How to add (not remove) C6+ to my Natural Gas

[sspence]

I have "dry" natural gas that I'm cleaning and selling to the pipeline, however the gas company wants a little Heavy Hydrocarbon (C6+) in my natural gas.
They state that the lack of heavy hydrocarbons can cause o-rings and other seals to shrink.

Any recommendations on how to add C6+ (in a controlled fashion) to my "dry" gas, which has no heavy hydrocarbon (methane only)?

 

[zdas04]

sspence,
Before you do anything check the contract. Typically gas-transport contracts specify zero liquid hydrocarbons and the potential for condensation is too real to be ignored--you could easily find yourself shut-in due to being a helpful individual (a good deed never goes unpunished).

I've doped a gas stream with CO₂ up to the limits for that component and/or total inerts to adjust the heating value, improve flow characteristics, and improve lubrication characteristics of 1.0 BTU/SCF natural gas. I've never heard of anyone doping with C6+.

If you do anything, get a contract intrepretation and/or revision prior to the change. You are probably paid on an MMBTU basis so the addition of heavies would sometimes be a wash--in some NGL markets, C6+ is worth more as a liquid than the gas MMBTU price.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Zoobie]

zdas04 makes some very good points.

We re-inject C3+ when the frac spread is not there. While there is some C6+ most of the liquids are C3 & C4. My approach has been like this:

1) Talk to the pipeline operator.
2) Check what your dew point would be with injected liquids (I use HYSYS for this). With C6+ you won't be able to put very much in.
3) Check your gas heat value (again I use HYSYS for this).
4) You should now know the upper limit of what you can add to the gas.
5) The injection system can probably be something as simple as a spray nozzle inserted into the middle of the pipe. A typical arrangement would be a tee with a blind flange. The blind flange would be rigged up with a nozzle so that you can easily insert the spray nozzle assembly. I always consult the nozzle vendor because nozzles can be funny little animals and there are lots to choose from....try BETE (manufacturer's name).
6) For control, I would think that a simple ratio controller would be fine....take the gas flow rate multiply it by a ratio set-point (determined from your checks of dew-point, heat value, and amount needed for lubrication) which will determine the set-point of your liquids injection stream. How good this part of the system works will depend heavily on installed hardware (meters, transmitters, valves) and tuning.

Having said all that, you may again want to consider zdas04's comments. You may be trying to control a very small dosage and you will probably find that it won't take a really big upset to send you off-spec.

 

[zdas04]

In the '80's a plant outside of Denver, CO sent a large batch of untreated (1,200 MMBTU/MCF) gas to a small town. The plant owner ended up buying several houses that had burners in water heaters and furnaces that couldn't take the heat and failed.

C6 is 4400 BTU/SCF so it won't take much to get to a really dangerous mixture. I put 94% C1, 2% C6, 2% C7, and 2% C8+ into my calculator and got 1,277 BTU/SCF which is really dangerous in a household system and probably not enough help to stop the drying effects the gatherer is trying to prevent.

David

 

[Zoobie]

Again zdas04 makes some excellent points. Using the pipeline specs in my area, you can only get about 0.3% hexane into pure methane to meet dew-point. Heating value is OK but you certainly can't put very much in. I don't know much about the drying effects but if it is as zdas04 has suggested then C6+ is not for you.

Any installed system though will need some interlocks.