1 reply to this topic

[shockerg]

hi to all

i am a final year chem eng student, and i am currently designing a separator for a benzyl alcohol plant.
i am just enquiring as to the easiest way and most cost effective way to separate methane from hydrogen on this plant ( the feed is hydrogen& methane in one stream and benzyladehyde in another)

my data are as follows:

mole fraction of hydrogen in feed stream 0.998732
mole fraction of methane in feed stream 0.001268


the mass flowrate of feed is 25.1 kg
the temp of feed © = 19.85
pressure = 1.5 bar


the only way which i believe is a little expensive is the pressure swing adsorbtion system, as i have a high concentration of hydrogen in the feed.

i would be most grateful for any feeback

ramy tolba

[Art Montemayor]

Ramy:

I believe you are correct in considering Adsorption as the Unit Operation to purify a Hydrogen stream even further than it is already. 99.87322% by volume is already considered very pure Hydrogen in many industrial applications. But I've produced "five nines" (99.999 % H2) without any problems using Molecular Sieves and PSA (Pressure Swing Adsorption). However, I would not do that today if I could employ TSA (Temperature Swing Adsorption). The PSA process wastes some H2 and I would price my 99.999+ H2 very highly.

You should have no problems with a 2-tower TSA system based on Molecular Sieves. You haven't said what purity you are targeting, but you will have difficulting in measuring 99.999+ so you should be OK in just sizing the adsorption beds such that there is never any breakthrough.

I don't know where you heard it or read it, but fixed-bed adsorption is not an expensive process. It is relatively very low cost in capital and more so in operating costs if you have waste heat such as steam. There are no moving parts! The maintenance is next to nothing. The Adsorbent will last for years and is dependent on the cleanliness of your feed and regeneration gases.

The adsorption application is a natural for this kind of separation. The only problem I see is the very low pressure. You are going to have to compress the gas up to a decent level to facilitate the Mol Sieve in doing it's work and reducing the amount of vessel and adsorbent. I would guess you would compress around 250 to 500 psig. You may have to do an optimization: test the effects of gas pressure on the amount of capital monies required. Normally when handling Hydrogen as a feed or a product you have to deal with it in a compressed, higher than normal pressure. Therefore, you might be adding value to the product by compressing the feed stream (to effect better adsorption) and obtaining a high pressure H2 product that you can store or pipeline to a consumer.

You will also have to come up with a regeneration gas if you use the TSA method. Normally, a stream of Nitrogen is used for this purpose.