Rough cut cyclone

Nearly every FCC unit employs some type of inertial separation device connected on the end of the riser to separate the bulk of the catalyst from the vapors. Most units use a deflector device to turn the catalyst direction downward. On some units, tlie riser is dhectly attached to a set of cyclones. The term rough cut cyclones (Figure 4-47)... 

Since the mid-1980s, FCC technology licensors and a number of oil companies have employed a number of RTD s to reduce non-selective post-riser cracking reactions. Two general approaches have been used to reduce post riser cracking. The most widely used approach is direct connection of the cyclones to the riser and on to the reactor vapor line. The second approach is quenching the reactor vapors downstream of the riser-cyclones (rough-cut cyclones). 

SWEC offers a reactor quench system rather than a closed cyclone system. Their typical RTD is an external, rough-cut cyclone (see Figure 9-7). The vapors from the rough-cut cyclone enter the reactor vessel. 

The recovered catalyst enters the reactor via an external dipleg. Aside from external rough-cut cyclones, SWEC also offers riser-cyclones, referred to as LD (Linear Disengaging Device), intended to separate catalyst from reactor vapors quicker than conventional cyclones (see Figure 9-7A). 

Reactor disengager rough-cut cyclones should be operated at about 50-55 ft/s ( 15-17 m/s) and primary cyclones at 60-65 ft/s ( 18-20 m/s). Disengager second stage cyclones should operate with an inlet vapor velocity of 60-75 ft/s ( 18-23 m/s). 

Mobil Oil developed and commercialized a "closed cyclone system" in which a rough cut cyclone was directly to the riser termination which dramatically reduced the residence time of hydrocarbon vapors in the dilute phase of the reactor vessel thereby dramatically reducing "post-riser cracking". 

In regenerators, sets of cyclones are operated as pairs in series to give a combined collection efficiency of about 99.998% at an overall pressure drop of about 1 psi. In modem reactors with riser separators, one stage cyclones are used with about a 99.995% collection efficiency and about 0.5 to 0.7 psi pressure drop. The modified API (1985) method can also be used for rough cut cyclones, i.e., cyclones attached to the end of feed risers as riser separators. 

Early fluid-catalyst units employed a bank of many small-diameter cyclones in parallel, but this practice was superseded by the use of a smaller number of large-diameter cyclones (97). These are typically 3 to 5 ft. in diameter and 10 to 15 ft. high, although rough-cut separators with diameters up to 8 ft. have been reported (272). Two stages of cyclones in series are ordinarily used in the reactor. In units without Cottrells, two stages are usually employed also in the regenerator. Three... 

To reduce riser-outlet-to-cyclone-inlet times to a minimum, the reactor cyclones can be directly connected to the riser outlet as shown in Figure 22. Both Mobil (through Kellogg) and UOP offer this type of technology. These are more sophisticated devices than simple rough cut riser cyclones (vapor from rough cuts passes into the reactor dilute phase much like the vapor from a... 

Experience shows that the approximate method (Eq. 3.2.7) comes out surprisingly accurate, even when the cut of the cyclone is far from sharp. This has very practical implications and is often used when one just needs a rough estimate of overall separation efficiency. 

Muschelknautz improved the model of Barth in a nnmber of ways. As we discussed in the previous chapter, he measured friction factors in both cylindrical and conical bodied cyclones and, on basis of this work, accounted for the effects of wall roughness and solids loading npon the cut-point and the pressure drop. We will discuss the latest version of his model for cyclone... 

In order to compute certain key cyclone characteristics, such as the internal spin velocity, vocs, or the particle cut size in the inner vortex core, X50, it is necessary to first compute the gas-phase and total gas-plus-solids wall friction factors, fair and /, respectively. Gas-phase wall friction factors for both cylindrical and conical cyclones as a function of body Reynolds number and relative wall roughness are presented in Fig. 6.1.3. Muschelknautz and Trefz define the cyclone body Reynolds number (compare with Eq. 4.2.8) as ... 

Unlike their gas-solids counterparts, one almost never knows fi om measurement the droplet size distribution feeding vapor liquid cyclones and, quite often, one is often not certain of the quantity of liquid feeding the cyclone. Still, the method presented below should provide some help as it provides a rough method for estimating the cyclone s cut size and, if the liquid loading and its approximate distribution is known, the overall collection efficiency. 

If geometrically similar cyclones or swirl tubes of different sizes are operated at the same inlet velocity, Vrcs and vecs will also be similar. The equation therefore shows that the cut size is roughly proportional to the square root of the vortex finder diameter. Thus, in geometrically similar cyclones, the cut size will be proportional to the square root of the characteristic cyclone dimension, say D. Incidentally, since vecs and Vrcs are proportional to the inlet and outlet velocities, it can be also observed from inspection of Elquation (5.2.1) that the cut size for geometrically similar cyclones is inversely proportional to the square root of any characteristic velocity such as the gas superficial inlet or outlet velocity. 

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