Control of gases and vapors

1. Control of gases and vapors • Major portion of the anthropogenic pollutants are gases. Colls, 2002

2. Percent contributions of different processes to the total EU emissions

3. Methods for gas control

4. ADSORPTION PHENOMENON • Adsorption is based on the capability of porous solids with large surfaces such as silica gel to selectively retain and relase compounds on the surface of the solid • Two phenomena are recognized in adsorption • Physical adsorption (van der Walls, low temperature) • Chemisorption (strong forces, high temperatures)

5. Intermolecular Forces • … are forces between molecules. • They determine melting points, freezing points, and other physical properties. • Types of intermolecular forces (van der Waals) include: • dispersion forces • Dipole–dipole forces • hydrogen bonding

6. Dispersion Forces • … exist between any two particles. • Also called London forces (after Fritz London, who offered a theoretical explanation of these forces in 1928). • Dispersion forces arise because the electron cloud is not perfectly uniform. • Tiny, momentary dipole moments can exist even in nonpolar molecules.

7. Dispersion Forces Illustrated At a given instant, electron density, even in a nonpolar molecule like this one, is not perfectly uniform.

8. Dispersion Forces Illustrated … the other end of the molecule is slightly (+). The region of (momentary) higher electron density attains a small (–) charge … When another nonpolar molecule approaches …

9. Dispersion Forces Illustrated … this molecule induces a tiny dipole moment … … in this molecule. Opposite charges ________.

10. Strength of Dispersion Forces • Dispersion force strength depends on polarizability: the ease with which the electron cloud is distorted by an external electrical field. • The greater the polarizability of molecules, the stronger the dispersion forces between them. • Polarizability in turn depends on molecular size and shape. • Heavier molecule => more electrons => a more- polarizable molecule. • As to molecular shape …

11. Molecular Shape and Polarizability … can have greater separation of charge along its length. Stronger forces of attraction, meaning … Long skinny molecule … … higher boiling point. … giving weaker dispersion forces and a lower boiling point. In the compact isomer, less possible separation of charge …

12. Dipole–Dipole Forces • A polar molecule has a positively charged “end” (δ+) and a negatively charged “end” (δ–). • When molecules come close to one another, repulsions occur between like-charged regions of dipoles. Opposite charges tend to attract one another. • The more polar a molecule, the more pronounced is the effect of dipole–dipole forces on physical properties.

13. Opposites attract! Dipole–Dipole Interactions

14. Adsorption • The large specific surface area is essential requirement for good adsorption. • Commonly used adsorbers such as silica gel and activated charcoal have surfave areas of up to 2km2/kg and adsorption capacities of up to 0.5 kg/kg.

15. Adsorption vs absorption Adsorption is a surface phenomena Physical adsorption chemisorption

16. Chemisorption • thephenomenon is characterizedbychemicalspecificity; • changes in theelectronicstatemay be detectablebysuitablephysicalmeans (e.g. u.v., infraredormicrowavespectroscopy, electricalconductivity, magneticsusceptibility); • thechemicalnature of theadsorptive(s) may be alteredbysurfacedissociationorreaction in such a waythat on desorptiontheoriginalspeciescannot be recovered; in this sense chemisorptionmay not be reversible; • theenergy of chemisorption is of thesameorder of magnitude as theenergychange in a chemicalreactionbetween a solidand a fluid: thuschemisorption, likechemicalreactions in general, may be exothermicorendothermicandthemagnitudes of theenergychangesmayrangefromverysmalltoverylarge; • theelementary step in chemisorptionofteninvolves an activationenergy; • wheretheactivationenergyforadsorption is large (activatedadsorption), trueequilibriummay be achievedslowlyor in practice not at all. • since theadsorbedmoleculesarelinkedtothesurfaceby valence bonds, theywillusuallyoccupycertainadsorptionsites on thesurfaceandonlyonelayer of chemisorbedmolecules is formed (monolayeradsorption)

17. PURIFICATION OF AIR, WATER AND OFF GAS · SOLVENT RECOVERY Activated Carbon for Solvent Recovery K. -D. Henning J. Degel Paper presented at the Meeting of the European Rotogravure Association Engineers GroupMulhouse/France. 20/21 March 1990

18. Adsorption is a useful technique when: • The pollutant gas is noncombustible or difficult to burn • The pollutant is sufficiently valuable to warrant recovery • The pollutant is in very dilute concentration in the exhaust system

19. The adsorbent must posses appropriate engineering properties • It must not offer too great pressure drop • Large surface area • Nor must be easily carried away in the flowing stream • Adequate strength • Not easily crushed

20. Differences between physical adsorption and chemisorption • Forces involved are different Physical adsorption:van der Waals forces (surface phenomena)=condensation Chemisorption:chemical reactions • The strength of binding between adsorbent and adsorbate Physical adsorption:van der Waals forces=heat of condensation Chemisorption:heat of reactions • Specificity of the process Physical adsorption:between any surface and any gas at sufficiently low temperatures Chemisorption:demands chemical affinity between adsorbent and adsorbate

21. Differences between physical adsorption and chemisorption • Rate Physical adsorption:Rate of adsorption is rapid Chemisorption: Energy of activation must be supplied before adsorbent-adsorbate complex can form • Physical adsorption:Multimolecular adsorption Chemisorption:Uni molecular adsorption • Physical adsorption:reversible Chemisorption:Not easy to remove chemisorbed material • Increase in TEMPERATURE decrease adsorpion but increase chemisorption

22. Mono vs multi layer • In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. • In multilayer adsorption the adsorption space accommodates more than one layer of molecules and not all adsorbed molecules are in contact with the surface layer of the adsorbent.

26. Selection or design of an adsorption equipment • Sufficientdwell time • Pretreatment of thegasstreamtoremovenonadsorbablematter • Pretreatmenttoremovehighconcentrations of competinggases • Gooddistribution of bed • Regenerationorrenewingadsorbentbedaftersaturation

27. Regenerative or nonregenerative? • If the concentration of pollutant is low in gas stream (1 or 2 ppm) adsorbent and absorbed material are discarded ıf the adsorbed material is non volatile • If the pollutant concentration is high (several ppm) regenerative type multiple systems might be used

28. Two unit fixed bed adsorber • Figure 6.1

29. Adsorption data • The amount of gas adsorbed per gram of adsorbent at equilibrium is a function of • Temperature • Pressure • Nature of adsorbent • Nature of adsorbate

30. Adsorption isotherm The plot of amount of adsorbed gas (a) versus pressure is an adsorption isotherm at constant temperature. a=f(P)

32. I= intercept S= slope

33. Adsorption isotherms of voc compounds

34. Heel effect • During regeneration trace amount of adsorbate retains in the absorbent (carbon) and this decreases the effective adsorption capacity. • In general activated carbon adsorption bed reach capacty at 30 to 40 percent of the equilibrium adsorption capacity.

35. Breakthrough point The concentration of pollutant at breakthrough point shold not exceed the emission standard

36. Regeneration of an adsorption bed • When breakthrough is reached flow should be directed to another unsaturated bed and saturated bed proceed with regeneration. • For regeneration purposes • Atmospheric air can be used (cold regeneration) • Steam or hot air can be used (hot regeneration). However bed must be cooled after regeneration before adsorption starts again.

37. Regeneration with steam

38. Absorption • Gas stream is in contact with absorbing solution by means of bubbling or spraying. • Absorption may be either chemical (reaction) or physical (by dissolution) • Water, mineral oils and aqueous solutions can be used as solvents. • HCl, H2SO4, HCN, H2S, NH3, Cl2, organic vapors (formaldehyde, ethylene, benzene etc) are commonly controlled by absorption. • Absorption process depends on the equilibrium between gas and the liquid and mass transfer between two.

39. Absorption Mass of pollutant needs to be transfered from one phase (gaseous) to another (liquid). Concentration gradients at the liquid gas interface is the driving force. Two phases must be in contact The mass transfer may take place with both streams flowing in the same direction (concurrent or co-current flow) OR Two streams may flow in opposite directions (countercurrent flow)

40. Absorption Absorption is widely used as a product-recovery method in chemical and petroleum industry. As an emission control technique, it is more commonly used for inorganic gases. HCl vapor in water Mercury vapor in brine and hypochlorite solution Hydrogen sulfide vapor in sodium carbonate and water Chlorine gas in alkali solution In all cases a suitable solvent which can be easily treated after it leave the process is desired.

41. Counter current flow Counter current flow

42. Combined counter current and co current operation

43. Mass transfer of gaseous pollutants to the liquid Is proportional to • Liquid-gas interface surface area Therefore absorber units should provide large surface liquid area with a minimum of gas pressure drop

44. Inert packing materials

45. What happens to clean gas and effluent liquid • Clean gas is vented through a stack to the atmosphere • The liquid leaving the absorber is either stripped of the contaminant gas Waste treatment or process use.