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Monitoring of freeze drying process with a TDL Hygrometer Disclosure Number: IPCOM000110546D
Publication Date: 2005-Mar-25
Document File: 7 page(s) / 94K

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Monitoring of freeze drying process with a TDL Hygrometer

Freeze drying is a process where low temperature and vacuum are combined to allow dehydration of food or pharmaceutical products.

The feed to be dehydrated is first frozen and sub-cooled  at atmospheric pressure, then water ice is sublimated under vacuum; the final step of dehydration is a secondary desiccation, occurring at temperature above 0°C.

A typical diagram of temperature and pressure is given fig 1.

For each type of product and each type of packaging (drugs are generally introduced on the shelves in their final individual packaging, bottles, bulbs, blisters), operating conditions are carefully optimized, leading to a referenced temperature profile that will have to be strictly respected during  industrial production.

It is noticeable that for large freeze dryers and high value pharmaceutical products, the “pay load”  in a freeze dryer may exceed $ 1 million , for a process that may last several days.

It is also noticeable that , for such a demanding process control, the only sensor is generally a reference thermocouple immersed in a reference sample.

Temperature is in fact only an indirect indication of the key parameter which is, obviously for a drying process, hygrometry, or  remaining water content in the product being dehydrated.

Direct measurement of  water partial pressure could give a valuable information on the process , that can be used to corroborate the temperature, and /or to generate indication that cannot be extracted from the temperature signal, such as the mass flow rate of water which is practically extracted as a function of time.

Monitoring the water content of the “average atmosphere” may also complement a temperature signal that only comes from a reference sample, compared to thousands of individual doses, on the tenth of square meters of different shelves.  Even if freeze dryers are designed to reduce discrepancies in temperature between the shelves, and between  different points of a shelf, one cannot guarantee that each individual dose is exactly at the same level  of dehydration at the end of the process : a global moisture signal may give a valuable information of the average state of dehydration of the whole load.

By using this moisture signal to determine the end-point of the dehydration process, the time allowed for the process may be optimized, providing a gain in throughput.

For that reasons, it is proposed and described here-after to use a TDL( Tunable Diode Laser ) hygrometer to record moisture content in a freeze drying process

Originally envisaged  for moisture analysis in ESG’s,  TDL based hygrometers have been extensively studied in the literature those last years.

Let’s remind that  the advantages that are currently claimed for TDL based hygrometers can be summarized as follows :

Flow through cell in process line

Low Detection limit  : 100 ppb

Short response time : 1 second

Wide operating pressure range: Atm to Vac...