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DEHYDRATION

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2The processdehydration

What you should get out of this course.

The purpose of dehydration

Hydrate formation

The composition of the gas

Principles

The different processes

dehydration by glycol absorption dehydration by methanol absorption

dehydration by adsorption

Diagram and location in the process

Operating a dehydration installation Problems encountered

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IntroductionWhen the gas is at reservoir temperature and pressure, it is

generally saturated with water.

Water is responsible for:

most types of corrosion when it is associated

- with acid gases (H2S and CO2)

- or salts (calcium carbonates)

hydrate formation

The gas therefore has to be processed to remove the water it

contains.

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Purpose:

Treat to obtain hydrocarbons which meet the specifications

Optimise recovery of the hydrocarbons

Discharge the non-marketable effluents

Protect persons and installations Facilitate transport in the pipelines (corrosion)

Prevent corrosion problems in the lift gas or injection gas

systems.

Prevent hydrate formation

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Hydrate formation

Definition:Crystal structures with a set geometry that require the presence of

water and components present in hydrocarbons, for their formation.

Hydrates are a major problem in the production and transport of natural

gas.

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Hydrate formation

How do they form? Presence of l iquid w ater

Example: liquid water released by the gas during a change in the

pressure or temperature conditions

Presence of l ight h ydrocarbons

Only the first four hydrocarbons (methane, ethane, propane, butane)

are likely to form hydrates in presence of liquid water (risk of hydrates

in the presence of CO2or dH2S)

Favourable condit io ns: temperature and p ressure

Hydrate formation conditions: pressure must be sufficiently high and

its temperature sufficiently low.

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Hydrate formation

How do they form? Certain factors contribute to hydrate formation

Vortices

Gas velocity

Bends, orifices, changes

High pressure

Self-amplifying effects

Low temperature

Each natural gas has its own specific hydrate formation range, which

depends on:

the density of the gas in relation to air (KATZ method)

its composition and solid-vapour equilibrium factors at P and T forthe most precise methods (CARSON and KATZ method).

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Hydrate formation

How do they form?

The area where T < T1, is governed by the hydrate formation

curve.

The area where

T > T1,

is governed by thedew point curve.

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Hydrate formation

Why is it a problem? Hydrate formation leads to:

blocking of the pipes and equipment

production shutdown

risks of overpressure in the installations.

water deposition due to condensation in the pipes or free water

from the reservoir may cause large pressure drops with risks of:

"water hammer" effects due to the liquid slugs

erosion

Water is responsible for most types of corrosion when it is

associated with

acid gases (H2S and CO2)

or salts (calcium carbonates).

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Hydrate formation

Why is it a problem ?

If a pipe becomes blocked by hydrates, the hydrate

block adheres so strongly to the pipe walls and is so hard that it

cannot be removed by any normal mechanical means.

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The gas composition

Commercial gas H2S content: 1.5 to 4 ppm

Total sulphur and contaminants: 50 to 150 mg/Sm3

CO2content: 2 to 3% molar mass

Water dew point: - 15C at 70 bar

Hydrocarbon dew point: - 2C at 70 bar

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The gas composition

examples of different natural gas compositions

Composition (% volume)

N2 1.50

H2O 1.00

H2S 15.30

CO2 9.30

C1 68.00

C2 3.00

C3 0.90

C4 0.50

C5 0.20

C6+ 0.30

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Principles

This solution consists of moving the hydrate formation curve

outside the facility's operating range. To achieve this, several

solutions are at our disposal.

Displacing the hydrate curve

inhibition by glycol or methanol. Case of uses considered:

inhibition by non-recoverable

methanol (without regeneration)

inhibition by methanol,

regenerated for re-use inhibiting with regenerated

diethylene glycol

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Principles

Displacing the operating range Maintain pressure

Increase temperature

reheaters

upstream

heat insulation

for the short pipes

Scope of application

short onshore gas

gathering systems.

heating upstream of the expansion nozzles (in certain cases).

acid gases or gases with non-negligible CO2 content.

Not recommended in the following cases

offshore high heating power

long distances.

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Principles

Displacement of the dew point curve

To avoid water condensation

in the dehydrating units

operating range by sufficient

gas dehydration.

Scope of application long distance transport of gas at

commercial specifications.

offshore: large subsea lines

carrying gas containing CO2 (corrosion)

upstream of the cooler units.

Contre-indications

short gas gathering lines.

short offshore inter-platform links.

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Principles

Displacement of the dew point curve

Advantages

no water condensation

no corrosion when there is H2S and/or CO2

good reliability

dew points obtained at commercial sales standards (-15 / -20C at

70 bar).

Disadvantages

relatively complex to install (investment)

safety problems if bare flame glycol reboiler used.

footprint not negligible (offshore)

continuous monitoring preferable.

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dehydration by glycol absorption

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dehydration by glycol absorption (cont'd)

Principle:

Absorption section

- The glycol absorbs water

- The gas circulates from

bottom to top- the regenerated glycol

is injected at the top

of the absorber

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dehydration by glycol absorption (cont'd) Principle:

Regeneration section

- water-laden glycol is

drawn off from the flash drum

- series of filters

- glycol flows downthrough the column

- exits the column towards

the reboiler for regeneration

- the water vapour exits

the distillation column inthe reverse direction

- the concentrated glycol

exits the reboiler via

a weir

Vapeur d'eau

Condenseur

de tte

Rebouilleur

Still columnBrleur

Glycol humide

StrippingFuel gas

Gazsec

Colonnede

stripping

Filtre

Stockage

Gazoline

Sparateur gazolineglycol

Glycol sec

Pompe glycol

Glycol

Gazde flash

Vapeur d'eau

Condenseur

de tte

Rebouilleur

Still columnBrleur

Glycol humide

StrippingFuel gas

Gazsec

Colonnede

stripping

Filtre

Stockage

Gazoline

Sparateur gazolineglycol

Glycol sec

Pompe glycol

Glycol

Gazde flash

arie

OW:

te parmi

rni

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dehydration by glycol absorption (cont'd) Performances

most commonly used process

dew point -15 to -20 C at 70 bars

use of TEG preferred (Triethylene glycol)

Scope of application

protection of treatment units by cooling protection of collection systems when there is no salt water

ingress or when there are WKOs at the well head.

protection on medium distance pipes.

subsea wells when there is no salt water ingress.

upstream of long-distance gas lines

protection of downstream lines

upstream of the turboexpander

presence of CO2--> corrosion

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dehydration by glycol absorption (cont'd)Not recommended in the following cases :

long lines subject to corrosion, sea lines,

long pipes with many low points (there is a danger of the glycol

being unevenly distributed over the whole of the facility).

production of salt water (contamination by salts from the DEG at

regeneration).

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dehydration by methanol absorption

Inhibition by methanol (not recovered)

Scope of application:

- small installations

- seasonal injection

- small quantity of gas- subsea wells

- short lines

- stand-alone installation

- commissioning after testing

Not recommended in the following cases:

- long lines

- prohibitive quantity to be injected

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dehydration by methanol absorption (cont'd)

Inhibition by methanol (regenerated)

Scope of application:

- developments with subsea wells

- long distances

Not recommended in the following cases:- lines which are impossible to repair

- prohibitive quantity to be injected

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property of certain solids (= desiccants) to fix certain molecules on

their surface.

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The main desiccants are:

Alumina: Good activity but becomes deteriorated by absorbing the

heavy hydrocarbons which are not eliminated by heating.

Silicagels: These are highly active amorphous substances, which are

easy to regenerate and which adsorb the heavy hydrocarbons to a

lesser degree. They are sensitive to liquid water.

Molecular sieves:These consist of zeolite crystals

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dehydration by adsorptionDifferences between the main desiccants:

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dehydration by adsorption (molecular sieve)

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Advantages and disadvantages of the various processes

Inhibition by glycol with regeneration

Advantages:

- low glycol consumption in simple regeneration (little

vaporisation in the gas) )

- no pollution problem (water eliminated during the vapour

phase).

- safe storage (low volatility product).

Disadvantages:

- presence of liquid in the transport facility (injection flow rate

higher than that of the methanol)- corrosion if H2S or CO2 present

- difficulties (or impossibility) to regenerate if salt water present

- gas does not meet the specifications

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Advantages and disadvantages of the different processes Inhibition by methanol (not recovered)

Advantages:

- simple to install

- low investments

- small equipment size

- good reliability

Disadvantages:

- creation of a two-phase flow

- corrosion if H2S or CO2 present

- high operating costs

- methanol supply?

- storage (safety)

- gas does not meet the commercial standards with respect

to water content.

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Advantages and disadvantages of the different processes

Inhibition by methanol with regeneration

Advantages:

- good reliability

- no water discharge

Disadvantages:

- presence of liquid in the lines

- corrosion if H2S / CO2 present

- loss of methanol (50%)

- complex to install

- gas does not meet specifications

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Representation and location in the process

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Location

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Location (Example: Girassol)

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Criticality

If the dehydration unit (TEG) shuts down, the methanol injection

is automatically opened at the column outlet.

If methanol injection is impossible, the following must be stopped:

gas-lift gas injection

which generates a loss of production

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Operating an installation

Absorption section

Parameters governing absorption Concentration of the regenerated glycol

The glycol's purity level depends on:

- The bathtemperature in the reboiler.

The higher the temperature, the more water is released by the

TEG.The limit is set at 204C because the TEG deteriorates above

215 C.

- The operating pressureof the distillation column

Operating below atmospheric pressure generates higher

concentrations at equivalent temperatures.

- The useof a dry gas stripping column.

With the stripping column, a level of 99.9% can be reached

(

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Absorption section

Parameters governing absorption

Gas temperature in the absorber

The dew point at the top of the absorber depends on the

temperature there. A reduction in the gas temperature at the inlet

to the unit reduces the dew point at the outlet.

Glycol circulation rate

- The minimum glycol circulation rate for a good glycol-gas

contact is approximately 15 litres per kg of water to be

removed from the gas.

- Average flow rate of 25 l/kg of water to be removed, for a

conventional installation..

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Absorption section Normal operation

Downgraded operation

Dehydration column by

passed MeOh pump operating

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Regeneration section

Regeneration makes use of the distillation principle by heating

the glycol - water solution in a reboiler whose energy is normally

supplied either by a fire tube, or by electric heating elements

The temperature of the glycol bath in the reboiler must be

maintained at 204C, for example, for the TEG.

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Recirculation system section

Pumps

Pumps are used to circulate the glycol through the regeneration

system

Filtration

The solid particles are stopped by the filters, which prevents them

being drawn into and deposited in the regeneration equipment by

the glycol.

The hydrocarbons present in the glycol are removed with an

activated charcoal filter which prevents foaming problems,

generally due to the presence of corrosion inhibitors, solid

particles, etc. in the crude. pH neutralisation equipment

A chemical injection unit is used to neutralise the pH of the glycol,

which must be maintained at 6 -7 to prevent foaming.

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Problems encountered

Operating problems in the regeneration section

Glycol oxidation

The oxygen, which penetrates into the system through the

atmospheric storage tanks and pump seals, can oxidise the glycol

and form corrosive acids.

The use of a gas atmosphere is recommended in the storage

tanks

Thermal breakdown

An excessive temperature in the reboiler can break down the

glycol and form corrosive products (the TEG decomposition

temperature is 215C). Local overheating may be caused by salt or bitumen deposits on

the fire tubes or heating tubes.

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Controlling the pH

The acidity of the glycol is due to the two points mentioned above

and to the presence of acid compounds in the gas to be treated

(H2S, CO2) which increase the equipment corrosion rate.

The glycol must be maintained at a level of pH = 7 - 8 by injecting

a pH neutraliser

Deposits

Good filtration and activated charcoal treatment prevents the solid

particles and bituminous hydrocarbons from being deposited.

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Operating problems in the regeneration section Foaming

Foaming may increase the glycol losses and reduce the capacity

of the equipment.

he causes of foaming are related to the presence of the following

in the glyco:

- liquid hydrocarbons,- corrosion inhibitors,

- salt,

- fine particles in suspension.

Presence of condensates The liquid hydrocarbons cause the glycol to foam.

They can be eliminated in the flash drum and in the activated

charcoal filters.

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Salt contamination

The salt deposits increase the equipment corrosion rate, and

reduce the heating tube heat transfers.

This salt is transported by a fine water vapour mist, which can be

trapped by demister at the separator.

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Glycol losses The glycol losses increase the operating costs of this type of unit. They

can be caused by:

- Vaporisation

These losses can be limited by sufficiently cooling the gas

upstream of the absorber.- Entrainment

The high points in the column are generally equipped with

internal systems (separator, demister, coalescer) designed to

prevent the glycol being mechanically entrained through the

system.- Mechanical leaks

Mechanical leaks can be reduced by keeping the pumps,

valves and other equipment on the lines correctly maintained

Documents

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