CO removal from a hydrogen stream

[renier]

We have a hydrogen stream being fed to our plant that is sometimes of spec on CO resulting in a loss of catalyst activity to our reactor for partially hydrogenating acetylene to ethylene.

Currently we are looking at using an activated carbon bed in a redundant drier vessel to protect the reactor. Would this be a good idea (best idea?). We would like to install a system that would be easy to regenerate (Not require extensive work to replace adsorbent with every incident where CO breaks through from the upstream PSA units.

 

[EGT01]

I don't have experience with your particular application so you may want to see what others have to say. About all I can do is suggest you call a carbon supplier but you probably already thought of that.

I think most suppliers will have a technical group that would be willing to assist you without cost. There are probably others you can call but recently (Nov-2002) I worked with Calgon which I found them to be very helpful. The technical rep I worked with was
Gilbert Palmgren
palmgren@calgoncarbon.com
Calgon Carbon
Pittsburgh, PA
412-787-6632

As additional info on carbon adsorbers, you may want to check the EPA website

http://www.epa.gov/ttn/catc/products.html

Look in the section for "Air Pollution Technology Fact Sheets and Technical Bulletins"

http://www.epa.gov/ttn/catc/products.html#aptecfacts

You'll find a technical bulletin for "Choosing an adsorption System for VOC"

 

[kenvlach]

I don't know much about your application, but in graduate school we removed O₂ from high-purity H₂ streams. Used a room-temperature, Pt-containing catalytic converter (sold by Linde or maybe Fischer Scientific) that converted the O₂ into H₂O for subsequent removal in a drying tube.
Working on the premise that it is easier to remove H₂O than CO, maybe you can use a catalyst for the reaction
CO + 3 H₂ = CH₄ + H₂O

delta G_{Rxn, 298K} = -33.984 kcal/mol

This is only useful if the CH₄ is not harmful to your process.

 

[parnulfos]

Quantum Group Inc. (QGI) has the ability to provide a regenerative CO scrubber for applications similar to the one you have.The product consists of a pellet filled container that has the ability to absorb CO upon contact with it. To regenerate the pellets, a stream of either clean air is run through it or if necessary, pure oxygen can be utilized to speed the regenerative process.

 

[Montemayor]

renier:

I've done the Steam-Reformer process for high-purity H2 production and fail to see what may be causing your problem from a supply standpoint. I assume you are purchasing the H2 as produced by another unit "across-the-fence". If so, I fail to understand why you haven't protected yourself contractually by forcing your supplier to do the actual "guard bed" operation. What you need is a guard bed of selective adsorbent downstream of the PSA (pressurized, selective adsorbption) unit and upstream of where you take custody. This guard bed doesn't necessarily have to be activated carbon (whose sorptivity for CO is not as good as the mol sieves in the PSA, I believe). A well-designed and operated PSA unit doesn't require a guard bed to produce high-purity H2 for hydrogenation purposes. However, your supply may be flawed in either (or both) design or operation.

Depending on the magnitude and duration of the CO breakthrough in the PSA unit, your guard bed may be a rather large one. Do you intend to regenerate, or will the adsorbent be "consumable" (not regenerated, but scrapped when saturated)? You say you would "like" to regenerate, but you don't make a firm stand on this. I would not accept this type of stance. There has to be an economic (or safety) driving force to protect the catalyst and the subsequent hydrogenation product, so you must quantify the amount of CO breaking through and relate this effect in an economic or operational manner. If it were my process, I would design a two-tower guard bed based on average break-through levels of CO and include at least a 50% safety margin on the adsorbent quantity and superficial bed velocity. I would regenerate with hot Nitrogen or steam with subsequent cooling by Nitrogen. One tower would be adsorbing while the other would be reactivating. I've done this in the past on other applications and it works very well. I still am reluctant to bite the bullet on accepting off-spec, high-purity H2 for hydrogenating purposes. I'm used to producing "5 nines" purity Hydrogen (99.999% vol.) without any problems - and that was over 25 years ago.

I hope my experience and suggestions help out.

Art Montemayor

 

[RogerH]

Renier
What is your spec on CO. I suspect you are in the lower ppm's correct.
Montemayor
I am interested on what levels you can achieve by regenerative CO absorption.
The reason why I ask is somewhat different from Reniers. I have experienced problems with CO coming from a CCR unit (not through PSA) to an Isomerisation unit where CO deactivates the catalyst, above a few ppm it is noticeable on activity. The Isomerisation unit do have dryers (molsieve) so there should be a potential to make use of these for (part) regenerative CO absorption.
If you have any more information on the subject I would appreciate it.
Thanks

 

[Montemayor]

RogerH (Petroleum):

This is a very important subject because it involves expensive catalyst savings, production time, and safety issues that necessitate the catalyst change-out in a Hydrogen environment. In some cases, it involves vessel entry by humans - a procedure that I always try to eliminate or reduce due to the hazards related to this procedure.

First, let me start by correctly identifying this Unit Operation as CO ADSORPTION, as opposed to CO ABSORPTION. The difference, while it may seem insignificant to some people, is important in that the Adsorbent's selective Sorptivity is involved. This Sorptivity is affected by the adsorbent's age, type, manufacturer, contaminants (poisons), superficial space velocity, channeling, adsorption temperature & pressure, regeneration levels, and cycle times. There are other factors, but what I want to make sure is that you appreciate that the manufacturer designs and controls the quality of the adsorbent (I presume we both agree on Molecular Sieves being the adsorbent of choice here) and, as such, should be the expert you rely on for selection and recommendations. I highly recommend that you work directly with pre-selected and qualified Mol Sieve manufacturers (notice I write "manufacturers" - and not distributors or sales offices). The manufacturer holds the key information on sorptivity design levels and expectations, once they receive your key information (ALL the key information; be honest and candid, don't hold anything back): suspected contaminants, operating procedures with temperatures & pressures, required CO removal rates and levels of concentration that must be achieved. Discuss your priorities in DEPTH with them and explain what you are prepared to trade-off and what you can't trade-off. They should be very detailed and explicit in explaining their recommended cycles, adsorbent and regeneration methods to you - complete with estimated adsorbent quantities, prices, regeneration requirements, and adsorber sizes.

Adsorbent manufacturers can also further recommend engineering and fabrication companies that will design, furnish and warrant a complete Adsorber Packaged unit that will do your specified job - on an automatic or manual operation cycle.

If your Isomerization unit stands to profit from a dehydrated feed stream that is also devoid of CO contamination, than I can save you some consulting bucks by freely advising you that you are headed in the precise and correct process direction when you propose to install a Mol Sieve Adsorption step prior to the Isomerization. I suspect that you are dealing with Big Buck $$savings when you start to economize on catalyst activity - not to mention the potential positive effect of completely dried feed stock. I have used Mol Sieves to reduce water moisture in gases down to 0.5 PPM - a level that is so hard to read and monitor that we finally didn't bother. It works, and it works well. Simultaneous (really, step-wise) adsorption of both water vapor and CO should be a snap. This is routinely done on PSA units (although more emphasis is made on CO2 + water vapor removal).

Molecular Sieves are human-engineered and designed adsorbents as compared with activated carbon, which is designed by Nature. There are Mol Sieves custom-made for CO removal - just as there are for CO2 and water.

I hope this information and experience is helpful.

Art Montemayor

 

[startupman]

I think you need to go back to the cause of your problem. If you are getting a CO breakthrough, of more than 10ppm, from your PSA unit, you are overloading the PSA and you will be contaminating your Molecular sieve. This will slowly but surely deactivate your PSA bed.

I suspect that you are running at more than 110% of the design capacity. Contact you PSA Unit supplier. Perhaps you can adjust the programming to get some extra capacity.