6 replies to this topic

[kirang11]

Sir, we have a naptha hydrotreator in which feed ( naptha + recycle gas) enters an exchanger train ( say A, B, C......H=total 8) shell side then to a furnace and then to a reactor and reactor O/L goes into tube side of the train( final tube side O/L is 102 deg celcius) and then to air fin cooler and a seperator (operating pressure=24.6kg/sq.cm)..Now we inject fresh makeup H2(300Nm3/hr) available at 37 Kg/sq.cm and a temp of 32 deg celcius at the Recycle gas suction KOD. can we inject fresh Hydrogen makeup at the exchanger tube side outlet ( upstream of air fin cooler at which temp is 102 deg celcius and a pressure of 26.5 Kg/sq.cm) instead of making up at Recycle gas compressor suction KOD? iam asking this because one exchanger (A) has been removed due to exchanger tube fouling and resulting in 102 deg at Upstream of air fin cooler which is difficult to cool by air fin cooler in summer.now air fin cooler outlet is 78 deg celcius. and there is no trim cooler between AFC and seperator.

Edited by kirang11, 03 June 2015 - 07:05 PM.

[juche]

Hello Kirang11,

 

Do you have a recycle gas Scrubber which uses Amine for scrubbing H2S from recycle gas or you have a continuous purge from recycle gas.

 

If you add makeup gas upstream of your air cooler installed upstream of separator and you have a continuous purge then you will loos  a good amount of high purity hydrogen from your purge line.

 

Second issue you may see is the erosion in the line downstream of the air cooler that line is designed for low velocity because H2S and NH3 combines to form some salts that can erode the line if velocity increases (that is expected to increase if you add makeup gas in upstream of the air cooler). These are two immediate problems i can see if this change is made.

 

can you confirm the metallurgy of the air cooler and down stream line.

[kirang11]

sir

we don't have amine scrubber in this section, yes we have a purge line upstream of a recycle gas compressor knock out drum but we don't purge in normal operation. what ever H2S formed due to reactions are stripped off in a stripper and sent to amine column for off gas treatment. 

Metallurgy of air fin cooler is killed carbon steel (std 3-11)

 

sir in our data sheet it is mentioned upstream of air fin cooler (  vapour vol flow is 5930Nm3/hr, liqd vol flow 76.5m3/hr, weight flow is 55264 kg/hr)

and at outlet of air fin cooler is  ( vapour vol flow is 5188Nm3/hr, liqd vol flow is 80.2 m3/hr , weight flow is 55264 kg/hr)

at present we are making up H2 at the upstream of recycle gas knock out drum around 300Nm3/hr. 

[juche]

See the Metallurgy for the air cooler rundown line is KCS, so you have to limit the velocity in the downstream line upto 6m/sec. Do some calculation if velocity after addition of Makeup gas at air cooler rundown line if it is less than 6m/sec, you can give a try.

 

And you also need to see how much drop in temperature are you expecting with the addition of hydrogen as i have seen the hydrogen flow is about 5-6% of the total flow.

 

And i would advice that before making this change, have a talk with your licencor what is their experience about the same. 

[kirang11]

thank you sir. i know the makeup H2 is lesser quantity. i want to reduce temperature at the inlet by atleast 10 deg celcius.

Edited by kirang11, 11 June 2015 - 11:12 AM.

[juche]

i was further thinking, what is the problem you are facing by this increase in Temp, I have seen in some cases hydrogen purity in recycle gas circuit is a problem. if you are getting hydrogen purity in the recycle gas circuit then you need not to change the location of hydrogen make up.

[kirang11]

sir,

problem is liquid carryover to Recycle gas compressor along with gases resulting in frequent draining of Recycle gas compressor suction bottle drain pot. liquid is getting filled up for 2 times in a hour. we want to condense all the hydrocarbons at the seperator. we are not getting liquid in compressor suction Knock out drum.

 

Regarding purity, we are getting around H2=75-80 vol%, and H2S= 200-500ppm in recycle gas circuit.