8 replies to this topic

[Chem01]

All,
In our plant a liquid (28 % K2CO3) expander is installed where pressure is being reduced from ~ 29 kg/cm2 to 5 kg/cm2. Expander is on the same shaft as of steam turbine and its load sharing with the turbine is understandable.

Further downstream of the process, on the gas side we have a cold box for separation of CH4 from process gas, rest being H2 & N2 mainly in the H2/N2 ratio of 2.0. The expander has DP of ~ 2.6 kg/cm2 and temperature drop from -128C to -134C as per data sheet.
I am not clear why expander is provided in original design, was it not possible to use throttle valve here? What is criteria to use expander instead of vale..?

The plant is ~ 40 years old relocated to our site; startup is still 6 months away. Expander has bypass valve also, and the operating manual states when expander trips bypass valve is used to safely shut down the cold box but it cannot be used to give cryogenic effect.

Some clarification would be helpful for better understanding, thanks.

[marthin_was]

have you consider about isentalphy issues in your case?

Just my thought to start further discussion.

thanks,
Marthin

[djack77494]

An expander will remove energy from your fluid and can result in much colder temperatures. Using an expander is considered an isentropic process. A throttle valve would change the nature of the process to an isenthalpic expansion. No energy is removed from the fluid and the resulting temperature will be warmer than the isentropic process.

[TechExpert]

DJack is exactly right. One can follow what he is saying.

An expander will remove energy from your fluid and can result in much colder temperatures. Using an expander is considered an isentropic process. A throttle valve would change the nature of the process to an isenthalpic expansion. No energy is removed from the fluid and the resulting temperature will be warmer than the isentropic process.

[Chem01]

Ok, In literature (see attachment) I found that expander comes in b/w of isenthalpic and isentropic process. Minimum temp can be achieved in case of Isentropic process, I am not sure if we can make expander process close to isentropic.
In industrial refrigerations loops up to -33C, I've seen valves instead of expanders, same is the case with home air conditioners no expander only throttle valve. Means we can decrease power requirement of inductrial ref. sysytems / home AC etc by the use of an expander? Keeping mechanical wear tear at a side.

Attached Files

•  Enthalpy__Entropy_diag.JPG   30.92KB   117 downloads

[djack77494]

Means we can decrease power requirement of inductrial ref. sysytems / home AC etc by the use of an expander?

Yes, you can reduce the power requirement if an expander is used. The biggest disadvantage of an expander is the cost. It is a piece of rotating equipment, and as such is much more complex and costly when compared to a simple throttling valve. As you note, an expander is less than completely efficient and therefore falls short of a true isentropic process.

[Satyajit]

Hi,
It looks to me that Chem01 is talking of a KBR based ammonia plant.
His question is right. Use of throttling valve is simply waste of pressure energy.
Use of expander helps to recover atleast part of the pressure energy as shaft power.

The hydraulic turbine in K2CO3 system is the perfect one.

In the cold box, I have seen many plants to recover the pressure energy for producing current.

You can think in that direction. There are other cold box designs availble where feed gas ( H2,N2,CH4 etc) pressure drop is just 0,5 bar; tail gas expander is used instead of feed gas expander.

The use of expander over bypass valve has an advantage of better stability, and less erosion. Expander is also helpful for fine control of exhaust temperature by using its oil break which is difficult in bypass or throttling valve.

Hope this replies your question.

Kind regards,

Satyajit

[Zauberberg]

I am not sure about your process, but you will often (or almost always) see a J-T valve installed in parallel with turboexpander, for startup purposes. This is the basic concept in cryogenic gas plants.

[Chem01]

@satiyajeet,
You r right, it is a kBR Ammonia plant.
Tail gas means waste gas?
What is oil break, how it is used for exhaust temp control..?
As earlier highlighted main reason for using Expander instead of throttle valve is the nature of process which is isentropic

I am in touch with the cold box for the first time, and interested to know more of it...

Regards,