In industrial spray drying, even a minor parameter deviation can compromise an entire production batch, leading to costly downtime and material waste. This article will introduce common spray drying operational errors and how to address them, helping you minimize losses.
8 Spray Dryer Common Problems
Wall Sticking
If you notice the accumulation of wet powder or caked material on the drying chamber’s inner walls, top, or cone section, it most likely indicates wall sticking or chamber deposit. This is one of the most common dryer issues.
There are many reasons for this. If your material contains high levels of sugar, fat, or heat-sensitive components, it may be because the material has softened and become sticky after being heated beyond its Tg.
It could also be due to an excessively low inlet temperature or an excessively fast feed rate, causing droplets to strike the chamber wall before their surface has fully dried. Another possible cause is a clogged or poorly designed hot air dispenser, which causes the airflow to deviate and blow the atomized droplets directly onto the chamber wall.
How to Deal with and Avoid
If wall sticking occurs, you need to immediately reduce the feed pump flow rate and moderately increase the air inlet temperature. If there is already a significant buildup of material, shut down the machine to clean it to prevent the material from falling into the finished product and causing contamination.
When sourcing a spray dryer, look for options like air sweep systems or jacketed wall cooling to reduce this issue. You can also add drying aids with a high glass transition temperature like maltodextrin. Be sure to calibrate the hot air distributor before starting up to ensure that a correct airflow is formed inside the tower.
Inconsistent Particle Size Distribution
Inconsistent particle size distribution is primarily caused by spray drying atomization issues. This may be due to unstable pressure at the pressure nozzle or fluctuating rotational speed of the rotary atomizer. It may also be caused by wear that enlarges the nozzle orifice or partial blockage, which disrupts the formation of a uniform liquid film.
If you have confirmed that the atomizer is functioning properly, the issue may be that the material was not thoroughly mixed, resulting in sedimentation or stratification. This causes the feed solution entering the atomizer to have inconsistent physical properties and influences the particle size.
How to Deal with and Avoid
You need to check the stability of the centrifugal atomizer bearings and the variable frequency drive, or replace any worn nozzles. If the issue is with the feed solution, you need to perform in-line homogenization or continuous stirring.
Preventing this situation is simple. You can install a high-precision peristaltic cavity pump to ensure a stable feed flow rate. You can also install an in-line filter and a homogenizer in the feed line. Additionally, regularly inspect the atomizer and nozzles and replace them promptly if damaged.
High Moisture Content in the Final Powder
If the powder feels sticky to the touch, tends to cake, and is highly prone to mold growth or degradation after storage, this indicates that the powder has a high moisture content.
There are three main reasons for this. First, the outlet temperature is too low, resulting in insufficient latent heat of evaporation and a reduced ability to remove moisture. Second, the feed rate is too high, exceeding the drying tower’s rated evaporation limit. Third, the inlet air humidity is too high, making it difficult to dry the powder completely.
How to Deal with and Avoid
If this occurs, you need to reduce the feed rate and increase the inlet air temperature within allowed limits to raise the outlet temperature.
You can prevent this situation by closely monitoring and maintaining a consistent outlet temperature. You can also install a dehumidification system at the air inlet to eliminate the effects of seasonal and weather variations on the inlet air humidity. For materials with high moisture content, you can consider conducting pre-treatment like dehumidification.
Thermal Degradation or Scorching
If you notice black specks or yellow spots mixed in the light-colored powder, accompanied by a burnt smell, or if active ingredients have been severely inactivated, this is most likely due to thermal degradation or scorching of the product.
This may be caused by an excessively high inlet temperature that exceeds the material’s tolerance limit. It could also be caused by the residence time being too long. Alternatively, it may result from inadequate filtration of the inlet air, allowing external dust or impurities to be heated, charred, and drawn into the powder.
How to Deal with and Avoid
If this occurs, immediately lower the inlet air temperature. In severe cases, suspend discharge, remove the contaminated batch of finished product, and check the hot air distributor at the top of the tower for material buildup.
If your material contains heat-sensitive components, you can use co-current drying to protect the material by the evaporative cooling effect. Install a three-stage filtration system at the air inlet to ensure the cleanliness of the incoming air. You can also perform mirror-finish polishing on the interior of the tower to reduce retention points.
Low Powder Yield
Low yield may be due to a mismatch between the cyclone separator size and the airflow rate, or air leakage from the feed hopper, which disrupts the vortex airflow. It may also be caused by poor system sealing, resulting in powder leakage. In addition, excessive atomization may produce overly fine particles that are difficult for the cyclone separator to capture, further reducing the powder recovery rate.
How to Deal with and Avoid
If you notice a low powder yield, promptly inspect the rotary valve or the seal ring on the material collection bin located below the cyclone separator. If is spray drying atomization problems, reduce the atomization pressure or the atomizer rotation speed to increase the particle size.
You can use a multi-stage recovery system to prevent this situation. Also, remember to perform regular leakage tests on the spray dryer to ensure the entire system is properly sealed.
Nozzle Clogging and Feed Line Blockage
If you notice a sudden spike in the feed pressure gauge reading, an abnormal spray pattern, a complete cessation of spraying inside the drying tower, or unusual noises coming from the pump, it is likely due to nozzle clogging or a blockage in the feed line.
This is often caused by excessively high viscosity of the feed solution or the presence of large solid particles or foreign matter in it. It may also be due to crystallization or caking resulting from the feed solution remaining in the piping for too long.
How to Deal with and Avoid
If a clog occurs, immediately switch to a spare nozzle or line. Alternatively, stop the pump and quickly flush the line with warm water or a solvent.
If your material is highly prone to clogging, we recommend filtering it through an 80-120 mesh dual filter before atomization. Be sure to flush the lines and nozzles before shutting down, and never allow the process fluid to remain in the lines and dry out.
Ignoring Regular Cleaning and Maintenance
Regular cleaning and maintenance are essential. If cleaning and maintenance are neglected, cross-contamination is likely to occur when replacing products. This can also lead to abnormal noises from equipment bearings, aging of seals resulting in air leaks, and increased exhaust resistance.
Be sure to follow standard operating procedures for proper CIP (Clean-in-Place) or manual disassembly and cleaning regularly. Regularly inspect and replace quick-connect clamp seals, filter bags, and lubricating oil for centrifugal atomizers.
Improper Operating Parameter Settings
There is a close relationship between the various parameters of a spray dryer, and any deviation in a parameter setting can disrupt both thermal and dynamic equilibrium. This section will explain the effects of errors in different parameters.
Choosing the Wrong Inlet Air Temperature
Setting the inlet air temperature too high may cause rapid case hardening of the droplets, preventing internal moisture from escaping and resulting in hollow particles. This can even lead to scorching and coking.
Setting it too low will result in insufficient evaporation capacity, which may cause severe sticking to the walls and wet powder. You should set the temperature based on the material’s TGA results and heat-resistance limits.
Ignoring Outlet Temperature
The outlet temperature directly reflects the final relative humidity inside the tower. If the outlet temperature is too low, the product’s moisture content will inevitably exceed the limit. If the outlet temperature is too high, thermal efficiency will be low, resulting in wasted energy.
You can integrate feed rate and exhaust air temperature with PID cascade control. When the exhaust air temperature falls below the setpoint, the feed pump will automatically slow down.
Mismatched Atomization Pressure and Feed Flow Rate
When the feed rate is high, but the atomization pressure or rotational speed is insufficient, the droplets will be too large to dry completely. When the feed rate is low, but the atomization pressure is too high, excessive fine powder will be produced, reducing the yield.
We recommend that you set a reasonable air-to-liquid ratio (ALR). Calibrate the parameters strictly according to the rotational speed/pressure-flow rate curve provided by the equipment manufacturer.
Ignoring Feed Liquid Viscosity
Pumping a high-viscosity feed liquid directly into the atomizer will cause uneven droplet size and may even result in the spray dryer nozzle clogging.
If you need to process a high-viscosity feed, you can reduce its viscosity through preheating. Remember not to set the temperature so high that it degrades the material’s activity. Or use an in-line viscometer to monitor the viscosity. You can add an appropriate amount of solvent to dilute the feed if necessary.
FAQ
How to Choose between a Rotary Atomizer and a Pressure Nozzle?
You can choose according to the material properties and product requirements. The rotary atomizer relies on centrifugal force generated by high-speed rotation for atomization. It is less prone to clogging and is suitable for processing high-viscosity, high-solids, or crystallizing materials. Pressure nozzles are better suited for low-viscosity materials and can produce powders with larger particle sizes, higher bulk density, and better flowability. If your material is prone to causing equipment wear or requires a narrow particle size distribution, choose pressure nozzles. If preventing clogging is your priority, a centrifugal atomizer is the better choice.
How Often Should Spray Dryer Nozzles Be Replaced?
There is no fixed standard for replacement intervals. When processing highly abrasive materials such as ceramic slurries or titanium dioxide, standard stainless steel nozzles may wear out within a few weeks, while carbide or boron carbide nozzles can last for several months. If the particle size distribution widens, the moisture content of the finished product increases abnormally, the feed pressure drops for no apparent reason, or the spray pattern becomes skewed. These indicate that the nozzle orifice has worn out and must be replaced immediately.
What Is the Difference between a Co-Current, Counter-Current, and Mixed-Flow Spray Dryer?
The main difference lies in the flow direction of the hot air and the droplets. In co-current flow, the hot air and droplets flow in the same direction, using the evaporative cooling effect to protect heat-sensitive materials. In counter-current flow, the hot air and droplets flow in opposite directions, resulting in thorough drying and high thermal efficiency, suitable for heat-resistant materials. The mixed-flow configuration combines the characteristics of both. The droplets are first sprayed upward and then settle downward, extending the residence time. It is suitable for drying coarse-particle powders.
How Do I Determine the Correct Chamber Size Based on My Required Hourly Evaporation Rate?
The tower dimensions must be calculated based on the target evaporation rate, the temperature difference between the hot air inlet and outlet, and the residence time. First, the required hot air flow rate and heat load are calculated based on the weight of water (or solvent) evaporated per hour to determine the tower diameter. Then, the required height is calculated based on the material settling velocity and distance. This helps the droplets become completely surface-dried before they strike the chamber wall. Spray dryer manufacturers usually use CFD simulations to determine the tower dimensions.
Can a Spray Dryer Be Integrated Directly into a Continuous Upstream Manufacturing Line?
Yes, modern industrial spray dryers often serve as a core component of continuous production lines. You can install an in‑line homogenizer, a dual‑port reversing filter, and a continuous feed buffer tank at the feed end, and also deploy an automated PLC/DCS control system. This configuration can deliver seamless integration with upstream equipment. In addition, the system can integrate an in-line viscometer and exhaust temperature-interlocked feed control to adjust operating parameters in real time to maintain stable powder quality and physical properties throughout the continuous production process.
How Do Electrical Heating, Steam Heating, and Gas-Fired Heating Compare in Terms of Operating Costs?
Electrical heating requires a low equipment investment and offers precise control. However, electricity costs are high, resulting in the highest operating costs, so it is primarily used in small laboratories or systems with low airflow. Gas-fired heating is extremely energy-efficient and has low operating costs. It is a good choice for medium- to large-scale industrial drying. Steam heating uses waste heat from the plant, resulting in low costs. However, it is limited by steam pressure, and the heating temperature is typically below 180°C. It is often used as a preheating stage in high-temperature gas-fired systems to reduce energy consumption.
What Documentation Package (IQ/OQ/PQ) Should Be Requested from the Manufacturer for Pharmaceutical Validation?
Pharmaceutical clients must request a complete set of validation documentation (3Q package) that complies with GMP and cGMP regulations. Installation qualification (IQ) includes material certificates for equipment, P&ID drawings, instrument calibration certificates, and a list of spare parts. Operational qualification (OQ) includes no-load operation tests, PLC software validation (in compliance with FDA 21 CFR Part 11), and safety alarm and power failure tests. Performance qualification (PQ) provides load operation data, process repeatability data, and supporting documentation for cleaning-in-place (CIP) validation.
Final Thoughts
As a professional spray dryer manufacturer, Pilotech has over twenty years of experience in the research, development, and production of spray dryers. Our engineers can recommend the right equipment based on your needs. We provide customized solutions for a wide range of industries, including food, pharmaceuticals, and chemicals. Click to chat with our team.