Resolution of Racemic Guaifenesin Applying a Coupled Preferential Crystallization-Selective Dissolution Process: Rational Process Development

Article abstract of DOI:10.1021/acs.cgd.8b01660

Preferential crystallization is a cost efficient method to provide pure enantiomers from a racemic mixture of a conglomerate forming system. Exploiting small amounts of pure crystals of both enantiomers, several batch or continuous processes were develope

Full text of DOI:10.1021/acs.cgd.8b01660

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Article
Resolution of racemic guaifenesin applying a coupled Preferential
Crystallization-selective Dissolution process - Rational process development
Erik Temmel, Matthias Eicke, Francesca Cascella, Andreas Seidel-Morgenstern, and Heike Lorenz
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01660 • Publication Date (Web): 01 May 2019
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Resolution of racemic guaifenesin applying a coupled Preferential
Crystallization-selective Dissolution process - Rational process development
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Erik Temmel , Matthias J. Eicke , Francesca Cascella , Andreas Seidel-Morgenstern , Heike
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Lorenz
1
Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstraße 1, 39106
Magdeburg, Germany
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Otto von Guericke University Magdeburg, Chair of Chemical Process Engineering, 39106
Magdeburg, Germany
Abstract
Preferential Crystallization is a cost efficient method to provide pure enantiomers from a
racemic mixture of a conglomerate forming system. Exploiting small amounts of pure crystals of
both enantiomers several batch or continuous processes were developed, capable to provide
both species. However, an intermediate production step has to be used when pure enantiomers
are not available. In such cases partially selective synthesis, chromatography or crystallization
processes utilizing chiral auxiliaries have to be used to provide the initial seed material.
Recently it was shown that a combined Preferential Crystallization-selective Dissolution process
(
CPCD) in two coupled crystallizers can be applied if at least one pure enantiomer is available
to produce both antipodes within one batch. The corresponding process is carried out in one
reactor (crystallization tank) by seeding a racemic supersaturated solution with the available
enantiomer at a certain temperature. The second reactor (dissolution tank) contains a saturated
racemic suspension at a higher temperature. Both reactors are coupled via the fluid phase
allowing for a selective dissolution of the preferentially crystallizing enantiomer from the solid
racemic feed provided in the dissolution vessel. The dissolution and crystallization processes
continue until the solid racemic material is completely resolved and becomes enantiopure. At
this point, both enantiomers can be harvested in their pure crystalline form.
For a specific pharmaceutically relevant case study, a rational process design and the applied
empirical optimization procedure will be described. The achieved productivities after
optimization show the great potential of this approach also for industrial applications. Also, a
strategy to control this process based on inline turbidity measurement will be presented.
Corresponding author:
apl. Prof. Dr. Heike Lorenz; lorenz@mpi-magdeburg.mpg.de
Max Planck Institute for Dynamics of Complex Technical
Systems; Sandtorstraße 1, 39106 Magdeburg, Germany
Tel: +49 391 6110293; Fax: +49 391 6110500
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Resolution of racemic guaifenesin applying a coupled Preferential
Crystallization-selective Dissolution process - Rational process development
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Erik Temmel , Matthias J. Eicke , Francesca Cascella , Andreas Seidel-Morgenstern , Heike
Lorenz1*
1Max Planck Institute for Dynamics of Complex Technical Systems, Germany
2
Otto von Guericke University Magdeburg, Germany
Abstract
Preferential Crystallization is a cost efficient method to provide pure enantiomers from a
racemic mixture of a conglomerate forming system. Exploiting small amounts of pure crystals of
both enantiomers several batch or continuous processes were developed, capable to provide
both species. However, an intermediate production step has to be used when pure enantiomers
are not available. In such cases partially selective synthesis, chromatography or crystallization
processes utilizing chiral auxiliaries have to be used to provide the initial seed material.
Recently it was shown that a combined Preferential Crystallization-selective Dissolution process
(
CPCD) in two coupled crystallizers can be applied if at least one pure enantiomer is available
to produce both antipodes within one batch. The corresponding process is carried out in one
reactor (crystallization tank) by seeding a racemic supersaturated solution with the available
enantiomer at a certain temperature. The second reactor (dissolution tank) contains a saturated
racemic suspension at a higher temperature. Both reactors are coupled via the fluid phase
allowing for a selective dissolution of the preferentially crystallizing enantiomer from the solid
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racemic feed provided in the dissolution vessel. The dissolution and crystallization processes
continue until the solid racemic material is completely resolved and becomes enantiopure. At
this point, both enantiomers can be harvested in their pure crystalline form.
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For a specific pharmaceutically relevant case study, a rational process design and the applied
empirical optimization procedure will be described. The achieved productivities after
optimization show the great potential of this approach also for industrial applications. Also, a
strategy to control this process based on inline turbidity measurement will be presented.
1
. Introduction
Enantiomers are molecules that are mirror images of each other (optical isomers) with the same
molecular structure and identical physicochemical properties. The stereoisomerism of two
enantiomers can cause different effects if they are applied to biological systems due to chirality
of enzymes, hormones etc. Nowadays, there is evidence that commonly only one enantiomer
induces the desired effect while the other is inactive or even harmful [1, 2]. Thus, if chiral
components are utilized as a racemic mixture (50/ 50 ratio of both enantiomers), half of the
applied material can be toxic, cause stress for the biological system or is wasted leading to
environmental pollution.
The development of processes to produce pure enantiomers is therefore a prerequisite for
economically and ecologically effective applications of chiral substances in e.g. the
pharmaceutical, food and agrochemical industries. Even though a direct synthesis of one single
stereoisomer is preferred, it is often expensive or impossible [3]. Alternatively, the substances
can be conventionally produced as racemic mixtures and separated subsequently with
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appropriate techniques exploiting chromatography, membranes or, like in the present study,
Preferential Crystallization [3-5].
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In order to make use of Preferential Crystallization, beside the basic requirement that the
racemate to be separated forms a conglomerate, a suitable amount of homochiral crystals is
initially necessary to gain reasonable yield and productivity. The availability of pure enantiomers
can be a bottleneck. In this study, only a very limited amount of one enantiomer of guaifenesin
was available from complex asymmetric synthesis [6]. Additionally, not much was known
regarding crystallization kinetics and also solubility data was insufficient.
An innovative approach is studied in this article to produce material for further investigations
that would circumvent possible loss or contamination of the valuable starting material, which can
be a result of unknown thermodynamic or kinetic information. The process is based on a
Preferential Crystallization of only one pure enantiomer combined with its selective dissolution
from a solid mixture of racemic composition utilizing two different tanks, which are coupled via
the liquid phase [7, 8]. Below we will designate this “Coupled Preferential Crystallization with
selective Dissolution Process“ as CPCD. It is ideally suited to safely amplify a small amount of
starting material while additionally gaining access to the counter-enantiomer in the form of
crystals.
1.1 Process principle
The process is carried out in two tanks that are initially filled with saturated solutions of racemic
composition (x , Fig. 1) at one specific temperature (T , Fig. 1). Solid racemic feed is provided
i,0
sat
in excess in the first tank, which remains at saturation temperature Tsat and serves as the
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subsequent dissolution stage (dissolution tank in Fig. 1 a). The second tank (crystallization tank
in Fig. 1 a) is cooled to a specific temperature Tcryst, where an appropriate driving force for
crystallization can be expected (ΔT = T -Tcryst) but primary nucleation does not occur.
sat
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Afterwards, the separation is initiated in this crystallization tank by adding a small amount of
pure crystals of one enantiomer (here (R)). These crystals will start to grow due to the present
supersaturation and thereby, selectively remove the seeded enantiomer from the liquid phase
indicated by its decreasing concentration in the mother liquor. The (R)-depleted liquid phase is
continuously transported to the dissolution tank that is still kept at the higher saturation
temperature T . As a result, the seeded (R)-enantiomer becomes undersaturated in the
sat
dissolution vessel which leads to its selective dissolution from the present racemic solid phase
(changing solid phase composition in the dissolution tank, Fig. 1 b), thus restoring the saturation
concentration at T . By coupling both reactors via their liquid phases, the mother liquors in both
sat
vessels remain at racemic composition if the exchange and the dissolution process are fast
enough to counterbalance the selective concentration decrease by the Preferential Crystallization.
The situation right after seeding is reflected in the left ternary phase diagram in Fig. 1 b).
Initially, only a small, enantiopure solid fraction is present in a supersaturated racemic liquid
phase at Tcryst, while in the dissolution tank, a large amount of racemic solid phase is in
equilibrium with a racemic liquid phase saturated at T . In the ideal case, the seeded enantiomer
sat
is preferentially crystallized and primary nucleation of the antipode is efficiently prevented,
which results only in an increasing solid mass in the crystallization tank while the liquid phase
composition stays racemic (composition x , Fig 1. b). This is depicted in the right ternary phase
i,0
diagram in Fig. 1 b), where the liquid and solid composition in the crystallization tank is
constant, while the overall state (reflecting the ratio between the masses of the liquid and solid
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phase) moves towards the corner of the pure (R)-enantiomer. By contrast, the solid mass
decreases in the other tank due to the selective dissolution that changes simultaneously the
enantiomeric ratio in the present crystal phase. Hence, the solid state (right phase diagram in Fig.
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b) of the dissolution tank moves towards the pure (S)-enantiomer and the changing overall state
reflects the loss of solid content. Also in this tank, the liquid phase is of racemic composition if
the selective dissolution counter-balances the preferential crystallization and the exchange rate
between both vessels is sufficiently high.
The process ends after the seeded enantiomer is completely dissolved from the racemic excess
material in the dissolution tank (Fig. 1 a). If the process is interrupted at exactly this time, only
the seeded enantiomer is present as solid in the crystallizer, while in the dissolution tank, only
the unseeded enantiomer as solid is left. Further crystallization leads to an enrichment of the
unseeded enantiomer in the crystallization tank and, similarly to conventional Preferential
Crystallization, increases the risk of primary nucleation.
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b)
Fig. 1: a) Schematic representation of the initial situation after seeding and the separation result
of the coupled Preferential Crystallization - Dissolution process (CPCD); b) Representation of
the composition evolution by means of a ternary phase diagram depicting the state changes in the
dissolution and crystallization tank for the ideal separation process. Right after seeding (t = tSeed
,
left figure) only a small amount of the solid (R)-enantiomer is present in the crystallization tank
while the dissolution tank contains a large amount of crystalline racemic mixture. Afterwards (t
>
tSeed, right figure), the mass of pure, solid (R)-enantiomer increases due to the Preferential
Crystallization and the overall state in this vessel moves towards the pure phase corner. Selective
dissolution of the (R)-enantiomer in the dissolution tank yields a decrease of the overall crystal
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fraction in suspension and an enrichment of the (S)-enantiomer in the solid phase, which moves
the solid state composition in this tank to the pure (S)-phase corner.
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.2 Separation problem
The API guaifenesin (3-(2-Methoxyphenoxy)-propane-1,2-diol, Fig. 2) was chosen as model
substance to demonstrate the strength of this process strategy given such difficult initial situation
(small amount of enantiopure seed material of only one antipode, little knowledge about the
substance system). Racemic guaifenesin is widely applied as an antitussive with an expectorant
and decongestant effect. Nevertheless, the substance is also a valuable precursor for other
pharmaceuticals like muscle relaxants (methocarbamol [9]), tranquilizers (mephenoxalone [9])
and β-blockers (levomoprolol [6]) that are partly used in enantiopure form in therapy. Some of
the latter substances form “true” racemates (racemic compounds) and are therefore not suitable
for a direct and effective resolution via Preferential Crystallization without previous treatment.
Guaifenesin in contrary, forms a stable conglomerate in water and a procedure to access the pure
enantiomers batch-wise from enriched and racemic solutions was already reported [6, 10].
Fig. 2: Chemical structure of (R)-guaifenesin.
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. Experimental section
.1 Substance system
Racemic guaifenesin was purchased from TCI Deutschland GmbH (purity >98%) and used
without further treatment. Water as solvent for the separation experiments and the washing
procedure was deionized with a milli-Q system (Merck Millipore, Milli-Q Advantage).
Pure (R)-guaifenesin was provided by the Arbuzov Institute of Organic and Physical Chemistry
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(Prof. Alexander A. Bredikhin, Russian Academy of Sciences, Kazan) with a total mass of 0.8 g.
Guaifenesin dissolved in water is known to form a conglomerate [10] without occurrence of
solvates or other polymorphs over the temperature and composition range of interest. It
crystallizes in an orthorhombic crystal lattice [11] (space group P2 2 2 ) with a needle-like habit
1
1 1
of the macroscopic particles (Fig 3 a).
a)
b)
Fig. 3: a) Microscopic image of pure (R)-guaifenesin after recrystallization. b) Solubility of
racemic (black and red squares) and (R)-guaifenesin (blue squares) taken from [10] in water in
comparison to the ideal solubility curve calculated according to Schröder - van Laar (black solid
line). Dashed lines are just guides for the eye.
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Solubility data of the racemic mixture and the (R)-enantiomer measured with a polythermal
method [10] and utilized for the design of the first experiment is shown in Figure 3 b). Both, the
solubility of the racemic mixture and the pure enantiomer show a strong non-ideal and non-linear
behavior, rather two lines with a bend in between, indicating a potential phase change. However,
during this study [10] only the known polymorph was found and hydrate formation was not
detected. Though the occurrence of a second liquid phase, i.e. a stable or metastable liquid-liquid
demixing was indicated [10]. Other physical properties of the substance system are listed in
Table 1.
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Table 1: Physical properties of solid racemic guaifenesin and a saturated solution at 36°C.
Solid state properties (safety data sheet [12])
Molar mass
Melting point
Boiling point
Density
198.22
78-83
215
g/mol
°C
°C
g/cm3
1.2
Solid state properties from own measurements
Melting point*
Enthalpy of fusion*
Density**
78.6
191.4
1.25
°C
J/g
g/cm3
Properties of a sat. solution (36°C, 0.198 gGua/ gSolution
)
Density***
Viscosity
pH
1.03
1.21
8.1
g/cm3
mPa∙s
-
*
Measured via DSC (Setaram, DSC 131)
*
*
Measured via volume expansion of a saturated solution by addition of a defined solid
mass
*
**
Measured via densitometry (Mettler Toledo, DE40)
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.2 Basic thermodynamic and kinetic investigations
Isothermal solubility measurements and small-scale cooling crystallizations of the racemic
guaifenesin were carried out to complement the data from [10] and to verify the evidence of a
potentially occurring second liquid phase.
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5 ml of suspension of water and racemic guaifenesin were prepared for each temperature to
investigate the solubility behavior. Each vial was sealed and equipped with a magnetic stirrer for
mixing. All samples were then placed in thermostated double-wall glass tanks and were initially
heated to dissolve the solid phase completely. The samples were recrystallized subsequently by
cooling to the temperature of interest and left for equilibration (t > 48 h). Afterwards, two to
three milliliters of liquid phase were withdrawn, weighed and placed in a vacuum oven for
drying. The mass of residual solid phase eventually yielded the solubility at the respective
temperature. At least three different samples were prepared for each temperature step
(temperature range 5-50 °C), where a part of the clear liquid phase of each sample was collected
at least two times for concentration analysis.
To investigate the metastable zone width and additionally the occurring phases, a multi-reactor
®
system was utilized (Crystalline , Technobis Crystallization Systems). This measurement system
enables polythermal experiments with overhead stirring and observation of the samples by
camera and light scattering. Samples were prepared in 6 ml glass vials for seven different
concentrations corresponding to saturation temperatures between 28-43 °C. The vials were
subsequently cooled linearly until the lowest possible temperature was reached (-20 °C) or when
a turbidity drop indicated the occurrence of a solid phase. Altogether, five different cooling rates
were investigated ranging from -0.5 K/h to -80 K/h. Image data around the identified nucleation
points was inspected afterwards with respect to the occurring phase and to possibly correct the
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turbidity measurement. All prepared concentrations were measured 5-20 times for each cooling
rate (except for -0.5 K/h where only 2 measurements were possible).
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2
.3 Experimental setup for resolution trials
All separation experiments applying the CPCD process were carried out in two 450 mL double-
wall crystallizers (Fig. 4) equipped with teflon coated propeller type stirrers (stirrer motor:
Heidolph, RZR 2102 control) and thermostats for temperature control (Lauda, Proline RP 845).
Both tanks are connected to a bypass for online analyses, which consist of a polarimeter (IBZ
Messtechnik, POLARmonitor) and a measurement of the total concentration. Two different
properties, density (Mettler Toledo, DE40) and UV-absorption (KNAUER, UV-Detector K-
2
501), were investigated for concentration analysis since no appropriate process online-
measurement for the considered concentration range of guaifenesin is reported up to now. Both
loops are fed with crystal-free liquid phase by peristaltic pumps (Heidolph, PD 5201 SP Quick)
at a rate of 3.5 mL/min.
The mother liquor exchange during the actual experiments was carried out by gear pumps
&
V
= 0-30 mL/min) connected to mass-flow controllers (Bronkhorst Maettig
(Tuthill, D-Series,
GmbH, mini Cori-Flow M14), which enable a precise volume control in both tanks as well as a
measure of temperature and density of the present liquid phase. All tubes are heated slightly
above saturation temperature via thermostats (Lauda, Ecoline RE 104, Julabo, FS 18) to prevent
blockage. The dissolution tank is additionally equipped with a turbidity probe (Mettler Toledo,
Trb. 8300) to follow the decrease in suspension density. Pt100 sensors were used to measure the
temperatures of each vessel and of the solution bypassed for online analyses.
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Fig. 4: Coupled dissolution/ crystallization plant consisting of two double-wall 450 mL tanks,
two online analytic bypasses and two liquid exchange sections. Frit - filter frit; M - stirrer motor;
MFM - mass-flow meter; T - Pt100; Turb. - turbidity probe.
2.4 HPLC analysis
Samples of the crystalline products at the end of each experiment were investigated by HPLC
with respect to purity and composition. A Chiralcel-OD-H (4.6x250 mm, particle size 5 μm)
column was utilized with an Agilent 1200 system (Agilent Technologies) and 65/35 n-hexane/
isopropanol as eluent. Flow rate, UV wave length, column temperature and injection volume
were 1 ml/min, 275 nm, 23 °C and 5 µl, respectively.
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2
.5 XRPD analysis
The solid products were investigated with an X’Pert Pro Diffractometer (PANalytical GmbH)
using Cu-Kα radiation and an X’Celerator detector in the 2-Theta range of 3-40° with a step size
of 0.017° and a step time of 50s per step
0
1
2
3
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8
9
0
No phase change was observed during the investigations and always anhydrous guaifenesin was
crystallized.
2
.6 Experimental procedure development
The initial testing experiment is shown Figures 5 and 6. A major challenge was to find suitable
analytics to track the evolution of the liquid phase composition as well as the expected reduction
in racemic suspension density in the dissolution tank caused by the selective removal of (R)-
guaifenesin. In earlier studies on, e.g. threonine [7], a combination of density and optical rotation
angle (ORA) measurement had proven as reliable techniques for process monitoring. However, it
was not possible to follow the process trend by these measurement techniques alone due to the
low specific ORA of guaifenesin and only slight liquid density changes during crystallization. A
semi-preparative UV-detector was therefore additionally used in the crystallization tank
providing a better resolution of concentration changes.
CPCD is a quasi-steady state process and hence no changes in the liquid phase composition are
expected as long as process conditions are chosen appropriately. Nevertheless, the process needs
to be interrupted when the provided solid material in the dissolution tank becomes enantiopure to
prevent nucleation in the crystallization tank and to ensure maximal yields. The process equals
afterwards a Preferential Crystallization characterized by a decreasing concentration and no
further removal of the initially racemic solid excess material in the dissolution tank. Hence,
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inline turbidity measurement was utilized in this tank to monitor and control the process. It is
able to indicate the quantitative selective dissolution of the crystallizing enantiomer and the
process end by reaching a lower plateau.
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8
9
0
Figure 5 shows the preparation phase of the first experiment. Based on initial solubility data [10],
solutions in both reactors were prepared to be saturated at 35 °C. The saturation point was
chosen to avoid an eventual liquid-liquid demixing but enabling crystallization at moderate
temperatures over a wide range of supersaturations (see Fig. 3 b). Five standards of racemic
guaifenesin were added to the crystallization tank for calibration, which is indicated by the
stepwise increase of the UV-signal (Fig. 5, right). Both tanks were kept at 45 °C during this
period to ensure complete dissolution of the feed material.
Fig. 5: Startup-phase and UV calibration during the trial experiment (T = 35 °C, T
= 30-34
sat
cryst
°
C, Exp. 1 in Table 3). Outliers or peaks in the UV and ORA signals in the crystallization tank
originate from measured air bubbles, which also refers to the following process trajectories
shown (Figs. 6,9 and 10).
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After cooling of both tanks to saturation temperature, at t ≈ 1.7 h, 20 g of solid racemate to be
separated were added to the dissolution tank resulting in a strong increase of the turbidity signal
0
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4
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9
0
(Figure 5, left). Liquid exchange was subsequently turned on at a rate of 5 mL/min to equilibrate
the solutions in both tanks. Unexpectedly, the added material dissolved quickly, which was
detected by the decrease in turbidity (black line, dissolution tank, Fig. 5, left) and the further
increase of the UV-signal at t ≈ 2 h (black line, crystallization tank, Fig. 5, right). With a
different process strategy (e.g., conventional PC or coupled PC [13]), the obvious inaccuracy of
the solubility data would have been left undetected and a loss of the pure seed material would
have been the result. An additional 50 g of racemate were added to the dissolution tank (t ≈ 2.8
h), which was estimated to result in a suspension. The stabilization of the turbidity signal after
≈
3.7 h indicated that no further dissolution took place. Nevertheless, both tanks were left
overnight, exchanging mother liquor at a rate of 5 mL/min to ensure saturation of the liquid
phases. Samples were withdrawn and analyzed gravimetrically to determine the actual liquid
phase concentration before restarting the plant.
After subcooling the solution in the crystallization tank by 0.5 K, 0.1 g (R)-guaifenesin seeds
were added to initiate the Preferential Crystallization process (black dashed line, crystallization
tank, Fig. 6, right).
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9
0
Fig. 6: Dissolution and crystallization phase of the trial experiment after start-up and calibration
shown in Fig. 5 (T = 35 °C, T
= 30-34 °C, Exp. 1 in Table 3).
sat
cryst
Subsequently, temperature was decreased step-wise to identify suitable crystallization
conditions. However, no significant changes in the signals were observed even at 30 °C
(supersaturation with respect to the racemic composition Srac = 2.16). The suspension of the
crystallization tank was finally harvested at t = 26.9 h, filtered and carefully washed with ice-
cold water. Altogether, 0.8 g of (R)-guaifenesin was entrained with a purity of 98.52% proving
the general feasibility of the process utilizing these conditions. However, the amount of solid
racemic excess material in the dissolution tank was too high to be purified successfully. It was
therefore reduced in subsequent experiments. Additionally, it was evident that guaifenesin
growth kinetics are rather slow. In order to improve the overall process performance, the initial
seed loading was also adapted to provide a larger crystallization surface area.
The experimental conditions found applicable during the trial experiment (Table 2), which
served as a starting point for further separation runs.
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1
1
1
1
1
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1
1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
4
4
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5
5
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5
5
5
5
5
5
5
6
Table 2: General process parameters for the CPCD separation experiments.
Parameter
Tsat
Value
35
Unit
°C
Tcryst
30
°C
0
1
2
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9
0
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Nstirr
200
400
53.3
2.16
rpm
mH2O
g
g
-
mGua (rac.)
Srac
mseed, R
msol. rac. feed
0.1 - 1.1
g
g
3.5 - 40
(dissolution tank)
퐕 퐞퐱퐜퐡퐚퐧퐠퐞
5 - 30
mL/min
3
. Experimental results
3
.1 Solubility and metastable zone width
In the preliminary experiment deviations from the polythermal solubility data obtained in [10]
were observed. Hence, additional isothermal solubility measurements were carried out following
the procedure described in section 2.2. Especially in the range of the steep increase of the
saturation curve between 30 °C and 45 °C, samples were prepared for temperature steps of 1 K.
Even though, solid phases were dissolved before equilibration at a higher temperature, a solid
phase recrystallized at the respective saturation temperature. Hence, no evidence was found
supporting a stable liquid-liquid phase separation.
The measured saturation concentrations are depicted in Figure 7 (black crosses and dashed line),
which show a similar trend as the data from [10] (black and red squares for polythermal and
isothermal data, respectively). Although the few isothermal data points from [10] perfectly agree,
the solubility curve is slightly shifted towards lower temperatures compared to results of
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polythermal studies reported in [10], which explains the unexpected dissolution during the trial
experiment. Particularly, in the range of the steepest slope of the solubility curve, deviations of
around 20 wt-% occur. The differences are probably caused by the very slow dissolution kinetics
of guaifenesin, which influences the accuracy of the polythermal method, even though, low
heating ramps of 0.05 K/min were utilized in [10]. Also no other than the known solid state form
was found by XRPD analyses. The isothermal solubility results from all samples are listed in the
Appendix (Table A1) together with the standard deviations of samples of one particular
temperature.
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1
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0
1
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4
5
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8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 7: Measured solubility curve of racemic guaifenesin (isothermal method) in comparison
with data from [10] and the ideal solubility line calculated according to the Schröder - van Laar
equation. The dashed line is just guide for the eye.
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6
Metastable zone widths were investigated for different concentrations with a polythermal method
and are shown in Figure 8. As expected, the metastable zone width increases with increasing
cooling rate. Further it can be seen, that for high cooling rates (-80 K/h and -20 K/h) no
correlation between nucleation points and the shape of the solubility curve is present. This can be
explained by the crystallization of water and guaifenesin together, i.e. crystallization below the
eutectic temperature, which is calculated to -4°C from intersection of the liquidus lines according
to the Schröder - van Laar equation. This explanation is supported by two different solid phases,
which were observed inside the samples right after nucleation. For cooling rates between -0.5
and -5 K/h, the metastable zone widths, or the supersolubility curves, follow the trend of the
solubility curve with possible subcoolings of the clear solutions ranging from 10-23 K (-0.5 K/h)
to 30-40 K (-5 K/h).
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9
0
The recorded image data during nucleation revealed that crystals were formed directly from the
clear solutions and no second liquid phase was observed. Thus, the hypothesis that a metastable
miscibility gap is the reason for the unusual solubility behavior cannot be supported by this data.
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8
9
0
Fig. 8: Identified nucleation points of solutions with different guaifenesin concentrations, which
were linearly cooled at different rates (0.5-80 K/h). Dashed and dotted lines are guides for the
eye for the solubility curve and the supersolubility curves at various cooling rates.
3
.2 Separation results
The results of all separation experiments, including Exp. 1 described before, are compiled in
Table 3. The essential initial parameters, the mass of seed material, mseed, and solid racemic feed,
msol. feed, are listed together with the produced solid phases in the crystallization and dissolution
tank, m and m . After HPLC analysis of the crystalline products, purity in both tanks, Pur and
R
S
R
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2
2
2
2
2
2
2
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5
6
Pur , was determined according to eq. A1 (see Appendix). The listed productivities, Pr and Pr ,
S
R
S
were calculated according to eq. A2 (see Appendix). The process time was defined as the time
between seeding the process and harvesting the product.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
It can be seen from Table 3, that in the first four experiments (Exp. 1-4), the mass of solid
racemic excess material, msol. feed, was adapted to achieve a complete separation and hence, pure
enantiomers in both tanks. With the process conditions listed in Table 2, 3.5 g of solid racemic
feed were purified (Exp. 4 in Table 3) and 1.8 g of (R)- and 1.2 g of (S)-guaifenesin with purities
>
95% could be harvested. The initial seed, mseed, and racemic solid excess masses, msol. feed, were
increased, subsequent to this successful separation, to enhance yield and productivity of the
process (Exp. 5-7). In the following experiments (Exp. 8-10), both enantiomers of guaifenesin
were produced with high purities for further studies. In the last experiment (Exp. 11), the
technical limit of the plant was investigated. An initial racemic solid excess mass was chosen,
which allowed just for an appropriate liquid phase exchange with respect to filter blocking and
exchange rate (technical limit 30 mL/min).
Table 3: Product purities, Pur and Pur , and masses, m and m , (dry product) as well as
R
S
R
S
process times, tproc, and productivities, Pr and Pr , of all CPCD experiments performed. Seeding
R
S
was always done with (R)-guaifenesin except for Exp. 9 where the (S)-enantiomer was used.
mseed
msol. feed
mR
PurR
mS
PurS
tproc
PrR
PrS
Exp.
[g]
[g]
[g]
[%]
[g]
[%]
[h] [g/(kg h)] [g/(kg h)]
4
6
7
6
5
3
.3
.3
.2
.7
.8
.5
0.36
0.45
0.68
0.55
1.49
9.31
-
1
2
3
4
5
6
0.1
0.1
0.1
0.1
0.3
1.1
(37)
20
4
0.8
1.4
99
98
97
98
98
80
-
-
-
-
-
0.17
0.37
1.26
6.63
2.4
0.6
1.2
3.5
11.2
91
95
97
98
3.5
10
20
1.8
4.3
19.6
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9
1
.9
3.3
.7
4.0
.0
14.27
14.03
8.15
9.39
5.47
10.89
7.55
8.39
7
8
9
1.1
1.1
1.1
1.1
1.1
30
30
30
30
40
13.8
31.6
11.3
16.1
32.7
97
69
98
93
97
8.8
8.9
98
98
98
98
98
2
14.5
14.9
16.9
7.67
1
0
1
4
16.86
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The online measurements of the first successful separation (Fig. 9, Exp. 4, see Table 3) show
similar trends compared to the trial experiment following the preparation procedure (dashed lines
at t = 0h in Fig. 9). The stable UV signal indicates constant concentration in both tanks (black
line, crystallization tank, Fig. 9, right). The small increase of the optical rotation signal between t
=
0…6h (blue dotted line, crystallization tank, Fig. 9, right) shows a slight enrichment of the
unseeded enantiomer in the liquid phase. Hence, the chosen liquid phase exchange rate of 10
mL/min was almost sufficient to counter-balance selective crystallization in the crystallization
tank. After seeding (dashed line in Fig. 9), the turbidity in the dissolution tank remains constant
for almost 2h indicating an initially slow Preferential Crystallization caused by the low seed
loading. The signal starts to decrease significantly at t ≈ 2 h due to distinct selective dissolution
in this tank and subsequently reaches a plateau (t ≈ 5 h). Both enantiomers could have been
harvested at this point already with high purity. However, to enhance productivity and yield, the
temperature in the dissolution tank was lowered to the crystallization temperature as well, which
transfers the CPCD process into a coupled Preferential Crystallization [13]. Hence, both solid
phases crystallize further and selectively remove the respective enantiomer from the liquid
phase. This is indicated by a slight increase in turbidity showing that the process can in fact be
monitored reliably with this method. Afterwards, both products were withdrawn, filtered and
washed.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 9: Trajectories of turbidity, optical rotation (optical rotation angle, ORA), UV absorption
and temperature of CPCD experiment 4 (T = 35°C, T
= 30°C, Exp. 4 in Table 3).
sat
cryst
Seed mass and solid racemic excess material were increased step-wise for the following
experiments to further enhance productivity. Figure 10 shows experiment 11 (compare Table 3)
where 40 g of racemic guaifenesin were separated. The larger mass of carefully ground seeds
decreased the initial startup period to 1 h and the overall process time until a complete
purification was achieved after 4 h. A liquid exchange rate of 30 mL/min between both tanks
was necessary to keep the mother liquor at racemic composition in this case. A slight decrease of
the UV signal can be seen in the last 0.5 h (black line in Fig. 10, crystallization tank, right) due
to the Preferential Crystallization of both enantiomers from the liquid phase at the end of the
experiment, which fits well with the final increase in turbidity in the dissolution tank. In total, 33
g (R)- and 17 g (S)-guaifenesin were produced with purities >97%. The high productivities,
which were achieved (Pr ≈ 17 g/(kg·h); Pr ≈ 11 g/(kg·h)), show the great potential of the
R
S
CPCD process also for industrial applications.
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Crystal Growth & Design
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 10: Trajectories of turbidity, optical rotation (optical rotation angle, ORA), UV absorption
and temperature of CPCD experiment 11 (T = 35°C, T
= 30°C, Exp. 11 in Table 3).
sat
cryst
The empirical process control strategy based on the utilized measurement techniques can be
summarized as follows. If the turbidity signal reaches a lower plateau and all other signals are
almost constant, the process can be interrupted since the seeded enantiomer is completely
removed from the solid excess material in the dissolution tank. An accelerating decrease of this
signal without any plateau indicates primary nucleation of the unseeded enantiomer in the
crystallization tank followed by complete loss of the solid phase in the dissolution stage. In this
case, the seed mass or the driving force needs to be lowered to reduce the rate of selective
entrainment in the crystallization tank. In the opposite case, if the turbidity signal stays constant,
supersaturation and seed loading have to be increased to enhance process performance. A change
of the optical rotation or the UV absorption in the crystallization stage indicates the selective
depletion of the seeded enantiomer from the mother liquor increasing the risk of primary
nucleation of the antipode. The liquid exchange rate between both tanks needs to be increased in
this case, to counter balance the entrainment by selective dissolution.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
It can be seen from the purities in Table 3, that primary nucleation of the counter-enantiomer
cannot always be prevented in the crystallization tank. If the process is optimized and higher
supersaturations are utilized, the probability of nucleation of the antipode increases rapidly. This
is caused by the inherently given non-robustness of the Preferential Crystallization process. A
successful separation is still possible even if nucleation of the unseeded enantiomer occurs in the
crystallization tank. In this case, the crystallization tank should become the dissolution tank and
vice versa until the nucleated crystals are dissolved again. However, this requires a precise
analysis of the solid phase composition in each tank.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Nevertheless, below a certain supersaturation no nucleation will occur in a finite time but at the
expense of process performance with respect to yield and productivity. In this case, a change of
the setup could be beneficial if, for example, a wet-milling process is introduced to generate
more crystal surface and, hence, increase the productivity even at low supersaturations.
By contrast, primary nucleation is completely avoided in the dissolution tank, thus pure product
will always be gained in this vessel if crystallization is carried out for a sufficiently long
duration.
Only a part of the product here can be lost if the unseeded enantiomer nucleates in the
crystallization tank. Hence, the process needs to be adapted to every substance system
specifically utilizing appropriate models or control strategies like the aforementioned empirical
control to achieve highest productivities and still meet the purity constraints in both vessels.
4. Conclusions
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9
The study has shown, that the presented CPCD process is capable to efficiently resolve racemic
guaifenesin providing both enantiomers simultaneously in a highly pure crystalline form. A
sufficient amount of pure material was produced in two connected lab-scale tanks for further
studies, starting from an initially small quantity of seeds even though slow growth kinetics
complicated the separations and deviations to the reported solubility curve were found. This
demonstrates the advantages of the applied quasi-stationary process. Crystallization conditions
can be chosen in a way that no or negligible nucleation of the unseeded enantiomer occurs to
keep the process metastable until a significant entrainment of the preferred enantiomer is visible.
An equilibration step during the preparation procedure of the process decreases the risk of
unexpected dissolution of expensive enantiopure seed material because of the presence of excess
solid racemic material. Problems arising from inaccurate solubility data or maybe also as a result
of different batches of starting racemate can thus be mitigated.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The solubility of racemic guaifenesin was determined with an isothermal method while
metastable zone width measurements were carried out using polythermal conditions. The trend
of the reported solubility data was verified but a small shift with respect to temperature was
found. It was shown, that guaifenensin/ water solutions have a rather large zone of metastability,
which increases strongly with faster cooling rates. Neither a stable nor metastable liquid-liquid
demixing was found during these investigations.
The process was designed rationally based on a few trial crystallizations, which yielded
appropriate operating conditions for a complete separation. Subsequently, the operating
conditions were adapted, mainly seed loading and preparation as well as the mass of solid
racemic excess material, to increase yield and productivity. In a final experiment a maximal
productivity of 16.9 g/(kg·h) in the crystallization and 8.4 g/(kg·h) in the dissolution tanks were
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2
3
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5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
achieved with purities exceeding 97%. From all observations of the performed experiments, a
simple, empirical process control strategy, which is mainly based on an inline turbidity probe,
was derived to enhance the applicability of this process to other systems.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
All separation experiments combined in this study yielded 136 g of the (R)-guaifenesin product
and 81 g of the (S)-guaifenesin product with average purities of 88% and 98%, respectively,
starting from only 0.8 g of pure (R)-guaifenesin. These values were improved to >99.9% by
simple recrystallization. The lower purity of the (R)-enantiomer results obviously from primary
nucleation of the counter-enantiomer showing potential to further improvement of the current
process.
Acknowledgements
The cooperation with the Arbuzov Institute of Organic and Physical Chemistry (Russian
Academy of Sciences, Kazan) is gratefully acknowledged. The authors thank especially Prof.
Alexander A. Bredikhin and Dr. Robert Fayzullin for providing ≈1g of pure (R)-guaifenesin.
This research received funding as part of a CORE project (October 2016−September 2020) from
the Horizon2020 Research and Innovation Programme of the European Union under Marie
Sklodowska-Curie Grant Agreement No. 722456 CORE ITN.
List of symbols
M
[-]
Motor
m
[g]
Mass
n
[rpm]
[V]
Stirrer speed
ORA
Signal of the optical rotation angle
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8
9
Pr
Pur
S
[g/(kg·h)]
[%]
[-]
Productivity
Purity
Relative supersaturation
Temperature
Turbidity
T
[°C]
[-]
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Turb
t
[h]
Time
x
[-]
Mass fraction
Signal of the UV-detector
UV
[V]
&
V
[mL/min]
Volumetric flow rate
Sub- & Superscripts
cryst
feed
gua
Crystallization
Feed
Guaifenesin
Water
H O
2
i
Component i
Process
proc
rac
racemic
sat
Saturation
Stirrer
stirr
seed
sol.
solution
0
Seed
Solid
Solution
Initial value
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