H2/H.C ratio and catalyst activity in VRDS 1

[1906]

Re the VRDS(residue hydrotreating) unit consists of 6 reactors with quench system in each reactor inlet, we always think H2/H.C ratio at the inlet of reactor and always try to maintain it above guide line such as 700Sm3/m3. However, as the unit goes into E.O.R condition, recycle gas molecular weight goes higher and it makes more compressing in recycle gas compressor thus much higher H2/H.C ratio at the reactor inlet. If we have enough H2/H.C ratio, keeping that far higher ratio is helpful to the catalyst activity or not ? Because of high gas needs much fuel and sometimes inducing channeling inside reactor. So, if you have good knowledge about catalyst activity vs. H2/H.C ratio(especially for much higher H2/ H.C ratio), please let me have those information.

 

[epoisses]

1906, I won't pretend to be able to answer your question without having much data better than you can do with data... but first just for my understanding, why does H2 over HC increase as the recycle gas MW goes up? Or do you mean recycle gas over HC increases?

 

[25362]

Qualitatively speaking, and assuming you speak of a fixed bed residue desulfurization process, the decline in catalyst activity at EOR is generally compensated by raising temperatures to offset the decline in activity caused by metal contamination and carbon deposition. This means an increase in the conversion to lighter products.

With high-sulfur feedstocks the level of hydrogen sulfide can build up to high values reducing the hydrogen partial pressure and inhibiting the desulfurizing reaction. Thus, the recycle gas from the HP separator is generally water-washed to remove ammonia preventing the formation of ammonium sulfide which might form blockages in the reactor effluent cooler, and scrubbed with amines to remove hydrogen sulfide.
Sometimes, depending on the unit, light hydrocarbons are removed to improve further the purity of the recycle gas.

The gas rate chosen is governed by economic considerations. Recycle is used to maintain the hydrogen partial pressure to ensure adequate desulfurization while minimizing carbon laydown. Above a certain gas rate, the change in hydrogen partial pressure will be relatively small. In general, higher gas rates than necessary incur extra heating and cooling rates, which may outweigh other advantages.

The run length is usually determined by the metals content of the feedstock. Hydrogen consumption varies depending on the sulfur content of the feedstock.

To get practical detailed answers it would be wise to ask the company who offered the process design.