We get a number of questions about the dehydration membrane separators and how they work. One of the confusing aspects of membrane dryers is that they do not process liquid water.
Sounds confusing – right? A dryer removes moisture. So why would a membrane dryer not remove water?
Gas separation membranes work at the molecular level. When a compressed air stream is being “dehydrated” the water vapor is actually being separated from the compressed air stream. This separation is the result of the very fast water molecule being able to diffuse into the membrane material and permeate out much faster than other air molecules like oxygen or nitrogen. This is not the same as a “filter” which blocks particulates and holds onto them. The smallest water droplets have a diameter of 0.01 to 1 micron, which is very tiny. But, water molecules are incredibly small with a typical diameter of 2.75 Angstroms (0.000275 microns). This makes the tiniest liquid phase water droplet 3600x larger than its vapor phase cousin. The droplets don’t have a chance of diffusing and permeating simply because they are too large.
Standard pretreatment is to ensure that all liquid water and entrained mists are removed. This also includes all traces of compressor oil, which will also cause damage. In properly designed systems, PRISM Membrane dryers will deliver consistently dehydrated compressed air (or gas) streams for years without the need to replace parts or consumable media.
Our nitrogen and air dehydration membrane separators were featured in last week’s publication of Compressed Air Best Practices Magazine. The end user company, Basil V.R. Greatrex, worked with our partner company, Titus Co, to incorporate PRISM membrane technology into a nuclear magnetic resonance spectroscopy operation. The project required both high purity nitrogen generation and compressed air dehydration to a dew point of -80°C. Our business was happy to partner with Titus Co to achieve the solution. Read the full article here.
Compressed air membrane dryers are typically smaller and lighter than desiccant and deliquescent dryers. More importantly, air dryer membranes require much less maintenance and attention than the other methods of dehydration. Check out our air dryer technology comparison below to learn more about the simplicity of membrane dryers.
We have received multiple requests recently for diagrams of our membrane separators. Here are some educational and simplistic diagrams to show you how membranes work in air separation, biogas upgrading, air dehydration, and hydrogen purification.
An interview with a dryer specialist from Air Products PRISM Membranes.
Q: What is the current level of involvement that Air Products has in train brake related applications? A: We supply our product line of PE air dryer membranes to several builders of air compressor systems that go on various types of trains. These customers are located in both Europe and North America, and all of our membrane products are manufactured in St. Louis, Missouri USA.
Q: Why must compressed air (on trains) be dried? A: Trains often use compressed air to power various components such as brake systems, pneumatic door systems, and even pantographs. Any humidity in the compressed air can cause deterioration and corrosion. Drying the compressed air with membrane dryers eliminates the risk of malfunctioning. Air dehydration is especially important when trains travel to areas with outdoor temperatures below freezing. In this case, the membrane dryer prevents moist compressed air from freezing and clogging the lines.
Q: What type of trains require these air dehydration systems? A: The majority of the trains that utilize our air dryer membranes are passenger trains. Some are high speed trains and even local street cars.
Q: Where are the membrane systems located in the train? A: That depends on the type of train. In some installations, the system is inside a sheltered area of a train car, while other installations have the membrane system located externally on the roof! This means that our membrane dryers may be exposed to very hot or very cold temperatures and even harsh weather.
Q: What about other drying technologies? A: Other compressed air drying technologies are common as well, but membrane dryers are often selected for their convenient smaller size, lighter weight, and mobility. Recently, a large, well-known compressor company had a prototype design that included a desiccant dryer. Maintenance technicians would have been required to use lift assist equipment to perform service on the dryer. Ultimately, the customer committed to membrane dryers in place of the desiccant due to the ease of accessibility and nearly 400 pound difference in weight.
Our Norway division is known for building world class nitrogen membrane systems for shipboard, offshore, and land based oil & gas projects. But you may be surprised to know that they also build membrane systems for air dehydration, hydrogen recovery, and CO2 capture. Each system application is detailed on their recently updated webpage. To learn more, meet the team at the upcoming SMM Maritime trade fair (booth 423) next month!
Does your facility use compressed air? For low volume compressed air dehydration, our smallest membrane dryer product is the PE1015. These small membrane dryers dehydrate compressed air for pneumatic tools, train brakes, and laboratory air compression systems.
PE1015s only weigh about one pound each and the rest of the dimensions and performance data are shared in this data sheet.
When installing membrane separators into a system, it is extremely important to position them properly. This PRISM Membrane Installation Guide provides recommendations for positioning the membrane vertically, horizontally, and also indicates airflow direction.
We offer three different product lines of membrane dryers, but most air drying applications can be served by our 2 common types of membrane dryers: Cactus, and PRISM PE dryers. The third offering is one of the more rare products at PRISM Membranes. The high pressure dryers, also known as interstage dryers AND also known as HD dryers, are rarer because they serve air drying applications up to 1200 psig.
Common compressed air drying activities for pneumatic tools or point of use applications are often taken care of by our lower pressure dryers which run at pressures up to 300 psig. However, for heavy duty locations like power plants or military ships, our high pressure dryers are the reliable solution. For dimension variations and operating limits of the high pressure membrane dryers, view this data sheet.