Desorbable water

Figure 16-49 [Engineering Data Book, 10th ed., Gas Processors Slippers Association, Tulsa, 1988, Sec. 20, p. 22] depicts the flow scheme for a typical two-bed TSA dryer system showing the auxiliary equipment associated with regeneration. Some of the diy product gas is externally heated and used couutercurreutly to heat and desorb water from the adsorber not currently drying feed. The wet, spent regeneration gas is cooled the water is condensed out and the gas is recycled to feed for recovery.

Fig. 1.71. Synopsis of Tict and desorption rates (DR) of the two tests in Fig. 1.63 (1) and Fig. 1.64 (4) and comparison with two other tests (2) carried out as (1) but with activated pressure control at 0.36 mbar and (3) only one tray used (instead of three trays in Fig 1.63), which has been placed at such a slope that the thickness of the product has been 0.5 cm at one side and 0.9 cm at the other. The course of the pressure (see Figs. 1.63 and 1.64) permits quantitative judgment of the SD. The DR data measure, independent of the chosen process data, the amount of desorbed water per hour in % of the solid content. It is visible, that a DR value of 5 %/h in test (4) is reached in 6.2 h, in test (2) in 10.2 h, in test (1) in 13.5 h, but in test (3) the time cannot be estimated. Because of the unequal product thickness, the DR values can change (9.5 h), the desorption process is not uniform for such a product.

The measuring of desorption rates can be used, as the above examples show, to determine the amount of desorbable water if the following prerequisites are fulfilled ... 

Fig. 1.73. Desorbable water in % of solids (dW) as function of the drying time. The dW-values have been calculated from the data of Fig. 1.72. In plot A, after 7.5 h only 1 % (of solids) water can be removed by further drying at this temperature. If e. g. 0.3 % are required, the drying can be terminated at 8.3 h.

The leak rate of the plant must be so small that a pressure rise due to the leak rate is also small compared with the pressure rise resulting from the desorbed water. 

The residual moisture content calculated by this method is called desorbable water, dW. 

Fig. 2.39. Desorbable water (dW) as a function of the drying time (run 3 not shown) (plots from measurements by AMSCO Finn-Aqua, D-50345 Hiirth).

In a freeze drying plant automated in this way, the desorption rates and the desorbable water content (in % of solids) can be measured, calculated and documented. 

During MD the Tke can be closely controlled and the change from MD to SD documented and, if required, automatically executed. During SD the pressure control can be switched of, the pressure will drop and the progress of SD can be followed by DR measurements. DRs give the amount of water desorbed/h in % of solids. By the integration over time (Eq. (20)) it can be decided, when e. g. 5 % desorbable water are reached and which result can be expected in another 48 h of drying. 

Hydrogen is not the only impurity frequently incorporated into SiNjHy films produced by PECVD. Oxygen is generally found in the films and is believed to come from desorbed water vapor from reactor walls or from small... 

---