3 replies to this topic

[RQA21]

Hello everyone,

 

I have several questions regarding the APCI C3MR liquefaction process and in particular the propane refrigeration loop. Unfortunately, I am unable to share any PFDs but I prepared a drawing which shows a much more simplified depiction of our propane refrigeration loop. I have been studying the process and looking for information to learn more about how the process is controlled and how the process variables affect each other. After searching the forum, I stumbled upon an old thread dealing with the basics of refrigeration. The feedback provided by Mr. Art Montemayor has been helpful in furthering my understanding of refrigeration loops. I will be quoting his explanation and will follow up with a series of questions to further my understanding of this topic. If at any point you feel that my understanding is lacking in some area, please feel free to recommend readings/books/resources that will help me get a better grasp on what I am trying to make sense out if - if that makes any sense!

 

1. The compressor discharge pressure is fixed by the temperature of the cooling medium. In our case, we use cooling water. The principle is that the compressor's discharge is designed to be totally condensed at saturated conditions. Note that our compressor is powered by a gas turbine.

 

Now let us consider the propane condenser (labelled as E002 in the attachment). Cooling water is supplied at 25 degC. According to the design, the approach temperature should be 15 degC. Now I will attempt to explain this: the propane at the condenser outlet should come out at 40 degC. This is the condensation temperature seeing as condensation is an isothermal process. The saturation pressure of propane at this temperature is around 15 bar. Therefore, the compressor K001 should discharge at 15 bar. If my explanation is correct, for this scenario:

1.1. What methods are employed to control the outlet pressure of the compressor? Does the temperature of the cooling water get used as an input to determine the "correct" pressure at which the compressor should discharge as explained above?

1.2. Let us assume that the temperature of my cooling water has increased due to an increase in ambient temperature (say 30 degC).This means that the propane has to condense now at a temperature of 45 degC, which corresponds to a higher compressor discharge pressure. Do we reduce the mass flowrate of propane through the inlet to the compressor to obtain a higher outlet discharge pressure from the compressor?

1.3. What does the temperature approach between the propane condenser outlet and the cooling inlet tell us? Assuming it exceeds design, can we infer that fouling has occurred? Doesn't the temperature approach indicate how much heat transfer driving force there is? so the bigger the difference the better? I know this contradicts with my previous statement so I am not sure which one is incorrect or if one is misleading.

1.4. Since our compressor is powered by a gas turbine, when the ambient temperature increases, the available power decreases and as such less compression power is available. Also, the cooling water supply temperature increases, meaning that we have to condense at higher pressure. With the limited power, we have to reduce the mass flow of propane through the compressor to meet the required discharge pressure. What would be a good way to quantify the effect of these parameters on the refrigeration loop performance? Are there any calculations that I can do to allow me to assess how well my system is doing? (e.g. consider the ratio of the refrigeration duties of all propane chillers to the compressor power used and trend over time)

 

2. The pressure in the evaporator is set by the capacity and controls on refrigeration compressor, not by the process requirement. If a colder evaporator is desired, the vapor pressure in the evaporator needs to be lowered by increasing the capacity of the refrigeration compressor.

 

2.1.  Lets say that I increased the plant load and now require more cooling. How can I lower the vapor pressure in the evaporator? This step involves drawing the generated vapors into the compressor, thereby reducing the amount of vapor in the chillers. My understanding is that, by lowering the pressure in the evaporator, evaporation will occur at a lower pressure. The latent heat of vaporization increases with decreasing pressure and thus more energy can be removed per unit mass flow of evaporating refrigerant. According to the above, the compressor will remove those vapors by drawing them into the suction drums. Wouldn't that increase the suction pressure in those drums? I recall reading that it is desired to have a low suction pressure into the compressor as this will reduce the compressor power. Is there a trade-off between increasing the chilling duty and maintaining a low suction pressure into the compressor?

2.2. Can we infer anything from the pressure of the compressor suction drum and the level of the liquid propane in the evaporator?

 

3. This part of my post is to do with the second attachment, where I am looking at a pressure-enthalpy diagram and trying to rationalize why operating a compressor at lower discharge pressure is more beneficial to obtain more refrigeration duty per unit mass of refrigerant. Note that this chiller receives saturated liquid propane from the accumulator, which has approximately the same pressure as that of the compressor discharge. Kindly let me know if what I am saying makes sense:

 

- Let us consider path A (starting at point 1). If I expand the saturated liquid from a pressure of 14 bar across the expansion valve to 7 bar (point 2), I will obtain a mixture that is roughly 23% statured vapor and 77% saturated liquid. At this evaporator pressure, I will be able to remove 270 kJ/s of heat from the process fluid for every kg/s of saturated liquid propane entering the chiller. This will be 77% of the mass flowrate of saturated liquid entering, as the vapor will not contribute to the heat transfer.

- Let us consider path B. The difference here is that I am expanding from a higher pressure of 18 bar but my target pressure is the same (7 bar). This will produce 30% vapor and 70% liquid and at this pressure, I will be able to remove 250 kJ/s of heat per kg/s of saturated liquid propane being vaporized.

By comparing these two scenarios, I can conclude that, by fixing the pressure of the evaporator (i.e. downstream pressure from the expansion valve), at a higher upstream pressure I will have less saturated liquid forming and more vapor will be generated. At a lower pressure, I benefit from more propane remaining as saturated liquid after expansion AND from the increase in latent heat brought about by the pressure reduction. Thus, it is more favorable to operate the chiller at lower pressure.

If I were to operate on the higher end of my compressor discharge pressure, I will have less saturated liquid going into my chillers, more vapors will be generated which will increase the suction drum pressures. Is it correct to conclude that increasing the compressor discharge pressure will proportionally increase my suction pressure over time?

 

 

I apologize for the structure of this post. I tried to organize my thoughts as logically as I could but due to my lack of experience they're all over the place. I would be very grateful if you can help me better my understanding. Your support is highly appreciated.

Attached Files

•  Process Sketch.pdf   715.4KB   30 downloads
•  Expansion.pdf   1.96MB   20 downloads

Edited by RQA21, 12 April 2021 - 07:28 AM.

[Pilesar]

I suggest you model this process with steady-state simulation software. You can then change parameters as much as you like to see how the process is affected. You will learn more this way than through verbal answers from this forum.

[Bobby Strain]

You should peruse P&IDs to see how the system is controlled. Or some document from APCI. If you want to see it in action, do as Pilesar suggests.

 

Bobby

[breizh]

http://www.jmcampbel...-what-is-it-and

 

You may find pointers in the link above.

Good luck

Breizh