Hydrocarbon Processing - December 2020 - GP-19

SPECIAL FOCUS: PLANT DESIGN AND
PROCESS CONTROL

Startup troubleshooting of amine
units in LNG export facilities
D. ENGEL, C. RIDGE, and S. WILLIAMS, Nexo Solutions, Amine Optimization Division,
The Woodlands, Texas

The production of LNG has gained
considerable importance in the past few
years because of the energy value contained in natural gas, the ability for export and its lower environmental impact
in terms of emissions. In addition, natural
gas is seen as a bridge fuel to cleaner energy sources.
LNG production (FIG. 1) begins with
inlet separation of contaminants from the
feed gas stream, followed by H2S and CO2
removal, water removal, mercury removal
and higher-molecular-weight hydrocarbon removal. The gas stream, which is
predominantly methane, is then liquefied
at -160°C in an aluminum cryogenic heat
exchanger. The liquefied product is normally transferred and stored in specialized LNG tanks onsite or on ships prior
to delivery to the final destination. All
stages of the LNG production process are
critical, but the removal of inlet contaminants is one the most important stages as
it can affect the entire LNG production
sequence at the facility.
Plant startup. Starting up a gas plant,
whether for the first time or after a
planned or unplanned shutdown, can be
a complex (and, in some cases, cumbersome) process that requires many tasks to
be performed in a coordinated manner. It
is important that a comprehensive procedure is in place and that everyone involved
has a shared understanding of the necessary responsibilities.
In a conventional gas plant, several steps
must be performed in a cautious manner.
Any vessels that were opened during shutdown must be purged to remove air prior
to startup. Any process with a heated liquid recycle stream, such as an amine or glycol unit, must be started up and brought in
line with normal operating temperatures.
Compressors must be started in recycle

FIG. 1. Simplified block process flow diagram of an LNG production plant.

mode and ramped up slowly. In many
situations, systems like these are kept in
recycle mode through short shutdowns
to avoid longer startup times. When gas is
brought in, the process should be opened
one valve at a time to make sure that all
valves were closed appropriately and that
no leaks or failures have occurred. In plants
with liquids processing, that portion of the
plant must be started up slowly as the front
end of the plant is brought online.
Relative to a conventional natural gas
processing plant, LNG facilities are even
more complex and difficult to start up due
to the time required and the sensitivity of
the process operations (FIG. 2). While a
conventional gas plant can be restarted after a shutdown in a few hours, LNG facilities can require 36 hr or more to return to
LNG producing conditions. The primary
issue with getting LNG plants running is
the very low temperatures required to liquefy natural gas. Even with the front end
of the plant running normally, with properly conditioned gas feeding the liquefaction process, the plant may need to recycle
gas continuously for a day or more before
LNG can be produced and stored.

It takes time for these low temperatures
to be reached because the main cryogenic
heat exchanger (MCHE) must be cooled
using mixed refrigerant, and the precooling systems in place (such as propane chilling systems) must be functioning properly
to deliver feed gas at an adequately low
temperature. Before the cooldown process
can take place, the system must be inert
with feed gas or nitrogen, and the system
must be warmed to remove any moisture.
After the proper conditions are in place
at the MCHE and the front end of the
plant, MCHE cooling can take place by
initially mixing precooled feed gas with
warm feed gas to slowly bring the MCHE
and mixed refrigerant systems to precooler
temperature. The system must be cooled
slowly to avoid thermally stressing the system, and it can take up to 12 hr to reach
precooler temperatures. After the MCHE
reaches precooler temperature, mixed refrigerant flow can be started. Once the
mixed refrigerant begins to liquefy, feed
flow to the MCHE can commence. The
gas exiting the MCHE will not be immediately liquefied, so the effluent must be
recycled, flared or sent elsewhere until

Gas Processing & LNG | NOVEMBER/DECEMBER 2020

Hydrocarbon Processing - December 2020

Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2020