Propositions belonging to the thesis of Y. Pérez de Diego:
Production of Controlled Drug Delivery Microparticles using supercritical C 02
1. Processes that use carbon dioxide as an antisolvent are at the moment the most promising supercritical fluid techniques for the production of controlled drug delivery polymer microparticles (Chapter I ofthis thesis)
2. The reason why predictions of recent models based on formation of droplets by a jet-breakup mechanism did not match experimental results [Rantakyla et al, Journal of Supercritical Fluids, 24, (2002), 251] is that the solvent and the carbon dioxide are completely miscible at the prevailing pressure and temperature [Sarkari et al, AIChE J., 46, (2000), 1850; Lengsfeldet al, J. Phys. Chem. B., 104, (2000), 2725].
3. A liquid-liquid phase split, induced by the carbon dioxide when brought into contact with a polymer solution, can explain the formation of droplets at conditions of complete miscibility between the solvent and the carbon dioxide. (Chapter 5 and 6 ofthis thesis). 4. To properly describe the mechanism of partiele formation in the PCA process,
hydrodynamics, mass transfer, thermodynamics and precipitation kinetics have to be considered simultaneously. (Chapter 2 ofthis thesis).
5. A major advantage of researching the field of supercritical fluids is that all the discoveries you make are novel. A major disadvantage is that in order to make these discoveries, novel tools are need to be developed.
6. Supercritical fluid technology can only be fully developed by means of interdisciplinary research, where chemists and physicists study the fundamentals of the processes and develop new theories and correlations and engineers develop new equipment and methods. 7. Although many individual scientists may consider it a waste of time, publishing failed
experiments on a regular basis would greatly enhance the efficiency of science as a whole. 8. Success in life or science, as in cards or board games, is not only about luck; an active
player with a clear strategy and goals is always much more likely to win.
9. A visit to Holland on "Koninginnedag" would teach many Spaniards that paying homage to the national flag or the national colours is not mcant to "hurt people's feelings", 'Lto divide the country" or "to glorify fascism".
10. Volunteers working for NGOs (Non-Governmental Organizations), with or without religious belief, understand the Christian principles and values better than many formal representatives of ecclesiastic institutions.
11. The best lottery you can win on life is to be born in a warm and caring family.
These propositions are considered defendable and as such have been approved by the supervisor, Prof. Dr.Ir. P.J. Jansens.
1. Processen die koolzuur als antisolvent gebruiken zijn, op dit moment, de meest veelbelovende superkritische technieken voor de productie van polymcer-microdeeltjes voor de gecontroleerde afgifte van geneesmiddelen (Hoofdstuk 1 van dit proefschrift).
I. De reden dat sommige modellen gebaseerd op de vorming van druppels door jet-brcakup niet overeenkwamen met experimentele resultaten [Rantakyla et al., Journal of Supercritica} Fluids, 24, (2002), 251], is dat het solvent en de koolzuur volledig mengbaar zijn bij de heersende druk en temperatuur [Sarkari el al, AlChE J., 46, (2000), 1850; Lengsfeld et al, J. Phys. Chem. B., 104, (2000), 2725].
3. Een vloeistof-vloeistof faseschciding, geïnduceerd door koolzuur wanneer bet in contact komt met een polymccroplossing, kan de vorming van druppeltjes verklaren bij condities van volledige mengbaarheid van solvent en koolzuur (Hoofdstukken 5 en 6 van dit proefschrift). 4. Voor een juiste beschrijving van het dccltjcsvormingsmechanismc in het PCA-proces moeten
hydrodynamica, stofoverdracht, thermodynamica en precipitatiekinctiek gelijktijdig beschouwd worden (Hoofdstuk 2 van dit proefschrift).
5. Een belangrijk voordcel van het onderzoeken van superkritische fluïda is dat alle ontdekkingen die je doet nieuw zijn. Een belangrijk nadeel is dat ook nieuwe hulpmiddelen nodig zijn om die ontdekkingen te kunnen doen.
6. Superkritische vlocistoftcchnologie kan alleen volledig ontwikkeld worden door interdisciplinair onderzoek, waarbij chemici en fysici de fundamentele aspecten van de processen onderzoeken en nieuwe theorieën en verbanden ontwikkelen, en ingenieurs werken aan nieuwe instrumentatie en methoden.
7. Hoewel veel individuele wetenschappers het als tijdverspilling beschouwen, zou het regelmatig publiceren van mislukte experimenten de effectiviteit van de wetenschap als geheel zeer ten goede komen
8. Zoals in kaart- of bordspellen is succes in het leven of de wetenschap niet alleen een kwestie van geluk; een actieve speler met een duidelijke strategie en doelen heeft altijd meer kans om te winnen.
9. Een bezoek aan Nederland op Koninginnedag zou veel Spanjaarden leren dat eer betonen aan je nationale vlag of nationale kleuren niet bedoeld is om "mensen te kwetsen", "het land te verdelen" of "het fascisme te verheerlijken".
10. Vrijwilligers die werken voor nict-gouvcrncmcntclc organisaties, met of zonder religieus geloof, begrijpen de Christelijke principes en waarden beter dan veel formele vertegenwoordigers van kerkelijke instellingen.
II. De beste loterij die je in je leven kunt winnen is geboren worden in een warme, zorgzame familie.
Deze stellingen worden verdedigbaar geacht en zijn zodanig goedgekeurd door de promotor, Prof. dr.ir. P.J.Jansens.
Microparticles using Supercritical C 0
2
TR diss
4415
Supercritical CO2
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op maandag 28 februari 2005 om 15:30 uur
door / >t- %\
/ H
-1
Yohana PEREZ DE DIEGOLicenciada en Quimicas, Universidad de Alcala de Henares geboren te Guadalajara, Spanje
Prof. dr. G.J.Witkamp
Samenstelling Promotiecommissie:
Prof. dr. ir. J.T. Fokkema , voorzitter
Prof. dr. ir.P.J.Jansens, Technische Universiteit Delft, promotor Prof. dr. G.J.Witkamp, Technische Universiteit Delft, promotor Prof. dr. ir. R. F. Mudde, Technische Universiteit Delft
Prof. dr.ir. J.T.F. Keurentjes, Technische Universiteit Eindhoven
Prof. dr. J. San Roman, Inst. de Ciencia y Tecnologia de Polimeros, Spanje Dr. ir. Th.W. de Loos, Technische Universiteit Delft
Dr.ir. F.E.Wubbolts, Technische Universiteit Delft
Dr.ir. F.E. Wubbolts heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen.
This research was financially supported by the European program Suprophar (G1RD-CT-2000-00164)
Cover design: M.C. Maas (SEMpicture created by P.F.M. Durville)
ISBN: 90-9019114-3
Copyright © 2005 by Y. Pérez de Diego Printed by: Febodruk B.V., Enschede
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the publisher.
"Cudn grande riqueza es, aün entre los pobres,
el ser hijo de buen padre"
1. Introduction 1
1.1. ControUed drug delivery systems 2
1.2 Supercritical Fluids 4 1.3. Production of microparticles using supercritical fluids 6
1.3.1. Rapid Expansion of Supercritical Solutions (RESS) 6 1.3.2. Precipitation from Gas Saturated Solutions (PGSS) 8 1.3.3. Supercritical Antisolvent Processes (GAS/PCA/SEDS) 9 1.4. Selection of the most appropriate supercritical technique 11
1.5. Purpose of this thesis 12 1.6. Thesis outline 12 1.7. References 15
2. Production of ControUed Drug Delivery Systems using the PCA process:
state of the art & challenges 19
2.1. Introduction 20 2.2. Supercritical processes using C02 as antisolvent 22
2.2.1 GAS 22 2.2.2 PCA, SEDS, ASES or SAS 23
2.3. Factors influencing the mechanism of partiele formation 24 2.3.1 Hydrodynamic Considerations: Droplet Formation Vs Mixing 25
2.3.2 Mass Transfer 28 2.3.3 Thermodynamic Behaviour 32
2.3.4 Precipitation Kinetics 36
2.4. Discussion 38 2.4.1 BelowThe Mixture Critical Pressure 39
2.4.2 Above Mixture Critical Pressure 41
2.5. Conclusions and perspectives 41
3. Measurement of the phase behaviour of the system dextran/DMSO/C02
at high pressures 49
3.1. Introduction 50 3.2. Theory 51
3.2.1 Effect of supercritical carbon dioxide on polymer -solvent mixtures 51
3.2.1 Ternary phase diagrams polymer-solvent-antisolvent 52
3.3. Experimental Methods 53 3.3.1 Materials 53 3.3.2 Preparation of the solutions 53
3.3.3 Variable composition view cell 54
3.4. Results 59 3.4.1 Measurements using the variable composition view cell 59
3.4.2 Measurements using the Cailletet tube 61
3.5. Discussion 67 3.6. Conclusions 71 3.7. Acknowledgements 72 3.8. References 72
4. Modelling mass transfer in the PCA process using the Maxwell-Stefan
approach 75
4.1. Introduction 76 4.2. Development of the model 79
4.2.1 Maxwell-Stefan Equations 81
4.2.2 Constraints 83 4.2.3 Component balances 83
4.2.4 Liquid (disperse) phase mass transfer coefficients 84 4.2.5 Vapour (continuous) phase mass transfer coefficients 85
4.2.6 Initial conditions 86 4.3. Physical data forthe simulations 86 4.4. Methodology of the computations 89
4.5. Results and discussion 89 4.5.1 Variation of concentration profiles, fluxes and droplet diameter with time 91
4.5.3 Influence of temperature 95 4.5.4 Influence of C02 to solvent molar ratio 97
4.5.5 Influence of initial droplet diameter 98
4.6. Conclusions 99 4.7. References 100
5. Improved PCA process for the production of nano- and microparticles
ofpolymers 103
5.1. Introduction 104 5.2. Theory 106
5.2.1. Phase diagram dextran-DMSO-C02 106
5.2.2. Mechanism of droplet formation 108 5.2.3. Classical nucleation theory 109 5.2.4. New device for controlling PCA process 110
5.3. Experimental methods 112 5.3.1. Materials 112 5.3.2. Partiele characterisation methods 112
5.3.3. Apparatus 113 5.3.4. Operational procedure 114
5.3.5. Experimental conditions 115
5.4. Results and discussion 116 5.4.1. Effect of fluid dynamic variables 117
5.4.2. Effect of residence time in the T-mixer 119 5.4.3. Effect of concentration of dextran in the DMSO 120 5.4.4. Concentration of C02 in the residence tube 121
5.4.5. Influence of density of the C02 123
5.4.6. Influence of temperature 126
5.5. Conclusions 126 5.6. Acknowledgements 127 5.7. References 128
6. Coprecipitation of PLLA and cholesterol with C02:
operating regimes and mechanism of partiele formation 131
6.1. Introduction 132 6.2. Theory 134
6.2.1 Binary P,x phase diagrams organic solvent-C02 134
6.2.2 Ternary phase diagrams polymer- solvent-C02 136
6.2.3 Phase behaviour of the studied systems 137
6.3. Materials, apparatus and methods 138
6.3.1 Materials 138 6.3.2 Partiele characterisation methods 138
6.3.3 Apparatus and procedure 138
6.4. Results 139 6.4.1 Below mixture critical pressure 140
6.4.2 Above Pc mixture 147
6.4.3 Spraying into liquid C02 149
6.5. Discussion 149 6.6. Conclusions 151 6.7. Acknowledgements 152 6.8. References 153
7. Opening new operating windows for polymer and protein micronisation
using the PCA process 155
7.1. Introduction 156 7.2. Materials and methods 157
7.2.1 Materials 157 7.2.2 Partiele characterisation methods 158
7.2.3 Apparatus 158 7.3. Results 159 7.3.1 Phase behaviour 159 7.3.2 PCA experiments 161 7.4. Discussion 169 7.5. Conclusions 172 7.6. Acknowledgement 172
7.7. References 173
8. Screening different PCA alternatives for coprecipitation of proteins and
biodegradable polymers 175
8.1. Introduction 176 8.2. Materials and Methods 180
8.2.1. Materials 180 8.2.2. Protein quantification method 180
8.2.3. Experimental procedure and equipment 181
8.3. Results on partiele formation 186 8.3.1. Precipitation from DMSO using conventional PCA process 186
8.3.2. Precipitation using the improved PCA process 188 8.3.3. Doublé Improved PCA process (using two mixing steps) 191
8.3.4. Conventional PCA process using water as a modifier of the phase behaviour 193
8.3.5. Experiments spraying over a Vapour- Liquid Antisolvent: 195
8.4. Co-precipitation efficiency test 200 8.5. Conclusions and perspectives 201
8.6. References 202
Appendices
I. Peng-Robinson Equation of State 207 II. PGSS process: micronisation of hvperbranched polymers 215
III. RESS process: micronisation of cholesterol 231
Summary 241
Samenvatting 245
Acknowledgements 249
About the author 251
Introduction
Micron-sized and microencapsulated drugs are desirable in the pharmaceutical industry for drug targeting and con trolled release
systems. The current methods available for micronisation and microencapsulation (e.g. coacervation, spray drying) however are limited by various factors such as the use of toxic organic solvents or high temperatures.
In the last decades, new processes based on the use of supercritical fluids are being extensively studied as a promising alternative to produce controlled drug delivery microparticles Due to its
non-toxicity, non-flammability, non-reactivity and inexpensiveness, carbon dioxide is the most commonly used supercritical fluid for pharmaceutical applications.
This introductory chapter provides background information about the importance of controlled drug delivery microparticles, the drawbacks of the conventional techniques, supercritical fluids and the opportunities that the use of C02 brings to the processing of
pharmaceutical products.
At the end of the chapter the aim and an outline of the thesis will be given.
1.1. Controlled drug delivery systems
Conventional means of administering drugs (i.e. pills, tablets) require successive doses to sustain the drug action for long periods of time. The concentration pro file for the effect of successive doses of a drug in a body compartment as a function of time is illustrated by Figure 1.1. Using the conventional dosage forms, an initial burst of the drug concentration in the body is observed. After some time, this concentration profïle decays below the therapeutic range and a new dose of drug needs to be administered to the body again (arrows below the time axis indicate a new drug dose). An important improvement in the treatment is obtained by the application of a device that releases the drug at a constant rate for long periods of time, the so- called controlled drug delivery systems [1].
Administration Conventional ■ " " Controlled Drug concentration Toxic level Therapeutic range
Figure 1.1. Comparison of the drug concentration profiles vs time using successive doses ofa conventional administration form and a controlled drug delivery form.
These controlled drug delivery systems improve patiënt compliance, and offer interesting advantages as the possibility of the location of the drug at the site of action and the protection of sensitive drugs from chemical or enzymatic degradation.
Biodegradable polymers are the most common matrix used to prepare controlled drug delivery systems [2,3]. The rate of drug release can be easily adjusted by modification of
polymer properties (i.e. molecular weight, copolymerization) and no surgery is needed to remove the polymer from the body after drug release, because biodegradable polymers naturally decompose to non-toxic products.
Micro and nanoparticles are the group of controlled drug delivery systems that is receiving the most attention over the last years [4] .These particles have potential applications for delivery of proteins and peptides, anti-cancer agents, contraceptives and vaccines [5].
The main requirements of the drug-loaded microparticles are:
Appropriate partiele size
o Oral administration: 1-50 \xm
o Inhalation and subcutaneous or intramuscular injection: 1-5 p.m o Injection in the systemic circulation system: < 1 um
Narrow diameter distribution to assure a constant release profile. Low organic solvent residual content
Linear release profile of the drug in time.
Controlled drug delivery microparticles can be prepared by various conventional methods [6].However, as shown in Table 1.1, those techniques have important limitations that include thermal degradation (spray drying), broad size distribution and large partiele size (milling), and the use of solvents (precipitation, coacervation), which are often toxic and hazardous either to the environment, to personnel involved in the manufacturing industry or to the fïnal user. New methods, based on the use of supercritical fluids, are a promising alternative to produce the controlled drug delivery systems [7].
Table 1.1. Process Milling, grinding, pulverization Lyophilization-sieving Spray drying Precipitation Coacervation Interfacial polymerisation Impregnation
Conventional techniques for
Driving force
Mechanical
the production of microparticles
freeze drying by solvent sublimation solvent evaporation temperature antisolvent addition solvent evaporation mechanical (stirring) coating hardened mechanical solvent evaporation drug diffusion Drawbacks
broad size distribution high temperatures loss of crystallinity broad size distribution broad size distribution high temperatures broad size distribution residual solvents agglomeration residual solvents
residual solvents and additives residual solvents
high temperatures
1.2. Supercritical Fluids
Supercritical fluids (SCF) are fluids at temperatures and pressures slightly above the critical point. At the critical point, the properties of the vapour and the liquid phase become equal and they merge into a new phase called supercritical. The position of the supercritical fluid region on a typical pressure- temperature phase diagram is illustrated in Figure 1.2.
The supercritical fluids exhibit attractive intermediate properties between those of liquids and gases (see Table 1.2). These fluids have a liquid-like density (and hence solvation power) and high compressibility, very low viscosity and high diffusivity. The first two properties make the solvent power of SCF easily controllable by changing pressure and/or temperature, while low viscosity and high diffusivity markedly enhance mass transfer phenomena and therefore, the kinetics of the process.
! Pressure
Temperature
*-Figure 1.2. General pressure-temperature phase diagram of a pure compound.
The most used supercritical fluid is C02. It is inexpensive, non-toxic, non-flammable and
chemically inert. Because of these properties it is especially attractive for pharmaceutical usage. It does not react with the processed compounds and it does not contaminate them because at the end of the process it is completely eliminated as a gas. Additionally, its low critical conditions (Pc= 71 bar, Tc= 304 K) allow to carry out the supercritical processes at
mild conditions, reducing process cost , simplifying process equipment and avoiding thermal or chemical degradation of the processed compounds.
Table 1.2. Comparison of properties ofgases, liquids and supercritical fluids [8J.
VAPOUR SUPERCRITICAL LIQUID Density (kg/m3) 0.6-2.0 200-1000 600-1600 Viscosity (mPa-s) 0.01-0.3 0.01-0.1 0.2-0.3 Diffusivity (m2/s) (1-4 HO"' (2-7)-10"7 (0.2-2.0)-10"9 supercritical fluid 'critical point vapour
1.3. Production of microparticles using supercritical fiuids
One of the most attractive properties of supercritical technology is the possibility of tuning system properties (i.e. solubilities, viscosities, densities, phase behaviour) by changing operating variables such as pressure and temperature. Separation between the processed microparticles and the supercritical fluid is easily achieved by depressurization. This produces a dried solvent-free product. Using low critical temperature supercritical fiuids (such as C02), the process can be performed at low temperatures, avoiding thermal
and chemical degradation of the processed materials. Due to the high driving forces involved in the precipitation processes, high supersaturations are quickly achieved and small particles with a narrow partiele size distribution are obtained. Moreover, recently it has been demonstrated that processes using supercritical C02 induce microbial
inactivation, so the product obtained after supercritical processing is effectively sterilised [9]. Supercritical fiuids can be used for the formation of microparticles of various sizes, shapes or structures mainly in three different types of processes: RESS [10], PGSS [11] and Antisolvent processes [12].
1.3.1 Rapid Expansion of Supercritical Solutions (RESS)
In the RESS process the supercritical fluid is used as a solvent for the solute. The supercritical solution is rapidly depressurised through a nozzle to precipitate the solute. Because expansion is a mechanical perturbation travelling at the speed of sound, uniform conditions are ensured within the solution. In this way, the solute nucleation, triggered by a sudden pressure decrease, takes place in an environment where high supersaturation ratios are uniformly reached. These two features lead to the formation of microparticles having a narrow size distribution. A schematic drawing of the equipment used to perform this supercritical process is shown in Figure 1.3.
Extractor Pre-expansion heater
4
jNozzle SoluteCO, EXTRACTION UNIT :•
Filter
\
r
Vent-line
> C02
PRECIPITATION UNIT
Figure 1.3. Schematic drawing of the RESS equipment.
The main advantages of the RESS process are its simplicity and the production of non-contaminated particles. Very small particles with a narrow partiele size distribution can be obtained. The feasibility of RESS to coprecipitate poly(DL-lactic acid) and lovastatin was shown by [13]. Poly(L-lactic acid) microspheres (10-90 urn) loaded with naproxen were obtained by Kim et al. [14] using a pre-expansion temperature of 391K and extraction pressures up to 190 bar. Mishima et al. [15] recently developed a new method called RESS-N (Rapid Expansion of Supercritical Solutions with a Non-solvent) to produce polymeric microcapsules of flavonoids without agglomeration. The particles do not adhere to each other because the ethanol (cosolvent) was volatile and acted as a non-solvent for the polymer (aminoalkyl methacrylic copolymer). They also produce polymer microparticles containing proteins such as lysozyme and lipase. The polymers they used are poly(ethylene glycol), poly (methylmethacrylate) and poly(propylene glycol).
The main limitation of the RESS process is that the most interesting pharmaceuticals, such as proteins and biopolymers, have usually high molecular weights and/or are polar and, hence, are poorly soluble in supercritical CO2, which is generally the fluid of choice.
So, the process is not economically viable if the solubility of these compounds in C02 is
not high enough (some mg per gram of C02). The pre-expansion temperatures associated
with this process can also be too high to enable the safe processing of thermally labile drugs and polymers with low glass transition temperatures.
1.3.2 Precipitation from Gas Saturated Solutions (PGSS)
In the PGSS process, the supercritical fluid is dissolved in a solution or melt at an elevated pressure. Expansion of this solution through a nozzle produces droplets that solidify by cooling. The presence of the supercritical fluid decreases the melting point of the solute (decreasing the operating temperature) and decreases the viscosity of the solution (aiding atomization). Furthermore, the decrease in temperature produced by the expansion of the supercritical fluid during the depressurisation aids the solidification of the droplets of molten solute atomized by the nozzle. A schematic drawing of the equipment used to perform this supercritical process is shown in Figure 1.4.
Pre-expansion heater
I
Vent-line > C02
PRECIPITATION UNIT
Figure 1.4. Schematic drawing of the PGSS equipment.
This process has a much lower C02 consumption than the RESS process, and can also be
variety of substances that melt in supercritical carbon dioxide. Co-precipitation of nifedipine, felodipine and fenofibrate with PEG 4000 has been described by Kerc et al. [16]. The high temperature required to melt some substances like semicrystalline polymers is the main limitation to apply the PGSS process to encapsulate thermolabile compounds. As the particles are produced by hydrodynamic atomization, another drawback of this process is the difficulty to produce submicronic particles and to control the partiele size distribution.
1.3.3 Supercritical Antisolvent Processes (GAS/PCA/SEDS)
In antisolvent processes, solutes of interest are first dissolved in a suitable organic phase. The addition of an antisolvent to this organic phase, decrease the solubility of the solute and induces its precipitation. Due to the beneficial transport properties of supercritical fluids, there is a fast contact between the antisolvent and the solute. High supersaturations are quickly achieved and very small particles with a narrow size distribution can be obtained. Nozzle i " ^
PCA
&*-+
Precipitator C02 StirrerB
SolutionHp)
Filter -cSEa—► GASFigure 1.5. Schematic drawing of PCA equipment (left) and GAS equipment (right).
There are different ways of mixing the supercritical fluid and the organic solution. In the GAS process (Gas Anti-Solvent), the solution is first charged into the precipitator. Then,
the compressed antisolvent is added, and the solute precipitates. In the PCA (Precipitation using a Compressed Antisolvent) the solution is dispersed into the supercritical phase using a nozzle. In the SEDS process (Solution Enhanced Dispersion by Supercritical Fluids) a coaxial nozzle is used. A schematic drawing of the GAS and PCA process is shown inFigure 1.5.
Table 1.3. Examples of production of controlled drug release microparticles using supercritical antisolvent techniques.
Drug+Polymer Process Reference 4-hydroxybenzoic + poly(lactide-co-glicolide)
4-hydroxybenzoic +poly(L-lactic acid)
Hydrocortisone+poly(DL-lactide-co-glicolide)
Insulin+ hyaluronic acid ethyl ester
Insulin+ poly(L-lactic acid)
Lysozyme+ poly(L-lactic acid)
Naproxen+ poly(L-lactic acid)
gentamycin, naloxone and naltrexone+ poly(L-lactic PCA acid)
budesonide + poly(L-lactic acid)
Supercritical antisolvent processes have to important advantages, they can be applied to a wide range of polymers and drugs (see Table 1.3) and, the processes are performed at mild temperatures. These two properties, together with the ability of producing solvent-free small size and narrow partiele size distribution products made of antisolvent processes the most promising supercritical processes for the production of controlled drug
SEDS SEDS SEDS GAS PCA PCA PCA PCA PCA 17 17 18 19 20 21 22 23 24
release systems (despite some disadvantages as the high C02 consumption and the use of
organic solvents).
1.4. Selection of the most appropriate supercritical technique to
produce controlled drug delivery microparticles
To choose the most appropriate supercritical technique to produce controlled drug delivery microparticles, the first issue to consider is the solubility of drug and polymer in the supercritical fluid (See Figure 1.6).
RESS-N/try other SCF
Does polymer easily melt into low viscosity solution?
q
PGSSIs drug soluble in SCF? RESS
Is polymer soluble in SCF? ^ B S T A R T I
Is drug soluble in SCF? YES) into low viscosity solution? Does polymer easily melt
d
PGSSPCA
PCA/try other SCF
Figure 1.6. Schema to select the most appropriate supercritical technique.
By modifymg the operating conditions and/or the supercritical fluid used, the solubility of drug and polymer can be tuned. Carbon dioxide is usually the supercritical fluid chosen to work with. Due to its simplicity, the RESS process must be considered at first, even if
drug and polymer solubility is very often too low to lead to a profitable process. PGSS process requires that the polymer melts at not very high temperatures producing a not very viscous solution. PGSS is the preferred process to produce larger particles. Finally, anti-solvent processes have to be considered. They allow the control of partiele size, shape and structure on a very wide range from nano-particles to micro-particles, even if they lead to high processing costs and are difficult to operate at large scale.
1.5. Purpose of this thesis
The aim of this research is to develop supercritical processes to prepare drug/polymer microparticles intended for controlled release. This thesis focuses on the understanding of the mechanisms of partiele formation and uses this knowledge to optimize the operating conditions and to develop new equipment and methods.
1.6. Thesis outline
Most of the semicrystalline polymers have high melting temperatures, are viscous as a melt and have a negligible solubility in supercritical carbon dioxide. As previously noted, these polymer properties are an important limitation to the use of the RESS and the PGSS process to produce controlled drug release microparticles. That is the reason why most of the work in this thesis is oriented to the comprehension, optimization and development of the supercritical antisolvent precipitation processes.
During the last years considerable work has been performed to study the feasibility of supercritical processes to produce controlled drug release systems. Chapter 2 presents the state of the art on the production of controlled drug release systems using supercritical antisolvent precipitation processes. It will be shown that there are some difficulties when applying this process to the micronisation of polymers, that there are apparent contradictions between the results found by different research groups, that it is not very
clear how the different operating parameters influence the precipitation process and, in short, that the mechanism of partiele formation during the PCA process is not completely understood. Hydrodynamic, mass transfer, thermodynamics and precipitation kinetics need to be studied in order to be able to control and/or design better ways to perform this process.
One of the mam factors that determine the viability of the supercritical process is the phase behavior of the polymers, drugs and additional solvents with the supercritical CO2. Especially when working with polymers, phase behaviour data are scarcely available in the literature and, the current thermodynamic models are not capable of accurate predictions of phase behavior. Hence, experimental work is needed to gain information about the behavior of these systems. Chapter 3 deals with measurements of the phase behavior of the system dextran-DMSO-C02, which was used in this thesis as a model
system to study the micronisation of polymers using the PCA process. It will be shown that a variable composition view cell method recently developed [25] to study solubilities of organic compounds, is a useful, easy and reliable method that can also be applied to study polymer systems.
Chapter 4 investigates the two-way mass transfer between a dichloromethane droplet and
carbon dioxide at conditions below their mixture critical pressure. A finite-difference approximation of the Maxwell-Stefan equations was used to simulate the effect of the different operating variables (i.e. pressure, temperature, droplet diameter, solution to CO2 flow ratio) on the droplet lifetime. Simulations show the variation with time of the droplet composition, the fluxes through the droplet-supercritical fluid interface, and the droplet diameter and provide a good insight to understand the two-way mass transfer process.
First trials to precipitate dextran, using the conventional PCA process revealed poor reproducibihty of the results and agglomeration of the particles. Chapter 5 introduces a new device and methodology that was developed to control the PCA process. In contrast
to other authors who tried to control the PCA process by influencing fluid dynamics or mass transfer mass the approach of this work is based on slowing down the kinetics of the precipitation process. This improved PCA process is able to control the process by separating the time scales of the different steps involved in the process: mixing of solution and CO2, liquid-liquid phase split and stripping of the solvent. It will be shown that the new device eliminates agglomeration of particles and yields reproducible results. The partiele size can be easily manipulated over a size range from several nanometers to tenths of microns by changing the operating conditions.
Chapter 6 deals with the micronisation of a biodegradable polymer (L-polylactic acid ) using the PCA process and shows experimental evidence about the existence of two different operating regions to perform the process (below and above the mixture critical pressure of the system solvent-C02). Two different mechanisms of partiele formation
have been proposed to explain why the morphology of the precipitated particles depends strongly on the region of operation. The possibility of encapsulating cholesterol in the L-PLA microparticles using this method was also demonstrated.
The use of the PCA process to precipitate proteins and polar polymers from DMSO using C 02 as antisolvent has many difficulties. In Chapter 7, an innovative solution, based on
the use of water to modify the phase behaviour of the system DMSO-C02 is presented.
The addition of water to the DMSO displaces the mixture critical pressure of the solution-C02 system to higher pressures and produces a new operating region in which the PCA
process can be carried out. Following this approach it was possible to develop a process to produce 1-10 um particles of N-trimethyl chitosan chloride (TMC) suitable for inhalation.
Finally, Chapter 8 explores the feasibility of the different supercritical antisolvent techniques (conventional PCA, improved PCA, doublé improved PCA and vapour-liquid PCA) to coprecipitate lysozyme and dextran. Controlled coprecipitation of both
compounds is achieved when the precipitation process is spatially confmed to the droplets produced by atomization in the nozzle.
Hyperbranched polymers as Boltorn H3200 are known to have a low viscosity. Appendix
II deals with the encapsulation of paracetamol into this polymer using the PGSS process. Appendix III describes the work performed on the precipitation of cholesterol using the
RESS process.
1.7. References
1. J. R. Robinson and V.H.L. Lee, Controlled Drug Delivery: fundamentals and applications, second edition, Marcel Dekker Ine, New York, USA, (1987).
2. R. S. Langer and N. A. Peppas, Present and future applications of biomaterials in controlled drug delivery systems, Biomaterials, 2, (1981), 201-214.
3. K.W. Leong and R. Langer, Polymeric controlled drug delivery, Advanced Drug Delivery Review, 1,(1988), 199-233.
4. M. N. V. Kumar, Nano and microparticles as Controlled Drug Delivery Devices, J. Pharm. Pharmaceut. Sci.,3, (2000),234-258.
5. L. Brannon-Peppas, Recent advances on the use of biodegradable microparticles and nanoparticles in controlled drug delivery, International Journal of Pharmaceutics, 116, (1995),l-9.
6. R. A. Jain,The manufacturing techniques of various drug loaded biodegradable poly(lactide-co-glycolide) (PLGA) devices, Biomaterials, 21, (2000), 2475-2490.
7. P.G. Debenedetti, J. W. Tom, S.-D. Yeo, G.-B. Lim, Application of supercritical fluids for the production of sustained delivery devices, Journal of Controlled Release,24, (1993), 27-44.
8. E. Stahl, K.W. Quirin, D. Gerard, Verdichte Gase zur extraction und raffination, Springer Verlag, Berlin., (1987).
9. S. Spilimbergo, N. Elvassore, A. Bertucco. Microbial Inactivation by High Pressure. Journal of Supercritical Fluids, 22, (2002), 55-63.
10. V. Krukonis, Supercritical nucleation of difficult to comminute solids, Annual AIChE Meeting, San Francisco, (1984).
11. E. Weidner, Z. Knez, Z. Novak, European patent EPO 744 992, February 1995, Patent WO 95/21688, July 1995.
12. E. Reverchon, Supercritical Antisolvent Precipitation of micro- and nanoparticles, Journal of Supercritical Fluids,15,(1999),1-21.
13. J.W.Tom, P.G.Debenedetti, S.-D.Yeo, G.-B.Lim, Application of Supercritical Fluids for the production of Sustained Delivery Devices, Journal of Controlled Release, 24, (1993), 27-44.
14. J-H. Kim, T. E. Paxton, D. L. Tomasko. Microencapsulation of Naproxen Using Rapid Expansion of Supercritical Solutions. Biothechnol. Prog. 1996,
12,650-661.
15. K.Mishima, K. Matsuyama, D. Tanabe, S. Yamauchi, T. J. Young , K. P. Johnston, Microencapsulation of Proteins by Rapid Expansion of Supercritical Solution with a Non solvent, AIChE Journal, 46, (2000), 857-865.
16. J. Kerc, S. Srcic, Z. Knez, P. Sencar -Bozic, Micronization of drugs using Supercritical Carbon Dioxide, International Journal of Pharmaceutics, 182, (1999), 33-39.
17. L. S. Tu, F. Dehghani, N. R. Foster, Micronisation and Microencapsulation of pharmaceuticals using a carbon dioxide antisolvent, Powder Technology, 126,(2002), 134-149.
18. R. Ghaderi, P. Artusson, J. Carlfors. A new method for preparing biodegradable microparticles and entrapment of hydrocortisone in DL-PLG
microparticles using supercritical fluids. European Journal of Pharmaceutical Sciences, 10,(2000), 1-9.
19. P.Pallado, L. Benedetti, L. Callegaro, Patent W096/29998,(l 996).
20. N. Elvassore, A. Bertucco, P. Calceti. Production of Protein-Loaded Polymeric Microcapsules by Compressed C02 in a Mixed Solvent, Ind. Eng.
Chem. Res., 40, (2001), 795-800.
21. T. J. Young, K. P. Johnston, K. Mishima, H. Tanaka, Encapsulation of Lysozyme in a Biodegradable Polymer by Precipitation with a Vapor-Over-Liquid Antisolvent, Journal of Pharmaceutical Sciences, 88, (1999), 640-650.
22. Y.H.Chou, D.L. Tomasko,GAS Crystallisation of Polymer-pharmaceutical Composite Particles, The 4th International Symposium on supercritical
Fluids, 11-14 May, Sendai, Japan, (1997), 55-57.
23. R. Falk, T.W. Randolph, J D. Meyer, R. M. Kelly, M. C. Manning, Controlled release of ionic compounds from poly(L-lactide) microspheres produced by precipitation with a compressed antisolvent, Journal of Controlled Release, 44, (1997), 77-85.
24. T. M. Martin, N. Bandi, R. Shulz, C. B. Roberts, U. B. Kompella, Preparation of Budesonide and Budesonide-PLA Microparticles Using Supercritical Fluid Precipitation Technology, AAPS Pharm. Sci. Tech., 3, (2002),articlel8.
25. F. E. Wubbolts, O.S.L. Bruinsma, G.M. van Rosmalen, Measurement and modelling of the solubility of solids in mixtures of common solvents and compressed gases, Journal of Supercritical Fluids, 32, (2004), 79-87.
Production of Controlled Drug Delivery Systems using
the PCA process: State of the art & Challenges
Micron-sized and microencapsidated drugs are desirable in the pharmaceutical industry for drug targeting and controlled release systems. During the last decades, researchers have been trying to develop new micronisation processes based on the use of supercritical fluids to overcome the shortcomings of conventional techniques.
A review of the state of the art in the formation of microparticles using supercritical fluids as antisolvent is presented. It focuses on identifying the gaps ofknowledge and the challenges that lay ahead. The effect of hydrodynamics, mass transfer, thermodynamics and precipitation kinetics on the partiele formation process has been analysed separately, However, to properly understand the mechanism of partiele formation and to analyse experimental results all these processes have to be taken jointly into account.
The main scientific contributions of this thesis to this field of the supercritical technology are also identified in this chapter.
2.1. Introduction
Microparticles for controlled drug delivery are often difficult to produce by currently available micronisation techniques. These techniques, such as spray drying, operate at temperatures that can thermally denature heat sensitive compounds, such as proteins. Milling produces powders with broad size distributions and solvent/emulsion evaporation techniques often require further processing stages to remove solvents. Furthermore, product drying can take several days until acceptable levels of residual solvent in the product can be achieved. During the last decades, research aimed at developing new micronisation processes based on the use of supercritical fluids to overcome the shortcomings of conventional techniques.
In 1984, Krukonis [1] proposed the RESS process (Rapid Expansion of Supercritical Fluids) as a promising process to tailor size and partiele size distribution of difficult to comminute solids. The solute to micronise is first dissolved in the supercritical fluid; then, this fluid is depressurised through a nozzle and the dissolved solid rapidly precipitates as very small particles. The main advantages of the RESS process are its simplicity and the production of solvent free products. However, its applications to the production of controlled drug delivery systems are limited by the reduced solubility of most polymers, proteins and polar pharmaceutical compounds in supercritical C02. Pre-expansion
temperatures associated with this process can also be too high to enable the safe processing of thermally labile drugs and polymers with low glass transition temperatures. Nevertheless, some work has been successfully carried out on this subject. The feasibility of RESS to coprecipitate poly(DL-lactic acid) and lovastatin was shown by Tom et al. [2]. Poly(L-lactic acid) microspheres (10-90 um) loaded with naproxen were obtained by Kim et al. [3] using a pre-expansion temperature of 391K and extraction pressures up to 190 bar. Mishima et al. [4] recently developed a new method called RESS-N (Rapid Expansion of Supercritical Solutions with a Non-solvent) to produce polymeric microcapsules of flavonoids without agglomeration. The particles do not adhere to each
other because the cosolvent (ethanol) was volatile and at the same time acts as a non-solvent for the polymer (aminoalkyl methacrylic copolymer). They also produce poly(ethylene glycol), poly (methylmethacrylate) and poly(propylene glycol) microparticles containing proteins such as lysozyme and lipase.
Another supercritical-fluid technique for partiele micronisation is the PGSS process (Particles from Gas Saturated Solutions) [5]. It is based on dissolving the supercritieal fluid in a molten solution, leading to a so-called gas-saturated solution that is further expanded through a nozzle with formation of solid particles or droplets. This process lowers the C02 consumption in comparison with the RESS process, can be also used to
produce solvent free particles and can be applied to process a great variety of substances that melt in supercritieal carbon dioxide. The high temperatures required to melt some substances like polymers are the main limitation to apply the PGSS process to encapsulate thermolabile compounds. Another drawback of this process is the difficulty to produce submicronic particles and to control partiele size distributions. Co-precipitation of nifedipine, felodipine and fenofibrate with PEG 4000 has been described by Kerc et al. [6].
Since the late 80's, supercritieal processes that use C02 as an antisolvent have been
extensively studied. A wide range of polymers and drugs can be micronised using these antisolvent processes and, in addition, the processes are performed at mild temperatures. These two features, together with the ability to produce solvent-free small-size partiele products possessing a narrow size distribution facilitated antisolvent processes as the most promising supercritieal processes for the production of controlled drug delivery systems (despite some disadvantages as the high C02 consumption and the use of organic
solvents).
This chapter reviews the work done so far on the production of microparticles for the controlled delivery of drugs using the precipitation with an antisolvent process. While earlier reviews [7] give an excellent extensive overview of the systems investigated, this
chapter is more focused on pointing out the state of the art in the PCA (Precipitation with a Compressed Antisolvent) process, the actual gaps of knowledge and the challenges that lay ahead.
2.2. Supercritical Processes Using CO2 as Antisolvent
In antisolvent processes, solutes of interest are dissolved or suspended in a suitable organic phase. This organic phase is then contacted with an antisolvent with a low affinity for the solutes and appreciable mutual solubility with the organic phase. Due to the fast transport properties of supercritical fluids, there is a fast contact between the antisolvent and the solute. High supersaturations are achieved and very small particles with a narrow size distribution can be obtained.
2.2.1 GAS
In the GAS (Gas Anti-Solvent) process, the precipitator is first partially filled with the solution of active substance (see Figure 2.1). Then, an antisolvent, e.g. C02, is introduced
in the precipitator, preferably from the bottom to achieve a better mixing of the solvent and anti-solvent, and pressure is increased to the desired value. The content of the reactor is stirred and, after a holding time, the expanded solution is drained under isobaric conditions. In order to obtain solvent- free product, the particles are rinsed with pure C02
prior to depressurisation.
Morphology can be controlled by manipulating process variables, such as temperature, flow rate, agitation rate, pressure or pressure build-up time. Crystal of small size and of narrow size distribution are usually obtained when solute are consumed mainly by nucleation, thus for concentrated solutions, short pressure build up time or large diffusion rates. On the other hand, crystals of larger average size and sizes distribution will be formed in conditions where only few nuclei are formed and grown.
Precipitator
Figure 2.1. Schematic drawing of GAS equipment 2.2.2 PCA, SEDS, ASES or SAS
In all these semi-continuous processes the solution is injected into the flowing dense gas using a nozzle (See Figure 2.2). The solution is continually dispersed in droplets. As the concentration of gas dissolved in the droplet is enough to induce the supersaturation of the solute, it precipitates. Precipitated particles are collected on the filter at the bottom of the precipitator. In order to obtain solvent- free product, the particles are rinsed with pure C02 prior to depressurisation. This operation is usually called PCA ( Precipitation with a
Compressed Antisolvent) and sometimes in the literature it appears as ASES (Aerosol Solvent Extraction System) or SAS (Supercritical Anti-Solvent). In SEDS (Solution Enhanced Dispersion by Supercritical Fluids) process the difference is that the solution is dispersed using a co-axial nozzle, by a large excess of the compressed gas instead of a hydraulic nozzle.
In this mode of operation the parameters that can influence the properties of the fmal particles are mainly flow rates and their ratios, nozzle design, solvent density, solution concentration, pressure and temperature.
Nozzle No
ra
co,
Filter \\i
0 0 0 0 0 0 PrecipitatorÖ
-Solution-<p)
| - & - ► PCAr*
CO, Filter \V . 1
1
0 0 0 0 0 0 Precipitator54-Solution
■f>
H & - *
SEDSFigure 2.2. Schematic drawing of PCA equipment (left) and SEDS equipment (right).
2.3. Factors influencing the mechanism of partiele formation during
the PCA Process
Although some trends in partiele size and morphology as a function of process conditions have been reported [8-14], conflicting observations abound in the literature. For example, considering the precipitation of poly(L-lactic acid) precipitated from dichloromethane using carbon dioxide as antisolvent : an increase of the antisolvent density (at constant temperature) has been reported to increase partiele size [8,16] , not to effect partiele size in the density range 0.22-0.79 g/ml [17], and to decrease partiele size if density was kept below 0.56 g/ml [19]. One of the reasons for this difficulty in the interpretation of the experimental results is the complexity of the partiele formation mechanism, in which phase behaviour, mass transfer, precipitation kinetics and hydrodynamics are strongly interlinked to each other.
A detailed analysis of the hydrodynamics of jet breakup, the mass transfer between droplets and their environment, the phase behaviour of the solvent-solute-antisolvent system, and the kinetics of the precipitation are needed to improve the understanding of the PCA process.
2.3.1 Hydrodynamic considerations: Droplet formation vs mixing
The earliest attempts for explaining the morphology of the particles precipitated by the PCA process used the hydrodynamic theory for conventional spray processes. These first studies [15] considered that solution droplets are formed as result of jet breakup and tried to relate the droplet size to the size of the final partiele by means of the Weber (NWe),
Reynolds (NRe) and Ohnesorge (N0h) numbers.
The Weber number is the ratio of the deforming external shear forces and the stabilizing surface tension forces experienced by a liquid droplet encountering flowing air. It is numerically defined by the following equation:
= pAu\d
a (2-1)
where pA is the antisolvent density, uR is the relative velocity droplet -surroundings, d
denotes the initial droplet diameter and G is the interfacial tension droplet-surroundings. When external shear forces are large compared to the surface tension forces (large NWe
values) the droplets break up into smaller droplets. In the PCA process, the Weber number is large due to low surface tension and the high density of the high surroundings. The Reynolds number is defined as:
N =
£S± (2-2)
Re M
The Ohnesorge number relates viscosity to surface tension forces and is expressed as:
Nok=-^=, jV/ 2 (2-3)
According to the conventional theories for spraying and atomisation [18], the larger the values of the Ohnesorge, the Weber and the Reynolds numbers the solution breaks into smaller droplets.
Spraying a polystyrene-toluene solution into C02, Dixon et al. [15] observed that the size
of the precipitated polystyrene particles decreased as the density of the C02 was raised.
They presumed that the rapid atomisation was controlled by inertial to interfacial forces. As the pressure increases, the compressed gas density increases and the interfacial tension droplet-surroundings decreases (due to the higher miscibility toluene-C02). So the
decrease in partiele size when the pressure (density) increases is explained by an increase in the Weber number. The same effect was observed by Thies and Muller [19] when spraying a solution of poly(L-lactic acid) in dichloromethane into C02.
However, other authors reported that for some experimental conditions, large changes in the Weber number (produced by changes in spraying velocity or nozzle geometry) did not affect the polymer partiele size [20]. Even more, different injection methods [8] (capillary, ultrasonic nozzles or hollow cone nozzles) all give similar results regarding partiele size in spite of differences in primary droplet size distributions.
To explain these conflicting results, different authors started to analyse the kinetic of the droplet breakup process at condition where the solution and the supercritical antisolvent were completely miscible.
Sarkari et al. [21] examined the kinetics of the spray process taking into account that the interfacial tension is a function of the varying concentration within the droplet. For completely miscible systems, in absence of solute, the interfacial density will vanish at equilibrium. When the Weber number is higher than the critical NWe (reported to range
between 6-50), the spray process is dominated by turbulent mixing and not by droplet formation. So, for miscible systems the Weber number is not an effective parameter for predicting partiele morphology.
Shekunov et al. [22] also defended the hypothesis that turbulent mixing and not droplet formation is the important mechanism for completely miscible systems. Nucleation and growth of the particles takes place in a single phase formed by the two miscible fluids.
They showed that Kolmogorov length scales for turbulent mixing, rather than droplet sizes, are relevant quantities in determining the length scales for partiele formation.
Lengsfeld et al. [23] developed a method for predicting the dynamic surface tension and combined this method with linear jet breakup equations to accurately predict the jet breakup lengths in immiscible and highly miscible systems. For miscible systems they found that if the distance in which the droplet surface disappears is shorter than the characteristic breakup length, distinct droplets never form. According to them, microparticle formation in the dilute polymer regime results from gas phase nucleation and growth within the expanding plume, rather than nucleation within discrete liquid droplets.
Despite these studies showing that droplet formation due to atomisation is not possible at conditions above the mixture critical pressure, some authors continue to interpret their results as an atomisation mechanism taking place in the miscibility region [24]. If the viewpoint of Lengsfeld et al. is adopted, it is not surprising that Rantakyla et al. [16] found no correlation between the partiele size predicted by their model and the experimental partiele size.
In 2000, Kerst et al. [25] reported an interesting work comparing the flow regimes of free jets at high pressures with the conventional correlations available for ambient conditions. Ohnesorge correlations were found not to be suitable to describe the jet flow regime transitions at high pressures. At high pressures, the properties of the gas phase as well as the high solubility of the gas into the droplets cannot be neglected. Kerst et al. proposed new correlations considering the reduced pressure and the Reynolds number to describe the flow transitions at high pressures.
Considerable work needs still to be done to develop correlations to predict the effect of the different operating conditions on droplet size.
2.3.2 Mass Transfer
In order to explain the experimental results that did not follow the trends expected for droplet formation by hydrodynamic break up (larger Reynolds or Weber numbers produce larger particles), some authors [8,10] proposed that mass transfer rates, rather than initial droplet size, were the dominant factors on determining partiele size. Factors favouring mass transfer induce a higher supersaturation and smaller particles would thus be obtained.
Randolph et al. [8] observed different trends in the variation of poly(L-lactic acid) partiele size with density depending if the experiments were carried out at subcritical or above critical conditions. The conclusion of those results was that there are two competing effects influencing partiele size: atomisation and diffusion. At subcritical temperatures the size of the particles is governed by atomisation. The atomisation becomes more intense as density increases and the partiele size decreases. At supercritical temperatures, mass transfer is the dominant effect. The diffusion coëfficiënt decreases when pressure increases. Smaller particles are thus produced at the lower pressures because higher mass transfer rates (higher diffusion coefficients) lead to quicker build-up of the supersaturation and exponentially increased nucleation rates.
In 1996, Mawson et al. [10] tried to achieve a better control of the precipitation process by influencing the mass transfer in the jet. They developed a coaxial nozzle where the polymer solution was sprayed through the core of the nozzle and the C02 through the
annulus. A high C02 velocity in the annulus lowered the relative velocity between the two
streams and atomisation was reduced. Due to the formation of larger droplets, the mass transfer rate was reduced and the precipitation was delayed. However, because of the much higher Reynolds number for the high velocity C02, recirculation and mixing of the
suspended droplets throughout the precipitator was enhanced. Particles produced using this coaxial nozzle were 3-8 times larger than when using a Standard nozzle and were less agglomerated.
Chattopadhyay and Gupta [26] proposed the use of ultrasonic nozzles to atomise the solution. The ultrasonic field generated enhances mass transfer and allows the formation of smaller particles. They obtained lysozyme particles up to ten fold smaller than those obtained by the conventional PCA process.
Besides the experimental work, some studies have also been done to simulate the influence of the different operating variables on the mass transfer between a solution droplet and the antisolvent surrounding.
Werling and Debenedetti [27] developed a mathematical model for the isothermal mass transfer between an isolated stagnant solvent droplet and carbon dioxide at conditions below the critical pressure of the mixture. They describe the mass transfer using time-dependent conservation equations and the modified Fick's law. Their simulations, focussed on the toluene-C02 system, show that the initial interfacial flux is always into
the droplet due to the high solubility of the carbon dioxide in the organic solvent; that low pressures and larger initial droplet diameters result in longer droplet lifetimes and that droplet lifetime increases sharply close to the mixture critical conditions. In a later article, they extended their work for conditions at which the solvent and the antisolvent are completely miscible [28]. Due to the inexistence of interface between the solvent and the environment they need to define the droplet as the region where the density is different from the environment. Their simulations indicate that droplets at conditions near the critical point experience greater swelling and longer lifetimes and are more sensible to the operating conditions than systems at temperatures and pressures far from criticality. At conditions beyond the mixture critical point droplet lifetime is shorter than in the subcritical region.
Lora et al. [29] developed a mass transfer model based on Fick's law that accounted also for the effect of the droplets velocity on the mass transfer rate. Their results were
evaluated in terms of variation of the liquid composition or the liquid flow rate as a function of the distance from the injector and time and not focussing on the analysis of what happens with individual droplets. Their model for the system toluene-C02 also
predicts that absorption of carbon dioxide is much faster than evaporation and that the solution initially swells. They extended their simulations for the quaternary system toluene-C02-phenanthrene-naphthalene to study the possibility of selective precipitation
by controlling the supersaturation values attained in the solution.
Recently, Mukhopadhyay and Dalvi [30] reported a simultaneous mass and heat transfer model based on the use of Fick's approach. Their simulations indicate that higher temperatures, lower pressures, higher flow rates of C02 and solution and, a lower ratio
C02-to-solution flow rate are desirable to perform the PCA process. According to their
results lower carbon dioxide density and a higher C02 flow rate facilitate the formation of
a smaller droplets and faster evaporation.
Within this frame of work, chapter 4 presents a model developed to estimate the effect of different operating variables on the droplet size and the drying time of solvent droplets produced by spraying dichloromethane into carbon dioxide at the typical operating conditions of the PCA process. In contrast to previous investigations, the mass transfer rates were modelled using a finite-difference approximation of the Maxwell-Stefan equations. Simulations show that in the studied pressure and temperature range (4-7.5 MPa and 308-328K) absorption of C02 into the liquid phase is always faster than solvent
evaporation. Droplets produced as result of atomisation in the nozzle will swell as soon as they get into contact with C02. Droplet lifetime can be reduced by an increase of the C02
to solvent molar ratio and by a decrease of the initial droplet diameter. The different factors enhancing/slowing down mass transfer have a complex dependence on pressure and temperature. At high pressures and low temperatures droplet lifetime is long due to larger diameter attained by the droplets as result of the high solubility of carbon dioxide
in the liquid phase; at very low pressures and high temperatures droplet lifetime increases due to the lower density of the vapour phase.
Elvassore et al. [31] have worked on modelling the mass transfer between a polymer solution droplet and a carbon dioxide supercritical environment at conditions of complete miscibility solvent-C02. They use the generalised Maxwell-Stefan diffusion equation to
describe the mass transfer and the perturbed-hard-sphere-chain-theory equation of state to estimate the equilibrium and volumetric properties of the system. Their simulations predict the formation of hollow microparticles if the solute has a low solubility in the solvent-antisolvent mixture whereas for solutes with a higher solubility in the solvent, dense particles are obtained.
So far, simulations relating to ternary systems neglect the effect of the solute on the diffusivities of carbon dioxide and solvent in the liquid. As a result of the solute precipitation in the outside layer of the droplet, a solid shell surrounding the droplet can be formed, thereby tremendously lowering the mass transfer rate. In the case of polymer solutions, the situation can become even more complex because the presence of carbon dioxide induces a liquid-liquid phase split. All these effects should be taken into account when simulating mass transfer in ternary systems. In the case of modelling mass transfer at conditions of complete solvent-antisolvent miscibility, formation of droplets is still considered as an assumption of the simulation. Future work should be oriented on describing mass transfer on basis of mixing scales and mixing times, a more appropriate approach to characterise these miscible systems. In addition, there are also scarce data of diffusivity values at high pressures, especially for binary or ternary systems. More work has to be performed on this subject.
2.3.3 Thermodynamic behaviour
For optimisation and understanding of the precipitation process it is essential to have information about the solubility of the solute of interest in the mixture solvent-antisolvent,
about how the solvent- antisolvent vapour-liquid equilibrium curve is affected by changes in composition, pressure and temperature and, about the number of phases present at the operating conditions.
Some useful reviews of the high pressure equilibrium data published in the last century are: Knapp et al. [32], covering the period 1900-1980; Fornari et al. [33], covering 1978-1987; Brunner and Dohrn [34], covering 1988-1993; and Christov and Dohrn [35], covering 1994-1999.
As shown by these reviews, phase behaviour data of binary mixtures of carbon dioxide and many common organic solvents are available. If data of the binary system of interest are not available in the desired pressure or temperature range, a group contribution equation of state, e.g. the Predictive-Soave-Redlich-Kwong model [36], can be used to predict binary vapour-liquid data at high pressures. The typical phase behaviour of the binary system carbon dioxide-organic solvent at constant temperature is represented in Figure 2.3. At low pressures, the vapour phase is formed for the most part by C02 and the
organic solvent is the predominant component of the liquid phase. As pressure increases, the solubility of carbon dioxide in the liquid phase increases while the solubility of the organic solvent in the vapour phase remains still very low. At a certain pressure, the mixture critical pressure (Pc,mixX the composition and properties of the vapour and the
liquid phase become identical and the two phases merge. Above this pressure, carbon dioxide and the organic solvent are miscible in all the composition range.
0.2 0.4 0.6 0.8 C02 molar fraction
Vapour
Figure 2.3. P,x phase diagram for the system dichloromethane-COi at 308 K modelled using the Peng-Robinson Equation of State.
In the case of the ternary system solute-solvent-antisolvent, phase behaviour experimental data are still scarce. Some thermodynamic models to calculate solid-liquid- vapour equilibria of solute-solvent-antisolvent systems at high pressures have been developed by Chang and Randolph [38], Dixon and Johnston [39] and Kikic et al. [40]. An example of this kind of diagrams at constant pressure and temperature is shown in Figure 2.4. Figure 2.4.a shows the ternary diagram at conditions above the mixture critical pressure of the system solvent antisolvent (when no solute is present, solvent and antisolvent are miscible in all the composition range). Point A represents the solubility of the solute in pure solvent. As the concentration of carbon dioxide in the system increases, the solubility of the solute in the liquid (supercritical) phase decreases. Discontinuous dotted lines represent the tie lines. Figure 2.4.b illustrates the ternary diagram at conditions below the mixture critical pressure of the system solvent-antisolvent. Depending on the composition, the number of phases in equilibrium will be one (vapour, liquid or solid), two (liquid-solid (L-S), liquid-vapour (L-V) or solid-vapour (S-V)) or three (solid-liquid-vapour (S-L-V)).
Figure2.4. Ternary phase diagram for the system solute-solvent-antisolvent measured at constant temperature and two different pressures.
a. above the critical pressure of the mixture solvent-antisolvent b. below critical pressure of the mixture solvent- antisolvent
If the solute of interest is a polymer, cubic equations of state, like the Peng-Robinson model, are not effective to describe the system, because the large difference in molecular size between polymer and solvent or antisolvent molecules is not properly accounted for. More recently developed equations of state are derived from statistical mechanical fluids theories and assume polymers to be chains of (mostly) spherical segments. While early models, like the Perturbed Hard Chain Theory (PHCT) [41], and the Sanchez-Lacombe theory [43] proved the potential of this approach a successful and more recent model is the Statistical Associating Fluid Theory (SAFT) [42] and its derivatives. Despite this progress however, the pure component parameters of polymers are non-trivial to obtain. Hence, experimental work is needed to gain information on the behaviour of these systems. That is the reason why the phase behaviour of the system dextran/dimethylsulphoxide/C02, which was used as a model system to study the micronisation of biopolymers using the PCA, needed to be measured as described in
Binodal Gelation line Tie line Liquid / / \ NV / t,iquid-Liquid Solvent Antisolvent
Figure 2.5. Temary phase diagram for the system polymer-solvent-antisolvent
measured at constant temperature andpressure.
The typical ternary phase diagram for the polymer-solvent-antisolvent system at conditions above the mixture critical pressure of the solvent-antisolvent system is represented in Figure 2.5 [10,15]. In this example, solvent and polymer are miscible in the complete range of composition. The addition of carbon dioxide to a polymer-solvent mixture induces a liquid-liquid phase split into a polymer-rich phase and a polymer-lean fluid (here termed liquid) phase. At high concentrations of polymer the liquid-liquid region is superposed with the gelation region, characterised by its high viscosity that hinders the movement of the polymer chains.
Ternary diagrams at constant pressure and temperature are very useful to understand the mechanism of partiele formation during the PCA process because they allow visualise the composition pathway foliowed by the mixture and the number of phases present. An important development to measure this kind of diagrams has been proposed by Wubbolts et al. [44], who developed a solubility cell to easily measure the phase behaviour of organic compounds in mixtures of a solvent and C02 at constant pressure and
temperature. Chapter 3 shows that this variable composition view cell method is a useful, easy and reliable method that can be applied to study also polymer systems.
