PRG.pr.GAS.0001 Gas Dehydraion

B. U. ONSHORE

DESIGN CRITERIA GAS DEHYDRATION

PRG.PR.GAS.0001 Rev. 0 January 2010

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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Sheet 2 (90)

INDEX

1.

SCOPE AND PURPOSE

5

2.

REFERENCE DOCUMENTS

5

3.

DEFINITIONS

5

3.1

Specific Definitions

5

3.2

Symbols and Abbreviations

8

4.

ACTIVITIES DESCRIPTION 4.1

Glycol Gas Dehydration

12 13

4.1.1

General

13

4.1.2

System Description and Process Flow Diagram

16

4.1.2.1

Gas Stream

17

4.1.2.2

Glycol Stream

17

4.1.2.3

Enhanced Regeneration Systems

19

4.1.3

Design Variables

22

4.1.3.1

Inlet Gas Flow rate

22

4.1.3.2

Inlet Gas Temperature

22

4.1.3.3

Inlet Gas Pressure

25

4.1.3.4

Lean TEG temperature

25

4.1.3.5

Lean TEG Concentration

25

4.1.3.6

Glycol Circulation Rate

26

4.1.3.7

Number of Stage in the Contactor

27

4.1.4

Equipment Design

27

4.1.4.1

Inlet Gas Cooler

27

4.1.4.2

Inlet gas Separator or Scrubber

27

4.1.4.3

Contactor

28

4.1.4.4

Glycol Flash Vessel

35

4.1.4.5

Regenerator and Reboiler

36

4.1.4.6

Surge Drum

44

4.1.4.7

Heat Exchangers

45

4.1.4.8

Condensed Overheads Separator

46

4.1.4.9

Filters

47

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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4.2

4.1.4.10

Glycol Circulation Pumps

49

4.1.4.11

Glycol StorageTank

49

4.1.4.12

Glycol Sump Vessel

50

4.1.4.13

Enhanced Regeneration Equipment Design

51

4.1.5

Materials

55

4.1.6

Operation and Maintenance

56

4.1.6.1

Oxidation

56

4.1.6.2

Ph Control

56

4.1.6.3

Salt Contamination

57

4.1.6.4

Hydrocarbon Contamination

57

4.1.6.5

Sludge Accumulation

57

4.1.6.6

Foaming

57

4.1.6.7

Acid gas solubilities and stripping

58

4.1.6.8

Mercury in Feed gas

58

Molecular Sieves Dehydration

59

4.2.1

General

59

4.2.2

System Description and Process Flow Diagram

65

4.2.2.1

Adsorption Mechanism

67

4.2.3

Design Variables

68

4.2.3.1

Gas Composition

68

4.2.3.2

Flow Rate

68

4.2.3.3

Pressure

68

4.2.3.4

Temperature

69

4.2.3.5

Further variables affecting the efficiency of regeneration

69

4.2.3.6

Utilities Availability and Local Conditions

69

4.2.4

Equipment Design

70

4.2.4.1

Scrubber

70

4.2.4.2

Gas Dehydrator

71

4.2.4.3

Regeneration Calculations

76

4.2.4.4

Special Bed Configurations

79

4.2.5

Operation and Maintenance

80

4.2.5.1

Operation Records

80

4.2.5.2

Good Operational Techniques

80

4.2.5.3

Depressurization / Repressurization

80

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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4.2.5.4

Analyzers

81

4.2.5.5

Insulation

81

4.2.5.6

Effects of contaminants on molecular sieves

81

4.2.6

Process Control and Safeguarding

82

5.

FLOW CHART

83

6.

APPENDIX

84

6.1

Appendix I – Water removal at different TEG concentration and TEG circulation rates

84

6.2

Appendix II – Reboiler Heat Duty

87

6.3

Appendix III – Molecular Sieve Equipment Sizing – Example

88

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1.

SCOPE AND PURPOSE

The purpose of this document is to provide the design guidelines for natural gas dehydration processes. For equipment mechanical details the relevant SAIPEM or Project criteria shall be considered. The application of this Design Criteria shall be subjected to accurate review of the results by a qualified and competent designer to avoid unnecessary over sizing or design not adequate to the scope. Moreover, margins indicated in the following paragraphs shall be considered as the minimum to be applied in case no indications are present in the Project documents or in other SAIPEM standards.

2.

REFERENCE DOCUMENTS

- PRG.PR.VES.0001

Guide to selecting and process sizing vessels

- PRG.PR.VES.0011

Guide to process sizing fractionating columns

- PRG.PR.GEN.0003

Guide to selecting standards for process equipment internals

- PRG.PR.HEB.0001

Guide to process selection and sizing of heat exchangers

- PRG.PR.MAC.0001

Guide to selecting and process sizing pumps

- PRG.PR.TUB.0001

Process piping sizing guide

- PRG.PR.HEB.0002

Guide to process sizing furnaces

- PRG.PR.MAC.0002

Guide to the process design of compressors

- GPSA Engineering Data Book - Natural Gas Conditioning Though Adsorption Technology (J. M. Campbell, W. P. Cummings)

3.

DEFINITIONS

3.1

Specific Definitions

Absorber

See contactor

Absorption Process

The attraction and retention of vapours (water) by liquids (glycol) / solids from a gas stream.

Actual Trays

The number of trays installed in a column or the equivalent number of actual trays for a packed column.

Bubble Cap Tray

Horizontal plate holding bubble caps and downcomers in the contactor.

Bubble Caps

Slotted metal caps attached over elevated nozzles (risers) on the bubble cap trays. The slots cause the gas to break up into small bubbles for intimate contact with the glycol.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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Condensate

Light hydrocarbon liquids.

Contactor (or Absorber)

A vertical pressure column where gas and glycol are intermingled counter-currently to remove water vapour from the gas. The contactor usually contains bubble cap trays, valve trays or structured packing.

Dehydration

Removal of water vapour from a gas or a liquid.

Design Pressure

The pressure used in the design of a vessel for the purpose of determining the minimum permissible wall thickness or physical characteristics of the different parts of the vessel.

Dewpoint

The temperature at which vapour begins to condense into a liquid at a particular system pressure. A natural gas stream exhibits both hydrocarbon and water dewpoints.

Dewpoint Depression

The difference in water dewpoint temperature between the gas entering and leaving the contactor.

Downcomer

The vertical conduit between trays which allows liquid to pass from tray to tray.

Flood

The condition wherein excess liquid hold-up occurs and normal counterflow action is prevented in the glycol contactor, regeneration still or stripping column. It is a design limit which when reached in operation causes an excessive loss of liquid from the top of the column.

Free Water

Liquid water which is not dissolved in any other substance.

Gas/Glycol Heat Exchanger

A heat exchanger employed to cool the lean glycol by the gas leaving the contactor before the glycol enters the contactor.

Glycol

A hygroscopic liquid. Mono-ethylene Glycol (MEG) and Diethylene Glycol (DEG) are commonly used in hydrate inhibition service and Tri-ethylene Glycol (TEG) is most common in gas dehydration service.

Lean Glycol (or Dry Glycol)

Glycol which has been regenerated and has a low water content.

Rich Glycol (or Wet Glycol)

Glycol which has absorbed water and thus has a high water content.

Glycol Flash Separator

A three phase separator which is used in the rich glycol stream to remove entrained gas and hydrocarbon liquids.

Glycol/Glycol Exchanger

A heat exchanger employed to recover heat from the outgoing hot lean glycol from the reboiler and for pre-heating the incoming cool rich glycol from the contactor.

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Heat Duty

The rate of heat absorption by the process.

Heat Flux

The average heat transfer rate to the fluid.

Inlet Gas Separator (Scrubber)

A separator which removes free liquids from the inlet gas stream. The separator may be included or not in the contactor.

Packing

Material installed in the contactor, still column or stripping column that provides a large surface area for merging liquid and vapour to facilitate mass transfer during absorption, distillation or stripping. Random packing consists of shaped pieces (e.g. rings, saddles) that have been dumped, not stacked, in the column. Structured packing is essentially a series of parallel formed metal sheets.

pH

Measure of the acidity of a liquid on a scale of 0 to 14 with 7 being neutral. 0 to 7 is acidic and 7 to 14 is alkaline.

Reboiler

A heat exchanger for boiling water out of the glycol.

Regeneration System

A unit including reboiler, still column and other related facilities to regenerate (or re-concentrate) rich glycol to lean glycol.

Reflux

Condensed liquid which flows back a column to maximise separation efficiency.

Saturated Gas (with respect to water)

A gas stream which contains the maximum amount of water vapour at a given temperature and pressure without condensing the water.

Sparging Tube

Internal pipe in the reboiler used to distribute stripping gas.

Standard (pressure and temperature)

Unit of gas volume at reference conditions of 1 bar and 15°C. Abbreviated: m3(st).

Still Reflux Column

Vertically mounted distillation (fractionation) column on top of the reboiler.

Stripping Column

A packed column where glycol from the reboiler flows downward to the surge drum while gas flows upward stripping the water from glycol.

Stripping Gas

Gas that is contacted with glycol to help remove water from the glycol.

Surge Drum

Reservoir for regenerated glycol which is included or not in the reboiler.

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Theoretical Tray

One in which the vapour and liquid leaving the stage are in equilibrium. The number of actual trays is equal to the number of theoretical trays divided by the overall tray efficiency.

Transfer Unit

The dimensionless distance within which the solute molecules transfer to the gas phase. A transfer unit can be calculated for a theoretical stage.

Tray Efficiency

The ratio between the number of theoretical and actual trays.

Valve Tray

Horizontal plate holding valves and downcomers in the contactor. A valve consists of a liftable metal plate which covers a hole in the tray, providing a variable area for gas flow.

3.2

Symbols and Abbreviations 2 [m ]

A

Area

BTEX

Aromatic components: benzene, toluene, ethylbenzene and xylene -

Cp

Heat Capacity

Css

Saturation correction factor for sieve

CT

Temperature correction factor

D

Inside diameter of column or vessel

[m]

DEG

Diethylene glicol

-

D

Nozzle inside diameter, or diameter (with subscript)

[m]

EG

Ethylene glicol

-

F

Packing factor

-1 [m ]

FF

Fraction of flood

-

G

Acceleration due to gravity

[9.81 m/s2]

G

Gas mass flowrate/unit area

[kg/(s.m2)]

H

Height

[m]

H

Henthalpy

[kJ/kg]

HC

Height of channel or riser height of distributor

[m]

[kJ/(kg K)]

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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HETP

Height equivalent to a theoretical plate

[m]

ILL

Interface level of condensate and glycol

[m]

L

Length of vessel between tangent lines

[m]

Ls

Length of racket bed saturation zone

[m]

HLL

High level pre-alarm

[m]

LLL

Low level pre-alarm

[m]

LCV

Level control valve

-

HHLL

High level trip

[m]

LLLL

Low level trip

[m]

m

Mass flowrate

[kg/s]

MEA

Methylethanol amine

-

MMSCF

Milion of standard cubic feet

-

MTD

Mean temperature difference

[°C]

MTZ

Mass Transfer Zone

-

N

Number of bubble cap

-

NLL

Normal level of liquid

[m]

NPSH

Net positive suction head

[m]

OVHD

Overhead

-

P

Pressure

[Pa]

PG

Propilene glicol

-

PSV

Safety relief valve

-

PVT

Pressare-Volume-Temperature

-

Q

Volumetric flow rate or Heat

[m3/s] or [kJ]

Re

Reynold's number

-

T

Temperature

[°C] or [K]

•

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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Sheet 10 (90)

t

Thickness

[mm]

TCV

Temperature control valve

-

TEG

Triethylene glicol

-

TREG

Tetraethylene glicol

-

TS

Tray spacing in contactor

[m]

TTL

Top tangent line on vessel

-

U

Overall heat transfer coefficient

2 [W/(m K)]

UCR

Unit circulation rate, volumetric flowrate of lean glycol per mass flowrate of water removed

[l/kg]

V

Velocità

[m/s]

W

Width

[mm]

Wr

Water removed per cycle

[kg]

Δ

Separation between plates in separator, difference in parameter values (as in Δρ)

[m]

ρ

Density

[kg/m3]

λ

Gas load factor:

[m/s]

μ

Dynamic viscosity

[Pa.s] or [mPa.s]

ψ

Ratio of ρ of water to ρ of glycol

-

ϕ

Flow parameter:

-

Greek Symbols

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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Sheet 11 (90)

Subscripts B

Bed

G

Gas (as in vg)

Hl

Heat loss

I

Initial

In

Inlet (as in vin, Yin)

L

Liquid

M

Mixture (as in vm)

Min

Minimum

Max

Maximum

Out

Outlet

Rg

Regeneration

Si

Sieve

St

Steel

Tr

Total regeneration

W

Water

Superscripts *

Density correction (e.g. in Q*max)

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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4.

ACTIVITIES DESCRIPTION

Gas Dehydration is the process of reducing the water content in the gas down to an acceptable level for the downstream systems. At the wellhead, reservoir fluids almost invariably contain water and, except for a few shallow wells, produced natural gas is saturated with water. The major reasons for dehydrating gas are: •

natural gas can combine with liquid or free water to form solid hydrates that can plug valves, fittings, equipment or even pipelines;

•

if not separated from the produced water, natural gas is corrosive, especially when CO2 and/or H2S are also present;

•

water can condense in the pipeline causing slug and possible erosion and corrosion;

•

water vapour increases the volume and decreases the heating value of the gas;

•

sales gas contracts and/or pipeline specifications have a maximum water content (usually 110 kg H2O / million (st)m3 i.e. 7 lb H2O per MMSCF) or a specified dew point value;

•

freezing in cryogenic and refrigerated absorption plants even in small quantities.

Natural gas is commercially dehydrated in one of the following methods: 1. Absorption

Glycol Dehydration

2. Adsorption

Molecular Sieve or Silica Gel

3. Condensation

Refrigeration with Glycol Injection

Glycol dehydration (absorption) is the most common dehydration process used to meet pipeline sales specification and field requirements (gas lift, fuel, etc.). Adsorption process are used to obtain very low water contents (0.1 ppm vol or less) required in low temperature processing such as deep NGL extraction and LNG plants. Condensation is commonly used as a dehydration process when the gas is employed in moderate levels of refrigeration or in pipeline transportation. An inhibitor such as ethylene glycol (EG) or methanol is used to prevent hydrate formation.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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4.1

Glycol Gas Dehydration

4.1.1

General

Glycol dehydration plants have many applications for natural gas dehydration. They are used to reach a water dew point in the range of -30 ÷ -40°C. They are preferred for the following reasons: •

low installation costs ;

•

operating costs similar to other dehydration systems;

•

glycol dehydration is continuous process rather than batch;

•

low pressure drop across the plant (0.35÷0.7 bar);

•

easiness to treat gas at low pressure;

•

possibility to receive gas at high feed temperature (till 66°C);

•

low costs for obtaining small reductions in the gas dew point.

The principle of glycol dehydration is contacting a gas stream with a hygroscopic liquid which has a greater affinity for the water vapour than does the gas. After contacting the gas, the water-rich glycol is regenerated by heating at approximately atmospheric pressure to a temperature high enough to drive off virtually all the absorbed water. The regenerated glycol is then cooled and recirculated back. All the following glycol types possess suitable properties to be used in a dehydration plant: •

Triethylene glycol (TEG) is the most commonly used dehydration liquid and it is the assumed glycol type in this process description because it is the most cost-effective choice. TEG is the most easily regenerated to a concentration of 98÷99.5% (using stripping gas) in an atmospheric stripper because of its high boiling point. This permits higher dew-point depression of natural gas in the range of 27÷66°C. Moreover, TEG has an initial theoretical decomposition temperature of 206.7°C, higher than other glycol types and it is not too viscous above 21°C.

•

Diethylene glycol (DEG) is sometimes used when hydrate inhibition is required upstream of dehydration or due to the greater solubility of salt in DEG. Moreover, DEG is somewhat cheaper to buy and sometimes is used for this reason. Compared to TEG, DEG has a larger carry-over loss, offers lower dew point depression, and regeneration to high concentrations is more difficult due to lower degradation temperature and consequent lower regeneration temperature.

•

Tetraethylene glycol (TREG) is more viscous and more expensive than the other glycols. The only real advantage is its lower vapour pressure which reduces absorber vapour loss. For this reason, it should only be considered for rare cases where glycol dehydration will be employed on a gas whose temperature exceeds about 50°C, such as when extreme ambient conditions prevent cooling to a lower temperature.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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•

Propylene glycol (PG) has been employed in some units and it is the least toxic glycol. It has a lower affinity for aromatics, but PG has a much higher vapour pressure than TEG, and a much lower flash point.

Physical properties of glycols are the following:

Glicol

MEG

DEG

TEG

TREG

Formula

HOC2H4OH

HO(C2H4O)2H

HO(C2H4O)3H

HO(C2H4O)4H

Molecular mass

62.07

106.12

150.17

194.32

Boiling point 101.3 kPa (°C)

197

245

287

327

Freezing point (°C)

-13

-8

-7.2

-6.2

Density 20°C (kg/m3)

1113

1116

1123

1246

Viscosity 20°C (mPa.s)

20.9

35.7

47.9

60.0

Degradation temperature (°C)

165

164

206

238

Flash Point (°C)

111

124

165

202

Auto Ignition temperature (°C)

410

229

370

358

Toxicity

YES

YES

YES

YES

Table 1 – Physical Properties of MEG, DEG, TEG and TREG

Glycols as a class are of a low order of toxicity (with the exception of oral toxicity). They do not vaporise readily at normal temperatures and, therefore, do not constitute a hazard from inhalation. They are also not active skin irritants. Glycols will burn and should be handled as if they were hydrocarbons. As they are highly miscible with water, alcohol type foam concentrate shall be used for extinguishing of a glycol fire. For extinguishing of small fires some dry powder types can be effective. TEG has been applied downstream of production facilities that use MEG or DEG as a hydrate inhibitor without apparently leading to contamination problems. Methanol used as a hydrate inhibitor in the feed gas to a glycol dehydration unit will be absorbed by the glycol, and according to the GPSA Engineering Data Book it can cause the following problems:

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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1. methanol will add additional reboiler heat duty and still vapour load and therefore increase glycol losses; 2. aqueous methanol causes corrosion of carbon steel. Corrosion can thus occur in the still and reboiler vapour space; 3. high methanol injection rates and consequent slug carry-over can cause flooding. Glycol entrainment may lead to the following downstream problems: •

coalescing and partial condensation in pipelines resulting in localised corrosion;

•

in cryogenic plants, particularly at temperatures below -25°C, freezing of TEG and plugging of equipment;

•

reduced performance of downstream adsorption plant, e.g. molecular sieves or silica gel.

Any entrained glycol should be removed upstream of cryogenic plant in high efficiency gas/liquid separators to prevent possible plugging. A range of lean TEG concentrations can be achieved with the basic regeneration flow schemes and various enhancements summarised in Table 2 and further described in the following paragraphs. It should be noted that the corresponding dewpoint depressions are approximate and achievable figures are affected by actual process conditions.

Regeneration Process Lean TEG concentration (wt%) Dewpoint Depression (°C)

Basic

Cold Finger

Vacuum

Stripping Gas

Azeotropic Stripping

98.75

99.5

99.9

99.96

99.99

45

60

65

70

100

Table 2 – Lean TEG Concentrations with Regeneration Systems

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

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4.1.2

System Description and Process Flow Diagram

The basic flow scheme without enhancements, such as stripping gas, is described below with reference to

Flash Gas 

Fig. 1, and follows the two main streams, gas and glycol. It is typical and many variations are possible.

Fig. 1 – Simplified Process Flow Diagram for TEG Dehydration System

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4.1.2.1

Gas Stream

Where feed temperatures are high, especially relative to ambient conditions, an inlet cooler may be used. Feed gas is scrubbed of free liquids (water and condensate) and solid particles before entering the glycol contactor. The separator may stand alone or form an integral part of the glycol contactor column. The latter solution is usually preferred for small gas flowrate. The saturated feed gas is introduced in the bottom of the contactor and rises up through the column where it contacts lean glycol which is injected in the top of the column. The contacting devices may be trays or packing. Dry gas leaves the column via a de-entrainment device, e.g. a demister mat, to remove entrained droplets. Flash gas from glycol flash vessel and off gas from still column overhead could have a high content of H2S, BTEX and volatile hydrocarbon and are usually routed to low pressure flare.

4.1.2.2

Glycol Stream

Lean glycol from surge drum flows through the rich/lean glycol heat exchanger to cool the lean glycol stream before entering the glycol circulation pumps. In some arrangements there will be two rich/lean glycol heat exchangers in series. Although there will be some pressure drop through the heat exchanger, due to the temperature reduction the glycol should not flash at this point. If there is insufficient NPSH for the glycol pumps in the location shown, they may be located between the surge drum and the lean/rich glycol heat exchanger. In this location the pumps will operate at a higher temperature. Sufficient NPSH can be created for the main glycol pumps by installing glycol booster pumps, if necessary. The lean glycol then flows to the final cooler, which is often an air-cooled heat exchanger but could also be a glycol-gas heat exchanger. In some cases (generally for low glycol circulation rate) lean glycol cooling coils are installed in the top section of contactor thus avoiding the installation of the gas/glycol heat exchanger. From the glycol final cooler, the lean glycol enters the top of the contactor. On its way down the column the glycol absorbs water and the rich glycol collects at the bottom of the contactor. The rich glycol passes from the contactor via a level control valve to a coil in the top of the regeneration still column, thereby providing reflux cooling in the still column. Rich glycol is heated to about 60°C or 70°C in the rich/lean glycol heat exchanger before it enters the glycol flash vessel. In this 3-phase separator, dissolved and entrained gas is removed from the glycol and liquid hydrocarbon condensate, if present, is separated from the glycol. These hydrocarbon components would flash in the regenerator and lead to an increased still column vapour load, a higher reboiler duty requirement, greater glycol losses and a loss of recoverable product. These components would also lead to coking of the reboiler heating elements, fouling and foaming.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 18 (90)

While this arrangement is typical (heating reduces glycol viscosity and makes gas and condensate separation easier), in some units the glycol flash vessel is located immediately downstream of the contactor, operating at a lower temperature. From the flash vessel the rich glycol flows through a full flow particle filter and an activated carbon filter often in slipstream service, to remove solids and dissolved hydrocarbons and degradation products, respectively, in order to prevent foaming and sludge build-up in the regenerator. The rich glycol is further heated in a second rich/lean glycol heat exchanger and then flows to the regenerator still column between two sections of packing. This is a typical arrangement, in some systems the glycol flows directly to the regenerator without passing through a second glycol/glycol heat exchanger. Heat is provided at the bottom of the regenerator evaporating water from the glycol. The reboiler may be directly fired or indirectly heated by electricity, hot oil or steam. Typical operating temperatures are up to 204°C, i.e. 2°C lower than degradation temperature (see Fig. 2). Water and volatile species present are evaporated from the rich glycol, the reflux is provided to reduce glycol losses. Because of the wide difference in volatility only a small reflux is needed to effect water/glycol separation. Off gas is normally cooled and sent to a three phase separator where condensate and oily water are recovered and sent to treatment/disposal. The regeneration units are designed to operate at prevailing atmospheric pressure and at temperature lower than the initial thermal decomposition temperatures of the glycols shown below:

GLYCOL

DECOMPOSITION

LEAN GLYCOL

SUGGESTED

TEMPERATURE

CONC., wt%

REGENERATION TEMPERATURE

EG

165°C

96.0

164°C

DEG

164°C

97.1

163°C

TEG

206°C

98.7

204°C

TREG

238°C

-

236°C

Table 3 – Glycol Decomposition Temperatures

These are the temperatures at which measurable decomposition begins to occur in the presence of air. In the units containing no air (oxygen), it has been found that the reboiler can be operated very close to the above temperatures without noticeable decomposition. The composition of the lean glycol is set by the bubble point composition at the regenerator pressure. Maximum concentration achievable in an atmospheric regenerator operating at decomposition temperature is also shown in the above Table 3.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

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Sheet 19 (90)

Fig. 2 – TEG concentration vs. reboiler temperature

If the lean glycol concentration required at the absorber (to meet the dew point specification) is higher than the maximum concentrations above, then some enhanced methods (see par. 4.1.2.3) of increasing the glycol concentration at the regenerator shall be incorporated in the unit. Generally all of these methods involve lowering the partial pressure of the glycol solution either by pulling a vacuum on the regenerator, or by introducing stripping gas into the regenerator.

4.1.2.3

Enhanced Regeneration Systems

A. COLDFINGER The "Coldfinger" method is a patented process which has been used in a number of locations, mostly in the USA, to give enhanced glycol regeneration. It consists of a heat exchanger tube bundle with a liquid collection through which is inserted into the vapour space of the surge drum. The heat exchanger tubes are fed with either rich glycol before flowing to the still column reflux, or with cooling water, which gives a lower temperature. This "cold finger" leads to condensation of some vapour which is richer in water than the regenerated TEG in the liquid space of the surge drum. The condensed vapours collected are continuously recycled back to the still column feed. The H2O partial pressure in the vapour space is thus lowered and the lean glycol concentration increased. Lean TEG concentration of 99.5÷99.9 wt% have been achieved in Coldfinger units without the use of stripping gas, although a small amount of gas is introduced into the surge tank for pressure balancing. A simplified scheme of this regeneration process is shown in Fig. 3.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 20 (90)

B. AZEOTROPIC STRIPING (DRIZO) The azeotropic striping "DRIZO", shown in Fig. 3, is a patented process to give enhanced regeneration of glycol. It utilises a circulating solvent, such as heptane or octane, to remove water by azeotropic stripping. The regenerator vapours are condensed in a 3-phase separator and condensed solvent is returned to the regenerator via a pump and solvent heaters. Some solvent is lost in the remaining vapour stream to vent. Depending on the composition of the gas being dried, sufficient heavy ends can be absorbed by the glycols in the contactor to more than compensate for these losses. Thus, an initial charge may only be needed for start-up and excess hydrocarbon liquid can be recovered as a product. This unit has the advantage of providing very high stripping gas rates with little or no venting of hydrocarbons. Glycol concentrations in excess of 99.99% have been achieved with the DRIZO process. It has an added advantage of condensing and recovering aromatic hydrocarbons from the still column overhead. In fact, these unit often operate with a stripping solvent which is not iso-octane but a mixture of aromatic, naphthenic and paraffin hydrocarbons in the C5-C8 range. C. STRIPPING GAS Glycol concentration can be increased by injecting a small quantity of stripping gas via a sparge pipe in the surge drum or via a packed stripping column with a counter-current glycol flow, as shown in Fig. 3. The latter alternative is preferred since it allows either a glycol concentration of 99.96 % instead of 99.9 % to be achieved or alternatively a reduction in the stripping gas rate. If introduced directly to the reboiler, it is common to use a distributor pipe along the bottom of the reboiler. Any inert gas is suitable as stripping gas: it may be drawn from the fuel gas system, from the gas being dehydrated or from exhaust gas from a gaspowered glycol pump if used. The use of stripping gas, however, is not recommended due to the increase of hydrocarbon emissions when vent gas from still column is routed to flare.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

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Sheet 21 (90)

Vent gas to flare or recycle

Vent gas to flare or recycle

Still Column

Rich TEG

Still Column

Rich TEG

Direct Fired

Direct Fired

Reboiler

Reboiler

Cooling Medium

Stripping Gas Surge Tank

Water/Rich TEG mixture to Still Column

Surge Tank

Lean TEG

Lean TEG

A) Stripping Gas

B) Coldfinger ®

Solvent Condenser

Vent

Still Column

Rich TEG

Drizo® Separator Direct Fired Reboiler Stripping Solvent

Water Solvent Vaporizer Excess Solvent

Surge Tank Lean TEG C) Drizo ®

Fig. 3 – TEG Regeneration Alternatives (Stripping Gas, Coldfinger® and Drizo®)

D. VACUUM The vacuum process utilises a low partial pressure over the glycol solution to achieve a higher glycol concentration. This is achieved by drawing a vacuum on the stripping column. These units are not common due to their high operating costs, control complexity and problems with glycol degradation due to air infiltration.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

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Sheet 22 (90)

4.1.3

Design Variables

A properly designed and operated TEG unit will dehydrate natural gas with only minor difficulties and require modest maintenance. High glycol losses, excessive TEG recirculation rates, improperly operating pumps, needless energy consumption, frequent plant shutdowns and excessive equipment replacement can lead to very high operating costs. The overall design of a glycol system requires optimisation of many variables that interact with each other. This paragraph explains the effect of process variables on TEG unit optimisation.

4.1.3.1

Inlet Gas Flow rate

The load (kg/hr H2O to be removed) varies directly with the feed gas flow rate. Normally the trays (bubble cap usually and valve type sometimes) are operating in a severe spray regime, i.e. very little liquid glycol compared to the gas flow rate. Increases in feed gas flow rate can exacerbate this delicate condition of “blowing flood” and can be very detrimental to contactor performance. Random or structural packing is not susceptible to this “blowing flood” because the liquid flows as wetted film on the packing surface. In addition, an increase in gas flowrate can lead to excessive glycol losses and overload of regenerator; of course the capability of the other units (i.e pumps and reboilers) must be considered. The lower flow limit is determined by the “overdesign” and the turndown ratio (depending on the type of the internals used).

4.1.3.2

Inlet Gas Temperature

Inlet gas temperature is a significant variable. Higher gas temperature requires more glycol circulation, since: -

there is an exponential increase in the water vapour content of the saturated gas (see Fig. 4);

-

a higher inlet dew point requires a higher dew point depression for a given outlet dew point;

-

a higher unit circulation rate (UCR) is required due to an increase in the equilibrium value of water dew point for a given lean glycol concentration.

Higher temperature also reduces the gas density leading to a higher volumetric gas flow and load factor. The operating inlet gas temperature shall be in the range 25÷50°C. Hydrates may form if the temperature of the water saturated gas is less than about 20°C, therefore it is desirable to have the inlet gas temperature at least 5°C above the hydrate formation temperature. It has to be noted that at very low inlet gas temperature (i.e. < 15°C) the increase of glycol viscosity will reduce the efficiency of absorption and will increase the foaming tendency.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 23 (90)

Gas cooling upstream of the inlet scrubber is generally recommended, especially for temperatures above 50°C, since it is usually more economic than providing increased glycol circulation and regeneration capacity. Reducing the glycol circulation rate is also preferred to reduce losses. Above 50°C the inlet gas contains too much water, and the drying ability of the glycol is reduced. Moreover, at higher temperature glycol vaporisation losses become appreciable. In addition, if the inlet gas temperature is much higher than ambient temperature, heavier hydrocarbons could condensate on the wall of the contactor. Glycol into contactor needs to be slightly warmer (3 ÷ 6°C) than the inlet gas to avoid hydrocarbon condensation.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 24 (90)

Fig. 4 – Equilibrium Water Dew Point Vs Temperature

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

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Sheet 25 (90)

4.1.3.3

Inlet Gas Pressure

The pressure effect is quite small. Operation is theoretically possible up to pressures of 150 to 200 bar. However, in practice operations at pressures above 135 bar, i.e. outside 900# ANSI pressure rating, are not recommended. The HETP increases with pressure and glycol vapour loss may become excessive at gas densities greater than about 100 kg/m3. At pressure below about 35 bar, the gas contains significantly more water vapour and the effect of pressure is quite appreciable.

4.1.3.4

Lean TEG temperature

While TEG can theoretically dehydrate natural gas at operating temperatures from 10°C to 55°C the preferred temperature range is 30 ÷ 55°C. The equilibrium water dew point decreases with decreasing temperature, but cooling below 20°C the glycol becomes too viscous. This reduces tray efficiency, promotes foaming, and increases glycol losses. Below 10°C the drop in dehydration efficiency is very pronounced. The inlet glycol temperature should be 3÷6°C higher than the inlet gas temperature. If the glycol enters cooler than the gas, the resulting chilling condenses hydrocarbons, which, in turn, promote foaming. If the glycol enters more than 8°C above the effluent gas temperature, TEG vaporization losses are increased unnecessarily.

4.1.3.5

Lean TEG Concentration

The lean glycol concentration has the greatest effect on the dew point depression highly affecting equilibrium water dew point of dry gas. For standard dehydration systems, the factors affecting concentration are the reboiler operating temperature and the quantity of stripping gas used, as discussed in par. 4.1.2.3. The drying ability of TEG increases rapidly with concentration as illustrated in Fig. 5.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 26 (90)

Fig. 5 – Equilibrium Water Dew Point Vs TEG Concentration

4.1.3.6

Glycol Circulation Rate

Total glycol circulation rate is a function of the total amount of water to be removed from the gas and the Unit Circulation Rate (UCR). As indicated in 4.1.3.2, a higher inlet gas temperature increases the water vapour content of the saturated gas, the dew point depression required, and the UCR, all of which compound total circulation requirement. Increasing the circulation rate, the dew point depression will increase by providing a higher mean difference between the operating and equilibrium lines. However, for UCR above 40 litres/kg, the improvement is usually small and the reboiler duty becomes excessive. Increasing the number of trays (or packing height) or the glycol concentration are generally more effective means to increase the dew point depression, especially when the percentage of the inlet water to be removed is high. A lower circulation rate will also reduce the sensible heat requirement of the system. A minimum design UCR of 18 litres/kg is recommended. Typically UCR varies between 25 and 40 litres of glycol per kg of water removed. Figures in Appendix I illustrate the effect of different TEG circulation rates on water removal, at different TEG concentration. Curves have been determined at fixed temperature and number of equilibrium stages.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

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Sheet 27 (90)

4.1.3.7

Number of Stage in the Contactor

At a fixed value of lean TEG concentration, it can be selected a contactor with several stages and a low circulation rate, or one with few stages and a high circulation rate. At fixed values of lean TEG concentration and circulation rate the water concentration in the treated gas has an asymptotic trend when the number of theoretical trays is 5 or higher. In conclusion, the number of trays in the Contactor is economically less important than other design variables (i.e. lean TEG concentration and circulation rate) in fact, due to the large difference between boiling point of glycol and water, few stages are required for dehydration; hence more efforts shall be made to optimise other process variables above mentioned.

4.1.4

Equipment Design

This section recommends the minimum requirements for the design, material selection and fabrication of glycol-type gas dehydration systems. The components of a basic system are described in a detailed manner. Supplementary design details for enhanced regeneration methods (see 4.1.2.3) are present. The equipments described in this section shall be equipped with piping, instruments, valve actuators, level shutdown devices and other accessories to make a complete and functional system. It shall be understood that sample connections, vents, low point drains and minor material items required to make the system functional are part of the assembly.

4.1.4.1

Inlet Gas Cooler

Whenever possible, the feed gas should be cooled by air or water ahead of the inlet separator, since this is generally a cheaper form of dehydration. The inlet cooler shall operate above the hydrate formation temperature, taking into account minimum ambient temperature scenario with all fans switch off.

4.1.4.2

Inlet gas Separator or Scrubber

An inlet gas separator with a minimum liquid removal efficiency of 99% of liquid droplets > 10 µm shall be provided upstream of the dehydration unit, even if it is downstream of a production separator. The need of removing entrainment from the gas stream before the contactor is emphasized by the fact that most of all gas dehydration problems are caused by inadequate scrubbing of the inlet gas. Five of the more common contaminants that impair the performance of the glycol systems are: 1. Entrained or Free Water. This water increases the glycol recirculation, reboiler duty, and fuel costs. If the unit becomes overloaded, glycol can be carried over from the contactor and/or the still column.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

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Sheet 28 (90)

2. Oils or Hydrocarbons. Dissolved oils (aromatic or asphaltic) reduce the drying capacity of the glycol and, with water, cause foaming. Undissolved oils coke on the heat transfer surfaces in the reboiler and increase the viscosity of the glycol. 3. Entrained Brine. These salts dissolve in the glycol, are corrosive to steels (especially stainless) and can deposit on the fire tubes in the reboiler, causing hot spots and fire-tube burnout. 4. Down-Hole Additives. They are for example corrosion inhibitors, acidizing and fracturing fluids. These materials cause foaming, corrosion, and hot spots if they deposit on the fire tubes. 5. Solids. They are for instance sand and corrosion products. They promote foaming, erode valves and pumps and eventually plug trays and packing. Heat tracing may be used to eliminate condensing liquids between the separator and the contactor. The separator can also be arranged inside the contactor column below the contacting section. The combination of the contactor and separator in one column offers savings in total weight, space and costs. Liquids that might otherwise condense in the piping between the separator and contactor are also avoided. The separator internals may either be of the demister type or a high efficiency type (High efficiency inlet device-mist mat-multicyclone separator). In the latter case the required diameter for separation will be considerably smaller than that required for contacting.

4.1.4.3

Contactor

As shown in Fig. 6, the absorber consists of an optional integral scrubber at the bottom, a mass transfer or drying section in the middle, and a mist extractor at the top. In smaller units, the lean glycol is cooled in a coil located in the contactor just below the mist extractor, while in larger units, a separate, external heat exchanger is used (see 4.1.4.7). Wet natural gas enters the integral scrubber tangentially, and then passes through a wire-mesh mist extractor that removes most of the remaining entrained liquid droplets. In the drying section, the gas flows upward and is contacted intimately by the descending glycol solution. This counter-current contact usually employs 4 to 12 bubble cap or valve trays or packed bed. Liquid glycol carryover should not exceed 16 kg/MMSm3 (1 lb/MMscf), therefore a mist extractor should be provided at the top of the absorber. Various configurations for the contactor are possible; the main variables are the choice of contacting internals, and whether or not the inlet gas separator is arranged in the column. The following Fig. 7 and Fig. 8 show the possible different configurations.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

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Sheet 29 (90)

Fig. 6 – TEG Contactor

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Jan 2010

Date

Sheet 30 (90)

Dry gas outlet

Dry gas outlet

0.15 D (min 0.15 m) 0.10 m

Demister mat

0.15 D (min 0.15 m) 0.10 m

Demister mat Manhole

2 TS

Glycol inlet

Manhole 2 TS

TS (0.6 m typical)

Glycol inlet TS (0.6 m typical)

Bubble cap trays (5 to 8 typical)

TS TS

Bubble cap trays (5 to 8 typical)

TS TS TS

Inlet gas distributor Wet gas inlet

min 0.4 m hc (0.8 m typical)

Chimney tray

0.5 d (min 0.3 m)

Glycol outlet

0.2 D (min 0.3 m)

d + 0.02 m

Demister mat

0.1 m d (min 0.3 m)

0.05 D (min 0.15 m)

LAHH

TS Riser cap Gas riser HC skimmer

HC skimmer

Inlet gas distributor

d + 0.02 m

Wet gas inlet

0.05 D (min 0.15 m)

LAHH Manhole Manhole

Vortex breaker

Vortex breaker Glycol outlet

Glycol outlet

Fig. 7 – Glicol Contacor with Bubble Cap Trays (left side) and with Bubble Cap Trays and inelt Separator (right side) Dry gas outlet

Dry gas outlet

0.15 D (min 0.15 m) 0.10 m

Demister mat Manhole (0.6 m typical)

0.6 m (or 0.3 m for a full dia. top flange)

Glycol inlet

0.4 m typically

Liquid distributor

Packed height 2.5 – 4.0 m typically

Structured packing

Riser cap Gas riser Chimney tray Drain pipe

min 0.4 m hc (0.25 m typical) 0.2 D (min 0.3 m)

Inlet gas distributor

d + 0.02 m

Wet gas inlet

0.05 D (min 0.15 m)

LAHH HC skimmer

0.15 D (min 0.15 m) 0.10 m

Demister mat Manhole (0.6 m typical)

0.6 m (or 0.3 m for a full dia. top flange)

Glycol inlet

0.4 m typically

Liquid distributor

Packed height 2.5 – 4.0 m typically

Structured packing

Riser cap HC skimmer Gas riser Chimney tray

min 0.4 m hc (0.25 m typical) 0.3 m

Glycol outlet

0.51 m 0.2 D (min 0.3 m) 0.10 m d (min 0.3m) d + 0.02 m

Multicycle trays Demister mat Inlet gas distributor Wet gas inlet

0.05 D (min 0.15 m)

LAHH Drain pipe

Manhole

Manhole

Vortex breaker

Vortex breaker Glycol outlet

Liquid outlet

Fig. 8 – Glicol Contacor with Structured Packing (left side) and with Structured Packing and Inlet Separator (right side)

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 31 (90)

Manufacturers shall be required to provide fully dimensioned and detailed drawings of the internals, including layout and location of gas risers, liquid inlet and, where applicable, drip pipes. A skim line is required from the bottom section of the contactor to prevent accumulation of condensate. This should be located such that condensate can be skimmed with glycol at the normal operating level. A check valve should be installed as close as possible to the injection point of the contactor to prevent a reversed flow and gas entering the glycol injection line in the event of a pump failure or line rupture. For design details of the internals refer to PRG.PR.VES.0011 and to PRG.PR.GEN.0003. •

Chimney Tray

A chimney tray is required immediately below the contacting section for all configurations except for trays with an external separator. The chimney collects the wet glycol and, for columns with an integral separator, it provides a liquid volume for level control. For columns with structured packing this tray also acts as a gas distributor. •

Contacting Internals

The options for the contacting internals include bubble cap trays, valve trays, random packing, structured packing and multicyclone trays. Since glycols tend to foam, the trays should be spaced at least 450 mm and preferably 600÷760 mm apart. The use of structured packing is recommended in view of its high specific gas capacity and low glycol entrainment characteristic, further reduction in column diameter can be achieved by using multicyclone trays.

A. Bubble Cap Trays Prior to the late eighties, most glycol contactors used bubble cap trays. They have proved effective and reliable and have good gas and liquid turndown ratios; the latter being limited primarily by the bypassing of gas. There is industry experience with columns up to 4.2 m in diameter with bubble cap trays. The gas turndown for bubble caps is superior to that for valve trays; turndown requirements should be stated in the specification. They are suitable for viscous liquid also. For gas flow rates below about 40 % of design, the tray efficiency may decrease. Blinding off several of the bubble caps has been proven as a cheap and effective way of overcoming problems at high turndown ratios. For design purposes most manufacturers use tray efficiencies in the range of 25 to 33 %, although actual efficiency may differ.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 32 (90)

Valve Trays Valve trays have about 10 to 15 % greater capacity than bubble cap trays for a given contactor diameter and, at design gas flow rates, possibly a little higher efficiency. However, due to their design, valve trays are more prone to weeping, i.e. liquid seepage through the valves. This is not significant at relatively high liquid rates, but with the low liquid rates normally encountered in glycol contactors, weeping causes the column to be inefficient unless the glycol rate is maintained at a high level.

B. Random Packing Random (or dumped) packings, consisting of ceramic saddles or pall rings, are not as well suited to glycol contactors as structured packings or trays due to the high gas-liquid ratios. They were previously used in preference to bubble cap trays, in small diameter columns less than 450 mm diameter due to easier installation and lower cost. For further information about turndown, please refer to Supplier data.

C. Structured Packing At low liquid loads such as encountered in glycol contactors, structured packings are superior to random packing because of their higher specific area and better mass transfer efficiency. The gas handling capacity of structured packings is about 150 % to 190 % greater than that of bubble cap trays, which allows smaller diameters and thus cheaper contactors. A gas turndown ratio of 10 to 1 can be achieved with structured packing, and the glycol losses due to carryover of liquid from the contactor are extremely low. The latter can be explained by the liquid film formed on packing, which is not as easily entrained as the liquid from a droplet bed on a tray. Structured packings can also be attractive for revamping existing columns in order to increase the gas handling capacity, to reduce the glycol carry-over from the contactor, or to improve the dew point suppression capability. The recommended minimum liquid superficial velocity based on contactor cross-sectional area for structured packing is 0.5 mm/s to ensure complete wetting of the packing surface area. Structured packings and their liquid distributors are more sensitive to liquid turndown (please refer to Supplier data). Packing, packing supports, hold-down grids and liquid distributor should be designed / verified by a single entity.

D. Multicyclone Trays Contacting multicyclone trays are a recent development that have been successfully field tested. Vanes are used to impart a rotation to the gas which forces the liquid droplets to the wall of the tube where they are collected.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 33 (90)

The gas handling capacity of multicyclone trays is of the order of four times that of bubble cap trays. At gas flows as low as about 30 % of design, contacting occurs with co-current upward flow of glycol and gas in the multicyclones. A gas turndown to 20 % is achievable in combination with a higher unit circulation rate. At the lowest gas flows contacting occurs with counter-current (downward) glycol flow. At maximum capacity glycol losses are extremely low, similar to structured packing. Multicyclone trays are more expensive than structured packing for the same capacity. However, for typical operating conditions and capacities of about 5 million m3(st)/day or more, the advantage consists of a substantial cost savings for the column shell. •

Column Diameter

The column diameter is governed by the gas load in the contacting section and is not affected by the glycol circulation rate. The design should be based on the operation mode under the severest conditions with the highest value of the volumetric load factor Q*, defined by:

Q * = Qg

ρg ρl − ρ g

[m3/s]

;

Eq. 1

where ρ is the density of the rich glycol leaving the contactor [kg/m3] l ρg is the density of the gas entering into contactor [kg/m3] Qg is the inlet gas volumetric flow rate [m3/s] Having identified the most severe loading from the highest value of Q*, it is then necessary to add a margin to give the value on which the design shall be based. This value, Q*max, should include margins for inaccuracies in basic data, for operational flexibility. The column inside diameter may be calculated from:

D=

* 4Qmax

πλmax

;

[m]

Eq. 2

in which λmax is the maximum allowable gas load factor. A λmax of 0.055 m/s should be used for bubble cap trays at 0.6 m spacing. For smaller tray spacings (TS) the capacity decreases as:

λmax

⎛ TS ⎞ = 0.055⎜ ⎟ ⎝ 0.60 ⎠

0.33

;

[m/s]

Eq. 3

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 34 (90)

With greater tray spacings only a marginal improvement is achieved. λmax is higher for structured packings and, depending on the type selected, the column diameter required is a factor of 1.4 to 1.25 smaller than that required for bubble cap trays. This is a simplified calculation method and final diameter shall be confirmed by trays / structured packing Manufacturer. A- TRAYS NUMBER The number of actual trays required is determined by the number of theoretical trays divided by the tray efficiency. The height of the contacting section follows from the actual number of trays and the tray spacing. An adequate space shall be taken into account for demister, distributor, chimney tray,… B- PACKING HEIGHT For structured packing the packing height is determined by the number of theoretical trays required for the process duty and the height of transfer unit given by Vendor, Supplier or Literature. Structured packing consists of prefabricated elements. The calculated packing height should be rounded up to a multiple of this height and the total packing height should not exceed 6100 mm or 10 times the column diameter whichever is smaller without redistribution of glycol in the tower. •

Liquid Distributor

A liquid distributor is required when packing or multicyclone are used. It consists of a number of liquid drip pipes and vapour risers evenly distributed over the column cross-sectional area. •

Pressure Drop

The pressure drop over a glycol contactor is the sum of pressure drops over the internals: -

inlet and outlet nozzles;

-

demister;

-

separation cyclones (if any);

-

chimney tray;

-

contacting section;

-

liquid distributor.

Contactor pressure drop shall take into account the overall system and should not exceed 0.5 bar. The following equations can be used for the calculation: Pressure drop over chimney tray (accurate figures should be obtained from tray Manufacturers):

ΔP = 2 ρ g v g2 *10 −3 ;

[kPa]

Eq. 4

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 35 (90)

Pressure drop over bubble cap trays: ΔP=0.01 bar/tray is typical. Accurate figures should be obtained from tray manufacturers. For pressure drop over structured packing refer to Manufacturers data. Pressure drop over the liquid distributor:

ΔP = 2 ρ g v g2 *10 −3 ;

[kPa]

Eq. 5

Actual gas velocity should be calculated through chimney tray and liquid distributor.

4.1.4.4

Glycol Flash Vessel

The glycol flash vessel is used to remove gaseous hydrocarbons that have been absorbed or entrained with the glycol. It also provides to separate any liquid hydrocarbons from the glycol to prevent them from entering the reboiler and causing fouling, foaming and flooding. Both sulphur compounds and carbon dioxide are very soluble in water/glycol mixtures and react to some degree with glycols. Degassing in the flash vessel before the still column reduces their concentration and reduces high temperature corrosion. Degassing is more efficient if the rich glycol is first pre-heated in the top of the still column and in the glycol/glycol exchanger. Pre-heating, to about 60 ÷ 70°C, also reduces glycol viscosity. However, increased temperatures increase the solubility of liquid hydrocarbons in the glycol. The pressure in the flash separator shall be sufficient to permit the exist glycol stream to flow through all downstream equipment, i.e. the heat exchangers and the filters. The general requirement for glycol vessel pressure is a maximum pressure of 15 % of the contactor operating pressure. Thus, for 70 bar contactor pressure the flash vessel should operate at a pressure lower than 10 bar. Flash gas may be routed to flare or to other Plant destination. In addition a blanket gas connection should be provided to ensure sufficient operating pressure in the glycol flash vessel It may be appropriate to design the piping and equipment upstream of the flash vessel for the design pressure of the contactor. The glycol flash vessel rich glycol LCV should be installed as close as possible to the still feed nozzle to minimise vaporisation. The vessel should normally be horizontal and sized for a residence time of 20 minutes. Since liquid capacity is limiting and gas rates are low, it should usually be designed to operate between about 60% to 80 % full. In order to separate liquid hydrocarbons from glycol, a bucket or trough and weir design is recommended due to the simplicity of the controls for the small density difference between the collected hydrocarbons and the glycol (for details refer to PRG.PR.VES.0001.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Jan 2010

Date

Sheet 36 (90)

4.1.4.5

Regenerator and Reboiler

The glycol regenerator includes a reboiler to supply heat for the glycol regeneration system and it is sized for the total heat demand. This is governed primarily by the total glycol circulation rate and the quantity of water removed, and secondly by the efficiency of heat exchange and system losses. Heat is required for vaporising the water to regenerate the glycol, for sensible heat lost to the treated gas stream and for system heat losses. Data on typical onshore TEG regenerators are given in the following table.

Rating [kW] Glyol rate [m3/hr] Reboiler dia & length [m] Storage dia & length [m] Firebox dia & length [m] Still dia & length [m] Skid size [m] Shipping weight [tonnes] Operating weight [tonnes] Glycol volume [m3]

23

51

110

150

220

290

370

440

590

0.15

0.32

0.68

0.95

1.4

1.9

2.4

2.8

3.8

0.4x3.0

0.5x2.7 0.6x3.0 0.76x3.0 0.86x4.0 0.86x4.9 0.92x5.8 1.07x6.1 1.14x6.6

Integral

Integral 0.5x3.0

0.6x3.0

0.6x4.0 0.6x4.9 0.76x5.8 0.76x6.1 0.91x6.6

0.1x1.2 0.15x1.5 0.2x2.7 0.25x2.6

0.3x3.4 0.3x4.3

0.4x5.4

0.5x5.8

0.15x2.0 0.2x2.1 0.25x2.4 0.3x2.4

0.4x3.0 0.45x3.0 0.45x3.7 0.5x3.7

0.6x3.7

1.2x3.7

1.7x4.6 1.7x5.8

1.5x3.6 1.7x3.7

1.7x3.7

0.3x5.2

2.0x6.7

2.1x7.0

2.3x7.6

64

107

164

220

310

410

540

640

850

83

144

251

350

490

640

880

1020

1370

0.17

0.33

0.80

1.2

1.7

2.1

3.1

3.9

4.7

Table 4 – Data on Typical Onshore TEG Regenerators

•

Reboiler

The reboiler should have a spill-over column or a weir to prevent the heating coils from becoming exposed. The spill-over column may be packed to serve as a stripping gas contact column. A separate stripping column is, however, recommended if stripping gas is required. The reboiler bundle should be easily removable.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 37 (90)

HEAT SOURCES Alternative heat sources for the glycol regenerator are: -

direct gas fired;

-

hot oil/steam;

-

electricity;

-

turbine or engine exhaust;

-

furnace flue gas.

Waste heat recovery, e.g. from engine or turbine exhaust with a heating fluid system, is most common in offshore oil applications and should in all cases be considered as the preferred option for environmental reasons. Where this is not possible or reasonably practicable, electrical heating is preferred to direct fired heating for safety. a)

Direct gas fired

Direct fired heaters are most common in onshore applications due to the lower cost. The source of permanent ignition shall always be installed in a non-hazardous area and fired heaters should be located as far as practicable away from a hazardous area. The minimum distance between a fired heater and a process equipment should be 30 m. They are not as inherently safe as waste heat recovery (except when hot oil is used) or electric heating units and need additional detection/protection measures. In common with all detection/protection systems, they require maintenance and adherence to procedures to ensure they work satisfactorily. b)

Hot oil/steam

In some location, such as offshore platform, indirect heating with oil or steam is required by fire code and prudent practice. A steam condensate trap and strainer with block valves and a bypass should be provided to drain the steam condensate. The reboiler should be sized as a heat exchanger using the U values. c)

Electricity

A minimum of three heating coils should be used. Over-temperature protection of the heater elements should be provided by means of at least two thermocouple elements clamped or welded to the heater sheath, located in an area of highest anticipated sheath temperature. Solid state proportional current control on part of the load is preferred to on-off control. d)

Turbine or engine exhaust

The use of exhaust gases form gas turbines and engines as the heating medium achieves substantial energy savings. e)

Furnace flue gas

For onshore locations, all flash gas and condensate may be incinerated in a furnace and the flue gas used as a reboiler heat source. This may be the most favourable alternative when it is necessary to dispose of all waste hydrocarbon streams.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 38 (90)

It is generally environmentally favourable to maximise the lean glycol concentration by using high reboiler temperatures since this will minimise the circulation rate or the volume of stripping gas required. HEAT DUTY The following maximum flux rates should not be exceeded at normal operating conditions:

Electric

12.5 kW/m

Direct fired

19 kW/m

Steam

24 kW/m

Hot oil

24 kW/m

2

2 2 2

For an example refer to Appendix II. The reboiler should typically be sized in compliance with PRG.PR.HEB.0001. •

Still Column

The column shall be located on top of the reboiler with a flanged connection. A direct acting PSV set at the design pressure of the regenerator should be installed on the vapour line between the still column and overhead condenser. The vapour line should be able to withstand full vacuum. Trays, structured packing and random packing may be used in still columns. Random packing is recommended in case of the small column size. With the larger surface areas recommended on the lean/rich glycol heat exchangers, some still feed vaporisation will occur. Feed piping to the still column should be designed for this two-phase flow to prevent surging which could upset the still operation, possibly causing flooding and glycol carry-over.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 39 (90)

DESIGN METHODS Column design for random packings, such as pall rings, is typically based on curves in Fig. 9.

Fig. 9 – Pressure Drop and Flooding Correlation

These curves give the generalised pressure drop correlation for packed column design. Packed columns are usually designed for 60 to 85 % of the flood point. However, for glycol units, care should be exercised in determining the appropriate mass flow rates, ml and mg. Due to the unsteady flows encountered in glycol units together with potential for overloads at start-up, still columns are usually designed at the lower end of the range, or about 60 % of the flood point. Columns designed in this manner are consistent with the sizes recommended by Manufacturers for onshore applications, see Table 5.

Form code: MDT.GG.QUA.0516 Sht. 01/Rev. 1.94 Cod.file: CRIDESBI.DOT Data File: PRG_PR_GAS_0001_E.doc CONFIDENTIAL document. Sole property of Saipem. Not to be shown to Thid parties or used for purposes other than those for which it has been sent.

PRG.PR.GAS.0001

Rev. 0

Date

Jan 2010

Sheet 40 (90)

Since regenerators usually operate below the design rate, this conservative design will result in a low velocity and a reduction of column efficiency. The wide difference between the boiling points of glycol and water makes separation easy even with a relatively short column, with a minimum of reflux and with reduced packing efficiencies. PACKING TYPES Table 5 gives recommended pall ring sizes and associated data for a range of still column diameters.

Column Diameter

Recommended pall ring size

Density [kg/m3]

HETP

[mm]

Packing Factor [m-1]

[mm]