Pressure control , main drying

Figure 1.70 shows the three times repeated measurement of desorption rates, without pressure control to demonstrate the reproducibility, and two measurements where the main drying has been pressure-controlled at 0.36 and 0.21 mbar. The process conditions for these five measurements correspond with those in Table 1.10.1. 

Figure 1.84 [1.69] summarizes the measurements of three runs of the product temperatures with RTD, Tice with BTM and of the pressures by CA. The plots show that the difference in pressures during main and secondary drying is largest with no pressure control and still clearly recognized with pc at 0.2 mbar in relation to an ice temperature of approx. -30 °C. 

The controlled operation pressure during the main drying is pc mbar -0 %, +10 % ). Tsh = -10 °C. With these data Tice has been -29.5 °C (standard deviation 0.38 °C) in the pilot plant. The production department must check the function Tlce = f (pc) for the production plant, and modify pc if necessary to achieve T-Kt = -29.5 °C(standard deviation < 0.4 °C)... 

The main drying is to be terminated automatically if the measured Tkt becomes (by two successive measurements) 2 °C smaller than max. Tice ave. At that time, the automatic pressure control is terminated and Tsh raised to +35 °C (+0 °C,-1 °C). After the pressure control has been terminated, the pressure must drop to pQl8 within 10 min. If the pressure remains higher, or it takes a longer time to reach this level, directive (j) must be followed. 

Typical laboratory plants can be used for different purposes if equipped accordingly. One example is shown in Figure 28. In this unit, small product quantities can be freeze-dried in ampules, vials, or trays, and also flasks can be connected to it. The laboratory plant as shown in Figure 29 is equipped for multiple purposes Shelves can be cooled down to -35°C for freezing the product. A valve is installed between chamber and condenser. Thus it is possible to measure the sublimation surface temperature by BTM, to determine the end of main drying and to control secondary drying. The pressure can be controlled, the temperature of the shelves or the product can be recorded, and vials can be closed with stoppers. 

Chamber pressure is measured by vacuum gauges and is controlled mainly during main drying. 

Main drying Shelf temperature, ice temperature, controlled operation pressure, maximum condenser temperature and desorption rates to determine the end of main drying. 

Heating of shelves to a preset temperature after a preset pressure in the chamber has been reached Pressure control during main drying... 

The main drying (MD) is, with some simplifications, governed by controlled operation pressure p, T ce and Tsh, or by and the temperature difference 2 sh - 2 ioe, called Tto here ... 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

Poor flow distributions may result in localized dry hotspots which, absent control of the temperature fluctuations, may cause rapid overheating. Temperature and pressure fluctuations, and poor flow distribution, are the main problems that accompany the use of two-phase micro-channels. 

Compressed air will be needed for general use, and for the pneumatic controllers that are usually used for chemical process plant control. Air is normally distributed at a mains pressure of 6 bar (100 psig). Rotary and reciprocating single-stage or two-stage compressors are used. Instrument air must be dry and clean (free from oil). 

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