Bringing Back Sponge Absorber To Service Evaluation

[mje03]

Greetings,

I'm currently doing an internship at a refinery. One of my projects is to evaluate the cost/benefits of bringing back the sponge absorber within the rheniformer/platformer back to service. The sponge absorber's purpose is to absorb some more propane, butanes and some C5+s from the recycle hydrogen stream produced by catalytic reforming (entering near bottom of the column), using lean oil entering the top of the column. Around 10 years ago, they decided to shut down the sponge absorber because they were losing the lighter ends composition in the lean oil to the overhead of the column.

I have looked at the historical data (L,V, and hydrocarbon compositions coming in and out of the column) and confirmed that the problem existed. However, I'm stuck on not knowing what the next steps should be like.

Any guidance towards solving the problem is much appreciated.

 

[georgeverghese]

Presume this sponge absorber is upstream of the recycle H2 compressor ?

 

[mje03]

It is downstream of of the recycle H2 compressor. A portion of the stream is used for recycle, the rest is pushed to the sponge absorber column. The lean oil is supposed to absorb propane + butanes and get routed back to the debutanizer column upstream (the overhead of the debutanizer column are light ends and are pushed to the light ends recovery unit).

 

[georgeverghese]

There may be one or more reasons why this is occuring. For one, it may be that recycle H2 to the sponge absorber is too hot. This would raise the lean oil temp to the extent that its light ends are vaporising into the over heads. What is the range in recycle H2 temp to this absorber from the compressor ? And how does this compare to cold feed lean oil to this absorber? Or is the lean oil feed too hot ?

 

[mje03]

Those are some great questions. I just checked the historical data and it looks like the normal temperature of the recycle H2 entering the sponge absorber is anywhere from 137F to 156.5F (average 149.7F). Lean oil is from 106.5F to 129.5F (average 117.9F). There is also a knockout drum downstream of the compressor but upstream (right before entering) of the absorber column to knock out some of the C5+ materials. Also, the data is within a 24-hour operating period.

 

[georgeverghese]

Feed rich H2 is considerably hotter than lean oil at 150degF. A water cooled HX on the compressor aftercooler should have brought this down to to 120degF or less. What is happening with your compressor aftercooler ?

See if you can run a process simulation model of this entire scheme and see what effect hotter and cooler H2 have on lean oil recirculation.

 

[georgeverghese]

Typically lean oil is at least a de iso C6 bottoms stream, or better yet, a de C6 bottoms stream. This lean oil you have now is from the de C4 bottoms, so it will contain volatile C4, C5, which is what is escaping into the H2 stream overheads from the lean oil at the lean oil absorber.

You can expect better recovery of light ends from the rich H2 with a de iso C6 bottoms lean oil. Right now, the c4 and c5 and c6 not stripped out from the rich H2 is going back to the platformer where it undergoes undesirable scission to lighter components.

Running this dec4 column in de iso c6 mode will require higher reboiler thermal load, more cooling capacity at the bottoms coolant exchangers, higher vapor traffic up the column, and more thermal duty at the column ovhd condensor. Does this deC4 unit have the margins to do this ?