Slurry settler

Early FCC units had soft catalyst and inefficient cyclones with substantial carryover of catalyst to the main column where it was absorbed in the bottoms. Those FCC units controlled catalyst losses two ways. First, they used high recycle rates to return slurry to the reactor. Second, the slurry product was routed through slurry settlers. 

Figure 2-6. FCC Process Unit Including Fractionator and Slurry Settler...

Indicated in Fig. 2 is a representative fluid catalytic cracking unit, comprising ( )a reactor (2) a regenerator (3) the main fractionator (4) an air blower or compressor (3) a spent-catalyst stripper (6) catalyst recovery equipment, including cyclones internal in the reactor and regenerator and slurry settler, and possibly an electrostatic precipitator and (7) a gas-recovery unit. The catalyst used is essentially u specially prepared composite of silica and alumina. 

There will generally be no chemical cleaning involved in the fractionator, cyclone separator, slurry settler, reactor, or regenerator. These areas will have coke, catalyst fines, and heavy sludge that can best be removed by hydrojetting. 

The early experiments on solvent extraction directly from leached pulp were beset with problems such as losses of solvent in the aqueous phase and the formation of emulsions. The use of mixer-settler, pump mixer, and internal mixer-settler type contactors on a laboratory scale (Gil) has demonstrated the feasibility of uranium extraction from desanded slurries with 5-1. )% solids and from high-density slurries with 48-60% percent solids. The deemulsification rate of a synthetic slurry as a function of the temperature of the system and the pH of the slurries (T12) and the effect of extractant entrainment in the aqueous effluent on solvent extraction of uranium from slurries containing more than 40% solids (E6) have been studied. 

This stream cools the vapors and scrubs the remaining catalyst out of the cracked products. Most of the slurry of catalyst in heavy oil withdrawn from the bottom of the tower is recirculated to the top of the disc-and-donut section, while a small portion is withdrawn for recovery of catalyst. The latter stream (slurry return) usually amounts to 3 to 10% of the volume of fresh feed to the reactor. Catalyst concentration in the slurry can be decreased by increasing the rate of withdrawal, and is usually maintained below 0.5 lb./gallon to avoid erosion of slurry pumps and valves. The slurry-return stream may be pumped to a separate settler (e.g., a Dorr thickener or a simple cone-bottom tank) or the settler may be incorporated in the bottom of the fractionating tower (25). About 70% of the heavy oil is removed from the settler as a clarified oil containing less than 0.01 lb. catalyst/gallon. The sludge is diluted with fresh feed and pumped to the reactor to return the catalyst to the system. 

About 10% of the raw rock, comprising silica and the like, does not dissolve. However, this material is readily removed by settling the dilute slurry in a thickener, sometimes aided by coagulants (Fig. 6.6). The clarified solution from the thickeners is then extracted with a C4, C5 alcohol mixture or trialk-ylphosphate in a series of three or more mixer settlers producing a solvent phase rich in phosphoric acid, and a raffinate of calcium chloride brine freed of phosphate. Presence of the calcium chloride salt in the aqueous phase undoubtedly assists in driving the phosphoric acid transfer to the organic phase by making the aqueous phase more polar. 

The oxidation and reduction steps in the RAQ/RAHQ cycle are performed in two separate reactors. A bubble column is applied for the oxidation of the RAHQ, during which HP is produced. For the Pd-catalyzed hydrogenation of the quinones, a slurry, fixed-bed or monolith reactor can be used. After the reactor and L/L settler, a diluted H P-containing water-methanol stream is finally obtained. After the epoxidation step, crude PO is separated and the water-methanol mixture is returned to the HP synthesis process, thus realizing an efficient process integration. 

Any suitable mixer and settler can be chosen for the individual units in a countercurrent leaching system. In those shown in Fig. 17.3 mixing occurs in the feed troughs (known as launders ) leading to each tank and also in the upper parts of the tanks themselves. Rakes B move settled solids to the discharge, and pumps C move the slurry from tank to tank. 

The loss of ammonia was reduced when the dehydration was carried out in a continuous manner. The slurry from the settler of the descaling step was continuously pumped into a reactor vessel fitted with a stirrer, thermometer, and heater in which the phosphates were dehydrated. The overflow was discharged by gravity into a filter or a settler. When this method of dehydration was used on a slurry containing 17% solids in sea water, the product was equivalent in composition to that obtained by the best batch procedures—i.e., with the more concentrated slurries. 

Most PTC reactions are carried out on an industrial scale in the batch mode in mixer-settler arrangements. In view of the reactor design in the liquid-solid-liquid PT-catalyzed reaction, Ragaini and coworkers [147-149] reported the use of fixed-bed reactors with a recycling pump or with a recycling pump and an ultrasonic mixer, and emphasized the importance of effluent recycle concept. Schlunt and Chau [150] reported the use of a cyclic slurry reactor, which allowed the immiscible reactants to contact the catalyst sites in controlled sequential steps. However, for triphase reactions in liquid-liquid systems where... 

---