Recovery of catalysts

An increase in the recovery of catalyst fines from the electrostatic precipitator or the tertiary separator... 

The methods reported in these and other patents are plagued by low yields furthermore they normally necessitate the use of high pressure technology. The expensive precious metal catalyst must be recovered and reused. In most cases, selectivity and reaction rates deteriorate when recycled catalyst is used. No reports of adequate recovery of catalyst activity have been found. 

In rhodium hydroformylations, highly efficient separation and recovery of catalyst becomes imperative, because of the very expensive nature of the catalyst. Any loss, by trace contamination of product, leakage, or otherwise, of an amount of rhodium equivalent to 1-2 parts per million (ppm) of aldehyde product, would be economically severe. The criticalness of this feature has contributed to some pessimism regarding the use of rhodium in large hydroformylation plants (63). However, recent successful commercialization of rhodium-catalyzed processes has proved that with relatively simple process schemes losses are not a significant economic factor (103, 104). 

Onium salts, crown ethers, alkali metal salts or similar chelated salts, quaternary ammonium and phosphonium are some of the salts which have been widely used as phase transfer catalysts (PTC). The choice of phase transfer catalysts depends on a number of process factors, such as reaction system, solvent, temperature, removal and recovery of catalyst, base strength etc. 

Perhaps the most viable short-term use for dendritic macromolecules lies in their use as novel catalytic systems since it offers the possibility to combine the activity of small molecule catalysts with the isolation benefits of crosslinked polymeric systems. These potential advantages are intimately connected with the ability to control the number and nature of the surface functional groups. Unlike linear or crosslinked polymers where catalytic sites may be buried within the random coil structure, all the catalytic sites can be precisely located at the chain ends, or periphery, of the dendrimer. This maximizes the activity of each individual catalytic site and leads to activities approaching small molecule systems. However the well defined and monodisperse size of dendrimers permits their easy separation by ultrafiltration and leads to the recovery of catalyst-free products. The first examples of such dendrimer catalysts have recently been reported... 

The difficulty in the recovery of catalysts from unreacted coal and minerals and the poor regenerability of used catalysts forces one to use disposable catalysts, especially in the primary stage. This increases the cost of coal liquefaction considerably. This section reviews the mechanism of catalyst deactivation, design of recoverable catalysts in the primary stage, and catalyst deactivation in the secondary stage. 

Additional washing tests with the peptone-poisoned catalysts showed a similar relationship. As seen in Fig. 15, the washed catalysts showed greatly improved activity compared to the result initially with the peptone-contaminated environment. It appears that the water washing of the catalyst improved the activity equivalent to an order of magnitude reduction in the peptone contamination. However, in this case, there is not a total recovery of catalyst activity to the precontaminated state. This recovery of activity indicates that a significant portion of the catalyst deactivation... 

In Chapter 7 we have already discussed the use of fluorous biphasic systems to facilitate recovery of catalysts that have been derivatized with fluorous ponytails . The relatively high costs of perfluoroalkane solvents coupled with their persistent properties pose serious limitations for their industrial application. Consequently, second generation methods have been directed towards the elimination of the need for perfluoro solvents by exploiting the temperature-dependent solubilities of fluorous catalysts in common organic solvents [42]. Thus, appropriately designed fluorous catalysts are soluble at elevated temperatures and essentially insoluble at lower temperatures, allowing for catalyst recovery by simple filtration. 

The second group of papers (chapters 13-20) discusses the more practical aspects of isobutane alkylation including mixing, reaction variables, computer modeling, recovery of catalyst, and an alternate fuel to alkylate. 

Recovery of catalyst from converted oil. Another way to process the residues is to add hydrogen to effect hydroconversion which avoids the formation of a large quantity of asphalt Solid catalyst is formed afterward by reaction. Membrane filtration is used to separate the converted oil from the catalyst This makes it possible to partially recycle the catalyst to the reactor. Alumina and zirconia membranes with pore diameters ranging from 30 to 600 nm have been tested for this application. The membrane with a pore diameter of 30 nm yields a stable flux and a catalyst retention better than 98% [Deschamps et al., 1989). Concentration polarization is significant and requires a high crossflow velocity and temperature to overcome it. 

U Recovery of Catalysts and Engineered Catalytic Reaction Zone... 

Recovery of catalyst. Many reactors use catalysts to increase the rate of the reaction. Catalysts are usually expensive, and the processes generally include provisions for recovering them from the product stream and recycling them to the reactor. They may be recovered with the unconsumed reactants or recovered separately in special facilities designed for this purpose. 

The mixture of catalyst and cracked-oil vapors passes overhead to cyclone separators for recovery of catalyst. Catalyst thrown to the walls of the cyclones by centrifugal force slides down into the spent-catalyst hopper, while the oil vapors leave through a central outlet pipe and pass to the fractionating tower. The small amount of catalyst powder still present in the oil vapors is recovered in the bottoms from the fractionating tower and can be returned to the reactor, if desired. 

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. 

A modem version of an old route to melamine involves treating the guanidine 184 with 1,1,1,3,3,3-hexamethyldisilazine (185), the latter acting as both catalyst and solvent. After refluxing for five hours at 120-130 °C, there is quantitative yield of melamine (Scheme 34). Good recovery of catalyst is claimed100. 

Catalyst cost Stability and Activity Use and Recovery of Catalyst... 

There are several intermediate separation and recovery operations within the conventional flowsheet, and a great deal of effort has gone into reducing the numbers of equipment items required to minimize feedstock consumption while maximizing recovery of catalyst, solvent, byproducts and energy in the most cost-effective way. For example, COMPRESS PTA incorporates these benefits ... 

In recent years, supported catalysts have become valuable tools for the simplified separation and recovery of catalysts from reaction mixtures. Commonly, the catalysts are attached covalently to a solid support. This covalent attachment of catalysts may lead to a partial loss of efficacy due to the decreased mobility. Alternatively, catalysts can be immobilized by non-covalent bonding through hydrogen bridges, or ionic, hydrophobic or fluorous interactions. Compared to covalent attachments, such non-covalent approaches increase the flexibility in the choice of the support material, reaction conditions and work-up strategies. 

As detailed in this overview, the non-covalent attachment of catalysts on a solid support is an important additional technique for the separation and recovery of catalysts from reaction mixtures. Such non-covalent immobilization strategies bring together a number of advantages of solution-phase chemistry and solid-phase supported chemistry. The catalysts can be separated from reaction mixtures by simple filtration. The pre-catalysts can be prepared and characterized in solution. The underlying principle is partitioning between a solid phase or a supported liquid phase and a liquid reaction phase of different solvating power. 

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