Water Vapor Transport during Drying

Transport out of the container (2) into the drying chamber produces no measurable pressure drop if the product surface is equal to the opening of the container (e.g. with trays). Vials without stoppers in the vial neck do not produce a measurable pressure 

Vapor transport into the condenser depends strongly on the geometric design of the plant. Under favorable conditions and including a valve between the chamber and condenser, a vapor speed of 60-90 m/s can be expected, resulting in a pressure drop between the chamber and condenser of a factor of 2, as an order of magnitude. 

With these estimated conditions and a condenser at e.g. -45 °C = 0.07 mbar, the following pressures can assumed  

If the freeze-drying conditions are extreme, namely small solid content and low sublimation temperature, e.g. 

If stoppers with more favorable channels are used, the vapor pressure in the vials could have been 0.09 mbar, leaving a Ap = pke - pF1 = 0.025 mbar, which is in agreement with b/p = 6.9 x 10 2 kg/h m mbar in this test. 

The water vapor transport in a freeze drying plant can be described schematically with the aid of Fig. 1.86 The ice (1) is transformed into vapor and has to flow out of the container (2) into the chamber (4). Between the chamber wall or any other limitation an area (3, FI) is necessary. The vapor flows then through the area (F2) into the condenser (7), having surface of (F3) on which the water vapor will mostly condenses. A mixture of remaining water vapor and permanent gas is pumped through (8), (9) and (10) by a vacuum pump (11). 

Frozen product 2, vial or the end of a shelf 3, open surface (FI) for the water vapor flow between 2 and 4 4, chamber wall 5, valve with an open area F2 6, condenser chamber 7, cooling and condensing surface in the condenser chamber having a surface of F3 8, vacuum pipe with the diameter d 9, stop valve 10, vacuum pipe with the length 1 (from 8 to II) 11, vacuum pump pjce, water vapor pressure at the sublimation front of the ice /, pressure in the vial pco, pressure in the condenser. 

Example ps at the sublimation front is 0.937 mbar (-21 °C) (see example in Table 1.9), in the chamber a pU20 = 0.31 mbar has been measured, resulting in a pressure difference of approx. 0.6 mbar. With these data, the water vapor permeability blp = 1.1 10 2 kg/h m mbar is calculated. With this data known, it is possible to calculate dp for different conditions, if the mass of frozen water miCL, the time /MD, the thickness (d) and the surface (F) are known. This dp depends from the amount vapor transported and thereby from the heat transfer (Table 1.9). In the examples given it changes between 0.17 mbar in a slow drying process (6 h) to 0.6 mbar for a shorter drying time, 2.5 h. 

These conditions remain comparable down to an ice temperature of approx. -40 °C, and drying performances can be expected to be reduced by a third by the stoppers. If the water vapor streams are smaller (the example used shows an upper limit), the reduction in performance may be smaller. 

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Malinin et al.[3.52] discussed the measurement of RM in freeze-dried bones and compared three methods gravimetry, Karl Fischer titration and NMR spectroscopy. The three methods are discussed in Section 1.3.1. All transplants in this comparison were frozen in LN2 and remained at this temperature for several weeks. The temperature of the condenser during freeze-drying was -60 to -70 °C. The shelves were kept at -30 to -35 °C for the first 3 days. During the last days of the drying the shelf temperature was raised to +25 or to +35 °C. The chamber pressure (p h) was 0.1 mbar. During the initial phase of the process the amount of water vapor transported to the... 

De Luca [3.5] recommends, furthermore, the addition of e.g. tert-butyl alcohol (TBA), to increase the transport of water vapor out of the product and to avoid collapse in sucrose, lactose and sorbitol solutions. Thereby, higher temperatures during drying (e.g. for hemoglobin in sucrose solution) can be applied. 

The cost of transporting wood chips by truck and by pipeline as a water slurry was determined. In a practical application of field delivery by truck of biomass to a pipeline inlet, the pipeline will only be economical at large capacity (>0.5 million dry t/yr for a one-way pipeline, and >1.25 million dry t/yr for a two-way pipeline that returns the carrier fluid to the pipeline inlet), and at medium to long distances (>75 km [one-way] and >470 km [two-way] at a capacity of 2 million dry t/yr). Mixed hardwood and softwood chips in western Canada rise in moisture level from about 50% to 67% when transported in water the loss in lower heating value (LHV) would preclude the use of water slurry pipelines for direct combustion applications. The same chips, when transported in a heavy gas oil, take up as much as 50% oil by weight and result in a fuel that is >30% oil on mass basis and is about two-thirds oil on a thermal basis. Uptake of water by straw during slurry transport is so extreme that it has effectively no LHV. Pipeline-delivered biomass could be used in processes that do not produce contained water as a vapor, such as supercritical water gasification. 

During secondary drying, the product temperature has to be raised to the maximum tolerable temperature of the dried product. This raise can be done as quickly as can be technically achieved in the plant. Raising it more slowly does not make the procedure safer. When there is no more ice in the product, the final temperature can be applied. The only exception may be in the case of very small dry product per vial. Water vapor desorbed at a high rate may take fine particles from the product and transport them to the chamber. The product temperature will approach... 

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