Process optimisation

Process optimisation (PAT-Tools)

Industrial-scale freeze-drying requires robust processes that are initially developed on pilot systems and then scaled up. The objective is to develop drying recipes (profiles of vacuum and shelf temperature versus time) that allow the best possible quality to be achieved in the shortest possible time.

Pilot systems must therefore have the same technology and geometry as the subsequent production freeze-dryer, such as rectangular product chambers and shelves, cooled and heated by liquid media.

The following table lists the parameters used by Christ for process control and monitoring. When used together with basic recipes stored in the freeze dryer control system for various products, an automatic, self-optimising process can be achieved.

 

FeatureFunctionArea of applicationComment
dTfreezingLowest product temperature before switching over to Freezing -> Main DryingReliable freezing,
No defrosting or collapse of the product in entry to main drying
Preset max. permitted T-difference between product and shelf, process will adapt auto­matically
dTmain dryingLowest product temperature before switching over to the next process stageReliable drying (main drying and final drying)Preset max. permitted T-difference between product and shelf, process will adapt auto­matically
T-sensor
wired
Product temperature measurement, traditional wiredProcess monitoring, determination of completion of dryingAvailable in laboratory freeze-dryers, pilot freeze-dryers and production versions, cannot be used for automatic loading/unloading
T-sensor
wireless
WTMplus
Product temperature measurement, wirelessProcess monitoring, determination of completion of dryingMore precise, easier to position, no power input via the sensor cable
Pilot freeze-dryers and production plants
Ice condenser temperatureDisplay and check whether desublimation is possibleOnset while large amounts of steam are present and stabilisation indicates completion of dryingStandard with all freeze dryers
Tshelf ON / OFFHeating fluid temperature in shelf, flow and returnDifference indicates energy consumption due to sublimation Standard with
Pilot freeze-dryers and
production plants
Lyo-RxMeasurement of electrical resistance shows freezing-/melting pointDetermination of freezing point,
self-optimising sharp cycle possible
Freeze dryer stops heating or cooling actively on the appearance of defrosting
pPiraniDrying vacuum, dependent on gas typeDisplay of drying vacuumTraditional measurement method, typically  used in the laboratory freeze dryers
pcapacitiveDrying vacuum, independent of gas type
(also known as p“MKS”)
Comparative pressure measurementMore precise measurement method,
which Christ also uses in the laboratory
psafetyPre-selectable safety pressureProduct protection,
Sharp cycle, limited by psafety
If specified value is exceeded (non-permitted pressure increase [=heating] of product), heating is switched off
pAlarmPre-selectable alarm pressureProduct protection,
Sharp cycle, limited by palarm
If specified value is exceeded (non-permitted pressure increase [=heating] of product), shelf cooling is activated, Pilot freeze-dryers and production plants
dp/dt
pressure increase test
Measurement of pressure increase in product chamber by closed intermediate valveDetermination of final dryingContinuous determination provides progressive information
dp/dt,
MTM
By measurement with rapid-closing intermediate valves, determination of T at the sublimation interface, and further thermodynamic characteristics (layer flow resistance etc.)Understanding the process, avoiding collapse

MTM = manometric temperature measurement

Pilot freeze-dryers

Dewpoint Moist gas volumeFinal drying of productFurther information can be derived
LyoBalanceMeasurement of product weight during dryingFinal drying of product, sublimation rateAdvantage of immediate measurement of the progress of drying
Pilot freeze-dryers
Camera monitoringVisualDrying quality, artefacts, faultsPilot freeze-dryers and
production plants
Controlled nucleationUniform freezing for homogenous drying and product properties Ice fog + over-pressure methodPilot freeze-dryers and
production plants
AnnealingAlso referred to as tempering (crystal growth by moderate heating of the frozen product)More rapid drying without collapse,
improved structure

Laboratory freeze-dryers,
pilot freeze-dryers and
production plants