PRO-2003 Natural Gas Processing Drying of Natural Gas

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1 PRO-2003 Natural Gas Processing Drying of Natural Gas
Adjunct Professor Jon Steinar Gudmundsson Department of Engineering and Safety University of Tromsø April 2014

2 Outline Drying (dewatering) means removing water vapour from natural gas. Carried out after sweetening (removal of sour gases) Water vapour condenses out from natural gas as liquid water, leading to the formation of solid gas hydrate (blocking of pipelines). Liquid water in the presence of acid gases is quite corrosive. Cooling (refrigeration and expansion), absorption and adsorption processes are used to remove water vapour from natural gas. Drying processes used at Kollsnes, Kristin and Melkøya are illustrated (based on Førde 2008). What about emissions from drying and sweetening processes? The chemical formulas depict the TEG molecule

3 TYPICAL SPECIFICATIONS
Transport Specification Hydrocarbon dew point, 5-10 C below ambient Water dew point, about 5 C below HC dew point Temperature, C Pressure, depends on receiving terminal Sales Specification (in addition to above) Heating value (GHV = Gross Heating Value), MJ/Sm3 Wobbe Index (WI = GHV/(specific density)0,5 Removal of non-HC gasser (inert gases)

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5 WATER VAPOUR IN NATURAL GAS
Produced natural gas (associated and non-associated) contains water vapour that condenses to liquid water at the surface. Liquid water in natural gas leads to the formation of solid gas hydrate that can block processing equipment and pipelines. Liquid water in the presence of carbon dioxide and hydrogen sulfide results in acid water that is quite corrosive. Too much water vapour in natural gas can lead to the condensation of liquid water in transmission and distribution pipelines. Water dew point is specified in natural gas sales contracts. Water dew point usually (arbitrarily) referenced to 70 bara, which is equal to about 1000 psia.

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7 METHODS TO DRY NATURAL GAS
Cooling by refrigeration (e.g. propane cycle) Cooling in expander (and Joule-Thomson valve) Absorption using glycol (usually TEG) Adsorption using molecular sieve New technology (Twister)

8 METODER FOR FJERNING AV VANNDAMP
 Direkte kjøling (bruk flytende propan, f.eks.), må injisere inhibitor  Absorpsjon (store volumer vann), væskefase, mest vanlig i industrien  Adsorpsjon (små volumer vann og/eller stor krav om tørking), faststoff  Ekspansjonskjøling (f.eks. Troll/Kollsnes). Ekspander isentropisk gir mer kjøling enn isentalpisk Joule-Thomson ventil.  Membranseparasjon, lite vanlig (brukes for CO2 fjerning, H2O fjernes samtidig)

9 Kocken (2013)

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11 COOLING CAN ACHIEVE THREE PURPOSES

12 Ideal Refrigeration Cycle

13 Førde 2008

14 Wickipedia (2013)

15 TRADITIONAL TEG-ABSORPTION AND STRIPPING

16 Førde 2008

17 KOLONNETEORI

18 DRIFTSLINJE OG LIKEVEKTSLINJE

19 ABSORPSJON OG DESORPSJON

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22 DUGGPUNKTKONTROLL MED TURBOEKSPANDER OG JOULE-THOMSON VENTIL
Trykk-tempratur for duggpunktkontroll med ekspander (TE) og Joule-Thomson ventil. (Naturgassemnet, Løvold 2008; fra Tufte 2007)

23 Nyhamna (Ormen Lange) Process

24 Gupta (2011). Feed gas is processed to remove heavier components for dew point control.

25 MASSEBALANSE ABSORBERKOLONNE
Renset gass ute Svak amin inn Rik gass inn Rik amin ute

26 Førde 2008 Førde 2008

27 COOLING IN SUPERSONIC FLOW (TWISTER)

28 TWISTER SUPERSONIC SEPARATOR
The Twister Supersonic Separator is a compact tubular device which is used for the removing water and/or hydrocarbon dewpointing of natural gas. The principle of operation is similar to the near isentropic Brayton cycle of a turboexpander. The gas is accelerated to supersonic velocities within the tube using a De Laval nozzel and inlet guide vanes spin the gas around an inner-body which creates the "ballerina effect" and centrifugally separates the water and liquids in the tube. Hydrates do not form in the Twister tube due to the very short residence time of the gas in the tube (around 2 milliseconds). A secondary separator treats the liquids and slip gas and also acts as a hydrate control vessel. Twister is able to dehydrate to typical pipeline dewpoint specifications and relies on a pressure drop from the inlet of about 25%, dependent on the performance required. The fundamental mathematics behind supersonic separation can be found in the Society of Petroleum Engineers paper (number ) entitled " Selective Removal of Water from Supercritical Natural Gas".

29 Twister – til duggpunktskontroll og væskegjenvinning
Figur 7.9 fra Kidnay

30 Nedkjøling ved ekstern kjølekrets (kompresjonssyklus)
Rojey & Jaffret (1997)

31 Nedkjøling ved isentalpisk ekspansjon (J-T ventil)
Rojey & Jaffret (1997)

32 Machado & Skouras 2012

33 European Gas Quality Parameters
Christensen & Skouras 2011

34 Summary Water vapour in associated and non-associated gas condenses out in oil and gas production, leading to the formation of solid gas hydrate and corrosive water (when carbon dioxide and hydrogen sulfide are present). Cooling (refrigeration or expansion), absorption and adsorption processes are used to remove water vapour from natural gas. In cooling processes, antifreeze needs to be added to prevent the formation of gas hydrate (separated after the cooling process). In absorption processes, TEG (triethylenglycol) is the most commonly used solvent. The process consists of an absorption column (water removed from natural gas) and a stripper column (water vapour distilled out from the glycol). Adsorption processes are used when very low levels of water vapour content are required, for example in LNG production.

35 References Aker Kværner (2008): CO2 håntering fra krafgenerering til havs, 20 sider. Kocken Sistemas de Energia (2013): Dehydration, 1 p. Machado, A.C. & Skouras, S. (2012): Gas Quality From Reservoir to Market. Guest Lecture, TPG 4140 Natural Gas, NTNU. Christensen, K.O. & Skouras, S. (2011): Gas Quality From Reservoir to Market. Guest Lecture, TPG 4140 Natural Gas, NTNU. Førde, T. (2008): Drying of Natural Gas, Guest Lecture, TPG 4140 Natural Gas, NTNU. Rojey, A. & Jaffret, C. (1997): Natural Gas – Production, Processing and Transport, Éditions Technip, 429 pp.

36 Fasediagram for en kondensatgass
Rojey & Jaffret (1997)

37 Fraksjonering ved lavtemperaturdestillasjon
(Her: NGL og metan) Rojey & Jaffret (1997)

38 Fraksjonering av gass

39 Fraksjonering av gass

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41 Dewatering Natural Gas by Cooling