Wednesday, February 4, 2015

Dehydrated fluids from compressor stations and where they go

This was from a private conversation I had with some friends.
I am sharing here in case this is useful for others. -- BH

The question is:
Do we expect to see liquid tanks (water/brine, and NGL condensate)
near compressor stations, and what do they do with these fluids?

First: These terms are all approximately synonyms:
Natural Gas Liquids, NGLs, Condensate.

Methane combined with NGLs is called "wet gas".

There is no precise definition of NGLs.
However they are generally not methane (C1)
or ethane (C2) on the light side
as these are typically gas at atmospheric pressure (1 ATM).

Propane (C3) and/or Butane (C4) and a few other
similar compounds, are considered NGLs, but are gasses at 1ATM.

But these can be bottled at low pressure and turn to a
liquid. Marketed either separately or mixed,
as LPG: Liquified Petroleum Gas.  Cigarette lighter fluid
is mostly butane. Pentanes (C5) and higher e.g. Octane (C8)
are often in the NGL mixture.

These are generally separated out with fractionation towers.
This is largely what a refinery does. PS: no fraction is every 100% pure.

NGLs absolutely must be removed before putting methane into a pipeline,
because pipelines are designed to handle liquids OR gasses. It is very hard to design a "multi-phase" pipeline, so they generally don't. When liquids enter gas pipelines, they turn into "slugs" which must be removed with pigs and other methods and can cause internal damage.

The Wet Gas region of the Marcellus is generally in SW PA, Eastern Ohio, and nearby WV. Most of the gas in NE PA (Susquehanna Co., Bradford Co) is mostly dry gas. I don't want to say that there are absolutely no NGLs in this region.
Let me show you some real world examples.

Williams Central Station, Brooklyn Township, Susquehanna County PA:

It's a beast, a mechanical thing of (my) nightmares.
A strange machine coming to destroy its human creators.

This facility is BOTH a compressor and also (being close to a production
field) has "natural gas processing", which means it is a mini-refinery.
(Some of you know Rebecca Roter, she lives less than a mile from this beast).

Let's try to figure out what's going on here. But first, a schematic
of what "processing means" (purifying the pipeline product and removing various contaminants)

Notice that from a functional/schematic view,
there is really not much difference between removing oil,
removing water, removing H2S, or Nitrogen, or NGLs.  Each stage
requires "bubble towers", which each contain ethylene glycol and ????)

Water removal requires ethylene glycol. I'm not sure about
each of the others. One question I was able to answer is
that glycol dehydrators remove "wet" hydorcarbons as well as water.
(see below).

But the goal is to take a stream of gas which is "dirty",
has many different kinds of impurities and separate them out.

Here is another way of visualizing what's happening:

Now lets look at a real facility, a different view of Williams Central Station, taken Nov. 2014:

One thing to notice is all of the emitters of air pollution!
There are 3 flares, 3 exhaust stacks for the compressor engines,
and also the Glycol Reboilers vent or flare gasses being removed.

Here's a great doc about reboilers:

QUOTE: Wet gas enters the integral scrubber at the bottom of the absorber tower. The gas ascends through a mist extractor where fine liquid particles are coalesced and removed. As the gas rises through the absorber tower's packing or bubble cap trays, lean glycol (water removed through regeneration process) continually pumped to the top of the tower, is distributed and descends while absorbing the water vapor from the gas. Dry gas exits the top of the absorber tower and passes through the glycol/gas heat exchanger to the gas outlet.

The rich glycol (wet with absorbed glycol) collects in the hat tray at the bottom of the tower and flows to the power side of the glycol pump. From the pump, the rich glycol flows through the reflux coil and then to the glycol/glycol heat exchanger where it is heated and passed to the flash gas separator. The flash gas separator separates gas and entrained hydrocarbons from the glycol. The gas then flows to the fuel gas scrubber and the lean glycol flows through a filter and into the reboiler.

As the reboiler drives off water vapor through the still column (and into the ambient air!!!), the hot, reconcentrated glycol flows from the reboiler to the sparger box to remove additional water vapor. The lean glycol flows to the storage compartment and then to the glycol/glycol heat exchanger for cooling. The cooled, lean glycol then flows through a glycol sock filter before passing to the glycol pump. Lean glycol and gas from the absorber together power the glycol pump, which pumps the glycol through a glycol/gas heat exchanger to minimize glycol loss and then to the absorber tower to continue the dehydration process cycle. ENDQUOTE

OK, finally to try to answer the question....

We have often wondered the same thing.
One piece of data is that Rebecca Roter and others who live
near some of these big compressors report strange/foul
smells at night, ranging from "burning rubber" to "paint thinner".

One theory is (posited by Leland Snyder, IIRC) is that it is NOT economic
to recover NGLs in sufficient quanties found in dry-gas regions like
NE PA, so whatever NGLs or oil which is extracted from the stream are
held in temporary tanks on-site, then flared off at night when no one is

Whether this is legal or permitted or not is highly questionable.
We really get the run-around from the PA DEP. Maybe things
will change with a new governor.

Hope this helps,

May you, and all beings
be happy and free from suffering :)
-- ancient Buddhist Prayer (Metta)

Don't forget to sign the
Pledge to Resist
the Constitution Pipeline:

1 comment:

Vera said...

thanks for investigating this and explaining it at length and gives us a better picture of the extent of toxins they are dealing with...
hope the New DEP Head will visit this and test the emissions at full operation.