Catalyst attrition rate

The improvements in the catalyst s binder properties will reduce the catalyst attrition rate thus, lowering the flue gas stack opacit This improvement allows refiners to use a harder catalyst without adversely affecting the catalyst s fluidization properties. 

The decomposition of the catalyst beads can cause a secondary air pollution emission consisting of the particulate dust generated by abrasion of the surface of the catalyst. Operating cost for catalyst replacement varies directly with catalyst attrition rate. The system can process waste streams with VOC concentrations of up to 25% of the lower explosive limit (LEL). The proprietary catalyst contains up to 10% chromium, including 4% hexavalent chromium. This could lead to the emission of hexavalent chromium in some applications of the technology. 

Catalyst-attrition rates as low as 0.7 ton/day for each 100 tons/hr. of circulation have been reported with bead catalyst (23b). In a 15,000-barrel/day unit, with a catalyst-circulation rate in the range of 270 to 350 tons/hr., this is equivalent to about 0.3 Ib./barrel of feed. 

Much effort has been made by catalyst manufacturers to improve catalyst atttition resistance and thus reduce the formation of fines (see Catalysts, supported). In the 10-year petiod from 1980 to 1990, most catalyst manufacturers improved the atttition resistance of their catalyst by a factor of at least 3—4. This improvement was achieved even though the catalyst zeoHte content duting this petiod was continually increasing, a factor that makes achieving catalyst hardness more difficult. As an example of the type of atttition improvement that has been achieved, the catalyst atttition index, which is directiy related to catalyst loss rate in a laboratory attrition test, decreased from 1.0 to 0.35 for one constant catalyst grade during 1989—1990 (37). 

The concentration of the ZSM-5 additive should be greater than 1% of the catalyst inventory to see a noticeable increase in the octane. An octane boost of one research octane number (RON) will typically require a 2% to 5% ZSM-5 additive in the inventory. It should be noted that the proper way of quoting percentage should be by ZSM-5 concentration rather than the total additive because the activity and attrition rate can vary from one supplier to another. There are new generations of ZSM-5 additives that have nearly twice the activity of the earlier additives. 

There is no general correlation available to date to predict the steady state attrition rates for various materials. Zenz and Kelleher (1980) gave a simple correlation to predict steady-state attrition rates for FCC catalyst and glass beads. This is an empirical dimensional equation as given by ... 

The following table gives the attrition-rate constant (Ka) for FCC catalyst as a function of particle size range (0-d) for upwardly-directed jets. 

The attrition-rate constant for 0-50 micron FCC catalyst (from Table) is... 

A lot of attempts have been made to describe the time dependence of the attrition rate in batch fluidized bed processes. Gwyn (1969) studied the degradation of catalysts in a small-scale test apparatus and defined the elutriated particles as the only attrition product. He described the increase of the elutriated mass, Wel, with time, t, based on the initial solid bed mass, Wbed 0, by the now widely known Gwyn equation ... 

Special attention has to be paid to a definition of attrition rates in the case of continuous processes where fresh solid material is continuously added. This is particularly the case in heterogeneously catalyzed fluidized bed processes where fresh make-up catalyst must be added to compensate for attrition losses. The fresh catalyst may contain elutriable fines which add to the measurable elutriation rate thus leading to an apparently higher attrition rate. 

Friability tests can be used for various purposes. They are widely used in quality control. Here, samples of produced material are subjected to a more or less arbitrary but well defined stress. The attrition extent is assessed by comparison with a standard value and a decision is reached whether the material meets the standard. Moreover, friability tests are often used for comparison of different materials to select the most attrition-resistant one. This is a usual procedure in the case of catalyst development. For example, Contractor et al. (1989) tested anew developed fluidized bed VPO-catalyst in a submerged-jet attrition test (described below). Furthermore, the specific attrition rate of a material in a certain process can be roughly estimated by friability tests. In this case the stress must be similar to that occurring in the process and the obtained degradation extent must be compared with those of other materials from which the process attrition rate is known. 

Modeling of Jet-Induced Attrition. Werther and Xi (1993) compared the jet attrition of catalysts particles under steady state conditions with a comminution process. They suggested a model which considers the efficiency of such a process by relating the surface energy created by comminution to the kinetic energy that has been spent to produce this surface area. The attrition rate, RaJ, defined as the mass of attrited and elutriated fines per unit time produced by a single jet, is described by... 

Figure 9. Influence of the jet velocity uor on the jet attrition rate of catalysts (dor = 2 mm. (Werther andXi, 1993.)...

Figu re 13. Variation of the steady-state attrition rate of a catalyst with superficial gas velocity fAVN 802, Ug nif = 0.002 m/s. (Xi, 1993.)... 

Kokkoris et al. (1991, 1995) suggested another method to impair catalyst attrition. In a small scale slugging bed they reduced the attrition rate of zeolites by addition of only very small quantities of various fine solid lubricants. In the case of graphite the reduction was up to 30%. The fines were assumed to reduce attrition by forming a protective coating that... 

The choice and properties of the aeration gas are important factors for maintaining stable standpipe operation. The condensate source for steam aeration can cause several problems. If the steam is not kept dry, the condensate can lead to stress cracking of the tap piping, plugging of the tap nozzle with mud, erratic aeration rates, orifice erosion, and potentially catalyst attrition. Similar problems can occur with wet fuel gas as an aeration source. When possible, dry air and/or nitrogen are preferred rather than steam as aeration media for standpipes. However, in actual... 

The attrition rate constant K describes in the first place material properties which may be influenced by, for example, the manufacturing process of the catalyst. The attrition rate also depends on whether the jet issues into a prefluidized bed or into a nonaerated bed. The attrition effect of an upward jet equals that of a horizontal jet, whereas the attrition effect of a downward jet is significantly higher. For the prediction of the attrition effect of a multihole gas distributor, a model has been developed that is based on a single jet attrition measurement [62]. 

To achieve these criteria, we needed to establish standard processing and characterization procedures for FCC catalysts. In particular, a process for making microspheres of controlled size distribution and shape, independent of composition, had to be defined. Also, an approach for obtaining the intrinsic attrition rate of commercial grade and experimental catalysts had to be adapted from a method for alumina. This paper describes these methods and shows that the substitution of CP alumina for part of the clay in a commercially viable FCC formulation can improve attrition behavior and enhance catalytic activity, especially in the presence of Ni+V poisoning. 

Response Curves for two commercial and three experimental catalysts are shown in Figure 5. Inspection shows that the period of near linear response was from 45 through 75 minutes. This linear response appears to be associated with a period when the dynamics of the bed are in equilibrium. This behavior demonstrates that the maximum attrition time for this class of samples could be reduced to 75 minutes. The shape of the Response Curve and the rate the collector fines were generated (as represented by the slope of the near linear portion) have the potential to be correlated with the catalyst makeup rate in commercial applications. 

As mentioned in above, the catalytic decomposition reaction S03(g) —> 802(g) + 02(g) is carried out at 850°C and above in order to obtain an appreciable conversion rate. The reaction is greatly enhanced by the application of a catalyst. However, catalysts have been observed to fail due to formation of volatile acid, which causes support poisoning and catalyst attrition at this temperature. If an optimum catalyst can be identified, the reaction temperature can be lowered. 

Figure 17.28. Reactors with moving beds of catalyst or solids for heat supply, (a) Pebble reactor for direct oxidation of atmospheric nitrogen two units in parallel, one being heated with combustion gases and the other used as the reactor (Ermenc, (1956). (b) Pebble heater which has been used for making ethylene from heavier hydrocarbons (Batchelder and Ingols, 1951). (c) Moving bed catalytic cracker and regenerator for 20,000 bpsd the reactor is 16 ft dia, catalyst circulation rate 2-7 Ibs/lb oil, attrition rate of catalyst 0.1-0.5 Ib/ton circulated, pressure drop across air lift line is about 2psi (L. Berg, in Othmer, 1956).

Each of the first units at Toledo was originally equipped with a single 19-in. catalyst-lift pipe, but it was later replaced by seven 8-in. pipes in parallel (62). This change reduced the catalyst attrition losses, at 200 tons/hour circulation rate, from 3.5-5.0 tons/day to 1.6-2.5 tons/day, the exact magnitudes depending upon the type of catalyst (43,62). In some units, a tapered decelerating section is employed at the top of the lift pipe (43). 

Attrition resistance Attrition rate <0.1%/hr Minimize catalyst consumption, maintain particle size distribution and particle shape... 

In response to this limitation, several processes have been developed that suppress catalyst deactivation through the use of slurry reactors, catalytic distillation, and an FCC-style fluidized bed with constant catalyst regeneration (6-8). These reactors achieve high I/O ratios and lower catalyst deactivation rates, but at a cost. Fluidized beds, for example, require specialized, attrition-resistant catalyst and extensive filtration equipment. 

---