Q & A > Question Details
Can anyone explain me what are all the parameters that affect H2 purity in Recycle gas in case of a Hydrotreater/Hydrocracker ?
24/11/2018 A: Abhay Raj Mishra, Iocl, mishraar@indianoil.in
There are many factors which are affecting the Recycle gas purity in the treating/ hydrocracking reaction loop. but few of them are most prominent.
1- The source itself( PSA) or the other source like CCRU off gas and CLPS off gases. The problem aggravated if the low purity Hydrogen( light hydrocarbon rich) is routed to Hydrogen header without proper purification from the source.
2- Reactor section pressure: lower the operating pressure lead reduction in purity due to higher hydrocarbon in the loop.
3. Low wash water flow leading concentration of ammonia in the loop.
4- High separator( HPS& RGC KOD) Temperature.
5-Very high conversion ( secondary cracking) is also one cause for low recycle gas purity in the loop.
23/11/2018 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
Parameters which affects H2 purity in Recycle Gas are
1. Fresh Makeup H2 purity
2. Purging gas rate (Purity vent gas rate at the compressor suction)
3. Separator inlet temperature (If separator inlet condenser temperature is low, purity will be higher). Providing water trim condenser at the inlet of Separator gives high recycle gas purity compared to providing trim condenser at separator vapor line of course more utility consumption in first case.
4. Availability of Recycle Gas Scrubber (Obviously purity will be higher if scrubber is available in reactor circuit)
5. If unit configuration contains Hot Separator, recycle gas from Cold Separator purity reduces slightly because of un availability of sponging affect in cold separator.
6. In case of lower cold separator pressure than design operating case because of some reason mainly during revamp case or other limitations, recycle gas purity reduces.
7. During EOR conditions, more cracking happens and hence recycle gas purity reduces
8. If catalyst is more acidic function, cracking happens and RG purity reduces.
9. High recycle gas flow generally reduces H2 purity slightly. It is mainly because of partial pressure effect of lighters at Separator. Hence it is always advisable to maintain recycle gas flow not more than design Heat and Material Balance values.
10. Absence of wash water at reactor effluent air condenser inlet also reduces rg purity slightly as NH3 circulation increases mainly during no scrubber case.
23/11/2018 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
The purity of hydrogen in the makeup gas
The concentration of non-condensible gases in the makeup gas, like nitrogen and helium.
The chemical consumption of hydrogen in the reactor
The yield of light gas components (H2S, CO2, C1-C4)
Whether or not you have an amine unit in the loop to remove H2S
Whether or not you have a purification step (e.g. PSA, membrane) in the loop
The balance of makeup gas rate and purge gas rate
The conditions in the effluent separators and how much of the light ends / hydrogen are removed from the loop in the liquid product to the stripping column
23/11/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
This answer does not speak to your flow scheme, which is unknown to me. In the H2 circuit, everywhere a vapor/liquid mixture flows through a “drum”, whatever its name, a flash occurs. The composition of the vapor from that “drum” is determined by three things, namely, inlet composition, temperature, and pressure. In practice, there is limited control of those factors. That is, it would be the case of the tail wagging the dog to adjust a “drum” temperature to increase the circulating H2 purity. An exception is when a “drum” temperature is increased to alleviate an emulsion problem. The composition of a two phase mixture entering a “drum” is affected by the following:
• Unit feed composition
• H2/H.C. ratio, which in turn is slightly influenced by the make-up H2 purity
• RIT (all reactors in the H2 circuit)
• Reactor(s) pressure(s)
• Catalyst condition and formulation
• Extent of any flow channeling in the reactor