In the world of compressed air and gas treatment, adsorption dryers play a critical role in ensuring dry, moisture-free air / gas at the outlet. Outlet moisture level varies depending upon the application and can even go down below 1 ppm.
Most of the application is to dry instrument air but dry gases/air also finds application in the process in industries like petrochemical, refineries, or food and pharmaceutical and others.
While most attention is usually given to the adsorption phase by specifying the adsorbent quantity, the regeneration process goes unnoticed but it plays an equally or more vital role in ensuring consistent dryer performance.
Why we need Regeneration Cooler? Why Cooling of Regeneration Air / Gas Matters Before Mixing with Inlet Stream?
Controlling the temperature of regeneration air / gas before it mixes with the wet inlet stream is critical for the long-term performance and reliability of the adsorption system.
If the regeneration air / gas are not sufficiently cooled before mixing with the incoming wet air / gas, even a slight temperature difference between the two streams can lead to undesirable condensation. As the hotter air / gas mixes with the cooler air / gas stream, the temperature of the combined stream may drop below its dew point, causing liquid water to condense. This condensate can migrate into the adsorbent bed and significantly shorten its life, leading to premature degradation, reduced adsorption capacity, and increased maintenance costs.
Moreover, the regeneration stream carries desorbed moisture from the previously saturated adsorbent bed. By cooling down to the flowing temperature of the inlet stream, the moisture in vapour phase is condensed and removed so that at the mixing point of both incoming and the regeneration streams have the same dew point.
Therefore, the cooling temperature is carefully selected so that the regeneration gas is in thermal equilibrium with the incoming inlet gas. This avoids re-condensation, protects the adsorbent, and ensures smooth, efficient operation of the dryer over multiple cycles.
To understand this better, let us explore the adsorption and regeneration process in detail in the following sections.
What is the Adsorption Process?
The adsorption process involves removing water vapour from compressed air or gases using a packed bed of adsorbents like activated alumina, silica gel or molecular sieves. As the moist gas flows through the bed, water molecules are adsorbed onto the surface of the material, resulting in a dry outlet stream. Since adsorbents have limited moisture-holding (equilibrium) capacity, this process cannot run continuously in a single vessel. Therefore, most commercial systems operate using two or more towers in a cyclic manner—while one tower dries the gas, the other undergoes regeneration. In a two-tower system, as shown in Figure 1, one tower is actively adsorbing moisture while the other is being regenerated. In a three-tower setup, shown in Figure 2, two towers remain in adsorption mode while the third is in regeneration. This arrangement ensures uninterrupted drying capability while allowing sufficient time for effective regeneration of the saturated bed.
Steps in Regeneration Cycle:
Step 1: Heating the Bed – Desorbing the moisture at certain temperature based on ISOSTERE of the respective adsorbent
The heating phase is initiated by introducing hot regeneration air / gas into the saturated tower. Initially, the heat energy is absorbed by the vessel wall, internals, and the mass of the adsorbent itself, gradually raising the overall temperature. Though the water start to vaporize at 100 °C, water starts to desorb from the surface of the adsorbent when the bed temperature reaches around 120°C (refer Figure 3, Curve 2 at Temperature T1), This marks the beginning of the desorption phase. At this phase, most of the heat is consumed in vaporizing the adsorbed moisture rather than increasing temperature, resulting in a temporary plateau in the temperature (refer Figure 3, curve 2 at interval B). Once the majority of the moisture has been desorbed—typically by the time the bed reaches around 125°C—the system enters in what is known as the trim heating phase. During this stage, the temperature is further elevated to let’s say 155°C (this temperature depends on type of adsorbent, type of gas and final moisture level required after the adsorption) ensuring that even the residual moisture is driven off and the adsorbent is fully regenerated.
Step 2: Cooling the Bed – Prepare for the Next Cycle
Once the heating cycle is completed, the adsorbent bed must be cooled before it can be reintroduced to wet process air/gas. This cooling is accomplished by stream temperature regeneration stream through the hot regenerated tower. During this phase, the heater is turned off, and air / gas flows through the bed, gradually reducing its temperature.
The cooling process isn’t meant to bring the bed temperature all the way down to the inlet air temperature. Instead, the target is usually 10 to 15°C above the inlet temperature to avoid pre-loading of the bed.
As the bed is cooled to 55 °C considering 40°C inlet temperature once the bed is switched there will be some temperature bump. Hence, cooling after heating is very important to avoid the temperature rise. Please refer our other insights for more details on effect of bed cooling and regeneration flow.
Selection of Heat Exchanger Type Based on Water Consumption Optimization:
Let us consider an example of an air dryer that handles 12100 Nm³/hr of air, fully saturated with water at 40°C and 7.7 barg at the inlet. The regeneration process is carried out using wet air regeneration in split stream mode. Heater set temperature is 1800C and the bed outlet temperature at the end of desorption is 1550C.
Now the cooling has two distinctive phases:-
Phae1 – To help desorption where the air/gas with moisture will come out at a temperature of 1200C (please refer figure-3)
Phase 2 – To cool the air/gas which is coming out during trim heating means above 1200C and bed outlet temperature (1550C) but without moisture load which is called trim heating.
Normally the phase 1 becomes the governing case but for low dew point units the trim heating may go up and the heat load comparison has to be done to arrive at the governing case.
Now let us understand how the exchanger is designed for the desorption case. The air/gas laden with moisture in vapour phase enters the exchanger at 1200C and leaves at 400C, which is the inlet temperature of the main stream.
In regeneration cooler for desorption , three types of heat transfer occur: sensible heat and latent heat.
Dry gas cooling: Mass of the air/gas without moisture which is 10010 kg/hr.
Sensible heat: Sensible heat is the heat that causes a change in temperature of a substance without changing its phase. Here this is the moisture and mass flow is 798.81 kg/hr which has to be cooled down to 400C.
Latent heat: Latent heat, on the other hand, is the heat exchanged during a phase change (like condensing of water) without a change in temperature. In this case the condensation will be 746.55 kg/hr.
So 52.26 kg/hr. moisture in vapour phase will go to mixer and then to the adsorption bed for adsorption.
The table below illustrates two design approach to optimise the water consumption:-
The two cases are highlighted in above table illustrates different flow configurations of the cooling water, largely dictated by the temperature approach.
Case 1:
In this case, the design is based on 1:2 approach (part co-current and part counter-current) means all TEMA configurations can be selected including AES configuration. Here the inlet temperature of cooling water is taken as 33°Cat inlet & outlet temperature of the cooling water is considered as 38°C to avoid any temperature cross, resulting in a low temperature difference (ΔT = 5°C), which leads to higher water consumption to achieve the desired heat removal.
Case 2:
In this case, inlet temperature of cooling water is 33°C & outlet temperature of the cooling water is 42°C, resulting in a higher temperature difference (ΔT = 9°C), which leads to lower water consumption—almost half of that in Case 1—to achieve the same heat removal.
To achieve this, a single-pass counter-current design is used and depending on heat load, the exchanger may get extend to 2 or 3 shell/tube passes in series. For execution, fixed tube sheet is the best choice and configuration can be AEL, BEM. Externally sealed floating tube sheet type AEW or floating head with backing device type AES can also be designed.
Floating head with backing device type AES is not recommended for manufacturing constraint and as the temperature difference is not that high and if required, this can be managed by AEW.
AES is recommended for toxic fluid, special gases whereas AEW is for non-volatile and non- toxic gases.
Even fixed tube sheet (AEL/BEM) with rod able cooler is a good choice from cost consideration.
Cooling water consumption in heat exchangers is closely linked to the temperature difference between fluids. A higher log mean temperature difference (LMTD) improves heat transfer efficiency, reducing the amount of cooling water needed. Conversely, a lower LMTD requires a larger surface area or higher water flow to achieve the same heat duty.
In practical terms, optimizing the exchanger design for temperature profile and flow arrangement is essential for minimizing water usage and improving energy efficiency. Counter-current configurations are generally preferred for their superior thermal performance and lower cooling water requirements.
The temperature profiles shown in Figure 4 illustrate different flow arrangements in heat exchangers and their impact on heat transfer efficiency. In counter-current flow (Figure 4.1), the hot and cold fluids flow in opposite directions, maintaining a nearly constant temperature difference across the length of the exchanger. This arrangement provides a higher log mean temperature difference (LMTD), which enhances heat transfer efficiency. As a result, less surface area is required for a given heat duty, and significantly less cooling water is needed compared to co-current flow systems.
In a 1:2 heat exchanger (Figure 4.2), the tube side fluid flows through the tubes twice, increasing the effective heat transfer surface area. Although the flow is mixed and not fully counter-current, the design still allows for relatively efficient heat transfer. This configuration helps optimize space while maintaining reasonable thermal performance and moderate cooling water consumption.
Figure 4.3 illustrates a temperature cross condition in heat exchangers, a scenario where the outlet temperature of the cold fluid exceeds the outlet temperature of the hot fluid — meaning 𝑡2>𝑇2. This phenomenon is typically avoided in co-current flow systems, as it indicates poor thermal performance and may lead to ineffective heat exchange. This can happen in a 1:2 heat exchanger in case we want to cool the air/gas to 40°C with a cooling water outlet temperature above 40°C.
The temperature cross generally avoided because it indicates a less efficient heat transfer process and can lead to wasted heat transfer area.
Conclusion:
Optimize Water Consumption:
Choose counter-current single pass heat exchanger configurations to achieve a higher temperature difference (ΔT), which enables effective heat transfer with lower cooling water flow rates, thus reducing water consumption and utility costs.
Balance both Efficiency and Cost:
For applications where space or budget is limited, multi-pass shell and tube exchangers like BEM or AEM can still be effective, though they typically require more water due to a smaller temperature gradient. These are better suited for compact setups.
Thermal Expansion:
For systems operating at high temperatures causing significant difference, floating head or floating tube sheet designs (TEMA S or W type) are recommended. These exchangers allow for thermal expansion and facilitate easy cleaning, making them suitable for long-term, and high-performance applications.
Simplify Maintenance and Cleaning:
Heat exchangers like AES or AEW come with removable bundles, which make them easy to maintain and clean but the maintenance cost goes up. This is helpful when the gas is dusty or contains impurities that might clog the exchanger.
In fact AEW can be a more economical option than AES, especially when temperature differences are not as severe and frequent cleaning is less critical.
For more details, reach out to us.
References:
- Gas Conditioning And Processing, Volume 2: The Equipment Modules, John M. Campbell
- Coulson & Richardson’s Chemical Engineering Design, Volume 6, R. K. Sinnott