3 replies to this topic

[LeoLeo]

Dear all,

 

I am trying to find a reference for straight length after gas-liquid inline mixer.

 

The problem is that we are injecting DEG in the natural gas (containing water and CO2) in duplex pipe. Duplex is chosen because it is resistant against sweet corrosion caused by CO2. Type of injection nozzle is Axisymmetrical.

 

Down stream of injection point, we want to change the material to CS. Therefore, it is important to know where I can get well mixed fluid. The references I have are applicable only for liquid-liquid and gas-gas mixing.

 

I know there are accurate ways to determine the straight length required for well mixed fluid such as CFD calculation. But, I am looking for a good estimate.

 

Thanks in advance,

 

ARM

[Art Montemayor]

 

ARM:

 

There is no such thing as “sweet” corrosion related to CO₂.  There is, however, carbonic acid corrosion – but the CO₂ must be mixed with LIQUID water (not water vapor) and that corrosion is simply due to the carbonic acid present.  Additionally, this carbonic acid corrosion takes place with static carbonic acid solution and not flowing.  The term “sweet” is given to the compound Hydrogen Sulfide (H₂S) – which you don’t mention.  Sweetening plants are units that remove H₂S – not CO₂.  Acid gas plants remove CO₂.

 

If there is no liquid water present (and I have to assume there is no liquid water in you gas line since you would only be wasting the DEG you are injecting), then there is no corrosion in carbon steel due to the presence of CO₂ and water vapor.  DEG is used to dehydrate the gas by absorbing the water VAPOR present and thereby reduce the dew point of the gas.  In my opinion you are wasting your money using Duplex Stainless Steel.  In fact, if you seriously believe that you are going to have corrosion, why not make the ENTIRE pipeline out of Duplex Stainless Steel?  Duplex SS is used to combat Stress Corrosion Cracking (SCC) and CO₂ does not cause SCC; H₂S and chlorides cause SCC.

 

It is very common and normal to use carbon steel for gases containing CO₂ saturated with water vapor.  In fact, I have designed and operated many PURE CO₂ plants with CO₂ coolers and pipelines flowing with CO₂ saturated with water vapor and they were always made of carbon steel.  They lasted well beyond their depreciated lifetime.  This is done in industry all the time.

 

Therefore, my recommendation to you is don’t waste your money and time on using duplex stainless steel and make the entire pipe section out of carbon steel.

 

[LeoLeo]

Dear Art,

 

Thanks for your reply. I am well aware of CO2 corrosion mechanism. Actually, in the plant we have the corrosion problem. No H2S exist in the gas. As you mentioned for the corrosion of CO2, we should have free water. We have condensation of water. Condensation of water mainly happens in the winter case.

 

It is the first time I hear that "Sweet Corrosion" is related to H2S and not CO2 !

 

Please refer to "Handbook of Corrosion Engineering", page 853.

 

"Sweet corrosion. Corrosion in CO2 gas wells can be divided into three temperature regimes. Below 60°C, the corrosion product is non-protective and high corrosion rates will occur. Above approximately 150°C, ...."

 

Only one of the features of Duplex is being resistance against SCC. The other feature of Duplex is higher PREN in compare to SS. PREN indicates that the material is resistance against pitting.

 

Anyhow, my question is about the required straight length downstream of injection point to be sure that we have well mixed fluid.

[curious_cat]

What are the flow rates? This sounds more an inter-facial mass transfer problem than a diffusion length issue. 

 

Won't the CO2 have to diffuse through your liquid droplets?