Ensuring Consistent Spray Dryer OperationEnsuring Consistent Spray Dryer Operation
Operational factors that must be managed to ensure a spray dryer performs consistently
January 15, 2025

The steadiness of your spray dryer operation has implications for efficiency, product quality and safety, and your bottom line. This article will discuss the operational factors that must be managed to ensure a spray dryer performs consistently.
Spray drying is a thermal process that transforms a liquid mixture into dry powder. The driving force for removing liquid from the feed is a heated gas with sufficient energy to evaporate water away from the solid particles. Most spray drying applications use water as the carrier liquid for the solid and air as the drying gas, though specific applications will require alternative carrier liquids and gases.
This physical process is a complex mass and heat transfer operation that is difficult to quickly optimize for most products. Pilot scale tests can provide directional control parameters, but significantly larger production equipment will likely operate at parameters far from a successful pilot operation. Creating the exact same atomization conditions, air/liquid interactions, and airflow profiles is difficult, if not impossible, to replicate from pilot to production scales.
The spray dryer is an empty vessel with the sole purpose of housing an evaporation operation. Its job is to provide enough residence time for the drying air to remove water out of the liquid feed. The residence time is different for each product dried – some products require eight seconds of residence time to dry while others require 30 seconds or more. This is due to each product’s unique drying curve, characterized by two different drying kinetics: constant rate and falling rate.
Constant rate drying occurs when the droplets have reasonably high moisture content, and the evaporation rate is limited by how quickly water is removed from the droplet’s surface. As the droplet loses surface water, the diffusion of internally bound water to the droplet’s surface is the rate limiting step, which slows the evaporation rate.
The main takeaway from the drying curve is that sufficient residence time must be given to allow the product to dry from its starting moisture to the desired moisture of the final powder. Many other factors influence the necessary residence time, such as drying air temperature, liquid feed temperature, and droplet size – hence one must be cognizant about how each input variable change affects the powder produced in the dryer. More information can be found in textbooks, such as Perry’s Chemical Engineering Handbook, Chemical Process Equipment – Selection and Design, etc.
Process Optimization – Drying Air
Most steady-state processes run better when the input variables are stable. For the spray dryer’s air supply, the main parameters to keep constant are airflow, inlet air temperature, and inlet air humidity (if equipped). For the spray dryer’s liquid supply, keep the liquid solids content, liquid feed temperature, and liquid feed rate steady. Additionally, changes to feed composition and pH can significantly impact dryer performance – see examples in the Case Studies section.
Most large spray dryers operate via a push-pull system, where one fan supplies air to the dryer while a second fan pulls air from the dryer. The dryer’s airflow is held constant by setting one fan to a fixed speed while using the other fan to control the dryer’s pressure to a slight vacuum. The inlet air temperature is controlled by the heater’s input energy, which is typically a natural gas burner, oil heater, steam heater, or electric heater. It is important to properly tune these control loops to provide 1) stable chamber pressure (+/-0.05 in. wc) and 2) steady inlet and outlet air temperature (+/-0.5˚F). Large oscillations in these control loops are almost certain to cause poor product quality control.
To control inlet air humidity, a dehumidifier removes ambient moisture from the drying air to mimic the same ambient conditions year-round; the dehumidifying agent is typically a desiccant or a cooling coil. When using a cooling coil, the water removed is limited by how cold the circulating fluid can get – which is impacted by ambient temperature, glycol concentration, chiller capacity, etc. The circulating fluid should be as cold as possible to create “dry conditions” in the summer. Note that dehumidifiers won’t do any work in the wintertime, when the dew point is less than the cooling coil.
The outlet air temperature is a function of the evaporation, i.e. any change to air flow, inlet air temperature, liquid feed rate, liquid feed solids, etc., will change the outlet air temperature. For example, if the liquid feed rate drops, less evaporation occurs, driving the outlet air temperature higher. If the feed solids drop, then the evaporation increases, driving the outlet air temperature lower (assuming sufficient evaporative capacity). Hence, it is important to keep the aforementioned variables consistent to repeatedly produce quality product.
To expand on outlet air temperature control, there are two options generally used across the industry:
1) Setting the liquid feed rate to a constant value and adjusting the inlet air temperature to maintain the outlet air temperature
2) Setting the inlet air temperature to a constant value and adjusting the liquid feed rate to maintain the outlet air temperature
There are pros and cons to each method but keeping the other inputs constant is the key to successful operation. In my experience, control scheme 1 maintains constant operation better, but any immediate disturbance is not quickly remedied by the control system, especially if using an indirect burner. Control scheme 2 makes it easier to adjust overall capacity and quickly remedy sudden disturbances but tends to oscillate the process.
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Inelt fan
Process Optimization – Feed System
The final optimization key is to standardize the atomization operation. The first step is to standardize the feed solids, as this will impact feed pressure, feed rate, particle size, density, etc. In many cases, the upstream processes determine the feed solids consistency, hence the importance of collaborating with those unit operations.
Optimization practices will differ between using a rotary atomizer and nozzle. A rotary atomizer’s speed can be adjusted real-time assuming a variable frequency drive (VFD) is used; this gives the operator an advantage in particle size control. However, specific parts of the atomizer may need to be changed if specific products require vastly different operating rates. For example, multiple sizes may be needed for the liquid distribution ring and rotating disc if atomizer speed alone can’t manufacture the desired product quality across multiple product lineups.
In contrast, a high-pressure nozzle cannot adjust particle size in situ – the swirlchamber and orifice combination are varied to give the desired product quality. The combination used is highly dependent on feed solids, rate, and pressure, which further stresses the importance of keeping those variables consistent. Additionally, the quantity of nozzles used in the drying chamber may need to be adjusted based on the desired rate. Further complicating the atomization process is the presence of a fines return system, typically used to increase agglomeration. Note there are some processes that use two-fluid nozzles, which are not covered here.
High-pressure feed system for a spray dryer (Caloris Engineering)
Maximizing Throughput
To maximize throughput, there are three general principles:
1) Maximize the airflow
2) Maximize the inlet air temperature
3) Maximize the feed solids
There is a limit to each of these principles, all of which are related to quality or safety. The maximized airflow must still allow enough residence time for the powder to dry. The inlet air temperature must not be near the minimum ignition temperature for the powder, nor should it cause the powder to discolor. Lastly, the maximized feed solids must be pumpable and able to be atomized. Note that adjusting these parameters will also impact the product density and particle size, which may push the operation outside of acceptable powder quality. Thus, you can’t continue to push capacity if the product is of unacceptable quality.
Ambient Conditions
Ambient conditions can have a drastic effect on the dryer throughput and quality, especially for dryers using air drawn from outdoors. For instance, the ambient humidity affects the spray dryer’s capacity as follows: higher ambient humidity decreases capacity since the drying air already contains additional moisture. To mitigate this effect, a dehumidifier can standardize the humidity content of the drying air.
Another example is ambient air temperature, which increases in density as temperature decreases. Since the fans move a fixed volume of air at a given speed, a greater mass of air will be moved at lower ambient temperatures, thereby increasing the drying air supply. The ambient temperature can also impact the dryer wall temperatures, especially for outdoor installations. Many operators will attest to dryer capacities drastically changing when sudden cold fronts and rainstorms hit the facility.
Case Studies
Below are a series of case studies to demonstrate various theories of dryer operation.
Feed composition issues: The author once worked with several organic powders composed of a carrier solid, oil, flavor, and water. Since these components did not form a nice slurry on their own, the concentrate required homogenization prior to drying to keep the feed components well mixed. One day, the operator forgot to set the homogenization pressure to 2,000 psig and instead left it at 0. The dryer proceeded to spit out the carrier solid and syrup that coated the chamber and cyclone walls rather than flavorful powder. While we unintentionally produced tasty molasses, the product was unsellable in this form.
The homogenizing pressure was not increased via the crankhandle to 2000 psig. Source: Caloris Engineering.
pH issues: A concentrate required pH adjustment to meet quality specification. The author learned that this pH range was very tight – anything outside of the pH range produced sticky powder that quickly clung to dryer walls and smoldered.
A powder produced at different pH levels (Caloris Engineering)
Variable solids issues: A new spray dryer had oscillating feed pressure to its rotary atomizer, which caused quality issues. After much troubleshooting, it was determined that the feed tank had insufficient agitation to keep the solids uniformly blended throughout the vessel. The agitator was upgraded with more impellers and a larger motor, which resolved the oscillating feed pressure issue.
Airflow control/training: A facility sought to increase capacity. An operator demonstrated how the dryer process variables were controlled, which was semi-automated. The outlet temperature was controlled by adjusting manual butterfly valves on the inlet and outlet fans. The fans were on VFDs and set to a specific speed. The author explained to the operator that he wasn’t necessarily controlling outlet temperature by throttling the butterfly valves but was instead adjusting the airflow through the system. After maximizing the airflow, adjusting the inlet and outlet air temperatures, and increasing the product feed rate, the drying capacity was doubled. However, this higher airflow was enough residence time for the product run that day, but not a different product. To successfully dry a second product, the airflow had to be decreased to allow enough residence time to properly dry.
Summary
Optimizing your spray drying operation is a delicate balance of minimizing variation for as many process parameters as possible. Process control engineers and technicians should finely tune the control loops, but they require repeatable runs to adequately perform this action. You should also have solid operating procedures to follow for consistent setup, startup, operation, and shutdown – machines don’t install the swirlchambers and orifices in the nozzles – people do. Lastly, upstream processes may cause spray dryer issues as well, so it is important that these operations are also optimized.
Kyle Mathis is a senior process engineer with Caloris Engineering (Easton, MD), a provider of membrane filtration, evaporation, and spray drying equipment, as well as rebuilds and optimizations of these units. For more information, call 410-822-6900 or visit caloris.com.
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