Desorption Rates

During secondary drying (SD), water that is removed is more less bound to solid molecules. The amount of water removed is small (e.g. 10% of solids), compared to 10 times the weight of solids during MD. The behaviour of water molecules close to a protein surface has been described by Bellissent-Funel and Teixeira. The water molecules are in a monolayer around the protein with a reduced mobility compared with bulk water.

The desorption of bound water can be measured during SD by measuring the pressure rise in the chamber after closing the valve to the condenser for 60-120 s. The length of time is not critical since the temperature does not change quickly in this phase. The pressure rise (dp s-1) can be converted into the desorption rate (DR) using:

where DR = desorption of water vapour in per cent of solids per hour; Vch = chamber volume (L); dp = pressure rise (mbar); dt = time of dp (s); and mso = mass of solids (g).

The course of DR describes not only the progress of the secondary drying quantitatively, but also reflects the structure of the frozen product as shown by Haseley and Oetjen in Figure 9. In Figure 9A DR data are shown for a 10% mannitol solution frozen in vials on the shelves of the freeze-drying plant at a rate of 0.5-0.8°C min-1. In Figure 9B the same solution in the same vials is frozen in liquid nitrogen and in Figure 9C the solution is frozen in liquid nitrogen but annealed before freeze-drying. From these figures the following conclusions can be drawn. (1) Slow freezing of 10% mannitol solutions results in structures in which the water is bound in several forms. The DR plots as a function of time are not single-valued. (2) Freezing at rates of more than 30°C min-1 produces structures with a more uniform desorption behaviour. DR plots show the influence of the operating conditions during MD: (1) run 6 in Figure 9B is collapsed, and the water has dissolved part of the solids, forming a sticky cake. The water vapour of 378 vials resulted in an unstable Tice, which is 5-7°C higher than in all other runs. (2) The systematic influence of Tsh is shown in Figure 9B and C. (3) The influence of pc is shown in Figure 9C, and the influence of annealing or low Tice is demonstrated by comparing Figure 9B and C: without annealing, DR plots bend between 3% and 5% per hour; with annealing this effect practically disappears. The exceptions prove the sensitivity of the measurements: run 1 (Figure 9B) is freeze-dried at Tice = — 36.9°C, others at approximately — 34.9°C; run 5 (Figure 9B) is annealed at — 40°C but for 8 h; runs 2 and 3 (Figure 9C) are annealed at a temperature 1°C too high and 1.5°C too low for 18 min. Annealing reduces the amount of unfrozen water.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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