WHAT IS ANFD OR AGITATED NUTSCHE FILTER DRYER:
ANFD is one of the most popular Filtration cum Drying equipment. In a typical Pharmaceutical API (Active Pharma Ingredients) production, after the reaction and crystallization stage, a free flowing slurry is formed which needs Filtration as well as Drying in order to produce a dry powder and granules which are free from impurities. To facilitate this, typically two or more small unit operations are carried out, including, Mechanical Filtration using Centrifuge or Filter Press, followed by Vacuum Or other type of Drying. This is highly time consuming, as well as, cumbersome (both ergonomically, as well as, economically). ANFD combines both these operations easily and effectively, thus producing dry cake with minimum moisture Or solvent in it.
Note: Please feel free to visit my blogpost on Lab Scale ANFD for R&D, skid mounted and Pilot Plant Model of ANFD.
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WORKING PRINCPLE OF ANFD:
ANFD consists of a vertical cylindrical stirred tank vessel (somewhat looking like a reactor), which has its shell divided into two parts viz. Main Shell and Bottom Shell, which are separated by means of a mesh Or sieve and a filter cloth covering the sieve. The excess water Or solvent from the wet mass is filtered through and collected in the bottom shell, whereas, the remaining major part of the water Or solvent is removed by heating and vacuum drying the wet mass.
In short, the solvent Or water to be removed, is separated and collected at two different places viz.
1. Bottom Shell: through mechanical filtration facilitated by:
(a) Air Pressure in the main shell Or
(b) by agitation created by the vertically adjustable specially designed Agitator Or
(c) by the Vacuum created in the bottom shell (which pulls the excess water from the mass towards the bottom shell).
2. Vacuum Trap Or Receiver: by means of condensation of solvent/water vapors generated inside the main shell.
MATERIAL OF CONSTRUCTION:
MAJOR COMPONENTS OF ANFD:
ANFD is basically is an integrated system consisting of various components. The brief listing and description of them is as follows:
1. Main Shell: This vertical cylindrical and Jacketed/ Limpted vessel is usually constructed larger in diameter and smaller in height. This is so because bigger is the diameter of the vessel, more is the filtration area, and hence, faster is the filtration. The main shell is insulated (and also cladded) with hot insulation material such as Mineral Wool Or Rock Wool.
2. Vertical Stirrer: A low RPM (rotations per minute) stirrer is specially designed to rotate both clockwise, as well as, anticlockwise. A set of hydraulic cylinders are provided on the top dish of the shell to lift the stirrer vertically and bring back down as per the requirement. Stirrer plays extremely important role is sqeezing the wet cake, mother liquor wash, disturbing the cake and in overall filtration and drying.
3. Bottom Shell along with a hydraulic cylinder, Mesh & Filter Cloth: Bottom shell, the vertical cylindrical vessel is nothing but the bottom part of the main shell. Both main shell and the bottom shell has the same diameter, hence are concentric with respect to the vertical axis. The part which separates the two vessels is the Mesh or Sieve or Perforated Plate, which is covered with a Filter Cloth. This mesh is made strong enough so that it can handle the cake weight easily. The filter cloth is usually PP (poly propylene) and is washable/ cleanable. The aperture of the filter cloth is such that it can pass the water/ solvent, however retains the wet cake. The mesh or perforated plate is used to hold and support the filter cloth. Bottom shell has small legs supports with castor wheels. Bottom shell is lifted Or lowered with the help of the hydraulic cylinder. While it is lifted, the special types of clamps hold it fastened to the main shell.
4. Top Dish with Motor, Gear Box, Mechanical Seal, and Hydraulic Cylinders: The top dish is usually torrisperical, however, is designed strong enough to handle the weight of the agitator, lantern and complete drive assembly, seal and coupling, hydraulic cylinders and nozzles and manholes etc. Motor is designed as per the torque calculations. Gearbox is selected accordingly keeping in mind the low rpm (between 6 to 12). Mechanical seal is designed keeping in mind the vacuum requirement, as well as, the temperature.
5. Cake Discharge Nozzle Or Size Discharge Valve with Hydraulic Or Pneumatic Cylinder: The dried cake is discharged through the lateral opening nozzle on the main shell, which opens through a hydraulic Or pneumatic cylinder. The rotating agitator facilitates pushing the cake out towards the discharge valve.
6. Condenser: Typically a Shell and Tube Type Heat Exchanger is used to condense the vapors coming out of the ANFD. Usually the process side fluid (vapors) are passed through tubes, whereas, on the Shell side the utility fluid (mostly chilled water Or cooling water) is circulated. However, it can be the other way around too (depending on the heat load calculations). Condenser can be insulated and cladded for avoiding heat loss to the ambience.
7. Receiver: A solvent recovery receiver is provided at the bottom of the STHE condenser. Receiver can be either Vertical Cylindrical with legs supports Or Horizontal Cylindrical with saddle supports. A vacuum breaking arrangement is provided with a Control/ On-Off valve on the receiver.
8. Vacuum Pump: A suitable capacity Vacuum Pump is connected to the system (mostly to the receiver), which creates vacuum inside the ANFD, and, also facilitates the extraction of the vapors, hence complements the condensation process.
9. Water Heating system Or Low Pressure Steam (for heating the ANFD main shell Limpet): Incase of a Hot Water System, a dedicated skid mounted vertical cylindrical vessel is provided with external heating i.e. either with steam (LPS) Or with Electrical Heaters. A circulation pump pushes the hot water into the Limpet, and the return water goes back into the Hot Water Generation tank. The level guage/switch on this tank ensure water doesn't spill over, and, make up water valves gets ON when level goes down. The Temperature sensor on the Hot Waters Tank, as well as, sensor on the Inlet of the Limpet coil together controls the flow of Steam (through PID control valve) Or the Electrical Heater, thus ensures the inlet water temperature stays in control. Incase of direct steam heating, the PID valve ensures steam is throttled and uniform temperature is maintained in the limpet.
10. Nitrogen Purging system/ Air Pressure system: A nozzle can be provided with control valve, that can facilitate the intermediate and periodic gushing or N2 gas, which pushes the vapors out towards the condenser. Same nozzle can be used for providing the Air Pressure to facilitate the filtration operation. Vacuum needs to be stopped temporarily while purging.
11. CIP Or Spray Ball arrangement: Optionally, spray balls can be provided on the top dish of the main shell, which can facilitate cleaning and washing of the main vessel post the batch operation is over.
TYPICAL SEQUENCE OF OPERATIONS IN ANFD:
A general sequence of operation can be described as per below. It is to be noted that the number of steps, sequence and other protocol can very from company to company and based on the product and most importantly the SOP.
STEP -1 FILTRATION: The wet mass before drying has to undergo filtration first. In this step, either Air Or Nitrogen pressure is applied to the main chamber Or Vacuum is applied to the bottom shell Or both of these done. The excess water or solvent it thus pushed and pulled to pass through the filter cloth. Heating may Or may not be needed in this case. However, the rotating agitator helps in pushing the material down towards the filter media.
STEP -2 SOLVENT/ WATER/ MOTHER LIQUOR WASH: Sometimes the process demands Process Water/ Solvent wash Or Mother Liquor wash to the wet cake. This action (Filtration - ML Wash- Filtration) may be repeated a number of times depending on the process. The agitator is pulled up with the hydraulic cylinder and after adding water/ML, it is taken down with opposite direction rotation which disturbs the settled cake and facilitate proper washing of the cake.
STEP -3 HEATING AND VACUUM DRYING: When the cake is well filtered (or filtered as per SOP), heating is carried under vacuum by circulating the hot utility in the limpet of the main shell. As per the process requirement, as well as SOP, the drying cycle can be repeated Or extended based on the in batch sampling results.
STEP -4 SOLVENT OR ML LIQUID COLLECTION: As mentioned earlier, the solvent or ML can be collected in both bottom shell, as well as, in the receiver.
STEP -5 DISCHARGING OF THE DRIED CAKE: When the cake is dried upto expectations, the vacuum is broken and side discharge valve is opened. The agitator pushes the dry powder towards the discharge valve.
FACTORS TO BE TAKEN INTO CONSIDERATION WHILE DESIGNING AN ANFD:
1. Orientation of ANFD, Available Floor Space, Availability of the Utilities and Floor Layout/ Elevation:
Many a times, project team overlooks the area and space available and later have to regret. Similarly, the utility equipments such as Hot Water Tank, Vacuum Pump, Chilled Or Cooling Water, Steam, Nitrogen and Electrical Panel etc if not planned favorably can cause a lot of trouble such as financial loss due to structural rearrangement work, major construction work and sometimes production time loss too. As a usual practice all these things are planned simultaneously and in sync with each other. Also, a GAD (General Arrangement Drawing) is superimposed on Autocad Layout Drawing to see the feasibility of installation during the design stage.
2. Material of Construction: As mentioned above, the right MOC will decide the adherence to the quality and process parameters, as well as, longevity of the ANFD.
3. Level of Automation: Depending on the needs of the Production Department, as well as, budget allocation, the level of automation can be decided.
4. Statutory and Safety compliance: Sometimes, due to company policy and FDA mandate, special features are incorporated such as Touch Screen HMI (Human Machine Interface) with PLC (Programmable Logic Control). In some cases, there can be further sophistication by adopting special Data Security measures such as 21 CFR Chapter 11 compliance, which allows password protected individualistic logins and maintains record of each activity by each user. Overall, Safety, GMP guidelines along with respective FDA mandates has to be followed while designing an ANFD.
5. Batch Size Calculations: A basic Process Calculations must be done including Heat Load Calculations. This helps in deciding the optimum batch size, as well as, the time cycle, hence monthly Or timely production throughput.
6. Product Specific Operational Challenges (based on the previous experience Or Studies): Since it is a capital purchase, the careful attention must be given to the specific needs of the Production Team, concerns and apprehensions of the Engineering and Maintenance, as well as, the Safety and Statutory department, which helps ensure the production goes smooth.
7. Mechanical and Process Design, as well as, Sizing of the Equipments: Stress analysis/ calculations must be done under varying parameters such as temperature, vacuum and pressure. Similarly, the thickness calculations must be done keeping in mind the static and dynamic loads.
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