Dew Point and Moisture Content 6 Maret, 2013Posted by husnukusuma in Upstream Oil & Gas.
Tags: Dew Point, dew point temperature, gravity separation, Moisture content, Natural Gas, Water, Water content
// I’m in an airport and I thought to do a quick writing about water dew point and water content.
We have a process condition that must have maximum water content of 3 lb/MMSCF. In one of our meeting, a site manager who was an ex-process engineer asked what that number is in term of dew point. Boy he was smart! (in my opinion). Probably not many engineers are aware that this number is interchangeable. I’m not a process engineer, but I have special interest in it. Let me explain:
Natural gas, as it comes out from the earth, may contain other elements, such as water, H2S, CO2, Nitrogen, and even Mercury (Hg). Normally water is always present (even just a trace). Water in hydrocarbon can be in two form, free water and water that are in the gas (vapor phase). Free water can be easily removed by simple gravity separation device. Just give it enough retention time and by gravity it will fall to the bottom of a vessel. Water that is in the gas is tricky to remove and need special treatment if we want to reduce it.
Water in the gas is a phenomenon that we encountered frequently. We know that air (gas) can contain water. We see it drops as dew in the morning on leaves or grass. They become water because the air can no longer retain them in gas form. Because the night temperature is low and reach its dew point.
So suppose we have an X mass of H2O in the air, on a certain temperature (let’s say M degree Celcius), that H2O will condense to become liquid. That temperature is called dew point. It is a point because we assume we hold a constant pressure. If we varied the pressure, it will become dew line. <if there is only one phase present, dew point is the same as bubble point>.
If we have more H20 (X + Δx mass of water), the dew point temperature will also increase to say M + Δm degree Celcius.
Now other hydrocarbon components (methane, ethane, propane, etc.) can also have dew point. It is often confused with water dew point. The important thing is: water and hydrocarbon is not miscible, they are immiscible. That means they will have different dew point.
If we try to create phase envelope for hydrocarbon that contain water, the simulator will give a warning like this:
When we have a gas composition analysis from lab, we normally don’t see any H2O (dry basis). That is because it has been removed so that the analyzer can work. What we did to see how many water are in there is to put water (in simulation) until the gas is saturated with water.
Now an amount of H2O in gas phase (vapor), at certain pressure, will correspond to a certain dew point temperature.
Why we are so worry about water? Because we don’t want them to condense in unwanted place, such as high pressure tanks, low points in pipeline (especially subsea), …. It will create trouble (hydrate, corrosion, etc.).
There are many technologies on how to remove water from gas stream. Glycol is the most common. Other technologies such as adsorption using molecular sieve, compress and cool are also available.
Once we can reduce water content, we will have low dew point temperature and along the way we might be quite confidence that temperature will not be reach.