Impurity effects, feed

The previous discussion has been concerned with impurities In feed streams and their effects on acid make-up requirement. 

The fourth option, shown in Fig. 4.4FEED-IMPURITY separation is expensive. To use a purge, the FEED and IMPURITY must be adjacent to each other in order of volatility (again, assuming distillation as the means of separation). Care should be taken to ensure that the resulting increase in concentration of IMPURITY in the reactor does not have an adverse effect on reactor performance. 

Only trace amounts of side-chain chlorinated products are formed with suitably active catalysts. It is usually desirable to remove reactive chlorides prior to fractionation in order to niinimi2e the risk of equipment corrosion. The separation of o- and -chlorotoluenes by fractionation requires a high efficiency, isomer-separation column. The small amount of y -chlorotoluene formed in the chlorination cannot be separated by fractionation and remains in the -isomer fraction. The toluene feed should be essentially free of paraffinic impurities that may produce high boiling residues that foul heat-transfer surfaces. Trace water contamination has no effect on product composition. Steel can be used as constmction material for catalyst systems containing iron. However, glass-lined equipment is usually preferred and must be used with other catalyst systems. 

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

Parallel feed involves the introduc tion of raw feed and the withdrawal of produc t at each effect of the evaporator. It is used primarily when the feed is substantiallv saturated and the product is a sohd. An example is the evaporation of brine to make common salt. Evaporators of the types shown in Fig. 11-122Z or e are used, and the produc t is withdrawn as a shiny. In this case, parallel feed is desirable because the feed washes impurities from the salt leaving the body. 

Performance information for the purification of p-xylene indicates that nearly 100 percent of the ciystals in the feed stream are removed as produc t. This suggests that the liquid which is refluxed from the melting section is effectively refrozen oy the countercurrent stream of subcooled crystals. A high-meltingproduct of 99.0 to 99.8 weight percent p-xylene has been obtained from a 65 weight percent p-xyfene feed. The major impurity was m-xylene. Figure 22-12 illustrates the column-cross-section-area-capacity relationship for various product purities. 

The circulating catalyst in the FCC unit is called equilibrium catalyst, or simply E-cat. Periodically, quantities of equilibrium catalyst are withdrawn and stored in the E-cat hopper for future disposal. A refinery that processes residue feedstocks can use good-quality F-cat from a refinery that processes light sweet feed. Residue feedstocks contain large quantities of impurities, such as metals and requires high rates of fresh catalyst. The use of a good-quality E-cat in conjunction with fresh catalyst can be cost-effective in maintaining low catahst costs. 

In other instances, reaction kinetic data provide an insight into the rate-controlling steps but not the reaction mechanism see, for example, Hougen and Watson s analysis of the kinetics of the hydrogenation of mixed isooctenes (16). Analysis of kinetic data can, however, yield a convenient analytical insight into the relative catalyst activities, and the effects of such factors as catalyst age, temperature, and feed-gas impurities on the catalyst. 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

Some deactivation processes are reversible. Deactivation by physical adsorption occurs whenever there is a gas-phase impurity that is below its critical point. It can be reversed by eliminating the impurity from the feed stream. This form of deactivation is better modeled using a site-competition model that includes the impurities—e.g., any of Equations (10.18)-(10.21)— rather than using the effectiveness factor. Water may be included in the reaction mixture so that the water-gas shift reaction will minimize the formation of coke. Off-line decoking can be... 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

Multiple reactions also can occur with impurities that enter with the feed and undergo reaction. Again, such reactions should be minimized, but the most effective means of dealing with byproduct reactions caused by feed impurities is not to alter reactor conditions but to carry out feed purification. 

Other options are possible. If the process produces a byproduct of reaction, then this can be recycled, provided it does not have an adverse effect on the reactor performance. If the feed enters with an impurity, then the impurity could also be recycled as a heat carrier, provided it too does not have an adverse effect on the reactor performance. 

Flotation properties of bastnaesite depend largely on the gangue composition of the ore and the impurities present in the mineral itself. Bastnaesite found in a carbonatite ore is recovered using fatty acid collector after heat pretreatment of the flotation feed. The effect of heat temperature on bastnaesite grade-recovery is illustrated in Figure 24.3. 

As was mentioned previously, an effective system, RNDS , has been developed to remove particular impurities from brine used in membrane electrolysis procedures. The basic concept of RNDS is to bring the feed brine into contact with an ion-exchange resin containing zirconium hydroxide for the adsorptive removal of impurities. For the removal of the sulphate ion from brine, commercial plants utilising RNDS are already in service. For the elimination of iodide and silica, pilot-scale testing is being planned. 

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