Diluent to Visbreaker Furnace
Diluent to Visbreaker Furnace
(OP)
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.
RE: Diluent to Visbreaker Furnace
RE: Diluent to Visbreaker Furnace
RE: Diluent to Visbreaker Furnace
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 315-370oC and the rate of primary decomposition doubles for every 15o increase in temperature at 450oC, 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(C5-165oC)/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>350oC) or vacuum residues (BP>550oC) as the case may be.
RE: Diluent to Visbreaker Furnace
Spaciba (that's thank you out here)
RE: Diluent to Visbreaker Furnace