[How To] Design a Scrubber [ Packed column ] (UPDATED) as on 07.02.2020

Hello Readers, Good day all.........!!!

After many days i'm back with much more awaited post, i.e., designing a Scrubber which shall be used for safe venting of vapours / gases from reactors / vessels.

This post was requested by Mr. Nitin kheese a long back ago, and may be i think he should have forgotten also, and Mr. Nitin sorry for being late over this query.

Before getting into the topic lets start with some basic stuff,





What is Scrubber & Whats its purpose ?

A scrubber is an object used for cleaning something unwanted thing, and in Pharma terminology i..e, our words, it is simply a combination of blower system and absorption system.

What is absorption ?

Absorption is simply grabbing a vapour/ gas or liquid into another liquid by means of holding them in the voids spaces available in this another liquid. Also there is some term which is called adsorption, which is same but instead of another liquid, a solid comes into play.

You may also like:

[How to] Calculate Product assay in liquid ? 

Alternative for Reaction calorimetry

What is blower ?

This term is the main heart of the scrubber, without a blower a scrubber is simply a re-circulation column. Blower is used to blow out the vapours / gases those are produced in a reaction reactor or to remove the unwanted vapours generated because of breathing losses in  storage vessels.

What are the major components of the scrubber ?

There are majorly 4 major components according to me,
1. Blower,
2. Re-circulation pump,
3. Column,
4. Collection / holding tank.

If anyone have any other major components, then kindly comment / message me, i'll add those here.





What is the principle of scrubber ?

May be i should have posted this question at the top, but sorry for not 😆, the principle of scrubber is gas liquid absorption.

What is Packing factor, Fp ?

Packing factor is the ration of packing surface area per unit volume to cube of the packing's void fraction. 
Holds the unit of m-1.

You may also like:

[How to] Calculate rate of distillation in batch reactor ? 

Calculate density of Reaction mass

What are the types of packing's used in scrubbers ?

Usually, there may be many packing's, but i know only three sort,

1. Super intallox,
2. Intallox saddles,
3. Pall rings.
4. Random packing's

Again there will be sub categories based on the MOC, like ceramic, metal, Plastic, etc.

By now, basic stuff is completed, 
Lets start out show, 





Design calculations of Scrubber,

Out of these let us start from the damn basic thing, i.e., Collection / holding tank.

Usually a collection tank is nothing but a vessel which contains water / caustic lye,
Directly i've mentioned water / caustic lye because usually in a pharma field we will use that.

If the fumes that were sucked by blower are acid then we will go for lye, and if other than that we will go with water, also the selection depends on the solubility.

Lets have an example, the reaction that's is carried out in the reactor generates 1000 Kgs of spent in form of vapour per hour and the component to be scrubbed is HCl vapour, which is produced at a rate of 70 Kg/hr and the volume percent of HCl vapour is ~6%(v/v). Scrubbing medium is 20% caustic solution.

Lets start,





For performing the design calculations, there will be some of the considerations that we have to made, especially for the packing materials
As the saddles would be cheap, for ensuring cost effectiveness, i'm considering intallox saddles.

Input Data:

Packing type                                   : Intallox saddles,
Packing size                                    : 25 mm,
Packing MOC                                 : PP,
Gas pressure drop per m of bed : 15 mmWC /  m of bed, [Considered Value]
Packing factor, Fp                          : 21 / m, [Considered value]
Characteristic packing factor, Cf : 32 [ refer below table ],

You may also like:

[How to] Prepare installation qualification ? 

[How to] Prepare Operational qualification ?

As the vapour requires a medium, we have to consider the carrier gas i.e., air along with the desired component which need to be scrubbed.

Carrier Gas properties :

Gas flow rate (Air)              : 1000 Kg/hr = 0.2778 Kg/sec,
Gas entry pressure             : 1.00 atm,
Gas temperature at entry : 30 degC,
Gas mol. wt.                        : 29 g/mole





Component to be scrubbed : 

Component flow rate              : 70 Kg/hr,
% component in gas                : 6%
Molecular wt. of component : 36.5

For scrubbing HCl fumes, i've selected Caustic solution 20%, but i should be able to calculate the theoretical amount of caustic required in this case. So, i would like to perform the material balance:

HCl	NaOH		NaCl	H2O
36.5	40		58.5	18.02
70.00	76.71		112.19	34.56
1.92	1.92		1.92	1.92

As per the material balance, HCl moles is 1.92, so theoretically caustic required is 1.92 moles, which is equivalent to 76.71 Kgs, so i'll round it off to 77 Kgs.
Scrubbing medium properties :

Scrubbing medium : 20% caustic,
Liquid flow rate       : 77 Kg/hr = 0.0214 Kg/sec
Liquid density          : 1100 Kg/Cu.m
Liquid Viscosity       : 3.5 Cp = 0.0035 N.Sec/Sq.m,






Gas Density Calculation :

Average molecular wt. of gas = 29.45 g/mole,
Gas in flow rate = 0.2778 / 29.45 = 0.0094 Kmol/sec,
= (Kmole/sec) x (T in K / 273.15) x ( 1 atm / pr. in atm) x (22.414/1),
= 0.234499 Cu.m/Sec.

You may also like:

[How to] Prepare design qualification ? 

[How to] Evaluate User Requirement Specification ?

So, now we have determined total of three flowrates for the gas,
1. Molar flow rate = 0.0094 Kmol/Sec,
2. Volumetric flow rate = 0.234499 Cu.m/Sec,
3. Mass flow rate = 0.2778 Kg/Sec,




Gas Density = Mass flow rate / Volumetric flow rate = 1.1846 Kg/Cu.m,

As of now basic detailing regarding flowrates is done, and we need to go calculating the cross sectional area of the tower /column.

Column Cross-Sectional area Calculation

Now, Assuming complete absorption of the desired HCl vapour into scrubbing medium,

Component removed = molar flow rate x % component x mol. wt. 
= 0.0094 x (6/100) x 36.5 = 0.0207 Kg/Sec,
Liquid leaving the packing (L') = inlet liquid flow rate + Component removed
= 0.0207 + 0.0214 = 0.0420 Kg/Sec.

(L' / G') x (ρg / ρl)^0.5 = (0.0420/0.2778) x (1.1846/1100)^0.5 = 0.00497,

Use the 0.00497 as ordinate, and pressure drop of 147.1 (N/Sq.m)/m,

find the abscissa from the below graph,

[(G' ^ 2) x Cf x (μl^0.1) x J ] / [ρg(ρl - ρg)gc] = 0.04 [from graph],

Now, Outlet flowrate of gas through the column (G') = [ 0.04 x ρg(ρl-ρg)gc/(Cf x (μl^0.1) x J)]^0.5,

= 1.6665 Kg/Sq.m.Sec,

** Pl don't confuse between the initial G' and the current G', the initial G' i.e., X-axis will represent the inlet flowrate of gas and the Y - axis G' will represent the outlet flowrate of gas.

Column cross section area = Gas mass flow rate / G' = 0.2778 / 1.6665 = 0.1667 Sq.m,





Column dia = 0.461 m,
That's it....!!!

07.02.2020
Due to receipt of continuous requests for the blower capacity, hydraulic power estimation, Flooding % & HETP calculations, extending the post.

Now i'm extending the post a little bit to further explain you to calculate the blower capacity, hydraulic power requirement, HETP and the amount of flooding.

Basically, the blower capacity and the hydraulic power is calculated based on the estimated pressure drop inside the tower,

Pressure Drop Estimation 

There will be total 4 types of considerable pressure drops happening inside the tower,
1. Pressure drop due to dry packing,
2. Pressure drop due to irrigation of liquid over dry packing,
3. Pressure drop due to the internals (packing supports + distributor plate),
4. Pressure drop due to contraction losses of gas.

Now we have to estimate all four, to get the overall pressure drop.

i. Pressure drop due to irrigated packing = Total packing height x Pressure drop across packing = 147.1 x 3 = 441.3 N/m2,

ii. For dry packing,
outlet gas flowrate = 
(Gas inlet flowrate - Component to be scrubbed) / Column cross-sectional area
= ( 0.2778 - 0.0207 ) / 0.1667 = 1.54 Kg/m2.sec.

Gas outlet pressure = 1 x 101325 - 441.3 = 100883.7 N/m2,

Outlet gas density = (100883.7 / 101325) x (36.5 / 22.414) x (273.15 / 303.15) = 1.46 Kg/m3;


Pressure drop due to dry packing (ΔP / z) = Cd x (G'^2/⍴g);

Cd for 25 NB intallox saddles is 96.7,  [Refer top table i.e., properties of packings],

ΔP / z  = 96.7 x (1.54^2 / 1.46) = 157.07 N/m2;

iii. Pressure drop due to internals,
Assuming 25 mmWC = 25 x 9.80665 = 245.17 N/m2.

iv. Pressure drop due to contraction losses,
Lets say, gas velocity be 6 m/sec,
Contraction losses = 1.5 x v^2/2gc = 1.5 x (6^2/(2 x 1)) = 27 N/m2.

Total Pressure drop = 441.3 + 157.07 + 245.17 + 27 = 870.54 N/m2.

Required hydraulic power = Pressure drop x (Total flow - Scrubbed gas flow) / Outlet gas density
= 870.54 x (0.2778 - 0.0207) / 1.46 = 153.29 N.m/sec = 0.1533 KW,

Considering 60% efficiency, 0.1533/0.6 = 0.255 KW = 0.34 HP.

Required blower capacity = Hydraulic power x 1714 / (0.1771 x blower efficiency)
= 0.34 x 1714 / (0.1771 x 0.6) = 5516.54 GPM = 5516.54 x 0.1605 = 885.41 CFM = 1504 m3/hr.

Flooding Estimation across column

Vapour liquid flow factor Flv = (Liquid flowrate/Gas flowrate) x (Gas density/Liquid density)^0.5
= (0.0214/0.2778) x (1.1846/1100)^0.5 = 0.0025
Norton's correlation curves - Generalized pressure curves

Using the above curve and plotting the flow-factor of ~0.01 at 15 mmWC curve(based on initial assumption),
K4 = 0.6, K4(flooding) = ~8;

Ideal flooding = (0.6 / 8)^0.5 x 100 = 6.25 %,

Mass flowrate of gas = [K4 x ρV x (ρL - ρV) / (13.1 x Fp x (µL / ρL)^0.1)]^0.5
= [0.6 x 1.1846 x (1100 - 1.1846) / (13.1 x 21 x (0.01/1100)^0.1]^0.5 = 3.01 Kg/m2.sec,

Ideal column cross sectional area = Inlet gas flow / mass flowrate = 0.2778 / 3.01 = 0.0922 m2,

Actual flooding = Ideal flooding x (Actual cross sectional area / Ideal cross sectional area)
= 6.25 x (0.1667/0.0922) = 11.3 %.
[For effective scrubbing the flooding should be less than 70%]

HETP Prediction

As per Norton correlation, Ln HETP = n - 0.187 x Lnσ - 0.213 x Ln μ
= 1.1308 - 0.187 x Ln 80 - 0.213 x Ln 1 [σ - Surface tension; μ - Viscosity in cP]
= 1.1308 - 0.819 - 0 = 0.311.

HETP = 1.365 ft = 0.416 m;

No. of theoretical stages required = Total height of packing bed / HETP = 3/0.416 = 7.21 ~8 stages. 


If any concern, pl do feel free to comment / contact, 

Comments are most appreciated.

Related Posts:

[How to] design a vapour column ? 

[How to] Select vacuum pump ?  

[How to] Calculate reactor heat transfer area ?  

[How to] Design a distillation column ?

About The Author

---

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.
Follow Me on Twitter AjaySpectator & Computer Innovations