Dried material layer

The two-zone model described above allows for the multiple mechanisms. The rules that govern these mass transfer operations are completely analogous to those governing heat transfer already discussed. The migration of vapor through the dried material layer can be expressed as... 

In this process, the material is first frozen and then dried by sublimation in a very high vacuum, 10-40 N/m2, at a temperature of 240-260 K. During the sublimation of the ice, a dry surface layer is left, though this is not free to move because it has been frozen,... 

To an ice-cooled, 250 ml, three-necked, round-bottomed flask equipped with reflux condenser, stirrer, and dropping funnel is added 23.6 gm (0.4 mole) of n-propylamine. Butyraldehyde (28.8 gm, 0.4 mole) is added dropwise at 0°C over a period of 2 hr. After the addition, 15 min is allowed to elapse and potassium hydroxide flakes are added. In about 10 min two layers appear and the organic layer is separated. The organic layer is added to a flask containing some crushed potassium hydroxide flakes and is placed in the refrigerator for a few hours. The dried material is distilled to yield 31.6 gm (70%), b.p. 120°-124°C, ni° 1.4149. 

Brattice driers, incorporating a device for removing the dried material (Fig. 44) have been used in Germany. The moist material, spread in a thin layer over cloth stretched on wooden frames (1), is dried in warm air supplied via the ducting (2) at a rate of about 0.5 m/sec. Next to the frame on which the material is dried there is a tin funnel (3) with a built-in sieve in its base. This funnel is connected with the ventilating duct by a flexible tube. Each frame (7) contains about 1.2 kg of fulminate (dry substance). To dry a batch of fulminate at 65-70°C takes... 

Since the sample as received will contain ca 40% water, and since some testing procedures require a dry material, transfer 5 g of composite" sample on a piece of filter paper placed in a Nutsch funnel, connected to a suction flask. Spread the sample evenly (using a stream of water from a suction bottle) and then apply the suction to air dry the material. Transfer the sample to a paper tray, spreading it in a thin layer, and then dry it in a hoc oven at 60° for a minimum of 16 hours. 

In drying, the temperature at the inputs of the sections is maintained within predetermined limits by changing the temperature of the heat-transfer medium. It is necessary to ensure a stable supply of moist material to the drier and unloading of dry material therefrom. Also, the height of the material layer in the drier should be constant. 

Feldman (FI6) explained the difference between the values for D-dried samples obtained with aqueous and other fluids. When the material is dried, the layers partially collapse, and some empty spaces remain between them. Water penetrates into this space, and the density obtained, after correction for CH and any other phases present, is that of the layers themselves. Organic liquids do not penetrate, and helium penetrates only slowly. The empty interlayer space is counted as part of the solid volume, with a consequent decrease in the measured density. 

Cohesion has often been attributed to the interlocking of fibrous or acicular particles. This could be important in the more porous parts of the material, but in the material as a whole, attractive forces between those parts of adjacent layers of C-S-H or other phases that are in contact are probably more important, both within particles and, in so far as the material is particulate, between them. The attractive forces could be direct, of the types mentioned above, or indirect, through interposed HjO molecules forming ion-dipole attractions and hydrogen bonds. Even for D-dried material, analogies with crystalline tobermorite and jennite indicate that much interlayer water is still present (Section 5.4). 

The sublimation step separates the ice crystals formed during the freezing step. When an ice crystal forms, that which remains is the concentrated solute phase called the dry layer . This will become the freeze-dried material at the end of the process. Immediately following passage through the interface, however, the solids in the dry layer still contain a substantial amount of water (about 25-30 g per 100 g of solids), which continues to be strongly bound to the solids. Most sample materials will not be structurally or chemically stable unless most of this water (called sorbed water ) is removed. The process... 

Layer dryers are very suitable for slow-drying materials requiring a long residence time. 

Celite was purchased from Fluka, Germany and dried in an oven for 4 hr at ca. 110°C. The dried material was degassed three times in an inert gas frit prior to use by an evacuation/argon purge cycle. After this procedure the Celite was compressed to a ca. 4-cm deep layer and then covered by an 1-cm layer of previously dried sea sand (Riedel de Haen, Netherlands) to avoid a disturbance to the Celite layer during manipulations. 

To 1.0 volume of serum or resin-treated urine, add 3.0 volumes of absolute ethanol in a centrifuge tube. Mix well and allow to stand for 10 minutes. Centrifuge any precipitated protein at 2000 rpm for 10 minutes. Decant off the ethanolic extract and add 3.0 volumes of chloroform. Mix well and allow to stand for 5 minutes. The aqueous layer on top of the chloroform-alcohol mixture contains the free amino acids and is pipetted into a separate tube. Aliquots of the extract of 0.10 ml and 0.20 ml are used for quantitative total amino acid analysis (Section 4.4). The remainder is evaporated to dryness in vacuo (40°) and the dried material is redissolved in 0.1 volume of water for use in spotting the plates for TLC amino acid analysis (Section 4.5). 

---