1

Gas Dehydration Using Glycol

Manning and Thompson, Volume IChapter 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 CO2 or H2S.– 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 lbmper 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 (200-800 m2/g) with very small radii of curvature (0.001-0.2 �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 lbm H2O per lbm

of CaCl2 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. 10-50 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

lbm H2O/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, H2S, CO2, 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

• Ethylene glycol (EG)• Diethylene glycol (DEG)• Triethylene glycol (TEG)• Tetraethylene glycol (TREG)• TEG has gained almost universal

acceptance as the most cost-effective 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.

EGDEG

TEG TREG

TEG dew point depressions range from 40 – 150 oF while inlet pressures and temperatures range from 75 –2500 psig and from 55 to 160 oF, respectively.

Flow Diagram for TEG Dehydration(Typical of Wellhead Unit)

Remove Liquid and

solids

Wet Glycol Needs Reconcentration

Remove Water Vapor

Preheat Rich Glycol & Cool Lean Glycol

Reboiler boils water out of

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

50% of All Dehydration

Problems are Caused by

Inadequate Scrubbing of

Inlet Gas

Absorber Section Usually Contains 4 to 12

Bubble Cap Trays

TEG Circulation Rates of 1.5 to 4 gal per lbm

water removed

Gas Glycol

9

Skimmer or Flash Tank

• Purpose: – Knock Condensate out of Glycol

• Operating Parameters:– Two-Phase Separator with 5-10

minutes retention time required.– Or Three-Phase Separator with 20-30

minutes liquid retention time.– Optimum Conditions are 100-150 ºF

and 50-75 psig.– Better condensate-glycol separation is

obtained with horizontal flash tanks; vertical separators require less platform space.

Rich Glycol & Condensate

Feed

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 drain-

down 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 6000-8000 Btu/hr-ft2.

– Water comes off as steam.

Instrumentation – Lean Design

LAH on integral scrubber in contactor

TAH on glycol temperature in reboiler OR on stack gas temperature

BAL on flame in main burnerLAL on glycol level in glycol flash tankLAH on glycol level in glycol flash tankShutdown PanelSDV on pilot fuel line (activated by shutdown panel)

SDV on fuel line to main burner (activated by shutdown panel)

PCV on fuel line to main burnerPI on fuel line to main burnerBSL flame sensor on burner (to shutdown panel)TSH on stack gas temperature (to shutdown panel)

TIC on glycol in reboiler connected to TCV on fuel gas to main burner

TI on glycol in reboilerTSH on glycol in reboiler (to shutdown panel)PSV on reboiler shellReconcentratorLC on contactorTI on contactorPI on contactorPC on exit gas lineContactor

CONTROLSITEM

High temperature shutdownTSH

Temperature indicating controllerTIC

Temperature indicatorTI

Temperature control valveTCV

High level temperature alarmTAH

Shutdown valveSDV

Pressure shutdown valvePSV

Pressure indicatorPIPressure control valvePCVPressure controlPCLow liquid level alarmLALHigh liquid level alarmLAHLevel controlLCBurner flame sensorBSLLow burner flame alarmBAL

LEGEND

12

Operating Temperatures

<200 (prefer 180)TEG entering pump

380 – 400 (prefer 380)350 yields 98.5 wt% TEG400 yields 99.0 wt% TEG

Reboiler

210190 with stripping gasTop of still

300 – 350Glycol into still

100 – 150 (prefer 150)Glycol into filters

100 – 150 (prefer 150)Glycol into flash separator or skimmer

5 – 15 warmer than gasGlycol into absorber

80 – 100Inlet gas

TEMPERATURE OR TEMPERATURE RANGE

(ºF)

PROCESS LOCATION

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 H2O/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 lbm H2O /MMscf at 1000, 800 and 600 psia.

• Entering TEG temperature and concentration:– The drying ability of the TEG is limited by the vapor-

liquid equilibrium of water between the gas phase and the liquid TEG phase.

Dew Point ChartTEG-H2O 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/lbm water removed is conservative but commonly used in the past.

– Recent energy conservation practices have lowered the rate to 2 gal/lbm 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 80-100 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 spargingstripping gas directly into thereboiler.

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-H2O-VLE Comparison

• Parrish et. al. (1986) compared existing VLE data for TEG-water-natural 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 98.5-99.9– Circ. rate, gal TEG/lb H2O 1.5-6– Temperature, ºF 80 and 100– Pressure, psia 300-400

19

Glycol Absorber Diameter

• Diameter of Absorber:

Vmax = maximum gas superficial velocity (ft/hr)

Ksb = Souders-Brown coefficient (ft/hr)= 660 ft/hr for towers 30” larger with

18” tray spacing..�L = Glycol density (lbm/ft3)�V = Gas density at column conditions

(lbm/ft3)

V

VLSBmax KV

�

����

maxVQ4D

�

�AVQ max�

Predicted Dew Point Depression1 & 1.5 Equilibrium Stages, 100 ºF and 600 psia

20

Predicted Dew Point Depression2 & 2.5 Equilibrium Stages, 100 ºF and 600 psia

Predicted Dew-Point Depression3 Equilibrium Stages – 100 ºF, 600 psia

21

Predicted Dew Point Depression1 & 1.5 Equilibrium Stages – 80 ºF, 600 psia

Predicted Dew Point Depression2 & 2.5 Equilibrium Stages – 80 ºF, 600 psia

22

Predicted Dew Point Depression3 Equilibrium Stages – 80 ºF, 600 psia

Glycol Pump

• 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� � � �

� � � �

hourperTEGgallonsgphpsiggph102.1kW

psiggph102.1BHP5

5

�

��

��

�

�

23

Glycol Flash Separator

• Wet glycol is flashed at 50-100 psia and 100-150 ºF.

• Liquid retention times are 5-10 min. for gas-glycol.

• Liquid retention times are 20-30 min. for gas-condensate-glycol.

• Vertical Separator:– Height (ft) = 3.4 + (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

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)

Q9D �

24

Glycol Reboiler

• Duty can be calculated as:

where Qr= regenerator duty Btu/lbm H2Om = gal TEG/lbm H2O

• 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-ft2 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.

m966900Qr ��

Glycol Heat Exchangers

Reflux condenser

glycol-glycol

Lean-glycol-dry gas

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 180-200 ºF down to 110 –120 ºF.