PRODUCT ISOLATION & CONCENTRATION (PART II)

1. PRODUCT ISOLATION & CONCENTRATION (PART II) ERT 320/ 3 BIO-SEPARATION ENGINEERING MISS WAN KHAIRUNNISA WAN RAMLI

2. Envochem Activated carbon Adsorption unit ADSORPTION

3. INTRODUCTION • ADSORPTION  molecules/ atoms/ ions in a gas/ liquid diffuse to the surface of a solid, where they bond with the solid surfaces or are held there by weak intermolecular forces. • ADSORPTION  based on the differential affinity of various soluble molecules for specific type of solids • Equilibrium is approached between a solid phase (stationary phase) and the soluble molecules in the liquid phase (mobile phase). • STATIONARY PHASE  refer to solid phase, packed in the fixed column • MOBILE PHASE  refer to liquid phase, flowing past the stationary solid phase • ADSORBATE  The adsorbed solutes, ADSORBENT  The solid material

5. INDUSTRIAL APPLICATIONS

6. ADSORBENT ACTIVATED CARBON • Porous material:Large surface area per unit mass,internal surface area greater than the external surface area,often 500 to 1000 m2/g. • Granular (50μm - 12 mm diameter), small pellets or beads • Suitable for packed bed use • Activated carbon, silica gel, alumina, zeolites, clay minerals, ion exchange resins • Separation occurs because differences in molecular weight, shape or polarity of components • Rate of mass transfer is dependent on the void fraction within the pores ZEOLITE

7. SILICA MICROSTRUCTURE ZEOLITE STRUCTURE DIFFERENT IN FORMULATION TO ACHIEVE SUITABLE STRUCTURE FOR HIGHER ADSORPTION RATE FOR DIFFERENT PROCESS

8. CLASSIFICATION

9. EQUIPMENT CONFIGURATIONS FIXED BED ADSORBER • Widely used with both liquid and gas feeds • Adsorbent particles size ranges from 0.05-1.2 cm • Factors to determine optimal particle size: Bed pressure drop, Solute transport rate • The flow of liquid/ gas feed always downward: to avoid jiggling or fluidizing the bed during adsorption process • The feed is switched to other bed when the concentration of solute in exit gas reaches a certain value. • The bed is regenerate by steam / hot inert gas.

10. REGENERATION METHOD THERMAL SWING ADSORPTION (TSA) • Adsorbent is regenerated by DESORPTION at T higher than that used during the adsorption step • T is increased commonly by heat transfer from inert, purge gas: i.e. steam • The bed is then cooled before the adsorption step is resumed • DISADV: heating & cooling takes time: hours to days • Only PRACTICAL for purification involving small rates of adsorption PRESSURE SWING ADSORPTION (PSA) • Adsorption occurs at elevated pressure whereas desorption (regeneration of adsorbent) occurs at near-ambient pressure • Used for bulk separation • The bed can rapidly pressurized & depressurized (sec to min)

11. ADSORPTION FROM LIQUID • Use of activated carbon to remove pollutants from aqueous wastes • Use carbon beds up to 10 m tall, several ft in diameter, several bed operating in parallel. • Tall beds are needed to ensure adequate treatment

12. ADSORPTION EQUILIBRIUM • A dynamic phase equilibrium is established for the distribution of the solute between the fluid & the solid surface • Expressed in terms of: concentration [mg/l]/ partial pressure of the adsorbate in fluid, solute loading on the adsorbent • ADSORPTION ISOTHERMS  equilibrium relationship between the concentration in the fluid phase and the concentration in the adsorbent particles • Limits the extent to which a solute is adsorbed from a given fluid mixture on an adsorbent of given chemical compositions for a given set of conditions

13. ADSORPTION ISOTHERMS FREUNDLICH LANGMUIR 5 types of adsorption isotherms for Pure gases Linear Isotherms: Adsorption amount proportional to the concentration in the fluid Irreversible Isotherms: Independent of concentration

14. DESIRABLE ISOTHERMS: strong adsorption TYPE I  unimolecular adsorption, characterized by a maximum limit on the amount absorbed, applies often to gases at T above Tcritical TYPE II  multimolecular adsorption, for gases at T below Tcritical & for P below & approaching Psaturation (Pvapor) UNDESIRABLE TYPE III  convex nature, adsorption is low except at high P, Eg: Adsorption of iodine vapor on silica gel CAPILLARY CONDENSATION TYPE IV  capillary condensation of TYPE II TYPE V  capillary condensation of TYPE III

15. ADSORPTION EQUILIBRIUM • Work by the differential adsorption of species to a resin surface, ligands, from a complex chemical mixture • The adsorption of a chemical species can be represented by the equilibrium reaction • C = dissolved chemical species, S = adsorption site & CS = chemical bound to the adsorption site, Keq = equilibrium constant governing the reaction. • The equilibrium constant, Keq for this adsorption is • In many cases, the concentration of the adsorption site is very much larger than the concentration of dissolved chemical species ([S] >> [C]), thus the equilibrium expression becomes

16. LINEAR EQUILIBRIUM • Or • LINEAR EQUILIBRIUM concentration of adsorbed species can be expressed as the multiple of the concentration of dissolved chemical species. • There is NO saturation limit of adsorption site will be reached. • This linear isotherm approximation is less practical in industrial scale adsorption. The most efficient process uses the entire adsorption site available, which the concentration of empty adsorption sites available cannot be ignored. • The total number of adsorption sites is measureable by defining the total site concentration, Stot: • By using the eqn for Stotand the linear approximation gives the LANGMUIR ISOTHERMS: LANGMUIR ISOTHERMS

17. LANGMUIR ISOTHERMS • Restricted to TYPE I isotherms • Often been used to correlate equilibrium adsorption data for protein. • Assumed chemisorption, the surface of the pores of the adsorbent is homogeneous, negligible interaction forces between adsorbed molecules • Isotherms that concave downward are considered to be favourable. • Where: [CS] = concentration of adsorbed species, [C] = concentration of dissolved species, Keq = equilibrium constant, Stot = the total no of adsorption sites • Keq[C] >> 1  the adsorption sites are saturated. • Langmuir isotherm  concave downward: having a linear slope in low concentration limit & plateau as the adsorption site becomes saturated

18. FREUNDLICH ISOTHERMS • Describe the adsorption of variety of antibiotics, steroids and hormones • High adsorption at low fluid concentration • Equation is empirical & nonlinear in P: • n > 1 • With n > 1, the isotherm is concave downward • Result of energetic heterogeneity of the surface of the adsorbent • Linearizethe equation: • Log [CS] = Keq+ (1/n) log [C] • Constant determined from experimental data by plotting log [CS]versus log [C] • Slope = 1/n, intercept = Keq

19. BASIC PRINCIPLES FIXED BED ADSORPTION • Possible to obtain nearly solute-free liquid/ gas effluent until the adsorbent in the bed approaches saturation • In fixed bed adsorption, the concentrations in the fluid phase and the solid phase change with time & the position in the bed. • At first, most of the mass transfer takes place near the inlet of the bed. • The fluid make contacts with the adsorbent. • After a few minutes, the solid near the inlet is nearly saturated. • Most of the mass transfer takes place farther from the inlet. • The concentration gradient become S-shaped.

20. MASS BALANCE To understand the dynamics of a fixed bed adsorption column, a mass balance is performed by considering a disk of cross sectional area (A) equivalent to that the column, but differ in thickness (Δx). A species to be separated (separand) flows in and out of the disk by convection and the combined effects of molecular diffusion and mechanical dispersion. • Convection rate into the disk  the interstitial velocity, the velocity of fluid in the void fraction, Ɛ. Convection rate = superficial velocity, vo • Within the volume of the disk, the separand may accumulate with both the mobile & stationary phases. • The mass balance of separand I can be written a follows: • (Rate of separand in)-(rate of separand out) = (rate of separand Accumulation) MASS TRANSFER of solute to average sorbent particle EQ. 1

21. ASSUMPTIONS • local equilibrium & negligible dispersion This allows us to focus on the velocity, ui at which the solute travel across the column. Using the equilibrium isotherm relationship, qi=f(ci), gives the mass balance of: • Where qi’ (ci) is the slope of the equilibrium isotherm at ci. • If the effective velocity of component i in the packed bed, ui Then, • This shows that effective velocity, ui is independent of concentration but inversely proportional to Keq,i. • However, for industrial processing, where the process desired to use high adsorbent loadings, the equilibrium is nonlinear and Langmuir isotherm is applicable. • For Langmuir isotherm, the shock wave velocity, ui,sh • is calculated: q’ (ci) = slope of [CS] vs [C] , then Δq = [CS]-[CS]o Δc = [C]-[C]o

22. ASSUMPTIONS • linear equilibrium isotherm & negligible dispersion The following equations described the adsorption assuming a linear adsorption isotherm, a linear driving force for the mass transfer rate, and negligible dispersion: • If axial dispersion is neglected (Deff = 0) and the velocity is constant, the equation can be expressed as a breakthrough curve is given by Klinkerberg (1948 dan 1954) as Where,cf is the feed concentration of the component and

23. The component concentration profile in the gas is shown in Figure 10. The breakthrough curve is given in Figure 11. Figure 11 The component gas concentration profile in the adsorber at different times.

24. Concentration-distance profiles t1 t2 Lf Ls At t1, no part of the bed is saturated At t2, the bed is almost saturated Mass transfer zone  Ls – Lf, where adsorption process occurs

25. BREAKTHROUGH CURVE • Prior tb, the outlet solute concentration is < than permissible value of 0.05 • At tb, this value is reached, adsorption step discontinued & regeneration of adsorbent initiated • t>tb, the outlet solute concentration would rise rapidly, approaching the inlet concentration as the outlet end become saturated • Steepness of breakthrough curve  capacity of adsorbent bed that can be utilized • Thus, shape of the curve is IMPORTANT to determine the length of the bed 0.5 t* tb = MTZ just reaches the end of the bed = BREAKTHROUGH POINT t* = the ideal adsorption time for a vertical breakthrough curve = the time when c/co reaches 0.50 Amount of adsorbed is proportional to the rectangular area to the left of the dashed line at t*

26. Scale up The scale up for a fixed bed adsorption focuses on the breakthrough curve for a single column. The Length of unused bed (LUB) method allows scale up based on data from laboratory columns, keeping the particle size & velocity constant. LUB METHOD It is necessary to define the break point time, tb & the adsorption time, t* on a breakthrough curve The break-point time, tb is usually taken at relative concentration Φ of 0.05-0.1. The ideal adsorption time  the time for breakthrough that would occur if the solute were in perfect equilibrium with the bed of adsorbent, which would give a vertical breakthrough curve. At Φ=0.5. The ideal adsorption time is given by: The amount of solute adsorbed at break point can be determined by integrating the breakthrough curve up to tb. ρb = bulk density of adsorbent qi,sat = weight of solute i/ weight of adsorbent

27. The width of the breakthrough curve defines the width of the mass transfer zone in the bed. The LENGTH OF UNUSED BED (LUB) is defined as: Where tb & t* are stoichiometric times determined by integration of the breakthrough curve: The length of column = LUB + Length assuming local equilibrium with shock wave concentration front.

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