Heat-sensitive materials

Separation of high-molecular-weight heat-sensitive materials. High-molecular-weight materials are often heat sensitive and as such are usually distilled under vacuum to reduce their boiling temperature. 

In summary, distillation is not well suited for separating either low-molecular-weight materials or high-molecular-weight heat-sensitive materials. However, distillation might still be the best method for these cases, since the basic advantages of distillation... 

When the dryer is seen as a heat exchanger, the obvious perspective is to cut down on the enthalpy of the air purged with the evaporated water. Minimum enthalpy is achieved by using the minimum amount of air and cooling as low as possible. A simple heat balance shows that for a given heat input, minimum air means a high inlet temperature. However, this often presents problems with heat-sensitive material and sometimes with materials of constmction, heat source, or other process needs. AH can be countered somewhat by exhaust-air recirculation. 

Sundower Seed. Compared to the FAO/WHO/UNU recommendations for essential amino acids, sunflower proteins are low in lysine, leucine, and threonine for 2 to 5-year-olds but meet all the requirements for adults (see Table 3). There are no principal antinutritional factors known to exist in raw sunflower seed (35). However, moist heat treatment increases the growth rate of rats, thereby suggesting the presence of heat-sensitive material responsible for growth inhibitions in raw meal (72). Oxidation of chlorogenic acid may involve reaction with the S-amino group of lysine, thus further reducing the amount of available lysine. 

The absolute pressure may have a significant effect on the vapor—Hquid equiHbrium. Generally, the lower the absolute pressure the more favorable the equiHbrium. This effect has been discussed for the styrene—ethylbenzene system (30). In a given column, increasing the pressure can increase the column capacity by increasing the capacity parameter (see eqs. 42 and 43). Selection of the economic pressure can be faciHtated by guidelines (89) that take into consideration the pressure effects on capacity and relative volatiHty. Low pressures are required for distillation involving heat-sensitive material. 

Gas flow in these rotary dryers may be cocurrent or countercurrent. Cocurrent operation is preferred for heat-sensitive materials because gas and product leave at the same temperature. Countercurrent operation allows a product temperature higher than the exit gas temperature and dryer efficiency may be as high as 70%. Some dryers have enlarged cylinder sections at the material exit end to increase material holdup, reduce gas velocity, and minimize dusting. Indirectly heated tubes are installed in some dryers for additional heating capacity. To prevent dust and vapor escape at the cylinder seals, most rotary dryers operate at a negative internal pressure of 50—100 Pa (0.5—1.0 cm of water). 

Principal advantages are high rate of heat transfer, no internal pressure drop, short time of contact (very important for heat-sensitive materials/ easy accessibihty to tubes for cleaning, and, in some cases, prevention of leakage from one side to another. 

Countercurrent flow of gas and sohds gives greater heat-transfer efficiency with a given inlet-gas temperature. But cocurrent flow can be used more frequently to diy heat-sensitive materials at higher inlet-gas temperatures because of the rapid coohng of the gas during initial evaporation of surface moisture. 

Comparison Data—Plate Dryers Comparative studies have been done on products under both atmospheric and vacuum drying conditions. See Fig. 12-79. These curves demonstrate (1) the improvement in drying achieved with elevated temperature and (2) the impact to the drying process obtained with vacuum operation. Note that cui ve 4 at 90°C, pressure at 6.7 kPa absolute, is comparable to the atmospheric cui ve at 150°C. Also, the comparative atmospheric cui ve at 90°C requires 90 percent more diying time than the vacuum condition. The dramatic improvement with the use of vacuum is important to note for heat-sensitive materials. 

P-type manifold—veiiical configuration. The P ring dryer (see Fig. 12-100) incorporates a single-stage classifier and was developed specifically for use with heat-sensitive materials. The undried material is reintroduced into a cool part of the dryer in which it recirculates until it is dry enough to leave the circuit. 

Heat sensitive materials need to be stored away from heat sources such heaters and windows where they are subject to solar radiation. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

A recent development of the insulated runner principle is the distribution tube system. This overcomes the possibility of freezing-off by insertion of heated tubes into the runners. However, this system still relies on a thick layer of polymer forming an insulation layer on the wall of the runner and so this system is not suitable for heat sensitive materials. 

In many eases the solid assumes the wet-bulb temperature of the gas stream (wet-bulb proteetion), partieularly if the solid is in the form of granules or is spray dried. This is partieularly useful for heat sensitive materials. So, at eonstant drying rate... 

The source of ignition of the polyethylene glycol was probably autoignition of the degraded material. The report recommends the use of nonabsorbent insulation for equipment containing heat-sensitive materials such as EO [19, 20]. 

Hattiangadi, V. S., Sizing Safety Valves for Heat-Sensitive Materials, Chem. Eng, Sept. 7, 1970, p. 96. 

The requirement of bottoms temperature to avoid overheating heat sensitive materials may become controlling. 

This system requires direct steam injection into the still with the liquid, all the steam leaves overhead with the boiled-up vapors (no internal condensation) in a steady-state operation, and system at its dew point. Steam is assumed immiscible with the organics. Steam distillation is usually applied in systems of high boiling organics, or heat sensitive materials which require separation at vacuum conditions. 

This technique is used for the production of radiation-cured coatings, adhesives, and inks. The process is not accompanied by the release of heat, which is particularly important in the case of heat-sensitive materials, e.g., wood, cardboard, paper, plastics, etc. Various radiation sources are used for this technique, among which UV and EB are most useful. 

Low-temperature steam with formaldehyde is of value in the disinfection/sterilization of some heat-sensitive materials (see also Chapter 20). 

Mode of operation Generic type Feed condition Specific dryer types Jack- eted Suitable for heat-sensitive materials Suitable for vacuum service Retention or cycle time Heat transfer method Capacity Typical evaporation capacity... 

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