Nickel Catalyst/Ethylene Dehydrogenation

[southtexashack]

We recently installed a nickel oxide catalyst bed to remove trace amounts of sulfur from a gas stream that is 65%Ethane/35%Ethylene at the following temp. & pressure:

P=360 psig (about 25 atmospheres)
T=60 F


We ran into numerous problems, (high exotherm, byproducts of hydrogen, CO, CO2, etc) and eventually had to change out the catalyst after one day and replace it with something else. When we dumped the catalyst, it appeared that there were alot of coke fines. The temperature probably got to at least 550 F, but we had no thermocouples in the vessel, so I'm not exactly sure how hot it got.

A few questions

1. What reaction may have caused the exotherm? OR Would there have been a strong reduction of the catalyst at 60F?

2. Does it make sense that ethane/ethylene could coke over this catalyst at these temperatures and pressures? If so, is there any publications that document this?

Any help would be appreciated

 

[25362]

The heading says dehydrogenation, you, on the other hand, speak of desulfurization. In what chemical composition does the S appear ? Qualityof hydrogen gas ?

 

[southtexashack]

It was meant to be a desulfurization catalyst, but it appears that it dehydrogenated the feed stream to the point of coking in a very short period of time. The coke was either in a dust form or a mass of black solids. The sulfur species are typically trace amounts (5 ppm)of light mercaptans, and the hydrogen content is lest than 0.5%.

 

[TD2K]

We used to hydrogenate actylene over a similar fixed bed though it was a platinum or palladium catalyst I remember. That bed had a temperature rise that we monitored carefully against the possibility of a runaway reaction and we were only hydrogenating less than a percent of acetylene if I remember correctly (I'll have to take a look at my notes tonight). H2 was fed to the bed under flow control and runaway reactions were known to be a very real possibility.

If you assumed the H2 all reacted, what sort of theoretical temperature rise could you see?

 

[southtexashack]

I believe that there was little or no hydrogenation going on, since we only had a little bit of hydrogen (less than 0.5%), but from the severely coked catalyst that we pulled out of the reactor, it appears that there was alot of de-hydrogenation. That's why I was thinking that the Nickel acted as a dehydrogenation catalyst, based on the large amount of coke that I found, but I'm still not sure. I was hoping to find some kinetic data showing the following reactions as a function of temperature & pressure:

1. C2H4 --> 2C + 2H2
2. C2H6 --> 2C +3/2H2

 

[25362]

I wonder whether somehow the highly exothermic polymerization of ethylene, initiated by free-radicals, catalyzed by nickel oxide, acted upon ethane facilitating its cracking with hydrogen and coke generation. It's only a thought.

 

[southtexashack]

That would probably make sense. Once the reaction kicked off, it was the classic runaway that I learned about in college. By the way, my reaction two doesn't balance, its supposed to be 3 hydrogens, not 3/2.

 

[25362]

How do you regenerate the catalyst ?

 

[southtexashack]

I don't know. This stuff may be beyond that point