[How To] Design Packed distillation column

Hiii Good Morning everyone …!! 

Hope everyone are doing good ….!!

From past many days, I’m receiving a query from a person and I don’t wanna disclose his name, the query is design of packed distillation column.

Most of us were well aware of tray/plate column design and operation of packed column, but not design of packed column. But many of us know that the packed column design is similar to tray column and instead of trays we’ll use packing.

Today I thought of posting this here. There wont be any Q & A session here.

In general packed columns are most common equipment’s used for distillation, gas liquid absorption, sometimes liquid – liquid absorption too. These will be having some pro’s & con’s for these packed columns when compared to tray column. Below are some of them:

i.      Packed columns are not effective for low liquid flow-rates,

ii.     Corrosive liquids can be handled in packed column effectively than plate column,

iii.   Liquid holdups in packed columns are low when compared with tray column, which can be profitable due to less inventory loss,

iv.   The pressure drop per equilibrium stage can be lower for packing than that of plates,

v.     Withdrawl of side streams can be made for tray column, but in-case of packed column it is not possible,

vi.   There might be chances of packing damages due to variation of pressures sometimes, which requires replacement and this will impact on principle costs,

vii.  Can be a better match to thermal sensitive liquids,

viii.  Packing are sensitive and the chances for getting damaged during installation itself are high.

Also Read:

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There will be two types of packing, as many of us knows these, Random and Regular packing.

Raschig rings, Pal rings, Berl saddle, Intallox saddle, Super intallox, Metal hypac are the once which will be commonly available. Below are the properties of them:

Packing type	Size		BD Kg/m3	Surface area (m2/m3)	Packing factor, m-1
	inch	mm
Ceramic Raschig rings	0.5	13	881	368	2100
	1.0	25	673	190	525
	1.5	38	689	128	310
	2.0	51	651	95	210
	3.0	76	561	69	120
Metal	0.5	13	1201	417	980
	1.0	25	625	207	375
	1.5	38	785	141	270
	2.0	51	593	102	190
	3.0	76	400	72	105
Pall Rings Metal	0.6	16	593	341	230
	1.0	25	481	210	160
	1.3	32	385	128	92
	2.0	51	353	102	66
	3.5	89	273	66	52
Plastics Saddles	0.625	16	112	341	320
	1	25	88	207	170
	1.5	38	76	128	130
	2	51	68	102	82
	3.5	89	64	85	52
Intallox Saddles Ceramic	0.5	13	737	480	660
	1.0	25	673	253	300
	1.5	38	625	194	170
	2.0	51	609	108	130
	3.0	76	577	78	72

Below are the packing sizes recommended ranges based on column dia:

Column Dia, m	Recommended Packing Size
< 0.3 m	< 1 in
0.3 m to 0.9 m	1.0 in to 1.5 in
> 0.9 m	2 in to 3 in

Let’s start the design part now and for your convenience I’ll explain it with a case study :

Design involves no. of stages, column dia, column height & Packing equivalent Dia.

Step – 1: Estimate no. of transfer units based on McCab thiele method

Saturated feed contains 50 % A, 50 % B. Required purity of top product A is 98 % and allowable A composition in bottom is 2 %. The relative volatility of A w.r.t. B is 3.

[Other input: Vapour density: 2 Kg/m3, Liquid density: 650 Kg/m3; Void fraction: ~20%; Vapour viscosity: 0.0000118 Pas.Sec; Liquid viscosity: 0.000314 Pas.Sec; Vapour flow rate: 500 Kmoles/hr; Liquid flow rate: 200 Kmoles/hr; Mol. wt. of A(Vapour): 50 g/mole, Mol. wt. of B(Liquid): 78 g/mole; Vapour diffusivity: 0.0000001 m2/sec]

Hence prepared a tabulation for Mole fractions X vs Y.

Mole fraction, X	Mole fraction, Y
0.0	0.00
0.1	0.25
0.2	0.43
0.3	0.56
0.4	0.67
0.5	0.75
0.6	0.82
0.7	0.88
0.8	0.92
0.9	0.96
1.0	1.00

The above tabulation done by using, Y = 𝛂 X / (1+(𝛂 - 1)X)

Based on the tabulation, plot the XY graph.

As per the feed details, it would be a saturated liquid, hence the feed line will be straight,

Below are some of the feed line positions for your reference:

Ours is a saturated liquid, hence the q line will be straight.

Also plotting the Xd, Xr points.

Now intersecting the extended q-line by drawing a line from Xd extrapolated to q-line point on the curve, which will result in enriching section operating line & similarly for Xw extrapolation to q-line point on curve, which will result in stripping section operating line.

Now drawing the steps from Xd to Xw over the enriching & stripping section lines by limiting inside the curve. Now the curve will look like this:

Now if we count the steps / stages, they are 18. So NTU is 18.

Also Read:

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Step – 2: HTU (Height of Transfer Unit) Calculation:

Let’s begin by selecting ceramic Saddles for the packing of Dia 1.5”.

For this we need to calculate the equivalent diameters (as these are non-spherical the diameter cannot be calculated, hence the equivalent diameter need to be estimated):

Equivalent Diameter, d = 4 x (Ɛ/ω);

Ɛ – Void fraction (~20%); ω – Surface area per unit column area (m2/m3)

d = 4 x (0.2/194) = 0.004 m.

Archimedes number, Ar = d³ x (ρ1 – ρ2) x ρ2 x g / µ₂² 

= (0.004)³ x (650 – 2) x 2 x 9.81 / (0.0000118)² = 5843728.

Reynolds number, Re = 0.15 x Ar^0.57 x (V/L)^0.43 = 0.15 x (5843728)^0.57 x (500/200)^0.43 = 1600.36.

Linear vapour velocity, V = Re x µ₂ / d x ρ2 = 1600.36 x 0.0000118 / 0.004 x 2 = 2.36 m/sec.

Mass transfer coefficient, K = 0.35 x Re^0.8 x D x (µ₂ / (ρ2 x D))^0.35 / d²

= 0.35 x 1600.36^0.8 x (0.0000001) x (0.0000118/(2 x 0.0000001))^0.35 / 0.004²

= 3.34 Sec^-1

Height of transfer unit, HTU = V / K = 2.36 / 3.34 = 0.71 m,

Step – 3: Column Height & Diameter estimation:

Column height, H = NTU x HTU =18 x 0.71 = 12.74 m;

Column Diameter, Dc = ((3 x ν)/(π x V))^0.5

For estimating the column diameter, we need to calculate the volumetric flowrate of vapour,

Volumetric flowrate, ν = molar flowrate x Vapour Mol. wt. / Vapour density

= 500 x 50 / 2 x 3600 = 3.472 m³/sec

Dc = ((3 x 3.472)/(3.141 x 2.36))^0.5 = 1.19 m.

So, finally we have derived our requirements,

Height of column is 12.72 m ~13 m,

Dia meter of column is 1.19 m,

Packing is ceramic super intallox and height of each packing is 0.71 m ~0.7 m with voidage of 20 %,

Dia of packing is 1.5" with interfacial surface area of 194 m2/m3 of column and

Equivalent diameter of each packing is 0.004 m 
[Don't miss-understand there is a difference between Diameter and Equivalent diameter, for spherical structures both are same, but for non-spherical they will vary.]

/* Also we need to confirm our vapour velocity is below flooding velocity, that i'll update later */

That's it ......!!!

If any queries, pl feel free to comment ....!!!

Comments are most appreciated.....!!! 

/* We are done, most of the input values are considered and some of them were grabbed from my Mass transfer operations lab observation notes */

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About The Author

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Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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