TECHNIQUE 9 Sublimation

In addition, the sublimation of ice in freeze-drying, discussed in Chapter 16, has become an important operation particularly in the biological and food industries. The various industrial applications of sublimation techniques are discussed by several authors(3,40 95 96 97), and the principles underlying vaporisation and condensation and the techniques for growing crystals from the vapour phase 99-1 are also presented in the literature. 

Lemberger, A.P., Higuchi T., Busse, L.W., Swintowsky, J.V., and Wurster, D.E., Preparation of some steroids in microcrystalline form by rapid freeze-sublimation technique, /. Am. Pharm. 

Amorphous Films. An amorphous film is generally prepared by solvent evaporation of a dry organic solution of Chi on a solid substrate surface. The vacuum sublimation technique, which is widely employed for most synthetic dyes, is not applicable to Chi due to possible thermal degradation of the pigment. 

Symmetrically substituted chloro-[184,185] and phenoxy-substituted [186] Pcs have been organized on Ag(l 11) and Au(l 11) by sublimation techniques. It has been demonstrated that in the case of the alkoxy-substituted Pc, the rotational degrees of the bulky phenoxy substituents imposes a bowl-like structure to the adsorbed Pc units, which in turn enables the interaction of the macrocycle core with the metal surface. 

AIN thermal conductivity was measured by Slack et al [12], using AIN single crystals obtained by the sublimation technique of Slack and McNelly [21]. The results of these measurements and the theoretical estimates of Slack are listed in TABLE 4. 

Solution growth utilises the ability of a solvent, such as Fe or Ca3N2, to take a solute into solution at a reduced temperature and precipitate the solute at an even lower temperature. The solution is contained in a crucible, and like the sublimation technique, the crucible generally fails during the lengthy growth experiments required to obtain crystals of appreciable size. 

Both Fe(C0)5 and (C I Fe were introduced Into several zeolites by sublimation techniques employing a vacuum line. The zeolite and either of the two above-mentioned iron compounds were introduced into two separate ends of an inverted U-tube which was connected to a vacuum line. The iron compounds were frozen with... 

Besides phthalocyanine pigments developed for office copiers, new polymorph materials for laser printer utilization have been prepared. These materials, whose absorption characteristics were extended to the near-infrared region, were prepared by vacuum sublimation techniques. Recently, for safety reasons and to reduce the cost of production, solvent-pigment interactions have received great attention [41]. 

Figure 9 NOx reduction efficiency of overexchanged Fe-MFI catalysts prepared by using ( ) a conventional ion exchange in an aqueous slurry(9,0,M) ci sublimation technique and (A,A) a solid-state ion exchange. The feed gas composition for (A,9,0,M) was NO, 2,000 ppm i-C4Hio, 2,000 ppm O2, 3% H2O 0 or 10% GHSV = 42,000 k, and that composition for (A, A) was NO, 1,000 ppm i-C Hu), 1,000 ppm O2,3% H2O 0 or 0.7% GHSV = 30,000 k. (Closed symbols) in the dry feed stream (open symbols) in the wet feed stream (adopted from refs. 55,136 and 139)...

Figure 22 Hydrothermal durability of Co-MFI-226 prepared by sublimation technique.

Distillation or sublimation techniques are commonly used to purify substances. For example, pure water as solvent is required in copious amounts the removal of many impurities is best accomplished by distillation. Iodine as a standard substance is best purified by sublimation. An important method for concentrating trace elements from water prior to analysis is by the technique of freeze-drying. 

The design of the precursor as well as the pyrolysis technique have to be optimized to obtain satisfactory selectivities and yields in the thermal conversions. The starting material has to be stable enough to be transferred into the gas phase without decomposition at temperatures up to 400 °C (depending on the vapor pressure and therefore on the molecular weight and structure) but also to possess an inherent reactivity which can be exploited to yield the desired products with high selectivities and yields under conditions as mild as possible. Thus, enthalpies of activation of 200-400 kJ mol 1 [31, 32] are representative for pyrolyses as synthetic procedures with moderate or high yields. Numerous special techniques have been developed to transfer thermally labile products into the gas phase (spray pyrolysis [33], solvent assisted sublimation techniques... 

Computer modeling of convection has had mixed success. Many convection problems, particularly those involving laminar flow, can readily be solved by special computer programs. However, in situations where turbulence and complex geometries are involved, computer analysis and modeling are still under development. Mass transfer analogies can play a key role in the study of convective heat transfer processes. Two mass transfer systems, the sublimation technique and the electrochemical technique, are of particular interest because of their convenience and advantages relative to direct heat transfer measurements. 

A comprehensive review of the naphthalene sublimation technique is given in Ref. 126. The naphthalene sublimation technique, commonly employed to measure convective transport phenomena, has several advantages over direct heat transfer measurement techniques. Tliese advantages are more detailed mass transfer distribution over the test piece (typically thousands of data measured points), avoidance of heat conduction and radiation loss, and better control on boundary conditions. 

In 1978, Tairov and Tsvetkov [24] reported a sublimation technique to produce SiC boules for device application. They produced an 8 mm diameter by 8 mm long boule of SiC on a seed crystal placed within a graphite crucible. In a further study, growth of SiC boules up to 14 mm diameter and 18 mm in length were obtained [15]. 

As a class, the azides present several problems when attempting to use the well-known techniques of crystal and thin-film growth. Their instability conflicts with the successful application of most thermal approaches, and their restricted solubilities limit the usefulness of growth from aqueous solutions. For instance, growth by sublimation techniques is not feasible because the azide ion decomposes at elevated temperatures. Melt growth is limited to the azides of K, Rb, Cs, and Tl, of which only KN3 is sufficiently stable at its melting point to permit routine success. Thus, while the azides must, in general, be crystallized from solutions, precautions are also necessary to avoid hydrolytic decomposition when water is used as the solvent. 

The sublimation temperature and the distance of the collecting surface from the material undergoing sublimation have a great influence on the form and size of the crystals produced. The occurrence of polymorphic modifications depends on the temperature of sublimation. In general, it may be assumed that unstable crystals form preferentially at lower temperatures, while at higher temperatures stable forms are to be expected. Nevertheless, mixtures consisting of several modifications are frequently found together. This is the case for barbital and for estradiol benzoate. It should be obvious that the sublimation technique is applicable only to those compounds that are thermally stable. 

Reviews of the industrial applications of sublimation techniques have been made by Kemp (1958) Holden and Bryant (1969) Mellor (1978) Kudela and Sampson (1986). The basic principles of vaporization and condensation have been discussed by Rutner, Goldfinger and Hirth (1964) Strickland-Constable (1968) and Mellor (1978). 

Sublimation techniques can be classified conveniently into three basic types simple, vacuum and entrainer. 

Low-temperature sublimation, which in some circumstances is termed freeze-drying, has been used to separate water, as ice, from biological fluids such as serum, urine, or saliva. The technique has been particularly useful in pediatric cases where sample volumes have been extremely low. Determinations have then been accomplished using infrared spectroscopy or mass spectrometry. Preparation of physiological samples for determination of deuteri-mn oxide has included sublimation techniques prior to spectrophotometric determinations. 

Polycyclic aromatic compounds have been separated using sublimation techniques from a variety of samples including coal, solids derived from oil, coal, and petroleum processing, and residues (soots) resulting from the use of such fossil fuels. 

Haider et al. [33] conducted experiments in a turbulent fluidized bed using naphthalene sublimation technique. Their results are plotted in Figure 7. Researchers [2] also calculated mass transfer coefficients from burning rate experiments in a hot turbulent bed. Mass transfer coefficients are obtained by using measured values of burning rates and chemical rate coefficient. 

Caffeine can be purified by sublimation (Technique 17, Section 17.5). Assemble a sublimation apparatus as shown in Figure 17.2A. Add approximately 0.5 mL of methylene chloride to the Erlenmeyer flask and transfer the solution to a clean, 5-mL, thin-walled, conical vial, using a clean and dry Pasteur pipette. Add a few more drops of methylene chloride to the flask in order to rinse the caffeine out completely. Transfer this liquid to the conical vial. Evaporate the methylene chloride from the conical vial by gentle heating in a warm water bath imder a stream of dry air or nitrogen. 

The technique for conducting a successful sublimation (Technique 17, Sections 17.3 and 17.4) is given below. Ask your instructor to demonstrate the sublimation procedure to the laboratory class before you start your sublimation. 

An example of the purification of a natural product, where the sublimation technique at the microscale level is effective, is the case of the alkaloid caffeine. This substance can be isolated by extraction from tea (see Experiment [IIB]). 

Following extraction and removal of the solvent, sublimation techniques are applied to the crude solid residues to purify the caffeine. This technique is espedaUy suitable for the purification of solid substances at the microscale... 

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