Catalyst density

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

A fluidized catalyst behaves like a liquid. Catalyst flow occurs in the direction of a lower pressure. The difference in pressure between any two points in a bed is equal to the static head of the bed between these points, multiplied by the fluidized catalyst density, but only if the catalyst is fluidized. 

The standpipe s height provides the driving force for transferring the catalyst from the regenerator to the reactor. The elevation difference between the standpipe entrance and the slide valve is the source of this pressure buildup. For example, if the height difference is 30 feet (9.2 meters) and the catalyst density is 40 Ib/ft (641 kg/m ), the pressure buildup is ... 

Assuming a stripper with a 20-ft bed level and a catalyst density of 40 Ib/ft, the static pressure is ... 

Reactor dilute phase (dome) pressure Reactor catalyst dilute phase bed level Reactor-stripper catalyst bed level Reactor-stripper catalyst density Spent catalyst standpipe elevation Pressure above the spent catalyst slide valve Spent catalyst slide valve AP ( 55% opening)... 

Catalyst density in the regenerator dense phase = 25 lb/ftV400 kg/m ... 

The pressure balance survey indicates that neither the spent nor the regenerated catalyst standpipe is generating optimum pressure head. This is evidenced by the low catalyst densities of 20 Ib/ft (320 kg/m ) and 25.4 Ib/ft (407 kg/m ), respectively. As indicated in Chapter 8, several factors can cause low pressure, including under or over ... 

In the regenerated catalyst standpipe, a 40 Ib/ft (640 kg/m ) catalyst density versus a 25.4 Ib/ft (407 kg/m ) density produces 3 psi (20,7 Kj,) more pressure head, again allowing an increase in circulation or a reduction in the regenerator pressure (gaining more combustion airi... 

Hopper entrance diameter Angie of cone Desired catalyst density Catalyst velocity... 

To retain fluidity of the catalyst and to maintain catalyst densities in the 35 to 45 Ib/ft (560-720 kg/m ) range (the fluid range), many standpipes require external aeration gas to be injected into the down-flowing... 

The pressure drop in the Y or J-bend section could be from improper fluidization or a flaw in the mechanical design. There are often fluffing gas distributors in the bottom of the Y or along the J-bend that are designed to promote uniform delivery of the cataly.st into the feed nozzles. Mechanical damage to these distributors or too little or too much fluffing gas affect the catalyst density, causing pressure head downstream of the slide valve. 

High pressure in the riser could also be due to insufficient fluidization gas in the base of the riser. Fluffing gas will vary the catalyst density more fluffing gas lowers the density in the system and the backpressure on the slide valve. 

Conduct a single-gauge pressure survey of the reactor-regenerator circuit. Using the results, determine the catalyst density profile. 

However, the intrinsic pseudohomogeneous rate used in Equation (10.39) is not identical to the rate determined from the CSTR measurements since the catalyst density will be different. The correction procedure is... 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

We first consider the stmcture of the rate constant for low catalyst densities and, for simplicity, suppose the A particles are converted irreversibly to B upon collision with C (see Fig. 18a). The catalytic particles are assumed to be spherical with radius a. The chemical rate law takes the form dnA(t)/dt = —kf(t)ncnA(t), where kf(t) is the time-dependent rate coefficient. For long times, kf(t) reduces to the phenomenological forward rate constant, kf. If the dynamics of the A density field may be described by a diffusion equation, we have the well known partially absorbing sink problem considered by Smoluchowski [32]. To determine the rate constant we must solve the diffusion equation... 

Stoichiometric coefficient for component i in reaction j. Catalyst density [gm/cnu]. 

In the recent past, commercial cracking catalysts have tended to be high in density (low in pore volume) in order to reduce attrition and avoid air pollution problems, which tend to relate to catalyst density. Catalysts for FCC application are not considered to be diffusion limited and, therefore, higher-density catalysts are quite acceptable. 

Assuming a catalyst density at flowing conditions in the standpipe of about 90% of the catalyst bulk density, the amount of excess gas above minimum fluidization that is entrained with the catalyst into the standpipe may be calculated. Sufficient aeration should be added to sustain minimum fluidization along the length of the standpipe. 

The circulating catalyst physical properties have a direct impact on fluidization and stable standpipe operation. Mechanical problems may cause a loss of catalyst fines, or a change in catalyst density both of which will impact fluidization and may require adjustment to the standpipe aeration. 

Overaeration of an unstable standpipe is a common response to process or catalyst changes not easily recognized. Factors influencing standpipe operation such as catalyst mass flux rate, catalyst density and PSD, standpipe aeration and pressure profile, and unit configuration should be thoroughly evaluated before making large adjustments to the aeration. 

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

Rh catalyst density (ppi) Pore diameter (pm) Surface area ( rnjg) Back-face temp (°C) Selectivity (%) ... 

Where pp is the catalyst density (kg/m3), dp is the catalyst particle size (in m) and dB is the bubble size (in m). To calculate the latter, the bubble hold-up, sg, is required. Both are usually estimated from the working regime of the impeller we will discuss this later. The above equations assume that catalyst particles and bubbles are spherical. 

Density of Steam Deactivated vs. Fresh Catalyst. The results of density measurements on the fresh parent of Catalyst A and on a portion of this catalyst that has been steam deactivated at 815°C for five hours (Table VIII) indicate a direct connection between the loss of crystalline microporous zeolite and increase in catalyst density. The major portion (67%) of the fresh catalyst is found in the density range 2.330 < d < 2.355 g/cc. By contrast, the major portion (87%) of the steam deactivated catalyst is found in the density range 2.372 to 2.394 g/cc. 

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