LNG – What Is Piping: All about Piping Engineering

Natural Gases from reservoirs can not be used in its raw form as it contain various contaminants. So natural gas must be treated before commercializing it and the process is known as gas treatment or natural gas treatment. Contaminants of Natural Gas Gas contains various contaminants Ammonia (NH3)Hydrogen Sulfide (H2S)Hydrogen Cyanide (HCN)Carbon Dioxide (CO2)Carbon Sulfide (COS)Carbon Disulfide (CS2)Mercaptans (R-SH)Water (H2O)Sulfur Dioxide (SO2) Gas contains other components and Heavy Hydrocarbons such as PentaneHexaneBenzeneTouleneXylene, etc Process Overview of Gas Treatment Hydrocarbons in production wells constitute three parts; Oil, Gas, and Water (Fig. 1). In a three-phase separator, all three elements are separated. The natural gas containing various contaminants is then subjected to gas treatment processes to sell or market it for the end-user. Fig. 1: Process Overview Reason for Natural Gas Treatment Removal of the above-mentioned contaminants are required for reason of : Corrosion ControlToxicityGas product specificationsTo prevent poisoning of catalyst in downstream facilitiesControl of the overall heating value of the natural gasTo meet environmental requirementsTo prevent freeze out at low temperaturesTo prevent hydrate formation CO2 & H2S are very common contaminants in Natural Gas System Reason for CO2 removal from Natural Gas: When combined with water forms Carbonic Acid which is corrosiveCO2 reduces the BTU value of the gasIf present in 2-3% gas is not saleable. Reason for H2S removal in Gas Treating: Extremely toxic gasHighly corrosive Reason for Mercaptants (R-SH) removal during Gas treatment: It has smellIf comes in contact with bacteria (inside the final product storage) then released H2S2R-SH Bacteria R-SSR + H2S Specification for Export Quality Natural Gas The following table shows the permitted quantities of elements in natural gas for export purposes. SpecificationUnitsMaximumMinimumGross Heating ValveBtu/scf1154940Carbon DioxideMole %1--NitrogenMole %2--Hydrogen Sulphideppm vol5--Mercaptan Sulphur Contentppm vol5--Oxygenppm25--Moisture Contentlb/MMSCF10--Max Water Dew Point°C2(at delivery pressure)--Max. Hydrocarbon Dew Point°C-2(at delivery pressure)--Table 1: Elements in Export Quality Natural Gas Typical Sales Gas Specifications Fig. 2 below shows the specification for shale gas and Fig 3 shows the specification for LNG products. Fig. 2: Sales gas Specification Fig. 3: LNG product Specification Natural Gas Treatment Steps Gas treatment is achieved in three steps Gas Sweetening (Removal of H2S, CO2, etc)Gas Dehydration (Removal of Water)Gas Dew Pointing (Removal of Heavy Hydrocarbons) The following Figure (Fig. 4) shows the steps involved in gas treatment process. Fig. 4: Gas Treating Gas Sweetening Process Various processes are used for Gas sweetening as listed below Chemical SolventPhysical SolventChemical + Physical SolventSolid BedDirect Conversion Acid Gas Removal Processes: Chemical SolventPhysical SolventDirect ConversionMEA (Mono Ethanol Amine)Selexol®Iron SpongeDEA (Di-ethanol Amine)RectisolStretfordTEA (Tri-ethanol Amine)PurisolUnisulfMDEA (Methyl Diethanol Amine)SpasolvTakahaxDIPA/Shell ADIP®Propylene CarbonateLO-CAT®DGA/Fluor Econamine®EstasolvenLacy-KellerProprietary AmineAlkazidTownsendBenfield (Hot Carbonate) SulfintCatacarb (Hot Carbonate) Giammarco-Vetrocoke (Hot Carbonate) Diamox Dravo/Still Table 2: Acid Gas removal Process Gases removed by different processes: ProcessGases Removed CO2H2SRHSCOSCS2SOLID BEDIron Sponge X SulfaTreat X Zinc Oxide X Molecular SievesXXXXXCHEMICAL SOLVENTSMEA - mono ethanol amineXX X*XDEA – diethanol amineXX XXMDEA - methyl diethanol amine X DGA – diglycol amineXX XXDIPA – di isopropanol amineXX X Hot Potassium CarbonateXX XXProprietary Carbonate Systems Table. 3: Gases removed by different process Process Capabilities for Gas Treatment Normally Capable of Meeting 1/4 Grain H2SRemoves Mercaptans and COS SulfurSelective H2S RemovalSolution Degraded (By)MonoethanolamineYesPartialNoYes (COS, CO2, CS2)DiethanolamineYesPartialNoSome (COS, CO2, CS2)DiglycolamineYesPartialNoYes (COS, CO2, CS2)MethyldiethanolamineYesSlightYesNoSulfinol®YesYesYesSome (CO2, CS2)Selexol®YesSlightYesNoHot Pot - BenfieldYesNoNoNoFluor SolventNoNoNoNoIron SpongeYesPartialYes––Mol SieveYesYesYes––StretfordYesNoYesCO2 at high concentrationsLO-CAT®YesNoYesCO2 at high concentrationsChemsweetYesPartial for COSYesNoTable 4: Process Capabilities of Gas Treatment Fig. 5: Gas Sweetening Gas Dehydration Glycol DehydrationSolid Desiccant Dehydration Fig. 6: Gas Dehydration Gas Dew Pointing for Gas Treatment The gas dew pointing is done in any of the gas treatment methods mentioned below: Propane RefrigerationJ-T RefrigerationTurbo Expander Fig. 7 shows typical schematic of Refrigerated J T Process Fig. 7: Refrigerated J T Process