INTRODUCTION TO FREEZE-DRYING

Freeze-Drying

Also called lyophilisation, it is a drying process where the wet product is first frozen to a solid phase and subsequently dried to vapour phase through sublimation, that is, without passing through the liquid phase, by exposing it to a low partial pressure of water vapour.

Lyo = Solvent Philo = Friend

The lyophilisation process makes the dried product “solvent loving”

Basic Concepts (1/4): Phase Diagram

Below the triple point co-ordinates, solidice can sublimate to vapour:

1. Liquid is frozen at atmospheric pressure

2. Pressure is reduced

3. Temperature is raised to promote sublimation from ice to vapour

4. Vapour travels to the ice condenser

where it is trapped as ice

5. When drying is finished, trapped ice is melted and drained

Basic Concepts (2/4): Vacuum

Vacuum: an enclosed region of space in which the pressure has been reduced (below normal atmospheric pressure)

Atmospheric pressure: 1.013 mbar

Rough vacuum: 1.013 mbar - 1 mbar

Medium vacuum: 1 mbar - 10-3 mbar

High vacuum: 10-3 mbar - 10-7 mbar

Ultra-high vacuum < 10-7 mbar

Lyophilisation is normally performed at medium vacuum

Basic Concepts (3/4): Eutectic

Above the freezing curve the solution has only one phase and it is liquid

Below the freezing curve there are two phases: pure ice + more concentrated solution

Below the eutectic, there is no liquid phase: everything is solid.

A similar curve is obtained with amorphous materials: eutectic is replaced by glass-transition temperature

Basic Concepts (4/4):Vapour Pressure

The vapour pressure of a liquid is the pressure exertedby its vapour when the liquid and vapour are in dynamic equilibrium.

If a substance is placed in an evacuated, closed container, some of it would vaporise. The pressure in the space above the liquid would increase from zero and eventually stabilise at a constant value: the vapour pressure.

This equilibrium pressure is a function of the temperature of the substance

Process Description

Lyophilisation consists in drying a substance removing the solvent (typically water):

-freezing the solution below the eutectic temperature (normally 10ºC below) - sublimating the solvent ice crystals

Freeze-Drying Steps

FreezingPrimary DryingSecondary Drying

Ancillary operations:

• Loading / unloading the product• Venting (re-establishing atmospheric pressure) / Backfilling• Defrosting• Cleaning of the unit (CIP)• Sterilisation of the unit (SIP)• HEPA filter integrity testing• Leak test of the unit

Freezing

PURPOSE: Cool (freeze) the product achieving a temperature below the eutectic point, getting a crystalline solid (glass state in case of amorphous materials)

An immobilisation of the product is also achieved, avoiding:

• Frothing under vacuum• Material shrinkage• Solvent migration• Concentration during drying• Temperature dependent reactions, specially those leading to loss of activity

Freezing Rate

It is important to have the correct freezing rate to allow the growth of DENDRITIC ICE CRYSTALS.

When sublimated, there are funnels, allowing vapour to escape.

Primary Drying

PURPOSE: remove the ice from the product with:

· Minimum damage to the product· Retention of shape

After the product is frozen at a temperature T1, the chamber is pumped down to a pressure Pc lower than the product saturation vapour pressure Pp.

Sublimation is an endothermic process, latent heat must be provided.

Condenser at a temperature T2 < T1, so that saturation vapour pressure at the condenser Pa < Pc (Pa < Pc < Pp)

Product temperature must be below the collapse temperature.

Primary drying is complete when all the ice has sublimed.

Moisture still remains bound to the product.

Switching from Primary Drying to Secondary Drying

Primary drying is complete when:

- The interface disappears at the vial base- Product and shelf temperature are identical- Chamber and condenser pressure are identical

Secondary Drying

PURPOSE: to remove residual and bound moisture (typically from 10% to less than 2%). Water in the product isn’t crystalline but adsorbed.

Residual water is removed by desorption. Secondary drying time is governed by the water desorption rate from the solid.

This process is temperature dependant and not time dependant. When equilibrium is reached increasing the time is useless.

Product temperature is raised as much as possible without having denaturalisation. Condenser pressure (and so its temperature) must be as low as possible.

Temperature probes now measure real value (but it is useless).

Why Freeze-Dry?

Minimum damage / loss of activity

Low final moisture

Long shelf life

Ease of reconstitution

Possibility of sterile manufacturing

Accurate dosing

When to Freeze-Dry?

Labile products

High value products

Sterile drying

Accurate dosing

Reconstitution is difficult

Long shelf life is required

Retention of form

Freeze-Drying Applications

Antibiotics

Antitoxins

Bacteria / Viruses

Blood coagulants

Enzymes

Fine chemicals

Hormones / Growth factors

Media

Pathological samples

Pharmaceuticals

Plasma

Reagents

Standards

Tissues

Vaccines

etc...

Simplified P&ID of a Freeze Dryer

R efrigera tingS ystem

S helf C oo ling / H eatingS ystem

Ice C ondenser

C ham ber + S he lves

V acuum P um pset