Diluent to Visbreaker Furnace 1

[KazakhJeff]

I'm working in Kazakhstan where the refinery uses a mix of gas oil/diesel as the diluent to the Visbreaker furnace, in place of steam/condensate. Has anyone else experience of this? If so, does it work in other locations (and what are the benefits)? Has anyone made the switch to H2O, and if so what were the improvements? Any other comments would be appreciated.

 

[StoneCold]

I have done some visbreaking research on the lab scale. There is a difference between standard visbreaking and what is called steam visbreaking. You can get slightly higher conversions with steam visbreaking when the conditions are compared to standard visbreaking (2% or so). You then need to separate the water off of the light ends. Foster Wheeler and Shell do most of the newer visbreaking designs. They could probably give you a better idea of what the improvement would be.

 

[mchohan]

We inject steam in VB furnace coil as velocity steam to reduce the furnace tube coking. It also surpresses the vapor pressure of heavy resid and causes to boils at lower temperature (stripping effect). It has helped us to improve the conversion and unit run length.

 

[25362]

By "breaking" fuel viscosities VB units are meant to reduce the quantity of fuel "cutter stocks", fuel diluents that could be diverted to more profitable outlets. Besides, a VB unit may produce gas, gasoline and a gas oil distillate side-cut that can be subsequently used as diesel or light heating oil. Thus, one may assume you are using recycle gas oil obtained from the fractionator's side-stripper, not virgin gas oil, in your heater.

Mchohan's experience with (high-pressure) velocity steam is a common approach, rather than to rely on the vaporization of recycled middle distillates at the prevailing pressures, to reduce coking.

On the other hand, soakers enable the use of lower temperatures for a given conversion level, and are therefore less prone to coke formation in the heater coils. However, with or without soakers, all VB units must be shut down periodically for decoking.

Higher vaporization rates may reduce the required residence time for visbreaking. Higher pressures, up to 20 bar, are then often preferred to provide greater control over residence time by minimizing vaporization, depending on the feed treated. Steam pressure would then have to be higher to enable its injection.

Thermal cracking reactions start at about 315-370^oC and the rate of primary decomposition doubles for every 15^o increase in temperature at 450^oC, and increases at higher temperatures. Temperatures must be closely controlled to avoid destroying the natural complex peptizing agents that keep asphaltenes in colloidal suspension. Without these agents asphaltenes precipitate rendering the VB residue unsuitable for sale as fuel oil. A problem usually referred to as fuel oil "instability". Bottom quenching with cooled VB residue is generally used to suppress further thermal cracking in the fractionator.

One should check whether injecting "velocity steam" to the heater would change the V/L loads on the fractionator, the overhead condenser, and the gas/gasoline(C₅-165^oC)/water separator, considering that steam to the gas oil product stripper would diminish due to the lower recycled gas oil circulating rates. More net steam would necessarily increase the amount of sour water produced.

I agree with BradStone's advice regarding consultation with one the large engineering companies, which could advise also on the overall economics of switching over to steam when treating topped (BP>350^oC) or vacuum residues (BP>550^oC) as the case may be.

 

[KazakhJeff]

Still in Kazakhstan, our refinery safety (pressure) relief valves are Soviet standard, with inherant problems of unreliability. The springs need changing annually, for instance. Can anyone suggest western manufacturers who we could contact for quotations and possible purchase?
Spaciba (that's thank you out here)

 

[TD2K]

Are you talking about potential suppliers of springs for your existing PSVs?