View Topic Comments

High H2S content in Vac slop and VR

Query Raised By: R K Kubade, Dy General manager, BPCL Mumbai Refinery

1. We are facing high H2S in Fuel oil and vacuum slops. VDU bottom level was decreased, VDU bottom temperature was decreased, stripping steam and coil steam increased. Even then vacuum slop is showing H2S presence of >15 ppm. RCO (Reduced crude oil –LR) also shows presence of H2S 2.3 ppm so stripping steam of CDU unit also increased and RCO result showed H2S presence as 0.5 ppm. Exchangers were also checked for leak and found normal. What could be the possible reason for high H2S in vacuum slops and Vacuum residue ?.

(1)  Comments

  • Subramoni Iyer M.R., CGM (Tech.), BPCL-Kochi(Expert)   -   23-Jul,2019 08:40

    Article Summary Selecting the right hydrogen sulphide scavenger is important, but appropriate treatment setup and tailored monitoring procedures are key to avoiding off-specs GIACINTO GRANDE and ALESSANDRA BERRA Chimec Scavenging hydrogen sulphide (H2S) from petroleum products may be easy, but doing it efficiently is much more challenging. Too often some important aspects are overlooked, causing extra costs in the best case and off-specs in the worst ones. Through the example of a case history we want here to show the impact that a proper approach in treatment management can have on dosage rates and therefore costs. H2S risk Risk is known to be the product of the probability of an event and its consequences, and H2S is a poisonous gas, lethal at certain concentrations, that can be present in petroleum products. This is why it represents a high risk for the petroleum industry. But there is more to this issue. H2S’s specific features make it worthy of special attention. Being a colourless gas, its presence can be identified only by smell which can be deceptive. H2S can be detected by a characteristic odour of rotten eggs when present in very low concentrations (1.4 μg/m3 or 0.0047 mg/kg), but the senses are soon saturated and deactivated by an increase in concentration, meaning that when the concentration exceeds 100 mg/kg humans are no longer able to smell it. This is perhaps one of the most dangerous aspects of this gas; its presence ceases to be perceptible by the human senses as its concentration becomes more dangerous. And exposure at a concentration equal to a few hundred mg/kg may already be lethal. H2S is present, in extremely variable amounts, in crude oils. Advanced refining processes generally allow for high, if not complete, removal of H2S from light and middle distillates, with rare but possible exceptions. The situation is different when it comes to heavy fuel oils. Generally speaking, H2S content in residues is the result of the sum of what remains trapped during the distillation processes, what is formed as a result of cracking phenomena, and what is removed by desulphurisation and/or stripping processes. The nature of the sulphur compounds present in the crude, and the severity of cracking applied, influence the amount of H2S that can be produced as a result of these phenomena. Conditions sufficient for the formation of H2S, for example, can be reached by the bottom of the vacuum distillation columns. The nature of residue refining processes is more oriented towards the formation of H2S (cracking) than its removal (desulphurisation) to the point that its content in the liquid phase can reach values even higher than 50 mg/kg even though it is mostly in the range 5-20 mg/kg. The values mentioned above may seem lower than the safety thresholds for human health but there is an important difference: the phase to which these concentrations refer. When we speak about petroleum products, H2S content is reported as concentration in the liquid phase. From this liquid, H2S is then released into the vapours above the oil, where it becomes dangerous to humans. The amount of H2S developed in the vapour phase depends on several factors, including the characteristics of the oil, conditions of temperature and pressure, and obviously the H2S concentration in the liquid. For residues and heavy fuel oils, the levels of H2S in the vapours are normally 50-100 times those Following are the views related to high H2S in vacuum slop & vacuum residue: • VDU heater Outlet temperature can be adjusted and check whether the H2S liberated in VDU overhead gas is coming down. This is done to check whether any excess thermal cracking is happening leading to High H2S liberation. • VDU vacuum can be improved, if possible by putting additional ejectors,etc., for better H2S stripping. • H2S scavengers shall be tried (as given in the attached reference)