Hydrocarbon Processing - December 2020 - GP-39
BACK TO BASICS
The natural gas dehydration process
L. MICUCCI, Siirtec Nigi SpA, Milan, Italy
Natural gas from wells invariably contains humidity. Gas also becomes wet in
the sweetening unit of a gas processing
facility when it comes into contact with
the aqueous solution of a solvent, such
as methyl diethanolamine (MDEA),
used to remove the acid gases contained
in the raw gas.
Water must be removed from natural
gas because its possible condensation
may result in several problems:
* Corrosion of pipelines in the
presence of CO2. CO2 dissolves in
condensed water, forming carbonic
acid (H2CO3), which dissociates in
bicarbonate (HCO3-), carbonate
(CO32-) and hydrogen (H+) ions.
The latter is reduced at the steel
surface to form H2 (2H+ + 2e- →
H2). The two electrons needed
for reducing H+ are donated by
iron (Fe), which dissolves into
the aqueous solution (Fe → Fe2+ +
2e-). For pipeline transportation,
the allowed residual humidity
generally ranges from 1 lb/
MMsft3-7 lb/MMsft3, depending
on the environmental conditions.
* Slugs formation. In two-phase
flow, liquid waves can reach the
top of the pipework encroaching
the gas cross-section. When this
happens, liquid slugs travel in the
same direction as the stream, with
relatively high velocity, albeit lower
than that of the gas. This results in
vibrations and mechanical shocks to
the piping fittings (e.g., bends and
valves). The fittings superimpose
a change to the direction of the
velocity vectors. The change of the
flow direction generates the thrusts
on the piping walls according to the
macroscopic momentum balance.
Since the slugs are randomly
formed, their impingement on
the piping/equipment wall may
result in dangerous vibrations and
mechanical stress on the materials
of construction.
* Hydrates formation. In natural
gas processing, hydrates are hard
crystalline structures wherein water
molecules encompass molecules
of CH4, C2H6, CO2 or H2S, which
are trapped in molecular " cages. "
The water molecules of the reticular
structure are tied to each other
by H2 bonds, and they are tied
to the guest molecules through
van der Waals interactions. These
solid structures are a source of
erosion and plugging. This latter
is particularly troublesome, as
decomposition of hydrates during
removal can generate large volumes
of gas with explosion hazards.
In addition to the above challenges,
water vapor increases natural gas volume
while decreasing heating value. It can also
cause plugging in cryogenic plants, which
call for a water dewpoint specification of
less than -100°C. These issues can be effectively addressed by removing the humidity contained in natural gas by means
of gas dehydration processes. These processes can be grouped into two categories: absorption on glycol and adsorption
on solid sorbent.
Absorption on glycol. The most-used
glycols are diethylene glycol (DEG) and
triethylene glycol (TEG). In the glycol molecules, hydroxyl groups (-OH)
and ether functional groups (-CH2-OCH2-) are present. These groups give
rise to H2 bonds, which explains their remarkable hygroscopicity and their desiccant properties.
Since their molecular weight is greater
than water, the glycol molecules boil at a
comparatively higher temperature. Glycols
can be easily regenerated by distillation. It
Offgas to
disposal
P = 1.1 barg abs
Dry gas
Still
separator
Glycol cooler
Still overhead
condenser
Reach glycol
PIC
Hydrocarbons
Wastewater
Still column
Flash
drum
Flue gas to
atmosphere
Contactor
LIC
Raw gas
Reboiler
Condensate
LIC
Stripping
column
Filters (*)
Wet gas
scrubber
P = 4-6 barg
Lean glycol
Condensate
surge drum
Water
Burner
TIC
Fuel gas
Set point max 204°C
for TEG
Lean/rich
heat exchanger
(*) Particle filter and active carbon filter
High-pressure pump
FIG. 1. The TEG dehydration process.
Gas Processing & LNG | NOVEMBER/DECEMBER 2020
39
Hydrocarbon Processing - December 2020
Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2020
Contents
Hydrocarbon Processing - December 2020 - Intro
Hydrocarbon Processing - December 2020 - Cover1
Hydrocarbon Processing - December 2020 - Cover2
Hydrocarbon Processing - December 2020 - Contents
Hydrocarbon Processing - December 2020 - 4
Hydrocarbon Processing - December 2020 - 5
Hydrocarbon Processing - December 2020 - 6
Hydrocarbon Processing - December 2020 - 7
Hydrocarbon Processing - December 2020 - 8
Hydrocarbon Processing - December 2020 - 9
Hydrocarbon Processing - December 2020 - 10
Hydrocarbon Processing - December 2020 - 11
Hydrocarbon Processing - December 2020 - 12
Hydrocarbon Processing - December 2020 - 13
Hydrocarbon Processing - December 2020 - 14
Hydrocarbon Processing - December 2020 - 15
Hydrocarbon Processing - December 2020 - 16
Hydrocarbon Processing - December 2020 - 17
Hydrocarbon Processing - December 2020 - 18
Hydrocarbon Processing - December 2020 - 19
Hydrocarbon Processing - December 2020 - 20
Hydrocarbon Processing - December 2020 - 21
Hydrocarbon Processing - December 2020 - 22
Hydrocarbon Processing - December 2020 - 23
Hydrocarbon Processing - December 2020 - 24
Hydrocarbon Processing - December 2020 - 25
Hydrocarbon Processing - December 2020 - 26
Hydrocarbon Processing - December 2020 - 27
Hydrocarbon Processing - December 2020 - 28
Hydrocarbon Processing - December 2020 - 29
Hydrocarbon Processing - December 2020 - 30
Hydrocarbon Processing - December 2020 - 31
Hydrocarbon Processing - December 2020 - 32
Hydrocarbon Processing - December 2020 - 33
Hydrocarbon Processing - December 2020 - 34
Hydrocarbon Processing - December 2020 - 35
Hydrocarbon Processing - December 2020 - 36
Hydrocarbon Processing - December 2020 - 37
Hydrocarbon Processing - December 2020 - 38
Hydrocarbon Processing - December 2020 - 39
Hydrocarbon Processing - December 2020 - 40
Hydrocarbon Processing - December 2020 - 41
Hydrocarbon Processing - December 2020 - 42
Hydrocarbon Processing - December 2020 - 43
Hydrocarbon Processing - December 2020 - 44
Hydrocarbon Processing - December 2020 - 45
Hydrocarbon Processing - December 2020 - 46
Hydrocarbon Processing - December 2020 - 47
Hydrocarbon Processing - December 2020 - 48
Hydrocarbon Processing - December 2020 - 49
Hydrocarbon Processing - December 2020 - 50
Hydrocarbon Processing - December 2020 - 51
Hydrocarbon Processing - December 2020 - 52
Hydrocarbon Processing - December 2020 - 53
Hydrocarbon Processing - December 2020 - 54
Hydrocarbon Processing - December 2020 - 55
Hydrocarbon Processing - December 2020 - 56
Hydrocarbon Processing - December 2020 - 57
Hydrocarbon Processing - December 2020 - 58
Hydrocarbon Processing - December 2020 - 59
Hydrocarbon Processing - December 2020 - 60
Hydrocarbon Processing - December 2020 - 61
Hydrocarbon Processing - December 2020 - 62
Hydrocarbon Processing - December 2020 - 63
Hydrocarbon Processing - December 2020 - 64
Hydrocarbon Processing - December 2020 - 65
Hydrocarbon Processing - December 2020 - 66
Hydrocarbon Processing - December 2020 - 67
Hydrocarbon Processing - December 2020 - 68
Hydrocarbon Processing - December 2020 - 69
Hydrocarbon Processing - December 2020 - 70
Hydrocarbon Processing - December 2020 - 71
Hydrocarbon Processing - December 2020 - 72
Hydrocarbon Processing - December 2020 - 73
Hydrocarbon Processing - December 2020 - 74
Hydrocarbon Processing - December 2020 - 75
Hydrocarbon Processing - December 2020 - 76
Hydrocarbon Processing - December 2020 - 77
Hydrocarbon Processing - December 2020 - 78
Hydrocarbon Processing - December 2020 - 79
Hydrocarbon Processing - December 2020 - 80
Hydrocarbon Processing - December 2020 - 80A
Hydrocarbon Processing - December 2020 - 80B
Hydrocarbon Processing - December 2020 - 81
Hydrocarbon Processing - December 2020 - 82
Hydrocarbon Processing - December 2020 - Cover3
Hydrocarbon Processing - December 2020 - Cover4
Hydrocarbon Processing - December 2020 - GP-1
Hydrocarbon Processing - December 2020 - GP-2
Hydrocarbon Processing - December 2020 - GP-3
Hydrocarbon Processing - December 2020 - GP-4
Hydrocarbon Processing - December 2020 - GP-5
Hydrocarbon Processing - December 2020 - GP-6
Hydrocarbon Processing - December 2020 - GP-7
Hydrocarbon Processing - December 2020 - GP-8
Hydrocarbon Processing - December 2020 - GP-9
Hydrocarbon Processing - December 2020 - GP-10
Hydrocarbon Processing - December 2020 - GP-11
Hydrocarbon Processing - December 2020 - GP-12
Hydrocarbon Processing - December 2020 - GP-13
Hydrocarbon Processing - December 2020 - GP-14
Hydrocarbon Processing - December 2020 - GP-15
Hydrocarbon Processing - December 2020 - GP-16
Hydrocarbon Processing - December 2020 - GP-17
Hydrocarbon Processing - December 2020 - GP-18
Hydrocarbon Processing - December 2020 - GP-19
Hydrocarbon Processing - December 2020 - GP-20
Hydrocarbon Processing - December 2020 - GP-21
Hydrocarbon Processing - December 2020 - GP-22
Hydrocarbon Processing - December 2020 - GP-23
Hydrocarbon Processing - December 2020 - GP-24
Hydrocarbon Processing - December 2020 - GP-25
Hydrocarbon Processing - December 2020 - GP-26
Hydrocarbon Processing - December 2020 - GP-27
Hydrocarbon Processing - December 2020 - GP-28
Hydrocarbon Processing - December 2020 - GP-29
Hydrocarbon Processing - December 2020 - GP-30
Hydrocarbon Processing - December 2020 - GP-31
Hydrocarbon Processing - December 2020 - GP-32
Hydrocarbon Processing - December 2020 - GP-33
Hydrocarbon Processing - December 2020 - GP-34
Hydrocarbon Processing - December 2020 - GP-35
Hydrocarbon Processing - December 2020 - GP-36
Hydrocarbon Processing - December 2020 - GP-37
Hydrocarbon Processing - December 2020 - GP-38
Hydrocarbon Processing - December 2020 - GP-39
Hydrocarbon Processing - December 2020 - GP-40
Hydrocarbon Processing - December 2020 - GP-41
Hydrocarbon Processing - December 2020 - GP-42
Hydrocarbon Processing - December 2020 - GP-43
Hydrocarbon Processing - December 2020 - GP-44
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