Increasing i-C4 production for alkylation unit feed

[Milutin]

Does anybody know ways how increase i-C4 production in refinery?

Our C4 mix from FCCU has about, 30mol% i-C4 and 55mol% C4 olefins. We have small i-C4 production from SR gas plant and it is not enough for alkylation unit continuous run.

 

[25362]

Catalytic reformers and hydrocrackers are also used as make up sources of isobutane.

Kindly note that isobutane consumption, m³/m³ of olefin is lower for pentenes than for lighter olefins, with a small penalty on yields and alkylate octane numbers.
The propylenes from the FCC tend to form isoheptane isomers (dimethylpentanes) having somewhat lower alkylate octane numbers.

Here is a table from literature:

Olefin C3 C4 C5 FCC C3-C4 stream

alk. yield (*) 1.76 1.77 1.7 1.76
i-C₄ consumption (*) 1.31 1.15 1.0 1.18
Alkylate RON 91 96 92 93.5
MON 89 94 90 91.5

(*) m³/m³ olefin

 

[epoisses]

Optimise your FCC, optimise iC4/nC4 fractionation (any idea how much you lose there?). I would tend to think it would not make economic sense to boost iC4 from the reformer at the expense of H2 and RON, but that needs to be backed up with a model run.
Talk with your Supply people about what can be done on the crude side.
Lastly you could do a screening on a Butamer unit.

 

[25362]

Beside n-butane isomerization, there are also processes to isomerize pentanes and hexanes providing high-octane isoparaffins in the lower boiling end of gasolines, often deficient in high-octane, in particular when speaking of those produced by catalytic reforming.

http://www.prod.exxonmobil.com/refiningtechnologies/pdf/2006_ARTCEMOGASColorforWeb.pdf

http://www.refiningonline.com/EngelhardKB/npra/NPR8851.htm

http://www.cheresources.com/refining.pdf

 

[Milutin]

Thank you for responses.

Investment in new processes is of course expensive and takes time.
In my opinion it is possible to squeeze more i-C4 from existing refinery processes.

I mentioned our SR gas plant, it actually takes feed from crude unit, reforming unit and diesel HDS, sorry for inaccuracy.

Current i-C4 spec for that unit is 95% i-C4 for top stream, in bottom stream of that column, n-C4 stream contains about 30% i-C4. As first measure I will try to increase i-C4 recovery from bottom stream, and as second lower i-C4 specification for top stream. In that way we will have more i-C4 for alky unit.
Higher content of n-C4 in alky feed cannot harm alky process?

My second idea is to try to introduce some quantity of C4 mix from FCCU as extra feed to DE i-C4 column on alky unit and separate additional quantity of i-C4 from C4 olefins. C4 olefins should be drawn as side stream together wit n-C4. Does anybody try this?

 

[25362]

To better understand your ideas on the effects of n-C4 as a diluent and the sidestream removal of C4's, could you please give some details on your alkylation process, such as the acid used, the refrigeration details, etc., as well as on your (recycling) de-isobutanizing tower ?

Some HF alkylation units actually operate their de-isobutanizer (aka Isostripper) in a way similar to your second idea: a propane-rich stream leaves at the overhead which is re-distilled to recover i-C4, an upper recycle i-C4 sidecut, a lower sidestream composed of n-butane, and an alkylate bottom stream.

 

[Milutin]

We have H2SO4 alkilation unit, with effluent cooling.

DE i-C4 column: top product is i-C4 used for recycle, n-C4 is drawn as side stream, bottom product is raw alkylate.

 

[25362]

My thinking:

[•] Feedstock with xs n-butane.

Because of the high viscosity of the acid in the temperature range of
3-12^oC, plus the high density difference between the acid and the reactants, the two phases need a vigorous mechanical agitation to get an intimate degree of mixing.

Any factor causing a reduction in mixing efficiency, and a drop in the high ratio of isobutane to olefin, may induce the formation of acid complexes and even polymerization reactions, which would lead to increased acid consumption, and a decline in alkylate quality.

If the refrigeration is caused by effluent partial vaporization, one should check whether excess n-butane could affect its performance.

[•] C4 fraction offtake as side stream.

If the de-isobutanizer capacity allows it your second idea could be practical as long as the overhead % of recycle isobutane and the quality of the bottom alkylate stream aren't degraded.

Good luck.

 

[epoisses]

Milutin, what is currently limiting the deisobutanizer?

 

[Milutin]

Nothing, as I know. Introducing C4 mix to deisobutanizer is not by design.

 

[Milutin]

Dear 25362

I'm not sure that my second idea, introducing C4mix in DE i-C4 column on alky unit, is good idea. On alkylation unit in some cases like emergency it is necessary to stop reaction in contactor. That is done by stopping C4 olefin feed, but in case if we have olefins in big i-C4 recycle stream from DE i-C4 column it is not possible.

Regards