Selective filtration of multicomponent aerosols

Vol. 22, No. 6, pp. 793 807, 1991. Printed in Great Britain.

0021 8502,/91$3.00+0.00 ;~S1991 PergamonPress plc.

d. Aerosol Sci.,

WORKSHOP REPORT

SELECTIVE F I L T R A T I O N O F M U L T I C O M P O N E N T A E R O S O L S B.A.T. Cigarettenfabriken G m b H , Hamburg, 3-4 September 1990 Organizing associations: Gesellschaft ftir Aerosolforschung (GAeF) B.A.T. Cigarettenfabriken G m b H

1. SPECIAL ASPECTS O F SELECTIVE F I L T R A T I O N W . SCHNEIDER B.A.T. Cigarettenfabriken G m b H , Bahrenfelder Chaussee 139, 2000 H a m b u r g 50, G e r m a n y

The term selective filtration here describes the effect whereby different chemical compounds in the particles of a multicomponent aerosol are reduced by different ratios with reference to the total mass of the particles, after the aerosol has been drawn through a filter. A quantitative expression for the 'selectivity', Sx, for the compound x, which is used in the area of cigarette smoke research (Davis and George, 1965; Morie et al., 1975) is given by Sx=(1 - RTPM)/(1 --Rx). R x p ~ and Rx are the fractional retention of TPM (total particulate matter) and compound x, respectively. The aspect of selectivity becomes especially important if a multicomponent aerosol is filtered, the particles of which contain different compounds with vapor pressures which differ by 4 orders of magnitude (as is the case, e.g. for cigarette smoke particles). The total process of selective filtration can be divided into the following processes: (A) the transport of particles to the filter surface; (B) the transfer of molecules between gas phase and filter surface and (C) the transfer of molecules between gas phase and particles. By far the largest portion of the publications on cigarette smoke filtration consider only process (A): the interaction of the filter material with the particles as rigid spheres--the mechanical filtration--is described by advanced theoretical treatment and found to be in good agreement with the corresponding experiments (see e.g. Kirsch and Fuchs, 1968; Keith, 1978; McRae, 1982; Hinds, 1981; Dwyer and Abel, 1986; Boldridge and Ingebrethsen, 1986). Very few publications are available on smoke filtration, which take into account the fact that a considerable amount of the smoke condensate consists of volatile and semi-volatile constituents (see e.g. Morie et al., 1975; Kirsch and Fuchs, 1968; Keith, 1978; McRae, 1981; Hinds, 1982; Dwyer and Abel, 1986; Boldridge and Ingebrethsen, 1986; Curran and Kiefer, 1973; Townsend, 1983). There are several routes which may possibly lead to a selective reduction of single compounds in the total particulate matter. (1) By means of particle size dependent mechanical filtration. This is only practical if: (i) the chemical composition of the particles is size dependent, and if (ii) the size dependence of the mechanical filtration efficiency is sensitive in the relevant particle size range. (2) By means of selective evaporation of molecules out of the aerosol particles within a time scale, which is given by the residence time of the particles in the filter. This time scale is 10-100 ms for cigarette smoke. 793

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'~A~ kshop t'eport

As for route (1) there are several publications on cigarette smoke which t'ek:l ~ 1he chemical composition as a function of particle size {see e.g. Jenkins ~i~ ~li.~ t979i. Investigations have shown that the particle size distribution is changed only slightty b3~ practicable cigarette fiber tihers, (McRae~ ~981). The special topics of this workshop are investigations which are relevant to route i2): the mechanisms which mainly o:,ntrol the selective evaporation, the time scale for selective evaporation dependent on the properties of the present compounds, and the degrees of selective reduction, which can be realized within the given time scales Investigations which contribute to these topics are performed in quite different areas of application, e.g. in waste gas control, optimization of fuel combustion, drying processes, formulation of medical aerosols, etc. The objective of this workshop is to benefit from these very different sources The single contributions have deliberately been chosen to emphasize different aspects of the whole problem: the tube flow: the effect of the properties of the multicomponent mixtures; surface layers; diffusion inside the droplet and electrolytic constituents. REFERENCES Boldridge, D. W. and Ingebrethsen, B. J. (1986) In Aerosols Formation and Reactivity, pp. 459-461, Pergamon Press, Oxford. Curran, J. G. and Kiefer, J. E. (1973) Beitr. Tabaklbr.sch. 7, 29. Davis, H. and George, W. (1965) Beitr. TabakJbrsch~ 3, 203. Dwyer, R. W. and Abel, S. G. (1986) Beitr. Tabakjorsch. Int. 13, 243. Hinds, W. C. (1982) Aerosol Technology. John Wiley, New York. Jenkins, R. W., Francis, B. W,, Flaehsbart, H. and St6ber, W. (1979) J. Aerosol Sci. 10, 355. Keith, C. A. (1971) Recent Adv. Tob. Sci. 4, 25~ Kirsch, A. A. and Fuchs, N A. (1968) Colloid J. USSR 30, 630. McRae, D. D. (1981) 35th Tobacco Chemists' Research Conference. Morie, G. P., Sloan, C. H. and Baggett, M. S. (1975) Bear. Tabakjbrsch. 8, 145. Townsend, D. E. (1983) 37th Tobacco Chemists" Research ConJOrence.

2. E V A P O R A T I O N A N D C O N D E N S A T I O N PROCESSES IN AEROSOLS O. PREINING Institute of Experimental Physics, University of Vienna, Vienna, Austria

Smoking a cigarette demonstrates the many facets of aerosols. The smoldering tobacco creates particles which, while passing through unburned tobacco, exchange gases with it, a process which continues after inhalation with the internal surfaces of the bronchial tree. The processes are governed by the vapor pressures over the plane walls altered by dissolved materials and sorbed films and by the vapor pressures over the spherical particle surfaces. The vapor pressure P over a plane clean surface depends on the temperature T relative to a standard state Po, To, and is given by a simple expression derived by integrating the Clausius-Clapeyron equation: P/Po = exp (r/R)(l/T0 - l/T), with r the evaporation enthalpy and R the universal gas constant. The vapor pressure Pc over a spherical droplet of diameter d is given by the Gibbs-Kelvin equation: P~/P = ex p (4s M/qr Td), with s the surface tension, M the molecular weight and q the density of the liquid. Integrating the steady state diffusion equation yields an evaporation (or condensation) rate (Maxwell): 1 = 2toDd(co- c, ),

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with D the binary diffusion coefficient and c o and co~ the vapor concentrations at the particle surface and at infinity, respectively. If the vapor can be considered an ideal gas the equation becomes:

I=(2zDd/RT)(Pc-Po~), with Pc and Po~ the vapor pressures at the particle surface and at infinity, respectively. For an evaporating droplet of an initial diameter do the lifetime t is derived by integrating the mass flux (sometimes called the Langmuir equation): qRd 2 t

8D(P/T-P~/To~)'

with Too the temperature at infinity. These straightforward classical theoretical developments give the overall qualitative understanding of the processes (see e.g. Fuchs, 1959; Davies, 1978; Hinds, 1982; Reist, 1984), but to achieve quantitative agreement between experiment and theory, corrective terms have to be added, which defy closed analytical solutions. Condensation or evaporation cause temperature changes of the droplet, hence not only the mass flux but also the heat flux must be correctly described; however, these fluxes interact mutually and nonlinear terms enter the numerical solutions. For smaller droplets of sizes comparable to the mean free path of the gas molecules, transition regime corrections have to be added additionally. Last but not least the surface properties of the droplets, the cover by adsorbed gas films and the accommodation coefficients of the molecule surface interactions also enter. In recent years, droplet growth rates were studied experimentally using the Constant Angle Mie Scattering method (Wagner, 1982, 1985). The complex theory, including the interacting heat and mass fluxes, was tested for defined systems (water and monodispersed condensation nuclei of 80 nm diameter). For a fast process in an expansion cloud chamber (large supersaturations 40-250%) good agreements were observed for accommodation coefficients of one; for slower processes in an expansion cloud chamber (small supersaturations--6-12%) agreements were only found using accommodation coefficients considerably smaller than one (e.g. 17%), as reported by Majerowicz and Wagner (1989). REFERENCES Davies, C. N. (1978) In Fundamentals of Aerosol Science (Edited by Shaw, D. T.), Wiley,New York. Fuchs, N. A. (1959)Evaporation and Droplet Growth in Gaseous Media, Pergamon Press, London. Hinds, W. C. (1982) Aerosol Technology. Wiley,New York. Majerowicz, A. and Wagner, P. E. (1989) In Atmospheric Aerosols and Nucleation (Edited by Wagner, P. E. and Valy, G., Lecture Notes in Physics 309. Springer, Berlin. Reist, P. C. (1984)Introduction to Aerosol Science. Macmillan, New York. Wagner, P. E. (1982) In Aerosol Microphysics II (Edited by Marlow, W. H.) Springer, Berlin. (1985). Wagner, P. E. (1985) Colloid Interface Sci. 105, 456.

3. EVAPORATION AND C O N D E N S A T I O N PROCESSES FOR AEROSOLS IN TUBE FLOWS C. F. CLEMENT Theoretical Studies 424.4, AEA Industrial Technology,Harwell Laboratory, Oxon OXll 0RA, U.K. 1. I N T R O D U C T I O N This paper summarises the main points of a presentation at the workshop, including relevant references, but without equations. For basic molecular theory and transport phenomena we refer to Hirschfelder et al. (1954) and Bird et al. (1960), and for a chemical engineering treatment of correlations, McAdams (1942).

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V~ ~rkq~op report

2 [ U BE FLOW The nature of the tube flow is determined by the Reynolds number, Re~ and is laminar for Re<2000 and fully turbulent for Re> ~0,000 (McAdams, 1942). For laminar flow the velocity is parabolic, and to treat heat or mass transmission, the approximation is ge~i~era[ly made of neglecting conduction or diffusion along the flow. A mathematical treatment of diffusion in steady tube flow has a fairly close analogy to the equation lk~r time-dependent diffusion in a one-dimensional cylindrical geometry~ For turbulent tube flow, heat transmission between the walls and bulk flow is specified by correlations for the Nusseh number, Nu, in terms of Re and the Prandtt number, Pr (McAdams, 1942), and mass transfer rates are given by Chilton-Coburn analogies (Bird, 1960) with the Sherwood number, Sh, and Schmidt number, Sc, replacing Nu and Pr. respectively. These correlations can fail when there is enough aerosol in the boundary layer (see sections 5 and 61.

3. MULTICOMPONENT DIFFUSION The basic theory of molecular multicomponent diffusion in gases is given in Hirschfelder et al. (1954) and Bird et al. (1960), especially p. 560, and leads to the Stefan-Maxwell

equations which give the relations between the mass currents, gradients in the molecular concentrations, and the two-component diffusion coefficients. Explicit calculations are complicated, even for three component systems. There is a substantial chemical engineering literature on the subject, and the possible physical phenomena of osmotic diffusion, a diffusion barrier, and reverse diffusion, all not possible with just two components, were pointed out by Toor (1957, 1964). The usual treatment for boundary layers is to use a film model originally due to Ackermann and Colburn and Drew, and developed by Krishna and Standart (1976), and Krishna and Panchal (1977) in a tube application. Heat and mass transfer in multicomponent condensation and boiling are reviewed by Sardesai et al. (1982). For aerosols, the problem is to solve the Stefan--Maxwell equations in a spherical geometry around a droplet, with possibly an associated internal mass transfer problem within a droplet. The literature on the subject is growing (Rosner and Chang, 1973; Landis and Mills, 1974; Ravindran and Davis, 1982; Taylor and Noah, 1982; Kreidenweis et al., 1987; Vesala et al., 1990). In recent work by Vesala (1990), results are obtained for methanol and ammonia droplets in humid air, where there is multicomponent diffusion of these vapours and water vapour in air. 4. BASIC PHYSICS OF EVAPORATION AND CONDENSATION The physics described concerns the heat and mass transfer and coupling between them involved in aerosol formation in a cloud (Clement, 1985, 1987). The driving force for evaporation and condensation is temperature differences from heat transfer, possibly augmented in the multicomponent case by differences in molecular surface compositions, In vapour-gas mixtures it is diffusion and conduction which lead to aerosol nucleation and growth, coupled with the nonlinearity of the equlibrium vapour pressure as a function of temperature. The amount and whereabouts of aerosol formation can be sensitive to the Lewis number, Le, which gives the ratio of the rate of heat transfer to that of mass transfer in a vapour-gas mixture (Clement, 1985). Because condensation releases latent heat, the proportion of condensation that takes place on an aerosol depends strongly on the condensation number, Cn, which may be looked as as the ratio of the rate of latent heat removal to that of mass transport (Clement, 1985). Unless C n > > 1, aerosol formation strongly couples heat and mass transfer, and uncoupled heat and mass transfer correlations cannot be used for boundary layers when the aerosol density exceeds a critical value. This value and alternative procedures to calculate heat and mass transfer have been given (Clement, 1987, 1988a, b), based on equations for the

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total heat transfer and for the supersaturation. The code RAFT (Im et al., 1985) does not have this coupling and is only applicable for Cn > > 1. The theory of Buckle (1986) couples the heat and mass transfer correctly, but makes the unjustified assumption that the aerosol produced moves with the vapour current, rather than being subject to all the forces, including gravity, thermophoresis and turbulence, which cause an aerosol to move relative to the gas. 5. AEROSOLS IN LAMINAR TUBE FLOW Detailed calculations for aerosol growth in cooled tube flow have been performed by Barrett and Fissan (1989) for water vapour-air (Cn < 1, Le = 0.85 < 1) and for DBP (dibutyl phthalate)-air (Cn > > 1, Le ~ 5-8 > 1). The coupled equations for heat, vapour and aerosol were solved with the assumption that the aerosol moves with the flow, and with the growth rate determined by the local supersaturation. As expected, aerosol condensation was found to dominate over wall condensation for DBP where, unless the initial aerosol density is high, large supersaturations build up and homogeneous nucleation (not included in the theory) might be expected. For water vapour air with Cn < l, wall condensation predominates, and the water aerosol tends to evaporate at the centre of the tube but grow near the wall. This effect of growth localisation arises because L e ¢ 1 and causes an initial monodisperse aerosol size distribution to become very disperse. Some water droplets were also found to pass through different regions of evaporation and condensation in their passage along the tube. 6. AEROSOLS IN TURBULENT TUBE FLOW Maximum aerosol densities which could be produced by cooling or evaporating into turbulent flows have been calculated by Clement and Ford (1989) assuming a uniform mixture, zero supersaturation, and a small pressure drop. The densities produced show a strong temperature dependence and are predicted quite well by a simple formula. When the presence of a boundary layer is allowed for and the vapour is allowed to be unsaturated or supersaturated, the situation becomes much more complicated in that there are several states of saturation in the main flow and boundary layer depending on the number density and size of the aerosol (Clement and Ford, 1988). Nucleation and growth can occur in the turbulent boundary layers and the timescales for these processes have recently been compared to those for turbulent bursts (Clement and Ford, 1990). Nucleation may sometimes be cut off by growth before ejection of aerosol into the flow takes place, showing that it may occur as an intermittant phenomenon in tube flow. The RAFT code is applicable to turbulent tube flow. Whilst boundary layers are not allowed for, it incorporates nucleation models and can treat a multicomponent gas-vapour phase with the species in local chemical equilibrium (Ritzman et al., 19881. 7. M U L T I C O M P O N E N T EVAPORATION AND CONDENSATION To treat multocomponent aerosols in tube flow requires not only taking over the effects necessary to describe a single component, but also the following specific multicomponent aspects: -

-

-

-

-

-

multicomponent gaseous diffusion; description of a multicomponent aerosol; and multicomponent transport within the aerosol.

These aspects must be coupled to the single component problems of: -

coupling to heat transfer; aerosol motion, especially for turbulent flow; and

798

~ o~kshop reporl

......... n u c l e a t i o n rates twhich can have a strong m u l t i c o m p o n e n t dependence). Although it has not yet been solved explicitly, the c o u p l i n g of heat iransfcr to m u l t i c o m p o n e n t v a p o u r a n d gaseous diffusion in aerosol f o r m a t i o n is a relatively ~traight. forward problem. F o r some types of m u l t i c o m p o n e n t aerosols such as liquid droplets., some progress has been made on descriptions of their surface properties and l r a n s p o r t within the aerosol. Also, the p r o b l e m of aerosol m o t i o n in t u r b u l e n t b o u n d a r y layers is being i a c k l e d However, the most difficult c o n c e p t u a l a n d practical p r o b l e m s of specifying the surface of a m u l t i c o m p o n e n t aerosol a n d its t r a n s p o r t properties are a long way from solution. The most acute difficulties will arise in the description of e v a p o r a t i o n from a m u l t i c o m p o n e m aerosol where positions of the types of molecules e v a p o r a t i n g must be specified. Clearly this would involve a knowledge of the radial d e p e n d e n c e of the c o m p o s i t i o n of the aerosol and ihus its history. T o all the p r o b l e m s associated with the specification of macroscopic multtcompo.o nent materials a n d their surfaces are added the necessity of averaging over a collection of particles or droplets in a cloud. REFERENCES Barrett, J. C. and Fissan, H. (1989) Wall and aerosol condensation during cooled laminar tube flows..L CoUoid Interface Sci. 130, 498. Bird, R. B., Stewart, W. E. and Lightfoot, E. N. (1960) Transport Phenomena. Wiley, New York. Buckle, E. R. (1986) Particle condensation in evaporative flows. Proc. Roy. Soc. Lond. A 40, 22Z Clement, C. F. (1985) Aerosol formation from heat and mass transfer in vapour-gas mixtures. Proc. Roy. Soc. A 398, 307. Clement, C. F. (1987) The supersaturation in vapour-gas mixture condensing into aerosols. Harwell report AERETP. 1223. Clement, C. F. (1988a) The formation of nuclear aerosols by evaporation-condensation processes. Harwelt report AERE-TP. 1285. Clement, C. F. (1988b) The calculation of aerosol growth rates in conjunction with heat and mass transfer. WaterCooled Reactor Aerosol Code Evaluation and Uncertainty Assessment, Proc. Workshop in Brussels, September 1987 (Edited by della Loggia, E. and Royen, J.L CEC OECD EUR 11351, pp. 156-169. Clement, C. F. and Ford, I. J. (1988) Aerosols formation in tube flow. Proc. European, Aerosol Conj.. Lund 1988. J. Aerosol Sci. 19, 817. Clement, C. F. and Ford, 1. J. (1989) Maximum aerosol densities from evaporation-condensation processes. J. Aerosol Sci. 20, 293. Clement, C. F. and Ford, 1. J. (1990) The physics of nucleation and growth in turbulent boundary layers. Proc. 3rd Int. Aerosol Conf, Kyoto 1990. Vol. I, pp. 221-224. Pergamon Press, Oxford. Hirschfelder, J. O.. Curtiss, C. F. and Bird, R. B. (1954) Molecular Theory of Gases and Liquids. John Wiley, New York. Ira, K. H., Ahluwalia, R. K. and Chuang, C. F. (1985) RAFT: a computer model for formation and transport of fission product aerosol in LWR primary systems. Aerosol Sci. Technol. 4, 125. Kreidenweis, S. M., Hagan, R. C. and Seinfeld,J. H. (1987) Evaporation and growth of multicomponent aerosols laboratory applications. Aerosol Sci. Technol. 6, 1. Krishna, R. and Panchal, C. B. (1977) Condensation of a binary vapour mixture in the presence of an inert gas. Chem. En 9. Sci. 32, 741. Krishna, R. and Standart, G. L. (1976) A multicomponent film model incorporating a general matrix method of solution to Maxwell-Stefan equations. AIChE J. 22, 383. Landis, R. B. and Mills,A. F. (1974)Effect of internal diffusionalresistance on the evaporation of binary droplets. In Fifth Intl. Heat Transfer Con[~ Proceedings 3-7 September 1974, Tokyo, Vol. 4, Paper B7.9. 345-349. Japanese Soc. Mech. Eng., Tokyo. McAdams, W. H. (1942) Heat Transmission, 2nd Ed. McGraw-Hill, New York Ravindran, P. and Davis, E. (1982) Multicomponent evaporation of single aerosol droplets. J. Colloid lnteUdce Sci: 85, 278. Ritzman, R., Rahn, F., Ahluwalia, R. and Irn, K. (1988) Recent improvements and applications of the RAFT code. Water-Cooled Reactor Aerosol Code Evaluation and Uncertainty Assessment, Proc. Workshop in Brussels,

September 1987 (Edited by della Loggia, E. and Royen, J.), CEC OECD EUR 11351, pp. 76-87. Rosner, D. E. and Chang, W. S. (1973) Transient evaporation and combustion of a fuel droplet near its critical temperature. Combust. Sci. Technol. 7, 145. Sardesai, R. G.. Shock, R. A. W. and Butterworth, D. (1982) Heat and mass transfer in multicomponent condensation and boiling. Heat Trans. En9. 3, 104. Taylor, R. and Noah, M. K. (l 982) Simulation of binary vapour condensation in the presence of an inert gas. Let cs. Heat Mass Trans. 9, 463. Toor, H. L. (1957) Diffusion in three component gas mixtures. AIChE J. 3, 198. Toor, H. L. (1964) Solution of the linearized equations of multicomponent mass transfer. AIChE J. 10, 448: Vesala, T. (1990) Binary evaporation. Report Series in Aerosol Science No. 13 Finnish Association for Aerosol Research, c/o Univ. of Hetsinki. Vesala, T., Kulmala, M., Majerowicz, A. and Wagner, P. E. (1990) Binary condensation and evaporation in the continuum regime. Proc. 3rd lnt. Aerosol Conf Kyto, 1990, Vol. I, pp. 176-179. Pergamon Press, Oxford.

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4. P R O B L E M S A N D A P P R O A C H E S T O G E T I N F O R M A T I O N ON THE CHEMISTRY OF MULTICOMPONENT AEROSOL PARTICLES IN SITU REINHARD NIESSNER Institute of Hydrochemistry, Technical University of Munich, MarchioninistraBe 17, D-8000 Mi.inchen 70, Germany

Aerosols are a rather complex mixture of thermodynamically labile materials. At present many processes are under investigation which need a continuous, that means 'on line' and because of instability an 'in situ' measurement for a more or less complete chemical characterization (Niessner, 1990). The chemical part of an aerosol characterization deals with the composition of gaseous phase as well as of particulate matter. An increasingly important parameter is the microstructure of particles. Microstructure of particles means the uneven distribution of elements or substances over the total particle cross-section. A typical example are particles or droplets which have picked up volatile and surface-active organic constituents from the surrounding atmosphere. These organic compounds may be enriched as mono- or multilayers on the droplet surface, and by this means may influence the phase transfer of reactive gases like NH 3 into the aqueous solution (Dfiumer et al., 1991). Results on the neutralization reaction of coated sulfuric acid droplets with ammonia were reported with regard to the influence of different coating substances, coating thickness and relative humidity on the reaction rate. Different techniques were combined which allowed the production of defined organic coatings on ultrafine monodisperse H2SO 4 droplets. The reaction with ammonia was measured as a function of time by chemical characterization of the particulate and gaseous phase before and after mixing in a flow reactor. A remarkable decrease in the reaction rate was observed, depending on the molecular structure of the organic coating substance and the thickness of coating. Straight chain molecules like n-hexadecanol and n-hexadecane strongly retarded the reaction due to the formation of a tight surface film which inhibited the transport ofNH 3 molecules to the acidic droplet nucleus. Branched chain coating substances (e.g. 1-hydroxymethyl-admantane) showed no significant effect because of their inability to form tight films. Naturally occurring coating substances like terpenes were investigated, too. To characterize the surface of ultrafine particles, their behaviour as condensation nuclei can be used. Monodisperse ultrafine particles with different surface structure were investigated by observing the onset of droplet formation at fixed electrical mobility diameter. Droplet growth was detected by application of a multistep condensation nuclei counter (Niessner et al., 1990). Especially hydrophobic organic monolayers encapsulating hydrophilic nuclei could be detected by the increased water supersaturation which was necessary to activate the particles as condensation nuclei. Aerosol photoemission (APE) is one technique for 'in situ' and 'on line' detection. The signal in aerosol photoemission is proportional to the photoelectrically active surface of the particles (like in photoelectron spectroscopy), while the total mass is proportionally to the volume. Plots of the ratio APE signal to mass concentration vs particle diameter clearly exhibited the lid dependence. Photoemissive materials are some heavy metal oxide aerosols, pure metal aerosols and polycyclic aromatic hydrocarbons (PAHs). In the meantime various applications with the APE technique are known: diesel exhaust control; waste combustion control; cigarette smoke analysis and workplace monitoring. Finally the detection and chemical characterization of PAHs by (Niessner and Lutz, 1991) means of laser-induced fluorescence was reported (Niessner and Krupp, 1991). Laserinduced and time-resolved aerosol fluorescence is introduced as an analytical technique for the qualitative and quantitative determination of surface-bound PAHs. The analytical figures of merit are evaluated by studying the fluorescent properties (emission wavelengths

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and temporal behaviour} of PAHs adsorbed on submicrometer NaCI particles. In a
5. S I M U L A T I O N O F T H E I N F L U E N C E O F T H E V A P O R COMPOSITION AND THE PROPERTIES OF THE SINGLE COMPONENTS ON THE EVAPORATION, CONDENSATION AND COMPOSITION OF A MULTI-COMPONENT PARTICLE W . SCHNEIDER,* K. ANDERS f a n d A. FROHN* * B.A.T. Cigaretten GmbH, Bahrenfelder Chausse 139, 2000 Hamburg 50, Germany t Institut ffir Thermodynamik der 1.uft- und Raumfahrt, Universit/it Stuttgart, Pfaffenwaldring 31, 7000 Stuttgart 80. Germany

A model for the simulation of the influences of several effects on the evaporation, condensation and composition of a multicomponent particle has been developed. The type of model has been determined by the following objectives and restrictions. (1) The first objective is to get an impression (by means of a sensitivity analysis) of which of the many parameters in the complex aerosol system has a notable influence on the composition of the particles within the given relevant interaction time. (2) The aerosol particles consist of a large variety of different substances. The vapor pressure of these substances ranges from less than 0.1 Pa to more than 2000 Pa at ambient temperature. (3) The relevant interaction time (tin,), which here means the residence time of the particle in a filter, is 1O-100 ms. The time elapsed after the formation of the aerosol is 0.1 1 s. (4) The values of the properties of many compounds in the particles considered here and the values of the coefficients, which are used in advanced multicomponent particle models (see e.g. Gelbard and Seinfeld (1978)) are not known. The model applied here is based on a quasistationary description. A quasistationary model, which describes the evaporation and condensation of a single multicomponent droplet in quiescent air, is presented, e.g. in Renninger et al. (1981). The assumption of quasistationary conditions means that the vapor field has been described at any instant of time by the solutions of the stationary diffusion equation. At each time step the enthatpy of evaporation or condensation equals the heat transferred from the ambient vapor. This implies practically constant droplet temperatures as long as only small changes of the composition occur. Knudsen corrections for the heat and mass transfer have been included. The first version of our model has been extended in order to release some of the restrictions made in previous models (Renninger et al., 1981) and to account for different additional phenomena not included in these models. The extended model has a modular composition. This allows us to study the influence of a particular elementary process as well as the interaction between different processes. All calculations have been made with 'substitute compounds'. The properties of the substitute compounds have been chosen according to the known properties of specific compounds which form larger fractions of the particle, or acording to the average values of similar compounds, which are grouped into the same

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Fig. 1. Droplet diameter and composition as a function of time for different particle densities n. Fig. la shows results for a single droplet in an infinite atmosphere. Figs l b ~ l give results for particles in a monodisperse ensemble with different particle number densities n. All results have been obtained for initial droplet diameters of 0.1/~m at an ambient temperature of 293 K. The droplets initial compositions were 52% H, 3% N, 10% P and 35% G. The saturation ratios S i in the surrounding atmosphere at the beginning of the process are S H= 60%, S N= 20%, Sp = 20% and S o = 20%. The properties of substitute c o m p o u n d H correspond essentially to those of water, which has a relatively high volatility. N and P are substitute c o m p o u n d s with a medium volatility and G is a substitute c o m p o u n d with a very low volatility. AS 2 2 : 6 - H

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category. This has been an iterative process because the criteria for the grouping have t'~eer~ found to be the result of the first simulations. For the first sensitivity amflysis the sai,.~.~':~fio~t ratios of the constituents of the gas phase were assumed to be constant It wa~..~ i',~;thcr assumed that the droplet consists of water and two ~semi-volatile' compounds, lhc sat t.,l :mot~ pressures of which are two orders of magaitude lower. Depending on the saturatio~:~ ~':~i~:s., on the initial composition of the droplet and on the properties of the m~ otved comp~mnds, the conditions are investigated which lead to a quasistationary composition within the giveu interaction time (t~,,), and the time necessary to reach a quasistationary comp,~r~.~.ition ('relaxation time t ~ ' ) and the values of the quasistationary composition are calcula~:d. Fhe degree of selective filtration can be determined in those cases with ~,.~-: ,~,,,,~ The following extensions ha~e been included. (1) To study transient behaviour of droplet temperature especially in the case o~ larger differences between droplet and ambient temperature the energy balance at the droplet surface has to be supplemented by a term accounting for the change of the internal energy of the droplet (Schneider et al., 1988~. As a consequence, the linear extrapolation of the vapor pressure at the surface based on the ambient temperature has been replaced by a ~nore precise calculation based on the droplet temperature. (2) A first step for the simulation of chemical reactions has been included, which uses rate constants to describe the conversion of a condensing compound into a volatile and a nonvolatile constituent within the droplet. (3) The influence of neighbouring particles has been included. The aerosol has been assumed to be monodisperse with the number density n. Figure 1 shows an example of the influence of the number densities on the diameter and the composition of a droplet. In all cases the initial conditions for droplet diameter and composition are the same. As can be expected (see e.g. Wagner (1982)), it can be seen clearly that the single droplet in an infinite constant atmosphere grows steadily, the same droplet within an ensemble reaches an equilibrium, the diameter decreases with increasing number density of the ensemble, and the final composition is affected by the presence of the other droplets. (4) The effects of adsorption layers on the surface of the droplets on the evaporation rate have also been included. According to La Mer (1962), it was assumed that the vapor pressure is not affected by adsorption layers. The resulting effects have been simulated, e,g. the amount of the reduction of the evaporation rate in a binary mixture has been calculaled, if the component with the lower volatility is accumulated in the surface layer, in this case the quasi-stationary composition of the droplet is not affected by the presence of a surface layer. REVERENCES Gelbard, F. and Seinfeld, J. H. (1978),L Colloid Inter.lace Sci. 68, 363 (t978}. La Mer, V. K. (1962) Retardation of Evaporation by Monolayers: Transport Processes. Academic Press, New York. Renninger, R. G., Hiller, F. C. and Bone, R. C. (1981) d. Aerosol Sci. 12, 505. Schneider, W., Anders, K. and Frohn, A. (1988) J. Aerosol Sci. 19, 845. Wagner, P. E. (1982) In Aerosol Microphysics II (Edited by Marlow, W. H.), pp. 129 -178, Springer, Berlin,

6. SELECTIVE E V A P O R A T I O N OF A T E R N A R Y M I X T U R E WITH A N O N - V O L A T I L E C O M P O N E N T IN D R Y I N G PROCESSES E.-U. SCHLONDER lnstitut fiir ThermiseheVerfahrenstechnik, Universit~itKarlsruhe (TH), KaiserstraGe 12, 7500 Karlsruhe, Germany In m a n y industrial applications the drying process is used not only to reduce the moisture of the drying goods but also to control the properties and the quality of the goods. This control

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is especially important if the moisture consists of mixtures. Examples are the conservation of the aroma in food, the removal of toxic components in tablets or the control of the tensile strength in paper drying processes. In all these cases the selectivity of the drying process is of special interest. The term selectivity means a preferential evaporation of certain components from the moist goods. The mechanisms and interactions, which give rise to the selectivity of the isothermal evaporation, have to be understood in order to determine the appropriate drying conditions. These mechanisms refer to the transport in the gas phase, to the transport in the liquid phase and to the vapor-liquid equilibrium. Because the moisture of many goods in industrial drying processes is formed by organic solvents, water and non-volatile substances, the evaporation of a ternary mixture with a non-volatile component has been investigated and is presented here. Even for this simple case all interaction phenomena of the multicomponent diffusion have to be considered. The results of the theoretical study show in detail the dependence of the selectivity of the evaporation on the evaporation conditions. Some of the theoretical results have been compared with measurements, which result from evaporation experiments with a ternary mixture of isopropanol, water and glycerol (Riede and Schlfinder, 1990). The theory bases on the fundamental experiments of Schlfinder (1988), where the evaporation into dry air has been investigated, and is an extension to those cases, in which the air is laden with one or two of the volatile components of the mixture. By means of mass balances, the transport equations in the gaseous and the liquid phase and the thermodynamic vapor-liquid equilibrium at the phase boundary an equation is derived, which allows us to calculate the selectivity. This selectivity is mainly determined by the following quantities: the ratio of the mass transfer coefficients at the gas side and the air flow rate; the ratio of the mass transfer coefficients at the liquid side; - - the relative volatility; the relative saturation ratios in the air; and -

-

the ratio of the mass transport resistances of the gas and the liquid side.

The azeotropic mixture, which exists for low glycerol contents, disappears with increasing glycerol contents. In the course of the evaporation the nonvolatile component glycerol is enriched in the mixture. By this enrichment the selectivity can be reversed during the evaporation process due to the thermodynamic equilibrium. Opposite the evaporation of a binary mixture, where with a dominating mass transport at the liquid side the evaporation remains non-selective, the evaporation of a ternary mixture can be selective even with a dominating mass transport at the liquid side (Riede and Schfinder, 1991). For high glycerol contents in the isopropanol water-glycerol mixture there is a preferential evaporation of the volatile alcohol in the total range of concentration, if the mass transport resistance at the liquid side can be neglected. If this resistance dominates, a preferential evaporation of water occurs, because of the faster diffusion of the water molecules in the boundary layer at the liquid side in comparison with the larger alcohol molecules. Because the diffusion coefficients decrease with increasing glycerol content, a reversal of the selectivity during the evaporation process is possible (Schwarzbach et al., 1987). If the air is already laden with one of the components of the mixture, the respective other component is preferentially reduced in the mixture. This is due to the reduced concentration gradient and the consequently reduced flow of evaporation of the prior component in the gas phase. With this mechanism the selectivity can be shifted into any direction, which is required. As the residual moisture of the goods is in equilibrium with the humidity of the air, a complete dehumidification is only realized by complete evaporation. At lower temperatures the remaining moisture is determined by the temperature of the liquid and the humidity of the air.

R t?~FERENCES Riede, Th. and Schlfindel. E-L. 11990) Selective e~aporation of a ternary mixture containing one n ~ ',cladk component with regard to drying processes. Chem. Eng. Process. 28, 151. Riede, Th. and Schl/inder, E.-U. (19911 Measurements of the diffusion coefficients in the teraary liqu.ld mix~Llrc 2 propanot-water-glycerol and calculation of three-component mass transfer in liquids. (In preparatio~ i Schlfinder, E.-U. (1986) Selective drying of mixture-.containing products. Proc. 6th Int. Drying Syrup, ~[)S 88, Versailles, France, 1988, KL. 923. Schwarzbach, J., Nilles, M. and Schlfinder, E.-U. (1987) Microconvection in porous media dm-ing perva p~,t~tiorJ of a liquid mixture an experimental study. Chem. Enq. Process. 22, 163.

7. S U B M I C R O N

MULTICOMPONENT PARTICLES CONTAINING ELECTROLYTIC CONSTITUENTS G A. FERRON

GSF-Forschungszentrum ffir Umwelt und Gesundheit, GmbH Projekt Inhalation, 8042 Neuherberg~ Germany

The sampling or filtration of aerosol particles is a function of the size of the individual particles of the aerosol. Electrolyte particles change their size in humid air caused by the uptake of water. The change in size can be up to a factor of 6 of the initially dry geometric particle size (Dautrebande and Walkenhorst, 1961). Hence, their sampling efficiency may change considerably. This contribution discusses the influence of different parameters on the change in size of electrolyte particles. The equilibrium size of a single electrolyte particle can be estimated by Raoult's law. The relative change of the vapor pressure of a solution is proportional to the mole fraction of the solute and the dissociation constant of the electrolyte. This mole fraction can be transformed to a mass fraction from which the dry and humid particle diameters can be calculated. The law of Raoult is correct for highly deluted electrolyte molecules (CibaGeigy, 1979: table on aqueous solutions NaC1 and glucose), where interactions between the molecules can be neglected. This is normally not the case, e.g. a 0.9% NaCI solution by weight has an effective dissociation constant of 1.8 instead of 2.0. The deviations of the Raoult's law are small ( < 10%) for mole fractions less than 1 mole per 100 ml of solvent; but may increase dramatically for large concentrations in either directions. A more realistic estimate of the size of a single electrolyte particle is obtained using experimentally determined osmotic coefficients or activity coefficients. Data are available for more than 100 different salts, acids, and bases as a function of the solute concentration (Robinson and Stokes, 1959: their Appendices 8.4-t0). Isotonic concentrations (corresponding to the freezing depression of a 0.9% NaC1 solution by weight) are available for more than 40 anorganic and 500 organic substances (Budavari, 1989: miscellaneous tables). These data show no fundamental difference between water soluble anorganic and organic substances. Theoretical data on different electrolyte solutions have been published by Hamer I1968L As a first estimate multicomponent solutions can be treated as the sum of the effects of the single components (Robinson and Stokes, 1959; their chapter 15). Different theories on the activities of aqueous solutions of multicomponent solutions indicated by RWR, RB, MK and ZSR have been reviewed by Sangster and Benzi (1971). The RWR method derives the osmotic coefficient of the multicomponent solution from the osmotic coefficients of each component. The MK methods calculate the activity coefficients of the multicomp0nent solution from the activity coefficients of the binary activity coefficients of the different components using the ionic strengths as weighting factors. The RB method is the same as the MK method except that the weight fraction of each component is used as the weighting factor. The ZSR method is the same as the RB method except that the activity coefficients are for the final strength of the multicomponent solution. The ZSR method has been used by Pilinis and Seinfeld (1987). Problem with these methods are the missing activity coefficients for most components, particularly for the organic components. Sangster and Benzi (11971) tested the precision of the different theories indicated by RWR, RB, MK and ZSR for eight -

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bi-component and two tri-component salt solutions. They found differences in the calculated and experimental water vapor pressures of less than 4% in most cases. The information given above concerns bulk solutions and may differ for aerosol particles. Measurement techniquess for aerosol particles have been reviewed by Davis (1983). Tang et al. (1977) measured the size of NaC1 droplets with initial crystal size less than 1 #m. Their results are well approximated by theory. Measurements on mixtures of KC1 NaC1 and H2SO4-(NH4)2SO 4 show phase transitions caused by crystallization of different salts (Tang, 1980). The dynamics of evaporation or condensation of a single droplet have been described in the literature (Fuchs, 1959; Mason, 1957; Wagner, 1979; Pruppacher and Klett, 1980). Recently Ferron and Soderholm (1990) calculated the evaporation of single pure water droplets for the size range of 0.1-50 #m and summarized the results (their Fig. 2, or their equations 25a-c). The stabilization of droplets containing electrolytes is well estimated by their results for pure water particles for values of r.h. less than 50%. For values of r.h. larger than 50% increases in stabilization times up to a factor of 4 has been indicated by these authors. The dynamics of the condensation of droplets is strongly depending on the activity of the solute in the solution. The time for stabilization of a droplet containing electrolytes is well described by the time to evaporated a pure water droplet with the same size as the equilibrium size of the electrolyte droplet (Ferron and Soderholm, 1990, their Fig. 5). The activity of submicron particles is reduced caused by the relative increase of the surface energy of the small particles and is described by the Kelvin correction (Skinner and Sambles, 1972). The Kelvin correction is primarily depending on the particle size, temperature, surface tension and density of solvent and secondarily on the change in density and surface tension of the solution by the solute (Nair and Vohra, 1976). The Kelvin correction is important for values of r.h. near to 100% and particle sizes less than 1/zm. Activation of particles with sizes below 0.01 ~tm by saturated air caused by the Kelvin effect is hardly possible. Submicron particles reach their equilibrium within 1 s. Impurities in water may reduce the evaporation rate. This has been shown for fatty alcohols (Archer and LaMer, 1955), cetyl alcohol (Derjaguin et al., 1966; Mansfield, 1968), and glycerol (Ray et al., 1989). REFERENCES

Archer, R. J. and LaMer, V. K. (1955) The rate of evaporation of water through fatty acid monolayers. J. Phys, Chem. 59, 200. Budavari, S. (1989) The Merck Index. Merck and Co., Rayway. Ciba-Geigy (1979) Wissenschaftliche Tabellen Geigy. Ciba-Geigy, Basel. Davis, E. J. (1983) Transport phenomena with single aerosol particles. Aerosol Sci. Technol. 2, 121. Derjaguin, B. V., Fedoseyev, V. A. and Rosenzweig, I. A. (1966) Investigation of the adsorption of cetyl alcohol vapor and the effect of this phenomenon on the evaporation of water droplets. J. Colloid Interfdce Sci. 22, 45. Ferron, G. A. and Soderholm, S. C. (1990) Estimation of the times for evaporation of pure water droplets and for the stabilization of salt solution particles. J. Aerosol Sci. 21,415. Fuchs, N. A. (1959) Evaporation and Droplet Growth in 9aseous Media (Edited by Bradley, R. S.). Pergamon Press, Oxford. Hamer, W. J. (1968) Theoretical mean activity coefficients of strong electrolytes in aqueous solutions from 0 to 100°C. NSRDS-NBS 24, Superintendent of Documents, U.S. Printing Office, Washington, D.C. Mansfield, W. W. (1968) The influence of monolayers on evaporation from water storages. I. The potential performance of monolayers of cetyl alcohol. Austr. J. appl. Sci. 9, 245. Mason, B. J. (1957) The Physics of Clouds. Clarendon Press, Oxford. Nair, P. V. N. and Vohra, K. G, (1975) Growth of aqueous sutphuric acid droplets as a function of relative humidity. J. Aerosol Sci. 6, 265. Pilinis, C. and Seinfeld, J. H. (1987) Continued development of a general equilibrium model for inorganic multicomponent atmospheric aerosols. Atmospheric Environment 21, 2453. Pruppacher, H. R. and Klett, J. D. (1980) Microphysics of Clouds and Precipitation. Reidel, Dordrecht. Ray, A. K., Johnson, R. D. and Souyri, A. (1989) Dynamic behavior of single glycerol droplets in humid air streams. Langmuir 5, 133. Robinson, R. A. and Stokes, R. H. (1959) Electrolyte Solutions. Butterworths, London. Sangster, J. and Lenzi, F. (1971) On the choice of methods for the prediction of the water-activity and activity coefficient for multicomponent aqueous solutions. Can. J. Chem. Eng. 52, 392. Skinner, L. M. and Sambles, J. R. (1972) The Kelvin equation a review. J. Aerosol Sci. 3, 199. Tang, I. N., Munkelwitz, H. R. and Davis, J. G. (1977) Aerosol growth studies--II. Preparation and growth measurements of monodisperse salt aerosols. J. Aerosol Sci. 8, 149.

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~,~ k s h o p repor~

'Fang, I. N. (1980) Deliquescence properties and particle size char, ge of hygroscopic: ael::osois. [n (;t.Yiu~..~[icJgi(3!: Aerosols and Facilities for Exposure Experiments {Edited by Willeke. K.), pp. 1513 167 Aim Arbo~ S , . %nr~ Arbor. Wagner, P. E. (1982) Aerosol growth by condensati~:m, ha Aerosol Vlicroph~sws, Vol. 11 (Edi{ed by Mar]~.,,,~. ~" [i L pp. 192-180. Springer, Berlin

8. INFLUENCE OF DIFFUSION INSIDE A MULTICOMPONENT DROPLET ON EVAPORATION AND COMPOSITION R.

KNEER,

M. SCHNEIDERand S. WITTm

Lehrstuhl und Institut fiir Thermische Str6mungsmaschinen, Universit~it Karlsruhe (TH), Karlsruhe, G e r m a n y

1, I N T R O D U C T I O N

The main requirements in the development of modern gas turbines, especially jet engines, are the simultaneous increase of the engine efficiency and the reduction of the emissions. The reduction is strongly coupled to the atomization of the liquid fuel and the combustion process inside the combustion chamber resulting in the demand for a well matched fuel-air mixing to ensure an appropriate evaporation and combustion behaviour. Since experimental investigations become increasingly expensive, the development of theoretical methods for the prediction of the spray evaporation and combustion is of considerable importance. The rate controlling parameter of the combustion process with gas phase reactions is generally the evaporation of the liquid fuel, which may be described by the evaporation behavior of sprays and in its simplest case by the evaporation of single droplets. Up to now, most of the theoretical models describing the fuel droplet evaporation do not account for the multicomponent nature of the fuels used. To illuminate some of the phenomena encountered in the evaporation of a multicomponent droplet, a model for the diffusion controlled evaporation of a binary droplet has been developed. 2. MODEL PRESENTATION AND RESULTS The situation considered is that of an initially cold binary droplet in a hot non-reactive environment. The model describes the heat conduction and diffusion processes inside the droplet by solving the corresponding partial differential equations. The gas phase is assumed to be quasi-steady and therefore an integral formulation is adopted. The assumption of spherical symmetry leads to a one-dimensional problem for the liquid phase although still

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present study

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Heptane / Dodecane Diffusion-Limit Decane

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..... Conduction-Limit Decane - - - Uniform-Temperature

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807

time-dependent. The simultaneous internal transport of heat and mass causes temperature and concentration gradients yielding gradients of the thermophysical properties. Therefore, a formulation of the governing equations has been developecl, considering variable properties (Schneider, 1989; Kneer et al., 1991). In Fig. 2 a plot of the total vapor mass fraction at the drop surface vs time shows a comparison of the results of the model presented and two simpler one-component evaporation models (Conduction-Limit and Uniform Temperature Model). The boundary conditions of the gas phase are given in the diagram. In the initial phase of the evaporation process the diffusion-limit model shows a much higher vapor mass fraction compared to the single component models due to the higher volatility of heptane. The behaviour of the more volatile fuel component described by the more accurate binary droplet model will have a considerable influence on the prediction of the ignition and spray combustion. 3. C O N C L U S I O N S The results obtained by the new variable property diffusion-limit model can be stated as follows. • A comparison with simpler models and also even more complicated models demonstrates the improved characteristics of the new approach. • If the species transport in the gas phase or the ignition are of interest, a variable property formulation of the model should be used. • The model can be applied for the conditions of elevated ambient pressures and temperatures and can be extended for the calculation of evaporating fuel sprays. • Due to the general formulation of the modified diffusion-limit model both evaporation and condensation can be calculated. REFERENCES Kneer, R., Schneider, M. and Wittig, S. (1991) Diffusion controlled evaporation of a multicomponent droplet: Theoretical investigation with variable liquid properties. (In preparation.) Schneider, M. (1989) Theoretische Untersuchung zur Verdunstung eines Mehrkomponenten-Fliissigkeitstropfens, Diplomarbeit, Lehrstuhl und Institut fiir Thermische Str6mungsmaschinen, Universit~itKarlsruhe (TH), o. Prof. S. Wittig.