Solids Refine

The parameterization schemes applied here to the core level shifts in Au—Sn and TiC are crude but, we believe, semiquantitative. Their essentials are estimates of Fermi level shifts, of relaxation, of change in atomic volume, and of interelectronic terms calculated, not for free atoms, but for the solid. Refinement is clearly in order. 

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

Solid phase peptide synthesis does not solve all purification problems however Even if every coupling step m the ribonuclease synthesis proceeded in 99% yield the product would be contaminated with many different peptides containing 123 ammo acids 122 ammo acids and so on Thus Memfield and Gutte s six weeks of synthesis was fol lowed by four months spent m purifying the final product The technique has since been refined to the point that yields at the 99% level and greater are achieved with current instrumentation and thousands of peptides and peptide analogs have been prepared by the solid phase method... 

As indicated in the previous section, the adsorption of a gas by a solid is the outcome of the forces of attraction between the individual molecules of the gas and the atoms or ions composing the solid. These forces have been studied theoretically over a number of decades, and though impressive advances have been made in recent years these remain more in the nature of refinements than of fundamental changes in the ideas themselves. And since. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Uracil exists as the dioxo tautomer (52 R = H) in the solid state on the evidence of refined X-ray analyses in which the positions of hydrogen atoms are determined directly... 

Tanks cool and partially freeze solids form a layer of self-insulation. This complex case, which has been known to occur with heavy hydrocarbons and mixtures of hydrocarbons, has been discussed by Stnhlbarg [Pet. Refiner,. 38, 143 (Apr. 1, 1959)]. The contents in the center of snch tanks have been known to remain warm and liquid even after several years of cooling. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Figure 22-8 shows the features of a horizontal center-fed column [Brodie, Au.st. Mech. Chem. Eng. Tran.s., 37 (May 1979)] which has been commercialized for continuous purification of naphthalene and p-dichlorobenzene. Liquid feed enters the column between the hot purifying section and the cold freezing or recovery zone. Ciystals are formed internally by indirect cooling of the melt through the walls of the refining and recovery zones. Residue liquid that has been depleted or product exits from the coldest section of the column. A spiral conveyor controls the transport of solids through the unit. 

Solvent-Refined Coal (SRC) This processing concept was initiated by the Pittsburgh Midway Coal Mining Co. in the early 1960s. The SRC-I process operating mode is designed to produce a solid fuel for utility applications. Typical operating conditions and product yields for SRC-I are shown in Table 27-14. 

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

In order to calculate the effect of several passes an iterative calculation is needed using the initial profile at each pass to represent Cq. Clearly for the second pass, die concentration profile given in the right-hand side of tire above equation must be used. It is clear that tire partition constant of the impurity between the solid and liquid is the most significant parameter in tire success of zone refining. 

The melt is heated by passing a large elecuical cunent between two electrodes, one of which is tire metal rod to be refined, and the otlrer is the liquid metal pool standing in a water-cooled copper hearth, which collects the metal drops as tlrey fall tluough the molten electrolyte. This pool tlrerefore freezes at the bottom, forming the ingot. Under optimum chcumstances tire product billet takes the form of a cylindrical solid separated from the molten salt by... 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

Fig. 4.4. Stages in zone refining o bar of impure silicon (a) We start with a bar that has a uniform concentration of impurity, Q. (b) The left-hand end of the bar is melted by o small electric tube furnace, making a liquid zone. The bar is encapsulated in a ceramic tube to stop the liquid running away. ( ) The furnace is moved off to the right, pulling the zone with it. (d) As the zone moves, it takes in more impurity from the melted solid on the right than it leaves behind in the freshly frozen solid on the left. The surplus pushes up the concentration of impurity in the zone, which in turn pushes up the concentration of impurity in the next layer of solid frozen from it. (e) Eventually we reach steady state, (f) When the zone gets to the end of the bar the concentrations in both solid and liquid increase rapidly, (g) How we set up eqn. (4.1).

The next stage in the zone-refining process is to move the furnace slowly and steadily to the right. The left-hand end of the bar will then cool and refreeze but with the equilibrium composition /cCq (Fig. 4.4c). As the furnace continues to move to the right the freezing solid, because it contains much less impurity than the liquid, rejects the surplus impurity into the liquid zone. This has the effect of inereasing the impurity concentration in the zone, which in turn then increases the impurity concentration in the next layer of freshly frozen solid, and so on (Fig. 4.4d). Eventually the concentrations ramp themselves up to the situation shown in Fig. 4.4(e). Flere, the solid ahead of the zone has exactly the same composition as the newly frozen solid behind the zone. This means that we have a steady state where as much impurity is removed from the... 

Figure 4.6 is interesting because it shows that for the best refining performance we need both a long zone and an impurity that is relatively insoluble in the solid (low k). Unfortunately long liquid zones can be destabilised by convection, and impurities with a low k do not come to order Commercial zone refining processes may therefore involve a large number of passes done one after the other (Fig. 4.7). This obviously adds a lot to the cost of the pure material, but the process can be speeded up considerably by using the multi-heater arrangement shown in Fig. 4.8.

Describe, using the copper-nickel ("monel") system as an example, the process of zone-refining. (Figure A1.51 shows a system with complete solid solubility.) Flow many phases are present in an alloy of 60 wt% Ni and 40 wt% Cu at ... 

Cs = concentration of impurities in refined solid Cq = average impurity concentration k = Csoua /Ciiquid, X = distance from start of bar / = zone length. 

Essentially, carbonization entails the heating of organic precursors in the absence of air. In so doing, a solid carbon residue along with gaseous and volatile hydrocarbons is created. Bituminous coals are used to make metallurgical-grade coke while wood and other similar substances make charcoal. The condensed volatile material can be further refined to yield chermcals, pitches, or other useful commodities. 

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