James:
Here are my responses--
1) A vent dryer can be reliable, depending on the amount of maintainence and upkeep that it receives; you don't describe what you mean by a "vent dryer" (it can mean several things) so I'll elaborate: your vent dryer is nothing more than a static bed of adsorbent (such as silica gel, activated alumina, etc.; it can also be made of a hydroscoptic substance -like a salt- that captures atmospheric water by hydration and drains the aqueous salty residue away). The drying substance (adsorbent or hydroscopic salt) doesn't regenerate by itself; it requires manual replacement on a timely and constant basis. This is a pain in the nalgas, as I've heard it described in Arizona. You have to furnish manual servicing and drying media to it constantly - and the quantity depends on the variations of atmospheric humidity where you are located. Additionally, I should warn you that the unit only "drys" the atmospheric air it comes in contact with -that which is the breathing air quantity that enters and exits due to thermal or filling effects in the tank(s). But the sad news is that the amount of "drying" it does can never compete with cryogen-sourced nitrogen. The vent dryer will dry to a very inefficient level as compared with any other type of dryer - so comparing it with nitrogen blanketing (not purging) is not an engineering logical alternative. There will be water vapor getting through the vent dryer and into the tank's vapor space allowing the Sulfuric Acid to slowly do its work in capturing it.
2) I don't have the slightest idea what you mean by "focus"; it doesn't work like a camera;
3) The only operational risks I have seen in the field are lack of operability due to little or no maintenance and subsequent severe corrosion from aqueous salt drainage due to using hydroscopic salt. This latter effect can be a mess.
4) I've never "used" them; I've found them in use (or rather, dis-use) where I've gone; I've stayed around long enough to see them eventually replaced - with a formal, engineered nitrogen blanketing system.
5) The suppliers don't claim to be experts; they'll sell you what you want. You have to specify to them what you want and they'll build, fabricate and furnish what you want. Actually, all it is is a static cannister of adsorbent sitting on a tank vent nozzle. If you correctly engineer your application based on the normal breathing and displacement needs (as well as the tolerable water content) you will, I believe, discover that the static cannister will be very large and almost impossible to safely maintain at an elevated height. This will force you to make a logical engineering decision and locate it at grade with large vapor piping, structural steel, ladders, platform, lights, OSHA handrails, etc. (see where I'm going?).
Unless you have the facilities to engineer it yourself and regenerate the adsorbent on an intermittant basis, I wouldn't recommend this system as a reliable, operative one.
Now, let's go back to something you wrote: "We use nitrogen to purge the tank." I wouldn't "purge" a Sulfuric Acid tank; I'd employ a constant, steady Nitrogen blanket on top of the fluid. That way, the only N2 consumption that I have to tolerate is the inventory displacement, breathing effects, and N2 leaks (if I allow them). If I balance my Nitrogen blanket between two or several tanks by filling into one while pumping out of another -while they're both connected to the same blanket header - I can reduce some of the displacement and breathing effects. I would not continuously run Nitrogen through the tanks and allow it to exit to atmosphere (called "purging"). I want to make perfectly sure that we understand the terms used to describe a system - otherwise, we'll find ourselves going around in circles trying to understand each other.
I hope this experience helps you out.
Art Montemayor
Spring, TX
