51
T
hemain cryogenic heat exchanger is critical to the operation of an LNG train. The refrigerant
used in this heat exchanger undergoes multiple stages of compression in its cycle. The low
pressuremixed refrigerant (MR) suction drum receives refrigerant vapour flow from the
main cryogenic heat exchanger and separates entrained liquids to protect theMR compressor from
damage. Operationally, the low pressureMR suction drum is characterised by high gas volume with
low liquid flow (typically less than 1%by mass). The high gas flow drives the size requirements of the
vessel and, as a result, these vessels tend to be quite large. As such, the low pressureMR suction
drum is a good candidate for optimisation through the careful selection of process internals.
Consider the design basis outlined in the ‘Low pressureMR suction knock out’ sidebar in this
article (p. 52). The resultant vessel diameter is nearly 34 ft.
High capacity double pocket vane
Sulzer’s Mellachevron
TM
H45D vane-typemist eliminator is capable of stable operation at
significantly higher gas load factors than knittedmesh pads, but it must be installed vertically, with
horizontal gas flow. For this application, the Souders-Brown k-value can be considered as 1.27 ft/sec.
This value is specific to the company’s mist eliminator, under the process conditions given in the ‘Low
pressureMR suction knock out’ sidebar. Substitution of the new k-value into these equations results
in a design velocity of 13.9 ft/sec. and a required flow area of 200 ft
2
. This approach reduces the
required flow area by a factor of 4.5. Note that this is the open flow area of the vane pack. This vane
will be contained in a housing structure that will block a portion of the perimeter. The total area will
be larger, in order to provide the required flow area.
John Brewer, Sulzer Chemtech, USA,
outlines the benefits of using
mist eliminators in low pressure mixed refrigerant suction drums.
