HEAT SENSITIVE PRODUCTS

The world value for end products using biological manufacturing methods was approximately 250 billion in 1980.This total value can be broken down as follows  

Many of these food and pharmaceutical products are heat sensitive that is, the finished product may be damaged or destroyed if it is exposed to too great a temperature over an extended period of time. Even common products like tomato catsup and penicillin spoil or lose their efficacy when exposed to ambient temperatures for long periods of time. The chemical reactions that limit shelf life are strongly temperature dependent. Some biological products may be handled at elevated temperatures in dilute solutions, but may degrade at the same temperature once the concentration exceeds a certain threshold value. 

it is not uncommon for heat sensitive products to be concentrated in two different evaporator types in series—in natural or forced circulation evaporators to perform the bulk of the solvent removal and in a low mean residence time evaporator to finish the concentration step. Because of the recycle and back mixing effects of a natural or forced circulation evaporator, the mean residence time of the average molecule can be several hours. Some evaporators operate without recycle these are called once-through or single 

An average residence time expressed as holding volume divided by discharge rate was frequently used in the past for both single-pass and recirculation evaporators. However, statistical analysis of several types of evaporators has revealed that the actual time of replacement of 97% of the 

The actual residence time achieved in any evaporator can be calculated from the equation below  

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Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

In steam distillation, steam is introduced into the column to lower the partial pressure of the volatile components. It is used for the distillation of heat sensitive products and for compounds with a high boiling point. It is an alternative to vacuum distillation. The products must be immiscible with water. Some steam will normally be allowed to condense to provide the heat required for the distillation. Live steam can be injected... 

A major breakthrough in separation of products from catalyst, in particular heat sensitive products, came with the discovery of the NAPS or Non-Aqueous Phase Separation technology. NAPS provides the opportunity to separate less volatile and/or thermally labile products. It is amenable to the separation of both polar [14] and non-polar [15] products, and it offers the opportunity to use a very much wider array of ligands and separation solvents than prior-art phase separation processes. The phase distribution characteristics of the ligand can be tuned for the process. Two immiscible solvents are... 

Operational category Evaporator type Feed condition3 Suitable for heat-sensitive products Retention timeb (s) Holding volume3 (m3)... 

The short residence-times in the reactor give less time-thermal-dependent degradation of heat-sensitive products and/or substrates. 

Figure 9.14. Spray dryer arrangements and behavior, (a) Spray dryer equipped with spray wheel straight section L/D = 0.5-1.0 (Proctor and Schwartz Inc.), (b) Spray dryer equipped with spray nozzle straight section L/D = 4-5 (Nonhebel and Moss, 1971). (c) Spray dryer for very heat sensitive products flat bottom, side air ports and air sweeper to cool leaving particles, (d) Distribution of air temperatures in parallel and countercurrent flows (Masters, 1976, p. 18, Fig. 1.5). (e) Droplet-forming action of a spray wheel (Stork-Bowen Engineering Co.).

Mechanically agitated liquid films in thin-film machines are commonly used for continuous processing of heat sensitive products of low... 

The drying at low temperatures reduces degradation of heat-sensitive products. 

The residence time characteristics in an extruder have a great influence on product quality, for instance when processing heat-sensitive products, in reactive processes, and in the case of dispersion and melting. As with all continuous processes, there is no precisely defined residence time in co-rotating twin screw extruders, but rather a residence time distribution. 

A pneumatic-transport device designed primarily for heat-sensitive products is shown in Fig. 11-64. This was introduced into the United States after 5 years use in Europe [Chem. Eng., 76, 54 (June 16, 1969)]. 

The range of sterilization specifications calculable by these various approaches is summarized in Table 2. It is apparent that very brief sterilization specifications (on the order of 2-3 min holding time at 121°C) are obtainable when the microbiological contamination is completely characterized in terms of numbers and thermal resistances. In practice, such limits on hold times could be difficult to control precisely, are probably insignificant in terms of thermal lethality compared with heat-up and cool-down times, and could prove difficult to sell to regulators. Without complete thermal characterization of thermal resistances, specifications calculable by the bioburden approach are hardly significantly shorter than overkill specifications. Thus, it probably makes practical sense in most cases to choose only between overkill cycles for thermally resistant products and aseptic manufacture for heat-sensitive products. 

Heat-sensitive products can break down to insoluble forms that deposit on the surface and inhibit heat transfer. For example, some soluble proteins, when heated, convert to an insoluble form. Change in the heat transfer characteristics of the tube surface can also be induced by corrosion. 

To sterilize heat-sensitive products at high temperature, drastically reducing the sterilization time. The logic behind this principle is that, as... 

Use cocurrent spray drying for heat-sensitive products of fine as well as coarse particle size, where the final product temperature must be kept lower than the dryer outlet temperature. 

Figure 12-93 shows a high-temperature chamber with the hot gas distributor arranged internally on the centerline of the chamber. Tne atomizer is rotary. Inlet temperature in the range of 600 to 1000 C can be utilized in the drying of non-heat-sensitive products in the chemical and mining industries. Kaolin and mineral flotation concentrates are typical examples. 

Figure 12-94 > shows a countercurrent flow chamber with pressure nozzle atomization. This design is in limited use because it cannot produce heat-sensitive products. Detergent powder is the main application. 

On the technical scale, different types of reboilers have been developed [38], Depending on the distillation requirements, various reboiler constructions can be used. The simplest ones are heat exchangers or double jackets evaporators constitute more elaborate technical constructions. The advantage of the latter systems is generally the short residence time and the handling of products with difficult physical properties, such as those with high viscosity or a tendency to crystallisation. These evaporators have very short direct contact times and, therefore, allow distillation of heat-sensitive products. 

Heat transfer rates in falling film evaporators are relatively high even at low temperature differences across the liquid film thus, these evaporators are widely used for heat sensitive products because of uniform temperatures cuid short residence times. Generally, moderately viscous fluids and materials with mildly fouling characteristics can easily be handled in falling film evaporators in series for heavy evaporation loads, and part of the liquid can be pumped and recycled to the top of the unit. 

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