INSIGHT - Comments
Vol. 1 No. 1 Misconceptions regarding the role of the condenser
Thanks for your latest Insight article on the role of the condenser in the lyophilization process. I often get asked questions about the insulating effect of ice build up on the condenser. Does that have an effect on the ability of the condenser to trap water vapor?
ST March 1998
Response by T.A. Jennings
While the thermal conductivity of ice is greater than water, a layer of ice on a condenser surface will have an insulating affect on the performance of the condenser. The ability of the condenser to trap water vapor will be directly dependent on the water vapor throughput to the condenser and on the surface area of the condenser and inversely dependent on the thickness of the ice. For a worse case scenario, e.g., a high vapor throughput to the condenser with a thick layer of ice, a temperature gradient will be formed across the ice thickness and the temperature of the surface of the ice will define the partial pressure of water vapor in the condenser chamber. If the increase in the partial pressure is significant, water vapor can flow past the condenser and into the pumping system and cause a decrease in pumping speed which would increase the pressure in the condenser chamber. An increase in condenser chamber pressure can also cause an increase in the drying chamber pressure. For a given shelf temperature, an increase in the drying chamber pressure would cause an increase in the product temperature (see April 1998 issue of INSIGHT) and open the possibility of collapse or even meltback of the product during the primary drying process. One should not operate a freeze-dryer under conditions that exceeds the equipment manufacturers specifications regarding either the water vapor throughput or the ice thickness on the condenser surface.
Vol. 1 No. 1 Misconceptions regarding the role of the condenser
We discussed your article on "Misconceptions of the role of the condenser" among four of us in the Plant. We have a doubt as to which interface are you referring to when the pressure in the drying chamber is evacuated to 150 mTorr .
Thank you for your answer to this question and for the excellent articles in lyophilization. We plan to discuss them in the working group as test cases for our education in freeze drying .
RG March 1998
Response by T.A. Jennings
The interface term that is used on the first page of the INSIGHT refers to gas-ice interface that would be associated with the product and not the condenser. For a frozen product matrix undergoing the primary drying process, the interface refers to the boundary between the frozen matrix and the newly formed cake.
Vol. 1 No. 1 Misconceptions regarding the role of the condenser
I agree completely with you for the role of the condenser which is sometimes easily overlooked and attributed to vacuum pump. Many times the role and importance of condenser itself is not recognized by users.
I wonder if you can discuss sometimes what happens after the formation of ice on condenser plates/tubes ? Because although the temperature you see are low say -50/-60, the temperature on surface of the ice may be lower.
Is it true that ice frozen below -20/-30 changes shape of the crystal and is very difficult to defrost ?
Hiren Shah September 1998
EUTECTIC TECHNOLOGIES
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Tel : 91 22 5139690 Telefax : 91 22 515 0744
E-mail : Hiren.Shah@Mailcity.com
Response by T.A. Jennings
Thank you for sharing your comments and questions concerning the role of the condenser in the lyophilization process.
In answer to your first question and comment regarding the surface temperature of the ice on the condenser, the temperature of the ice surface, especially during the primary drying process, will be higher than that of the metal surface of the condenser. The reason for the higher temperature is that in order for the water to condense and form ice it must give up energy nearly equivalent to that of the heat of sublimation (about 675 calories per gram). In order for the condenser to dissipate such energy, there must be a temperature differential between the temperature of the ice surface and that of the metal condenser surface. Since the direction of the energy flow must be from the ice surface to the metal surface, the temperature on the ice surface must exceed that of the metal surface of the condenser.
There are times, however, when the temperature of the ice surface can be lower than that of the metal surface of the condenser. Such a condition can occur during the final stages of the secondary drying process when there is no pressure control and the pressure in the dryer will be determined by the pumping speed of the vacuum pump(s) and the real and virtual leak rates of the dryer.
Consider the example when the condenser metal surface temperature is at -50 C and the pressure in the chamber is at 10 mTorr. At a temperature of -50 C, the vapor pressure of ice is about 30 mTorr while the temperature of ice at 10 mTorr is given as -58 C. Since the number of degrees of freedom for the phase diagram of ice at temperatures lower than 0 C is 1, the temperature of the ice surface must be -58 C. In this example, the direction of the energy flow will be from the metal surface to the ice surface. As a consequence, sublimation will occur at the ice surface. If for an extended period of time, the condenser plate remains at -50 C and the pressure in the chamber continues to be maintained 10 mTorr, there will be no ice layer on the surface of the condenser. Where did the ice go - into the vacuum pump.
In answer to your second question regarding the defrosting of ice crystals formed at temperatures lower than -20 C, the structure of the ice on the condenser will have no affect on the melting temperature. The melting temperature of ice at pressures greater than 4.58 Torr will still be 0 C. However, the melting temperature of the ice can be increased if a gas hydrate is formed. If a significant partial pressure of methane were present during the ice formation then there would be an increase in the melting temperature. It is doubtful, although there is no supporting data, that the effect of methane would increase the melting temperature of ice by more than 1 C.
Vol. 1 No. 1 Misconceptions regarding the role of the condenser
Dear Dr. Jennings,
I have been receiving "Insight" for 2 years now and have enjoyed and learned from it tremendously. I have a question though that I can not find the answer to and was hoping you could help or point me in the right direction.
If you have two lyophilizers and one has a great condenser system and one has a good at best condenser system, how would you compensate?
For example: one lyophilizer can maintain a condenser temperature throughout the cycle of -82 C to -85 C and the other lyophilizer maintains a condenser temperature throughout the cycle of -69 C to -75 C. Should you adjust the vacuum pressure or the shelf temperature?
If the vacuum pressure is maintained the same and the shelf temperature in the two lyophilizers are maintained the same and the duration of primary dry cycle was the same, what would you think the affect of just the condenser temperatures differences have on the final cycle/product?
Would one have a higher moisture content then the other?
If you have any thoughts you can share with me I would greatly appreciate it. I am developing cycles for mixed solvent solutions. One day I hope to be at a point to write something that you would publish in an "Insight" article.
Thank you,
Bill Reilley February 2000
Biomira USA
Response by T.A. Jennings
You should always maintain pressure control during a drying process. It makes transfer of the process from one dryer to another easier and simplifies the process validation.
In your example, let us maintain the pressure at 200 mTorr. I selected 200 mTorr to reduce the effects of the backstreaming of hydrocarbons from the vacuum system. However, your product will need to have a collapse temperature equal or greater than -30 C. Let us say that the shelf-surface temperature during primary and secondary drying are the same for each dryer. Then under these condition, the condensers will be of no factor in the properties of the final product. We are assuming that you have used the same freezing function in both dryers.
The condenser only comes into play when it can affect the chamber pressure. The vapor pressure of ice at -69 C is lower than 10 mTorr so unless you are drying at very low pressures the condenser temperature will not be a factor. The condenser merely acts as a water vapor pump and does not suck water from the product. The water vapor is driven out of the product by energy supplied by the shelves.
