Hiii all............!!!
Back after some time, can't find time for blogging these days, received a good response for the last post " Vapour column height calculation ", Thankful to the one who requested the post.
Yesterday i've received a mail requesting a post related to Filtration and its relevant equipment selection. So by now i think you came to know about today's post.
Before going deep into the topic, lets have some look at the basics of filtration,
What is Filtration ?
Filtration is a technique to separate the solids present in the liquid(basic definition).
Now here i'll show you how legends define filtration, " Filtration is physical / biological operation which is intended to separate the solid content from either gases form of matter or liquid matter by arranging a filter medium through which the fluid can permeate leaving the solid content as retentate ".
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What are the types of filtration ?
One can classify filtration based on many reasons, but in our pharma field we can simply classify filtration into two categories.
1. Product filtration,
2. Bi-product filtration.
Within these two, we may classify based on our knowledge,
Now i'm getting somewhat deep, better follow my frequency (since i donno your frequency 😁), if we are trying to filter the reaction mass at reaction temperature after reaction, it is intended mostly for separating the bi-products which were formed during the reaction. In my view this is simply Bi-product filtration. Mostly we can observe this in intermediate stage of API manufacturing.
The second thing mostly we will see in API stage, while we try to transfer the reaction mass in hot condition into rooms where pressure is controlled(to avoid any form of foreign particles entering the final product), and while separating the isolated mass from mother liquor(shortly ML's). This falls under product filtration.
If i'm right, after this classification the most probable question is
'' Which equipment's comes under those two categories ? "
So here i'm prepared for that, in product filtration as like mentioned above there are again two types,
1. For removal of extraneous matter,
2. For separation of isolated material.
Basically removal of extraneous matter majorly involves two operations, i.e., Re-circulation & filtration.
The re-circulation comes into frame if we need to decolourize the intermediate material, i.e., adsorption. The most suitable equipment's used for this operation were sparkler filter, leaf filter, bag filter, module filter, and sometime we may use candy filter(if the load is low), based on our requirement.
Coming to filtration equipment's, most of the times we use it to make our reaction mass particle free i.e., free from foreign particles. This case is somewhat easy to explain as it doesn't need high filtration areas because the desired retentate shall be of very low volume, sometimes it will be zero. We will use candy filter, micron filter, capsule filters in this case. If we are not satisfied with one filter then multiple filters shall be used in the filtration train to de-risk the product quality parameters.
So, by now as a process engineer we have gone through the types of filtration, and if any fresh engineer is reading the post and felt unsatisfied kindly go through the "Introduction to Chemical Engineering" book by Dutta & Goshal.
Now Lets start the show,
Types of Filtration Equipment:
Product Filtration : Micron filter, Candy filter, Module filter, sparkler filter, leaf filter, ANFD, Centrifuge, Pressure nutsche filter, bag filters, Nutsche filter, Filter press, Rotary pressure filter etc.
Bi-product filtration: ANFD, Bag filters, Nutsche filter, Pressure nutsche filter, etc.
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Selection of Filtration Equipment's :
Here we need to discuss specifically about filtration equipment's, because selection of wrong equipment's will leads to hell while execution.
ANFD:
Most commonly used filters in pharma industry are ANFD, Centrifuge, Nutsche filter, Pressure Nutsche filter, Filter press.
ANFD shall be used mostly because of safety reasons, where the evaporation of solvents will be low when compared to that of centrifuge.
The advantage in using the ANFD is we can perform press dry and suck dry with external torque included i.e., agitation, We can maintain inert atmosphere, Solvent recovery will be high(when compared to others), operator safety can be ensured at any time.
The specifications of the most ANFD shall be as like presented below:
Now i need to clarify you on what bases the ANFD size is selected.
Lets suppose the reaction mass volume is 300 L(solid content shall be about 50 Kg) and i need to select an ANFD for filtration, and 2 ANFD were available with capacities 2 KL and 6 KL.
Usually there will be a gap of 1"(2.5 cm / 25 mm) between the agitator and the filter screen,
The gap of 2.5 cm will have a volume of 50 L (2 KL ANFD) , 142.5 L (6 KL ANFD).
The final wet material will be around 50 Kg (as mentioned above),
Let the bulk density of the material be 0.3 Kg/L.
The volume occupied by the 50 Kg wet material shall be 50 / 0.3 = 166.7 L
That means when you compare the volume occupied by the gap and the wet cake, it is clear that the filtration(press dry) operation can be effectively executed in 2 KL ANFD when compared to the 6 KL ANFD.
This is how you select the ANFD capacity for filtration.
CENTRIFUGE:
Some times, there may be cases where we cant filter the reaction mass while using the ANFD, due to several reasons like sticky nature, due to high viscous etc. then we need to neglect some safety considerations, solvent losses and should move towards Centrifuge.
Centrifuge will have disadvantages like solvent losses and safety issues, as if the integrity of the centrifuge legs is not ensured, (i mean the positioning) then trust me it will become a flying saucer for sure.
Most of the cases centrifuge can settle the issues of filtration because of its working principle(i.e., centrifugal action).
There are 3 types of centrifuges available(as per my knowledge),
1. Basket type centrifuge, ----- Regularly used in pharma
2. Bottom dish centrifuge, ----- Randomly used in pharma
3. pusher centrifuge. ------- Majorly used in solvent recovery systems
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| Basket Type Centrifuge |
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| Bottom Discharge Centrifuge |
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| Pusher Centrifuge Continous |
Data Sheets for these Centrifuges:
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| BAsket Type Centrifuge |
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| Bottom Discharge Centrifuge |
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| Pusher centrifuge |
So as per my knowledge, centrifuge can be applicable in almost all cases, if safety is neglected, if we need to consider safety then centrifuge shall be used based on solvent volatility.
If solvent is high volatile, then better to avoid centrifuge, if solvent is low volatile and the material is sticky then 100% we need to consider centrifuge.
How to Scale-up in a Centrifuge ?
Like cake height in ANFD and power per unit volume for reactor mixing, there will be an hidden factor which shall be used to scale-up centrifuge i.e., Relative Centrifugal Force.
During our schooling we should have gone through the Relative Centrifugal force, the same can be employed here, i.e., 11.2 x r x (RPM/1000)^2.
Let's say, we feed our centrifuge at 400 RPM and the wet cake weight would be 100 Kgs, and the basket dia be 800 mm,
Relative Centrifugal force = 11.2 x (800/10) x (400/1000)^2 = 143.36 RCF
Total force per Kg of wet weight would be 143.36 / 100 = 1.43 RCF/Kg
This would be the scale-up factor that we have to consider, and now lets scale-up to a case where the wet weight would be 200 Kgs and the basket dia is going to be 1000 mm
Relative Centrifugal force = 11.2 x (1000/10) x (RPM/1000)^2 = 1120 x (RPM/1000)^2.
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FILTER PRESS:
Another filtration equipment that is mostly used in these recent times is Filter press(based on membrane technology).
This is some what innovative and the filtration capacity is higher( i mean the output that we get will be higher when compared to centrifuge), It will completely work on hydraulics, this is used very rarely and the cycle time will be low when compared to other filters. I'll explain its working here now,
Note: It is majorly applicable to the slurries where solid concentration is less than 5%.
The mechanism involved is surface filtration,(mentioning this because some of our readers shall be reading this for interview purpose)
The major components for filter press is Pusher Cylinder, bomb doors, Membranes which will arranged alternatively.
If you can see the filter press, for every filter plate one side cloth will be fixed and on the other side a membrane shall be fixed, feeding shall be done from the center and the ML's shall be collected from the corners and the flow direction of Feed and the ML's shall be counter current.
Initially the cylinder will squeeze the filter plates with a pressure of around 200-240 Kg/cm2, after then the feed to be passed through the squeezed filter plates with some external pressure (ex. through pump).
After completion of feeding, air shall be passed through the filter plates where the filter cloth and membrane attached to the filter plate will bulge expelling the ML's completely and the material in between the filter plates will be free from ML's and will press dry with the air pressure , after complete expelling of ML's the cylinder pressure shall be released and the wet material between the plates shall be collected. This technology reduces time cycle.
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| Filter Press Specification |
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| Filter Cloth Specification |
The major disadvantage of the Filter press is high investment and high labour involved.
SPARKLER FILTER:
This is majorly used for decolourization process in pharma field, it will have a better filtration area when compared to its size, since it will have a series of plates arranged with inlet & outlet from bottom. Usually the sparkler filter shall be used with cellulose pads inside, but based on requirement carbon pads can be used instead of cellulose pads to avoid any form of disturbance while handling the carbon.
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| Zero holdup sparkler filter |
The sparkler filter can be used in both forward and reverse flow pattern based on requirement.
Forward & Reverse flow pattern: Usually the inlet and outlet shall be given at the bottom of the filter, and if the flow enters from the center of the series of plates arranged, then it is reverse flow pattern, the vice versa of the reverse flow pattern is forward flow pattern.
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| Reverse flow pattern |
How to replace the carbon quantity used in a process with carbon pads ?
This is a thumb rule that i came to know in the recent times, and may be it will help you some time, because handling carbon is difficult when compared to filtering the reaction mass through sparkler filter with carbon pads.
1 Kg of activated carbon used in the process can be replaced with 1 sq.m of carbon pad.
i.e., lets suppose, we are using 10 Kg of carbon in a process and the carbon need to be replaced with a set of carbon pads, then the filtration area of the carbon pads shall be 10 Sq.m.
Let the dia of the plate used in the sparkler filter be 18", so the number of carbon pads required to replace the 10 Kg of carbon shall be:
10 Sq.m / ( 18 x 2.54 / 100 ) = 21.87 ~ 22 plates = 22 carbon pads need to be used instead of 10 Kg carbon.
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Leaf Filter :
Leaf filter also holds similar duty like sparkler filter but the filtration area increases with increase in number of leaf's used in the filter, It consists of a longitudinal drain screen covered with a filter cloth
Usually there will be 3 to 10 leafs inside a leaf filter. The higher the filtration area the higher the leaf's.
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| Leaf filter |
In the above shown leaf filter there are 5 no's of leaf's included.
To increase the filtration rate, pressure can be applied and the mechanism behind the leaf filter is surface filtration.
That't it.......!!! These are the majority things which are to be considered while selecting a filtration equipment .....!!!
If any queries feel free to comment / message.....!!!
Comments are appreciated .............!!!!
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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.
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