0263–8762/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part A, November 2004 Chemical Engineering Research and Design, 82(A11): 1505–1510
CHEMICAL PRODUCT DESIGN A New Challenge of Applied Thermodynamics J. ABILDSKOV and G. M. KONTOGEORGIS CAPEC and IVC-SEP Engineering Research Center, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark
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hemical products involving specialty chemicals and microstructured materials are often multicomponent systems. A number of five to 20 molecules is not unusual, comprising a range of different chemical compounds e.g. polymers, surfactants, solid particles and water. Milk is an example of such a product involving both solid – liquid phases and (non-equilibrium) metastable states. Thus, many of these products are colloidal systems of different types, e.g. liquid – liquid emulsions, suspensions, powders, solid and liquid dispersions, aerosols and sprays. The physical chemistry (thermodynamics, stability) of such products is often as important as their manufacture, while a number of non-traditional manufacturing/separation processes are of relevance, e.g. emulsification, foaming, gelation, granulation and crystallization. Today, serious gaps exist in our thermodynamic modelling abilities when we try to describe and understand chemical products with traditional thermodynamic models, typically applicable to problems of petrochemical industries. The purpose of this article is two-fold: first to present some current and future challenges in thermodynamic modelling towards chemical product design, and then to outline some specific examples from our research activities in the area of thermodynamics for chemical products. The examples cover rather diverse areas such as interrelation between thermodynamic and engineering properties in detergents (surfactants), paint thermodynamics and the development of models for gas solubility in elastomeric polymers. Keywords: product design; applied thermodynamics; detergents; paints; gas solubilities; polymers.
FUTURE NEEDS IN THERMODYNAMICS FOR CHEMICAL PRODUCT DESIGN It is often stated that chemical engineering is changing in many directions, in the context of post-modernism (Prausnitz, 2001), biotechnology (life sciences; Prausnitz, 1989, 1995), knowledge-based materials and chemical product engineering (Prausnitz, 1999; Rainwater et al., 2001). Future developments in thermodynamics should be expected to follow these trends and thus not solely address problems towards the design of traditional petrochemical systems and conventional separations (distillation, absorption, extraction). Even in these ‘traditional’ areas there are still ‘thermodynamic’ challenges (Zeck, 1991), especially in relation to condensed phases (multiple liquids and solids), but the directions of biotechnology, materials and chemical products are expected to exert even more influence on applied thermodynamics. These ‘newer’ directions often require highly multidisciplinary thinking and developments, where thermodynamics is coupled with
Correspondence to: Dr J. Abildskov, CAPEC, Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark. E-mail:
[email protected]
other fields, especially transport phenomena, mathematical modelling and kinetics, e.g. in the design of biodegradable polymers such as hydrogels for use in controlled drug release devices (Prausnitz, 1999). Complex chemical products are multicomponent materials, many of which are classified as ‘soft materials’ (colloidal dispersions); solid particles, surfactants, water (or other solvents) and polymers are often present. Such materials range from pharmaceuticals and paints to food, detergents and chemical devices and thus the needs in thermodynamic modelling are also very diverse. Despite their multicomponent character, the presence of (often multiple) liquid and solid phases as well as the fact that they often exist in metastable states (emulsions, gels) complicates the picture significantly (Favre and Kind, 1999). It is clear that development of thermodynamic tools to quantitatively describe such materials may require much more advanced techniques than those used for calculation of vapour –liquid and liquid – liquid equilibria for ‘ordinary’ (low molecular weight) mixtures, of interest to petrochemical industries (Villadsen, 1997). We outline below some areas where serious gaps in existing thermodynamic knowledge exist and where, hopefully, much promise exists for future developments, based on current research activities.
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Structured Polymers and Complex Interactions An area where much research is being done involves new polymeric structures such as dendrimers, hyperbranched polymers, inorganic polymers used in antifouling paints and new complex blends and co-polymers. Thermodynamic and transport models should include the effect of structure in the properties, especially swelling and permeability of gases. Moreover, in most cases, models have been independently developed for polymer –solvent systems and electrolytes. Such division is done for simplicity, but should be abandoned (Chen and Mathias, 2002), and new tools should focus on materials where both charged molecules (interactions) and polymeric structures are present, such as polyelectrolyte hydrogels (Prausnitz, 1989). Surface Phenomena Surface phenomena are very important in numerous materials and processes (surfactants, emulsions, adhesion) and joint developments of advanced models, e.g. SAFT (statistical association fluid theory) with frameworks such as the DFT (density functional theory) in order to predict surface and interfacial tensions are highly desirable. Such work is in progress, but it is only just beginning (Gloor et al., 2002). Multicomponent, Multiphase Mixtures and Complex Phase Diagrams This is a general need that spans the description of water –oil –surfactant – co-surfactant mixtures of importance to the design of microemulsions up to the complex paint formulations containing mixed solvents. Existing models are typically not adequate and advanced theories need to be developed that can, at least qualitatively, predict the multiple liquid or solid phases present. These models should be formulated in such a way that they can be easily extended to multicomponent, multiphase systems, and much focus should be given to distribution coefficients (rather than absolute values). Models expected to treat both polymers and electrolytes should be extended to cover surfactants as well. Polar interactions, hydrogen bonding and association in all forms (including intramolecular association) are all expected to play an important role in these materials. When even ternary liquid –liquid equilibria (LLE) for a system as simple as water – methanol –benzene is a challenge today for advanced thermodynamic models, we understand that even the ‘simple microemulsion’ ternary system water – alkane –nonionic surfactant might be a serious challenge to be met in future developments, but could at least serve as the first milestone. Pharmaceuticals The presence of solvents is essential in all steps of pharmaceutical processes (reaction, separation and formulation). For toxicological reasons, drug manufacturers are increasingly required to minimize the amounts and number of solvents employed in processing. The selection of solvents is an important area of applied thermodynamics in industrial companies. It has been estimated (Kolar et al., 2002) that 30% of the work of an industrial thermodynamic group can
be related to various aspects of solvent selection. The molecules dealt with in the pharmaceutical area are substantially different from common petrochemicals. The high polarizability, the presence of heteroatoms and the existence of multiple functional groups in pharmaceuticals make the molecules liable to a wide variety of specific interactions with polar solvents. For example, protonation, hydrogen bonding, specific solvation, etc. Systematic approaches to solvent selection in the area of synthetic pharmaceuticals or at least prototypes of moderately flexible, highly functionalized molecules could serve as a first milestone. Polymorphism The ability to predict what polymorph precipitates under a given set of conditions is highly relevant in the food and pharmaceutical industries (Karaborni, 2003; Karamertzanis and Pantelides, 2004). Crystal morphology is an important factor that determines a product’s quality (e.g. dissolution rate), processing characteristics in downstream units (filtration, washing, drying) as well as formulation possibilities. For example, monoglycerides are often crystallized in an a-form and upon storage a recrystallization is initiated, resulting in a change in crystal form (a ! b). If the energy release connected with this transition happens in the final packaging, the result is the formation of one big lump of material rather than a free-flowing powder, as there is no way of escaping the heat release. There is currently no way to predict this behaviour, which is highly dependent upon the solvent medium, the influence of which does not reduce to a simple set of rules. Most synthetic pharmaceuticals are medium-sized molecules containing 10 – 50 non-hydrogen atoms. The molecules are typically composed of several interlinked aromatic cores and multiple substituents containing heteroatoms N, P, O, S and X ¼ F, Cl, etc. Owing to the presence of the aromatic delocalized p-electrons and the electronegative heteroatoms, the molecules are highly polarizable so there is a strong interplay between the solute and solvent properties. Furthermore, in most cases, the solutes are conformationally flexible. By the possibility of exposing certain parts of the molecules, the conformational flexibility may affect both the reactivity and solvation behaviour of the molecules, and also the formation of polymorphs. The identification of conformers has been intensively studied using molecular simulation and new methods for conformer searches in flexible molecules are being developed. The above represent only a small portion of phenomena related to chemical product design where thermodynamic developments are both desired and hopefully also feasible. Many more areas within materials/chemical products require thermodynamic knowledge, which is either more or less mature (metal alloys) or extremely complex, such as food (Moorwood, 2001). However, even in the area of food products some first steps have been reported (Bruin, 1999), including success stories, e.g. the phase behaviour of margarine and solidification of chocolate (Moorwood, 2001). THERMODYNAMICS OF DETERGENTS Commercial detergents are mixtures of surfactants, builders and other materials. The Danish Environmental
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Figure 3. Relationship between CMC and Krafft points for sodium alkyl sulfonates (open squares) and sodium alkyl sulfates (solid squares).
Figure 1. Key thermodynamic and engineering properties in surfactant science (Cheng, 2003).
Research Programme (www.mst.dk) has defined as priority surfactants those belonging to the families of anionic sodium alkyl sulphates, alkyl ether sulphates and the nonionic alcohol ethoxylates (Madsen et al., 2001). In a recently completed PhD project (Cheng, 2003), we have looked that the relationships between the thermodynamic and engineering properties of surfactants. The thermodynamic properties such as the octanol – water partition coefficient (Kow) and the critical micelle concentration (CMC) can be, in principle, predicted from a thermodynamic model, whereas relationships between the ‘practical’ engineering properties and the thermodynamic ones could be established (Figure 1). We have worked in these directions. Thermodynamic modelling was performed with suitably modified group contribution models (UNIFAC, Cheng et al., 2002) and with available engineering software. Examples of the clear correlations that exist between thermodynamic and engineering properties can be seen in Figures 2 and 3 for three important families of anionic surfactants (Cheng, 2003). Similar correlations have been developed for other families of surfactants as well as for other properties
Figure 2. CMC of polyoxyethylene sodium alkyl sulphates as a function of HLB.
(aggregation number, etc.) as well as between Kow and HLB, the hydrophilic –lipophilic balance of a surfactant (Cheng, 2003). Somewhat less clear (but still useful) correlations have been established (Cheng, 2003) between thermodynamic properties (Kow) and properties used in the environmental assessment of surfactants such as the bio-concentration factor (BCF) and their toxicity to rat, as shown in Figures 4 and 5 for alcohol ethoxylates (experimental BCF and toxicity data are from Madsen et al., 2001).
Figure 4. Correlation between BCF of alcohol ethoxylates in fish and Kow.
Figure 5. Correlation between Kow and LD50 of alcohol ethoxylates in rat.
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Figure 6. Experimental and predicted Kow values for the non-ionic alcohol ethoxylate surfactants C6En (Cheng, 2003).
However, care should be exercised when using octanol – water partition coefficients for surfactants. As these are surface-active compounds, they tend to partition on the octanol – water interface, which complicates the measurements; thus, experimental data are scarce and often doubtful. This is also verified by the results shown in Figure 6 for alcohol ethoxylates having six carbon atoms and varying number of ethoxylates. VLE1 is the UNIFAC method, while the ACD, ClogP and KowWin are all different commercial software packages for Kow-calculations. These have been developed for non-surfactant molecules and provide qualitatively different results with each other; actually only KowWin seems to follow the trend of the experimental data (UNIFAC does also). More information on these commercial packages, including ordering information, is available from Sangster (1997). THERMODYNAMICS OF PAINT-RELATED SYSTEMS Polymer thermodynamics (phase equilibria) often plays an important role in many paint-related applications, e.g. formulation (selection of suitable solvents), control/design of drying/curing and estimation of solvent emissions, protection against external factors, e.g. swelling, understanding of adhesion phenomena and development of novel antifouling paints. The systems of interest are complex
Figure 8. Ternary LLE with PC-SAFT for PS(300,000)–acetone– methylcyclohexane. Calculation is based on parameters obtained from the binary data.
polymers and all types of solvents. Specific interactions such as hydrogen bonding are often present. Mixed solvents and multicomponent systems are the typical case. Typically in the literature few data and theoretical investigations have been reported for multicomponent polymer systems. Even fewer models have predictive capabilities. We have developed a theoretically based thermodynamic model (simplified PC-SAFT), which is an equation of state that accounts explicitly for the chain and hydrogen bonding effects in polymer solutions (Von Solms et al., 2003). We have proposed simplifications which can ensure computational efficiency retaining the good results, and applied PC-SAFT to various types of complex phase equilibria (both vapour – liquid and liquid – liquid equilibria) for binary and ternary polymer–solvent systems, including complex hydrogen bonding solvents and systems of relevance to the manufacturing process of Nylon-6 (Kouskoumvekaki et al., 2004a, b; Lindvig et al., 2004). Sample results are shown in Figure 7 (a binary VLE system) and Figure 8 (a ternary LLE system). More results were presented in the publications mentioned previously. The results demonstrated that PC-SAFT can be potentially employed for the problems of interest to the paints and coatings industry. Future planned applications include polymer blends, aqueous polymer systems and gas solubilities in polymers. GAS SOLUBILITIES IN ELASTOMERIC POLYMERS
Figure 7. VLE prediction (kij ¼ 0) with PC-SAFT for PVAC(170,000) – acetone at three temperatures.
Thermopane windows provide thermal insulation and at the same time allow entrance of light. For insulation, most windows consist of two or more panes of glass glued with one or more sealants, and separated by a gasfilled gap, as shown in Figure 9. An ideal filling gas has low thermal conductivity for effective insulation; thus freons were widely used in the past. After environmental constraints have phased out freons, argon has been used as an alternative, even though the thermal conductivity of argon is not much different from that of air. In addition to the choice of filling gas,
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where V2L (d1 d2 )2 F21 RT FFV FFV ¼ ln 2 þ 1 2 x2 x2
ln g2R ¼ ln gCþFV 2
Figure 9. Window construction.
a choice is to be made as to what sealant should be used to glue the glass panes together. Polymers such as silicone rubbers and polyisobutylene have been proposed as sealants. Sealant selection has several aspects: water vapour can diffuse through the sealant into the gas gap and via condensation it can fog and essentially destroy the window. In addition, the filling gas (argon or other) can leak out faster than air leaks in, causing the panes to bend together. Satisfactory sealants have therefore the main functions of sealing the panes together and keeping argon in and water out. Proper design should search for materials that satisfy both demands. The first is related to adhesion while the second is related to gas permeability (i.e. solubility diffusivity). Sealant design requires a predictive method for estimating permeability of various gases in various polymers. This requires both solubility and diffusivity. Whereas diffusivity is largely a function of geometric gas – polymer relationships (and therefore reasonably easy to scale for a given polymer or gas), solubility represents a more complicated phenomenon if we take a scientific point-of-view in trying to obtain a reliable correlation. For design purposes, it is important that reliable solubility predictions can be made with a minimum of experimental data. Several models have been proposed for predicting the solubility of gases in polymers. However, most of them are merely correlations of gas – polymer solubility (directly or via a Henry’s law constant) data as a function of some characteristics of the components. Others (Bithas et al., 1995) are thermodynamic models. These, however, require parameters fitted to the individual system and have thus little predictive value. Cubic equations of state using the advanced EoS/GE mixing rules have been successfully used for gas solubility in ordinary liquid solvents (Dahl and Michelsen, 1990), but have not been systematically extended to polymers (Zhong and Masuoka, 1998). That would probably involve addition of a free-volume activity coefficient term, requiring hypothetical ‘liquid’ volumes for gases. That would be a quite time-consuming effort. Most predictive attempts are based on Hildebrand solubility parameters, which are also hypothetical quantities for gases. We extended an activity coefficient model (Prausnitz et al., 1986) for the estimation of gas (filling gases and air gases) solubility in polymers ln gi ¼ ln giR þ ln gCþFV i
(1)
(2) (3)
where d1 is the solvent solubility parameter, d2 is the solubility parameter of the gaseous solute, x2 is the mole fraction of gas in the liquid/polymer and F2 is the ‘apparent’ volume fraction of solvent. The model requires hypothetical values for the gas ‘liquid’ volumes and solubility parameters. The model is based on a previously proposed free-volume equation suitable for polymer solutions (Elbro et al., 1990) and the Hildebrand solubility parameters. Despite the simplicity of the approach, good results have been found in many cases, provided that suitable input parameters (hypothetical ‘liquid’ volumes of the gases) are available. Two methods were developed, each with different merits, i.e. . the Hildebrand-entropic-FV-1 [equation (1)] using V L correlated by means of an equation of critical properties, V L ¼ 1.7553 Vc þ 85.093; and . the Hildebrand-entropic-FV-2 [equation (1)] using V L obtained from fitting to solubility data. Detailed results and discussion are provided by Thorlaksen et al. (2003). CONCLUSIONS Several future challenges and needs in thermodynamics in relation to the design of complex chemical products, as well as some examples from our research activities in this field, have been outlined. The economic impact of chemical product development and the challenges of establishing a model-based understanding of chemical products will continue to inspire thermodynamic research programs in years to come, and will point to gaps in current modelling capabilities. The cases vary in complexity, and in the degree of multidisciplinarity. The approaches—tools, which will be used and/or are emerging—vary, but there is some general consensus. Recent reviews from both industry (Gupta and Olson, 2004) and academia (Arlt et al., 2004; Sandler, 2003) reveal much hope of meeting ambitious challenges such as these discussed here. Solutions to these challenging problems may, in some cases, be obtained from extensions of group-contribution/solubility parameter techniques, but are most likely to come from the developments in statistical theories such as SAFT-type approaches, in molecular simulation and other computational tools based on quantum mechanics such as the COSMO method (Klamt and Eckert, 2000). Advanced equipment will assist in the assessment of the complex intermolecular forces, e.g. the use of light scattering in proteins and the use of AFM in the experimental determination of interparticle forces. REFERENCES Arlt, W., Spuhl, O. and Klamt, A., 2004, Challenges in thermodynamics, Chem Eng Process, 43(3): 221–238.
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Bithas, S., Kontogeorgis, G.M., Kalospiros, N., Fredenslund, Aa. and Tassios, D., 1995, Correlation and predicition of Henry constants for liquids and gases in five industrially important polymers using a CS-type correlation based on the van der Waals equation of state. Comparison with other predictive models, Fluid Phase Equilibria, 113: 79–102. Bruin, S., 1999. Phase equilibria for food product and process design. Fluid Phase Equilibria, 158 –160: 657 –671. Chen, C.C. and Mathias, P.M., 2002, Applied thermodynamics for process modeling, AIChE J, 48(2): 194–200. Cheng, H.Y., 2003, PhD Thesis. Institut for Kemiteknik, Technical University of Denmark. Cheng, H.Y., Kontogeorgis, G.M. and Stenby, E.H., 2002, Prediction of micelle formation for non-ionic surfactants through UNIFAC method, Ind Eng Chem Res, 41: 892. Dahl, S. and Michelsen, M.L., 1990, High-pressure vapor–liquid equilibrium with a UNIFAC-based equation of state, AIChE J, 36(12): 1829–1836. Elbro, H.S., Fredenslund, Aa. and Rasmussen, P., 1990, New simple equation for the prediction of solvent activities in polymer solutions, Macromolecules, 23(21): 4707– 4714. Favre, E. and Kind, M., 1999, Formulation engineering: towards a multidisciplinary and integrated approach of the training of chemical engineers. In 2nd European Congress of Chemical Engineering, Montpellier. Gloor, G.J., Blas, F.J., de Rio, E.M., de Migeul, E. and Jackon, G., 2002, A SAFT-DFT approach to the vapour– liquid interface of associating fluids, Fluid Phase Equilibria, 194 –197: 521 –530. Gupta, S. and Olson, J.D., 2004, Industrial needs in physical properties, Ind Eng Chem Res, 42(25): 6359– 6374. Karaborni, S., 2003, Computer simulations of nanostructures in the petrochemical and pharmaceutical industries, in FOMMS 2003, Plenary Lecture. Karamertzanis, P.G. and Pantelides, C.C., 2004, Optimal site charge models for molecular electrostatic potentials, Molecular Simul, 30(7): 413– 436. Klamt, A. and Eckert, F., 2000, COSMO-RS: a novel and efficient method for the a priori prediction of thermophysical data of liquids, Fluid Phase Equilibria, 172: 43–72. Kolar, P. Shen, J.-W., Tsuboi, A. and Ishikawa, T., 2002, Solvent selection for pharmaceuticals, Fluid Phase Equilibria, 194–197: 771– 782. Kouskoumvekaki, I.A., von Solms, N., Michelsen, M.L. and Kontogeorgis, G.M., 2004a, Application of the simplified Perturbed Chain SAFT equation of state to complex polymer solutions, Fluid Phase Equilibria, 215(1): 71– 78. Kouskoumvekaki, I.A., Krooshof, G., Michelsen, M.L. and Kontogeorgis, G.M., 2004b, Application of the simplified PC-SAFT equation of state to the vapor–liquid equilibria of binary and ternary mixtures of polyamide 6 with several solvents, Ind Eng Chem Res, 43(3): 826– 834. Lindvig, Th., Michelsen, M.L. and Kontogeorgis, G.M., 2004, Liquid– liquid equilibria for binary and ternary polymer solutions with PC-SAFT, Ind Eng Chem Res, 43(4): 1125– 1132.
Madsen, T., Boyd, H.B., Nyle´n, D., Pedersen, A.R., Petersen, G.I. and Simonsen, F., 2001, Environmental and health assessment of substances in household detergents and cosmetic detergent products, Environmental Project no. 615, Danish Environmental Protection Agency, Copenhagen; www.mst.dk. Moorwood, T., 2001. CAPE tools and techniques for the 21st century. Properties of materials and mixtures—where do we need to be 10 years from now? Report, Eureka Project 3211. Prausnitz, J.M., 1989, Biotechnology: a new frontier for molecular thermodynamics, Fluid Phase Equilibria, 53: 439 –451. Prausnitz, J.M., 1995, Some new frontiers in chemical engineering thermodynamics, Fluid Phase Equilibria, 104: 1 –20. Prausnitz, J.M., 1999, Thermodynamics and the other chemical engineering sciences: old models for new chemical products and processes, Fluid Phase Equilibria, 158– 160: 95–111. Prausnitz, J.M., 2001, Chemical engineering and the postmodern world, Chem Eng Sci, 56: 3627–3639. Prausnitz, J.M., Lichthenthaler, R.N. and de Azevedo, E.G., 1986, Molecular Thermodynamics of Fluid-Phase Equilibra, 2nd edn (PrenticeHall, Englewood Cliffs, NJ, USA). Rainwater et al., 2001, Report on Forum 2000: fluid properties for new technologies—connecting virtual design with physical reality. NIST Special Publication 975; http://Forum2000.Boulder.NIST.Gov/ NISTSP975.pdf Sandler, S.I., 2003. Quantum mechanics: a new tool for engineering thermodynamics. Fluid Phase Equilibria, 210(2): 147–160. Sangster, J., 1997, Octanol–Water Partition Coefficients: Fundamentals and Physical Chemistry, (Wiley, Chichester, UK). Thorlaksen, P., Abildskov, J. and Kontogeorgis, G.M., 2003, Prediction of gas solubilities in elastomeric polymers for the design of thermopane windows, Fluid Phase Equilibria, 211: 17–33. Villadsen, J., 1997. Putting structure into chemical engineering: proceedings of an industry/university conference, Chem Eng Sci, 52: 2857. Von Solms, N., Michelsen, M.L. and Kontogeorgis, G.M., 2003, Computational and physical performance of a modified PC-SAFT equation of state for highly asymmetric and associating mixtures, Ind Eng Chem Res, 42: 1098–1105. Zeck, S., 1991. Thermodynamics in process development in the chemical industry—importance, benefits, current state and future development, Fluid Phase Equilibria, 70(2–3): 125 –140. Zhong, C. and Masuoka, H., 1998, Modeling of gas solubilities in polymers with cubic equation of state, Fluid Phase Equilibria, 144: 49 –57.
ACKNOWLEDGEMENT The authors gratefully acknowledge the contributions of Drs Thomas Lindvig, Nicolas von Solms, Hongyuan Cheng and Irene Kouskoumvekaki. The manuscript was received 23 June 2004 and accepted for publication after revision 3 September 2004.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A11): 1505–1510