14.gas dehydration

DEHYDRATION

2The process – dehydration

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


The purpose of dehydration

• 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.


The purpose of dehydration

• 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


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.


Hydrate formation

• How do they form? Presence of liquid water Example: liquid water released by the gas during a change in the pressure or temperature conditions

Presence of light hydrocarbons 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 CO2 or d’H2S)

Favourable conditions: temperature and pressure Hydrate formation conditions: pressure must be sufficiently high and its temperature sufficiently low.


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 for

the most precise methods (CARSON and KATZ method).


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 the dew point curve.


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).


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.


The gas composition

• Commercial gas H2S content: 1.5 to 4 ppm

Total sulphur and contaminants: 50 to 150 mg/Sm3 CO2 content: 2 to 3% molar mass

Water dew point: - 15°C at 70 bar Hydrocarbon dew point: - 2°C at 70 bar


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


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


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.


Principles

• Displacing the operating range Advantages

– simple to install and implement.– no water condensation – no corrosion when there is H2S and/or CO2 present in the gas.– low investment costs when no major heat insulation on the

downstream line.– moderate operating costs.

Disadvantages– safety problems if bare flame equipment is used on gas

installations.– footprint and weight not negligible (offshore). high costs when heat

insulation is necessary – need for a reliable fuel gas supply or another source of heat – gas does not meet commercial standards with respect to water

content.


Principles

• Displacement of the dew point curve To avoid water condensation

in the dehydrating unit’s 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.


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 / -20°C at

70 bar). Disadvantages

– relatively complex to install (investment) – safety problems if bare flame glycol reboiler used.– footprint not negligible (offshore) – continuous monitoring preferable.


The different processes

• dehydration by glycol absorption



• 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



• dehydration by glycol absorption (cont'd) Principle:

– Regeneration section

- water-laden glycol is drawn off from the flash drum

- series of filters

- glycol flows down through the column

- exits the column towards the reboiler for regeneration

- the water vapour exits the distillation column in the reverse direction

- the concentrated glycol exits the reboiler via a weir

Vapeur d'eau

Condenseur

de tête

Rebouilleur

Still columnBrûleur

Glycol humide

StrippingFuel gas

Gazsec

Colonnede

stripping

Filtre

Stockage

Gazoline

Séparateur gazoline glycol

Glycol sec

Pompe à glycol

Glycol

Gaz de flash

Vapeur d'eau

Condenseur

de tête

Rebouilleur

Still columnBrûleur

Glycol humide

StrippingFuel gas

Gazsec

Colonnede

stripping

Filtre

Stockage

Gazoline

Séparateur gazoline glycol

Glycol sec

Pompe à glycol

Glycol

Gaz de flash

•Anne-Marie

WAWRZKOW:

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manquante parmi

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•Anne-Marie

WAWRZKOW:

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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



• 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).



• 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



• 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



• dehydration by adsorption

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

their surface.



• dehydration by adsorption

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



• dehydration by adsorptionDifferences between the main desiccants:



• dehydration by adsorption (molecular sieve)



• 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



• 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.



• 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


Representation and location in the process

• REPRESENTATION PFD (Process Flow Diagram):

this document, which is issued during the project phase, shows the main process lines and tanks and their main operating parameters



• Representation P&ID (Piping & Instrumentation Diagram)

This document, which is issued during the project phase, shows all the process lines and tanks and their main operating parameters in a much more complex format than the PFD.



• Location



• Location (Example: Girassol)



• 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


Operating an installation

• Absorption section Parameters governing absorption

– Concentration of the regenerated glycol The glycol's purity level depends on:

- The bath temperature in the reboiler.

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

The limit is set at 204°C because the TEG deteriorates above 215 C.

- The operating pressure of the distillation column

Operating below atmospheric pressure generates higher concentrations at equivalent temperatures.

- The use of a dry gas stripping column.

With the stripping column, a level of 99.9% can be reached (<98.7%).



• 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..



• Absorption section Normal operation

Downgraded operation – Dehydration column by

passed – MeOh pump operating



• 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 204°C, for example, for the TEG.



• 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.


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 215°C).

– Local overheating may be caused by salt or bitumen deposits on the fire tubes or heating tubes.



• Operating problems in the regeneration section 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.



• 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.



• Operating problems in the regeneration section 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.



• Operating problems in the regeneration section 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

14.gas dehydration

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