Eutectic columns

FIGURE 23. Gas chromatograms of (a) a whole crude oil from the Altamont Bluebell Field, Unita Basin, and (b) saturated hydrocarbons from a Canadian Arctic Island sedimentary rock, GC on eutectic column ... 

The reaction of chlorine gas with a mixture of ore and carbon at 500—1000°C yields volatile chlorides of niobium and other metals. These can be separated by fractional condensation (21—23). This method, used on columbites, is less suited to the chlorination of pyrochlore because of the formation of nonvolatile alkaU and alkaline-earth chlorides which remain in the reaction 2one as a residue. The chlorination of ferroniobium, however, is used commercially. The product mixture of niobium pentachloride, iron chlorides, and chlorides of other impurities is passed through a heated column of sodium chloride pellets at 400°C to remove iron and aluminum by formation of a low melting eutectic compound which drains from the bottom of the column. The niobium pentachloride passes through the column and is selectively condensed the more volatile chlorides pass through the condenser in the off-gas. The niobium pentachloride then can be processed further. 

Center-fed columns can he adapted for hoth eutectic and solid-solution systems. End-fed columns are inefficient for separation of solid-solution systems. 

Most investigators have focused their attention on a differential segment of the zone between the feed injection and the crystal melter. Analysis of crystal formation and growth in the recoveiy section has received scant attention. Table 22-4 summarizes the scope of the literature treatment for center-fed columns for both solid-solution and eutectic forming systems. 

Column crystalhzers of the end-fed type can be used for purification of many eutectic-type systems and for aqueous as well as organic systems (McKay loc. cit.). Column ciystaUizers have been used for xylene isomer separation, but recently other separation technologies including more efficient melt ciystaUization equipment have tended to supplant the Phillips style ciystaUizer. 

As described in Section 15.2.1, eutectic systems can be purified in theory by single-stage crystallisation, whereas solid solutions always require multistage operations. Countercurrent fractional crystallisation processes in column crystallisers are described in Section 15.4.3. 

Figure 7,8 Gibbs free energy curves and T-X phase relations for an intermediate compound (C), totally immiscible with pure components. Column 1 Gibbs free energy relations leading to formation of two eutectic minima separated by a thermal barrier. Column 2 energy relations of a peritectic reaction (incongruent melting). To facilitate interpretation of phase stability fields, pure crystals of components 1 and 2 coexisting with crystals C are labeled y and y", respectively, in T-X diagrams same notation identifies mechanical mixtures 2-C and C-1 in G-X plots.

For the separation of benzene and thiophene that form a solid solution, a tray efficiency of more than 40% could be realized. Flow rates of 100-1000 kg/m2 hr have been tested. The residence time of crystals was about 30 min per stage. Eutectic systems also have been handled satisfactorily. A column 500 mm dia and 3 m long with 19 trays has been built it is expected to have a capacity of 300 tons/yr. 

The liquid eutectic goes to a distillation column where it is separated into fractions at a pressure in the receiver of about 10 mm Hg and at 180°C at the bottom of the column the temperature in the dephlegmator is 100-120°C, The first fraction. 

Separation of isomeric nitrotoluenes, chloronitrobenzenes, etc. is easily accomplished with columns of this type. Some mixtures, whose constituents have boiling points very close to each other, can be fractionated provided that the substances crystallize so that the pure compounds can be separated from the eutectic mixture by centrifuging. An example is the mixture of nitrochlorobenzenes, described on page 90. 

The experimental apparatus used for our column studies consisted of a 3/4-inch i.d. by 36-inch glass column filled with Na-Y sieve containing about 12 wt % water. The DMN heart-cut containing eutectic 2,6-DMN and 2,7-DMN is pumped into the bottom of the column. At the end of this DMN charge, desorbent is pumped into the bottom of the column. The desorbent pushes out a 2,6-DMN enriched raffinate and desorbs the 2,7-DMN enriched adsorbate. When the last of the adsorbate is removed from the bottom portion of the sieve bed, the column is ready... 

Kennedy and Buse used a LiC104-LiC103 eutectic mixture as a molten salt phase in a GLC column at 280 °C for rearrangement of a few model epoxides. Those with tertiary centers gave lower product selectivity than in the typical solution reaction, but the harsher conditions did cause cyclohexene oxide to rearrange, giving cyclopentanecarbaldehyde as the major product. 

---