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Development of a complete process cycle from lab to pilot scale and beyond

In the previous blogs the focus was put on screening the parameters completely to let a molecule to crystallize to its desired critical material attributes (CMAs), most notably polymorphism, size and size distribution, and shape. Please note that our clients also investigate at this level the appearance of the powder to already give qualitative impressions on the flowability of the obtained powder, filtration behavior, compaction of the powder, etc. to already gain insight in the processability of the material. In general, you can create the following matrix valid for antisolvent crystallization with all parameters included.

SCT-LAB Parameters
Solution concentration
Solution temperature
Antisolvent
Antisolvent temperature
Flow rate solution
Flow rate antisolvent
Mixing insert
Reactor volume
Post treatment
Growth temperature
Growth time
Filtration conditions
Results
Crystal size
Crystal appearance
Crystal shape
Crystal size distribution
Powder appearance
Filterability
Powder flow appearance

 

Table 1: Parameter settings and results table for crystallization tests using the SCT-LAB equipment

Once this has been set, the user is ready for long-term testing at the pilot scale. One of the major benefits of Secoya’s Crystallization Technology is that the upscaling on the SCT-PILOT is performed using exactly the same parameter settings as for the SCT-LAB tests as pointed out in table 1. In this blog post, we elaborate on the example of lactose production, work that has been published as well: https://pubs.acs.org/doi/abs/10.1021/acs.oprd.3c00132

Lactose is extremely soluble in water, the solubility curve is shown in Figure 1. Several tests using our SCT-LAB instrument have demonstrated that the metastable zonewidth is very wide, at a concentration of 700 mg/mL nulceation has been observed inside the reactor at 5°C. This corresponds to a supersaturation value s of about 4. Compared to other molecules, this is not an extreme value, however, due to the large solubility of lactose, one cannot use a wide variation in temperatures to find different nucleation conditions: concentration higher than 1000 mg/mL are not workable, temperatures lower than 0°C become so viscous they do not run through the reactors anymore. Therefore the operational zonewidth is a small triangle between the orange dotted MSZW, 1000 mg/mL in concentration and about 2°C in temperature, indicated with the green zone in Figure 1. Nonetheless, using a specific execution of the reactor to cope with the high viscosity of the solution at low temperatures at 5°C nucleation temperature and a solution concentration of 850 mg/mL nucleation conditions have been found that result in the desired particle size and shape as was verified with laser particle sizing by our client. Microscopic images of the obtained crystals are shown in Figure 2. In Table 2 the corresponding SCT-LAB results table is found. This table is usually based on several repetitions of the same set of parameters to check the robustness of the methodology at lab scale.

Figure 1: Solubility of lactose in water as a function of temperature (blue), experimentally validated metastable zonewidth inside our reactors in orange.
SCT-LAB Parameters
Solution concentration
850 mg/mL
Solution temperature
85 °C
Flow rate solution
20 mL/min
Mixing insert
Specific execution
Reactor volume
7 mL
Reactor temperatue
5 °C
Post treatment
Vessel at 60 rpm
Growth temperature
5 °C
Growth time
18 hrs
Filtration conditions
No washing at present
Filter funnel
Results
Crystal size
D10
32 µm
D50
60 µm
D90
90 µm
Standard deviation in between repeated tests
2.1 µm
Crystal appearance
Individual crystals Distinct aspects Transparent surface
Crystal shape
‘tomahawk’ shaped crystals
Powder flow appearance
Some agglomeration
Fluffy powder
Filterability
Vacuum filtration fast
Powder flow appearance
Nicely flowing powder

This exact configuration was used for pilot testing, identical reactor conditions were set out on the pilot unit by connecting the SCT-PILOT unit to a IKA Easysyn 2000 system, set at 5°C, see Figure 3. The connection between both is simply made by putting the end of the Secoya reactor into the vessel and let the vessel fill up during production. 2 liter productions were performed a the start. Once all material was collected, the vessel was continuously stirring at 60 rpm during 18 hrs. At that point in time, the material was filtered using a stainless steel Nutsche cel from DrM, the lactose was washed with ethanol and evaluated for size and shape.

Figure 3: SCT-PILOT setup (left), IKA Easysyn 2000 unit (middle), and lab scale 100 mL Nutsche cell from DrM.

Weight calculation measurements of dried samples resulted in a 93% yield, holding the solubility of lactose in water into account. In the publication the study to reach equilibrium can be read in further detail. In Figure 4 two microscopic mages are shown for two different pilot productions. The typical tomahawk shaped crystals can be seen, there seems to be some coagulation, due to the large remaining part of lactose in solution. Washing with ethanol was performed to reduce this effect. Another test was performed as well to cross-check the effect of Secoya’s reactors. Identical quantities of product were dissolved and cooled down in the same vessel with the same residence time afterwards. The overall size of the product increased drastically to 90 µm average size, so 50% larger crystals are obtained. So, even though you can nucleate lactose in a vessel by cooling it down, the quality of product and the control in size is only guaranteed by the passage through the Secoya’s reactor. This size variation can be seen in Figure 5 where the lab tests, pilot test and batch test are compared in size, the d10, d50 and d90 values are shown in a spider-plot. This kind of plot makes it very easy to see true outliers of any test, and visibly the batch test is the one where the sizes are completely different as opposed to the tests using Secoya’s reactors, whether with the lab tests or pilot tests.

Figure 4: Microscopic image of two distinct sample originating from two different pilot test using the parameter settings from Table 2.
Figure 5: Spider-plot showing particle size attributes for different tests on lactose crystallization: LAB indicates test on the SCT-LAB instrument, PILOT on the SCT-PILOT unit, BATCH indicates the cross-test without a passage through our reactor, at pilot scale.

Further upscaling is then performed by placing the reactors parallel to each other. As the parameter settings at lab scale are completely identical to pilot scale productions, one just has to connect a second, third, fourth, or more reactor with its own flow control. In the next blog more information on this final step is shown.

Discover the Secoya Crystallization Technology

Versatile equipment for multiple crystallization modes and particle sizes range

The versatility of the Secoya crystallization platform

Secoya’s crystallization technology allows a very wide range of different particle sizes whilst maintaining the same production yield. Moreover, all different important screening parameters for continuous nucleation can be performed on one single instrument, the SCT-LAB. And, all these parameters can be extrapolated to the same configuration at pilot scale and industrial production scale.

The SCT-LAB equipment for example, Figure 1, is accommodated to heat solution and antisolvent in place (and agitated) up to 85°C, which covers most of the organic solvents and solubilities of small molecules in solution. The syringes of the system are set at the same dissolution temperature, but also the antisolvent syringe can be cooled to 5°C. then, the inserts that are available for both antisolvent and cooling crystallization are placed on the SCT-LAB, it is a software choice to use only the solution syringe when cooling crystallization is performed. Also the different existing reactor types can be easily installed and uninstalled on the SCT-LAB so that in between testing different combinations of insert and reactor can be generated to help faster screening of different conditions.

Figure 1: The SCT-LAB instrument

These combinations have led to the development different crystallization processes and amorphous precipitations between several hundreds of micrometers and sub-micrometric particles. Evidently, to reach submicrometric particle sizes in suspension surfactants are needed to prevent aggregation. However – to the benefit of the technology – these surfactants can be mixed with either the solution phase or antisolvent phase. Thanks to the intense interaction between surfactant and recently created particles inside the mixers a number reported drastic drops in used surfactants as opposed to other ways fabrication have been published (https://doi.org/10.3390/pharmaceutics16030376).

Impact of nucleation temperature

Example 1: COOLING-0 + 3 mL reactor

We have developed a setup that enables both the finalization of the acetylation step of salicylic acid into acetyl salicylic acid or Aspirin. To do so a mixture of salicylic acid  and acetic anhydride was preheated and flushed through a tubing at 90°C in oder to fully converse the reaction. Then, at the outlet of the heated reactor a mixture is obtained of 844 mg aspirin/mL acetic acid. With the used flow rate of 10 mL/min, the nucleation reactor of 3 mL was placed in water bath at different temperatures in between different productions. The crystal growth temperature was set at 25°C. this simple change in nucleation temperature has led to the crystal size curve as a function of the different nucleation temperatures, see Figure 3.

While the crystal growth phase is performed at room temperature there is a clear different between the tests performed at higher nucleation temperatures and the lower nucleation temperatures. One would expect that when the nucleation cannot take place at temperatures above 25°C, then these samples would all result in the same average particle size. However, the passage through the reactor enables indeed reaching spontaneous nucleation conditions; once again in the collection vessel the created crystals grow to equilibrium. Further lowering the nucleation temperature enables reaching very small particles. Details of this work can be found: https://doi.org/10.1039/C8RE00313K.

Figure 2: Aspirin crystal size as a function of nucleation temperature as dissolved in acetic acid in a 844 mg/mL concentration (https://doi.org/10.1039/C8RE00313K)

Example 2: CROSS FLOW MIXING INSERT + 0.5 mL reactor

An important feature of the use of the antisolvent mode is the possibility of the use of a cooled antisolvent to be mixed with the solution subsequently followed by a passage at cold temperatures inside the nucleation reactor. With Cross flow setup, cooled hexane at 5°C was mixed directly with a Brivaracetam/IsoPropyl Acetate (IPAc) solution at 30°C at different concentrations. Brivaracetam is known t have different polymorph of which one is an unwanted solvated structure, kinetically favoured at higher nucleation temperatures (https://pubs.acs.org/doi/abs/10.1021/acs.cgd.8b00928). Thanks to the mixing with cold hexane, the temperature is drastically decreased and therefore the wanted polymorphic form nucleates, as this is the metastable form at lower temperatures (https://pubs.acs.org/doi/abs/10.1021/acs.cgd.8b00930). The nucleation rate that was obtained is very high and therefore at very short residence times, small enantiopure crystals are obtained at different conditions (figure 3).

Temperature evolution inside the cross-mixer for a Brivaracetam solution
Figure 3: Temperature evolution inside the cross-mixer for a Brivaracetam solution mixed with hexane at identical flow rates (https://pubs.acs.org/doi/abs/10.1021/acs.cgd.8b00930)
Crystallization mehtod
Initial concentration
Reactor volume
Average particle size
mg.mL-1
mL
μm
Casted
150
0.5
10 ± 5
200
0.5
9 ± 5
Filter paper
150
0.5
9 ± 4
200
0.5
7 ± 3
300
0.5
8 ± 3

Table 1: Average particle sizes for antisolvent crystallization of Brivaracetam.

Impact of the nature of mixing together with antisolvent

Example 3: CO-FLOW MIXING INSERT + 1mL REACTOR

An important feature of the use of the antisolvent mode is the possibility of the use of a cooled antisolvent to be mixed with the solution subsequently followed by a passage at cold temperatures inside the nucleation reactor. With Cross flow setup, cooled hexane at 5°C was mixed directly with a Brivaracetam/IsoPropyl Acetate (IPAc) solution at 30°C at different concentrations. Brivaracetam is known t have different polymorph of which one is an unwanted solvated structure, kinetically favoured at higher nucleation temperatures (https://pubs.acs.org/doi/abs/10.1021/acs.cgd.8b00928). Thanks to the mixing with cold hexane, the temperature is drastically decreased and therefore the wanted polymorphic form nucleates, as this is the metastable form at lower temperatures (https://pubs.acs.org/doi/abs/10.1021/acs.cgd.8b00930). The nucleation rate that was obtained is very high and therefore at very short residence times, small enantiopure crystals are obtained at different conditions (figure 3).

particle size diagram
microscopic images of the Terephtalic acid powder

Use of different flow rates inside antisolvent mixing conditions

Example 4: CO-FLOW MIXING INSERT + 1mL REACTOR

While using antisolvent additions many experimenters need to vary the particle size. It has been observed that the particle size can be altered by using higher flow rates inside the cross-flow mixer. The cross-flow mixer allows mixtures up to 1/3 to 1/4 in flow rate ratio between solution/antisolvent. Once the rate of the antisolvent goes higher, blockages may occur. To demonstrate this, in Figure 6 the flow simulations of a 1/1 and 1/2 flow rate ratio of solution and antisolvent are shown. Already at the 1/2 mixing ratio, the higher flow rate occupies the space inside the T-mixer. When this flow rate goes up, the solution end risks of becoming stagnant, which at the interface between both liquids provokes in stagnant solids that are formed with blockages as a result.

Figure 6: Flow simulations inside the Cross-flow mixer for a 20 and 20 mL/min and 20 and 40 mL/min flow rates for solution and antisolvent.
Figure 6: Flow simulations inside the Cross-flow mixer for a 20 and 20 mL/min and 20 and 40 mL/min flow rates for solution and antisolvent.

To enable higher mixing ratios, we recommend highly the use of the co-flow mixer, schematized in Figure 5, where the mixing is not chaotic as in the cross-flow mixer but purely diffusive as both solvents will gradually dissolve into one another. Playing with flow rate ratios there help to have a large excess in antisolvent but also aid in the particle migration towards the antisolvent rich phase (arrest of growth) or solution rich phase (resulting in larger particles). The example shown for Terephtalic acid is an example where the particles remain longer in the solution rich phase (1/2 mixing ratio) and therefore ‘larger’ crystals are found. In the next example, see Figure 7, a steroid compound with very low solubility of 20 mg/mL at room temperature was investigated, using the co-flow mixer and a 1 mL reactor in plain air. Also the antisolvent was kept at room temperature. Where the mixing in a 1/1 mixing ratio gave a wide variety in particle sizes, increasing the mixing ratio to 1/4 already drastically decreased the particle size to sub-20 µm particle sizes. A flow rate ratio of 1/5 and 1/6 not only decreased further the overall crystal size but it also homogenized the batch to batch variability. Then, with a mixing ratio of 1/8 particles of 3 µm in average size and low span value (indicator for diffence in smallest to largest particles). Please note that with the use of surfactant also submicrometric particles were retrieved.

Co-fLow mixer insert crystallizations
Figure 7: Co-fLow mixer insert crystallizations of a steroid compound at a concentration of 20 mg/mL, average particle size with standard deviations shown as a function of the mixing ratio.

Practical examples for the crystallization process development using the SCT-LAB instrumentation

A rapid process development using one single reactor configuration

As the technology is based on the use of one single reactor configuration from the lab equipment to the production scale, all exploration of process conditions is performed on the SCT-LAB equipment and then held constant. As such, all process development is done with a minimum quantity of product involved. At later stages of the increase in scale these process parameters are simply copy-pasted and used as such on both the PILOT unit as well as the industrial scaled ICE unit.

These parameters are:

  •  Selection of mixer and reactor
  •  Solution concentration and temperature
  • Flow rates of solution and antisolvent, when the latter is required
  • Nucleation temperature
  • The first filtration and drying steps.

To do so, a wide variety of different conditions is to be tested on the SCT-LAB equipment. A swift parameter testing sweep, typically performed on filtered and dried crystals gathered for each different test gives a in-depth insight in how the molecule is nucleating inside the reactor, and what sizes can be obtained by changing the thermodynamic conditions, i.e. playing around in the phase diagram, as shown here below schematically.

Once the optimal zone is reached, which means that upon retesting the exact same product is obtained, and this with a variety of different combinations of flow rate, residence time and nucleation temperature, one can fix the thermodynamic route for crystallization. Then, and this is a true benefit of the approach for controlled nucleation as done with this technology, one can use the influence of hydrodynamics to further refine the product up to final particle size that is desired. As an example for this approach for cooling crystallization, the molecule Brivaracetam was nucleated inside of these reactors to find an optimal size of 50µm average particle size with a d90 below 100 µm.

Brivaracetam has a particular behavior as it forms very easily a solvated crystal structure with different solvents. Nonetheless, a route was found in order to avoid the formation of the unwanted solvated polymorph using our technology.

To do so the following thermodynamic pathway was followed: using the
previously discussed fast cooling rates inside the reactors, the heated
solution is rapidly cooled down to temperatures inferior to the
enantiotropic point between 15 and 20°C, as shown with the blue arrow
numbered “1” in the next graphic.

Once the targeted nucleation temperature between 0 and 15 °C is reached, then we stay inside of the reactor at temperature to maximalise the nucleation rate. As such the overall concentration of solutes in solution drops as we are creating there already solid nuclei (arrow number 2 in the graphic). Then, the product exits the reactor. In this particular case, the product slurry has to be gathered at room temperature, therefore the slurry heats up to room temperature while still growing (arrow number 3). However, as the slurry has exited the reactor, no more nucleation of neither the wanted polymorph (cubic blocks as shown in the next figure)  or the solvated structure occurs due to a lack of shear rate to overcome the metastable zonewidth as previously discussed.


At equilibrium, the product slightly grows for a few minutes to reach equilibrium state as stated by point number 4. In this way, the optimized thermodynamic route using the parameter settings as shown in the table is obtained.

Parameter
Optimal value
Concentration
400 mg/mL
Solution temperature
45 °C
Flow rate
20 mL/min
Reactor volume
7 mL
Reactor temperature
5 °C
Residence time inside reactor
21 s
Hydrodynamic coupling
Straight
Obtained polymorphic form
100 % pure anhydrous, non-solvated structure
Obtained average particle size
190 µm
Calculated nucleation rate
10 000 crystals/mL/s

Once this was achieved, we documented (doi/abs/10.1021/acs.cgd.8b00928) the use of static mixers before entering the cooling phase. This change un hydrodynamic conditions, as discussed previously has a drastic effect on the nucleation rate: up to a million of crystals is produced per mL per secod of slurry. Due to the conservation of the overall mass balance, this results then in a drastic decrease of the average particle size down to a average size of 45 µm and a very narrow particle size distribution as indicated by the error bars in the next graph where we have applied from 1 to 4 of these static mixers prior to cooling as opposed to the datapoint with no use of these mixers (point 0):

Therefore, for any crystallization based on cooling crystallization we recommend to first aim for a construction of the optimal thermodynamic pathway using our reactors focusing on stable and reproducible results. Once this is optimised, there is always the hydrodynamic optimisation that can be applied in order to finetune the eventual result.

A second degree of freedom in playing around with the hydrodynamics to have a big impact on the final crystal size is the use of antisolvents. A very good example is the crystallization of itraconazole, where an injectable formulation has been generated using our antisolvent setup in the SCT-LAB environment. While maintaining excellent crystal properties our technology allow to drastically reduce the used solvents and especially the surfactants needed to stabilize the product. Pinpointing the excellent mixing capacities achieved in our technology. This paper is open access and can be downloaded by clicking on this DOI: https://www.mdpi.com/1999-4923/16/3/376.

 

Screening crystallization parameters for targeted particle size distribution

Watch the video

Directly achieving the optimal crystal size for a solute in solution

Introduction

The development of a completely new technology does of course not come by itself. Many years on research on a specific molecule, Brivaracetam, has led to the creation of the understanding how to generate crystalline material while flowing heated solution through narrow capillaries. A large part of this research journey has been documented and published in Crystal Growth & Design, also a patent application WO2019215022A1 was submitted based on these results.

In general, our findings pointed out that solutes in solution can start to nucleate very rapidly inside reactors with reduced internal volume. More specifically, as Brivaracetam is a rather complex molecule to crystallize, we observed the possibility of selection a certain polymorphic form of Brivaracetam to nucleate preferentially. The benefits of these long, elongated reactors with narrow inner diameters and even smaller wall thickness are that the solutions are cooled down in a very controlled way from soluble conditions, through the metastable zone, and into the zone where spontaneous nucleation – whether homo- or heterogeneous – may take place. This cooling down is not affected by the fact that we use flow velocities inside these reactors from 10 to 100 cm/s; hence, the use of relevant flow rates – to obtain desired production rates of APIs – becomes possible. Together with the defined cooling of liquids flowing through our reactors, we have also observed to beneficial influence of occurring shear rate inside our reactors on the width of the metastable zone, resulting in a drastic increase in nucleation rates at identical conditions when increasing the shear rates inside the reactors. A side-effect was the observation of the absence of fouling during long-term productions of Brivaracetam from the start. On a side note, we have not considered fouling during the first years of research as we never encountered any blockages of the tubing, even during long runs. In the paragraph discussing fouling and blockages we will go deeper into the reasons behind the fact that blockages inside our reactors will only occur when several critical factors are or are not considered.

Developing a way to crystallize directly the correct crystal size for a solute in solution

From the start, our philosophy was based on the control of the number of nuclei appearing inside a crystallizing slurry. We had observed in literature several attempts to control crystal growth using difficult crystal growth models in different axes of a crystal, the use of CSTR tanks, etc. None of them we found satisfactory for this goal as it complexifies the process and you always rely on the use of solid seeds of some form and shape that results in a redundant process step if crystals are generated spontaneously. On the other hand, if one would be able to control how much nuclei are generated homogeneously while passing through a reactor, then in a simple, maybe even naïve, calculation, one would be able to predict their size once they have grown to their equilibrium state.

Suppose that you would have an initial solution concentration of 400 mg of solute per mL of solvent. And that at a low temperature solubility is 100 mg of solute per mL of solvent. That would mean that per mL of solvent you can crystallize – when performing for this example a cooling crystallization – 300 mg of crystals. Then, if you have 100 crystals in this slurry, or hundred thousand of crystals, this would then result immediately in completely different particle sizes once these crystals are fully grown.

 

 

Table 1: estimation of particle size of crystals as a function of the total amount of crystals present in a slurry for a given concentration and crystal density.
Mass to crystallize
300 mg
Density of a crystal
1 g/mL
1 mg/µL
1 mg/mm3
Number of crystals
volume / mm3
Spherical average length / µm
Cubic average length / µm
100
3
895
1442
1000
0,3
415
669
10000
0,03
193
311
100000
0,003
89
144
1000000
0,0003
42
67
10000000
0,00003
19
31

So, for an oral dosage form with average sizes preferentially below the 100 µm mark, anywhere between 105 and 106 crystals produced per mL solvent is to be achieved. Please note that in the above table we assumed both spherical and cubic particles leading to different expected crystal sizes. If we look at estimated nucleation rates for homogeneous crystallization at different conditions we will describe here below, in theory (Mersmann, Crystallization Technology Handbook, 2nd edition, ISBN 0-8247-05828-9, chapter nucleation) a nucleation rate can be expected in between 1012 and 1020 per cubic meter per second, which equals to 106 and 1014 of crystals produced per mL. Comparing both values indicates that whenever we can reach nucleation conditions as set out in Figure 1 and control their evacuation out of the reactors to let the freshly generated nuclei grow in non-nucleating conditions, we would be able to separate nucleation and growth of crystals and target exact reproducible size whenever the passage through our reactors is as homogeneous as possible.

 

Figure 1: Expected nucleation rates for homogeneous nucleation Bhom as a function of supersaturation and solubility, for molecules n=1, taken from 1

 

As stated in above figure, solubility and supersaturation are important parameters that rule the occurrence of nucleation. In fact, for a crystalline structure to appear, a treshold energy barrier needs to be overcome in order for the structured material to appear instead of molecules remaining disordered inside the solvent. The nucleation theory is of course already well described, but in general, for a molecule to start nucleating, this energy barrier is most easily depicted using the phase diagram of the solute in solution, as shown in Figure 2. This means that for a solute in solution, to the right of the solid line in Figure 2, needs to be cooled down in order to nucleate. The energy barrier is the most wide for primary homogeneous nucleation, which is nucleation from the solution without any other foreign material present. In the presence of foreign material – impurities, surfaces, etc. – but without any solute-own crystals, primary heterogeneous nucleation takes, with a lower energy barrier to be overcome as for homogeneous nucleation. Once crystals are present in the slurry, these may brake up due to fractures, shear effect, collisions, etc. These are grouped together as secondary nucleation – so with the presence of solute-own crystals in the slurry. Logically, these come at the lowest energy barrier to be overcome.

Figure 2: Solubility C* as a function of temperature T and the metastable zonewidths indicated for primary homogeneous and heterogeneous nucleation and secondary nucleation. Figure related to 1.

 

Following our philosophy to control the formation of crystals and to maintain the quantity of crystals nucleated, inside our reactors we aim for primary nucleation, whereas the gathering of the slurry coming out of the reactor has to be done in such a way that secondary nucleation is avoided.

To express this metastable zonewidth, at isothermal conditions supersaturation s is defined as the difference between the concentration C of a system and the solubility concentration at that temperature C*. To create a dimensionless number, this difference between C and C*, is divided by C*:

Once the metastable zonewidth is crossed for primary nucleation, the kinetics of nucleation – the number of nuclei generated – is increased by further cooling, generating a higher supersaturation value. As an example, in Figure 3 the difference in metastable zonewidth is shown between the compounds Brivaracetam and Aspirin in their proper solvents2.

Figure 3: Solubility diagrams of Brivaracetam (left) and Aspirin (right), with solubility (orange), experimentally established metastable zonewidth (blue) and indicated supersaturation at which crystallization tests are conducted .

[1] Rimez at al. J Flow Chem 2020, 10, 241-249 (doi: 10.1007/s41981-019-00069-2)

The supersaturation value needed to obtain primary nucleation for Brivaracetam is of 0.6, and experiments can be conducted at concentrations as high as 400 mg/mL, resulting in very high supersaturation values and a large variability in crystal sizes while playing with different conditions can be obtained as published earlier (Rimez et al., Crystal Growth & design 2018, 18, 6431-6439 (doi: 10.1021/acs.cgd.8b00928)). For Aspirin, the metastable zonewidth when dissolved in water/ethanol is much larger, a supersaturation value of 5.7 needs to be overcome to observe nucleation in our reactors, making it for this example difficult to generate working conditions at supersaturations above the optimal value of Brivaracetam (σ = 6.7 instead of 4.6). However, both obtained supersaturations are high and it can indeed be expected that predicted nucleation rates and hence refined particles sizes can be obtained for both examples, with reported particle sizes between 50 and 200 µm for Brivaracetam3 and in used solvent between 400 and 800 µm for Aspirin2. Hence, the key factor to crystallize directly the desired particle size is to nucleate a certain amount of nuclei inside the solution and gather this liquid to let the crystals grow to their equilibrium state, independent of the time required to do so, as long as primary nucleation event only take place inside the specific nucleation reactor. As a side-note: all solution passing through such a reactor needs to undergo the exact same cooling pattern as to homogenize maximally the nucleation rate for the complete production liquid envisaged.

The use of capillary reactors to control the nucleation rate while maintaining uniformity

In order to cool a liquid down not only very rapidly but also in a uniform way, we opted for millifluidic tubing with maximally 1 mm inner diameter. We conclude this after an experimental study on Aspirin (Rimez et az., J Flow Chem 2019, 9, 237-249 (doi : 10.1007/s41981-019-00042-z)) where we experimentally and by simulation demonstrated how a liquid cools down when it is injected at a constant rate inside a tubular reactor of 1 mm having a wall thickness of 150 µm and being placed in a water bath of at least 100 times the volume as opposed to the volume of the reactor. For the study on Aspirin, we used a reactor of 3 m length which corresponds roughly to 3 mL of volume; this reactor was placed inside a 1 L water bath that was set at the desired temperature. Then we measured using thermocouples at different distances the temperature inside the reactor while liquid was flowing through the reactor. This liquid was preheated at the desired solution temperature for the Aspirin solution in ethanol/water (50/50 v/v). this overlapped with the simulate values for the tube center temperature. In the next figure, we show the simulated values for both the tube center and the simulated temperature at the tube wall.

In Figure 4 the liquid entering the reactor is indicated at distance zero. The temperature of the liquid drops drastically at the tube wall (dashed black lines), followed by the temperature in the center (solid black line). The dots in orange indicate when the liquid passes the solubility temperature of the concentration of 200 mg/mL; the blue dots correspond to the metastable zonewidth. It can therefore be observed that after a distance shorter than 50 cm inside the tubing the solution is already in conditions where spontaneous nucleation will take place, the remainder of the time inside the tubing, 4.7 s in total at ruling flow velocities, renders the possibility to produce a large quantity of nuclei in order to reach the desired amount of crystals. These short residence times prevent crystals to grow to sizes that might become difficult to handle.

 

Figure 4: Calculated spatial and temporal evolution of temperature (black lines) and supersaturation values s (green lines) at the centerline (solid lines) and at the wall (dashed lines) for an Aspirin solution of 200 mg/mL 50/50 vol% ethanol/water flowing at a rate of 30 mL/min inside a 1 mm ID tubing sitting in a water bath held at 10°C. Starting solution temperature was set at 60°C.

Any crystal growth is therefore also conducted outside of our reactors. In general, the next schematics represent very well how we perform crystallizations, see Figure 5: a heated liquid is injected at a constant rate inside a tubular reactor sitting at a low temperature at which the nucleation should take place. Before this cooling sown there is a possibility to alter flow hydrodynamics inside the tubing or add an antisolvent – these topics will be discussed later. While flowing through the tubing all liquid is always cooled down in exactly the same way: laminar flow conditions are reached for the combination of flow rates and tubing diameter we apply, therefore plug flow conditions are reached and all material is constantly moved forward through the tubing. Inside the phase diagram we go as fast as possible to the desired nucleation temperature (horizontal arrow from high to low temperatures in the phase diagram). At this point, the nucleation is already started, indicating a drop in solute concentration (vertical arrow in the phase diagram) while the liquid is still flowing throught the tubing. Then, at the end of the tubing, the slurry containing fast growing crystals can be gathered inside a waiting tank for the crystal growth to be fulfilled or, when the growth kinetics allow this, directly on a filtration unit. For crystal growth conditions, any temperature might be selected.

Figure 5: Schematic representation of the cooling crystallization approach of Secoya's Crystallization Technology (SCT).

Benefits of capillaries: solid particle mobility and shear rates

The uniformity in the cooling of liquids flowing through the tubing but also the development of a laminar flow regime inside the capillaries is beneficial for SCT. As the estimated Reynolds numbers are well below 4000, see Table 2, generally considered the threshold for turbulent flow, laminar flow behavior is to be expected. On the other hand, capillary reactors possess other beneficial aspects that are in use inside our reactors.

Table 2: typical flow rates with corresponding flow velocity inside capillary reactors and estimated Reynolds number using water at room temperature as basis for calculation.
Flow rate
mL/min
10
20
30
Flow velocity
cm/s
21,2
42,4
63,7
Reynolds number
20
400
600
Shear rate
s-1
1698
3395
5093

At usual flow rates inside the reactors, shear rates at the tube are above 1000 s-1. For other types of solid particles flowing through narrow channels under laminar flow conditions and elevated shear rates, it is known that these particles do not migrate towards the channel wall. Most famously is the Fåhræus–Lindqvist effect that describes the migration of erythrocytes to the center of the vessel, leaving only plasma neat the wall of the vessel (https://en.wikipedia.org/wiki/F%C3%A5hr%C3%A6us%E2%80%93Lindqvist_effect). In Figure 6 an artificial vein is shown with hemoglobin flowing through, showcasing the absence of hemoglobine at the tube wall.

 

Figure 6: Hemoglobin flowing through an artificial vein, image taken from Carboni et al. (doi : 10.1016/j.bpj.2016.08.026

When simulating the preferential position of solid particles inside circular tubings under constant flow conditions, Javier Rivero-Rodriguez and Prof. Benoit Scheid of the Université libre de Bruxelles (Rivero-Rodriguez and Scheid, J Fluid Mech 2018, 842, 215-247 (doi: 10.1017/jfm.2018.78)) discovered that small particles, when not subjected to gravity, will migrate under laminar flow conditions to a preferentially eccentric position, away from the tube center. At the tube wall the lowest probability to retrieve a solid particle is observed. This preferential position is shown in Figure 7. It is demonstrated that the difference in flow velocity in the laminar flow regime pushes the particle away from the tube center. On the other hand, the particles bounce off the tube wall, generating an equilibrium position off center. Therefore, as long as the particles are not subjected to gravity, the flow conditions inside of the reactors prevent collisions and deposition on the tube wall, resulting on blockages of the tubing.

Figure 7: Preferential position (black line) of a neutral bubble - solid particle - with particle diameter d (y-axis) inside a tubing with diameter ε (x-axis). Taken from Rivero-Rodriguez and Scheid (doi: 10.1017/jfm.2018.78).

Using Secoya’s Crystallization Technology, a large amount of nuclei are however formed at different conditions, which may provoke coagulation of these particles, very fast crystal growth, etc. Therefore, the use of a certain length for the reactor depends on the trade-off between sufficiently high nucleation rates and avoiding the possibility to block the tubing during operation due to too intense growth for example. The study to find the optimal reactor dimensions is an integral part of the development cycle of the process at the laboratory scale.

 

Another benefit of the tubular reactors is the high shear rates obtained inside. Mura and Zaccone have described mathematically the preferential nucleation conditions for particles under flow conditions and high shear rates (Mura and Zaccone, Phys Rev E 2016, 93, 042803 (doi: 10.1103/physRevE.93.042803)). in a joint study between Zaccone, ULB and Secoya, experimental data further evidenced this beneficial effect of shear rate in flow on the nucleation rate of a solute in solution (Debuysschère et al., Cryst Growth Des. 2023, 23, 4979-4989). In this work a cooling crystallization of Glycine in water was conducted. All Thermodynamic variables were kept constant (solution concentration and temperature, residence time at low temperature, nucleation temperature, etc.). The only variable was a change in shear rate while flowing through the reactor. This was achieved by varying the capillary diameter, flow rate and reactor lengths to obtain always the same thermodynamic conditions. The result was that the measured nucleation rate, shown in Figure 8a goes through an optimal value at a shear rate of 3000 s-1 and Reynolds number of 340 for 98% pure Glycine, whereas 99.5+ % pure Glycine had its maximum at a slightly lower shear rate of 1700 s-1. Other evidences on the influence of more localized shear rates generated inside the tubing in different studies (different studies already referenced here above)  lead to further evidencing the large impact of shear rates on the nucleation rate of a solute in solution while cooling down. For any cooling crystallization study, the shear rate influence is part of the development cycle for optimization of the crystallization of any molecule.

Figure 8: a) Normalized nucleation rates (Jexp-Jthe) as a function of ruling normalized shear rate for 98% pure Glycine (left) and 99.5+ % pure Glycine (right). b) microscopic images of exemplary crystals obtained during experiments. Taken from Debuysschère et al. (DOI:10.1021/acs.cgd.3c00232)

Conclusions

Secoya’s Crystallization Technology is based on the adaptation of the size and shape of tubular reactors to the speed of nucleation of solutes in solution. The overall short residence times and uniform cooling conditions inside the reactors result in the control of primary nucleation, enabling a prediction of the final particle size of crystalline material after complete growth of the crystals, outside the reactor in a collecting vessel. Focusing only on nucleation in flow conditions also avoids blockages during production, as nuclei inside the reactors will not be able to migrate to the tube wall resulting in blocked tubings. The high shear rates obtained inside the reactors have an extra beneficial effect on the nucleation rate.

Understanding API crystallization process and its challenges

What is API Crystallization?

API crystallization refers to the process of transforming the active pharmaceutical ingredient (API) of a drug from a solution into a solid crystalline form. This is a crucial step in drug manufacturing because the crystalline form of an API can significantly influence its stability, solubility, uptake in the human/animal body and overall effectiveness. Crystallization also bridges the drug substance manufacturing and the drug product manufacturing towards its final application as a medicine. The process involves carefully controlling parameters such as temperature, solvent composition, and concentration to ensure the formation of crystals with desired properties (ex. crystals size, distribution).

Why crystallization is crucial
in drug product manufacturing?

Crystallization is crucial as the final step in drug product manufacturing because it directly affects the purity, stability, and reproducibility in production.

Crystallization helps in purifying the active pharmaceutical ingredient (API) by removing impurities. It ensures that the drug substance is consistent in its chemical composition, which is essential for the safety and efficacy of the medication

Crystalline forms of APIs are generally more stable than their amorphous (non-crystalline) counterparts. A stable crystalline form ensures that the drug has a longer shelf life and maintains its therapeutic effectiveness over time. Also the selection of the most stable polymorphic state – when more than one crystalline structure exists for one single molecule – is important to reach the highest possible shelf life.

Crystallization processes can be standardized to produce consistent batches of the API. This reproducibility is critical for ensuring that every dose of the drug performs the same way, which is a key requirement in pharmaceutical manufacturing.

Batch crystallized drug substances from solution

Crystallization in a large batch reactor in a manufacturing process is conducted in such a way that any variability in between two produced batches is reduced to an absolute minimum. In order to achieve this feature inside large tank sizes, the API is dissolved at elevated temperatures. While gently cooling, a preset quantity of solid API is added as very fine sized particles – called “seeds” – already having the features of the desired polymorphic form. During cooling, the API in solution will adhere to the crystalline seeds to generate grown crystals at the lowest possible temperature. To maximalize the efficiency of the process at the end usually an antisolvent is added to promote adhesion of the remaining API in solution to the crystals.

If this is executed in a very standardized way, the variability in between different produced batches is acceptable to be further treated downstream in the drug product manufacturing process steps.

What are the important parameters for the crystalline API in drug product manufacturing?

Once the crystalline API from the drug substance production is generated, during the drug product manufacturing, focus is set on the actual application of the API and its formulation. Other parameters come into play like the physical properties size and shape of the crystals, making a large part of the critical material attributes (CMA) of a drug product. Size, size distribution and shape of the crystals determine the actual drug application, dissolution and bioavailablity, dose uniformity, processability of the drug product:

The size of the crystals determines how quickly they dissolve in the body. Smaller crystals generally dissolve faster due to their larger surface area relative to their volume, which can improve the drug’s bioavailability—how effectively it is absorbed and used by the body. A uniform size distribution ensures that the dissolution rate is consistent, leading to predictable drug performance and therapeutic outcomes.

In drug manufacturing, consistent crystal size and distribution help ensure that each dosage form (like a tablet or capsule) contains the same amount of active pharmaceutical ingredient (API). If the crystals vary widely in size, it can lead to uneven distribution of the API in the dosage form, resulting in variability in drug potency and potentially impacting patient safety.

Crystals with a narrow size distribution are easier to handle and process. They flow more uniformly through manufacturing equipment, mix evenly with excipients (inactive ingredients), and compress more consistently into tablets. This reduces the risk of production issues like segregation (where particles separate based on size), uneven tablet weights, or poor compaction, leading to more efficient and reliable manufacturing processes.

The size and distribution of crystals can affect the physical and chemical stability of the drug. For example, larger or irregularly shaped crystals might be more prone to breaking down or undergoing unwanted transformations, which could compromise the drug’s stability and shelf life. Consistent size and distribution help maintain the integrity and stability of the drug over time.

Crystallization conditions, including the size and distribution of crystals, can influence which polymorphic form of the API is produced. Different polymorphs can have different solubility, stability, and bioavailability. A controlled crystallization process that produces a uniform size distribution helps in consistently obtaining the desired polymorphic form, which is critical for maintaining drug efficacy and safety.

Challenges for batch produced crystalline API substances towards the change to drug product

While seeded batch crystallized APIs are in the correct polymorphic state and production is sufficiently consistent, the produced drug substance usually needs to be re-engineered towards its desired size and shape for its application. Hence, the addition of one or more additional process steps besides classic seeding, growing, filtration and drying are necessary: changes in shape and size are done with different grinding and milling techniques- wet or dry – followed by seeving steps to obtain the desired size and particle size distribution, avoiding a too large variability in between the smallest and largest crystals present in the drug product powder prior to final formulation.

Although applied consistently for nearly every produced API substance, any additional process step is very costly due to inherent losses of drug product for each different additional step. Also recycling of lost material during these steps comes at a high cost to work up this product to desired properties, if even possible.

Secoya Crystallization Technology’s unique proposition

At Secoya, we have developed a unique crystallization process that allows to reduce the total number of process steps to its bare minimum from drug substance to drug product. We bring the drug substance in solution, and we generate the final CMAs of the drug product in one single process step; followed by filtration and drying. No hustle anymore to generate correctly sized seeds, no more grinding, milling and sieving of the drug products. As a true benefit, our technology bridges completely the drug substance manufacturing with the drug production facility with the direct use of the manufactured API powder.

Discover the Secoya Crystallization Technology

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