Heat sensitive plate

Figure 22.2. Schematic diagrams of (A) a power compensation DSC and (B) a heat flux DSC. Heat sensitive plate heat flux plate. (Reproduced with permission from Hatakeyama and Quinn, 1994.)...

Quantitative differential thermal analyser Instrument for measuring the difference temperature between a sample and reference in response to a temperature programme. Knowing the heat capacity of the heat-sensitive plate as a function of temperature, this instrument can be used to estimate the enthalpy change... 

Apparatus—Heat-flux DSC is illustrated schematically in Figure 2.9. The sample holder unit is placed at the center of the furnace. The sample is sealed in a sample pan and placed in one side of the holder, and an inert reference material (exhibiting no phase change over the entire temperature range of the experiment) is likewise inserted in the other side of the holder. The temperature difference between the sample and reference is measured by thermocouples, attached to the base of each side of the holder, as a function of the furnace temperature. A second series of thermocouples measures the temperature of the furnace and of the heat-sensitive plate. 

DTA curve—Heating the furnace at a programmed rate raises the temperature of the sample and the reference. When the sample undergoes a phase transformation, energy is absorbed or liberated, altering the heat flux through the heat-sensitive plate and inducing a temperature difference between the sample and the reference. A DTA curve plots this tempera-... 

MiscelUneous. Mahc acid is used in pharmaceuticals (qv), cosmetics (qv), dentifrices (qv), metal cleaning, electroless plating (46), wash-and-wear textile finishing (47—49), for stabilization of heat-sensitive copying paper (50), as an inhibitor of gelation, livering, and agglomeration in cellulose nitrate Hqueurs, and in many other appHcations. 

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. 

Two heat-sensitive organic liquids of an average molecular mass of 155 kg/kmol are to be separated by vacuum distillation in a 100 mm diameter column packed with 6 mm stoneware Raschig rings. The number of theoretical plates required is 16 and it has been found that the HETP is 150 mm. If the product rate is 5 g/s at a reflux ratio of 8, calculate the pressure in the condenser so that the temperature in the still does not exceed 395 K (equivalent to a pressure of 8 kN/m2). It may be assumed that a = 800 m2/m3, /x = 0.02 mN s/m2, e = 0.72 and that the temperature changes and the correction for liquid flow may be neglected. 

APV, whose Paraflow plate heat exchanger is illustrated in Volume 1, Chapter 9, supply climbing and falling-film plate evaporators with evaporative capacities up to 10 kg/s. Such units offer the advantages of short contact and residence times and low liquor hold-up, and hence are widely used for the concentration of heat-sensitive materials. 

High sensitivity DSC (HS-DSC) instruments are used to measure small heats of transition. They were developed originally for measuring to heat of denaturation of biopolymers in dilute solution (Hatakeyama and Quinn, 1994). The sensitivity of a heat flux instrument, for example, can be improved by increasing sample size, using multiple serially-connected thermocouples to measure both sample and reference temperatures, and increasing the heat sink capability of the heat flux plate (Hatakeyama and Quinn, 1994). 

Also propellant charges for rockets and guns have also been developed by compounding solid explosives such as nitramines (e.g. -> Cy-clonite) with plastics. Plastic explosives and plastic propellants are of interest, if low thermal and impact sensitivity is needed (- LOVA -> Armor Plate Impact Test -> Friction Sensitivity -> Heat Sensitivity - Impact Sensitivity -> Projectile Impact Sensitifity - Susan Test). 

Centrifugal evaporators are used on an industrial scale for gentle distillation of temperature-sensitive materials (Fig. 2.49). Here, a thin film is evenly spread on a heated conical plate by centrifugal force. This technique reduces hold-up, contact time and foaming of the liquid on the heated surface considerably. Furthermore, the centrifugal force immediately throws the condensed steam away from the rotor s heating surface (dropwise condensation). This results in a uniformly heated rotor surface with a high overall heat transfer coefficient up to 30,000 kJ/(m h K). This heated evaporation surface is located opposite to a cooled box which results in the distillate removal. 

The volume of product holdup) in the evaporator is very small in relation to the large available heat transfer sur ce. Plate-and-ffame evaporators can generally handle the evaporation of heat sensitive, viscous, and foaming materials. They permit fast start-up and shutdown and are quite compact, so little head room is required. They are easily cleaned and readily modified. 

Tray dryers may be operated under vacuum, often with indirect heating. The trays may rest on hollow metal plates supplied with steam or hot water or may themselves contain spaces for a heating fluid. Vapor from the solid is removed by an ejector or vacuum pump. Freeze-drying is the sublimation of water from ice under high vacuum temperatures below O C. This is done in special vacuum tray dryers for drying vitamins and other heat-sensitive products. 

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