2 replies to this topic

[vicki15]

I'm a final year chem eng student at Nottingham Uni, Uk, currently undertaking my final design project on a cryogenic air separation plant. Unfortunately, due to the extreme temperatures involved i have come across a few problems. Firstly, in mechanical design of my columns any design stresses I need for my materials (304 stainless steel) are only listed to 0 C in the literature, unfortunately not as low as the -185 C i require.
The second issue is in terms of insulation. I have yet to find a suitable material and equation that will allow the necessary thickness to be designed at such extreme temperatures.
The third and final (for the moment!) is the internal design of my distillation column. I used hysys, then standard equations to determine basic data such as height of packing and diameter of column, but unfortunately I am finding it very difficult to find any information on either methods of design the other necessary column internals (packing support, liquid distributors, hold down plates etc), or even standard values for example items such as dimensions and weight, basically the elements needed to complete my internal structure of the column and complete my mechanical design.
If anyone has any advice or any good places for me to look i would greatly appreciate it.
Thank you

[Art Montemayor]

Vicki:

Unlike you, I had my first exposure to an Air Separation Unit on my initial job straight out of university as a 23-yr old Chemical Engineering graduate. I was assigned to manage and operate an industrial gases complex outside Kingston, Jamaica in a small township called “Ferry”. Like you, I was taken aback by the tremendous and awesome low temperatures we had to achieve just to liquefy the air. Then I had to conceive how the distillation was taking place inside a “Cold Box” --- and that’s all I could see: nothing but an external box. It seemed like a weird situation for me and I thought I would never get an opportunity to see what was actually inside this box and whether people were lying to me about what was inside.

It didn’t take long for me to have to get to the bottom of things and have to literally dismantle the entire “box” and get to the guts of the situation. Since then, I worked with other Air Separation units and in all I’ve worked both with Copper and Stainless Steel as the prime materials of construction. I like Copper better; it’s easier to work with and simple to comprehend. Since then I’ve learned that the softer the metal is at ambient conditions, the tougher it will be at the cryogenic level – at least as a “general” rule. I know this works for Copper, Lead, Brass, and Stainless. Therefore, if you employ the ambient temperature Stainless yield stress values, you will be conservative in your calculations. Nevertheless, there are stress values for Stainless – of that I’m sure. I know I had some data on the subject but I don’t have it at hand at present. What you can do is look through your telephone directory or “Google” on the net and locate a liquid Oxygen or liquid Hydrogen tank fabricator and ask them for the information. I’m sure that you can obtain the information that way – especially in the UK, where people are so cordial.

The insulation that was in vogue in my time was “rock” wool or perlite. Both were “poured into the cold box and I remember the itchiness that the rock wool caused. I would be scratching for days after a maintenance job on one of the air columns. This is very cheap and ordinary material and is very effective. You should be able to find this information on the Web. Don’t forget that the insulation within the cold box is constantly purged with Nitrogen in order to keep out any external humidity which will nullify any insulation within a couple of days due to the formation of water ice inside the cold box.
You don’t need any fancy –schmancy insulation materials other than the type I’ve describded. What really does the insulation effect is not the material but the trapped, static air or Nitrogen within the material. I remember a thickness of 12” to 18” as being enough to insulate the column.

I don’t understand what you need regarding the internal design of the column. Do you mean Process Design or Mechanical Design? Why don’t engineers (or soon-to-be engineers) specify what type of design they mean? In my time Air Separation columns were designed by the McCabe-Thiele graphical method. There were no computers then – not even hand calculators. We used slide rules. So it can’t be that tough! I would never employ any packing in a cryogenic column. The expansions and contractions that the equipment has to endure would crush the cylindrical walls when they tried to contract around the internal packing. Trays have been traditionally used. Perhaps structured packing has been used, but I am unaware of that.

Don't be too scared with the ultra-low temperatures. The application is actually an easy one, as you will soon find out. For example, there is no cleaner fluids than the cryogenics. There is no fouling or foreign objects to worry about. The stuff is so clean that it follows the theoretical predictions very closely. You can solder or weld in shut every equipment and pipe. This makes for a simple and straight-forward piping application. You have a minimum of pumps involved: only those to pump out the products. Everything else inside the cold box takes place due to gravity flow. I think the Prof likes you. You got an easy and fun assignment.

I hope these comments help you out somewhat.