New Separation Processes: Questions and Answers

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1 New Separation Processes: Questions and Answers Questions 1: GE in general, Phase equilibrium 1. A general process scheme of a SF process is given in the figure. Write down in the figure typical operating conditions for a process with supercritical fluids (e.g. the extraction of caffeine from green coffee beans) and for the process step of precipitation/regeneration, if this step is carried out by pressure reduction only MPa K 5 8 MPa 290 or 310 K 2. What for do we need the knowledge on phase equilibrium in context with a process using supercritical fluids as solvent? The equilibrium state provides information about: the capacity of a supercritical (gaseous) solvent, which is the amount of a substance dissolved by the gaseous solvent at thermodynamic equilibrium, the amount of solvent, which dissolves in the liquid process streams, and the equilibrium composition of the liquid phase; the selectivity of a solvent, which is the ability of a solvent to selectively dissolve one or more compounds, expressed by the separation factor as defined by: yi / y j, 3.1 x / x i j with = separation factor, x i, x j = equilibrium concentrations of component i and j in the liquid phase, in mole or mass fractions, y i, y j = equilibrium concentrations of component i and j in the gaseous phase, in mole or mass fractions, the dependence of these solvent properties on conditions of state (P, T), the extent of the two-phase area, as limiting condition for a two-phase process like gas extraction. 1

2 If capacity and selectivity are known, a good guess can be made about whether a separation problem can be solved with the separation process of gas extraction and on its mode of operation. 3. Draw a rough sketch of a generalized solubility diagram (solubility versus temperature) of a solid in a supercritical solvent (e.g. like SiO 2 in H 2 O, naphthalene in ethylene, caffeine in CO 2 ). Include lines of constant density. Solubility of naphthalene in ethylene. 4. Indicate in the P,x-diagram, shown below, possible operating conditions for a process with a supercritical solvent and possible conditions for regeneration of the solvent and precipitation of the extract. P,x-diagram of oleic acid - ethylene 2

3 5. Explain the characteristic features of the solvent power of a supercritical fluid! Nonpolar supercritical fluid (like CO 2 ) dissolves preferably nonpolar substances. A polar supercritical fluid (like NH 3 ) dissolves preferably polar substances. The solvent power of a nonpolar SF decreases with increasing polarity of the solute. Polar subcritical fluids added to a SF enhance polarity and increase solvent power for polar substances. A supercritical fluid with low critical temperature (like nitrogen) added to a SF will reduce the solvent power. Solvent power of a SF increases with density. Solvent power of a SF is high at supercritical temperatures near the critical temperature. Different SF have a different solvent power for the same substance. 6. In the Figure given below you see typical diagrams for binary systems including the SF region. Indicate the following features in the diagrams: Vapour pressure (VP), critical points (CP), solubility of low volatile compound in the gaseous phase, solubility of SF in the liquid phase. Are the critical points in binary systems with respect to pressure (P,x-diagram), or with respect to temperature (T,x-diagram) always at maximum or minimum condition? P,x-diagrams of n-heptane ethane T,x-diagrams of n-heptane - ethane 7. Separation of a (quasi-) binary system with a SF involves three compounds. Indicate in the ternary diagram given below on the left, the solubility of low volatile compounds in the gaseous phase, the solubility of the SF in the liquid phase, and the probable place of the critical point. Purification of the di-/triglyceride fraction is limited to the extent of the two phase region. Referring to the figure below on the right, indicate what change in operating conditions is necessary in order to being able to purify the di-/triglyceride fraction to higher values than possible with the equilibrium given in the figure on the left! 3

4 Phase diagram of a ternary system at constant pressure and temperature Generalized phase diagram of a ternary system at constant pressure 8. Compounds added to SF influence the solvent power and the solubility behaviour of solutes. Draw a rough sketch for the solubility of a low volatile compound in scco 2 a) if ethanol is added, b) if nitrogen is added. Solubility Ethanol added Nitrogen added Increasing amount of added substance 4

5 F 9. Explain the phase behaviour according to the figure given below if increasing amounts of gas (SF) are added to the initial liquid feed mixture F. 10. For separating mixtures into fractions or pure substances, the separation factor x ij is used to characterise the possibility of a separation. Answer the following questions: Yes No The separation factor does not depend on concentration The separation factor is equal to 1 for x i or x j = 1 The separation factor may have a maximum The separation factor may be > 1 in one region of concentration, and < 1 in another concentration region s fi T s T P T x i lim 0 i i is Ki v, v, reines i i x i 0 i P i P lim K 1 i s 1 f H lim1 x1 12 P H s 1 sf P s 1 s 2 P2 lim0 x1 P 12 s 1 1 s P2 5

6 Questions: Extraction from Solids 11. Draw a rough sketch of an integral extraction curve: amount of extract versus extraction time. Indicate regions of constant mass transfer and of transport resistance in the solid. The extraction of components from solid material is carried out by contacting the solid substrate with a continuous flow of the supercritical solvent. The solid substrate in most cases forms a fixed bed, through which the supercritical gas flows and extracts the product components until the substrate is depleted. For the solid as well as for the solvent, this is an unsteady process. The course of the extraction process can be followed by determining the amount of extract against time of extraction. Internal mass transport resistance Mass transfer rate constant 12. Draw a rough sketch of extraction rate (amount of extraction per unit of time) versus amount of solvent. Indicate regions of constant mass transfer and non stationary mass transfer. Curve 1 represents the extracted quantity per unit of time, the extraction rate, in case of a high initial concentration of extract in the solid substrate, and for an extract which is readily accessible for the solvent. During the first part of the extraction, mass transfer is then constant (at constant operating conditions) and determined by the transition at the interface between the solid and the fluid. The part of the fixed bed of solids, for which constant mass transfer can be assumed, travels with increasing time of extraction through the fixed bed. In the second part of the extraction, two effects may add and cause a declining medium concentration of extract in the outflowing solvent: 1. The extract in the solid substrate near the interface solid/gas is depleted for most of the solid substrate. Transport of the extract within the solid to the interface then adds an additional transport resistance. 6

7 2. The length of the fixed bed containing the initial content of extract is not long enough to enable the maximum loading of the solvent. Curve 2 represents the extraction rate in cases of a low initial concentration of extract in the solid substrate, or an extract not readily available for the solvent, so that transport within the solid to the interface solid substrate-fluid solvent is dominating mass transport from the beginning. Curve 2 also corresponds to the second part of curve 1, since a depletion phase always follows the first extraction phase of constant concentration at the outlet of the supercritical solvent. 13. List the process parameters which influence SF extraction from solids. Explain the influence shortly. P, T, density, solvent ratio, type of solvent, size of particles P: higher pressure, higher solubility. T: higher temperature: higher mass transfer, in most cases also higher solubility. Density: higher density, higher solubility. Solvent ratio: higher solvent ratio: higher mass transport rates, higher amount of extract per unit of time. Size of particles: smaller particles, shorter extraction times. Limit: Hindered flow through fixed bed! 14. List as many important parameters (process parameters plus other parameters which influence extraction. Fluid phase (extract phase) Concentration of the extract downstream of the extraction vessel: accumulated quantity of extract; quantity of extract per unit of time; composition of the extract in dependence of time. Concentration of the extract in the extraction vessel: medium concentration over the total volume; concentration throughout the extraction vessel for plug flow; 7

8 local concentrations considering radial distribution (no backmixing, but no plug flow); local concentrations considering radial and axial distributions (backmixing). Solid phase (raffinate phase) Concentration of the extractible substances in the bulk solid: accumulated depletion of the solid (mean value for the extraction vessel); depletion of the solid related to the remaining content of extractible substances (mean value for the extraction vessel); remaining concentration of extractible substances: radial and axial distribution. Concentration of the extractible substances in single particles: mass transport by diffusion; mass transport resistance by chemical reactions and/or phase transitions; simple geometric particles, complex shape of particles; monodispersity of the solid particles (size), multidispersity of the solid particles (size distribution). Operating parameters Pressure, temperature, density of the fluid, quantity of solvent per unit of time and mass of solid (solvent ratio); chemical composition of the extracting solvent. Pretreatment of the solid size reduction and enlargement of surface; destruction of the plant cells adjustment of the water content; chemical reactions for setting free the extract compounds. 15. Write down a simple differential equation for modeling extraction from solids (quasi steady state approach). What assumption has to be made that integration can be carried out easily? d m dcm m ms, dt dt m = mass of extract components; m s = mass of the solid substrate; c m = mean concentration of extractible components in the solid. The extract is transported within the solid to the interface solid-fluid and from there into the bulk of the fluid: m s Acm c 0, m F Ac0 c. Total mass transfer resistance is given by the equation. below, if no other transport resistances must be taken into account and there is no phase transition at the interface solid-fluid (or the equilibrium distribution coefficient is set to 1) , k s F s F = mass transfer coefficient in the solid phase; = mass transfer coefficient in the fluid phase; 8

9 k c 0 c A = total mass transfer coefficient; = initial mean concentration of extractible components in the solid; = concentration of extracted components in the bulk of the fluid; = mass transfer area. If a phase transition occurs, the equilibrium at the interface has to be taken into account in addition. The differential equation can be integrated if the total mass transfer coefficient k is assumed to be constant. For k = const., Eq. 7.5 for the mean concentration of the extract components in the solid can be obtained: c m c k A exp t. 7.5 c0 c ms c0 16. Write down the general equation for mass transfer for the constant mass transfer rate period. ( Sh =...) Sh 2 1.1Re Sc for 3 Re 3000, with F d u d Sh, Re, Sc, D D F d D G u G = mass transfer coefficient solid-fluid; = diameter of a volume equivalent sphere; = self diffusion coefficient of the fluid; = viscosity of fluid; = linear flow velocity of fluid. G 17. Flow of a (supercritical) solvent through a fixed bed of solids can not be considered as plug flow. Write down the reasons for that deviation. In supercritical solvent extraction from solid substrates an axial dispersion of fluid flow can be observed (often indirectly by the result of the extraction). In a fixed bed, in most cases plug flow is assumed. Since hydrodynamic flow across the fixed bed is not uniform, an axial dispersion is the result. The reasons are: nonuniform distribution of the solvent at the entrance of the extractor; radial distribution of the porosity; radial distribution of the viscosity of the solvent (due to concentration and temperature gradients); non steady or non stabile hydrodynamic flow. 18. Write down at least three industrial commercial applications for the SF extraction from solids. Why are they economical competitive to normal solvent processes? Decaffeination of green coffee beans Decaffeination of black tea leaves Extraction of acids from hops. 9

10 Competitive because solvent cycle is cheap (decaffeination), and because of better extract quantity and quality. 19. Draw a sketch how you can achieve a simulated countercurrent extraction with fixed beds. 20. A customer wants to extract a valuable compound from a plant material. You are the one who is responsible for the offer to be delivered in two weeks time. What data do you need for your design of a SF solids extraction? Product (extract) quantity and quality. Utilities available, and a selection of the parameters of influence to the extraction process: (as given above in question

11 Questions: Gas cycle and solute/solvent separation. 21. Draw a sketch of a gas cycle for supercritical fluid extraction. Name the process elements and write down their function. Flow scheme of solvent cycle in a gas extraction process. 22. Given is a T,s-diagram of CO 2. Draw the process steps of a gas cycle with solute recovery by pressure reduction. Write down the process steps and mark them in the diagram. Compressor process of a solvent circuit of a gas extraction process in a T,s-diagram. 1 = precipitation of extract and regeneration of solvent; 2 3 = isobaric cooling; 3 = extraction; 3 4 = isenthalpic throttling; 4 5/1 = evaporation; 1 2 = isentropic compression. 23. What is the difference between a compressor-process and a pump-process? Can a compressor-process be operated by a pump? 11

12 Caffeine Loading in SCF Phase [mg/kg] Solubility [mg/kg CO 2 ] The main difference in solvent cycles is whether the solvent is cycled in the supercritical (gaseous) or subcritical (liquid) state. In both cycles the solvent can be driven by a pump or by a compressor. Pump cycle: The main difference of this solvent circuit to the compressor cycle is that process steps are carried out in a clockwise direction in a T,s-diagram. 24. In a compressor-process with pressure reduction to subcritical pressure, we need electrical energy (for the compressor) and thermal energy for evaporation. Can we make use of some of this energy (energy recovery!)? Yes. Compression of the gas increases the temperature of the gas. This energy can be used for evaporation. 25. List at least 4 methods for separation solvent andsolute in a supercritical fluid process. P, T, absorption, adsorption, membrane separation, adding an anti-solvent, deentrainment. 26. In the diagram below solubility of caffeine is plotted versus density. What can your do to precipitate caffeine from compressed CO T = 313 K T = 318 K T = 333 K Density [kg/m 3 ] Solubility of Caffeine in supercritical CO 2 as a function of density and temperature 27. In real processes, caffeine is recovered by absorption in water. Why? P = 19 MPa T = 343,1 K T = 323,1 K P = 28 MPa T = 343,1 K Caffeine Loading in Water Phase [mg/kg] Caffeine loading in supercritical fluid phase as a function of caffeine loading in the water phase for different pressures and temperatures 12

13 X [kg/kg AC] 28. Solutes can also be recovered by adsorption. Draw a sketch of a Langmuirisotherm. What loading of caffeine can be achieved by activated carbon at a level of about 100 ppm caffeine in the supercritical solvent CO 2? T=318 K P=13 MPa 0.2 P=20 MPa P=30 MPa 0.1 P=13 MPa Langmuir P=20 MPa Langmuir P=30 MPa Langmuir Y [mg/kg CO 2] Loading of caffeine on activated carbon as function of supercritical fluid loading and pressure 20 to 40 % 29. Explain why adding hydrogen to CO 2 can be used for recovering the solute? The solvent power of supercritical carbon dioxide is drastically reduced by adding hydrogen. 30. Why would a membrane be an ideal means for recovering solutes from supercritical fluids? Explain! From a homogeneous gaseous phase, separation of the extract components is possible with membranes, since the difference in molecular weight between supercritical solvent and the extract compounds is sufficiently high. Typically, the molecular weight of the solvent is about 50 kg/mole, while the molecular weight of the solute compounds is in the range of 200 to 800 kg/mole. Membranes allow the operation of the solvent circuit at low pressure differences of some MPa. Membranes for separation of extract and solvent in gas extraction have been proposed by Gehrig and are under experimental investigation on laboratory scale. 13

14 Questions Countercurrent Separation 31. Draw a rough sketch of a basic flow scheme of a countercurrent separation (column) using a supercritical solvent. Name the various process steps. 32. List the goals of design for a countercurrent separation using a supercritical solvent, give an example of operating conditions. Number of theoretical stages (or number of transfer units), Separation performance (Mass Transfer), Height (Size) of a separation device, Capacity of a separation device, throughput, - diameter. Operationg conditions: Fatty acid esters: P = 120 bar, 70 o C,. S/F = 40. Tocopherol purification: P = 250 bar, T = 80 o C, S/F = List the basic equations for modeling the countercurrent separation using a supercritical solvent. Indicate the parameters which in particular are sensitive within this separation process. Mass balances: dl dvi 0, dz d z Enthalpy balances: i L L, V V. i i H L dh V d i d z V d z q 0. 3 Equilibrium relations: Ki V V i Li. L Rate equations for mass transfer: dvi d z k a P V i Vi, V G i Brunner CC-GE: Basic Equations TUHH-GE 14

15 34. What data are needed for designing a countercurrent separation? List these data or parameters according to the proceeding in the design method. Stress in particular the various aspects of equilibrium data needed. Data on the mixture, data on product specification, production rate. Equilibrium data (Extension two phase area, K-factors, separation factors, all in dependence on concentration of the key components, and the other components. Mass transfer data or height of a theoretical stage or height of a transfer unit. Capacity data: Flooding point, limiting gas velocities, void volume of mass transfer equipment. 35. Short cut methods for binary separations are an excellent tool for getting an insight in a countercurrent separation. Draw a sketch of the McCabe-Thiele method and the Ponchon-Savarit (Jänecke-diagram) as used for countercurrent separations with supercritical fluids. 36. What do we understand as a separation analysis? What are the limiting values of parameters which have to be determined? Determination of number of theoretical stages in dependence on reflux ratio, and in dependence on process conditions (separation factor). Minimum number of theoretical stages (infinite reflux ratio) and minimum reflux ratio (infinite number of theoretical stages). 15

16 37. Most real separations are multicomponent separations. How can you break down a multicomponent mixture suitable for binary process analysis? Individual components, key components, pseudo-components. 38. Mass transfer determines the height of a countercurrent column. What does the height of a theoretical stage mean? Give an example of the height of a theoretical stage. The same for HTU. Height of column to achieve one equilibrium stage. For low viscous liquids: 0.25 m, for high viscous and aqueous systems: 0.5 to 1.0 m. 39. What is a flooding point. Draw a rough sketch of a flooding point diagram and list the principal parameters on the axes. Guess the maximum amount of supercritical solvent which can be flown through a countercurrently operated gravity driven column of 100 mm diameter kgco 2 /(h m 2 ); 46 mm linear gas velocity; 785 kg CO 2 /h (100 mm diameter). 40. List some applications (possible) of countercurrent separation with supercritical gases. What is the main advantage against vacuum distillation? Essential fatty acids (DHA, DPA, DHA, Arachidonic acid etc. Purification and enrichment of vitamins Ethanol purification Deterpenation of citrus oils Main advantage: Low temperature, multistage separation. 16

17 Questions Supercritical Fluid Chromatography (SFC) 41. What is the basic principle of a chromatographic process? Explain the process of elution chromatography. The word chromatography summarises processes by which a separation of a mixture of compounds takes place by repeated redistribution between a mobile phase and a stationary (liquid or solid) phase, while the mobile phase is moving across the surface of the stationary phase. In the elution mode (elution chromatography) the feed mixture is fed to the chromatographic unit as a batch. The compounds of the mixture are detected and/or collected individually and sequentially at the outlet of the chromatographic unit. 42. Explain the characteristic differences between gas chromatography, liquid chromatography and supercritical fluid chromatography. GC: mobile phase is a (normal) gas; the feed mixture is evaporated and mixed with the mobile phase. Phase change, high temperatures, polarity has to be introduced by a modifier (polar compound of high polarity added to mobile phase in low concentration), compounds of low to medium molecular weight can be separated. Separation tuned by temperature ( temperature program ) LC (HPLC): The mobile phase is a liquid or a mixture of liquids. The feed mixture is dissolved in the liquid mobile phase. No phase change, low temperature, mobile phases of high and varying polarity possible, compounds of high molecular weight can be separated. Separation tuned by polarity which is changed by varying the composition of the mobile phase. SFC: The mobile phase is a supercritical fluid. The feed mixture dissolves in the supercritical fluid. Phase change, but low temperature, polarity has to be introduced by a modifier (polar compound of high polarity added to mobile phase in low concentration), compounds of low to high molecular weight (not as haigh as with LC) can be separated. Separation tuned by density (in most cases: pressure at constant temperature). What chromatographic process would you apply for (analytical) chromatographic separation of the following mixtures: 17

18 Hydrocarbons up to 10 carbon atoms: GC Enzymes (polar organic molecules of substantial size): HPLC Total analysis of fats and the by-compounds: SFC 43. What is used as mobile phase in supercritical fluid chromatography? Explain the role of a modifier. The mobile phase is a supercritical fluid. Phase change, but low temperature. Polarity has to be introduced by a modifier. A modifier is a (in most cases) polar compound of high polarity added to mobile phase in low concentration. Separation in SFC is tuned by density (in most cases: pressure at constant temperature). The modifier may also be used for tuning (retention time and resolution). 44. What materials, shape of materials, and sizes of particles are used as a stationary phase in packed column supercritical fluid chromatography? Silica-Polymers, (sometimes metal oxides), spheres, 3 5 micrometer for analytical purposes, micrometer for preparative and production purposes. Silicagel, modified and unmodified. 45. List length and diameter of columns used in supercritical fluid chromatography for analytical, preparative and production purposes. What are the diameters of the particles for the stationary phases? Analytical: L = mm; d = mm. 3 5 micrometer. Preparative: L = mm; d = mm. 20 micrometer. Production: L = < mm; d = 40 - > 100 mm. 20 to 50 micrometer. 46. In packed columns with diameters > 10 mm for supercritical fluid chromatography, the fixed bed of the stationary phases is disturbed by pressure variations. Draw a sketch and describe a solution which avoids this problem. Keep the stationary phase (fixed bed) at a static pressure which is higher than the highest occurring operating pressure. Mechanical solution: Cylinder with piston. 18

19 47. For scale up, the loading of the mobile phase must be enhanced. This results in nonlinear adsorption isotherms. What is the influence of an Langmuir type and an anti- Langmuir type adsorption isotherm in the nonlinear range on the shape of the elution peaks? 48. Elution chromatography is a batch process. A continuously operated countercurrent process would have several advantages. List the principal possibilities to operate fixed bed column chromatography in a continuous mode. What are the advantages, what are the disadvantages? Rotate column, feed and product entries are fixed. Rotate feed and product entries, column is fixed. Simulate movement of the column by switching functionality of the columns or colulmn sections (simulated moving bed). First two possibilities: mechanically difficult, in particular at higher pressure. SMB possible and known up to production scale (preparative scale in SFC). Continuous contercurrent process: Less mobile phase (solvent), higher product concentration, higher productivity. Only binary separation possible. Relatively complicated. Must be controlled by computer which has been programmed by well trained professionals. 49. A continuous chromatographic process can be carried out by a simulated moving bed operation. How many zones does such an operation need? How many columns are usually applied? 4 zones: 2 separation zones (enriching or extraction zone, stripping or raffinate zone), 2 desorption or cleaning zones. Usually 2 columns per zone: 8 columns as minimum. 19

20 Considering separation of a multicomponent mixture, what is the difference between batch elution chromatography and continuous simulated moving bed chromatograhy? Batch elution chromatography can separate a multicomponent mixture into individual components in one run, while continuous chromatography can only perform a binary separation. 50. Separation processes normally can be characterised by data specific for a special process. In chromatography, specific productivity and cost are the most interesting parameters. What are these values? Specific productivity SFC: 1.5 g of product per liter of stationary phase per hour. Costs are in the range of 200 to 500 DM per kg of product for low quantities (up to a few tons per year), but are much lower for bulk products (thousands of tons per year). Scaled up costs usually decline by a power law with an exponent of 2/3. (Careful: This is only a rule of thumb and must be checked for the individual case!). Questions: Membrane Separation (Gas Permeation) 51. Explain the mechanism of transport for organic liquid and gaseous compounds through polymeric membranes. Why is a separation of two different components possible? Separation: Different transport velocities through the membranes. Mechanism of transport (model): Solution-diffusion-mechanism. Gaseous (liquid) compound dissolves in membrane - compound diffuses through membrane - compound desorbes from membrane. 52. Write down the transport equations (basic differential eq. [Fick s law], integral transport equation for solute and solvent). c c D, (one dimensional in x-direction); t x x D diffusion coefficient m 2 s Steady state: c 0 t ; dc Integrated in x : j D ; (1), dx D j ( C 1 C2 ) (2) Z Henry s law: C D H ( 2 ; Z H p : j p 1 p ) 20

21 P With permeability coefficient P = D H: j ( p 1 p2 ) Z 53. Transport through composite membranes: p j Pi X i i p P L 54. What properties of the membranes are essential for separation of gases? High selectivity (20-40), Applicable for high pressures and high pressure differences, Maintaining properties for a long time. 55. What advantages can be expected from a membrane process for gas separation? Lower amount of energy needed, Easier to control, Various separation principles of the membranes (size, shape, polarity, chemical interactions), 56. What assumptions are made for modeling the transport through membranes? Thermodynamic equilibrium at the membrane surfaces, Henry s law applicable, Diffusion coefficient constant. Questions: Separation by (enzymatically) catalysed reactions 57. Compare the reaction rates of enzymatically catalysed reactions in aqueous and non aqueous systems. What measures can be taken to enhance the reaction rate in non aqueous systems? Reaction rates in non aqueous systems are lower by a factor of about three orders of magnitude. Enhance temperature (if possible, e.g. with Lipase Novozym 435 up to > 100 o C). Enhance pressure (only applicable if volume of transition complex is lower than the volume of the educts). 58. Why are reactions in a non aqueous environment useful? Hydrolysis of products in aqueous environment. 59.Do you know of a process in competition with enzymatically catalysed reactions for separation of enantiomenrs? 21

22 Chromatography, in particular simulated moving bed chromatograhy. 60. Do you know something about the stability of enzymes at elevated pressure: Hydrostatic pressure (aqueous environment, no pressure fluctuations); Gas pressure (CO2) with and without pressure fluctuations; Many enzymes can endure very high hydrostatic pressures up to 300 to 400 MPa. Enzymes are stable up to several hundred bars of static gas pressure. They tend to be inactivated by pressure changes. 61. What is the basic principle of the separation of enantiomers by enzymatically catalysed reactions? Give an example! One isomer reacts preferently with an educt compound which is added to the educt mixture in order to yield a product compound with the reacting isomer which can be separated much easier from the initial educt isomer. 62. What is essential for a competitive and an economic favourable enzymatically catalysed reaction for separation? The separation of the resulting product mixture must be much easier than the separation of the isomeric educt mixture. 63. Why has the reaction in scco 2 the potential of being competitive and economic? By means of scco 2 the reacted isomer in the product mixture or the non reacting isomeric compound in the educt mixture may be easily removed (dissolved, removed from the reactor and precipitated or recycled - in the simplest case). 22