End point determination in primary drying in Lyophilization

Hi Folks,

Primary drying is longest step of lyophilization process. So, it has to be optimized to make the shortest possible lyophilization recipe for the product.

Till previous posts, we came upto setting the conditions for primary drying step of Lyophilization process.
Once the parameters were set, how long the step of primary drying shall be continued???

The answer is till the frozen ice is removed completely (Some authors describe till removal of unbound moisture) from the formulation.

ok, fine. How we will come to know that there is no ice left in the formulation???

The question need an exact answer. It requires evaluation of quantity of ice in lyophilization chamber.
Now the question is How it can be measured?

For all the above questions, there is multiple answers together called as 'Determination of End point of primary drying".

Here it is.....

The primary drying time is directly related to the ice sublimation rate and is determined by numerous factors, including chamber pressure, shelf temperature, heat transfer coefficient of vials, fill volume, and product resistance.

As per the literature, The end point of primary drying can be detected by several different methods. At the end of primary drying, there is no ice present in vials (i.e., no ice sublimation and no heat
removal by sublimation); thus, the product temperature increases to the shelf temperature, and the vapor composition in the freeze-drying chamber changes from essentially all water vapor during primary drying to mostly air or nitrogen. Thus, product temperature data indicate the end point of
primary drying when the product temperature approaches the shelf temperature. Normally, the product temperature as a function of time shows a steep increase in temperature at the end of primary drying, followed by a plateau.

Techniques based on gas composition in the product chamber:
1. Comparative pressure measurement (i.e., Pirani vs.capacitance manometer)
2. Dew point monitor (electronic moisture sensor)
3. Process H2O concentration from tunable diode laser absorption spectroscopy (TDLAS)
4. Lyotrack (gas plasma spectroscopy)

Others:

5. Product thermocouple response
6. Condenser pressure
7. Pressure rise test (manometric temperature measurement (MTM) or variations of this method)


Comparative pressure measurement (i.e., Pirani vs.capacitance manometer): 
Comparative pressure
measurement also works well. The thermal conductivity pressure gauge (Pirani pressure gauge) is calibrated against air and shows higher vapor pressure during primary drying because the thermal conductivity of water vapor is about 1.5 times that of air or nitrogen. At the end of primary drying, the pressure difference between the thermal conductivity pressure gauge (Pirani gauge) and capacitance pressure gauge (MKS Baratron gauge, which measures actual pressure) decreases and approaches zero. The point where the Pirani pressure starts to sharply decrease (i.e., onset) indicates that the gas composition is changing from mostly water vapor to nitrogen; i.e., sublimation is “essentially” complete.

Dew point monitor (electronic moisture sensor):
 Dew point sensors, which can detect the vapor composition change or the relative humidity in the freeze-drying chamber, shows a sharp dew point decrease at the end of primary drying due to the vapor compositions in the chamber changing from almost 100% water vapor to essentially 100% nitrogen.

Process H2O concentration from tunable diode laser absorption spectroscopy (TDLAS): 
Tunable diode laser absorption spectroscopy (TDLAS) directly measures the water vapor concentration (molecules/ cm3) in the duct connecting the chamber and the condenser. The TDLAS unit is commonly installed with two laser beams, one directed with and the other directed against the vapor flow. TDLAS works on basic spectroscopic principles measuring absorption of radiation by water vapor to monitor the trace concentration of water vapor during primary drying that are used to determine the end point of primary drying. The point where water concentration starts decreasing sharply (i.e., onset) indicates that the gas composition is changing, and hence sublimation is

“essentially” complete.

 Lyotrack (gas plasma spectroscopy): 
This method is the latest addition to the online monitoring devices for freeze-drying and is manufactured by Alcatel Vacuum Technology, France. Lyotrack is based on optical emission spectroscopy and measures water vapor concentration during the drying process.

Product thermocouple response:
The end point of primary drying can also be determined from the product thermocouple response, assuming the vials containing the thermocouples are representative of the batch as a whole. Product temperature approaching the shelf temperature set point  is commonly taken as an indication of the end of primary drying.

Condenser pressure: 
During primary drying, most of the gas in the chamber is water vapor, and because the total
vapor flux is high, a high ΔP (difference between chamber and condenser pressure) develops to remove the water from the chamber. However, once primary drying is over, ΔP. decreases (i.e., condenser pressure (Pcond) increases since chamber pressure (Pc) is held constant). The condenser
pressure reflects mostly the partial pressure of nitrogen in the condenser. The purpose of the nitrogen bleed is to control the chamber pressure at the desired set point. The point where condenser pressure starts increasing (i.e., onset) indicates that the sublimation is “essentially” over since the high mass transfer portion of the process (i.e., sublimation) is largely over (Fig. 2). A capacitance manometer installed in the condenser reads the condenser pressure.

Pressure rise test (manometric temperature measurement (MTM) or variations of this method):
MTM is a procedure to measure the product temperature during primary drying by quickly isolating the chamber from the condenser for a short time (≈25 s) and analyzing the pressure rise during this period. This analysis yields vapor pressure of ice at the sublimation interface, the product temperature, and the mass transfer resistance of the dried product.

However, the data obtained measure the vapor pressure of ice accurately only as long as the system remains in primary drying. At the end of primary drying, there is little or no pressure rise because all ice is gone, and hence the calculated “vapor pressure of ice” becomes equal to the chamber pressure. Thus, a close approach of the calculated vapor pressure of ice to the chamber pressure forms the basis of the criterion for end of primary drying. The vapor pressure of ice determined by a fit of pressure rise data to the MTM equations approaches the chamber pressure when no ice remains (Refer: X. C. Tang, S. L. Nail, and M. J. Pikal. Freeze drying process optimization by manometric temperature measurement, 2001 AAPS Annual Meeting, Denver, Colorado, 2001).


Thats all about the methods for determination of end point of primary drying.


With regards,
Teja Ponduri

Pressure in Primary drying phase of Lyophilization

Hi Folks,

Now Let's have look on Primary drying phase aka sublimation phase that will follow when the frozen material, placed under vacuum, is progressively heated to deliver enough energy for the ice to sublime.

During this very critical period a correct balance has to be adjusted between heat input (heat transfer) and water sublimation (mass transfer) so that drying can proceed without inducing adverse reactions in the frozen material such as back melting, puffing, or collapse.

A continuous and precise adjustment of the operating pressure is then compulsory to link

the heat input to the “evaporative possibilities” of the frozen material.

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. Chamber pressure should be well below the ice vapor pressure at the target product temperature to allow a high sublimation rate.

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

temperature (i.e., given ice vapor pressure), the smallest chamber pressure gives the highest ice sublimation rate.

So, as per the above concept let us set the vacuum set point for primary drying step. (temperature selection done as per previous posts).

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

Using the vapor pressure of ice table is a scientific way to determine an appropriate pressure for freeze drying. 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. 

Herewith attached the vapor pressure of Ice table.

During the Initial days of the Lyophilization utilization, it is believed that higher the vacuum the better the sublimation process could be. 

However, very low chamber pressure may cause problems, such as contamination of product with volatile stopper components or pump oil, and also produce larger heterogeneity in heat transfer, thereby giving larger product temperature heterogeneity between vials. In most applications of practical interest, the chamber pressure varies from 50 to 200 mTorr. It is difficult to maintain consistently chamber pressure much below 50 mTorrr, and there is little reason to use pressures much higher than 200 mTorr. It has been reported that moderate chamber pressure (100–150 mTorr) gives optimal homogeneity of heat transfer in a set of vials.Therefore, the optimum chamber pressure is a compromise between high sublimation rate and homogenous heat transfer.

Thats how vacuum is set for primary drying folks.

Yours,
Teja Ponduri

Determining conditions for Primary drying of Lyophilization- Part III (FDM)

Hi Pharma folks,

A Sample done with DSC, the result tells us, what is the thermal event , is it a glass transition, is it eutectic melt, and what are the critical temperatures that are associated with it. Glass transition  doesn’t mean that’s going to collapse the sample.Sometimes we may have a glass transition that occurs, and we see no collapse. So this is the beauty of using both the microscope and the DSC, in that the microscope complements and supports the data that we get from the DSC.

In Freeze dry microscopy tells us  where that sample physically loses structure.

What actually happens in Freeze dry microscopy?

The sample upon exposure to heat during DSC, may undergoes various thermal events. But, we cannot view/visualize  those events. There comes Freeze dry microscopy (FDM). With FDM, we can view the various thermal events of sample directly with change in temperature as well as pressure combination. Nothing but, we can simulate the conditions of Freeze drying at micro level and see what happens to the sample.

In FDM, very tiny sample quantity (typically Less than 2 µL) will be placed on thermally controllable stage and completely enclosed in a chamber in order to hold the vacuum applied. (just imagine a lyophilizer chamber with shelves). The sample will be undergone cooling and followed by applying vacuum and further increasing the temperature at a slow phase to see at what temperature the collapse of the sample is visible.

FDM is nothing but, a direct examination of stages of freeze drying using a special microscope and a thermal stage (Thermally controllable stage as mentioned in above paragraph).

What a Freeze dry microscope contains?

Sample Preparation:

The bulk solution which has to be lyophilized should be the sample for Freeze dry microscopy. The sample which has to be lyophilized should be utilized in FDM. Below diagram depicts the sample preparation at a glance.

Initially, the sample stage will be like this.

Now, let us know about parts of a stage.

The sample stage is provided with a vacuum ports, and for stage adjustment purpose will have X and Y manipulators visible on exterior view and also a sample holder will be present inside which will be place onto the stage with the help of sample door lock.

Coming to the interior of stage, a 22 mm silver block containing 1.3 m  light aperture onto which the sample will be placed, and is connected with below parts as provided in the picture.

• Liquid nitrogen inlet
• Liquid nitrogen outlet
• Thermocouple leads (for temperature determination)

Before sample loading, below procedure shall be followed.

• Ensure silver block is cleaned and have enough silicone oil on the aperture. 
• Place a 16 mm cover slip and a sample separator and then 2 µL bulk solution followed by 13 mm cover slip.

Once sample is prepared, the stage is closed with a lid and is ready for the evaluation.

Operation:

• Initially the Freeze dry microscope should be calibrated with known concentration of calibrating substance such as NaCl. 
• Once, the collapse temperature was attained within a specified range, then actual sample shall be evaluated.
• The sample will be ran through the steps of freezing similar to lyophilization recipe but at a faster rates during freezing. 
• Once sample was frozen completely, then focus has to be adjusted such that sample was clearly visible clearly. 
• Then vacuum pump was switched on and slightly temperature was increased to start primary drying at a rate that all the thermal events are clearly recorded. 
• If any rough idea about the range of the transition temperature then at that temperature, drying at a slow rate shall be performed.
• The images will be captured such that at material changes at all temperature points with each 0.1°C for accurate recording of collapse.    
• Once collapse is identified, it can be taken a screenshot and video/images can be recorded.
• Then the sample shall be brought back to room temperature and   vacuum pump to be swithced off, and vacuum to be made to atmospheric pressure. 
• Stage shall be cleaned and closed with the lid. 
• The example FDM data is provided below which helps to interpret the data.

  

Note: 

Why colors ?

When we use a Ist order red compensator, depending on the material whether Isotropic or an-isotropic in presence of polarized light, the images obtained are colored as per the orientation of ice crystals frozen direction (IF ice crystal formed parallel to the direction of light path will show one color and if in perpendicular to light path then other color).

For complete concept please refer (Practical Application of FD Microscopy in Product Thermal Characterization, by Ruben Nieblas).

Thats all for the FDM, will post on primary drying concept in the next post.

Till then, take care.

Yours, 

Teja Ponduri

Determining conditions for Primary drying of Lyophilization- Part II (DSC-II)

Hi folks,
In the last blog "Determining conditions for Primary drying of Lyophilization- Part I" we have seen the utilization of Differential scanning calorimetry (DSC) to determine the glass transition temperature of formulation, that helps us to set shelf temperature during primary drying of lyophilization of pharmaceuticals.

Now lets see other useful information that can be obtained from DSC.

If the sample run through DSC, it makes sense that every possible information shall be obtained. Hence, lets have a glance on those information.

Crystallization: 
A Sample after glass transition, substances will have a lot of mobility and never stay in one position for very long time. But when they reached a specific temperature, it will give off enough energy to move into very ordered arrangements, which are called crystalline substances and they release heat. So it doesn't have to put out much heat to keep the temperature of the sample pan rising. This drop in the heat flow as a big peak in the plot of heat flow vs. temperature.

The temperature at the highest point in the peak is usually considered to be the crystallization temperature, or Tc. Also, the area of the peak can be measured, which tells us the latent energy of crystallization of the substance. But most importantly, this peak tells us that the substance can in fact crystallize. If 100% amorphous polymer is analysed, like polystyrene, this peak cannot be obtained, because such materials don't crystallize also, because the polymer gives off heat when it crystallizes, called as crystallization is an exothermic transition.

Also, liquid formulation during freezing step once crystallization occurs that may result in little sharp peak from base line of a DSC as shown in above figure.

Now lets see, what happens when the substance gets heated beyond its crystallization temperature. The resulting thermal transition is called Melting.

Melting:

When substance's melting temperature is reached, polymer crystals begin to fall apart, that is they melt. It comes out of their ordered arrangements, and begin to move around freely that can be spotted  on a DSC
plot The heat which polymer give off when crystallized is absorbed when reached at Tm. That is a latent heat of melting like latent heat of crystallization. When the polymer crystals melt, they must absorb heat in order to do so. Melting is a first order transition. This means that at the melting temperature, the polymer's temperature won't rise until all the crystals have melted. The heater under the sample pan has to put a lot of heat into the polymer in order to both melt the crystals and keep the temperature rising at the same rate as that of the reference pan.  This extra heat flow during melting shows up as a big dip on DSC plot, like this:

In a DSC curve, all the three phase transitions seen above Glass transition, Crystallization, melting are denoted in a single curve as shown below.

 Before concluding the topic of DSC, for a better understanding on variations in DSC curves due to  crystallization and melting process are provided below.

Conclusion:  
Even though DSC is useful for Tg determination, it should not be the one and only evaluation to determine the primary drying temperature. As any method has its own uses and limitatiions, DSC has below limitations.

• can not really control the rate of experiment (can only be checked but cant controlled)
• Dependent on too many parameters (Thorough understanding of analysis is required for analyst)
• Very sensitive to any changes 
• Result depends a lot from the operator (Well trained operator required)

That's all for today folks. Hope the content was useful.

Take care......

Regards,
Teja Ponduri

Determining conditions for Primary drying of Lyophilization- Part I (DSC-I)

Hi folks,

In the previous post, we have seen about sublimation science, which happens in primary drying. Now lets go for establishing the conditions for sublimation.

It is well known that primary drying shall be done below the critical temperatures.The critical temperature is nothing but the Collapse temperature. Determination of the critical collapse temperature of a product is an important step in establishing and optimizing a freeze drying process. This critical temperature determines the maximum temperature that the product can withstand during primary drying without it melting or collapsing.

The critical temperature depends on the nature of the formulation.

Crystalline products have a well defined “eutectic” freezing/melting point that is its collapse temperature.

Amorphous products have a corresponding “glass transition” temperature and they are much more difficult to freeze dry. The collapse temperature of amorphous products is typically a few degrees warmer than its glass transition temperature.

A eutectic is an intimate physical mixture of two or more crystalline solids that melts as single pure compound. So at Eutectic temperature, is the temperature where the mixture melts. When the solids melt and ice get sublimed, the melted solutes will remain and that is called collapse or melt back. 

In Amorphous system, glass transition temperature corresponds to a change in the viscosity of solution from a viscous liquid to a glass or an essentially solid solution of solute in water.

So, Eutectic temperature is the critical temperature for crystalline systems. For Amorphous systems, critical temperature is few degrees warmer than glass transition temperature. 

Usually, the critical temperatures are identified by thermal analysis of the formulation. Currently, major techniques that are using in Differential scanning Calorimetry (DSC) & Freeze Dry Microscopy (FDM).

In the current post, we will have a look at utilization of Differential scanning Calorimetry (DSC) to identify the critical temperatures.Before going to know how Tg is determined by Differential scanning Calorimetry, it makes sense to know basics of it.

DIFFERENTIAL SCANNING CALORIMETRY (DSC)

Differential scanning calorimetry (DSC) is one of the popular and most used thermo-analytical techniques. Thermo= Heat, Analysis= detailed examination. Means thermo-analytical techniques like DSC, are analytical techniques that examine the properties of materials which change with temperature.

Differential scanning calorimetry (DSC) measures enthalpy changes in samples due to changes in their physical and chemical properties as a function of temperature or time.

Enthalpy is defined to be the sum of the internal energy E plus the product of the pressure p and volume V. Using the symbol H for the enthalpy:

H = E + p * V

( Enthalpy is similar to energy, but not the same. When a substance grows or shrinks, energy is used up or released. Enthalpy accounts for this energy. Because of this, scientists often calculate the change in enthalpy, rather than the change in energy.) 

DSC is a technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment.

Principle of DSC: 

Differential scanning calorimetry (DSC) measures the difference between the heat flows from the sample and reference sides of a sensor as a function of temperature or time.

When a sample undergoes a physical transformation such as a phase transition, more or less heat will need to flow to it than to the reference (typically an empty sample pan) to maintain both at the same temperature. Whether more of less heat must flow to the sample depends on whether the process is exothermic or endothermic.

For example as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. 

If a sample undergoes exothermic processes such as crystallization, less heat is required to raise the sample temperature by observing the difference in heat flow between the sample and reference, DSC is able to measure the amount of heat absorbs or release during such transition.

A typical Differential scanning calorimeter set up in a laboratory is provided below.

The various kind of sample pans/ crucibles for using in DSC are provided below.

The Differential scanning calorimeter and its componets shown in a  block diagram

Before going to know about DSC thermograms and their interpretation, lets go through some key terminology which helps to understand the DSC curves.

Heat flow: 

The heat flow is the amount of heat supplied per unit time. Heat flow = heat(q) / time (t) = q / t
This is the parameter that is plotted on Y-axis against temperature on X-axis of result of DSC which is represented in DSC curve i.e., thermogram. 

Endothermic Event: 

It is a thermal event of a material where energy is absorbed by the material, i.e. melting.

Exothermic Event : 

It is a thermal event of a material where energy is expelled by the material, i.e. crystallization.

Glass Transition (Tg): 

It is an endothermic event, a change in heat capacity that is depicted by a shift in the baseline. It is considered the softening point of the material or the melting of the amorphous regions of a semi-crystalline material. 

Scanning: 

Heating or cooling at a controlled rate.  

How the sample is loaded in DSC: 
Please refer the link provided https://youtu.be/aTWVCfRlMX8

How the result from a Differential scanning calorimeter represented:

Differential Scanning Calorimetry (DSC) is a method involving the measurement the difference of heat flow between a tested sample and a reference sample which is generated by the temperature control system.
The result of calorimetric measurements is a DSC curve shown as the temperature/time dependence on the heat flux (per time unit) and is provided below.

After sample was loaded and scanning ( Heating or cooling at a controlled rate) was done and no transformations/reactions occurs in the sample then, a 'baseline' forms and is represented below.

In the above example plot of heat flow versus temperature for a material that does not undergo any changes during the heating.

Any material when subjected to heat i.e., heat is given to the material or taken away from the material, then those changes are captured in DSC curve as represented below.

An endo- and exothermic peaks are recorded on these curves, which result from the temperature differences between a tested sample and a reference sample, showing negative or positive deviations from the baseline.

Now lets have a look on various changes in materials which are either endothermic or exothermic.

Endothermic events:

Melting
Sublimation
Desolvation chemical reactions

Exothermic events: 

Crystallization
Decomposition chemical reactions

Now, guess what kind of change a glass transition is???????

Actually glass transition is a shift in the baseline rather than an event that slightly endothermic in nature.

Glass Transition: 

Glass transition is physical state conversion of glassy material to a rubbery material. 

Glass transition temperature is the temperature, below which the conversion of glassy material to a rubbery material. 


At this point the mechanical properties of the material change from those of an elastic material to those of a brittle one due to changes in chain mobility.

A typical example of a heat flow versus temperature plot at a glass transition temperature is shown below.

The glass transition results in a kink in the heat versus temperature plot due to the change in heat capacity (A). 
In a plot of heat flow versus temperature it is a gradual transition that occurs over a range of temperatures (B). 

The glass transition temperature is taken to be the middle of the sloped region. The temperature in the middle of the inclined region is taken as the Tg.

The heat capacity of the material is different before and after the glass transition temperature. 

The heat capacity Cp of material is usually higher above Tg.

The value of the glass transition temperature depends on the strain rate and cooling or heating rate, so there cannot be an exact value for Tg.

When a certain amount of heat is transferred to the sample, its temperature increases by a certain amount, and the amount of heat it takes to get a certain temperature increase is called the heat capacity.  (The heat capacity (Cp) of a system is the amount of heat needed to raise its temperature 1◦C. )

Glass Transitions characterized by change in heat capacity (no heat absorbed or evolved). 

* Transition from a disordered solid to a liquid

* Appears as a step (endothermic direction) in the DSC curve

* Gradual enthalpy change may occur, producing an endothermic peak superimposed on the glass       

   transition.

(The glass transition (Tg) has been called the “melting of amorphous material” and as unscientific as that is, it's an adequate description. Amphorus material such as glass has no organization in the solid state – it is random. This gives it the transparency that glass has, among other properties. As you warm it up, its heat capacity increases. At some point you have enough energy in the material that it can be mobile. This  requires a fair amount of energy compared to the  baseline increase, although much less energy than the melting point does. This energy normally appears as a step change in the instrument baseline – pointing up in heat flow instruments and down in heat flux.)

For better understanding, few DSC curves showing glass transition temperature from Literature are provided below. 

It is important to note that the transition does not occur suddenly at one unique temperature but rather over a range of temperatures. 

That's about determination of glass transition through DSC. 

Till then, Bye...Bye.....

Your's

Teja Ponduri

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.............