Chemical Engineering September 2014 - 46

Cover Story
Feature Report
Part 1
Modern Water-Treatment
Challenges
Heavy
hydrocarbon
recovery unit
Acid gas
scrubber
LNG and other facilities that are expanding
due to the shale-gas boom face specific
challenges when it comes to ensuring the
purity of the inlet and outlet water
Brad Buecker and Brian Clarke
Kiewit Power Engineers
W
ater and wastewater treatment
are extremely important
at liquefied natural
gas (LNG) facilities
and other industries that are blossoming
due to the shale-gas boom.
Engineering concerns are similar
to those of other industrial watertreatment
and power-generation
systems, but with the added issue
of adequate treatment for entrained
hydrocarbons. With proper system
design and operation, plant personnel
can plan for, and react to, issues
that could otherwise cause unit
shutdown, environmental hazards
or safety problems.
The enormous growth in shalegas
production has led to an expansion
in the number of LNG distribution
facilities in the U.S. These
facilities require power and steam
to compress and cool incoming natural
gas, and also to scrub the gas
of impurities that would otherwise
interfere with the liquefaction process.
Economically, it is often logical
to use a portion of the incoming
natural gas as a source of energy for
onsite combined-cycle power and
steam generation.
However, a number of specific
engineering challenges confront
personnel who must work with
makeup water, process water and
wastewater treatment for such
complex and integrated facilities.
Dehydration
unit
Raw
gas
Pre-cooling and
liquefaction units
Nitrogen
stripper
LNG
storage
Figure 1. The major unit operations involved in the treatment
of natural gas for conversion to LNG are shown here.
In order, the processes remove acid gases and sulfur, water,
heavy hydrocarbons and nitrogen
Many of the water-treatment processes
discussed in this article are
also applicable to other industries
that are benefitting from the shale
gas boom, such as production facilities
for ammonia and urea, ethylene
and other petrochemicals.
Processing natural gas
Natural gas is primarily composed
of methane (CH4), but the methane
content is never 100%. Rather, impurities
typically exist in most natural
gas supplies (Table 1).
Ethane, heavier hydrocarbons
and impurities must be removed to
ensure successful production and
delivery of the required end product.
The impurities will negatively
impact the liquefaction process, and
some may cause corrosion in the
equipment. Figure 1 shows many
of the fundamental unit processes
that are used to treat natural gas
prior to LNG production.
The acid-gas scrubber typically
uses an aqueous amine solution
to remove carbon dioxide and sulfur
compounds. This is followed by
dehydration to remove water that
would otherwise freeze during liquefaction.
Next is a heavy-hydrocarbon-recovery
unit (HRU), which
extracts ethane and other higher
hydrocarbons (these are recovered
for further processing). It should
be noted that ethane is becoming
an increasingly popular feedstock
for ethylene production, with a new
technology - " oxidative dehydroge46
ChemiCal engineering www.Chemengonline.Com September 2014
nation " - gaining interest as an alternative
to standard cracking [2].
The remaining steps in LNG preparation
are liquefaction and nitrogen
stripping to produce the final liquid
product for storage.
The waste- and product-recovery
streams from the LNG-production
facility are typically treated in
various heat exchangers and other
unit operations. Among the several
water and wastewater streams
around the plant, the following are
very likely:
* High-purity makeup water for
the power- and steam-production
units. A water-cooled thermal oxidizer
may also be present to destroy
some organics
* Makeup water for the amine solution
for acid-gas scrubbing
* Recovered water from the dehydration
unit
* Returned condensate from other
unit processes, including heavyhydrocarbon
recovery
* Makeup water for a possible cooling
tower
Treating each of these streams presents
new challenges beyond those of
standard water purification.
Makeup water treatment. Common
for new LNG plants and related
facilities, such as petroleum
refineries and petrochemical plants,
is high-pressure steam generation,
often with auxiliary power production.
Ensuring proper makeup
water treatment for this process is
critical [3]. For high-pressure steam
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Chemical Engineering September 2014

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