Agitated Thin Film Dryer (ATFD) Working & Design Calculations

Credits: Economy Suppliers

Hiii Amigos...!!! Good day all.......!!!!

After a big gap i'm posting here. Today i'm gonna type something regarding ATFD, as i'm having a deep attachment and memories with this machine.

Being an Rookie engineer in Technical Services department, i've involved in the commissioning of this equipment along with fellow engineers and after successful commissioning operated for around 1 year. In this 1 year i've gained decent knowledge regarding the design and working of this beast. And i would like to recall my memories and want to say that i'll be grateful to my former boss Mr. Y L N Reddy, who has given me a life in Pharma and to Mr. K K Kumar, who has trusted me and given me a chance in commissioning of this equipment, which is fully wrapped up of sophisticated technologies. Also i'm quite happy to have worked with Mr. Srikanth & Mr. G. Anand(Currently associated with Hetero Labs), Mr. Jaisai Reddy(Currently associated with Cipla) & Mr. Murali B(Currently associated with Auroindo Labs) during commissioning.

Well, it's time to go into the topic. 




Working Principle:

Agitated Thin Film Dryer (ATFD) is a very modernized and a dynamic equipment. This shall be used majorly for product layer concentrations and in ETP(Effluent treatment plants). The working principle of this equipment is vaporization under vacuum.

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Constriction & major components of ATFD:

ATFD majorly has  components, 

1. Cyclone separator,
2. Feed distributor,
3. Cylindrical shell containing shaft & blades,
      a) Hinged blades / immovable blades,
      b) Scrapper blades / movable blades,
4. Motor & Mechanical seal,
5. Gear-box,
6. Powder collection area.
Cyclone separator:
Usually cyclone separator is used to reduce the flow of particles through it by restraining them in its body.

Feed distributor:
A feed distributor is something like a plate located on top section, the plate will be a fixed element having nozzles through which feed enters the cylindrical shell. 

Cylindrical shell:
The cylindrical shell volume depends on the capacity of the ATFD, the cylinder contains a shaft and blades.
Shaft:  Usually the shaft will occupy 70-75% of the cylindrical structure rotating with an higher RPM.



Hinged blades / immovable blades: Te hinged blades are static and they move along with the shaft, 

Scrapper blades/Movable blades: The scrapper blades will swing upon the heat transfer area, as the rotor/shaft will rotate with an high rpm, the scrapper blades will swing in a single direction. Because of the swinging effect of the scrapper blades the scaling effect will be very low.

Side view of Scrapper

The shaft of the ATFD will look like a rod having uniform distribution of spikes.

Motor & Mechanical Seal:
Motor calculation is presented below.




Powder Collection area: 
At the end of the cylindrical structure a collection bowl shall be fixed to collect the dry material.

Now, i think all of you have a clear idea of the ATFD components. And i'll present you the Con's and Pro's of the machine.

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Pro's:

1. The scaling will be very minimal and sometimes negligible.
2. Dry material with minimal LOD can be achieved.
3. Solvent recovery will be high.
4. Few seconds of residence time.
5. Single stage stripping.
6. Great heat conductivity.
7. Zero deadzone - thermal accumulation / overheating is negligible.
8. One pass evaporation.

Con's:

1. Requires high vacuum.

2. Cleaning is very difficult.

3. Dangerous w.r.t. operation(rotating equipment with very high RPM).

Orientation of ATFD:
ATFD's are distinguished based on orientation, 
1. Horizontal, 
2. Vertical.

Horizontal ATFD's are used when the input feed are of high viscous slurries, and when the solids concentrations are high.

Vertical ATFD's can be used, when the input feed are of low viscosity and when the solids concentrations are low.




Working of ATFD:

ATFD shall be fed from top and the desired product shall be collected at bottom under vacuum application.
I'll make it clear, just scroll down.

1. Vacuum: Maximum possible vacuum need to be maintained in the system

2. Heating: Pre-heating to be given to the jacket shell,

3. Feeding: Feed to be pumped into the system based on a flow rate consistently and the feed will be distributed uniformly through a distributor which shall be located above the shaft.

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4. Film formation: The fed feed will get into contact with the hinged blades attached to the shaft, the high RPM blades shall splash the feed onto the surface. The splashed feed will form a film over the surface of the cylinder and due to the gravity effect, film will slide over the cylindrical vessel.




5. Heat transfer and Vaporization: The heated surface will transfer the energy to the falling film and as the film gets slided over the heat transfer area, the volatile matter will be vaporised and from point to point the volatile content reduces.

Credits: Pharma Engineering owned

6. Powder Collection:  The dried material shall be collected in the bottom section of the ATFD.

Once again i'll give you a short note, the main working of ATFD is film formation at the top section and as the film passes to the bottom the volatile matter will be vaporised and dry powder shall be collected at the bottom.

Now lets start the calculation part,

Lets have a case study, we have a feed containing around 25% of solids(product) with a bulk density of 0.4 Kg/L and the ATFD capacity be 5 Sq.m.
Now we need to know what might be the clearance between the scrapper blades and the cylindrical shell, usually as per my knowledge it will be very minimal and stays around ~4 mm.
Let the diameter of the shell be 0.6 Sq.m

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Feed Rate Calculation:

So, we have to decide the feed rate now.

Input data:
Heat transfer area of ATFD ( A ): 5 Sq.m
Diameter of ATFD ( d ):  0.6 m; Radius ( r = d/2): 0.3 m. 
Solids bulk density ( BD ): 0.4 Kg/L
Solids concentration ( C ): 25 %
Clearance between scrapper blades and cylindrical shell ( R ): ~4 mm.

Output details:

Height of the cylindrical heat transfer surface ( L ): 2.7 m
Formula used: Cylindrical surface area ( A): 2 x 𝛑 x r x L

Volume of the cylindrical vessel ( V ): 0.75 Cu.m. = 75 Lts
Formula used: Cylindrical vessel volume:  𝛑 x (r^2) x L

Clearance volume between the shaft and the cylindrical vessel ( Vc ): 0.01 Cu.m = 10 Lts
Formula used: Clearance volume: V - ( 𝛑 x ((r-R)^2) x L ).

Material crystallization height ( H ): 0.531 m
Formula used: Material will be crystallized after film crossing the 80% of HT area, 0.8 x L.

Available clearance volume after phase transition ( Va ): 0.002 Cu.m = 1.993 Lts 
Formula used: ( H x Vc ) / L = 0.531 x 10 / 2.7,

Average Film residence time ( t ): 30 secs.
Note: Residence time includes splashing of feed onto the surface, film formation, complete film sliding over the HT area.

Total volume that crosses the post phase transition area in one hour ( Vn ): 239.2 L
Formula used: ( 3600 x Va )  / t = 3600 x 1.993 / 30,

Total mass that can be obtained in one hour ( m ): 95.68 Kgs
Formula used: Vn x BD = 239.2 x 0.4,




Total reaction mass that can be fed into ATFD ( Vg ): 382.7 L
Formula used: m / C = 95.68 / 0.25

So, it is clear that the ATFD of 5 sq.m can be operated at a feed rate of 382.7 L /hr of feed that contains 25% of solids which have 0.4 Kg/L of bulk density.

Now, lets perform the material balance for the ATFD,

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Material Balance:

Feed rate is 380 L /hr ( Vg )
Let the solvent be methanol, Feed rate in Kgs be 300 Kgs/hr ( Mf ),
Feed contains 25% solids, lets the product be AJ(solids).
Operating temperature is 50°C = 273.15+50 = 323.15 °K,

Material Balance

From the material balance, it is clear that 

95.68 Kgs of solids shall be formed ( Ms ) and 
204.32 Kgs of methanol shall be vaporized ( Mv ).

Now lets enter the energy balance,




Energy Balance:

( Mf x Hf ) + ( ms x hs ) = ( Mv x Hv ) + ( Ms x Hs )

Hf = (Cpf) x Tf
Feed contains, 
Methanol & Solids(AJ), 
Methanol Specific heat is 0.61 KCal/Kg. °C,
Solids Specific heat be 0.5 KCal/Kg. °C.

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Weight fraction of methanol = 204.32/300 = 0.681
Weight fraction of solids = 95.68/300 = 0.319

Average Specific heat of feed (Cpf) = ( 0.681 x 0.61 ) + ( 0.319 x 0.5 ) = 0.574 KCal/Kg. °C.

Hf = 0.574 x ( 323.15 - 373.15 ) = -28.7 KCal/Kg.

Latent heat of steam at 50°C, hs = 569.1 KCal/Kg (Refer Steam tables)

Let the Specific heat of dry solids be 1.2 KCal/Kg °C,

Hs = Cps x Ts = 1.2 x ( 323.15 - 373.15 ) = -60 KCal/Kg.

Hv = 264 KCal/Kg [ Latent heat of Methanol - Refer Solvent properties]

Energy Balance

Now lets get back to main equation,




( Mf x Hf ) + ( ms x hs ) = ( Mv x Hv ) + ( Ms x Hs )

( 300 x -28.7 ) + ( ms x 569.1 ) = ( 204.32 x 264 ) + ( 95.68 x -60 )

-8160 + 569.1 ms = 53940.48 - 5740.8,

ms = 99.03 Kgs/hr.

Hence, our energy balance is stating that ~100 Kgs/hr of steam is required for the process.

Now lets move to another interesting topic,

i.e., Design of agitator.

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Agitator Design:

Feed rate = 383 Lts/hr.

Cross sectional area ( Acs ) = 3.141 x ( 0.6 ^ 2 ) / 4 = 0.282 Sq.m

Velocity of feed stream ( v ) = Vg / v = 0.383 / 0.282 =  1.358 m/sec 

Reynolds number ( Re ) = D x v x 𝝆 / 𝝁 = 0.6 x 1.358 x 0.92 / 0.192 = 3.904

Power number ( Np ) = 203 /  Re  = 203 / 3.904 = 51.99

Power ( P ) = Np x 𝝆 x ( N^3 ) x ( D ^5 ) = 51.99 x 792 x ( (150/60)^3) ) x ( 0.5^5 ) = 20105 watts.

[** Shaft dia considered as 0.5 m]

= 26.95 HP.

Now, i would like to go still deep calculating the shell and jacket thickness, if you would like to join, please, or else better scroll to post end and download the sheet.

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Design of shell:

Let the working pressure be 2.5 Kg/cm2 + Vacuum.

As per theory, thickness can be derived using the formula,

T = P x R / [ S x E - 0.6 x P ]

P - design pressure, R - Inner dia, S - allowable stress, E - Joint efficiency.

Let the design pressure be 30% excess to the working pressure,

P = 1.3 * 2.5 = 3.25 Kg/cm2.

For SS304 grade, S = 1400 Kg/cm2, E = 0.9.

R  = 0.55 m = 55 cm

T = ( 3.25 x 55 ) / ( 1400 x 0.9 - 0.6 x 3.25 ) = 0.142 cm = 1.42 mm.

If fouling / corrosion allowance is considered as 2 mm, 

then the thickness would be 2 + 1.42 = 3.42 mm ~4mm

Jacket design:

Lets suppose the jacket design pressure be 4 Kg/cm2, and 

Gap between jacket and shell be 50 mm,

Internal dia = Shell internal dia + ( 2 x Shell thickness ) + ( 2 x jacket-shell clearance )

                   =  550 + ( 2 x 4 ) + ( 2 x 50 ) = 658 mm = 65.8 cms

Internal Radius = 65.8 / 2 = 32.9 cms

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Jacket thickness = ( 4 x 32.9 ) / ( 1400 x 0.9 - 0.6 x 4 ) = 0.105 cms = 1.05 mm

As Jacket would be more vulnerable to corrosion, the allowance the would consider is ~3 mm,

Hence the jacket thickness would be 3 + 1.05 = 4.05 mm ~5 mm.

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

We are done, Hope you are clear.....!!!

If you are having nay queries, feel free to comment / message me.

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

Automated Excel sheet shall be updated within a week..... STAY TUNED.....!!!!

I've never thought of posting this in recent days, but i got a request from one of my friend, but i'm unable to help him with this at right moment and i don't want some other to face the same, hence posted. Sorry Buddy 😐😐😐😐........!!!!!


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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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