5 replies to this topic

[Milutin]

In our refinery we buy pyro gasoline from petrochemical complex, we treat pyro gasoline on hydrotreating unit to remove olefins and after hydrotreating it is sent to Sulfolane plant, aromatics extraction.

Usually pyro gasoline color is between +30 and +20 saybolt, but we started to receive pyro gasoline with poorer color, even +4. In both cases color of pyro gasoline after hydrotreating is +30.

I’m afraid that we receive some impurities which can harm hydrotraeter catalyst. To try find cause of color changes we treat pyro gasoline with active charcoal, but no change in color. What is possible cause of pyro gasoline color change, or what is your suggestion for lab analysis to tray find reason for color degradation.

Regards,

Milutin

[P.K.Rao]

Pyro gasoline turns dark on storage without anti-oxidant due to oxidation of diolefins which are invariably present in it. Diolefins are very reactive. Please check whether you received stored material.

By hydrotreating, these oxidized products are converted to saturated compounds due to which the color improves depending on the severity of the treatment. Water is the by-product from hydrotreating the oxidized hydrocarbons which may reduce the activity of the hydrotreater catalyst. You may check bromine number of the gasoline before and after hydrotreatment and compare with the bromine number of fresh pyrogasoline (color +30) after hydrotreatment.

Regards

P.K.Rao

[Milutin]

Dear P. K. Rao

First hydrotreating stage is in petrochemical complex (selective hydrogenation), designed to saturate diolefins. After first stage it is stored in tanks and pumped to refinery tanks. All tanks are without nitrogen blanketing, but they are small and we frequently switch them. Tanks have floating roofs. In refinery we have second stage hydrotreating designed to saturate olefins and to desulphurize pygas.

Few days after we introduced pygas color improve to +20, but we have problem with high dP across reactor. dP increase rapidly but when we introduce kero it drops significantly. I understand that when we introduce kero we have different reactor hydraulics, and that can be reason for lower dP, but when we again introduce pygas dP started from low value and continually increases.

It looks like reactor is regenerated after kero processing. I expect if we contaminated reactor with coke, gums or some other dirt it is not possible to rejuvenate catalyst without classic regeneration.

Besides bromine number what is other pygas analysis necessary to have good control of pygas coming from petrochemical complex?


Regards,

Milutin

[P.K.Rao]

It is true that PyGasoline is selectively hydrogenated to remove diolefins. Still then, it can turn dark if stored without antioxidant for long periods because mono olefins can also undergo oxidation. It all depends on the type of material you are getting.

Existent gum can be helpful in knowing the age of pygas. If the pygas is stored for long periods without antioxidant, existent gum will increase and color will be darker. You can check fresh and stored pygasolines.

Regarding dP, perhaps the oxidized gummy material of the pygas is getting deposited on the catalyst due to which dP is increasing. Kerosene (perhaps you are working at higher temperature) might be removing this material due to which dP is decreasing. A careful examination of the unit conditions may throw some light.

Regards

P.K.Rao

[Milutin]

Pygas analysis

We analyzed hydrotreater feed and products (pygas).

First set of data
Pygas - feed:
Existing gums: 0.9 mg/100ml
Potential gums: N/A
Induction period: 150 min.
Bromine number: 51.8

Hydrotrated Pygas:
Existing gums 0.5
Potential gums: N/A
Induction period: >360 min.
Bromine number: 0.13

Second set of data.
Pygas - feed:
Existing gums: 0.8 mg/100ml
Potential gums: 303.4 mg/100ml
Induction period: >360 min.

In my opinion quality of feed pygas doesn’t look so bad.

Regards,

Milutin

[goroseca]

Hi Milutin (zdravo Milutine)!

I have same problems (diolefines, colour, reactor dp, ...) in my unit in Pancevo refinery. Reactor dp is usualy increase 0.2-0.3 bar/day (last two cycles even 0.5 bar/day). We had earlier experience while we were producing euro diesel and kero. When produce euro diesel and dp reach certain level we had prattise to change feed to kero and work utnil dp is on minimum level. After that we change feed to euro diesel and worked several weeks. With pygas "procedure" is same, just when dp reach ~6-6.5 bar "wash"catalyst weth kero, but with increase reactor temperature of 5-10 °C. Reactor dp is decreasin 0.2-0.3 bar/day up to ~5 bar when we change feed to kero+LGO (euro diesel feed). In that moment dp decrease almost 2.5 bar in one day. We work like this few days and then return to pygas. With this procedure cicle with pygas is around 12-15 days, depending on feed.

Regarding all this I couldn't find any data about max diene number for this tipe of catalyst. Some recomendations is max 1, bat we have usualu betwine 2 and 3, sometimes even 6. If zou have any information please send it to me.

Best regards

Goroseca