How does ethylene production work?

[BingoBongo18]

Hi everyone,

as an engineer at a rotating equipment vendor, I am curious about what my machines are actually doing in ethylene production. I have read the Wikipedia article on the topic. However, the article does not go into greater detail when it comes to compressor operation. Can anyone explain to me what the purpose of charge gas compression is? And what do the ethylene and the propylene refrigerant compressors do? Are those loops for process cooling only and thus closed loops, never in contact with process gas? Or do the machines compress process gas?

Thanks a lot in advance for any clarification.

 

[TD2K]

My experience is many years out of date in an ethane cracking ethylene plant.

Cracked gas from the furnaces is compressed by the charge gas compressor. We had a 5 stage centrifugal, 50 kPa suction to about 4000 kPa discharge with interstage cooling. From that point, the cracked gas went through a caustic scrubber to remove traces of CO2 before it was dried (we used mol sieves) and started down the cold train and then the fractionation towers.

The propylene and ethylene compressors were purely refrigeration units, there was no direct connection to the process.

The ethylene compressor discharge was (I think condensed) against propylene coolant (this was just one propylene load, propylene refrigeration was used in several other services) and was the second lowest level of refrigeration. I think we had 3 different levels of ethylene refrigeration (3 compressor suctions) but it may have been 2 (2 compressor suctions). Hp was in the area of 15,000 Hp, condensing turbine.

The propylene compressor was the larger of the two (ours was originally designed as propane but we used the propylene/propane product as makeup). Again, 3 suctions with the lowest just above atmospheric. Compressor discharge was condensed against cooling water. I don't remember the discharge pressure, it would be in the area of the saturation temperature of propane at about 80F I'm guessing. Ours had 2 drivers, a gas turbine with a net output of about 25,000 Hp and a 12,000 Hp steam turbine helper, 600 psig to 50 psig. The steam turbine provided the majority of 50 psig steam used in the ethylene plant.

Lowest level of refrigeration was provided by the discharge gas from the turbo expander before it was recompressed and sent to fuel gas.

 

[BingoBongo18]

Thank you very much so far.

One nice source for information on the operation of ethylene plants with ethane feed gas is this brochure on one of Exxon's plant:

http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CC8QFjAA&url=http://

http://www.exxonmobil.co.uk/UK-Engl...KWiwnatZP-uJ5uQ_m9DALAw&bvm=bv.58187178,d.Yms

According to this brochure, the major process steps are:

1) Preparation and storage
2) Steam cracking
3) Cooling
4) Compression
5) Separation

I am now wondering how the three compressors (cracked gas compressor, ethylene refrigerant compressor, propylene refrigerant compressor) fit into this. My understanding is that step 5, separation, is done by overpressure destillation with the overpressure being supplied by the CGC. After compression by the CGC, the cracked gas will be cooled and condensed in the cold box (the cold box being cooled by the ethylene and propylene loops with the two other compressors). Now a liquid, it will be routed to the distillation columns where the different components will be separated. Overpressure distillation is essential in order to separate the boiling points of the different components from each other far enough. This ensures that only the desired component will be boiled off (e. g. methane and hydrogen in the first step). After several other process steps, pure ethylene is left.

Is my understanding correct? If so, one more question: Is compression and distillation done repeatedly or only once? From the sources I have found this is not so clear.

 

[intaim13]

These are continuous processes...