2.0 Natural Gas Processing natural gas is gaseous form of petroleum mostly methane (C 1

Transcription

1 2.0 Natural Gas Processing natural gas is gaseous form of petroleum mostly methane (C 1 ), some ethane(c 2 ), propane (C 3 ), butanes (C 4 ), pentanes (C 5 ), hexanes (C 6 ) and C 7 + as of year-end 2005 Canada + US proved natural gas reserves = 7.43*10 12 m 3 (262.3 Tcf) 5.78 Tcm (204.4 Tcf) in US and 1.64 Tcm (57.9 Tcf) in Canada. Gas transported via pipeline, LNG, and in future as CNG or transformed via GTL Pipeline advantageous when infrastructure in place and close to market Remote or stranded gas better served by LNG, CNG, and GTL

2 LNG proven technology, reduction 600 times volume, can meet pipeline specs BUT contaminants (e.g. water) must be removed prior to liquefaction (T<- 160C), liquefaction plant (large and expensive $ billion), regasifcation unit required, requires proven reserves 20 years, NIMBY CNG 200 times volume reduction, smaller reserves possible, limited contaminant removal required, less expensive as no plant required BUT limited knowledge on quality of gas, no commercial apps, heating/cooling GTL (gas to liquids) CH O 2 = CO H 2 O CH 4 + ½ O 2 = CO + 2 H 2 low temp Fischer Tropsch nco + (2n+1)H 2 = C n H (2n+2) + nh 2 O high temp Fischer Tropsch nco + 2nH 2 = C n H 2n + nh 2 O nco + 2nH 2 = C (n-1) H (2n-1) CH 2 OH + (n-1)h 2 O - Resulting liquid fuel has good combustion efficiency and easy to transport BUT taking high H;C fuel and converting to low H;C fuel and process requires severe operating conditions and many unit operations

3 North American Natural Gas Supply/demand Bcf Bcm Gulf Onshore 1 6, Gulf Offshore 2 2, Total Gulf 9, Wyoming 1, New Mexico 1, Oklahoma 1, Alaska Other US 3 3, Total US Production 18, Western Canada 4 5, Scotian Shelf Total Canada Production 5 6, Total N.A. Production 24, US Gross LNG Imports US Gross Mexican Imports 12 0 US Supplementals Total N.A. Supply 25, Total N.A. Demand (20% residental,14% commercial, 59% industrial/electricity) 24, Sources: EIA, StatsCan, NRCan estimates.

4 C 1, C 2, H 2 S, CO 2 etc.. Acid Gas Removal C 1, C 2, H 2 O Dehydration/ Compression gas from wells Inlet Separators Condensate Stabilization condensate (C 2 -C 5+ ) C 3, C 4 H 2 S, CO 2 Sulphur HC, SO2, CO 2 Recovery Propane/Butane Processing e.g. deep cut, turboexpansion S C 5 + Simplified PFD for Sour Gas Processing Plant

5 Point of processing is to meet pipeline/storage/use specifications Pipeline Specification (Typical) Oxygen 10 ppm Nitrogen 3 % CO 2 2-3% pipeline to 100 ppm for LPG plant feed H 2 S low as 4 ppm (0.25grains/100 scf) for pipeline higher for fuel gas CS 2, COS, RSH 20 grains/100 scf Natural Gas Liquid (NGL) Specifications: H 2 S, Sulfurs Pass Copper Strip, ASTM D-2420 CO2 varies 0.35 LVP of Ethane content 1000 ppm or less, depends on application

6 acid gas Acid Gas Injection Gas Sweetening Dehydration Dewpoint Control and Compression gas from wells Inlet Separation light gases natural gas to market C 2 -C 5 + Condensate Stabilization C 5 + Simplified PFD for Sable Island

7 C 5 +,C 3 -C 4 C 3 -C 4 C 1, C 2 some C 3 -C 4 +contaminants Inlet Sep C 1, C 2 some C 3 -C 4 +H 2 O Amine plant acid gas dehy Claus Plant comp SO 2, CO 2, CO etc.. S C 1, C 2 some C 3 -C 4 Condensate stabilization/ fractionation C 5 + Sour Gas Plant in AB

8 2.1 Auxiliary Equipment a) fired equipment - heat exchangers throughout plant, furnaces used in utility and SRU 2 types i. direct fired - combustion gases heat process stream which is contained in pipes ii. fire tube - combustion gases are surrounded by a liquid that either is used as a heat transfer medium or is the process stream itself application characteristics direct fired regeneration gas heaters more equip/controls amine reboilers higher η thermal lower space forced/natural combust firetube line heaters low heat duty C3+ vaporizers skid mount gly/am reboilers forced/natural combust low P steam gen less hot spot

9 b) HE - discussed in section 1.3 c) cooling towers - detail in section purpose cool process water by ambient air achieved by maximize evaporation of H 2 O in droplets exposed to maximum air flow over longest time (picture) - mech draft fans move air and natural draft use density d) pumps/turbines - mostly centrifugal type due to lower cost, smaller space, and low maintenance e) compressors/expanders - compressors used inlet and sales gas to boost pressure + displacement dynamic thermal

10 f) refrigeration - used in: NGL/LPG recovery HC dewpoint control reflux condensation for light HC fractions LNG plants - refrigerant type selected by T requirements, availability, economics, previous experience e.g. natural gas plant may use C 2 and C 3 while due availability and economics olefin plant may use ethylene and propylene i. mech refrigeration - most common - simple cycle of expansion, evaporation, compression, condensation ii. Absorption Refrigeration - if low cost of n.gas, low level heat source, and electricity rates

11 C D A B from GPSA Handbooks

12 2.2 Inlet Separators discussed fractionators in general, separator is like one stage of a fractionator where adjust P of incoming gas to separate v and l a) 4 major sections A. primary section sep main portion of free l by abrupt change in momentum or direction (nozzle) B. secondary or gravity sectn use gravity to enhance sep of entrained droplets gas moves at low velocity w/ little turbulence C. coalescing sectn coalescer (wire, mesh, vane elements, cyclonic passage) or mist extractor removes droplets can t be sep by gravity by impingement on surface limits l carryover into gas (<0.013 ml/m3) D. sump/l collection recover l from ii and iii provides surge V for degassing a slug catching b) orientation vertical high v:l ratio or total gas V low horizontal used large V total fluids and large amounts of dissolved gas in l spherical occasionally used where high P and compact size needed, l volumes are small new are small valve types on platforms

13 from GPSA Handbooks

14 2.5 Fractionation separate gas mixtures into individual products in next section discuss bulk separation of NGLs from gas which differs from this discussion absorption type units also used use trays/packing a)types of fractionators at gas plants demethanizer product bottom is C 2+, OH is C 1 deethanizer - product bottom is C 3+, OH is C 1 /C 2 commercial C 3, C 3 /C 4 (LPG), C 4, C 4 /gasoline, natural gasoline e.g. at gas plant in AB deethan run depending price butane depropanizer debutanizer

15 from GPSA Handbooks

16 from GPSA Handbooks

17 b) Product specs material balance around column is 1 st step in design calcs need to assume product stream compositions defined in terms of % recovery of component in OH or bottom OR composition of component in either product OR specify physical properties (Pvap) in either product c) design in fractionation there usually 2 components which are key in separation lightest component in bottom (LK) heaviest component in OH (HK) these components are adjacent to each other in volatility in hand calcs make the assumption all components heavier than than heaviest in OH are in bottoms

18 2.3 dehydration dehydration or removal of water from gas stream is necessary to prevent hydrate formation and increase the heating value of the gas a) water content of gas f(t,p,composition) amount gas can hold increases with pressure sour and acid gases can hold more water (increased solubility of water) e.g. 100% 500 kpa 1000 mg/sm 3 wet gas 30% C1 60% CO2 10% H2S 1500 mg/sm 3 wet gas 100 % CO mg/sm 3 wet gas - to determine H2O content requires experiment/gas analysis

19 b)hydrates crystalline ice-like structures, water lattice where CO2, HC, N2, H2S occupy cavities (diagram) crystalline molecular complexes formed from mixtures of water and suitably sized gas molecules water (host) molecules, upon hydrogen bonding, form unstable lattice structures with several interstitial cavities gas (guest) molecules occupy lattice cavities and when minimum number cavities occupied crystalline structure becomes stable solid gas hydrates forms even at temperatures well above the ice point. 3 recognized structures (so far) i. structure I body centred cubic w/ smaller molecules (C1, C2, CO2, H2S) ii. II diamond lattice, larger molecules (C3,C4) iii. III most HC>C4 don t form hydrates or stable lattice but some isoparrafins and cycloalkanes > C5 can form stable - gas composition determines structure, e.g. mixed gases typically form II - structure doesn t affect appearance or properties of hydrate but does affect T and P where hydrates occur e.g. S II more stable than I C3/C4 form hydrates at higher T than light H2S shifts hydrate formation to higher T at given P

20 in general hydrate formation is time dependent and the rate is f(gas comp, presence nucleation sites in l phase, degree of agitation) primary considerations effect hydrate formation which first l forms) 1. gas or or below dew pt 2. T, P, composition secondary considerations mixing, kinetics, physical site for nucleation (pipe elbow, orifice, dead space), salinity in general hydrates prone to form at high P or low T own figures Pressure (kpa) Temperature (C)

21 Phase Envelope (inlet gas mixture of 62% C1, 15% C2, 16% C3+, 4% H20, balance H2S/CO2/N2) Pressure (kpa) bubble pt curve dew pt curve hydrate line Temperature (C)

22 c) Hydrate inhibition options to gas dehydration if not practical or feasible try to inhibit the formation of hydrate by adding chemical which shifts the phase diagram away from hydrate (think adding salt to roads) or decrease T hyd form inject glycols or methanol - combines w/ condensed aqueous phase decreases T hyd form - chemical recovered with aqueous phase at separators d)gas Dehydration i. glycol units glycol is a l (DEG, TEG most common, tetraethylene glycol TREG) applications where T DP depression of C required usually preceded by inlet gas scrubber to prevent slugging (H2O, HC, treatment chem)

23 regenerated glycol enters top tray of absorber (contactor), absorbs H2O in gas as flows down and gas goes up. Water rich glycol passes thru reflux condenser, soluble gas is flashed in flash tank, glycol/h2o heated in rich/lean HE, sent to regeneration unit where heated at atm P to drive off water from GPSA Handbooks PROBLEM aromatics very soluble and can be significant absorption based on eqm constants (K) = y aro /x aro 10-30% of BTEX in gas can be absorbed higher the P and lower T increased absorption aromatic absorption is f(circulation rate) higher the rate the higher the absorption but independent of # of contactors therefore to minimize absorption must minimize circ rate and increase size of tower (decrease P)

24 Enhanced glycol concentration processes standard designs limited to 98.6% TEG purity by reboiler op T (204 C at atm P) number processes increase purity by reducing the PP H2O in vapour space of reboiler so get higher [glycol] at same T e.g. DRIZO, COLDFINGER, PROGLY from GPSA Handbooks

25 general considerations for glycol units if inhibitor present 40-60% absorbed in glycol which increases duty on reboiler and added volume load glycol losses mechanical carryover from contactor (13 L/10 6 Sm 3 ), vapours from contactor/regenerator, foaming in absorber/regen, low P and high T (40 L/10 6 Sm 3 ), losses glycol of gas w/ CO 2 is higher than n.gas at P>6200 kpa becomes corrosive w/ prolonged exposure to O high T (>200C) decomposition low ph decomposition

26 ii. Solid Dehys comprise of 2 or more towers (one on, one off) - more expensive than glycol units therefore used when: high H2S lower dew pt regs simultaneous control of H2O and HC dew pt from Norwegian University of Science and Technology (NTNU) O2 containing gases where CH3OH not favoured both dry/sweeten NGL bed on line 8-24 hours regenerated by heating to C (w/ waste heat) gas enters top (down flow) prevent fluidization, regen gas is up flow general 3-5 yr life, regen cycle is depressure/repressure sometimes see CaCl 2 (consumable) for dehy n.gas remote gas well

27 3 types 1. gels alumina or silica gels (SiO2) v and l dehydrated and HC recovered for natural gas (ic5+) hydrocarbon recovery units (HRU), outlet dew pts ~-60C 2. Alumina hydrated form Al2O3 (alumina oxide), TDP~-70C, less heat required than mol sieve and T regnerator lower 3. molecular sieve aluminosilicates, high H2O capacity and produces lowest T DP ~-100C, can sweeten and dry gases and liquids (fig 20-69) H2O capacity less dependent on ambient T and relative humidity expensive commonly used ahead NGL plants to recover C2 from GPSA Handbooks

28 iii. Membranes separate gas from H 2 O, CO 2, HC according to permeability where dissolve/diffuse through membrane driving force is differential PP across membrane CO 2 /H 2 O permeate thru membrane permeate at reduced P while P slightly<p feed C 1+ in permeate f( P, SA membrane), 5-10% carryover only applicable to plants use low P natural gas fuels

29 e) Dehydration of Liquid Phase HC typically amount of water in HC l is low, even at saturation i. gas stripper counter current stripper w/ dry gas, used offshore, trayed contactor and stripper low cost, simple need dry n.gas stream, waste stream of VHC from condensate ii. solid desiccant activated alumina, T dp ~-70C, absorbs heavy HC CaCl 2 brine has neg effect MS too expensive for H 2 O removal iii.distillation fractionation columns for use in dehy of NGLs higher energy requirements