Page 72 - Industrial Plants 2014

Fig. 1 - Block diagram
for the SNG
production standard
process

As well as in all methanation processes, syngas This reaction is highly exothermic, therefore it is
(mainly composed of CO and H , but also H O, CO , carried out across a series of reactors with interstage
2 2 2
CH and N ) must be purified before the methanation heat recovery. Reaction runaway conditions, in
4 2
process, to remove organic contaminants (e.g. tar), methanation process, are the major concern. In the
inorganic contaminants (H S, NH etc.) and VESTA process this risk is avoided by the presence
2 3
particulate matter. This purification is aimed at the of CO that acts as a thermal flywheel and moderates
2
removal of all contaminants with particular attention temperatures. Temperature control is one of the
to the separation from syngas of sulphur (H S or reasons why Foster Wheeler chose to remove the
2
COS), the presence of which leads to the irreversible CO not upstream but downstream of methanation
2
deactivation of the catalysts used in the downstream section. This strategy eliminates the need for
processes [3]. In contrast, in the VESTA process, reaction gasses recycle (solution proposed and
unlike competing technologies, CO removal from used by competitors) and its associated compressor.
2
syngas is not required; the CO can be left in the This choice reduces the high capital and operating
2
process gas, so that it may act as temperature costs associated with the recycle compressor;
moderator, and be removed only after the SNG furthermore, the CO recovered downstream of the
2
production section (figure 2). methanation section could have a higher purity than
Sulphur-free syngas from AGR unit is routed to that recovered from the syngas upstream and may
sweet shift reactor in which the high temperature be used for other industrial purposes.
water gas shift reaction is accomplished on
®
ShiftMax 120 catalyst beds (3): Process description
The heat recovered by the cooling of the reaction is
CO + H O ? CO + H 2 (3) exploited for the production of steam. By thermal
2

2
integration, the steam required for the process
The stream leaving the shift reactor is sent to the (sweet shift reactor) is produced, but a significant
methanation reactors where the reaction (4) takes amount can be exported. The thermal integration is
place over Clariant’s SNG 5000 catalyst beds: designed and optimized in order to meet the
customer’s requirements: the production of medium
CO + 3 H ? CH + H O (4) or high pressure (saturated or superheated) steam
2 4 2
and low pressure steam at the same time, if

Fig. 2 - Conceptual
scheme of the
Foster Wheeler
VESTA process

IndustrIal Plants - May 2014
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