Desulphurisation Processes And Recovery Of Sulphur
The level of sulphur in the past two decades has steadily increased due to use of more and more heavier crude, use of cheaper high sulphur crude which has forced the refining industry to go for additional facilities like ultra-desulphuristion for gasoline and diesel to meet the requirement of the stringent sulphur emission standards. Requirement of sulphur content for MS and HSD is given in Table M-VI 8.1.Sulphur is one of the major impurities in heavy crude resulting higher concentration of sulphur compounds in the un-desulphurised product stream. Sulphur content in the crude varies widely depending on the origin. Table M-VI 8.2 shows sulphur content in crude oil.
The variation is considerable and this impacts the processing scheme as well as the product slate. Sulphur content of commonly used sweet and sour crudes. Due to increasing environmental concerns, stringent limits on sulphur levels in fuel are being implemented world over to achieve target of sulphur below 100 ppm, deep hydrodesulphurization is required which is an additional capital cost as well as an energy intensive step. Table M-VI 8.3 given the details of reactivity of sulphur compounds present in crude oil
Table M-VI 8.1: Sulphur Requirement in Different Gasoline & Diesel in PPM
Table M-VI 8.2: Sulphur Content in Crude Oil
‘S’ Content, (wt%)
Table M-VI 8.3: Reactivity of Sulphur Compounds Present in Crude Oil
Relative reaction rate
Dib enzothi ophene
Di methyl dib enzothi ophene
trimethy l dib enzothi ophene
Sulphur distribution in typical refinery is given in below
Future Demand in Indian Refineries
Figure M-VI 8.1: Major sources of Sulphur & Recovery Processes in Refinery
Sulphur Recovery Units Characteristics – Refineries
Source: Tarapdar, T., Sulphur recovery technologies- Present and future development June 2011 Petrotech
Sulphur Recovery Unit
Sulphur recovery unit consist of recovery of sulphur from H2S present in acid gas from Amine Treating/ Regeneration unit and H2S from sour water stripper section Hydrogen sulphide content of the feed gas is converted to elemental sulphur. Typical sulphur recovery unit is shown in Figure M-VI 8.2.
Amine absorption and Regeneration: Absorption of H2S bearing stream and regeneration of amine. H2S rich stream from amine regeneration is sent to sulphur recovery unit.
Sour Water Stripping: Sour water is tripped off its sulphur and recycled. H2S is sent to sulphur recovery unit.
Amine Absorption Unit: Various hydro desuplhurisation processes in the refinery and hydrocracker unit generate large quantity of H2S. H2S bearing gases from various unit is sent to Amine treating unit which uses amine as a solvent for absorbing H2S and subsequently releasing H2S as H2S Rich stream in the amine generator.
Merox (Mercaptan Oxidation Unit)
Merox process is used in the refinery for controlling the mercaptan sulphur in gases, LPG, naphtha and other petroleum fractions. The Process is used for the chemical treatment of LPG, gasoline and distillates from FCCU, OHCU etc to remove mercaptans.
Mercaptans are either extracted from the stream or sweetened to acceptable disulphides. For treatment of light feed stocks such as LPG, no sweetening is required as mercaptans are nearly removed by extraction. However, feed containing higher molecular weight mercaptans and may require a combination of Merox extraction and sweetening using catalyst. Catalysts promote the oxidation of mercaptans to disulphide using air as the source of oxygen. Merox treatment can in general be used in following ways
Pretreatment(Remove H2S and Naphthenic Acids by dilute Alkali Solution)
Extraction (Remove Caustic soluble Mercaptans)
Sweetening(Oxidation of mercaptans to disulphides)
Post Treatment (Remove Caustic Haze)
(Caustic Settler, Wash Water, Sand, Clay Filters
Sulphur Recovery from H2S
Sulphur recovery now has become one of the most critical aspects of sulphur management and affects emission sulphur dioxides significantly in the refinery. There are two sulphur recovery processes are
Conventional Claus process has only 99% sulphur recovery. In order to meet the sulphur emission standards now Claus process has been improved substantially to meet the standards. Modern claus process is shown in Figure M-VI 8.3. New processes are characterized by
Figure M-VI 8.2: Typical Sulphur Recovery Unit
Figure M-VI 8.3: Modified Claus Process
Super Claus Process
The SUPER CLAUS process was developed to catalytically recover elemental sulphur from H2S containing Claus tail gas to improve the overall sulphur recovery level. The SUPERCLAUS process was commercially demonstrated in 1998, and today now more than 160 units are under license and over 140 are in operation. SUPERCLAUS process achieves high sulphur recover levels by suppressing SO2 formation in claus stages and selectively oxidizing H2S in presence of oxygen using proprietary catalyst .
A typical SUPER CLAUS sulphur recovery unit consist of following sections:
Function of Claus reactors:
(Where x= 6 and 8 mainly)
Function of Super Claus reactor
H2S + 0.5O2 → l/8S8 + H20 + 208kJ
SUPER CLAUS Process use selective oxidation catalyst minimizes side reactions & increase sulphur recovery
Claus Process Limitations:
The big difference between SUPER CLAUS catalyst and Claus Catalyst is that the reaction is not equilibrium based. Therefore, the conversion efficiency is much higher than the equilibrium limited Claus reaction. SUPER CLAUS is a non-cyclic process that has repeatedly shown simplicity in operation, high online reliability and sulphur guarantees up to 99.3percent
Super Sour Process: Stringent environmental regulations have necessitated higher recovery of H2S from sour water stripper unit design. Super Sour process ensures minimum H2S loss. the process employ additional hot feed flash drum upstream of cold feed surge drum. The H2S rich vapours from hot feed flash drum upstream of cold feed surge drum is routed to a small amine scrubber to absorb liberated H2S. The H2S lean gas containing primarily hydrocarbons is then routed to incinerator of the sulphur recovery unit. The absorbed H2S rich amine is recovered in the amine regenerator and is fed to the sulphur unit for converting it to sulphur.
INDE Treat and INDE Sweet Technology [Indian oil Technologies 2001]: INDE Treat and INDE Sweet Technology is based on the Continuous Film contactor(CFC) for effective removal of undesirable compounds at lower cost. It can remove H2S from LPG, Mercaptans from LPG, naphtha, gasoline and ATF/Kero, naphthenic acid from diesel, acid gases from natural gases, fuel gases and can regenerate spent caustic if required. CFC technology which is the heart of process.Salient features of CFC are
Merichem Fibre film Contactor Technology: The process is based on Continuous Film contactor (CFC) Fibre film Contactor technology for removal of impurities from hydrocarbon streams. The process achieves non-dispersive phase contact without problem inherent in conventional dispersive mixing devices.