7 replies to this topic

[h2hydrogenz]

Hello everyone,

I would like to know regarding compressibility factor. In one article about CO2 pipeline, there is a figure which describes the relationship between temperature, pressure and compressibility factor (x-axis is the pressure line and y-axis is compressibility factor line). In that figure, there is a significant drop in compressibility factor for pressure below 86 bar and temperatures above 35 oC, while for temperatures below 35 oC, there is no significant decrease in compressibility factor. However, all the curves in figure illustrate that compressibility factor increases gradually along with increasing pressure. Then the author give a statement as follows:

"To reduce difficulties in design and operation, it is generally recommended that a CO2 pipeline operate at pressures greater than 8.6 MPa where the sharp changes in compressibility of CO2 can be avoided across a range of temperatures that may be encountered in the pipeline system"

My question is what kind of the design and operation difficulties that will be encountered when using the minimum pressure of 80 bar for example, where there is drastic change in compressibility factor? The only thing that cross my mind is that probably it is something to do with the accuracy of the pressure information on the instrument that is attached on the pipeline.

Thank you for your kind attention and look forward for your favorable reply.


PS: Critical Pressure of CO2 is 73 bar. The figure that I mention above is using Peng-Robinson as its equation of states.

[djack77494]

For a CO₂ pipeline, it is good to operate above the critical pressure, and it is even better if you can maintain a comfortable margin above the critical pressure. Why? Because impurities in the CO₂ can alter the cricondenbar of the CO₂ mix to a higher value. (Not totally accurately, I tend to think of the cricondenbar as the equivalent of critical pressure for a mixture.) If you fall below the cricondenbar, you risk being in an area where two phases may co-exist. And you definitely do not want two phases to co-exist in your pipeline. As to the characteristics of the compressibility vs. pressure curve, what can anyone say? It is what it is. I also don't know about operating in regions where significant changes in say compressibility can occur over small ranges in pressure. Just in general, I like to operate in well-behaved regions where I don't get unexpected results, even if they can be predicted.

[h2hydrogenz]

Thank you very much for giving one piece of the puzzle djack77494

[Art Montemayor]

h2hydrogenz:

Doug has given you some practical and expert advice.

I'd like to add some additional information for your use and enlightment:

• When you use the term "pipeline" in english, I believe that Doug (as myself) immediately think of a long-distance pipe duct - and not a normal process line in an operating plant. This is a significant piece of information in your query because long distance transport of CO₂ concerns an optimization of the transport system in order to maximize the efficiency and minimize the operating costs.
• For pipelining CO₂, you will find that transporting the fluid in the SUPERCRITICAL PHASE is of great importance. You fail to mention that term (supercritical) and I point it out because it is the basis of what Doug has stressed. You should endeavor to maintain the fluid in the supercritical phase when you pipeline it.
• The principal reason for keeping the fluid in the supercritical phase is because the fluid exhibits a minimum of viscosity when in that phase region. This phenomenom greatly reduces the pressure drop through the pipeline and, as a result, yields a minimum of transport power requirements.

Therefore, if you are dealing with the supercritical phase, the concept and application of a "compressibility factor" is of absolutely no concern to the designer. There simply is NO GAS PHASE PRESENT - only the supercritical phase (which is unique, and neither liquid nor gaseous).

I hope this helps to convince you that there is no "puzzle" at all.

[h2hydrogenz]

Hi Art Montemayor,

Thank you for your additional information. I am sorry if I did not explain it precisely. I would try my best to explain it more precisely for my next post.

The term "pipeline" that I used here is for long distance pipe. Therefore the CO2 is in high density state or above critical pressure or above cricodenbar in order to avoid two phase flow. I did not explain about the term in this state because personally I am still a little bit confuse about the term in this region. DNV, Union and some other articles including the article that I quoted here, state that there are two regions separated by critical pressure and temperature. Those regions are liquid and supercritical although there are no phase change between them. There is also term called dense phase which is introduced by Americans who is pioneered in this CO2 transportation. Therefore I am kinda hoping that the reader would understand about the CO2 state that I am working on. Once again I am sorry.

When I read that sentence that I quoted here, I have a lot of question in my mind. Is compressibility factor in this transportation matter? Is it because these region can still be considered as a gas (compressibility factor at 100% of CO2, 30 oC and 80 bar is 0.225). Is it going to disturb the instrument accuracy?

However thank your time to explaining it to me much better. I really appreciated.


PS: Do not hesitate to criticize me

[Art Montemayor]

h2hydrogenz:

The intent of the Forum senior members is never to criticize, but rather to correct when we believe there are errors or misconceptions of scientific and logical engineering practice.

For example, please go to: http://en.wikipedia...._carbon_dioxide

Pay particular attention to and study the CO₂ Pressure versus Temperature chart given there. You should also study all the extensive material and writings on supercritical fluids in the internet. By doing so, you will find out that there are four (4) basic phases:

• the gaseous phase;
• the liquid phase;
• the solid phase; and,
• the supercritical phase.

From the research that you do, you should be able to confirm all the information that both Doug and I have written in this thread. I don't know where you picked up the term "dense" phase, but the only phases I am aware of are the ones listed above. The supercritical phase has only been subjected to deep study and research in the past 40 years and the work is still in progress. There is much that remains to do in studying and identifying the unique properties of this phase phenomena.

I hope this helps to orient you and starts to familiarize you with the supercritical phase and the challenges involved in working with it.

Good luck.

[katmar]

Disclaimer: I have no experience in pipeline transportation of supercritical CO2, so this is a theoretical observation only.

When transporting a gas like air at moderate pressures a potential problem is that as the air travels along the pipe the pressure decreases and the air density decreases as well, so the air velocity increases as it travels down the pipe. This decrease in density is basically proportional to the pressure because the compressibility remains approximately 1.0

Supercritical CO2 still follows the PV = ZnRT relationship, but you have the added complication that as the pressure changes the compressibility (Z) can change quite rapidly in some regions. So in a pipeline you can have the situation where the CO2 is accelerating not only with the decrease in pressure along the line, but also because the compressibility is changing as well.

According to NIST, at 40 deg C as the pressure decreases from 90 bar to 80 bar the density decreases by 43%. If the compressibility was constant over this range the decrease would be only 11%. This could make design and operation quite tricky with the widely varying velocities. I suspect is the basis for the word of caution in the cited reference.

[h2hydrogenz]

Thank you for all your responds. I really appreciated