Uniform freezing rates

Convection-Governed Impurity Distribution with Uniform Freezing Rate. The diflFusion-controlled solute build-up at the interface can be minimized by stirring the solution. Burton, Prim, and Slichter (17) showed that, in this case, a steady state will be reached in which... 

When the vials are pushed on to the cold shelves, e.g. -40 °C, the product will start freezing, e.g. with a freezing rate of 1 °C/min between +10 and -35 °C. The product in the first vials will be frozen down to -35 °C in 45 min and approach -38 °C in the time thereafter. If the loading time is, e.g., 5 h, the freezing of the last vials to -35 °C will take 5 h 45 min. The first vials have been kept for 5 h at = 35 °C. It must be tested whether the structure of frozen product in the vials is uniform enough to obtain... 

An adjustable rate of brine circulation in the shelves could be necessary (especially for smaller plants), to obtain a measurable temperature difference (dT) between inlet and outlet during freezing dt should not be too large (uniform freezing), a maximum of 3-4 °C (see Figure2.86) is a practical range. 

C. Isolation and purification of XK-62-2 100 g of the white powder obtained in the above step B are placed to form a thin, uniform layer on the upper part of a 5 cm0X 150 cm column packed with about 3 kg of silica gel advancely suspended in a solvent of chloroform, isopropanol and 17% aqueous ammonia (2 1 1 by volume). Thereafter, elution is carried out with the same solvent at a flow rate of about 250 ml/hour. The eluate is separated in 100 ml portions. The active fraction is subjected to paper chromatography to examine the components eluted. XK-62-2 is eluted in fraction Nos. 53-75 and gentamicin Cja is eluted in fraction Nos. 85-120. The fraction Nos. 53-75 are combined and concentrated under reduced pressure to sufficiently remove the solvent. The concentrate Is then dissolved in a small amount of water. After freeze-drying the solution, about 38 g of a purified preparate of XK-62-2 (free base) is obtained. The preparate has an activity of 950 units/mg. Likewise, fraction Nos. 85-120 are combined and concentrated under reduced pressure to sufficiently remove the solvent. The concentrate is then dissolved in a small amount of water. After freeze-drying the solution, about 50 g of a purified preparate of gentamicin Cja (free base) is obtained. 

The Sol.—Five grams of commercial glucose dissolved in 25 g. of water is made alkaline with a 10 per cent solution of sodium hydroxide. The solution is stirred in a freezing mixture while 100 cc. of a 6 per cent solution of potassium permanganate is slowly added (during 10 min. and at a uniform rate). 

C) The purpose of the outer test tube is to slow down the rate of temperature decrease to allow for more uniform cooling and also greater accuracy of the temperature at the freezing point. Submerged directly in the water, the test tube with the solution will cool too rapidly. 

Undercooling is the driving force in freeze drying. An aqueous salt solution is introduced dropwise into an immiscible liquid (hexane or a petroleum fraction such as kerosene) cooled below 243 K. The individual droplets are frozen instantaneously and the solid particles are decanted or filtered. The frozen particles are then sublimed in a vacuum to obtain a homogeneous powder of fairly uniform particle size. Important parameters in freeze drying are the final temperature of the salt solution and the cooling rate. These can be controlled to some extent, but only on a small scale. Hence the method is not very suited for large-scale manufacture of catalysts. 

Controlled-release biodegradable PLG polymers loaded with parathyroid hormone were formulated as a freeze-dried form with particle size ranging from 27 to 47 i. The freeze-dried method did not alter the surface morphology, particle size, and parathyroid hormone content or release rate of the microspheres. The freeze-dried microspheres resuspended very rapidly and uniformly in solution. In vitro release studies indicated that except for a slight early burst ranging from 4-18%, release of parathyroid hormone from the nanoparticles was very slow over the first 14 days. At 15 days, release of parathyroid hormone accelerated rapidly. 

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