Dehydration

(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.

(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 CO₂or 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 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

(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 mm)

–

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 H₂O per lb_m

(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

lb_m 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.

(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.

EG

DEG

TEG TREG

TEG dew point depressions range from 40 – 150 o_{F while}

inlet pressures and

temperatures range from 75 – 2500 psig and from 55 to 160

o_{F, 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

(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.

•

– 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.

•

– 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.

•

– 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.

•

– 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 burner LAL on glycol level in glycol flash tank LAH on glycol level in glycol flash tank Shutdown Panel

SDV 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 burner PI on fuel line to main burner

BSL 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 reboiler

TSH on glycol in reboiler (to shutdown panel) PSV on reboiler shell

Reconcentrator

LC on contactor TI on contactor PI on contactor PC on exit gas line Contactor CONTROLS ITEM High temperature shutdown TSH Temperature indicating controller TIC Temperature indicator TI Temperature control valve TCV

High level temperature alarm

TAH

Shutdown valve SDV

Pressure shutdown valve PSV

Pressure indicator PI

Pressure control valve PCV

Pressure control PC

Low liquid level alarm LAL

High liquid level alarm LAH

Level control LC

Burner flame sensor BSL

Low burner flame alarm BAL

(12)

Operating Temperatures

<200 (prefer 180) TEG entering pump

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

210

190 with stripping gas Top of still

300 – 350 Glycol into still

100 – 150 (prefer 150) Glycol into filters

100 – 150 (prefer 150) Glycol into flash

separator or skimmer

5 – 15 warmer than gas Glycol into absorber

80 – 100 Inlet 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

(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 lb_mH₂O /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 Chart

(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 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.

(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

(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

(18)

TEG-H

₂

O-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

(19)

Glycol Absorber Diameter

•

Diameter of Absorber:

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.. rL = Glycol density (lbm/ft3)

rV = Gas density at column conditions

(lb_m/ft3₎ V V L SB max K V r r -r = max V Q 4 D p = A V Q₌ _max