area is generally associated with the catalyst activity. This relationship is dependent on the catalyst type, so a correlation between surface area and activity may not be consistent between different catalyst types.
The total surface area (in meters squared per gram), includes area associated with both the zeolite and the matrix. Zeolitic surface area primarily cracks gas oil range material in the feedstock and is a major factor in determining product selectivities of the catalyst. Matrix surface area is important for cracking heavy molecules and for precracking oil to "feed" the zeolite.
Surface area loss is accelerated by high regenerator temperatures, steam in the regenerator, and catalyst poisons such as vanadium and sodium (see chemical properties). A loss of 10 meters squared per gram of surface area can drop the MAT activity by as much as 5 numbers. Matrix activity can trap some contaminants and protect the zeolite, resulting in a higher MAT activity. The 90% confidence limit for total surface area is +/- 6.
Apparent Bulk Density
The apparent bulk density (ABD) is a function of the catalyst raw materials and manufacturing process, and it does not usually change unless the catalyst type changes. Very high regenerator temperatures can sinter the catalyst matrix, which can show up as a drop in surface area along with an increase in bulk density. A higher density can increase pressure drop across slide valves and improve catalyst circulation. It can also reduce catalyst losses in the regenerator. The 90% confidence limits on ABD are +/- 0.04 gram per cubic centimeter.
The chemical properties section reports amounts of the most common contaminants found on the equilibrium catalyst. Metals present in the feed normally deposit quantitatively on the catalyst, so the level of metals on the ecat depends on the amounts in the feed and the rate of fresh catalyst make up. Metals deposited on the ecat affect both the activity and selectivity of the catalyst.
Copper is present in some feed stocks. It catalyzes both hydrogenation and dehydrogenation reactions, leading to increased production of hydrogen and coke.
Nickel acts similarly to copper, but it is probably the most active dehydrogenation agent on a typical ecat. Nickel deposition increases the production of hydrogen, dry gas, olefins and coke, while it reduces gasoline production. Its effects become noticeable above about 500 ppm on the ecat, and become significant above 1000 ppm. Passivating agents are commonly used to reduce the catalytic effects of nickel.
Vanadium is present in the heavier organometallic compounds in many FCC feeds. It forms a low-melting eutectic which collapses the zeolite and deactivates the catalyst. The deactivation potential becomes noticeable at about 1000 ppm and significant at about 2000 ppm. The dehydrogenation activity of vanadium is about one quarter that of nickel.
Most of the iron on ecat typically comes from equipment scale and not the FCC feedstock. This tramp iron is not chemically active, although the iron present in the feed is active.
The dehydrogenation activity of nickel, vanadium, copper and iron can be expressed as "equivalent nickel" as follows (all values are ppm):
"Equivalent Nickel" = Ni+Cu+V/5+Fe/10
Sodium is usually present as a salt in the feed, most likely as sodium chloride. Changes in the level of sodium in the FCC feed can occur as a result of operational problems at the crude distillation units or desalter. Purchased gas oils which are run straight to the FCC can also contribute to sodium contamination of the ecat. Sodium is a potent poison that deactivates the catalyst almost immediately. An increase in sodium of 0.1 wt % can decrease activity by 1-3 numbers, depending on the individual unit. Sodium in combination with vanadium will increase catalyst deactivation.
The level of carbon on the regenerated catalyst provides an indirect measurement of the coke deposited on the catalyst. This information can provide insight into the reactor-regenerator operation. Higher levels of temperature and oxygen partial pressure in the regenerator promote oxidation, leaving less carbon on the regenerated catalyst. Typical carbon levels on ecat from a full combustion FCC are 0.05-0.15 wt %. Typical levels from a partial burn unit are 0.10-0.40 wt %. Each 0.10 wt % of coke on the regenerated catalyst lowers the effective catalyst activity by about 3 MAT numbers.
The ecat may be analyzed for other properties as required to track the performance of individual units. Results for these tests are included in this section of the ecat sheet. Some of the most common properties are noted below.
Alumina is found in both the zeolite and matrix components of the catalyst. The practical minimum amount of alumina in a low matrix catalyst is about 27 %. As more matrix material is added to the catalyst, the alumina level can increase up to about 55%.
The bulk silica/alumina ratio in the catalyst does not correlate with the Si/AI ratio in the zeolite. And since different catalyst types have different alumina contents, the catalyst alumina content can not be used to infer the relative matrix content of different catalysts.
Rare Earth Oxides (REO)
The rare earth content of a catalyst affects both its activity and its selectivity. As the rare earth content of a catalyst increases, its activity increases.
At the same time, an increase in rare earth content of the zeolite shifts the catalyst product selectivity toward more gasoline and less LPG. An increase in zeolite rare earth content also reduces the gasoline octane potential. An increased rare earth content also increases the coke making tendency of the catalyst. This can show up as an increase in delta coke on the unit, which tends to lower the catalyst circulation rate and increase the regenerator temperature.
It should be noted that the important measure for rare earth is the amount of rare earth on the zeolite, while the quantity reported on the ecat sheet is rare earth on the catalyst. Therefore the rare earth content of different catalyst families (particularly if one contains rare earth which is not in the zeolite) may not be directly comparable. Other potential sources of REO on the catalyst include additives for vanadium trapping, combustion promotion and SOx reduction.
Antimony is an additive used in some refineries to passivate the effects of nickel. The amount of passivation can be tracked by the level of antimony and nickel on the ecat. Typically the antimony level on the ecat is on the order of one third to one half the level of nickel. Antimony passivation can decrease the selectivity of the catalyst for hydrogen and coke production.
Data Sheet Use for Unit Troubleshooting
The information on the equilibrium catalyst data sheet provides an excellent summary of the ecat properties. This information can be a valuable tool to assess unit performance by monitoring trends in the properties of the ecat.
A troubleshooting reference for commercial ecat properties is provided in Tables 2 thru 5 below. Typical commercial values and ranges for these selected properties are provided. For each property listed, possible causes for a change in these properties are also given.