Gas Dehydration. Chapter 11 Based on presentation by Prof. Art Kidnay

Transcription

1 Gas Dehydration Chapter 11 Based on presentation by Prof. Art Kidnay

2 Plant Block Schematic 2

3 Reasons for Gas Dehydration Field Operations Prevent hydrate formation Minimize corrosion Need to dry gas to dew point below lowest operating temperature Plant Operations Need 4 to 7 lb/mmscf (85 to 150 ppmv) in pipeline Glycol dehydration most common to produce water contents down to 10 ppmv Need to have less than 1 ppmv H 2 O in gas to cryogenc units Glycol dehydration cannot get to these low water levels mole sieves used for this service 3

4 Topics Water Content of Hydrocarbons Gas Dehydration Processes Absorption processes Adsorption processes Non regenerable desiccant processes Membrane processes Other processes Comparison of dehydration processes Safety and Environmental Considerations 4

5 Water Content of Hydrocarbons

6 Equilibrium considerations Equal fugacities for each component in each phase. Between gas & water phases: vap P il, ip i vil, yi xk i i where Ki exp dp iv, iv, P sat RT P i For a gas in contact with pure water: y H2O P P vap H2O since x 1 H2O Formation of the water phase will control the water content in the gas phase Increasing water in the feed increases the amount of free water, not the concentration of water in the gas. Can decrease the gas water content by adding compounds that are water soluble 6

7 Water content of natural gas Based on typical gas composition Separate corrections for actual composition & acid gas content Takes into account non-idealities Take care if gas is specified as wet or dry basis dry basis does not include the amount of water in the MMscf N / M Wet Basis: X y / M Dry Basis: H2O H2O H2O H2O H2O NHC NH2O X H2O N / M y / M N 1 y H2O H2O H2O H2O When less than 5,000 lb/mmscf the wet & dry values are within 0.5% HC H2O Fig. 20-4, GPSA Engineering Data Book, 13 th ed. Figure 11.1 in Kidnay et. al. text book 7

8 Water content of natural gas typical pipeline specs GPSA Engineering Data Book, 13 th ed. 8

9 Water content of natural gas GPSA Engineering Data Book, 13 th ed. Figure 11.1 (b) & (c) in Kidnay et. al. text book 9

10 Applicability of dehydration processes 10

11 Dehydration by Absorption

12 Equilibrium considerations Glycols tend to be only in the water phase (i.e., non-volatile & very low solubility in the hydrocarbon liquid phase) For a gas in contact with water/glycol mixture: y H2O x H2O P P vap H2O Water content in the gas phase is less than that for a pure water phase since x H2O < 1 Away from glycol, must reduce temperature to create a free water phase. 12

13 Typical Glycols Name EG DEG TEG Ethylene Glycol Diethylene Glycol Triethylene Glycol Formula C 2 H 6 O 2 C 4 H 10 O 3 C 6 H 14 O 4 Molecular Weight Boiling Point ( F) Vapor 77 F (mmhg) 0.12 < 0.01 < F (lb/gal) F (cp) Decomposition temperature ( F) Fig , GPSA Engineering Data Book, 13 th ed. 13

14 Glycol molecular structure Ethylene glycol HO-CH 2 -CH 2 -OH Diethylene Glycol HO-CH 2 -CH 2 -O-CH 2 -CH 2 -OH Triethylene Glycol HO-CH 2 -CH 2 -O-CH 2 -CH 2 -O-CH 2 -CH 2 -OH Chemical structures drawn using 14

15 Equilibrium water content above TEG solutions Based on Fig GPSA Data Book 13 th ed. & Figure 11.3 in Kidnay et. al. text book Based on 1,000 psia contactor pressure 15

16 Example Equilibrium water content above TEG solutions Operate a TEG 100 o F & 1,000 psia with 99.9 wt% TEG introduced at the top Dried gas is protected to a dew point of -40 o F Fig , GPSA Engineering Data Book, 13 th ed. Figure 11.3 in text book 16

17 Equilibrium water content for TEG solutions 17

18 Typical Glycol Dehydration Unit System 2 5 gal TEG per lb water removed Absorber / Contactor o F inlet Can operate up to 2,000 psia Typically 4 10 bubble cap trays 25 30% efficiency 5 10 psi pressure drop Flash tank minute residence time 150 o F, psig Regenerator Packed equivalent to 3 4 trays o F Fig , GPSA Engineering Data Book, 13 th ed. Basis for Figure 11.2 in text book 18

19 Typical Glycol Dehydration Unit System 2 5 gal TEG per lb water removed Absorber / Contactor o F inlet Can operate up to 2,000 psia Typically 4 10 bubble cap trays 25 30% efficiency 5 10 psi pressure drop Flash tank minute residence time 150 o F, psig Regenerator Packed equivalent to 3 4 trays o F y P x P H2O vap H2O H2O 19

20 Example based on GPSA Data Book example MMscfd of a 0.65 gravity natural gas enters a TEG contactor at 600 psia and 100 o F. The outlet water content specification is 7 lb H2O/MMscf and the TEG circulation rate is 28 lb TEG/ lb H2O absorbed (3 gal TEG/lb H2O). How much water is to be absorbed? What is the rich TEG concentration? What is the lean TEG concentration? Water content at inlet conditions? 70 lb/mmscf How much water is removed? lb H2O lb TEG day lb H2O lb TEG day 20

21 Example based on GPSA Data Book example MMscfd of a 0.65 gravity natural gas enters a TEG contactor at 600 psia and 100 o F. The outlet water content specification is 7 lb H2O/MMscf and the TEG circulation rate is 28 lb TEG/ lb H2O absorbed (3 gal TEG/lb H2O). How much water is to be absorbed? What is the rich TEG concentration? What is the lean TEG concentration? How much TEG is circulated? lb H2O lb TEG day lb H2O lb TEG day lb H2O gal TEG day lb H2O hr min day hr gal TEG 3.9 min 21

22 Example based on GPSA Data Book example (#2) 30 MMscfd of a 0.65 gravity natural gas enters a TEG contactor at 600 psia and 100 o F. The outlet water content specification is 7 lb H2O/MMscf and the TEG circulation rate is 28 lb TEG/ lb H2O absorbed (3 gal TEG/lb H2O). How much water is to be absorbed? What is the rich TEG concentration? What is the lean TEG concentration? Dew point temperature at the contactor pressure (600 psia)? ~ 24 o F 22

23 Example based on GPSA Data Book example (#3) 30 MMscfd of a 0.65 gravity natural gas enters a TEG contactor at 600 psia and 100 o F. The outlet water content specification is 7 lb H2O/MMscf and the TEG circulation rate is 28 lb TEG/ lb H2O absorbed (3 gal TEG/lb H2O). How much water is to be absorbed? What is the rich TEG concentration? What is the lean TEG concentration? What is the minimum TEG concentration for a 24 o F dew point & the contactor temperature (100 o F)? ~98.5 wt% Lean TEG has 806 lb/day water Rich TEG content (after absorbing the water from the wet gas) lb TEG day lb H2O lb TEG day day 95.2 wt% H2O 23

24 Solubility of hydrocarbons in glycol solutions GPSA Engineering Data Book, 13 th ed. Methods to control BTEX emissions from Regenerator Condense overhead & recover Burn vent gas through flare or thermal oxidizer Recycle back to process 24

25 Field Glycol Dehydrator stripper contactor reboiler glycol pump Inlet separator gas burner heat exchanger, surge tank Flash separator 3-phase, gas,glycol,condensate From Sivalls, Glycol Dehydration Design, LRGCC,

26 Common Operational Problems Contactor foaming Contaminates: hydrocarbons, salts, particulates, inhibitors, O 2 Poor dehydration (from source other than foaming) Gas rate too low - 80% flow reduction = 20 % tray eff Glycol rate low - 75% flow reduction = 33% tray eff Glycol inlet temperature too high Flash drum / Foaming in Still Presence of heavy hydrocarbons 26

27 Dehydration by Adsorption

28 Absorption vs Adsorption Absorption Adsorption 28

29 Physical absorption 29

30 Adsorption fundamentals Two types of adsorption Chemisorption Chemical interaction between adsorbate and adsorbent May not be completely reversible Physical adsorption Only physical interaction between adsorbate and adsorbent Completely reversible -ΔH Chem >> -ΔH Phys 30

31 Physical Adsorption Fundamentals Factors affecting selectivity Size adsorbent pore diameter major factor Volatility less volatile displaces more volatile (e.g., C 3 displaces C 2 ) Polarity For desiccants, more polar displaces less polar (e.g., CO 2 displaces C 2, MeOH displaces CO 2, water displaces MeOH) 31

32 Adsorption Isotherms Lb Water Adsorbed / 100 lb Activated Adsorbent From UOP 32

33 Solid Desiccant Dehydrator Twin Tower System Fig , GPSA Engineering Data Book, 13 th ed. 33

34 Typical Vessel Loading Sample packing of catalyst/dessicant on top of supports Model prepared by Enterprise Products Possible configuration for drying 100 MMscfd to a dew point of -150ºF, adsorption time ~12 hours

35 Concentration Profile Equilibrium Zone (Saturated) Mass Transfer Zone (Partially saturated) Active Zone (Unsaturated) 35

36 Concentration Profile y In y Out 36

37 Regenerating Bed Temperature History Heat On Inlet Temperature Outlet Temperature Desorption Bed Heating Bed Cooling Time, Hours

38 Regenerating Bed Temperature History Heat On Inlet Temperature 250 Temperature, ºF Outlet Temperature Temperature, ºC Desorption Bed Heating Bed Cooling Time, Hours 38

39 Common Adsorbents for Drying In order of increasing cost: Silica gel (SiO 2 ) Min exit water content 10 to 20 ppmv (~-60 o F) Inert and used for inlet concentrations of > 1 mol% Activated Alumina (Al 2 O 3 ) Min exit water content 5 to 10 ppmv (~-100 o F) High mechanical strength but more reactive Molecular Sieve (4A and 3A) Min exit water content below 0.1 ppmv (~-150 o F) Highest surface area Composite of sieve and clay binder 39

40 Design steps Determine size of vessels for adsorption Determine the bed diameter based on superficial gas velocity / allowable pressure drop Too small pressure drop will be too high & can damage the sieve Too large need too high a regeneration gas rate to prevent channeling Typically use (-P/L) < 0.33 psi/ft with a total pressure drop of 5 8 psi max Choose an adsorption period & calculate the mass of desiccant Sets the bed height contributions from saturation zone & mass transfer zone heights 8 to 12 hour periods with 2 or 3 beds are common Regeneration o Too long more desiccant & larger vessels needed than necessary o Too short a time shorter desiccant life Calculate heat required to desorb water while also heating the desiccant & vessel Total amount of regeneration gas flow calculated based on heating phase about 50-60% of total regeneration time Regeneration gas flowrate should give a pressure drop gradient of at least 0.01 psi/ft 40

41 Design equations (#1) Determine gas velocity for bed diameter Modified Ergun equation for pressure drop P B V C V L 2 Viscosity [cp] & density [lb/ft³] determined at inlet conditions Solve quadratic equation for maximum superficial velocity (V max [ft/min]) for 0.33 psi/ft pressure drop Pressure drop gradient in units of psi/ft Minimum diameter D min 4 m V max Adjust diameter upwards to nearest ½ foot increment Recalculate superficial linear velocity & pressure drop using adjusted diameter 41

42 Design equations (#2) Determine bed length (method 1) Amount of desiccant in saturation zone S m 4 S L 0.13 SS T water sat sat sat 2 C C D bulk Assumes 13 lb water per 100 lb dessicant Amount of desiccant in the mass transfer zone (MTZ) (GPSA EDB method) C ln %sat SS L MTZ ft V ft/min C Z where C Z is 1.70 ft for 1/8 inch sieve & 0.85 for 1/16 inch sieve or Trent method for MTZ L MTZ ft V ft/min C F T 42

43 Design equations (#3) Determine bed length (method 2) Calculate effective desiccant capacity which includes the MTZ effect, temperature, and relative humidity corrections. An effective capacity of 8 10% is typically assumed. S m 4 S L water bed bed bed 2 Ceff D bulk Finalize bed length Total bed height (L sat +L MTZ or L bed ) but should not be less than the bed diameter or 6 ft, whichever is greater Total bed pressure drop should be 5 8 psi max If too large increase the bed diameter Determine vessel height & weight Total bed height plus other allowances at least 3 ft (for inlet distributor on top and bed support & hold down balls underneath) 43

44 Design equations (#4) Regeneration calculations Used to determine the required regeneration gas flow & the fuel gas requirements If regeneration gas recycled back to inlet of mole sieves then you must add this rate to that of the feed gas for the bed calculations Heat loads Heat to desorb water increase water to its desorption temperature, break adherence to surface, & vaporize o Use 1,800 Btu/lb water adsorbed for conservative design Heat to increase sieve to regeneration temperature Heat to increase vessel to regeneration temperature Heat losses typically estimated as 10% 44

45 Design equations (#5) Regeneration Calculations (cont.) Calculation of vessel weight for heating calculations 12 DPdesign tin and msteel lb155t 0.125Lvessel 0.75 D D P design where the 0.75D term accounts for the weight of the vessel heads Design pressure in psig. Usually 10% greater than operating pressure (minimum 50 psig) Usually have to heat the regeneration gas 50 o F hotter than the desired regeneration temperature (e.g., 500 o F gas needed to regenerate at 450 o F) Total regeneration load 2.5 times the minimum load Assumes only 40% of the heat is transferred from gas to mole sieve system. The remainder exits as hot gas. Need to size downstream coolers appropriately. Regen gas flowrate. Check that pressure drop gradient at least 0.01 psi/ft m Q m V Total Regen rg Regen Gas rg 2 CP Thot Tbed rg D 4 45

46 Example based on GPSA Data Book example MMscfd natural gas (molecular weight of 18) is water saturated at 600 psia and 100 o F & must be dried to 150 o F dew point. Determine the water content of the gas (inlet & outlet) & amount of water that must be removed. Do preliminary design of a molecular-sieve dehydration system consisting of two towers with down-flow adsorption in one tower and up-flow regeneration in the other. Use 4A molecular sieve of 1/8 beads (i.e., 4x8mesh). The regeneration gas is part of the plant s residue gas (at 600 psia and 100 o F) & has a molecular weight of 17. The bed must be heated to 500 o F for regeneration. Base this on a 24-hour cycle consisting of 12 hours adsorbing and 12 hours regenerating (heating, cooling, standby, and valve switching; the heating time is 60% of the regeneration time). 46

47 Example based on GPSA Data Book example (#2) 100 MMscfd natural gas (molecular weight of 18) is water saturated at 600 psia and 100 o F & must be dried to 150 o F dew point. Determine the water content of the gas (inlet & outlet) & amount of water that must be removed. Water content at inlet conditions? 70 lb/mmscf Water content at outlet conditions? Essentially 0 lb/mmscf How much water is to be removed? 70 0 lb/mmscf100 MMscfd 7, 000 lb/day 47

48 Example based on GPSA Data Book example (#3) Do preliminary design of a molecular-sieve dehydration system consisting of two towers with down-flow adsorption in one tower and up-flow regeneration in the other. Use 4A molecular sieve of 1/8 beads (i.e., 4x8 mesh) Determine bed diameter. Velocity criteria not given so determine from allowable pressure drop (0.33 psi/ft max) Ideal gas flowrate at inlet conditions (600 psia and 100 o F) V IG ft 14.7 psia R 6 ft day 600 psia R day Real gas flow much different? Estimate: Z=0.93 V act ft 6 ft ft ZV IG day day min 48

49 Example based on GPSA Data Book example (#3) Do preliminary design of a molecular-sieve dehydration system consisting of two towers with down-flow adsorption in one tower and up-flow regeneration in the other. Use 4A molecular sieve of 1/8 beads (i.e., 4x8 mesh) Determine bed diameter. Velocity criteria not given so determine from allowable pressure drop (0.33 psi/ft max) Real gas density at inlet conditions (600 psia and 100 o F) lb R ft PM ZRT Gas viscosity at inlet conditions (600 psia and 100 o F). Estimate cp. Velocity vs. pressure gradient. For given beads & gas properties: P B u C u L u u u 41.4 min 5 2 ft 49

50 Example based on GPSA Data Book example (#4) Do preliminary design of a molecular-sieve dehydration system consisting of two towers with down-flow adsorption in one tower and up-flow regeneration in the other. Use 4A molecular sieve of 1/8 beads (i.e., 4x8mesh) Determine bed diameter. Velocity criteria not given so determine from allowable pressure drop (0.33 psi/ft max) Minimum diameter is ratio of volumetric flowrate to maximum velocity. Scale up to next 6. A ft D V min act Dmin 7.2 ft D=7.5 ft 4 u ft 41.4 min 50

51 Example based on GPSA Data Book example (#5) Do preliminary design of a molecular-sieve dehydration system consisting of two towers with down-flow adsorption in one tower and up-flow regeneration in the other. Use 4A molecular sieve of 1/8 beads (i.e., 4x8mesh) Determine bed diameter. Velocity criteria not given so determine from allowable pressure drop (0.33 psi/ft max) Determine actual gas velocity & pressure drop in absorbing bed 3 V 4V ft /min u 38.5 ft/min 2 2 A D 7.5 ft P Bu Cu L psi ft 51

52 Example based on GPSA Data Book example (#6) Base this on a 24-hour cycle consisting of 12 hours adsorbing and 12 hours regenerating (heating, cooling, standby, and valve switching; the heating time is 60% of the regeneration time). Since the overall removal rate is 7,000 lb/day we must have enough adsorbent to safely contain 3,500 lb of water (corresponding to the adsorbing time). No other criteria given for amount of water to be contained by desiccant determine size using the zone analysis (method 1) Size saturation zone to contain all water for the cycle. Use a typical sieve bulk density of 45.0 lb/ft3 S L m water sat 0.13 CSS CT sat sat 2 2 D bulk S ft 28,600 lb sieve 52

53 Example based on GPSA Data Book example (#7) Base this on a 24-hour cycle consisting of 12 hours adsorbing and 12 hours regenerating (heating, cooling, standby, and valve switching; the heating time is 60% of the regeneration time). determine size using the zone analysis Add appropriate length for the mass transfer zone (MTZ) to ensure no breakthrough of water. C Z =1.7 for this size sieve L MTZ u 38.6 CZ ft Total bed height is the sum of these two zones. Total vessel height adds 3 ft for supports, L L L ft L L ft Bed sat MTZ vessel Bed 53

54 Example based on GPSA Data Book example (#7) Base this on a 24-hour cycle consisting of 12 hours adsorbing and 12 hours regenerating (heating, cooling, standby, and valve switching; the heating time is 60% of the regeneration time). determine size using the zone analysis Check that the bed length is at least the bed diameter (here 7.5 ft) or 6 ft, whichever is greater. o This bed depth does not need to be adjusted Check that total pressure drop is 5 8 psi. If too small, add bed height; if too large, add diameter p p Lbed =4.7 psi (close enough) L 54

55 Example based on GPSA Data Book example (#8) The regeneration gas is part of the plant s residue gas (at 600 psia & 100 o F) & has a molecular weight of 17. The bed must be heated to 500 o F for regeneration Determine amount of heat needed for regeneration Heat to desorb water Q m H ,300,000 Btu w w w Heat the sieve to regeneration temperature Q m C T T C T T 4 4 3,070,000 Btu 2 DL si si p, si regen ads p, si regen ads bed bulk

56 Example based on GPSA Data Book example (#9) The regeneration gas is part of the plant s residue gas (at 600 psia & 100 o F) & has a molecular weight of 17. The bed must be heated to 500 o F for regeneration Determine amount of heat needed for regeneration (cont.) Heat the steel to regeneration temperature t steel 12 DP design in Pdesign lb vessel 0.75 Q m C T T steel steel p, steel regen ads , 430, 000 Btu m t L D D in lb 56

57 Example based on GPSA Data Book example (#10) The regeneration gas is part of the plant s residue gas (at 600 psia & 100 o F) & has a molecular weight of 17. The bed must be heated to 500 o F for regeneration Determine amount of heat needed for regeneration (cont.) Total regeneration heat needed Q Q Q Q Q Q Q Q f 1 6, 300, 000 3, 070, 000 2, 450, regen w si steel loss w si steel loss 13, 002, 000 Btu Determine amount & rate of regen gas needed Heat that must be transferred to the regeneration gas Q rg 2.5Q , 002, 000 =32, 505, 000 Btu regen 57

58 Example based on GPSA Data Book example (#11) The regeneration gas is part of the plant s residue gas (at 600 psia & 100 o F) & has a molecular weight of 17. The bed must be heated to 500 o F for regeneration Determine amount & rate of regen gas needed (cont.) Determine amount regen gas needed o o C, 0.65 Btu/lb F (based on Fig in GPSA EDB averaged between 100 & 550 F) m prg rg Qrg 32, 505, ,100 lb C T T prg, rg cold Determine rate of regen gas needed m rg mrg 111,100 15, 430 lb/hr 257 lb/min t

59 Example based on GPSA Data Book example (#12) The regeneration gas is part of the plant s residue gas (at 600 psia & 100 o F) & has a molecular weight of 17. The bed must be heated to 500 o F for regeneration Verify there is sufficient pressure drop during regeneration to prevent channeling (i.e., pressure drop is above 0.01 psi/ft) For the hot regen gas (@ 550 o F): PM lb rg ZRT ft u rg V rg 4 m ft/min A 0.94 rg D 2 rg cp (from Fig in GPSA EDB) P psi B u C u L ft Flow rate is sufficient 59

60 Common Mole Sieve Operational Problems Loss of bed capacity Aging, rapid initial loss then gradual loss over years Coking by partial oxidation of heavy hydrocarbons Coking by conversion of H2S to elemental sulfur Poor regeneration Increased pressure drop Attrition Caking at top of bed Fines Attrition Failed bed support COS formation Chemical equilibrium H 2 S + CO 2 COS + H 2 O 60

61 Other Dehydration Processes

62 Other processes Consumable salts (CaCl 2 ) Refrigeration with MEOH addition, more complex Membranes, ideal for remote sites when low pressure permeate gas can be used effectively If drying high pressure gas: Vortex tube one application known Simple but poor turndown ratio and efficiency Twister Supersonic Separator one known offshore application Simple, poor turndown ratio but better efficiency 62

63 Twister Operating Principle Acceleration to Mach >1 cools gas (typically o C) ΔP = 30% Cooling causes condensation (water and heavier hydrocarbons) Swirl centrifuges liquid droplets to the tube wall Drainage section removes liquid film from the wall + ~20% gas Diffuser section recompresses the gas 63

64 Comparison of Dehydration Processes For < 1 ppmv H 2 O need mole sieve. For higher concentrations: Glycol (usually TEG) widely used Minimal manpower requirements High turndown Regenerative desiccants (silica gel, alumina) more costly Membranes, and Twister(?) where pressure drop acceptable Nonregenerative desiccants (CaCl 2 ) for remote, low water content gas 64

65 Summary

66 Summary Water content can be estimated from Fig Units of lb/mmscf Wet & dry bases essentially the same below 5,000 lb/mmscf Three primary separation technologies Bulk removal by cooling & separation TEG dehydration to pipeline specs (4 7 lb/mmscf) Mole sieves required upstream of cryogenic applications 66

67 Supplemental Slides

68 Glycol Dehydration Unit stripping still contactor reboiler 68

69 Glycol Dehydration Unit stripping still contactor reboiler 69

70 Mole Sieve Dehydration Unit 70

71 Zeolite structures Zeolite A Zeolite X 71