FLUID BED DRYER OR FLUIDIZED BED DRYER (FBD):

MEANING OF FBD OR WHAT DOES FBD STAND FOR:

FBD Or Fluid Bed Dryer Or Fluidized Bed Dryer, is considered to be one of the most popular, reliable and economic drying equipments used in Pharmaceutical Granulation process. Apart from granulation, it is also used in Pharma - API production, Chemical and Food industry.

PRICIPLE OF WORKING: 

FBD works on the Principle of Fluidization. Fluidization is a phenomenon in which the packed bed of wet powder particles Or particulate solids, when are exposed to a thrust and flow of hot fluid (in this case Hot Air), flowing from the bottom of the bed upwards, forces it to form a flexible bed (which appears like a Bed of Boiling Water), and the bed of floating particulate solids is called as Fluidized Bed. This is achieved because the velocity of the hot air is more than the terminal settling velocity of the particulate solids. In other words, the pressure created by draft of hot fluid (in this case Hot Air) is able to not only balance the weight of (as well as gravitation force exerted by) the wet mass, but also, is able to lift it upwards against the gravitational force.

As per the theory of Fluidization, there can be various stages of fluidization viz. Minimum Fluidization, Smooth Fluidization, Bubbling Fluidization, Turbulent Fluidization, Lean Phase Fluidization etc.

However, for our purpose of Fluidized Bed Drying, the two most important factors to be considered while designing are (a) Minimum Fluidization Velocity, (b) Heat Transfer by Convection (between Hot Air and Wet Particles). Which essentially means that we are merely using the fluidization for the purpose of better area of heat transfer. Hence, the flow rate of Hot Air is chosen accordingly (taking into consideration the wet material properties and the drag force created).

ADVANTAGES OF FBD OVER OTHER DRYERS:

• Fast Drying with Minimum Heat: FBDs are usually operated with Hot Air as a drying media. Because of the fluidization, the hot air can easily cover the large surface area of each granule Or powder particle, hence, the drying becomes highly effective in a minimum span of time.
• Economic Operation with Minimum Energy Consumption:   FBD uses Heat energy provided mostly by Low Pressure Steam or Electric Heaters. Flow of air is induced with the help of a blower fan operated by AC frequency motor. Apart from these, hardly any other sources of energy are needed. 
• Possibility of De-Humidification: Incase of coastal areas Or area where the Air carries a lot of humidity, an inline de-humidifier can be added in the air suction line Or inlet AHU (inlet Air Handling Unit), which drags the moisture from the Air by condensing it and producing Dry air (suitable for heating and drying purpose).
• Ease of Operation: All the important parts of the FBD viz. Inlet AHU, Plenum Chamber, Product Container, Expansion and Retarding Chamber, Filter Bag assembly, Exhaust AHU and blower etc are all accessible and hence cleanable. Some of the parts like Finger Bag assembly and External AHU may require ladder Or other means to access as they are may be placed high.
• Minimum Powder Loss Or Maximum Drying Yield: Due to the presence of the Flinger Bag Filters, as well as, optional HEPA filter in exhaust line, the entrained powder particles are arrested and hence, maximum dry powder is collected by end of the operation.
• Easy availability of Spares, Gaskets and Filters: All of the spares used in FBD are easily available in the market (and Majorly available in almost all countries where Pharma Production industry exists)
• Possibility of fully automated operation with optional 21 CFR: FBD operation can be made fully automated with HMI and PLC and/or SCADA as per the appropriate Zone Classification, as well as, integration of 21 CFR chapter 11.

DISADVATAGES OR LIMITATIONS OF FBD:

• Not Suitable for Air Sensitive Or Hazardous Or Inflammable Products: Certain products due to their properties, cannot accept direct contact with air Or blow of air. Also, there can be products which are either hazardous Or may have low flash point of the solvent to be removed. While, there can be some other products which are simply not suitable for FBD. All these products are usually dried under Vacuum.
• Not suitable for Too Wet Or Very High density powder particles: It is difficult to create fluidization effect with Too Wet Or Very High Density products (ex. Metal Powders), as they may require very high air thrust and controlling their velocities can be very challenging.
• Possibility of frequent Chocking of the HEPA filters: usually, the finger bag are +not less than 10 micron, hence, highly fine particles may escape and get arrested by HEPA. If dryer yields more such fine particles, the HEPA may get chocked and mat need frequent replacements.
• May need frequent Finger Bag Cleaning: Usually the pneumatic cylinder attached to the finger bags holder shakes the bags periodically. However, if the powder is sticky Or hygroscopic, there are chances of powder forming sticky layer on the finger bag surface, hence, merely shaking by the pneumatic cylinder may not help.
• Inlet and Exhaust AHU needs bigger space: The space needed for suction Air Handling Unit as well as Exhaust Air Handling Unit is usually high. Also, the connecting ducting too requires a lot of space. Incase of an Inlet AHU, usually it is placed in the technical area Or non-operational area, whereas the FBD is in the clean room Or operational area. The ducting has to pass through the cleanroom wall.  Similarly, the exhaust AHU along with its blower and motor must be kelp outside of the cleanroom and is usually kept in the open space. The ducting sometimes has to travel in a really zigzag fashion, because, many a times the technical area has no space left due to HVAC ducting. One has to keep these challenges in mind when planning the FBD.
• Proper Insulation needed for safety purpose, as well as, to avoid heat loss: FBD inlet ducting, as well as, sometimes the exhaust ducting requires proper insulation and cladding as it carries the Hot Air. This is important in both safety point of view, as well as, for  preventing the heat losses. 

MATERIAL OF CONSTRUCTION:

Since Pharmaceutical, Food and even Cosmetics follows GMP guidelines (sometimes due to statutory mandates), usually the Contact Parts are SS316 Or Higher Grade, and Non Contact Parts are SS304. Non - Contact can be of Mild Steel, in case of Non-GMP set up. Further, just like other Process Equipments, incase of Highly Corrosive chemicals, the FBD Contact Parts CAN BE Hastelloy (C22 Or C276), Or Halar Coated SS304 Or PU/ PFA Coated SS304. However, usually such expensive metals are not used for fabrication of FBDs, making it an Economic Option.

SURFACE FINISH: 

As per the GMP (Good Manufacturing Practice) guidelines, the product contact surface needs to be extremely smooth. This also helps in preventing the surface corrosion due to electrolytic cell formation. Hence the Product Contact Surface usually is minimum 240 grit Mirror Finish and Non Contact Surface is 180 grit matt finish. Incase of a Non-GMP set up, if Mild Steel is used (for Product Non Contact in some cases), the surface is either epoxy painted Or coated with protective layer.

MAJOR COMPONENTS OF A FBD:

For the sake ease, lets review the components in three parts viz. 

1. Inlet AHU, and its Ducting

2. FBD with its operating Panel 

3. External AHU, and its Ducting

 

1. Inlet AHU: The Rectangular AHU box is built in either SS304 Or in Mild Steel. It's components can be described as follows:

A. Pre Filter Assembly: Usually PP Air Filters of mesh size of 10 to 15 microns are used to filter the inlet air and arrest the suspended Or unwanted solid particles.

B. Dehumidification Assembly (Mostly Optional): Incase of Humid Air, the Dehumidifying coils are used with the chilled water Or brine solution circulating from within the coils. This drags and condenses the water Or moisture from the air and makes it dry. The chilled water piping (supply and return line), as well as, the control valve are outside of the AHU.

C. Fine Filter Assembly (Mostly Optional): PP Air Filters of mesh size of 3 to 5 microns are used to filter the inlet air (already passed through the Inlet Filter and/or over the Dehumidifier) and arrest the suspended Or unwanted solid particles (if any).

D. Steam Heated Coil Or Electrically Heated Coil Assembly: Usually Galvanized Steel Tubes Or SS304 Tubes are used to form the coil with fins (welded over the coil) to increase the Heat Transfer Area. The coil carries the Low Pressure steam. When the clean dry air passes over the coil and fins, it gets heated. The temperature of the Hot Air is controlled through the PID operated gate valve.

E. HEPA FIlter (Mostly Optional): Depending on the process requirements, sometimes, the inlet air stream needs to pass through the HEPA filter assembly. However, HEPA filter  due to its highly fine mesh ( micron) increases the pressure dropo across it and hence the higher capacity suction blower might be needed to creat fluidization effect.

F. Ducting (Optional): Inlet Air ducting sometime is needed  if the inlet AHU is kept away from the FBD. Usually, Pharma uses either SS304 Or SS316 ducting with Or witout insulation. 

G. Instrumentation: Instruments like DPG Or DPT (Differential Pressure Guage/ Transmitter), PG/PT (Pressure Guage/ Transmitter), Temperature Indicator/ Transmitter etc are generally mounted on the AHU and forms an important part of the control philosophy.

2. FBD with Operating Panel: The major parts of the FBD can be described as under:

A. Plenum Chamber Or Bottom Chamber: The bottom-most part of the FBD assembly, which is connected with the inlet AHU is known as Plenum Or Bottom Chamber. The utility of Plenum Chamber is to ensure the Hot Air is passed upwards through the bed of wet powder in the product container. Plenum chamber is interlocked and sealed with Product Chamber with the help of an inflatable gasket (usually food grade silicon).

B. Product Chamber: The Product Chamber is nothing but a detachable container which has swivel wheel mounted leg supports. Product chamber is taken off to load and unload the wet and dry powder respectively. While, the bottom of the chamber contains a mesh/ perforated sheet for passage of of hot air from the plenum chamber, the top is directly connected to the bottom of the Fluidization Chamber (also known as Expansion Chamber) with the help of a D type hollow tube inflatable gasket. 

C. Fluidization- Expansion-Retarding Chamber:  Expansion and Finger Bag chamber can be the same, which facilitates Fluidization, as well as, arrests the powder from flowing out by means of the finger bag filters. The fluidization chamber has a inspection window Or a view glass.  The bottom of the chamber and the top of product container are sealed by a D type hollow tube inflatable gasket. As a safety measure, the chamber can be provided with rupture disk Or pressure release valve. Also, static charge remover must be attached to the body of the chamber. The chamber also carries a series of finger bags, which are usually Anti Static Polypropylene bags, whose area of cross section, length, diameter and number of fingers etc are decided based on the process data viz amount of air to be dissipated, quantum of the powder to be handled, volumetric flow of air etc.

3. Exhaust AHU: Just like the Inlet AHU, in some cases, there can be an Exhaust AHU required too (box built in either SS304 Or in Mild Steel). The components of Exhaust AHU can be described as follows:

A. Fine Filter Assembly (Mostly Optional): PP Air Filters of mesh size of approx. 3 to 5 microns are used to filter the inlet air (already passed through the Inlet Filter and/or over the Dehumidifier) and arrest the suspended Or unwanted solid particles (if any).

B. HEPA FIlter (Mostly Optional): Depending on the process requirements, the exhaust vapors and hot air streams need to pass through the HEPA filter assembly. HEPA filter due to its highly fine mesh ( 0.3micron) increases the pressure drop across it and hence the higher capacity suction blower might be needed.

C. Ducting (Optional): Exhaust Air ducting is needed whenever the exhaust AHU is kept away from the FBD. Usually, Pharma uses either SS304 Or SS316 ducting with Or without insulation. 

D. Instrumentation: Instruments like DPG Or DPT (Differential Pressure Guage/ Transmitter), PG/PT (Pressure Guage/ Transmitter), Temperature Indicator/ Transmitter etc are generally mounted on the AHU. Furhter, solid flow monitor is optionally mounted on the exhaust ducting to identify the leakage Or damage to the finger bags, which can lead to powder loss.

E. Finger Bag cleaning device by shaking them: Finger bags are mounted on a circular stainless steel ring, which has a hooking arrangement of each finger bag. The pneumatic cylinder attached to this ring facilitates the periodic shaking of the rings and hence the fingers and shed the powder blanketing formed on the surface of the finger bags.

Safety Features and Instrumentation: 

The common safety features includes -

A. Inflatable gasket and blower motor interlock

B. Rupture disk and and blower motor interlock

C. Differential Pressure Transmitter across the HEPA and sometimes across the Fine filters.

D. Broken Bag detector and Alarm

E. Earthing element

Instruments such as PT 100 sensors, PID for steam assembly, FLP Or Non FLP earthing arrangements, Timer, Pressure Gauges/ Transmitters, Temperature Gauges/ Transmitters are used.

Further, the operating panel can be either relay based push button type panel Or  HMI (Human Machine Interface) operated PLC (Programmable Logic Control) based panel and can be always suitable to appropriate Zone classification. 

FACTORS TO BE TAKEN INTO CONSIDERATION WHILE DESIGNING A FBD:

1. Orientation of FBD, Available Floor Space, Availability of the Utilities and Floor Layout/ Elevation: This is one of the most overlooked Or underestimated point. As a usual practice, FBD with its AHUs, Blowers, Ducting etc must be planned 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:  The appropriate MOC selection is very important with respect to the adherence to the quality and process parameters, as well as, longevity of the FBD.

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 FBD.

5. Batch Size Calculations: A basic Process Calculations must be done including Heat Load, Drag Force and hence Volumetric Air Flow rate 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 carried out under varying parameters viz temperature and pressure. Similarly, the thickness calculations must be done keeping in mind the static and dynamic loads.

HOW TO SELECT THE RIGHT FBD MODEL/ FBD VENDOR (Tips and Tricks):

Here are a few tips that will help you choose/ select the most appropriate FBD brand Or Vendor who will manufacture the FBD for you. These tip and suggestions are over and above your Vendor Qualification protocols and procedures, and purely focuses on the Product/ Machine under discussion.

1. Drying Capacity Calculations: As far as possible, we must see to it that the vendor's design fulfills Or matches our calculated container volumes, thicknesses, finger bags area, heat load, blower fan dia and flowrate and other design parameters. If vendor has their own calculations available, we must see and evaluate them on our specifications and requirements.

2. Material of Construction,  Quality of Gaskets and Makes of BoughtOuts,  Adherance to GMP guidelines: FBD is otherwise a commonly manufactured products and hence a lot of vendors claim to have made outstanding FBDs, Therefore, apart from all other criteria, we must evaluate them on the selection of Material of Construction, Quality of Gaskets and Other BoughtOut items, Aesthetically Pleasing Quality and Surface finish etc. Pharma's requirement of Validation Documentation along with the Test Certificates too is an additional and important parameter.  

Please feel free to contact me regarding any questions, queries Or additional assistance needed on Design, Sales, Qualification or other aspects at   shg.techo@gmail.com