Topic 10: design of packed columns Flashcards
are liquid and vapourflow rates fixed?
the required liquid and vapour flow rates are typically fixed by the process specifications and the equilibrium curve
what do superficial linear velocities depend on?
the superficial linear velocities of the phases scale inversely with column area Ac
what is the Ac chosen based on
ac should be chosen to satisfy additional design criteria relating to;
liquid hold up in the column
avoidance of flooding
attainment of an acceptable mass transfer efficiency
overall column productivity (not overdesigning to column)
compare pressure drops in packed columns and plate columns
pressure drops are smaller in packed columns than in plate columns
what are the flow parameters defined for the region of satisfactory operation?
FL=pL^(1/2)uL=Lw/(ApL^(1/2))
Fv==pv^(1/2)uv=Vw/(Apv^(1/2))
uL,uv are the superficial vapour and liquid velocities
what is low gas velocity?
uv=0 apprix.
film thickness determined by liquid load only
what is loading gas velocity
uv,load = 0.7 .uv,fl approx.
veloxity vanishes at the liquid-gas interface, liquid filmstarts to thicken
what is flooding gas velocity?
uv=uv,fl
velocity gradient vanishes at the packing-liquid interface, liquid hold-up rises exponentially, mass transfer efficiency falls to zero
what are indicators of hydraulic performance?
liquid hold up hL - defined as the fraction of the packing void space filled by liquid
pressure gradient dP/dz, in the vertical direction
loading and flooding points
what are indicators of mass transfer efficiency?
height of a theoretical transfer unit HTU, HOG, for common distillation, absorption or stripping processes
height equivalent of a theoretical plate HETP, HTP, for absorption or stripping of dilute solutes or distillation of ‘standard’ test mixtures
what graphs are available for parameters that depend upon gas and liquid flow rates and their relation to loading and flooding points of the column
liquid hold up hL (linear axes)
pressure gradient dP/dz (log log axes)
mass transfer efficiency (linear axes)
flodding curve (log log axes)
when does the loading point occur?
loading point occurs at about 70% of flooding vapour load
what is the generalised pressure drop correlation (GPDC)
a widely used alternative design tool for packed columns
plot gives the capacity parameter K ( is directly proprtional to the vapour flow parameter Fv) as a function of the flow parameter Flv for various pressure gradients dP/dz
pressure gradient at flooding also cprrelated with Fp
K is the capacity factor not equilibrium ratio
what is the generalized pressure drop correlation GPDC method
- compute Flv from the mass flow rates Vw and Lw
- compute the pressure gradient at flooding (dP/dz)fl
- either choose a desired pressure drop (dP/dx), (dP/dz)fl and read off K or choose a desired K
what can packing-specific mass transfer charts be used for?
data like these can be used for rough comparisons between different packing types
detailed mass transfer and hydraulic modelling is preferred for process design
what design margin is typically applied to a process design?
a design margin of 20% to 50% is typically applied to the packing height
describe the 8 components of a packed column configuration
feed pipe to liquid distrubutor liquid distributor structured packing structured packing support packing hold down grid random packing random packing support gas/vapour inlet
name some column internals
slot type liquid distributor, pan typw liquid distributor, spray type liquid distributor, random packing support, structured packing support, mist eliminator, gas inletm splash plate gas inlet, two phase inlet
what are the process specifications for packed bed distillation
zF feed composition q feed state F feed flow rate P column pressure xD distillate composition xB bottoms composition R/Rmin reflux requirements
what are the process specifications for packed bed absorption?
YB feed composition V' feed flow rate T process temperature P process pressure YT product composition XT solvent composition L'/L'min solvent flow ratio
what are the process specifications for packed bed stripping?
XT feed composition L' feed flow rate T process temperature P process pressure XB product composition YB gas composition V'/V'min gas flow ratio
what is the column design workflow
- assemble design specifications
- calculate L/V or L’/V’ based on R/Rmin, L’/L’min or V’/V’min
- specify the vapour load as a fraction of flooding
- calculate the required column diameter
- obtain KOG from a detailed mass transfer model, prior experience or pilot-scale experimetns. hence obtain HOG
- calculate NOG by graphical or analytical method as appropriate
- calculate the required height of packing: H=HOG.NOG
- specify all column internals
- determine the pressure gradient and total column pressure drop
for distillation columns size each section separately
what factors favour choosing a packed column?
wide choice of packing materials (important in corrosive service)
lower pressure drop (important in vacuum distillation)
less liquid entrainment
low liquid hold-up, especially suitable for thermally-sensitive fluids
foaming liquids handled more readily
higher capacity and often higher mass-transfer efficiency
what factors are against choosing a packed columnn?
weight of packing can be problematic
difficulty of cleaning packing
good liquid distribution critical
what factors favour choosing plate columns?
fouling service ( plate columns are easier to clean)
robust construction less affected by temperature extremes
ability to install cooling coils inside the columns in exothermic absorption
high liquid residence time possible
very low liquid load possible
taller columns possible
what factors are agaimst plate columns?
higher pressure drop
less able to handle foaming systems
what is the benefit of GPDC?
the GPDC can be used as a quick alternative to detailed models to detemine the required column diameter
