How Different Types of Spray Systems in Fluid Bed Equipment Work (Specific Functions &Applications)

Fluid bed machines come in a wide range of designs and configurations that suit various applications in the pharmaceutical industry.

However, one distinct feature is always the exact location of the spray system.

Depending on the application or position of their spray system, you may classify the available fluid bed equipment as:

• Fluid bed drying machine
• Fluid bed granulation machine
• Fluid bed coating machine

With the advancement in technology, at times, we may combine two functions in one machine. Here are good examples:

• FBG Fluid Bed Dryer Granulator; the machine can dry and granulate powders at the same time.
• DPL Fluid Bed Dryer Granulator and Coater; the machine can coat and dry materials.

Having said that, now let’s focus on what makes these machines different based on the position of their spraying system.

In fluid bed processor systems, we have three distinct types of spray systems:

• Top spray
• Bottom spray
• Tangential spray

I will review all the three types of spray systems, while discussing their specific functions and applications at the same time.

This information is fundamental both in understanding the working principle of fluid bed equipment, besides choosing the right machine for material processing.

The image below shows a practical action of different sprays in fluid bed equipment:

These are different patterns of fluid bed processing techniques. Image source: Research on Fluid Bed Technology by S. Saurabh & M. Garima

1. Top Spray in Fluid Bed Systems

Can you see the exact location of the spray system as shown in the above image?

This is exactly what I am talking about here:

A top spray system

Of course, nothing much changes from other material processing procedures. That is, the entire process will still involve:

• Charging material into the processing chamber
• Fluidization process – forcing air to suspend the material
• Spraying process from the top: The discharging nozzle is designed to atomize air and spray liquid. It sprays the binder solution or active ingredient downwards.
• Drying the material
• Turning off fluidization process
• Discharging the final product

Normally, this process may vary slightly depending on the specific use of the fluidized bed equipment.

Functions of Top Spray in Fluid Bed Equipment

Like I said earlier, a fluid bed process will perform different task depending on the exact location of the spraying systems.

Under normal circumstances, we use the top spray in the following key applications:

• Drying wet granules and powder
• Granulation
• Coating (not very common)

So let’s discuss each of these processes based on the top spray technique:

· Drying Wet Granules and Powder

In most cases, you’ll find that the drying function in fluid bed technology can be found in a granulation or coating machines.

This is because the position of the spraying nozzle does not affect its how it works.

Technical drawing of a fluid bed dryer

The working principle of fluid bed dryers is simple and straightforward.

It involves the following key stages:

• Machine introduces hot air/gas from the bottom section of the system
• Increasing velocity of air causes an upward drag force that suspends materials you intend to dry (fluidizing the drying bed).
• Normally, cold air from the surrounding environment pass through heaters in the fluid bed dryer machine
• As hot air passes between the moist particles, it sucks the moisture leaving material dry
• Air leaving the fluid bed dryer passes the filter where tiny particles of materials you were drying are recovered.

Below is a complete video showing how the process works:

Clearly, this is a simple procedure that can easily be integrated with other material processing techniques.

Take for instance our Mini Laboratory Fluid Bed Dryer Granulator and Coater.

NOTE: The position of the spray nozzle does not play a critical role in fluid bed dryer machines.

· Top Spray Fluid Bed Granulation Machine

As I had mentioned earlier, you can integrate the drying and granulation processes in a single machine. Normally, top spray is widely used in fluid bed granulation machines.

This is due to its efficiency and reliability in the entire process. Let’s have an in-depth insight of fluid bed granulation machine.

A close analysis of our fluid bed equipment has revealed a number of factors that you need to consider when buying any of these machines.

You can watch this video first to have a clear picture of what I intend to discuss in this section.

From the video above, it is quite clear that the entire system may be categorized as having these key sections:

• An exhaust system
• Air handling system
• Product container; also called the expansion chamber

All these sections control different process variables that affect quality of the final product. I’ll make this simple for you by highlighting all these fundamental aspects below:

Key Process Variables in Top Spray Granulation

By adjusting certain process variables, you can achieve the desired agglomeration specifications.

Our machines offer reliable and practical mechanisms to adjust all parameters.

This is a summary of fluid bed granulation process. Source: ResearchGate

Remember, all these variables affect the nature of top spray granulation end product. Some of these variables include:

• Inlet air temperature
• Fluidization air volume
• Atomization air pressure
• Rate of adding liquid

In a research paper that was published in 2010 in the International Journal of Pharmaceutical Sciences and Drug Research.

It’s quite evident that:

• Increasing inlet air temperature reduces the average granule size
• Fast flowing binder solution causes formation of less-friable and larger than average granules.
• Increasing atomization pressure decreases size of the granules

As you can see, every process variable determines quality of the final product.

However, atomizing air pressure and adding more liquid still remain critical parameters.

Just in case you’re new to the fluid bed granulation process, you can contact our technical team.

We will explain everything you need to know about these machines.

Particles float in the air freely as they are sprayed with appropriate binder

It is for this reason that top-spray fluid bed process is suitable for:

• Fluid bed spray drying
• Granulating with binders
• Granulating with water

Clearly, you can use a top spray granulator to agglomerate fine and free flowing granules easily.

· Top Spray in Fluid Bed Coating Machine

The application of top spay fluid bed process is quite limited despite the fact that is a popular and simple process.

For instance, top spray fluid bed coating is not suitable for applications where quality and uniformity of film thickness are of essence.

How a top spray fluidized bed works

For a fact, a top spray coating fluid bed process is common in fine chemical industry, animal feed and food processing.

NOTE: Don’t expect a perfect membrane when you use top spray in fluid bed coating.

For high quality fluid bed coating, you need a machine whose spray nozzle is located in a different location.

This is why as fluid bed equipment manufacturers we recommend bottom spray nozzle.

2. Bottom Spray in Fluid Bed Systems

Unlike in top bed fluid bed equipment, here a spraying nozzle is in the bottom section.

This is exactly what I am talking about:

Liquid spray is in the bottom section

In this machine, hot air flows through the bottom section of its column, blowing all powders/granules upwards.

As these materials move upwards, the bottom nozzle sprays them homogeneously.

A bottom spray fluid bed process is commonly used in powder and granules coating in the pharmaceutical industry.

It provides a uniform and excellent coating on every material.

Bottom spray coating is uniquely identified with the following features:

• Spraying nozzle is located at the bottom of a fluidization chamber
• Minimal spray drying due to the short distance between nozzle and particles
• High coating efficiency and uniformity

Again in this process, to ensure efficiency and high quality of the final product, it is important to monitor all process variables.

This helps to maintain them to a certain level where an optimal result can be realized.

A fluid coated particle

You can use this process to coat both granules and powders.

Remember, the bottom spray in a fluid bed system ensures an even and uniform coating, hence, an optimal film quality.

This is the main reason why as fluid bed coating machine manufacturer, we prefer doing the spraying process flow below.

We ensure the spraying nozzle is integrated within the bed system such that; it is completely surrounded by the product you intend to coat.

In short, with a bottom spraying in fluid bed coating equipment you should expect

• Uniform coating
• Homogeneous and high film quality
• Particles don’t stick together due to high kinetic energy the system generates
• No premature evaporation of carrier solution

NOTE: For any agglomeration or granulation process, I will recommend top spray coating while for fluid bed coating you should consider bottom spray.

Important Process Variables of Bottom Spray Process

Apart from the key variables I had mentioned earlier in the top spray granulation process, there are certain fundamental aspects to consider.

• Nature of air distribution; air distribution plates come in a wide range of sizes and shapes to accommodate different fluidization processes.
• Partition height; a range of factors determine partition and these include the size of particle, set velocity and material density

At times, you may find a top spray fluid bed machine that can be modified to become bottom spray fluid equipment.

This will be at the discretion fluid bed machine manufacturer.

Apart from these two, we have adopted side or tangential spray mechanism in fluid bed systems.

Let’s see what this process has to offer.

3. Side/Tangential Spray in Fluid Bed Equipment

This is an innovative technology that is slightly different from the top and bottom spraying.

The good thing with tangential fluid bed equipment is that you can comfortably use it as a:

• Fluid bed granulation machine
• Fluid bed coating machine

Don’t worry, I will give an in-depth analysis of each process shortly.

But first, I want you to have an overview of this technology.

You can compare the diagrams below:

This is an image of the three main types of spray nozzle positions in fluid bed system. Image source: ResearchGate

Clearly, from the above image, you can see the nozzle is positioned on the side of the fluidized bed chamber.

For a comprehensive understanding of this concept, let’s analyze these two cases:

· Side/Tangential Spray in Fluid Bed Granulation

This process is slightly different from the other two fluid bed processes in the sense that

• Nozzle is positioned on the side of product container
• It has a spinning, variable disc

As a result, whenever this machine is operating, the product will be subjected to a wide range of forces

• Centrifugal force due to the spinning disk
• Lifting force due to air volume through the adjustable disk
• Gravity force

All the three forces help to move the product to ensure homogenous granulation.

A fluid bed granulator system

Still, to achieve this, you need to control the centrifugal force by determining the disk speed and air volume/velocity. You can do this by varying the disk gap.

Remember, it is the air volume and velocity that lift particles.

As you can see, this process tends to be complex compared to the top spray in fluid bed system.

But, under what circumstances should you opt for tangential spray fluid bed granulation machine?

Side/Tangential Spray vs. Top Spray Fluid Bed Granulation Technique

Now, let’s have a quick comparison of these two processes in fluid bed granulator systems:

 

Still, I can recommend tangential spray fluid bed system for granules that should be coated.

This is because you can perform all the operations within the same machine without making complex adjustments to it.

Of course, you can remember in the early stages of this article, I did indicate that top spray fluid bed processor is suitable for powder granulation.

· Side/Tangential Spray in Fluid Bed Coating Equipment

This process is quite similar to the bottom spray coating except for the existence of a rotor disc that is driven by an electric rotor.

For instance

• No possibility of particles granulating due to high rolling motion
• Coating substrate is sprayed at the same time as product rotates

However, when you choose this process; it is important to consider the following key aspects that may affect the quality of your coated products.

• Side spray fluid bed coater has a high kinetic energy that makes it difficult to coat small particles.
• It’s difficult to coat very large and non-spherical products; there is a high chance of destroying them.
• Always take care of high mechanical forces that are produced during the process

At times in pharmaceutical and food industries, you’ll have no choice but to use side spray fluid bed processor.

Good examples of such situations are:

• When processing compounds whose exposure to solvents or water should be minimized; you can connect powder feeder, which is not the case in both bottom and top spray systems.
• Producing pallets or granules that require subsequent coating.

I am sure with all these, you can explain how different types of spray systems in fluid bed equipment work.

More importantly, you can choose an appropriate machine for a suitable application.

Conclusion

You should use top spray for agglomeration/granulation, bottom spray for coating and side/tangential spray for processes that require granulation and coating at the same time.

Of course, there are certain exceptions that I have mentioned in the article.

You need to consider such rare situations. If any of these prove difficult, then I suggest you contact us for further clarifications.

References

• Fluidized Bed– By: L.M.College of Pharmacy
• Fluid Bed Bottom Spray Pellet with Particle Coating– By: FN Christensen
• Lab Fluid Bed Granulator Coater Video– By SaintyCo
• A Guide to Spray Technology for Fluid Bed Applications
• Comparative Study of the Fluid Dynamics of Bottom Spray Fluid Bed Coaters– By L.W. Chan & S.K. Tang