4. Natural Gas Liquid Processing and Sulfur Recovery

Teknologi Pemrosesan Gas (TKK 564)Instructor: Dr. Istadi(http://tekim.undip.ac.id/staf/istadi )Email: istadi@undip.ac.idid

Instructor’s t Background• BEng. (1995): Universitas Diponegoro• Meng. (2000): Institut Teknologi Bandung• PhD. (2006): Universiti Teknologi Malaysia• Specialization:• Catalyst Design for Energy Conversion• Process Design for Energy Conversion• Combustion Engineering• Computational Fluid Dynamic (CFD)

Course Syllabus: (Part 2)1. Hydrocarbons Recovery (Pengambilan Kembali Hidrokarbon)2. Nitrogen Rejection/Removal (Penghilangang g Nitrogen))3. Trace Component Removal (Penghilangan Komponenlainnya)4. Natural Gas Liquid Processing and Sulfur Recovery(Pemrosesan Cairan Gas Alam dan Penghilangan Sulfur)5. Gas Transportation and Storage (Transportasi danPenyimpanan Gas)6. Liquified Natural Gas #1 (Gas Alam Cair)7. Liquified Natural Gas #2 (Gas Alam Cair)8. Second Assignment9. Ujian Akhir Semester

This chapter discussesprocessing the hydrocarbon liquidsLiquid stream proceed in a glass planCondensateprocessingNGL processing

CONDENSATE PROCESSING

Two steps of CondensateProcessing• Water washing to remove salt andadditives• Condensate stabilization • remaining lighter hydrocarbons are strippedand recombined with the gas that leaves theinlet receiver• to produce a bottom product that hasspecifications to be sold as “natural gasoline”or “slop oil”

Sweetening for Liquid Processing• Ideally, the sulfur compounds go with the light gases strippedin the stabilizer, so that they can be removed ed in gas treating.• Most of the H2S and CO2 will be removed along with COS,CS2, and mercaptans.• To enhance acid gas removal, sweet natural gas can be usedas a stripping gas.• This process can reduce H2S levels to the 10 ppmv range.However, the cost is increased recycle compression.• If the natural gasoline is sour, the plant can either treat it ortake a price penalty and sell the liquid as sour crude.• The price differential between sweet and sour crude drives theneed for sweetening.• In many cases, the liquid volumes are low and sweetening atthe plant is not justified.

NGL Processsing

Sweetening Method in NGLProcessing• Amine TreatingMost remove only CO2 and H2S . Two amines, DGA and DIPA, will removeCOS, but none remove CS2 and merchaptans. Amine treating is often usedupstream of caustic treaters to minimize caustic consumption caused byirreversible reactions with CO2.• AdsorptionAble to remove sulfur compounds down to low levels when no water ispresent, which makes it attractive for treating demethanizer bottoms.• Either promoted alumina, 13X or 5A molecular sieve will remove H2S, COS,and mercaptans from LPG.• The larger pore 13X is usually preferred because it has a higher capacity• Caustic TreatingRegenerative = sodium hydroxide.Nonregenerative = ZnO, KOH

Regenerative caustic wash andwater wash.

Chemistry involved in Washing• Air is added to the rich caustic, with a catalyst tooxidize the mercaptan to a disulfide:2RSNa + 1/2 O2 + H2O → RSSR + 2NaOH

Dehydration Process in NGL• Adsorption Processes• Desiccant Dehydration• Gas Stripping• Distillation• Absorption

• Currently only two methods are available for dealingwith large quantities of H2S:1. Disposal of the gas by injection into undergroundformations2. Conversion of the H2S into a usable product,elemental sulfur

Properties of Sulfur

SULFUR RECOVERY PROCESSES1. Claus Process• All Claus units involve an initial combustionstep in a furnace. The combustionproducts then pass through a series ofcatalytic converters, each of whichproduceselemental sulfur.

• Basic ProcessH2S + 3/2 O2↔ H2O + SO22H2S+SO2↔ 2 H2O + (3/x) SxThe first reaction is a highlyhl exothermic combustionreaction, whereas the second is a more weakly exothermicreaction promoted by a catalyst to reachequilibrium.• KohlandNielsen(1997)pointoutthatatasulfurpartialpressure of 0.7 psia (0.05 bar) and temperaturest bl below700°F (370°C), the vapor is mostly S6 and S8, but at thesame partial pressure and temperatures over approximately1,000°F (540°C), S2 predominates.

Process

2. CLAUS TAIL GAS CLEANUPBefore sulfur emissions restrictions wereimposed, the offgas from the Claus unit wasflared to convert the remaining H2S to SO2.Now, environmental agencies demand highersulfur recovery than can be achieved with astandard Claus unit, and additional treating of theClaus tail gas is needed. The tail gas cleanupentails either an add-on at the end of the Clausunit or amodification of the Claus unit itself.

• three categories for final sulfur removal(Kohl and Nielsen, 1997):• Direct oxidation of H2S to sulfur• Sub-dew point Claus processes• SO2 reduction and recovery of H2S

Direct Oxidation of H2S to SulfurCatalytic Reaction :2H2S + O2 → 2S + 2H2OThe selective oxidation catalyst in the third reactordoes not promote the reaction2H2S + 3O2 ↔ 2SO2 + 2H2Oor the reverse reaction of sulfur with H2O3S + 2H2O ↔ 2H2S + SO2Total revovery rate = 99%

Sub Dew‐Point Claus Processes

SO2 reduction and recovery of H2S

Sulfur Storage• Sulfur from the Claus unit is withdrawn as a liquid and isgenerally stored and transported in the molten state.• A number of potential problems are associated with sulfurstorage, including release of H2S dissolved in the moltensulfur and the possibility of sulfur fires, which will producehighly toxic SO2.• Sulfur fires. Uncommon, but they can produce large amounts ofSO2.• H2S. Any H2S dissolved in the molten sulfur from the condensersmay be a significant hazard if appropriate p degassing g techniquesare not used.• Corrosion. A wet sulfidic atmosphere can lead to severe corrosionof carbon steel.• SO2. Highly toxic and it forms highly corrosive sulfurous acid inthe presence of water.• Static discharge. Because of the excellent insulating properties ofmolten sulphur, static discharge may occur under certainconditions and lead to possible fires or explosions.