3 replies to this topic

[jzl558]

I'm going to try to put things in terms most people can understand, so sorry if anything sounds too simple or unnecessarily detailed.

 

Background:

I am doing an internship with a natural gas midstream company. Gas is given to us by Production, and we put the gas through compressor stations to ship it over a few hundred miles to sell to local distributors. I am working almost exclusively with mechanical engineers (ME's); they specifically wanted a CHE intern to help them with a chemistry-related problem none of them know how to solve.

 

The problem:

Every so often, a slug of water gets into the stations, and this water usually has a lot of salt in it. As soon as gas enters the station, it goes through a knockout and filter; the knockout gets rid of the water, but the salt apparently makes it through the filter. From here, the gas/salt goes through a compressor to increase its pressure (for transport), and then it goes through a TEG (glycol) dehydration tower (aka dehy) to remove any remaining water. The problem is mostly in the reboiler: a layer of salt, often over 1 cm thick, builds up along the walls and heat exchange in the reboiler. Similar problems occur in the cylinders of the compressors. This salt buildup renders the reboiler useless until it is cleaned.

 

Attempted solutions:

So far, the ME's solution has been to shut down the entire station for a day and hire contractors to power wash the units to remove the salt. This is outrageously expensive, and obviously this does nothing to prevent the problem in the future. They're ME's, so you certainly can't blame them for not being able to come up with a solution to a CHE problem.

 

My idea:

I have no idea if this would work, so this is one of the major areas where I'm looking for feedback.

 

1) Firstly, I'd like to install a sidestream at the inlet of one of the compressor stations to provide me with gas I can use for testing. (My mentor thinks this is a good idea, because their approach to fixing problems is usually to install something that costs a lot of money, and if it doesn't work, they try something else that costs more money. He feels there is a desperate need for experimentation before implementation.) I could run the gas through my second idea (below) before committing to a huge project.

 

2) To remove the salt, I'm wondering how feasible it would be to just blow the gas directly into a tank of water immediately before (or after) the inlet knockout/filter. This would obviously make the gas pretty wet, but the salt removal seems to be a bigger concern than dehydration. The remove some of the water vapor from remaining in the stream to the compressor, I'm wondering if you could just put it through a very tall column that's cooled toward the top (in an attempt to condense water vapor and let the gas escape through the top, like fractional distillation).

 

I should also point out that I have a BS in chemistry, but the only CHE course I have taken thus far is thermo, so if this is a really stupid idea, please don't shun me too badly.

 

So far, I have collected a sample of the salt to determine its exact composition. (Results should be in this week.) Also, tomorrow I am taking gas samples before and after the inlet knockouts at two compressor stations with notorious salt problems, and I'm having them tested for the amount of salt being inlet and outlet of the knockout. I'm not sure how the sampling company is going to test for salt from a gas cylinder, but they assured me they'd find a way.

 

Thoughts? Questions or comments? (Should this be in the industry forum?...)

 

Thanks for any help!

[ankur2061]

jzl558,

 

What you probably need is a gas scrubber upstream of the knockout and filter. Also known as a water wash column it would be a counter current packed tower with random or structured packings with gas fed from the bottom and water (salt-free) sprayed from the top. If properly designed this would rmove most of the salt in the effluent water from the tower bottom.

 

In all probability the water content of the gas exiting the scrubber would have higher water content than what is presently entering your knockout drum. This may require modification to you current knockout drum and downstream filter to handle the additional water load. If you are using a conventional filter my suggestion would be to replace it with a high efficiency filter-coalescer. These are effective to control wet salt-laden gas entering the compressor. One such vendor that supplies high-efficiency filter-coalescers is "PECOFacet". In fact they may be able to supply you a complete solution for water / salt removal from your inlet gas to the compressor station.

 

Refer the links below:

 

http://www.pecofacet...iniPuraSep.aspx

 

 

http://www.pecofacet...eminiVideo.aspx

 

 

Hope this helps.

 

Regards,

Ankur.

[neel_avi]

Hello everyone,

 

I am curious to know the process by which salt goes into the natural gas stream from the knock out drum outlet. Salt can go with natural gas either because the knock out drum is not sized to handle the flow of salt containing water which is carried over into the gas steam, or the drop in temperature during the transportation causing the salt to separate out and get into the gas stream.

 

regards

Avijit

[jzl558]

Ankur

 

It turns after some extensive discussion with other engineers with our company that, not only do we already have a PEACH in place, but it also is effective in removing nearly all of the small quantities of salt that used to accumulate in the equipment. So this is no longer an issue.

 

There is, however, a bigger problem with salt, and it occurs when Production pumps the wells full of some kind of soap or foaming agent that sends thousands of gallons of water to us, and the water brings the agent with it. This flood cakes salt on the inside of our dehys several inches thick, and it also forces us to change out our glycol. My suggestion for this issue is that Midstream (for whom I work) and Production need to better communicate when such a process is going to occur. But even if we know all this water is coming, what can we do about it?