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Separation Processes 1 Resit > Topic 10: design of packed columns > Flashcards

Topic 10: design of packed columns Flashcards

1
Q

are liquid and vapourflow rates fixed?

A

the required liquid and vapour flow rates are typically fixed by the process specifications and the equilibrium curve

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2
Q

what do superficial linear velocities depend on?

A

the superficial linear velocities of the phases scale inversely with column area Ac

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3
Q

what is the Ac chosen based on

A

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)

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4
Q

compare pressure drops in packed columns and plate columns

A

pressure drops are smaller in packed columns than in plate columns

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5
Q

what are the flow parameters defined for the region of satisfactory operation?

A

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

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6
Q

what is low gas velocity?

A

uv=0 apprix.

film thickness determined by liquid load only

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7
Q

what is loading gas velocity

A

uv,load = 0.7 .uv,fl approx.

veloxity vanishes at the liquid-gas interface, liquid filmstarts to thicken

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8
Q

what is flooding gas velocity?

A

uv=uv,fl

velocity gradient vanishes at the packing-liquid interface, liquid hold-up rises exponentially, mass transfer efficiency falls to zero

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9
Q

what are indicators of hydraulic performance?

A

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

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10
Q

what are indicators of mass transfer efficiency?

A

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

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11
Q

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

A

liquid hold up hL (linear axes)
pressure gradient dP/dz (log log axes)
mass transfer efficiency (linear axes)
flodding curve (log log axes)

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12
Q

when does the loading point occur?

A

loading point occurs at about 70% of flooding vapour load

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13
Q

what is the generalised pressure drop correlation (GPDC)

A

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

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14
Q

what is the generalized pressure drop correlation GPDC method

A
  1. compute Flv from the mass flow rates Vw and Lw
  2. compute the pressure gradient at flooding (dP/dz)fl
  3. either choose a desired pressure drop (dP/dx), (dP/dz)fl and read off K or choose a desired K
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15
Q

what can packing-specific mass transfer charts be used for?

A

data like these can be used for rough comparisons between different packing types
detailed mass transfer and hydraulic modelling is preferred for process design

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16
Q

what design margin is typically applied to a process design?

A

a design margin of 20% to 50% is typically applied to the packing height

17
Q

describe the 8 components of a packed column configuration

A 
feed pipe to liquid distrubutor
liquid distributor
structured packing
structured packing support
packing hold down grid
random packing
random packing support
gas/vapour inlet
18
Q

name some column internals

A

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

19
Q

what are the process specifications for packed bed distillation

A 
zF    feed composition
q       feed state
F      feed flow rate
P     column pressure
xD      distillate composition
xB      bottoms composition
R/Rmin    reflux requirements
20
Q

what are the process specifications for packed bed absorption?

A 
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
21
Q

what are the process specifications for packed bed stripping?

A 
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
22
Q

what is the column design workflow

A
  1. assemble design specifications
  2. calculate L/V or L’/V’ based on R/Rmin, L’/L’min or V’/V’min
  3. specify the vapour load as a fraction of flooding
  4. calculate the required column diameter
  5. obtain KOG from a detailed mass transfer model, prior experience or pilot-scale experimetns. hence obtain HOG
  6. calculate NOG by graphical or analytical method as appropriate
  7. calculate the required height of packing: H=HOG.NOG
  8. specify all column internals
  9. determine the pressure gradient and total column pressure drop

for distillation columns size each section separately

23
Q

what factors favour choosing a packed column?

A

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

24
Q

what factors are against choosing a packed columnn?

A

weight of packing can be problematic
difficulty of cleaning packing
good liquid distribution critical

25
Q

what factors favour choosing plate columns?

A

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

26
Q

what factors are agaimst plate columns?

A

higher pressure drop

less able to handle foaming systems

27
Q

what is the benefit of GPDC?

A

the GPDC can be used as a quick alternative to detailed models to detemine the required column diameter

Separation Processes 1 Resit (7 decks)

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