Cleaning and Troubleshooting of Perma Pure Nafion™ Gas Dryers
Performance of Perma Pure’s MD, PD or ME Series gas dryers may decline over time due to buildup of residues on the inside or outside of the Nafion™ tubing within the dryer. This procedure describes the various steps required to identify the possible cause of the performance decline and offers options to clean the dryer in order to restore it to normal performance.
Disassembly of the PD-Series Dryer
- Disconnect the sample inlet and sample outlet lines from the dryer. If adequate access to the dryer is available, it is unnecessary to disconnect the purge inlet and outlet lines.
- Holding the coupling, remove the front nut on each end.
- Loosen the one set screw located near the threads in the coupling.
- Repeat for the other side.
- Remove the snap ring and the visible o-ring from both sides.
- Wearing rubber gloves, gently pull the element header out of the housing on one side. Be careful not to rotate more than 10 degrees in either direction.
- Remove inner o-ring.
- Gently pull the element out from the other side of the housing.
Visual Inspection of Nafion™ Dryer Elements
Although Nafion™ is not hazardous, the nature of any sample residues within the dryer is unknown. Harsh solvent(s) are used to clean the Nafion™ tubing. Wear proper protective equipment, including at a minimum protective eyewear and rubber gloves. If acids are used to clean the tubing, a lab coat or coverall is recommended.
Visually inspect the Nafion™ tubing element. Look for discoloration of the Nafion. Nafion™ tubing will discolor over time in normal operation, turning yellow, then progressively darker. This is normal, and does not seriously affect performance. The following is a chart illustrating some of the common problems, their causes and Perma Pure’s recommended cleaning method. See the note below for the ME Series.
|Observance||Likely Cause||Repair Method (See note below for ME Series) |
|Collapsed Tubing||Negative sample pressure||Methanol|
|Black, Brittle Tubing||Overheating||Acid Wash may be used but it is probable that the element will need replacing and the temperature adjusted. Note Anodized Aluminum shell is recommended if dryer is to be heated.|
| ||Ammonium salts commonly formed when ammonia is injected into stack gas exhaust as part of the DeNOx process.||De-ionized Water|
| ||Indication of contamination with salts from corroded metal upstream of the dryer. Red for copper, blue or green for stainless steel (chromium salts).||Acid Wash|
| ||Most likely caused by oils or tars (residues of organic compounds)||Acid Wash|
|Dark Oily Liquids||High-boiling liquid residues may be deposited within the dryer. Dark oily liquids are likely to be sulfuric acid residues.||Acid Wash|
|Bent Tubing||Twisting of element when assembling, disassembling or rotating fittings.||Methanol|
|Broken Tubing||Over-twisting of element, liquid water entering the tubing or excessive sample pressure and temperature||None – replace with new dryer|
- Remove any solids or other visible deposits from the outside of the tubing and from the epoxy header.
- Brush or scrub away any solids deposited on sample header.
- Wipe off any dust or liquid residues from the tubing and header with a clean soft cloth.
- Blow clean, dry air through the sample outlet end of the header to remove any entrained particles.
- To dissolve any salt deposits within the tubing, briefly inject DI water into tubing bundle with syringe or plastic squeeze bottle. Do not immerse the headers of the dryer element in water or excessively wet the tubing inside the headers. Nafion™ tubing swells when exposed to water. If the tubing inside the epoxy headers becomes too wet, it may swell sufficiently to crack the rigid epoxy header.
Caution: De-ionized water must be used. Nafion™ is a cationic exchange material, and when exposed to liquid water will exchange the hydrogen from the sulfonic acid with any cations in the water.
- Blow dry, oil-free compressed air through the dryer element tubing to remove any residual water.
- Begin blowing dry air through the dryer element quickly after wetting the element. This will minimize the risk of damage to the dryer due to swelling of the tubing.
- Continue for several minutes. This will reduce any swelling and restore the dryer to normal length.
- If the element appears to be free of all salt deposits and remains relatively clear in color, proceed to Reassembly of the dryer.
When Nafion™ comes into contact with an alcohol (ie. Methanol), it will swell up to 188% of its original size, both in diameter and overall length. It is because of this characteristic that kinks in tubing can be repaired.
For collapsed tubing;
- Remove the coupling from the shell.
- Reinstall the element into the coupling only.
- Slide element into coupling through threaded end
- Push until snap ring seats
- Reinstall the front nut.
- Connect dry air to the sample inlet port, do not turn on flow.
- Gently apply methanol to the element, do not go closer than 1.5 inches from the header.
- Slowly begin to flow dry air through the tubing and allow to expand.
- If there are still collapsed sections, apply methanol to those areas while air is flowing.
- Once tubing is back to the proper shape, continue to flow dry air for several minutes.
- Proceed to Reassembly of the PD-Series Dryer.
If oily residues are present, the dryer should be washed with acid to remove the residues and to recondition the Nafion™ (restore it to its original sulfonic acid form).
- Prepare a bath of 10% nitric acid (HNO3) in a suitable container. The container should withstand exposure to nitric acid and heating.
- Immerse the Nafion™ tubing into the nitric acid bath taking care not to wet the epoxy headers. About 1.5 inches (4cm) of tubing as well as the header should remain out of the acid.NOTE: Do not sharply bend the Nafion™ tubing during this cleaning process. The bath should be large enough to allow the Nafion™ to flex gradually. A magnetic stirring hotplate should be used if available. If not, the acid container should be heated in some fashion and stirred periodically.
Caution: Heating of the acid bath will generate noxious fumes. The cleaning process should be performed in a fume hood or very well ventilated area. Covering the container with a loose plastic lid will minimize the fumes.Caution: Rubber gloves and protective eyewear must be worn. Nitric acid at this concentration will stain and/or irritate the skin and injure the eyes if splashed.
- Heat the acid bath to 70°C to 80°C, and soak the Nafion™ dryer element for one hour with occasional stirring (a magnetic stirring bar set on low if possible).
- Remove the dryer from the acid bath. The acid bath may be covered and reused until it shows significant discoloration.
- Rinse the outside of the tubing and headers with de-ionized (DI) water at room temperature to remove the nitric acid residues.
- Briefly force De-ionized water through the inside of the tubing. This may be accomplished with a plastic squeeze bottle or syringe. Do not immerse the headers of the dryer element in water or excessively wet the tubing inside the headers. Nafion™ tubing swells when exposed to water. If the tubing inside the epoxy headers becomes too wet, it may swell sufficiently to crack the rigid epoxy header.
- Quickly blow clean, dry air through the dryer element to remove any residual water.
- Continue to blow air through the dryer for several minutes. This will reduce any swelling and restore the dryer to normal length.
- Install all (4) o-rings onto element.
- Slide element into shell and coupling.
- Install snap ring onto one end.
- Push element into shell until o-ring seats.
- Repeat for other end.
- Tighten smaller set screw located in coupling threads, older models only.
- Replace front nut.
- Connect the sample inlet and outlet lines.
- If necessary, reconnect the purge lines. Note that the purge inlet is at the opposite end from the sample inlet.
Effects of Liquid Water and Methanol / Alcohols on Perma Pure ME Nafion™ Dryers
Because of the unique construction of the ME Series, Perma Pure strongly recommends against using liquid water or alcohols to clean them. Both liquid types have the tendency to swell the Nafion, possibly causing the material to deform and kink within the protection braiding during full contact, restricting gas flow and reducing drying performance. In situations where this type of cleaning is deemed necessary, in order to limit the swelling effect, Perma Pure recommends that the cleaning process be done quickly and that clean dry air be used immediately following the cleaning to quickly remove the moisture from the tubing after contact with liquid water or alcohol.