Drying of Fuel Gas

[asghar]

Wellhead panels operate in many cases on fuel gas. In several cases the separation of condensate or oil from the fuel gas supply has proved inadequate resulting in damaging the equipment or malfunction.

It is desired to improve the fuel gas by knocking down the heavy hydrocarbons and moisture (if present) at remote location.

The typical composition of gas at wellhead is here under:

Component Mole %
>H2S 0.0
>CO2 1.43
>N2 14.9
>C1 51.24
>C2 7.72
>C3 8.25
>C4 4.01
>C4 5.69
>C5 3.12
>C5 2.07
>C6 1.27
>C7 + 0.3
>Total 100%

Operatining pressure minimum 85 Max. 100 psig
>Design pressure 150# rating
>Design Temp 150 deg. F
>Operating TEmp 115 deg. F
>Liquid Spec. Gr 0.79 w.r.t water
>Gas Sp. Gra 0.65 w.r.t air
>Gas Flow Rate 2000 scfh
>Liquid flow rate 5 gallons/day (Approximately)
>Micron of Gas required 0.03 to 0.08
>
NOTE-1: The Micron of Gas I assume considering the fact that Tobacco Smoke particales are in the range of 0.01 to 0.1

Note-2: No electricity available at site

In my opinion insllation of a filter separator will knock the liquid and solid particles down to 5 microns. Addition of a coalescer will bring the particle size to <1 micron. Beyond this it is realy a difficult task.

Do you think the addition of a vessel containing a mixuture of activated Charcoal and physically drying agent(Type: Alumina)will improve the fuel gas quality?

Since, the possibility of carry over of Charcoal particles and drying agent is there therefore, a <0.1 micron or smaller filter would work to safeguard the instruments from being damaged?


I would like your comments and recommendations to come up with practical and economical solution.

 

[eyadamk]

I suggest u to install one of the following on the main line of fuel gas:
1- 'zigzag' valve if the pressure of gases is not high, the zigzag shape pull liquids down & allow vapors to go through.
2-a cheep method ( which I reccomend) is the using of Mesh (either iron mish or polyprobelen) they are cheep, available,& u can make them by ur self, it is a complex network.
3-or by applying means of cooling (turbine finned-fan cooler since u do not have elect.) the passing them to a presurized receiver, where heavy components condensate down from the vessel & light gases flow up.

*charcoal may adsorb hydrocarbons!!! but water????
* silica gell.... I am not sure for hydrocarbons!!!! I have never heard about silica for hydrocarbon removing... i believe that if u applied to HC it is not possible to regenerate eazily.. I advise the study of the 3 points above.


hope this will help

 

[StoneCold]

I would go with a coalessing filter/demister pad. Make the coalessor vessel large enough that the gas velocity drops below the entrainment velocity if possible. Koch-Glitch make demister pads that could probably solve the problem very easily. You should talk to them.

 

[asghar]

eyadamk/BradStone,

Thanks for valuable tips. However, the solution you presented are very basic and can remove Heavy hydrocrbons upto certain limit, say 10 microns. The problem come as you down to less than 1 micron.
Secondly, the Alumina or Silica Gel is for water absorbing and not H/C inhallation.

Brad, I shall be talking to Kotch-Glitch and would post if some concrete concept is developed.

Any other Suggestion shall be highly appreciated.

 

[MortenA]

asghar

Since you have no electricity you also have to consider what to do with the liquids that you knock out.

Best regards

Morten

 

[tickle]

Ashgar - in your questino you said that your fuel gas was used for the panel. Do you mean that you use fuel gas as instrument gas? The pressures could be instrument gas pressures.

If not and it is fuel gas that you want to treat. There are numerous companies out there that can supply filter coalescers, Pall Peco.... And Aker Kvaerner are developing/releasing a cyclonic separator.

Also ask yourself this &quot;how can the vendor prove that the particle size of liquids in the gas is less than 0.01 micrometre?&quot;

 

[asghar]

tickle,
You are right, at well site normally the same gas (light Hydrocarbons) is used for instrument operation. Presently, Filter Separator are in operation at well site for this purpose but there is big Hue and Cry by the insturment department regarding the quality of gas. I agree to there yelling and have to come up with a solution before they burry me in their .......

In response to your question, I have to rely on vendor data. In my opinion they carry out some kind of test that gives them micron size. I have no way of confirming the micron size.

MortenA..........Your point is in the back of my head and at present want to keep it there as the priority is on gas conditioning. Thanks anyway

 

[eyadamk]

Asghar... have u contacted 'zigzag' valves (filter) vendor to assure thier capabilities??? they are used to settle the per min. moisture droplets in instrument air... I have experienced them...

 

[asghar]

eyadamk,
Yes, I did today after you pointed out. It is Phillip if I remember correctly. Let us see how they respond. I will post there response soon I receive it
Thanks

 

[1969grad]

You say you are using fuel gas for instrument gas. If so, run the fuel gas line through the heater after regulation to heat it above the hydrocarbon and water dewpoints. This techique is especially useful for water bath type heaters. You may have to add a fast loop to insure constant flow.

Also, if you are taking separator gas select a regulator which protrudes into the vessel to reduce the amount of cooling from the pressure regulation.

Suggest you contact one of the desicant manufacturers to help you select the proper desicant. However, you should realize that the desicant will have to be replaced on a regular basis if you cannot regenerate it.

 

[asghar]

1969grad
Thanks for your contribution. The hot bath you suggested is a good idea but at a remote location one does not keep such facility due to safety reasons and there is no electricity as well.

 

[TD2K]

I've worked at one facility which used fuel gas as instrument gas very much as you have described Asghar.

High pressure/high temperature gas from the wellhead (60 bar +/50 to 80C) was let down through two pressure regulating valves to instrument gas pressure. Two regulators were required to avoid problems with hydrates. The gas from the first valve would pick up enough ambient heat to avoid hydrates as it passed through the second valve. The pressure drop across each valve had to be adjusted to avoid having hydrate problems with either valve.

Significant changes in ambient temperature also wasn't a problem as this was in Indonesia.

Basically, the gas exiting the second regulator, relative to ambient temperatures, was cool so the gas was effectively superheated as it came up to ambient temperatures traveling to the various users. I don't know if you can duplicate this. Can you cool off the gas before the let-down regulator (so it will exit the valve below ambient) without having problems with hydrates?

For a field installation, I don't think you will have much luck with activated carbon. It will absorb heavy hydrocarbons but monitoring it when it's saturated is difficult and the cost to change it out on an ongoing basis might be prohibitive. I'd look at coalescing filters with a good capacity right before each user. You want a good capacity so the liquids can build up until someone visits those remote sites and can blow down the liquids. Assign one wellhead as a test site and try out difficult filters/setups until you find one that works for you.

Have you got the vendors involved? They will have some expertise with using instrument gas (it's definitely not new technology) and can help you out other than telling you to supply dry gas ;-).

 

[asghar]

TD2K
I agree to your point of view but interestingly the pressure at well site are low, in the range of 150-250. Had the pressure in the range of 1000 psig the J-T impact would have worked effectively.

The well sites already have filter separators but not effective. The coalescing may work to certain extent but some airborn heavy hydrocarbon are to be absobed. For this I am proposing activated Charcoal bed and then a silica bed to absorb water moisture.

I have contacted the vendors as well but they are not taking up the challange.

Thanks for your input

 

[Gabamco]

Seems like you have a problem associated only with inst reading using fuel gas.

Q1: What is the flow rate of this gas in the instrument lines that you need to read instruments off. Are these lines dead ended or there is some purge and if so, how much (scfm)?

Q2: Your analysis does not show water content and nor is there any comment that it is saturated? Is it or is it not?

However, from your discussion it seems that you know what to do, and that is you want the particle size to be @ .03 to .08 micron. The only way to measure this may be by using laser isokenetic technology, it will measure particle size distribution, and efficiency of separation.

I have never come across a case where I would require that kind of particle size control, but there are vendors that can provide internal centrifuges and will also do CFD analysis to predict the separation. I am writing from home and I do not have the vendor names, hopefully when I get to the office I will try to get that information.

I feel that your problem is not with the particle size, but with stagnation in instrument lines...leading to drop out of liquid, and how that happens we can discuss if necessary.

 

[dcasto]

To get the water out, either inhibit/remove with methanol or use Calcium Chloride. For the hydrocarbons, the gas can be warmed up after large pressure cuts with catalytic heaters made for the oil field. There is even a company packaging a vortex shedder normally used as an &quot;air conditioner&quot; for people working in encapsilated suits.

 

[Chuck56]

For drying gas for instrument air use in a remote location (i.e., not electricity, etc.), a pressure swing dryer using activated alumina is recommended. You want to deal with heavy hydrocarbons first, since if these condense on the alumina you will not be able to drive them off. Filtration ratings seem a bit extreme. Typically, 0.3 micron filters are used for instrument air, though special instrumentation might need a tighter spec. As was mentioned, it is difficult to accurately measure smaller particles, so perhaps the point is moot.

A number of packaged system vendors that produce air dryers should be helpful in your assessment of your needs.