Saturated Gas Plant 2

[Favy]

I'm trying to find some informations about refinery gas plants that process saturated fuel gas from various refinery units (Crude Distillation, HDS, Hydrocrackers,Catalytic Reformer) to recover LPG and Gasoline. I found that most of the times they are formed by an absorber/stripper (that has the function of a deethanizer) and a series of fractionation columns to recover propane, butanes, and gasoline. I'm searching more detailed information about them and I'd like to know if there is some alternative process besides the simple lean oil absorption. For example in natural gas processing there are a lot of different processes like refrigeration, or turboexpander and I don't know if there is some similar process developed for refinery fuel gas.
Thanks

 

[mbeychok]

Favy:

To my knowledge, most gas recovery plants (saturated and unsaturated gas) in an oil refinery are as you described, namely a reboiled absorber system or reboiled deethanizer followed by a distillation train ... probably because: (1) the economics favored that instead of using refrigeration and (2) the refinery is not interested in trying to recover ethane and ethylene.


Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JJPellin]

We have a fuel gas liquids recovery unit at our refinery. It uses refrigeration and has a turbo-expander. However, the unit is not economic to operate and has been shut down for more than 2 years. The last time I asked, I was told that it is unlikely that it will be restarted in the foreseeable future.

 

[dcasto]

The type of plant Favy is looking for is called a Lean Oil plant. The process uses a fairly flat boiling curve. If the gases are contacted (absorbed) at nearly atmoshereic temperature, the lean oil has a avergae mole wt of over 160 (number 2 desiel works). Normally, the gas and oil are chilled down to temperatures as low as -40F. At these lower tempers the lean oil will have a molecular wt of 90 or close to gasoline.

The lean oil is distilled and the unwanted methane, ethane are cooked out in a rich oil de-ethanizer (ROD). Then another column distills the oil to remove the propane and heavier in a distillation called the still. With heavier oils, the still may have steam stripping.

Lean oil plants fell from favor mainly because of capital costs. The cryogenic plant with a turboexpander is cheaper to in install and maintain. The expander consumes less fuel than a lean oil plant on a per unit recoverd, however, if there is no ethane market, a lean oil plant is just as fuel efficient as a cryogenic plant.

 

[Favy]

Thanks for your help, I already thought that lean oil absorption was the most used process and your confirm is very useful to me. dcasto, I think that you were referring to natural gas processing but I thank you for your interest.
However, for who is interested I found that the Advanced Extraction Technologies uses a propane refrigerated process for recovering C3+ from refinery fuel gas.
Another refrigerated process is the U.S. Patent 6,405,561 that is dedicated to refinery gas streams too.

 

[dcasto]

Recovering hydrocarbons from refinery gas is exactly the same process as from a refinery. The gas processing industry has even recovered valuable ethylene from refinery gases and separated it and sent the ethylene on into the PVC plants.
You can use straight refrigeration to cool the gas to -40F very easily and there is not patent on the process. I've even operated a cascade refrigeration with propane condensing ethylene and cooling the gas to -121 F. The refinery industry could learn alot about the light end streams from gas processors.

 

[Favy]

I'm working on a project for a new refinery and I didn't understand why in the project wasn't even considered the use of a refrigerant for the gas plant.I completely agree with you and my post was intented to understand why refinery gas processes haven't had the same evolution of the ones in gas processing industry. I wanted to understand if where I work (as you can imagine I've not much experience) there were really incompetent people or if my doubts and ideas were totally senseless. I think that as mbeychok said, there are economical reasons that favor the use of not refrigerated systems, especially if ethane or ethylene recovery is not required. However I think that the use of refrigerant should be at least taken in consideration in a project for a refinery gas plant because, as your examples demonstrate, it's not such an absurdity even if there are cases(look at JJPellin)in which the application of natural gas processes to refinery gas streams wouldn't result economic.

 

[dcasto]

I find it hard to believe that burning propane, butane, and pentanes is cheaper than burning methane in the fuel system. There has to be a problem with the burners requiring higher BTU/CF gas (what we call the WOBBE index). I know at some chemical plants they can only consume only so much hydrogen in the burners so they have to limit their recovery of the heavier hydrocarbons in their fuel system. If your refinery doesn't want to to build and run the fuel gas plant and there is sufficient recoverable propanes and heavier, there are gas processors that would provide the service. The refinery and gas processor would share the upgrade.

The refrigeration plant can be skid mounted up to 200 MMSCFD very easily. The plant requires very little attention. Good luck and keep trying to break the conventional wisdom mode.

 

[mbeychok]

dcasto:

I agree with you that the conventional wisdom may not always be correct. However, I would like to point out that most well-designed refineries do not burn butanes and pentanes (other than insignificantly small amounts), nor do they burn the bulk of their propanes. A well designed reboiled absorber (aka reboiled deethanizer) can recover at least 70-75 percent (or even more) of the propane without using refrigeration. I would also point out that most refinery gases contain enough hydrogen and methane so that they are no burner problems with "high Btu gas".

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[dcasto]

My sentance didn't come out as I planned about WOBBE index, the high H2 also gives most burners a problem, one plant exported a lot of H2 because they could not get it into their fuel. If there was no export market at the time, they mixed it with ethane and burned it.

 

[Jason5000]

In the modern refinery with lots of hydrotreater and hydrocracker capacity it wouldn't be a bad idea to consider a cryogenic hydrogen gas recovery unit. Using a cold box it is possible to process hydrogen rich off gas into a relatively pure hydrogen stream, fuel gas, and a C3+ mix for further processing in a traditional gas plant. A well designed plant of this type can run with relatively little attention, and this type of gas plant carries with it the benefits of recovering hydrogen for reuse that would otherwise be burned as fuel gas, lowering the hydrogen content of refinery fuel gas, and debottlenecking the traditional sat gas plant somewhat. One drawback to this type of unit is the higher pressure required, but this can be offset to a degree by incorporating a power recovery turbine into the design. Additional cons for this unit are the necessity to sweeten and dry the gas streams before sending them to the cold box to avoid hydrate formation. This design is definitely worth weighing as an option if you plan of having a lot of hydrotreater stripper/frac, separator, and purge gas in your off gas system.