1 reply to this topic

[mike.h]

hi there,

i am currently in the process of designing a cryogenic air separation plant but i having some problems with the design of the pre purification unit. I am to remove water carbon dioxide and some hyrdocarbons but i am unsure which method to use. I have read up on thermal swing adsorption, pressure swing adsorption and molecular sieves but different texts reccommend different methods. I am to achieve 99% nitrogen oxygen and argon products. the flowrate goin into the PPU is 7000kgmole/h.

does anyone have any advice on what method i should use and where i can find information on the design and calculations for the process?

thankyou to anyone for their help, i appreciate it.

michael

[Art Montemayor]

Mike:

I started my engineering career in compressed gases, operating air separation units in 1960. The process hasn’t changed very much since then. The first rule to follow is to avoid ingesting any hydrocarbons into the process as mixed with your air feed. Common sense tells you that if you do, you are creating your own potential destruction. Air separation units are designed to create concentrated oxygen. If that concentration of oxygen is contaminated with any hydrocarbon (a combustible!), you will obtain the inevitable explosion and you don’t want to be around.

Water and CO2 are two other contaminants that cause a plugging of the process by literally freezing into the solid state while inside the cold box. This stops the entire process and can’t be tolerated. Those are the reasons for designing an efficient pre-purification unit.

I don’t know why you accept the contamination of hydrocarbons. No one today would do that. The location of the air intake is designed such that there is no possibility of that occurring. Furthermore, a special molecular sieve adsorber is located in the cold box to selectively “trap” any hydrocarbon – especially acetylides – that may migrate into the cold box. This adsorber treats the crude liquid oxygen coming out of the bottom of the high pressure column.

The most positive and efficient way to remove the water and CO2 is to employ a TSA (Temperature Swing Adsorption) Unit on the high pressure air prior to chilling it and introducing it into the cold box. PSA (Pressure Swing Adsorption) employs a regeneration method that is wasteful of the produced energy and regeneration air. Besides, PSA cannot produce the extremely low levels of water and CO2 (less than 1.0 ppmv) that are required in the product air stream.

There are several text books and references that you should be using. Kohl and Nielsen’s “Gas Purification” is just one. You haven’t mentioned what you are using, so why make me detail out what you already have or know? You should tell us ALL the basic data – and that includes all your background and preparation.