Closed cyclones

Mobil started installing closed cyclone systems in its FCC units. 

Riser termination. Good riser termination devices, such as closed cyclones, minimize the vapor and catalyst holdup time in the reactor vessel. This reduces unnecessary thermal cracking and nonselective catalytic re-cracking of the reactor product. The benefits are a reduction in dry gas and a subsequent improvement in conversion, gasoline octane, and flexibility for processing marginal feeds. 

In KBR closed cyclone technology, each set of riser and upper reactor cyclones is connected via the use of a slip joint conduit. The stripper steam and hydrocarbons, as well as dome steam, exit the reactor housing by entering through this conduit as shown in Figure 9-5. 

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. 

FIG U RE 7.3 Closed cyclone system. (Reprinted from Jack Wilcox, R., Published in Petroleum Technology Quarterly, Troubleshooting Complex FCCU Issues, http //www.ePTQ.com, Q3, 2009. With permission.)... 

In the process, a residuum is desulfurized and the nonvolatile fraction from the hydrodesulfurizer is charged to the residuum fluid catalytic cracking unit. The reaction system is an external vertical riser terminating in a closed cyclone system. Dispersion steam in amounts higher than that used for gas oils is used to assist in the vaporization of any volatile constituents of heavy feedstocks. 

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". 

Challenges, 6th. International Conference on Fluidization, Banff, Alberta, May 7-12 (1989). Avidan, A.A., F. J. Krambeck, H. Owen and P. H. Schipper, FCC Closed Cyclone System Eliminates Post-Riser Cracking, NPRA Annual Meeting, San Antonio, Texas, March 25-27 (1990). 

Avidan A.A., foambeck, F.J., Owen, H. and Schipper P.H., "FCC closed-cyclone system eliminates post-riser cracking", Oil and Gas J., Mar 26, 56-62 (1990b). 

The diameter of the air core varies with the feed volumetric flow rate. If the rate is too low, there is no air core and all of the pulp leaves the cyclone as underflow if the rate is too high, the air core expands, closing off the apex and forcing all of the pulp to leave the cyclone as overflow. Consequently there is a minimum and maximum volumetric feed rate. Because the pressure drop is proportional to the square of the volumetric feed rate, the minimum and maximum rates can be monitored by the pressure drop. The ratio of the maximum pressure drop to the minimum pressure drop should be less than 4, meaning the maximum to minimum volumetric feed rate should be less than 2. 

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Ball mills or tube mills can be operated in closed circuit with external air classifiers with or without air sweeping being employed. If air sweeping is employed, a cyclone separator may Be placed between mill and classifier. (The principles of size reduction combined with size classification are discussea under Characteristics of Size Classifiers. ) Likewise other types of grinding mill can be operated in closed circmt with external size classifiers (Fig. 20-12), as will be described at appropriate places on succeeding pages. However, many types of grinders are air-swept and are so closely coupled with their classifiers mat the latter are termed internal classifiers. 

FIG. 20-45 Closed -circuit wet grinding with liquid-solid cyclone. [Tonty, Pit Quarry (Fehtuaty-March L95.9).]... 

Figures 20-43 to 20-44 illustrate single-stage and two-stage closed-circuit wet-grinding systems. The circuits or Fig. 20-59 may also be used as a closed-circuit wet-grinding system incorporating a hquid-solid cyclone as the classifier.

Cyclone Separator with Separate Catch Tank This type of blowdown system, shown in Fig. 26-17 and 26-18, is frequently used in chemical plants where plot pan space is hmited. The cyclone performs the vapor-liquid separation, while the catch tank accumulates the hquid from the cyclone. This arrangement allows location of the cyclone knockout drum close to the reactor so that the length of the relief device discharge hne can be minimized. The cyclone nas internals, vital to its proper operation, which will be discussed in the following sections. 

Cyclonic filters (and closely related designs such as U-tubes) are employed as an initial gas cleanup step in most gasifier systems because they are effective and relatively inexpensive to operate. In circulating fluidized-bed or entrained-bed gasifiers, cyclones are an integral part of the reactor design, providing for separation of the bed material and other particulates from the gas stream. 

At the bottom of the vortex, there is substantial turbulence as the gas flow reverses and flows up the middle of the cyclone into the gas outlet tube. As indicated above, if this region is too close to the wall of the cone, substantial reentrainment of the separated solids can occur. Therefore, it is very important that cyclone design take this into account. 

The vortex of a cyclone will precess (or wobble) about the center axis of the cyclone. This motion can bring the vortex into close proximity to the wall of the cone of the cyclone and pluck off and reentrain the collected solids flowing down along the wall of the cone. The vortex may also cause erosion of the cone if it touches the cone wall. Sometimes an inverted cone or a similar device is added to the bottom of the cyclone in the vicinity of the cone and dipleg to stabilize and fix the vortex. If it is placed correctly, the vortex will attach to the cone and the vortex movement will be stabilized, thus minimizing the efficiency loss due to plucking the solids off the wall and erosion of the cyclone cone. 

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