The Ice Condenser

The same amount of heat that is required in the drying chamber for the sublimation of the water is subsequently liberated as the water vapor condenses at the ice condenser, and must be removed by means of refrigeration units. Therefore, three basic conditions have to be fulfilled in order to carry out the freeze-drying process  

The temperature of the material must be regulated in such a way that on one hand, thawing is avoided, and on the other hand, vapor pressure above the material does not sink due to overcooling. 

The water vapor molecules escaping from the frozen material should be removed in such way that saturation of vapor pressure above the substance to be dried is avoided. 

The process should take place in a vacuum so that removal of water molecules may not be impeded by the presence of residual gases. 

In constructing any freeze-drying apparatus, care should be taken to obtain optimum values of all the above-mentioned conditions. A freeze-drying plant should contain a drying chamber, an ice condenser, a refrigeration unit, and a vacuum pump. 

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Process documentation 2, drying chamber with shelves 3, operation control 4, ice condenser 5, vacuum pump with exhaust filter 6, refrigeration machine for the ice condenser 7, refrigeration machine for the shelves 8, circulation pump for the brine 9, heat exchanger (Lyovac GT 6, AMSCO Finn-Aqua, D-50354 Hiirth, Germany). 

Figure 2.8.1 shows a typical installation for flasks and other containers in which the product is to be dried. The condenser temperature for this plant is offered either as -55 °C or as -85 °C. For this type of plant, a condenser temperature of -55 °C is sufficient as this temperature corresponds with a water vapor pressure of approx. 2.1 10 2 mbar, allowing a secondary drying down to approx. 3 10-2 mbar. This is acceptable for a laboratory plant, in which the limitations are not the condenser temperature but the variation of heat transfer to the various containers, the rubber tube connections and the end pressure of the vacuum pump (2 stage pump, approx. 2 10 2 mbar). Figure 2.8.2 shows that these units are designed for very different needs. The ice condenser in this plant can take up 7.5 kg of ice at a temperature down to -53 °C. 

The ice condenser is a horizontal condenser, ft has a storage capacity of approximately (provide value) kg. The condenser cooling capacity has... 

The ice condenser valve is a hydraulically operated valve (provide DN requirement standard) with stable positioning and vacuum tight on both sides against atmosphere. The piston rod seal in the condenser has a double seal ... 

The refrigeration system is required to cool the ice condenser and the heat exchanger for the heat transfer medium. The system consists of four separate two-stage compressors with water-cooled liquehers. 

They can be easily enriched with flavourings before freezing or freeze drying. In this context, it is often advantageous to concentrate the defrosting-water of the ice-condensers, which contains volatile flavour constituents, and to add it to the granules. 

Shown in Figure 8-7 is a 12-port drying chamber. It is made of stainless steel, usually weld free to minimize vacuum leaks, and contains an inner chamber about 7-10 cm smaller in diameter. The inner chamber is filled with dry ice and acetone or isopropanol and is the chamber usually used to freeze the sample when it is first prepared. It is about 30 cm high and 22-25 cm in diameter. The progress of the sublimation can be followed roughly by watching the dry ice-acetone (isopropanol) mixture. As the ice condenses, the coolant will boil off the COj in it - sometimes so vigorously early in the process that the pressure may have to be increased a bit to slow down the sublimation. 

The ice condenser is a pipe coil system or in the case of smaller apparatus, a cold trap, fitted in a vacuum-tight container, which is connected to the drying chamber. 

In smaller laboratory devices, the ice condenser is normally cooled by means of carbon dioxide or liquid nitrogen, while in larger plants, by refrigeration units. 

The ice condenses the water vapor inside. Since the water is still hot, it will begin to boil at lednced pressure. (Be sure to drive out as much air in the begiiming as possible.)... 

To achieve these pressures, the ice condenser has to have a temperature for with the two model substances of T o < —65°C and <—70°C respectively. The permanent gas pressure should be small compared with the total pressure of 0.1 to 0.2pc, e.g., in the region of 10 mbar. 

Sublimation, which is the first drying step of the lyophilization process, consists in the direct transformation of the free water of the frozen matrix into vapor which is then trapped on the ice condenser of the... 

To achieve this step of the process, the product is heated at a temperature inferior to the lowest eutectic point to avoid any remelting, usually around —35°C, and the ice condenser is cooled down at a temperature well below the shelf temperature (usually —55°C) and dependent on the pressure (vacuum) level requested in the chamber. 

The difference of temperature between the shelves and the condenser results in a vapor pressure difference above the ice, also called driving force, which is an essential factor in the sublimation rate. The colder is the ice condenser, the higher is the driving force for the same shelf temperature and pressure (vacuum) set point. This also means that for an identical shelf temperature profile and pressure (vacuum) set point, the sublimation rate can vary in relation to the condenser temperature. 

Traditionally the condenser cooling and freezing is achieved by direct expansion of frigorific fluids (HFCs allowed by the Montreal protocol), or nowadays liquid nitrogen (LN2), in the coils or the plates of the ice condenser. 

The purpose of this question is to find out whether any passive sink for the iodine has been provided to supplement the natural iodine removal mechanisms, like deposition, adsorption, chemical reaction, mass transfer into the water pool or into the droplets, and pool scrubbing, etc. Table 6 presents a summary of the responses. Except the borax used in the ice condenser of the Loviisa units (Finland) no other passive means have been reported. 

Borax in the ice condensers makes the ice rather basic, and thus the ice removes quite efficiently also gaseous iodine (I2) from the gas flows. 

Malinowski, D. D. Iodine removal in the ice condenser system. Report WCAP-7426 (1970) Marimuthu, K., Mitragori, D. S., Sundararajan, A. R. Radiolysis of aqueous cesium iodide. 

HgO changes its surface tension. 12.108 66.8%. 12.110 1.69 g/em. 12.112 When water freezes, the heat released proteets the fruits. Also, the ice forms help to insulate the fruits by keeping the temperature at 0°C. 12.114 The water vapor generated from the combustion of methane condenses on the eold beaker outside. 12.116 The ice condenses the water vapor inside. Beeause the water is still hof it will begin to boil at redueed pressure. (Air must first be driven off in the beginning.) 12.118 6.019 X 10 ... 

A suitable method to measure the temperature during the drying process is to determine the saturation vapour pressure. If the flow of water vapour between the chamber and the ice condenser is interrupted for a short period of time, a saturation vapour pressure in accordance with the water vapour partial pressure diagram will settle in the chamber. The corresponding ice temperature can be taken from the phase diagram (Figure 14.2). This type of measurement is called a... 

The drying chamber is separated from the ice condenser by a valve so that while the ice condenser is being defrosted, the chamber can already be charged again. This chamber-condenser valve is used for BTM and for Pressure Rise Measurements (PRM) to determine the end of the drying process. 

Mostly two-stage, semi-hermetic refrigerating units are chosen to cool the ice condenser andthe shelf heat transfer system. With the presently available lowtem-perature refrigerating agents, R 404 A and R 402 A, the following parameters can be reached ... 

Such systems are called Clean-In-Place (CIP). They consist of a spray nozzle system installed in the drying chamber and possibly also in the ice condenser and connected to a cleaning system. The spray nozzles are positioned in such a way that all the surfaces in the areas to be cleaned are reached (Figure 14.14). 

In the plant, this separation is accomplished with an isolation valve that separates the ice condenser from the drying chamber this valve is also able to permit (a) the pressure rise test at the end of the freeze drying cycle, (b) the simultaneous discharging and loading of the product and condenser defrosting, and (c) the reduction of cross-contamination between batches to a minimum. All the internal parts of the freeze dryer are of stainless steel type AISI 304L or 316L with a finished surface of 300 mesh or more. In the modern plants, the internal sterilization of the... 

The variable pZ is taken to represent the water vapor pressure in the drying chamber (the external mass transfer resistance is taken to be insignificant) and its value is determined by the design and the operational temperature of the ice condenser. Thus, Ah may be changed by changes in pZ (pZ could be changed by changes in the temperature of the ice... 

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