ZnO of H2 plant 1

[skvishwa]

hi guys,
i wnat to know whether Zinc oxide used in Hydrogen plant for desulphurizing of feed naphtha adsorbes any light organic compound or not?
regards,
santosh

 

[Yordif]

Zinc Oxide in the front end of a hydrogen plant is typically used in a hot design, after some HDS catalysts (CoMo or NiMo). The Zinc Oxide really is best at picking up H2S and the HDS converts the mercaptans to H2S. It will also convert organic chlorides to HCL so that alumina can be use to remove that. HDS requires that some hydrogen be in the feed, either recycle or with the incoming feed. Sulfur must also be present for the conversion to take place and care must be taken to insure that the catalyst does not become desulfied.

Some cold designs don't have enough temperature for the HDS conversion. In these cases Zinc can pick up some of the light mercaptans but it is not very effient. The theoritical maximums for H2S for the Zinc is about 32%, for the light mercaptans you drop down to only a couple of percent.

If you must go to a design with out a hot HDS front end, you should look at a metal oxide blend for the absorbent. A copper oxide, Zinc Oxide combination has shown very effective at removing mercaptans all the way down to ambient temps. These are optimized products but are simliar to the way the modern low temperature shift catalysts can poison.

 

[skvishwa]

thanks yordif,
actually the como bed in our HDS section of hydrogen plant is showing slippage of organic sulphor to the zinc bed in the downstream,as you mentioned it is a hot design , operating temperature being in the range of 380 deg c and the temperture at the zinc bed being abround 370 deg c.
aould zinc oxide be of any use as far as adsorption of mercaptons is concerned?
santosh

 

[Yordif]

ZnO has a small ablity to pick up mercaptans. The lighter the mercapton the higher the ablity. You can only get a couple of weight percent pick up with the light mercaptan species. You may see some cracking of the mercaptans at these temps over the binder in the ZnO which will help on the sulfur pick up in the later section of the bed.

Sulfur slip is bad for the h2 plant as you probably know. Sulfur is a temporary poison for the SMR in that it will reach an equilibrium loading. If you can get the como bed active again, the you have a chance of minimizing the damage to the SMR. If the sulfur is on for a long time, it will cause the nickel crystalites to cinter and you will have long term damage to the SMR.

Any Sulfur that slips the SMR will find its way to the LTS bed ( if you have one) which is an extremely effective sulfur trap.

 

[skvishwa]

thanks yordiff,
regards,
santosh