4 replies to this topic

[Art Montemayor]

Dattatreya:

Your queries seem to be more in the light of a Student Forum subject, rather than Industrial Forum material. I don’t write this to demean or to categorize the importance of the matter; rather, I point this out to qualify the detail employed in answering and the degree of explanations given. I don’t have a good idea of your receptiveness, experience, or preparation in this subject. Therefore, I’m going to play it safe and go into larger detail than I normally would.

First, let me point out that, in the process of planning or operating an air separation plant, one should not have to take into consideration the inclusion of such “exotic” impurities as SO2, SO3, or NH3. The siting of an air separation plant is a serious and methodical procedure that always takes in considerations to avoid any inclusion of the sulfur compounds, ammonia, and especially any acetylene in the vicinity. If these compounds already appear in your air intake, you’re in bad trouble.

1) From a practical standpoint, it is foolish to rely on any CO2 being removed in the water condensate collected in the air compressor intercoolers. The quantity of CO2 and the completeness of Henry’s Law is just not worth the bother to calculate. If you are asking a theoretical, student-type of question then yes, there should be some CO2 dissolved in the condensate. But the quantity and the rate is of no consequence for the design of the purification train downstream.

2) As stated above, you are “spinning your wheels” trying to calculate the trace amount of CO2 dissolved in the water condensate. In obvious accordance with Henry’s Law, the less the condensate temperature and the higher the pressure, the more CO2 will dissolve in the water condensate. Old style, Ammonia plants used to employ a simple, high pressure, water absorption process to remove the CO2 produced in the Hydrogen reformer. They would put the high pressure, CO2-rich water through an expansion turbine and flash the product into a cooling tower. The stripped, “lean” water would then be pumped back to the CO2 absorber by using the work produced in the expansion turbine, plus an incremental motor load. However, this is no longer done due to economic and process disadvantages.

Total reliance on producing a clean, filtered, and conditioned air feed into the air separation column should rest on a correctly designed Adsorption unit – usually employing Molecular Sieves. Some of the CO2 load can be removed from the Molecular Sieve bed if prior treatment with a caustic wash tower is made. However, today a caustic wash is not considered as optimal from a cost or safety viewpoint. Molecular Sieves can be easily designed to form the bulk of the purification train in one unit, mutually adsorbing CO2, water, or other contaminants. Every effort is made to reduce the load of contaminants adsorbed by the Mol Sieves to only CO2 and water. And this is done primarily by correctly siting the plant in a location free of the SO2, SO3, and NH3.

I hope this answers your queries.

[Art Montemayor]

Dave:

I understand your situation as one that has inherited a poor site for an air separation unit to be operating in. However, I do not agree with your explanation of rationalizing that this can't be avoided. I believe it most certainly can be. The solution to a prior bad siting decision may be expensive, but solutions to ignorant engineering decisions usually are. This is not suprising.

What is also not surprising is for management of a badly situated plant site to be demanding quick, easy, inexpensive, and efficient resolutions to their prior mistakes. I was a victim of these type of stupid management decisions when I was a young graduate engineer. It is unfortunate and regrettable that this is still an existing characteristic of current management organizations - but it serves to point out the hurdles that a young engineer must endure and overcome in order to continue on his career. It is one thing to try to resolve a prior stupid decision by others; it is an entirely different thing to try to resolve it in an unsafe manner - one that can affect the health and lives of people working in that environment. The safe, rational, and correct engineering decision may not be to the liking of present management - but it still stands as safe, rational, and correct.

Accepting a situation where SO2, SO3, NH3, (& possible other dangerous impurities) can be fed into an air separation column without paying the trade-off of expensive purification steps and controls is asking for trouble and allows a serious situation to arise in the future. The impurities mentioned can be successfully addressed and resolved through a properly designed adsorption purification train. But the adsorption unit must be specifically designed and fabricated for that purpose. This, as I state above, is going to cost money and effort to incorportate and operate - something not to the liking of any present manager. However, in my opinion, there is no other sane or rational answer to the inherited problem. As mentioned in another thread, "there are no free rides or lunches". You have to pay a price.

If you are also seeking incremental additional Oxygen and/or Nitrogen capacity out of your present air separation unit, you will have to confront incremental power and refrigeration requirements - as well as purification needs. I'm going to assume you have confirmed your cold box can handle the additional liquid air feed and both LP and HP columns can also handle the tray and condenser loadings. I'm also assuming you have the additional power and refrigeration needs covered as well. That being the case, you can aleviate the existing purification load on your adsorption unit (I'm assuming you are operating with an adsorption unit for your principal CO2 and water removal) by designing and installing a new, smaller and specific adsorption unit upstream of the existing one and operating both in series. This may not be pretty or efficient, but it can serve the purpose of dealing with increased impurities loading as well as increased capacity. From the basic data you've furnished, this is the principle and main option that I would study first if it were my project. It can be undertaken locally, using national resources (if you originally imported all your technology and hardware) and be done and completed while your present unit is operating. You can startup the added purification train while still on stream with the original one and not have to wait for a turn-around to do so.

I also assume you have presently installed in your cold box adequate, and operating, acetylide adsorbers treating the crude liquid oxygen stream within the cold box. This is something I would recommend to any air separation operator - especially one who recognizes that his air intake is ladened with excess impurities.

I've done the above, so I know it can be carried out in the field - even in so-called developing countries. I hope this experience is of some value to you.

Art Montemayor