Natural Gas

NATURAL GASCHE252INDUSTRIAL PROCESS

What is natural gas?A mixture of HC compounds & small quantities of various non HC (eg. N2 & CO2) existing in the gaseous phase or in solution with oil in natural underground reservoirs at reservoir conditionsNatural gas is so called because it occurs naturallyFormed from sediments rich in organic matter which in the past have been treated to high T & PFound undergrd structures similar to crude oilMay or may not be associated wt accumulation of oilVirtually all oil accumulation have nat. gas associated wt them but many gas accumulation exist independently of oil accumulation

Composition and TerminologyNG consists of varying proportions of HC of paraffin series wt CH4 predominatingDry gas little or no pentane & heavier fractions (C5+) presentWet gas significant amt of C5+ presentOther component : CO2, N2, H2S, He,Ar, Hg, H2O, solid particles etc present Condensate/natural gasoline - C5+ fractions liquid phase at atmos. T & PNGL ethane, propane and butanesLNG natural gas mainly methane liquefied by cooling to minus 161.5oC at atm. PressureLPG mixture in either liq or gas phase of propane n butane

ExplorationOil & gas accumulate undergrd if the following geological conditions are met: presence of reservoir rock containing interconnected pores eg. Sands and sandstones or limestones presence of cap rock eg. Clays, anydrite, salt or shales presence of a trap ie. Geometrical configuration of rocks and seal that prevents lateral escape

Exploration methodsGravimetric Measuring slight variations in force of gravity at surface influenced in magnitude & direction by distribution of rock of diff density underlying the areaGravity surveys carried out by air n particularly useful for locating salt structures

MagneticMeasuring variations in intensity of earths magnetic fieldMethod give indication of presence n thickness of sedimentary formation since sedimentary rocks are non magnetic while igneous & metamorphic rocks are magnetic

SeismicMost important & expensiveWorks on principle of low frequency sound waves generated from sources on or just below the land or water surface being reflected & refracted as they pass thru rock layers & measuring time taken for sound waves to travel back to surfaceReturning sound waves pick by geophones on land surface or hydrophones in water converted to electric signals & stored on magnetic tape for digital computer processingDespite all the latest tech. available, only way to confirm whether structure contain oil or gas is to drill a wellIn addition, drilling provides more information on further exploration and future field development plan

DrillingTechniques same whether on land or seaPenetration of subsurface achieved by a steel, tungsten carbide or diamond bit attached to screw-jointed pipes, rotated at surface

Objective of gas processingTo remove any impurities presentTo extract valuable component of natural gas (C3H8 n C4H10)To ensure customers specification for quality gas are met

Schematic LNG plant: main components

Gas receiving & meteringFeed gas regulated at satisfactory P for later removal of HC which otherwise cause problems in downstream processesFor high P gas - P let down station installed For low P gas compressors usedMetering gas quantity supplied to plant necessary if gas supplier(s) is owner of the plant which is often the case

Acid Gas Removal (Sulfinol Unit)CO2, H2S & other sulphur compounds are removed to acceptable levels to avoid freezing out in liquefaction unitCO2 : 50 ppm by volumeSulphur compounds < 3 ppmInvolved a solvent which absorbs the impurities & releases them in a regeneratorIf sufficient H2S present, a sulphur recovery unit installed to convert sulphur compounds into elemental sulphur, a saleable prodt

Mercury RemovalAllowable limit: 0.1 g/Nm3 Reason: to prevent corrosion of pipework and equipment made of aluminium or alloysAchieved by passing feed gas thru sulphur impregnated carbon, whereby Hg reacts (chemisorption) wt sulphur to form stable mercuric sulphide or adsorption onto macroporous aluminaAdsorbents not regenerable, disposal of spent chemical which contain certain %tage Hg has to be controlled for safety and environmental reasons

DehydrationTreated sweet gas leaving acid gas removal unit is saturated wt water since solvents are aqueous solutionBef. Cooling below 0oC, gas must be dried to avoid icing up in downstream processDone in 2 stages1st: water condensed n separated from gas stream by cooling2nd: balance water vapour removed by adsorption on molecular sievesAcceptable limit: < 1 ppmUsu. 3 driers required: 1st drier in service2nd drier being regenerated by heated stream of dry gas3rd on standby

Heavy Hydrocarbon separationDesign of units depends on:a. quantities of CH4 n other heavier HC present in NGb. the need to extract heavy HC that can freeze out in liquefaction unitc. the need to meet LNG specs as agreed by customersAchieved usg cryogenic scrubbing involved cooling NG against boiling C3 down to abt -30oC liq. mixture goes to fractionation unitStripped NG to liquefaction for further cooling

Fractionation- Involved a series of DCs: de-methanizer, de-ethanizer, de-propanizer, de-butanizerEach stripping out one component from its top endBottom stream of each column fed to the next so that fr last column flows pentanes & heavier HCs known as gasoline or NGLStreams of C2, C3 and C4 from tops of columns are either reinjected into liquefaction unit

Liquefaction3 main processes:a. pure refrigerant cascade processb. mixed refrigerant processc. combination of both ie. the pre-cooled mixed refrigerant process- Operates similar to refrigerator at home

Storage & loading facilities- NG is stored in liquefied form above ground or in-ground tanks