ABSORPTION AND STRIPPING IN PACKED TOWERS column is filled with an inert packing material packing →large surface area per unit of volume, wetted by liquid as completely as possible. Internals must offer minimum resistance to gas flow. designed for maximum mass transfer between the vapour and the liquid and for low-pressure drop. The vapour and liquid compositions vary continuously with packing height rather than discretely as in trayed columns.
Advantages of packed columns For corrosive liquids cheaper than plate column. liquid hold-up is lower than plate column - important bec. inventory of toxic or flammable liquids needs to be kept as small as possible for safety reasons. more suitable for handling foaming systems. pressure drop can be lower for packing than plates; and should be considered for vacuum columns. Packing should always be considered for small diameter columns, less than 0.6 m, where plates would be difficult to install, and expensive. Packing materials: 1) random (dumped) packings, and 2) structured (arranged, ordered, or stacked) packings
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Materials: metal plastic ceramic Choice depend on the corrosiveness of system and cost Provide large interfacial surface between liquid and gas Possess desirable fluid-flow characteristics Be chemically inert to fluids being processed Have structural strength to permit easy handling and installation low cost
random (dumped) packings
structured packings
• Low pressure drop for the gas • Greater possible fluid flow rates • Usually at the expense of more costly installation than random packings
In designing a packed tower once the packing has been chosen it is necessary to know the column diameter and the height of packing needed. The column diameter is sized on the basis of either the approach to flooding or the acceptable pressure drop. Methods for Packing Height (Z) Packing height can be found either from an equilibrium stage analysis or from mass transfer considerations. The equilibrium stage analysis using the height equivalent to a theoretical plate (HETP) procedure
HETP - approach In practice, packed columns are often analyzed on the basis of equivalent equilibrium stages using a Height Equivalent to a Theoretical Plate (HETP):
Knowing the value of the HETP and the theoretical number of stages n of a trayed column, we can easily calculate the height Z of the column : Z = HETP x n Height Equivalent to a Theoretical Plate Represents the height of packing that gives similar separation to as a theoretical stage. HETP values are provided for each type of packing The HETP concept has no theoretical basis. HETP values can only be calculated using experimental data from laboratory or commercial-size columns.
MASS TRANSFER APPROACH (HTU, NTU) Preferable to determine packed height from a more theoretically based method using mass transfer coefficients. Interphase Mass Transfer Theory mass transfer from the bulk of one phase to the interphase surface and then from the interphase to the bulk of another phase is called interphase mass transfer. Example absorption of SO2 from air by water. SO2 diffuses through air, then passes through the interface between air and water, finally diffuses through the adjacent immiscible water phase. mass transfer occurs in each phase because of concentration gradient till an equilibrium state exists at the interface between the phases
Two-film Resistance Theory Consider mass transfer of solute from the bulk of a gas phase to the bulk of a liquid phase. This can be shown graphically in terms of distance through the phases as shown.
conc. of solute in main body of the gas is y mole fraction, falls to yI at the interface. In the liquid, the conc falls from xI at interface to x in the bulk. No resistance to solute transfer across the interface separating the phases. Only diffusional resistances are residing in the fluids. The equilibrium concs yI and xI are obtained from the system’s equilibrium distribution curve. This called the “two-resistance theory”. For steady state mass transfer, rate at which solute reaches the interface from the gas must be equal to the rate at which it diffuses to the bulk liquid. Therefore the mass transfer flux of solute in terms of k, the volumetric mass transfer film coefficient for each phase, can be written as: r = mass transfer rate per unit volume, mol/(m3·s) ky = gas phase mass transfer coefficient a = surface area per unit volume of packing
rearranging: relative resistance of mass transfer between the two phases
a straight line of slope -kxa/kya, drawn from the operating line at point (y,x) intersects the equilibrium curve at (yI,xI). The slope -kxa/kya determines the relative resistances of the two phases to mass transfer. AE is the gas-phase driving force (y-yI), while AF is the liquid-phase driving force (xI-x). If resistance in gas phase is very low, yI ≈ y → resistance resides entirely in the liquid phase, absorption of a slightly soluble solute in the liquid phase, liquid-film controlling process. If resistance in liquid phase is very low, xI ≈ x, absorption of a very soluble solute in the liquid phase, gas-film controlling process. Important to know which of the two resistances is controlling so that its rate of mass transfer can be increased by promoting turbulence in and/or increasing the dispersion of the controlling phase.
Overall mass transfer coefficients mass transfer film coefficients ky and kx difficult to measure the overall mass transfer coefficients Ky and Kx are measured on the basis of the gas phase or the liquid phase. The entire two-phase mass transfer effect can then be measured in terms of gas or liquid phase molar fraction driving force as: Ky = overall driving force for the gas phase, mole/m2.s and y* is the fictitious vapour mole fraction in equilibrium with the mole fraction, x, in the bulk liquid; and x*is the fictitious liquid mole fraction in equilibrium with the mole fraction, y, in the bulk vapour. combining, overall coefficients can be expressed in terms of separate phase coefficients: for dilute solutions when the equilibrium curve is a nearly straight line through the origin,
where Henry’s law is applicable K= H/P
