Droplet size, feed emulsion

The following section covers the general aspects of solvent losses by crud its formation and characteristics and its treatment and prevention. Losses attributed to emulsion and crud formation, can in part be related to (1) nature of feed (2) reagent choice (3) equipment selection and (4) method of operation, such as the droplet size, continuous phase, excessive turbulence, etc. 

Borel, P., Armand, M., Pasquier, B., Senft, M., Dutot, G., Melin, C., Lafont, H., and Lairon, D. (1994). Digestion and absorption of tube-feeding emulsions with different droplet sizes and compositions in the rat. J. Parenter. Enteral. Nutr. 18, 534-543. 

Formation of spray in small drops (i.e. 30 microns, with a narrow size distribution) using a rotary disc or nozzle, to enhance the product-air interface. The preparation of the feed (dry matter content, composition, temperature, mixing) must facilitate pumping and spraying with minimal modification (partition of constituents, droplet coalescence in emulsions). 

The feeds for spray dyers with nozzle atomizers are normally solutions, emulsions, and dilute suspensions. As with rotary atomizers the product particle size of a nozzle plant depends on primary droplet size. For a given feed and nozzle type and size, the droplet diameter is inversely proportionate to the liquid pressure to the power of e.g. 0,3 and directly proportionate to the square root of the orifice diameter. Typical liquid pressures are 5 to 50 bar and the orifice diameter is between 1 and 4 mm. With these... 

Fig. 6.22 Effect of feed emulsion droplet size 1 43 on the retention ofo-limonene during spray drying (Carrier materials blend of CA-MD, blend ofSSPS-MD, V blend of HI-CAP lOO-MD).

Fig. 6.23 Changes in distribution of emulsion droplet size before and after the atomization with a rotary atomizer, (a) Coarse emulsion in feed liquid, (b) fine emulsion in feed liquid.

The problem is to prevent instability, not only to maintain the appearance of the emulsion, but so that the characteristics of the emulsion and of medicaments dissolved in the emulsion are as little changed on ageing as possible. As an example, ageing might alter the absorption of heparin from O/W emulsions where absorption of heparin appears to be directly related to the particle size and total surface area of the oil droplets [11]. Fat emulsions are used extensively in intravenous feeding [12] where it is vital that particles remain below 1 in diameter to avoid thrombophlebitis and other complications, but the state of the art is exemplified by the statement [13], that the emulsions must be stored in a refrigerator and no antibiotics, vitamins or potassium supplements added because they may break the emulsions . Lynn [14] reports some experiments on the addition of disodium carbenicillin and sodium cloxacillin to intravenous lipid emulsions which verify this statement. The special case of intravenous emulsions is dealt with in Section 8.7.2. 

Operation of the virgin and modified AI2O3 manbrane was compared by the separation of an oil-water emulsion, which consisted of 20 engine oil (1 g/L), Tween 80 (0.5 g/L), Span 80 (0.5 g/L), and distilled water. Stable oil-water emulsion had an average droplet size of 1.79 pm, and 90% of the oil droplets were between 0.67 and 7.4 pm. Filtration tests were conducted at the transmanbrane pressure of 0.16 MPa, feed flow rate of 5 m/s, and at an operating temperature of SO C. The membrane was back-flashed at the interval of 10 min. For the virgin AljOj manbrane, the flux declined sharply in the first 60 min, from 446 to 159 L/m h. The steady flux was only about 30% of the initial flux, implying that the AljOj membrane was seriously fouled. However, the flux of the modified membrane quickly reached a constant in 10 min. The flux declined from 506 to 441 L/m h, which is 88% of the initial flux. 

Colloidal suspensions, emulsions and solid dispersions are produced by means of colloid mills or dispersion mills. Droplets or particles of sizes less than 1 (im may be formed, and solids suspensions consisting of discrete solid particles are obtainable with feed material of approximately 100-mesh or 50 p,m in size. 

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer characteristics and other operating variables on spray dryer performance are summarized in Table 9.18. A detailed survey of theory, design and performance of atomizers is made by Masters (1976), but the conclusion is that experience and pilot plant work still are essential guides to selection of atomizers. A clear choice between nozzles and spray wheels is rarely possible and may be arbitrary. Milk dryers in the United States, for example, are equipped with nozzles, but those in Europe usually with spray wheels. Pneumatic nozzles may be favored for polymeric solutions, although data for PVC emulsions in Table 9.16(a) show that spray wheels and pressure nozzles also are used. Both pressure nozzles and spray wheels are shown to be in use for several of the applications of Table 9.16(a). 

An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-in-water (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled aiKl morphology controlled fine particles such as metals, carbonates/ and oxalates.Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant. In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination. 

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