Separation of catalysts

After exiting tlie riser, tlie catalyst enters the reactor. In modern FCC operations, the reactor serves two basic functions as a disengaging space for the separation of catalyst and vapor, and as the housing for the reactor-internal cyclone. 

In most of today s FCC operations, the desired reactions take place in the riser. In recent years, a number of refiners have modified the FCC unit to eliminate, or severely reduce, post-riser cracking. Quick separation of catalyst from the hydrocarbon vapors at the end of the riser is extremely important in increasing the yield of the desired product. The post-riser reactions produce more gas and coke versus less gasoline and distillate. Presently, there are a number of commercially proven riser disengaging systems offered by the FCC licenser designed to minimize the post-riser cracking of the hydrocarbon vapors. 

Purifying detergent alkylates Separation of catalyst sludge by intro- 140... 

Since butyraldehyde has a low boiling point (75 °C) separation of catalyst from both reactants and product is straightforward. Most of the rhodium remains in the reactor but prior to recovery of propene and distillation of crude product the gaseous effluents from the reactor are passed through a demister to remove trace amounts of catalyst carried over in the vapour. This ensures virtually complete rhodium recovery. 

When colloids are involved in catalysis, the separation of catalyst by means of solvent evaporation becomes inappropriate because the agglomeration of particles is then favoured [3]. Soft methods like filtration or centrifugation turn out to be more convenient. 

Of course, homogeneous catalysis of course also has disadvantages. The main problem is the separation of catalyst and product. This is often only feasible for low molecular weight products. The u.se of solvents requires an additional separation step. 

Figure 3.7 (Plate 6) Separation of catalyst from products using a short column of FRPSG (Photograph by Ben Croxtall)...

For example, conversion after two hours reaction time (T=70°C, p(H2/CO) = 50 bar) dropped from 99% in the absence of CO2 to 66.4% at a density d(C02) = 0.35 g mL to 0% at d(C02) = 0.57 g mL The observation of a yellow-orange solid precipitated during the addition of CO2 confirmed the efficient separation of catalyst and substrate. The same separation was induced in the product mixture if CO2 was introduced after the hydroformylation was completed. The mixture of regioisomeric nonanals formed during the reaction was extracted quantitatively with SCCO2 leaving an active hydroformylation catalyst behind in the reactor. 

They can serve as soluble or insoluble carriers for catalytically active metal complexes. Separation of catalysts of this kind can be effected by dialysis, ultrafiltration, simple filtration or sedimentation. 

Separation of catalysts from high-value products such as fine chemicals or pharmaceuticals is often accomplished by precipitating the catalyst from the product solution. Recycling of these catalysts is feasible, provided that they do not decompose. In industry, catalyst recovery by means of catalyst precipitation is applied only in relatively small batch processes. An example of such a process is the production of (—)-menthol (id) in which an Rh-BINAP isomerization catalyst converts the allylic amine substrate into (R)-citronellal (after hydrolysis of the enamine) in high yield (99%) and with high enantioselectivity (98.5% ee). After distillation of the solvent (THF) and product, the catalyst is recovered from the residue by precipitation with -heptane. 

The concept of this biphasic catalysis implies that the organometallic catalyst is soluble in only one phase whereas the reactants and products are confined almost entirely to the other phase. In most cases, the catalyst phase can be reused, and the products and reactants are simply removed from the reaction mixture by decantation. In successful processes involving biphasic catalysis, the advantages of homogeneous catalysis listed above may be realized without the disadvantages of expensive separation of catalysts from products. 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

Catalyst-supporting materials are used to immobilize catalysts and to eliminate separation processes. The reasons to use a catalyst support include (1) to increase the surface area of the catalyst so the reactant can contact the active species easily due to a higher per unit mass of active ingredients (2) to stabilize the catalyst against agglomeration and coalescence (fuse or unite), usually referred to as a thermal stabilization (3) to decrease the density of the catalyst and (4) to eliminate the separation of catalysts from products. Catalyst-supporting materials are frequently porous, which means that most of the active catalysts are located inside the physical boundary of the catalyst particles. These materials include granular, powder, colloidal, coprecipitated, extruded, pelleted, and spherical materials. Three solids widely used as catalyst supports are activated carbon, silica gel, and alumina ... 

Apart from its use for solvation and separation of catalysts, the fluorous phase can also be used advantageously for separation processes during workup. Strategic synthesis planning is facilitated by tagging with fluorous residues to overcome the frequently limiting recovery and purification difficulties [1], As in solid-phase syntheses, an excess of components can be used to drive the reactions to completion. Side products can easily be separated if, for example, only the product is tagged with fluorous alkyl residues and therefore precipitates from the reaction mixture or is extracted with the fluorous phase. 

Heterogeneous chiral catalysts are useful because of the easier separation of catalyst from the product and recovery process than homogeneous chiral catalysts. Heterogeneous chiral catalysts supported on polystyrene-type resin catalyse the highly enantioselective addition of dialkylzincs to aldehydes.17... 

Soluble polymers have attracted recent attention in catalysis and combinatorial chemistry.1 3 When used in catalysis by organometallic compounds, soluble polymer ligands offer the following advantages The reaction is homogeneous in nature and separation of catalysts can be easily achieved by filtration or precipitation. We have now developed a new class of polymer ligands based on fluoroacrylate-arylphosphine copolymers for catalysis in periluorocarbon solvents and supercritical C02 (scC02). 

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