Sublimation science of Primary Drying in Pharmaceutical Lyophilization

In continuation from INTRODUCTION TO PRIMARY DRYING IN LYOPHILIZATION (https://pharmaperception.blogspot.com/2019/03/introduction-to-primary-drying-in.html)


Once freezing (and annealing if necessary) are complete, as we kick start the vacuum pump, ice sublimation begins, and the cycle moves into the primary drying phase. Primary drying will continue until all of the pure ice, surrounding the solute components of formulation is removed.

Hence, Primary drying is nothing but sublimation. Hence, it makes sense to understand Sublimation science to know about primary drying in lyophilization.

Sublimation is when a solid (ice) changes directly to a vapor without first going through a liquid (water) phase. Sublimation is the direct conversion of a solid to a gas or vapour. 

Sublimation in freeze drying process requires,
i) A Completely frozen / solid formulation
ii) Vacuum below triple point of water
iii) Sufficient heat to provide energy for sublimation


As lot of literature available on this topic, lets have a brief  look into it.

The above phase diagram is taken from SP scientific website and it figured out the behavior of substances under various circumstances. 

Initially, let us start with Liquid phase (Water) as the formulation we prepare will be in liquid (bulk solution). The temperature of bulk solution will be reduced to sub zero levels during freezing step. But keep in mind that, it will be performed at atmospheric pressure only. This results in freezing of water in the formulation. This is the conversion of liquid phase to solid phase.

Now, we have solid ice. Unless the freezing step, where pressure is not the major concern, sublimation requires vacuum depending on the vapor pressure of solvent. From the phase diagram provided above, once freezing occurs, by reducing the pressure to below atmospheric pressure, sublimation gets initiated. At a pressure of 4.58 torr / 0.006 Bar/ 0.006 Atmospheric pressure ice begin to convert as vapor. Remember, this happens at low pressure.

The next transition is condensation where vapor gets converted to liquid. The vapor collected during sublimation step (Primary drying) will go to condenser and converted to liquid (But as condenser is at the sub zero temperatures, the water immediately gets converted to ice which is later removed through defrosting). 

For much clarity, pls have a glance https://www.youtube.com/watch?v=HEzkHqWIiKM.

So far, we have seen the phase transitions in related to freeze drying. Now let us know more about sublimation behavior. 

Sublimation always starts at an open surface and then moves inwards into the sample. After some of the ice is sublimated, the sample exhibits two distinct regions, namely: the dry layer (from which ice crystals have sublimated) and the frozen layer (where ice crystals are still present). These two regions meet at the so-called “ice interface”, “Sublimation interface”, “freeze-drying interface” or, simply, “interface”. 

The sublimation step requires very careful control of two of the key variables of the freeze-drying process: temperature and pressure.

While sublimation can occur at atmospheric pressure, the process is rather slow because the gas molecules from the ice must find their way through the atmospheric gases that are bombarding the surface of the ice. This slow process by which the water molecules leave the ice surface is known as “diffusion”.

The rate of sublimation of ice from the frozen product depends on the difference in vapour pressure between the product and the ice collector. It can be increased by decreasing the pressure over the ice surface. This can be accomplished by placing the frozen material in an evacuated chamber, where molecules will migrate from the sample to an area of lower pressure. 

Although the extent to which the pressure is reduced increases with increasing vacuum in the chamber, there is relatively little change in sublimation rate of the water from the ice surface. Only when the pressure in the chamber becomes less than the pressure of the ice is there a marked increase sublimation rate. The vapor pressure of ice is dictated by the ice temperature.

That's it for the day folks...

In the next blog, we will meet with concept of determination of critical temperatures. 

Till then, TATA.....

Yours,

Teja Ponduri....................

INTRODUCTION TO PRIMARY DRYING IN LYOPHILIZATION

Hi pharma folks,

Lets proceed towards our topic of perception……PRIMARY DRYING in Lyophilization.

Till now we have gone through the freezing step, where In the freezing stage, the solution or product to be processed is cooled down to a temperature where all the material is in a frozen state.

Once the formulation was frozen completely, then it further proceeds for drying. Actually any pharmaceutical formulation that has to be lyophilized, has to pass through two stages of drying namely primary drying and secondary drying.

The frozen water (if aqueous formulation) or solvent (If non aqueous formulation) is removed through the process called sublimation (A process in which the ice crystals / frozen solvent gets converted to vapour without passing through the phase of liquid). Once there is no ice is available to sublime (at the sublimation interface) then it is called end of primary drying step.

So, as a formulator, you may have some questions…

• ·         What is the temperature to be chosen for primary drying?
• ·         For how much time it must be carried out?
• ·         How do we know that this is the time to stop the primary drying step?
• ·         What are all the equipment/instrument we should have for determination of primary drying end point?
• ·         What to do if we don’t have any equipment/instrument for determination of primary drying end point?
• ·         Does pressure have role in primary drying? If so, what is the set point?
• ·         What are the other factors affecting the primary drying process?

So, lets imagine, we have a frozen formulation and a lyophilizer with us. Now first I must know the answer for basic needs to proceed for primary drying that I can control. They must be set as a recipe in the lyophilizer right at the beginning itself.

• ·         Shelf set temperature
• ·         Chamber vacuum
• ·         Condenser temperature
• ·         Duration of primary drying

If we know the answers for setting above parameters, then the job is almost done.

Shelf set temperature:

The temperature is required for Sublimation, as it requires heat energy to drive the phase change process from solid to gas.

Before setting the shelf temperature, let’s have an idea about the shelf. 

Shelf in any lyophilizer act as a heat exchanger, removing energy from the product during freezing, and supplying energy to the product during the primary and secondary drying segments of the freeze-drying cycle. 

This energy exchange is traditionally done by circulating a fluid through the shelves at a desired temperature. 

Usually in many lyophilizers, shelves will be connected to the silicone oil system through either fixed or flexible hoses. 

The temperature is set in an external heat exchange system consisting of cooling heat exchangers and an electrical heater. 

The fluid circulated is normally silicone oil. 

This will be pumped around the circuit at a low pressure in a sealed circuit by means of a pump.

A commercial lyophilizer shelf was shown below. 

So, to maintain the product at a desired temperature, set the shelf temperature as closer as possible to the required product temperature. 

But keep in mind that, required product temperature is always 2-3 °C colder than the critical temperature or collapse temperature. 

(Remember that, Each product has a unique critical temperature. It is necessary to keep the product temperature safely below this critical temperature during primary drying to avoid collapse. we will have a broader look at it in later posts).

For example, If my product critical temperature (Collapse temperature) is -22°C. so, I should maintain my product at around -22°C. It means, the desired product temperature should be 2-3° colder than -22°C to prevent the product from collapse. Hence, I may chose to dry my product at a shelf temperature set at -24°C to -26°C.

Note: Sometimes, the product can be dried at a shelf temperature equal to collapse temperature also.

Chamber pressure:

Primary drying is carried out at low pressure to improve the rate of ice sublimation. 

The chamber pressure impacts both heat and mass transfer and is an important parameter for freeze-drying process design.

Hence, during primary drying, the chamber pressure is well below the vapor pressure of ice, and ice is transferred from the product to the condenser by sublimation and crystallization onto the cold coils/plates (<−50°C) in the condenser. 

The sublimation rate is proportional to pressure difference between the vapor pressure of ice and the partial pressure of water in the chamber, this difference being the driving force for ice sublimation. The partial pressure of water in the chamber is essentially the same as chamber pressure during primary drying.

A recommended approach is to first set the chamber pressure using the vapor pressure of ice table.  

A general guideline is to choose a system pressure that is 20% to 30% of the vapor pressure of ice at the target product temperature. 

When the vacuum level set point is deeper than the vapor pressure of ice at the current product temperature, sublimation can take place. 

Typically, vacuum levels for freeze drying are between 50mTorr and 300mTorr with 100mTorr to 200mTorr being the most common range.

For an example, our desired product temperature for primary drying is -24°C. then, the vapour pressure of Ice at -24°C is 524.30 mTorr or 0.7 mBar. 

Hence, as a rule of thumb, I will choose a chamber pressure as mentioned below.

Product Temperature	Vapor pressure of ice at temperature	20% of Vapor pressure of ice at temperature	30% of Vapor pressure of ice at temperature
-24°C	524.3  mTorr	104.9 mTorr	157.3 mTorr
Chosen chamber vacuum: 105 mTorr

Condenser temperature:

The condenser is also called as a cold trap. 

It is designed to trap the solvent, which is usually water, during the drying process. 

The process condenser will consist of coils or sometimes plates which are refrigerated to allow temperature. 

These refrigerated coils or plates may be in a vessel separate to the chamber, or they could be located within the same chamber as the shelves.

     

            

The condenser temperature required is dictated by the freezing point and collapse temperature of the product. 

The refrigeration system must be able to maintain the temperature of the condenser substantially below the temperature of the product.

Thus, the condenser temperature is always less than the shelf temperature / product temperature.

As rule of thumb, during drying the condenser temperature shall be set below -50°C.

Duration of primary drying:

Usually when we have to detect the end point of primary drying (using various techniques/devices to determine the Barometric Endpoint Determination).

If we don’t have any external devices/instruments to help in this regard, then the end of primary drying is identified by the product temperature reaching the set shelf temperature.

The temperature or literally energy we are providing to the product during primary drying for sublimation is taken up by ice. 

So, this loss of temperature to sublimation ensures that always colder product than shelf set temperature during the primary drying.

Once all the ice is sublimed and there is no ice left, then the temperature reaches directly to product and that results in increase inn product temperature. 

This increase in temperature continues till the maintenance of equilibrium of  shelf and product temperatures.

Another way to determine the end point of primary drying is by pressure rise. 

The pressure is lyophilizers at laboratory scale is by Vacuum Gauges  (Pirani & Capacitance Manometer).

Capacitance Manometers  (Give true vacuum readings and are not erroneously influenced by water vapor).

Pirani Gauges (Give artificially high readings proportional to the amount of water vapor present in the lyophilizer chamber).

With decrease in water vapour in the chamber, pirani gauge shows decrease in chamber pressure. At one point, both pirani and capacitance manometers will show same pressure (a single line in the lyo graph).

So once we got confirmation about any of the indicators (temperature /pressure) regarding the end of primary drying then it is the time to proceed for the secondary drying.

That's it for the day folks...

will go a bit deeper into the aspects of primary drying in further posts...

till then take care, by bye....

Yours. 

Teja Ponduri.............

Overview of Freezing in Freeze drying

Hi pharma folks,

Till now we have gone through the freezing concept in freeze drying of pharmaceuticals.

Lets have a glance on what we have covered so far.

 If you can freeze it, you can freeze dry it. That is the importance of freezing in lyophilization.

Freezing phase: The principal dehydration step. Most of the solvent (typically water) is separated from the solutes to form ice.

Ok, lets freeze our formulation (In the form of aqueous bulk solution) by cooling in a lyophilizer at sub zero temperatures. 

As we all known from school days, water freezes at 0°C, let us keep the bulk solution on the shelves whose temperature is around -10°C and hold it for some time lets say 60 min. 

Guess, what might be the observation???

There is partially frozen vials.........

Why??? Because of freezing point depression.

The pure water freezes at 0°C. But the addition of Active ingredient, excipient (Bulking agent, preservative, pH adjuster, Buffer etc) may cause the depression of freezing point of the resulting bulk solution which is Pure water +Active Ingredient+ Excipient. 

How the freezing point depression happens?

As we all know that, Freezing point depression is a colligative property observed in solutions that results from the introduction of solute molecules to a solvent. The freezing points of solutions are all lower than that of the pure solvent and is directly proportional to the molality of the solute. 

Freezing point depression is nothing but difference between freezing point of the pure solvent and solution. It is calculated by the formula provided below,

 ΔT_f =T_f(solvent)− T_f(solution)=K_f×m 

Where,

ΔT_f  is the freezing point depression,

T_f  (solution) is the freezing point of the solution,

T_f  (solvent) is the freezing point of the solvent,

K_f  is the freezing point depression constant, and

m is the molality of non volatile solutes.

The freezing point depression constant of solvents per mole of a solute dissolved were provided below from literature. 

Solvent	Kf
Water	1.86
Acetic acid	3.90
Benzene	5.12
Phenol	7.27

The freezing point of a solution is less than the freezing point of the pure solvent. This means that a solution must be cooled to a lower temperature than the pure solvent in order for freezing to occur.

The freezing point of the solvent in a solution changes as the concentration of the solute in the solution changes (but it does not depend on the identity of either the solvent or the solute(s) particles (kind, size or charge) in the solution). 

Another reason for the freezing of pharmaceutical formulations at very low temperatures than 0°C is Super cooling.

·         Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid. 

·         A liquid below its freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form.

·         In a clean room environment of pharmaceuticals, there is less probability of seed crystal (sub micron particles, undissolved drug particle etc).

·         Also this is the reason for difference in freezing temperature of lab scale and pilot scale.

·         This difference of temperature between  equilibrium freezing point and actual nucleation temperature (The point at which ice crystals begins to form) is known as Degree of super cooling. 

·         The degree of super cooling may change with cooling rate (low freezing rates, higher super cooling). Degree of super cooling  at Lab scale typically, in the range of -5°C to -15°C (Particulate contamination), and at Production scale Can be as low as -40°C (sterile environment). 

·         Also, there is impact of Rate of freezing on Crystal size of Ice. 

·         The crystal size formed during freezing can significantly affect the dissolution rate of the dried material.

A fast ice growth also helps to prevent the denaturation of proteins (If present) which may result from prolonged exposure to strong concentrations of salts because of slow ice growth.

Rapid cooling results in small ice crystals, useful in preserving structures to be examined microscopically, but resulting in a product that is, more difficult to freeze dry.

Slower cooling results in large ice crystals and less restrictive channel in the matrix during the drying process.

The main pores in the solid residue after freeze drying are those left by the sublimation of pure ice and they form the principal channels for the escape of vapor. 

After completion of freezing, that is after complete solidification of formulation, once ice sublimation begins, then the remaining structure should be maintained intact and should not collapse. 

This depends on the size of ice crystals (Large/small) and type of solutes in the formulation (Crystalline/amorphous/ partially crystalline). 

A frozen formulation contains, Ice crystal and solutes. The space between ice crystals and solutes is called Interstitial space (some authors described as  Interstitial state also). 

For the crystalline solutes, the interstitial material consists of a mixture of eutectic ice and crystalline solute. when ice is removed by sublimation, a crystalline solid with very little water is left.

For amorphous system, the interstitial glassy material must be rigid enough to support its own weight after the ice is removed in order to keep the micro-structure established during freezing. 

Examples of Crystalline substances:

• Glycine
• Mannitol
• Sodium carbonate
• Dibasic sodium phosphate
• Citric Acid

Examples of Amorphous substances:

• Dextran
• Fructose
• Gelatin
• Sorbitol
• Maltose
• Trehalose
• Lactose
• Glucose

For some solutes, it is observed that, there is incomplete crystallization of solutes during freezing stage and the complete crystallization occurs at sublimation step. This results in the breakage of vials. Examples Formulations containing Mannitol, glycine.

For those formulations, it is required to allow them for sufficient time at sufficient temperature to crystallize completely during freezing step only.   

Technically the phenomenon of holding the product at a temperature above the final freezing temperature for a defined period to crystallize the potentially crystalline components (usually, crystalline bulking agent) in the formulation during the freezing stage.

The temperature of Annealing is always above (Warmer) the Glass transition temperature of formulation and below (Colder) the Melting temperature (Eutectic melt) of formulation. 

Literature reveals that annealing has impact n primary drying time and product appearance, reconstitution time etc..

That’s it guys, this is my perception on freezing in pharmaceutical freeze drying.

Lets close today's write up and will catch you soon.

Till then,

Teja Ponduri signing off…….