Before the Membrane Dryer: The Value of Pre-Drying
Membrane dryers are a pretty simple and effective way to remove moisture from a gas stream.
- They don't have a lot of moving parts
- They don't require a regeneration cycle
- They can run continuously with relatively little maintenance.
The membrane does its job by allowing water vapor to pass through the membrane while retaining most of the gas. As the gas moves through the system, moisture is removed and the gas leaving the dryer is significantly drier.
Where things can get interesting is the amount of moisture the membrane has to deal with.
If you're starting with a gas stream that has a high moisture load, you're asking the membrane to handle all of it. In some applications, it makes more sense to take some of that moisture out before it ever reaches the membrane.
That's where a pre-dryer can help.
What does a pre-dryer do?
A deliquescent dryer is one option for removing moisture upstream. It uses a moisture-absorbing media that pulls water out of the gas. As the media absorbs moisture, it eventually dissolves and is drained from the dryer.
The benefits of this technology include:
- zero emissions
- zero moving or wetted parts
- minimal purge loss compared to most other systems
The deliquescent dryer isn't there to replace the membrane. It's there to take some of the workload away from in the first stage rather then send the entire moisture load directly into the membrane:
Wet Gas → Membrane Dryer → Dry Gas
The membrane is doing all of the drying work. Without any protection leaving the dryer exposed to incoming conditions.
Wet Gas → Pre-Deliquescent Dryer → Membrane Dryer → Dry Gas
Now the pre-dryer (deliquescent dryer) takes care of a good portion of the moisture, and the membrane handles the final drying. Extending the life of your membrane and protecting it from any particulates or oils that could possibly exist in your systems pre dryer.
You're not necessarily asking the membrane to do something it couldn't do before. You're just keeping it from having to handle the entire moisture load.
Where Does a Pre Drying System Actually help?
Take a natural gas application as an example.
Let's say you have a gas stream coming in with a fairly high moisture content, but the gas leaving the system needs to meet a much lower moisture requirement.
You could size a membrane dryer to handle the entire job Or, depending on the conditions, you could put a pre-dryer ahead of it. If the membrane is exposed to a heavy moisture load for years, you have to consider what that could mean for membrane life, maintenance, and replacement.
And replacing a membrane isn't always as simple as buying a new part. Depending on where the equipment is located, there can be labor, shipping, and downtime involved as well.
A pre-dryer gives you another layer of protection. By removing moisture before it reaches the membrane, you're reducing the load that membrane has to deal with every day.
That doesn't mean a pre-dryer will automatically double membrane life or eliminate maintenance. Every application is different. But if the membrane is an expensive component to replace, it's worth looking at whether using a deliquescent dryer as a sacrificial pre dryer could help reduce your cost long term.
What does this look like in an actual system?
Using the earlier natural gas application where the inlet gas has a high moisture content.
A basic system might be:
Gas → Filter/Separator → Membrane Dryer → Dry Gas
Add a pre-dryer and you might have:
Gas → Filter/Separator → Deliquescent Dryer → Membrane Dryer → Dry Gas
The filter and separator take care of solids and free liquids. The deliquescent dryer removes a good portion of the moisture, and the membrane finishes the drying process.
You can use the same basic approach with compressed air or other gas applications where moisture is a concern.
The Case for Pre-Drying
At first, adding another dryer can seem like an unnecessary expense. If the membrane can dry the gas, why add something else?
The answer is really about what happens over time. By removing much of the moisture before it reaches the membrane, a pre-dryer can help reduce the workload on the membrane and protect it from the conditions it's being asked to operate in. That extra investment upfront may save you money when it comes time for maintenance or membrane replacement.
It All Comes Down to the Application
A membrane dryer may be all you need for some applications. But when you're dealing with a high moisture load, it’s worth looking at what the gas is bringing into the system before deciding on the dryer.
How much moisture is coming in? Are you seeing free liquids? What contaminants are present? And what dew point do you need on the outlet?
Those details can make a big difference in how you approach the system.
In applications with heavier moisture loads, adding a deliquescent dryer upstream can be a simple way to remove bulk moisture before it reaches the membrane. That means less work for the membrane and an added layer of protection for a component that can be expensive to replace.
The right combination of pre-drying and membrane drying can give you the performance you need today while helping protect the system for the long haul.
For more information on our deliquescent dryers, and how they may help your application contact us at 814-774-2631.