Gas Dehydration Using Glycol

Transcription

1 Gas Dehydration Using Glycol Manning and Thompson, Volume I Chapter 8 Outline Introduction Process Description Design Methods Design Examples Troubleshooting

2 NATCO Glycol Dehydration Unit The NATCO glycol dehydration process removes water vapor from natural gas. Removing water vapor prevents hydrate formation and corrosion, and maximizes pipeline efficiency. 1.4 Bscfd Glycol Dehydration Plant

3 Why Should We Dehydrate Gas? If left in gas, water can cause: Solid hydrate formation under certain conditions. Corrosion, especially in the presence of CO 2 or H 2 S. Slugging (two-phase flow) and erosion. Increase in specific volume and decrease in the heating value of gas. Freezing in cryogenic and refrigerated absorption plants. Sales gas contracts and/or piping specifications have a maximum water content (typically 7 lb m per MMscf). Methods of Dehydration Liquid Desiccants (glycols): Desiccant is substance that has an affinity for water Usually the choice of dehydration method is between glycol and solid desiccants. Glycol dehydration is by far the most commonly used process.

4 Methods of Dehydration Solid Desiccants (alumina, silica gel, molecular sieves): Characterized by porous structure that contains very large internal surface areas ( m 2 /g) with very small radii of curvature ( m) Strong affinity for water Capacities between 5-15% by weight Can dry gas to less than 0.1 ppm of water or a dew point of 150 F. Methods of Dehydration Expansion Refrigeration: Also known as low-temperature extraction (LTX). Employs Joule-Thompson expansion (isothermal expansion) to dry the gas and recover condensate. J-T expansion requires large pressure drops. Because of large pressures drops, LTX is used only when the prime objective is condensate recovery. Calcium Chloride: Anhydrous calcium chloride absorbs 1 lb m H 2 O per lb m of CaCl 2 before becoming brine.

5 Glycol vs. Solid Desiccants Advantages of glycol over solid desiccants: Lower installed cost (Kohl and Riesenfeld, 1979) 50% less at 10 MMscfd 33% less at 50 MMscfd Lower pressure drop (5-10 psi vs psi for dry desiccants). Glycol dehydration is continuous rather than batch. Glycol makeup is easily accomplished. Glycol units require less regeneration heat per pound of water removed. Glycol units can typically dehydrate natural gas to 0.5 lb m H 2 O/MMscf Glycol vs. Solid Desiccants Disadvantages of glycol over solid desiccants: Water dew points below -25 ºF require stripping gas and a Stahl column. Glycol is susceptible to contamination. Glycol is corrosive when contaminated or decomposed.

6 Comparison Continued Advantages of solid desiccants: Dew points as low as 150 ºF. They are less affected by small changes in gas pressure, temperature and flow rate. They are less susceptible to corrosion or foaming. Comparison Continued Disadvantages solid desiccants: Higher capital cost and higher pressure drops. Desiccant poisoning by heavy HC s, H 2 S, CO 2, etc. Mechanical breaking of desiccant particles. High regeneration heat requirements and high utility costs. Bottom Line: Glycol dehydration is by far the most commonly process.

7 Choice of Glycol EG DEG TEG TREG TEG dew point depressions range from o F while inlet pressures and temperatures range from psig and from 55 to 160 o F, respectively. Ethylene glycol (EG) Diethylene glycol (DEG) Triethylene glycol (TEG) Tetraethylene glycol (TREG) TEG has gained almost universal acceptance as the most costeffective choice because: TEG is more easily regenerated TEG has a higher decomposition temperature of 404 ºF while DEG is 328 ºF. Vaporization losses are lower than EG or DEG TEG is not too viscous above 70 ºF. Flow Diagram for TEG Dehydration (Typical of Wellhead Unit) Remove Liquid and solids Remove Water Vapor Reboiler boils water out of Glycol Wet Glycol Needs Reconcentration Preheat Rich Glycol & Cool Lean Glycol

8 Flow Diagram for Glycol System Skimmer Added to Remove Condensate Additional Heat Exchangers Added to Reduce Fuel Consumption & Protects Glycol Pump Glycol Absorber with Integral Scrubber TEG Circulation Rates of 1.5 to 4 gal per lb m water removed Absorber Section Usually Contains 4 to 12 Bubble Cap Trays Gas Glycol 50% of All Dehydration Problems are Caused by Inadequate Scrubbing of Inlet Gas

9 Skimmer or Flash Tank Rich Glycol & Condensate Feed Purpose: Knock Condensate out of Glycol Operating Parameters: Two-Phase Separator with 5-10 minutes retention time required. Or Three-Phase Separator with minutes liquid retention time. Optimum Conditions are ºF and psig. Better condensate-glycol separation is obtained with horizontal flash tanks; vertical separators require less platform space. Rich Glycol to Reboiler Filters Purpose: Prevent pump wear, plugging of heat exchangers, foaming, fouling of contactor trays, cell corrosion and hot spots on the fire tubes. Operating Parameters: Keep solids below 100 ppm Sock filter designed to remove 5 micron and larger particles Sock filters are designed for an initial pressure loss of 3 to 6 psi and change out at 15 to 25 psi. Activated charcoal filters used to remove condensate, surfactants and treating chemicals.

10 Glycol Pump Purpose: Returns LP lean glycol to HP contact tower. Operating Parameters: Contains only moving parts in unit A spare pump should be provided since dehydration stops when glycol circulation stops. Typically a positive displacement (PD) pump. Can be HP gas, HP liquid, or electric motor driven. Surge Tank Purpose: Reservoir to handle a complete draindown of TEG from the absorber-tower trays. Operating Parameters: Should be designed to operate at half full under normal operation. A gas blanket is recommended to prevent oxygen contamination.

11 Reboiler Purpose: Provides heat necessary to boil the water out of the rich or wet glycol. Operating Parameters: Direct fired heaters often used onshore. Indirect heating offshore. TEG does not undergo thermal decomposition if temperature is kept below 400 ºF. U-shaped fire tube should be sized for Btu/hr-ft 2. Water comes off as steam. Instrumentation Lean Design ITEM Contactor Reconcentrator Shutdown Panel CONTROLS PC on exit gas line PI on contactor TI on contactor LC on contactor PSV on reboiler shell TSH on glycol in reboiler (to shutdown panel) TI on glycol in reboiler TIC on glycol in reboiler connected to TCV on fuel gas to main burner TSH on stack gas temperature (to shutdown panel) BSL flame sensor on burner (to shutdown panel) PI on fuel line to main burner PCV on fuel line to main burner SDV on fuel line to main burner (activated by shutdown panel) SDV on pilot fuel line (activated by shutdown panel) LAH on glycol level in glycol flash tank LAL on glycol level in glycol flash tank BAL on flame in main burner TAH on glycol temperature in reboiler OR on stack gas temperature LAH on integral scrubber in contactor BAL BSL LC LAH LAL PC PCV PI PSV SDV TAH TCV TI TIC TSH LEGEND Low burner flame alarm Burner flame sensor Level control High liquid level alarm Low liquid level alarm Pressure control Pressure control valve Pressure indicator Pressure shutdown valve Shutdown valve High level temperature alarm Temperature control valve Temperature indicator Temperature indicating controller High temperature shutdown

12 Operating Temperatures Inlet gas Reboiler PROCESS LOCATION Glycol into absorber Glycol into flash separator or skimmer Glycol into filters Glycol into still Top of still TEG entering pump TEMPERATURE OR TEMPERATURE RANGE (ºF) warmer than gas (prefer 150) (prefer 150) with stripping gas (prefer 380) 350 yields 98.5 wt% TEG 400 yields 99.0 wt% TEG <200 (prefer 180) Process Operation Contactor or Absorber: Operating efficiency depends on the inlet gas flow rate, temperature, and pressure and also the lean glycol concentration, temperature, and circulation rate. Inlet Gas Flow Rate: Load (lbs water to be removed/hr) varies directly with feed gas flow rate. Most contactors have been designed conservatively and can handle flow rates 5 to 10% above capacity. Lower flow limit set by 5 to 1 turndown ratio of the bubble caps.

13 Process Operation Inlet Gas Temperature: Inlet gas may be assumed to enter the absorber saturated with water vapor. McKetta and Wehe s correlation shows that at 1000 psia, the water content increases from 33 to 62 to 102 lb H 2 O/MMscf as the temperature increases from 80, to 100 to 120 ºF. Pressure is not as severe: at 100 ºF, the water content is 62, 72 and 87 lb m H 2 O /MMscf at 1000, 800 and 600 psia. Entering TEG temperature and concentration: The drying ability of the TEG is limited by the vaporliquid equilibrium of water between the gas phase and the liquid TEG phase. Dew Point Chart TEG-H 2 O system

14 Process Operation (cont d) Glycol Circulation Rate: The water picked up by the glycol increases with inlet glycol concentration, decreasing glycol temperature, higher circulation rates, and the number of contactor trays. A glycol circulation rate of 3 gal/lb m water removed is conservative but commonly used in the past. Recent energy conservation practices have lowered the rate to 2 gal/lb m of water removed. Process Operation (cont d) Dehydration Temperature: While TEG can dehydrate natural gas at operating temperatures from 50 ºF to 130 ºF, the preferred temperatures range is ºF. Below 70 ºF, glycol is too viscous. Above 110 ºF, the inlet gas contains too much water and the drying ability of the glycol is reduced. Reconcentrator: Usually operated at atmospheric pressure. Temperature ranges from 350 to 400 ºF.

15 Boiling Point of TEG Solutions Normal range for Reboiler Stripping Column Purpose: Increase glycol concentrations up to 99.6 wt% by sparging stripping gas directly into the reboiler.

16 Optimum Values for Glycol Analysis Design Method Obtain Design Information Select an appropriate combination of: Lean glycol concentration Circulation rate Absorber trays Establish the required balances: Material Energy Size Equipment

17 Required Information Inlet gas flow rate, pressure & temperature Required water dew point or water content of exit gas Inlet gas analysis or inlet gas gravity & acid gas content Required Information Other important considerations: Available utilities Safety & environmental regulations for discharging stripper overhead

18 TEG-H 2 O-VLE Comparison Parrish et. al. (1986) compared existing VLE data for TEG-waternatural gas and found considerable disagreement. Dehydrated natural gas leaving absorber cannot contain less water than that which would be in equilibrium with entering lean glycol. Equilibrium is never reached. In practice, the water dew point of dried gas leaving the absorber is 5-10 ºF higher than equilibrium dew point. Rule of thumb, dew-point depression is 60 ºF for first four trays and 7 ºF for each additional tray. Glycol Absorber (Contactor) Sizing the absorber involves specifying: Type and number of trays The TEG circulation rate The column diameter Sizing can be done by charts such as Sivalls (1976) or Worley (1987) or more recently by Olbrich and Manning (1988): Actual trays: 4-12 Lean glycol conc., w/o Circ. rate, gal TEG/lb H 2 O Temperature, ºF 80 and 100 Pressure, psia

19 Glycol Absorber Diameter Diameter of Absorber: V max K SB Q V max L V V A D 4Q V max V max = maximum gas superficial velocity (ft/hr) K sb = Souders-Brown coefficient (ft/hr) = 660 ft/hr for towers 30 larger with 18 tray spacing.. L = Glycol density (lb m /ft 3 ) V = Gas density at column conditions (lb m /ft 3 ) Predicted Dew Point Depression 1 & 1.5 Equilibrium Stages, 100 ºF and 600 psia

20 Predicted Dew Point Depression 2 & 2.5 Equilibrium Stages, 100 ºF and 600 psia Predicted Dew-Point Depression 3 Equilibrium Stages 100 ºF, 600 psia

21 Predicted Dew Point Depression 1 & 1.5 Equilibrium Stages 80 ºF, 600 psia Predicted Dew Point Depression 2 & 2.5 Equilibrium Stages 80 ºF, 600 psia

22 Predicted Dew Point Depression 3 Equilibrium Stages 80 ºF, 600 psia Glycol Pump BHP gph psig gph psig kw gph gallons TEG per hour Sizing Pump: Use Reciprocating pump Assume pump efficiency of 70-80% Calculate temperature rise based on converting mechanical work into enthalpy change. Can use quick estimate for pump break horsepower

23 Glycol Flash Separator Wet glycol is flashed at psia and ºF. Liquid retention times are 5-10 min. for gasglycol. Liquid retention times are min. for gas-condensate-glycol. Vertical Separator: Height (ft) = (0.4) (gpm) Where gpm = gal TEG circulated/min Minimum height =4 ft Maximum height =10 ft Minimum diameter =1.5 ft Horizontal Separator: L/D ratio = 3 Min. length = 3 ft Min. diameter = 2 ft Glycol Stripping Still Computer programs usually consider the stripping column as three theoretical trays: Reboiler Packed stripping column Reflux condenser Diameter of stripping column is based on the required vapor and liquid loads at the base of the column. An approximate diameter equation is D 9 Q where D = Still diameter (in) Q = TEG circulation rate (gpm) Conservative design and field test data dictate that the packed section should be at least 4 ft high, and that this height be increased to 8 ft for a 1 MMBtu/hr unit (Sivalls, 1976)

24 Glycol Reboiler Duty can be calculated as: Q r m where Q r = regenerator duty Btu/lb m H 2 O m = gal TEG/lb m H 2 O A more detailed procedure is illustrated in the design example below. Design duty is calculated requirement duty plus 5% of condenser and glycol exchanger duties. Vapor disengagement area is based on 14,000 Btu/hr-ft 2 heat flux across the vapor liquid interface. Reboiler shell L/D ratio is 5. Minimum D is 1.5 ft, minimum L = 3.5 ft. Glycol Heat Exchangers Lean-glycol-dry gas Reflux condenser glycol-glycol

25 Glycol Heat Exchangers Reflux Condenser Exchanger: Design duty plus 5% for fouling. Seider-Tate correlation used for the heat transfer coefficient. Glycol-glycol: Design duty + 5% for fouling. Entering temperatures for the lean and rich streams known. Set the approach or lean glycol in rich glycol out = 60 ºF to minimize preheat of the rich glycol. Two or more heat exchangers should be placed in series to avoid any temperature cross. Lean glycol cooler: Lean glycol outlet temp. should be 5-10 ºF hotter than the inlet gas to absorber. Therefore, the lean glycol is cooled from ºF down to ºF.