Demystifying Dehumidification and Drying Part 4: Dehumidification in Spray Drying
- Posted on 15th May 2025
- in Dehumidification, Dryers, Spray Dryer
- by John Real
Welcome back to Part 4 of our multi-part series on Dehumidification in Dryers! After an excruciatingly five-year-long wait, this part of the series is where we finally apply dehumidification to spray drying. If you missed our previous parts where we discussed why you would want to consider dehumidification in spray drying, our primer on psychrometrics, or a comparison of the two main dehumidification technologies used in spray drying, be sure to check those out!
In the first part of this series, we looked at the impact of the time of the year on a spray drying facility in Wisconsin. A spray dryer designed to achieve the desired production rate must be designed with summer conditions in mind.
However, by designing the spray dryer to meet production requirements in the summertime, the spray dryer ends up being oversized for the wintertime. In this case, about 13% oversized in the wintertime.
Let’s have a look at how dehumidification impacts the spray drying curve.
Cooling-Based Dehumidification in Spray Drying
In part three, we discussed how cooling-based dehumidification works by cooling the air to the saturation point and then further cooling to remove water from the air stream.
For our spray dryer in Wisconsin, our outside air condition of 83°F and 72% relative humidity is where we start.
We cool the air stream down to 72°F to achieve 100% relative humidity and then continue cooling to 45°F. By doing this, we remove 80 grains of water per pound of dry air, reaching an absolute humidity level of 46 grains/pound compared to where we started at 126 grains/pound.
Once we reach our 45°F, we will then heat the drying air to our desired inlet temperature for our spray dryer of 360°F. Once we reach our inlet temperature, we dry our product to our desired outlet temperature and humidity.
By removing the moisture from the air through cooling dehumidification and targeting the same outlet relative humidity, we can achieve a 13.5% increase in water evaporation per pound of drying air in our spray dryer system.
Let’s see how this compares to a desiccant system for dehumidifying our process air.
Desiccant-Based Dehumidification in Spray Drying
Starting at the same inlet conditions to the spray dryer, we know from part three of this series that the adsorption process increases the process temperature when we remove water from air.
From 83°F and 72% relative humidity, we dry the air to 14 grains per pound, the minimum for a desiccant system we would want to target for a spray drying system. In the adsorption process of drying the air to our 14 grains, we increase the temperature of the air along the enthalpy line to 156°F as the latent heat energy stored as water vapor in the air is converted to sensible heat energy.
Once we have dried our air, we heat the air to our desired spray drying temperature of 360°F and then dry our product to our desired outlet temperature and humidity conditions.
By utilizing the desiccant rather than the cooling dehumidification, we can theoretically gain 15.2% over our original design condition for the summer.
Combination of Desiccant and Cooling-Based Dehumidification in Spray Drying
Desiccant dehumidification however has its limits, and based on the type of silica or desiccant used in the system the maximum level of moisture removal from the air is generally accepted to be not more than 15 grams of water per kg of dry air or 105 grains of water per pound of air, but generally 12 grams is what most desiccant manufacturers can reliably deliver. This would mean that in our Wisconsin example, starting at 18 grams of absolute humidity in the summertime, we could likely achieve a lower of 3-6 grams of water in the air.
By combining cooling-based and desiccant-based dehumidification, you can exploit the strengths of both technologies to achieve lower humidity and potentially achieve higher production than you may be able to achieve with either technology alone.
Cooling the air to 45°F prior to introducing the air to a desiccant wheel brings the total moisture content of the air down from 18 grams to ~6.6 grams at 100% humidity or the saturation point. Desiccant wheels are more efficient the closer that the air is to the saturation point, and therefore the wheel itself can be smaller. The desiccant then begins the spray drying process, reducing the moisture content from 6.6 grams to the desired end point of 2 grams.
Climate Impacts Throughout the Year
While Figure 1 focuses on the average summertime and wintertime conditions and their effect on the spray drying curve, the reality is that there is a gradient of conditions throughout the year that impact the spray dryer operation. Figure 5 displays a graph and table of the average absolute incoming moisture conditions for our central Wisconsin Spray Drying plant throughout the year.
When we introduce dehumidification into a spray drying system, we can eliminate some, if not all, of the yearly production variability that is a result of the changes in the climatic conditions through the seasons.
If this is incorporated into an existing plant, this can result in increased annualized throughput for the facility as well as stabilizing of the process, resulting in less downtime due to fouling and plugging that can be caused by weather upsets or operator inexperience.
When these technologies are incorporated right from the beginning of the design process, they can result in an overall reduction in the spray dryer system size, as we do not need to consider ensuring production rates during the more difficult drying times of the year.
Contact us for more information on adding dehumidification to your spray drying processes!
John is a Process Industry Expert in Spray Drying and Evaporation, responsible for sales and business development for custom process solutions in the sanitary food, beverage, and dairy industries. John has a wide range of project execution-related experience, including, but not limited to, process engineering, system installation and commissioning support, documentation control, purchasing, and equipment quality control. His hands-on experience ensures that he seeks the best solution for our clients.
In addition, John leads a small team of engineers and designers to ensure the efficient and timely delivery of process engineering, equipment, system changes, and optimization efforts, helping our customers maximize the return on their capital investments.
Tags: Dehumidification, Dryer, Dryers, Powder production