Sublimation entrainer

In entrainer sublimation, an entrainer gas is blown into the vaporisation chamber of a sublimer in order to increase the vapour flowrate to the condensing equipment, thereby increasing the yield. Air is the most commonly used entrainer, though superheated steam can be employed for substances such as anthracene that are relatively insoluble in water. If steam is used, the vapour may be cooled and condensed by direct contact with a spray of cold water. Although the recovery of the sublimate is efficient, the product is wet. The use of an entrainer gas in a sublimation process also provides the heat needed for sublimation and an efficient means of temperature control. If necessary, it may also provide dilution for the fractional condensation at the desublimation stage. Entrainer sublimation, whether by gas flow over a static bed of solid particles or through a fluidised bed, is ideally suited to continuous operation. 

A general-purpose, continuous entrainer-sublimation plant is shown in Figure 15.34. The impure feedstock is pulverised in a mill and, if necessary, a suitable entrainer gas,... 

The product yield from an entrainer-sublimation process may be estimated as follows. The mass flowrate G of the inert gas and the mass sublimation rate S are related by ... 

The theoretical maximum yield from an entrainer sublimation process is the difference between the calculated sublimation rates corresponding to the conditions in the vaporisation and condensation stages. 

Quenching the vapour with cold air in the chamber may increase the rate of heat removal although excessive nucleation is likely and the product crystals will be very small. Condenser walls may be kept free of solid by using internal scrapers, brushes, and other devices, and all vapour lines in sublimation units should be of large diameter, be adequately insulated, and if necessary, be provided with supplementary heating to minimise blockage due to the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the risk of explosion since many solids that are considered safe in their normal state can form explosive mixtures with air. All electrical equipment should therefore be flame-proof, and all parts of the plant should be efficiently earthed to avoid build-up of static electricity. 

Condensers usually are large air-cooled chambers whose walls are kept clear with brushes or scrapers or even swinging weights. Scraped or brushed surface crystallizers such as Figure 16.10(a) should have some application as condensers. When a large rate of entrainer gas is employed, a subsequent collecting chamber will be needed. One of the hazards of entrainer sublimation with air is the possibility of explosions even of substances that are considered safe in their normal states. 

If the phase change is solid to vapor, then this is a sublimation but the compound is lost. The preferred technique is solid to vapor back to solid, so the product can be collected. If the vapor is carried away from the sample surface as soon as it forms, so as to reduce the vapor pressure immediately above the sample, then this is called an entrainer sublimation, also in Chapter 7. If a liquid is solidified, changed to a vapor, and then to a solid, this is called freeze drying. This is discussed in Chapter 8. 

The types of distillation processes to be examined will be simple, fractional, steam, immiscible solvent, azeotropic, extractive, vacuum, molecular, entrainer sublimation, and freeze drying. 

Figure 7-18, p. 82, is a diagram of a simple entrainer sublimator. If the tube that collects the sublimate is cooled to different temperatures at successive regions along the length, then it is possible to fractionally sublime a mixture of compounds. The separation in this case is usually not clean, but it can be used in some situations. Commercially, coffee is decaffeinated by entrainer sublimation. 

Figure 7-18. Entrainer sublimators Top, air cooled. Bottom, liquid cooled multiple condensers.

The separation of caffeine from coffee by entrainer sublimation... 

Your instructor has been given a new and simple entrainer sublimator that it is supposed to work quite well. You are to test it for him by separating caffeine from coffee. First you are to determine if anything can be sublimed from coffee. If there is any sublimate, you will try to identify it. 

The use of an entrainer in a sublimation process has many desirable features. It enhances the flow of vapours from the sublimer to the condenser, as already mentioned it also provides the heat needed for sublimation, and thus an efficient means of temperature control is provided. The technique of entrainer sublimation, whether by gas flow over static solid particles or through a fluidized bed, is ideally suited to continuous operation. 

The purification of salicylic acid provides a good example of the use industrially of entrainer sublimation. Air may be used as the carrier gas but as salicylic acid can be decarboxylated in hot air, a mixture of air and CO2 is often preferred. The process shown in Figure 8.30 is carried out batchwise. A 5-10 per cent mixture of CO2 in air is recycled through the plant, passing over heater coils before over the containers, e.g. bins or trays, holding the impure salicylic acid in the vaporizer. The vapours then pass to a series of air-cooled... 

Figure 8.30. An entrainer sublimation process used for salicylic acid purification Air To exhaust...

For example, salicylic acid (Ms = 138) is to be purified by entrainer sublimation with air (Mq = 29) at 150 °C. The vapours pass to a series of condensers, the internal temperature and pressure of the last being 40 °C and 1 bar (10 Pa). The air flow rate is 2000 kg h and the pressure drop between the vaporizer and the last condenser is 15 mbar. The vapour pressures of salicylic acid at 150 °C and 40 °C are 14.4 and 0.023 mbar, respectively. Therefore, assuming saturated conditions ... 

Fig. 7-42. Entrainer sublimator with rotating disc on top of each other and divided into sections to receive solids feed. Entrainer or carrier gas is recycled by a turbine. Entrainer flows counter-currently to the solid which is passed from tray to tray from the top to bottom. Sublimed vapor is picked up by the entrainer. Separation of entrainer and vapor takes place in the desubli-mator. The entrainer then recycles to the sublimator.

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